Time-space Representation of Warm-season Rainfall in Weather Prediction Models

Chris Davis, Kevin Manning and John Tuttle

We discuss the verification of climatological aspects of numerical weather prediction (NWP) model forecasts of warm-season rainfall. Time-longitude (Hovmoller) diagrams of warm season rainfall over central North America reveal distinct diurnal patterns whose maxima are temporally in phase with surface temperature and coincident with orography. In addition, a propagating axis of nocturnal rainfall dominates the signal between the cordilleras and leads to a semi-diurnal temporal behavior over the Great Plains of the U.S. Analysis of the forecasts of warm-season rainfall from two NWP models, the NCEP Eta and the Weather Research and Forecast (WRF) model reveals that neither model correctly captures the propagation. Both models miss the nocturnal rainfall maximum over the central U.S. Furthermore, zeroth-order errors in the diurnal cycle of rainfall are apparent. Despite their dismal performance mentioned above, current NWP models are relatively adept at forecasting the approximate latitudinal belt of rainfall at lead times of about 1 day.