Stratosphere-Troposphere Exchange Associated With Gravity Waves

The characteristics of an orographic gravity wave event on May 1 st,2008 and its impacts on stratosphere and troposphere exchange(STE) were examined, using NASA MERRA (Modern-Era Retrospective Analysis for Research and Applications) and European Centre for Medium-Range Weather Forecasts(ECMWF) Interim reanalysis data and Moderate Resolution Imaging Spectroradiometer(MODIS) data, together with the Weather Research and Forecasting model (WRF). On the basis of these studies, the annual/seasonal variation of gravity waves and its impacts on STE in UTLS(Upper Troposphere/Lower Stratosphere) over northern hemisphere mid-latitude regions, especially the Tibet plateau, were examined by the viewpoint of climatology, using NCEP CFSR(Climate Forecast System Reanalysis) reanalysis data together with stratospheric tracers(N2O, CO and CH4) derived from MLS(Microwave Limb Sounder) and HALOE(Halogen Occultation Experiment) observation data. The impacts of gravity waves on B-D circulation were also examined, through result analysis of WACCM3 (Whole Atmosphere Community Climate Model) simulation in which the strength of gravity wave source were changed.(1) Using WRF-ARW, together with MERRA reanalysis data and MODIS data, an orographic gravity wave and its impacts on STE were examined. The result demonstrates that: The horizontal wavelength is about 600 km throughout the troposphere and stratosphere, close to the terrain width, and the vertical wavelength is about 3 km in troposphere. The wave structure shows a westward tilt with height. Orographic wave propagate upward into the stratosphere and breaks near 150 hPa, caused by jet stream, leading to a strong attenuation of momentum flux and the release of energy into basic flows. Meanwhile, vertical turbulent mixing is extremely increased and turbulent exchange coefficient enhances by more than eight times during a short period (within 1 hour). Large turbulent mixing process causes air transports from the troposphere to the stratosphere, corresponding to the low value of ozone, potential vorticity and buoyancy frequency in the lower stratosphere.(2) Gravity wave drag derived from CFSR reanalysis data from 1979 to 2009 and stratospheric tracers(N2O, CO and CH4) derived from MLS and HALOE observation data were used to study the annual/seasonal variation of gravity waves and its impacts on STE in UTLS(Upper Troposphere/Lower Stratosphere) over northern hemisphere mid-latitude regions by the viewpoint of climatology. The result demonstrates that:The gravity waves hotspots in global UTLS area is distributed mainly over mountain terrains. The UTLS gravity wave activities over north hemisphere mid-latitude, especially the Tibet plateau, are strong in winter and weak in summer. The UTLS gravity wave activities and Troposphere-Stratosphere Transport(TST) have a good correlationship over Tibet plateau. Gravity waves enhanced the TST process, which is even more significant in DJF and SON.(3) Using simulation data derived from two WACCM3 experiments, the impacts of gravity waves on B-D circulation were also examined. The result demonstrates that:Increase in the gravity wave source tends to strengthen the down-ward residual velocity of B-D circulation over the Tibet plateau. However, TST caused by turbulent mixing during gravity wave breaking plays the main role in STE associated with gravity waves.