Microphysical and Dynamical Properties of Drizzling Marine Boundary Layer Stratocumulus Clouds

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. Recent model simulations made using parameterizations that have unified treatment of cloud and turbulence processes have highlighted the need for improved quantification of sub-cloud layer drizzle evaporation and its coupling with turbulence. In this work we have used the data collected at the ARM Eastern North Atlantic (ENA) site during drizzling stratocumulus cloud conditions to 1) characterize the drizzle microphysical and boundary layer properties, and 2) to assess the impact of drizzle evaporative cooling on boundary layer turbulence.

In a companion study we present the retrieval algorithm that was used to retrieve the cloud and drizzle macro- and micro-physical properties by combining the data collected by vertically pointing cloud radar, laser ceilometer, and microwave radiometer. The algorithm was applied to 12 (288 hours) closed cellular (CC) cases and 19 (437 hours) open cellular (OC) cases observed at the ENA site. We combined the retrieved vertical air motion in drizzle shafts with that reported by the vertically pointing Doppler Lidar in the clear parts to estimate the vertical air motion below the cloud base. Hourly values of vertical velocity variance, updraft and downdraft strengths were calculated from these estimates.

The average ceilometer cloud fraction during CC cases was 96%, while the same during OC cases was 76%. On average the CC cases had shallower boundary layers (cloud top height of 1258 m) and thinner clouds (liquid water path of 90 g/m2), compared to the OC cases that had cloud top heights of 1709 m and liquid water path of 266 g/m2. The averaged rain rate at the cloud base was 2.86 mm/day for CC cases, with negligible rain reaching the surface, while for OC cases the averaged rain rate was 4.47 mm/day with the rain rate at the surface being 1.30 mm/day. Collectively our initial results suggest the stratocumuli associated with CC mesoscale structure to be thinner and weakly precipitating than their OC counterparts.

The retrieved microphysical properties will be used as an input to a 1-dimensional radiative transfer model (RRTM) to simulate profiles of radiative fluxes and heating rates. We will examine the relationship between cloud top radiative flux divergence and vertical velocity variance in the boundary layer to gage the impact of drizzle evaporation on turbulence. Additional attempts will be made to classify the data by boundary layer depth, LWP, sub-cloud layer relative humidity to gain insights on prominent processes governing drizzle evaporation.