Abstract:
In this talk the representation of African easterly waves in analyses and forecasts from four operational numerical weather prediction centers (NCEP, ECMWF, UK Met Office and Météo-France) are explored and compared. This is accomplished using synoptic analysis, temporal and spatial filtering techniques within an objective identification framework. Analysis products were consistent between the forecast centers in terms of trough location and propagation characteristics. All analysis products were successful in capturing the subtle phase relationship between the easterly waves and convection in which the maximum in precipitation and curvature vorticity varies according to longitude, consistent with observations from previous studies. The most significant difference in the analysis product was noted in the mean intensity, measured by the area mean 700hPa curvature vorticity value.
Forecasts of African easterly waves, up to five days lead time, were examined using the same techniques. Large diversity was noted in the forecast behavior with three of the four models examined showing persistent westward drift of synoptic activity from the analysis time forwards. This suggests that the forecasts struggle to generate new AEWs, instead tending to advect those present in the initial conditions westwards with the mean flow. The most significant forecast errors were observed in the forecast precipitation field due to both forecast drift and persistent errors in the mode of precipitation (i.e. synoptic, diurnal etc). Composite forecasts, created using lag regression techniques show the forecast models are unable to capture the phasing of precipitation with respect to the easterly wave trough axis, suggesting that these modeling systems do not fully capture the physical coupling between easterly waves and precipitation.