GRAPES模式中的坡地辐射方案及其对短期天气过程模拟的影响

为了改进太阳短波辐射参数化, 在非静力中尺度模式GRAPES (全球/区域同化和预报系统) 中考虑了坡地辐射 (SLOPE), 并着重讨论其在不同分辨率情况下对我国短期天气过程的影响.数值试验结果表明: 地形坡度和坡向对地表短波辐射 (GSW) 计算有较大的影响, 与没有考虑坡地作用的参考运行 (FLAT) 相比, 早晨和傍晚的GSW最小可减小60%以下, 最大也可增加60%以上, 比一天中其他时间和平均值大得多.在江淮流域一次梅雨锋降水过程中, 坡地辐射参数化方案的引入对降水模拟有较明显的影响, 特别在高分辨率 (6 km) 情况下, 降水的增加和减小不仅发生在GSW改变比较大的地区, 且与复杂地形有较好的相关性.另外, 由于地形坡度和坡向引起地表温度的改变与地表热通量的变化一致, 且也发生在地形起伏的多山地区.因此, 当水平分辨率较高且地形陡峭起伏时, 应当包含坡地辐射.

Slope Irradiance Scheme in GRAPES and its Effect on Simulation of Short-Range Weather Processes

Solar radiation is obviously an important factor in many aspects of surface forcing.Currently short wave radiation scheme in GRAPES(Global/Regional Assimilation and PrEdiction System) is based on Dudhia(1989)and taken from MM5.It is a simple downward integration of solar flux,accounting for clear-air scattering(Lacis and Hansen 1974),water vapor absorption,cloud albedo and absorption,and uses look-up tables for clouds from Stephens(1978).Short wave radiation at the ground surface(GSW) is originally calculated under the assumption of horizontal surface.This method of estimating the GSW may lead to considerable errors when the model resolution becomes high and the model terrain steep.To improve solar short wave radiation parameterization,slope irradiance has been implemented into nonhydrostatic mesoscale model GRAPES.Topography height is taken from NCEP with a resolution of 1 km,and the slope and orientation of terrain are calculated by the staggered difference scheme.The focus of this paper is slope irradiances and the effect on the simulation for short range weather processes in the different resolutions,especially on the simulation of a Meiyu front precipitation process.The result shows that the influence of slope and orientation of terrain on the calculations of GSW is large.The radiation minima's are reduced up to 60%,and the maximums are up to 60% larger compared to the reference run,which are much larger than at other time during the day and the 12 or 24 h average.The impact of model resolution on GSW patterns has been investigated,the areas where the differences of GSW between two schemes are larger mostly occur in undulating mountains for high resolution(6 km),but there is no such connection for lower resolution(60 km).The result also shows that during a Meiyu front process in the Yangtze River and Huaihe River valleys the increase or decrease of precipitation accords with the decrease or increase of GSW due to slope irradiance,but it is difficult to see such relations with the complex terrain at a lower resolution(60 km).At a higher resolution(6 km) the increase or decrease of precipitation not only occurs in the areas where GSW changes are larger,but also has better correlation with the complex terrain,which indicates that the slope irradiance has the obvious effect on the simulation of precipitation.To investigate the effect of GSW on meteorological conditions in the planetary boundary layer,the surface energy budget has been studied.The heating or cooling takes place whenever net incoming fluxes are positive or negative,and the surface heat flux(SHF) is thus naturally important in the energy budge.The result shows that the changes of surface temperature due to the slope and orientation of topography accord with the SHF,there is better correlation between them,and also occur in undulating mountains at a higher resolution(6 km).Therefore,slope irradiance can normally be neglected in numerical models when the horizontal model resolution is low and the slopes are moderate,and should be included when the resolution becomes high and the terrain steep and undulating.