Modulation of Tropical Cloud Clusters’ Zonal Propagation By Super Cloud Clusters

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, O(100km)] embedded in the eastward-moving SCCs [O(1000km)]. A careful visualization of sequences of precipitation and OLR fields in the tropical region yields several recognizable patterns of propagation that cannot be reconciled with the generally accepted idea of negative zonal phase speeds for all CCs. These features are better discernible in longitude-time diagrams: CCs travel westward relatively fast ahead of the SCC axis. However these clusters slow down and sometimes become stationary when they develop near the center of the SCC, and by its rear section their motion can even become eastward. Evidence of this behavior has been identified in a wide range of idealized simulations (permanent-equinox, fixed SSTs, aquaplanet-like configurations) as well as in observations using TRMM data. In this work, the underlying dynamics of this velocity modulation in CC propagation is explored, and two major factors are identified: these are the magnitudes of both the background mid-tropospheric flow and the zonal wind perturbations associated with the SCCs. When both contributions are strong, the westerlies can locally counteract and even dominate over easterlies, producing “S-shapes” of precipitation on the x-t space. An algorithm is especially designed for the identification and tracking of CCs providing a quantitative analysis of tropical CC motion. Finally a conceptual model is proposed.