Diurnal Cycle of Tropical Deep Convection and Anvil Clouds: Global Distribution Using 6 years of TRMM radar and IR data

Before the launch of the TRMM satellite in late 1997, most studies of the diurnal cycle of tropical convective clouds used IR data only. A typical metric would be the area covered by clouds whose outgoing long wave radiation (or IR brightness temperature) was less than some specified value. The area covered by cold cloud often was used as a proxy for coverage of deep or intense convection, or for rainfall. The association is a logical one, because high cold clouds are usually anvil clouds produced directly by deep precipitation convective clouds.

Using the Precipitation Radar (PR) and the Visible and Infrared Scanner (VIRS) on the TRMM satellite, it is possible now to view the high cold anvil clouds, and at the same time the structure of the precipitating convection, and do so globally. This study combines the PR and VIRS measurements, focusing on the diurnal cycle of anvil clouds identified by VIRS, the diurnal cycle of deep convection observed by the PR, their similarities and their differences.

After matching 7 years TRMM PR and VIRS measurements within the PR swath, the Precipitation Features (PFs) and Cold Cloud Features (CCFs) are identified independently. The former are defined by contiguous PR raining areas; the latter by contiguous areas of cold VIRS 11 micrometer brightness temperature (colder than 210 K and 235 K). The global distribution of areas of deep convection and areas of anvil cloud are related, but there are important regional differences shown. The composite life cycle of tropical deep convection is demonstrated by showing the time lag between the maximum phase of the diurnal cycle, differently defined. First we observe a peak in the population of PFs, dominated by small showers. Next in quick succession comes the peak in area of PFs reaching 14km, reaching 10 km, then CCFs area < 210 K and finally CCFs area < 235 K . The cycles over land and ocean are shown separately.