EFFECTS OF HORIZONTAL ORIENTATION ON THE RADIATIVE PROPERTIES OF ICE CLOUDS
Huang Eunheng(Institute of Atmospheric Physics, Academia Sinica)Liou Guonan (Department of Meteorology, University of Utah. VSA)
Transfer of radiation through cirrus consisting of non-spherical ice crystals randomly oriented in a horizontal plane (2D model) is solved by using the discrete-ordinates method. The dependence of the scattering parameters on the incident angle caused by horizontal orientation of ice crystals is approximated by the Legendre polynomial expansion of zero and second order. The scattering phase function is represented by means of the analytic Henyey-Green stein function with an asymmetry factor dependent on the incident zenith angle. 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 abledo computed from the two models can be as large as 8% for a cirrus of 2 km thickness. On the thermal infrared side, the difference in the upward radiances using 2D and 3D models is also significant. However, the IR flux emission for cirrus is less affected by the orientation of ice crystals. In the light of these results, it appears that the preferential crystal orientation may be one of the dominate factors determining the solar albedo of cirrus and may have significant effects in the sounding of the cirrus composition and structure from infrared wavelengths.