Modeling Water Vapor in the Upper Troposphere and Lower Stratosphere

Abstract

Upper troposphere and lower stratosphere (UTLS) water vapor is investigated using a general circulation model, the Community Atmosphere Model 3.0 (CAM3.0). Seasonal variability in UTLS water vapor, temperature and zonal wind, based on model simulation results for the period 1991 - 2000, are analyzed. Results are validated against satellite data from the Halogen Occultation Experiment (HALOE) and ERA-40 reanalyzes from ECMWF. The model captures the seasonal cycle in temperature as well as water vapor. The zonal wind deviates from the reanalysis data in the tropics as the model is not able to reproduce the Quasi-Biennial Oscillation (QBO). Outside the tropics, the zonal wind corresponds very well with the ERA-40 zonal wind. The model is able to reproduce the seasonal signal in the tropical stratospheric water vapor; i.e., the Tape Recorder Signal. This indicates a realistic Brewer-Dobson circulation in the model. However, the Tape Recorder signal attenuates too fast compared to the HALOE data, suggesting a too strong horizontal mixing between the tropical stratosphere and mid latitudes. CAM3.0 shows considerable improvements in UTLS temperatures as well as water vapor compared to earlier generations of the NCAR general circulation models.

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