A Vertically Nested Regional Numerical Weather Prediction Model with Second-Order Closure Physics

The model we describe involves a unique strategy in which a high vertical resolution grid is nested within the coarse vertical resolution grid of a regional numerical weather prediction (NWP) model. Physics computations performed on the high vertical resolution grid involve time-dependent solution of second-order turbulence equations, the transfer equations for long- and shortwave radiation, and moist thermodynamic calculations which include liquid water content and fractional cloudiness. The dynamical computations involving advection, pressure gradient, and Coriolis terms are performed on the regional model grid. The two grids interact fully each model time step. This approach represents an extension into NWP of the general practice of supplying coarse large-scale dynamical forcing to high-resolution boundary layer models. Aside from the computational savings of performing dynamical calculations only at the coarser resolution, we also avoid difficulties which can arise with high vertical-resolution dynamical computations in regions of significant topography. This model can, however, be easily made to take on the appearance of a standard, nonnested model by specifying everywhere one fine grid paint per coarse grid layer. Several preliminary model forecasts are presented. The first is a 36-hour forecast over the Mediterranean and adjacent regions during midsummer. This provides a good test of the model's ability to develop a realistic cool marine mixed layer over the Mediterranean, while properly treating the extreme diurnal variations in the boundary layer over North Africa. Our second numerical forecast takes place in a much more active synoptic regime involving a wintertime frontal passage at a weather station ship in the North Atlantic.