The Clouds of the Middle Troposphere: Composition, Radiative Impact, and Global Distribution

The clouds of the middle troposphere span the temperature range where both ice and liquid water in a supercooled state can exist. However, because one phase tends to dominate, of the two midlevel cloud types, altostratus are deep ice-dominated, while altocumulus are shallow water-dominated, mixed-phase clouds with ice crystal virga typically trailing below. Multiple remote sensor examples of these cloud types are given to illustrate their main features, and the radiative consequences of the different cloud microphysical compositions are discussed. Spaceborne radar and lidar measurements using the CloudSat and CALIPSO satellites are analyzed to determine the global distributions of cloud frequencies and heights of these clouds. It is found that together these little-studied clouds cover ~25% of the Earth's surface, which is about one-third of the total cloud cover, and thus represent a significant contribution to the planet's energy balance.