Mesoscale Modeling Development at the National Centers for Environmental Prediction : Version 4 of the NAM Forecast System and Scenarios for the Evolution to a High-Resolution Ensemble Forecast System

The version 4 upgrade to the North American Mesoscale Forecast System (NAMv4) features many changes to its model and data assimilation components that serve to improve forecast performance for both the parent 12 km domain and the 5 nested domains. Grid-spacing was also adjusted in two of the primary nested domains; the 4 km CONUS and 6 km Alaska nests were both refined to 3 km.

The most significant data assimilation enhancement is the replacement the 12-h NAM Data Assimilation System (NDAS), which runs 3-h analysis/forecast segments with the 12 km parent domain, with a 6-h assimilation system with hourly analysis updates for three domains (12 km parent and 3 km CONUS/Alaska nests) with radar and lightning data initialization for the 12 km parent and 3 km CONUS nest. Numerous model changes were made, resulting in the improvment of high-threshold QPF biases for both the 12 km parent and 3 km nest domains, while also improving stratiform precipitation.

Details on these and other NAM version 4 changes will be discussed along with future plans for the evolution of NCEP's mesoscale modeling/assimilation systems towards a high-resolution ensemble forecast system.