Analysis of Multiple Stable Layers in Cold-Air Pools

Cold-air pools in the Salt Lake Valley during the 2010-2011 winter are examined in terms of high vertical resolution GPS soundings available twice daily from the Salt Lake City airport as well as over 100 research soundings taken during the Persistent Cold-Air Pool Study (PCAPS). An objective technique is used to identify the occurrence of multiple stable layers within those cold-air pools as well as others present during the winters from 1989-2012. This objective technique relies on Rising Motion Inhibition (RMIN), a variant of Convective INhibition (CIN), to assess the theoretical amount of work required to overcome the negative buoyancy of parcels lifted a fixed distance. The base (top) of stable layers are evident in terms of minima (maxima) in RMIN.

The conditions leading to multiple stable layers are well recognized but their frequency of occurrence and characteristics have rarely been documented. Multiple stable layers are common during the PCAPS period as well as during other winters as a result of temporally evolving mesoscale conditions (e.g., descending stable layers associated with rapid warming above the basin's crest) combined with turbulent and radiative processes below crest level. Operational numerical forecasts fail to capture the occurrence of these multiple stable layers in most instances and hence miss the vertical structures that control vertical mixing of pollutants within the basin.