Characterising the shape, size and orientation of cloud-feeding coherent boundary layer structures

TL;DR

This paper introduces two novel techniques for analyzing the shape, size, and orientation of boundary layer structures feeding clouds, using large-eddy simulations to understand their dynamics under different shear conditions.

Contribution

It develops generalized correlation and decomposition methods to characterize coherent boundary layer structures, revealing their shape, size, and orientation, and quantifies effects of ambient shear.

Findings

Structures are plume-like and rise vertically without shear.

Ambient shear causes planar stretching and tilting of structures.

Techniques are applicable for future boundary layer pattern studies.

Abstract

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 decomposes the vertical transport by coherent structures by the shape, size and orientation of these structures. It is found that the structures dominating the vertical flux are plume-like in character (extending from the surface into cloud), show small width/thickness asymmetry and rise near-vertically in the absence of ambient wind. The planar stretching and tilting 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.