Role of drizzle in modulating turbulence in stratocumulus-topped boundary layers

 

Submitter:

Ghate, Virendra Prakash - Argonne National Laboratory
Cadeddu, Maria Paola - Argonne National Laboratory

Area of research:

Cloud Processes

Journal Reference:

Ghate V and M Cadeddu. 2019. "Drizzle and Turbulence Below Closed Cellular Marine Stratocumulus Clouds." Journal of Geophysical Research: Atmospheres, 124(11), 10.1029/2018JD030141.

Science

Marine boundary-layer (MBL) stratocumulus clouds cover vast areas of eastern subtropical oceans and persist for long timescales. These clouds reflect a much greater amount of solar radiation back to space than the ocean surface, making them an important component of the Earth’s radiation budget, and necessitating their accurate representation in Earth System Models (ESM). Many ESMs, however, have difficulty in accurately representing stratocumulus clouds due to inaccurate representation of the associated dynamic, thermodynamic, and radiative processes and their coupling. In this study we investigate the coupling between boundary-layer turbulence and drizzle microphysics below the cloud layer. The observations made at the ARM Eastern North Atlantic (ENA) site during stratocumulus cloud conditions are used to characterize general features of the stratocumulus-topped boundary layer, properties of the drizzle shafts, and the effect of drizzle evaporation on boundary-layer turbulence.

Impact

The study combined data from multiple ground-based sensors to retrieve boundary-layer turbulence and radiative properties, along with cloud and drizzle macro- and microphysical properties. The results showed that precipitation in these systems evaporates almost entirely before reaching the surface, impacting turbulence immediately below cloud base. The quantification of this effect determined from the data analysis can help better represent these processes in models.

Summary

This study combined data from several instruments at the ARM ENA observatory to characterize the dynamic, thermodynamic, and radiative properties of completely overcast precipitating stratocumulus clouds. We also reported cloud and drizzle macrophysical and drizzle microphysical properties. Weak drizzle at the cloud base that seldom reaches the ground was ubiquitous in these cloud systems, with average rain rates of about 1 mm/day and drizzle diameter of 139 micrometers. On average the drizzle shafts were 28 km wide and the evaporating drizzle below the cloud cooled the sub-cloud layer by about 28 W/m2. The primary forcing for turbulence within these clouds is radiative cooling at the cloud top and, for a similar amount of radiative cooling at the cloud top, the turbulence below the cloud layer was about 16% lower during relatively heavily precipitating conditions as compared to weakly precipitating conditions. As most of the drizzle evaporated within a few -100 m of the cloud base, the reduction in turbulence was far greater (~27%) in that layer.

Overall, our results reaffirm the notion that most of the precipitation from stratocumulus clouds evaporates before reaching the surface, thereby cooling and moistening the sub-cloud layer. This substantially impacts below-cloud turbulence, and hence needs to be adequately represented in in ESMs.