Characterizing subsiding shells in shallow cumulus using Doppler lidar

Science

Shallow cumulus clouds—the low-altitude puffy clouds—are represented in Earth system models (ESMs) by assuming that the effects of these clouds can be separated into two parts: the clouds themselves and the cloud-free air surrounding them. Recent observations of a moist, subsiding buffer region between the cloud and its surrounding environment complicates this assumption. This buffer region, commonly referred to as the subsiding shell, directly impacts cloud-edge mixing processes and the thermodynamic properties of the air that is drawn into the cloud. As ESM model grids become finer and finer, the traditional cloud-environment partitioning becomes increasingly ill-posed, and the influence of the subsiding shell needs to be accounted for. To this end, we characterized the subsiding shell behavior in shallow cumulus using high-resolution, large-eddy simulation combined with Doppler lidar observations from the ARM Southern Great Plains atmospheric observatory and from a secondary site in western Germany.

Impact

If we wish to understand how shallow cumulus interact with their environment for their improved simulation in climate models, we must understand the structure of the subsiding shells that envelop them. We show that the shells are only on the order of 100 m wide and have a vertical asymmetry between the upwind and downwind edges of the cloud, which impacts mixing between the cloud and its environment. We find that the asymmetry reflects fundamental cloud structures and is not simply a consequence of the sampling method, requiring further study to understand.

Summary

Shallow convective mixing remains a major source of uncertainty in climate projections, and our standard approach to parameterizing shallow convection neglects subsiding shells. Doppler lidars indicate the presence of a vertically asymmetric subsiding shell, with the back edge (edge of cloud that passes over the lidar last) of the subsiding shell being more vertically oriented and descending several hundred meters farther into the sub-cloud layer compared to the front edge. High-resolution (10 m) large-eddy simulation was used to determine the impact of transient evolution during the time it takes a cloud to pass over the lidar. Model results suggest the observed asymmetric shell structure is dynamically driven and not simply an artifact of cloud evolution.

This multi-institutional study began as a group project in the 2018 ARM Summer Training and Science Applications Event that took place in Norman, Oklahoma during July 14-21, 2018. This program brought together graduate students, early career scientists, and established researchers to provide experience in both the theoretical underpinnings and practical use of ground-based remote-sensing observations.