Reflection and transmission of solar light by clouds: asymptotic theory

[摘要] The authors introduce a radiative transfer model CLOUD forreflection, transmission, and absorption characteristics ofterrestrial clouds and discuss the accuracy of the approximationsused within the model. A Fortran implementation of CLOUD isavailable for download. This model is fast, accurate, and capableof calculating multiple radiativecharacteristics of cloudy mediaincluding the spherical and plane albedo, reflection andtransmission functions, absorptance as well as global and diffusetransmittance. The approximations are based on the asymptoticsolutions of the radiative transfer equations valid at cloudoptical thicknesses larger than 5.

While the analytic part of the solutions is treated in the code in anapproximate way, the correspondent reflection function (RF) of asemi-infinite water cloud R_∞ is calculated using numericalsolutions of the radiative transfer equation in the assumption ofDeirmendjian's cloud C1 model. In the case of ice clouds, the fractal icecrystal model is used. The resulting values of R_∞ with respect tothe viewing geometry are stored in a look-up table (LUT).

The results obtained are of importance for quick estimations ofmain radiative characteristics of clouds and also for the solutionof inverse problems.

[学科分类] 大气科学