تحليل آماري - همديدي پديده گرد و غبار در نيمه غربي ايران

Statistic and Synoptic Analysis of Dust Phenomena in West of Iran

هدف اين مقاله بررسي چگونگي شكل‌گيري پديده گرد و غبار و همين‌طور شناخت مناطق منشا و مسير ورود آنها به نيمه غربي ايران است. در اين تحقيق يك دوره زماني 30 ساله كه از 1979 تا 2008 را پوشش مي‌دهد، استفاده شده است. روش تحقيق تركيبي از تحليل‌هاي آماري- همديدي و دورسنجي است. فراواني وقوع، تدوام و شدت اين پديده با استفاده از داده‌هاي ساعتي گرد و غبار ايستگاههاي سينوپتيك (8 نوبت در 24 ساعت) در محيط ARC GIS و SPSS بررسي شد. وضعيت پايداري جو، با استفاده از داده‌هاي جو بالا در محيط نرم افزار RAOB محاسبه و تحليل شده‌اند. بارزسازي گرد و غبار روي تصاوير MODISبا استفاده از ويژگي‌هاي دماي درخشايي طول موجهاي 11 و 12 ميكرومتر در محيط ENVI4.5 انجام شد، محاسبه و نمايش پهنه‌اي داده‌هاي تراز بالايي در محيط برنامه GrADS انجام شده است. نتايج تحقيق نشان مي‌دهد كه بيشينه روزهاي همراه با گرد و غبار در ماههاي مه، ژوين و ژوييه و كمينه آن در ماههاي دسامبر و ژانويه است. همچنين بيشينه رخداد گرد و غبار در ساعت‌هاي بعد‌از ظهر به وقت محلي و كمينه آن مربوط به ساعت 0.0UTC (5/3 به وقت محلي) است. با بررسي تصاوير ماهواره‌اي همراه نقشه‌هاي جوي و مقايسه آنها طي سالهاي مختلف، شرق سوريه، عراق و نيز بيابان‌هاي عربستان به‌عنوان چشمه‌هاي گرد و غبار شناسايي شدند. بررسي نقشه‌هاي همديدي نشان مي‌دهد كه در دوره گرم سال فرايندهاي ترمودينامكي و در اواخر دوره سرد سال و ماههاي گذر، فرايندهاي ديناميكي مهم‌ترين نقش را در شكل‌گيري و انتقال اين پديده ايفا مي‌كنند.

Dust storm is a meteorological phenomenon common in arid and semi-arid regions. Iran and particularly its west and south-west parts, periodically faces with the dust phenomenon and its problems. In this investigation two dust samples has been selected and studied that they has sustained over three days with visibility less than 1000m in west of Iran. The first one occurred during 18 - 21 April 2008 and the second one occurred in 4 -11 Jun 2008. For this study were used combined statistics, synoptic and remote sensing analysis methods. Daily dust storm data in period of (2008-1979), from 61 selected stations in western half of Iran was obtained. After primary investigation, due to the large data volumes, 20 stations were selected and compared. Also temperature, moisture, precipitation and dew point data to check the status of surface stations were analyzed. Also upper atmosphere radio sound data was collected for western half stations. Software such as RAOB was used for plotted Skew-T diagrams and GrADS were used for prepared weather charts that were received from NCEP-NCAR with net CDF format. By MODIS satellite images and using ENVI4.5 via NDDI equation was extracted dust phenomena over study area. Results show that in the past, daily dust storm frequency in the selected stations that have been located in the path of dust was more than today. Annual Surveying of data shows the dust phenomenon in all of the stations have two peaks times; the first was during the period of 1982-1990 and another was from 2000- 2008. The difference between them is in the intensity, persistence and the number of days with dust. Comparing of satellite images over different time periods show that the number of dust sources has increased in recent decades, that they are Include: East of Syria and West and North-West Iraq. Finally synoptic maps show that during the dust storm phenomena can see a low pressure over Persian Gulf and West of Iran and a high pressure on Saudi Arabia. Pressure gradient between mentioned areas created a suitable condition to move atmospheric patterns towards the study area.