Extended Abstract (140K)3C.1
Prediction of African easterly waves by The FSU Superensemble
Robert S. Ross, Florida State University, Tallahassee, FL; and T. N. Krishnamurti
The performance of The Florida State University (FSU) Superensemble (SE) in forecasting the 850 hPa wind field and precipitation for African easterly waves (AEWs) during June-October 2001 is evaluated. Wind field forecasts are evaluated by root mean square error (RMSE) and systematic error, while precipitation forecasts are examined through use of RMSE and correlation coefficient. The SE is found to be the best model for forecasting the winds and precipitation associated with AEWs, in comparison to all ensemble member models and the ensemble mean.
Two AEW regimes in the lower troposphere over North Africa are identified, one to the north of the African easterly jet (AEJ) and coincident with the surface position of the monsoon trough, and the second to the south of the jet and coincident with the equatorial rain belt. The northern wave regime, comprised of relatively dry waves driven by baroclinic dynamics, is found to be generally well forecast by the SE and the ensemble member models. The southern wave regime, where the waves draw their energy from a complex interaction of barotropic dynamics and latent heat release, is generally not well forecast by the ensemble member models, presumably due to errors in the feedback between latent heating/precipitation and the wind flow in the model. The SE is found to provide better forecasts of waves in the southern wave regime, since the technique forecasts wind and precipitation independently of each other, based on separate training procedures for these variables.
It is demonstrated that the two wave regimes can interact and that the northern wave regime can be important in tropical cyclogenesis over the Atlantic. Wave energy originating to the north of the AEJ crossed to the south of the jet and developed into two tropical storms and one depression in 2001, representing half of the AEWs that developed into depressions and named storms in that year. More studies are needed to better understand the complex interactions between the northern and southern wave regimes and of the dynamics of those waves from the northern wave regime that develop into depressions and named storms.
Session 3C, Tropical Cyclogenesis III
Monday, 3 May 2004, 1:30 PM-3:15 PM, Napoleon II Room
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