Liquid water determination by airborne millimeter cloud radar and in-situ size distribution measurements

Abstract

Liquid water content (LWC) in clouds determines the precipitable water of clouds, which is a crucial factor for aircraft safety and weather modification operations. More importantly, it influences the optical depth of clouds in the visible wavelength range, thus determining their climate cooling effects. Identifying and quantifying the LWC in mixed-phase clouds via remote sensing techniques remains challenging owing to the large variability of hydrometeor sizes in the cloud. In this study, we used in-situ aircraft measured full size distributions and collocated airborne radar reflectivity (Z) to explicitly fractionate the contributions of hydrometeors (cloud liquid droplets, ice, and precipitation particles) at different size ranges from the measured total Z. A linearly decreasing contribution of non-precipitation hydrometeors with increasing total Z was discovered for a range of cloud types, including cumulus, status, and deep convection clouds. The relationship between the mass and Z for each type of hydrometeor derived from in situ measurements was then applied. This approach of apportioning the contribution of cloud liquid droplets from the measured total Z as the first step significantly reduced the scattering of the correlation between the LWC and Z, as presented in previous studies; thus, the LWC was more accurately determined. The derived liquid water path exhibited high agreement with the microwave radiometer measurements. Our method of deriving the LWC from the total Z stemming from the in situ measured size distributions may be applied in other situations to derive the cloud liquid droplets, ice, and precipitation masses for clouds with a given radar Z.