Climatology of Upper-Tropospheric Relative Humidity from the Atmospheric Infrared Sounder and Implications for Climate

Recently available satellite observations from the Atmospheric Infrared Sounder (AIRS) are used to calculate relative humidity in the troposphere. The observations illustrate many scales of variability in the atmosphere from the seasonal overturning Hadley–Walker circulation to high-frequency transient variability associated with baroclinic storms with high vertical resolution. The Asian monsoon circulation has a strong impact on upper-tropospheric humidity, with large humidity gradients to the west of the monsoon. The vertical structure of humidity is generally bimodal, with high humidity in the upper and lower troposphere, and a dry middle troposphere. The highest variances in humidity are seen around the midlatitude tropopause. AIRS data are compared to a simulation from a state-of-the-art climate model. The model does a good job of reproducing the mean humidity distribution but is slightly moister than the observations in the middle and upper troposphere. The model has difficultly reproducing many scales of observed variability, particularly in the Tropics. Differences in humidity imply global differences in the top of atmosphere fluxes of ~1 W m-2.