An Objective Analysis of Tropical Cloud Cluster Motion Associated with Super Cloud Clusters

The nature and propagation characteristics of Super Cloud Clusters (SCCs), also known as Convectively Coupled Kelvin Waves, have been studied extensively. A systematic result has been the westward propagation of short-lived cloud clusters (CCs) within the SCC due to an easterly low-level flow. Some cases of eastward-moving CCs have also been reported, typically embedded in a slow-moving, large-scale MJO envelope. A careful visualization of sequences of precipitation and OLR fields in the tropical region of aquaplanet-like simulations yield several recognizable patterns of propagation that cannot be restricted to the generally accepted idea of westward-moving CCs within eastward-moving SCCs. These features are better discernible in longitude-time diagrams, with clear “S-shaped” signals in OLR and precipitation: the westward-moving CCs are driven by easterlies ahead of the SCC axis, but they are stationary when they develop nearly collocated with it, while behind the SCC axis the CC motion is typically eastward due to the associated westerly wind bursts. Since the eastward-propagating CCs are typically weaker precipitation events, they might be not clearly depicted, masked by the strongest and deepest convection area within the SCC. On the other hand, a reduced OLR or precipitation threshold will likely highlight the eastward-moving SCC rather than individual westward-moving CCs. This work studies the tropical CC propagation characteristics in association with SCCs. An algorithm is especially designed for the identification and tracking of CCs, also called “time-clusters” in time-longitude coordinates. In successive steps, the OLR or precipitation thresholds are changed to accurately assess CC motion in both regions of enhanced and relatively suppressed convection. Then, all detected CCs as well as their tracks are combined for a systematic analysis, in which results are categorized according to variables such as: location and width of tropical band, CC position relative to the SCC axis, stage of the SCC life cycle, vertical profile of zonal wind, CC size, CC lifespan, travelled distance, etc. The objective of this work is therefore to present an objective and robust analysis of CC propagation, especially focusing on the influence of easterlies versus westerlies and their correlation with the magnitude of the CC phase speed. This work will help elucidate aspects of the SCC-CC interaction over the tropical ocean.