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G. D. Nastrom

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G. D. Nastrom

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Analytic solutions of a balloon's response to simultaneous variations of the vertical wind speed and the density are obtained using Fourier expansion methods. It is assumed that density and wind are in phase quadrature. The balloon's response depends on wave period, density and wind amplitudes, and static stability. When density and wind amplitudes are related as in a gravity wave the amplitude of the balloon's velocity response varies from about 75 to 105% of the wind amplitude as the lapse rate condition varies from isothermal to adiabatic, for a typical wave period (13.8 min) and air motion amplitude (30 cm s−1). Further, balloon phase leads air motion phase by ∼30° for adiabatic lapse, but lags the air motion by ∼25° for isothermal lapse conditions at this wave period. For very long period waves the balloon asymptotically approaches a true isopycnic tracer (with some phase shift) for high static stability, but not for low static stability.

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G. D. Nastrom

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G. D. Nastrom

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Ozone measurements taken from commercial airliners (GASP data) are used to estimate the vertical and horizontal fluxes of ozone near the tropopause. The annual average flux of O3 into the troposphere at 30–50°N is 7.8 × 1010 molecules cm−2 s−1, which is nearly the same as indirect estimates based on surface O3 data, thus supporting the hypothesis that the amount of ozone in the troposphere is essentially controlled by injection from the stratosphere. The present GASP estimates of the total flux of O3 into the troposphere verify the model results of Cunnold et al. (1975), although the distribution of flux between mean motions and diffusion is different and thus suggests that models with coarse horizontal resolution must continue to parameterize much vertical transport by diffusion coefficients. A significant variation in the east-west spatial autocorrelation function of O3 near the tropopause is found to be about 1900 km. Monthly estimates of the horizontal transient eddy flux of ozone are generally smaller than seasonal or yearly results based on ozonesonde data. This is perhaps because the present estimates are made over monthly periods to reduce the influence of correlation between the annual variations in ozone and meridional wind. The available data support the hypothesis that transient eddy fluxes of O3 have large longitudinal variations.

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R. Rüster, G. D. Nastrom, and G. Schmidt

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Measurements using the Sounding System (SOUSY) VHF radar in the Harz Mountains, Germany, were carried out in the troposphere and lower stratosphere in the summer of 1996. To study the structure and the dynamics with high temporal and spatial resolution, the beam was pointing continuously in the vertical direction, and a 75-m range resolution was used. Three case studies are analyzed in detail: one within a frontal passage, one during a developing ridge, and one during a thunderstorm.

Fine structures in the echo intensity that could not be resolved with the standard resolution of 300 m are observed during the frontal passage and are related to humidity fluctuations. Integrating the high-resolution velocity data with time makes it possible to estimate vertical displacements that are used to indicate vertical transport. The velocities are analyzed harmonically and interpreted as waves with periods of about 6 h and amplitudes of a few centimeters per second.

Stable layers evident in the troposphere and lower stratosphere during the approach of a building ridge of anticyclonic flow have slopes with time reminiscent of the latitudinal pattern of the isentropes of the larger-scale flow field.

Observations during the thunderstorm reveal strong convective cells with periods of about 10–20 min, associated up- and downdrafts, and a series of persistent stable layers in the troposphere ahead of the thunderstorm.

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G. D. Nastrom, R. Rüster, and G. Schmidt

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The perturbations to the static stability (and hence to the radar reflectivity) and to the velocity in a vertically propagating gravity wave are correlated, and the sign of the correlation depends on whether the wave is propagating upward or downward. The wave-induced correlation between radar reflectivity and vertical velocity is the basis of a hypothesis to explain the downward bias in long-term averages of the vertical velocity seen at extratropical sites by wind profiler radars, and for predictions of biases in the horizontal wind speeds and in the vertical momentum flux seen by profiler radars. In this study, the hypothesis that mean vertical velocity is related to the correlation between perturbations to vertical velocity and signal power is tested. Observations with very high time and vertical resolution from the SOUSY VHF radar are used. It is found that the mean vertical velocity in the midtroposphere (2.4–6.3 km) is downward (upward) when the perturbations to vertical velocity and to backscattered power over this height range are negatively (positively) correlated. Similar results are found during summer and winter periods. Results are also similar when the radar was upwind of the nearby Harz Mountains compared to when the radar was downwind of the mountains.

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G. D. Nastrom and A. D. Belmont

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The 12-year mean temperature and the amplitude and phase of the quasi-biennial oscillation (QBO) and first three harmonies of the annual wave are presented on height-latitude sections, 20 to 65 km, 80°N to 30°S. New features include adjusting the long-term mean temperature for errors due to solar radiation effects and for biasing by the diurnal tide. Due to the longer period of record used here, the extratropical QBO differs from that reported previously in the literature. Amplitudes of the annual wave at 30°S are larger than those at 30°N at all levels; the amplitude ratio is greatest near 50 km. The largest amplitude (7°C) of the semiannual wave in the stratosphere or mesosphere is near 75°N at 32 km. The terannual wave's amplitude near 35 km at 55°N is as large as the amplitude of the semiannual wave there and is larger than the well-known tropical %semiannual wave. These thermal properties of the upper atmosphere require theoretical explanations, stratosphere modelers should be able to reproduce them, and continued observations are needed to describe their hemispheric differences at high latitudes and altitudes.

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G. D. Nastrom and A. D. Belmont

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The diurnal component in meridional wind is estimated for each season at twelve rocket stations. Amplitudes and phases are presented as a function of height-latitude or as vertical profiles. Many of the gross features of the tide persist throughout the year, but as they migrate in height and latitude the amplitude or phase at a given location may undergo large changes with season. Longitudinal variations in the diurnal tide are found in the mid-stratosphere, and it is suggested they are coupled with longitudinal variations in the tropospheric temperature structure.

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G. D. Nastrom and F. D. Eaton

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The mean vertical profiles of the winds from about 5 to 20 km at White Sands Missile Range, New Mexico, are described. The variability of wind speed, spectral width, volume reflectivity calibrated as C N 2, and vertical wind shear are documented as functions of season and of time of day using observations taken from 1991 through April 1994 with the 50-MHz profiling radar. The mean meridional winds are from the south at about 1-3 m s−1 during every season except autumn, and mean zonal winds have a broad jet near the tropopause with maximum speed over 30 m s−1 during the winter. The mean vertical velocity is downward at about 5 cm s−1 in the troposphere and is weakly upward in the lower stratosphere. The shear of the mean wind and the mean wind shear have small interseasonal variability. The variance over 1-h periods of all three wind components, the spectral width, and C N 2 have lognormal frequency distributions. The variance of the meridional wind speed is greater than that of the zonal wind speed in the troposphere, but in the stratosphere during winter and spring the variance of the zonal wind speed is greater. The mean profiles of logC N 2 in the stratosphere are nearly constant with altitude and from season to season, ranging over only a few decibels. Diurnal cycles of wind speed have amplitudes on the order of 1 m s−1, but the phases are highly variable with height and season, suggesting strong local topographic control of the observed diurnal cycles. The diurnal cycles of C N 2, spectral width, and of the variance of the vertical velocity have the largest amplitudes in the troposphere where the daily maxima are during the afternoon.

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G. D. Nastrom and F. D. Eaton

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Doppler spectra taken with the VHF Doppler radar at White Sands Missile Range are used to describe the winds and turbulence for 10 days in March–April 1991. The large power aperture product of this radar provides excellent data coverage in 150-m layers over the entire height range used, about 5–20 km. The results show that gravity-wave activity and small-scale turbulence are significantly enhanced at all levels during times when wind speeds in the troposphere, near 5.6 km (about 500 hPa), are strong. Largest enhancements are found in the lower stratosphere, near 16–18 km, where the mean log C 2 N is increased by over 10 dB during times of strong winds at low levels. Mean winds, wind shears, and static stability in the lower stratosphere were found to be nearly the same, regardless of wind speeds at low levels. The authors conclude that the enhanced turbulence is due to an effect not described by the local background wind and static stability, and suggest that this effect is upward-propagating gravity waves launched in the troposphere during the periods of strong winds.

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