Tropical Dendritic Cumulus: An Observational Analysis

Since the launch of the TIROS Satellite, many mesoscale convective structures have been studied. One such structure, shallow cumulus arranged in a dendritic pattern, has however been overlooked despite its frequent occurrence over tropical oceans. The existence of dendritic arrangements of shallow cumulus clouds provides a new example of mesoscale structure in the lower troposphere. A hypothesis is presented for how self-organization via modification of the background flow by secondary circulations can cause shallow convective clouds form into large, complex structures ranging from a hundred kilometers to a few thousand kilometers across. Analysis of a two and a half years sample of Moderate-resolution Imaging Spectroradiometer (MODIS) imagery and the corresponding NCEP/NCAR reanalysis data has shown that dendritic cumulus clouds are a common occurrence over the tropical oceans. Quantitative results indicate that they occur during cold air advection with light to moderate wind speeds and wind shear in the 850 mb to surface layer. Thus, they appear to represent a transitional stage between random, cellular cumulus convection and cloud streets.