High humidities and subvisible cirrus near the tropical tropopause

Measurements of water vapor from the Microwave Limb Sounder on the Upper Atmospheric Research Satellite show that humidities in the extreme tropical upper troposphere are often near saturation with respect to ice. These high humidity values are supported by the available accurate in situ water vapor measurements near the tropical tropopause. A nearly saturated upper troposphere reduces the outgoing longwave radiative flux at the top of the atmosphere by about 1-2 W-m(-2) compared to the radiative flux calculated using a standard atmospheric humidity profile. Subvisible cirrus near the tropopause reduce the outgoing longwave radiation an additional few W-m(-2) as well as increasing the radiative heating near the tropopause. Changes in the humidity frequency distribution in this region of the atmosphere and the associated changes in subvisible cirrus frequency would substantially impact the radiative budget of the tropics.