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.
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