Understanding and representing the effect of wind shear on the turbulent transfer in the convective boundary layer

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Abstract

Goal of this study is to quantify the effect of wind shear on the turbulent transport in the dry Convective Boundary Layer (CBL). Questions addressed include the effect of wind shear on the depth of the mixed layer, the effect of wind shear on the depth and structure of the capping inversion, and the effect of wind shear on the entrainment of free tropospheric air into the mixed layer. Following previous research, we use numerical experiments performed with the DALES LES-model varying both the strength and vertical profile of the geostrophic wind. In contrast to previous investigations, the effect of wind shear is analysed using a General Structure Model (GSM), instead of a more traditional zero-order or first order approach. We have selected the GSM framework because it allows for a more detailed assessment of the vertical structure of the capping inversion. Concepts derived within this framework can therefore be more easily transfered to global and meso-scale models turbulent transport parameterizations, as especially in conditions with high wind shear the capping inversion is often of such thickness that it can comprises multiple model layers in state-of-the art meso-scale and Numerical Weather Prediction (NWP) models. The effect of wind shear on the parameterized turbulence transport in NWP and meso-scale models is examined using the Eddy Diffusivity Mass-Flux (EDMF) approach. In contrast with traditional approaches based on K-theory, the EDMF framework acknowledges that turbulent transfer in the CBL can be decomposed into transport due to vertical advection by confined updrafts and diffusive transport (K-theory) within the updrafts and the environmental air. The DALES LES-model results are used to evaluate the effect of wind shear on the different parameters of the EDMF approach. Parameters include the release height and the excess temperature of a parcel that is employed to monitor the updrafts, the vertical velocity of an updraft parcel as it rises through the CBL and penetrates into the capping inversion, and the fraction of air occupied by the updrafts. This study thus extends previous research showing the performance of the EDMF approach in representing turbulent transfer in both clear and cloud-topped boundary layers without wind shear.
Original languageEnglish
Title of host publication20th Symposium on Boundary Layers and Turbulence/18th Conference on Air-Sea Interaction, American Meteorological Society (9-13 July 2012, Boston, MA)
Place of PublicationBoston
PublisherAmerican Meteorological Society
Pages4.1A
Publication statusPublished - 2012
Event20th Symposium on Boundary Layers and Turbulence/18th Conference on Air-Sea Interaction - Boston, United States
Duration: 9 Jul 201213 Jul 2012

Conference

Conference20th Symposium on Boundary Layers and Turbulence/18th Conference on Air-Sea Interaction
CountryUnited States
CityBoston
Period9/07/1213/07/12

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