Maintenance of a mountain valley cold pool: Numerical study

A persistent cold pool, which formed in the Yampa Valley of northwestern Colorado, was simulated with the Pennsylvania State University - National Center for Atmospheric Research Mesoscale Model version 5 (MM5). The observed cold pool, which was identified by a line of surface stations ascending the western side of the valley, remained in place throughout the day of 10 January 2004. Initial simulations at 1 km resolution did not match the strength of the observed cold pool and did not retain it throughout the day. Varying the planetary boundary layer (PBL) parameterization, increasing the vertical resolution, and increasing the model spin-up time did not significantly improve the simulation.

The absence of snow cover significantly degraded the forecast, but with snow cover present, the valley retained a near-isothermal structure. The simulated cold pool increased in strength and was retained throughout the day when the horizontal resolution was increased to 333 or 111 m. In addition to gaining a sufficient number of grid points for feature resolution, increasing the horizontal resolution improved the horizontal diffusion calculated along sigma levels and captured a flow field with less drainage out of the valley. Calculating the horizontal diffusion along constant height levels also improved the simulation at 1 km resolution.