The annual cycle of stratospheric water vapour in a general circulation model

Abstract. The application of general circulation models (GCMs) to stratospheric hemistry and transport both permits and requires a thorough investigation of stratospheric water vapor. The National Center for Atmospheric Research as redesigned its GCM, the Community Climate Model (CCM2), to enable studies of the chemistry and transport of tracers including water vapor; the importance of water vapor to the climate and chemistry of the stratosphere quires that it be better understood in the atmosphere and well represented in the model. In this study, methane is carried as a tracer and convened to water; this simple chemistry provides an adequate representation of the upper stratospheric water vapor source. The cold temperature bias in the winter polar strato-sphere, which the CCM2 shares with other GCMs, produces excessive dehydration in the southern hemisphere, but this dry bias can be ameliorated by setting a minimum vapor pressure. The CCM2's water vapor distribution and seasonality compare favorably with observations in many re-spects, though seasonal variations including the upper stratospheric semiannual oscillation are generally too small. Southern polar dehydration affects midlatitude water vapor mixing ratios by a few tenths of a part per million, mostly after the demise of the vortex. The annual cycle of water vapor in the tropical and nonhem midlatitude lower stratosphere isdominated by drying at the trop-ical tropopause. Water vapor has a longer adjustment time than methane and had not reached equi-librium at the end of the 9 years simulated here. 1