A numerical study of a persistent cold air pool episode in the Salt Lake Valley, Utah

Abstract

The Weather Research and Forecasting (WRF) model is used to simulate a persistent cold air pool (CAP) episode observed in December 2010 during the Persistent Cold Air Pool Study (PCAPS) field campaign in Salt Lake Valley, Utah. The availability of intensive observations from PCAPS, especially the added vertical profiles, allows for an in-depth analysis of CAP structure and evolution and model evaluation beyond what was done in similar work before. Comparisons of the WRF simulation with surface observations and upper air soundings indicate that WRF is capable of simulating the observed spatial and temporal variation of the valley atmosphere during the CAP episode. The model also successfully captures the formation and removal of the persistent temperature inversion. Process analysis helps quantify the contribution of various physical processes to the buildup and breakup of the inversion. The inversion developed due primarily to a rapid warming above the valley by synoptic subsidence and warm advection and to strong radiative cooling within the valley under clear sky conditions. The inversion was lifted when strong southerly winds entered the valley from the gap mixing out the cold air in the valley. Synoptic-scale cold advection above the valley contributed to the inversion removal by weakening the inversion from aloft. Sensitivity experiments suggested that the WRF simulation of the CAP episode is more sensitive to large-scale initialization fields with North American Mesoscale Model outperforming North American Regional Reanalysis and National Centers for Environmental Prediction (NCEP) Eta analysis than to the choice of boundary layer parameterizations and land surface models.

Evaluation of Surface Fluxes in the WRF Model: Case Study for Farmland in Rolling Terrain

“…The statistics showed that in comparison with NARR cases, WRF runs using the NAM input forcing data with a finer spatial resolution (12 km) had better agreement with WS10 observations. This is consistent with the result from Lu and Zhong [8] when modeling a persistent cold air pool event in Utah.…”

“…The model was initialized at 0000 UTC 21 September 2014, where the first 32 h were considered as the model spin-up period and removed from subsequent analyses. Forty-five vertical terrain-following levels, extending to 100 hPa, were employed in all domains with increased Atmosphere 2017, 8,197 4 of 23 resolution in the lowest part of the boundary layer (i.e., nine layers below 1 km). The middle level of the lowest model layer height was 10.5 m above ground.…”

“…Atmosphere 2017, 8,197 4 of 23 spin-up period and removed from subsequent analyses. Forty-five vertical terrain-following levels, extending to 100 hPa, were employed in all domains with increased resolution in the lowest part of the boundary layer (i.e., nine layers below 1 km).…”

“…Atmosphere 2017, 8,197 11 of 23 variation and magnitude, indicating that land-use datasets had a negligible influence on the simulation of the surface layer shear stress at the flux tower site. The temporal variations of sensible heat fluxes were similar in all WRF scenarios and the observational dataset (Figure 3), except that on average the value simulated by WRF was up to two times larger than the observations (NARR-YSU-M).…”

Simulations of a cold-air pool associated with elevated wintertime ozone in the Uintah Basin, Utah

“…Numerical studies have examined the lifecycle of CAPs for a variety of idealized (Zängl, 2005a;Katurji and Zhong, 2012; and actual topographic basins (Whiteman et al, 2001;Clements et al, 2003;Zängl, 2005b;Billings et al, 2006;Reeves and Stensrud, 2009;Reeves et al, 2011;Lareau et al, 2013;Lareau and Horel, 2014;Lu and Zhong, 2014). However, relatively few studies have examined the impact of snow cover, clouds, and cloud microphysics on CAP formation and evolution.…”