Atmospheric flow is significantly influenced by orography creating lift and friction forces. The representation of orography and its influence in numerical weather prediction (NWP) models, as well as other physical processes, are necessarily divided into the resolvable scales of motion and treated by primitive equations. Physical processes on scales less then the model truncation are parameterized or emulated as well as possible to account for the influence of unresolvable scales on the larger scales of the model. In terms of large scale NWP models, wind flow around sub-grid scale orography is a process that retards motion at various model vertical levels near or in the boundary layer. It is thought that the flow around the mountain encounters larger frictional forces by being in contact with the mountain surfaces for longer time as well as the interaction with vortex shedding which occurs in numerous observations and simulations. The National Weather Service (NWS) global spectral model (GSM) shows improvement after incorporating enhanced friction due to sub-grid scale mountain blocking processes, following the work of Lott and Miller (1997), in conjunction with sub-grid scale vertically propagating gravity waves, following the work of Alpert et al (1996): the flow is expected to go around the mountain below a dividing streamline above which gravity waves are potentially generated and propagate vertically, dependent on the stable stratification. In addition to a general improvement in northern hemisphere model skill scores using the GSM, at T62, there is a reduction in the number of very poor 5 day forecasts, and a too low bias in geopotential height is somewhat alleviated. Results at T62 and T254 resolutions will be presented and implementation of the mountain blocking scheme in the operational GSM, after suitable testing at higher (T254) resolution, may be scheduled. Case studies of the flow around Hawaiian islands with the NCEP 10km regional spectral model (RSM), to simulate atmospheric flow around an obstacle, will be used as a laboratory to compare to the larger scale modeled sub-grid scale mountain blocking parameterization at various resolutions will be discussed.

Lott, F and M. J. Miller: 1997, “A new subgrid-scale orographic drag parameterization: Its formulation and testing”, QJRMS, 123, pp101-127.

Alpert, J. C., S-Y. Hong and Y-J. Kim: 1996, Sensitivity of cyclogenesis to lower troposphere enhancement of gravity wave drag using the EMC MRF”, Proc. 11 Conf. On NWP, Norfolk, 322-323.