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

Two techniques are presented 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 the orientation can be extracted. The second technique identifies individual coherent structures and decomposes their vertical transport by the shape, size, and orientation of these objects. The bulk-correlation technique is shown to capture the elongation and orientation of coherence by ambient wind, but is unable to characterise individual coherent structures. Using the object-based approach, it is found that the individual structures dominating the vertical flux are plume-like in character (extending from the surface into cloud) rather than thermal-like, 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.

Automated summary

This study analyzes the shallow cumulus clouds using two techniques to characterize cloud-feeding coherent boundary-layer structures. The first technique extracts the correlation length-scale and orientation of the structures, while the second technique decomposes the vertical transport of individual coherent structures. The results show that the bulk-correlation technique captures the elongation and orientation of coherence by ambient wind, but cannot characterize individual structures. The object-based approach reveals that the dominating structures have plume-like characteristics and rise vertically in the absence of ambient wind.