فهرست مطالب

پژوهش های اقلیم شناسی - پیاپی 27-28 (پاییز و زمستان 1395)

نشریه پژوهش های اقلیم شناسی
پیاپی 27-28 (پاییز و زمستان 1395)

  • تاریخ انتشار: 1395/12/13
  • تعداد عناوین: 9
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  • شاهرخ فاتح *، علی اکبر رسولی، بهروز صاری صراف، غلامعلی کمالی صفحات 1-9
    شناخت و آگاهی از چگونگی تغییر فرآیندهای کشاورزی می تواند برنامه ریزان بخش کشاورزی را در روند شناخت راهکارهای مناسب برای کاهش خسارات احتمالی این چالش یاری نماید. چگونگی تغییرات فصل رشد محصولات کشاورزی از جمله گندم درکشور یکی ازمهمترین مقوله های پژوهشی است که باید مورد بررسی قرارگیرد. در این مطالعه، ضمن تعریف فصل رشد، چگونگی تغییرات آن در طول رشد گیاه گندم به عنوان یکی از محصولات استراتژیک کشاورزی با روش های مختلف از جمله تجزیه و تحلیل داده های آب و هوایی و با استناد به اندازه گیری های فنولوژیک گندم دیم استخراج خواهند شد. در بین عناصر اقلیمی دما از اهمیت خاصی برخوردار است و به این دلیل بررسی تاثیراتی که دما در رشد و نمو موجودات بویژه گیاهان می گذارد، ضروری می باشد. هدف کلی این مقاله بررسی و پهنه بندی واحدهای گرمایی طول فصل رشد گیاه گندم بر اساس پایه دمایی صفر درجه سلسیوس در 27 ایستگاه سینوپتیک ایران در طول دوره آماری مشترک 30 ساله با استفاده از شاخص های دمایی طی سال های 2010-1981 بوده است. به این منظور به منظور محاسبه واحدهای گرمایی ابتدا طول فصل رشد برای آستانه دمائی صفر درجه سلسیوس بدست آمد و پس از محاسبه واحدهای گرمایی نقشه پهنه بندی واحدهای گرمایی در GIS تهیه شد. میزان واحدهای گرمایی طول فصل رشد در پایه دمایی صفر درجه سلسیوس از جنوب به شمال و از غرب به شرق کشور می یابد.
    کلیدواژگان: طول فصل رشد، واحدهای گرمایی طول فصل رشد، آستانه های دمایی، پهنه بندی اقلیمی، ایران
  • سعید رنجیر، غلامعلی کمالی *، هوشمند عطایی، امیر گندمکار صفحات 11-24
    آب و هوا تاثیر بسیار مهمی در حیات موجودات زنده کره زمین دارد. وزش باد با دما اثر تلفیقی احساس محیط را ایجاد می نماید، بطوریکه به کاهش دما همراه با باد، سوزباد و برعکس آن گرم باد اطلاق می شود. در این تحقیق سوزباد دمای کمینه در استان اصفهان مورد بررسی قرار گرفت. دمای کمینه و سرعت وزش باد برای 10 ایستگاه همدیدی استان طی دوره آماری 1372 تا 1394 از سازمان هواشناسی کشور تهیه گردید. نتایج نشان داد که مهر و اردیبهشت را می توان به ترتیب به عنوان ماه های شروع و پایان سوزباد در استان اصفهان در نظر گرفت. میانگین کاهش دمای کمینه به واسطه وزش باد به میزان 1/4 درجه سلسیوس در بهمن بوده است که بیشترین مقدارطی ماه های تحت مطالعه است. بیشترین و کمترین فراوانی وقوع سوزباد در استان به ترتیب به ایستگاه فرودگاه اصفهان با 2196 مورد و ایستگاه خورو بیابانک با 228 مورد تعلق دارد. سوز باد در ماه دی بیشترین سطح استان را در بر می گیرد. بیشترین تاثیرگذاری باد بر روی دمای کمینه مربوط به مناطق مرکزی و غربی استان به ویژه ایستگاه های فرودگاه اصفهان، داران، اردستان و نائین بوده است.
    کلیدواژگان: سوز باد، دمای کمینه، ایستگاه همدیدی، باد، احساس محیطی
  • محمود خسروی، محسن حمیدیان پور، صالح کردی تمین صفحات 25-42
    کم فشار پاکستان یکی از سامانه های شبه دائمی مهم و تاثیرگذار در فصل گرم سال در جنوب آسیا است و به دلیل نقش آن در اقلیم تابستانی منطقه تحت نفوذ خود از اهمیت بالایی برخوردار است. در پژوهش حاضر به تحلیل تغییرات زمانی این کم فشار به روش سینوپتیک - آماری پرداخته شده است. در این مطالعه متوسط متغیر فشار تراز دریا طی دوره آماری 2015-1980 در محدوده مکانی 5/62 تا 5/72 درجه طول شرقی و 5/22تا 5/32 درجه عرض شمالی مربوط به دوره ی گرم سال (ژوئن، جولای، اوت و سپتامبر) از داده های شبکه ای ERA-Interim مرکز پیش بینی میان مدت اروپایی استخراج شده است. تغییرات زمانی کم فشار پاکستان به صورت سری زمانی سالانه و ماهانه فشار مرکزی کم فشار و همچنین نابهنجاری های دهه ای آن مبتنی بر فشار میانگین درازمدت بررسی شد. برای بررسی روند این تغییرات از آزمون های ناپارامتریک من-کندال و شیب سن استفاده شد. نتایج پژوهش نشان داد که فشار مرکزی این سامانه به طور متوسط 31/997 (ه.پ) است و بیشینه و کمینه فشار مرکزی این سامانه به ترتیب در سال 1984 و 2015 با فشار مرکزی 996 (ه.پ) و 999 (ه.پ) رخ داده است. به لحاظ فشار مرکزی سامانه در مقیاس سالانه از روندی افزایشی برخوردار بوده و این شرایط در مقیاس ماهانه برای ماه های ژوئن و اوت نیز قابل مشاهده است. هرچند سطح معنی داری آزمون روند کمتر از 90 درصد بوده است. بررسی ناهنجاری های دهه ای فشار مرکزی سامانه بیانگر این مطلب است که فشار مرکزی این سامانه در دو دهه ی اول نسبت به متوسط درازمدت کاهش یافته و در دهه ی سوم و پنج سال پایانی بر میزان فشار مرکزی این سامانه افزوده شده است. از دیگر نتایج این مطالعه این است که کم فشار پاکستان یک سامانه ی دوقطبی با هسته قوی در شرق پاکستان(دره سند) و هسته فرعی ضعیف تر بر روی دشت مارگو(جنوب غرب افغانستان) است. درنهایت فشار مرکزی کم فشار پاکستان طی بازه ی زمانی موردمطالعه افزایش داشته، بنابراین می توان نتیجه گرفت که سامانه با شیب ملایمی رو به تضعیف است
    کلیدواژگان: کم فشار، ناهنجاری، جنوب شرق ایران، موسمی، تحلیل روند
  • سحر تاج بخش* صفحات 43-56
    نظر به اهمیت کاهش دید افقی ناشی از وقوع مه برای عملیات فرود هواپیما در یک فرودگاه ، در این مطالعه کوشش شده است ضمن بررسی آماری باد، ارتفاع کف ابر و دید افقی کمتر از حداقل دید فرودگاه در موارد فرودگاه های مهم و پرترافیک کشور شامل مهرآباد تهران، مشهد، رشت، تبریز، اهواز، کرماشاه و بندر عباس، الگوی میانگین جوی شاخص های فشار تراز دریا، باد و مقدار ابر تعیین و با استفاده از الگوریتم شناسایی مه، انواع مه ایجاد شده در فرودگاه های منتخب و در دوره ی 1995-2014 (20 سال) شناسایی شوند. مهمترین نتایج این بررسی نشان می دهد در ماه های سرد سال (دسامبر، ژانویه و فوریه) بیشترین رخداد مه در کل کشور گزارش شده است. غالب گزارش ها قبل از طلوع و بعد از غروب خورشید و به ترتیب در ساعت های 03، 00 و UTC21 دیدبانی شده اند. الگوی میانگین سمت وسرعت باد در ارتفاع 10 متری نشان دهنده ی هوای آرام در غرب کشور و بادهای غربی با سرعت میانگین 5 تا 10 نات در نواحی مرکزی و شرق کشور دیده می شوند که در حاشیه شمال شرق ایران گردش چرخندی کوچکی را نشان می دهند. بیشترین آمار رخداد مه در فرودگاه های اصفهان، کرمانشاه و رشت مربوط به مه تابشی نوع دوم ( وزش باد ملایم و ارتفاع کف ابر 300 پا یا کمتر) می باشد. مه تابشی نوع یک (وزش ملایم باد و آسمان بدون ابر) بیشترین هوای مه آلود را در فرودگاه های به خود اختصاص داده است. در فرودگاه های مهرآباد تهران و تبریز ضمن اینکه تعداد رخداد ها کمتر از سایر فرودگاه ها می باشد، بیشترین رخداد به مه ناشی از پایین آمدن ارتفاع کف ابر برمی گردد. این امر می تواند در اثر وقوع بارش جبهه ای ایجاد شود. مه فرارفتی تنها در فرودگاه مشهد قابل بررسی و توجه می باشد. به نظر میر سد گردش چرخندی در الگوهای میانگین باد موجب شده شکل گیری مه فرارفتی که نیازمند وزش بادهای حداقل 6 ناتی می باشد، در فرودگاه مشهد بیشتر از دیگر فرودگاه ها باشد که نیازمند بررسی های موردی سایر عوامل در این فرودگاه می باشد.
    کلیدواژگان: مه، دمه، حداقل دید فرودگاه و ارتفاع کف ابر
  • نوید حیدری، رضا دوستان*، مجید حبیبی نوخندان صفحات 59-75
    بررسی پدیده های جوی، ارائه گزارش وضعیت جوی مطابق با استاندارد ها و استخراج میانگین عناصر اقلیمی در ناحیه شهری، به جهت شناخت الگوی آب و هوای محلی و اقلیم شهر، مستلزم ایجاد شبکه منظم از ایستگاه های هواشناسی شهری می باشد. این مساله با توجه به شرایط خاص در نواحی شهری، با وجود اختلاف فیزیکی سطح و اقلیمی شهرها با محیط های مجاور شان، اهمیت دارد. بنابراین این پژوهش با بررسیپارامترهایموثربرمکان یابی ایستگاه های هواشناسی شهریتحتتاثیرخصوصیات شهری و ویژگی های منطقه مورد مطالعه، شاخص های مکان یابی شامل: پراکنش عناصر اقلیمی در سطح شهر، پراکنش خصوصیات فیزیکی شهر (معابر و کاربری ها) و پراکندگی مخاطرات محیطی مرتبط با اقلیم شهر انتخاب شدند و پس از جمع آوری داده های مورد نیاز هر شاخص شامل: داده های اقلیمی، داده های رقومی خصوصیات فیزیکی شهر و مخاطرات شهری، لایه های مورد نیاز با استفاده از سیستم اطلاعات جغرافیایی(GIS) استخراج شد. تعیین اولویت شاخص ها نسبت به یکدیگر با نظر کارشناسان خبره و با بهره گیری از تکنیک تحلیل سلسله مراتبی و مقایسه زوجی انجام شد. اوزان سلسله مراتبی بدست آمده با لایه های نرمال شده منطق فازی با روش جمع وزنی تلقیق گردید. در نهایت مناسب ترین نقاط شامل، 11 نقطه اولویت بندی شده با توجه به جهت گسترش شهر مشهد و با فاصله مناسب از ایستگاه اصلی، برای جانمایی ایستگاه های هواشناسی شهری مکان یابی گردید. این موقعیت ها به ترتیب اولویت: 1-بلوار توس/ بزرگراه میثاق.2- خواجه ربیع/ موسوی قوچانی/ بلوار امامیه.3-ابتدای هفت تیر/ خیابان سلمان فارسی/ قلعه ساختمان.4-خیابان امام رضا/ بلوار مجلسی/ جاده سیمان می باشند.
    کلیدواژگان: اقلیم شهر، ایستگاه هواشناسی شهری، تحلیل سلسله مراتبی، مکان یابی، GIS
  • بابک شاهی نژاد، رضا دهقانی * صفحات 75-86
    دمای هوا که در ایستگاه های هواشناسی استاندارد اندازه گیری می شود یکی از توصیف کننده های اصلی وضعیت محیط زمین است. بنابراین برآورد و تخمین دقیق دمای متوسط روزانه در هر منطقه یکی از پیش نیازهای مهم برای برنامه ریزی کشاورزی و نیز مدیریت منابع آب می باشد که به روش های مختلفی همچون مدل های تجربی، نیمه تجربی و هوشمند قابل انجام است. در این پژوهش کاربرد شبکه عصبی موجک به منظور برآورد متوسط دمای روزانه هوا در ایستگاه ساری مورد بررسی و ارزیابی قرار گرفته و کارایی آن با مدل شبکه عصبی مصنوعی مقایسه گردید. جهت مدل سازی از داده های دمانگار ایستگاه هواشناسی ساری واقع در استان مازندران استفاده شد. پارامتر رطوبت نسبی، دمای بیشینه، دمای کمینه، سرعت باد و تبخیر در مقیاس زمانی روزانه در طی سال آبی (1382-1392) بعنوان ورودی شبکه و دمای متوسط روزانه هوا به عنوان خروجی شبکه انتخاب گردید. معیارهای ضریب همبستگی، ریشه میانگین مربعات خطا و ضریب نش ساتکلیف برای ارزیابی و مقایسه عملکرد مدل ها مورداستفاده قرار گرفت. مقایسه نتایج نشان داد مدل شبکه عصبی موجک عملکرد بهتری نسبت به مدل شبکه عصبی مصنوعی در مدل سازی دارد، بگونه ای که مدل شبکه عصبی موجک با بالاترین ضریب همبستگی (999/0)، ریشه میانگین مربعات خطا (001/0) و نیز بیشترین معیار نش ساتکلیف (998/0) در مرحله صحت سنجی در اولویت قرار گرفت. در مجموع نتایج نشان داد مدل شبکه عصبی موجک در تخمین بیشتر مقادیر دقت بالایی از خود نشان داده است.
    کلیدواژگان: تخمین، دمای هوا، شبکه عصبی مصنوعی، شبکه عصبی موجک، ساری
  • زهرا سادات حسینی*، شهریار خالدی، عبدالمجید نادری بنی صفحات 87-98
    به منظور مطالعه پوشش گیاهی گذشته در تالاب دشت اژن مغزه ای به طول 960 سانتیمتر از این تالاب برداشته شد. در ابتدا، مغزه تحت اندازه گیری پذیرفتاری مغناطیسی قرار گرفت. سپس نمونه ها باز شده و با توجه به پذیرفتاری مغناطیسی از آنها زیر نمونه تهیه شده، برای مطالعات گرده های گیاهی آماده سازی شد. چهار نمونه تکه گیاهی هم در طول مغزه برداشته شده و برای سن سنجی به آزمایشگاه رادیوکربن پزنان لهستان ارسال گردید. نتایج مطالعات سن سنجی بازه زمانی 15000 ساله را نشان داد. از 15000 سال تا11000 سال قبل، پلیستوسن پسین پوشش گیاهی استپی با آب و هوای سرد و نسبی خشک غالب بوده است. گونه کوزینیا و خانواده اسفناجیان و درمنه که در اوایل این دوران به وفور مشاهده شده است حکایت از شرایط سرد پایان دوره یخبندان در این دوران دارد. گونه های درختی از جمله جنگل های بلوط و پسته-بادام تقریبا 10000 سال قبل ظهور کرده و در هولوسن میانی گسترش پیدا کرده اند. احتمالا در این دوران بارش فصل گرم با افزایش نسبی بارش و دما شرایط مناسب برای رشد بلوط را فراهم آورده است. از 10000 سال قبل بیشتر گونه های گندمیان در منطقه غلبه داشته اند. حدود 5700 سال قبل هولوسن میانی پوشش درختی غلبه پیدا کرده و الگوی تقریبا منظمی را تا عهد حاضر طی می کند. این مشاهدات تایید کردند که اگر چه نوسانات در اقلیم درطول هولوسن میانی و پسین رخ داده است، اما الگوهای بارشی در این منطقه از 6000 سال قبل به بعد به الگوی تقریبا ثابت امروزی رسیده است و در این دوره گونه های درختی بلوط و جنگل های پسته- بادام دراین محیط گسترده شده اند. جنگل های بلوط از حدود 5700 سال قبل شروع به رشد کرده اند. فاز دوم گستردگی در گونه های بلوط حدود 3000 سال قبل اتفاق افتاده، ظهور درخت گردو در این دوره نیز قابل توجه است و نشان دهنده فعالیت انسانی در این دوره می باشد.
    کلیدواژگان: دیرینه اقلیم، گرده های گیاهی، ضریب مغناطیسی، تعیین سن
  • عباس رنجبر*، فائزه نوری صفحات 99-114
    عوامل بزرگ مقیاس جوی نقش غالب و تعیین کننده شرایط وضع هوا و رخداد پدیده های هواشناختی ازجمله پدیده گرد و غبار می باشند. ایران به دلیل شرایط گردش کلی جو در منطقه خشک و نیمه خشک جهان قرارگرفته بطوریکه کانون های گرد و غبار متعددی در این عرض های جغرافیایی وجود دارد که تحت برخی از عوامل بزرگ مقیاس جوی فعال شده و می توانند نواحی گسترده ای از غرب آسیا، ازجمله مناطق غربی کشور را تحت تاثیر قرار دهند. هدف از این مطالعه تعیین ارتباط بین عوامل کنترلی جوی با رخداد گرد و غبار در نواحی غربی کشور است. برای این منظور ابتدا با استفاده از داده های سینوپتیک ایستگاه های هواشناسی، تعداد روزهای همراه با پدیده گرد و غبار، طی دوره 22 ساله (2013 -1992) که دید افقی به کمتر از 5 کیلومتر رسیده تعیین گردید. سپس با استفاده از داده های NCEP/NCAR، الگوهای ترکیبی میانگین ماهانه و بی هنجاری کمیت های بارش، فشار تراز دریا، دما و ارتفاع ژئوپتانسیل ترازهای 850 ، 700 و 500 هکتوپاسکال و شاخص نوسان اطلس شمالی بررسی شد. نتایج مقایسه این کمیت ها برای دوره های بیشینه و کمینه رخدادهای گرد و غبار، نشان داد که تقویت کم فشار گرمایی در فلات مرکزی ایران و عراق، گرادیان شدید پربندی ترازهای پایین جو در نواحی شمالی ایران و عراق و شرق سوریه، تقویت و گسترش نصف النهاری پر ارتفاع تراز 500 هکتوپاسکال از نواحی مرکزی مدیترانه به روی اروپا، استقرار ناوه پربندی در شرق مدیترانه، کاهش قابل ملاحظه بارش فصل زمستان وکاهش غیرعادی شاخص نوسان اطلس شمالی (که منجر به انتقال مسیر حرکت توفانها به نواحی جنوبی اروپا و شمال آفریقا می شود)، از مهمترین عوامل تشدید و توسعه این پدیده در نواحی غربی کشور می باشند.
    کلیدواژگان: عوامل کنترلی بزرگ مقیاس جو، گرد و غبار، غرب ایران
  • فخرالدین ایران پور*، سعید باقری صفحات 115-131
    پدیده تگرگ یکی از بلایای جوی خطرناک است که اغلب همراه با طوفان های تندری بوده و از ناپایداری جو باروکلینیک در مقیاس همدیدی ناشی می شود. پژوهش حاضر در یک دوره آماری 23ساله (2015-1992) داده های مربوط به توفان های تگرگ (که کدهای 27، 87، 88، 89، 96، 94، 93 ،90، 99 را شامل می شود) از مرکز تحقیقات هواشناسی همدان دریافت گردید. در این پژوهش رخ دادهای تگرگ با استفاده از نرم افزار Spss خوشه بندی گردید، سپس از روزهای موجود، دو روز را که دارای ناپایداری شدیدتری بود، به عنوان روزهای نماینده انتخاب گردیدو سپس با استفاده از داده های سایت2 NCEP/NCAR نقشه های همدیدی فشار سطح دریا (Slp)، ژئوپتانسیل، شاخص امگا، نم ویژه در ترازهای دریا،850 و 700 هکتوپاسکال و نقشه وزش باد در تراز 850 با استفاده از نرم افزار گرادس3 در روز اوج بارش برای دو نمونه انتخابی،ترسیم و مورد تحلیل قرار گرفت. نتایج حاصل از پژوهش نشان داد که ایستگاه همدید ملایر بیش ترین فراوانی وقوع بارش تگرگ را به خود اختصاص و بیشینه وقوع این پدیده نیز در ماه های آوریل و مارس، در ساعت های 09 و 12 گرینویچ و بیشینه فراوانی آن در بعد از ظهر و اوایل شب رخ داده است. بررسی الگوهای جوی نشان داد که بارش تگرگ در منطقه مورد مطالعه، نتیجه گسترش سامانه ی کم فشار واقع بر روی دریای سرخ وسودان است که در روز بارش تگرگ منطقه در جلو ناوه(تراف) قرار داشته و سامانه پرفشار بر روی دریای عرب سبب انتقال هوای گرم و مرطوب با امتداد جنوبی- شمالی به درون منطقه شده و ضمن تقویت سامانه کم فشار سودانی شرایط مناسبی برای ایجاد ناپایداری و ریزش تگرگ فراهم کرده و بررسی نقشه های امگا نیز همگرایی تراز 1000 هکتوپاسکال و واگرایی قوی در ترازهای 850 و700 هکتوپاسکال را نشان داد، به طوری که جریانات بالاسوی قوی هوای گرم و مرطوب را صعود داده و زمینه ایجاد ناپایداری های شدید را فراهم کرده است.
    کلیدواژگان: تگرگ، الگوی همدید، ویژگی های ترمودینامیکی، توزیع زمانی و مکانی، همدان
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  • Shahrokh Fateh *, Aliakbar Rasouli, Behroz Sari Saraf, Gholam Ali Kamali Pages 1-9
    Climate is a major environmental factor that controls all aspects of life. Among the climatic elements temperature is the important factor and because of this, study the effects of temperature on the growth of organisms, especially plants will be necessary. If temperature to be below the critical level of growth, plant growing stages will be stop and may hurt plant. The critical level temperature is different and dependent on the kind of plants. The majority of biological changes in plants is dependent on Growing Degree-days. In the present study, it has been achieved Growing Degree Days (GDD) Zoning with using temperature indicators. According to these factors, it has been determined capabilities and limitations of agricultural climate in different plant species adaption in country. Wheat is one of the strategic agricultural products of Iran. This variety of wheat has the best yield and quality in north-western provinces of country, including West and East Azerbaijan, Ardabil, Zanjan, Kordestan and Hamedan. Despite the fact that the mentioned region has the area of 17578000 acres or nearly 16.3% of the country area, the area under cultivation of winter wheat in these provinces consist about 40% of total wheat cultivating farms in the country or about 2411000 acres. This implies that the region is prone of cultivating wheat (and also other agricultural products) about 2.3 times more than other parts of country.
    Another fact that verifies this is fraction of area under cultivation of wheat per total province area. Among 31 Iranian provinces, ranking of this statistic shows that all the provinces in selected area rank below 10, including first rank in country, Hamedan Province with 21.2% of its area under cultivation of wheat, Second rank Ardabil Province with 20.0%, rank 4 Kordestan province with 18.4%, rank 6 Zanjan provinces with 14.1% and finally, rank 9 and 10 East and West Azerbaijan provinces with 9.7% and 9.6% respectively.
    Although the most important source of impact on yield of wheat, especially in dry farming, is adequate and timely precipitation, the climate change and global warming plays an undeniable role on long-term trends and attitudes toward choice of new places for building farms to answer the consistently increasing demand for this strategic product.
    The trend in four phenological stages beside total Lenth of Crop Season (LCS) as a measure of intensity of the effect of climate is calculated in 27 stations in the region.
    Materials and methods For this research, it has been used daily temperature at 27 Synoptic Stations during 1981 to 2010 periods. For calculating heat units, it was used the thresholds of 5 and 10 ° C as the lower threshold and 30 and 35 C °for the upper thresholds. For determining of hit units, at first, the dates of starting and end of 5 and 10 C° temperatures were extracted and then heat units during the growing season was calculated from mean daily temperature minus the base temperature and zoning heat units maps in GIS were prepared.
    Result and discussion According to this research, the growing season length of 5 and 10 C° were divided into five categories in Iran. In the research, the areas of the country that Thermal Units based on 5 and 10 were homogeneous had taken in a group. Heat units in the base of 5 C° are divided into five groups, including 2000-2500, 2500-3000, 3000-3500, 3500-4000, 4000> (GDD) and for 10 C° are divided into four group including 500-1500, 1500-2500, 2500-3500, 3000>.(GDD).
    Conclusion Growing Season Degree-Days for the basis of 5 and 10 C° are declining from South to North and East to West in the country. The Heat Units amounts during the growing season at 5 C° are increased from South to North and West to East. These amounts are decreased from South to North for the base temperature 10 C° and are increasing from West to East. According to the obtained results, it can be suggested that in areas where the growing season is reducing, farmers by reducing crop yield and also products that have not been well face. That is better farmers used early growing plants until they got heat energy to supply in the plant during the growing season.
    Keywords: Keywords: Growing Season Length, Growing Degree Days, Base Temperatures
  • Saeed Ranjbar, Gholam Ali Kamali *, Hoshmand Ataei, Amir Gandomkar Pages 11-24
    Introduction Temperature is a factor feels by human body and body sense changes when the outside temperature changes for the reason of the heat transfer between body and the environment.
    Wind chill is caused by the wind in lower temperature and the higher the wind speed and the lower the temperature, the wind chill and its effects on the creatures would increase. The WCT is important, since it’s an index which shows people how to dress up or to be optimum on energy consumption. When it feels cold due to the wind chill, it could contribute greatly to decision making in urban design and fair and optimum distribution of fuel. The main purpose of this paper is to study the effect of wind chill on reducing the temperature that body feels, also when this done, it could help decision makers in social, political issues as well as peoples clothing and etc based on the wind chill index.
    Material and Methods This research has been done about the determination of the temperature which our body feels based on the wind chill index in Isfahan.Isfahan with an area of about 105937 square kilometers has been placed between 30 degrees and 43 minutes to 34 degrees and 27 minutes of north latitude of equator and 49 degrees and 36 minutes to 55 degrees and 31 minutes of east longitude of Greenwich meridian. In this article we can calculate the wind chill index (WCT) from equation 1.
    WCT=13.12.6215T-11.37V0.16.3965TV0.16 (1)
    V: wind velocity in terms of km (hr) and T: air temperature in terms of °c
    Results and discussion From the studies we have done the table results indicated that the month of October is the beginning of the wind chill and the end of it would be in May. But since the frequency of occurrence of this phenomenon in these two months is much lower than other months, will not be discussed in this article. Therefore April, November, December, January and February and March were considered to study. The occurrence of wind chill at Knur Biabanak station was 228 times during 23 years (8395 days). The highest of the wind chill occurrence was 2257 times in February and its lowest was 39 times in May. The wind chill has occurred in Naein station 432 times in the month of February. And this number is the highest occurrence of wind chill in a month.
    Conclusion In this article, we have studied the effect of wind speed on the minimum temperature reduction in Isfahan province. Wind chill reduces the emotional temperature on the human body. The results of this research showed that the wind chill factor has been felt 2196 times at Isfahan airport station and 228 times at Khur station as the highest and the lowest numbers recorded, respectively. In the month of February we had 2257 times of wind chill occurrence in the statistical period that has been the most frequency of wind chill occurrence and the lowest number was 39 times in May. The minimum temperature and wind chill in the month of January respectively was -1.6 and -5.6 degrees Celsius. On average, wind blowing will cause the minimum temperature to reduce 4 degrees in this month. The zoning results of the minimum and the wind chill temperatures in Isfahan province indicate that the vulnerable areas of the wind chill in Isfahan are located in the center and western half (overlooking the mountains) of the province . In these regions some stations due to the topographic conditions are more vulnerable to the wind chill. These stations include Isfahan airport, Daran, Ardestan and Naien that they are the most vulnerable areas of the province against the wind chill. Also the month of February is considered as the most critical month in terms of wind chill effects on human body and the most widespread zone of wind chill happens in this month.
    Keywords: Wind Chill, Minimum Temperature, synoptic station, Wind, Environmental Feeling
  • Mahmood Khosravi *, Mohsen Hamidianpour, Saleh Kordi Tamin Pages 25-42
    Introduction Long-term synoptic maps related to sea levels show the presence of a semi-permanent, low-pressure system in the warm season in the Indian subcontinent, southern Afghanistan, Pakistan and Central Plateau of Iran, known as Low-Pressure Pakistan or Gang. The above mentioned low pressure refers to thermal low pressure created on Sindh basin in Pakistan due to the entry of solar radiation in May and June. Since the daily radiation is more than the nightly radiation, this thermal low pressure has become a permanent summer system (Boucher, 1975, 132; Kaviani and Alijani, 1999, 192). Pakistan thermal low pressure is an important and effective system in the climate of Pakistan, the northwest of India, the southwest of Afghanistan, and the southeast and east of Iran. Therefore, extensive studies have been conducted to examine the effect of this system on other parameters and climatic phenomena such as the studies by Iqbal and Jawaid (2008), Faisal and et al., (2013); Alijani et al., (2011); Khosravi et al., (2011). The study by Bollasina and Nigam (2011) which is a complete study on Pakistan low pressure, on the origins and changes of summer low pressure in Pakistan and northwest India, concluded that Pakistan-India low-pressure current influence two regional and remote forces. In regional scale, the effects of Hindu Kush Mountains were stronger than those of Earth surface and talent temperatures. The effects caused by remote factors, rooted in convection currents in Gulf of Bengal and East India in June and July, are also of importance. According to the importance of this system and its effect on the Iranian climate especially in the southeast, it is essential to study the tempo spatial variation. Therefore, the main questions outlined here are as follows: What are the changes of Pakistan low pressure during the statistical period? How are the spatial changes over time? How are the changes in terms of intensity over time?

    Data and Methodology ECMWF (ERA-Interim Version) with a spatial resolution of 1*1° was used in the geographical area (Latitude of 20-50° N and Longitude of 20-90° E) for the warm seasons (June to September). The statistical period 1980-2015 was taken into account. In order to extract the central pressure, the longitude of 62.5° to 72.5°E and latitude of 22.5°to 32.5°N were defined. First, the annual minimum pressure was extracted in the form of time series. Man-Kendal non-parametric trend tests were employed to study the variations.
    Results and Discussion 36-year medium maps (1980-2015) for sea level pressure over the warm seasons show the deployment of the low-pressure system with two closed cells. As it is clear in the literature review, the system is a feature of warm seasons usually created as a result of extreme heats of underlying atmosphere along with the dynamical subduction originating from the West Indies in the south of Pakistan (Ramage, 1966). According to the time-series results, the lowest pressure was reported in 1984 with the central pressure of 996, and the maximum was reported in 2015 (999). A review of monthly Pakistan low pressure shows that the central pressure of this system has different fluctuations. It experiences a rising trend over all months. The increase was evaluated at three confidence levels (90%, 95%, and 99%). They are not significant, however. The greatest rising trend was observed in June and lowest in July. As a result, the system experiences a slightly increasing trend. Decade anomaly of central pressure was also studied in monthly and yearly scale. Table 1 shows the results. According to the results, the central pressure of the system is weakening and the last decade has the highest central pressure.
    Table 1: Anomaly of Central Pressure of Pakistan Low Pressure (1981-2015), Based on Spatial Period (Latitude of 22.5*32.5°N and of Longitude 72.5*62.5° E)
    Period
    Pressure
    Difference
    Total Average (1981-2015)
    997.31
    -
    First decade
    997.26
    -0.05
    Second decade
    997.19
    -0.12
    Third decade
    997.29
    -0.02
    Five years of latest
    997.63
    0/32
    Finally, Hoff Muller Maps were used to describe the static continuity variations, that is, spatial changes of this low-pressure region during two periods (1980-1989 and 2005-2016). The results showed that Pakistan low pressure has no specific spatial extent or displacement of the East or the West direction. In other words, the system has equal extent over the initial and closing decades.
    Conclusion Pakistan low pressure is an important, effective system in South Asian regions. It is of great importance in the region. This article aimed to study the tempo variations of this system using Man-Kendal nonparametric test and the Sen's estimator of slope. The monthly and annual review show that this system has a rising central pressure trend. In other words, it is weakening. Note that the increase is slight and is not significant at 95%, 90%, and 99%. Anomaly map review shows verified it, too so that the greatest central pressure was observed in the recent decade. The system did not show any specific spatial variation.
    Keywords: Southeast of Iran, Low pressure system, Anomalies, Trend analysis, Monsoon
  • Sahar Tajbakhsh* Pages 43-56
    Introduction Fog is among the most important weather hazards from the aviation perspective. This phenomenon can lead to horizontal visibility reduction. Therefore, accurate prediction is essential for flight safety and easing air traffic. Fog consists of a weather condition in which water drops and ice crystals reduce the horizontal visibility to less than 1000 meters. Various methods are suggested for fog forecasting. Numerical and statistical methods, experimental approaches, and very short range fog forecasting are some of the most common methods. Experimental methods are commonly used for first guess in forecasting centers. Saunders technique is one of the forecasting methods for radiation fogs using radio sounds data. Although this technique goes back to many years ago, it is being used in many parts of the world, including UK Met Office, and is recommended by World Meteorological Organization.
    Material and method Present study tries to evaluate the performance of two experimental methods using real data after studying synoptic condition of fog occurrence in two selected airports. The validity of them is measured then with the real occurrence in a number of case studies of fog occurrence for the selected airports using the bias technique in order to choose the more appropriate method. In the next step, the more appropriate method is administered using the numerical prediction model output and is again evaluated with the bias technique. In both these methods, an index called fog point temperature has been used, and the fog occurrence has been determined by calculating this temperature and comparing it with the minimum temperature. The selected airports are Mehrabad Airport, Tehran and Shahid Hasheminezhad Airport, Mashhad, which have been chosen because of high flight traffic (in Tehran) and high fog occurrence (in Mashhad). Experimental methods examined in this study are Saunders and Prichars-Crodack techniques, which 25 case studies in selected Airports tried to offer the best results for first guess of fog occurrence. The accuracy of these relations was evaluated comparing real conditions using Bias technique. After choosing the more appropriate method, a similar process has been gone through using numerical prediction model of WRF for the next 12 hours.
    Results and discussion Results of synoptic evaluations show that high-pressure systems are a major factor in creating coldness in lower levels of the atmosphere. Evaluation of pressure field in this study doesnt show figures below 1020 hPa. Specific humidity values were 6-8 g/kg and 4-6 g/kg for 1000 and 925 hPa levels respectively. Winds are frequently northern or eastern and cold weather advection is seen in selected stations.
    In Saunders technique, using radio sound data of 1200 UTC in 25 case studies for mentioned airports, the fog point is calculated. This temperature is then compared with next day's minimum temperature and if the difference is less than -2°C, fog occurrence would be ruled out. Saunders considers this method mostly useful for radiation fog. In Crodack-Prichars technique, which is performed by creating a regression association between temperature and dew point temperature, the fog point temperature is determined. Here again, fog is not formed If the temperature difference is less than -2°C.
    After calculating fog point temperature using Saunders technique and comparing it with actual observation, it was found that among 25 cases, 15 fog observations were consistent with Saunders technique calculations. In the five cases of fog nonoccurrence, the results of this method were consistent with reality. So, Bias evaluation technique shows 75% for probability of detection.
    The same process has been gone through for Crodack-Prichars technique. In this method, a linear relationship exists between temperature, due point temperature, and fog point temperature. Wind condition and cloudiness are also presented experimentally in the form of a table. For different amounts of these two factors, a numerical amount of 1.5 to -1.5 is added to the fog point temperature. Fog occurrence is determined by calculating fog point temperature using Crodack-Prichars technique and comparing it with the minimum temperature according to table 2. This evaluation showed that in 13 of 20 fog occurrence cases, the right answer were obtained, and 5 cases of fog nonoccurrence, were consistent with reality.
    Conclusion Therefore, POD index was reduced to 65%. Based on the results, Saunders technique has been considered as the more appropriate method for initial guessing in fog forecasting in the airports under study. Now, the amounts of temperature and due point temperature were determined for the next 12 hours using WRF numerical prediction model, and Saunders technique was used again for predicting fog (using predicted data). The results of this evaluation were also investigated using Bias evaluation technique, which were not so agreeable, so that it was consistent with reality in 50% of cases. Hence, it seems that careful consideration of numerical prediction models output is needed.
    Keywords: fog, Sanders Method, fog point, WRF numerical weather prediction
  • Navid Heydari, Reza Doostan *, Majid Habibi Nokhandan Pages 59-75
    The proper study of atmospheric phenomena and reporting the weather in accordance with current standards and extracting the average of climatic elements in different periods in a region in order to recognize local climate and micro climate of a geographical units, requires building a network of enough weather stations. This problem is very important, due to creating exclusive conditions in urban areas that are created because of the physical difference between cities with their surrounding environment. Therefore in this research after studying effective parameters on urban weather stations site selection under mentioned circumstances and the qualities of the studied area, positioning indexes including: dispersion of climatic elements in the city, dispersion of physical qualities of the city (crossings and land uses) and dispersion of environmental jeopardies in relation with the climate of the city were selected and after gathering required information of each index such as: climatic data in reported form, matrix data related to physical qualities of the city and environmental jeopardies reports, required layers were extracted using GIS. Setting priorities of indexes related to each other was done using experts’ verdicts and analytical hierarchy process technique and binary comparison. Then, the obtained weights of layers normalized with Fuzzy membership function in AHP are compounded and overlapped using weighted average approach and the weight of the layers were determined and finally the most appropriate parts including 11 parts are prioritized and located with a suitable distance from the main station in order to expand Mashhad and replace urban weather stations .
    Keywords: local climate, urban weather station, Analytical Hierarchy Process, Site Selection, GIS
  • Babak Shahinejad, Reza Dehghani * Pages 75-86
    Introduction The average air temperature prediction great importance in the field of water resources management, agriculture, water and a lot of things everyday. Air temperature is also one of the components of the hydrological and ecological models is input, as well as land evaluation models. On the other hand, because the weather has a significant impact on social life, and individual centers worldwide scientific research on climate issues raised as a priority, nearly as fundamental. Quantitative prediction of air temperature is one of the most important elements in managing and programming of surface water resources, especially take suitable decisions in occurrence of drought event.
    Objective
    In this study, recorded data sets in Sari Station (located in Mazandaran province), were used to investigate the precision of different Air temperature prediction models. The wavelet neural network model and artificial neural network models selected for modeling of daily Air temperature, and the results were compared to examine the accuracy of studied models.
    Methods
    Daily Air temperature were selected and observed of this basin in the Sari station that were applied for calibration and validation of models. For this purpose, at first 80 % of daily Air temperature data (2002-2010) were selected to calibrate selected models, and 20 % of data (2010-2012) were used to validate models. For modeling, meteorological data from sari synoptic station (Mazandaran Province) were used. Humidity parameters, maximum temperature, minimum temperature, wind speed and evaporation in daily time scale during a ten year time period (2002-2012) are inputs of the network, and mean temperature as network output is selected. Wavenet called wavelet-based neural network which combined with wavelet theory and neural networks have been created.It also have supportive of the benefits and features of neural networks and charm and flexibility and strong mathematical foundations and analysis of multi-scale wavelet . a combination of wavelet theory with neural network concepts to the creation of wavelet neural network and feedforward neural shock can be a good alternative for estimating approximate nonlinear functions .Feedforward neural network with sigmoid activation function is in the hidden layer While at the nerve shocked wavelet ,wavelet functions as activation function of hidden layer feedforward networks are considered, In both these networks and scale wavelet transformation parameters are optimized with their weight.
    Artificial neural networks inspired by the brain's information processing systems, design and emerged intoTo help the learning process and with the use of processors called neurons trying to understand the inherent relationships between data mapping between input space and optimal space. Hidden layer or layers, the information received from the input layer and output layer are the processing and disposal.Based on the artificial neural network structure, its major features high processing speed, the ability to learn the pattern,The ability to extend the model after learning, flexibility against unwanted errorsNo disruption to error on the part of the connection due to weight distribution network. The first practical application of synthetic networks with the introduction of Multilayer Perceptron network wasConsultants. for training this network back propagation algorithm is used.The basis of this algorithm is based on error correction learning ruleThat consists of two main routes.By adjusting the parameters in the MLP model error signal and input signal occurs.Determine the number of layers and neurons is the most important issues in simulation with artificial neural network. Criteria of correlation coefficient, root mean square error and Nash Sutcliff coefficient were used to evaluate and compare the performance of models. . Coefficient of correlation, root mean square error and Nash Sutcliffe coefficient was used to evaluate and compare the performance of the models.
    Results
    Results showed that all two models (in a structure), consisting gives better than results any other structure. And also, based on results according to the evaluation criterion, the models was used to wavelet neural network model, most accurate (R=0.999), and the lowest root mean square error (RMSE=0.001) and the highest standards Nash Sutcliffe (NS=0.998) the validation phase is capable.
    Conclusions
    Finally, wavelet neural network model outperformed the artificial neural network. So, wavelet neural network model can be effective in forecasting the daily Air temperature and in turn facilitate the development and implementation of Prevent drought will be useful. , and the use of wavelet neural network model can estimate the drought effectively, Which in return facilitates the development and implementation of management strategies to avoid drought.
    Keywords: Estimation, Air temperature, Artifical Neural Network, Wavelet neural network, Sari
  • Zahra Sadat Hoseini *, Shahriar Khaledi, Abdolmajid Naderi Bani Pages 87-98
    Introduction Complex of climate related to topography and general circulation atmospheric interaction in Iran have been unintelligence. Modern climate in southwest Iran is governed by Red sea Trough and westerly depression and Siberian high pressure and in order to understanding of this complex we need to obtain the climate conditions of past. One of the best method for reconstruction of paleoclimate change based on reconstructs of vegetation dynamics by pollen count. This method the first time have applicateed by Van Zist and Bottema in northwest of Iran and continued by Stevense et all,2001 and 2006, 2012, and Djamali and et al, 2009,2012,2015 and Joens et al,2015. Safaei Rad, 2014 in Persian and Davoodi and et al, 2015 in Persian and Azizi et al, 2012 in Persian and Akbari et al,2016 in Persian.
    Arzhan wetland located on ′ 56 °51 ÊÇ ′ 59 °51 in sout west of zagros Fars province Iran. The climate parameter of this region show in table (1)
    The number of frost days
    Minimum absolute temperature
    The maximum absolute temperature
    The average temperature of the coldest month
    Average temperature of the warmest month
    Annual average température
    124 days
    15˚C
    38˚C
    2.1˚C
    21.1˚
    11
    Table (1) Modern climatic factors
    Materials and methods In order to understand past climate and environment change and reconstruct at Arzhan wetland SW of Zagros Iran 960 Cm core were taken and a pollen diagram was derived through late Pleistocene and Holocene. We performed magnetic sensibility analysis in primary step and the second we initially to subsampled based on magnetic sensibility measure and in thirst we have prepare our samples according to Moor et al, 1995 methods. In this step we added Lycopodium to our samples in order to counting and frequency of pollen grain in samples than we counted pollen and derived pollen diagram during late Pleistocene and Holocene of Southwest of Iran( Figure 1)
    In other hands for dating, we extracted four samples of piece of plant for obtained date and after preparing sent to Radio Carbon laboratory Poznan Poland.(Figure 2)
    Results and discussion Radio carbon dating represent 15000Cal. BP that covered late Pleistocene and Holocene at the region. The pollen diagram shows at late Pleistocene covered by cold and dry steppe with Cousinia and Artemisia whit dry summer. Represent of cousinia refer to cold and dry in the upland of Zagros mountain (El Moslemany, 1986 and 1987) also Djamali believe that cousinia pollen describe a dry conditions( Djamali et all, 2012) arboreal pollen represent during Holocene around 10000Cal B.P. and start to spread in this region. Quercous woodland and Pistasio- Amygdalus shrubs have spread from around 6000 years ago during middle Holocene. A maximum of spread of Oka woodland represent in 4000 and 3000 Cal.B.P so we referred them to increase of moisture of summer rainfall. Appearance of Guglans in 3000 years ago is important and refer to human activity in this region
    Conclusion The pollen records started at around 15100 Cal. BP and show an absence of arboreal plant in area during late Pleistocene up to 11000 Cal. BP. Pollen records in this zone present a cold steppe that indicated by Cousinia, Artemisia Centaura and Chenopodiaceae pollen with decrease of Poaceae. Younger Dryas in this part of Zagros have not presented by spatially pollen grain. It seems due to upland and local characteristic the basin has not experienced Younger Drays as other region of Zagros. From 10000 Cal. BP arboreal plant have begun to spread as Quercous woodland and Pistacio- Amygdalus scrub that dominated the area during last Holocene. At 8300Cal.BP Quercous woodland and Pistacia- Amygdalus scrub degraded during 600 years up to 7700 Cal BP after this phase Quercous woodland start to spread until 6300 Cal. BP. Middle Holocen at 6300 years ago Quercous woodland have degraded and start to spread. At 5000 Cal.BP. Through Holocen at this period Quercous woodland, have maximum extended. Gaglans has appearance at 3000 years ago and continued to present. Based on pollen diagram we reconstructed climate of past in this region. It seems the atmospheric circulation and climate regime have been stable and closed to modern climate in general.
    Keywords: Paleo environment_Pleistocene_Holocene_Pollen_Dasht E Arzhan
  • Abbas Ranjbar Saadatabadi *, Faezeh Noori Pages 99-114
    The western area of Iran is subjected often to dust, which reduces horizontal visibility to 5 km, and sometimes even to less than 1 km. In this study, dust datasets are developed based on the visibility less than 5 km. for each 3-hour spell through 22 years (1992-2013). Dust data comprised monthly frequency for the period, in 6 stations across the west and south west of Iran. It consists of monthly dust frequency (06-code of synoptic reports) in the 22-year period from the meteorological synoptic stations. Then monthly and seasonal pressure patterns and their anomalies were studied for the months that had minimum or maximum of dust frequency during 22 years (1992-2013). Thus, in this case study the annual and interannual occurrences of dust events based on 22 years of 3-hours visibility. An analysis of large-scale circulations over the lower and middle of troposphere, for the two months (July of 2004 and 2009), was done using NCEP-NCAR reanalysis data to link their roles in and contributions to the raising frequency of dust across the western Iran. For this rezone we investigated the main factors to describe the relationship between effective Large Scale Atmospheric Circulation (LSAC) and dust frequency over western Iran, using the available meteorological data from synoptic meteorological stations and the data set includes monthly sea-level pressure, and 850, 700 and 500mb geopotential heights, wind, Temperature fields and their anomalies relative to long term mean (1981-2010). Finally the relationships of dust frequency with precipitation and North Atlantic Oscillation (NAO) have been investigated.
    Surveys showed that in recent years the event frequency, intensity and the extent of the region affected by dust have been increasing. The monthly data indicate that the dust season starts at February and ends in September, with a maximum in June and July. More than 70% of the total annual dust was recorded between March and July, the 'high dust season'. The monthly and annual total dust frequencies vary considerably from year to year. In the period study, the maximum and minimum dust frequencies have been occurred in 2009 and 2004 years respectively, where the ratio of dust frequency in 2009 to 2004 was more than 31 times.
    During in the years that precipitation decreases, the occurrence of dust event increases; thus, there was a meaningful relation between precipitation and the decreasing of the frequency of dust event. A highly significant negative correlation (more than -0.50) was found between dust event and precipitation in the west and south west of Iran (Table 3). Also, a negative correlation was found between dust event and the intensity of the NAO in the some stations (Table 3), which modulates the cyclonic activity over Europe and the northern Mediterranean.
    Maximum numbers of dust events usually occurred in the summer. Because of strengthening the subtropical high pressure from north east Africa to Iran, the waves of Mediterranean have been delivered to northern latitudes, and there will be no possibility of moisture feeding and no precipitation. While low pressure system moves to the east, the conditions for high pressure system's deployment located in the west of upper trough, by convergence of upper flows, leads to its creation and strengthen of high pressure. So, intensive pressure gradient has created between low and high pressure systems which leads to creation of strong winds and causes dust storm over Iraq and east of Syrian. However, geographic condition might have an effective role in creating strong winds. Some potential predictors are found through detecting the atmospheric circulation factors in qualitative analyses. These are as following order: 1- In poor dust (rich dust) periods, ridge (trough) and positive (negative) geopotential height anomalies over east of Mediterranean Sea, were relatively strong compared to those in normal years, while over the west of Mediterranean Sea was standing or developing Vice versa conditions.
    2- Another reason for rich dust periods in the west of Iran was developing of heat low pressure in the central of Iran and this region that is usually at the same time along with developing low pressure on north of Iraq. In poor dust periods, the situation was reversed, especially, strengthening high pressure system over the Caspian Sea and developing ridge of high pressure in the central of Iran.
    3- Variation of precipitation in the region: The scavenging of summer dust particles by winter precipitation is a major process of dust removal on the region. The results showed the negative correlations between the winter precipitations and summer dust levels for the all stations. A high correlation was found between them with an R2 value of -0.71. Therefore, variation of winter precipitation in the region could affect summer dust levels.
    4- A negative correlation was found between dust event and the intensity of NAO in the some stations, which modulates the cyclonic activity over Europe and the northern Mediterranean.
    Keywords: Large Scale Atmospheric Circulation, Dust, West of Iran
  • Fakhr-Al-Din Iranpour *, Saeed Bagheri Pages 115-131
    Introduction
    Hail is one of the most destructive weather phenomena causing damage in many different sectors, including losses in the agricultural sector. Hail is linked to the atmospheric elements and geographical factors ; whenever atmospheric conditions are suitable for physical processes to be combined with geo-location, the intensification of this phenomenon happens. Depending on the size and severity of the storm, damage hail is different (Dong et al.,2006,p.193)Hail made more than a billion dollars damage in agriculture in China in 2004. Hail or ice grains are composed of particles with a diameter of 5 to 50 mm and sometimes more. Severe and frequent hail are resulted from vertical movement of air in the cumulonimbus clouds causing the sperm of water droplets around them to be absorbed and freezing (Alijani & kavyany, 2008,p.264). (Iran Pour et al.,2015,p.115)In relation to the synoptic and thermodynamic analysis of thunder storms in Hamadan, it has been concluded that the pressure center for cool and cold air to the middle layers of the atmosphere and the low pressure in the North West South underlying currents of warm and humid climate of the region is located in Saudi Arabia, where the growth of cumulus and cumulonimbus clouds create thunderstorms, hail, and torrential rain . Many studies have been conducted in the country in the field of hail. (Fryzbay,1961,p.350,)To study the relationship between type of damage caused by hail in the plains of synoptic patterns, United States of America showed that the speed of movement of synoptic systems has a significant impact on the degree of damage from hail on the ground. (Hough,1961,p.242)In a 50-year period, the distribution of thunderstorms, rain, maximum temperature, dew point and air fronts and their influence on the distribution of hail in Illinois have been examined.It is believed that distribution of hail in a small area can be affected by various elements of climate. (Ezzati,2003,p.121). The role physical processes in atmospheric instabilities resonance is studied.
    The studied area
    Hamadan province with approximately 19545/82 square kilometers, is located on (33 degrees 33 minutes) to (35 degrees and 38 minutes) north latitude, between (47 degrees and 45 minutes) meridians and (49 degrees and 36 minutes) east longitude.It is surrounded by Zanjan and Qazvin from north, Lorestan from south, Markazi from east and Kermanshah and Kordestan from west, and based on the last country division, it includes 9 states, 29 cities, 25 districts, 73 rural districts, and 1210 villages. The average height of sea level in this region is about 1800 meters. The highest point in Hamadan province is Alvand 3584 meters high and the lowest point is Omar Abad 1600 meters high. Specifications of the studied stations are given in the following table.
    Table (1): Specifications of studied synoptic stations
    Station name
    Longitude
    Latitude
    Altitude
    Hamadan Airport
    48.32
    34.52
    1741.5
    Nojeh in Kaboudar-Ahang
    48.71
    35.20
    1679.7
    Malayer
    48/51
    34/19
    1725
    Nahavand
    48/24
    23/22
    1644
    Materials And Methods
    To investigate the occurrence of hail, data of statistical synoptic station were used four times during a period of 23 years (1992-2015). In this study, the primary data from surface meteorological research center, Hamedan in the period 1992 to 2015 (the first half of 2015) were collected and then initial test and extract data were controlled and a database of hail from present and past weather codes were prepared (codes 87.27, 88, 99,96,94,93,90, 89 to be included) at intervals of 3 hours . Of the 100 present weather codes, the codes of hail phenomena that contain different intensities were considered to include any emergence scout in hours and three hours before . In order to identify patterns that cause hail and identify the index days , a cluster analysis was carried out on 63 days of hail using software Spss in the study area and the days were clustered. Then, of the days,and the hours when occurrence, severity and duration of hail and rain showers were remarkable, two days was chosen, as the representative days (the tenth day of April 2002 and April 2007) and then other elements of climate, such as rainfall, temperature, pressure, humidity, wind direction and velocity and study phenomena such as thunderstorms (cumulonimbus clouds that arise), the date mentioned were studied using data from NCEP site / NCAR, maps synoptic pressure at sea level (Slp), geo potential indicator of omega-Nam, especially in the levels sea, 850 and 700 hp and map wind at 850 using the software GRADS on the peak month for the sample, and were analyzed. To analyze data and maps software packages such as, Grads Excel, ArcGIS, and Spss were used.
    Discussion
    According to statistical analysis, the frequency of hail events in the synoptic stations was different from one another.Due to non-uniform distribution of hail during the period of 23 years (1992, 2015) studied at the station, due to its geographical location and topography , the greatest hail synoptic frequency stations has been 22 times in Malayer and the lowest synoptic frequency in Nojeh 10 times . Maximum occurrence of hail occurred in April and March at 09 and 12 o`clock (all hours in this study are in GMT) and evaluating changes in hours of hail indicates that the maximum number of storms may occur in the afternoon and early evening. In this period , Code 27 and Code 89 had the highest and lowest frequency , respectively.The greatest occurrence of hail , for a 23-year period, was from 9 to 15 o`clock reaching the peak at 12 o`clock and from 15 it declines rapidly. This shows the increase of atmospheric instabilities in these hours with increased atmospheric radiation energy received by the atmosphere and land surface features, atmospheric instability is perfectly amplified and appears to emerge at 9 to 12 because of the warming climate. The highest frequency of hail occurred respectively in April, March and February and the hail was not reported in July.
    In June, December, August and September there was the lowest frequency and in March and April as the transitional warm (spring) months and, because of rapid warming of the Earth's surface and lower layers of the atmosphere (the day elongation and Sun approaching to vertical line ) and high air humidity , the instability increases and convective flows are accelerated and under the right conditions, convective clouds are grown making thunderstorm (Ezzati, 1382,p.121). The frequency of hail in the region in terms of year indicates that 1994 and 2009 experienced the greatest frequency and in 1995 and 1999 there was no hail. Spring had the highest frequency of hail and summer the lowest frequency . More than half of the hail events occurred were reported in the spring, followed by winter,.
    Conclusion
    The province of Hamedan is among the areas where due to the occurrence of hail, in a lot of crops are destroyed , and there may also appear casualties. Therefore, understanding the mechanism of formation, development and prediction of hail and events resulting from it such as thunderstorms , heavy torrential rain, lightning and strong wind can provide great help in order to reduce losses. Based on the results obtained, the highest frequency of hail occurred in April and March. In the spring, sun warms the Earth's surface and sea level in the study area is also relatively and high latitude of region strengthens the conditions for south low pressure system at this time of year . Investigating the synaptic systems along with hail showed that location of low pressure center on Sudan and extension of its edge to higher latitude in eastnorth-westsouth has covered all the srea study.On hail day, an edge of high specific humidity of the water resources in the southern Red Sea, Arabian Sea and the Sudan, penetrates the high latitudes. On this day at the same time a strong negative core Omega 850 and 700 hp on the studied area is the convergence at 1000 hPa with strong divergence in the upper levels of the atmosphere is closed and warm, moist air advection divergent and the release of latent heat of abundant, high flows and create the appropriate convective instability has provided. The wind field maps of the day, hail, can be seen on the meridional wind in South and West orbital wind is weak.
    Concurrently, a core of negavtive strong omega at level of 700 and 850 hectopascal was investigated on the study area where the convergence was seen in 1000 hectopascal in higher levels of strong convergence and the convicted warm and wet air becomes divergent .With the abundant latent heat released , suitable upward flows provide the ground of convection instabilities.Examination of wind field maps on hail days shows that the dominant wind is on the south equator region and the western wind is weak.
    This confirms moisture advection over warm seas. With the establishment of a high pressure system in the South East of Saudi Arabia and on the Arabian Sea, warm, moist air transfer through divergent flux over Oman and Arabian Sea has been convicted to the center of the low pressure system located on the Red Sea and Sudan by the spinner on region. Penetration of warm low pressure edges from southern Iran on the West with vertical development of clouds (cumulus and cumulonimbus clouds), high humidity, convection and intense ascending and mountain conditions in the West have been an important factor for instability and hail.
    Keywords: hail, synoptic pattern, thermodynamic characteristics, temporal, spatial distribution, Hamedan