Boundary layer ventilation by mid-latitude cyclones
Ventilation of the boundary layer has an important effect on both local and
regional air quality and is a prerequisite for intercontinental pollution
transport. If pollutants remain within the boundary layer they can easily be returned to the surface by dry deposition and are more likely to remain in the local area. However, once pollutants are ventilated out of the boundary layer and into the free troposphere they can be lifted to the mid to upper troposphere and can be transported hundreds of kilometres away from their original source.
This project aims to understand which physical processes are important for ventilating pollutants out of the boundary layer, and to identify dynamical parameters that control the amount of ventilation. These questions are being addressed using an idealised modelling approach. We have simulated baroclinic life-cycles in idealised GCM. To reduce the degree of complexity of a real mid-latitude cyclone while still retaining the essential ingredients the only parameterisation scheme we have included is a boundary layer scheme. To represent pollutants we have included a passive tracer, emitted at the surface, which is acted upon by the resolved wind and the boundary layer turbulence. Results so far have shown that both boundary layer processes (turbulence, Ekman motions) and large scale processes (advection by conveyor belts) are equally important for boundary layer ventilation. Current work is focusing on identifying boundary layer and large scale variables that controls the amount of ventilation.