Estimating the fluctuations in droplet fall velocity via in situ measurements in stratocumulus: an essential step in airborne Doppler radar turbulence measurements

During the DYCOMS-II (Dynamics and Chemistry Of Marine Stratocumulus) field project, the Wyoming cloud radar was mounted on the NCAR C130, with one of two beams looking downward. An essential step in order to study turbulence in the boundary layer with this radar is to estimate the contribution of the fall velocity of the hydrometeors to the fluctuations of the Doppler velocity. Therefore, in situ microphysics probe measurements of drop counts collected during this project with the NCAR C130 within the stratocumulus boundary layer are used to estimate fluctuations in the reflectivity-weighted fall velocity of the drops. The departure of the drop count distribution from Poisson statistics allows us to to estimate the actual (non-Poissonian) fluctuation in counts due to cloud heterogeneities. The drop count distribution measured by the microphysics sensors is well fitted by a lognormal distribution of the Poisson counting rate; i.e. a lognormally varying Poisson distribution. Using the estimates of the non-Poissonian fluctuations in drop counts from all bins of the microphysical probes, we deduce the variance of the reflectivity-weighted fall velocity. Radar reflectivity and Doppler velocity measurements confirm the feasibility of using in situ microphysics measurements to estimate the effect of variations in the terminal velocity of the scatterers on Doppler velocity derived turbulence measurements.