Coincident In Situ and W-Band Radar Measurements of Drop Size Distribution in a Marine Stratus Cloud and Drizzle

Abstract

Investigation of precipitation formation requires measurements of the drop size distribution in a cloud. These measurements have usually been made using ground-based radar systems or aircraft in situ probes. Difficulties encountered in practice using these systems include accounting for the air motion at points remote from the radar systems and small sample volumes measured using the aircraft probes. An airborne W-band radar system provides a measurement from a much larger sample volume, close to the aircraft, with a correction for air motion possible using the data from the aircraft inertial navigation system. The Coastal Stratus Experiment conducted off the coast of Oregon in late 1995 provided W-band radar and microphysical probe data sampled from much of the same region of a marine stratus cloud. The unique combination of cloud probes and W-band radar on board the University of Wyoming King Air allowed the radar sampling to be only 60 m away from the probe sampling region. Doppler spectrum data from the W-band radar were used to produce estimates of the drop size spectrum density N(D). These estimates were compared to measurements of N(D) taken by the Particle Measuring Systems forward scattering spectrometer, 1D, and 2DC probes. This comparison suggests that a vertically pointing airborne W-band radar is a viable remote sensing tool for measuring N(D) in clouds and precipitation. This radar provides information on drop size distribution variation on a much smaller horizontal scale than the probes as a result of the much higher sample rate and larger measurement sample volume.