به‌‌كارگيري مدل تابشي‌فرارفتي براي برآورد سرمايش شبانه در يك حوضه محصور به توپوگرافي (حوضه رفسنجان)

A radiative-advective model for estimation of nocturnal cooling in a basin surrounded by topography (Rafsanjan basin)

سرمايش در كف حوضه محصور به توپوگرافي، تابع عوامل متفاوتي است كه مهم‌ترين آنها فرايندهاي تابشي و فرارفتي محليِ ناشي از جريان هاي زهكشي است. در اين پژوهش از مدل بسته هوا براي توصيف شيب باد استفاده شده و با فرض صفر بودن شار گرماي نهان در حوضه موردنظر، اثر شيب باد بر انتقال گرماي محسوس، در آن مورد بررسي قرار گرفته است. بر اين اساس مدلي عددي براي برآورد ميزان سرمايش شبانه در طول يك شب آرام و صاف در حوضه اجرا و توزيع قايم ميانگين دماي پتانسيلي و سرعت شيب باد برآورد شد. نتايج نشان مي‌‌دهد كه افت دما در يك عمق ثابت در حوضه مخروطي نسبت به حوضه كاسه اي و در حوضه كاسه اي نسبت به حوضه هموار و همچنين در حوضه هاي عميق تر بيشتر است. همچنين حوضه اي با خاك خشك نسبت به حوضه اي با خاك مرطوب متحمل سرمايش شبانه بيشتري مي شود و بيشترين عامل موثر در دماي صبح روز بعد مقدار دماي پتانسيلي اوليه در هنگام عصر، ارزيابي شد. براي راستي آزمايي نتايج مدل، حوضه آيزو (واقع در كشور ژاپن) و براي پيش‌بيني سرمايش شبانه بر اثر آن، حوضه رفسنجان (واقع در استان كرمان) مورد بررسي قرار گرفت. بررسي ها در ماه هاي انتقالي نشان مي دهد كه تقريباً در 65 % روزهايي كه در عصر آنها دمايي كمتر از C ?5/9 در ايستگاه رفسنجان تجربه شده اند، دماي هوا در صبح روز بعد به زير صفر رسيده است.

In the nighttime, drainage flow occurs along the basin sideslope and advects cold air to the boundary layer over the basin bottom (BBL), intensifying the cooling rate of the layer. A nocturnal cold air lake develops in the basin, attaining a depth nearly equal to the topographical depth of the basin. Heat budget analysis of the whole basin surface shows that net radiative flux closely balances with sensible heat flux and ground heat conduction. In the daytime, the BBL is warmed not only by sensible heat flux from the surface of the basin bottom, but also by local subsidence heating. This local subsidence above the basin bottom depresses development of the convective boundary layer until the nocturnal cold air lake vanishes completely. The subsidence velocity increases with time after sunrise. Over the whole basin surface, net radiative flux closely balances with sensible and latent heat fluxes. Cooling in an enclosed basin surrounded by topography, is a function of different factors and most notably the local processes of radiation and advection due to drainage flows. In this study, radiative cooling with an air parcel model for down-slope winds with zero latent heat flux assumption, are used to build a numerical scheme for estimating nocturnal cooling in such basin. The meteorological condition is assumed to be calm which is often the case for topographically surrounded basins for the area. The model requires a prescribed potential lapse rate during the night. For validation of the model the data of the Aizu basin in Japan with a good set of measurements is used. For typical model basins, the dependence of the nocturnal cooling on topographic parameters are obtained as follows: (i) The governing parameters are the depth of the basin and a shape parameter. The conical basin with a small shape parameter has more air cooling and a weaker slope wind than a flat bottom basin with a large shape parameter; (ii) Mean sensible heat flux during the night is almost proportional to a cube root of the depth of the basin, but little affected by the shape parameter. Sensitivity to radiational condition, the thermal constant of the ground, and surface roughness are also examined in this study. The results show that for a conical shaped surrounding the temperature drop during the night is more than a case with bowel like shape with the same depth. Also as the depth of the basin increases this temperature drop is large. Also dryer surface of the basin leads to larger radiative cooling and hence lower temperature in comparison to the wetter case. It is also found that the slope of the surrounding slopes does not affect the cooling rate as long as the depth of the basin is kept constant. The results of the model also show that often ( > 65% of the times) the morning temperature of the basin surface can reach zero degrees centigrade if the evening temperature is about 9.5 degrees centigrade or less. Thai can be used to issue warnings to farmers in such areas in order to avoid frost damage to crops. The model is used for Rafsanjan city to predict nocturnal temperature drop in spring seasons. This area with vast pestasous farms is prone to frost damage in spring time.