Wintertime cold air pools in the Columbia Basin

An ongoing field investigation of wintertime cold air pools that form at the lowest elevations of the Columbia Basin near Pasco, Washington, is described. Cold pool episodes are a significant feature of the wintertime climatology of this basin, affecting air pollution dispersion, transportation safety, and the frequency of occurrence of stratus, fog, and freezing rain and drizzle. Because no rawinsondes are launched from the Pasco area, our investigation uses wind and temperature soundings from a 915 MHz radar wind profiler and a Radio Acoustic Sounding System, as well as pseudo-soundings of temperature from lines of temperature data loggers exposed on open mountainsides.

The presentation begins with a proposed definition of cold pools, pointing out the differences between diurnal and persistent cold pools. Using this definition, we develop an initial climatology of persistent wintertime Columbia Basin cold pools from 11 years of surface meteorological data from mountain top and basin floor locations. The signatures of these cold pools in hourly temperature and wind records illustrates the broad range of mechanisms that lead to their formation and dissipation. These mechanisms are illustrated using two episodes that occurred during our field investigations on 24-25 December 1998 and 4-7 January 1999. Analyses center on the roles of pre-existing diurnal inversions, fog and stratus, cold and warm air advection aloft, chinooks, and turbulent erosion in their formation and dissipation. The December cold pool was destroyed during a warm frontal passage, while the January cold pool was destroyed by cold air advection aloft and boundary layer warming following the breakup of stratus. Computer animations of the evolution of the vertical temperature structure in the basin are used to illustrate the breakup mechanisms.

A separate presentation by Zhong et al. will illustrate the use of numerical simulations to investigate further the physical processes leading to cold pool evolution.