Abstract
AbstractThis 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.
Highlights
Reasonable estimation of the vertical velocity within parameterized shallow cumulus clouds in climate models is critical for representing the atmospheric convective mixing that is responsible for half of the spread in model climate sensitivity (Sherwood et al, 2014)
This study focuses on a large eddy simulation of shallow cumulus clouds using the Met Office-Natural Environment Research Council (NERC) Cloud model (MONC; Brown et al, 2015; Brown et al, 2018), based on the Barbados Oceanographic and Meteorological Experiment (BOMEX)
Our study demonstrates that the thermodynamic component of pressure drag in the vertical momentum budget of a cloud ensemble has a similar vertical distribution to the buoyancy source but with the opposite tendency (Figure 1c)
Summary
Reasonable estimation of the vertical velocity within parameterized shallow cumulus clouds in climate models is critical for representing the atmospheric convective mixing that is responsible for half of the spread in model climate sensitivity (Sherwood et al, 2014). A steady-state vertical momentum equation (Simpson & Wiggert, 1969) is vertically integrated to diagnose the in-cloud vertical velocity and has been widely used in many convection parameterizations (Bretherton et al, 2004; Cheinet, 2003; de Rooy & Siebesma, 2010; Gregory, 2001; Kim & Kang, 2011; Neggers et al, 2009; Pergaud et al, 2009; Rio & Hourdin, 2008; Siebesma & Teixeira, 2000; Siebesma et al, 2007; Soares et al, 2004; Sušelj et al, 2012). de Roode et al (2012) found that it is the pressure gradient
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