My research interests focus on wave dynamics, airglow chemistry, and energetics in the Mesosphere and Lower Thermosphere (MLT) region. I’m a theorist in Aeronomy, specializing in analytical approach, numerical simulation, and data analysis.
Gravity wave forcing has been an intense research topic in the field of Aeronomy ever since it was found that gravity waves were responsible for the ionospheric disturbances. Gravity waves are important because they can transport energy and momentum to higher altitudes; deposit energy and momentum through wave-mean flow interactions; alter gas concentration distributions by the combined effects of vertical transport of species and chemistry; induce exothermic heating variations. They play an important role in the dynamics, chemistry, and energy budget in the MLT region.
In addition to gravity waves, airglow in the MLT region is another important research topic in Aeronomy. It is important because the variations in the airglow intensity can oftentimes be used to deduce characteristics of gravity waves or other types of waves that caused the variations. Observations of airglow intensity have shown that gravity waves are a ubiquitous feature in the atmosphere. The presence of wave like patterns in the airglow imagery arises from the coupling of wave dynamics and airglow chemistry in that region. Just like the ionospheric disturbances, airglow intensity variations can be a manifestation of wave modulation in the airglow intensity.
Below is a list of projects that I have been working on.
The Dynamics, Chemistry, and Energetics in the MLT Region
A spectral full-wave model and a 2-D, time-dependent, fully nonlinear chemistry model were developed and used to investigate the latitudinal variations of the wave effects on the minor species in the OH chemistry in the MLT region. Secular variations of minor species and OH airglow along with the airglow intensity-weighted temperature were 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. The models can also be used to investigate how gravity waves affect the energy budget in the MLT region by studying the induced variations in the exothermic heating in that region. This work is in collaboration with Dr. Michael Hickey of Embry-Riddle University.
Lightning-Induced Transient Emissions (LITEs) in the Mesosphere
Sprites and elves are two different kinds of Transient Luminous Events (TLEs) in the MLT region, discovered around 1990. Sprites are the optical emissions of tens of milliseconds in duration. Their shapes come in various forms. They can appear like columns, carrots, inverted trees, or fireworks. Elves appear disk-shaped in less than a few milliseconds. The Imager of Sprites and Upper Atmospheric Lightning (ISUAL) instrumentation consists of a CCD camera, a spectrophotometer (SP), and an array photometer onboard the FORMOSAT-II satellite that was launched in 2004. The ISUAL’s TLE observations are unique in that their observations are the first from a satellite, thus they are devoid of atmospheric absorption of these TLEs.
I have used their data and proposed mechanisms to explain the phenomenon of sudden brightening in the OH nightglow layer. We estimated the N21P intensity enhancements to be ~ 65% and 53% of the total intensity enhancements for the two events we analyzed using the imager and the SP data in conjunction with an ELVE model. Our analysis indicates that there is still somewhat considerable intensity enhancement (~ 1.25 kR) unaccounted for after the N21P contribution has been removed. Our study suggests that there might be OH emissions in ELVES and that OH species might also be involved in the lightning-induced process to contribute to the intensity enhancements that we observed. Further work on how lightning induces other observed optical emissions in the MLT region is in progress. This work is in collaboration with the ISUAL team in Taiwan.
Gravity Wave Effects in the MLT Region with a 2-D, Nonlinear, Multiple-airglow Chemistry-dynamics Model:
A 2-D, nonlinear, Multiple-Airglow Chemistry-Dynamics (MACD) model has recently been developed to investigate gravity wave effects in the MLT region. Building on the success of the previous 2D, OH chemistry-dynamics model [Huang and Hickey, 2007; 2008], the MACD model will be even more versatile and powerful. It consists of a gravity wave dynamics model and a chemistry model for three airglow emissions – OH, greeline O(1s), and O2(b1) atmospheric (0,1) band.
The 2-D, MACD model can be used to study the following topics: 1) quantitatively investigating gravity wave effects on airglow emission layers, 2) comparing the model simulation results from a variety of different wave characteristics, 3) investigating wave-induced secular variations of the airglow intensities and the intensity-weighted temperatures of these airglow layers, 4) calculating the Krassovsky’s ratios with the intensity and temperature from these airglow layers, 5) investigating wave-induced secular variations of gas species concentrations, and 6) investigating wave-induced exothermic heating with different gravity waves. The amplitude growth factor from the temperature amplitude ratio of the O2 to OH will be calculated for a variety of gravity waves to better understand the interactions of gravity waves with the airglow layers.
Airglow Intensity Variations Caused by Global Climate Change in the MLT Region
Using airglow intensity variations as a proxy for global climate change is proposed for the investigation. The impacts of global warming in the lower atmosphere on the MLT region due to atmospheric CO2 increases are 1) enhanced gravity wave activity, and 2) increased airglow Volume Emission Rates (VERs). A gravity wave propagation model (GROGRAT) coupled with an airglow model is proposed to investigate such changes in the MLT region. Yearly global maps of Convective Available Potential Energy (CAPE) and rainfall rate from the Tropical Rainfall Measuring Mission (TRMM) will be used to identify regions of strong convection. Background winds and tidal forcing will be included in GROGRAT. Airglow measurements onboard the Upper Atmosphere Research Satellite (UARS) satellite will be used to find locations of enhanced gravity wave activity for a global study. Ground-based airglow measurements will be used for the regional case study. The surviving waves at the airglow altitude from GROGRAT will be input to the airglow chemistry model to study the intensity variations induced by the passage of waves. Background atmospheric parameters (number density and temperature) will be obtained from TIME-GCM runs with the annual mean CO2 concentration input to TIME-GCM. Trends in the VERs will be deduced from airglow observations and simulations to better understand the state of the atmosphere in the MLT region. This work is in future collaboration with Prof. Tim Kane of Penn State-University Park.
Nonlinear Response of Minor Species in the MLT region
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. We 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. Using the perturbation method to treat the response of minor species to a small-scale gravity wave, the first and second order perturbation terms can be found. In the previous work, minor species’ number density profiles are described by a Chapman function. It is planned to use the observed number density profiles to study how the number densities would be perturbed by a monochromatic gravity wave.
Mesospheric Temperature Inversion Layers
A sudden mesospheric heating event was observed during the ALOHA-93 campaign. We proposed that the heating was due to a gravity wave breaking event. Further work aims to advance our understanding of this phenomenon by numerical simulation of the mesospheric temperature inversion layer with a full-wave model. 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 in the simulations.