Reflection of a Long-period Gravity Wave Observed in the Nightglow over Arecibo on May 8–9, 1989?

During the Arecibo Initiative for Dynamics of the Atmosphere (AIDA) campaign in 1989 a characteristic of gravity wave perturbations observed in mesopause region airglow emissions was that airglow brightness fluctuations and airglow-derived temperature fluctuations often occurred either in phase or in antiphase. This stimulated the development of a theory suggesting that such in-phase fluctuations were most probably the result of strong reflections occurring in the mesosphere and lower thermosphere region. Recent examination of a particular wave event and application of simple WKB-type theory has appeared to support this hypothesis. Here we use a full-wave model and a WKB-type model, each coupled with a chemical-airglow fluctuation model describing O2 atmospheric and OH Meinel airglow fluctuations, to assess the strength of wave reflection and also to explicitly calculate the phase difference between the airglow brightness and the temperature fluctuations. Our results suggest that reflection is not strong for the particular wave event, and the model produces fairly large phase differences between the airglow brightness and the temperature fluctuations (∼35° and ∼134°–165° for the O2 atmospheric and OH airglow emissions, respectively). These results are not particularly sensitive to the nominal mean winds used in the simulations. There is an instance when a region of minimum refractive index occurs directly above a region in which reflection is strongest, suggesting that the two are related. However, the reflection does not appear to be strong. Our results suggest that chemical effects can account for the inferred phases of the observed airglow fluctuations and that effects associated with wave reflection appear to play a relatively minor role in the airglow fluctuations

Unmanned Aerial Systems Research, Development, Education and Training at Embry-Riddle Aeronautical University

With technological breakthroughs in miniaturized aircraft-related components, including but not limited to communications, computer systems and sensors and, state-of-the-art unmanned aerial systems (UAS) have become a reality. This fast growing industry is anticipating and responding to a myriad of societal applications that will provide either new or more cost effective solutions that previous technologies could not, or will replace activities that involved humans in flight with associated risks. Embry-Riddle Aeronautical University has a long history of aviation related research and education, and is heavily engaged in UAS activities. This document provides a summary of these activities. The document is divided into two parts. The first part provides a brief summary of each of the various activities while the second part lists the faculty associated with those activities. Within the first part of this document we have separated the UAS activities into two broad areas: Engineering and Applications. Each of these broad areas is then further broken down into six sub-areas, which are listed in the Table of Contents. The second part lists the faculty, sorted by campus (Daytona Beach---D, Prescott---P and Worldwide--W) associated with the UAS activities. The UAS activities and the corresponding faculty are cross-referenced. We have chosen to provide very short summaries of the UAS activities rather than lengthy descriptions. Should more information be desired, please contact me directly or alternatively visit our research web pages (http://research.erau.edu) and contact the appropriate faculty member directly

Effects of Eddy Viscosity and Thermal Conduction and Coriolis Force in the Dynamics of Gravity Wave Driven Fluctuations in the OH Nightglow

Recently, Walterscheid et al. (1987) have described a dynamical-chemical model of wave-driven fluctuations in the OH nightglow which incorporated a five-reaction photochemical scheme and the dynamics of linearized acoustic-gravity waves in an isothermal, motionless atmosphere. The intensity oscillation (δI) about the time-averaged intensity (I0) and the temperature oscillation (δT) about the time-averaged temperature (T0) were related by means of the complex ratio η ≡ (δI/I0)/(δT/T0). One of the main conclusions of their work was that the inclusion of dynamical effects is absolutely essential for a valid assessment of η at any wave period. In this paper the model of Walterscheid et al. (1987) is modified to include in the gravity wave dynamics the effects of eddy viscosity, eddy thermal conduction, and Coriolis force (with the shallow atmosphere approximation), and calculations are performed for the same nominal case as used by these previous authors (i.e., λx = 100 km and atmospheric conditions pertinent to 83 km altitude), but only gravity wave periods are considered. It is found that for wave periods greater than some 2 or 3 hours the value of η is greatly modified by the inclusion of eddy thermal conduction. Although when acting alone the eddy viscosity is relatively unimportant, it significantly modifies the results when acting in conjunction with the eddy thermal conduction. The inclusion of the Coriolis force is found to be insignificant at any wave period. Although it is for the longest-period waves that the values of η are most modified by the inclusion of dissipation, this dissipation may be severe enough to place an observational constraint on such waves. Results of Walterscheid et al. (1987) suggest that η is virtually independent of horizontal wavelength (λx), but it is indicated here that the inclusion of dissipation is likely to make η highly dependent on λx and to complicate comparisons which have been made between observation and theory. The effects of varying the Prandtl number are also discussed

Airglow Variations Associated with Nonideal Ducting of Gravity Waves in the Lower Thermosphere Region

A numerical full-wave model is used to study the response of the O2 atmospheric airglow to ducted gravity waves in the mesopause region. For an isothermal, quasi-adiabatic, and motionless background atmosphere the calculated phase differences between airglow brightness fluctuations and fluctuations of temperatures derived from the airglow, as given by Krassovsky\u27s ratio, are in good agreement with the predictions of published theory. Significant departures from the predictions of the basic theory are obtained when we consider ducting in the presence of the eddy and molecular diffusion of heat and momentum in a nonisothermal background atmosphere. Wind shears also affect the phase difference between airglow brightness fluctuations and temperatures derived therefrom. Nonisothermal effects and the effects of diffusion and winds are largest for the slower waves we consider. Only the fastest of the ducted waves considered conform to the basic theory, while the airglow signatures associated with slower, more weakly ducted waves may be easily misinterpreted as being due to propagating waves. We conclude that for the short horizontal wavelength waves observed in the airglow, the phase of Krassovsky\u27s ratio may be useful to identify wave ducting only for the shortest period, fastest waves. Therefore identification of ducted waves using Krassovsky\u27s ratio will be difficult even if the required high temporal resolution measurements become available

Wavelength Dependence of Eddy Dissipation and Coriolis Force in the Dynamics of Gravity Wave Driven Fluctuations in the OH Nightglow

The theory of Walterscheid et al. (1987) to explain internal gravity wave induced oscillations in the emission intensity I and rotational temperature T of the OH nightglow was modified by Hickey (1988) to include the effects of eddy dissipation and Coriolis force. In the theory of Walterscheid et al. (1987) the ratio η = (δI/I0)/(δT/T0) (δ refers to a perturbation quantity, and a zero subscript refers to an average) was found to be independent of horizontal wavelength at long periods, while in the extended theory of Hickey (1988) some such dependence was inferred. In the present paper the horizontal wavelength dependence of η is examined. It is found that values of η will be dependent on both wave period and horizontal wavelength, meaning that in order to compare measurement with theory, horizontal wavelengths will need to be measured in conjunction with the OH nightglow measurements. At long periods the modifications to η by the inclusion of eddy dissipation are much larger for the shorter horizontal wavelength waves, although such modifications may be more observable for some of the longer horizontal wavelength waves. The Coriolis force is important only for waves of very large horizontal wavelength

Time-resolved Ducting of Atmospheric Acoustic-gravity Waves by Analysis of the Vertical Energy Flux

A new 2-D time-dependent model is used to simulate the propagation of an acoustic-gravity wave packet in the atmosphere. A Gaussian tropospheric heat source is assumed with a forcing period of 6.276 minutes. The atmospheric thermal structure creates three discrete wave ducts in the stratosphere, mesosphere, and lower thermosphere, respectively. The horizontally averaged vertical energy flux is derived over altitude and time in order to examine the time-resolved ducting. This ducting is characterized by alternating upward and downward energy fluxes within a particular duct, which clearly show the reflections occurring from the duct boundaries. These ducting simulations are the first that resolve the time-dependent vertical energy flux. They suggest that when ducted gravity waves are observed in the mesosphere they may also be observable at greater distances in the stratosphere

A Full-wave Investigation of the Use of a ‘‘Cancellation Factor’’ in Gravity Wave–OH Airglow Interaction Studies

Atmospheric gravity waves (GWs) perturb minor species involved in the chemical reactions of airglow emissions in the upper mesosphere and lower thermosphere. In order to determine gravity wave fluxes and the forcing effects of gravity waves on the mean state (which are proportional to the square of the wave amplitude), it is essential that the amplitude of airglow brightness fluctuation be related to the amplitude of major gas density fluctuation in a deterministic way. This has been achieved through detailed modeling combining gravity wave dynamics described using a full-wave model with the chemistry relevant to the airglow emission of interest. Alternatively, others have employed approximations allowing them to derive analytic expressions relating airglow brightness fluctuations to major gas density fluctuations through a so-called ‘‘cancellation factor’’ (CF). The effects of these approximations on the derived CF are investigated here using a full-wave model describing gravity wave propagation in a nonisothermal, windy, and viscous atmosphere. This numerical model combined with the chemical reaction scheme for the OH (8, 3) Meinel airglow emission is used to derive fluctuations in the OH* nightglow from which an equivalent CF is calculated. Comparisons are made between the analytically derived CF’s and the numerically derived CF’s based on using different approximations in the latter model. Differences exist at most wave periods, but they also depend on the horizontal wavelengths of the gravity waves considered. In addition to these different model comparisons, the sensitivity of the numerically derived CF to specific physical processes is examined exclusively using the full-wave model. These sensitivity tests show that the effect of eddy diffusion marginally influences the calculated CF’s only for the very slowest gravity waves. Accounting for the effects of a nonisothermal mean state has a significant influence on the calculated CF’s, and the CF’s calculated assuming an isothermal mean state can be as much as a factor of 2 smaller than those calculated assuming a nonisothermal mean state. The effects of background mean winds also influence the derived CF’s, which then become dependent on the azimuth of propagation. In this case the calculated CF’s can vary by a factor of ~2 from their windless values for gravity waves of short horizontal wavelength with phase speeds less than 100 m s¯¹. Finally, reflection from the lower and middle thermosphere in the full-wave model leads to undulations in the calculated CF’s as a function of phase speed for gravity waves with horizontal wavelengths of 100 km and phase speeds greater than about 100 m s¯¹. These effects that are not reproduced in the analytic model lead to large differences between the CF’s calculated with and without winds, but they only occur for fast gravity waves that are not usually observed in the airglow

Numerical Modeling of a Gravity Wave Packet Ducted by the Thermal Structure of the Atmosphere

A time-dependent and fully nonlinear numerical model is employed to solve the Navier-Stokes equations in two spatial dimensions and to describe the propagation of a Gaussian gravity wave packet generated in the troposphere. A Fourier spectral analysis is used to analyze the frequency power spectra of the wave packet, which propagates through and dwells within several thermal ducting regions. The frequency power spectra of the wave packet are derived at several discrete altitudes, which allow us to determine the evolution of the packet. This spectral analysis also clearly reveals the existence of a stratospheric duct, a mesospheric and lower thermospheric duct, and a duct lying between the tropopause and the lower thermosphere. In addition, we determine the spatially localized wave kinetic energy density and the horizontally averaged, time-resolved, normalized vertical velocity. Examination of these diagnostic variables allows us to better understand the process of wave ducting and the vertical transport of wave energy among multiple thermal ducts. The spectral analysis allows us to unambiguously identify the ducted wave modes. These results compare favorably with those derived from a full-wave model

Gravity-wave-induced Variations in Exothermic Heating in the Low-latitude, Equinox Mesophere and Lower Thermosphere Region

We investigate gravity-wave-induced variations in exothermic heating in the OH nightglow region at a latitude of 18° in the Northern and Southern Hemispheres during March. An OH nightglow chemistry model with gravity wavefields from a spectral full-wave model is used for the investigation. Our simulation results show that the wave packet induces a large secular increase in the number densities of the minor species involved in the OH chemistry, a 50% increase in O3, 42% in O, and 29% in OH (v= 8), and the ultimate driver for these increases is the wave-driven downward transport of O. We find that the total exothermic heating rates have increased by ~44.2% for 18°S and ~30.9% for 18°N by the end of the simulation time. Also, the peak values of the mean wave-induced total exothermic heating rates are significant, ~2.0 K d¯¹ at the peak altitude of 88 km and ~2.2 K d¯¹ at 89 km for 18°S and 18°N, respectively. The major reactions contributing to exothermic heating rates are the three-body recombination O + O + M and the H + O3 reaction. The hemispheric asymmetry in the heating rates is mainly due to the different atmospheric conditions at 18°N and 18°S since the same wavefields are used in the numerical simulations

A Note on Gravity Wave-driven Volume Emission Rate Weighted Temperature Perturbations Inferred from O₂ Atmospheric and O I 5577 Airglow Observations

A full-wave dynamical model and chemistry models that simulate ground-based observations of gravity wave-driven O₂ atmospheric and O I 5577 airglow fluctuations in the mesopause region are used to demonstrate that for many observable gravity waves modeling is required to infer temperature perturbation amplitudes from airglow observations. We demonstrate that the amplitude of the altitude-integrated volume emission rate weighted temperature perturbation differs by at least about 30% from the amplitude of the temperature perturbation of the major gas in the vicinity of the peak of the airglow volume emission rate for gravity waves with horizontal phase speeds less than about 150 m s¯¹ and vertical wavelengths less than about 50 km and that the amplitude of the altitude-integrated volume emission rate weighted temperature perturbation differs considerably from the amplitude of the temperature perturbation averaged over the vertical extent of the emission layer for waves with horizontal phase speeds less than about 65 m s¯¹ and vertical wavelengths less than about 20 km. For waves with phase speeds less than about 100 m s¯¹ and vertical wavelengths less than about 30 km the amplitude of the altitude-integrated volume emission rate weighted temperature perturbation differs by at least about 30% from the altitude-integrated mean volume emission rate weighted temperature perturbation, demonstrating that the nonthermal fluctuation contribution to the former (involving volume emission rate perturbations) needs to be included in such modeling. We conjecture that the observed brightness perturbation is a simpler and better quantity to simulate using detailed modeling than the observed airglow temperature perturbation for the determination of wave amplitude in cases where nonthermal effects or cancellation effects (for short vertical wavelengths) are strong