S100 Do Clouds Feel Drag? Insights into Atmospheric Convection

Sunday, 6 January 2019
Hall 4 (Phoenix Convention Center - West and North Buildings)
Brett A. McKim, University of California, Santa Barbara, Santa Barbara, CA; and N. Jeevanjee, D. Lecoanet, N. Tarshish, and L. Donner

Among atmospheric processes, convection is a large source of uncertainty in climate models, Sanderson et al., 2008. A basic question is how to model the vertical momentum equation for convecting clouds, Donner et al., 2016, and in particular whether such clouds experience drag and what its origin might be. Early work such as Escudier and Maxworthy, 1973 suggests that a cloud experiences little to no form drag as it rises through the atmosphere. We test this assumption about the form drag by analyzing full resolved direct numerical simulations of dry clouds in a neutrally stratified environment. These clouds are represented as discrete, spherical masses known as “thermals”. Form drag can be calculated from thermals by numerically solving for the dynamic pressure via a Poisson equation specified in Jeevanjee and Romps, 2015 and then integrating the dynamic pressure gradient over the thermal’s volume. We expect that the calculated drag force will be small when compared to to the buoyant forces acting on the thermal, as calculated in Tarshish et al., 2018.
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