كليدواژه :
اثر غيرمستقيم هواويزها , سپيدايي ابر , طرحوارۀ خردفيزيك كپّه‌اي دو مؤلفه‌اي , هسته‌هاي ميعان ابر
چكيده فارسي :
هواويزها از طريق تغيير تعداد و اندازۀ قطرك‌هاي ابر، اثرهاي پيچيده‌اي بر خواص تابشي ابرها دارند كه تعادل تابشي زمين و در نتيجه دماي هوا را تغيير مي‌دهند. با استفاده از آزمايش‌هاي عددي، اثر غيرمستقيم هواويزها بر تابش طول موج كوتاه، بلند و خالص براي يك سامانۀ ابر همرفتي مورد مطالعه قرار گرفته است. براي اين منظور، سه آزمايش عددي (مرجع، پاك و آلوده) با غلظت‌هاي متفاوتي از هواويزها و استفاده از مدل WRF و به‌كارگيري يك طرحوارۀ خردفيزيك كپّه‌اي دو مؤلفه‌اي اجرا شد. براي آزمايش مرجع، غلظت هواويزها از شبيه‌سازي‌هاي جهاني مدل GOCART استخراج شد، درحالي‌كه در آزمايش‌هاي پاك و آلوده، غلظت هواويزها به 2/0 و 5 برابر غلظت آن‌ها در آزمايش مرجع تغيير يافت. در آزمايش آلوده افزايش غلظت هواويزهايي كه به عنوان هسته‌هاي ميعان عمل مي‌كنند، باعث افزايش سپيدايي ابر مي‌شود؛ بنابراين تابش طول موج كوتاه كمتري به سطح زمين مي‌رسد. در مقابل، در آزمايش پاك كاهش غلظت هواويزها، كاهش سپيدايي ابر را در پي دارد؛ بنابراين تابش طول موج كوتاه بيشتري به سطح زمين مي‌رسد. برخلاف تفاوت قابل ملاحظۀ واداشت تابشي طول موج كوتاه ابر، تغيير در تعداد و اندازۀ هسته‌هاي ميعان ابر، تأثير اندكي بر واداشت تابشي طول موج بلند ابر مي‌گذارد، به نحوي كه واداشت تابشي خالص ابر، سرمايش زمين- جوّ براي شرايط آلوده است. مقايسۀ دماي هوا در نزديكي سطح زمين نشان داد كه افزايش و كاهش سپيدايي ابر در آزمايش‌هاي آلوده و پاك، به ترتيب كاهش و افزايش دماي هواي سطحي را در پي دارد.
چكيده لاتين :
Through modifying the number concentration and size of cloud droplets, aerosols have complex impacts on radiative properties of clouds, which consequently change the radiation balance of the Earth, and modify the atmospheric air temperature. By conducting numerical experiments for a mid-latitude cloud system in April, the indirect effects of aerosols on shortwave and longwave radiation, and subsequent impacts on the near-surface air temperature are investigated over Tehran. To this end, three numerical experiments (control, clean and polluted) with initial identical dynamical and thermodynamic conditions, but different cloud-nucleating aerosol concentrations were conducted using the Weather Research and Forecasting (WRF) model. Simulations were conducted over three nested domains with two-way interactions (nesting ratios: 1:3:3; horizontal resolutions: 21, 7 and 2.333 km). A two-moment aerosol-aware bulk microphysical scheme, recently developed, discussed and tested by Thompson and Eidhammer (2014), was used. In the control experiment that represents conditions of the current era in terms of the aerosol number concentrations, concentrations of atmospheric aerosols were derived from 7-yr global simulations of the Goddard Chemistry Aerosol Radiation and Transport (GOCART) model which include mass mixing ratios of sulfate, dust, black carbon (BC), organic carbon (OC), and sea salt. Hygroscopic aerosol number concentrations were reduced to one-fifth in the clean experiment, and increased by a factor of 5 in the polluted experiment. The meteorological initial and lateral boundary conditions in the three experiments were derived from the National Center for Environmental Prediction final analysis (NCEP/FNL) data at 1˚ horizontal resolution and 6 h temporal intervals. Results indicate that increasing (decreasing) cloud-nucleating aerosol concentrations in the polluted (clean) experiment is associated with more (less) numerous cloud droplets of overall smaller (larger) size. Indeed, mean cloud droplet number concentrations (effective radius of cloud droplets) in cloudy grid points averaged over the innermost domain and during the simulation period were found to be approximately 46, 158 and 417 cm-3 (8.5, 6.1 and 5.2 μm) in the clean, control and polluted experiments, respectively. Thus, the total droplet cross-sectional area of cloud droplets increases in the polluted experiment, leading to an enhancement in the shortwave cloud radiative forcing (or cloud albedo), such that less shortwave radiation reaches to the Earth surface. In contrast, the total droplet cross-sectional area of cloud droplets decreases in the clean experiment, leading to a reduction in shortwave cloud radiative forcing (or cloud albedo). In contrast to the significant changes in the shortwave cloud radiative forcing by aerosols, results indicate that changing the number and size of cloud condensation nuclei in the polluted and clean experiments has little impact on longwave cloud radiative forcing. Values of shortwave and longwave cloud radiative forcing indicate that as the positive longwave cloud radiative forcing in all experiments are nearly half of the negative shortwave cloud radiative forcing, clouds have an overall cooling effect on the climate system, counteracting the warming caused by increases in concentrations of the atmospheric greenhouse gases. Comparing the near-surface air temperature of the three experiments reveals that the enhancement of cloud albedo in the polluted experiment leads to a reduction in the near-surface air temperature, while reduction of cloud albedo in the clean experiment leads to the enhancement of the near-surface air temperature.
