Role of the residual layer and large-scale subsidence on the development and evolution of the convective boundary layer

E Blay-Carreras,J Vilà-Guerau De Arellano,D Pino,M Lothon,F Lohou,O Hartogensis,A Van De Boer,H Pietersen,O De Coster,C Darbieu

doi:10.5194/acp-14-4515-2014

Abstract

Abstract. Observations, mixed-layer theory and the Dutch Large-Eddy Simulation model (DALES) are used to analyze the dynamics of the boundary layer during an intensive operational period (1 July 2011) of the Boundary Layer Late Afternoon and Sunset Turbulence campaign. Continuous measurements made by remote sensing and in situ instruments in combination with radio soundings, and measurements done by remotely piloted aircraft systems and two manned aircrafts probed the vertical structure and the temporal evolution of the boundary layer during the campaign. The initial vertical profiles of potential temperature, specific humidity and wind, and the temporal evolution of the surface heat and moisture fluxes prescribed in the models runs are inspired by some of these observations. The research focuses on the role played by the residual layer during the morning transition and by the large-scale subsidence on the evolution of the boundary layer. By using DALES, we show the importance of the dynamics of the boundary layer during the previous night in the development of the boundary layer at the morning. DALES numerical experiments including the residual layer are capable of modeling the observed sudden increase of the boundary-layer depth during the morning transition and the subsequent evolution of the boundary layer. These simulations show a large increase of the entrainment buoyancy flux when the residual layer is incorporated into the mixed layer. We also examine how the inclusion of the residual layer above a shallow convective boundary layer modifies the turbulent kinetic energy budget. Large-scale subsidence mainly acts when the boundary layer is fully developed, and, for the studied day, it is necessary to be considered to reproduce the afternoon observations. Finally, we also investigate how carbon dioxide (CO2) mixing ratio stored the previous night in the residual layer plays a fundamental role in the evolution of the CO2 mixing ratio during the following day.

Highlights

The atmospheric boundary layer, characterized by a clear diurnal cycle, has been intensively studied since the 1970s
We demonstrate the importance of residual layer (RL) during the morning transition and of subsidence during the afternoon by analyzing the observed and simulated evolution of the boundary-layer depth, potential temperature, turbulent kinetic energy budget and its influence on the evolution of CO2 mixing ratio
By using mixed-layer theory, if heat advection is considered negligible due to the low winds recorded, the time evolution of the mean potential temperature in the mixed layer (θ) in convective conditions is driven by surface and entrainment heat fluxes, and reads

Summary

Introduction

The atmospheric boundary layer, characterized by a clear diurnal cycle, has been intensively studied since the 1970s. The physical processes associated to the CBL development have been extensively studied. Sorbjan (1996), Sullivan et al (1998) and Conzemius and Fedorovich (2006) studied the role of the entrainment processes; Moeng and Sullivan (1994), Fedorovich et al (2001), Fedorovich et al (2001), Pino et al (2003), Pino et al (2006a) and Pino and Vilà-Guerau de Arellano (2008) the contribution of shear in the generation and maintenance of CBL. Several methodologies have been used to study the CBL: large eddy simulation (LES) numerical experiments (Moeng, 1984; Nieuwstadt and Brost, 1986; Cuijpers and Duynkerke, 1993; Sorbjan, 2007), mixed-layer model (MLM) (Tennekes, 1973; Tennekes and Driedonks, 1981; Fedorovich, 1995; Pino et al, 2006a), observations (Kaimal et al, 1976; Angevine et al, 1994; Cohn and Angevine, 2000) and Published by Copernicus Publications on behalf of the European Geosciences Union

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