مطالعه موردي تأثير برخي خصوصيات ديناميكي و خردفيزيكي ابر بر آذرخش درون‌ابري با استفاده از مدل WRF

Case study of the impact of some of dynamical and microphysical properties of cloud on the intra-cloud lightning using WRF model

از مشخصه­ هاي بارز توفان­ هاي تندري، آذرخش است. فعاليت آذرخش معمولاً 10 تا 20 دقيقه زودتر از بارش صورت مي­گيرد و همچنين آذرخش­ هاي درون­ابري زودتر از آذرخش­ هاي ابر به زمين رخ مي­دهند. در اين پژوهش به‌منظور مطالعه اثر برخي خصوصيات ديناميكي و خردفيزيكي ابر بر ميدان الكتريكي و آذرخش درون­ابري از مدل WRF براي شبيه­ سازي دو رخداد توفان تندري با مقادير CAPE متفاوت در منطقه تهران استفاده شد. در اين راستا، طرح‌واره تفكيك بار بر اساس نظريه غير القايي پيشنهادي توسط ساندرز و همكاران (1991) تهيه و در مدل جايگذاري شد و شدّت ميدان الكتريكي درون­ابري محاسبه شد. نتايج حاصل از شبيه‌سازي‌ها و بررسي نيمرخ قائم نسبت­هاي آميختگي گويچه برف و بلور يخ نشان­ دهنده‌ي حضور گويچه ­هاي برف در ترازهايي پايين­تر از تراز تجمع بلورهاي يخ بود. همچنين، نيمرخ قائم بار كل انتقال­ يافته به گويچه برف نشان داد كه بيشينه بار منتقل­ شده به گويچه برف با تراز تجمع گويچه­ هاي برف هم­ خواني دارد. بررسي سري زماني پارامترهاي ذكر شده نيز نشان داد زمان­ هايي كه سرعت قائم بالارو افزايش پيدا مي­كند، نسبت آميختگي گويچه برف و همچنين بار انتقال­ يافته به آن افزايش پيدا كرده است. مقايسه دو مطالعه موردي نشان داد كه قرارگيري بيشينه مقادير سرعت قائم بالارو مابين ارتفاع بيشينه تجمع گويچه ­هاي برف و بلورهاي يخ تأثير بيشتر و بارزتري بر بار انتقال­ يافته و ميزان فعاليت آذرخش درون­ابري دارد. بررسي و تحليل قطبش بارها نيز نشان داد كه ساختار غالب بارهاي الكتريكي عمدتاً به‌صورت دو قطبي بوده و اين نوع قطبش بيان مي­كند كه در موردهاي بررسي شده، عمدتاً آذرخش­ هاي درون­ابري رخ داده است.

Lightning is one of the distinct characteristics of thunderstorms. This phenomenon is the most important natural hazards for the power stations, the airline industry, wind farms, forestry management and public due to the high current and temperature at lightning channels. Lightning activity usually occurs 10 to 20 minutes earlier than precipitation and so is needed for the skill of short-term forecasts. In this research, to study some of dynamical and microphysical properties of intra-cloud electric field and lightning, the Mesoscale WRF model was used for simulation of two thunderstorm events (on 15 and 17 April 2012) with different CAPE in Tehran area. It is noteworthy that these case studies have been chosen according to data taken from the Iranian Meteorological Organization (IRIMO) and Lightning Imaging Sensor (LIS). Simulations were conducted using the WRF model initiated by FNL data and are provided in 1 degree space and 6-hour time resolution. Each of the simulations was performed for 36 hours and the first 12 hours of simulation were considered as the spin-up time. It uses one-way nesting for 3 meshes of 27-, 9- and 3-km horizontal grid spacings. Thirty-five vertical levels with a maximum height of 50 hPa were used for all domains. Then, the charge separation scheme was coded based on Saunders et al. (1991) relations. Then, the intensity of the intra-cloud electric field was calculated using outputs of the WRF model simulations and Saunders‘s scheme for two selected case studies. Comparison of the intensity of the intra-cloud electric field and threshold electric field, resulted the time of occurrence of intro-cloud lightning. Also, the effect of maximum values of graupel and ice mixing ratios and maximum values of vertical velocity on charge separation was investigated. To verify the results, output of LPI index was compared to LIS data. Vertical profiles of graupel and ice mixing ratios showed the presence of graupels in the lower levels compared to ice crystals. Also, vertical profile of the charge transferred per collision showed that the maximum values of that is consistent with the presence level of graupels. Moreover, time series of above mentioned parameters showed that the increase of vertical velocity lead to not only the increase of the graupel mixing ratio but also the increase of charge transferred per collision. The results also showed that the time of lightning occurrence well matched with the occurrence time of maximum values of the above mentioned parameters. The LPI index well predicted the time evolution of lightning activity in the study area despite of a relative inability to predict the likely area of lightning activity. Comparison between two case studies showed that the presence of updraft core between the core of graupels and ice crystals had more influence on charge transferred and intro-cloud lightning activity. Analysis of charge polarization also showed that the co-existence of graupel and ice crystal was necessary for charge separation. It also showed that the dominant structure of the electric charges were mainly bipolar, and this kind of polarization resulted in the occurrence of intro-cloud lightning based on previous researches.