地表太阳辐射参数化改进在高原山区WRF模式降水模拟中的应用

Improvement of surface solar radiation effect parameterization and its application in WRF precipitation simulation in plateau mountainous areas

区域气候模式对高原降水模拟存在系统性高估。该文从热力学角度考虑了局地(小尺度)地形对陆气过程的影响,以提高区域气候模式高原降水模拟能力。通过在天气研究与预报(weather research and forecasting, WRF)模式陆面过程模块耦合地形热力效应方案,将二维地形太阳辐射方案改进为三维地形太阳辐射方案,根据山区太阳辐射理论对太阳辐射直射、辐射散射、地形反射过程进行改进,得到改进方案;基于改进方案在亚东河谷开展高分辨率WRF模式降水模拟研究。结果表明:采用改进方案模拟的太阳辐射、降水结果与实测结果更接近,能够降低河谷降水高估的现象。改进方案能够体现出更多地形变化对地表辐射分布的影响:日间,山体坡面由于接收更多辐射得到加热(0.09~0.20℃),产生上升气流,在坡面形成更强上坡风(0.02~0.08 m/s),将河谷水汽携带到坡面以上,水汽在坡面以上区域爬升并形成降水,导致河谷区域降水减少(-4.54~-3.34 mm)、坡面以上区域降水增多(0.59~2.82 mm)。该研究对提高区域气候模式对山区降水的模拟效果具有重要意义。

[Objective] The parameterization schemes for the surface solar radiation effect in the climate model primarily include the plane-parallel radiative transfer scheme, two-dimensional(2-D) solar radiation effect scheme, and three-dimensional(3-D) solar radiation effect scheme. Because the plane-parallel radiative transfer scheme assumes a flat topography, it cannot ascertain the impact of terrain on radiation, which results in systematic biases in simulating the land surface processes of the climate model. The 2-D scheme only considers the impact of the terrain slope and aspect on the solar incident angle. However, it overestimates the surface radiation during early morning and late afternoon, leading to systematic biases in simulating precipitation over mountainous areas. In contrast, the 3-D scheme is based on the solid physical foundation of the mountain radiation theory, which considers the radiation impact of 3-D structures along the sub-grid terrain on the surface radiation. The weather research and forecasting(WRF) model incorporating the 2-D scheme in simulating land surface processes also encounters problems related to the precipitation overestimation and cold bias over the Tibetan Plateau. [Methods] This study coupled the WRF model with the 3-D scheme to accurately represent surface radiation processes over complex terrains such as the Tibetan Plateau, thus improving the model performance in simulating surface energy balance and precipitation. Based on the mountain radiation theory, the WRF model was incorporated with the following three types of incoming solar radiation: the solar radiation flux, diffuse radiation flux, and solar radiation flux reflected by the surrounding terrains. The influence of local(small-scale) topography on the land-atmosphere process was considered from the perspective of thermodynamics to improve the simulation ability of a regional climate model for precipitation over the plateau. Accordingly, a high-resolution WRF simulation study was conducted in the Yadong Valley, located in the central Himalayas. [Results] The results showed that:(1) The improved scheme reduced the precipitation overestimation in the Yadong Valley. The simulated surface solar radiation and precipitation were closer to the measured data. The precipitation simulated using the default scheme and that using the improved scheme were 15.7 and 12.1 mm, respectively. In addition, the correlation coefficient between the precipitation simulated using the improved scheme and the measured data was 0.46. Moreover, the improved 3-D scheme reduced the overestimation of simulated solar radiation during sunrise and sunset and increased the surface solar radiation at noon, while the correlation coefficient between the solar radiation simulated using the improved scheme and the measured data was 0.95.(2) The improved 3-D scheme better reflected the impact of complex terrains on the surface radiation distribution. During the daytime, the mountain slope was heated by a large amount of incident radiation(0.09-0.20 ℃). The generated updrafts and subsequent formation of uphill winds(0.02-0.08 m/s) on the slope carried the water vapor in the valley to the upper slope. The water vapor rose and formed precipitation over the slope, which resulted in a decrease in precipitation in the valley(-4.54--3.34 mm) and an increase in precipitation on the upper slope(0.59-2.82 mm). [Conclusions] This research can explain the mechanism behind the influence of terrain on precipitation in mountainous areas, making it a reference for future research in precipitation simulation.