Radiative impacts of clouds in the tropical tropopause layer

We quantify the seasonal and spatial variations of cloud radiative impacts in the

tropical tropopause layer (TTL) by using cloud retrievals from Cloud-Aerosol Lidar and

Infrared Pathfinder Satellite Observation (CALIPSO), CloudSat and International Satellite Cloud

Climatology Project (ISCCP). Over the convective regions including Western Pacific,

Africa, South America and South Asia, we find pronounced solar heating and infrared

cooling in the lower part of the TTL. The solar heating weakens above 16 km and

nearly diminishes at 18 km whereas the infrared cooling extends vertically throughout

the TTL. The net cloud radiative forcing, which is the summation of cloud solar and

infrared radiative forcing, has heating below ~16 km and turns to mostly cooling above

17 km. The net cloud radiative heating over the convective regions is mainly

contributed from solar radiation whereas the weak net cloud radiative heating

surrounding these regions is due to infrared heating.

To further examine the impacts of different cloud types in the TTL, we classified TTL

clouds in terms of cloud optical depths as thin cirrus (τ< 0.3), thick cirrus (0.3≤τ< 3)

and opaque clouds (τ≥3). In the solar part, thin and thick cirrus play a relatively small

role and the impact of cloud free air above clouds is negligible. The solar heating is

dominantly contributed from the solar absorption near the top of opaque clouds. In the

infrared part, the thick cirrus heating is mainly confined over the convective regions in

the lower part of TTL while the thin cirrus infrared heating is more prevalent both

vertically and horizontally in the TTL, which is the dominant infrared heating source.

The infrared cooling in cloud free air above clouds is dominant above 17 km whereas

the infrared cooling near the top of opaque clouds is dominant below. Despite the

infrared heating effects of thin and thick cirrus clouds, the infrared cooling from the

opaque cloud top and cloud free air above clouds outweighs the heating effects so

that the ensemble mean cloud infrared radiative forcing is mostly cooling except

outside the convective regions.