فهرست مطالب

فیزیک زمین و فضا - سال چهل و پنجم شماره 2 (تابستان 1398)
  • سال چهل و پنجم شماره 2 (تابستان 1398)
  • تاریخ انتشار: 1398/05/01
  • تعداد عناوین: 15
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  • ساهره گلریز، امیرپیروز کلاهی آذر* صفحات 247-267
    در این مطالعه سعی شده است تا میزان پیچیدگی توزیع زمانی لرزه خیزی در پهنه زمین ساختی زاگرس مورد بررسی و ارزیابی قرار گیرد. برای این منظور از راه کار تجزیه و تحلیل فرکتالی مبتنی بر تبدیل موجک استفاده شده است. در این خصوص سری های زمانی لرزه خیزی برای تمامی زیرپهنه های گستره زاگرس تهیه شده و سپس مورد تجزیه و تحلیل فرکتالی قرار گرفته اند. نتایج به دست آمده نشان دهنده ماهیت چندفرکتالی و مستقل از مقیاس توزیع زمانی لرزه خیزی در تمامی قسمت های پهنه مورد مطالعه است. با این وجود ویژگی های فرکتالی زیرپهنه های مختلف یکسان نبوده و هر یک از آنها دارای سرشتی متفاوت هستند. در نواحی ایذه و زاگرس مرتفع که دارای میزان تمرکز تنش به نسبت بالایی هستند، توزیع زمانی لرزه خیزی دارای ماهیتی پادهمبسته و تا حدی ساده است. این در حالی است که در زیرپهنه های لرستان، فروبار دزفول و کمان فارس، توزیع مذکور به نسبت پیچیده بوده و ماهیتی نسبتا همبسته و گاها تصادفی دارد. همچنین یافته های این تحقیق نشان می دهد که در گستره زاگرس لرزه خیزی مستقل نسبت به لرزه خیزی وابسته دارای توزیع زمانی ناهمگن تری است. هر چند که این ناهمگنی در بخش های مختلف پهنه مذکور یکسان نبوده و متغییر است.
    کلیدواژگان: تبدیل موجک، طیف تکینگی، چندفرکتال، لرزه زمین ساخت، لرزه خیزی مستقل، لرزه خیزی وابسته
  • امین افتخاری، حمیدرضا سیاهکوهی* صفحات 269-281
    لرزه نگاری سه مولفه ای از امواج لرزه ای در هر ایستگاه در سه راستا نمونه برداری کرده و اطلاعات امواج P و S را به صورت همزمان برداشت می کند. در برخی کاربردها استفاده از اطلاعات موج S به همراه اطلاعات موج P برآورد درستی از خصوصیات مخزن به دست می دهد. چون در برداشت های لرزه ای گاهی به علت شرایط منطقه یا خرابی دستگاه ها، نمونه برداری مکانی یکنواخت از امواج لرزه ای میسر نیست، باید از روش های درون یابی استفاده کرد تا ردلرزه های برداشت نشده بازسازی شوند. روش های مرسوم بازسازی ردلرزه های گم شده در لرزه نگاری سه مولفه ای، معمولا با اجرای روش روی هر مولفه به طور جداگانه انجام می شود، که می تواند به ویژگی های ظریف در رکورد صدمه بزند. در این تحقیق روشی برای بازسازی همزمان هرسه مولفه به یک باره ارائه می شود (برگرفته از ایده استنتون و ساشی) که با استفاده از تبدیل فوریه کواترنیون و الگوریتم تصویر بر روی مجموعه های محدب (QPOCS)، انجام می شود. نمایش برداری داده مختلط در حوزه فرکانس می تواند به وسیله قراردادن بخش های حقیقی و موهومی هر مولفه در آرگومان های یک کواترنیون، به دست آید. این روش اجازه می دهد تا عملگرها بر روی هر دو مولفه به طور همزمان اعمال شوند. مزیت این روش به خاطر همپوشانی طیفی مولفه ها در حوزه فرکانس-عددموج می باشد. نتیجه حاصل از کاربرد این روش بر داده های سه مولفه ای مصنوعی و واقعی، با نتیجه حاصل از کاربرد الگوریتم POCS بر روی هر مولفه به تنهایی، مقایسه شده که نتایج بازسازی با استفاده از الگوریتم QPOCS دارای کیفیت بازسازی بهتری می باشند.
    کلیدواژگان: درون یابی سه بعدی، لرزه نگاری سه مولفه ای، تبدیل فوریه، کواترنیون، مجموعه محدب
  • محمد احسان حکمتیان* صفحات 283-298
    در جوامع معمولی آماری، اغلب وضعیت توزیع عادی یا نرمال حاکم است و لذا در آنها استفاده از تابع چگالی احتمال گوسی یا نرمال و استفاده از میانگین گیری حسابی یا معمولی کار صحیحی می باشد. اما اگر جامعه آماری از تعدادی جهت دلخواه فضایی تشکیل شده باشد، وضعیت توزیع عادی یا نرمال حاکم نمی باشد. در این شرایط از تابع چگالی احتمال فیشر ومیانگین گیری برداری می توان بهره برد. یکی از جوامع آماری جهتی، جامعه آماری جهت های مختلف مغناطیدگی سنگ ها است. در این مقاله پس از یک مقدمه، برای درک بهتر تفاوت جامعه آماری معمولی و جامعه آماری جهتی، هم پراکندگی نرمال و هم پراکندگی فیشر (که برای جامعه جهتی استفاده می شود) مورد بحث قرار می گیرد. در ادامه الگوریتم محاسبه جهت میانگین مجموعه بردارها مطرح می شود. سپس به یک برنامه رایانه ای دارای توانایی میانگین گیری برداری که در جریان همین پژوهش تولید شده، اشاره شده است و بعد میانگین گیری برداری و حسابی با استفاده از داده های مغناطیدگی مقایسه شده اند. در این پژوهش معلوم شد که یک ضعف در میانگین گیری برداری وجود دارد و آن اینکه در بعضی شرایط جواب میانگین گیری برداری یکتا نمی باشد. راه حل ارائه شده در این پژوهش برای رفع این ضعف این است که در کنار میانگین گیری برداری، مناسب است که میانگین گیری معمولی یا حسابی هم صورت بگیرد تا در مواردی که جواب میانگین گیری برداری چند جهت متفاوت است، بتوانیم میانگین برداری صحیح را تشخیص دهیم.
    کلیدواژگان: میانگین گیری برداری، میانگین گیری حسابی، جهت های مغناطیدگی
  • مرتضی فتاحی*، کوین وودبریج، مارک دبیتمن صفحات 299-311
    تراس های آبرفتی اطراف رود کارون به صورت توام تحت تاثیر آب و هوا و تکتونیک بوده و می باشند. سازندهای سطحی تراس های آبرفتی عموما محیط های مناسبی برای تجمع انسان ها بوده اند، لذا تحت تاثیر انسان ها نیز قرار گرفته اند و بررسی آنها از دیدگاه های مختلف از جمله زمین شناسی، ژئومورفولوژی، دیرینه اقلیم شناسی و باستان شناسی ضروری است. در خوزستان -تراس های آبرفتی متعددی وجود دارد که مطالعه آنها اطلاعات مهمی را در اختیار می گذارد. یافتن سن رسوبات نقشی کلیدی در این مطالعات دارد. به عنوان مثال می توان به کمک آنها نرخ بالا آمادگی را تخمین زد. این مقاله ارائه کننده تکنیک هایی است که جهت اندازه گیری سن هفت نمونه برداشت شده از تراس های رودخانه ای کارون انجام شده است. کوارتزهایی از این نمونه ها استخراج شد و با استفاده از روش دانه درشت و تک الیکوتی سن یابی شدند. به منظور اندازه گیری سن، نمونه ها به روش لومینسانس نوری تحریک شد و با دو پارامتر دز معادل، دز طبیعی و دز سالانه اندازه گیری شد. با تقسیم این دو پارامتر سن نمونه ها تخمین زده شد. دزهای معادل به دست آمده برای هر نمونه پراکندگی زیادی را نشان دادند که موجب شد تا سن های مختلف با توجه به مدل های مختلف برای هر نمونه به دست آید. اطلاعات کامل در مورد محل تراس ها و محل نمونه برداری ارائه شده است تا این سن ها بتواند جهت تحقیقات بیشتر مورد استفاده قرار گیرد.
    کلیدواژگان: سن یابی، لومینسانس نوری، تراس رودخانه ای، کارون
  • احسان لیموپرورجهرمی، بنفشه حبیبیان دهکردی*، بهروز اسکوئی صفحات 313-324
    تفسیر داده های مگنتوتلوریک در حضور اعوجاج های گالوانی ناشی از ساختارهای کوچک مقیاس نزدیک سطحی می تواند به نتایج نادرست منجر شود. ساده ترین شکل ظهور این اعوجاج ها درحالی که فقط به تغییر دامنه میدان های الکتریکی محدود می شوند، جابه جایی قائم منحنی های مقاومت ویژه ظاهری یا همان پدیده جابه جایی ایستا (static shift) است؛ که مقدار آن تنها با استفاده از داده های تانسور امپدانس قابل محاسبه نیست و در واقع جزء بخش های غیرقابل تعیین ماتریس اعوجاج است. در این تحقیق از توابع تبدیل ژئومغناطیسی (تیپر و داده های مغناطیسی افقی) برای برآورد نسبی میزان این جابه جایی و بازیافت مقاومت ویژه ظاهری معوج نشده یا منطقه ای مود TE بر اساس قانون القاء فاراده استفاده شده است. با لحاظ کردن تغییرات افقی مولفه های افقی میدان مغناطیسی، روش مورد نظر بر روی دو مدل مصنوعی اعمال شده است. به دو روش محاسباتی مختلف، اغتشاشات ناشی از ساختارهای کوچک مقیاس سه بعدی شبیه سازی شده و به مدل های مورد استفاده اضافه شدند. نتایج به دست آمده، موید دقیق تر بودن میزان جابه جایی برآورد شده نسبت به حالتی است که از توابع تبدیل مغناطیسی افقی صرف نظر و تنها توابع تبدیل مغناطیسی قائم مد نظر قرار گیرند.
    کلیدواژگان: مگنتوتلوریک، توابع تبدیل، جابه جایی ایستا
  • سعید فرزانه*، کمال پروازی صفحات 325-342
    روش برآورد مولفه های واریانس کمترین مربعات زمانی که تنوع مشاهداتی در شبکه وجود داشته باشد کارایی خوبی از خود نشان می دهد. با استفاده از این روش برای هر دسته از مشاهدات مختلف یک ضریب مقیاس محاسبه می شود. در این تحقیق از روش وزن دهی برآورد مولفه های واریانس کمترین مربعات استفاده شده است. این بهبود دقت برای مختصات نقاط شبکه به نحوی است که مقدار نیم قطر بزرگ بیضی خطای مطلق نقاط در حالت استفاده از برآورد مولفه های واریانس کمترین مربعات برابر 29 میلی متر، در حالی که با استفاده از روش فاکتور وریانس ثانویه این مقدار به دو برابر افزایش می یابد. علاوه بر این در هنگام استفاده از روش برآورد مولفه های واریانس کمترین مربعات اثر ماتریس کوواریانس مجهولات برابر 8/0 میلی متر می باشد که نسبت به روش فاکتور وریانس ثانویه مقدار آن به اندازه دو برابر کاهش می یابد. در واقع مزیت روش برآورد مولفه های واریانس کمترین مربعات برآورد واقع بینانه ای از دقت پارامترهای مدل و ابعاد بیضی خطای مطلق می باشد.
    کلیدواژگان: برآورد مولفه های واریانس کمترین مربعات، فاکتور وریانس ثانویه، شبکه های ژئودتیک، عدد آزادی
  • سعید نگهبان*، حمید گنجائیان، عطرین ابراهیمی، کامیار امامی صفحات 343-354
    شهرنشینی یکی از عوامل انسانی مهم و تاثیر گذار بر کاربری اراضی و همچنین تغییردهنده ویژگی های مختلف سطح زمین است. با توجه به روند رو به رشد نواحی سکونتگاهی و افزایش میزان تخریب اراضی مستعد، این پژوهش سعی دارد تا روند تغییرات مناطق مسکونی در شهر سنقر را مورد ارزیابی قرار دهد و همچنین بر مبنای تغییرات صورت گرفته بین سال های 2000 تا 2012، روند این تغییرات برای سال های 2025 و 2040 پیش بینی کند. درواقع هدف اصلی تحقیق حاضر آگاهی از شرایط آینده کاربری اراضی در صورت ادامه یافتن روند موجود است. روش کار به این صورت است که پس از تهیه تصاویر ماهواره ای و پیش پردازش تصاویر، کاربری اراضی محدوده مطالعاتی برای سال های 2000 و 2012 تهیه و با استفاده از مدل LCM میزان تغییرات کاربری اراضی آنالیز شده است. سپس بر اساس مدل زنجیره مارکوف میزان پتانسیل تغییر هر کاربری به کاربری سکونتگاهی سنجیده شده است. پس از محاسبه پتانسیل انتقال هر کاربری به کاربری سکونتگاهی با استفاده از داده های توصیفی موردنظر، نقشه پیش بینی سخت کاربری اراضی برای سال های 2025 و 2040 تهیه شده است. نتایج حاصله بیانگر این است که نواحی سکونتگاهی محدوده مطالعاتی از 3/8 کیلومترمربع در سال 2000 به 6/12 کیلومترمربع در سال 2012 رسیده است که این مقدار بیانگر رشد قابل توجه نواحی سکونتگاهی دارد. همچنین نتایج حاصل از پیش بینی بیانگر این است که میزان گسترش نواحی سکونتگاهی تا سال 2025 و 2040 به ترتیب به 2/18 و 1/24 کیلومترمربع خواهد رسید.
    کلیدواژگان: سکونتگاه، سنقر، کاربری اراضی، تغییرات، LCM
  • پیمان محمدی احمدمحمودی، اسداله خورانی* صفحات 355-371
    هدف از انجام این پژوهش تجزیه و تحلیل روند تغییرات پوشش برف در محدوده رشته کوه زاگرس جهت واکاوی نوسانات پوشش برف طی دوره زمانی 2016-2001 است. در این پژوهش از داده های روزانه سنجنده MODIS جهت استخراج و بررسی تغییرات سطح پوشش برف (SCA) و برفمرز (SL) برای ماه های دارای پوشش برف طی سال های 2016-2001 شده است. برای ارزیابی صحت تغییرات پوشش برف از آمار میانگین بارش (بدون تفکیک برف و باران) ایستگاه های سینوپتیک قرارگرفته در ارتفاع بالاتر از 1500 متر استفاده شد و جهت ارزیابی دقت تصاویر MODIS در استخراج سطح پوشش برف از 12 فریم تصویر سنجنده ASTER به عنوان نقشه صحت زمینی استفاده شد. تحلیل روند تغییرات پوشش برف و برفمرز با استفاده از آزمون من-کندال نشان می دهد که پوشش برف و برفمرز به طور کلی چه در سطح معناداری 99 درصد و چه 95 درصد فاقد روند است. بررسی آمارهای پوشش برف هرماه نشان می دهد که سال 2009 نقطه عطفی در پوشش برف زاگرس به شمار می رود و از این سال به بعد تا سال 2016 برای تمامی ماه ها به جز ماه نوامبر تغییرات پوشش برف کاهشی است. بررسی تغییرات میانگین سطح پوشش برف طی سال های 2016-2009 نسبت به سال های 2008-2001 نشان می دهد که ماه ژانویه با بیشترین کسری پوشش برف به میزان 11/55- درصدی در دامنه ارتفاعی 1162-4212 و کسری بارش 85/45- برای ایستگاه های سینوپتیک مواجه بوده است. ارزیابی دقت تصاویر پوشش برف MODIS نسبت به تصاویر ASTER نیز نشان می دهد که مقادیر ضرایب R2، RMSE و MAPE بین سطح پوشش برآوردی توسط هر دو تصویر به ترتیب 99/0، 3/49 کیلومترمربع و 51/21 درصد بوده است.
    کلیدواژگان: پوشش برف، زاگرس، MODIS، منابع آب
  • امیر باقری مصلح آبادی، عباسعلی علی اکبری بیدختی*، مریم قرایلو، رضا خلیفه صفحات 373-382
    تشکیل مه باعث کم شدن دید افقی می شود و دید افقی پایین می تواند باعث بروز مشکلاتی عمدتا در فرودگاه ها و جاده ها شود. از این رو با تشکیل مه مصنوعی در آزمایشگاه، تاثیر برخی هواویزها بر روی مه بررسی می شود. ذرات آب دوست می توانند به عنوان هسته های میعان با افزایش شعاع موثر و کاهش غلظت قطرک ها، در تعدیل مه موثر باشند. در این مطالعه، آزمایش ها بر روی سدیم هیدروکسید، اوره و نمک انجام شده است که با هواویزهای زمینه مقایسه شده اند. ابتدا عمق نوری مه محاسبه و سپس توزیع اندازه قطرک های مه تخمین زده می شود. همچنین دیگر پارامترها مانند غلظت قطرک های مه، محتوای آب مایع و زمان ماندگاری مه، مورد ارزیابی قرار می گیرد. نتایج حاکی از آن است که ذرات آب دوست به عنوان هسته های میعان، نقش به سزایی را در تعدیل مه ایفا می کنند. به طوری که در حضور این ذرات، غلظت قطرک های مه در مقایسه با هواویزهای زمینه کاهش یافته است و مه رقیق تر شده است. در حضور ذرات سدیم هیدروکسید غلظت کل قطرک ها کاهش زیادی داشته است و مدت زمان ماندگاری مه به 31 ثانیه رسیده است که نشان می دهد این ترکیب برای تعدیل مه عملکرد بهتری داشته است. همچنین هنگامی که ذرات سدیم هیدروکسید درون محفظه وجود داشتند، به طور مشاهداتی قطرک های بسیار کوچکی که در انتهای فرآیند سه آزمایش دیگر بر روی پرتوی لیزر نوسان می کردند، برای این ذرات وجود نداشتند.
    کلیدواژگان: ذرات آب دوست، بررسی تجربی، قطرک ها، تعدیل مه
  • آذر جلیلیان، حسن ذوالفقاری*، جلیل صحرایی صفحات 383-400
    در این پژوهش برهم کنش جت های جنب حاره و جبهه قطبی در بارش های سنگین بهار و پاییز ایران (2017-1988) بررسی شده است. برای نیل به این منظور، پس از شناسایی منطقه بیشینه تغییرات جت ها، با استفاده از تحلیل عاملی به روش مولفه مبنا (PCA) بر روی داده های ارتفاع تراز 500 هکتوپاسکال، چهار مولفه اصلی با تبیین 82 درصد از پراش داده ها تعیین شد. سپس با استفاده از روش تحلیل خوشه ای، روزهای واقع در هر خوشه و با روش همبستگی درون گروهی، روز نماینده هر خوشه مشخص شد. همچنین با دریافت داده های واگرایی، سرعت قائم جو (امگا)، تاوایی، رطوبت نسبی، رطوبت ویژه، دما و میانگین فشار تراز دریا از مرکز مطالعات میان مدت اروپا (ECMWF, European Centre for Medium-Range Weather Forecasts)، اثرات الگوهای برهم کنش جت ها بر رخداد بارش سنگین بررسی شد. نتایج پژوهش نشان دهنده تاثیرات متفاوت سه الگوی برهم کنش جت ها بر بارش سنگین است. به طوری که اثرات برهم کنش مستقیم ناشی از انطباق گردش واچرخندی جت جنب حاره بر گردش چرخندی جت جبهه قطبی باعث تمرکز بارش در ایستگاه های محدودی شده است؛ اما تاثیر برهم کنش غیر مستقیم ناشی از انطباق گردش چرخندی جت جنب حاره بر گردش وا چرخندی جت جبهه قطبی، بارش های گسترده ای را در پی داشته است. همچنین برهم کنش متقابل حاصل مجاورت گردش های وا چرخندی جت جنب حاره و چرخندی جت جبهه قطبی، بر تشدید فرآیند جبهه زایی و رخداد بارش در امتداد این جبهه ها موثر بوده است. مجاورت گردش های چرخندی جت جنب حاره و واچرخندی جت جبهه قطبی، برهم کنشی در پی نداشته و رخداد بارش سنگین در این الگو، فقط از گردش چرخندی جت جنب حاره متاثر شده است.
    کلیدواژگان: بارش سنگین، جت جنب حاره، جت جبهه قطبی، برهم کنش، بهار و پاییز، ایران
  • منصوره عبداللهی، فرهنگ احمدی گیوی*، محمد میرزائی صفحات 401-421
    در این مطالعه،نقش بی هنجاری مثبت تاوایی پتانسیلی (PV) در وردسپهر زبرین بر جبهه زایی سطوح زبرین و زیرین در منطقه ی خاورمیانه و ایران مورد بررسی قرار گرفته است. برای این منظور، ابتدا سه سامانه همراه با تاشدگی وردایست و جبهه ی قوی انتخاب شد. سپس، حذف بی هنجاری تاوایی پتانسیلی مربوط به تاشدگی وردایست با روش میانگین گیری مداری PV و وارون سازی آن انجام و میدان های هواشناختی تغییریافته به دست آمد. در ادامه، دو شبیه سازی، یکی بدون تغییر داده ها و دیگری با داده های تغییریافته با استفاده از مدل WRF انجام شد. در انتها، جملات تابع جبهه زایی با به کار بردن خروجی مدل، محاسبه و مورد بررسی قرار گرفتند. نتایج نشان داد که با حذف تاشدگی وردایست، کاهش شیوهای افقی و قائم دما، تضعیف سرعت افقی و سرعت قائم منفی در سطوح زیرین و زبرین و افزایش سرعت قائم مثبت در سطوح زیرین رخ می دهد. به طور کلی جملات دگرشکلی و جبهه زایی قائم و کج شدگی در سطوح زیرین و زبرین با علامتی مثبت، کاهش می یابند. جمله کج شدگی با علامتی منفی در سطوح زیرین، مطابق تغییر الگوی سرعت قائم، افزایش می یابد. در بررسی سه سامانه به طور کلی، با حذف تاشدگی وردایست، جبهه زایی کل، به خصوص در سطوح زبرین، مانند الگوی جبهه زایی قائم در پایین دست ناوه و نزدیک به مرکز حذف تاشدگی وردایست کاهش می یابد؛ تنها در یکی از سامانه ها دورتر از مرکز حذف تاشدگی وردایست و در سطوح زیرین، مقدار جبهه زایی کل افزایش می یابد. بنابراین، می توان نتیجه گرفت که تاشدگی وردایست نقش بارزی در تضعیف و تقویت جبهه های سطوح زیرین و زبرین وردسپهر ایفا می کند.
    کلیدواژگان: تاوایی پتانسیلی، تاشدگی وردایست، جبهه زایی، شبیه سازی، جبهه های سطوح زیرین و زبرین
  • آمنه ملاشریفی، علیرضا محب الحجه*، فرهنگ احمدی گیوی صفحات 423-440
    نوسان اطلس شمالی (NAO) به عنوان مهم ترین عامل اثرگذار بر وضع هوای اروپا و مدیترانه و همچنین موقعیت مکانی و جهت مسیر توفان مطرح است، به طوری که مسیرهای توفان تحت تاثیر این پدیده دورپیوند هستند. هدف این پژوهش، نگاهی نو به اثر NAO بر مسیرهای توفان اطلس و مدیترانه از دیدگاه انرژتیک با استفاده از مجموعه داده های بازتحلیل JRA-55 و مقایسه نتایج آنها با داده های بازتحلیل NCEP/NCAR است. بدین منظور، نقشه ترکیبی جمله های مهم معادله گرایش زمانی انرژی جنبشی پیچکی (EKE) و تولید کژفشار برای ماه های بحرانی مثبت و منفی NAO، در فصل زمستان برای بازه زمانی 1959 تا 2017 محاسبه شد.
    نتایج هر دو مجموعه داده نشان می دهند عمدتا همگرایی شار انرژی کل نسبت به عوامل دیگر نقش مهم تری در تقویت EKE دارد و نقش شار آزمینگرد در تقویت مسیر توفان مدیترانه بیش از مسیر توفان اطلس است. با وجود سازگاری نسبی نتایج و الگوهای به دست آمده از دو مجموعه داده، نتایج JRA-55 حاکی از قوی تر بودن تمام جمله های انرژی، در هر دو فاز به ویژه در فاز منفی، هستند. از طرفی، مراکز واگرایی و همگرایی شار انرژی در مسیر توفان مدیترانه حاصل از NCEP/NCAR حساسیت کمتری به تغییر فاز NAO نشان می دهند. برخلاف مطالعات پیشین که بیانگر قوی تر بودن هسته مسیر توفان اطلس در فاز مثبت NAO هستند، نتایج JRA-55 اختلافی بین فازهای مثبت و منفی نشان نمی دهند. به علاوه، با وجود آن که نتایج JRA-55 نیز نحوه ارتباط دو مسیر توفان در دو فاز را تایید می کنند، ولی در فاز منفی ارتباط کمتری بین مسیرهای توفان، نسبت به نتایج NCEP/NCAR، مشاهده می شود.
    کلیدواژگان: نوسان اطلس شمالی، مسیر توفان، انرژی جنبشی پیچکی، تولید کژفشار، NCEP، NCAR، JRA-55
  • فاطمه بهمن زاده، سرمد قادر*، سیدعباس حق شناس، دانیال یازجی صفحات 441-458
    در پژوهش حاضر شبیه سازی پیش یابی و ارزیابی میدان باد تراز 10 متر و همچنین دمای ‏تراز دومتر با مدل میان مقیاس ‎WRF‎‎‏ نسخه 1. 7. 3 در منطقه دریای عمان و دریای عرب انجام شده است. تمامی شبیه سازی ها برای دو پیکربندی متفاوت از دیدگاه پارامترسازی فیزیکی که به نام های Phys1 و Phys2 معرفی شده اند‏، انجام شده است. ارزیابی نتایج شبیه سازی با استفاده از داده های ایستگاه های همدیدی در دسترس و همچنین داده های دو سنجنده ماهواره ای ‎ASCAT‎‏ و ‎OSCAT‎‏ انجام شده است. برای بررسی عملکرد مدل عددی از سنجه های آماری مانند ضریب همبستگی پیرسون، ریشه میانگین مربعات خطاها و نمودار آماری تیلور استفاده شده است. نتایج نشان می دهند که در بخش ارزیابی‎ با داده های ماهواره ای باد تراز 10 متر نتایج عملکرد مدل‏، قابل قبول و مناسب بوده است. در این بخش شبیه سازی های برای مناطق دور از ساحل که به کمک داده های ماهواره ای ارزیابی شده اند‏، نسبت به داده های ایستگاه های همدیدی ساحلی و نزدیک ساحل، به مقادیر مشاهدات نزدیک ترند. نتایج حاصل از ارزیابی دمای تراز دو متر نیز گواه عملکرد مناسب مدل در این بخش است.
    کلیدواژگان: مدل WRF، داده ماهواره ای، سنجنده ASCAT، سنجنده OSCAT، دریای عمان، دریای عرب
  • نفیسه پگاه فر* صفحات 459-472
    پدیده چرخند حاره ای از دیدگاه های متفاوتی توسط پژوهشگران بررسی شده و هدف مشترک تمامی پژوهش ها، ارتقاء پیش بینی شدت و مسیر این پدیده بوده است. در این راستا پارامترهای مختلفی برای شناخت دینامیک و ترمودینامیک چرخند حاره ای معرفی و بررسی شده اند. در این پژوهش با تاکید بر پارامتر ترمودینامیکی آنتروپی سعی شده تا شار سطحی آنتروپی، شار قائم آنتروپی (طبق چارچوب نظری تانگ و امانوئل، 2010) و شار جانبی آنتروپی در دوره عمر چرخند حاره ای گونو که سواحل جنوبی کشور ایران را متاثر کرد، محاسبه و بررسی شود. همچنین گستره قائم و شدت درون شارش ها و برون شارش ها نیز در دوره مورد نظر محاسبه و تحلیل شده است. برای این منظور از داده های باز تحلیل ERA-Interim با توان تفکیک 125/0 درجه در راستای طول و عرض جغرافیایی و داده های مرکز هواشناسی هند در بازه های زمانی 6 ساعته استفاده شد. برای بررسی همدیدی، پارامترهای مذکور در منطقه ای دایره ای به مرکز چرخند گونو با شعاع 500 کیلومتر محاسبه شد. نتایج نشان داد که در مقیاس همدیدی مقدار تجمعی شارهای قائم، جانبی و سطحی آنتروپی قبل از اینکه شدت چرخند بیشینه شود، با تقدم های زمانی متفاوت به مقدار کرانگینه خود رسیده اند. همچنین قوی ترین درون شارش و برون شارش به ترتیب قبل و بعد از بیشینه شدت چرخند گونو روی داده است. قابل توجه است که نسبت به الگوهای مشاهده شده در ابتدای دوره عمر چرخند گونو، در بازه زمانی که چرخند شدت دسته-5 و بیشتر را تجربه کرد، هم درون شارش گستره قائم کمتری داشته و هم برون شارش از ترازهای پایین تری به بالا توسعه یافته بود.
    کلیدواژگان: چرخند حاره ای گونو، شارهای (سطحی، جانبی و قائم) آنتروپی، سرعت شعاعی، درون شارش، برون شارش
  • حامد امینی، اباذر اسمعلی عوری*، رئوف مصطفی زاده، معراج شرری، محسن ذبیحی صفحات 473-486
    جریان رودخانه ای یکی از مهم ترین منابع آبی است که برای مصارف مختلف حائز اهمیت بوده و روند تغییرات آن در مدیریت منابع آب ضروری است و از طرفی، جریان رودخانه ای توسط سدها، انحرافات، جریان های برگشتی و توسعه شهری تحت تاثیر قرار گرفته و منجر به مسائل مختلفی از جمله وقوع خشکسالی، کاهش آبدهی و مسائل زیست محیطی می شود. هدف تحقیق حاضر، ارزیابی اثر احدات سد یامچی و سد سبلان بر تغییرات رژیم جریان و خشکسالی رودخانه های پایین دست آنها می باشد. در مطالعه حاضر با استفاده از حد آستانه فصلی مشخصات خشکسالی ایستگاه بالادست و پایین دست هر یک از سدها ارزیابی و سپس شدت های خشکسالی در تداوم های مختلف در نرم افزار Easy Fit با دوره بازگشت های متفاوت محاسبه شد. با استفاده از نمودارهای شدت خشکسالی در دوره بازگشت های متفاوت و نمودارهای SDF و نمودار درصد تغییرات دبی جریان در ایستگاه های بالادست و پایین دست سدهای مذکور، اثر احداث این سدها بررسی شد. با توجه به نتایج تحقیق، می توان نتیجه گرفت که احداث سد یامچی باعث کاهش رژیم جریان در پایین دست شده و افزایش وقوع رخدادهای خشکسالی هیدرولوژیک را سبب شده است درحالی که سد سبلان باعت تعدیل و تنظیم رژیم جریان در پایین دست شده و از شدت خشکسالی های هیدرولوژیک، کاسته و نقش بیشتری در تنظیم جریان داشته است.
    کلیدواژگان: شدت خشکسالی، نمودارهای SDF، حد آستانه، تنظیم جریان، سد یامچی، سد سبلان
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  • Sahereh Golriz, Amir Pirooz Kolahi, Azar * Pages 247-267
    In this research it is tried to examine the fractal complexity of seismicity temporal dispersion in the Zagros Mountain range. The Wavelet Transform Modoulos Maxima (WTMM) as an innovative strang attractor formalism has been utilized for the multifractal investigation. Earthquakes that occurred from December 2003 to May 2016 have been collected from the master catalog of the International Institute of Earthquake Engineering and Seismology (IIEES). As all events in the master list are reported based on the local magnitude (ML), the achieved catalog is already homogeneous. ML is saturated for the earthquakes with magnitude greater than 5.5, so they are converted to the moment magnitude (MW) using some empirical relations. For a reliable and comprehensive seismicity examination, the Gutenberg–Richter analysis is performed over the cumulative distribution of events, and the minimum magnitude of completeness (MC) has been obtained. For MC calculation, the maximum curvature method is used and an overall Mc=3.1 is computed for the attained earthquake catalog. To complete the catalog, all events with MW<MC have been removed from the earthquakes list. As the occurrence time is the most reliable seismicity parameter, the time-series are prepared as interevent times between the consecutive earthquakes for the different subzones of the Zagros region. The WTMM technique has been applied to each of the time-series and their fractal characteristics are gaind from the attributes of the related scaling and singularity spectrums. The obtained results revealed that the seismicity is scale invariant; however, its multifractal nature is not constant. There are some differences among the fractal aspects of seismicity temporal changes in the different portions of the belt. Chronological distribution of earthquakes in the simply-folded belt and Dezful embayment are remarkably more complex than the other portions of the Zagros Mountain range. Dezful embayment as an indenter plays an important role on deformation style in the Zagros Mountain. It causes crust materials to escape from the frontal regions toward the Fars-Arc and Lorestan side-salients. Our findings indicate a relatively complex and heterogeneous temporal variation of earthquakes in the salients and Dezful indenter with respect to those in high-Zagros and Izeh frontal subzones. Abadan plain is the quietest subzone seismically and it shows the least amuont of temporal complexity. From the dependency point of view, the seismicity of high-Zagros, Izeh, and Abadan plain has an anticorrelate sharing. On the contrary, Fars-Arc and Lorestan salients have correlated seismic activities and in Dezful embayment the seismicity behaves in a random (stochastic) manner. These findings reveal that the seismicity offers relatively inconsistent configuration in regions with a high-stress concentration and in contrary, earthquakes work dependably in other calm areas. Generally, in the Zagros region independent (scattered) earthquakes are more heterogeneous with respect to the dependent (clustered) seismicity. In other words, the Zagros tectonic setting is such that the independent earthquakes have more intricate temporal spreading with respect to the affiliated temblors. The results of this study are in agreement with Kolahi-Azar and Golriz (2018) examination. In the mentioned work topography complexity has been measured for the different subzones of the Zagros region. Assuming the topography is affected by the superficial tectonic processes; they concluded shallow tectonic processes that act more intricately in Dezful embayment, Fars-Arc, and Lorestan side-salients. Similarly, our results show the more intricate temporal distribution of seismicity for the same regions. The fractal study of seismicity temporal distribution is a useful tool for the better understanding of the geodynamic conditions in a region. This approach reveals new seismotectonic aspects of the Zagros region which has not been addressed from this point of view.
    Keywords: Wavelet transform, Singularity spectrum, Multifractal, Seismotectonics, independent seismicity, affiliated seismicity
  • Amin Eftekhari, Hamid Reza Siahkoohi * Pages 269-281
    Three-component (3C) seismic data acquisition method samples seismic wave field at each station along three Cartesian coordinates, simultaneously. Many reservoirs have been discovered and determined by the generation and recording of P waves only, but the P wave alone cannot provide a comprehensive description of the reservoir characteristics. In some studies, S-wave information is required in addition to P-wave information to get a correct estimation from reservoir properties. By the three component seismic acquisition, P and S waves’ information can be recorded simultaneously. More often in seismic surveys, one cannot sample seismic wave field uniformly the along spatial direction due to environment limitations or instrument malfunctions; inevitably we have to use interpolation methods for reconstruction of missing traces. Reconstruction of missing or noisy traces is done using the projection onto convex sets (POCS). The POCS algorithm is a simple algorithm which is suitable for reconstruction of irregularly lost traces in a regular grid using multiple repetitive Fourier transforms. Conventional methods for reconstruction of missing traces in three component acquisition is usually done by implementation of POCS on each component separately, which could damage any subtle features in the record. This research introduces a method to reconstruct all three components at once using the quaternion Fourier transform and Projection onto Convex Sets (QPOCS).Quaternions in mathematics are a commutative numbers system that extend the complex numbers system. As the ordinary complex numbers can be displayed on two dimensions, these numbers can also be displayed on four dimensions. Quaternions were first introduced by William Rowan Hamilton when looking for a way to extend complex numbers to three dimensions. He knew how to sum and multiply three-dimensional numbers, but he was looking for a way to divide these numbers into each other. In 1843, Hamilton discovered that the division of quaternions requires a fourth dimension. Quaternion Algebra is often shown with H (in honor of Hamilton). The two-component data vector representation in the frequency domain can be obtained by putting the real and imaginary parts of each component in the arguments of a quaternion. This method allows operators to apply both components simultaneously. Quaternions are converted to Frequency-wavenumber domain by Quaternion Fourier Transform (QFT) and a single domain spectrum for both components is defined using the polar representation of the Quaternions. Quaternions have other applications in seismic data processing such as computing spectral attributes, multi-component velocity analysis and multi-component deconvolution. The advantage of this method is because of the spectral overlapping of the components in the frequency-wavenumber domain, thus the perpendicularity of input components is preserved (signals are not interconnected) and similarities between components are maintained that helps improve the quality of reconstruction. The coding of this method has been done in MATLAB environment and results of applying the proposed method on 3-component synthetic and real seismic data are compared to that of the POCS algorithm when applied on each component separately. The results of reconstruction using QPOCS algorithm indicate a better quality for reconstructed seismic data and in the output data, the percentage of produced artifacts is lower than that of the POCS algorithm on each component alone.
    Keywords: Three-dimensional interpolation, Three-component seismic acquisition, Fourier transform, Quaternion, Convex set
  • Mohammad Ehsan Hekmatian * Pages 283-298
    In statistical common population, common or normal distribution is often governed and so that using Gaussian or normal probability density function and arithmetic averaging is appropriate. But if the statistical population has been formed from a number of spatial arbitrary directions, then common or normal distribution is not governed. In this condition Fisher probability density function and vector averaging can be used (Fisher is the name of the scientist who proposed the mentioned density function for the first time). In this function, each direction is shown as a point on a sphere with unit radius. The mentioned function shows the probability of having a particular direction in unit angular area of a particular area that has a definite central direction. This central direction shows the angular difference with the real average direction. In Fisher function, the distribution of the azimuth angles around the real vector average direction is symmetrical. The azimuth and the declination angles are the same and being symmetrical around the their distribution of the real average direction is logical. One of the statistical directional populations is the statistical population of different directions of the magnetization of rocks (Each magnetization direction is specified by two angles. First the angle between the magnetization direction and the surface of the horizon (inclination angle) and second the angle between the magnetization direction projection on the surface of the horizon and the geographic north direction (declination angle)). In this paper after an introduction, both normal and Fisher distributions (the latter is used for directional population) are discussed for better understanding of the difference between normal and directional statistical populations. Then the algorithm for calculating the vector averaging is presented. After that a software having vector averaging ability that is produced in this research is presented and then the vector and arithmetic averages are compared for magnetization data. During this research, it is clear that there is a weakness in the vector averaging and that weakness is that in some conditions the result of the vector averaging is not unique (this non uniqueness is because of the functions used in vector averaging algorithm). For example for calculating the declination angle, the function arc-tangent is used and we know that the result of this function is not unique. For example arctan (0.5637) is equal to both 29.41 and -150.9 degrees). The proposed method for the treatment of this weakness in this research is that, it would be proper to perform an arithmetic averaging beside the vector averaging and by which in the cases of having non unique results for vector averaging, the true result can be detectable (The result of the arithmetic averaging is unique) Between different results of the vector averaging, that result is true which is more similar to the arithmetic averaging. For example if there is a directional population which their declination angles are between -170 to -140 degrees and their arithmetic average is -150.67 degrees and the results of their vector averaging are 29.41 and -150.59 degrees, then the correct vector average is -150.59.
    Keywords: vector averaging, arithmetic averaging, directions of magnetizations
  • Morteza Fattahi *, Kevin P. Woodbridge, Mark D. Bateman Pages 299-311
    The River Karun in lowland Khuzestan, SW Iran is influenced by various factors including tectonics, human activities, climate, and relative sea-level changes. Therefore, it is necessary to study these features from different aspects such as geology, geomorphology, paleoclimatology and Archeology. Disentangling these influences can be improved by investigating where river channels incise across active folds to produce river terraces. Determining the age of river terrace deposits has a fundamental role in these studies; especially since average rates of river incision since the time of terrace deposition can be a guide to average rates of tectonic uplift, particularly over longer timescales of thousands or tens of thousands of years where the influences of changes in aggradation and incision due to changes in sediment supply tend to be evened out (Bull, 1991; Burbank and Anderson 2012). River terraces of the Karun river system were found associated with active folds in the Upper Khuzestan Plains. These folds were mostly asymmetric detachment folds and fault bend folds trending approximately NW-SE, with a more steeply dipping fore-limb to the south-west and a more gently dipping back-limb to the north-east (Blanc et al., 2003). Woodbridge (2013) described these river terraces, and assigned each terrace a new name (from a nearby village or fold). As shown in Figure 1, four river terraces were associated with the Naft-e Safid Anticline: the 'Dar Khazineh terrace', the 'Batvand terrace', the 'Naft-e Safid terrace' and the 'Abgah terrace', on the fold fore-limb and back-limb. One river terrace was associated with the Sardarabad Anticline: the 'Kabutarkhan-e Sufla terrace', and one river terrace was associated with the Shushtar Anticline: the 'Kushkak terrace'; both on the fold back-limb Sediment samples were collected from the river terrace deposits and subjected to Optically Stimulated Luminescence (OSL) dating (Woodbridge and Frostick, 2014; Woodbridge et al., 2016). OSL dating was performed in the luminescence laboratory at the University of Sheffield, U.K. Both the palaeodose (De) and the dose rate was determined to derive an OSL age. For De Determination the procedure outlined in Bateman and Catt (1996) was employed. The single aliquot regenerative (SAR) approach (Murray and Wintle 2000), was used for De determination.  All the samples showed a weak naturally OSL decay curves. Many aliquots failed to show good growth curves. All aliquots where the recycling ratio exceeded 10% of unity were excluded from further analysis. Thus, only around 10-20 percent of measured aliquots for each sample passed the criteria of the SAR protocol and their De are reported. The most appropriate preheat temperature for each sample was selected using a dose recovery preheat plateau test. This resulted in selection of preheat temperatures of 220 °C for 10 seconds and cutheat of 200 °C for 10 seconds, which were applied to each sample prior to OSL measurement to remove unstable signal generated by laboratory irradiation.  Analyst software was used for De determination. All samples demonstrated a high amount of replicate scatter with a large range of De values. Some of the distribution shape may reflect the limited population size of replicates but it also may reflect incomplete bleaching. Typically, poorly bleached sediments retain a significant level of residual signal from previous phases of sedimentary cycling, leading to inherent inaccuracies in the calculation of a palaeodose value. This is difficult to establish with any certainty from OSL data and should be taken in consideration with the site stratigraphy. In principle a well bleached unpost-depositionally disturbed sample should have replicate palaeodose (De) data which is normally distributed (See Bateman et al. 2003, Fig 3). By plotting the replicate data for each sample as a probability density function, some assessment of where older or younger material has been included in the sample measurements can be made. However, by determining the De of aliquots that contains 1000-2000 grains any heterogeneity in De that individual grains have may still be masked. We tried to overcome this problem by using smaller aliquots or at the single grain level. However, for these particular samples the weak OSL signal and low sensitivity to laboratory dose prevented such analysis. In order to calculate an age, different models can be used. Woodbridge and Frostick (2014) and Woodbridge et al. (2016) published the age for each sample based on the mean De value determined by Finite Mixture Modelling or the Central Age Model. This paper provide the technical information behind dating these samples and provide all ages based on different models and without any judgement about partial bleaching, bioturbation or cryoturbation. Ages are quoted in years from the present day (2010) and are presented with one sigma confidence intervals which incorporate systematic uncertainties with the dosimetry data, uncertainties with the palaeomoisture content and errors associated with the De determination.
    Keywords: Dating, Optically stimulated luminescence, river traces, Karun
  • Ehsan Limooparvar Jahromi, Banafsheh Habibian Dehkordi *, Behrooz Oskooi Pages 313-324
    Interpretation of magnetotelluric data in the presence of galvanic distortions, caused by small-scale near-surface structures, can lead to unreliable results. The simplest manifest of these distortions, while limited only to the changes in the amplitude of electric fields, is vertical displacement of the apparent resistivity curves or static shift phenomenon that constitutes non-determinable part of the distortion matrix. Due to the boundary conditions governing components of the electric and magnetic fields, the occurrence of charge accumulation and therefore the static shift of apparent resistivity curves, affects only TM-mode data in the case of two-dimensional models. Thus, we can use the information available in the TE-mode- impedance phase (tipper and horizontal magnetic data) which are independent of this phenomenon. In this study, geomagnetic transfer functions have been used to estimate this displacement and recover the undistorted TE-mode apparent resistivity based on the Faraday induction law. Ledo et al. (2002) show that tipper data can be used to estimate static shift of magnetotelluric data, if the horizontal variations of the horizontal components of the magnetic field can be ignored. This assumption may be violated in complex situations. We estimate static shift while incorporating such variations and taking into account the horizontal magnetic transfer functions. Estimation of static shift through mathematical methods is only relatively possible and requires the selection of a reference station that has the minimal effect of galvanic distortion. The relations between different components of electric and magnetic fields are integrated and characterized by their mean values. To incorporate the horizontal magnetic tensor, array magnetotelluric data are required, so that components of the magnetic field at the reference and measurement sites are simultaneously provided. Considering two consecutive sites, impedance tensor at one site is written in terms of tipper and horizontal magnetic tensor at that site and the impedance at the adjacent site. By ignoring other types of distortions that can generally exist and using some algebra, the problem of determining the frequency-independent static shift factor becomes a linear fit problem. A set of data points covering different frequency ranges is selected and the quality of their linear fitting is examined through soling procedure. Considering the horizontal variations of the horizontal components of the magnetic field, the method has been applied to two synthetic models. Using two different approaches, the distortions caused by small-scale three-dimensional structures are simulated and added to the model responses. In the first approach, the distortion matrix is considered as the product of four parameters of gain factor, anisotropy, and twist and shear angles in the decomposition model then the distortion simulation is performed by selecting some numerical values of these four parameters and multiplying the resulted distortion matrix by the impedance tensor. In the second approach, some part of the top-layer of the model is replaced by a Gaussian distribution of resistivity with known selected mean and standard deviation. In this way, the effects of various geological processes, such as weathering, erosion- and to some extent- the nature of the deposited sediments are involved. The obtained results confirm that the estimated static shift parameter is more accurate than that of the case in which horizontal magnetic transfer functions are ignored and only the vertical magnetic transfer functions are considered.
    Keywords: magnetotelluric, transfer functions, static shift
  • Saeed Farzaneh *, Kamal Parvazi Pages 325-342
    Any infrastructure such as dams need constant monitoring for the detection of risks of failure and/or to plan civil engineering maintaining work. A recent approach considers precise geodetic instruments and satellite-based geodetic monitoring as a method to estimate potential deformation of such structures. A growing need for a fully automated and continuous monitoring of structural and ground deformations has created new challenges for design and analysis of the monitoring schemes, where multi-sensor geodetic systems can provide essential aid. Combination of different geodetic data helps determining displacements with high precision, hence, the risk of damages is reduced. Corresponding authorities of large man-made structures are faced with the safety problem, as all have aim to reduce risk and cost. Designers try to design large structures to tolerate against different forces like wind, traffic load, temperature, flood, earthquake, land uplift etc. Using geodetic instruments and techniques, we are able to monitor the deformation behavior or deflection in the mentioned structures and eventually provide a structural failure alarm capability (Andersson 2008). It is important to select appropriate sensor and methods to detect the deformation. Slow deforming dams require sub-millimeter to millimeter level accuracy to monitor the displacement and deformation (Lindenbergh et al. 2005). Reaching this level of accuracy is not costly, if geodetic sensors are integrated with other sensors (e.g. geotechnical sensors, and precise total stations, see Hwang et al. 2012). It might be to implement other sensors (e.g. laser scanner and Total Station). Using point clouds data for deformation monitoring is almost new. Gonzalez et al. (2012) studied on point clouds accuracy for applications in civil engineering e.g. deformation monitoring. They showed that the results appear suitable for deformation monitoring, with accuracies less than 1 mm. Bagherbandi et al. (2009) studied on various techniques to find the optimal design of a deformation network using various criteria such as precision, cost and reliability. Better results can be achieved using the control network, provided that an optimal network design is performed for detecting deformations (Kuang 1996). In addition, the methods of geodetic network process can affect the results (Bagherbandi 2016). The aim of this study is primarily to evaluate different deformation monitoring methods and possibilities to physically interpret the deformation and evaluate the risk of failures. In this research, the idea of assigning weights for the observations by least square variance components estimation (LS-VCE) is used (Amiri-Simkooei 2007; Teunissen and Amiri-Simkooei 2008) in order to improve accuracy of adjustment results, which differs from the applied method in Bagherbandi (2016) to determine the variance components. Some issues and parameters should be investigated in LS-VCE such as the effect of variance components estimation on the observations final accuracy, the absolute error ellipsoid estimation, the study of the necessary conditions in a network to achieve higher accuracy and its effect on obtaining real results from the reliability matrix. All results obtained from adjustment by element, LS-VCE, and Tikhonov regularization are compared using a simulated geodetic network and real data. Results from this study provide important information in studying deformation that can be used to interpret the deformation mechanism, which may reduce the risk of potential disasters in large structures. We will evaluate the above-mentioned methods in Jamishan dam in Iran and utilize the geodetic techniques and observations to monitor the deformation of the dam.
    Keywords: Geodetic Network, least squares variance component estimation, Deformation
  • Saeed Negahban *, Hamid Ganjaeian, Atrin Ebrahimi, Kamyar Emami Pages 343-354
    Considering the ever-increasing changes in land uses and the need for managers and experts to know how changes have taken place in policy and options for solving the existing problems. Detection of changes to determine the trend over time seems necessary. On the other hand, modeling future changes is important for understanding the quality of future changes. Therefore, the full recognition of land use, its past changes and the prediction of future changes plays an important role in the sustainable management of resources. Modeling land use processes is an important tool in optimizing land use and land use planning. One of the models used to predict landslide changes is the model of artificial neural networks and Markov chain analysis. The features of the artificial neural network include the ability to learn and generalize and process information in parallel. Considering the goal of urban development during the years 2000 to 2012, satellite imagery of the years 2000 and 2012 in June has been used. After the preparation of satellite imagery and pre-processing of images, the landuse in the study area for the years 2000 and 2012 has been prepared. Then useing the LCM model landuse change patterns of changes were analyzed. Then, based on the Markov chain model, the potential for changing each use to residential use is measured. This means that each pixel was capable of showing change the image from one land use to another. Then, based on the major changes in the region in the survey, three sub-models of shifting change were identified as transforming pastures into habitat areas, converting agricultural production into settlements, and transforming dryland farming into settlements. After calculating the potential for the transfer of any land use to a settlement using descriptive data, a plan for predicting the use of land for 2025 and 2040 was then provided. Given that the purpose of the present study was to assess the development of residential areas, the extent of changes in these areas were assessed during the years 2000 to 2012. The results indicate that the residential areas increased from 8.3 square kilometers in 2000 to 12.6 square kilometers in 2012, according to the land use map, and mostly changes in the urban area of Songhor area have been made. The results of the assessment of changes indicate that the land use change from irrigated agricultural to residential use during the 12 year period was 1.9 km2, which for dryland agriculture it was 0.6 kilometers, Also 1.8 km2 of rangelands has become residential. The results of this study indicate that the irrigated agricultural lands of the city of Sangar, especially the southern regions and pastures near the urban area, have had most changes. Among the changes in other uses, about 11.5 km2 of the rangeland has been converted into rainfed farming, and about 12.3 km2 of land has also become rangelands and also, about 4.7 km2 of irrigated agricultural has become arable land or Bayer land and about 1.5 km2 of rangelands has become irrigated agricultural land. The growing population has led to an increase in the number of habitat areas and, as a result, agricultural lands and pastures have undergone changes. The growing trend of settlement development varies from region to region, and in the urban area of Songhor more are moving toward the southern regions of the urban area. Considering the geomorphologic status of the study area, a large part of the range is covered by rangelands. Irrigated agricultural lands which have a significant share, are located on the outskirts of the city of Songhor, which are undergoing further changes. According to the main objective of the research, based on descriptive data such as distance from communication, distance from urban boundaries, elevation and slope, the amount of development of residential areas for 2025 and 2040 is also projected. The results of the forecast indicate that in the case of the growing trend, the development of the settlements will reach about 18.2 km2 in 2025, and will reach 24.2 km2 in 2040, due to the high potential of the southern regions of the city of Songhor, the highest rate of development of settlements will be towards these areas. The results indicate that the increasing number of settlements in the city of Songhor will lead to the degradation of high-quality agricultural lands and pastures. If the trend is continued, the irrigated agricultural around of the city of Songhor will reach the lowest level by 2040. Also most of the pastures will also be degraded. Hence, it is necessary to identify areas suitable for the development of a settlement before increasing of rate the destruction occur, so that less prone areas for agriculture and pastures can be degraded.
    Keywords: Settlement, Songhor, Land Use, Changes, LCM
  • Payman Mohammadi Ahmadmahmoudi, Asadollah Khoorani * Pages 355-371
    Snow cover (SC) is the main part of global cryosphere that covers about 46 million square kilometer of earth surface. Seasonal SC and snow storage provides the required water of around 1/6 of world population. About 98 percent of global SC is in the northern hemisphere. Southern and southwestern mountain range of Iran, called Zagros is located in the mid-latitudes that is exposed to the northern hemisphere westerlies resulting in high amount of rain and snowfall. This region contains 40 percent of Iran's water storage. Monitoring and evaluation of seasonal SC is essential for managing winter tourism, transportation, environment, land cover, water and irrigation and also essential in runoff modeling. In our study area snow gauges are insufficient and sparse, therefore monitoring spatial changes of Snow Cover Area (SCA) is difficult. MODIS images lunched on TERRA and AQUA platforms are suitable data source for estimating SC, because they provide daily images with 500 meter spatial resolution. The aim of this study is detecte and analyze SC variations and trends in Zagros mountain range using daily images of MODIS in order to determine SC variation. In this research, 348 daily products of MODIS for first days of snowy months (Months with some snow on the ground in Zagros; Dec, Jan, Feb, Mar, Apr, May, Jun) are obtained from National Snow & Ice Data Center (NSIDC) to calculate SCA. Digital Elevation Model (DEM) is used to determining snow line (SRTM 90m from Earthexplorer.usgs.gov) and total monthly precipitation of weather stations above 1500 meter elevation of the same period with SC data is used. Snow line is extracted by overlaying DEM on SCA. Then the snow line pattern is determined as the average of snow line for each day during 2001 to 2016. SCA pattern is also extracted as the average of each day during the same period. In order to calculate trends in SCA and Snow Line Man-Kendal test is used. Man-Kendal test is less sensitive too extreme values of time series. The trends of SC are investigated using Man-Kendal test results show a decreasing trend in January, May and June in 2007-2016. The highest amount of SCA decrease and snow line retrogression for January, May and June are recorded in 1250-4500 m, 2300-4500 m and 3500-4500 m altitude belts, respectively. Based on SC departure from the average, two periods are recognizable: first, from 2001 to 2008, in this period SC is above the average and is more similar to the snow cover pattern of the whole period (2001-2016). Second period from 2009 to 2016, in this period SC is more below the average and is not similar to the snow cover pattern of the whole period (2001-2016). In the second period mean SC reduction compared to the first period in Dec, Jan, Feb, Mar, Apr, May and June are 4, 67.6, 24.5, 21, 10, 47.3 and 48.4 respectively. Mean monthly precipitation changes of weather stations above 1500 meter elevation is similar to the changes of SC changes.
    Keywords: Snow Cover, Zagros, MODIS, Water Resources
  • Amir Bagheri Mosleh, Abadi, Abbas Ali Aliakbari, Bidokhti *, Maryam Gharaylou, Reza Khalife Pages 373-382
    Fog formation reduces the visibility, and low visibility cause problems mainly in airports and roads. Fog modification or even clearance can reduce such problems. Here, in the laboratory the effects of some aerosols on the modification of artificial fog have been investigated. This research is carried out using the method used in the work of Vajaiac et al. (2014), for hydrophilic particles. These particles, as the fog modification factor, can be effective as condensation nuclei by increasing the effective radius of the droplets and reducing the concentrations of the fog droplets in the fog.. We used a glass chamber with a height of 41cm and diameter of 28 cm in which a red laser and a detector were placed on both sides of the chamber. We dissolved water-soluble compounds in water and by pumping the air into water of the chamber, the bubbles are formed and their breaking led to formation of particles. Then the particles are dispersed inside the chamber, while the pressure inside the chamber was increased with an air pump, and so the temperature was rised. Then suddenly or quasi-adiabaticaly, the pressure is released and hence, the air temperature was dropped creating the fog (cloud). The cloud droplets are mainly formed on these particles. With the formation of droplets, the received laser signal was decreased and after falling all of droplets, the received laser signal reached its primary level. Hence, by measuring the level of drop in the signal and its duration some properties to the cloud structure can be found. In this study, we used sodium hydroxide, urea, and salt particles that was compared with background aerosols affecting the cloud. First, the optical depth of fog was calculated and then size distribution of fog droplets was estimated. Also other parameters like the concentration of the fog droplets, liquid water content and fog lifetime were evaluated. The results show that hydrophilic particles as condensation nuclei played a significant role in fog modification. So, in the presence of these particles, the concentration of fog droplets in comparison with that of the background aerosols, was decreased and the fog was diluted significantly. The maximum of fog lifetime was observed in the presence of background aerosols as 63 s, also these particles have had the lowest effective radius of 7.77µm. In the presence of sodium hydroxide particles, the concentration of total droplets was decreased significantly. In addition, the maximum effective radius was 10.48 µm for these particles. Also, fog lifetime was reduced to 31s which suggests that this component for fog modification has a better performance. The area under the curve’s of the size distribution of droplets for salt and urea were nearly identical, that indicated the concentration of the droplets was close to each other. Parasitic structures might appear in curves of the size distribution of droplets that were smoothed. As the time passed all of the effective radius curves showed decreasing trends due to the fact that, the larger droplets can droped out of the laser beam, so gradually the effective radius should be decreased. In comparison with other compounds, when sodium hydroxide particles were used in the cloud chamber, observationally small droplets that moved up an down in the laser beam weren’t present, hence no oscillations were seen in the laser signal. Considering experimental errors, while neglecting the process of coalescence that might occur in the falling stage of the droplets, when the droplets were passing the laser beam, they appeared at larger sizes, which could lead to an overestimation. The signal noises and non-uniform distribution of condensation nuclei could also be considered as experimental errors.
    Keywords: Hydrophilic particles, Experimental study, Droplets, Fog modification
  • Azar Jalilian, Hasan Zolfaghari *, Jalil Sahraei Pages 383-400
    The narrow bands of strong wind (≤30ms-1) in the upper levels of the atmosphere called jet stream. The formation of these streams in the maximum gradient zone of the atmosphere (the location of the maximum temperature and energy transition) has caused the establishment and movement of their speed nuclei at the western extremities of the equatorial fringes and on the polar front in the extraterrestrial region and the formation of western margin and polar fronts.  This winds blow from west to east, but they often shifts to the north and south of seasonal average position. In this case, jets may be deployed simultaneously in one region and occurrence the interaction between them may affect the heavy rainfall. Based on several definitions that have been presented with heavy rainfall according to different criteria and thresholds, Cumulative precipitation of more than 30 mm in 24 hours, causing severe financial losses to various urban and rural areas, is called heavy rainfall. In This article, shifting and intensity of subtropical - polar front jet stream in the mid-latitude region and that’s effects on heavy precipitation in Iran has been studied. In this study, the effects of interactions patterns subtropical-polar front jet stream on heavy spring and autumn rainfall in Iran (1988-2017) with Atmospheric circulation to environment Approach has been investigated.  Therefore after determining the maximum coefficient of variation of this jet streams, in order to determine the effective circulation patterns in these synchronies, by applying factor analysis by PCA on the data of a height of 500 hp, four main components with The explanation of 82% of data diffraction was determined.Then, by using cluster analysis method, the days in each cluster and by intra-group correlation, the representative day of each cluster was determined.After drawing different patterns of simultaneous deployment in the Surfer software, four synchronous patterns were determined in the range of maximum coefficient of variation of jet stream (27-40°N, 36-56 ° E). Finally, the effects of each of these patterns on the occurrence of heavy rainfall were determined by obtaining divergence, vertical velocity, vorticity, relative humidity, specific humidity, temperature and mean sea pressure from the ECMWF center. The results of the study show the effects of three interaction patterns on heavy rainfalls. The effects of direct and indirect jet streams interactions in the first and second patterns, with the change in the thickness of the polar fronts and the interaction of jets in the third model, has been affected on atmospheric conditions and occurrence of heavy rainfall. Despite the direct interactions of the first pattern of jet stream synchronization in the rainfall concentration, the indirect interactions effect of the second pattern on the cyclone circulation boost of the polar jet stream has led to the occurrence of extensive rainfall. Also, the mutual interactions of the jet stream in the third pattern, with the intensification of the frontogenesis process, have been effective in the occurrence of precipitation along these fronts, while the proximity of these opposite turns in the fourth jet streams concurrency pattern is not effective on rainfall, and the occurrence of heavy rainfall has been affected only from the cyclonic circulation of the subtropical jet stream.
    Keywords: heavy rainfall, polar front jet stream, subtropical jet stream, interaction, spring, autumn, Iran
  • Mansooreh Abdollahi, Farhang Ahmadi, Givi *, Mohammad Mirzaei Pages 401-421
    Tropopause folds are intimately linked to upper level frontogenesis and jet stream dynamics. They play an important role for stratosphere-troposphere exchange, the dynamical coupling of upper and lower tropospheric levels, and for generating severe weather events. In this study, the effect of upper-level positive potential vorticity (PV) anomaly on upper- and lower-level frontogenesis over the Middle East and Iran is investigated. In this regard, first three frontal systems associated with deep tropopause folding and strong fronts were selected by using the ECMWF data with the horizontal resolution of 0.75×0.75 degrees on the latitude and longitude. Then, PV anomaly was removed by replacing the zonal mean of PV in the study area and inverted to obtain the modified fields. To do this, a program package (PV inversion), comprising several different steps, is used which allows to isolate PV elements and then to study their impact on the atmospheric flow field as well as the temperature distribution. In the next step, the weather research and forecasting (WRF) model was applied by using the ECMWF data to perform two simulations with real (unchanged) and modified data, as initial conditions, in two domains with 9km resolution for the inner domain. Finally, the frontogenesis function terms, including deformation, tilting, diabatic heating and vertical frontogenesis were computed, using the WRF outputs. By comparing the results of the two simulations, we can determine the effect of tropopause folding on the frontogenesis function terms in the upper- and lower levels throughout the lifecycle of the fronts. Results show that in the absence of tropopause folding, the horizontal and vertical temperature gradients, horizontal velocity, as well as negative vertical velocity are decreased significantly in the upper and lower levels. Also, positive vertical velocity is increased and its pattern is changed mainly in the lower levels. Generally, large positive values of deformation and vertical frontogenesis terms are collocated well with the gradient of potential temperature (frontal zone), and these terms are declined due to decreasing of horizontal velocity and temperature gradient when the fold of tropopause is removed. The tilting term does not have a fixed pattern in the upper and lower levels, and it follows the vertical velocity pattern. By removing the tropopause folding, vertical velocities are changed, thereby having variable effects on the tilting term and the gradient of potential temperature. The diabatic heating term produced by the release of latent heat intensifies ascending motions, and so affects the gradient of potential temperature (frontogenesis) in the lower levels. This term is also increased in the absence of tropopause folding in the lower levels. The other noticeable point is that the diabatic heating term does not significantly affect the frontogenesis in upper levels, because heating mostly takes place in the lower half of the troposphere far to the east of the upper level front. The total amount of frontogenesis function follows the vertical frontogenesis term in the upper and lower levels, and vertical gradient of potential temperature is generally very large. Although, the amount of the diabatic heating term is larger than the vertical frontogenesis term in the lower levels, but it is limited to a small area. Removal of the tropopause fold causes the total amount of frontogenesis function, similar to the vertical frontogenesis term, to decrease throughout the region of the upper-level front, especially in the downstream of the upper-level trough close to the center of tropopause folding omission. The results of the three cases studied here indicate that frontogenesis function terms are considerably more intense in the presence of tropopause folding. Therefore, it is concluded that tropopause folding has a remarkable positive effect on the formation and intensification of the upper-and lower-level fronts.
    Keywords: Potential vorticity, tropopause folding, frontogenesis, simulation, upper-, lower-level fronts
  • Amene Mollasharifi, Ali Reza Mohebalhojeh *, Farhang Ahmadi, Givi Pages 423-440
    The weather and climate are affected significantly by the storm tracks, which play an important role in midlatitude atmospheric dynamics. There are indications that the Mediterranean region and its downstream areas like the western Asia are affected by the North Atlantic (NA) storm track. As NAO is the most important factor affecting weather in Europe and the Mediterranean region by altering the location and direction of storms, the NA and Mediterranean storm tracks are influenced by this teleconnection. On the basis of the monthly index of the NAO, the critical positive (negative) months, denoted by NAO+ (NAO-), are defined as the ones with the NAO index greater (less) than the long-term mean of the NAO index, plus (minus) one standard deviation. The aim of this research is to study the impacts of the NAO on the NA and Mediterranean storm tracks from the energetic point of view by the JRA-55 reanalysis dataset and to compare the results with those previously obtained by the NCEP/NCAR reanalysis dataset. To this end, composite maps of the most important terms of the time tendency equation of eddy kinetic energy (EKE), as well as the baroclinic generation term, were computed for the critical positive and negative months of the NAO using both datasets in 1959–2017 period for the winter.
    Based on the both dataset’s results, although though baroclinicity plays the most important role in increasing EKE in the western NA, it is not very important in Western Europe and the Mediterranean region. Instead, convergence of total energy flux is the dominant factor in producing EKE in the latter areas. Interestingly, the ageostrophic flux has a greater role in strengthening the Mediterranean storm track compared to the NA storm track.
    Results of the JRA-55 are generally consistent with the results of the NCEP/NCAR, and the two datasets lead to almost similar patterns for energetics but some differences are also observed in terms of magnitude and extension of the main centers. In almost all the fields, the JRA-55 results show higher values with more details. The JRA-55 results display stronger and more extended maxima for the NA and Mediterranean storm tracks in both phases. The difference between the storm tracks in the two datasets, in terms of EKE amplitudes, is more clear in the negative phase of NAO. In NAO-, the maxima of the NA storm track is about 10% stronger in the JRA-55 results than that of the NCEP/NCAR. Also, both the northern and southern branches of the NA storm track are more intense with a greater eastward extension in the JRA-55. In a similar way, the Mediterranean storm track obtained from the JRA-55 dataset is also more intense with a greater extension, especially over the east and south of the Mediterranean and its downstream regions like the Middle East, than the corresponding results by the NCEP/NCAR. In the negative phase, the difference between the maximum of divergence over the Mediterranean Sea and the maximum of convergence over the Red Sea is larger in the JRA-55. In NAO- this can play an important role in making the strength and spread of the Mediterranean storm track in the JRA-55 greater than those of the NCEP/NCAR. In addition, the impact of NAO phases on the intensity of the divergence (convergence) center over the Mediterranean (Red) Sea is greater when the JRA-55 data are being used.
    Unlike the previous results on the NA storm track as being stronger in the positive phase of the NAO, results of the JRA-55 dataset do not exhibit a noticeable difference between the two phases. In addition, although being consistent with the NCEP/NCAR results in the way the Mediterranean storm track is related to the NA storm track in each phase, however the JRA-55 results show weaker linkage between the two storm tracks in NAO- when compared to previous studies.
    Keywords: NAO, storm track, eddy kinetic energy, baroclinic generation, NCEP, NCAR, JRA-55
  • Fateme Bahmanzade, Sarmad Ghader *, Seyed Abbas Haghshenas, Daniel Yazgi Pages 441-458
    Reliable and sufficient information of 10-m wind and temperature fields over open seas and near coastlines is a necessary and important data that has impact on many marine activities. Assimilation of NWP models can be used to assess an estimation of these fields. This study reports the performance of the weather research and forecasting (WRF) model to hindcast 10-mwind and temperature fields that were evaluated under two different physical options of planetary boundary layer (PBL) and surface layer (SL) for an area over the Gulf of Oman and the Arabian Sea. The case study includes 16 simulations of 8 different days from WRF model version 3.7.1. The WRF model is configured with two nests. Parent nest has 0.3 degree and the inner nest has 0.1 degree horizontal grid resolution. The grid spacing of the inner domain is almost 11-km. The Lat-Lon (latitude-longitude) method is used as the map projection method. For all domains and all runs 39 terrain following vertical levels are set. The validation of the simulated fields is done considering two observational datasets (the weather stations for 10-m wind and 2-m temperature and satellite instruments just for 10-m wind). Near-surface observations of 2-m temperature and 10-m wind speed and direction are collected from 55 weather stations, located within the chosen area. The measurements from satellite instruments have become an important source of data in the regions that in-situ observations are sparse like seas and oceans, hence observations from two different scatterometers (ASCAT and OSCAT) are also used to evaluate 10-m wind simulations. Moreover, in order to better understand the model performance for different choices of the physical schemes, sensitivity of the model has been investigated. There is plenty of choices for the combination of parameterization schemes available for WRF model; for the current study two configurations are taken from other’s previous published research works. The physical parameterization that used in this study are Revised MM5 and Monin-Obukhov for surface layer and MRF and MYJ for planetary boundary layer. These choices are used to create two different configurations called Phys1 and Phys2. Comparison between winds from satellite scatterometer and simulated winds show very little difference and hence good agreement with observations. Acceptable accuracy has been obtained from statistical analyses. These analyses demonstrate that the maximum average RMSE of wind field is 2.39 m/s, based on results of comparing with ASCAT data and it is 2.37 m/s, based on results of comparing with OSCAT data. The analyses also show that simulation of wind fields have better results over offshore regions than coastlines weather stations. The outcome shows that the simulated 10-m ‎wind‎ present acceptable general skills over the sea. The validation of 2-m temperature presents that the model has a proper estimation about temperature field over the coasts and near coastal station within the simulation domain. The maximum average RMSE of temperature field is 2.6 degrees of centigrade. Finally, without any justification to run WRF for longer periods from a quantitative and qualitative assessment of the results, it can be concluded that for the WRF model has an acceptable performance to simulate 10-m wind and 2-m temperature over the Gulf of Oman and the Arabian Sea. It should be noted that to verify these results for longer periods more similar experiments must be tested.
    Keywords: The WRF model, Satellite data, ASCAT, OSCAT, Oman Gulf, Arabian Sea
  • Nafiseh Pegahfar * Pages 459-472
    Tropical cyclones (TC) have been investigated from different points of view. Development of forecast of TC intensity and its track is often the shared purpose of all previous researches. To this aim, various empirical indices and different frameworks, based on various parameters, have been defined to provide deep knowledge of TC dynamics and thermodynamics. In this research, using the thermodynamic parameter of entropy, entropy fluxes (including surface, lateral and vertical fluxes) have been calculated. A theoretical framework based on hypothesized mechanism, introduced by Tang and Emanuel (2010), has been used to calculate the vertical flux of entropy. This ideal framework used a set of rigid assumptions including steadiness, axisymmetry and slantwise neutrality to assess the effects of vertical entropy flux on TC intensity via the possible pathway of downdrafts outside the eyewall. The lateral entropy flux has been computed based on radial component of surface wind. Azimuthal average of lateral entropy flux has been calculated to analyze vertical extension and strength of inflow (in the lower part of boundary layer) and also outflow (in the upper part of troposphere). Here, Tropical Cyclone Gonu (TCG) has been focusedon. TCG, formed at 18:00 UTC 1 June 2007 and decayed on 7 June, passed intensity of Saffir-Simpson Category-5 and affected southern coast (Makran) of Iran. All above parameters have been computed and analyzed during TCG lifetime using (1) Era-Interim reanalysis data (from European Center for Medium Range Weather Forecast) with 0.125 degree horizontal resolution, 12 vertical levels from 1000 to 200 hPa and 6-hour time intervals, and (2) data produced by India Meteorological Department. The variables were used both at the surface and also at pressure levels, the surface values were temperature and humidity (both at 2 m height), wind vector (at 10 m height), mixing ratio and sea level pressure. Synoptic–scale analysis has been done using data in a circular region centered by TCG center with a radius of 500 km. Results of horizontal patterns and time series of radial and tangential components of wind vector indicated that the value of radial component was maximized simultaneously with maximum activity of TCG. At TCG peak activity time, the tangential component had a comparatively minimum value embedded between two relative maximum values. Time series analysis showed that the integrated values of all three parameters of surface, vertical and lateral entropy fluxes experienced their extremum values before TCG reached its maximum intensity. It is worthwhile to be noted that their lead time varied from 6 hours (for surface entropy flux), 18 hours (for lateral entropy flux) to 30 hours (for vertical entropy flux). A comparative analysis between the values of entropy fluxes during TCG and those for Haiyan Tropical Cyclone (TCH, the strongest TC formed over the Pacific Ocean), reported by Pegahfar and Gharaylou (2019), indicated that entropy surface flux and lateral entropy flux during TCG were respectively two and one order of magnitude larger than the related values during TCH. In contrast, TCG experienced entropy vertical flux with two orders of magnitude smaller than that during TCH. Hence it can be concluded that the accumulation of energy helped TCG to travel to the higher latitudes. Moreover, the strongest inflow and outflow occurred before and after TCG maximum intensity, respectively. In a period that TCG reaches category-5 intensity and more, vertical extension of inflow layer was minimized while outflow layer started from the lower levels, comparing with results from the beginning of TCG life cycle. Conclusively, findings of this research showed that surface, vertical and lateral entropy fluxes, even in synoptic scale, have the ability to be served as empirical indices and also need to be focused in theoretical, computational and practical frameworks, for all prognostic purposes of TC intensity.
    Keywords: tropical cyclone Gonu, (surface, lateral, vertical) entropy fluxes, radial wind, inflow, outflow
  • Hamed Amini, Abazar Esmali, Ouri *, Raoof Mostafazadeh, Meraj Sharari, Mohsen Zabihi Pages 473-486
    Due to the importance of rivers in water supply, it is crucial to understand the flow condition and river regime. The river flows are affected by dam construction, climate change, flow diversion, backwater and urban development, which cause various problems, such as drought, flow reduction and environmental issues. Hydraulic structures have great impacts on the hydraulic conditions of rivers downstream. The reservoir dams as the most important hydraulic structures constructed on rivers, have considerable changes on flow regime conditions, therefore, drought and river flow drying area are major impacts of dams on downstream rivers. One of the common types of droughts is the hydrological drought, which is followed by meteorological drought with a delayed time. The most commonly definition of hydrological drought is based on a truncation level. The hydrological drought occurs when the river flow falls below a defined threshold level. The Yamchi and Sabalan dams are two main important dams in Ardabil Province due to the water supply for drinking water and irrigated agricultural lands.
    Due to the consequences of short and long-term socio-economic and environmental effects of droughts as a natural disaster, it is necessary to study the frequency, severity, and spatial extent of drought spells. Accordingly, the drought characterization plays an important role in macroeconomic planning and decision-making to achieve sustainable development goals. The purpose of this study is to evaluate the effect of the Yamchi and Sabalan dams on the flow regime and hydrological drought occurrence of the downstream rivers, areas.
    The used data in this study were the daily discharges, which were obtained from four hydrometric stations located upstream and downstream of the dams under the study. In this study, the seasonal threshold of was chosen and drought characteristics of the upstream and downstream stations of each of the dams were evaluated and then the drought severity at different durations was calculated using the Runs theory. Then, the Easy Fit software was employed to calculate the drought intensity at different return periods. The drought severity amounts were plotted over different return periods and the Severity-Duration-Frequency curves that were drawn for the upland and downstream of the constructed dam reservoirs. The percent change in discharge at the upstream and downstream stations of the studied dams were calculated to quantify the degree of influence.
    According to the results, the highest drought occurrence in both studied dams were observed at 1-month duration and at Pole-almas and Dost-beiglou river gauge stations. It can be concluded that the severity of the observed hydrological droughts increases under the influence of both Yamchi and Sabalan dams as the return period increases. Also, the most severe drought spells occurred is related to the Sabalan dam and its upstream station (Arbabkandi). According to SDF curves, the drought severity increases in higher drought durations in the upstream and downstream stations of the Yamchi dam. According to the results, the percentage of changes in the downstream hydrometery station of the Sabalan dam is positive in almost all water years, which confirms the positive effect of Sabalan Dam on hydrological drought reduction and flow regulation. The highest percentage of changes in both of these dams is for to June, which indicates the difference in discharge values in upstream and downstream of the dams. In other words, this indicates that both of the studied dams, especially the Sabalan Dam, have increased the monthly base flow rate over the downstream reaches.
    It can be concluded that the construction of the Yamchi Dam caused a decrease in the flow amount in the downstream area and increased the occurrence of hydrological drought events. While, the Sabalan Dam regulated the flow regime of the Garesou river in the dam downstream and reduced the severity of hydrological drought spells and modifies the river flow regime for all months of the year.
    Keywords: Drought Severity, SDF curves, Threshold level, Flow Regulate, Yamchi Dam, Sabalan Dam