Dendritic Patterns in Tropical Cumulus: An Observational Analysis

Abstract

An observational analysis of the structure and synoptic setting of tropical dendritic cumulus formations was undertaken using 30 months of global data from the Moderate Resolution Imaging Spectroradiometer aboard the National Aeronautics and Space Administration Terra satellite, the Quick Scatterometer aboard the SeaWinds satellite, and the National Centers for Environmental Prediction global reanalysis. This analysis yielded 1216 cases of tropical dendritic cumulus formations of which 61 were randomly selected for quantitative study. From these sample cases, it was found that dendritic patterns in shallow cumulus occurred over warm tropical oceans in response to cold air advection. They typically dissipate downstream in regions of cooler water, neutral to warm advection, or deep convection. Moreover, shallow cumulus formations take on a dendritic pattern only in areas where the background wind velocity is between 1.5 and 13 m s?1 in the surface to the 850-mb layer and a shallow layer of conditional instability is present. Individual cumulus clouds in these dendritic formations are arranged in a compound, hierarchical branching pattern in which each element of the pattern takes the form of a Y-shaped cloud line. Analysis of the cloud pattern observations in conjunction with the scatterometer-derived surface winds and the lower-tropospheric wind profiles from reanalysis data reveals that the individual Y elements are aligned closely with the surface wind direction, as linear cloud streets would be. These Y elements are oriented so that their forked end aligns as closely as possible with the surface-to-850-mb shear vector, even when this conflicts with the surface wind direction. A formation mechanism is hypothesized by which the secondary circulation of a towering cumulus line modifies the shear and stability profiles in the adjacent areas to favor shallower cumulus lines oriented at an angle to itself, thus forming a hierarchical branching structure. This hypothesis is supported by stability profiles from the reanalysis data.