The Moist Boundary Layer under a Mid-latitude Weather System

Mid-latitude weather systems are key contributors to the transport of atmospheric water vapour, but less is known about the role of the boundary layer in this transport. We expand a conceptual model of dry boundary-layer structure under synoptic systems to include moist processes, using idealised simulations of cyclone waves to investigate the three-way interaction between the boundary layer, atmospheric moisture and large-scale dynamics. Forced by large-scale thermal advection, boundary-layer structures develop over large areas, analogous to the daytime convective boundary layer, the nocturnal stable boundary layer and transitional regimes between these extremes. A budgeting technique demonstrates the key role of boundary-layer processes in the transport of moisture. Moisture is evaporated from the ocean behind the cold front and in the high-pressure part of the wave, and transported large distances within the boundary layer into the footprint of the warm-conveyor belt. The warm-conveyor belt forms one of the two main processes of boundary-layer ventilation, with shallow cumulus convection being of similar importance.