THE MIXED SPECTRAL FINITE-DIFFERENCE (MSFD) MODEL - IMPROVED UPPER BOUNDARY-CONDITIONS

The Mixed Spectral Finite-Difference (MSFD) model for neutrally strati fied, turbulent surface-layer flow has been shown to produce accurate, computationally economical predictions of flow in complex terrain as long as certain conditions are met. Since the model is based on linear ized equations, variations in the surface roughness must not be extrem e and terrain slopes must, in practice, be of order 0.3 or less. This paper summarizes modifications to the MSFD model since the original de scription appeared in Beljaars et al. (1987a) and Karpik (1988). A sig nificant improvement is the formulation of the upper boundary conditio ns. This new approach follows from an asymptotic form of the model equ ations where vertical diffusion terms are neglected. The resulting dif ferential equations have an explicit, analytically expressible solutio n. The asymptotic solution derived in this way is shown to match very well with the solution to the full equation set within the computation al domain. We illustrate that the improved upper boundary conditions p ermit the use of a computational top boundary that is lower than was p reviously possible, thus increasing computing efficiency without sacri ficing accuracy. The paper concludes with a brief description of MSFD- PC, the personal computer version of the model, and an outline of futu re development plans for the MSFD model.