An Evaluation of Gravity Waves and Gravity Wave Sources in the Southern Hemisphere in a 7-Km Global Climate Simulation

Abstract

In this study, gravity waves in the high-resolution GEOS-5 Nature Run are first evaluated with respect to satellite and other model results. Southern Hemisphere winter sources of nonorographic gravity waves in the model are then investigated by linking measures of tropospheric nonorographic gravity wave generation tied to precipitation and frontogenesis with absolute gravity wave momentum flux in the lower stratosphere. Finally, nonorographic gravity wave momentum flux is compared to orographic gravity wave momentum flux and compared to previous estimates. The results show that the global patterns in gravity wave amplitude, horizontal wavelength, and propagation direction are realistic compared to observations. However, like other global models the amplitudes are weaker and horizontal wavelengths longer than observed. The global patterns in absolute gravity wave momentum flux also agree well with previous model and observational estimates. The evaluation of model nonorographic gravity wave sources in the Southern Hemisphere winter shows that strong intermittent precipitation (greater than 10 mm per hr) is associated with gravity wave momentum flux over the South Pacific, whereas frontogenesis and less intermittent, lower precipitation rates (less than 10 mm per hr) are associated with gravity wave momentum flux near 60 degrees South. In the model, orographic gravity waves contribute almost exclusively to a peak in zonal mean momentum flux between 70 and 75 degrees South, while nonorographic waves dominate at 60 degrees South, and nonorographic gravity waves contribute a third to a peak in zonal mean momentum flux between 25 and 30 degrees South