A novel meteorological method to classify wintertime cold-air pool events

Abstract

Cold-air pools (CAPs) are common in mountain valleys throughout the world (e.g., western North America, Himalayas, Alps, etc.) during winter months. Weak surface winds, cold temperatures, high humidity, and snow cover presence are common characteristics of CAPs, and in populated areas there is an increase in air pollution concentrations. Previous methods for identifying CAP events and determining their strength often rely on a combination of, radiosonde data, air pollution concentrations, and/or surface meteorological datasets. Ambient air pollution concentrations vary by location based on the local emissions sources and continually change due to regulations and human activity patterns, therefore they are unreliable for consistent CAP quantification. Here, the bulk atmospheric stability is calculated as the valley heat deficit (VHD) using radiosonde data for 12 locations in the western U.S. over 16 winters. A new CAP classification method is developed and compared to three existing CAP classification methods. Results indicate that the new method agrees well with existing approaches but provides a more robust CAP classification because it is solely based on meteorology and not air quality. For all locations, 00Z (afternoon/early evening) radiosondes account for roughly 8–41% of all CAP occurrences (12Z and 00Z), independent of the method used. Meaning that the stable boundary layer persists throughout the daytime in these cases often leading to persistent CAP events (PCAP). While PCAP length varies across locations, they are a similar order of magnitude because synoptic conditions that span the entire western U.S. govern CAP onset and PCAP length. Additionally, several locations (e.g., Reno, Elko, Spokane, Riverton, and Grand Junction) release radiosondes from a site located at an elevation above the valley floor, often underestimating the bulk atmospheric stability by more than 30%. A method to incorporate surface station data in the bulk atmospheric stability calculation is given to reduce this underestimation.