Coherent Structures in the Boundary and Cloud Layers: Role of Updrafts, Subsiding Shells, and Environmental Subsidence

Pierre Gentine,Seung-Bu Park,Marie Farge,Kai Schneider

doi:10.1175/jas-d-15-0240.1

Abstract

Abstract Coherent structures, such as updrafts, downdrafts/shells, and environmental subsidence in the boundary and cloud layers of shallow convection, are investigated using a new classification method. Using large-eddy simulation data, the new method first filters out background turbulence and small-scale gravity waves from the coherent part of the flow, composed of turbulent coherent structures and large-scale transporting gravity waves. Then the algorithm divides this coherent flow into “updrafts,” “downdrafts/shells,” “subsidence,” “ascendance,” and four other flow structures using an octant analysis. The novel method can systematically track structures from the cloud-free boundary layer to the cloud layer, thus allowing systematic analysis of the fate of updrafts and downdrafts. The frequency and contribution of the coherent structures to the vertical mass flux and transport of heat and moisture can then be investigated for the first time. Updrafts, subsidence, and downdrafts/subsiding shells—to a lesser extent—are shown to be the most frequent and dominant contributors to the vertical transport of heat and moisture in the boundary layer. Contrary to previous perspective, environmental subsidence transport is shown to be weak in the cloud layer. Instead, downdrafts/shells are the main downward transport contributors, especially in the trade inversion layer. The newly developed method in this study can be used to better evaluate the entrainment and detrainment of individual—or an ensemble of—coherent structures from the unsaturated boundary layer to the cloud layer.

Highlights

Clouds are one of the biggest uncertainties in climate prediction using general circulation models (GCMs), and these uncertainties are partly attributed to the incomplete parameterization of convection (Bony et al 2006)
Most entrainment and detrainment parameterizations are based on bulk mass flux approaches, which assume a top-hat distribution of velocity and scalars with one uniform value given to the updraft and one uniform value given to the environment (Gregory 2001; de Rooy et al 2013)
A well-mixed boundary layer of huli and hqtoti extends up to z 5 ;0.6 km, and a cloud layer appears from the top of the boundary layer up to z 5 ;1.44 km (Figs. 3a,b) and is capped by the ‘‘trade inversion’’ layer, which extends up to z 5 ;2 km

Summary

Introduction

Clouds are one of the biggest uncertainties in climate prediction using general circulation models (GCMs), and these uncertainties are partly attributed to the incomplete parameterization of convection (Bony et al 2006). Most entrainment and detrainment parameterizations are based on bulk mass flux approaches, which assume a top-hat distribution of velocity and scalars with one uniform value given to the updraft and one uniform value given to the environment (Gregory 2001; de Rooy et al 2013). By definition, those approaches do not consider subsiding shells or other surrounding vertical motions (Tiedtke 1989; Bechtold et al 2008) and could be misleading (Heus and Jonker 2008; Abma et al 2013). Dawe and Austin (2011) confirmed that the larger entrainment and detrainment found in direct measurements of entrainment (Romps 2010) could be related to the presence of subsiding shells

Methods

Results

Conclusion