Microphysical and Dynamical Properties of Drizzling Marine Boundary Layer Stratocumulus Clouds, PART I: Retrieval Development.

Description

Marine boundary layer stratocumulus clouds cover vast areas of the Earth’s surface and hence have a significant impact on the radiation budget and climate. Drizzle is known to be ubiquitous in these clouds affecting the boundary layer turbulence through evaporation and water budget through depletion. In this work we have combined measurements from several instruments to investigate microphysical and dynamical properties of drizzling marine stratocumulus clouds at the ARM Eastern North Atlantic (ENA) site. In Part 1 of this work we present the framework that combines data from several instruments to retrieve radiatively consistent estimates of cloud and drizzle microphysical properties. In a companion work we use the retrievals to characterize the drizzle microphysical properties and study the impact of drizzle evaporation on boundary layer turbulence. Recent studies (Cadeddu et al., 2017) have shown the Liquid Water Path (LWP) retrievals are sensitive to the scattering by drizzle drops larger than 100 microns. It was also shown that higher frequencies contain information useful to separate, to a certain degree, cloud from drizzle liquid water in lightly precipitating clouds. Retrievals of LWP from ARM microwave radiometers neglect scattering effects due to hydrometeors, however, even when precipitation is light and does not affect the quality of the measurements, the departure from the Rayleigh approximation (due to the increased size of precipitating hydrometeors) requires the inclusion of cloud microphysical properties, such as the drop size distribution in the radiative transfer calculations. To account for the scattering in the retrieval of LWP, we combined the data from vertically pointing Ka-band Doppler cloud radar and ceilometer to retrieve drizzle size distribution below the cloud base. The drizzle drop diameter and liquid content profiles were then used in the retrieval of total, cloud and drizzle LWP from the MWR3C using a scattering radiative transfer model in an optimal estimation framework. The retrieval technique was applied to 288 hours of data collected during closed cellular stratocumulus cloud conditions and 437 hours of data collected during open cellular stratocumulus cloud conditions at the ENA site. Our results indicate significant impact of drizzle drops on the brightness temperature, and hence the LWP, at 90 GHz even during light drizzling conditions with drizzle completely evaporating before reaching the surface.