Atmosphere Ocean Science Colloquium

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Modeling the Effects of Continental U.S. Convection on the Dynamics of the Troposphere and Stratosphere

Speaker: Claudia Stephan

Location: Warren Weaver Hall 1302

Date: Wednesday, December 2, 2015, 3:30 p.m.

Synopsis:

Convection is known to be an important source of gravity waves with short periods and small horizontal wavelengths. It is commonly understood that their impacts on the global-scale circulation are important, but capturing their effects in large-scale models remains an ongoing challenge. Convective cells, the source of these waves, are highly variable in both space and time and cannot be resolved in coarse-grid models. Instead, simplified parameterizations are used to include gravity wave effects on model winds. These parameterizations are poorly constrained and tuning parameters have orders of magnitude uncertainty. Cloud-resolving full-physics models, on the other hand, cannot accurately reproduce the locations, timing, and intensity of individual convective rain cells, limiting the validation of simulated waves. We present a new modeling approach that retains the spatial scope of larger scale models but permits direct validation of the modeled waves. It reproduces instantaneous satellite-observed features of the gravity wave field above storms with great accuracy. Furthermore, this approach permits the study of regional circulation influences of these small-scale waves. These are virtually unknown because it is extremely difficult to separate wave effects on circulation from other large-scale dynamical or cloud-radiative influences in full-physics models or observations. The relative simplicity of the new model allows us to simulate the continental U.S. for a full month of June at a high horizontal resolution of 4km. We relate the variability in summer U.S. convection to properties of the associated waves and quantify the wave-driven forcing in the middle atmosphere, as well as its spatial and temporal intermittency in the tropopause region and stratosphere. In comparing to the parameterized wave forcing in global models we diagnose deficiencies in their representation of gravity wave drag and suggest avenues for potential improvements.