Cirrus spectral optical depths retrieved from solar transmittance measurements

Current multi-channel solar radiometers now allow for near-simultaneous collection of transmitted solar radiation in multiple spectral bands. Such simultaneous measurements are necessary for understanding the spectral nature of highly temporal phenomena such as cirrus clouds. Measurements of spectral optical depth from several locations in the desert southwest have been made by the University of Arizona for more than 10 years under both clear and cloudy conditions. Studies of the data collected under cirrus conditions indicate, as expected, that the transmittance through cirrus is nearly spectrally flat over a range of optical thicknesses. A recent data set collected at Ivanpah Playa, Nevada included measurements through a layer of cirrus clouds of varying thickness for a period of nearly three hours. These data were preceded by a two hour period of cloudless conditions with relatively stable atmospheric aerosol loading. Results from this data set indicate that the cirrus optical depth is spectrally flat to better than 6% over a range of optical depths from 0.10 to 0.30. Because of the spectrally flat nature of the cloud radiative properties, it is possible to use these clouds as band-to-band radiometric calibration targets for multichannel radiometers and two-channel lidar systems. The possible accuracy of such calibrations is presented based on the historical measurements of cirrus cloud transmittance.