بررسي همديدي و شبيه‌سازي توفان خاك استان خوزستان در بهمن ماه سال 1395

Synoptic analysis and simulation of dust storm in Bahman 1395 in Khuzestan province

هر ساله وقوع رخدادهاي گردوخاك ساكنان بخش‌هاي وسيعي از كشور از جمله استان خوزستان را با مشكلات جدي مواجه مي كند. در روزهاي 7 تا 10 بهمن ماه سال 1395، وقوع هم زمان پديده گردوخاك و بارش باران در شهر اهواز منجر به قطع برق و به دنبال آن آب در اين شهر به مدت دو شبانه­روز و لغو پرواز هواپيماها گرديد. بررسي نقشههاي همديدي نشان مي‌دهد در نقشه فشار سطح زمين، كل منطقه تحت تأثير يك سامانه كم فشار ديناميكي قرار دارد. چرخش خطوط هم‌فشار، تغيير جهت باد و وجود كژفشاري در منطقه از علائم وجود جبهه است. اجراي مدل HYSPLITدر تراز 500متر به خوبي مرز جبهه سرد و تغيير جهت باد در منطقه را نمايش مي‌دهد. مقادير كد هواي حاضر ايستگاه همديدي اهواز بيانگر وقوع توفان، پديده گردوخاك محلي و غير محلي و بارش باران در اين دوره بوده و كمترين ميزان ديد افقي برابر 100 متر گزارش شده است. شبيه‌سازي اين رخداد گردوخاك با مدل WRF-Chem نشان داد كه مدل، گسيل گردوخاك از عراق و جنوب خوزستان را به خوبي نشان داده و بار گردوخاك خروجي مدل با تصاوير ماهواره‌اي در توافق است. با مقايسه كمّي غلظت خروجي مدل با مقادير مشاهداتي مي‌توان نتيجه‌گيري كرد كه مدل در شبيه‌سازي غلظت ذرات عملكرد قابل قبولي دارد. مدل در شبيه سازي جهت باد و بارش، عملكرد بهتري دارد و الگوي بارش و ميزان آن در شهرهاي مختلف را به خوبي نشان داده است. همچنين وجود جبهه و تغيير جهت ناگهاني باد توسط مدل شناسايي شده است.

Dust storms damage peoples’ lives and cause many financial problems every year. Dust storm is one of the most important natural phenomena and a kind of natural disasters that influence many parts of the world. Dust storms cause many problems in different regions of Iran, in particular Khuzestan province every year. This province is affected by internal and external dust sources because of some arid and semi-arid areas inside and outside of Khuzestan province, including Iraq, Syria and Saudi Arabia. Therefore, many researches have been conducted to analyse and simulate dust storms. A severe dust storm happened in the west and south west of Iran in 7-10 Bahman 1395 (26-29 January 2017). Dust particles were drenched on the power transmission lines in Ahwaz city because of low precipitation on the 8th Bahman, resulting electricity cut off in the city. This event made many problems for residents of Ahwaz city. Synoptic analysis showed that, the existence of a cold front in the southeast of Iraq caused severe winds in the area on the 8th of Bahman. In Khuzestan province, ahead of the cold front, the south easterly winds triggered the activation of local dust hotspots in the south of this province. The baroclinicity in the front area was quite apparent at the 850hPa level, and there was a deep height trough at the 500hPa. The output of the HYSPLIT model in Khuzestan region near the surface showed the south-easterly wind flow. Also, the change in wind direction in the front area was clearly seen in the model output at 500m level. Present weather codes reported from Ahvaz synoptic station showed local and non-local dust storm and precipitation altogether. The visibility reduced under 100 meters in Ahvaz on the 8th of Bahman 1395 (January 27, 2017). The WRF-Chem model with the GOCART emission scheme was implemented for simulation of the dust event. The model outputs showed that the emission of dust particles from Iraq and the south of Khuzestan was correctly simulated, and they were in agreement with the satellite images; although the dust mass was not detected well, due to cloudiness in the region. The PM10 concentration of the model output and the measured values at the air pollution monitoring stations showed that the model has a satisfactory performance in the estimation of the PM10 concentration. The model simulated the dust concentration trend correctly, however, some errors in the output were seen. These errors were expected due to the complex topography of Khuzestan province. The model had a better performance in simulation of wind direction and rainfall. The model also identified the existence of the front and the sudden change of wind direction.