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.
Highlights
Water vapor is a key tracer in the upper troposphere and lower stratosphere (UTLS) region
From the H2Omix observations given in Figs. 5 and 6, we find that tropical tropopause water vapor starts to decrease in December with its lowest values occurring in January and February located slightly to the north of equator
When the ERA-40 dataset is analyzed we find four westerly quasi biennal oscillation (QBO) periods for 1991 - 2000
Summary
Water vapor is a key tracer in the upper troposphere and lower stratosphere (UTLS) region. Due to very weak horizontal mixing in the tropical lower stratosphere the seasonal signal in water vapor is preserved following the upward and poleward propagation of the Brewer-Dobson circulation (Brewer 1949). The tape recorder signal is suitable for transport analyzes in observational data (Clark et al 2001; Randel et al 2001), and for transport validation of general circulation models, GCMs. The quasi biennal oscillation (QBO) as well as the El Niño/Southern Oscillation (ENSO) are important factors for the interannual variability of stratospheric water vapor. Since the tropical tropopause temperature is one of the important factors controlling the amount of water vapor that enters the stratosphere, these cold biases influence stratospheric water vapor significantly. This in turn affects the heating rates in the models. Controlling factors for water vapor like temperature, zonal wind and stratospheric transport are validated against observational data and reanalyses
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