A Simple Model for Mesoscale Effects of Topography on Surface Winds

Abstract

This paper describes a diagnostic, one-level, primitive equation model for computing mesoscale influences of orography, friction and heating on surface winds, given large-scale data from synoptic or prognostic charts. The model is capable of simulating phenomena such as orographic channelling, effects due to changes in atmospheric stability, land- and sea-breezes, and anabatic and katabatic winds. It probably works best for orographic channelling. The model has been applied to Juan de Fuca and Georgia Straits in British Columbia using a grid size of 10 km. Compared to simple methods of computing surface winds, use of the model typically reduces the difference between computed and reported directions by 10–15° for land stations and 25° for exposed lighthouses. Improvement in cases of strong orographic channelling is much greater. One interesting result is the sensitivity of wind directions in Juan de Fuca and Georgia Straits when the geostrophic direction is in the ranges 220–250° or 030–060°. Slight variations of the geostrophic direction from these critical values results in abrupt 180° reversals in surface winds.