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G. D. Nastrom and T. E. VanZandt

Abstract

Radar reflectivity for Bragg scattering is proportional to the atmospheric static stability. In vertically propagating gravity waves the perturbations to the static stability and to the winds occur either in phase or out of phase, so that the radar reflectivity and the wind perturbations are correlated. This correlation leads to a bias in winds observed by radars and any other remote sensors that rely on Bragg scattering. The magnitude of the biases of the vertical and horizontal wind components due to a monochromatic wave propagating in the radar beam are found to be proportional to the amplitude squared of the gravity wave, and for a spectrum of waves they are proportional to the spectral energy. For radar systems with two coplanar beams, the bias to the observations of horizontal wind speed is about 0,2 m s−1 for gravity wave amplitudes typically encountered over flat terrain at midlatitudes and increases to 1 m s−1 or more for wave amplitudes seen over mountainous terrain and in the vicinity of fronts, etc. For radars with only one oblique beam, the magnitude of the bias in the horizontal wind speed due to waves with typical amplitudes ranges from near zero to several meters per second, depending on wave amplitudes and on the zenith angle of the beam. The bias to the mean vertical velocity is a few centimeters per second for similar wave conditions. Variances of velocities along oblique beams also have a bias due to vertically propagating gravity waves; ranging from near zero to about 0.5 m2 s−2 depending on the zenith angle of the beam and on the ratios of the radar vertical range-gate size and temporal averaging period relative to the wave vertical wavelength and the wave period. The observed vertical momentum flux is about 20% smaller than the true momentum flux due to this bias effect.

The theoretical predictions of biases due to gravity waves are compared with observations from the Flatland 50-MHz radar, located in the very flat terrain of central Illinois. It is found that the magnitude and the sign of the observed differences between eastward- and westward-directed beams are about the same size as expected for gravity waves with amplitudes typically observed at Flatland. The mean momentum flux for all cases combined is also consistent with the predictions of this theory for wave energy propagation upward toward the east, whereas the momentum flux for those cases with large variances in the midtroposphere at Flatland is about −0. 1 5 m2 s−2 and is consistent with wave energy propagation downward toward the cast (or upward toward the west).

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G. D. Nastrom and T. E. VanZandt

Abstract

Radar wind profilers have been used to measure directly the vertical motion above the radar site. Mean values of vertical motions in the troposphere and lower stratosphere reported at sites in and near mountains are often several centimeters per second and have often been attributed to the effects of quasi-stationary lee waves. However, observations now available at sites in the plains, far from any mountains, also show mean values of several centimeters per second. For example, monthly mean values seen by the Flatland VHF radar near Champaign-Urbana, Illinois, range from about −3 to −7 cm s−1, with largest magnitudes during the winter. The authors examine several of the hypotheses that have previously been advanced to explain these observations and find that each is inconsistent with the observations in some respect, except that quasi-horizontal flow along gently sloping isentropic surfaces leads to mean downward motion as large as 1–2 cm s−1. In this paper the authors suggest that the effects of vertically propagating gravity waves can account for most of the mean downward motions measured with radars, and the measured mean vertical motions can aptly be termed “apparent” mean vertical motions. In gravity waves with downward phase propagation (upward energy propagation), the perturbations to the static stability and to the vertical velocity are negatively correlated. Since the radar reflectivity is proportional to the static stability, regions of the radar sampling volume with downward (or less strongly upward) vertical air motion due to gravity waves are weighted more heavily. A model incorporating this suggestion is first developed for a monochromatic gravity wave and is then expanded to a spectrum of gravity waves. This model predicts a correlation between the magnitude of the downward motion seen by the radar and the gravity wave energy density; the predicted relationship is verified by the observations from the Flatland radar. Statistical analysis of data from Flatland suggests that in the midtroposphere about 60% of the gravity wave energy is contained in waves with downward propagation of phase. The present model for w applies to the reflectivity from any refractive-index irregularities that can be treated as passive scalars, whether they are in the neutral atmospheric density, aerosol density, or plasma density and whether they arise from isotropic turbulence, an-isotropic turbulence, Fresnel scattering etc.

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G. D. Nastrom and T. E. VanZandt

Abstract

Solutions of the nonlinear, coupled equations of horizontal and vertical balloon motions are obtained using standard expansion methods. Comparison of the results of these solutions with those from numerical integration and from solutions of the approximate, linear versions of the equations of motion shows that 1) the linear solutions provide reliable estimates of the transfer function for horizontal air motions, and 2) the nonlinear solution provides the salient features of coupling between horizontal and vertical motions. For example, a balloon's mean ascent rate will diminish slightly if the horizontal winds are fluctuating rapidly, while in no case do the nonlinear effects lead to significant frequency mixing of horizontal balloon motions.

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Daren Lu, T. E. VanZandt, and W. L. Clark

Abstract

The Platteville VHF Doppler radar, located on the Colorado piedmont near Platteville, Colorado, continuously measured the vertical wind velocity during a 12-day period in late July and early August 1981. Measurements were made every 2.5 min on the average with range gates centered at 3.3, 5.7, 8.1, 10.5, 12.9, 15.3, 17.7, and 20.1 km above sea level.

Periods of active thunderstorms were identified from the PPI maps from the National Weather Service 10 cm weather radar at Limon, Colorado. When no thunderstorm activity was present, the vertical velocity fluctuations were small and erratic. But a few hours after strong thunderstorm activity began, large quasi-sinusoidal wave trains with periods of about 40 min were observed. Power spectra of the vertical velocity time series showed enhancements at all frequencies during thunderstorm activity, but for periods longer than 30 min the enhancements were larger, particularly for the mid-tropospheric range gates from 5.7 to 12.9 km.

Some of the implications of these observations on the relations between thunderstorms and buoyancy waves in the free atmosphere are discussed.

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T. Tsuda, T. Inoue, S. Kato, S. Fukao, D. C. Fritts, and T. E. VanZandt

Abstract

We present vertical wavenumber spectra of mesoscale wind fluctuations using data observed in the troposphere, lower stratosphere and mesosphere by the MU radar at 35°N in Japan in October 1986 and June 1987, as well as lower stratospheric spectra obtained by the Arecibo UHF radar at 18°N in Puerto Rico in June 1983. These spectra are much more homogeneous than previously available spectra since all of the data were observed by the same radar technique, the data in the different atmospheric regions were taken essentially simultaneously, and all of the spectra were analyzed using very similar methods. In the large-wavenumber ranges of the observed spectra, the asymptomatic slopes and amplitudes agree well with the saturated gravity wave spectral model developed by Dewan and Good (1986) and Smith et al. (1987), which has a slope of −3 and a spectral amplitude proportional to the buoyancy frequency squared. The good agreement between the model spectrum and the observed spectra from different altitudes, different reasons, and two different stations located at 35° and 18°N suggests that the model is essentially correct, in spite of the heuristic nature of some of its assumptions.

The spectral densities of the zonal and meridional components are similar at large wavenumbers, while the meridional spectrum has larger energy density at small wavenumbers where the spectrum is not saturated. The dominant vertical scales of the gravity wave field in the mesosphere, lower stratosphere, and troposphere are estimated to be >10 km, 2.2 to 3.3 km, and ≥3.3 km in october and ≥4.5 km in June, respectively, consistent with determinations from previous studies.

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T. E. VanZandt, S. A. Smith, T. Tsuda, T. Sato, S. Fukao, S. Kato, and D. C. Fritts

Abstract

We present in this paper a study of the azimuthal anisotropy of the motion field observed during a six-day campaign in March 1986 using the MU radar in Shigaraki, Japan. The radial wind velocity was observed at 20° zenith angle, at every 30° of azimuth during four days, and at every 45° during two days. A jet stream was present during the entire six days. The average radial velocity variance from 10.4 to 19.2 km was calculated every four minutes and then averaged over 20 minutes or one hour.

The average variance was found to be a strong function of both azimuth and time. The azimuthal variations were analyzed in terms of the mean and the first and second harmonics. The mean is proportional to the kinetic energy per unit mass of the radial wind fluctuations, and the first harmonic is proportional to the vertical flux of horizontal momentum per unit mass. The strong azimuthal variation was usually dominated by the second harmonic; i.e., with two peaks, but was occasionally dominated by the first harmonic, with one peak. The phase of the first harmonic was usually westward, but the phase of the second harmonic was quite variable.

It was shown by a development of gravity wave theory that all of the observed azimuthal variations could probably be caused by a gravity wave field whose parameters vary with time.

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G. D. Nastrom, M. R. Peterson, J. L. Green, K. S. Gage, and T. E. VanZandt

Abstract

Observations of vertical velocity made with the Flatland VHF radar located in the extremely flat terrain near Champaign, Illinois, are used to study sources of enhanced variance. The variance is used as an indicator of gravity wave activity. In contrast to sites in or near mountains where lee wave activity often masks signals due to other sources, at Flatland we find that all episodes of enhanced variance are correlated with synoptic or mesoscale weather events, such as the passage of fronts or jet streams and convection. Case studies are used to characterize the sources of variance in the data, with specific examples from the spring of 1987. Also, summaries from data collected over the entire period March 1987 through May 1988 are presented. It is found that largest variances of vertical velocity are associated with low stability in the lower troposphere; most often indicated by clouds and convection and less frequently due to a dynamic feature such as strong winds or a front. 11 is found that wave activity is about 50% greater in the troposphere and lower stratosphere in the cloudy skies ahead of midlatitude storm systems than in the clear skies above the stable air behind storms.

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F. Einaudi, W. L. Clark, J. L. Green, T. E. VanZandt, and D. Fua

Abstract

In order to gain insight into the complex dynamics of a convective system interacting with a gravity wave train, we have carried out an experiment in northeast Colorado during July and August, 1983, utilizing data from several program areas in NOAA. Pressure data from the PROFS mesonetwork of microbarograph stations were combined with velocity profiles from the Wave Propagation Laboratory UHF wind profiler (ST) radar at Stapleton Airport in Denver and convective cell location data from the NWS Limon weather radar. Several events were clearly visible in the microbarograph data, from which four (called Events A, B, C and D) in late July were selected for further study. These events differed from each other in fundamental ways.

In each event the waves represent oscillations of a substantial depth of the troposphere and seem to appear and disappear together with the convective cells. In Events A and B the waves have a critical level and are probably unstable modes generated by wind shear in the jet stream, from which they extract energy. We suggest that the convective cells cause the selection of some modes over others in a system that is initially dynamically unstable. In Event A the wave appears to be locked together with the convective cells, which move at the same velocity as the phase velocity of the wave. The wave and the cells seem to grow and evolve synergetically. In Event B the wave and convective cells commence at about the same time, but the cell velocities are quite different from the wave phase velocity. The cell velocities vary substantially over the time of the event and appear to be controlled by the local winds.

In the Events C and D, the waves move faster than the maximum wind in the jet and at least twice as fast as the convective cells. It is suggested that these are nonsingular neutral modes whose excitation depends on a number of mechanisms, such as vertical convective motions and acceleration in the jet flow.

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G. D. Nastrom, W. L. Clark, K. S. Gage, T. E. VanZandt, J. M. Warnock, R. Creasey, and P. M. Pauley

Abstract

The hypothesis that temporal averages of vertical motions over a single radar station are representative of weather systems large enough to be resolved by the radiosonde network is tested using data from the Flatland VHF radar, located in the very flat terrain of central Illinois. Six-hourly means of radar data were compared with four separate estimates of the synoptic or subsynoptic-scale vertical motions computed using the dynamical equations with unsmoothed rawinsonde data and with NMC gridded analyses. Spring and fall cases of large upward and downward vertical motions were selected for study. During the course of this study it was found that contamination of the Doppler radar spectra by heavy or moderate precipitation must be taken into account during analyses of VHF radar data in the troposphere.

The signs of the vertical-motion estimates from the indirect schemes in the extreme cases selected for study here nearly always agree, although the magnitudes often differ by a factor up to about 4. The adiabatic method was found to be unrepresentative due to the large time separation of radiosonde measurements. The 6-b average radar observations usually fall within the envelope of estimates from the various indirect methods. The major source of statistical uncertainty of the temporal means of the vertical motions seen by the radar is the mesoscale structure seen in shorter-period averages and not completely filtered out during averaging. Such structure is not resolved by the radiosonde network data and analyses.

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S. Grivet-Talocia, F. Einaudi, W. L. Clark, R. D. Dennett, G. D. Nastrom, and T. E. VanZandt

Abstract

This paper presents a climatology of coherent disturbances detected during 1991–95 by a network of barometers with a diameter of about 50 km located in a very flat terrain centered on the Flatland Atmospheric Observatory in east-central Illinois. An automatic, wavelet-based adaptive filter is used to extract the waveforms of all disturbance events with amplitudes larger than a frequency-dependent threshold. The extracted events cover characteristic temporal scales from about 30 min to 6 h, that is, the range that includes mesoscale disturbances that affect the weather and the forecasts.

The analysis resulted in two classes of events. One class, called coherent events, or CEs, consists of disturbances that propagated coherently through the barograph network and for which the phase propagation velocity, dominant period, and horizontal wavelength could be estimated with good accuracy. The propagation directions of 97% of the CEs were between 0° and 180° (i.e., had an eastward component) and the speeds of 96% were between 10 and 50 m s−1 with a mode at 25–30 m s−1. The other class, called incoherent events, or IEs, consists of disturbances that had significant amplitudes but that did not propagate coherently across the network, so that the propagation velocity could not be estimated. This class consists of localized disturbances and wave packets with short periods and/or wavelengths, or with pressure signatures that were too different at the network stations. The extracted events are attributed to gravity waves, wave packets, gravity currents, pressure jumps, solitary waves, bores, etc.

The rate of occurrence of events had a strong seasonal dependence, with a maximum in fall and winter and a minimum in summer. The CEs occurred about 20%–21% of the total time in fall and winter and 12% in summer, while all events occurred 34% in both fall and winter and 23% in summer. The seasonal dependence of events confirms the strong relation of these disturbances to the baroclinicity of the atmosphere.

Concurrent vertical velocity fluctuations observed by the 50-MHz radar at the Flatland Atmospheric Observatory showed that many of the large-amplitude events extended up to at least 7 km, the highest altitude reliably observed by the radar.

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