On moistening of the tropical troposphere by cirrus clouds

Sublimation of ice crystals in cirrus clouds is often invoked as a moistening mechanism for the free troposphere, particularly in the tropics. An alternative mechanism is investigated here: "pumping" of water vapor into the cloudy atmospheric column from surrounding regions by diabatic transports resulting from the local radiative effects of the cloud. The effectiveness of this mechanism is computed for a variety of cirrus cloud types, using a simple model. For nonprecipitating cirrus clouds it is found that the pumping mechanism can import much more vapor mass into the cloudy column than the clouds contain as ice, especially as the clouds become optically thin. The advected vapor per unit ice mass (or "pumping ratio") also varies with cloud height, is proportional to cloud lifetime, and (for thin clouds) is approximately inversely proportional to effective particle radius. Thin cirrus above 300 hPa can pump vapor rapidly enough to sustain itself against dessication. The overall results are supported by the limited information available from observations and indicate that at sufficient distances from active convection, direct moistening of the environment by the sublimation of ice has significantly less impact on vapor distributions than cloud-radiative-dynamic effects.