a One-Level Mesoscale Model for Diagnosing Surface Winds in Mountainous and Coastal Regions

A computationally inexpensive, easy to use, one -level mesoscale model ("Mesmod") for diagnosing surface winds in complex terrain has been developed and compared to observations and to other models. Mesmod explicitly solves momentum and temperature tendency equations in pressure-sigma coordinates only at the surface. Mesmod has no mass budget, but it parameterizes the interaction between the surface wind field and mass field aloft. It accomplishes this by very simply parameterizing the vertical temperature profile. Under the hydrostatic assumption, the vertically-integrated temperature profile determines the surface pressure. Mesmod also parameterizes friction, subgrid-scale horizontal diffusion, the diabatic heating/cooling. The only initial data that Mesmod requires are terrain data, synoptic-scale temperatures and geopotential heights from a reference pressure (e.g., 850 mb), and a representative reference lapse rate from a radiosonde sounding. Mesmod is integrated from an initially unbalanced state to a nearly steady state without diabatic forcing, and is then integrated as long as desired, driven by a specified, time-dependent diabatic heating or cooling term. Mesmod cannot reproduce vertically-propagating gravity waves, but at internal Froude numbers (LESSTHEQ) 0(1) Mesmod approximately reproduces the surface pressure pattern normally associated with gravity waves in flow past three-dimensional topography. Mesmod is less successful with flow past two-dimensional topography. Experiments with Mesmod showed that rotation and nonlinear advection can create a line of convergence downstream of isolated, mesoscale mountains. Mesmod was compared to observations in five case studies from the Pacific Northwest in which the estimated Froude numbers were (LESSTHEQ) 0(1). Mesmod sometimes failed to reproduce certain details of the observed wind fields, but it usually reproduced the general wind pattern very well. In particular, Mesmod successfully reproduced pronounced lines of convergence observed in three of the cases downwind of the Olympic Mts., an isolated mesoscale mountain range.