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

Abstract

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 ϳ1 W m Ϫ2 .

Variability of subtropical upper tropospheric humidity

“…Fig. 2 of Gettelman et al, 2006). The analysis performed extends that of Waugh (2005) by considering a longer data record, variability in the middle and upper troposphere, and the impact of subtropical anticyclones and the MJO as well as Rossby wave breaking.…”

“…Rind et al, 1993;Soden and Bretherton, 1993;Bates et al, 1996;Read et al, 2001;Gettelman et al, 2006). These measurements have also enabled examination of the processes causing variability in the water vapor distribution.…”

“…Also, the distribution and synoptic variability in upper tropospheric humidity in the summer subtropics has been linked to the deep convection and transport associated with summer monsoons (e.g. Randel and Park, 2006;Gettelman et al, 2006). Another process linked to variability in the subtropical humidity field is Rossby wave breaking along the tropopause (Waugh, 2005).…”

“…This dataset enables spatial and temporal variations in UTH to be examined (e.g. Gettelman et al, 2006). The majority of the analysis examines the northern subtropics during winter, but we also briefly consider the southern subtropics in winter.…”

Tropical upper tropospheric humidity variations due to potential vorticity intrusions

“…However, this is one of the least well-measured parameters and significant uncertainties in the measurements remain a barrier to making reliable prediction about future climate change. In the past, satellite measurements, namely, the Stratospheric Aerosol and Gas Experiment, the Television Infrared Observation Satellite (TIROS), operational vertical sounder, 6.7 µm brightness temperature from a single Geostationary Operational Environmental Satellite, Microwave Limb Sounder, and Atmospheric Infrared Sounder (AIRS) have contributed significantly to the retrieval of water vapour in the upper troposphere from the radiances and also to the understanding of its variabilities (Rind et al, 1993;Soden and Bretherton, 1993;Soden and Fu, 1995;Liao and Rind, 1997;Gettelman et al, 2006). These humidity observations from satellites are important data sources for humidity assimilation in global data assimilation systems (Chen et al, 1999).…”