My research focuses on dynamics, chemistry, and energetics in the Mesosphere/Lower Thermosphere region. I'm a theorist in Aeronomy, specializing in analytical approach, numerical simulation, and data analysis. The projects that I have been involved are about, but not limited to, exothermic heating, secular variations of minor species and airglow emissions, wave ducting, the formation of Mesospheric Inversion Layers (MILs), tidal variation of atomic oxygen, nonlinear response of minor species, and lightning-induced transient emissions (LITEs).
The first noted accomplishment of my career at Lehigh Valley is that my first NSF proposal as a sole PI was awarded in 2004. It is also the first research NSF proposal ever funded at this campus. Our local newspaper, The Morning Call, wrote an article on this grant; additionally, a few articles in Penn State publications featured my work. With the NSF grant and funding from the campus and University, I have employed several undergraduate research assistants to help me carry out several research projects outlined below. Most of the students presented or co-presented the research findings at national and international professional meetings and in-house conferences. I enjoyed working with the students, and they were also excited about being able to work on challenging research projects.
The mesosphere and lower thermosphere (MLT) is a region full of active photochemical and dynamical interactions. It is also a region where waves and wave-dissipative processes can play an important role in the energetic, dynamics, and chemistry of the atmosphere. There are waves of various kinds propagating through the atmosphere like gravity waves, tides, and planetary waves. My research primarily focuses on the effects induced by gravity waves. Most of gravity waves originate from the lower atmosphere. They act as a vehicle for energy and momentum transport to the middle and upper atmosphere. Therefore, understanding gravity waves is essential in understanding atmospheric dynamics and long distance energy transport.
The physical explanation for the existence of gravity waves is that when the force of earth's gravity and the stabilizing restoring force produced by the atmospheric density gradients become comparable with compressible forces, the resultant disturbances are gravity waves. They can be generated by many sources like jet streams, tidal waves, earthquakes, volcanic eruptions, nuclear explosions and thunderstorms.
One of my research topics is about understanding the mechanism for the occurrence of the mesospheric temperature inversion layers recorded by the better and improved instruments. After analyzing both wind and temperature data collected in the ALOHA-93 campaign, we developed a theory to explain the peculiar phenomenon, i.e., the sudden temperature increase in a few hours. We proposed that the formation of the inversion layer (the temperature increase) is due to the gravity wave-critical layer interactions. To the best of our knowledge, we were the first to link wind and temperature together and developed a model that successfully explains the phenomenon. Waves encounter critical layers when the waves' phase speeds equal the background wind speeds. When a wave approaches the critical layer, much of its energy and momentum is deposited to the atmosphere causing the atmosphere to be unstable. The temperature rise could result from the turbulence instigated by the gravity wave. In addition, when temperature increases, the density of the atmosphere decreases, thus creating a "hole" or "trough" in the atmosphere. Such holes pose possible danger for the reentry of a space shuttle in the atmosphere. Thus understanding the mechanism for the formation of an inversion layer is critical for maneuvering space vehicles and assessing its impact on the temperature structure of the atmosphere.
Another research topic that I have been working on is numerical simulation and theoretical modeling of wave-induced secular variations of minor species and OH airglow emissions. The OH airglow emissions come from the de-excitation of the excited OH species that have been primarily created from the H + O3 chemical reaction. When a gravity wave propagates through the atmosphere, major gas species like N2 and O2 are modulated by the passage of the wave, therefore, they are forced to oscillate with the wave. Not only that, the temperature of the atmosphere is also subject to wave modulation. Minor gas species interact with major gases through chemical reactions, hence they are affected by the presence of the wave as well. Since airglow emissions coming from the chemical reactions involving these minor species, they are affected by the presence of waves. It is therefore not surprising to see wave signatures existing in the minor species profiles and in the airglow emissions when a wave passes through the atmosphere. Note that for the major gas species, the wave effect is dynamical. However, for the minor species, the effect is more complex. The effect is a result of the coupling between chemistry and dynamics. I simulate wave effects on the vertical distribution of minor species and OH airglow emissions, given a specified gravity wave packet, with a 2D OH chemistry-dynamics model. Wave parameters like its period, phase speed and wavelengths can be extracted from the observations of airglow intensity.
Below are the summaries of my recent research topics:
The Dynamics and Chemistry of the Mesosphere: An analytical solution to the continuity equation of minor species was for a long time obtained only with a linear treatment owing to the fact that secularity would arise when a direct perturbation expansion was applied to the equation. Recently, Huang et al. (2003) applied the Krylov-Bogoliubov-Mitropolsky (KBM) averaging method to remove the higher-order secular terms in the perturbation expansion series, and what remained in the series were the terms that oscillate at frequencies that are an integer multiple of the forcing wave frequency. Following my previous work, a coupled chemical-dynamical gravity wave model with the application of the KBM method has been constructed to investigate the wave-induced nonlinear response in the presence of wind. The work on coupling wave dynamics with chemistry is currently underway.
The Chemistry and Energetics of Transient Luminous Events(TLEs): This is the topic of my latest research interest. My previous simulation results, ignoring dynamical effects, indicate that the column-integrated OH brightness shows a significant increase when temperature is at a higher value. Latest simulation results using a more realistic chemical set have shown that a moderate temperature increase, say 10 K, could produce measurable intensity enhancements. The latest satellite observations of TLEs do show an enhancement at the OH airglow altitude. However, most people think the enhancements are due to N2 1P emission, not the OH emission, and that the enhancements occur at the OH altitude is purely coincidental. I am currently undertaking the task to investigate the real mechanisms that are responsible for the observed enhancements. This work is in collaboration with the ISUAL principal investigators at Cheng Kung University in Taiwan who provide their satellite observations of TLEs for the investigation.
The Dynamics and Chemistry of Minor Species and OH Airglow: A spectral full-wave model and a 2-D, time-dependent, fully nonlinear chemistry model are used to investigate the latitudinal variations of the wave effects on the minor species in the OH chemistry in the mesosphere/lower thermosphere region. Secular variations of minor species and OH airglow along with the intensity-weighted temperature are also investigated. The wave packet causes non-periodic secular variations of the minor species densities and OH airglow as a consequence of violation of the non-acceleration conditions due to wave transience and dissipation. These secular variations of OH airglow could be mistaken as long-period or short-period waves in the airglow observations. Therefore, care must be taken when analyzing the data from observations. This work is in collaboration with Dr. Michael Hickey at Embry-Riddle University.
Mesospheric Temperature Inversion Layers: Following my previous work of developing a mechanism of temperature inversion layer formation, this project aims to further our understanding of this phenomenon by numerical simulation of the mesospheric temperature inversion layer observed during the ALOHA-93 Campaign with a full-wave model developed by Dr. Michael Hickey. The simulation results look promising in that they capture the key features of the observations. More work on adjusting the parameters in the numerical program is planned to see how they affect the wind profiles from the simulations.