基于新的多形状冰云参数化方案和BCC_RAD辐射传输模式,详细分析了在辐射传输算法中通常使用的HG(Henyey-Greenstein)近似对冰云短波辐射传输过程的影响。对于单个冰晶粒子光学性质,HG近似对相函数勒让德展开式第3和第4项系数计算造成了较大误差,最大误差分别为-0.28和-0.33,最大相对误差分别为-55.7%和-73.8%。第4项系数的误差绝对值和相对值均高于第3项系数。进一步分析发现,HG近似在可见光区造成的误差高于近红外区。对于冰云体积光学性质,HG近似同样对第3和第4项系数造成了较大的误差,最大误差分别为-0.18和-0.22,最大相对误差分别为-27.9%和-37.1%。第4项系数的误差高于第3项系数,可见光区误差高于近红外区,与对单个冰晶粒子光学性质的影响一致。HG近似也对有云大气的辐射传输过程计算造成了较大的误差。对短波向下辐射通量造成的最大误差为-2.78 W·m ^-2,对短波向上辐射通量造成的最大误差为-1.06 W·m ^-2,对短波加热率造成的最大误差为0.27 K·d ^-1。HG近似低估了短波辐射通量,高估了云内加热率。因此,在冰云辐射传输计算中需精确描述相函数。

Based on a new multi-shape ice cloud radiative parameterization scheme and the Beijing Climate Center radiative transfer mode (BCC_RAD), the effects of the Henyey-Greenstein (HG) approximation on the shortwave ice-cloud radiation are analyzed in details, in which the HG approximation is widely used in radiative calculations. To the optical properties of the single ice crystal, the HG approximation causes large errors on 3 rd and 4 th coefficients of the Legendre expansion of the phase function, and the maximum values of the errors are -0.28 and -0.33, respectively, while the maximum values of the relative errors are -55.7% and -73.8%, respectively. The errors of 4 th coefficients are higher than those of 3 rd coefficients, and the errors in near-infrared waves are higher than those in visible waves. To the bulk optical properties of the ice clouds, the HG approximation causes large errors on 3 rd and 4 th coefficients, and the maximum values of the errors are -0.18 and -0.22, respectively, while the maximum values of the relative errors are -27.9% and -37.1%, respectively. The errors of 4 th coefficients are higher and so as in visible waves, which are same to the single ice crystal. The HG approximation also causes large errors on the radiative calculations. The maximum value of the errors on the shortwave downward fluxes is -2.78 W·m -2, and on the shortwave upward fluxes is -1.06 W·m -2, while on the shortwave heating rates is 0.27 K·d -1. The HG approximation underestimates the shortwave downward fluxes and overestimates the shortwave heating rates. Therefore, phase function accurately describing on ice clouds radiation transfer is necessary.