ساختار گردش جوي در طي بارش سنگين 21 دسامبر 1992 برروي شيراز (30 آذر 1371)

Atmospheric circulation structure during heavy precipitation of 21th December 1992 over the City of Shiraz

در طي بررسي الگوي گردش جوي در زمان وقوع بارش هاي سنگين برروي شهر شيراز در فصول پاييز سال هاي 1371 تا 1378 به موردي برخورديم كه با ديگر موارد تفاوت هايي داشت و ما را به بررسي جداگانه آن واداشت. در اين مطالعه براي تعيين بارش هاي سنگين از روش آماري بروكس استفاده گرديد. جهت تعيين ساختار گردشي جو در زمان وقوع بارش سنگين در اين تاريخ كميت هاي هواشناختي از قبيل فشار تراز دريا، ارتفاع ژئوپتانسيل، رطوبت نسبي، واگرايي، و سرعت قائم در ترازهاي 850 و500 هكتوپاسكال، دما در تراز 850 هكتوپاسكال و سرعت باد افقي در تراز 300 هكتوپاسكال مورد بررسي قرار گرفته اند. نقشه هاي مورد نياز با استفاده از نرم­افزار GRADS و داده هاي مركز ملي پيش بيني هاي محيطي (NCEP) امريكا ترسيم گرديد. الگوي جوي روز 21 دسامبر نشان مي دهد يك سامانه كم­فشار كه از روزهاي قبل برروي عربستان و درياي سرخ شكل گرفته با حركت به سمت شمال شرق در اين روز به داخل ايران نفوذ كرده و در برخورد با پرفشار مستقر برروي اروپا كه به سمت جنوب غرب حركت مي كند شرايط مناسب براي ناپايداري را برروي جنوب غرب ايران ايجاد كرده و منجر به همگرايي در سطوح پايين برروي منطقه به ميزان S-1 5-^10*2 - شده است . در سطح 500 هكتوپاسكال منطقه واگرايي شديد ناوه (S-1 -5^10*1) برروي غرب و جنوب غرب ايران قرار دارد كه باعث تقويت حركات صعودي به ميزان PaS-1 0/4 و تشديد كم فشار سطحي مي شود. جت جنب حاره نيز با توجه به نحوه قرارگيريش نسبت به كم فشار سطحي نقش مهمي در توسعه ناپايداري ايفا كرده است. در روز بعد از وقوع بارش سنگين سامانه كم فشار در حال خارج شدن از سمت شمال شرق ايران است و سامانه پرفشار با طي مسير شمال شرق جنوب غرب برروي درياي مديترانه مستقر مي شود.

Introduction Heavy precipitation are among those natural events that may lead to disasters like flood and landslides or both simultaneously. One of the occasions that these types of precipitations occur is after a prolonged drought. Heavy precipitation normally  occur during transient seasons like autumn and spring. However, they may occur in winter or even in summer. Considering the atmospheric circulations diversity at different times of year, it does not seem wrong if suppose there must be significant differences between weather conditions that led to such precipitations would be of great assistance to weather forecasters to issue weather warnings that may help to prevent or reduce the impacts of those precipitations. To identify heavy precipitations, Brook’s statistical method has been used. In the course of the study upon the structure of atmospheric circulations during heavy precipitations over the city of Shiraz during autumns of 1371 to 1378(1992 and 2000), we came across with an interesting situation on 21th of December 1992, that is the subject of this article. Its atmospheric circulation structure examined by means of the meteorological quantities such as sea level pressure, geopotential height, relative humidity, horizontal wind divergence, vertical­ motion at 850 and 500 hPa surfaces, temperature at 850 hPa surface and horizontal wind speed at 300 hPa  surface. Required data have been acquired from  NCEP(National Center for Environmental Prediction (website and maps have been plotted by means of Grads software. Results showed that a low pressure system that was formed over the Saudi Arabia and Red Sea some times before 21th of December 1992, entered Iran on that day. During its course towards the northeast of the country, it met with a high pressure system entering the country from Europe. This situation created a suitable condition for development of instability over the southwest of Iran. At 500 hPa surface intensive divergence area was located over west and southwest of Iran, supporting the vertical motion and the intensification of surface low pressure system. Subtropical Jet relative location to surface low pressure played important role in the development and intensification of the instability. In day after precipitation event the pressure system exits from northeast of Iran and high pressure system with moving in direction of northeast to southeast locates over the Mediterranean Sea.   Materials and methods The data related to daily precipitation on 1371 to 1378 (19922000) of Shiraz station is received from IRIMO, the country meteorological organization. The data related to meteorological parameters is extracted from National Center for Environmental Prediction site (NCEP). In this research the heavy precipitations of shiraz on 1371 to 1378 are determined by the Brooks statistical method. In this method the precipitation are divided to different groups according to the relations of (1) , (2) that the first group precipitation as known as heavy precipitation.   The number of groups = 5Log N                     (1) The space of groups = maximum precipitation – minimum precipitation / number of groups                                                          (2) That N is the number of days with precipitation on autumn. According to the first group results that only have 75 mm precipitation, this number is known as heavy precipitation on autumn. This amount of precipitation occurs on the 30 day of Azar (21 December 1992). To determine dominant atmospheric circulation of them, distribution of temperature, pressure and moisture quantities and their changes on the earth surface, levels of 850, 500 and 300 hPa on an area between the longitude of 20 degree of west to 90 degree of east and the latitude of 10 to 70 degree of north were investigated. In the dynamical analysis the distribution of pressure, temperature, wind and moisture fields and the relation between them were investigated by using quantities like geopotential height, horizontal wind divergence and vertical­ motion. Distribution of above quantities will clear the atmospheric conditions that change of them by passing of time shows the way of forming, strengthening and weakness of systems and atmospheric circulations. The maps were prepare one day before to the day after the heavy precipitation on the time of 00UTC and 12UTC. The maps were prepared by programming on the GRADS of studied geographical area.   Results and Discussion The atmospheric pattern of 21 December shows that a low pressure system that is formed on Arabia and Red Sea some days ago, has moved toward the north of Iran and penetrate inside of Iran. Then by encountering to a high pressure system entering the country from Europe moving toward the southwest, the system has made a proper condition on the southwest of Iran for instability and causes the value of    2*105 S1 convergence on low levels of the area. On the 500 hPa level of area intensive divergence area (1*105 S1) has located on the west and southwest of Iran that causes the strengthening of vertical motions about 0.4 PaS1 and intensification of low pressure. Subtropical Jet also according to its position relative to surface low pressure played important role in the development and intensification of the instability. According to the high relative humidity on both levels (90 %and 75%), we will see heavy precipitation on this day. In the next day of heavy precipitation, the low pressure system is going out of Iran from the northeast side and the high pressure system is still on the Mediterranean Sea and its effect has increased on the northwest of Iran. Since the convergence on the low levels is less than divergence on the high levels on last day, the vertical motion on this day is weaken. The temperature at the beginning of day has increased as compared with the last day and regarding that weakening of vertical motions on this day there will be less precipitation.   Conclusion Results showed that a low pressure system that was formed over the Saudi Arabia and Red Sea some times before 21th of December 1992, entered Iran on that day. During its course towards the northeast of the country, it met with a high pressure system entering the country from Europe. This situation created a suitable condition for development of instability over the southwest of Iran. At 500 hPa surface intensive divergence area was located over west and southwest of Iran, supporting the vertical motion and the intensification of surface low pressure system. Subtropical Jet relative location to surface low pressure played important role in the development and intensification of the instability. In day after precipitation event the pressure system exits from northeast of Iran and high pressure system with moving in direction of northeast to southeast locates over the Mediterranean Sea.