Momentum transport in shallow convection

TitleMomentum transport in shallow convection
Publication TypeTechnical memorandum
Date Published2016
Secondary TitleECMWF Technical Memoranda
AuthorsSchlemmer, L, Bechtold, P, Sandu, I, Ahlgrimm, M

The vertical transport of horizontal momentum by convection has an important impact on the general circulation of the atmosphere as well as on the life cycle and track of cyclones. So far it has mostly been studied for deep convection, whereas little is known about its properties and importance in shallow convection. In this study convective momentum transport by shallow convection is investigated by analysing both data from large-eddy simulations (LES) and simulations performed with the Integrated Forecasting System (IFS) of the European Centre for Medium-Range Weather Forecasts (ECMWF). Thereby the IFS is run in 3D mode as well as in a single column model (SCM) version. In addition, the central terms underlying the bulk mass-flux parametrisation of the IFS convection scheme are evaluated offline. The analysed cases exhibit shallow convective clouds developing within considerable low-level wind shear. Analysis of the momentum fluxes in the LES data reveals significant momentum transport by the convection in both cases, which is directed down-gradient despite substantial organisation of the cloud field. A detailed inspection of the convection parametrisation reveals a very good representation of the underlying entrainment and detrainment rates and an appropriate representation of the convective mass and momentum fluxes. To determine the correct values of mass-flux and in-cloud momentum at the cloud base in the parametrisation yet remains challenging.

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