Factors Governing Cloud Growth and Entrainment Rates in Shallow Cumulus and Cumulus Congestus During GoAmazon 2014/2015

Abstract

Shallow cumulus and cumulus congestus clouds play an important role in the large-scale tropical circulation by mixing heat and moisture vertically and preconditioning the environment for deeper convection. Different representations of these shallow clouds also account for much of the spread in General Circulation Models (GCMs) climate sensitivity. In particular, GCMs typically struggle representing low-cloud cover and the diurnal cycle in cloud properties. One of the reasons for these shortcomings may be how entrainment is represented in GCM parameterizations. Entrainment is a first-order mechanism that governs the depth to which shallow cumulus and congestus penetrate. This study uses observations from the Department of Energy's Atmospheric Radiation Measurement (ARM) mobile facility deployed at Manacapuru, Brazil during the Green Ocean Amazon (GOAmazon) 2014/2015 Campaign. Environmental thermodynamic profiles and observations of cloud-top height are used to constrain an entraining plume model to estimate bulk entrainment rates. A new and improved best-estimate of cloud-top height is obtained from a combination of vertically-pointing W-band ARM cloud radar (WACR) and 1290-MHz Radar Wind Profiler (RWP) observations. A combination of radiosonde, microwave radiometer profiler (MWRP) and microwave radiometer (MWR) observations provide a new and improved best-estimate of the environmental thermodynamic state. We quantify uncertainty in entrainment rates considering uncertainties in estimated cloud top height, environmental thermodynamic properties, and assumed initial parcel characteristics. We evaluate several entrainment closures that are in current use in atmospheric models or have been proposed based on theory or large-eddy simulation results. Entrainment rates for cumulus congestus clouds are weakly correlated with low-level buoyancy, cloud depth, and cloud size. For shallow cumulus, we find a modest correlation between entrainment rate and low-level relative humidity. Additional relationships between entrainment rate, vertical velocity, and environmental thermodynamic variables are also presented.