Cloud Top Observations of the Droplet Size Distribution in Drizzling Stratiform Clouds

Abstract:

Some processes that result in the rapid onset of precipitation in shallow stratiform clouds are regarded as an uncertainty in cloud physics. Many studies have investigated the role turbulent processes have in enhancing the rate of formation of large droplets and find that they can enhance collision efficiency by a factor of 2-4 enabling precipitation to form on timescales that better align with observations. However, the need for additional observational studies linking microphysical processes and precipitation is identified as a key requirement needed to improve and incorporate additional model parameterizations of precipitation-related processes.

In this study, the authors investigate transitions and connections of cloud-top droplet size distributions (DSDs) with precipitation in open-cell stratiform clouds. Features of the DSD exhibit a bimodal pattern as the aircraft transects precipitating cells. The larger mode reaches a maximum size near the center of the precipitating cell, while the smaller mode remains nearly constant in size through the transition. We discuss possible explanations of this feature that include turbulent effects such as inhomogeneous entrainment, convective outflow, in-cloud nucleation and other possible explanations. We review similarities between our observations, droplet growth processes and their connections with precipitation onset. We then estimate sedimentation rates using cloud top DSDs and compare results with retrieved precipitation rates. We examine how changes in the liquid water path and droplet concentrations may act to enhance or suppress precipitation rates.

The observations of stratiform clouds were made during NASA’s Observations of Aerosols Above Clouds and their Interactions (ORACLES) campaign. Cloud top DSD retrievals are made using the Rainbow Fourier Transform technique implemented using the Research Scanning Polarimeter. This technique does not require assumptions of the DSD functional shape and can retrieve monomodal and multi-modal DSDs. Precipitation rates and rain water paths are retrieved from the W-band plus Cloud Optical Depth product using Airborne Precipitation Radar - 3rd Generation (APR-3) reflectivity measurements.