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.

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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

The physical properties of meteoroids producing meteor plasma non-field-aligned irregularities (NFAI) in the Earth’s atmosphere are poorly known. Here we report a complete picture of a fireball and mesospheric NFAI that it produced for the first time. Simultaneous radar and optical observations were made by the recently completed facility, Meteor and ionospheric Irregularity Observation System. The observations show that the mesospheric NFAI were clustered into three patches where the optical meteor flares took place, instead of being generated continuously along the whole meteor path. It is very likely that nanometer-or-larger-sized dust particles could be directly generated via meteoroid fragmentation at the flaring points and thus promote the generation of NFAI patches. The properties of the parent meteoroid show a chondrite type and a Jupiter family comet orbit, with Na/Mg and Fe/Mg intensity ratios of 1.5 and 1, respectively, photometric mass of about 4 g, and fragmentation strengths of around 10–74 kPa. The results suggest that the direct generation of dust particles, which was previously observed in the atmospheric disintegration of a kiloton-scale meteoroid, could be extended to the much smaller gram-scale meteoroids. Since meteoroids having such characteristics or more fragile material are not unusual, further studies leading to a better understanding of meteor mass deposition in the upper atmosphere, which consider the dust particles directly generated via gram-scale meteoroid fragmentation, are extremely important.

The Spectrum and Orbit of a Fireball Producing Mesospheric Irregularity and Implications for Meteor Mass Deposition

There has been a sequence of experiments aimed at improving our understanding of tropical thunderstorms. For example, an experiment combining polarimetric radar and multiple frequency wind profiler observations was performed in 1997 near Darwin, northern Australia with the aim of characterizing the vertical motion and microphysical structure of storms and comparing polarimetric (see Keenan et al., 1998) and profiler observations of quantitative rainfall, drop size and mixed phase (May et al., 2001). A weakness of these experiments has been the lack of thermodynamic data to support the kinematic storm analyses, To try and address this, there have been two experiments aimed at extending these studies to include thermodynamic information through the use of 50 MHz RASS (Radio Acoustic Sounding System) observations in January/February 2000 and 2001. This allows the direct measurement of the virtual temperature profile within the convection. This combined with the precipitation and vertical motion information gives a fairly complete picture of the internal structure of the storms. One case from each of these periods will be discussed. The first case is observations of a cell that in a squall line that was decaying as it moved over the profiler and the second was an isolated active deep cell.

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Wind Profiler/RASS observations of deep convection

: All-sky meteor radars have become a reliable and widely used tool to observe horizontal winds in the mesosphere and lower thermosphere (MLT) region. The horizontal winds estimated by conventional single-station radars are obtained after averaging all meteor detections based on the assumption of the homogeneity of the horizontal wind in the meteor detection area (approximately 200-300 km radius). In this study, to improve the horizontal winds, we apply a multistatic meteor radar system consisting of a monostatic meteor radar in Mengcheng (33.36 °N, 116.49 °E) and a bistatic remote receiver in Changfeng (31.98 °N, 117.22 °E), separated by approximately 167 km to increase the number of meteors by at least 70% and provide two different viewing angles of the meteor echoes. The accuracy of the horizontal wind measurement depends on the meteor number in time and altitude intervals. Compared to typical monostatic meteor radar, our approach shows the feasibility of estimating the two-dimensional horizontal wind field. The technique allows us to estimate the mean

Tracked changes Multistatic meteor radar observations of two-dimensional horizontal MLT wind

Reply [to Comment on Paper: Trends of airglow imager observations near Adelaide, Australia by J.

There is a growing need to detect and identify the multitude of space objects orbiting Earth, largely driven by the increase in space debris competing with a drive to launch more satellites into orbit. Radar sensors are arguably under-utilised for observing unique characteristics of space objects, offering well-known advantages over more commonly used optical sensors, including the capability of operating both day and night and in all weather conditions. Although radar systems are adapting to perform observations of space objects, micro Doppler analysis has the potential to provide a unique RF signature based on rotational motion. Observed satellite rotational motion is presented in this paper for the first time using a VHF radar system, and we discuss the processing requirements to extract the micro Doppler information. Using a simple computer aided design (CAD) model of a satellite and electromagnetic simulation software, we gain insight into the ideal micro Doppler signature in isolation from ionospheric effects at VHF. The simulations not only provide assistance with interpreting the real data but are also validated for the potential use in automatic classification algorithms.

Iain M. Reid

Papers

The Spectrum and Orbit of a Fireball Producing Mesospheric Irregularity and Implications for Meteor Mass Deposition

Wind Profiler/RASS observations of deep convection

Tracked changes Multistatic meteor radar observations of two-dimensional horizontal MLT wind

Reply [to Comment on Paper: Trends of airglow imager observations near Adelaide, Australia by J.

Analysis of RF Signatures for Space Domain Awareness using VHF radar