A Regional Model Study of Synoptic Features over West Africa

Abstract

Synoptic weather features over West Africa were studied in simulations by the regional simulation model (RM) at the NASA Goddard Institute for Space Studies. These pioneering simulations represent the beginning of an effort to adapt regional models for weather and climate prediction over West Africa. The RM uses a Cartesian grid with 50-km horizontal resolution and 15 vertical levels. An ensemble of four simulations was forced with lateral boundary conditions from ECMWF global analyses for the period 8?22 August 1988. The simulated midtropospheric circulation includes the skillful development and movement of several African wave disturbances. Wavelet analysis of midtropospheric winds detected a dominant periodicity of about 4 days and a secondary periodicity of 5?8 days. Spatial distributions of RM precipitation and precipitation time series were validated against daily rain gauge measurements and International Satellite Cloud Climatology Project satellite infrared cloud imagery. The time?space distribution of simulated precipitation was made more realistic by combining the ECMWF initial conditions with a 24-h spinup of the moisture field and also by damping high-frequency gravity waves by dynamic initialization. Model precipitation ?forecasts? over the central Sahel were correlated with observations for about 3 days, but reinitializing with observed data on day 5 resulted in a dramatic improvement in the precipitation validation over the remaining 9 days. Results imply that information via the lateral boundary conditions is not always sufficient to minimize departures between simulated and actual precipitation patterns for more than several days. In addition, there was some evidence that the new initialization may increase the simulations' sensitivity to the quality of lateral boundary conditions.