Graduate Exam Abstract
July 30, 2009, 10 am - 12 pm
Effects of background winds and temperatures on bores, strong windshears and concentric gravity waves in the mesopause region
Abstract: Using data from the CSU sodium Doppler lidar and Kyoto University OH airglow imager at Fort Collins, CO (40.6N, 105W), supplemented by meteorological data in the lower atmosphere, this thesis provides a comprehensive, though qualitative, understanding for three different yet related observed fluid-dynamical phenomena in the mesopause region.
The first project involves the convection-excited gravity waves observed in the OH airglow layer at 87 km. Two case studies on May 11, 2004 and September 9, 2005 are discussed in detail along with statistical studies and a ray-tracing modeling. A single convection source matches the center of the concentric gravity waves. The horizontal wavelengths and periods of these gravity waves were measured as functions of both radius and time. These results agreed reasonably well with the internal Boussinesq gravity wave dispersion relation with an assumed zero background wind. The weak mean background wind between the lower and middle atmosphere (10 to 90 km) determines the penetration of the gravity waves into higher altitude.
The second project involves mesospheric bores observed by the same OH imager. The observation on October 9, 2007 reveals the close correlation between breaking large-scale gravity waves and mesospheric bores. It suggests that when a large-amplitude gravity wave is trapped in a thermal or shear duct, its wave front could steepen and forms bore-like structure in the mesopause. In turn, the large gravity wave and its bore may significantly impact the background. Studying all ~8 observed cases reveals the possible link between the jet/front system in the lower atmosphere and the propagation and ducting of large-scale gravity waves and associated bores in the mesopause region of the atmosphere.
The third project involves the relationship between large wind shear generation and sustainment and convective/dynamic stabilities measured by the sodium lidar at the altitude of 80-105 km during 2002-2005. A large wind shear could substantially affect the electrodynamics in the ionosphere through Lorentz force acting on ionized particles. The correlation between windshear, S, and Brunt-Vaisala frequency, N, as observed by lidar suggests that the maximum sustainable windshear is determined by the necessary condition for dynamic instability of Richardson number R=0.25, leading to the result that the maximal windshear occurs at altitudes of lower thermosphere where the atmosphere is convectively very stable (large value of N2), ~ 100 km in winter and ~ 90 km in summer. Through a case study and more than 20 examples (not shown), we conclude that at the temporal and vertical resolution of 15 min and 2 km, the dominate source for sustainable large windshears appears to be the semidiurnal tidal-period perturbations with shorter vertical wavelengths and greater amplitude, compared to the typical solar tides.
Though the observations of concentric gravity waves, mesospheric bores and large windshears were made previously at other locations, our long-time studies with 5 to 10 times more observational data have enabled us to draw new comprehensive conclusions.
Adviser: Chiao-Yao (Joe) She
Co-Adviser: Steven C. Reising
Non-ECE Member: David A. Krueger (physics)
Member 3: Steven Reising
Addional Members: NA
Program of Study: