The Sensitivity of Numerical Simulations of a Persistent Wintertime Cold-Air Pool to Updated Land and Snow Cover

Persistent cold-air pools (CAPs) are observed frequently during the winter in Utah's Salt Lake Basin, as cold air becomes trapped within the basin by the surrounding topography. When CAPs last more than several days, they often result in prolonged periods of poor air quality. The land surface state plays an important role in the lifecycle and low-level boundary-layer characteristics of CAPs, which in turn affects the concentration and dispersion of pollutants trapped within them. In this study, the sensitivity of Weather Research and Forecasting model simulations to variations in the land cover and snow cover is examined for a persistent CAP that occurred from 1 January 2011 to 8 January 2011.

The impacts of improvements in land cover and associated land use, and snow cover initialization fields on the CAP numerical simulation are studied. Simulations are compared using the 1993 United States Geological Survey land cover dataset and an updated version of the more recent 2011 National Land Cover Database that better prescribes the areal extent of the seasonally-varying Great Salt Lake. Improved snow analyses derived from observations were used instead of the snow cover provided by the North American Mesoscale Reanalysis, which over predicted snow depth in the Salt Lake Valley by interpolating mountain snow observations onto lower elevation locations. The model simulations were found to be sensitive to land cover, land use and snow cover initialization, resulting in substantive changes in simulated thermally driven flows (down/up slope/valley and lake/land breezes) and vertical profiles of boundary-layer temperature, wind, and moisture.