Tai-Yin Huang

Sudden narrow temperature‐inversion‐layer formation in ALOHA‐93 as a critical‐layer‐interaction phenomenon

A sudden and dramatic mesospheric heating event was observed over Haleakala, Maui, on October 21, 1993 (day 294), during the ALOHA-93 Campaign. It consisted of a persistent, narrow temperature-inversion layer about 3-4 km wide near 87 km altitude with a peak temperature rise approaching 40 k, lasting for about 3.5 hours. Owing to the large number of ground-based instruments recording the event, it is possible to attempt to seek out a physical explanation for this temperature rise. There is powerful evidence to suggest that the temperature-inversion layer is associated with energy deposition, direct and indirect, resulting from gravity wave/critical layer interaction. Indeed, lidar wind profiles and mesospheric wave structures simultaneously observed with CCD imagers reveal the presence of a critical layer at the appropriate altitude. The data also show a narrow unstable background wind profile over a 2-3 km thickness in the immediate vicinity of the critical layer. We will show quantitatively that there is sufficient energy available from the dissipation of the observed gravity wave and from the observed instability in the background winds to account for the heating. After the critical layer has disappeared, the temperature rise subsides, and the background wind again becomes stable at all height levels of interest.

Application of the dispersion formula to long‐and short‐period gravity waves: Comparisons with ALOHA‐93 data and an analytical model

During the Airborne Lidar and Observations of Hawaiian Airglow (ALOHA) Campaign on October 10, 1993, a wave structure with a sharp front covering significant parts of the sky was observed to move with a phase velocity of 76 m/s at a period of 4 to 5 min. Simultaneous observations of the Green line and the OH emission brightness showed a 180° phase diffeence. We have two objectives. (1) By using a dispersion formula, we first separated the true partial wave guidance for long-period waves from the background effects and showed explicitly that many partial modes previously thought to exist do not in fact exist. In this way, we can show that two very different methods for computing partial modes which previously gave very different results can now be reconciled. We showed that by lowering the period the modes became, first, virtually guided and then fully guided. (2) We then showed that the long-period S 2 partial mode eventually became a two-node fully guided mode, which at periods in the range of 4.6 min, had not only phase velocities comparable to the observed result, but also a phase reversal (from the two-node structure) at altitudes in the vicinity of the two airglow emission peaks. At those periods, the dispersion curves were shown to be out of the range of the nearest kissing mode and were thus largely unaffected by interference from the stratospheric duct. The results agree well with an analytic model. The wave explanation may in some way complement the other explanations (such as the bore, or soliton) for the same event mentioned in presentations elsewhere.

Further investigations of lightning‐induced transient emissions in the OH airglow layer

[1] A previous study of lightning-induced transient emissions in and below the OH airglow layer using observations by the Imager of Sprites and Upper Atmospheric Lightning (ISUAL) CCD camera onboard the FORMOSAT-II satellite showed that intensity enhancements occurred more frequently in the OH airglow layer. Here we show the results of new observations made in December 2009 and January 2010 using a narrowband 630 nm filter and spectrophotometer and present further analysis. We estimated the N 2 1P intensity enhancements to be ~65% and 53% of the total intensity enhancements for the two events we analyzed using ISUAL and the spectrophotometer data in conjunction with a model for emissions of light and VLF perturbations from electromagnetic pulse sources (elves). Our analysis indicates that there is still somewhat considerable intensity enhancement (~1.25 kR) unaccounted for after the N 2 1 P 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 and contribute to the intensity enhancements that we observed.

Simulations of gravity wave‐induced variations of the OH(8,3), O2(0,1), and O(1S) airglow emissions in the MLT region

We investigate gravity wave-induced airglow intensity variations of the OH(8,3), O2(0,1) atmospheric band, and O(1S) greenline emissions in the mesosphere/lower thermosphere (MLT) region with two two-dimensional, time-dependent, nonlinear models—an OH Chemistry-Dynamics model and a Multiple Airglow Chemistry-Dynamics model. Our simulation results of the wave effects by a small-scale 30 km wave packet show that it induces an ~22% secular increase in the OH(8,3) airglow intensity, a 30% increase in the O2(0,1) atmospheric band, and a 33% increase in the O(1S) greenline. The largest wave-induced airglow intensity fluctuation amplitude is seen in the OH(8,3) emission (~2.4%), followed by the O2(0,1) atmospheric band and O(1S) greenline with comparable maximum wave-induced airglow intensity fluctuation amplitudes of 1%. Our study also shows that the production of atmospheric bands and O(1S) strongly depends on the rate coefficients in the three-body recombination reactions.

Simultaneous observations of storm‐generated sprite and gravity wave over Bangladesh

We report simultaneous observations of sprite and gravity wave generated by a storm over Bangladesh. The origin of a concentric gravity wave can be traced to the storm region on Apr 27, 2014 over Bangladesh with a low cloud top surface temperature (175 K). After data analysis, the time period of the concentric gravity wave is found to be 8.8-8.9 minutes. The horizontal wavelength is found to be ~50 km for red emissions (~55 km for green emissions) and the horizontal phase velocity is 94.4 ± 31.7 m s-1 for red emissions (102.6 ± 29.4 m s-1 for green emissions). Using the dispersion relation of gravity wave, the elevation angle of wave propagation direction is found to be ~ 53.3 °. The sprite associated with the gravity wave was also recorded at 1534 UT on Apr 27, 2014. The initiation time of storm-generated gravity wave is estimated to be 1454 UT at which lightning activity was relatively low using lightning data. At time 1534 UT of the recorded sprite, the lightning rate was close to its maximum value. The storm-generated gravity wave could be thought as a precursor phenomenon for lightning and sprites since one of the necessary conditions for gravity wave, lightning and sprites is strong convection inside storms.

Investigations of Lightning‐Induced Sudden Brightening in the OH Airglow Layer Observed By ISUAL Onboard FORMOSAT‐II Satellite

We report observations of lightning‐induced transient emissions (LITEs) in or below the OH airglow layer observed from the Imager of Sprites and Upper Atmospheric Lightning (ISUAL) onboard the FORMOSAT‐II satellite. The ISUAL team conducted a nine‐day campaign in January 2007 using both the broadband and narrowband filters on the CCD imager. It is found that LITEs occur more frequently in the OH airglow layer than below. It is also found that the observations of LITEs below the OH airglow layer by the narrow‐band filter are rare. The observations of LITEs by the narrowband filter are the first of its kind in existence. Our data analysis shows that there is a significant intensity increase in the OH airglow layer when there is lightning and a residual intensity increase post the lightning activity. We propose that LITEs in the OH airglow layer could be due to OH species.

Investigations of Wave-induced Nonlinear Response of Minor Species with the KBM Averaging Method

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. A vertical drift velocity demonstrated to be the Eulerian drift was required to remove the secular terms. Following the previous work, a coupled chemical-dynamical gravity wave model with the application of the KBM method is presented here to investigate the wave-induced nonlinear response. The significance of the higher-order nonlinear response will be investigated. Results indicate that higher-order terms are significant when the scale height of the minor species is small.