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NASA NTRS1984

Excerpts from the paper: Research Status and Recommendation from the Alaska Workshop on Gravity Waves and Turbulence in the Middle Atmosphere, part 1.3A

Internal gravity waves are disturbances whose intrinsic frequencies k(c - u) are smaller than the Brunt-Vaisala frequency (N). Their importance arises because: they are the major components of the total flow and temperature variability fields of the mesosphere (i.e., shears and lapse rates) and hence constitute the likely sources of turbulence; and they are associated with fluxes of momentum that communicate stresses over large distances. For example, gravity waves exert a drag on the flow in the upper mesosphere. However, in order for gravity waves to exert a net drag on the atmosphere, they must be attenuated. There are two general types of processes that seek to attenuate gravity waves: dissipation and saturation. Dissipation is any process that is effective independent of the wave amplitude, while saturation occurs when certain wave amplitude conditions are met. Radiative damping is an example of dissipation, while convective overturning is an example of saturation. The two processes are not mutually exclusive.

Fritts, D. C.↗
NASA NTRS2019

Satellite Estimates of Momentum Fluxes from High-Impact Gravity Wave Events in the Stratosphere and Their Effects on Circulation

Recent assessments of chemistry-climate models (CCMs) reveal biases in temperatures and winds in, especially but not limited to, the Southern Hemisphere stratosphere, where winds are generally too strong and temperatures too cold. The reasons for these biases are not completely understood, but it is thought that missing wave drag in models is a major culprit. Observational and modeling studies support this idea by elucidating the role of infrequent but very high-impact gravity wave events in the stratosphere. These highly intermittent gravity wave events with large momentum fluxes are the most important drivers of circulation and transport in the stratosphere, yet they are not treated correctly in most global models. This has implications for the cold pole problem in the Southern Hemisphere and the global Brewer-Dobson circulation in general. In this presentation we show results combining HIRDLS and AIRS to derive detailed gravity wave properties and obtain new quantitative estimates of the local and intermittent gravity wave drag in the stratosphere. The combination of high-vertical resolution (1 km) and near-global (60°S to 80°N), close horizontal sampling (100 km) makes HIRDLS temperatures the best available dataset for retrieving gravity wave properties needed to diagnose gravity wave effects on circulation. We further exploit the close zonal sampling of HIRDLS near the turnaround latitude in the Southern Hemisphere to obtain estimates of the missing drag. We combine the HIRDLS results with AIRS brightness temperature images, which reveal high-spatial resolution detail of long vertical wavelength waves, to obtain 3-D, day-to-day variability in gravity wave properties and attribute the wave events to wave sources. The AIRS and HIRDLS datasets complement each other well since the two instruments have very different resolutions and horizontal sampling.

Holt, Laura↗
NASA NTRS1991

A simple nearly analytic model of a gravity wave driven middle atmospheric circulation

A nearby analytic model of a gravity wave driven middle atmosphere circulation is formulated. The simplified model represents the one-dimensional interaction of a single gravity wave mode with a zonal-mean flow. Solutions of this model are shown to agree very well with numerical results from a more complete mean-flow/gravity-wave model (essentially the model developed by Holton, 1982). A single nondimensional parameter, the ratio of a mean flow radiative forcing parameter, and the gravity wave momentum flux incident into the middle atmosphere from below largely determine the solution of the model. For typical middle atmosphere parameter values, an approximate solution can be obtained, and this solution permits the parameter dependence of the circulation to be characterized very simply. The gravity wave driven circulation extends downward from the level (the breaking level, approximately) where the momentum flux equals the mean flow radiative forcing.

Barnes, Jeffrey R.↗
NASA NTRS1986

Gravity waves in severe weather

With a view to determining the role of severe weather in producing gravity waves, two tests were made. In the first, the wind speed measured at two nearby radiosonde stations, Peoria and Salem, was correlated with the stratosphere gravity-wave intensity at Urbana. Although the gravity-wave intensity fluctuated greatly from day to day, these is little if any correlation with the stratospheric wind speed. This suggests that orographic forcing is not a factor in generating gravity waves in Urbana. On the other hand, a clear correlation is found between cloud to heights exceeding 20,000 ft and an increased gravity-wave amplitude in the stratosphere.

Bowhill, S. A.↗
NASA NTRS1986

Effects of anisotropy on the frequency spectrum of gravity waves observed by MST radar

In the investigation of gravity waves using mesosphere-stratosphere-troposphere radar data, model gravity-wave spectra have been used. In these model spectra, one usually assumes azimuthal symmetry. The effect of spectral anisotropy on the observed spectrum is studied here. It is shown that for a general Garrett-Munk-type spectrum, the anisotropy does not affect the frequency spectrum observed by the vertically beamed radar. For the oblique beam, however, the observed frequency spectrum is changed. A general gravity wave spectrum including azimuthal anisotropy is considered.

Liu, C. H.↗
NASA NTRS1993

Generation of pseudo-waves in the middle atmosphere by flow accelerations due to breaking gravity waves?

Low frequency, large amplitude waves are often observed at mesospheric heights in radar and lidar wind measurements. A series of individual profiles of 1-hr averages of velocity data that were taken in October 1981 using the MST radar at Poker Flat, Alaska are presented. The vertical wavelength of this wave-like event is about 15 km and has a period close to 10 hours. A clear downward phase propagation can be seen, and so these oscillations are usually, and probably correctly, interpreted as being gravity wave flow perturbations. We investigate an alternative explanation that may also be possible; specifically, we investigate the possibility that the perturbed flow is a pseudo-wave structure produced by mean flow acceleration due to gravity waves propagating upward from below and breaking in the mesosphere. The question is whether effects similar to those that produce the much longer period Quasi-Biennial Oscillation (QBO) in the equatorial stratosphere can produce mesospheric pseudo-waves (MPW) at much shorter periods.

Jacobson, Raymond A.↗
NASA NTRS1974

Acoustic-gravity waves in the upper atmosphere

In this paper we review the theory of acoustic-gravity waves, the interaction of such waves with the ionosphere, the experimental support for the existence of such waves in the upper atmosphere, and the role played by acoustic-gravity waves in thermospheric dynamics. After a thorough discussion on the properties of acoustic-gravity waves in an ideal isothermal atmosphere, the effects produced by horizontal winds, sharp boundary discontinuities, and dissipative processes are discussed. The generation of these waves by stationary or moving sources is then treated. It is shown that the atmospheric response to a stationary impulse source can be described by the emission of three waves: acoustic, buoyancy, and gravity. These discussions are then followed by reviewing propagation effects in a realistic atmosphere for both free waves and guided waves. Recent numerical results are given. When acoustic-gravity waves propagate through the ionosphere, interaction between the wave and the ionosphere will take place. The physical processes involved in such an interaction are examined.

Yeh, K. C.↗
NASA NTRS1984

Propagation and selective transmission of internal gravity waves in a sudden warming

Longitudinally asymmetric features of gravity wave propagation in a sudden warming are examined theoretically, using observed geostrophic wind fields in the stratosphere for three days of winter 1979. It is shown that the wind patterns accompanying a sudden warming act to reduce, but not eliminate, quasi-stationary gravity wave propagation to the mesosphere. The onset of large-amplitude planetary waves leads to the formation of propagating zones and forbidden zones for gravity waves of intermediate horizontal scale (50-200 km). Lateral ray movement and horizontal refraction are secondary but observable effects for these waves. To the extent that these waves are excited isotropically in the troposphere, it is possible to evaluate the direction and magnitude of the average wavevector reaching the mesosphere as follows. Stationary waves with wavevector orthogonal to the local mean flow are selectively absorbed in the stratosphere, implying that for these waves the average wavevector transmitted to the mesosphere is antiparallel to the average of the mean flow orientation extrema in the underlying stratosphere.

Dunkerton, T. J.↗
NASA NTRS2003

Sensitivity of Middle Atmospheric Temperature and Circulation in the UIUC Mesosphere-Stratosphere-Troposphere GCM to the Treatment of Subgrid-Scale Gravity-Wave Breaking

The sensitivity of the middle atmospheric temperature and circulation to the treatment of mean- flow forcing due to breaking gravity waves was investigated using the University of Illinois at Urbana-Champaign 40-layer Mesosphere-Stratosphere-Troposphere General Circulation Model (MST-GCM). Three GCM experiments were performed. The gravity-wave forcing was represented first by Rayleigh friction, and then by the Alexander and Dunkerton (AD) parameterization with weak and strong breaking effects of gravity waves. In all experiments, the Palmer et al. parameterization was included to treat the breaking of topographic gravity waves in the troposphere and lower stratosphere. Overall, the experiment with the strong breaking effect simulates best the middle atmospheric temperature and circulation. With Rayleigh friction and the weak breaking effect, a large warm bias of up to 60 C was found in the summer upper mesosphere and lower thermosphere. This warm bias was linked to the inability of the GCM to simulate the reversal of the zonal winds from easterly to westerly crossing the mesopause in the summer hemisphere. With the strong breaking effect, the GCM was able to simulate this reversal, and essentially eliminated the warm bias. This improvement was the result of a much stronger meridional transport circulation that possesses a strong vertical ascending branch in the summer upper mesosphere, and hence large adiabatic cooling. Budget analysis indicates that 'in the middle atmosphere the forces that act to maintain a steady zonal-mean zonal wind are primarily those associated with the meridional transport circulation and breaking gravity waves. Contributions from the interaction of the model-resolved eddies with the mean flow are small. To obtain a transport circulation in the mesosphere of the UIUC MST-GCM that is strong enough to produce the observed cold summer mesopause, gravity-wave forcing larger than 100 m/s/day in magnitude is required near the summer mesopause. In the tropics, only with the AD parameterization can the model produce realistic semiannual oscillations.

Yang, Fanglin↗
NASA NTRS1985

Gravity Waves

In recent years it has been widely recognized that vertically propagating gravity waves play an important role in the dynamics of the middle atmosphere in partly determining the large scale temperature and wind structure through their energy and momentum transport. In particular, the meridional temperature distribution and the mean wind profile in the upper mesosphere are considered to be largely affected by the turbulence diffusion and friction that gravity waves induce through their breaking. For an understanding of the middle atmosphere circulation, it is therefore very important to know the global distribution and temporal variation of gravity waves in a climatological sense. The results of the statistics for gravity wave activity in the stratosphere and mesosphere are presented as a function of latitude and month, with the aid of meteorological rocket network observations.

Hirota, I.↗
NASA NTRS2000

Long-term Global Morphology of Gravity Wave Activity Using UARS Data

An extensive body of research this quarter is documented. Further methodical analysis of temperature residuals in Cryogenic Limb Array Etalon Spectrometer (CLAES) Version 8 level 3AT data show signatures during December 1992 at middle and high northern latitudes that, when compared to Naval Research Laboratory/Mountain Wave Forecast Model (NRL)/(MWFM) mountain wave hindcasts, reveal evidence of long mountain waves in these data over Eurasia, Greenland, Scandinavia and North America. The explicit detection of gravity waves in limb-scanned Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA) temperatures is modeled at length, to derive visibility functions. These insights are used to convert CRISTA gravity wave temperature residuals into data that more closely resemble gravity wave fluctuations detected in data from other satellite instruments, such as Microwave Limb Sounder (MLS), Limb Infrared Monitor of the Stratosphere (LIMS) and Global Positioning System/Meteorology (GPS)/(MET). Finally, newly issued mesospheric temperatures from inversion of CRISTA 15gin emissions are analyzed using a new method that uses separate Kalman fits to the ascending and descending node data. This allows us to study global gravity wave amplitudes at two local times, 12 hours apart. In the equatorial mesosphere, where a large diurnal tidal temperature signal exists, we see modulations of gravity wave activity that are consistent with gravity wave-tidal interactions produced by tidal temperature variability.

Eckermann, Stephen D.↗
NASA NTRS2002

Sensitivity of Middle Atmospheric Analyses to the Representation of Gravity-Wave Drag in the DAO's Data Assimilation System

The damping of mesoscale gravity waves has important effects on the global circulation, structure, and composition of the atmosphere. A number of assimilation and forecast experiments have been conducted to examine the sensitivity of meteorological analyses and forecasts to the representation of gravity wave impacts in a data assimilation system (DAS). The experiments were conducted with the Finite-Volume (FV) DAS developed at NASA's Data Assimilation Office (DAO), The main purpose of this research is to determine the optimal combination of wave number, phase speed, wavelength, etc. for representing gravity-wave drag (GWD) in FVDAS. The GWD included in FVDAS includes a spectrum of waves, as would be forced by topography and transient motions (e.g., convection) in the troposphere. The sensitivity experiments are performed by modifying several parameters, such as the number of waves allowed, their wavelength, the background stress amplitude, etc. The results show that the assimilated fields are very sensitive to the number of gravity waves represented in the system, especially at high latitudes of the middle and upper stratosphere and mesosphere in winter. The analyzed stratopause temperature varies by up to 10K when the GWD scheme is modified from a multiple-wave scheme (using a stationary wave and waves with phase speeds of 10, 20, 30 and 40 m/s in each direction) to a single, stationary wave. Insight into the reality of the various versions of the GWD can be obtained by examining the "Observation minus Forecast" residuals from the FVDAS.

Li, Shuhua↗
NASA NTRS2008

Role of Gravity Waves in Determining Cirrus Cloud Properties

Cirrus clouds are important in the Earth's radiation budget. They typically exhibit variable physical properties within a given cloud system and from system to system. Ambient vertical motion is a key factor in determining the cloud properties in most cases. The obvious exception is convectively generated cirrus (anvils), but even in this case, the subsequent cloud evolution is strongly influenced by the ambient vertical motion field. It is well know that gravity waves are ubiquitous in the atmosphere and occur over a wide range of scales and amplitudes. Moreover, researchers have found that inclusion of statistical account of gravity wave effects can markedly improve the realism of simulations of persisting large-scale cirrus cloud features. Here, we use a 1 -dimensional (z) cirrus cloud model, to systematically examine the effects of gravity waves on cirrus cloud properties. The model includes a detailed representation of cloud microphysical processes (bin microphysics and aerosols) and is run at relatively fine vertical resolution so as to adequately resolve nucleation events, and over an extended time span so as to incorporate the passage of multiple gravity waves. The prescribed gravity waves "propagate" at 15 m s (sup -1), with wavelengths from 5 to 100 km, amplitudes range up to 1 m s (sup -1)'. Despite the fact that the net gravity wave vertical motion forcing is zero, it will be shown that the bulk cloud properties, e.g., vertically-integrated ice water path, can differ quite significantly from simulations without gravity waves and that the effects do depend on the wave characteristics. We conclude that account of gravity wave effects is important if large-scale models are to generate realistic cirrus cloud property climatology (statistics).

OCStarr, David↗
NASA NTRS1981

Theoretical and lidar studies of the density response of the mesospheric sodium layer to gravity wave perturbations

The density response of atmospheric layers to gravity waves is developed in two forms, an exact solution and a perturbation series solution. The degree of nonlinearity in the layer density response is described by the series solution whereas the exact solution gives insight into the nature of the responses. Density perturbation in an atmospheric layer are shown to be substantially greater than the atmospheric density perturbation associated with the propagation of a gravity wave. Because of the density gradients present in atmospheric layers, interesting effects were observed such as a phase reversal in the linear layer response which occurs near the layer peak. Once the layer response is understood, the sodium layer can be used as a tracer of atmospheric wave motions. A two dimensional digital signal processing technique was developed. Both spatial and temporal filtering are utilized to enhance the resolution by decreasing shot noise by more han 10 dB. Many of the features associated with a layer density response to gravity waves were observed in high resolution density profiles of the mesospheric sodium layer. These include nonlinearities as well as the phase reversal in the linear layer response.

Shelton, J. D.↗
NASA NTRS1986

Mesospheric wave number spectra from Poker Flat MST radar measurements compared with gravity-wave model

The results of a comparison of mesospheric wind fluctuation spectra computed from radial wind velocity estimates made by the Poker Flat mesosphere-stratosphere-troposphere (MST) radar are compared with a gravity-wave model developed by VanZandt (1982, 1985). The principal conclusion of this comparison is that gravity waves can account for 80% of the mesospheric power spectral density.

Smith, S. A.↗
NASA NTRS1986

Possibility of measuring gravity-wave momentum flux by single beam observation of MST radar

Vincent and Reid (1983) proposed a technique to measure gravity-wave momentum fluxes in the atmosphere by mesosphere-stratosphere-troposphere (MST) radars using two or more radar beams. Since the vertical momentum fluxes are assumed to be due to gravity waves, it appears possible to make use of the dispersion and polarization relations for gravity waves in extracting useful information from the radar data. In particular, for an oblique radar beam, information about both the vertical and the horizontal velocities associated with the waves are contained in the measured Doppler data. Therefore, it should be possible to extract both V sub Z and V sub h from a single beam observational configuration. A procedure is proposed to perform such an analysis. The basic assumptions are: the measured velocity fluctuations are due to gravity waves and a separable model gravity-wave spectrum of the Garrett-Munk type that is statistically homogeneous in the horizontal plane. Analytical expressions can be derived that relate the observed velocity fluctuations to the wave momentum flux at each range gate. In practice, the uncertainties related to the model parameters and measurement accuracy will affect the results. A MST radar configuration is considered.

Liu, C. H.↗