فهرست مطالب

فیزیک زمین و فضا - سال چهل و چهارم شماره 1 (بهار 1397)
  • سال چهل و چهارم شماره 1 (بهار 1397)
  • تاریخ انتشار: 1397/02/09
  • تعداد عناوین: 15
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  • مصطفی موسی پور یاسوری، وحید ابراهیم زاده اردستانی * صفحات 1-20
    تخمین عمق ساختارهای زمین شناسی یکی از مهم ترین اهداف مطالعات ژئوفیزیکی است. واهمامیخت اویلر (اویلر استاندارد) یکی از روش های معروف و پرکاربرد در تخمین عمق است. بر پایه اویلر استاندارد روش های متنوعی برای کاهش خطای تخمین عمق معرفی شده است. در این پژوهش از روش جدیدی به نام اویلر RDAS استفاده شده است. این روش با استفاده از اویلر استاندارد به دست می آید و بر پایه گرادیان اول قائم و مشتقات سیگنال تحلیلی استوار است. بررسی داده های گرانی مصنوعی نشان می دهد که اویلر RDAS، در تخمین عمق این داده ها، خطای کمتری از اویلر استاندارد دارد. همچنین از این دو روش برای تخمین عمق داده های گرانی ناشی از توده هماتیت، واقع در استان کرمان، استفاده شده است. جواب های اویلر RDAS در مقایسه با اویلر استاندارد انطباق بیشتری با مرز آنومالی ها دارد و همچنین برای هر آنومالی، جواب ها در بازه قائم کوچک تری قرار دارند که می تواند ملاکی برای دقیق تر بودن جواب های اویلر RDAS باشد. برای بررسی بیشتر با استفاده از وارون سازی به روش کاماچو داده های گرانی معدن هماتیت مدل سازی شده است. افزون بر این، نتایج مدل سازی با نتایج تخمین عمق اویلر مقایسه شده است. در این مقایسه 10 نقطه برروی آنومالی ها مشخص شده و عمق دقیق آنومالی ها برای این نقاط نشان داده شده است. خطای میانگین مجذور مربعات بین جواب های اویلر و مدل سازی محاسبه شده است. این خطا، بین اویلر RDAS و مدل سازی، کمترین مقدار است که نشان می دهد جواب های اویلر RDAS به جواب های مدل سازی نزدیک است. بنابراین می توان گفت جواب های اویلر RDAS و مدل سازی از اویلر استاندارد دقیق تر است.
    کلیدواژگان: اویلر استاندارد، تخمین عمق، داده های گرانی، سیگنال تحلیلی، ضریب ساختاری، واهمامیخت اویلر
  • محمدهادی رضایی، ناصر خاجی * صفحات 21-38
    ارتباط تنگاتنگ طراحی سازه ها در برابر زلزله با شناخت خصوصیات لرزه ای محل ساختگاه و برآورد زلزله طراحی در ساختگاه مورد نظر، دانش مهندسی را بر آن داشته تا گام در عرصه شناخت زلزله و عوامل ایجاد کننده آن گذارد و دانش لرزه شناسی را با دیدگاه ها و نیازهای مهندسی همگام سازد. بی شک رسیدن به پیش بینی قابل اطمینان از حرکات زمین، ناشی از وقوع زلزله در یک ساختگاه مشخص، بدون داشتن شناخت صحیح از سازوکار تولید امواج لرزه ای، عوامل ساختاری اثرگذار بر این امواج در مسیر انتشار و شناخت شرایط فیزیکی و ویژگی های ساختاری محل ساخته شدن سازه ها میسر نخواهد بود. مدل حائل های ویژه که از مشهورترین مدل های شبیه سازی زلزله است، گسل را به عنوان مجموعه ای از ترک های دایره ای در نظر می گیرد. گسیختگی به صورت افت تنش های موضعی در این ترک ها فرض می شود و توقف ها و شروع های مکرر گسیختگی، عامل اصلی تولید امواج فرکانس بالا در این مدل است. استفاده از دوایر یکسان و نیز استفاده از توابع چگالی احتمال با توزیع یکنواخت که به منظور ساده سازی های اولیه در مدل اولیه استفاده شده است، با خاصیت ذاتی زلزله مبنی بر تصادفی بودن این رخداد مغایرت دارد. از این رو، در این مطالعه سعی شده با پیشنهاد روش جدید استخراج توابع چگالی احتمال رسید امواج لرزه ای، توابع زمانی متناسب با موقعیت هر ایستگاه تولید شود. روند به دست آمدن توابع چگالی احتمال یاد شده مبتنی بر هندسه گسل و نیز موقعیت مکانی ایستگاه های گیرنده امواج می باشد که باعث می شود طیف های چشمه به دست آمده از شبیه سازی به واقعیت نزدیک تر شوند.
    کلیدواژگان: گسلش، مدل سینماتیکی زلزله، مدل حائل ویژه، طیف چشمه زلزله، توابع زمانی
  • سعید حاجی آقاجانی، یزدان عامریان * صفحات 39-52
    در این مقاله به بررسی میزان کارآیی روش نوین ردیابی پرتو سه بعدی (3D Ray tracing) در تصحیح اثر وردسپهر (Troposphere) در تعیین موقعیت مطلق دقیق با استفاده از سیستم های تعیین موقعیت جهانی (Global Positioning System: GPS) پرداخته شده است. بدین منظور با انتخاب دو ایستگاه تبریز و ابرکوه در کشور ایران و استفاده از داده های هواشناسی ERA-Interim و مشاهدات فاز (Phase) و کد (Code) ایستگاه های GPS، تصحیحات وردسپهری با استفاده از روش ردیابی پرتو سه بعدی، ردیابی پرتو دو بعدی (2D Ray tracing) و مدل سستامینن (Saastamoinen) محاسبه شد. در ادامه تصحیحات وردسپهری به دست آمده از روش های فوق بر مشاهدات GPS اعمال شده و تعیین موقعیت در این سه حالت انجام گرفت. یک بار نیز با استفاده از نرم افزار Bernese موقعیت دو ایستگاه ابرکوه و تبریز با مجهول در نظر گرفتن تاخیر مربوط به لایه وردسپهر تعیین شد. معیار قرار دادن موقعیت محاسبه شده از نرم افزار Bernese و مقایسه آن با موقعیت به دست آمده از سه روش فوق، نشان دهنده این است که موقعیت به دست آمده از روش ردیابی پرتو سه بعدی در ایستگاه تبریز به اندازه 017/0 متر دقیق تر از موقعیت به دست آمده از روش ردیابی پرتو دو بعدی است و همچنین 049/0 متر دقیق تر از موقعیت به دست آمده در حالت استفاده از مدل سستامینن می باشد. در عین حال در ایستگاه ابرکوه نتایج سه روش تفاوت چندانی ندارند. این موضوع را می توان به تغییرات و اندازه بیشتر بخارآب در ایستگاه تبریز و در نتیجه اهمیت استفاده از روش های نوین و دقیق تصحیح خطای وردسپهری در این گونه مناطق نسبت داد.
    کلیدواژگان: بخارآب، تعیین موقعیت مطلق، ردیابی پرتو، وردسپهر، ERA، Interim
  • مصطفی یاری، مجید نبی بیدهندی *، ظاهر حسین شمالی، لقمان نمکی صفحات 53-69
    در این مقاله برای تصویرسازی هم زمان مجموعه داده های لرزه شکست مرزی و مقاومت ویژه الکتریکی روش وارون سازی توامان دو بعدی تکرار شونده انتخاب شده است. در این الگوریتم، تابع گرادیان های متقاطع به عنوان عامل پیونده دهنده ساختاری به مسئله وارون اضافه شده است. نتایج حاصل از وارون سازی توامان داده های زمان سیر موج S (برشی) و مقاومت ویژه که در امتداد یک پروفیل برداشت شده اند، تشخیص دقیق مرز لایه های تشکیل شده از مواد نرم و سخت را برای ما آسان تر می کند و علاوه بر آن ارتباط ضعیف ساختاری که بین مدل های سرعت موج برشی و مقاومت ویژه وجود دارد را تقویت خواهد کرد. در مدل های بازسازی شده از الگوریتم توامان می توان بسیاری از ناهمگنی های نزدیک سطح (از جمله زون های پر سرعت و کم سرعت) را که در مدل سرعتی حاصل از وارون سازی منفرد پدیدار نمی شوند آشکار کرد. این تکنیک جدید به طور موفقیت آمیزی بر روی داده های مصنوعی (به عنوان آزمون اعتبارسنجی) و همچنین بر روی داده های صحرایی که در سواحل جنوبی ایران اندازه گیری شده ، اعمال شده است. با مقایسه نتایج حاصل از هر دو الگوریتم، مشاهده شد که مدل های به دست آمده از روش وارون سازی توامان نسبت به مدل های حاصل از روش وارون سازی منفرد دارای تشابه ساختاری بهتری هستند؛ بنابراین، این انطباق ساختاری قابل ملاحظه، شناسایی ناهمگنی های کم سرعت و یا پرسرعت را راحت تر می کند.
    کلیدواژگان: وارون سازی توامان، ناهمگنی نزدیک سطح، لرزه شکست مرزی، مقاومت ویژه، تابع گرادیان های متقاطع
  • اسماعیل دودانگه، کاکا شاهدی *، کریم سلیمانی صفحات 71-88
    شرط اصلی استفاده از یک مدل هیدرولوژیک در یک حوضه آبخیز، ارزیابی عملکرد آن مدل است. در تمام معیارهای معمول در هیدرولوژی از جمله ضریب ناش- ساتکلیف (E)، ضریب همبستگی (R)، مجذور میانگین مربعات خطا (RMSE) و شاخص تطابق (d) به مقایسه خروجی مدل با داده های مشاهده ای پرداخته می شود و از توزیع احتمالاتی خروجی مدل و رابطه آن با توزیع مقادیر ورودی غفلت می شود. این مسئله به ویژه زمانی که هدف، استفاده از خروجی مدل در تحلیل فراوانی هیدرولوژیک باشد، حائز اهمیت است. در این مطالعه کاربرد جدیدی از تئوری کوپلا جهت ارزیابی عملکرد مدل در شبیه سازی صحیح رابطه احتمالاتی بارش-رواناب معرفی می شود. بدین منظور مدل هیدرولوژیکی IHACRES انتخاب گردید و جهت شبیه سازی دبی روزانه حوضه آبخیز طالقان به کار رفت. مدل مذکور در دوره پنج ساله 1995-2000 واسنجی شد و سپس جهت شبیه سازی دبی روزانه دوره پنج ساله 2000-2005 به کار رفت. نتایج ارزیابی عملکرد مدل با استفاده از معیارهای ارزیابی عملکرد معمولی از جمله ضریب ناش- ساتکلیف (75/0)، ضریب همبستگی (87/0)، RMSE (2/7) و شاخص تطابق (93/0) بر عملکرد مطلوب مدل دلالت دارد؛ با این حال برازش توابع کوپلا به داده های شبیه سازی شده و مشاهده ای بارش-رواناب نشان داد توابعی با خصوصیات متفاوت به داده های شبیه سازی شده و مشاهده ای برازش یافتند. تابع کوپلا کلایتون و گوس با دارابودن حداقل مقادیر AIC، BIC و LL به ترتیب به داده های مشاهده ای و شبیه سازی شده برازش داده شدند که دلیل این امر ناتوانی مدل در شبیه سازی دبی های حداکثر است که در انتخاب نوع تابع کوپلا نمود پیدا کرده است.
    کلیدواژگان: کوپلا، کلایتون، گوس، مدل IHACRES
  • حجت تابان، نرگس ظهرابی *، علیرضا نیکبخت شهبازی صفحات 89-102
    در این تحقیق تاثیر عدم قطعیت ناشی از مدل های گردش کلی (GCM) مورداستفاده، روش های ریزمقیاس نمایی و همچنین سناریوهای انتشار گازهای گلخانه ایبر رواناب حوضه دزعلیا در دوره 2069-2040 بررسی شد. برای این کار از شبیه سازی دما و بارش حاصل از 10 مدل GCM و دو روش ریزمقیاس کردن و سه سناریوی انتشار (A1B و A2 و B1) و از مدل آماری LARS-WG و روش عامل تغییر استفاده شد و جهت شبیه سازی بارش-رواناب، شبکه های عصبی مصنوعی به کار گرفته شد. ابتدا مدل بارش- رواناب برای دوره پایه 2000-1971 واسنجی و صحت یابی شد. سپس با ریزمقیاس کردن داده های اقلیمی به دو روش عامل تغییر و مدل آماری، 10 مدل منتخب GCM برای منطقه مطالعاتی تعیین شدند. سپس با معرفی جداگانه هریک از آن ها به مدل بارش-رواناب، محدوده تغییرات رواناب حوضه در دوره 2069-2040 تحت سه سناریوی انتشار مشخص شد. نتایج نشان داد که درصد تغییرات درازمدت بارش منطقه در دو روش ریزمقیاس، اختلافی حدود 4/4 درصد دارند و در بیشتر ماه ها درصد میانگین درازمدت بارش حاصل از روش آماری (27/1- درصد) در مقایسه با روش ریزمقیاس عامل تغییر (52/7- درصد) کاهش کمتری دارد. اختلاف درصد تغییرات در رواناب بلندمدت ماهانه شبیه سازی شده طی دو روش ریزمقیاس، 11/5 درصد است. همچنین بیشترین اختلاف در فصل تابستان با 58/30 درصد و در ماه آگوست با 55/78 درصد وجود دارد. دبی میانگین ماهانه حاصل از داده های ریزمقیاس شده با روش آماری، کاهش 63/2 درصدی دارد و برای روش تناسبی این مقدار 66/21 درصد است. نتایج نشان داد که بارش متوسط در بقیه فصول کاهش می یابد؛ رواناب حوضه دزعلیا عدم قطعیت زیادی دارد؛ محدوده درصد تغییرات بارش برای سه سناریوی انتشار متفاوت است و این اختلاف برای ماه های سال، روند یکسانی ندارد. نتایج مقایسه سناریوهای انتشار در استفاده از میانگین 10 مدل اقلیمی نشان داد که روند اختلاف محدوده درصد تغییرات در سه سناریوی انتشار برای ماه های مختلف هماهنگی نزدیکی با یکدیگر داشته است. بررسی توام نشان داد که عدم قطعیت های ناشی از مدل های اقلیمی مختلف به کاررفته در این تحقیق بیش از عدم قطعیت روش های ریزمقیاس نمایی و سناریوهای انتشار است.
    کلیدواژگان: تغییر اقلیم، دزعلیا، ریزمقیاس نمایی، رواناب، سناریوی انتشار، عدم قطعیت
  • الهام قاسمی فر، منوچهر فرج زاده *، یوسف قویدل رحیمی، عباسعلی علی اکبری بیدختی صفحات 103-124
    تاکنون، تحلیل تغییرات فضایی- زمانی ابرناکی بر پایه ویژگی های جغرافیایی(عرض جغرافیایی، توپوگرافی و پوشش گیاهی) در کشور وسیعی نظیر ایران از سوی پژوهشگران آب و هواشناسی بررسی نشده است. هدف پژوهش حاضر فراهم کردن نمای کلی از درصد ابرناکی بر پایه این ویژگی های جغرافیایی طی دوره 2001-2015 (ترا) و 2002-2015 (آکوا) برای صبح هنگام و عصرهنگام است. ارزیابی داده های ماهانه سنجنده مادیس شامل دو ماهواره ترا (صبح هنگام) و آکوا (عصرهنگام) با استفاده از داده های بازتحلیل ERA-interim و ایستگاهی انجام گرفت. نتایج، دقت بیشتر از 80 درصد را برای ماه های سرد سال و صبح هنگام با استفاده از ضرایب تعیین پلونومیال نشان داد، در صورتی که این دقت برای ماه های گرم به ویژه ژوئن و ژوئیه بسیار کم بود. نتایج حاصل از بررسی ماهانه درصد ابرناکی، ابرناکی بیشتر از 65 درصد را برای ماه های سرد به ویژه فوریه و ژانویه و 7 تا 25 درصد را در ماه های ژوئن تا سپتامبر نمایش داده است. نتایج بررسی عرض جغرافیایی نشان داد که با حرکت به سمت عرض های جغرافیایی پایین از مقدار ابرناکی در همه فصول به جز تابستان کاسته شده است. در این فصل نقش سیستم مونسون، این الگو را متفاوت کرد و بیشترین ابرناکی در نواحی جنوب شرق و سواحل شمالی کشور قابل مشاهده بود. وادا شت های توپوگرافی نقش بسیار مهم همرفت دامنه ای را در فصول پاییز و بهار آشکار کرد که بیشترین ابرناکی در محدوده با میانگین ارتفاع 500 تا 1000 متر ثبت شده بود. پوشش گیاهی همبستگی مثبت و ناکاملی (به طور مکانی) را با درصد ابرناکی نشان داد. تغییرات زمانی ابرناکی با استفاده از مقدار انحراف معیار، بیشترین پراکندگی در درصد ابرناکی را در فصل پاییز و در صبح هنگام نشان داد. در تغییرات زمانی ماهانه درصد ابرناکی روند قابل ملاحظه ای دیده نشد و تنها ماه دسامبر بیشترین روند کاهشی سالانه را با مقدار 2 تا 3 درصد ابرناکی، طی دوره مطالعاتی نشان داد.
    کلیدواژگان: ایران، ابرناکی، تغییرات فضایی و زمانی، داده های سنجش از دور، ویژگی های جغرافیایی
  • سارا کرمی، عباس رنجبر سعادت آبادی، علیرضا محب الحجه *، محمد مرادی صفحات 125-146
    امروزه پدیده گردوخاک در بسیاری از مناطق دنیا به ویژه خاورمیانه به یکی از مشکلات بزرگ تبدیل شده است. در این مطالعه، به منظور شبیه سازی انتقال و نهشت ذرات گردوخاک با استفاده از تعدادی از طرحواره های روش حجم متناهی، مدلی ارائه و خروجی هریک از این طرحواره ها برای دو مطالعه موردی بررسی شده است. همچنین به منظور بررسی درستی خروجی ها و ارزیابی نحوه شناسایی چشمه گسیل گردوخاک و گستردگی مناطق تحت پوشش آن، خروجی طرحواره UNO مرتبه دوم به عنوان نمونه با تصاویر ماهواره و خروجی مدل HYSPLIT مقایسه شده است. داده های موردنیاز برای اجرای مدل از جمله داده های باد از خروجی مدل WRF استخراج و برای محاسبه گسیل سطحی گردوخاک از طرحواره GOCART استفاده شده است.
    مقایسه خروجی این مدل با تصاویر ماهواره نشان می دهد که مدل در هر دو توفان گردوخاک مناطق تحت پوشش و چشمه گسیل را به درستی شبیه سازی کرده است. همچنین الگوی گردوخاک به دست آمده توسط طرحواره های مختلف با توجه به اینکه میدان باد و طرحواره گسیل مشابه است، شباهت قابل قبولی با یکدیگر دارند و از مقایسه کمی طرحواره ها نتیجه گیری می شود که طرحواره UNO مرتبه دوم پس از طرحواره پادجریانسو، کمترین زمان اجرا را دارد و در مقایسه با سایر طرحواره های مرتبه اول و دوم دارای کمترین میرایی است. مقایسه کمی خروجی طرحواره ها با خروجی مدل WRF-Chem نشان می دهد که این طرحواره از نظر جذر میانگین مجذور خطا و ضریب همبستگی نیز در مقایسه با سایر طرحواره های مشابه عملکرد بهتری دارد.
    کلیدواژگان: پدیده گردوخاک، حجم متناهی، شبیه سازی، طرحواره، مدل، مقایسه کمی
  • شعیب آب خرابات * صفحات 147-161
    در این پژوهش این فرضیه مورد آزمون قرار گرفت که باد سیستان (باد 120 روزه سیستان)، خصوصیت رودبادهای تراز پایین را داشته و پس از در نوردیدن شرق و جنوب شرق ایران، وارد بخش شمالی دریای عرب شده و در تقابل با بادهای تجاری جنوبی، منطقه ای از همگرایی را به وجود می آورند. در این راستا با اجرای آزمون تحلیل عاملی و خوشه بندی بر روی داده های شبکه ای ارتفاع ژئوپتانسیل تراز 850 هکتوپاسکال، طی دوره 2012-1993 (از اول خرداد تا آخر شهریور)، 5 خوشه شناسایی شد و از این میان در دو الگوی سینوپتیکی وزش این بادها تشخیص داده شد. در ادامه به محاسبه تابع دینامیکی واگرایی افقی در ترازهای 700- 1000 ه.پ و همچنین محاسبه واگرایی افقی نیمرخ قائم جو پرداخته شد. در پایان نیز مشخص گردید که این جریان وزش باد، از خصوصیات رودبادهای تراز پایین برخوردار بوده و هسته اصلی آن غالبا در تراز 850 ه.پ مستقر است و ادامه سرعت آن به ترازهای پایین تر و سطح زمین نیز کشیده می شود که به نام باد سیستان معروف گردیده است. این رودباد پس از گذر از مناطق شرقی ایران، وارد بخش شمالی اقیانوس هند شده و در برخورد با بادهای تجاری جنوبی، منطقه ای از همگرایی را در این منطقه به وجود می آورند و غالبا از سطح زمین تا تراز 750 ه.پ گسترش ارتفاعی دارد. همچنین شدت/ ضعف رودباد تراز پایین سیستان، سبب جابه جایی منطقه همگرایی دریای عرب می شود. به طوری که در زمان شدت رودباد تراز پایین سیستان، منطقه همگرایی نیز در عرض های جغرافیایی جنوبی تر و در زمان ضعف آن، در عرض های شمالی تر مستقر می شود.
    کلیدواژگان: بادهای تجاری، رودباد تراز پایین سیستان، سینوپتیک، منطقه همگرایی دریای عرب
  • مسعود خوش سیما *، رضا امجدی فرد، ساسان زمانی مقدم، سجاد غضنفری نیا صفحات 163-177
    تابش های محیط فضا بر عملکرد و طول عمر قطعات اپتیکی و الکترونیکی محموله ماهواره ها تاثیرگذار است.در این پژوهش، اثرآسیب تابشی دز یونیزان کل بر عملکرد زیرسیستم لیزر محموله لیداریک ماهواره، در مدار 500 کیلومتری، با ماموریت سه ساله، شبیه سازی شده است. شبیه سازی های تابشی بر روی دو قسمت محیط بهره و پمپ با استفاده از برنامه نویسی C++ در جعبه ابزار GEANT4انجام گرفته است. نتایج شبیه سازی ها نشان می دهد دز تابشی دریافت شده در قسمت محیط بهره، در حالت بدون محافظ، برابر با rad 1951 است و این در حالی است که دز القایی با در نظر گرفتن شیلد آلومینیومی با ضخامت2 میلی متر به حدود rad 275 برای محیط بهره و حدود rad 623 برای پمپ لیزر کاهش می یابد. همچنین براساس محاسبات، با در نظرگرفتن مدت زمان ماموریت و مساحت جانبی، حجم حساس به تابش تعداد ذرات برخوردکننده برابر با 1012×7 خواهد بود. نتایج شبیه سازی نشان می دهد اثر تابشی دز یونیزان کل بر بخش لیزر باعث افزایش15 درصدی جریان آستانه و کاهش توان اپتیکی می شود.
    کلیدواژگان: تابش های فضایی، دز یونیزان کل، لیزر، لیدار، محیط بهره
  • سکینه خان سالاری *، علیرضا محب الحجه، فرهنگ احمدی گیوی صفحات 179-198
    رویدادهای بارش سنگین اغلب باعث به وجود آمدن خسارات جانی و مالی جبران ناپذیر می شوند. از این رو توانایی و امکان پیش بینی صحیح وقوع این رویدادها برای سازمان های هشدار سریع کشورهای مختلف بسیار مهم است. کاربست روش جداسازی عامل ها در مطالعه رویدادهای بارش سنگین ضمن اینکه امکان تفکیک و بررسی دینامیکی عوامل وردسپهر زبرین و زیرین را در یک نمونه واقعی می دهد، تخمینی کمی نیز از سهم تعاملی و خالص هریک از عوامل فراهم می سازد. یکی از این عوامل، نفوذ زبانه ای از هوا با مقادیر بزرگ تاوایی پتانسیلی (PV) مثبت، موسوم به کشانه PV، از عرض های بالا درپوشن سپهر به عرض های پایین تر در وردسپهر است که موجب کاهش پایداری ایستایی در وردسپهر و شکل گیری همرفت می شود. بررسی تاثیر این عامل مهم بر میدان های هواشناختی با کاربست فن وارون سازی تاوایی پتانسیلی صورت می گیرد. از دیگر عوامل مهم، اثر کوهستان است که چگونگی این اثر بستگی به آن دارد که جریان هوا به وسیله مانع، سد شود یا از آن عبور کند. در این پژوهش، با استفاده از داده های بازتحلیل ERA-Interim و مدل پیش بینی عددی میان مقیاس WRF، تاثیر چهار عامل شامل بی هنجاری PV ترازهای زبرین (کشانه PV) و میانی وردسپهر و دو رشته کوه البرز و زاگرس در رویداد بارش سنگین برف منطقه تهران در روز ششم ژانویه 2008 مطالعه و ارزیابی شده است. نتایج نشان می دهد که در بارش این روز کشانه PV با 5/76 درصد دارای بیشترین تاثیر بوده و پس از آن سهم مشارکت رشته کوه البرز قرار دارد و اثر رشته کوه زاگرس و بی هنجاری PV تراز میانی وردسپهر ناچیز است.
    کلیدواژگان: بارش سنگین، تاثیر کوهستان، روش جداسازی عامل ها، کشانه تاوایی پتانسیلی، منطقه تهران، وارون سازی تاوایی پتانسیلی
  • سمیه رفعتی *، مصطفی کریمی صفحات 199-214
    در این بررسی همگنی داده های دمای میانگین ماهانه 33 ایستگاه همدید در ایران باکاربرد الگوریتم PMFred مورد آزمون قرار گرفت. روند خطی تغییر، پیش و پس از همگن سازی داده ها، برآورد و تاثیر همگنی داده ها بر نتایج حاصل از روند ارزیابی شد. همچنین به منظور تسهیل در تشخیص معنا داری روند تغییر، مقادیر شیب رگرسیون با تقسیم شدن بر نصف فاصله اطمینان (در این مطالعه %95) استاندارد شدند.
    نتایج نشان داد، به کارگیری یک آزمون مطلق همگنی در صورت نبود فراداده کامل و قابل اطمینان، بدون دخالت اطلاعات حاصل از ایستگاه های دیگر و دیدگاه های کارشناسی ممکن است با خطا همراه باشد. همچنین ناهمگنی یا جهش های مصنوعی در متغیرهای دمای میانگین ماهانه همان طور که انتظار می رود به مقدار زیادی برآوردهای روند خطی را منحرف می کند. بعد از اصلاح ناهمگنی ها مشخص شد روند افزایشی میانگین دمای ماهانه در بیشتر نقاط کشور معنا دار است و در مناطق نسبتا محدودی نیز دما افزایش معنا داری نشان نمی دهد. علاوه بر آن الگویی نواری با جهتی تقریبا شمالی – جنوبی در شدت افزایش دما در ایران قابل مشاهده است.
    کلیدواژگان: الگوی تغییر دما، روند تغییر دما، همگنی داده ها، الگوریتم PMFred
  • سید حبیب الله حسینی، محمد اکبری نسب *، محمدرضا خلیل آبادی صفحات 215-225
    محیط دریا، محیطی تلاطمی است. پدیده های مختلفی همچون امواج درونی، جبهه ها و جزر و مد و غیره که باعث تغییراتی در سرعت شاره در محیط می شوند در نتیجه باعث ایجاد تغییرات در انتشار صوت می شوند. در این مطالعه، با استفاده از مدل MITgcm به صورت دو بعدی، در شرایط غیرهیدرواستاتیک و کاملا غیرخطی شکل گیری این امواج داخلی به واسطه جزر و مد داخلی در حضور تپه در خلیج عمان بررسی شد. سپس با استفاده از خروجی هیدرو فیزیکی مدل (دما و شوری)، در یک دوره جزر و مدی، پارامتر سرعت صوت (رابطه مکنزی) محاسبه گردید. نتایج تاثیر پرتوهای موج داخلی منتشر شده و بازتابیده شده بر پروفایل سرعت صوت را می توان به صورت اغتشاشات بین لایه ای در امتداد این پرتوها نشان داد. برای بررسی اثر فرکانس های مختلف در یک سیکل جزر و مدی، تلفات انتقال آکوستیکی مدل سازی شد و نتایج نشان داد که در فرکانس پایین، انرژی صوتی بیشتر در محیط نفوذ می کند و در فرکانس بالا اثرات زیادی مشاهده نمی شود. نتایج نشان داد که حضور جزر و مد داخلی باعث فشردگی و باز شدگی پرتوهای صوتی در محیط و افزایش برد امواج آکوستیکی در منطقه در برخی زمان ها جزر و مدی می شود.
    کلیدواژگان: امواج درونی ناشی از جزر و مد، تلفات انتقال، دریای عمان، مدل MITgcm
  • سیاوش غلامی، سرمد قادر *، حسن خالقی زواره، پروین غفاریان صفحات 227-243
    در این مقاله عملکرد مدل میان مقیاس پیش بینی وضع هوای WRF با هسته دینامیکی ARW برای شبیه سازی میدان باد در منطقه خلیج فارس تحت شرایط مرزی و اولیه مختلف ارزیابی و بررسی شده است. برای این منظور از سه نوع مجموعه داده ERA-Interim، NCEP-FNL و NCEP-R2 برای تامین شرایط اولیه و مرزی مدل استفاده شده است. سه نوع شبیه سازی مختلف WRF در این مقاله انجام گرفت و برای مقایسه خروجی باد مدل تحت شرایط مرزی و اولیه متفاوت از مشاهدات ایستگاه های همدیدی در محدوده شمالی خلیج فارس، داده های ماهواره QuikSCAT و داده های ماهواره ASCAT استفاده شد. بر اساس ارزیابی های انجام گرفته در این تحقیق هم برای جهت و هم تندی باد مجموعه داده ERA-Interim در مقایسه با NCEP-FNL و NCEP-R2 می تواند شبیه سازی باد نزدیک تر به واقعیت داشته باشد. در رتبه دوم داده های NCEP-FNL قرار دارد که در غیاب ECMWF ERA-Interim می تواند جایگزین مناسبی برای تامین شرایط اولیه و مرزی مدل WRF باشد اما داده بازتحلیل NCEP-R2 خطای زیادی در تخمین باد به خصوص اندازه آن (تندی) ایجاد می کند.
    کلیدواژگان: داده های بازتحلیل، شرایط اولیه، خلیج فارس، میدان باد، مدل WRF
  • حسین نجفی، علیرضا مساح بوانی *، پرویز ایران نژاد، اندرو ویلیام رابرتسون صفحات 245-264
    هدف از این پژوهش، ارزیابی روش تحلیل همبستگی متعارف (CCA)در ارائه پیش بینی های فصلی به صورت مقیاس کاهیشده در یک دوره بلندمدت 30 ساله است. این بررسی در غرب کشور ایران و با استفاده از برونداد بارش سامانه های پیش بینی فصلی همادی آمریکای شمالی انجام شد. بدین منظور، در ابتدا بارش شبکه بندی شده بر اساس اطلاعات سنجش ازدور(PERSIANN-CDR)با داده های ثبت شده از 23 ایستگاه همدیدی ارزیابی شد. ضریب همبستگی PERSIANN-CDRبا داده های ایستگاهی همدیدی بین 7/0 و 95/0 محاسبه شده است. سپس اریب داده های سنجش ازدور به نسبت داده های ایستگاه های همدیدی تصحیح و در انتها هر دو مجموعه داده (سنجش از دور- ایستگاه) تلفیق شدند. از مجموعه داده تلفیق شده به عنوان بارش مرجع در ارزیابی سامانه های پیش بینی فصلی با تفکیک مکانی 1 و 25/0 درجه (برونداد مستقیم و پس از کاربست CCA) استفاده شد. مدل های پیش بینیفصلی به صورت انفرادی و وزن دهی شده (سامانه های همادی متشکل از 2 تا 8 مدل) مورد استفاده قرار گرفت. برای ارزیابی مهارت این مدل ها، معیارهای ارزیابی شامل معیارهای پیوسته و طبقه بندی شده است که در دوره صحت سنجی محاسبه شده است. در این دوره، همبستگی اسپیرمن به عنوان شاخص نیکویی برازش، بیشینه شده است. شاخص های ارزیابی به صورت برونداد مستقیم و تصحیح شدهمقایسه شدند. نتایج نشان می دهد که همه شاخص ها پس از اعمال CCA بهبود می یابند. لذا روش شناسی پیشنهادی در مقیاس کاهی و پس پردازش سامانه های پیش بینی فصلی در محدوده مورد مطالعه کارا است. همچنین، سامانه همادی سه مدلی متشکل از CCSM4، CMC2، CFSv2 دارای مهارت بیشتر در مقایسه با همادی هشت مدلی و سایر مدل های انفرادی است. این سامانه که دارای همبستگی اسپیرمن بیش از 6/0 با داده های مرجع می باشد، به عنوان مدل برتر با بیشترین نیکویی برازش در محدوده مورد مطالعه است. در اکثر محدوده مورد مطالعه، GFDL-aer04و سامانه های همادی چند مدلی توانسته اند در 80 درصد از سال هایی که بارش زیرنرمال اتفاق افتاده، بارش زیرنرمال را به درستی پیش بینی نمایند. یافته های این پژوهش، کاربست روش شناسی پیشنهادی در پیش بینی خشک سالی هواشناسی به صورت زمان واقعی در فصل اکتبر- دسامبر در محدوده غرب کشور ایران را آشکار می سازد.
    کلیدواژگان: مدل های همادی آمریکای شمالی، مقیاس کاهی، حوضه کرخه، همادی چند مدلی، پیش بینی فصلی بارش
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  • Moustafa Mousapour Yasoori, Vahid Ebrahimzadeh Ardestani * Pages 1-20
    Depth estimation of geological structures is one of the most important objectives in geophysical studies. Euler deconvolution (standard Euler) is a well-known method in the depth estimation. Based on standard Euler, various methods are introduced to reduce error of the depth estimation. In this study we have used a new method called Euler RDAS. This method is based on the standard Euler. In this method derivatives of analytic signal and first vertical are used in Euler equation. Applying derivatives of analytic signal for depth estimation is better than the analytic signal. There is no problem of choosing structural index in this method which increases accuracy in the depth estimation. To examine the performance of this method, depths of several synthetic models are estimated and their results are compared to that of the standard Euler. In all models, results of the RDAS Euler shows fewer errors in high depth model in comparison to that of the standard Euler. This method was tested on synthetic data with noise. RDAS Euler sensitive to noise due to usage of high-degree derivatives is less than the standard Euler. Study shows that if the noise in the data is reduced by methods such as filter upward, it can provide an appropriate estimate of the depth. In this study, the depth of gravity anomalies caused by the masses of hematite located in Kerman, has estimated using standard Euler and RDAS Euler. Upward continued by 3 m has been used for reducing noise in this data. There are 3 possible hematite masses in residual map of the study area. The minimum of high depth anomalies indicates masses of hematite, which is calculated using RDAS Euler and was about 5 meters and a maximum of high depth is about 20 meters. The minimum depth obtained using standard Euler for this anomaly is about 5 meters and the maximum depth is about 40 meters. Responses of RDAS Euler is more compatible that of standard Euler with the boundary anomalies and has smaller vertical interval which can be a criterion for more precise solutions of the RDAS Euler. For further examination, the gravity data of hematite mine is used with inverse modeling of Camacho method. Minimum and maximum upper depths obtained for these anomalies are 5 to 35 meters, respectively. In addition, the modeling results is compared with the results of depth estimation of Euler. To this, 10 points in the anomalies area are pointed and the calculated depth of these points using standard Euler, RDAS Euler and modeling are shown. Root mean square error (RMS) between Euler’s and modeling results is calculated. In comparison of the results in different methods, which are not standard, results of two methods that have the lowest RMS error is considered as the selection criteria. RMS for standard Euler with MATLAB code, Geosoft, and RDAS Euler are equal to 11.43, 8.5, and 2.66 respectively. The results of two methods among these three methods which are used to estimate the depth of hematite masses are close hence they can be more reliable results. Therefore, due to fewer errors of RMS of RDAS Euler and modelling results are more accurate than that of the standard Euler.
    Keywords: Euler deconvolution, standard Euler, analytic signal, depth estimation, structural index, gravity data
  • Mohammad Hadi Rezaei, Naser Khaji * Pages 21-38
    A reliable physical modeling of strong ground motion is required to examine the three crucial seismic parameters: seismic source specifications, wave propagation path and seismic site effects, which are all very important in seismic source simulation. Among various seismic source specifications, a more physically based realistic source model is the specific barrier model (SBM). The SBM is specifically more suitable for regions with poor seismological data bank and/or ground motions from large earthquakes with large recurrence intervals. In order to simulate seismic ground motions from a specific earthquake source model in an efficient way, the stochastic modeling method has been widely used. An essential part of the seismological model used in this method is the quantitative description of the far-field spectrum of seismic waves emitted from the seismic source. Since shear wave is one of the main factors of earthquake damages, the application of stochastic approach of the SBM is focused for on the far-field shearwave spectrum, in which two corner frequencies of the observed earthquake are represented. The ‘two-corner-frequency’ shows two considerable length-scales of an earthquake source: a length-scale that quantifies the overall size of the fault that ruptures (e.g., the length of a strike-slip fault) and another length-scale that measures the size of the subevents. Associated with these length-scales are two corresponding time scales: (1) the overall duration of rupture, and (2) the rise time. The SBM has a few main source parameters which have been calibrated by the parameters of earthquakes of different tectonic regions.
    The SBM may be considered as a general idealization of the faulting process of an earthquake. For example, a uniform probability density function (PDF) of ‘arrival times’ is assumed in the SBM. In this paper, the effects of various PDFs of arrival times on the far-field source spectrum of the SBM are studied. For this purpose, direct simulations of ground motion records for an earthquake source, which have fractally-distributed subevent sizes, is used. So, in this research, a new non-uniform more realistic PDF of arrival times for seismic waves corresponding to the fault’s geometry is derived to reach desirable time functions. To this end, the appropriate PDF of arrival time is simply computed by making various zones on the fault, based on their distance from a given receiver on the ground surface. Therefore, a large number of points on the ground surface is chosen as receiver positions, for which the PDFs of arrival time are obtained. To divide the fault to various zones with the same distance from a given receiver, several spheres of the various radius are drawn, whose centers are located on the mentioned receivers. Consequently, a group of different curves is produced by the intersection of different spheres and the fault plane. All points in the region between the semi-parallel adjacent curves are considered to have the same distance from the receiver (the center of various spheres). This means that all points surrounded by two adjacent curves have equal chance to get to the receiver position. As a result, for creating the PDF of the arrival time of seismic waves, the chance of arriving seismic wave in specific time window should be determined, based on the above mentioned simple assumption. By changing distance parameter to time parameter, the PDF of the arrival time of seismic waves may be easily obtained. Afterwards, by using the proposed time functions, the effect of site position to the fault on source spectra, as well as the effect of distance of site to the fault on time functions, are investigated.
    Keywords: Faulting, Kinematic models of earthquake, specific barrier model, source spectra, time function
  • Saeid Haji Aghajany, Yazdan Amerian * Pages 39-52
    Earth's atmosphere has a series of layers, each with its own specific traits. Moving upward from ground level, these layers are named the troposphere, stratosphere, mesosphere, thermosphere and exosphere. The exosphere gradually fades away into the realm of interplanetary space. The troposphere is the lowest layer of our atmosphere. Starting at ground level, it extends upward to about 10 km above sea level. Humans live in the troposphere layer, and nearly all weather occurs in this layer and affects their activities. Ninety nine percent of the water vapor in the atmosphere is found in the troposphere; therefore most clouds appear in this layer. Air pressure and temperature drops in the troposphere with height. The tropospheric path delay is one the main error sources in Global Navigation Satellite System (GNSS) such as Global Positioning System (GPS) observations and reduces the accuracy of GNSS point positioning. Accurate estimation of tropospheric path delay in GNSS signals is necessary for positioning and also its meteorological applications. The tropospheric delay is divided into the dry (hydrostatic) and wet (non-hydrostatic) parts. The dry tropospheric delay depends on the pressure variations between satellite and station on the Earth’s surface and can be determined accurately using experimental models. The wet delay can be determined by subtracting the dry delay from the total GPS derived delay. In this paper the efficiency of 3D ray tracing in increasing the accuracy of point positioning is investigated. The 3D ray tracing technique based on Eikonal equation is the strongest and newest ray tracing method. These equations are solved in order to get the ray path and the optical path length. The Eikonal equation itself is the solution of the so-called Helmholtz equation with respect to electro-magnetic waves. In this method the ray paths are not limited to a certain azimuthally fixed vertical plane. In 2D methods the ray paths are forced to stay within a vertical plane of constant azimuth. European Center for Medium Range Weather Forecasting (ECMWF) is currently publishing ERA-I, a global reanalysis of the meteorological data. This reanalysis provides values of several meteorological parameters on a global gride ∼75 km. The vertical stratification is described on 37 pressure levels. Tropospheric corrections were calculated using 3D ray tracing, 2D ray tracing and Saastamoinen methods in Tabriz and Abarkuh stations using ERA-I meteorological parameters. These corrections were applied to the GPS observations and the stations coordinate were computed. Furthermore, these stations coordinates were determined twice using Bernese GPS processing software, one time the tropospheric delay was not canceled from observations and second time it was considered as unknown parameter and evaluated with stations coordinates. The result of this process was considered as a reference to evaluate the three prescribed correction methods. These comparisons indicate that the correction computed from 3D ray tracing is more efficient than that of 2D ray tracing and Saastamoinen model corrections. Also the correction amount in Tabriz station is meaningful with respect to Abarkuh station, which can be attributed to small variations of water vapor in Abarkuh station.
    Keywords: ERA, Interim, Precise point positioning, Ray tracing, Troposphere, Water vapor
  • Mostafa Yari, Majid Nabi-Bidhendi *, Zaher Hossein Shomali, Loqhman Namaki Pages 53-69
    Establishing the precise relationship between electrical resistivity and seismic shear (S) wave velocity in heterogeneous near-surface materials is a fundamental problem in geophysics and can complement petrophysical measurements for improved subsurface characterization. The relevant data from two-dimensional (2-D) electrical resistivity and seismic refraction investigations of the near-surface have jointly been inverted leading to accurate models. Nevertheless, Joint 2-D resistivity-velocity inversion is a difficult task since there is no established analytical relationship between resistivity and velocity. There are different approaches to 2-D joint inversion of disparate data with varying degrees of success. These can be classified into (1) petrophysical approach and (2) structural (or geometrical) approach. The petrophysical approach are based on the fact that for some specific geological environments, multiple geophysical parameters can be correlated via physical or empirical relationships. In the structural approach, both methods of geophysical are sensing the same underlying geology which in turn structurally controls the distribution of petrophysical properties. In this paper, we select the structural approach and posit that petrophysical information may be derived from the resultant models.
    When there is no special analytical relationship between the physical properties that have been extracted by different geophysical methods, we can estimate the models that there are good structural agreement between the physical properties, by means of joint inversion techniques. Gallardo and Meju (2003) by introducing cross-gradientsfunction that is defined in the form the cross product of the gradients, estimated the structural resemblances between the resulting images from joint inversion. The cross-gradients function is incorporated as a constraint in a nonlinear least squares problem formulation, which is solved using the Lagrange multiplier method. When the value of this function is zero, images of models will completely be similar in structure. Being zero of this function requires that the simultaneous spatial changes of different geophysical models, independent of the amplitude, should be collinear. In term of geology this means that if the changes of the physical properties for both methods are collinear then can characterize boundaries of layers and structures using the images obtained from structure‐coupled inversion.
    For the DC resistivity, forward problem is used by the procedure given by Perez-Flores et al. (2001). In this approach, the resistivity forward calculation is stated as a linear problem, which is based on the nonlinear integral equations for electromagnetic inverse problems of Gomez-Trevino (1987). The resistivity response of the model using the forward codes of Perez-Flores is approximated because of its computational speed of this approach we have employed it in the two dimensional joint inversion procedure. For estimating the first-arrival times from source to receiver, we adopt the approach of pseudo-bending technique (Um and Thurber, 1987). Compared to the earlier bending methods (Julian and Gubbins, 1977), the pseudo-bending technique is much rigorous and faster. For a cell with constant velocity, the Jacobian matrix is simply the length of the linear segment of the ray in the cell in transit, which is calculated impressively by ray tracing through the field of travel times that are generated during the forward modeling process.
    Incorporating auxiliary factors such as, Levenberg-Marquardt (LM) stabilization factor and smoothness constraints in the inverse problem assures convergence and stability of solutions much more and can resolve the nonuniqueness that has been taken place because of data error. Joint two-dimensional (2-D) inversion scheme is used to a test data sample (to validate inversion technique) and also to a field data set that has been recorded along a profile. The obtained results from this method is compared with the conventional separate inversion results and we conclude from this comparison that the joint inversion scheme is more powerful than the traditional separate inversion in illustrating the structural similarities between seismic velocity and resistivity models. As a result, this gained structural conformity of the cross-gradients inversion models can help us in better characterization of heterogeneous near-surface materials.
    Keywords: Joint inversion, near, surface heterogeneity, seismic refraction, resistivity, cross, gradients
  • Esmaeel Dodangeh, Kaka Shahedi *, Karim Soleimani Pages 71-88
    Hydrologic modeling plays an important role in hydrologic response prediction for water resources managements, flood control and soil and water conservation. Performance evaluation is necessary before using a hydrologic model in a watershed. Various traditional evaluation criteria such as Nash-Sutcliff (E), correlation coefficient (R), Root Mean Square Error (RMSE) and index of agreement (d) are commonly used for the performance evaluation of hydrologic models. All these criteria compare the model output with observed data, however they cannot capture all the features reproduced by the hydrologic models such as information content of data and stochastic relationship between rainfall and run-off. This is doubly important when carrying out frequency analysis on the model output. This study aims at introducing a new application of copulas for the performance evaluation of hydrologic models in accurate simulation of stochastic relationship between rainfall and runoff. To this end, IHACRES hydrologic model was selected for daily flow simulation in the Taleghan watershed. The selected model was calibrated for the 5-year period of 1995-2000 and evaluated for 5-year period of 2000-2005. The non-linear module calibration optimized the parameters C, τw and F as 0.002 mm, 22 days and 4 degree Celsius respectively. The linear module parameters was also optimized as τq = 0.02 days, τs = 41.25 days and vs = 1. Performance evaluation of the model via Nash-Sutcliff (E=0.75), Pearson correlation coefficient (R= 0.87), root mean square error (RMSE=7.2) and index of agreement (d=0.93) indicated a good performance of the model for the evaluation period. Although the numerical performance evaluation criteria show satisfying results, however visual inspection of the scatter plot of observed and simulated flow showed that the model significantly underestimated the peak flows in spring. In contrast, the winter flow rate is noticeably overestimated. Regarding the Mediterranean climate of the study area, the greater portion of the winter precipitation falls as snow which piles up in mountains without draining into the river network. With the onset of spring and warming weather, the snow pack melting accompanied with spring precipitations lead to the peak flow generation in spring. As the snowmelt simulation module is not included in the model, the model considered all precipitation as rain in January, February and March. This lead to the overestimation of winter flow rate. On the other hand, the spring peak flows is underestimated because of neglecting the snowmelt runoff. In the next step various copula functions with different tail dependence structure in the upper and lower tail including Archimedean copulas (Clayton, rotated Clayton, Gumbel, rotated Gumbel, Frank) and elliptical copulas (Gaussian, t-copulas) were fitted on observed and simulated rainfall-runoff time series. Various goodness-of-fit test criteria AIC, BIC and Log likelihood (LL) were employed to choose the best fit copula functions for the observed and modeled time series. Results showed that the Clayton copula with the lowest AIC, BIC and LL values (AIC = –236.28, BIC = –236.28 and LL = –118.13) best fitted observed rainfall-runoff time series. Gaussian copula with lowest AIC, BIC and LL values (AIC = –217.08 BIC = –217.08 and LL = –108.54) was also selected as the best fitted copula for modeled rainfall-runoff time series. Nevertheless, the poor performance of the model in simulating the spring peak flows due to snowmelt runoff, the model has been approved by traditional performance evaluation criteria. However fitting the observed and simulated rainfall-runoff time series with Clayton and Gaussian copulas with different tail dependencies indicated inability of the model to properly simulate stochastic relationship between rainfall and runoff.
    Keywords: IHACRES model, Copula, Clayton, Gaussian
  • Hojjat Taban, Narges Zohrabi *, Ali Reza Nikbakht Shahbazi Pages 89-102
    Increase greenhouse gases in the Earth's atmosphere have led to imbalances in the phenomenon of climate over the past decades, defined as Climate Change. Studies show that climate change can have negative effects on water resources, agriculture, environment, health, industry and economy. Global warming and climate change is happening and changing weather and climate volatility is associated with greater risk of damage. Since increasing the likelihood of future climate change could have devastating consequences for human societies, it is essential to examine the drought situation in the future periods in this area. For climate change effects on various resources in the future, climatic variables affected by greenhouse gases should be determined. Different techniques are available to simulate the future climatic variables under climate change effects; the most reliable data is atmospheric general circulation models. GCM models are three-dimensional models of the physical relationships that govern the atmosphere, crysphere, biosphere and hydrosphere. One of the weaknesses of GCM models is large spatial and temporal scales of the climatic variables. Therefore variables regarding hydrological and water resources studies are not sufficiently accurate. It should be downscaled by various techniques. Since different methods are available for downscaling, the uncertainty associated with these methods must be investigated. Various uncertainties affect the final outcome runoff simulation in a basin under the impact of climate change. The credibility of the results by ignoring any of these uncertainties would be reduced.
    In this study, the GCM models uncertainty, methods of downscaling climate models and the SRES emission scenarios over the period 2069-2040 on Dez Ulya basin runoff were examined. For this purpose, the simulated temperature and precipitation of 10 GCM models, including BCM2.0, CGCM3T63, CNRMCM3, CSIROMK3.0, GFDLCM2.0, GISS-ER, HADCM3, INMCM3.0, IPSLCM4, MIROC3.2MEDRES, with two downscaling methods (Change factor and statistical using LARS-WG software) and three emission scenarios (A1B and A2 and B1) and artificial neural network model were used to simulate rainfall-runoff model. LARS-WG (Long Ashton Research Station Weather Generator) is a stochastic weather generator which can be used for the simulation of weather data at a single site, under both current and future climate conditions. These data are in the form of daily time-series for suitahle climate variables, namely, precipitation (mm), maximum and minimum temperature (°C) and solar radiation (MJm-2day-1). Stochastic weather generators were originally developed for two main
    Purposes
    1) To provide means of simulating synthetic weather time-series with statistical characteristics corresponding to the observed statistics at a site, but which were long enough to be used in an assessment of risk in hydrological or agricultural applications.2) To provide means of extending the simulation of weather time-series to unobserved locations, through the interpolation of the weather generator parameters obtained from running the models at neighboring sites. It is worth noting that a stochastic weather generator is not a predictive tool that can be used in weather forecasting, but is simply a means of generating time-series of synthetic weather statistically ‘identical’ to the observations. New interest in local stochastic weather simulation has arisen as a result of climate change studies. At present, output from global climate models (GCMs) is of insufficient spatial and temporal resolution and reliability to be used directly in impact models. A stochastic weather generator, however, can serve as a computationally inexpensive tool to produce multiple-year climate change scenarios at the daily time scale which incorporate changes in both mean climate and in climate variability. It utilizes semi-empirical distributions for the lengths of wet and dry day series, daily precipitation and daily solar radiation. The rainfall-runoff models for the base period (2000-1971) has been calibrated and verified, then by downscaling of ten GCM-AR4 climate models for the study area and take into account each of them separately for rainfall-runoff models, changes of runoff in the period 2069-2040 under the three scenarios (A1B and A2 and B1) were determined.
    Results from downscaling models showed that the rainfall for some models increase and others decrease in the future, compared to the base periods. Changing factors in downscaling method showed more decrease than statistical method. Results showed that the percentage change in long-term monthly simulated runoff for the two downscaling methods is about 5.11 percent, while a decreasing trend in the future compared to the base runoff was seen. Runoff simulation scenarios relative to each other in different months had the same difference. The results showed uncertainty in climate models used in this study is more than of uncertainty according to downscaling methods and emission scenarios.
    Keywords: Climate change impact, Downscaling models, Uncertainty, Emission scenarios, Dez Ulya basin Runoff
  • Elham Ghasemifar, Manouchehr Farajzadeh *, Yousef Ghavidel Rahimi, Abbas Ali Ali-Akbari Bidokhti Pages 103-124
    Clouds cover major portion of the earth’s surface and play an important role in climatic system. Clouds affect the radiation energy balance of the earth’s climate system by absorbing or scattering solar radiation and long wave radiation and emitting thermal radiation. Cloud properties are closely related to cloud cover patterns, a shift in cloud regime would result in changes in cloud fraction and the cloud microphysical properties and both of these (cloud fraction and cloud microphysics) influence the radiation forcing (Rapp, 2015). Clouds have a strong effect on precipitation distribution, tropospheric temperature profile, climate change, radiation budget, global hydrology budget. Thus, they have an important influence on global climate. The purpose of this research is the study of this variety of cloud fraction in Iran during all months over 2001-2015 with respect to the latitude, topographic forcing, and vegetation cover. Latitude, altitude, slope, aspect and vegetation are geographic characteristics in an area which determine and control many climatic parameters such as temperature, precipitation and etc. Analysis of spatiotemporal variations of cloud fraction based on the characteristics in a vast country like Iran has not been considered by researcher. Satellite imagery is one of the most efficient data source to monitor cloudiness. The spatial and temporal variation of cloud type Ping as deep cloud (Ping et al., 2014), stratospheric clouds (Pitt et al., 2007) and different cloud type (Halladay et al., 2012) have been studied by researchers over the world. Some researchers consider relation between cloud fraction and climate and geographic parameters e.g. Sato et al., 2007. Some other researchers reviewed cloudiness studies e.g. Bromwich et al., 2012. Iran is located between 24.5 to 39.5 north latitude and has topographic range between -28 to 5595m. NDVI value reaches a maximum value in June (0.897) during 2001-2015 time period. This research uses DEM 30 meter and Normalized Difference Vegetation Index in order to analyzie the effets of geographic parameters on cloud fraction. Monthly mean values of cloud fraction are extracted from MOD08/MYD08 MODIS products. We have then validated accuracy of MODIS mean monthly of cloud fraction aboard the Terra and Aqua using ERA-Interim and station data. Results show that there is a good agreement between them but the data is more accurate in cold month, and in the mornings, so that, polynomial coefficients of determination are higher against the stations data and in the morning times due to hourly stations weather data which corresponded to the satellites overpass. The geographic characteristics results showed that cloud fraction increases with increase in latitude except summer seasons due to monsoon system. In order to showing topographic forcing on cloud fraction, this parameter is divided into intervals of 15% for each months and then altitude, slope and aspect that were extracted for each interval. Topographic forcing presents the interesting role of slopped convection in mountain area in average elevation (500-1500 meter) over spring and autumn. Vegetation also has nearly direct relation with cloud fraction. Investigation of temporal variations of cloud fraction showed that the maximum value of STD is obtained in autumn for both satellites. Furthermore, significant trend was not observed in many months, but month of December showed decreasing trend by 2 to 3 annually. This research is the first attempt in the field of cloud climatology in recent decades and further analysis are needed to show the ongoing climate change effects on cloud climatology in this region. Study of cloud vertical profiles can be the next research in this field.
    Keywords: Spatial, temporal variations, remote sensing data, geographic characteristics, Iran
  • Sara Karami, Abbas Ranjbar Saadatabadi, Ali Reza Mohebalhojeh *, Mohammad Moradi Pages 125-146
    Since dust phenomenon has become one of the major problems in many parts of the world, a large number of models have been developed in order to predict the concentration of dust particles in the atmosphere. The current study is devoted to present a model to simulate transport and deposition of dust particles using several schemes of the finite volume method. The outputs of each of these schemes are compared with each other, both quantitatively and qualitatively in two case studies. The data needed to run the model, including the wind are data derived from the Weather Research and Forecasting (WRF) model output and the GOCART emission scheme is used to calculate the vertical dust flux from surface. Comparing the model outputs with the satellite images available and the HYSPLIT model output show that both the areas covered with dust and the emission of dust are simulated correctly. The dust patterns obtained using the various schemes examined are reasonably similar to each other, considering the fact that the wind field and the emission scheme are similar.
    Two cases of the dust storm that affected the extensive regions of Iraq and Iran are studied and simulated in this paper. In the first case, the dust emission starts from the eastern part of Syria on the 18th of June 2012. According to the satellite images, the dust that transported southeastward enters Iraq and subsequently affects the west and southwest regions of Iran, including parts of the Persian Gulf. The second case is related to a strong dust storm which sets up in Iraq, due to a synoptic system active on the 31st of August 2015. Dust emission is seen in the satellite images over Syria–Iraq border on the 31st of August 2015. The dust intensity increased during the next 24 hours, and was observed like a cyclone in the eastern and central parts of Iraq. Afterwards, the dust entered Iran’s borders and was extended towards the Persian Gulf, and finally covered all parts of the Persian Gulf.
    The performance of the flux limiter, the second- and the third-order UNO, the second- and fourth-order Bott finite volume schemes have been examined in terms of numerical accuracy and computational cost. The numerical accuracy has been determined by comparing the dust concentrations obtained by the model with the corresponding results of the WRF-Chem. The dust concentration patterns obtained by all of the schemes are in overall agreement with each other even after 72 hours of integration, the differences being mainly in the damping caused by the schemes and their computational costs. Whereas the highest damping is observed for the upstream scheme, the fourth-order Bott exhibits the least damping followed by the third-order UNO. The latter two schemes are, however, involved high volumes of computation and may not be cost effective. Considering both numerical accuracy in terms of damping and the computational cost, the second-order UNO scheme offers promising results. With the quantitative comparison carried out, it can be concluded that the second-order UNO scheme shows the most correlation coefficient with the WRF-Chem model output and is the most appropriate scheme, among the schemes examined, for the dust operating model.
    Keywords: dust phenomenon, model, Simulation, scheme, finite volume, quantitative comparison
  • Shoaieb Abkharabat * Pages 147-161
    Sistan wind is one of the most important atmospheric phenomena of warm period of year in Iran plateau which creates environmental different impacts on its dominated region. In the northern hemisphere during summer monsoon, ITCZ moves to higher northern geographic latitude. South Asia summer monsoon system is one of the phenomena which arises from this movement and a tongue extension of Gang low moues towards west, its domination over southern region of Iran, and its penetration to Khozestan plain and Mesopotamia is also one of this movement outcome. In the meantime, in atmospheric low level, Azores high tongue, in this region of the world also has to remain in north territories of Iran (that is over Caspian sea and its surrounding) more than world-wide average. So that, with the pressure gradient between the Caspian sea high and Gang low generates the Sistan wind system, in a way that can be said that this is northern trade wind which blows in the region. The period used in this study was for 2480 days in 22 years (2012-1993) from May until end of September. The atmospheric circulation types were extracted using daily mean of the 850 hPa geopotential height data for these days between 15°–80°E, 5°–50°N, with a 2.5° (lat) × 2.5° (lon) spatial resolution. These data were retrieved from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis archive. Then the agglomerative hierarchical cluster analysis with the ward algorithm and Euclidean distance were used to identify atmospheric circulation types over Iran in mentioned period of years. Because using hierarchical cluster analysis can take maximized within-group similarity and minimized between-group similarity in data, the groups of days with similar characteristics were determined. Then the calculated within-group correlations were used to identify representative days. The day with highest within-group correlation was representative day of atmospheric circulation types. Finally, 5 atmospheric circulation types were identified in this period in which 2 cluster showed the mechanism of in Sistan winds in the southeast of Iran. Then for representative day of atmospheric circulation types in 2 mentioned clusters, wind speed and direction, as well as the wind convergence in levels of in 1000, 925, 850 and 700 hPa and also the convergence in atmospheric vertical profiles were analyzed. This paper identifies that, this wind has the features of LLJ and its core is often at 850 hPa level which is titled and is known as Sistan LLJ. Its speed continuation also stretches to low levels and to the earth surface, as a result it creates Sistan wind near the surface. On the other hand, along with ITCZ belt anomalous movement to northern hemisphere, the southern trade winds also enter northern hemisphere to reach ITCZ belt and after acquiring humidty from Indian ocean and Arabian sea, they enter Indian subcontinent. Now, the assumption is tested that between these two kinds of blowing systems (Sistan LLJ and southern trades) a region of convergence must be created, then with the continuation of the above convergenced, wind sysrem is identified over north of Arabian sea, Pakistan and Iran coasts. This convergence region which establishes at the time of Sistan LLJ balances with southern trade winds, which determines its location geographic latitude. So that, if Sistan LLJ blows slowly, this convergence region moves to more southern geographic latitudes. also its vertical extension in atmosphere is to the extent that its generating factors (Sistan LLJ and southern trades) exist and when one of these factors, does ascend to the higher level of atmosphere, this convergence region disappears. So that, naturally its establishment can be observed from the earth surface to 750 hPa level, and it often disappears above this level. This convergence region can also be a reason for the issue that during the fact Sistan LLJ the northern trade winds converge with southern trades on Arabian sea. The other indications of these winds are being originated from subtropical high and blowing toward tropical convergence region, so that the Indian monsoon is in fact the southern trades and Sistan LLJ that blow from two sides towards ITCZ.
    Keywords: Sistan LLJ, Trade winds, Arab Sea Convergence Region, Synoptic
  • Masoud Khoshsima *, Reza Amjadifard, Sasan Zamani Moghadam, Sajjad Ghazanfarinia Pages 163-177
    Space environment radiation affects the operation and life-time of optical and electronic devices on satellites payloads. LIDAR payloads include electro-optical components such as laser section. LIDAR payloads are usually setup on various platforms. Space-borne remote sensing technology provides global data set of uniform quality and rapid data acquisition and also specially provides key information for evaluating the local, regional of aerosol irradiative forcing in global climate system, cloud properties and precipitation, Air pollution and hydrologic cycle. Also, global coverage collected data is updated in every time range. On the other hand, high expenditures of design, manufacture and using advanced technology are caused an increase in design and manufacture of high reliability LIDAR payloads, the mission duration and its quality in space. Due to space standards of NASA and ESA, one of the effective factors on degradation of space systems is radiation damages. In this research, effect of Total Ionizing Dose (TID) radiation on the operation of LIDAR subsystem orbiting 500 km altitude orbit with a 3-year mission is simulated. Radiation simulations on gain medium (Nd:YAG) and pump (laser diode) are performed by GEANT4 and C programming.
    Laser subsystem in LIDAR payload includes 3 different parts. These parts are Nd:YAG, mirrors and laser diodes pump. Space radiation sources are classified in three types: Van Allen radiation belts, Galactic cosmic rays (GCR), periodic gradual activities of sun. The sun emits neutralized plasma (almost protons and electrons) to the space. Some energetic particles originated from solar winds and GCRs are trapped in Earth magnetic field. These belts are expanded from 1000km to 65000 km and consist of electrons up to 7Mev and protons up to 300Mev. GCR particles are mainly protons with very high speed originated from out of the galaxy. The most populated particles in 500km altitude are electrons and protons. In order to simulate the radiation effects, Input data such as average flux number of incident particles, total number of particles passing through the sensitive volume are needed. These input data will be entered in GEANT4 toolkit by C. Most important input data are flux, total number of incident particles during the mission, Energy range, geometry, materials definition and their properties, sensitive volume definition are needed physics and initial position of particles. The most important radiation damages related to lasers and solid state optical devices are total ionizing dose and displacement damage. TID is resulted from passing charged particles through matter, while displacement damage is due to collision of passing particles with atoms of matter. TID effects depend on two factors. The first factor is generation of electron-hole pairs in dielectric layers (such as oxides). The second factor is trapping sites (oxide and interface). This damage results in degradation and possible failure, such as threshold voltage shift, decrease in drive current, switching frequencies, leakage current, noises, etc. In bipolar transistors, hfe degradation, leakage current, offset voltage, changes in offset current, bias current and gain degradation in analog devices are possible effect. In other devices, frequency shift in crystals, mechanical degradations and changes in dielectric parameters are considered.
    The results demonstrated that the absorbed dose from different space radiation sources for Nd:YAG and laser diode are important. Also, differences between the absorbed doses in 2 cases were simulated. The first case was absorbed dose with no shield against space radiation and the second one was calculating the absorbed dose with 2 mm Al7075 shielding the sensitive volumes against incoming particles. GEANT4 simulations determined that the absorbed dose in Nd:YAG when no shield was used to protect the laser parts against space radiation, was reached 1951 rad. In order to decrease the absorbed dose, the shield was used and it made considerable changes. The absorbed dose was reduced to 275 rad. Calculation showed that approximately 7×1012 protons would pass through the Nd:YAG, during the mission period. The number of passing particles through the sensitive volume depends on particle flux rate, sensitive volume sizes, mission duration. This value for GCR particles was 4×108 particles in each square centimeter. Results related to simulation of inner Van Allen radiation belt particles effect on Nd:YAG showed that the absorbed doses from the inner belt electrons and protons were about 68 rad and 187.5 rad respectively while these values for GCR protons and alpha particles were about 11 rad and 8.5 rad respectively. These values were about 481 rad, 128.5 rad, 7.5 rad and 6 rad for laser diode respectively. The total values for Nd:YAG and laser diode reached 275 rad and 623 rad. It is estimated that absorbed doses for radiation sensitive parts will be increased about 15 percent and according to the threshold current, the optical power versus threshold current curve will be changed.
    Keywords: Laser, Nd:YAG, Space radiation, Total Ionizing Dose, LIDAR
  • Sakineh Khansalari *, Ali Reza Mohebalhojeh, Farhang Ahmad-Givi Pages 179-198
    Heavy precipitation events often cause irreparable damage to human and economy. The ability to accurate prediction of the occurrence of such phenomena is very important especially for early warning systems run by operational centers. Identifying the main factors involved in heavy precipitation can result in more accurate predictions. The “factor separation method” (FSM) is used for this purpose. Based on the previous studies on factors affecting heavy precipitation especially in the Alpine region of Europe, and with regard to the geographical location of the Tehran Province, four factors are included in the application of the FSM. These factors are (1) the potential vorticity (PV) anomalies in the upper troposphere manifested in the form of PV streamers, (2) the PV anomalies in the middle troposphere, (3) the Alborz mountain range, and (4) the Zagros mountain range. The contributions of these factors were assessed in the heavy snowfall event of the 6th of January 2008 in Tehran by applying the FSM using the ERA-Interim data and the WRF (Weather Research and Forecasting) model. To apply the FSM, 16 simulations, which cover all possible combinations of the four factors, were performed by the WRF model using 3 nested domains with horizontal resolutions of 45, 15, and 5 kilometers, respectively, and 60 vertical levels. These 16 simulations are: the control run (1), the removal of PV anomalies of the upper (2) and the middle (3) levels of the troposphere, the reduction of the heights of the Alborz (4) and Zagros (5) mountain ranges, the removal of two-factor combinations (6 to 12), the removal of three-factor combinations (13 to15), and the removal of the four factors (16). To carry out the experiments involving the removal of PV anomalies, a PV inversion procedure was used to construct the initial states.
    On the 6th of January 2008, a case of heavy snowfall occurred in a significant part of the country from Tehran to the west in the south of the Alborz mountains. The case involved the intrusion of cold and dry air with large positive PV anomalies from the stratosphere to the upper troposphere in the form of a PV streamer. The PV streamer acts to substantially decrease the static stability in the lower to the middle troposphere. The resulting PV anomalies are associated with low-level winds, which can provide the low-level moisture supply for the precipitation event. The results show that the PV streamer is the factor with the highest impact. The independent contribution of the PV streamer is about 76.5% as obtained by the detailed analysis of the FSM using the outputs of 16 simulations. Another factor to consider is orography which can block air flows or lead to substantial ascent or descent. The magnitude of the impact due to orography depends on the characteristics of the flows over and around the mountains. The determining properties of the flows are the angle of hitting the mountains and the intensity of the flow. In those simulations that the altitude of the Alborz mountain range was reduced, the dominant air flow was from the windward to the leeward side of the mountain. As a result of the increase in the penetration of the cold air from the windward to the leeward of the Alborz mountain range, the pressure on the windward (leeward) of the mountains reduces (increases). In other words, the pressure gradient is reduced in the Tehran area, when the height of the mountains is reduced. So the warm front in the southern slopes of Alborz mountain ranges is weakened. The results of the corresponding simulations and the application of the FSM show that the contribution of the Alborz mountain range in this case is next to the upper-level PV anomalies. Quantitatively, the absolute contribution of the Alborz-mountain factor is about 48% in the case of the heavy snowfall that occurred in Tehran. Finally, it is worth mentioning that the Zagros mountain range and the PV anomalies of the middle troposphere play negligible role in this case.
    Keywords: heavy precipitation, potential vorticity streamer, mountain effect, factor separation method, potential vorticity inversion
  • Somayeh Rafati *, Mostafa Karimi Pages 199-214
    Climate data series usually contain artificial shifts due to inevitable changes in observing instrument or observer, location, environment and observing practices/procedures taking place during the period of data collection. Data discontinuities also arise from the continuously evolving technology of climate monitoring. It is important to detect artificial changepoints in climate data series, because these artificial changes could considerably bias the results of climate trends and variability analysis. Thus, corrections and homogenization of climate data are imperative for the assessment of observed climate trends.
    In this study homogenization of mean monthly temperature was assessed for 33 synoptic stations in Iran using PMFred algorithm and also linear trend estimates were obtained using this algorithm. The p value of the linear trend was determined by the t-test statistic of the slope parameter. The p value is the probability for an estimated positive trend to be greater than zero, or for an estimated negative trend to be smaller than zero. The probability for the estimated trend to be within these intervals is 95%. Linear trend was estimated for raw and homogenized data in order to evaluation of homogenization effect on trend analysis. Linear trend was normalized via half of confidence interval (95% confidence level) so that absolute value of significant trend (at this confidence level) would be greater than one. Then distribution map of mean monthly temperature trend was provided.
    This study showed that assessment of homogenization using an absolute test can lead to wrong results without the usage of adjacent stations data comparison, if there is no complete and reliable metadata. Because absolute homogenization tests could not realize between natural and artificial shifts and thus should not be used automatically and without subjective qualitative check. Thus adjacent stations data along with metadata (if it exists) was used for the detection of artificial shifts. Mean monthly temperature data was recognized homogeneous in Tehran, Shiraz, Esfahan, Hamedan-Nojeh, Tabriz, Khoy, Oromieh, Sabzevar, Shahrood, Babolsar and Bandar-Anzali stations and it was recognized inhomogeneous in Zanjan, Saqez, Sanandaj, Kermanshah, Khoram-Abad, Shahrekord, Ahvaz, Abadan, Yazd, Bandar-Abbas, Bam, Kerman, Zahedan, Zabol, Mashhad, Torbat-Heydarieh, Gorgan, Ramsar, Rasht, Qazvin, Birjand and Arak Stations. The results showed that the estimates could be biased by the unaccounted shifts in the series as expected. In the other words, it was observed negative trend before adjustment in mean monthly temperature in many stations which have inhomogeneous data, while they showed positive trend after adjustment (Torbat-Heydarieh, Birjand, Zabol, Gorgan, Bandar-Abbas, Khoram-Abad, Shahrekord, Ahvaz, Zanjan, Rasht, Qazvin, Saqez stations). Estimation of linear trend for homogenized data revealed that mean monthly temperature has increased significantly in most stations in Iran. Also, it has not been increased significantly in northwest, except Tabriz station and in Sabzevar- Shahrud to Bandar-Abbas, in a north-south direction. Also a north-south pattern was observed in intensity of increased trend in Iran. That is temperature has not increased in the northwest, while it has increased in north to central and southwest of Iran relatively severely (about 0.003 degrees Celsius in each month). It has not increased significantly in east of this region. Also, it has increased in east of Iran severely.
    Keywords: Temperature trend, Data homogenization, PMFred algorithm, Temperature trend pattern
  • Seyed Habibollah Hosseini, Mohammad Akbarinasab *, Mohammad Reza Khalilabadi Pages 215-225
    The ocean is a random medium having both deterministic and nondeterministic characteristics. This behavior often leads to the difficulty in performing such underwater applications as telemetry and tomography. Propagation of acoustic rays in the ocean depends on temperature, salinity and density (Frosch 1964). While pressure is primarily controlled by depth, temperature and salinity variations in the ocean due to currents, the surface mixed layer, eddies, internal waves and other oceanographic features. These features affect the structure of the temperature and salinity fields, which in turn determines the sound velocity fields. Furthermore, these features change both in time and space, modifying the temperature, salinity and sound velocity fields. Other oceanographic features which affect acoustic propagation are internal tides and waves. Internal tides are internal waves in the ocean with tidal frequencies. As they propagate they alter the temperature structure and consequently the sound velocity fields. A primitive-equation model (MIT General Circulation Model (MITgcm)) with tidal forcing provided the temperature and salinity fields, from which the horizontal and vertical dependence of sound speed fields of the Oman Gulf were generated. This model solves the fully nonlinear, non-hydrostatic Navier-Stokes equations under the Boussinesq approximation for an incompressible fluid with a spatial finite volume discretization on an orthogonal computational grid. The model formulation includes implicit free surface and partial step topography. The Makenzi formula for sound velocity was used to calculate the sound speed from the potential temperature, salinity and pressure fields. Using these sound speed fields and the Bellhop acoustic ray tracing software, the effect of internal tide on sound propagation was investigated. Both ray paths and Transmission Loss (TL) were analyzed for dependencies on the tidal cycles. This program traces acoustic rays along a 2-D sound speed field, which varies both horizontally and vertically. It was designed to “achieve fast, accurate wavefront, and eigenray travel time predictions” and is based on Bowlin’s RAY program (Dushaw and Colosi 1998). In the sill region, the topography is supercritical with respect to the M2 internal tides. The calculations of the sound field were performed for a harmonic source operating at frequencies of 100, 400,800 and 1500 Hz at a depth of 350m. In the different scenarios of simulations of propagation sound, the calculations were performed during a period tide cycly (at hours 3, 9, 15 and 21).
    The results of the modeling of sound propagation with nonlinear internal waves impact on the sound propgation during the period tide is as follows (Freitas, 2008): 1- Propagation of the sound during a period of internal tide leads to energy of sound being expanded and compressed at some points. At a frequency of 300 Hz, the sound scattering occurs intensively in the environment, due to fact that the wavelength source of acoustic is order of wavelength of internal tide (fine structure). As a result, fewer blind spots are seen in the environment.
    2- During a period of the internal tide, the basic structure of the sound velocity profile is not similar for all hours.
    3- Internal tidal waves in the hours of 9, 15 and 21 over the hill lead to the, intensity of acoustic pressure increase and leading to the convergence of sound beams in this region.
    Keywords: internal tide, Transmission Loss, Oman sea, model of MITgcm
  • Siavash Gholami, Sarmad Ghader *, Hasan Khaleghi Zavareh, Parvin Ghafarian Pages 227-243
    The Persian Gulf with subtropical climate is located between latitudes of 23-30 degrees, whit its coast adjoiner to Iraq, Kuwait, Saudi Arabia, Qatar and United Arab Emirates from one side and to Iran from the other side. The Persian Gulf’s width in widest part is 370 km and its length is 990 km. For people living near the sea and the surrounding coastal area as well as construction of onshore and offshore structures, knowledge and understating of the wind field and its variability is essential. In addition, the most common effect of wind is seen in wind driven currents, swells, upwelling and down-welling systems. Moreover wind stress plays a key role in the modeling of air sea interaction phenomenon, e.g., determination of the drag coefficient. Although other factors such as wind palfern and oceanic current are also contributing to the in determination of this coefficient but the wind is the main governing factor. In other words, any inaccuracy in determination of the sea surface wind field could cause a large over or under estimations in atmospheric and oceanic estimations.
    This work is devoted to verify the effects of different initial and boundary condition in global data (using reanalysis and analysis datasets) on numerical simulations of WRF (Weather Research and Forecasting) model over the Persian Gulf area. The main obstacle in the verification and evaluation of a model simulation is the lack of observation data with fine spatial and temporal resolution, in particular for offshore areas. Fortunately, in onshore areas, the existence of weather stations has solved the problem somehow, and for offshore areas satellite data are found to be the best dataset considering the spatial coverage. In the present study, simulations of WRF model are compared with different type of observation data. In this research, two types of data are used for verifying the model, i.e. Synoptic stations data and Satellite data (QuikSCAT and ASCAT).
    The WRF model version 3.4.1 is employed with ARW dynamical core for simulating of sea surface wind field over the Persian Gulf region. Considering the connection and information exchange between the domains, a two way nesting method is applied in simulations. As the goal was just to verify the effects of different initial and boundary conditions on simulations, therefore, for all the simulations the number of domains and their analogous grid sizes are considered the same. For these simulations three domains are considered the main domain approximately covers the whole are of the Middle East, from West and some parts of Far East with 36km spatial grid spacing. First nested domain covers the southern half of Iran along with marginal countries of Persian Gulf, with a 12km spatial grid resolution and ultimately, the innermost domain of the Persian Gulf that also includes some parts of Oman Sea with 4km grid spacing. The time step for simulations is assumed 216seconds and the time period for each simulation is 30hours, from which the first 6 hours are assumed as spin-up time. To provide the initial and boundary conditions three datasets of ERA-Interim (ECMWF Re-Analysis Interim), NCEP-FNL and NCEP-R2 are employed.
    Along with lots of effective factors, results from this research show that one of the sources of error in the WRF model wind simulations is the selection of initial and boundary conditions (input data). The obtained results of this work reveal that for the surface wind hindcast simulations over the Persian Gulf using WRF model, the ECMWF ERA-Interim data is a more suitable dataset to provide the initial and boundary conditions, rather than the NCEP-FNL and NCEP-R2 data. However, the NCEP-FNL is an alternative data set when the ERA-Interim data has some lacks.
    Keywords: Wind field, WRF model, Initial condition, Reanalysis, Persian Gulf
  • Hossein Najafi, Ali Reza Massah Bavani *, Parviz Irannejad, Andrew Viliam Robertson Pages 245-264
    The aim of this research is to evaluate a statistical method for downscaling the precipitation output of a number of Coupled General Circulation Models issuing seasonal forecasts 9 month in advance. Canonical Correlation Analysis (CCA) is applied for post-processing precipitation from the North American Multi-model Ensemble (NMME) project. The analysis is done for a long-term period (1986-2015) in the west of Iran. The area under study includes Karkheh River Basin where a significant reduction in renewable water resources has faced policymakers with challenges in water resources allocation and provision of environmental requirements to Hoor-al-Azim marshland downstream. PERSIANN-CDR biases are computed and corrected against in-situ observations by applying the multiplicative method. Bias corrected Satellite-based rainfall data merged with 23 gauge-based data. The approach for merging station-satellite-based rainfall estimation includes a spatio-temporal LM method which fits linear regression to the deterministic part of universal variation. It exhibits appropriate performance in terms of Correlation, Nash-Sutcliffe Efficiency and mean absolute error and multiplicative bias. After merging, correlation coefficients between the merged data and gauge-based rainfall are between 0.92 and 0.98 for all stations whereas it was between 0.7-0.95 for PERSIANN-CDR. The merged precipitation grided dataset is then used as the reference to evaluate NMME seasonal forecasting systems October-December being the target season. Forecasts initialized on the early October, September and August (lead time-0, lead-time-1 and lead-time-2 months, respectively) are evaluated for individual raw model outputs. Multi-Model Ensemble is also developed by assigning equal weights to individual models. Multi-model Ensemble which consists the 3 best individual models (CCSM4, CMC2 and CFSv2) outperforms all other MME which consist 2 to 8 models (ρ=0.560). It also outperforms CCSM4 which has the highest Spearman correlation of 0.486 among all models. Canonical Correlation Analysis (CCA) is then applied to individual and MME seasonal mean precipitation forecasts to correct biases in the position. Probabilistic forecasts are produced based on the best-guess forecast estimated by regression model (CCA). Predictand is transformed to normal distribution before performing the calculations. Then the forecast is transformed back to the empirical distribution. By assuming that the errors in the best-guess forecast are normally distributed, the variance of the errors is defined by the sampling errors in the regression parameters, and by the variance of the errors in the cross-validated predictions. Then the probabilities of exceeding the various thresholds (below normal, normal and above normal terciles) are calculated for issuing probabilistic forecast from 1986-2015. The goodness index is improved for all models after performing CCA especially for GFDL-aer04 and CMC1 having the most correctable systematic biases. 3 model-based MME is recognized to have highest skill (Spearman correlation=0.623) at 0-month lead time. The models also show high skill for initializations made in the early August and early September. ROC-area for below-normal precipitation is more than 0.5 for almost all models which shows the skill of NMME seasonal forecast systems in meteorological drought prediction. The skill of NMME in forecasting October-December precipitation in the west of Iran can help decision makers in real-time water resources and agricultural planning before water-year starts (In the late September).
    Keywords: North America Multi, model Ensemble (NMME), Downscaling, Karkheh River Basin, Multi, Model Ensemble, Seasonal Precipitation Forecasts