Synoptic-Scale Flow and Valley Cold Pool Evolution in the Western United States

Abstract

Valley cold pools (VCPs), which are trapped, cold layers of air at the bottoms of basins or valleys, pose a significant problem for forecasters because they can lead to several forms of difficult-to-forecast and hazardous weather such as fog, freezing rain, or poor air quality. Numerical models have historically failed to routinely provide accurate guidance on the formation and demise of VCPs, making the forecast problem more challenging. In some case studies of persistent wintertime VCPs, there is a connection between the movement of upper-level waves and the timing of VCP formation and decay. Herein, a 3-yr climatology of persistent wintertime VCPs for five valleys and basins in the western United States is performed to see how often VCP formation and decay coincides with synoptic-scale (;200-2000 km) wave motions. Valley cold pools are found to form most frequently as an upper-level ridge approaches the western United States and in response to strong midlevel warming. The VCPs usually last as long as the ridge is over the area and usually only end when a trough, and its associated midlevel cooling, move over the western United States. In fact, VCP strength appears to be almost entirely dictated by midlevel temperature changes, which suggests large-scale forcing is dominant for this type of VCP most of the time.

Understanding high wintertime ozone pollution events in an oil- and natural gas-producing region of the western US

“…Cold-pool-type inversion layers, occurring within basins and valleys in the western US, have been studied extensively (Banta and Cotton, 1981;Clements et al, 2003;Lareau et al, 2013). Such valley cold pools (VCP) are identified when a radiosonde indicates the presence of an inversion below the maximum crest height of the surrounding mountains, and average wind speeds beneath the inversion top that are less than 5 m s −1 (Reeves and Stensrud, 2009). The VCPs form as an upper-level ridge approaches the western US and in response to strong mid-level warming, and they can exist several days until being removed, usually by advection or turbulent erosion processes (Reeves and Stensrud, 2009).…”

Sensitivity of local air quality to the interplay between small- and large-scale circulations: a large-eddy simulation study

“…This can lead to a transport of pollutants from high emission areas onwards to inhabited areas (Gariazzo et al, 2007) or a closed recirculation and hence accumulation of pollutants (Crosman and Horel, 2016;Lo et al, 2006;Rimetz-Planchon et al, 2008). Most of these works have in common that the background circulation reduces the relevance of the local effects (Crosman and Horel, 2010).…”

“…The most prominent examples of such local stagnation are typically observed in stably stratified cold air pools trapped in valleys and other orographic depressions (e.g. Reeves and Stensrud, 2009;Hoch et al, 2011;Sheridan et al, 2014;Hughes et al, 2015).…”

Dynamically Induced Displacements of a Persistent Cold-Air Pool

“…The strength and longevity of persistent CAPs are modulated by the existing synoptic conditions, the surface energy budget, and subsequent interactions with advecting weather systems (Wolyn and McKee 1989;Whiteman et al 1999Whiteman et al , 2001Reeves and Stensrud 2009;Gillies et al 2010;Zardi and Whiteman 2013). CAPs most often form during the warming aloft accompanying the arrival of anticyclones.…”

“…CAPs most often form during the warming aloft accompanying the arrival of anticyclones. Weak disturbances may then temporarily perturb a CAP, whereas more vigorous baroclinic troughs are likely to completely destroy them, especially those accompanied by strong cold-air advection (Whiteman et al 1999Zhong et al 2001;Reeves and Stensrud 2009;Zardi and Whiteman 2013).…”