Realistic simulations of gravity waves over the continental US using precipitation radar data

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Monday, 5 January 2015: 4:45 PM
212A West Building (Phoenix Convention Center - West and North Buildings)
Claudia Christine Stephan, University of Colorado, Boulder, CO; and M. J. Alexander

Direct interactions between the troposphere and the middle atmosphere involve primarily Rossby waves and small-scale gravity waves. While Rossby waves are included in global models, grid spacings still remain too coarse to capture the physics of small-scale gravity waves. The use of gravity wave drag (GWD) parameterizations in general circulation models (GCMs) has permitted realistic modeling of the background state of the middle atmosphere as well as variations such as the SAO and QBO in the equatorial stratosphere. On the other end of the spectrum, high-resolution models like WRF-ARW have become more sophisticated and now include a multitude of interactive physics packages that can be customized to yield accurate simulations of sub-cloud scale processes. Numerous studies have successfully used such models to relate convective properties to the generated gravity wave spectrum. This has led to source parameterizations for GCMs that deliver a momentum-flux spectrum that depends on the latent heating properties of the underlying convection and the background wind.

However, large uncertainties remain. In coarse-resolution models convection itself is a sub-grid-scale process such that the true extent, depth and amplitude of convective cells are unknown. Establishing appropriate values for these quantities is challenging as they are only loosely constrained by observations. Full physics high-resolution model simulations are of case-study nature and lack general applicability. Other common assumptions include that waves propagate conservatively, within one time step, and within the same column, to the height where they deposit their momentum and create drag. For these reasons all GWD parameterizations include adjustable parameters that scale the wave drag or impact the height where the wave dissipates. Tuning these parameters to obtain desired middle-atmosphere circulations in global models is common practice.

We present a new modeling approach that combines the realism of local full-physics simulations with the spatial and temporal scope of larger scale models to study gravity wave effects on regional circulation patterns. An idealized dry version of the WRF model is forced with a three-dimensional time-varying heating field. We introduce an algorithm to derive this heating field from local precipitation rates and show that conventional precipitation radar data is suitable to drive the model. The idealized model is capable of simulating stratospheric gravity wave spectra that reproduce the shape, magnitude and complexity obtained from full-physics models. Focusing on the continental summer US, where convection is the main source of gravity waves, we present examples of simulations using radar precipitation data to force the model. Radar products can accurately capture the high spatial and temporal variability in occurrence and strength associated with convective sources of gravity waves.