Persistent Cold-Air Pools in Mountainous Areas: Distribution and Simulation

Persistent cold air pools (PCAPs) are conditions when stagnant air are trapped within the valley for more than one diurnal day. PCAPs are usually associated with limited surface radiation, large-scale subsidence, or cold front passages. Due to the stable meteorological condition during PCAPs, air pollutants tend to accumulate within the valley, which impacts human health. PCAPs can be identified using the method of valley heat deficit (VHD) threshold that is calculated based on vertical profiles of temperature and wind speed. We propose a revised method of PCAP classification by appending meteorological data from surface weather stations to radiosonde profiles to have more complete vertical profiles above the valley. The spatial and temporal distribution of PCAPs in the western U.S. are given based on this revised method. We also evaluate the numerical model performances during PCAPs. Simulation results from Weather Research and Forecasting (WRF) model and estimated VHD are compared with observations during PCAPs. Results in our case study show that the WRF model underestimates VHD during PCAPs. We also investigate the Community Multiscale Air Quality Modeling System (CMAQ) performance and find that the underestimation of PCAP strength (quantified by VHD) makes a major contribution to the underestimated air pollutant concentrations from CMAQ model. This work provides more accurate information regarding to PCAPs distribution and highlights the importance of meteorology model improvement during PCAPs.