The Seasonal Cycles in the Distribution of Precipitation around Cyclones in the Western North Pacific and Atlantic

Abstract

The seasonal cycles in the distribution of precipitation around the western North Pacific and Atlantic cyclones have been examined by compositing quantitative estimates of the precipitation rate relative to cyclone centers. The precipitation data sources considered include estimates produced by the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) project, the satellite-based daily precipitation estimates produced by the Global Precipitation Climatology Project, and estimates derived based on observed weather reports contained in the Comprehensive Ocean?Atmosphere Data Set (COADS). Results from all three datasets suggest that for Pacific cyclones, substantially more precipitation is found in the warm sector in fall than in winter and less precipitation is found behind the cold front in spring and summer than in winter. The seasonal cycle for Atlantic cyclones is found to be distinctly different. The distribution in precipitation around cyclones in fall and winter are not very different, while in spring and summer less precipitation is found over much of the cyclone. The implications for the observed seasonal cycles are discussed. The seasonal cycle for Pacific cyclones suggests that diabatic contributions to the generation of eddy available potential energy (APE) due to latent heat release should be maximal in fall with a relative minimum in midwinter, while for Atlantic cyclones diabatic generation of eddy APE in fall and winter is nearly the same. This is suggested to be one of the factors that can contribute to the observed midwinter minimum in the Pacific storm track, and the absence of such a minimum in the Atlantic. Possible reasons contributing to the differences in the seasonal cycle between the two basins are discussed. Preliminary analyses suggest that differences in static stability, availability of moisture, as well as dynamical forcing may all be contributing factors. Issues on estimating rates of precipitation based on ship reports are addressed in appendix A. It is argued that it may be a good time to recalibrate existing schemes.