A Climatological Study of Thermally Driven Wind Systems of the U.s

MAY 2002 AMERICAN METEOROLOGICAL SOCIETY | T he complex topography of the western United States produces a variety of thermally and dynamically driven wind systems. Historically, the knowledge and understanding of these systems has been limited by a number of factors, including a lack of observational data. In the Intermountain West (IW, Fig. 1), the elevated semiarid area between the Cascade–Sierra and eastern Rocky Mountains, dry air, dry soil, and limited cloud cover promote intense diurnal fluctuations in sensible heat flux, a large diurnal range in air temperature, and strong thermally driven winds (Carter and Keislar 2000). Thermally driven wind systems in the mountainous IW consist of three major wind circulations: plain–mountain winds, valley winds, and slope winds. The different phasing and superposition of the valley and slope wind systems produces clockwise and counterclockwise diurnal rotations on the right and left banks (or sidewalls) of a valley, respectively, when facing down valley (Hawkes 1947; Whiteman 1990). In some regions of the IW, such as the Wasatch Front valleys (WFV) of northern Utah (Fig. 2a), mountains are located adjacent to lakes, and a fourth diurnal wind system is apparent: the lake–land breeze. The interaction of these four wind systems creates complex flow patterns that are a part of the everyday winds in complex terrain. Over the past few years, high-density data from more than 70 independent meteorological networks have been gathered as part of MesoWest, a collection of cooperative mesonets in the western United States (see Horel et al. 2002). In this paper, we will use MesoWest data from 1997 to 2000 to illustrate the diurnal evolution of thermally driven winds in the IW.