Pressure Drag for Shallow Cumulus Clouds: From Thermals to the Cloud Ensemble

Abstract

This study takes the first step to bridge the gap between the pressure drag of a shallow cloud ensemble and that of an individual cloud composed of rising thermals. It is found that the pressure drag for a cloud ensemble is primarily controlled by the dynamical component. The dominance of dynamical pressure drag and its increased magnitude with height are independent of cloud lifetime and are common features of individual clouds except that the total drag of a single cloud over life cycle presents vertical oscillations. These oscillations are associated with successive rising thermals but are further complicated by the evaporation-driven downdrafts outside the cloud. The horizontal vorticity associated with the vortical structure is amplified as the thermals rise to higher altitudes due to continuous baroclinic vorticity generation. This leads to the increased magnitude of local minima of dynamical pressure perturbation with height and consequently to increased dynamical pressure drag. Plain Language Summary Shallow cumulus clouds play a crucial role in the Earth's energy budget by vertically redistributing momentum, heat, and moisture from the surface to the free atmosphere, but they cannot be explicitly resolved in current numerical models. Therefore, a reasonable parameterization of vertical velocity of shallow cumulus clouds is critical for improving weather forecast and climate projections. Recent studies suggest that these clouds are composed of sticky rising thermals whose vertical velocity is mainly controlled by the buoyancy source and the drag due to the pressure perturbation. However, little is known about how the pressure drag of thermals can be related to that of a large ensemble of clouds, which is the focus of convection parameterization in climate models. This study finds that the pressure drag of a cloud ensemble increases with height and is dominated by the dynamical component. The pressure drag of individual clouds shares similar features except that there are frequent oscillations in the vertical related with successive rising thermals. The increased pressure drag with height is due to the continuous baroclinic generation of horizontal vorticity along with the rising thermals. These findings could provide useful insights for a reasonable representation of pressure drag in a parameterization. where w c and B c are in-cloud vertical velocity and buoyancy, respectively, and is the fractional entrainment rate. The basic assumption is that the buoyancy source is primarily offset by a sink associated with entrainment processes. The effects of non-hydrostatic pressure perturbations and subplume fluctuations are taken into account by the coefficients a and b. However, de Roode et al. (2012) found that it is the pressure gradient RESEARCH LETTER

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Evaluation of a New Approach for Entrainment and Detrainment Rate Estimation

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