Estimation of cloud attenuation from dual beam airborne radar measurements

Stereoradar measurements can yield 2D cloud and precipitation velocities by observing particles from two different directions with a Doppler radar. Stereorad, a similar dual-beam measurement scheme can also provide the attenuation and the true reflectivity field. To test and to further develop this technique, MIRSL joined UWyo and CETP to collect airborne dual-beam radar data during the TRAC98 experiment, conducted near Paris, France in June 1998. The Wyoming Cloud Radar (WCR) was mounted in the French research aircraft ARAT and was connected to a pair of 30 cm antennas mounted in a pod on the underside of the aircraft. One antenna was nadir looking and the other pointed 42 degrees forward of nadir along the aircraft centerline. The WCR is a 95 GHz polarimetric Doppler radar capable of measuring dual reflectivity and dual Doppler velocity measurements. The Stereorad measurement scheme samples the same cloud volume through different propagation path lengths and consequently the difference between the observed reflectivity in the two beams will be related to attenuation. The authors propose an optimal estimation algorithm, based on a Kalman filter, which makes use of this difference in the apparent reflectivity to estimate the attenuation field without any assumptions on the cloud droplet size distribution or K-Z (attenuation-reflectivity) relation. They also present images of the attenuation and true reflectivity fields obtained by applying this algorithm to data collected during the TRAC98 experiment