Tim Palmer

Abstract:

The topics discussed include 1) The evolution of extended-range systematic error and skill in forecasting large-scale weather regime transitions; 2) The dependence of extended-range systematic error and skill on model horizontal resolution; 3) Monthly mean forecasts of tropical rainfall; 4) Tropical/extratropical interaction, and the influence of tropical low-frequency variability on extratropical forecast skill; 5) Ensemble forecasting, including the impact of ensemble averaging on forecast skill, and ensemble dispersion as a measure of forecast reliability; and 6) Probabilistic forecasting using phase-space cluster analysis. We believe that operational extended-range forecasting using the ECMWF model may be viable to day 20 - and possibly beyond - following further research on techniques for Monte Carlo forecasting, and when model systematic error in the tropics has been reduced significantly. -from Authors

Abstract:

First, an empirical orthogonal function (EOF) analysis is made of outgoing longwave radiation (OLR) in the tropics over seven winters. Having removed the seasonal cycle and interannual variability, the two leading EOFs describe the 30-60 day oscillation. A composite of extratopical 500 mb geopotential height correlated simultaneously with this mode of tropical variability is constructed. The 500 mb height composite is compared succesfully, with the Simmons, Wallace and Branstator (SWB) mode of barotropic instability, which has similar periodicity and similar spatial structure in both its phase-quadrature components. In the final phase of this study, the ECMWF model has been integrated over four wintertime 20-day periods. -from Authors