Cloud Optical Depth Retrieval from Cloud Radar and Microwave Radiometer Measurements

摘要

Radar and microwave radiometer measurements of clouds were taken along the California Coast at Vandenberg AFB on 10 Sept. and 18 Oct 2003 in conjunction with Minuteman III and Titan II rocket launches. The purpose was to characterize the clouds to derive an estimate of the optical depth at the time of the launches. These measurements support the Signature Exploitation Program development effort at AFRL to detect rocket emissions through optically thick clouds. The instruments used were the Air Force Research Laboratory Ka-band (35 GHz) Doppler radar, termed the Air Force Cloud Profiling Radar and a Radiometrics Corp. model WVQ-l500, 5-channel (22-3O GHz) water vapor profiling microwave radiometer. A variation of the technique developed by Frisch et al. used radar backscatter and microwave radiometer derived total integrated liquid water to yield an estimate of the cloud droplet number density and the vertical profile of the droplet effective radius. The cloud droplet distribution is approximated by a lognormal distribution. Climatology is used to select the distribution width. The retrieved cloud characteristics are applied to the Wiscombe Mis scattering codes augmented for optical depth calculations. An optical depth of 68 was obtained near the time of the 10 Sept launch. No useful retrieval resulted from the 18 Oct case because the skies were clear overhead.