中国西部陆面过程次网格地形参数化的改进对区域气温和降水模拟的影响研究

地表作为大气模块的下垫面,为大气模块提供边界条件,地形对于模式结果的准确性起到至关重要的作用。现有的陆面过程模式在陆面同一网格内的次网格单元采用相同的大气强迫量,没有考虑次网格地形对网格内大气强迫量的影响,这关系到模式对气象要素和陆气交换量的模拟水平。本文在陆面模式NOAH处理次网格单元的同时,将输入的大气强迫量根据其与地形高度的关系进行修订,提出新的次网格地形的参数化方案,并引入到WRF（Weather Research and Forecasting）模式中进行数值试验,通过3组数值模拟试验,与未改进的方案和细网格方案分析比较,探讨新参数化方案对WRF 模式模拟结果的影响。结果表明:地形越复杂区域,次网格地形的影响越大。本文引入的新陆面次网格地形方案对天山山脉和昆仑山脉以及青藏高原南部的地表气温的模拟有较大改善,模拟的地表气温在大范围区域内都更贴近细网格方案。虽然新陆面次网格地形方案和细网格试验都对温度的模拟结果都有改善,但新陆面次网格地形方案对降水的模拟改善甚微,而细网格试验对降水模拟却有改进,这是由于细网格试验在陆面和大气网格都进行了细化,而新陆面次网格地形方案只考虑了陆面次网格的影响。具体来说,新陆面次网格地形方案对温度的模拟结果改进是通过改变地表向上长波和地表感热实现的。而细网格试验由于同时细化了大气和陆面的空间网格,对降水和温模拟的改进是通过综合改变地表能量平衡实现的。

Effects of Improvement of Land Surface Subgrid Topographic Parameterization on Regional Temperature and Precipitation Simulation in Western China

The land surface provides underlying lower boundary conditions for atmospheric models. Topography plays a crucial role in the accuracy of the model results. Most of current land surface models use the same atmospheric forcing in subgrid units within the same model grid, and don’t not consider the influence of subgrid topography on atmospheric forcing that would impact the simulation of meteorological elements and land-atmosphere interaction. In this paper, a modified Land Surface Subgrid Topographic Parameterization (LSSTP) is proposed to revise the input atmospheric forcing according to its relationship with the subgrid terrain height in the NOAH land surface model . The LSSTP is then introduced into the WRF (Weather Research and Forecasting) model for numerical experiments. Three groups of numerical experiments have been conducted to investigate the effect of this improvement on the simulation results of the WRF model. It is found that the new LSSTP introduced in this paper has clearly improved the simulation of 2m air temperature in the surface over the Tianshan Mountains, the Kunlun Mountains and the southern Qinghai-Tibet Plateau. However, the new LSSTP shows little improvement on precipitation simulation. In contrast, the experiments with higher resolution can well simulate precipitation, which is attributed to the fact that the experiment with higher resolution has been refined in both the land and the atmosphere, while the new LSSTP only considers subgrid effect in the surface. Temperature simulations using the new LSSTP are improved by revising surface upward long-wave flux and surface sensible heat flux, while the experiment with higher resolution has simultaneously refined the grids of both the atmosphere and land, and the improvement of precipitation and temperature simulation is achieved by comprehensively changing the surface energy balance.