Examining the Connections between Observed Atmospheric Gravity Waves and Convective Clouds for Improved Climate Simulations

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Waves in which buoyancy is the restoring force are referred to as gravity waves, and such waves are ubiquitous in the atmosphere. They occur over a broad range of spatial scales but are generally smaller than the frontal weather systems seen on weather maps. They can be generated by a variety of mechanisms including air flow over mountains, the formation of frontal systems, and the vertical motions that accompany convection. Despite their relatively small size, they are thought to play an important role in the atmospheric circulation due to their vertical flux of horizontal momentum, through which they drive the quasi-biennial oscillation in the equatorial stratosphere, modulate the strength of the midlatitude jet streams, and alter the strength of the stratospheric polar vortices, thereby playing a role in the seasonal evolution of the Southern Hemisphere ozone hole. But the small size of the waves and their relatively rapid propagation make it difficult to observe them and examine their generation, propagation, and impacts on the mean flow. Moreover, the waves cannot generally be simulated by global weather and climate models due to their small size, and instead they must represented through parameterizations which estimate their aggregate effects as a function of the resolved flow. While these parameterizations have become quite sophisticated, their validity is difficult to establish, and common circulation biases in models are often ascribed to inadequacies of the gravity wave parameterizations.Work under this award specifically addresses gravity waves generated by convection, motivated by recent observations from balloons and satellites suggesting that that large amplitude gravity waves, of the sort that come from vigorous, small-scale deep convection, account for a larger fraction of the gravity wave momentum flux than previously assumed. Further motivation comes from the availability of the record of tropical convection from the Tropical Rainfall Measurement Mission (TRMM), a satellite record which is now over a dozen years long and can be used to estimate the generation of gravity waves by tropical and subtropical convection. The data is used in conjunction with the gravity wave parameterization from the Community Atmosphere Model, along with observational wind and atmospheric stability data, to estimate the generation, propagation, and momentum flux of convectively generated gravity wave activity. The momentum flux is then compared with estimates calculated from the satellite record of atmospheric temperature from the satellite record of the Atmospheric Infrared Sounder (AIRS). The goal of this effort is to reconcile the large-scale patterns in gravity wave activity observed in the tropics and subtropics with existing knowledge of their convective sources, based on a theoretical understanding of their propagation and mean flow interaction. The reconciliation is expected to require some ad hoc adjustment of free parameters in the parameterization scheme, and the PIs will conduct further experiments to determine how this "tuning" of the parameterization affects the large-scale atmospheric simulation produced by the model. Some work will also consider the generation of gravity waves by convection occurring in the storm tracks of the middle latitudes, using other data sources to estimate convection.The work has broader impacts due to the need for accurate gravity wave parameterizations in weather and climate models. These models are widely used as research tools for a variety of applications, and are also used to provide information to first responders, decision makers, and the general public. In addition, the project supports and trains a graduate student, thereby providing for the future workforce in this research area.