تشخيص منابع تأمين رطوبت و مسير دقيق حركت توده‌هاي هواي مرطوب مؤثر بر بارش‌هاي سيل‌آسا در غرب و جنوب ايران (مطالعه موردي: سيل 7 و 20 آبان 1394)

Determination of humidity sources and accurate trajectory of moist air mass effective on heavy rainfalls in west and south of Iran case study: flooding events of October and November 2015

هدف مطالعه حاضر، بررسي منابع رطوبت و مسير حركت آن براي دو رويداد سيل شديد در غرب و جنوب ايران است. براي اين هدف از الگوريتمي استفاده شده است كه در سال­ هاي اخير براي بررسي انتقال رطوبت از نواحي حاره ­اي به عرض­ هاي مياني توسعه داده شده است. بارش­ هاي سيل‌آساي منتخب در غرب و جنوب ايران به‌ ترتيب روزهاي 6 تا 9 آبان 1394 (28 تا 31 اكتبر 2015) و 19 تا 21 آبان 1394 (10 تا 12 نوامبر 2015) رخ‌ داده ­اند كه خسارت­ هاي سنگيني را برجاي ­گذاشتند. داده ­هاي مورد استفاده در اين پژوهش، داده­ هاي تحليل شدهGFS با قدرت تفكيك نيم درجه در راستاهاي طول و عرض جغرافيايي بودند كه براي محاسبه انتگرال قائم بزرگي شار افقي بخار آب به‌ كار ­رفتند. علاوه‌ بر‌ اين، مؤلفه ­هاي مداري و نصف‌النهاري شار نيز جهت بررسي بيشتر ترسيم و تحليل شده ­اند. نتايج نشان داد كه براي سيل غرب ايران، درياي عرب و درياي سرخ تقريباً سهم يكساني در تأمين منبع رطوبت بارش رخداده دارند ولي مسير حركت بيانگر اين است كه منبع اصلي و مسير حركت رطوبت از سوي درياي سرخ است. اما براي سيل جنوب ايران، منبع اصلي تأمين رطوبت در روزهاي قبل از بارش، نواحي جنوبي درياي سرخ است ولي در روزهايي كه با بارش همراه بوده است، خليج فارس نقش اساسي در تأمين رطوبت آن دارد و درياي سرخ و درياي عرب در زمان بارش نقش مهمي در تأمين رطوبت ندارند.

The aim of the present study is detecting the moisture sources of the heavy floods in the western and southern parts of Iran using hydrometeorological analysis. For this purpose, two heavy floods happened during 28th to 31th October and 10th to 12th November 2015 in the western and southern parts of Iran were selected. The Integrated Vapor Transport (IVT) algorithm was used to study the transfer path of the tropical moisture to the mid-latitudes region. This algorithm is also useful for detecting the moisture source of the heavy rainfall and floods. In mid-latitude locations, most of the heavy precipitation and flood events are related to intense vertically integrated horizontal water vapor transport. The Integrated Vapor Transport algorithm is in the base of accumulated atmospheric moisture transport from 1000 hPa to 300 hPa levels which is computed using the Global Forecast System (GFS) analyzed data with 0.5°0.5° resolution for all days of October to November from 2007 to 2015. Then threshold values of the IVT were computed for the selected western and southern flood events separately as median value of the maximum IVT along a meridional line at 45E between 20N to 45N for the western flood case and a diagonal line between 42E, 23N and 52E, 33N for the southern flood event one. To detecting the role of humidity associated with the Persian Gulf in the southern flood event, an additional diagonal line was also considered in the northern parts of the Persian Gulf between 56.5E, 25N and 47E, 34.5N. To detect the source and path of the moisture for the selected floods, the IVT algoritm (as mentioned in the main text) is used. In addition to the IVT, the zonal and meridional components of integrated vertical moisture flux vector were computed and analyzed to determine the direction of the moisture flux in all grid points. The results showed that the IVT algorithm is a useful tool for finding the atmospheric moisture sources of the floods and the path of the moisture for the studied area. The algorithm indicated that the Red Sea and the Indian Ocean participate equally in moisture supply for the studied flood in the western parts of Iran. But for the peak rainfalls, the results showed that the center of the Red Sea is the main moisture source. For the selected heavy rainfall and flood in the southern parts of Iran, the IVT analysis showed that in days before the flood, the end edge of the moisture path is located over south of the Red Sea but in day with heavy rainfall and flood, the moisture path is limited to the Persian Gulf and surrounding area. So, it seems that for the selected southern flood, the main source of the moisture is the Persian Gulf and the other moisture sources have lesser role in the moisture supply.