一维辐射对流模式对云辐射强迫的数值模拟研究

A Study of the Radiative Forcing of Clouds by Using a One-Dimensional Radiative-Convective Model

利用一维辐射对流气候模式,详细研究了云量、云光学厚度以及云高等要素的变化对大气顶和地面太阳短波辐射和红外长波辐射通量以及云的辐射强迫的影响,给出了计算这些物理量的经验拟合公式。结果表明,云具有极为重要的辐射气候效应。云量、云光学厚度以及云高即使只有百分之几的变化,所带来的辐射强迫也可以与大气二氧化碳浓度加倍所产生的辐射强迫(3.75 W/m2)相比拟。例如,当分别给它们+3%的扰动时,即取云量变化0.015,云光学厚度变化0.27,以及云高变化0.15 km时(在实际的地球大气中,这种尺度的变化是完全可能发生的),那么,可以得到地气系统的太阳短波辐射强迫-3.10 W/m2以及红外长波辐射强迫-1.77 W/m2,二者之和为-4.78 W/m2,已经完全可以抵消大气二氧化碳浓度加倍所产生的辐射强迫。但是,当云量、云光学厚度以及云高向相反方向产生类似扰动时,所产生的辐射强迫可能极大地放大二氧化碳浓度增加所产生的增强温室效应。因此,研究结果揭示出,不管是为了解释过去的气候变化,还是预测未来的气候变化,亟待加强在一个变化了的气候环境(例如地面温度升高)下,云将发生何种变化的研究。

By using a one-dimensional radiative-convective climate model （RCM）, the effects of the changes in cloud amount, cloud optical thickness and cloud height on the short-wave and long-wave radiation fluxes at surface and top of the atmosphere （TOA） and radiative forcing of clouds have been investigated in detail. Based on the numerical results, the experiential formulas for the relationship among these physical quantities are proposed. For the radiative transfer calculations the absorptions due to O3, H2O, O2 and NO2, are included in the model for the solar regime while the absorbers in the thermal regime are mainly H2O, CO2, O3, CH4 and N2O. Rayleigh scattering and the absorption and scattering by clouds have also been incorporated into the model. Clouds are classified as water cloud and ice cloud according to the temperature of model atmospheric layer where cloud is located. When the ambient air temperature is below 253 K, the cloud is assumed to be an ice cloud otherwise a water cloud. Optical parameters such as single scattering albedo and asymmetry factor of clouds are pre-calculated based on the cloud type. Lacis and Hansen＇s （1974） scheme is used for multiple scattering calculations while a k distribution method is adopted in the model to calculate the thermal infrared radiation where the clouds are assumed to be a black body. In order to speed up the convergence of the model, a combination of time marching and Newton-Raphson iteration is used to reach the equilibrium temperature profile. The results show that clouds have extremely important radiation-climate effect. Radiative forcing caused by only few percentage changes in the amount, optical thickness and height of clouds will be comparable with that caused by a doubling of carbon dioxide concentration in the atmosphere, which is 3. 75 W/m^2 in the present. For instance, given a 3 percent disturbance in the above three parameters of clouds, that is, taking a cloud amount change of 0. 015, optical thickness of 0. 27, and cloud height of 0. 15 km, respectively, the solar short-wave radiative forcing and infrared long-wave radiative forcing could reach up to -3. 10 W/m^2 and -1. 77 W/m^2, respectively. Their sum is -4. 78 W/m^2 , which can completely counteract the radiative forcing due to a doubling of carbon dioxide concentration. However, when the disturbances are reversed in direction, the resultant radiative forcing caused by these three factors would substantially amplify the greenhouse effect caused by increasing carbon dioxide. Therefore, it implicates that the climate effects caused by the change in cloud should be seriously considered in the climate change studies for both explaining the past climate change and predicting the future climate change.