Radiative heating and upwelling by different types of TTL clouds over the tropical convective regions

Abstract

Tropical tropopause layer (TTL) clouds, defined as clouds whose tops are located above the tropopause level, play important roles in Earth's radiation balance and troposphere-to-stratosphere transport. Examining how different TTL cloud type gives influence on the radiative heating, we classified TTL clouds into three types (i.e., deep convective cloud, optically thick convective anvil cloud, and optically thin cirrus cloud) over the convectively active four tropical regions (i.e., Central Africa and South America continental regions and the Central and Western Pacific oceanic regions), using the combined Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) and CloudSat data. Using the classified cloud data, we first examined characteristics of the macrophysical and microphysical properties of the classified cloud types. Amongst three types of TTL clouds over selected four regions, thin cirrus clouds are most abundant (almost four-fifths), and approximately 7% and 17% of TTL clouds are deep convective cloud and thick convective anvil clouds, respectively. It is noted that the land–ocean contrast in TTL clouds appears weak in spite of well-known land–ocean contrast in the convection intensity, suggesting that TTL cirrus clouds may be dominated by in situ cirrus clouds. We further examined radiative heating over the TTL altitudes by each cloud type and the associated radiatively-driven upwelling motion. It is revealed that the dominant contribution to the overall radiative heating is from thin cirrus clouds (∼48.4%) because of their abundance. Nevertheless, convective anvil clouds and deep convective clouds are also found to give significant contribution to the radiative heating. Such cloud radiative heating tends to lower the level of zero net radiative heating rate found in the clear sky by ∼400–900 m. The TTL clouds play an important role in the secondary circulation in the TTL by inducing the upwelling motion throughout the TTL, supporting the mechanism that the radiative effect of TTL clouds can enhance the mass flux from the troposphere to the stratosphere.