Multiseason Verification of the MM5. Part I: Comparison with the Eta Model over the Central and Eastern United States and Impact of MM5 Resolution

Abstract

This paper describes the multiseason verification of the fifth-generation Pennsylvania State University?National Center for Atmospheric Research Mesoscale Model (MM5) and the National Centers for Environmental Prediction (NCEP) Eta Model over the eastern two-thirds of the United States and surrounding coastal waters during the cool (1 November?31 March) and warm (1 May?30 September) seasons from the autumn of 1999 through the summer of 2001. Verification statistics are calculated by interpolating model forecasts to the observation sites. The horizontal and vertical distributions of model errors are presented as are the diurnal and intraseasonal trends. During the cool season, both the MM5 and Eta have a low-level cool and moist bias over land, a significant surface warm bias over water, and surface winds that are too strong over land to the east of the Rockies and too weak over water. The low-level cool and moist bias is maximized during the day, and the cool bias is largest during late winter. During the warm season, the MM5 and Eta have little temperature bias over water and a negative wind speed bias over the Rockies. Both the MM5 and Eta have a surface dry and warm bias over land during the warm season; however, the Eta warm-season biases were reduced between 2000 and 2001 because of recent improvements to the land surface model and soil moisture initialization. Using the NCEP Aviation Model rather than the Eta to initialize the MM5 during the 2000/01 cool season resulted in slightly better sea level pressure forecasts over the Northeast on average, but not for wind and temperature. In order to quantify the impact of increased resolution, the MM5 was verified down to 4-km grid spacing around coastal southern New England. For 32 objectively identified sea-breeze events, the 12-km MM5 has significantly greater wind and temperature skill along the coast than the 36-km version, but there is little improvement from 12 to 4 km. The sea breezes in the MM5 are too early on average and are associated with a late afternoon cool bias. Many of the MM5 and Eta errors have slowly evolving intraseasonal trends. In particular, the cool bias amplifies during the winter and the summer dry bias in the MM5 increases during prolonged wet periods. The sea level pressure errors are episodic, with clusters of negative and positive mean errors lasting approximately 3?6 weeks, thereby suggesting a dependence on the large-scale flow.