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David C. Fritts and Robert A. Vincent

Abstract

We present here the results of an analysis of gravity wave momentum fluxes in the mesosphere and lower thermosphere, inferred using a dual-beam Doppler radar near Adelaide, Australia during June 1984. Our analysis reveals that over 70% of the momentum flux and of the inferred zonal drag was due to gravity waves with observed periods less than one hour. This suggests that it is the gravity waves with high intrinsic frequencies and small horizontal scales that are most effective at transporting momentum into the middle atmosphere.

The temporal variations in the momentum flux and flux divergence due to high-frequency motions were also examined in detail. In addition to daily variability, a strong diurnal modulation was observed to occur. This was found to be correlated with the phase of large-amplitude diurnal tidal motions. As a result of these observations, a gravity wave–tidal interaction model was proposed which accounts for all of the major features of the observed data, including a reduction in the inferred diurnal tidal amplitude and an advance of its phase with time. We believe that this gravity wave–tidal interaction model may explain a majority of the tidal variability observed worldwide.

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Robert A. Vincent and David C. Fritts

Abstract

A statistical study of gravity wave motions in the mesosphere and lower thermosphere measured with a MF partial reflection radar located at Buckland Park new Adelaide (35°S, 138°E) in the period November 1933 to December 1984 is presented. The analyses am confined to waves with ground based periods between 1 and 24 h. Time-height cross sections show that the mean square amplitudes u2 and v2, of the zonal and meridional perturbation velocities, respectively, vary in a predominantly semiannual manner such that the minima in wave activity coincide with the reversals in the zonal circulation in the middle atmosphere. In most instances, v2 is greater than u2 which, together with the small but nonzero uv fluxes shows that the gravity wave field is partially polarized. A technique similar to that used to analyse partially polarized electromagnetic waves suggests that on a seasonal basis, the wave field is polarized by about 10% to 20% but for shorter periods the degree of polarization may be significantly higher. It is found that the polarized waves with periods greater than 1 h are probably propagating in directions essentially east of south in summer, while in winter the 1–8 h waves are propagating towards the southwest and the 8–24 h waves are propagating towards the northwest.

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A Hole in the Weather Warning System

Improving Access to Hazardous Weather Information for Deaf and Hard of Hearing People

Vincent T. Wood and Robert A. Weisman

In this article, the problems deaf and hard of hearing people experience when attempting to access the weather warning systems in Oklahoma and Minnesota are documented. Deaf and hard of hearing people cannot hear Civil Defense sirens, cannot listen to local radio stations that are broadcasting emergency information through the Emergency Alert System, cannot access weather warnings through conventional National Oceanic and Atmospheric Administration (NOAA) Weather Radio, and often have problems obtaining weather information from local television stations due to the lack of text information. These problems had forced deaf and hard of hearing people to rely on looking at the sky or having hearing people alert them as their primary methods of receiving emergency information. These problems are documented through the use of a survey of277 deaf and hard of hearing people in Minnesota and Oklahoma as well as specific examples.

During the last two years, some progress has been made to “close this hole” in the weather warning system. The Federal Communications Commission has approved new rules, requiring that all audio emergency information provided by television stations, satellite, and cable operators must also be provided visually. In addition, the use of new technology such as pager systems, weather radios adapted for use by those with special needs, the Internet, and satellite warning systems have allowed deaf and hard of hearing people to have more access to emergency information.

In this article, these improvements are documented but continuing problems and possible solutions are also listed.

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Stephen D. Eckermann and Robert A. Vincent

Abstract

Four extended observational campaigns were conducted during August and November 1988 with an ST (stratosphere–troposphere) radar in southern Australia during the passage of cold fronts over the system, giving around 30 days of three-dimensional wind measurements with 15-min time and 0.5-km height resolution over the 2–11.5-km height range. Order of magnitude increases in the variance of time-fluctuating wind velocities were measured during frontal passages, which are definitively ascribed to gravity waves. The time–height morphology of the horizontal- and vertical-velocity fluctuations differed. Bursts of horizontal-velocity variance u2 + υ2 ∼ 10–100 m2 s−2 arose at upper levels about a day before the frontal boundary arrived, and this activity gradually extended to lower heights as the front neared. The arrival of the frontal boundary marked a sudden reduction in this activity. After the frontal boundary passed, reduced activity persisted for ∼ 12 hours, after which bursts in u2 + υ2 returned at upper levels and persisted typically for about a day. These bursts arose in regions of high mean wind speeds (∼20–50 m s−1), and analysis associates this activity with a spectrum of many saturating inertia–gravity waves with long horizontal wavelengths and large ground-based phase speeds. Strong interaction between the waves and the mean flow is likely. In contrast, bursts in vertical-velocity fluctuations, w′, were confined almost entirely to the troposphere and were quasi-sinusoidal in appearance. These fluctuations are ascribed to gravity waves with high intrinsic frequencies. Significant w′ amplitudes were evident both after and prior to frontal passage, but the largest amplitudes (w′ ∼ 0.5 m s−1) occurred with the onset of strong vertical circulation when the frontal boundary arrived. The smaller w′ amplitudes observed in the stratosphere are due in part to the more oblique propagation of wave energy in this more stable environment, but may also reflect vertical ducting of this activity at altitudes of small static stability just below the tropopause. Two clear cases of ducted w′ oscillations are identified with the aid of radiosonde temperature data from a nearby site. Comparisons between these measurements and the limited numerical modeling of frontal gravity waves show some similarities in wave characteristics.

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Robert Davies-Jones, Vincent T. Wood, and Mark A. Askelson

Abstract

Two accepted postulates for applications of ground-based weather radars are that Earth’s surface is a perfect sphere and that all the rays launched at low-elevation angles have the same constant small curvature. To accommodate a straight vertically launched ray, we amend the second postulate by making the ray curvature dependent on the cosine of the launch angle. A standard atmospheric stratification determines the ray-curvature value at zero launch angle. Granted this amended postulate, we develop exact formulas for ray height, ground range, and ray slope angle as functions of slant range and launch angle on the real Earth. Standard practice assumes a hypothetical equivalent magnified earth, for which the rays become straight while ray height above radar level remains virtually the same function of the radar coordinates. The real-Earth and equivalent-earth formulas for height agree to within 1 m. Our ultimate goal is to place a virtual Doppler radar within a numerical or analytical model of a supercell and compute virtual signatures of simulated storms for development and testing of new warning algorithms. Since supercell models have a flat lower boundary, we must first compute the ray curvature that preserves the height function as the earth curvature tends to zero. Using an approximate height formula, we find that keeping planetary curvature minus the ray curvature at zero launch angle constant preserves ray height to within 5 m. For standard refraction the resulting ray curvature is negative, indicating that rays bend concavely upward relative to a flat earth.

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Deepak K. Rajopadhyaya, Peter T. May, and Robert A. Vincent

Abstract

A technique is described that allows estimates of the raindrop size distribution to be obtained from the Doppler spectra measured by wind-profiling radars. The method makes no a priori assumptions regarding the shape of the drop size distributions. To test the accuracy of the technique, artificial data with realistic statistical properties have been generated and the shape of the model drop size distribution varied, The analysis technique obtains an accuracy of around 10% in the drop size range between 1 and 4 mm for data consistent with typical 50-MHz observations averaged over 5–10 min. There are limitations outside this range and the physical reasons for these are discussed. Simulations with multiple-peaked distributions show that the technique can also well resolve complicated distributions

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Deepak K. Rajopadhyaya, Peter T. May, and Robert A. Vincent

Abstract

Recently, the authors observed significant echoes from precipitating ice particles above the freezing level in the stratiform region of tropical squall lines with a 50-MHz wind-profiling radar. A technique is described that allows ice particle size distributions to be obtained from the 50-MHz wind-profiling radar spectra. A model ice echo is developed in which it is assumed the number density of ice particles is exponentially distributed. A composite spectrum of dendrites, plates, columns, and bullets is assumed, and good fits to the observed spectra are obtained. To test the reliability and stability of the retrieval technique, simulated data with realistic statistical properties were generated and the shape of the model size distribution varied. Only two types of ice particles are considered. It is shown that the population spectra are recovered well for a wide range of exponential slopes. Relative precision of about 10% is obtained when the clear-air spectral width is 0.1 m s−1, and is about 30% when the spectral width is 0.3 m s−1. However, when the spectral width is large, such as 0.5 m s−1, the relative error can exceed 100%. Spectral widths of about 0.3 m s−1 are typically observed in the trailing stratiform region of tropical squall lines.

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Christopher Lucas, Peter T. May, and Robert A. Vincent

Abstract

An algorithm to detect frontal zones in time–height cross sections of horizontal wind from wind profiler measurements is described. The algorithm works by identifying regions with 1) a strong horizontal temperature gradient, estimated by using a quasigeostrophic thermal wind retrieval, 2) a strong temporal increase in the signal-to-noise ratio at a given range gate, and/or 3) a strong temporal shift in the horizontal winds at a given range gate. The type (e.g., cold or warm) of front is determined by examining the advection field and the characteristics of the boundary. Most weight is given to the horizontal temperature gradient component of the algorithm.

A springtime frontal system and an associated baroclinic wave over South Australia are examined using both routine synoptic observations and analyses as well as data from the profiler. Synoptic observations depict a prefrontal trough and two cold fronts at the surface and a deep trough in upper levels. The tropopause is identified at ∼6 km in one sounding. The algorithm successfully identifies the one main cold front and the lowered tropopause in the polar air. There are also hints of a prefrontal trough and a descending tropopause with the onset of the main cold front. After the passage of the upper trough, the ascending tropopause and the so-called jet front or trailing front are also identified by the algorithm. The latter represents the passage of the upper-level baroclinic wave and the reappearance of a strong jet stream.

Other regions are spuriously identified as fronts. These regions could be the reflection of some short-term meteorological phenomena, such as gravity waves; deviations from the assumed quasi-geostrophy; or simply reflections of noise in the analysis. An examination of the effect of random measurement uncertainties on the frontal analysis gives an estimate of error of around 2 K (100 km)−1 in the horizontal temperature gradient calculations for typical wind errors. The errors on the retrieved advection vary, depending on the wind speed, but are around 25 K day−1 for a ∼20 m s−1 wind speed. These values are typical of the noise in those fields, suggesting that the spuriously defined fronts likely reflect uncertainties in the data rather than actual meteorological phenomena.

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Gillian Boccara, Albert Hertzog, Robert A. Vincent, and François Vial

Abstract

A methodology for estimating gravity wave characteristics from quasi-Lagrangian observations provided by long-duration, superpressure balloon flights in the stratosphere is reviewed. Wavelet analysis techniques are used to detect gravity wave packets in observations of pressure, temperature, and horizontal velocity. An emphasis is put on the estimation of gravity wave momentum fluxes and intrinsic phase speeds, which are generally poorly known on global scales in the atmosphere. The methodology is validated using Monte Carlo simulations of time series that mimic the balloon measurements, including the uncertainties associated with each of the meteorological parameters. While the azimuths of the wave propagation direction are accurately retrieved, the momentum fluxes are generally slightly underestimated, especially when wave packets overlap in the time–frequency domain, or for short-period waves. A proxy is derived to estimate by how much momentum fluxes are reduced by the analysis. Retrievals of intrinsic phase speeds are less accurate, especially for low phase speed waves. A companion paper (Part II) implements the methodology to observations gathered during the Vorcore campaign that took place in Antarctica between September 2005 and February 2006.

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M. Joan Alexander, Toshitaka Tsuda, and Robert A. Vincent

Abstract

Knowledge of the latitudinal variations in the occurrence of gravity waves is important for their parameterization in global models. Observations of gravity waves with short vertical scales have shown a pronounced peak in wave activity at tropical latitudes. In this paper, it is shown that such a peak may be a natural consequence of the latitudinal variation in the Coriolis parameter, which controls the lower limit for gravity-wave intrinsic frequencies ω̂. Two distinct but related effects of this parameter on observations of gravity-wave activity are explained and explored with a simple model. The results are also compared to observed latitudinal variations in gravity-wave activity. The authors formally distinguish between observed gravity-wave spectra and what is called gravity-wave “source spectra,” the latter being appropriate for input to gravity-wave parameterizations. The results suggest that the ω̂ −5/3 dependence of the gravity-wave energy spectrum commonly assumed as input to parameterizations is likely too steeply sloped. Much more shallowly sloped spectra for gravity-wave parameterization input ∝ω̂ −0.6ω̂ −0.7 show better agreement with observations. The results also underscore the potential importance of intermittency in gravity-wave sources to the interpretation of gravity-wave observations.

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