واكاوي همديد پيدايش و گسترش توفند حاره اي فت در جنوب غرب آسيا

A Synoptic Analysis on Occurrence and Development of PHET Tropical Cyclone’s in Southwest of Asia

در خرداد 1389 منطقه خاورميانه توسط توفند حاره اي موسوم به فت مورد تهاجم قرار گرفت كه خساراتزيادي را به سواحل كشور عمان، پاكستان و به مقدار كمتر بر سواحل جنوب شرقايران وارد ساخت. هدف اين پژوهش واكاوي چگونگي پيدايش چرخند فت و تعيين مسير حركت آن براساس قاعده كشش و رانش، به همراه سنجش اثرات آن بر دو فراسنج بارش و دما است. روش تحقيق توصيفي تحليلي با استفاده از واكاوي نقشه‌هاي همديد مراكز /NOAA3ESRLبه همراه تفسير تصاوير سنجنده‌هاي تي، آر، ام در باندهاي مرئي و تركيبي در طول حيات توفان فت است. نتايج پژوهش نشان داد كه توفند حاره اي فت در جنوب­ غرب شبه قاره هند با فشار مركزي 1002 هكتوپاسكال در اطراف هسته آبگرم در مركز اقيانوس هند شمالي در درون شارش بادهاي موسمي شكل گرفته است. همچنين در واكاوي مسير حركت اين توفند مشخص شد، كه جابجايي پهنه هاي تغييرات فشار در ترازهاي پاييني و مياني جو و ارتباط آن با ميزان حركات قائم (امگا) در ترازهاي مذكور نقش موثري در تعيين مسير حركت آن داشته است، به شكلي كه با تحليل ارتباط ميان اين دو مولفه (تغييرات امگا و فشار) مي‌توان گذرگاه توفند فت را تعيين نمود. در بررسي اثرات توفند مشخص گرديد ناهنجاري هاي منفي دما تا تراز 850 هكتوپاسكال به سبب فرآيند تبخير شديد ناشي از همرفت قوي هواي مرطوب به 8 درجه سلسيوس نسبت به شرايط نرمال رسيده است، اما در تراز 500 هكتوپاسكال به علت آزاد شدن انرژي نهان ناشي از بخارآب ناهنجاري دما، مثبت و در حدود 4+ درجه سلسيوس بوده است. از طرف ديگر پهنه‌هاي بيشينه بارش، همخواني زيادي با مراكز صعودي هوا در لايه‌هاي پاييني و مياني جو دارد، به شكلي كه جابجايي هسته‌هاي بارشي با ميزان حركت و قدرت صعودي سامانه‌ در تراز 500 هكتوپاسكال ارتباط زيادي را نشان مي دهد .

Introduction A tropical cyclone (TC) is a lowpressure system that forms on the warm waters of the tropical and subtropical oceans. Intensive tropical cyclones are called “Hurricanes” and are hazardous for marine transportation, ports and even to aircrafts and so on. Pressure at the center of these systems reduces to 1005 hPa and lesser. Wind velocities of hurricanes may exceed 33 m/sec. In general, there is a theory that says a tropical cyclone (TC) occurs in two stages: Stage 1: TC occurs when the mesoscale convective complexes (MCC) produce a mesoscale vortex. Stage 2: TC occurs when a second blow up of convection at the mesoscale vortex initiates the intensification process of lowering central pressure and increasing swirling winds.  The Main regions for TC (and hurricane) generation are the Gulf of Guinea toward the Carrabin Sea, eastern parts of the Pacific Ocean in Philippines and yellow Sea and south of Indian peninsula in Maldives isles toward west. Most of the TCs in Indian Ocean move toward west and northwest. In rare conditions the country of Oman, southeast of Iran and south of Pakistan are affected by a TC (hurricanes) impacts. Return period for this movement of the TCs (and hurricanes) towards the north is very long. The Gonu and Phet TCs were two extraordinary tropical storms that reached to the north coasts of the Indian Ocean.  It is suggested that Global Warming might be a main cause for the next unexpected behaviors of TCs   Materials and methods In this study, data and climatic information of NOAA/ESRL were used to provide Maps of the Sea surface temperature (SST) changes and its anomalies, pressure changes in the 3 levels of 850,500 and 250Hpa to study horizontal wind speed within the lower, middle and upper troposphere and investigate causes of the PHET TC occurrence. Then, to determine motions of the Phet in its life,   two main parameters namely changes of Omega and geopotential height in lower and middle troposphere were evaluated on the basis of Attraction and Buoyancy rules. To determine changes due to storm movement in affected areas by the Phet, we selected parameters of Temperature and accumulated rainfall and analyzed rainfall from satellite images field measured by the TRMM and temperature anomaly field from synoptic charts of 850 and 500 hPa levels (difference from longtime average).   Results and discussion Results of this study show that increase of north Indian ocean SST up to 32Co associated with SST of the Bay of Bengal and Regressive movement of STHP from the Indian peninsula, Pakistan, the Persian Gulf due to onset of the monsoonal southwesterly flows from equator are the basic factors for onset and development of the TCs and hurricanes such as the Gonu or the Phet. Also inspections of storm tracks show that they follow the rule of “Attraction and Buoyancy. According to the rule the Phet from the beginning stage until 4th June moved toward the Oman coasts due to far effects of the Balkan surface low. But northward expansion of the STHP in that region (moving to the south coasts of Iran) by reinforcing closed cells of the monsoon on Pakistan from 4 June onwards, the storm’s path was deviated toward southeastern coasts of Iran and then toward south parts of Pakistan.  Geopotential heights variations analysis in 850 and 500 hPa levels on 1 to 6 June, better show the STHP effects on passage tracks and intensity of storm. Polar satellite images show the most severe storm and rainfall (600 mm) in west of the Indian ocean on 2 and 3 of June. Generally, movements of rainfall cells of the mentioned storms have a close relation with the 500 hPa motion field of weather patterns.   Conclusion Forecasting of movement tracks for tropical cyclones is very important in weather forecasting.  Predictions of tropical cyclone tracks in north of the Indian Ocean may be based on the rule of “Attraction and Buoyancy”, therefore it can determine movement tracks of storms in this region. The rule is based on relation and reaction among changes of the pressure and omega parameters in low, middle and upper levels of the atmosphere. Timely predictions can reduce damages due to these risks. Damages caused by tropical cyclones because of their heavy rainfall and severe winds, have placed the climate system in the class of horrible natural hazards. For Exle PHET cyclone damages in Oman were more than 600m dollars. Also 10 thousand people in Pakistan have been moved and 16 killed. On the southeastern coast of Iran, on the fourth and fifth days of its formation, particularly in the Chabahar Port, the Sea wave height raised to 13m and dust storms with visibility reduced to less than500m. Over the recent decades, frequency of the tropical cyclones in the North Indian Ocean has shown a significant increasing trend. Based on this trend, in future occurrence of such storms in the northern coastlines of the Golf of Oman will be more possible