Orographic Gravity Waves and Their Diagnosed Effects on Transport in High-Resolution Models and Satellite Observations

Orographic gravity wave drag is a key process in the winter stratosphere that controls temperature and wind biases in global chemistry-climate models. Without orographic gravity wave drag, such models would have cold temperature and westerly wind biases that would result in major errors in the timing and strength of seasonal ozone depletion. Because gravity waves remain unresolved at today’s chemistry-climate model resolutions, parameterizations of orographic gravity wave drag are tuned to control these biases. The tuning has remained stubbornly difficult to constrain using observations. Global, high-resolution, and 3-dimensional observations are needed to constrain the gravity wave fluxes that drive the drag forces, while observing the drag itself is essentially impossible. Hope lies in utilizing very high-resolution models whose gravity waves are carefully constrained by multiple satellite sensors.

Our work utilizes very high-resolution forecast and assimilation models developed at the Global Modeling and Assimilation Office (GMAO) at NASA’s Goddard Space Flight Center, combined with NASA observations from three satellite instruments: High Resolution Dynamics Limb Sounder (HIRDLS), Microwave Limb Sounder (MLS), and Atmospheric Infrared Sounder (AIRS). Each sensor observes different components of a wave event at similar but different times. Waves in southern hemisphere winter are relatively easy to observe, and have been reported in many studies in the literature. Northern hemisphere waves are in contrast more difficult to observe due to the much higher variability in wind conditions and wave observability, but focusing on these events offers an opportunity to study wave-mean flow interactions in greater detail.

We describe a case study of a 10-day series of orographic waves above Norway. Both observations and high-resolution simulations display clear 3-day variability in wave momentum fluxes in this period. Our study disentangles wind effects on wave visibility from wave effects on the mean flow and derives new and tighter constraints on gravity wave drag in the stratosphere during these events. Previous work highlighted how stronger orographic gravity wave drag in global models leads to compensation in Rossby wave drag and little net change in zonal-mean drag or mean-meridional transport. However, the mixing effects of gravity waves and Rossby waves are quite different. Our results provide new insights for vertical and latitudinal mixing during orographic gravity wave breaking events.