CIRRUS INFRARED PARAMETERS AND SHORTWAVE REFLECTANCE RELATIONS FROM OBSERVATIONS

A summary of experimental observations and analysis of cirrus from hig h-altitude aircraft remote sensing is presented. The vertical distribu tion of cirrus optical and infrared cross-section parameters and the r elative effective emittance and visible reflectance are derived from n adir-viewing lidar and multispectral radiometer data for observations during the 1986 and 1991 FIRE cirrus experiments. Statistics on scatte ring and absorption cross sections in relation to altitude and tempera ture are given. The emittance and reflectance results are considered a s a function of solar zenith angle. Comparative radiative transfer cal culations based on the discrete-ordinate method were carried out for t hree representative cloud phase function models: a spherical water dro plet, an ice column crystal cloud, and a Henyey-Greenstein function. T he agreements between observations of the effective emittance and shor twave reflectance and the model calculations were a function of the so lar zenith angle. At angles between 54 degrees and 60 degrees a Henyey -Greenstein (HC) function with an asymmetry factor of 0.6-0.7 produced the best comparison. At 66 degrees-72 degrees the ice column model wa s equally comparable to observations. Comparisons to the water cloud m odel were poor in all cases. The effects of ice crystal microphysical variations on the observed results were not generally apparent, but on e dramatic example of difference was found. In order to explain the va riations noted for solar zenith angle, an instrument-the Tilt Scan CCD Camera radiometer-was developed to directly observe the shortwave bid irectiond reflectance function for 1991 measurements. The results indi cate a characteristic angular function of the visible reflectance of c irrus that is flatter than predicted by the ice column scattering mode l, but the overall asymmetry factor is comparable. The good agreement with values from an HG function at some angles is not generally applic able. The characteristics of the observed cirrus angular reflectance p attern correlate well with, and are explained by, the results that wer e found for the solar zenith angle dependence of the emittance and ref lectance.