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Leonard M. Druyan, Matthew Fulakeza, Patrick Lonergan, and Mahaman Saloum

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.

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The JET2000 Project: Aircraft Observations of the African Easterly Jet and African Easterly Waves

Aircraft Observations of the African Easterly Jet and African Easterly Waves

C. D. Thorncroft, D. J. Parker, R. R. Burton, M. Diop, J. H. Ayers, H. Barjat, S. Devereau, A. Diongue, R. Dumelow, D. R. Kindred, N. M. Price, M. Saloum, C. M. Tayor, and A. M. Tompkins

Scientific background and motivation for the JET2000 aircraft observing campaign that took place in West Africa during the last week of August 2000 are presented. The Met Research Flight CI30 aircraft made two flights along the African easterly jet (AEJ) between Sal, Cape Verde, and Niamey, Niger, and two “box” flights that twice crossed the AEJ at longitudes near Niamey. Dropsondes were released at approximately 0.5°–10° intervals. The two box flights also included low-level flights that sampled north–south variations in boundary layer properties in the baroclinic zone beneath the AEJ.

Preliminary results and analysis of the JET2000 period including some of the aircraft data are presented. The JET2000 campaign occurred during a relatively dry period in the Niamey region and, perhaps consistent with this, was also associated with less coherent easterly wave activity compared to other periods in the season. Meridional cross sections of the AEJ on 28 and 29 August (after the passage of a mesoscale system) are presented and discussed. Analysis of dropsonde data on 28 August indicates contrasting convective characteristics north and south of the AEJ with dry convection more dominant to the north and moist convection more dominant to the south. The consequences of this for the AEJ and the relationship with the boundary layer observations are briefly discussed.

Preliminary NWP results indicate little sensitivity to the inclusion of the dropsonde data on the AEJ winds in European Centre for Medium-Range Weather Forecasts (ECMWF) and Met Office analyses. It is proposed that this may be due to a good surface analysis and a realistic model response to this. Both models poorly predict the AEJ in the 5-day forecast indicating the need for more process studies in the region.

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