تحليل همديدي سامانه‌هاي شرجي در استان خوزستان

Synoptic Analysis of Sultry Phenomena in Khuzestan Province

پديده ي شرجي يكي از ويژگي‌هاي اقليمي بارز در استان خوزستان بوده كه در اثر افزايش هم زمان رطوبت و دماي روزانه حادث مي شود. هدف اصلي اين پژوهش تحليل همديد شرجي هاي استان خوزستان است. در اين پژوهش پس از استخراج موج هاي شرجي طي دورۀ آماري (2008-1994)، موج هاي شرجي شاخص به دو دورۀ گرم (بهار و تابستان)  و سرد (پائيز و زمستان) تفكيك‌شده براي هر دوره موج هايي كه بيشترين شدت و فراگيري را داشته انتخاب شدند. سپس با استفاده از نقشه هاي تركيبي ارتفاع ژئوپتانسيل تراز 500 هكتوپاسكال و سطح متوسط تراز دريا، نقشه وزش جريان رطوبتي تراز دريا و جريان هوا در دو سطح 500 هكتوپاسكال و سطح متوسط دريا در محدودۀ 10 تا 50 درجه شمالي و 10 تا 70 درجه شرقي براي دو سامانه شرجي شاخص انجام‌شده است. نتايج تحقيق نشان مي­دهد كه در دورۀ گرم سال در تراز 500 هكتوپاسكال استيلاي كامل پرفشار ديناميكي جنب حاره، گسترش و تقويت نصف النهاري آن موجب تقويت و تداوم پايداري بر فراز جو منطقه شده است. تأثير هماهنگ  كم فشارهاي حرارتي سطح زمين بخصوص كم فشار شبه‌جزيره عربستان زمينه مناسب را براي انتقال رطوبت به منطقه فراهم مي آورد. در دورۀ سرد نيز تسلط مركز فشار زياد جنب‌حاره‌اي بر جنوب غربي ايران و استقرار محور پرفشار منطبق بر آن، مهم‌ترين عامل در هدايت جريان هاي نصف النهاري مثبت (جنوبي) و انتقال رطوبت از درياهاي گرم جنوب به سمت استان محسوب مي شود. مهم‌ترين منابع رطوبتي مؤثر بر پديده شرجي استان خوزستان، درياي عرب،درياي عمان و خليج فارس بوده كه محتوي رطوبتي جريانات درگذر از خليج­فارس تقويت مي‌شود.. مسيرهاي ورودي رطوبت نيز عمدتاً جنوبي و جنوب شرقي مي‌باشد.

Introduction Composition of temperature and humidity is a climate phenomenon in southern coast of Iran. Humidity and hot weather create sultry sense. Human are very sensitive and vulnerable against climate change. Iran has a significant temperature and humidity change, therefore during a year, the comfort conditions are limited. A sultry phenomenon is a basic factor in limitation comfort statues in the coastal zone. So the control of humidity and temperature is very important. Severity of this condition is increased with increasing temperature and humidity. The south coast of Iran has a varied climate that is affected by its widespread, variation in topography and location of Persian Gulf in southern boundaries. Because of high temperature and humidity in Khuzestan province, Sultry is seen in more than a half year and create an uncomfortable bioclimatic conditions in human population. Unfavorable climatic conditions, lack of suitable knowledge of the special features of the formation and development of localized climate in Khuzestan province has caused that many of Khuzestan climate characters are unrecognized. The main purpose of this study was to identify the structure and nature of the synoptic scale circulation components in the lower and middle levels of the troposphere, which control the sultry phenomenon. The identifying sultry weather in this region is of considerable importance in order to better plan and find better ways for decreasing environmental problems. Finally recognizing this phenomenon allows us to decrease their harmful effects.   Material and methods In this paper, to identify the formation structure of sultry phenomena in Khuzestan province, 10 synoptic stations which had a suitable spread and without a gap in climate variables were selected. Daily data of mean temperature and humidity from 1994/01/01 till 2008/12/30 were used. Sultry days were extracted by using lancasterkarston index. Because of spatial distribution and temporal changes, the days with simultaneity in 5 stations were selected. This research has done after extracting 14 distinct sultry waves and dividing them to both warm and cold periods of the year. Finally synoptical analysis has been done by using Geopotential height in level of 500hpa and sea level pressure through compositional maps, streamline in two levels of 500hpa and sea level and the map of surface level  humidity advection in the range of 10 to 50 north degrees and 10 to 70 east degrees for two distinct sultry waves.   Result and discussion The study of Geopotential height in level of 500hpa and sea level pressure through compositional maps, streamline in two levels of 500hpa and sea level and the map of surface level humidity advection showed that subtropical high pressure have moved to the 45 degrees south of the circuit in the warm period of the year. Air dynamic subsidence of the subtropical high pressure#039;s eastern section causes a warm and dry weather in the study area. Earth#039;s surface thermal low pressure systems, especially Iraq lowpressure system would be one of the sultry creating factors. Meridional flows were southern. Subtropical high pressure#039;s axis moving, backing on its two southern latitudes and dominate of the high pressure ridge in the study area are the main factors of creating south advections in the cold period of the year. The humidity advection maps showed that the most important humidity resources entered into Khuzestan province include Arabian Sea and Persian Gulf. Moisture of Arabian Sea was strengthened in the Persian Gulf, entered to study area. Humidity advections were south and southeast.   Conclusion The results of synoptic analysis were shown that the distinct waves with complete hegemony of subtropical high pressure in 500hpa level, its meridional extension and support causing the stability atmosphere support and continues over the region. The impact of harmonizing heat low pressure, especially Arabia low pressure, provides a suitable condition for humidity transferring to the region. Domination of subtropical high pressure and the settled manner of its axis during the cold period of the year is the most important trends in south stream from warm southern seas to the study area. The main resources of humidity and sultry conditions of Khuzestan province include Arabian Sea, Gulf of Oman, and Persian Gulf that the moisture contents of currents strengthened over the Persian Gulf. Humidity entry routes are mainly from the south and southeast.