Influence of anthropogenic greenhouse gases on the propensity for nocturnal cold-air drainage

Cold-air drainage and pooling can have wide-ranging impacts, including affecting ecosystem processes and agricultural crops, and contributing to decreased air quality associated with temperature inversions. Future climate changes may alter both the frequency and intensity of cold-air drainage. This study estimates the response of nocturnal cold-air drainage to warming resulting from anthropogenic greenhouse gases, specifically C­ O2, considering radiative and thermodynamic effects but not changes in background air flow (dynamic effects). A simple index is proposed to represent the propensity for clear-sky nocturnal cold-air drainage as a function of air temperature and humidity near dusk. Decreases in this index with increasing atmospheric emissivity due to increasing anthropogenic greenhouse gas concentrations imply a weakening of cold-air drainage. The magnitude of the decrease in the index is positively related to the initial background temperature and humidity: Warm regions are more sensitive than cold regions, and humid regions are more sensitive than dry regions, implying that warm and/ or humid regions are more at risk of decreases in cold-air drainage. Under atmospheric ­CO2 concentrations consistent with Representative Concentration Pathway (RCP) 8.5, the magnitude of decrease in the index indicates that nocturnal cold-air drainage intensity may decline by at least 10% by 2100 CE (compared to 1979–1990) with larger decreases in warm and humid regimes. The index should be tested with intentionally designed field or lab experiments, and the relative effects on cold-air drainage of changes in radiative, sensible, and latent heat fluxes, and atmospheric circulation, should be compared.