Numerical modeling study of boundary‐layer ventilation by a cold front over Europe

Ventilation of the boundary layer has an important effect on regional air quality and the long‐range transport of pollutants and their chemistry, as well as climate change. It is very difficult to quantify the ventilation of the polluted boundary‐layer air in field experiments because sources are not well known and there are many mechanisms that can contribute to the ventilation. In frontal cyclones ventilation can occur by advection by the large‐scale flow, slantwise and upright convection, and turbulent mixing. In order to quantify and partition the different mechanisms, numerical modeling experiments have been performed for a cold front that passed over Europe during the EXPORT (European Export of Precursors and Ozone by long‐Range Transport) field experiment. The numerical experiments have been performed by initializing the Met Office Unified Model with passive tracers with two different lifetimes which can be used to mimic a wide range of pollutants and estimate the age of the ventilated air. The spatial distribution of the estimated age of air is consistent with Lagrangian trajectory results and observations. Budget calculations over the model domain show that 33% of the longer‐lived tracer is exported from the boundary layer to the free troposphere by advection only and 68% by advection, convection and mixing. Therefore much younger air is found in and above the warm conveyor belt than would be found if advection was the only transport mechanism. The results also emphasize the importance of convection and mixing acting together, particularly for short‐lived tracers.