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Distinguished Lecture

Distinguished Lecture - ST

Hanli LIU
National Center for Atmospheric Research

Tue- 30 Jul, 12:00-12:30pm
Nicoll 2, Level 3

"The Power of the Powerless--The Significant Role of Mesoscale Processes in the Atmosphere"

It is well established that the atmospheric kinetic energy spectrum follows a power law in the mesoscale range with a slope of ~ -5/3. This makes the mesoscale processes, likely gravity waves, appear quite powerless in the overall energy budget. On the other hand, gravity waves are thought to strongly affect the circulation pattern in the middle and upper atmosphere, though there are large uncertainties in quantifying such effects in global models because mesoscale waves are not well resolved. Even with increasing spatial resolutions and more realistic and detailed representation of the atmosphere, the forcing on the mean circulation by resolved waves in general circulation models (GCMs) is still not converging. This suggests the importance of unresolved, smaller-scale waves in the global momentum budget, which still defies physically consistent quantification. In this talk, I will present a recent study of the zonal spectra of momentum flux and wave forcing over the resolved mesoscale range, which shows that they display scale invariance and have shallow slopes. By assuming the same statistical distribution throughout mesoscale range and with the knowledge of the resolved portion, it is possible to quantify the forcing by the portion that is unresolved in global models. The result reveals the leading order importance of this forcing in comparison to that by the resolved waves--thus the power of these small-scale waves--and it is in general agreement with that required for obtaining the zonal mean wind climatology. It is also found that wave and mean flow interaction is important in maintaining the rather robust spectral structure. This method may provide a strategy to design physically consistent and scale-aware parameterization schemes for scale invariant quantities, when a model has sufficient resolution to partially resolve their power-law spectra.

Dr. Hanli Liu is a senior scientist at the High Altitude Observatory, National Center for Atmospheric Research. He received a B.S. in Fluid Mechanics from the University of Science and Technology of China, and a Ph.D. in Atmospheric and Space Physics from the University of Michigan. After two year of postdoctoral training at the Observatory, he joined the scientific staff. His research includes: theoretical, numerical, and interpretive studies of the dynamics, structure, and variability of the Earth's middle and upper atmosphere; coupling of different atmospheric regions on global and regional scales, including impacts of lower atmospheric forcing on space weather; atmospheric waves and geophysical turbulence. He leads the development of the thermosphere/ionosphere extension of the Whole Atmosphere Community Climate Model (WACCM-X), and received the 2018 NSF/CEDAR Prize for his work.