Meterological processes affecting the evolution of a wintertime cold air pool in the Columbia Basin

Meteorological mechanisms affecting the evolution of a persistent wintertim e cold air pool that began on 2 January and ended on 7 January 1999 in the Columbia basin of eastern Washington were investigated using a mesoscale nu merical model together with limited observations. The mechanisms include su rface radiative cooling and heating, large-scale subsidence, temperature ad vection, downslope warming in the lee of a major Mountain barrier, and low- level cloudiness. The cold pool began when cold air accumulated over the basin floor on a cle ar night and was maintained by a strong capping inversion resulting from a rapid increase of air temperatures above (lie cold pool. This increase of t emperatures aloft was produced primarily by downslope warming associated wi th strong westerly winds descending the lee slopes of the north-south-orien ted Cascade Mountains that form (lie western boundary of the Columbia basin . While the inversion cap at the top of the cold pool descended with time a s the westerly flow intensified, the air temperature inside the cold pool e xhibited little variation because of the fog and stratus accompanying the c old pool. Although the low-level clouds reduced the diurnal temperature osc illations inside the pool, their existence was not critical to maintaining the cold pool because surface radiative heating on a midwinter day was insu fficient to completely destroy the temperature deficit in the persistent in version. The presence of low-level clouds becomes much more critical for th e maintenance of persistent cold pool.,, in the spring and, perhaps, the fa ll seasons when insolation is much stronger than in midwinter. The cold poo l was destroyed by cold air advection aloft, which weakened and eventually removed the strong inversion cap, and by art unstable boundary layer that g rew upward from the heated ground after the dissipation of low-level clouds . Finally, erosion of the cold pool from above by turbulent mixing produced by vertical wind shear at the interface between quiescent air within the p ool and stronger winds aloft was found to be insignificant for this case.