Characterising the shape, size, and orientation of cloud‐feeding coherent boundary‐layer structures. (5th January 2022)

• Record Type: Journal Article
• Authors: Denby, Leif; Böing, Steven J.; Parker, Douglas J.; Ross, Andrew N.; Tobias, Steven M.
• Is Part Of: Quarterly journal of the Royal Meteorological Society. Volume 148:Number 742(2022)
• Page Start: 499
• Page End: 519
• Publication Date: 2022-01-05
• Subjects: coherent structures -- convective triggering -- methodology -- moist convection -- structure characterisation; Meteorology -- Periodicals; 551.5
• DOI: 10.1002/qj.4217
• Languages: English
• ISSNs: 0035-9009

Abstract

Abstract: 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. Abstract : Under the presence of ambient shear, coherent boundary‐layer structures that dominate vertical moisture transport are stretched planar to be more sheet‐like. This can be captured succinctly by computing the planarity and filamentarity of these coherent structures to measure how elongated ( pencil ‐like) or planar ( pancake ‐like) individual coherent structures are.