水平取向对冰云辐射传输的影响

本文用分离坐标法求解辐射通过卷云的传输,该卷云由在水平面内随机取向的非球形冰晶所组成,即“二维卷云模式”。文中用零阶和二阶Legendre多项式的展开式来近似表示由于冰粒子水平取向所引起的散射参数随入射角的变化。散射相函数由解析的Henyey-Greenstein函数表示,其不对称因子也与入射天顶角有关。文中用这种模式计算了太阳短波以及地球红外热辐射通过卷云时的辐射通量传输性质以及方位平均强度的分布。将上述二维模式的计算结果与通常的三维模式(即假定冰粒子在三维空间随机分布)相比较,表明冰晶的优先取向对卷云的反照率有重要影响,两种模式的反照率可以相差8％。另一方面,对于红外辐射而言,两个模式的向上辐射强度有显著的差别,但是冰晶的取向状况对红外辐射通量影响较小。上述结果表明,冰晶的优势取向可能是决定卷云反照率的一个重要因子,同时它对于利用红外辐射探测卷云的组成和结构来说有重要的影响。     

REMOTE SOUNDING OF THE CIRRUS OPTICAL DEPTH AND TEMPERATURE FROM 3.7 AND 11 MICROMETER WINDOWS

On the basis of forward IR radiation transfer analyses for an atmosphere containing semi-transparent, non-black cirrus, parameterization equations are derived for the retrieval of the cloud optical depth and cloud temperature utilizing AVHRR 3.7 and 10.8 μum channels. The retrieval techniques developed involve the use of either dual-frequency or dual-scanning angle radiance observations. We show that the cloud optical depth and cloud temperature may be inferred successively from the observed brightness temperature differences using these two techniques. Numerical experiments anderror analyses demonstrate that the dual-frequency method is specifically appropriate for optically thin cirrus cases (τ <1). In case the optical depth of cirrus is close to 3, combination of dual-frequency and dual-scanning angle methods is shown to give reasonable accuracy for the cloud optical depth and temperature retrieval.     

用二维模式研究非洲气候的生物地球物理学反馈作用

本文用二维气候模式研究了非洲气候的生物地球物理学反馈作用。利用气候-植被耦合模式做了两个数值试验,一个是撒哈拉沙漠消失;另一个则将撒哈拉沙漠扩展。试验结果表明地面状况的变化对气候系统产生重要的反馈作用。在考虑撒哈拉沙漠时模拟出的温度和纬向风同非洲干旱年份的气候资料相一致。     

六角柱冰晶的光散射函数的计算

六角柱冰晶是卷云中粒子的基本形态。本文着重讨论了利用作者已发展的模式,对有吸收的红外波段冰晶的散射矩阵的计算结果。对比了可见光的计算结果,也对比了长六角柱、扁六角柱以及球状、旋转椭球状冰晶的散射。发现吸收的影响主要造成冰晶的侧向和后侧向散射能量减小以及因折射和多次内反射引起的晕峰和“雨虹峰值”减弱。不同形状冰晶的散射函数虽然总的情况颇一致,但与六角柱冰晶相比,球状冰晶的侧向散射较弱,而椭球状冰晶则表现出后向和后侧向散射较弱。这些特征无疑地都将影响激光探测卷云和光传输过程。     

EFFECTS OF HORIZONTAL ORIENTATION ON THE RADIATIVE PROPERTIES OF ICE CLOUDS

Transfer of radiation through cirrus consistingofnon-sphericalice crystals randomly oriented in a plane (2D model) is solved by using the discrete-ordinates method. The model is employed to determine the radiative flux properties and the intensity distribution of cirrus for both solar and thermal infrared radiation. Comparison of the 2D cloud model with the conventional 3D cloud model, i.e., randomly oriented in a three-dimensional space, shows that the preferential orientation of ice crystals has a substantial effect on the cloud solar albedo. The difference in the cloud albedo computed from the two models can be as large as 8% for a cirrus of 2 km chicknss. On the thermal infrared side, although the flux emission for cirrus is less affected by the orientation of ice crystals, the difference in the upward radiance using 2D and 3D models is also significant.     

CALCULATION ON THE LIGHT SCATTERING FUNCTION OF HEXAGONAL ICE CRYSTALS

Hexagonal ice crystal is a basic form of the particles in cirrus. The emphasis in this paper is on discussing the computational results of scattering phase matrices for infrared absorptive band by making use of the model developed by the authors. Comparisons have been made between infrared and visible light as well as between hexagonal columns and plates and equivalent spheriods and spheres. It is found that the effects of light absorption on phase function may cause the decreases of scattering energy in side direction and the directions near backscattering, and weaken halo and rainbow peaks which are induced by the refractions and internal reflections. There is a general agreement in the phase functions for ice crystals with different shapes. However, the scattering intensity in side direction for spheres and in backward and backside directions for spheroids is weaker as compared with hexagons. It is believed that these features would, doubtless, have influences on the detection of cirrus by laser and