Clustering, randomness, and regularity in cloud fields: 3. The nature and distribution of clusters

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Abstract
This study applies the K‐nearest‐neighbor (KNN) statistic as well as a morphological filtering approach for the identification and analysis of cloud clusters. These approaches first are evaluated by application to synthetically generated normally distributed clusters within a random background. They are found to provide conservative estimates of cluster size while accurately estimating the percentage of clouds in clusters. In Landsat scenes, typical clusters are found to contain four to five clouds and typically two to three clouds per cluster in advanced very high resolution radiometer (AVHRR) data. In the Landsat imagery the KNN analysis finds that about 15% to 50% of the total cloud field participates in clusters. These clusters are found to be concentrated in less than 2% of the total cloud field area. In AVHRR scenes the KNN method yields from 1.5% to 16% of the cloud field participating in clusters, concentrated within 1% of the total cloud field area. Mean cluster radii are typically 300 m to 600 m in the Landsat scenes and less than 3 km in the AVHRR images. Both the cloud cluster centers and the non‐participating background clouds are randomly distributed within the cloud field. Less than 10% of the cloud clusters form superclusters (i.e., clusters of clusters). When clouds are segregated into classes of uniform size, the clustering signal weakens with increasing cloud size. When the scene as a whole is divided into 30‐km subregions, the clustering signal is relatively homogeneous throughout the scene in spite of significant fluctuations in cloud number density within the various subregions. These results show that the physical mechanism underlying this clustering signal, although producing clusters on scales less than 3 km, is operative over the entire field. A nearest‐neighbor angle statistic is applied to search for regularity at the subkilometer scale. The angle statistic results are consistent with normally distributed point processes and inconsistent with any form of regularity at this scale. Presently, no method is presented which is effective at probing the cloud field structure at scales intermediate to 2 km and 30 km. Therefore cloud field regularity cannot be ruled out at these intermediate scales.

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