NMMB Model Changes as Part of the NAMv4 Upgrade

The following changes were made to the Nonhydrostatic Multi-scale Mesoscale Model on the B-grid (NMMB) as part of the version 4 upgrade to the North American Mesoscale system (NAMv4; see Rogers et al., 2017; Carley et al., 2017), some of which were necessitated by model failures in the operational and real-time parallel CONUS nests associated with hurricane Joaquin forecasts from last fall. (i) The frequency in which specific humidity was updated by advection was increased from every other time step to every time step in order to maintain numerical stability. (ii) The frequency in which moist physics processes were calculated was increased from once every four time steps (or more) to once every other time step. (iii) Mixing was increased near the top of the PBL in areas where the equivalent potential temperature decreased with height and the air was near saturation. (iv) Cloud condensation was calculated at times between physics calls in order to eliminate supersaturations with respect to water, which frequently developed in moderate-to-strong updrafts, as a result of advecting temperature and moisture every time step. (v) Superadiabatic lapse rates, which developed largely in response to vertical oscillations that formed in areas of strong updrafts, were eliminated by mixing out the layers of instability.

Examples will be shown from 5-min output to demonstrate the impacts of these changes on forecast soundings.