The Topographically Forced Diurnal Circulations of Western Washington State and Their Influence on Precipitation

Abstract

Western Washington State is a region where both land-water and topographic contrasts result in diurnal circulations that produce significant diurnal variations in parameters such as wind speed and direction, cloudiness and precipitation. These circulations are most apparent from late spring through early fall and are manifested on several interacting scales. In this paper resultant winds are examined both at the surface and aloft in order to describe the origin, structure and effects of Washington State's diurnal circulations and their influence on forecasting. It is shown that during the summer months the surface winds undergo a large diurnal variation with onshore and upslope winds during the day and offshore and downslope winds at night. This diurnal evolution, which is large only in the lowest 50 mb, results in a diurnal variation of low-level divergence in Puget Sound and produces a substantial modulation of precipitation frequency. It is shown that the winter months also possess diurnal wind variations but of far lesser magnitude. Finally, the relative roles of sea breeze and topographic effect are discussed.