Enhanced vertical mixing associated with a nocturnal cold front passage and its impact on near-surface temperature and ozone concentration

A sudden rise in surface temperature is sometimes observed during the nighttime hours with the passage of cold fronts. The physics contributing to such nocturnal warming events and their potential impacts on atmospheric chemistry are not yet fully understood. In this study, a nocturnal warming event associated with a cold front passage in Oklahoma on 3 April 2006 is simulated with the Weather Research and Forecasting model with Chemistry (WRF/Chem). During the prefrontal period under clear-sky and calm conditions, surface radiative cooling resulted in a decoupled shallow surface layer in which air temperature and wind speed decreased quickly and ozone was removed efficiently by chemical reactions. During the passage of the cold front, strong wind shear enhanced turbulent mixing, which weakened the temperature inversion near the surface. Warmer and ozone-richer air from aloft was mixed downward to the surface. Thus, a sudden warming and nocturnal secondary ozone maxima were observed near the surface. Dry deposition of ozone at the surface was also enhanced in this warming event.