The Parameterization Of Surface Radiation Budget In Climate Model

Parameterization for the solar radiation at the earth surface is very important for improving the surface radiation budget calculations. The previous studies on the surface radiation budget are mostly based on the analysis for radiation data observed at radiation stations that are averaged over a period of 10 day or month to establish a climatological method to calculate radiation budget. This study is focusing on the investigation of how to calculate instantaneous radiation budget and apply the method to large scale numerical model for calculation of the surface radiation budget.The radiative process is an important physical process that must be considered in a large-scale model. Due to its complicated and expensive nature, the radiative transfer process in the atmosphere must be simplified in terms of parameterization. Even if using a parameterization approach, the computational burden use in the radiation calculations is on the top of all physical processes. In order to reduce the computational burden and satisfy the requirement of real time Numerical Weather Prediction (NWP), the time step used for radiation calculation is much larger than that used in model integration. Although the computational burden is effectively reduced in this way, the treatment has a negative effect on the diurnal variation of radiation, which in turn influences the simulations of convection and precipitation. The purpose of this study is to enable the surface radiation budget to be calculated simultaneously with the model time integration. This will at least improve the surface radiation calculation, which may help in improving the simulation for the surface processes and consequently improving the simulations for the convection and precipitation.We first use a radiation model to calculate the total solar radiation under clear sky at the surface for various atmospheric conditions, with the mid-latitude summer atmosphere and a given surface albedo. The results are used as benchmarks against which the parameterization is developed. The effect of the surface albedo is considered secondly as an error correction so as to obtain the net solar radiation at various surface albedos.Based on the parameterization under clear sky condition, the parameterization under cloudy condition is further investigated. We first parameterized total solar radiation under overcast condition in terms of cloud optical deptli and cloud layer pressure. The effect of cloud amount is introduced as an error correction. The parameterization was developed for ice, water and mixed-phase cloud separately.The parameterization schemes obtained from this study are simple, easy to use and very accurate compared with the benchmarks. The mean relative errors for the total and net solar radiation under clear sky condition are 0.31% and 0.30% respectively. The mean relative error for the total solar radiation under cloudy condition is 0.5%. The required input to the parameterization includes precipitable water, concentration of carbon dioxide amount, ozone amount, cloud amount, cloud visible optical depth, solar zenith angle and surface albedo. The cloud visible optical depth can be determined using cloud liquid (ice) water content and effective radius. All these variables are available in most NWP or climate models. Therefore, the method can be easily used in any large-scale model.