Comparison of NCEP Stage IV precipitation composites with ECMWF model

Comparison of NCEP Stage IV precipitation composites with ECMWF model
Technical memorandum
Date Published
Secondary Title
ECMWF Technical Memoranda
Philippe Lopez
Abstract A systematic comparison of ECMWF precipitation short-range forecasts with NCEP Stage IV rainfall (NEXRAD) composites over the U.S.A. was performed over the period January 2002 to June 2014. Statistics show that the match between the model and NEXRAD observations has been regularly improving over the years, particularly in terms of mean biases, but also correlations and threat scores. Correlations exhibit a strong seasonal cycle with maximum values in winter and minimum values in summer, in all regions but the West Coast. Besides, precipitation tends to be slightly over-predicted all day long during the cold season. Conversely, the warm season is characterized by a strong under-prediction in the first half of the night and a strong overestimation in the morning, except over the West Coast. A spin-down of precipitation during the first 12 hours of the forecast could be identified over the southern states in the summer. However, this spin-down does not change the sign of the precipitation mean biases between model and NEXRAD when comparing forecasts started at 0000 and 1200 UTC. Lastly, it was confirmed that the recent changes applied to the convective parametrization of the model (cycle 40r1) helped to reduce the long-standing large phase advance in the diurnal cycle of the model's summer precipitation over most regions.