There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

[1] Atmospheric gravity waves have been a subject of intense research activity in recent years because of their myriad effects and their major contributions to atmospheric circulation, structure, and variability. Apart from occasionally strong lower-atmospheric effects, the major wave influences occur in the middle atmosphere, between ∼ 10 and 110 km altitudes because of decreasing density and increasing wave amplitudes with altitude. Theoretical, numerical, and observational studies have advanced our understanding of gravity waves on many fronts since the review by Fritts [1984a]; the present review will focus on these more recent contributions. Progress includes a better appreciation of gravity wave sources and characteristics, the evolution of the gravity wave spectrum with altitude and with variations of wind and stability, the character and implications of observed climatologies, and the wave interaction and instability processes that constrain wave amplitudes and spectral shape. Recent studies have also expanded dramatically our understanding of gravity wave influences on the large-scale circulation and the thermal and constituent structures of the middle atmosphere. These advances have led to a number of parameterizations of gravity wave effects which are enabling ever more realistic descriptions of gravity wave forcing in large-scale models. There remain, nevertheless, a number of areas in which further progress is needed in refining our understanding of and our ability to describe and predict gravity wave influences in the middle atmosphere. Our view of these unknowns and needs is also offered.

/pdf/gravity-wave-dynamics-and-effects-in-the-middle-atmosphere-4huexaq7y8.pdf

Gravity wave dynamics and effects in the middle atmosphere

The influence of breaking gravity waves on the dynamics and chemical composition of the 60- to 110-km region has been investigated with a two-dimensional dynamical/chemical model that includes a parameterization of gravity wave drag and diffusion. The momentum deposited by breaking waves at mesospheric altitudes reverses the zonal winds, drives a strong mean meridional circulation, and produces a very cold summer and warm winter mesopause, in general agreement with observations. The seasonal variations of the computed eddy diffusion coefficient are consistent with the behavior of mesospheric turbulence inferred from MST radar echoes. In particular, it is found that eddy diffusion is strong in summer and winter but much weaker at the equinoxes and that this seasonal behavior has important consequences for the distribution of chemical species. Comparison between computed atomic oxygen and ozone, and the abundances of these constituents inferred from the 557.7-nm and 1.27-μm airglow emissions, reveals excellent agreement. The consistency between model results and these diverse types of observations lends strong support to the hypothesis that gravity waves play a very important role in determining the zonally averaged structure of the mesosphere and lower thermosphere.

The effect of breaking gravity waves on the dynamics and chemical composition of the mesosphere and lower thermosphere

Systematic westerly biases in the northern hemisphere wintertime flow of the Meteorological Office 15-layer operational model and 11-layer general circulation model are described. Evidence that the failure to parametrize subgrid-scale orographic gravity wave drag may account for such biases is presented. This evidence is taken from aircraft studies, surface pressure drag measurements, and studies of the zonally averaged momentum budget. A parametrization scheme is described in which the surface stress is proportional to the near-surface wind speed and static stability, and to the variance of subgrid-scale orography. The stress is absorbed in the vertical by considering the influence of such gravity wave activity on static stability and vertical wind shear. A Richardson-number-dependent wave breaking formulation is devised, and the vertical stress profile determined by a saturation hypothesis whereby the breaking waves are maintained at marginal stability. It is shown that wave breaking preferentially occurs in the boundary layer and in the lower stratosphere.

Results from a simple zonally symmetric model show how the adjustment to thermal wind balance with a wave drag in the stratosphere, warms polar regions by adiabatic descent, and decelerates the mean westerlies in the troposphere.

The influence of the parametrization scheme on integrations of the 11-layer model is described, and found to be generally beneficial.

In a discussion of the reasons why this problem has only recently emerged, it is suggested that the satisfactory northern hemisphere winter circulations of previous, coarser general circulation models were due to a compensation implied by underestimating both the surface drag, and the horizontal flux of momentum by explicitly resolved large-scale eddies.

Alleviation of a systematic westerly bias in general circulation and numerical weather prediction models through an orographic gravity wave drag parametrization

This paper provides a review of recent advances in our understanding of gravity wave saturation in the middle atmosphere. A brief discussion of those studies leading to the identification of gravity wave effects and their role in middle atmosphere dynamics is presented first. This is followed by a simple development of the linear saturation theory to illustrate the principal effects. Recent extensions to the linear saturation theory, including quasi-linear, nonlinear, and transient effects, are then described. Those studies addressing the role of gravity wave saturation in the mean circulation of the middle atmosphere are also discussed. Finally, observations of gravity wave motions, distribution, and variability and those measurements specifically addressing gravity wave saturation are reviewed.

Gravity wave saturation in the middle atmosphere: A review of theory and observations

[1] The ability of all-sky interferometric meteor radars to measure mean wind and high-frequency gravity wave wind perturbations from meteor radial velocities is assessed. A Monte-Carlo technique that models line-of-sight meteor wind measurements with realistic errors in angle-of-arrival and range is used to investigate uncertainties in mean wind and gravity wave wind parameters as a function of meteor echo rate. It is shown that mean horizontal wind speeds are recovered with reasonable accuracy at meteor rates as low as 10 hr−1. Mean-square horizontal wind perturbation can be derived with relatively little averaging, but momentum fluxes are recovered with much less accuracy, which means that considerable averaging is required to produce meaningful values. Results are illustrated using meteor wind radar observations taken over a 30-day period in January–February 2006 in Northern Australia.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2010GL044086

Gravity wave flux retrievals using meteor radars

[1] We studied the behavior of intraseasonal oscillations (ISO) of the zonal wind velocity in the equatorial mesosphere and lower thermosphere (MLT) using simultaneous observations during 500 days from 1 January 1996 with a meteor radar at Jakarta (6°S, 107°E) and two medium-frequency radars at Pontianak (0°N, 109°E) and Christmas Island (2°N, 157°W). Cross-correlation analysis of the ISO at 88 km indicated that the phase difference of the ISO between Pontianak and Christmas Island is, on average, small in spite of the longitudinal difference of ∼90°. Therefore the ISO in the equatorial MLT region seems to be a variation of zonal mean flow. At Jakarta the amplitude of the ISO of the zonal wind at 88 km was somewhat smaller than that observed at the other two equatorial sites. The peak of the ISO amplitude appears at ∼88 km at both Pontianak and Christmas Island, but the peak seemed to be slightly lower at Jakarta. We also investigated long-term variation of the ISO of the zonal wind at 84 km using Jakarta meteor radar data observed from January 1993 to October 1999. Biennial variations are dominant in the ISO amplitude of the mesospheric zonal winds and the zonal amplitude variations of the diurnal tide. A relation between the ISO of the zonal wind and tidal amplitude is suggested. However, gravity wave activity in the MLT region did not correlate with the ISO in the zonal wind. We employed the outgoing longwave radiation (OLR) in the tropical troposphere as an index for excitation intensity of the atmospheric waves. The long-term variations of the ISO components in the OLR over the western Pacific were characterized by the tropospheric biennial oscillations, which seems to correlate with the ISO of the mean zonal winds and diurnal tides in the mesosphere.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2003JD003378

Intraseasonal oscillations of the zonal wind near the mesopause observed with medium‐frequency and meteor radars in the tropics

We conducted meteor echo observations using the Buckland Park MF radar (35°S, 138°E) at 00:40–05:45 LT on October 22, 1997 In addition to the usual full correlation analysis (FCA) technique to measure horizontal wind velocities from 60 to 100 km MF radars have a potential to detect meteor echoes and infer winds through their Doppler frequency shifts Because of the relatively low radio frequency employed MF radars have a great advantage of providing meteor wind well above 100 km altitude, where very few techniques can measure wind velocities There is a limitation which should be noted as well The observations are possible only during night time when the electron density of E-region is low enough for the radio wave to penetrate into the upper region We detected 233 underdense meteor echoes from 80 km to 120 km with a mean height of 1044 km Although the transmitting antenna beams were steered toward off-zenith angles of 25°, almost all the echoes were received outside of the main lobe, indicating that conventional MF radar systems with a broad transmitting beam can work well for meteor observations Bi-hourly wind profiles were obtained from 94 to 114 km altitudes The profiles revealed a clear wave structure with a downward phase progression with time FCA winds from 80 to 100 km were also estimated, and a continuous wind structure was obtained from FCA to meteor heights Note that the present observations happened to be conducted during a major meteor shower activity However, a majority of the underdense echoes were from non-shower meteors, and observations during non-shower periods will also yield enough information

/pdf/meteor-observations-with-an-mf-radar-1ph59h7uon.pdf

Meteor observations with an MF radar

The Buckland Park Aerial Array (35° S, 138° E) is situated about 40 km north of Adelaide, Australia, on a flat coastal plain. It was designed by B. Briggs with assistance from G. Elford and constructed between 1965 and 1968. The first results were published in the late 1960s (Briggs and Elford, 1968; Briggs et al., 1969). Since then some 250 papers have been published using data from the array. Some aspects of the history of the array are described by Briggs (1993). A new medium frequency (MF) Doppler radar utilizing the array has been developed, and this paper describes some of the technical details of this new facility.

Iain M. Reid

Papers

Gravity wave flux retrievals using meteor radars

Intraseasonal oscillations of the zonal wind near the mesopause observed with medium‐frequency and meteor radars in the tropics

Meteor observations with an MF radar

The new Adelaide medium frequency Doppler radar

Observations of mesospheric wind velocities 2