فهرست مطالب

فیزیک زمین و فضا - سال چهل و سوم شماره 3 (پاییز 1396)
  • سال چهل و سوم شماره 3 (پاییز 1396)
  • تاریخ انتشار: 1396/07/29
  • تعداد عناوین: 15
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  • معصومه فرزانه، سید رضا مهرنیا، تقی نبئی* صفحات 461-472
    معدن سرب و روی مرجان آباد تحت تاثیر زون های گسله و تکتونیکی بوده وکانی سازی درامتداد زون های گسله قرار دارد. سنگ های منطقه بیشتر از نوع ریولیت، داسیت و به مقدار کم آندزیت و بازالت و از نوع آلکالن تا کالکو آلکالن هستند. از جمله بافت های موجود در این کانسار، بافت رگه ای است. بی هنجاری های مغناطیسی و گرانی در این منطقه مشاهده شده است که ناشی از آثار تکتونیکی محیط و کانه زایی در بین رگه ها هستند. اغلب سنگ های این منطقه در محدوده سنگ های اسیدی قرار گرفته اند. با استفاده از داده ها و مطالعات صحرایی ژئوفیزیکی چهار منطقه امیدبخش معدنی مشخص شده اند که با یکدیگر هم پوشانی دارند. هم پوشانی نتایج بی هنجاری های مغناطیسی وگرانی در این چهار منطقه می تواند مهم باشد، زیرا هرجا دو بی هنجاری مغناطیسی وگرانی هم پوشانی داشته باشند، احتمال پیدایش کانسار بیشتر است. در مجموع در تحلیل اطلاعات ژئوفیزیک هوابرد این چهار منطقه نتایج مثبتی ارائه شده که عبارت اند از: دست یابی به الگوی گسل های منطقه و معرفی مناطق امیدبخش معدنی بر اساس بی هنجاری ها؛ زیرا دراین مناطق با توده های آذرین ساب ولکانیکی رو به رو هستیم که براثر فعالیت های پساماگمایی سنوزوئیک تحت تاثیر محلول های هیدروترمال قرارگرفته و ضمن پیدایش هاله های دگرسانی، موجب شکل گیری کانه ها در امتداد رگه ها شده است. تیپ این کانسار به تیپ کانسار دره می سی سی پی شبیه است.
    کلیدواژگان: بی هنجاری، سرب و روی، گرانی، می سی سی پی، مرجان آباد، مغناطیسی
  • سعید فرزانه، محمدعلی شریفی*، منا کوثری صفحات 473-487
    در این مقاله نقش پارامترهای گوناگون مربوطه در افزایش قابلیت ماموریت های هم مدار پس از GRACE بررسی خواهد شد. این باز طراحی با توجه به توسعه دانش و فناوری روز بر اساس استفاده از فناوری لیرزی به جای طول موج باند K، کاهش ارتفاع زوج ماهواره با فرض استفاده از فناوری پیشرانه فعال و موارد دیگر صورت خواهد گرفت. این نوع ماموریت ها علاوه بر پر کردن خلا ناشی از پایان عمر ماموریت GRACE، در بهبود مدل سازی میدان ثقل نیز کمک شایانی خواهند داشت. در این مقاله پارامترهای طراحی یک ماموریت هم مدار شامل طراحی مدار، تعیین وضعیت زوج ماهواره نسبت به هم (فاصله بین زوج ماهواره) و دقت سیستم مشاهداتی بر اساس فناوری موجود و قابل دست یابی، بررسی خواهد شد. در همین راستا، در تعریف مدار ماهواره، شرط مدار تکرارشونده نیز در نظر گرفته شده است. در این شبیه سازی ارتفاع زوج ماهواره را با فرض مجهز شدن به سامانه های پیشرانه ای فعال تا 350 کیلومتر کاهش دادیم. علاوه بر آن با توجه به مزیت اصلی سیستم های لیزری (افزایش دقت تا دو رقم اعشار) و محدودیت این سیستم (برد محدود تا 100 کیلومتر)، جایگزینی آن سیستم را با سیستم فاصله سنجی باند K به عنوان یک سوال مهم مطالعه و بررسی کرده ایم. نتایج شبیه سازی نشان داد که با توجه فناوری موجود، استفاده از ابزارهای لیزری باوجود دقت بیشتر تا دو رقم اعشار، به دلیل برد محدود به 100 کیلومتر نمی تواند نتایجی بهتر از ابزار سنجش تغییرات فاصله به کمک طول موج باند K داشته باشد.
    کلیدواژگان: سیستم های فاصله سنجی، طراحی مدار ماهواره، مدل سازی میدان ثقل، ماموریت های فضایی جاذبی، ماموریت های هم مدار، ماموریت GRACE
  • محمدعلی شریفی، محمدرضا سیف صفحات 489-499
  • امین عباسی* صفحات 501-520
    مکان یابی قابل اعتماد زمین لرزه یکی از مهم ترین معیارها در بررسی لرزه خیزی، لرزه زمین ساختی و تحلیل خطر لرزه ای است؛ در این بین، تعیین ژرفای رویدادها، چالش برانگیزتر و البته مهم است. در این مطالعه، لرزه های ثبت شده همزمان توسط شبکه محلی موقت لبه جنوبی البرز مرکزی در سال 2006 و شبکه منطقه ای لرزه نگاری تهران، ارزیابی شده اند. برای این کار، رویدادهای ثبت شده در هر دو شبکه، بدون تصحیح زمانی در فایل گزارش داده ها (S-files)، در نرم افزار سایزن (SEISAN) و با کمک مدل ساختار سرعتی پوسته و نسبت سرعت امواج لرزه ای (Vp/Vs) یکسان، دوباره تعیین محل (relocation) شده و پارامترهای وابسته به مکان یابی رویدادهای دارای ثبت زمانی مشابه، مقایسه شده اند. نتایج این بررسی نشان می دهد رویدادهای دارای ویژگی های دقت مناسب در خواندن فاز(معمولا بیش از 8 فاز)، حداقل فاصله از ایستگاه ها (Dmin) کمتر از 25 کیلومتر، ثبت در بیش از 6 ایستگاه (NST)، پوشش آزیموتی مناسب (180°≥Gap ) و خطای باقیمانده زمانی (RMS) کمتر از 35/0 ثانیه در هر دو شبکه، دقت و برازش بهتری داشته اند. میانگین خطای مکان یابی در رومرکز و ژرفای رویدادهای انتخابی مشابه، در شبکه منطقه ای عموما بیش از 6 و 15 کیلومتر و در شبکه محلی به ترتیب کمتر از 3 و 5 کیلومتر بوده است. همچنین با وجود اهمیت خطای باقیمانده زمانی، کاهش این متغیر به تنهایی برای بهبود تعیین محل رویداد، کافی نبوده است. در حالی که کاربرد مدل پوسته همراه با سایر پارامترهای مناسب دیگر، به ویژه میانگین فاصله ایستگاهی و کاهش همزمان خطای باقیمانده زمانی، موجب افزایش دقت (و کاهش خطای) مکان یابی شده است. در نتیجه، معیارهای مکان یابی در شبکه محلی بهتر از شبکه منطقه ای بوده است.
    کلیدواژگان: داده های زمانی مشابه، مکان یابی، محلی، منطقه ای
  • تقی شیرزاد* صفحات 521-529
    امروزه تداخل سنجی لرزه ای به عنوان روشی متداول برای تعیین تابع گرین تجربی بین دو گیرنده استفاده می شود، به طوری که یک ایستگاه به عنوان چشمه و ایستگاه دیگر به عنوان گیرنده به کار برده می شود. در این روش، توابع گرین با استفاده از همبسته سازی سیگنال های ثبت شده در دو ایستگاه و سپس برانباشت آن ها حاصل می شود. در مواردی توابع همبسته شده و به تبع آن تابع گرین بین ایستگاهی، به دلایلی نظیر عدم پوشش متقارن چشمه های لرزه ای و توزیع غیریکنواخت انرژی در محیط، به درستی/ به طور صحیح تعیین نمی شود. در این موارد می توان از ماتریس توابع همبسته شده با مرتبه کمتر، با استفاده از روش جدایش مقدار تکینه، برای بهبود این ضعف ذاتی روش تداخل سنجی بهره جست. روش جداسازی مقدار تکینه، امکان جداسازی سیگنال های همدوس با فازهای ایستا را از سیگنال های غیرهمدوس با فازهای غیرایستا فراهم می کند. دراین مطالعه برای بررسی این امکان جداسازی در راستای بهبود تابع گرین بین ایستگاهی، از سه دسته داده مصنوعی استفاده شده است. این داده ها، شامل داده های ثبت شده حاصل از چشمه های داخل منطقه فرنل با انرژی یکسان، چشمه های خارج منطقه فرنل با انرژی یکسان و چشمه های داخل و خارج منطقه فرنل با انرژی متفاوت هستند. نتایج مطالعات این سه دسته از چشمه ها، امکان تعیین صحیح تابع گرین تجربی را برای محیط همگن و همسانگرد با چشمه هایی در داخل منطقه فرنل (حتی با انرژی کمتر از چشمه های خارج منطقه فرنل) تایید می کند. همچنین نتیجه این مطالعه نشان می دهد که برای حالت هایی که صرفا دارای چشمه هایی خارج از منطقه فرنل هستند، امکان بازسازی صحیح توابع گرین بین ایستگاهی غیرممکن است.
    کلیدواژگان: تداخل سنجی، جدایش مقدار تکینه، سری زمانی مصنوعی
  • امین اسماعیلی، محمد جواد کلایی* صفحات 531-537
    اخیرا با مطالعه آماری مشاهدات ماهواره کلاستر (CLUSTER satellite system) یک لایه گذار در داخل غلاف مغناطیسی گزارش شده است. این لایه باMA=1 (Alfven Mach) به جریان کلی ذرات (Bulk flow) اجازه می دهد که از فراآلفونی به فروآلفونی از طرف خارج کاسپ (Cusp) بیرونی به سمت داخل کاسپ تبدیل شود و در حالت باد خورشیدی با جهت میدان مغناطیسی بین سیاره ای (Interplanetary Magnetic Field (IMF) شمالی مشاهده شود، اما نتایج شبیه سازی وجود آن را در شرایط باد خورشیدی با جهت میدان مغناطیسی بین سیاره ای جنوبی در طول تمام مرز مغناطوسپهر نیز نشان می دهد. نقش این لایه به منظور درک تغییرات جریان کلی ذرات در نزدیکی مرز کاسپ بیرونی مهم است. به منظور تحلیل این لایه، از یک شبیه سازی سه بعدی ذره ای با اندازه بزرگ مقیاس برای برهم کنش باد خورشیدی با مغناطوسپهر زمین استفاده شده است. ما IMF را از جهت شمال به سمت جنوب چرخانده ایم تا دینامیک لایه گذار آلفونی (ATL) را با توجه به چرخش IMF بررسی کنیم. اگرچه در مطالعات آماری کلاستر (CLUSTER) تنها منطقه محدودی از مغناطوسپهر برای این لایه آلفونی مطالعه شده است، ما این لایه را برای کل مغناطوسپهر بررسی کرده و دینامیک آن را با توجه به چرخش IMF از شمال به سمت جنوب بررسی کرده ایم و در نهایت نتایج به دست آمده از شبیه سازی را با مشاهدات تجربی مقایسه کرده ایم. نتایج حاصل از شبیه سازی برای جریان های موازی ذرات و انتقال به ارتفاعات کمتر لایه گذار آلفونی با مشاهدات حاصل از ماهواره کلاستر برای حالت IMF جنوبی سازگار است.
    کلیدواژگان: کاسپ در ارتفاع بالا، لایه گذار آلفونی، ماهواره کلاستر، مغناطوسپهر
  • محمدعلی نصر اصفهانی*، علیرضا محب الحجه، فرهنگ احمدی گیوی صفحات 539-552
    در این مطالعه با استفاده از داده های بازتحلیل NCEP/NCAR و شاخص چندمتغیره نوسان مدن-جولین تلاش شده است تا آثار فازهای مختلف این پدیده بر توزیع برخی از کمیت های مهم هواشناختی در منطقه خاورمیانه، بررسی و برای آن توضیح فیزیکی ارائه شود. به این منظور دوره های بحرانی MJO از سال 1974 تا 2015 براساس شاخص آن تفکیک شده ومیانگین و بی هنجاری کمیت های منتخب به دست آمده است. نتایج نشان داد که اثر هم شاری و واشاری ناشی از MJO روی اقیانوس هند تا خاورمیانه و شرق دریای مدیترانه نیز گسترش می یابد؛ به این صورت که ترابری جرم از مرکز همرفت به سوی شرق دریای مدیترانه در ترازهای بالای وردسپهر در فاز چهار سبب حرکت های فروسو در این منطقه می شود. این چرخه در فازهای هفت و هشت MJO عکس می شود و بی هنجاری حرکت های فراسو و واگرایی (همگرایی) در ترازهای بالای (پایین) وردسپهر در شرق مدیترانه را به وجود می آورد. جابه جایی جرم در این دو فاز در نهایت سبب ایجاد شرایط مناسب چرخندزایی در شرق مدیترانه در فاز چهار و از بین رفتن آن در فاز هشت می شود.
    کلیدواژگان: پتانسیل سرعت، تابش موج بلند، چرخندزایی، مدیترانه، نوسان مدن-جولین، واگرایی
  • فاطمه کماسی، عباسعلی علی اکبری بیدختی، سمانه ثابت قدم* صفحات 553-568
    در مقاله حاضر، تغییرات مکانی ارتفاع لایه آمیخته جوی شهر تهران در دو مطالعه موردی در فصل تابستان و زمستان با استفاده از مدل پیش بینی عددی WRF بررسی شده است. تغییرات محلی نیمرخ قائم دمای پتانسیلی و نسبت آمیختگی بخار آب در پنج نقطه مختلف از حوزه چهارم مدل که شامل نقاط شهری و ارتفاعات شمالی تهران هستند، همچنین تاثیر عوامل سطحی نظیر ارتفاع زمین، دما و رطوبت نسبی دومتری و شار گرمایی سطحی بر ارتفاع لایه مرزی مطالعه شده است.
    نتایج بیان گر وابستگی قابل توجه میان ویژگی های سطحی و ارتفاع لایه مرزی هستند. شبیه سازی نیمرخ قائم دمای پتانسیلی و نسبت آمیختگی، بیشینه ارتفاع لایه مرزی را در بخش های مرکزی منطقه شهری تهران نشان می دهد؛ یکی از دلایل احتمالی آن بیشتر بودن اثر جزیره گرمایی در بخش های مرکزی تهران می تواند باشد. از سوی دیگر، در ارتفاعات شمالی شهر به دلیل جریان های ناشی از ناهمواری های سطحی و همچنین فرارفت دمای پتانسیلی تغییرات زیادی برای این کمیت در تابستان مشاهده می شود. به طوری که اختلاف بین کمینه و بیشینه ارتفاع لایه مرزی برای تابستان در حدود 1700 متر و برای زمستان در حدود 1000 متر است. بیشینه مقدار ارتفاع لایه مرزی تابستان در حدود 2400 متر و در بخش های مرکزی تهران و برای زمستان در حدود 1200 متر و در بخش های غربی و مرکزی تهران است. بر اساس نتایج، ارتفاع سطح زمین، دما و رطوبت نسبی دومتری در هر دو فصل، فرارفت دمای پتانسیلی در فصل تابستان و شار گرمایی سطحی در فصل زمستان، نقش موثری در تغییرات مکانی ارتفاع لایه مرزی دارند.
    کلیدواژگان: لایه آمیخته، منطقه شهری، WRF، تهران
  • امید علیزاده چوبری*، محمد سعید نجفی صفحات 569-584
    در این پژوهش با استفاده از داده های هواشناسی 15 منطقه ایران از سال 1951 تا 2013، روند تغییر دما و بارش تجمعی سالانه مطالعه شده است. نتایج نشان داد که دماهای کمینه، بیشینه و میانگین روزانه در بیشتر مناطق ایران روندهای افزایشی داشته اند. روند افزایشی دمای کمینه بیشتر از روند افزایشی دمای بیشینه بوده است که عمدتا ناشی از گسترش شهرسازی است که از طریق مسدود کردن تابش طول موج بلند خروجی نقش موثرتری در افزایش دمای کمینه ایفا می کند. همچنین، آلودگی هوای شهری باعث کاهش تابش خورشیدی رسیده به سطح زمین می شود؛ بنابراین روند افزایش دمای بیشینه را کند می کند. دما در ایران عمدتا در دهه های 1980 و 1990 دچار یک جهش شده است، به نحوی که متوسط دما در دوره بعد از جهش بیش از دوره قبل از جهش بوده است. بیشتر مناطق ایران در طول دوره مورد مطالعه روند کاهشی را در مقدار بارش تجربه کرده اند، اگرچه نقطه تغییر معناداری در داده های بارش تجمعی سالانه وجود نداشت. کاهش مقدار بارش، به همراه افزایش دمای هوا بیانگر این است که ایران طی چند دهه گذشته به طور تصاعدی خشک تر شده است. روند کاهشی بارش در ایران برخلاف میانگین جهانی است که افزایش بارش را به واسطه افزایش دما نشان می دهد. اقلیم گرم مناطق خشک و نیمه خشک جنب حاره ای که بیشتر مناطق ایران را تحت تاثیر قرار می دهد، باعث می شود که زمان بیشتری برای اشباع جو از بخار آب و شروع بارش صرف شد. بنابراین، بخار آب بیشتری از طریق گردش کلی جو و قبل از شکل گیری بارش به عرض های جغرافیایی بالاتر منتقل می شود.
    کلیدواژگان: تغییر اقلیم، روند تغییر دما و بارش، مناطق جنب حاره ای
  • محمود صفر، فرهنگ احمدی گیوی* صفحات 585-600
    هدف این پژوهش بررسی و پاسخ به این سوال است که «آیا می توان با استفاده از داده های سنجش از دور مانند برگشت پذیری قطبش افقی رادار، بدون درگیر شدن با حجم بسیار بالای داده پردازی در روش های داده گواری، روند اجرای مدل های پیش بینی عددی وضع هوا را تسهیل کرد و دقت پیش بینی را افزایش داد؟» برای دست یابی به این هدف، علاوه بر طراحی و توسعه یک نرم افزار تحلیل داده های راداری، مدل پیش بینی عددی میان مقیاس وضع هوا WRF به نحوی توسعه یافته است که بر مبنای خروجی فراهم شده توسط مدل راداری و همچنین با نوآوری در بخش کنترل همرفت، تعیین بهترین طرحواره همرفت در حین اجرای مدل پیش بینی عددی، امکان پذیر باشد. مدل عددی توسعه یافته برای یک بازه زمانی 12 ساعته به منظور بررسی چگونگی پیش بینی بسیار کوتاه مدت اجرا شده است. این آزمون با استفاده از 8 پیکربندی طرحواره های فیزیکی و همچنین واردکردن داده های راداری انجام گرفته است که در مجموع 40 اجرا را شامل می شود. به علاوه، در مطالعه موردی نیز رخداد یک تندوزه نسبتا قوی در منطقه تهران در ساعت UTC 2330 روز 30 مارس 2009 بررسی شده است.
    بررسی نتایج با استفاده از شاخص های ریشه میانگین مربعات خطا و همچنین همبستگی بین مقدار بارش پیش بینی شده با مقادیر ثبت شده دیدبانی، بیانگر بهبود پیش بینی بارش بسیار کوتاه مدت 6 ساعته برای کل منطقه مورد مطالعه است. در این ارزیابی ها، آزمون انطباق الگوی بارش پیش بینی شده با بارش دیدبانی نشان داد که الگوی بارش پیش بینی شده تا حد زیادی با دیدبانی هم خوانی دارد و روش های آماری نیز موید افزایش همبستگی بین بارش پیش بینی شده به مقدار 15/0 برای اجرای مرجع و دیدبانی و همچنین کاهش ریشه میانگین مربعات خطا به مقدار 2/0 است. به علاوه، برای ایستگاه هواشناسی مهرآباد تهران، سری زمانی بارش، تهیه و تحلیل و ارزیابی شد که نتایج حاکی از تاثیر بسیار خوب داده های راداری بر کاهش سری زمانی ریشه میانگین مربعات خطا است.
    کلیدواژگان: رادار هواشناسی، مدل پیش بینی عددی وضع هوا WRF، داده گواری، بارش پیش بینی شده، طرحواره همرفت، درستی سنجی
  • غلامرضا روشن* صفحات 601-621
    دما به عنوان یکی از مولفه های تاثیرگذار بر عرضه و تقاضای انرژی است؛ بنابراین تعیین یک دمای پایه مناسب به منظور محاسبه تقاضای انرژی سرمایشی و گرمایشی سکونتگاه ها می تواند در مدیریت انرژی نقش قابل توجهی ایفا کند. بنابراین این تحقیق در نظر دارد دمای پایه ده ایستگاه کشور را که معرف شرایط متنوع آب و هوایی و جغرافیایی ایران هستند، با استفاده از شاخص های آسایش گرمایی Cooling power index(CPI)، Temperature-Humidity index (THI) وPredicted Mean Vote (PMV) مورد بازنگری قرار دهد. مولفه های اقلیمی استفاده شده در این پژوهش در مقیاس روزانه شامل دمای متوسط، رطوبت نسبی، سرعت باد و ابرناکی برای دوره آماری 1960 تا 2010 هستند. نتایج توزیع فراوانی رخدادهای زیست اقلیمی ایستگاه ها برای دو شاخص THI و CPI نشان می دهد که تمرکز غالب فراوانی ها متعلق به طبقات گرم تا داغ بوده ولی برای شاخص PMV تمرکز رخدادها منحصرا برای این آستانه ها نیست. یافته ها نشان داد که بر اساس شاخص های CPI، THI و PMV به ترتیب شیراز با 6/33 درصد، همدان با 6/26 درصد و بندرعباس با 2/15درصد، بیشینه فراوانی طبقه آسایش اقلیمی را تجربه کرده اند. در ادامه به طور جداگانه با استفاده از هرکدام از شاخص های مختلف زیست اقلیمی و به کارگیری روش صدک ها، آستانه های دمای پایه آسایش از محدوده طبقه آسایش اقلیمی استخراج شد. از جمله ضعف هر روش پهنای زیاد دامنه آسایش، دوربودن از مقادیر استاندارد مرسوم یا منطبق نبودن با شرایط آب و هوایی به عنوان ایرادهای آن ها تشخیص داده شد که بیشینه ضعف به شاخص بیکر اختصاص یافت. بنابراین با ترکیب همه شاخص ها، ضعف های فوق، برطرف و مقادیر دمای پایه آسایش جدید و مطلوبی با توجه به وضعیت و الگوی بیوکلیمایی هر ایستگاه ارائه شد.
    کلیدواژگان: تنوع اقلیمی، تقاضای انرژی، دمای پایه، درجه- روز سرمایشی و گرمایشی، شاخص های آسایش حرارتی
  • سونیا مهری، رئوف مصطفی زاده*، اباذر اسمعلی عوری، اردوان قربانی صفحات 623-634
    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. هدف تحقیق حاضر، ارزیابی تغییرات زمانی و مکانی جریان سطحی و جریان پایه رودخانه های حوضه استان اردبیل در 22 ایستگاه هیدرومتری است. بدین منظور از آمار دبی روزانه و بارندگی با دوره آماری مشترک از سال 1368 تا 1390 استفاده شد. پس از محاسبه رواناب کل، دبی پایه به روش حداقل محلی و مقادیر شاخص جریان پایه (BFI)، محاسبه و تغییرات آن ها درون یابی شد. سپس دوره های آماری موجود به دوره های پنج ساله، تقسیم و تغییرات مقادیر جریان پایه و BFI و نیز ارتباط آن با مقادیر رواناب کل و بارندگی تحلیل شد. براساس نتایج، تغییرات کاهشی دبی در تعداد بیشتری از ایستگاه ها مشاهده شد. مقایسه مقادیر بارش با تغییرات دبی پایه، حاکی از تاثیر اندک بارش بر روند کاهشی دبی ایستگاه ها است. دامنه تغییرات شاخص BFI، از 05/0 تا 88/0 متغیر است. علاوه بر این الگوی تغییرات مکانی رواناب کل، دبی پایه و شاخص BFI مشابه بوده و بیشترین مقادیر در هر متغیر به محدوده غرب و جنوب غرب استان مربوط است. براساس مقایسه دبی پایه، رواناب کل و شاخص جریان پایه، بیشترین دبی پایه با مقدار 30 متر مکعب بر ثانیه به ایستگاه دوست بیگلو و بیشترین مقدار شاخص دبی پایه به ایستگاه گیلانده مربوط است. در مجموع در دوره های موردمطالعه مقدار جریان پایه و BFI کاهش یافته و شدت کاهش جریان پایه در سه ماه فصل پاییز بیشتر بوده است.
    کلیدواژگان: استان اردبیل، تغییرات مکانی دبی پایه، تغییرات فصلی، دبی پایه، شاخص جریان پایه
  • فرشته کاویانی، محمدحسین معماریان *، محمد اسلامی کلانتری صفحات 635-650
    حوادث هسته ای چرنوبیل و فوکوشیما نشان دادند که حادثه هسته ای مسئله ای در مقیاس منطقه ای نیست و هسته های پرتوزا تحت شرایط جوی، صدها و هزاران کیلومتر پخش و انتقال می یابند و تاثیر درازمدت نامطلوب و خطرناک روی محیط زیست و سلامتی انسان ها می گذارند. با توجه به اهمیت این موضوع، در این مقاله به شبیه سازی انتقال، پخش جوی و نهشت آلاینده های هسته ای رهاشده از حادثه فرضی نیروگاه بوشهر با استفاده از مدل HYSPLIT پرداخته شده است. حادثه فرضی نیروگاه بوشهر براساس بزرگ ترین حادثه هسته ای جهان، حادثه چرنوبیل، طراحی شده است. در این پژوهش، فرض شده که حادثه نیروگاه هسته ای بوشهر در اول دی ماه 1393 اتفاق افتاده و مهم ترین هسته های پرتوزا یعنی سزیم-137، ید-131، استرانسیم-90، پلوتونیم-239 و تلوریم-132 در این شبیه سازی استفاده شده است. نتایج شبیه سازی در بازه زمانی یک ماه نشان می دهد که مواد هسته ای رها شده از این حادثه شهرستان های تنگستان، دشتی، دیر، کنگان، جم و عسلویه را بیشتر از مناطق دیگر آلوده می کنند و بیشینه نهشت آلاینده های هسته ای در بخش ساحلی شهرستان تنگستان است. آلودگی هسته ای ناشی از این حادثه تاثیر زیادی روی مناطق جنوبی، جنوب شرقی، شرقی و بخشی از مناطق شمال شرقی ایران می گذارد و در این مدت نه تنها ایران بلکه اکثر کشورهای آسیایی، بخشی از قاره آفریقا، آمریکا، اروپا و حتی استرالیا را نیز به مقادیر کمتر، تحت تاثیر قرار می دهد. در ایران بیشینه غلظت آلاینده های هسته ای در شهرهای لامرد و بندرعباس است. در اولین روز حادثه، بیشینه غلظت مواد پرتوزای ورودی به لامرد در ارتفاع 10 متری و بیشینه نهشت آن ها در این شهر است. همچنین در این مقاله، حادثه فرضی راکتور بوشهر در اول دی ماه 1393 با حوادث فرضی راکتور بوشهر در اول فروردین، اول تیر و اول مهر 1394 مقایسه شده است. به دلیل تاثیر تقریبا یکسان سامانه های فشاری بر نیروگاه بوشهر، پخش و انتقال مواد هسته ای رها شده از حادثه اول تیر و حادثه اول مهر در جهت جنوب شرقی نیروگاه بوده که با حادثه اول دی تقریبا مشابه است. با گذشت زمان، پخش و انتقال هسته های پرتوزا متفاوت می شود اما در هر سه حادثه بیشترین آلودگی برای مناطق جنوبی ایران است. از نتایج این پژوهش، تحت شرایط جوی مشابه می توان در فوریت های هسته ای نیروگاه بوشهر استفاده کرد.
    کلیدواژگان: آلاینده های هسته ای، پخش جوی، شبیه سازی، نیروگاه بوشهر، HYSPLIT
  • علی محمد خورشیددوست، عباس مفیدی*، علی اکبر رسولی، کامل آزرم صفحات 651-671
    هدف از تحقیق حاضر، ارزیابی حساسیت مدل RegCM4 به طرحواره های بارش همرفتی در مدل سازی بارش های بهاره در محدوده خاورمیانه و شمال غرب ایران است. از مجموعه خروجی های حاصل از اجرای مدل، مقادیر بارش بزرگ مقیاس و مقادیر بارش همرفتی به عنوان متغیرهای اصلی جهت بررسی تاثیر انواع طرحواره های همرفتی شامل امانوئل، گرل و کو انتخاب شدند. ارزیابی خطای مدل سازی، از طریق مقایسه مقادیر بارش بزرگ مقیاس و بارش همرفتی برآورد شده توسط مدل با پنج دسته داده بارش رقومی شامل: داده های پرشین، آفرودیت، چیرپس، آگمرا و ERA-Interim که از تفکیک افقی بالایی برخوردارند، به انجام رسید. نتایج بیانگر آن است که مدل RegCM4 حساسیت زیادی به انتخاب نوع طرحواره همرفت در مدل سازی بارش های بزرگ مقیاس و بارش همرفتی دارد و انتخاب یک طرحواره بارشی مناسب به منظور برآورد هرچه دقیق تر بارش های بهاره از ضروریات است. ارزیابی عملکرد طرحواره ها در برآورد بارش بزرگ مقیاس و همرفتی، نشان از برتری نسبی طرحواره کو به طرحواره های امانوئل و گرل دارد. مدل، اندازه بارش ماه های نزدیک به دوره سرد سال را بهتر از ماه های نزدیک به دوره گرم سال مدل سازی می کند. در واقع با نزدیک شدن به دوره گرم سال، مدل عدم قطعیت بیشتری می یابد. مقدار خطا و اریبی مقادیر بارش همرفتی نیز حاکی از آن است که هر سه طرحواره مدل در برآورد مقدار بارش همرفتی، هم در مقیاس بزرگ تر بر روی خاورمیانه و هم در مقیاس کوچک تر بر روی شمال غرب ایران از اریبی منفی برخوردار هستند.
    کلیدواژگان: بارش شبیه سازی شده، شمال غرب ایران، طرحواره های پارامترسازی همرفت، طرحواره کو، مدل RegCM4، نمودار تیلور
  • طیب رضیئی* صفحات 673-695
    در این پژوهش به منظور شناسایی رژیم های بارشی ایران از داده های بارش ماهانه 155 ایستگاه همدیدی پراکنده در سطح کشور در دوره آماری 1990 تا 2014 استفاده شد. با تحلیل مولفه های اصلی بر روی ماتریس میانگین درصد بارش ماهانه ایستگاه ها در دوره آماری مورد نظر، 5 مولفه اول انتخاب و سپس با خوشه بندی وارد بر روی ماتریس نمره استاندارد مولفه های انتخابی ایستگاه های مورد مطالعه به 10 منطقه همگن بارشی گروه بندی شدند. رژیم های بارشی آذربایجان شمالی و جنوبی با بیشینه بارش در مه و آوریل بخش هایی از شمال غرب ایران را پوشش می دهند. رژیم بارشی جنوب-جنوب غربی با بیشینه بارش در ژانویه، پس کرانه های خلیج فارس و رژیم بارشی کوهستانی غربی با بیشینه بارش در ماه مارس، بخش کوهستانی غرب ایران را در بر می گیرند. رژیم بارشی خزری هم با بیشینه پاییزه و توزیع تقریبا مناسب بارش در سال کرانه های دریای خزر را شامل می شود. بخش مرکزی-شمال شرقی و بخش مرکزی- شرقی ایران هم دارای رژیم بارشی مرکزی-شمال شرقی و رژیم بارشی مرکزی-شرقی هستند که در آن ها زمستان پربارش ترین فصل و مارس پربارش ترین ماه سال است اما فصل بارش در رژیم بارشی مرکزی-شرقی کوتاه تر است. کرانه های دریای عمان و بخش بزرگی از جنوب شرق ایران هم به ترتیب دارای رژیم بارشی ساحلی جنوب شرقی و رژیم بارشی موسمی جنوب شرقی است که در آن ها بارش های موسمی تابستانه قابل توجه است. ارتفاعات البرز مرکزی و پس کرانه های دور شرق دریای خزر نیز دارای رژیم بارشی کوهستانی البرز مرکزی است که در آن بارش به طور تقریبا منظمی در همه ماه های سال توزیع شده است.
    کلیدواژگان: ایران، تحلیل مولفه های اصلی، خوشه بندی، رژیم بارشی، مناطق همگن
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  • Masomeh Farzaneh, Seyed Reza Mehrnia, Taghi Nabaei * Pages 461-472
    The Marjan Abad zinc and lead mine has been affected by fault and tectonic zones and the mineralization are mainly located along the fault zones. Rocks of the area are mainly rhyolite, dacite with small amounts of andesite. Basalt consists of kind of alkaline to calc-alkaline. streak texture which is the main texture of this deposit. Gravity and magnetic anomalies are observed in this area that are the result of effects of tectonic structures and mineralization in between streaks. Rocks in the region are often acidic. As it is recognized, using geophysical data and field studies for mineral prospecting can be important, as magnetic and gravity anomalies overlap in four regions. The deposits usually have overlapping magnetic and gravity anomalies.
    The mineral deposits, found by the geophysical anomalies, are due to post-magmatic activated in Cenozoic, as the area includes sub-volcanic igneous masses. The area has been affected by hydrothermal solutions with alteration halos that led to the formation of minerals along the streaks.
    There deposits are similar to MVT deposits as there are extremely low intensity areas of magnetic and gravity anomalies, shown on GIS maps prepared for the region. Such maps are overlaid with geological maps with faults locations to find the areas with mineral deposits, that include four regions. The areas of the deposits are found from the spatial intensity changes of gravity and magnetic anomalies. The lineaments based on magnetic anomalies have two directions, namely along the northwest-southeast and northeast-southwest. The gravity anomalies however show an east-west direction.
    According to the pictures, evident polarization in the magnetic field and less tangible polarization in the gravitational field do not show obvious changes in the contour maps. The first derivative operations highlight the high and low-intensity areas in the magnetic field and the gravitational field. According to this image the first derivative operations have similar results on both magnetic and gravity anomalies. The first derivatives of the anomaly fields show the spatial distributions of gravity and magnetic fields. By putting these images on the pattern of faulting and geological maps, it is clear that multiple gravity and magnetic lineaments match with the locus of regional faults. There is also a significant relationship between the mineral masses and the magmatic rocks in the area.
    Keywords: Marjan Abad, Lead, Zinc, MVT, Anomaly, Magnetic, Gravity
  • Mohammad Ali Sharifi *, Mohammad Reza Seif Pages 473-487
    In recent years, strong scientific interest has been generated in a better understanding of the physical system of the Earth. It has been heightened the need for improving our knowledge of the gravity field of the Earth, both in terms of accuracy and spatial resolution. this could be globally and homogeneously possible only by means of space gravity missions. Nowadays, it is becoming increasingly difficult to ignore the widely used applications of the satellite gravity mission's information in studying the Earth system. For example, the application of the gravity information in geophysical and geotechnical research, is a new dimension for geodynamic research and seismology, Oceanography and determining ocean circulation, Hydrological research, Ice mass balance and sea level study.
    There have been many motivations behind launching the space gravity missions. After the first gravity mission CHAllenging Minisatellite Payload (CHAMP) launched in 2000 for the gravity and atmosphere applications, Gravity Recovery And Climate Experiment (GRACE) mission was launched to improve the temporal and spatial resolution for hydrological and geophysical studies. As a new space-born gravity mission, Gravity field and steady-state Ocean Circulation Explorer (GOCE) was designed based on the gradiometry observation in height about 250km. Because of the short life-time of GOCE, this mission was designed to determine static gravity field and the temporal gravity field modeling was assigned to other space-born missions like GRACE. The GRACE was designed to determine and interpret the temporal gravity variations. By the help of GRACE monthly solutions, it is easily possible to extract the periodic and quasi-periodic signals of the gravity. It allows researchers to interpret the time gravity variation as the mass redistribution in the Earth dynamic system. The temporal gravity variation might be caused the global water cycle, ice mass loss in the poles, the glacial isostatic adjustment, the earthquake subsequences and geodynamic activities.
    A follow-on mission to GRACE is desirable to bridge the gap in the time-series of the monthly gravity models. After the successful GRACE mission, in order to minimize the cost and technical risk, the same space mission has been proposed to measure the gravity field variations. Then, the GRACE follow-on mission will be a rebuild of the original GRACE with a few developments. Laser interferometry will be tested as a new experiment to improve the ranging precision whereas, the mission will be equipped with the microwave ranging system similar to the GRACE. As the first mission in the planetary science, the lunar GRAIL mission was proposed as a pair co-orbiting spacecraft similar GRACE. GRAIL that was launched in 2011 to improve our knowledge about the moon's gravity field.
    In this study, we investigate the role of various parameters of the co-orbiter mission design to improve the gravity field modeling for post-GRACE missions. A proper definition of these parameters will have a large effect to improve the gravity field modeling. The redesign has been carried out based on the science and technology developments in recent years. Using laser ranging system instead of K-band ranging system, and decreasing the satellite height (assuming the use of active propulsion system) are new suggestions. The mission could not only improve the quality of the gravity modeling, but also bridge the gap in the time-series of the monthly solutions. In the co-orbit missions, the mission design commonly consists of designing the orbit and satellite separation. Assuming the mission is equipped with an active propulsion system, the height of the satellite pair will be reduced to 350 km. Laser interferometry will be tested as a new experiment to improve the ranging precision by considering its advantage and disadvantages. The results show that using the mission equipped with laser ranging system could not improve the quality of the gravity modeling, because of the limitation in average range
    Keywords: Earth's gravity modeling, Space Gravity Mission, Orbit Design, Co-orbiter formation, Ranging System
  • Amin Abbasi * Pages 501-520
    The reliable location of earthquakes is one of the most important criteria in the assessments of the seismicity, seismotectonics and seismic hazard analysis and focal depth accuracy has been more questionable. In this study, archived data by local temporary seismological network at the southern edge of Central Alborz (Abbassi et al., 2010; Abbassi et al., 2012; Tatar et al., 2012) and by Iranian Seismological Center (IrSC) at the same times are evaluated. The most basic goal of seismic networks is the determination of accurate earthquake locations (Havskov et al, 2012). The local network (TEFIR) has 47 short period stations that are installed at a spacing of 7 to 15 km and worked at 22 weeks in June to November 2006 in area with 35.4° to 36.1° N and 51.5° to 53° E coordinates which operated in continuous and trigger modes at a sampling rate of 100 up to 125 Hz by International Institute of Earthquake Engineering and Seismology (IIEES). In TEFIR network, the time was calibrated every hour by a GPS receiver connected to each station.
    The IrSc network is comprised of 17 telemetries, 1-s short-period stations at a spacing of 35 up to 90 km in region with 35° to 37° N and 50° to 54° E coordinates which are operated in continuous mode at a sampling rate of 50 Hz by Institute of Geophysics, University of Tehran (IGUT).
    The location improvement for few events in IrSC catalogue, even if being for the limited area and time period, is one of the main goal. Attention to the seismic network, especially station intervals to modify the standards of location criteria is another aim of this attempt.
    To do this study, the events reported in S-files format by the both local and regional network catalogues are relocated with Vp/Vs ratio and one dimension crustal velocity structure (Abbassi et al., 2010). This relocation process has been done in SEISAN software (Havskov and Ottemöller, 2005) and Hypocenter program (Lienart et al., 1986; Lienart, 1991; Lienart and Havskov, 1995). Then relevant location parameters for events that are recorded commonly by the two networks at the same times and different only up to 5 seconds, have been compared carefully.
    Average error estimations for the same events at all azimuthal gap (Gap) was up to 1.1 seconds in origin times, from 5.3 to 6.9 kilometers for latitudes and longitudes respectively and more than 15 kilometers for focal depths that recorded by the regional network. These estimated errors for the same events in the local network are less than 0.52 second in origin times, 1.9 kilometers for latitudes and longitudes and up to 5.2 kilometers for focal depths (TEFIR201 and IrSC201 in the table 4). Then 27 anticipated well location events have been selected and analyzed based on Gap ≤ 180° simultaneously from the 201data sets in both relocated catalogues (TEFIR27 and IrSC27).
    Location criteria for the events in TEFIR27 data set that have proper mean minimum distances to the recorded stations (Dmin) were being more reliable locations. While the expected improve ment in locations for the IrSC27 data set, despite proper Dmin was not satisfactory.
    The relocated same events have shown good relative fitness in location dependent parameters such as root mean square residual timing (RMS), Gap, Dmin, number of stations (NST) and number of phases. Also, this proportionality for calculated latitude and longitude coordinates and origin times, is more than for the calculated depths.
    This approaches show that the uncertainties have specific dependencies on the input data files, especially phase readings and phase pickings of the events. As were observed in some seismic records by the regional network at the stations and the reread of the Pg and Sg phases by the local network, some tendency to RMS reduction in the regional operation alone, and intern phases of the far stations to the event locations scheme were being some sources of location errors. Reading the Pn phase instead of the Pg at under the critical distances to receiver station for example less than 180 kilometers, and phase detecting error for few reported events were most erronious cases.
    Obviously, reliable crustal model and Vp/Vs ratio in addition to other appropriate location parameters with a simultaneous decrease RMS causes better location accuracies. Then application of the proper criteria such as Dmin, Gap, NST and corrected phase pickings may improve related location parameters, containing RMS, spatial and time accuracies under the other restrictive network conditions. Among these criteria, Dmin has an important role in determination of the seismological network geometries.
    The results of this study, despite the fundamental difference in the processing methods and data with the work of Bond´ar et al. (2004), have a good agreement in the discussed criteria and analytical issues related to reliable locations. Also the consistency of this work may be useful for confidence of the location certainties at least for some earthquake occurred in the studied area at the time span.
    Keywords: Earthquake location, same origin time data, Local, Regional
  • Taghi Shirzad * Pages 521-529
    Various studies have shown that the cross-correlation (Wapenaar, 2004), cross-convolution (Slob and Wapenaar, 2007) and de-convolution (Wapenaar et al., 2008) can provide empirical Green’s functions (EGFs) between receiver pairs. These approaches, which are attributed to seismic interferometry, assume that one of the receiver acts as a source, whereas the other one is instated as a virtual receiver. The resulted EGFs allowed many studies to be applied in different regions even though (including) areas with low seismicity.
    The main assumption of interferometry approach is based on completely diffuse signals which are generated by a closed surface of sources (Schuster et al. 2004). In other word, the distribution of sources and theirs energy in a medium are usually uniform. This condition ensured that inter-station EGF is extracted accurately. In general, the sources (left and right of the receivers) located on or near lines which is passed through both receivers are in the stationary region, and the sources above and below are in non-stationary regions. Also, Snieder (2004) indicated that the Fresnel zone of receivers surrounded all the sources which are located in stationary region. In this study, we referred to these sources as stationary sources. Consequently, all sources outside the Fresnel zone were referred to as non-stationary sources. It is generally accepted that the stationary sources play a major role/contribution to retrieve the inter-station EGF. Stationary sources and their energies are characterized by coherency and small wavenumber. In contrast, non-stationary sources and their energies are characterized by incoherency, larger wavenumber. We used this difference in order to separate stationary and nonstationary sources.
    In the Earth, distribution of noise sources and theirs energy are strongly non-uniform, which contravenes the theoretical interferometry requirements (Stehly et al. 2006). In other words, cross correlations from non-stationary sources in stacking procedure do not cancel completely if the source coverage is incomplete. Consequently, incomplete source coverage leads to retrieve unreliable inter-station EGF.
    In this study, we used 144 sources on circle environment (r=40 km) surrounded by two receivers which are located/installed in A(-4 , 0) and B(4 , 0) as shown in Figure 1. Moreover, synthetic time series were generated using Mexican-hat source time function (see left panel of Figure 2). All recorded waveform signals of these sources in station A and B are shown in middle and right panel of Figure 2, respectively. After the preprocessing procedure and cross-correlation performances, we constructed a cross-correlogram matrix, which is called CC, using a collection of cross-correlation function signals (see left panel of Figure 3). The dimensions of this matrix include time (number of point in signal data set, npts) and source counter/numerator. In brief, inter-station EGF is retrieved using stacking the cross-correlogram matrix signals along the source counter dimension. We followed the analysis and preprocessing of the cross-correlogram before stacking outlined in Poliannikov and Willis (2011). Thus, we decompose cross-correlogram matrix using singular value decomposition (SVD) technique to separate the stationary and non-stationary energies. This idea illustrates/explains that the cross-correlogram matrix could be calculated by its eigenvalues and eigenvectors. Poliannikov and Willis (2011) indicated that the large eigenvalues (singular values) are associated with events which is located in Fresnel zone. Afterward, we constructed lower-rank approximations of the cross-correlogram matrix using two larger eigenvalues, which is called CC2, and then stack CC2 along the source counter dimension to retrieve inter-station EGF (see Figures 4, 5 and 6).
    Keywords: Synthetic time series, Interferometry, Singular Value Decomposition
  • Amin Esmaeili *, Mohammad Javad Kalaee Pages 531-537
    Space weather is the concept of changing environmental conditions in the space from the solar atmosphere to the geospace. Much of space weather is driven by energy carried by solar wind through the interplanetary space from regions near the surface of Sun and Sun’s atmosphere. Space weather has two focal points: scientific research and applications. Space weather research touches upon the large-scale energy and matter eruptions from the solar activity region, the propagation and evolution of the interplanetary disturbances, the response of the geospace to the interplanetary disturbances, the significant changes of the geospace environment, and finally the impact on the human activities. The magnetosphere is the outermost layer of the geospace, and the interaction of solar wind with the magnetosphere is the key element of the space weather cause-and-effect chain process from Sun to Earth, which is one of the most challenging scientific problems in the geospace weather study. The nonlinearity, multiple component, and time-dependent nature of the geospace make it very difficult to describe in terms of the physical process in geospace using traditional analytic analysis approach. Numerical simulations, a new research tool developed in recent decades, have a deep impact on the theory and application of the geospace. The low- and middle-altitude cusp regions of Earth’s magnetosphere have been widely studied in the past. However, the high-altitude cusp region was not explored extensively in the early days of space science, with the notable exception of the HEOS-2 spacecraft which undertook the first detailed investigations of this part of the magnetosphere. More recently, major advances on the role of this key region, which is in direct interaction with the solar wind, have been provided by the Polar and four-spacecraft Cluster missions. Much of this work has been focused on analysis of individual crossings, but a few statistical studies of the high-altitude cusp region have been undertaken using data from the HEOS-2, Hawkeye, Polar and Interball data set. The actual flow pattern of the exterior cusp region is poorly known. A transition layer inside the magnetosheath near the outer boundary of cusp has been clearly evident during statistical experimental observation of the cusp boundaries from CLUSTER mission. Because of its high-quality data and its suitable orbit the four-spacecraft Cluster mission is perfectly adapted for high-altitude cusp investigations and therefore for large scale statistical studies. This layer characterized by MA=1 (Alfvenic Mach) allows the bulk flow to transit from super-Alfvenic to sub-Alfvenic from the exterior to the interior side of the outer cusp and has been observed experimentally mainly during Northward IMF case, however our simulation results show its existence during southward IMF as well. The role of this layer is important in order to understand the flow variations near the outer cusp boundary. In order to analyze this layer, we have used a large 3D PIC simulation of solar wind-magnetosphere interaction. We turned the IMF from Northward to southward direction to investigate Alfvenic Transition Layer (ATL) dynamics according to the IMF rotations. Although, in CLUSTER statistical studies, only a limited area of magnetosphere studied for ATL, we reported this layer for the whole magnetosphere during all simulation and have investigated its dynamics with IMF rotation. Finally we compared our results with experimental observations. Our results had a good agreement with the observational data for the parallel currents and also ATL observed in lower altitudes for the southward IMF case.
    Keywords: Magnetosphere, High Altitude cusp, Alfvenic Transition Layer, CLUSTER satellite
  • Mohammad Ali Nasr-Esfahany *, Alireza Mohebalhojeh, Farhang Ahmadi-Givi Pages 539-552
    In this investigation, some aspects of the impact of the Madden-Julian Oscillation (MJO) on the subtropical region of the Northern Hemisphere together with the underlying mechanisms are studied using NCEP/NCAR reanalysis data. The data cover winter months (December to February) from 1974 to 2015. The main method used is that of averaging and analyzing of meteorological parameters associated with convection over the Indian Ocean and the mid-latitude large scale motions in the eight phases of MJO. The indices of MJO provided by the Australia's National Weather, Climate and Water Agency (BMRC) are used to identify the MJO phases. The averaging is carried out over the periods when MJO index is higher than unity and stays in the same phase for at least 5 days. The selected parameters are the “outgoing longwave radiation” (OLR), velocity potential and divergent component of the horizontal wind at 200 hPa level and vertical component of velocity in pressure coordinate denoted by . These parameters have been selected based on their potential to unravel the interaction between tropical and subtropical tropospheric circulations.
    The average of OLR in the selected period shows clear movement and amplification of convection cells associated with MJO from the Western Indian Ocean to the east. This confirms that the periods have been selected properly.
    The distributions of averaged OLR, divergence at 200 hPa level and at 600 hPa level show that the southwest Asia is significantly affected by MJO. Over the Indian Ocean, convective cells of MJO are strengthened from the phase 1 to phase 4 while anomalous convection at 200 hPa level and the associated downdraft at 600 hPa level in the southwest Asia are manifested. During the phases 3 and 4 of MJO, the convection cells associated with MJO exhibit the strongest anomalies over the east of Indian Ocean. The results thus suggest that the atmospheric circulation pattern provides adverse conditions for cyclogenesis and cyclone development in the southwest Asia and especially over Saudi Arabia and the south of Iran. On the contrary, all anomalous patterns are reversed in the phase 6 to phase 8 in the tropical and subtropical region. In these phases, anomalous convergence at 200 hPa and updraft motion at 600 hPa seen during the phases 3 and 4 in the southwest Asia are replaced by anomalous divergence and updraft motion, respectively. The change is such that the atmosphere circulation provides suitable conditions for cyclogenesis and cyclone development at the downstream end of the Mediterranean storm track.
    The current study shows that confluence and diffluence associated with MJO are extended from the Indian Ocean to the Middle East and the east of Mediterranean Sea. The extension is such that the movement of mass from the Indian Ocean to the Middle East at the upper troposphere in the phase 4 results in the formation of a downdraft motion in the east of Mediterranean Sea. The reverse circulation seems to prevail in the phases 7 and 8 of the MJO. Anomalous updraft motion with divergence (convergence) at the upper (lower) troposphere in the east of the Mediterranean Sea are seen when convection is suppressed in the Western Indian Ocean. Another interesting point is that the convergence and divergence in the east of the Mediterranean Sea are dominantly due to variation of wind speed (and not confluence and diffluence), which may be caused by the effects of topography or interaction with mid-latitude flow. Finally, the distribution of OLR confirms the results of the dynamical analysis in the sense that in the Middle East, positive (negative) anomalous values of OLR are seen in the phases 3 and 4 (7 and 8) suggesting less (higher) than normal cloudiness and precipitation. This study shows that confluence and diffluence associated with MJO expand from Indian Ocean to Middle-East and east of Mediterranean Sea. So that mass movement from Indian Ocean to Middle-East in upper troposphere in phase 4 causes to formation a convection center and downdraft motion in the east of Mediterranean Sea. The reverse circulation seems to occur in the phase 7 and 8 of the MJO. Anomalous updraft motion with divergence (convergence) in upper (lower) troposphere in the Eastern Mediterranean Sea are seen when suppression of convection exists in the Western Indian Ocean. Another point is that convergence and divergence in east of Mediterranean Sea is due to the variation of wind speed (not confluence and diffluence) that may becaused by topographic effects or interaction with mid-latitude flow. Distribution of OLR confirms the results of this study so that in the Middle-East, positive (negative) anomalous value of OLR is seen in phase 3 and 4 (7 and 8) which suggests less (more) than normal cloudiness and precipitation.
    Keywords: Cyclogenesis, Divergence, Madden-Julian Oscillation, Mediterranean, Outgoing Long-wave Radiation (OLR), Potential velocity
  • Fatemeh Komasi, Abbasali Aliakbari-Bidokhti, Samaneh Sabetghadam * Pages 553-568
    The atmospheric boundary layer (ABL) is the lowest part of the atmosphere and it plays an important role in the assessment of air quality, transport process in the lower troposphere and climate change. The thickness of ABL varies in time and space. The Atmospheric boundary layer over an urban area with complex horizontal topography horizontally can be highly non-homogeneous. Daytime ABL, known as mixed layer (ML) can be affected by sloped surfaces and urban roughness therefore it assumes a complex structure, contrary to what happens over a flat terrain. In a non-homogeneous ML, not only turbulent convection is important, but also advection and venting to the free atmosphere is important. Such advections are mainly caused by the sloped surfaces and valley circulations in mountainous areas as that of the north of Tehran. Convection and advection by mountain forcing in the boundary layer can be important for the air pollution problem and even transport processes around cities as for the Tehran urban area.
    In this paper the structure of ML over the urban area of Tehran and its surroundings, has been studied using numerical simulations. The simulations have been done with WRF (Weather Research and Forecast) model with four nested, one way grids. The simulations of the finest inner grid with horizontal resolution of 1.33 km and varied vertical grids (typically 70 m near the surface to 600 m in the free atmosphere), with two boundary layer schemes, namely YSU (Yonsei University) and MYJ (Mellor Yamada Janjic) which are non-local and local respectively, are here presented. Radiosonde measurements as well as surface meteorological data at the Mehrabad synoptic station are used to evaluate the performance of the boundary layer schemes. Comparison of surface and boundary layer observations with WRF simulations show that the YSU scheme, which is a nonlocal closure scheme, gives more realistic results. In any case, both schemes gives cooler and more moist boundary layers than observations, as also have been observed in other studies.
    The ML height over the area varies and depends on surface parameters as surface elevation, 2m temperature and humidity and surface heat fluxes. Potential temperature advection affects the boundary layer height over slopes, especially in summer season. Cold advection of potential temperature caused by the up-slope stream, decreases instability of air near the surface and thus lowers the ABL height. ML height, derived from the simulated potential temperature and moisture profiles over the city, shows a maximum value near the center of the area and a minimum value near the mountain tops. Results also show that in areas where vertical wind speed and moisture have relatively high values, the potential temperature and water vapor mixing ratio profiles do not show the same values for boundary layer height. Typical ML heights for the summer and winter days are respectively 2400 and 1200 for the central part of the city. Maximum ML height variations over the area for summer and winter times are about 1700 m and 1000 m respectively. The main determining parameter in ML height spatial variations is surface heat flux for winter, while the thermal advection is more important in summer days.
    Keywords: Mixed layer, Urban Area, WRF, Tehran
  • Omid Alizadeh-Choobari *, Mohammad Saeed Najafi Pages 569-584
    Observations unequivocally show that climate change is happening in most regions of the globe. Warming which has been observed in most regions of the globe, particularly in recent decades, is the best manifestation of the climate change. In contrast to the warming of the most regions of the world, many places across the globe have experienced different changes in the amount and intensity of precipitation, such that under the global warming both increases and decreases of precipitation have been reported. Using meteorological records of fifteen ground stations across Iran for a 63-yr period from 1951 to 2013, trends of the minimum, maximum and daily mean near-surface air temperatures and annual accumulated precipitation are examined. Results indicated that the annual minimum, maximum and daily mean near-surface air temperatures in most regions of Iran have experienced increasing trends. Thus, Iran, like most regions of the world, has been rapidly warming over the past few decades. The observed increasing trend in air temperature is mostly attributed to the increase of the greenhouse gases due to human activities. In most regions of Iran, the increasing trends of the minimum temperature have been greater than those of the maximum temperature, the feature which has been mostly attributed to the urbanization development. Indeed, through blocking the outgoing longwave radiation, the urbanization development has effectively contributed to the more significant increase of the minimum temperature than the maximum. In addition, the urban air pollution decreases the incoming shortwave radiation reaching to the Earth surface; thereby partly contributes to the less increase of the maximum temperature compared to the minimum. As a result, a decreasing trend in the diurnal temperature range (the difference between the daytime maximum and nighttime minimum temperatures) is identified. Temperatures in most regions of Iran have experienced a changing point either in 1980s or 1990s, such that the mean temperature of the all regions during the period after the changing point was approximately 1.2 ˚C greater than the mean temperature during the period before the changing point. Under the warming, most regions of Iran have experienced decreasing trends in the annual accumulated precipitation, although most of the trends have not been statistically significant. The decrease of precipitation, and the increase of air temperature imply that Iran has become drier and more vulnerable to drought over the past few decades. The observed decreasing trend in precipitation over Iran is in contrast to the trend of global mean precipitation, for which the increase of precipitation under the global warming has been noted. Indeed, previous studies have indicated that 1K rise in temperature is associated with 2 percent increase in the global mean precipitation. However, the results of the present study are consistent with previous studies conducted over the subtropical regions. In a warmer climate, saturation of the atmosphere takes alonger time, which delays the onset of precipitation. Thus, in the arid and semi-arid regions of Iran with the dominant subtropical climate, more water vapour can be transported to higher latitudes by the general circulation of the atmosphere before precipitation can form. In contrast, previous studies have indicated that precipitation increases in both subpolar and tropical regions. We, therefore, argue that depending on the geographical location, the intensity and frequency of precipitation vary in response to the warming of the climate.
    Keywords: Trends of temperature, precipitation, Climate change, Subtropical regions
  • Mahmoud Safar, Farhang Ahmadi-Givi * Pages 585-600
    One of the challenges facing meteorologists in recent years is to improve the quality and accuracy of weather nowcasting for limited areas and in this regard various methods based on the statistics principles, such as data assimilation and ensemble forecasting methods, have been used in numerical weather prediction models. In the data assimilation methods, by transferring and collecting different data including atmospheric measurements by observational stations, satellites and radars, the process of rectifying the results of numerical models is performed statistically. The aim of this research is to investigate and address the question of whether it is possible to facilitate a cycle of numerical weather prediction and improve the prediction accuracy using remote sensing data, without involving very large computational effort required in the data assimilation techniques.
    To reach the objectives of this research, we first designed and developed a software for radar data analysis. Second, based on both the output data provided by the radar model and the innovative changes in the relevant part of the main model for controlling convection, the Weather Research and Forecasting (WRF) model was modified in such a way that the best convection scheme is chosen during the execution of the model. In fact, the appropriate convection scheme is chosen automatically by the capability of the radar system in detecting convection within the execution of the model. To evaluate the results, the modified model was used for a region of Iran in such a way that the site of Tehran weather radar was located in the center of the simulation domain. Before carrying out the simulations, two necessary actions were taken. First, the sensitivity of the results provided by the WRF model to the initial input data was examined. In this stage, using two reanalysis datasets of the NCEP-FNL (Final) on 1°×1° grid prepared operationally every six hours and the ECMWF dataset gridded to a horizontal resolution of approximately 80 km at 6-h intervals, a selected case was studied and the results were compared with the observational data. Then, the processing algorithms necessary to identify and remove radio electromagnetic interference (RFI) noise from radar returns were prepared.
    The modified WRF model was run for 12 hours to evaluate its ability and the quality of prediction for very short time periods. In total, forty experiments were carried out using eight configurations of physical parameterization schemes as well as inclusion of the radar data. In addition, the modified model was implemented for a severe squall line that occurred in Tehran’s area at 2330UTC on the 30th of March 2009 and was detected by the Tehran weather radar.
    Results for the root mean square error index and correlation between the forecasted and the observed precipitation showed that the accuracy of precipitation forecast in the study area for very short time, e.g. 6 hours, was increased when the modified model was carried out. The comparison between the forecasted precipitation patterns and the observations confirms higher consistency for the modified model’s results. Also, evaluating the results by the statistical methods, it is seen that the correlation between the forecasted precipitation and the observed values is increased significantly and the root mean square error is decreased. In addition, the time series of the precipitation data for Mehrabad synoptic station in Tehran was investigated for which the root mean square error in the precipitation time series was decreased when the Tehran radar data was included in the working of the WRF model.
    Keywords: weather radar, WRF, data assimilation, nowcasting, convection scheme
  • Gholamreza Roshan* Pages 601-621
    One of the today`s world issues is saving energy that is not renewable and the use of natural energy not only can make the living environment a comfortable atmosphere but it also greatly reduces the energy consumption. Determining the extent of thermal comfort and the perfect base and in other words, the perfect base temperature to calculate the energy demand for cooling and heating can have a crucial role in energy saving, sustainable development and land use planning. In this paper, it was assumed that the base comfort temperature for each area similar to other climatology phenomena are affected by climatology patterns and behaviors of that area by sticking to a global standard temperature cannot justify regional and station conditions. The crucial point in this study is not only determining bioclimatic climatic comfort but also comfort temperature threshold was modified. In other words, the methodology of this work is such that it does not only search to identify base temperature outside the comfort zone, but in this zone base temperature values have been modified according to the regional climate patterns. In this study, a temperature was introduced as the base temperatures that have played roles in interacting and being influenced by other climatic factors affecting thermal comfort. The main goal of this study is to review and modify the proper base temperature for calculating heating and cooling energies. To determine the new temperature thresholds for providing climate comfort conditions 3 thermal comfort indices of Biker, THI and a temperature-physiological index entitled Predicted Mean Vote have been used. Each of these indicators is presented below. But it should be noted that all climatic data used in the study are daily data from 1960 to 2010 that include wind speed, daily temperature, relative humidity and cloudiness. It must be noted that reconstruction of missing data was performed by linear regression and the results were confirmed after validation of reconstructed data. The monitoring data are being random and their homogeneity were investigated by using run-test and drawing histograms. Importantly, since Iran has different climatic diversity, 10 stations representing different climatic conditions were selected and analyzed. The method of this study to determine and review base temperature that was based on the principle that according to each of the heat and physiological-temperature indices, after gaining the output of each of these models, days in comfort zones were determined. Then separately for each bioclimatic index, the temperatures of days in comfort zone were selected. But despite that all these screened thermal temperatures are in the comfort zone, the methodology was not based on this principle that from the range of extracted thermal temperatures, the minimum and maximum temperatures are introduced as base temperature for calculating HDD and CDD respectively. In the next step, for calibrating this temperature range, percentile method was used. In the following, using various bioclimatic indicators, base temperature values were modeled to calculate HDD and CDD. The main weakness of Biker index was in determining the band and the wide width of thermal comfort range that apart from the base temperature values, it has shown a significant difference compared to the existing standards. About the THI index, although the range of comfort zone reduces comparing to CPI but the long distance of comfort range of Tehran and Babolsar from the existing standards have been identified as the weaknesses of this index. Considering PMV index, it is noteworthy that the range of thermal comfort had little sway compared to the CPI. And it had greater overlap with THI method. One of its strengths is modeling of Tehran`s base temperature which is close to the existing standards. But the weaknesses of this modeling is in determining the base temperature for calculating HDD specifically for 80P and Ahvaz , Babolsar, Rasht, Mashhad. As it was seen, any of the bioclimatic indicators had weaknesses and strengths in modeling of base temperature. This study tried to eliminate these weaknesses by combining the results of all three indicators. The results of combined indices showed that not only the range of fluctuations and on the other hand the values of modeled base temperature are consistent with traditional standards of 18 to 22 or 18 to 24, but also the derived values of this modeling have been able to apply the impact of weather condition of each area in this modeling and provide more realistic results.
    Keywords: Thermal comfort index, base temperature, climatic variability, energy demand, cooling, heating degree day
  • Sonia Mehri, Raoof Mostafazadeh *, Abazar Esmaliouri, Ardavan Ghorbani Pages 623-634
    River flow is one of the most important water resources for uses in agriculture, industry and sanitary sectors. In this regard, analysis of temporal trend in river flow is essential in management of water resource. Better understanding of base flow can be used in analysis of river flow pattern and discharge regime and management of water resources in the low flow periods. Water withdrawals alter the natural patterns of stream flow regime and flow conditions. The main aim of the present research is to investigate the spatio-temporal variations of surface flow and base flow over Ardabil Province through 22 river gauge stations considering rainfall variability. Therefore, the daily discharge and rainfall records were analyzed over the period 1989-2012. A common approach to quantify the base flow components of stream flow is to use a graphical hydrograph separation approach, as the discharge data is the only requirement. The stream discharge records are widely available, and therefore, the graphical approach can be employed in different spatial scales. The base flow component of the hydrograph was estimated at a daily time step for the period of record at each station. The amount of surface run off was calculated and the local minima technique was used to separate the base flow contribution and then the base flow index (BFI) values were calculated in each station and were interpolated to detect the spatial variation pattern. The recorded data length were divided into 5-year periods and the variations of base flow and BFI were analyzed and its relationship with total runoff and rainfall amounts was investigated to detect the temporal changes in the base flow amount and base flow index. The results showed that the decreasing changes was dominant through study area. Also relationships of monthly precipitation and base flow amount indicated that the precipitation variations had low impact on the base flow decreasing pattern. Based on the results, the spatial pattern of total runoff, base flow and base flow index follows a similar regime and the highest values in three variables is related to West and South-west parts of the Ardabil Province and the South-East parts of the study area had the lowest values of interpolated amounts. The highest values of base flow index varies from 0.05 to 0.88 in the study area and highest values are concentrated in the West part of the study area. It was determined that the highest values of base flow is related to the amount of 30 cubic meter per second in the Doost Beigloo station. Also, it can be concluded that the maximum of base flow was observed in Gilandeh station. It can be noted that the West parts of the study area is snowmelt-dominated rivers where there is a persistent flow discharge during snowmelt in late spring or early summer and low-flow periods. The impacts of rainfall variations and water uses on the river flow is clearly reflected in the timing and quantity of river flow. It should be noted that the base flow and base flow amount have decreased and the rate of decreasing intensity was more intense in three months of autumn season.
    Keywords: Base Flow, Spatial variations, Base Flow Index, Seasonal Change, Ardabil province
  • Fereshteh Kaviani, Mohammad Hossein Memarian *, Mohammad Eslami-Kalantari Pages 635-650
    Chernobyl and Fukushima nuclear accidents showed that nuclear accident is not an issue of regional scale and radioactive nuclei disperse and transport faraway under atmospheric conditions. They have a lot of damage, undesirable and dangerous long term effects on the environment and human health. Given the importance of this issue, in this paper, the simulation of atmospheric dispersion, transport and deposition of nuclear materials released from a hypothetical accident in Bushehr nuclear power plant have been studied using HYSPLIT. The hypothetical accident at Bushehr nuclear power plant is based on world's largest nuclear accident, the Chernobyl accident. In this study, it is assumed that the accident in Bushehr nuclear plant occurred on 22 December 2014. Most important radioactive nuclei namely Cesium-137, Iodine-131, Strontium-90, Plutonium-239 and Tellurium-132 are used in this simulation. Release of the radioactive materials is different into the environment. So that, 30% of the cesium-137, 55 % of the Iodine-131, 5 % of the Strontium-90, 3.5 % of the Plutonium-239 and 40 % of the Tellurium-132 contained in the reactor core are released. The accident is controlled for three days. The emission rate of radioactive materials is also different in these days. So, 60 %, 30 % and 10 % of nuclear pollutants are released into the environment, respectively in the first day, the second day and the third day. Given speed and energy of pollutants exit from reactor, they are placed in different heights. The approximate release heights are determined based on the Chernobyl accident. The release heights are 225, 425, 715, 1090, 1575 and 2225 meters. The amount of pollutants that are placed at any height is also different.
    Simulation of atmospheric dispersion, transport and deposition of radioactive materials released in the accident is performed for 1, 10, 24, 48, 168, 336 and 720 hours at different levels. In this paper, the atmospheric dispersion, transport and deposition of nuclear pollutants is investigated over Bushehr province, Iran and the world separately. In addition, the hypothetical accident of Bushehr reactor in 22 December 2014 are compared with the hypothetical accidents of Bushehr reactor in 21 March, 22 June and 23 September 2015.
    The simulation results show that in the period of one month, the nuclear materials released from the accident polluted Tangestan, Dashti, Dayyer, Kangan, Jam and Asaloyeh more than other regions in Bushehr province and the maximum deposition of nuclear pollutants is in the coastal areas Tangistan.
    The simulation results show that a week after the accident, nuclear pollutants are polluted southern, southeastern, eastern and part of northeastern areas of Iran thoroughly and they also affected neighbouring some countries. On the other hand, nuclear materials polluted Red Sea. Two weeks after the event, part of the pollutants entered the continent of America after crossing East Asia and they are deposited in North America. Another part of the pollutants also dispersed in the African continent and after a month of the event they are deposited in South America.
    In Iran, Lamerd and Bandar Abbas have the maximum of nuclear pollutants concentration. In Lamerd, on the first day, the maximum concentration of radioactive materials at ground level and the heights 10 meters is and , respectively.
    Comparing the hypothetical accident of Bushehr reactor in 22 December 2014 with the hypothetical accidents of Bushehr reactor in 21 March, 22 June and 23 September 2015 show that the dispersion and transport of radioactive materials released from accident of 22 December 2014 is almost similar with the hypothetical accidents of Bushehr reactor in 22 June and 23 September 2015, because the pressure systems have the same effects on Bushehr plant. With passing time, the dispersion and transport of nuclear pollutants is different. But in every three incidents, the most pollution is over the southern regions of Iran. The results of this study under similar atmospheric conditions will be applicable to use in Bushehr nuclear plant emergencies.
    Keywords: Nuclear pollutants, Atmospheric dispersion, Bushehr Power Plant, Simulation, HYSPLIT
  • Ali Mohammad Khorshiddoust, Abbas Mofidi *, Ali Akbar Rasouli, Kamel Azarm Pages 651-671
    In order to implement Limited Area Models (LAMs) for a new area, an appropriate domain size, an ideal horizontal and vertical resolution, an appropriate Initial and lateral boundary conditions, and an appropriate Convection Parameterization Schemes (CPSs) are the most important challenges that should be considered. RegCM4 model provides several options, in relation to convection and land surface schemes. Therefore, one of the very important issues is the choice of an appropriate CPS in the model. In fact, precipitation is the most challenging variable which should be considered in numerical modeling. In this regard, using different CPSs could have a significant impact on precipitation characteristics such as intensity, frequency, and spatiotemporal variations. Various studies have been done to evaluate the sensitivity of convective schemes on the simulated variables, particularly precipitation. Suffice to say that all of the studies reveal the importance and impact of CPSs on the simulation results.
    In this study, the Regional Climate Model version 4 (RegCM4.1) was used to evaluate types of CPSs to simulation of springtime precipitation in the North West of Iran (NWI). For this purpose, all conditions such as domain size, vertical and horizontal resolution, initial and lateral boundary conditions have been set uniformly and three experiments were conducted with three CPSs with a 20km horizontal resolution for the Spring 2004 (March, April, May and June) as a typical Spring season. One-month spin-up was considered for the simulation and then at each simulation the model was run at the beginning of February 2004. In order to analyze the effects of the boundary conditions (BCs) on the model simulations, the ERA-interim data with a horizontal resolution of 1.5°´1.5° were employed to provide the lateral boundary conditions for RegCM4. Various types of data were used to evaluation the RegCM4 performance for large-scale and convective precipitation. The monthly and seasonal precipitation of APHRODITE, AgMMERA, CHIRPS and PERSIANN CDR gridded data were used for validation of large-scale precipitation. ERA-Interim data with a horizontal resolution of 0.25°´0.25° spanning 4 months from March to June 2004 were used for validation and evaluation of convective precipitation over two different domains, i.e. the Middle East (ME) and NWI. For evaluation of most appropriate CPS, quantity correlation coefficient, standard deviation, and root mean square error (RMSE) has been demonstrated using Taylor diagrams, and also the bias or average error of precipitation estimation has been calculated
    The results show that overall precipitation patterns demonstrated by four Satellite / Rain gauge based gridded precipitation data are coincided to each other. The maximum amount of precipitation in the area has occurred between the Caspian Sea and the Black Sea in accordance with the North Caucasus. The highest amount of seasonal rainfall by the CHIRPS data is demonstrated in the eastern edge of the Black Sea and the southern coast of the Caspian Sea and the lowest amount in the same regions are related to the APHRODITE data. In terms of spatial distribution, AgMMERA and CHIRPS show more details of precipitation occurrence in compared to PERSIANN CDR and APHRODITE data is estimating total precipitation of four months with three Convective schemes, it becomes clear that, there is a high relative agreement between simulated and observed precipitation, in terms of spatial distribution. The highest estimates of total precipitation are related to Kuo scheme which occurs in a small region of the Black Sea coast and the total amount is up to 1600 mm. Evaluation of CPSs in the estimating of convective precipitation has revealed that they have different simulations on the smaller domain of NWI than to larger domain (i.e. ME). Generally, Emanuel scheme has simulated the large-scale precipitation with highest overestimates while the Kuo Scheme has simulated more balanced conditions of the precipitation. Calculation of the error and bias for convective precipitation amounts revealed that all three schemes had a negative bias in simulating of convective precipitation both for ME and NWI. Generally, the model has simulated an underestimation of convective precipitation in comparison with observations. Results also indicate that the RegCM4 model is not very sensitive to the type of CPS to simulate large scale and convective precipitation in the two first months of Spring (i.e. March and April), while the model is more sensitive to simulate the precipitation for May and June. Evaluating the CPSs performance in the simulation of large-scale and convective precipitation indicates that the Kuo scheme has a relative superiority in comparison to Emanuel and Grell schemes. Also, the model simulation with Kuo scheme over the larger domain (i.e. ME) and Emanuel scheme in the smaller domain (i.e. NWI), have the least bias in simulating the total spring convective precipitation. Due to the relatively higher performance of the Kuo Scheme compared to others, this scheme was chosen as the most appropriate way to simulate the large scale and convective precipitation of the Spring over the study area.
    Keywords: Convection Parameterization Schemes (CPSs), Kuo Scheme, Simulated precipitation, North West of Iran, RegCM4, Taylor Diagram
  • Tayyeb Raziei * Pages 673-695
    Delineation of precipitation regimes is very important for large countries such as Iran which is characterized with complex topography and different climates. The very rare previous studies on precipitation regimes of Iran have used very limited and unevenly scattered stations across the country; thus making it necessary to identify the most realistic precipitation regimes for Iran using as much as available stations. Henco, the data of 155 synoptic stations with relatively regular distribution over Iran; mostly having full data records for the common period of 1990 to 2014, were used for identifying the updated precipitation regimes for the country. For each station, the percentage of monthly precipitation in relation to total annual precipitation was computed for all the time period and the mean of the time period was considered for the analysis. A principal Component Analysis (PCA) was applied to the inter-stations correlations matrix (155×12) that is composed of 155 stations and 12 mean monthly percentage of precipitation for each station. The computed Kaiser-Meyer-Olkin measure of sampling adequacy for the considered matrix with the value of 0.79 indicates that the considered matrix is approximately meritorious for a PCA application. The first 5 leading significant PCs were considered for further analysis based on the Scree plot and the sampling errors of the PCs (North et al., 1982). The retained PCs were then rotated using varimax orthogonal and promax oblique criterion. The PC scores of both rotated solutions and un-rotated solution were separately used as input for Cluster Analysis (CA) to partition the considered stations into distinctive clusters. Moreover, all agglomerative CA methods as well as K-means CA were examined to find out the most appropriate method for partitioning the data. The cophenetic correlation coefficient was used to measure how well the hierarchical dendrogram of a given CA candidate represents the relationships within the input data. The results indicate that all the clustering approaches well represented the inherent structure of the input data, but the Ward method was selected as the most appropriate method since it resulted in much realistic clusters that well matched the topographic and geographical features of the country. The correct number of clusters was also selected based on the Silhouette index (Rousseeuw, 1987) that measures how well objects lie within their cluster, and which ones are merely somewhere in between clusters. The average silhouette width provides an evaluation of clustering validity, and might be used to select an ‘appropriate’ number of clusters. Computing the index for a set of predefined cluster numbers (2 to 15 clusters) suggests that 9 clusters is the most appropriate cluster number that better represents the inherent structure of the data. As such, all 155 stations were classified into five clusters applying Ward CA method on the 5 leading un-rotated PC scores. However, the 8th cluster that grouped stations from two distant areas into a single cluster was subjectively partitioned into 2 distinctive clusters to better represent the precipitation regimes of these two areas. Moreover, the 5 leading varimax rotated PC scores were also mapped to present spatial variability of seasonal precipitation across the country.
    The maps of varimax rotated PC scores well represent areas characterized with seasonal precipitation maximum. For example, summer precipitation in the coastal areas of the Caspian Sea and south eastern Iran are presented by the rotated PC score 1 while the rotated PC score 2 points to the spring precipitation maxima in north western Iran. By taking into account the seasonal displacement of maximum precipitation across the country in the Ward clustering, the identified clusters labeled with its core geographic position or the season of the maximum precipitation well portrait the Iranian precipitation regimes. The Caspian Sea precipitation regime is the most humid precipitation regime in the country with relatively well distributed precipitation during the year that maximizes in autumn. The northern and southern Azerbayjan in northwestern Iran are represented by two distinct precipitation regimes, both being characterized with relatively uniform precipitation distribution during the year but getting their maximum precipitation in a different month of the spring. The south-eastern monsoon precipitation regime featured south-eastern Iran where summer monsoon precipitation has a considerable contribution in annual total precipitation. Similarly, the southeastern coastal precipitation regime characterizing coastal areas of Oman Sea that benefits from summer monsoon but with a lesser magnitude and duration. The western mountainous regime is characterized with a precipitation regime spanning from October to May that maximizes in March. The south-western precipitation regime that encompasses south-western and southern Iran along the Persian Gulf is characterized with a winter rainy season that maximizes in January. Central-eastern and central-northeastern Iran also exhibit two distinct precipitation regimes, both getting their maximum proportion of precipitation in winter but the rainy season is much shorter in central-eastern Iran. And finally, central Alborz is characterized with a precipitation regime in which summer precipitation is relatively high.
    Keywords: Precipitation regime, principal component analysis, cluster analysis, homogeneous regions, Iran