The effect of ambient shear on coherent boundary layer structures

This paper presents two techniques for characterisation of cloud-feeding coherent boundary layer structures through analysis of large-eddy simulations of shallow cumulus clouds, contrasting conditions with and without ambient shear. The first technique is a generalisation of the two-point correlation function where the correlation length-scale as well as orientation can be extracted. The second technique consists of decomposing the boundary layer air into individual coherent structures and thereafter characterising each object by size, orientation and moisture flux carried. Using these techniques it is found that the majority of vertical moisture flux is carried by plume-like structures with volume scaling with the height of the boundary layer. This plume-like structure is in apparent contrast with the assumptions of some modelling systems, that fluxes are carried by thermals, and therefore the conclusions are significant to parametrisation development for weather and climate models. The elongation and orientation of boundary layer structures caused by the introduction of ambient shear is also quantified, demonstrating the general applicability of the techniques for future study of other boundary layer patterns.