A33L-0360
Differences in surface wind speed between observations, reanalyses and climate models: why do they matter for dust emission simulations in the Sahel?
Wednesday, 16 December 2015
Poster Hall (Moscone South)
Françoise Guichard1, Yann Largeron1, Dominique Bouniol1, Fleur Couvreux1, Laurent Kergoat2, Cathryn Birch3 and Beatrice Marticorena4, (1)CNRM (CNRS and Météo-France), Toulouse, France, (2)GET Géosciences Environnement Toulouse, Toulouse, France, (3)University of Leeds, Institute for Climate and Atmospheric Science, School of Earth and Environment, Leeds, United Kingdom, (4)University Denis Diderot Paris VII, Paris Cedex 13, France
Abstract:
The Sahel is prone to intense soil erosion, and dust emission is highly sensitive to surface wind speed. In this study, we use high-frequency observations acquired across the Sahel over five years or more to assess the ability of atmospheric models to simulate the observed surface wind events that are critical to wind erosion. These notably includes morning wind speed maxima resulting from convective mixing of the nocturnal low-level jet and wind gusts associated with deep convective events.
The surface wind fields provided by three global reanalyses, ERA-interim, MERRA and NCEP-CFSR are first assessed. The strongest wind speeds, observed in the morning and during deep convective events, are systematically underestimated in reanalyses (Largeron et al., Geophys. Res. Lett. 2015). As analyzed wind fields are one of the main inputs of many dust emission models, their too low fraction of high wind speeds is expected to lead to major errors and/or compensating errors in dust emission simulations.
The results of eight 30-year long CMIP5 climate simulations which provided high-frequency outputs (namely for each model time step at a few Sahelian sites, cfSites) are then compared to observations in more details. After comparing diurnal cycles and probability distributions of surface wind speeds and nocturnal low level jets, we further make use of composites centered on convective events, both in observations and models. Large differences are also found among climate simulations and between simulations and observations, and these differences again are found to matter for the simulation of dust emission in the Sahel.
Finally, based on the analyses of observations and high-resolution convection-permitting simulations (CASCADE), we present a new parametrization which aims to improve the representation of surface wind speeds during convective events in coarse-resolution atmospheric models.
