Cloud microphysics, dynamics, and free tropospheric water vapor

The probability distribution of local relative humidity (RH) in the free troposphere is explored by comparing a simple theoretical calculation with observations from the Global Positioning System (GPS) and the Microwave Limb Sounder (MLS). The calculation is based on a parcel of air that conserves its composition while warming, until it is resaturated by randomly entering a convective system. This leads to a probability density for RH proportional to RH^r-1, where r is the ratio of time scales associated with subsidence drying and random moistening. The observations support this prediction remarkably well from 600 hPa to 200 hPa. Thus, humidity is likely set by quasi-conservative processes.

We also explore the sensitivity of relative humidity to cloud microphysics and dynamics using a simple 2-D humidity model and various configurations of the NCAR CAM3 General Circulation Model (GCM). The GCM is run in one standard and two aquaplanet configurations. In the last of these, we impose surface temperatures that vary weakly with latitude and stipulate that clouds not interact with radiation. This eliminates the Hadley and Walker circulations and almost eliminates the main westerly jet, creating instead a homogeneous ``boiling kettle'' world in low- and midlatitudes and an enhanced polar cell/jet system. We find that the convective organization that characterizes Earth's atmosphere buffers relative humidity against large microphysically-induced changes that could occur otherwise, and conclude that the crude representation of cloud physics in current models is unlikely to cause their water vapor feedback to depart from reality by more a few percent. Dynamical uncertainties may pose a larger problem, however.