Abstract
Abstract An evaluation of precipitation and evapotranspiration simulated by mesoscale models is carried out within the African Monsoon Multidisciplinary Analysis (AMMA) program. Six models performed simulations of a mesoscale convective system (MCS) observed to cross part of West Africa in August 2005. Initial and boundary conditions are found to significantly control the locations of rainfall at synoptic scales as simulated with either mesoscale or global models. When initialized and forced at their boundaries by the same analysis, all models forecast a westward-moving rainfall structure, as observed by satellite products. However, rainfall is also forecast at other locations where none was observed, and the nighttime northward propagation of rainfall is not well reproduced. There is a wide spread in the rainfall rates across simulations, but also among satellite products. The range of simulated meridional fluctuations of evapotranspiration (E) appears reasonable, but E displays an overly strong zonal symmetry. Offline land surface modeling and surface energy budget considerations show that errors in the simulated E are not simply related to errors in the surface evaporative fraction, and involve the significant impact of cloud cover on the incoming surface shortwave flux. The use of higher horizontal resolution (a few km) enhances the variability of precipitation, evapotranspiration, and precipitable water (PW) at the mesoscale. It also leads to a weakening of the daytime precipitation, less evapotranspiration, and smaller PW amounts. The simulated MCS propagates farther northward and somewhat faster within an overall drier atmosphere. These changes are associated with a strengthening of the links between PW and precipitation.
Highlights
Mesoscale convective systems (MCSs) are still poorly handled by large-scale models (e.g. Lebel et al 2000). This is of concern because mesoscale convective system (MCS) are major contributors to the rainfall amounts over West Africa (Mathon et al 2002), where they lead to local flooding every year
This study aims to document this issue using results from an intercomparison of mesoscale models conducted within the African Monsoon Multidisciplinary Analysis (AMMA) program (Redelsperger et al 2006)
These results provide guidance on what is to be expected in terms of agreement between land surface models (LSMs) seeing the same forcing, and they help in the interpretation of results obtained when LSMs are part of a coupled land surface–atmosphere mesoscale modeling system
Summary
Large-scale model simulations of rainfall over West Africa suffer from major weaknesses, in both numerical weather prediction (NWP) systems. Mesoscale models may be more appropriate than large-scale models for studying hydrology This raises other issues though, as discussed by Davis et al (2003) and Moncrieff and Liu (2006) for simulations over North America using horizontal resolutions of about 10 to a few 10s of kilometers. It is currently unknown how accurately mesoscale models depict water budgets over West Africa, and how they compare to each other in their simulations of individual MCSs. This study aims to document this issue using results from an intercomparison of mesoscale models conducted within the African Monsoon Multidisciplinary Analysis (AMMA) program (Redelsperger et al 2006).
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