Shallow Cloud Field Organisation has Four Interpretable Dimensions

Abstract:

Spatial patterns in mesoscale shallow trade wind cloud fields ("organisation") might play an important role in the shallow cloud feedback on climate. However, effectively encoding cloud patterns is not trivial. Therefore a large number of "organisation metrics" have been proposed. In this work, we synthesise these metrics to construct an effective description of organisation. We compute 21 frequently used or recently developed organisation metrics for 5000 satellite-observed shallow trade wind cloud fields. By projecting this dataset onto its principal components, we show that the 21 metrics primarily vary along only four, interpretable dimensions: 1) The dominant scale of cloud objects, 2) the size of voids between clouds, 3) the degree to which clouds align with a dominant direction and 4) cloud-top height variance across the cloud field. Linear combinations of these dimensions form an excellent description of organisation. The corresponding organisation distribution over our dataset is unimodal, continuous and does not possess distinct classes. We further explore if relating this description to cloud physical processes from LES and cloud-controlling variables from reanalysis reveals emergent properties of organisation.