Meteorological processes leading to cold pool formation and destruction in the Columbia Basin

Persistent cold pools are a prominent feature of the wintertime meteorology of many basin areas, leading to high static stability conditions that can trap air for many days and allow pollutants to build up to high levels. Forecasting the buildup and breakup of these cold pools is difficult because the mechanisms leading to their development, maintenance and destruction are not well identified. In the winter of 1999, a persistent cold pool episode was documented with enhanced observations in the lower Columbia Basin in eastern Washington. This paper presents a high-resolution numerical study of this episode focusing on the identification of physical mechanisms responsible for the buildup, maintenance, and breakup of the cold pool. The mechanisms are examined in detail, including low-level cloudiness, synoptic warm and cold advection, downslope adiabatic warming, and radiative heating and cooling. The results indicate that downslope warming on the lee-side of the Cascades played a major role in the formation of the strong capping inversion that helped to trap cold air inside the basin. Low-level clouds, a feature frequently associated with persistent wintertime cold pools, were found to suppress diurnal temperature variations inside the pool, but their presence was not critical for maintaining the strong inversion. Cold air advection above the pool was the key process responsible for removing the capping inversion and, ultimately, destroying the cold pool. The implications for air pollutant transport and dispersion during cold pool episodes will also be discussed. A related paper by Whiteman et al. will present analyses of observations of the cold pool evolution in the Columbia Basin.