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) 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.