Waves From Planetary to Gravity: Modeling and Observations in the Atmosphere

Marvin Geller was modeling stationary planetary waves, the dominant feature of the winter tropospheric and stratospheric circulation over three decades ago, discovering their forcing role and structure. At that time, when numerical general circulation and operational models were coarse, and computing resource was a fraction of what is available now, one needed to be clever to sift out answers to the questions of the day. For example, the importance of topographic versus thermal forcing that gives rise to stationary planetary wave structures in the atmosphere. The thread that runs through many of Marvin's studies is to ask and then answer the question of the day. For example, making the first comparison between gravity wave momentum fluxes in models and those derived from observations. Gravity waves (GW) that are generated in the troposphere drive circulation changes in the stratosphere and mesosphere, and are one of the important unresolved atmospheric processes in global models that must be treated with physical parameterization. GWs transport energy and momentum from lower atmospheric sources to drive the quasi-biennial and semi-annual wind oscillations (QBO and SAO), mean circulation mixing and transport. Including GW physics in operational models can modify the stratosphere-troposphere exchange, and provide better extended forecasts of the Artic Oscillation (AO) and North Atlantic Oscillation (NAO) patterns as well as play a component role in the forcing of the QBO with the planetary wave dynamics. Although the scales of the individual GW's are small, collectively, they demonstrate important global-scale effects on the circulation. Orographic gravity wave drag in global circulation models has long been recognized as one of the important physical parameterizations in the extra-tropics. Waves from other sources, so-called non-orographic gravity waves, have long been recognized as controlling winds at mesospheric heights, and now known to be important throughout the stratosphere, particularly in the tropics for driving the prominent QBO and SAO. The simplified set of parameters used to tune parameterizations cannot completely represent the true variability of waves in nature, so a great deal of freedom remains in the tuning process when considering how to achieve a model simulation and minimum bias. Thus, we are in need of Marvin's above mentioned comparison as NCEP implements non-orographic gravity wave drag in the Global Forecast System operational model. Results from a collaborative project, suggested by Marvin, between CU-CIRES and NOAA/NWS is under way to implement non-orographic gravity wave physics.