The Nested Domains of Version 4 of the NAM Forecast System

Version four of the North American Mesoscale Forecast System (NAMv4) features many changes to its model and data assimilation components that serve to improve forecast quality from its convection-allowing, nested domains.  A subset of these changes include: (a) updates to the Ferrier-Aligo microphysics scheme to reduce noted high-precipitation biases and improve stratiform precipitation, (b) improved consistency between model dynamics and physics through calling the physics routines more frequently, and (c) advecting humidity every dynamics timestep.  In addition, the CONUS, Alaska, Hawaii, and Puerto Rico nest grid-spacings have all been unified at 3 km while the requestable, on-demand Fire Weather domain will use a grid-spacing of 1.5 km.

Prior versions of the NAM forecast system have featured a three hourly forecast-analysis assimilation cycle done just on the 12 km parent domain.  The NAMv4 package has been updated with an hourly forecast-analysis assimilation cycling period, which includes radar and lightning observations, prior to initializing the free forecasts at the traditional times of 00, 06, 12, and 18Z.  Furthermore, the NAMv4 features a distinct data assimilation cycle for its 3 km CONUS and Alaska domains in addition to the traditional 12 km North American domain.

This presentation will discuss the updates and changes associated with the nested domains of version four of the NAM upgrade.