Identifying and characterizing drizzle distributions within marine stratocumulus using W-band radar reflectivity Wang Jingyun, Bart Geerts University of Wyoming

In situ airborne measurements of drop size distribution give 1-D information. This study uses the 95 GHZ (3 mm) Wyoming Cloud Radar (WCR) reflectivity profiles to discriminate drizzle in marine stratocumulus, and to describe 2-D (horizontal) drizzle patterns in optically rather homogeneous Sc cloud layers observed in 1999 off the Oregon coast. A field project was carried out on Oregon coast during August 1999 to study the coastal stratus with Wyoming Cloud Radar (WCR) aboard the Wyoming King Air, combining the measurements of in-situ cloud probes and a 95Hz radar allows a threshold value of radar reflectivity be given to decide the presence of drizzle. The 2DC probe on the King Air was used to determine the presence of drizzle. Based on the distribution of 2DC measurements, drizzle was considered to be present when a minimum of 200 drops, in diameter ranging from 100¦Ìm to 500¦Ìm, were counted in a m3. Reflectivity measurements of the side-looking WCR, at the closest reliable range of 90 °À 15 m to the right of the aircraft, were compared against in situ drizzle data, for 3 days in August 1999. Independent WCR measurements have shown that the WCR reflectivity in marine stratocumulus is largely due to drizzle: while the liquid water content at a given level within the cloud layer is spatially rather uniform, the reflectivity is not, nor is drizzle. In the current study, strong correlation was found between the presence of drizzle (2DC) and the presence of echoes exceeding some threshold intensity in cases where 2DC measurements indicate drizzle areas larger than 90m. Calculated results show out a relation between reflectivity and drizzle. Further studies are done to get a threshold value of reflectivity to decide drizzle. Correlation coefficients with different reflectivity threshold values are calculated, -16.0dB seems to be a reasonable threshold value. Calculations show that as the value of reflectivity increases, the frequency of drizzle increases sharply. 25%~30% of observed drizzle is associated with reflectivity less than ¨C16.0dB. A reflectivity threshold, corrected for attenuation, could be applied to all radar gates and drizzle can be mapped out using this reflectivity threshold value.

References Vali, G., R.D.Kelly, J. French, S. Haimov, D. Leon, R.E. McIntosh and A. Pazmany, 1998: Fine-scale structure and microphysics of coastal stratus. J. Atmos. Sci., 55, 3540-3564.