Synoptic controls on moist boundary layer structure

The structure of the atmospheric boundary layer is often thought of in terms either the diurnal cycle or its interaction with a changing surface. But synoptic scale meteorology also forces coherent boundary layer structure through large scale advection. Here we use idealised simulations of the development of a mid-latitude cyclone and anticyclone to determine the canonical structure of the mid-latitude moist boundary layer and the boundary layer clouds. The simulations show the characteristics of real weather systems, with strong frontal cloud along a warm conveyor belt, shallow convection behind the fold front and dry convective boundary layers in the anticyclone. A budgeting technique is used to calculate the pathways of moisture transport within the boundary layer, and reveal systematic patterns across the synoptic system: moisture is evaporated from the surface behind the cold front and in the anticyclone, it is then carried via Ekman transport into the footprint of the warm conveyor belt where it is ventilated into the troposphere. Scaling arguments are presented to demonstrate how the strength of the synoptic forcing, and the boundary layer processes control these pathways. The results indicate (i) how the boundary layer structure can be forced coherently across synoptic scales, and (ii) the subtle interplay between synoptic scales and boundary layer scales that determine the rate of ventilation of moisture from the boundary layer into the troposphere. These processes are important if we are to understand how the moisture cycle in mid- latitudes might change in a warming climate.