PNNL

Abstract

Meteorological processes affecting the evolution of a persistent wintertime cold air pool that formed in the Columbia Basin of eastern Washington were investigated using a mesoscale numerical model. The limited observations suggested that various mechanisms, including surface radiative cooling and heating, large-scale subsidence and temperature advection, and low-level cloudiness may have played a role in the formation and destruction of this cold pool episode that began on 2 January and ended on 7 January 1999. The evolution of the simulated temperature and wind fields agreed reasonably well with the observations, and a series of numerical simulations were carried out to isolate individual mechanisms. The cold pool was initialized by radiative cooling on a clear night and was maintained by a strong inversion cap at the top of the cold pool. Large-scale subsidence plus strengthening downslope flow in the lee of the Cascades produced the inversion cap that descended with time as the subsidence and downslope flow increased. Weak warm advection aloft helped to maintain this capping inversion. Fog and stratus reduced the diurnal temperature oscillations inside the pool, but their existence was not critical to maintaining the cold pool because surface radiative heating on a midwinter day was insufficient to completely destroy the temperature deficit in the persistent inversion. The presence of low-level clouds becomes much more critical for the maintenance of persistent cold pools in the spring and, perhaps, the fall seasons when insulation is much stronger than in midwinter.