The variability of the ice crystal mass-size relationship in trailing stratiform precipitation systems using airborne Doppler cloud radar and in-situ microphysical observations

The importance of clouds on the evolution of climate through their direct effect on the earth radiation budget and water cycle is well recognized. The evaluation of the representation of clouds in global and high-resolution models requires observational references. Cloud radar-lidar retrieval techniques allow for the characterization of the vertical distribution of the macrophysical and microphysical properties of clouds. However, as recently demonstrated using intercomparison exercises and radiative closure, state-of-the-art ice cloud microphysical retrieval techniques still produce very different microphysical properties Furthermore, the shortwave and long-wave cloud radiative effects derived using these microphysical properties in radiative transfer models do not compare well with reference ground radiation measurements. Many studies suggest that the discrepancies are due to assumptions made in ice cloud microphysical retrievals. Specifically, the statistical relationship between mass, projected area (which relates to particle habit), fall speed, and maximum dimension of ice crystals, are not sufficiently constrained by currently available in-situ observations. Another possibility is that the natural variability of these statistical relationships is too large to be represented with a single relationship for all ice clouds. The objective of this paper is to report on the natural variability of the mass-size relationship we have derived from collocated airborne multi-beam Doppler cloud radar and in-situ microphysical measurements collected over West Africa and the Maldives Islands in trailing stratiform regions of mesoscale tropical convective systems. The implications in terms of robustness of the assumptions currently held in ice cloud retrieval techniques will also be discussed and a path forward will be suggested.