Identifying drizzle in marine stratus with W-band radar reflectivity

Wang Jingyun

MS Thesis Defense

Monday 19 August 2002, 3 pm

Wyoming King Air (UWKA) cloud probe data are used to discriminate drizzly regions from drizzle-free regions in marine stratus, assuming a threshold diameter of 50µm for drizzle drops. The measured drop size distribution (DSD) is found to be presented well by a three-modal lognormal function, with parameters based on observations on three summer days off the Oregon Coast during Coastal Stratus 1999. This function is used to calculate the radar reflectivity separately for drizzly and for drizzle-free regions, assuming Rayleigh scattering. The calculated reflectivity (Z_c) is generally larger in drizzly regions, but the threshold reflectivity Z_t is found to be strongly dependent on altitude within the stratus layer. Statistical methods are applied to obtain an empirical reflectivity threshold, as follows,

where f is the normalized cloud altitude in warm marine stratus. To derive this threshold, both calculated (Z_c) and measured (Z_m) radar reflectivities are used. Z_m is from the nearest gate of the 95 GHz Wyoming Cloud Radar beam, 75-90 m to the side of the UWKA.

Liquid water content (LWC) cannot be estimated from reflectivity in drizzle regions, because small variations in the size and the amount of drizzle dominate the reflectivity but are insignificant in terms of LWC. Both Z_c and Z_m and in-situ data are used to obtain the following relationship between LWC M (g m^-3) and radar reflectivity Z in drizzle-free regions in marine stratus: