85th AMS Annual Meeting

Thursday, 13 January 2005: 2:00 PM
Recent West African hydrologic anomalies in the NCEP coupled forecast system
Wassila M. Thiaw, NOAA/NWS/NCEP, Capm Springs, MD; and K. C. Mo
The performance of seasonal mean precipitation forecasts from the NCEP Climate Forecast System (CFS) and the seasonal forecasts SFM over West Africa are evaluated against the CMAP precipitation data. The 6-hr forecasts from the reanalysis 1 (R1) and 2 (R2) are also examined. Preliminary results show that the climatological mean rainfall in the SFM features higher intensity of rainfall over the Guinean highlands than observed. This is also the case for R2 which exhibits a rainfall pattern over the Guinean highlands quite similar to the OLR pattern. Rainfall intensity in R1 over the Guinean highlands appears to be reasonably well represented. The annual cycle forecasted by the SFM shows a lag in the northward progression of the rains and an early withdrawal throughout the rainy season. Circulation features are examined to account for the bias in the rainfall forecasts. It is shown that SFM produces stronger winds in the low-level southwesterly flow, a stronger and diffuse African Easterly Jet (AEJ), and a stronger Tropical Easterly Jet (TEJ) compared to observations. Thus, the bias in the SFM can partially be attributed to a poor representation of the AEJ, which is a response to the north south moisture gradient across West Africa. Results from the CFS are discussed and forecast skill for the period 1997-2004 are presented.

Supplementary URL: