G. D. Nastrom

A Preliminary Climatology of the Spectrum of Vertical Velocity Observed by Clear-Air Doppler Radar

Abstract Multiheight time series of atmospheric vertical velocities in the troposphere and lower stratosphere observed by clear-air Doppler radar are presented at various locations around the world. Frequency spectra of vertical velocities determined from these data sets are compared with the objective of developing a preliminary climatology. We emphasize the nearly universal shape and magnitude of spectra observed during low-wind conditions. These spectra are quite flat for frequencies between the buoyancy and inertial frequencies, and they closely resemble the internal wave spectra observed in the ocean. Spectra observed under strong wind conditions, on the other hand, are greatly enhanced in magnitude, approaching the f−5/3 spectral slope observed for the spectrum of horizontal motions. Finally, spectra determined from both quiet and active periods at Poker Flat, Alaska, possess spectral slopes and amplitudes intermediate to those spectra determined solely from quiet or active periods at other locations.

Preliminary estimates of the vertical profiles of inner and outer scales from White Sands Missile Range, New Mexico, VHF radar observations

There are very few reliable results of the inner and outer scales of turbulence in the remote atmosphere. Knowledge of these parameters is of high interest to the propagation and remote sensing communities. Seasonal profiles from 5 to 20 km above mean sea level of the inner scale have been estimated based on the kinematic viscosity and eddy dissipation rates which were determined from 5 years of nearly continuous 49.25-MHz radar observations at White Sands Missile Range, New Mexico. Inner scale values were found to increase from about 1 cm at 5 km to near 7 cm at 19 km altitude. Outer scale profile determinations were made using a method proposed by Tatarskii [1971] that involves vertical wind shear and the eddy dissipation rate, both derived from the longterm VHF radar measurements. The outer scale decreased from about 60 m at 5 km altitude for all seasons to 12–20 m at 15 km (depending on season) and then increased to 22 m at 19 km. Seasonal differences in the inner and outer scales and background meteorological conditions are also presented and discussed.

Sources of Gravity Wave Activity Seen in the Vertical Velocities Observed by the Flatland VHF Radar

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 w...

Frequency spectra of vertical velocity from Flatland VHF radar data

The vertical wind velocity over very flat terrain was observed every 153 s in the troposphere and lower stratosphere by the Flatland radar, near Champaign-Urbana, Illinois. Several hundred frequency spectra were calculated from all accepted 6-hour time series from March through May 1987. By stratifying the spectra in various ways we find the following: (1) The spectra were independent of altitude within the troposphere or lower stratosphere, but the spectra in the two regions differed in amplitude and frequency; (2) At a given altitude the spectra were independent of the wind shear dū/dz, the buoyancy frequency N, and the maximum wind speed below 16 km; (3) The change of spectral shape and amplitude with increasing background wind speed ū was much less than at stations near mountains. The variance of the spectra, equal to twice the vertical kinetic energy per unit mass, roughly doubled as ū increased by 10 m s−1; (4) The spectra were consistent with being due to a spectrum of gravity waves, as indicated by the sharp drop in spectral amplitude near N at small ū and by the fact that the observed change of shape with increasing ū was quite consistent with the change of shape of model Doppler-shifted gravity wave spectra; (5) The results of comparison between the observed and model spectra are consistent with an intrinsic gravity wave spectrum that is invariant with ū, dū/dz, etc., contrary to expectations from gravity wave theory; (6) The results are insensitive to the azimuthal distribution of gravity wave energy, as long as the distribution is roughly symmetrical relative to the mean flow; (7) The resulting characteristic horizontal phase velocity c* of the intrinsic frequency spectrum was about 6 m s−1 in both the troposphere and the stratosphere. The corresponding characteristic vertical wavelengths were about 3300 and 1800 m, respectively, consistent with previous estimates.

Quasi‐monochromatic inertia‐gravity waves in the lower stratosphere from MST radar observations

[1] The frequency of occurrence of quasi-monochromatic oscillations with periods near the inertial period is examined using hourly mean wind observations from the MST radars at White Sands Missile Range, New Mexico, and Vandenberg Air Force Base, California, spanning 6 years and 4 years, respectively. Power spectral analyses show that the energy levels near the inertial frequency during summer are nearly constant with altitude from about 12 km up to the highest altitude available, about 20 km, while energy levels at lower frequencies decrease with altitude. This decrease leads to a relative enhancement of energy near the inertial frequency. During winter the relative enhancement near the inertial frequency is much smaller. Results from least squares curve fitting used to find the percent of wind variance explained (PEV) by a single wave over data blocks 72 hours long and 2 km deep indicate that a quasi-monochromatic oscillation is present when PEV > 25%. During summer in the stratosphere over 50% of the cases have PEV > 25%. The best fit waveform has mean period near 21 hours and vertical wavelength near 3 km. The wind vectors exhibit anticyclonic rotation in time and with height, consistent with upward propagating gravity waves. The mean ratio of the intrinsic to the inertial frequency is about 1.3 in this data set, and the associated mean horizontal wavelength of these waves is slightly over 1000 km.

Tropopause Folding and the Variability of the Tropopause Height as Seen by the Flatland VHF Radar

Abstract The Flatland radar, a VHF wind profiler located near Urbana, Illinois, has been used to study the variability of the tropopause over the period March 1987–April 1988. The vertically directed radar beam provides an indicator of tropopause height as well as measurements of the vertical velocity of the air. The sudden tropopause height changes previously seen in special campaigns are found to be relatively common features. Several case studies made under varying synoptic conditions are used to show that thew discontinuities are associated with tropopause folding events as revealed by cross-section analyses of potential temperature and potential vorticity based on radiosonde data. The vertical velocity in a typical tropopause fold is downward at up to 15 cm s−1. Finally, applications of the continuous data from VHF radars to develop quantitative climatologies of tropopause folding events are presented.

A comparison of gravity wave energy observed by VHF radar and GPS/MET over central North America

Monthly mean values of the kinetic energy at frequencies associated with gravity waves based on wind observations made with the VHF radar at White Sands Missile Range, New Mexico, are compared with potential energies determined using GPS/ MET soundings. The monthly mean curves of E k and E p are highly correlated, and both show minimum values in the summer season with summer to winter increases of about a factor of 2. The observed ratio E k /E p agrees very closely with the prediction of a linear gravity wave model. These observations support previous comparisons of E k with E p made using the middle and upper atmosphere radar in Japan.

MST Radar Observations of Gravity Waves and Turbulence near Thunderstorms

The effect of deep convection on the intensities of gravity waves and turbulence during the summer at White Sands, New Mexico, is investigated using 50-MHz mesosphere‐stratosphere‐troposphere (MST) radar observations and surface weather reports. Radar data taken at 3-min intervals from the summers of 1991 through 1996 (with occasional gaps of varying length) are used to construct hourly means, medians, and standard deviations of wind speed, spectral width ( ), and backscattered power calibrated as the refractivity turbulence 2 s turb structure constant ( ). The hourly variance of the vertical velocity is used as an indicator of high-frequency 2 2 C s n w gravity wave intensity. Surface observations taken near the radar site are used to identify periods marked by convection at or near the radar. During cases in which no convection is reported, the median hourly is nearly 2 s w constant with altitude (about 0.04 m 2 s 22 below and 0.03 m2 s 22 above the tropopause). Values of , , and 22 s C wn are significantly enhanced from no-convection cases to thunderstorm cases. Largest increases are about 12 2 s turb dB relative to the no-convection cases at about 11 km for , about 9.5 km for , and about 7.5 km for 22 ss w turb . The relatively lower height for the maximum of is likely due to the influence of humidity advected 2 2 CC n n upward during convection on the mean gradient of the refractive index. The probability density distributions of and near their levels of maximum enhancement are unimodal, with the modes steadily increasing with 22 C s n turb increasing proximity of convection. However, the probability density distribution of is bimodal in all instances, 2 s w suggesting that there can be enhanced wave activity even when visible convection is not present and that the presence of a thunderstorm at the station does not necessarily indicate greatly enhanced wave activity.

Radar and aircraft observations of a layer of strong refractivity turbulence

The Air Force Research Laboratory at Kirtland Air Force Base examined the characteristics of the refractive index structure parameter (C2n) within a strong and persistent layer of turbulence using observations obtained from a 49.25 MHz radar and an instrumented C-135E aircraft at the Atmospheric Profiler Research Facility at White Sands Missile Range, NM on January 23, 1997. The aircraft measurements sensed the atmospheric temperature structure parameter (C2T) with fine-wire aerothermal probes for deriving C2n while the radar measurements provided C2n from Bragg scatter at turbulent scales in the clear air. The aircraft results provide horizontal spatial information at the specific altitudes flown while the radar-obtained values show temporal profile information. Flight legs approximately 200 km long were flown along the wind direction at eight different altitudes from 11.01 km to 12.21 km MSL. The turbulent layer and direction of flights were selected from the VHF radar-obtained C2n and wind measurements prior to take-off. Presentations include a range-height display of the patterns of refractivity turbulence obtained from the aircraft measurements and a range-height display derived from the radar observations corresponding to the aircraft results. Both range-height displays were produced by assuming Taylors hypothesis and applying the actual wind profile to the time-height data. The evolution and persistency of features is discussed. A statistical evaluation comparing the two different methods of sensing C2n is presented. Salient features of the aircraft sensors and radar are discussed.

The Coupling of Vertical Velocity and Signal Power Observed with the SOUSY VHF Radar

Abstract 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 ba...