Abstract
AbstractA modelling case study designed from observations from the African Monsoon Multidisciplinary Analysis (AMMA) is presented and discussed. It aims at investigating the issue of initiation of convection in a semi‐arid environment. This case corresponds to the development of local daytime convection mainly controlled by boundary layer characteristics rather than by atmospheric synoptic scales. A high‐resolution three‐dimensional simulation is presented and extensively evaluated against the numerous observations available for 10 July 2006 from the AMMA campaign. The simulation, run over a domain of 100 × 100 km, is able to represent main boundary layer structures and processes leading to deep convection initiation as well as the formation of density currents. Sensitivity tests point to the key role of the sensible heat flux, the humidity of low to mid levels, the lapse rate at low levels and of a mesoscale ascent to initiate deep convection in those semi‐arid conditions, while evaporation of precipitation is shown to play a minor role. This study thus provides a case to investigate the ability of parametrizations to handle the initiation of convection in a semi‐arid environment. Copyright © 2011 Royal Meteorological Society
Highlights
Current climate and Earth system models exhibit large biases in the Tropics, among them an incorrect representation of the diurnal cycle of precipitation over continents (Yang and Slingo, 2001) and a poor representation of the location, intensity distribution and diurnal cycles of precipitation in monsoon regions (Kang et al, 2002; Janowiak et al, 2007)
We present a large-eddy simulation (LES) of the case of L11 over a large domain in order to capture the interaction between the boundary layer turbulence and moist convection
All those conditions may not appear favourable for moist convection development and suggest a key role of the boundary layer to explain the initiation of convection as observed
Summary
Current climate and Earth system models exhibit large biases in the Tropics, among them an incorrect representation of the diurnal cycle of precipitation over continents (Yang and Slingo, 2001) and a poor representation of the location, intensity distribution and diurnal cycles of precipitation in monsoon regions (Kang et al, 2002; Janowiak et al, 2007). As suggested by Findell and Eltahir (2003), over dry soils convective initiation will be favoured for initial conditions presenting a relatively large convective triggering potential (small potential temperature lapse rate), i.e. where the boundary layer will grow significantly In such a semi-arid environment, it may be expected that knowledge resulting from studies documenting the humid Tropics and the midlatitudes cannot be directly transposed. We take advantage of the very well-documented case of 10 July 2006 over Niamey (Lothon et al, 2011, L11 in the following) observed during the African Monsoon Multidisciplinary Analysis (AMMA) campaign (Redelsperger et al, 2006) This set of data is unique, as a convective system is observed during most of its life cycle at high resolution in time and space in an intensively instrumented site.
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