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Study Investigates Ascent Rate of Moist Deep Convective Thermals for Improving Cumulus Parameterizations

Since the 1950s, two primary conceptual models have served as a basis for the scientific community’s understanding of the structure and behavior of atmospheric moist convection: the steady-state entraining plume model, and the non-steady thermal model entraining buoyant bubble rising through a fluid. The plume and thermal models predict markedly different structure and flow characteristics.

To gain a better understanding of thermal dynamics, and its practical importance for improving modeling results, Dr. Paul Lawson, Senior Research Scientist at SPEC Incorporated, USA and a second cycle awardee of the UAE Research Program for Rain Enhancement Science (UAEREP), published a recent study on the ascent rate of moist deep convective thermals and the maximum vertical velocity within them.

The study focused on the dynamics of moist thermals, and employed a simple treatment of the microphysics that included only cloud liquid condensation and evaporation. Large-eddy simulation (LES) models were also used to investigate detailed aspects of moist convective behavior and structure.

The study utilized a simple modeling framework that facilitated a direct quantitative comparison with the theoretical expressions. High resolutions of 100-m horizontal and vertical grid spacings allowed the model to capture details of the flow including the thermal toroidal circulation. While simple, this framework captured key features of moist convective thermals in previous LESs.

The results of the study suggest that nonzero buoyancy within moist convective thermals, relative to their environment, fundamentally alters the relationship between the maximum vertical velocity and the thermal-top ascent rate compared to nonbuoyant vortices. The results also have implications for convection parameterizations and interpretation of the forces contributing to thermal drag.

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