تأثیر هواویزها بر بارش در شرایط رطوبت‌های نسبی متفاوت: مطالعه موردی

نوع مقاله: مقاله تحقیقی‌ (پژوهشی‌)

نویسندگان

گروه فیزیک فضا، مؤسسۀ ژئوفیزیک دانشگاه تهران، تهران، ایران

چکیده

تغییر تعداد هواوبزهایی که به‌عنوان هسته‌های میعان درون قطرک ابر فعال می‌شوند، تأثیر قابل ملاحظه‌ای بر ویژگی‌های خردفیزیک ابر می‌گذارند، به‌نحوی که می‌توانند مقدار و الگوی بارش را تغییر دهند. در این مطالعه با استفاده از طرح‌وارۀ خردفیزیک ابر تامپسون موجود در مدل WRF، تأثیر هواویزها بر بارش در یک رخداد توفان تندری بررسی شد. داده‌های مربوط به هواویزها از مدل جهانی GOCART استخراج و به مدل WRF خورانده شد، درحالی‌که برای شرایط اولیه و مرزی هواشناسی از داده‌های FNL استفاده گردید. دو آزمایش عددی که معرف هوای پاک و آلوده هستند انجام گرفت که در آن‌ها تعداد هواویزهای آب‌دوست به‌ترتیب به 2/0 و 5 برابر غلظت استخراج‌شده از مدل GOCART تغییر یافت.
نتایج شبیه‌سازی‌ها نشان داد که توزیع مکانی بارش در دو حالت پاک و آلوده متفاوت است، به‌نحوی که در جوّ آلوده در برخی مناطق فراهنج‌های شدیدتری وجود دارد که بارش‌های شدیدتری را نیز در پی دارد. افزایش فراهنج‌ها در این مناطق سبب می‌شود که زمان رشد آب‌شهاب‌ها طولانی‌تر و اندازه‌شان بزرگ‌تر گردد؛ درنتیجه زمانی که از پایۀ ابر فرو می‌افتند کمتر تبخیر و ذوب می‌شوند و از این‌رو افزایش بارش سطحی را در این مناطق موجب می‌شوند. از طرفی کاهش بارش در حالت آلوده در مناطق پایین‌دست جریان باد شبیه‌سازی شد؛ که دلیل آن کاهش شعاع بلورهای یخ است که به کاهش فرایند یخ‌زدگی و تولید گویچۀ برف منجر می‌شود. همچنین، بررسی آهنگ ساعتی بارش نشان داد در ساعت‌هایی که رطوبت نسبی جوّ زیاد است و بخار آب به‌اندازۀ کافی در جوّ وجود دارد، افزایش تعداد هواویزهای آب‌دوست سبب افزایش بارش سطحی می‌شود. در حالی که در ساعت‌هایی که رطوبت نسبی جوّ کم است، کاهش بارش و گاهی توقف کامل بارش وجود دارد.

کلیدواژه‌ها


عنوان مقاله [English]

Aerosol impact on precipitation under different relative humidities: A case study

نویسندگان [English]

  • Fatemeh Zarei
  • Maryam Gharaylou
  • Omid Alizadeh-Choobari
Department of Space Physics, Institute of Geophysics, University of Tehran, Tehran, Iran
چکیده [English]

Although cloud properties and precipitation formation are primarily affected by atmospheric dynamics, cloud microphysical features also play key roles. The aerosol number concentration strongly influences cloud microphysics and precipitation formation, mainly through affecting the formation of cloud droplets and ice crystals.
In the current research, using the Thompson aerosol-aware microphysics scheme implemented on the Weather Research and Forecasting (WRF) model, the effects of aerosol number concentration was investigated on the precipitation formation of a heavy rainfall in Tehran. The aerosol number concentrations were obtained from the Goddard Global Ozone Chemistry Aerosol Radiation and Transport (GOCART) model, while the National Center for Environmental Prediction Final Analysis (NCEP/FNL) dataset was used for the initial and lateral boundary conditions. Two numerical simulations were conducted, referred to as the clean and polluted experiments. The initial hygroscopic aerosol number concentrations, compared to the values obtained from the GOCART model, were reduced to one-fifth and increased by a factor of 5 in the clean and polluted experiments, respectively. The model simulations were run with three nested domains, with horizontal resolutions of 21, 7 and 2.3333 km, and 45 levels in the vertical position, reaching up to the 50 hPa level. Simulations were conducted for 30 hours, starting from 18:00 UTC  April 13, 2012, from which, the first 6 hours were considered as the model spin-up. The Rapid Radiative Transfer Model (RRTM; Mlawer et al., 1997) was used for the shortwave and longwave radiation, respectively. The land surface scheme and surface layer scheme were based on the five-layer thermal diffusion and the revised MM5 similarity theory, respectively (Zhang and Anthes, 1982). The non-local Yonsei University (YSU) scheme was employed for the parameterizations of the boundary layer processes (Hong et al., 2006). The Kain-Fritsch scheme (Kain, 2004) was used to parameterize moist convection in the mother and first nested domains, while it was explicitly modelled in the innermost domain.
Results indicated that changes in the aerosol number concentration are associated with changes in the spatial distribution of precipitation. Stronger updraft cores were found in the polluted experiment, entailing higher precipitation,  longer growth times, and  larger sizes of hydrometeor; accordingly, more raindrops survived from the evaporation after falling from the cloud base, increasing the surface precipitation. On the other hand, surface precipitation decreased in the downstream, primarily due to the decrease in the effective radii of ice crystals, reducing the riming processes and the amounts of graupels. Results further indicated that the increase in the aerosol number concentration is associated with the increase in the rate of precipitation under high relative  humidities, while the reverse  is true when the available water vapour  is relatively low.

کلیدواژه‌ها [English]

  • aerosol
  • Precipitation
  • cloud condensation nuclei
  • cloud microphysics scheme
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