An important constraint on tropical cloud ‐ climate feedback

Tropical convective anvil clouds detrain preferentially near 200 hPa. It is argued here that this occurs because clear‐sky radiative cooling decreases rapidly near 200 hPa. This rapid decline of clear‐sky longwave cooling occurs because radiative emission from water vapor becomes inefficient above 200 hPa. The emission from water vapor becomes less important than the emission from CO 2 because the saturation vapor pressure is so very low at the temperatures above 200 hPa. This suggests that the temperature at the detrainment level, and consequently the emission temperature of tropical anvil clouds, will remain constant during climate change. This constraint has very important implications for the potential role of tropical convective clouds in climate feedback, since it means that the emission temperatures of tropical anvil clouds and upper tropospheric water vapor are essentially independent of the surface temperature, so long as the tropopause is colder than the temperature where emission from water vapor becomes relatively small.