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M. F. Larsen

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M. F. Larsen
and
J. Röttger

Abstract

Atmospheric backscatter or partial reflections observed with VHF radars are strongly enhanced for angles within a few degrees of the vertical, although the strongest echoes are not necessarily exactly from the vertical direction. Consequently, a nominally vertical beam can actually receive the strongest echoes from a direction slightly off-vertical. The radial velocities measured in the vertical beam, then, are not true vertical velocities but have contributions from both the projection of the vertical and the horizontal velocity along the effective beam direction. We present measurements of the tropospheric and lower stratospheric incidence angles obtained over a period of four days with a spaced antenna radar system and calculate the true vertical velocities by means of a correction derived by combining the incidence-angle measurements, the radial velocities in the vertical beam, and the horizontal winds obtained by the standard spaced antenna method. Typical values for the incidence angles are less than 2° when averaged over 8 min or more. The vertical velocity corrections are typically 5%–200% of the magnitude of the vertical-beam radial velocity. The corrected vertical velocities are found to be in better qualitative agreement with expectations based on the meteorological conditions for the period.

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M. F. Larsen
and
J. Röttger

Abstract

We compare observations of thunderstorms made with two radars operating at different wavelengths of 70 cm and 5.67 m. The first set of observations was made with the UHF radar at the Arecibo Observatory in Puerto Rico, and the second was made with the Max-Planck-Institut für Aeronomie VHF radar in the Harz Mountains in West Germany. Both sets of observations show large echo strengths during periods of convective activity. We show, based on the observational data and calculations, that precipitation completely dominates the UHF signals. In fact, a sensitive UHF radar such as at the Arecibo facility is a good tool for investigating cloud droplet distributions in the upper parts of the clouds. The signal at VHF has contributions from both precipitation and the turbulent scatter, and the two contributions can easily be separated since the droplet fall velocity and the updraft velocity are different, except for the smallest drop sizes. Our results show that VHF and UHF are potentially a good combination of frequencies for cloud physics research.

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M. F. Larsen
and
J. Röttger

Abstract

The Doppler method for radar wind profiling of the troposphere and stratosphere has been the standard technique used for operational and most research applications of the instrumentation. An alternative approach for measuring winds, by making use of scattering and reflection from variations in the refractive index, is what is known as the spaced antenna technique. This technique has received little attention so far, and few comparisons of the two techniques have been carried out, in spite of the fact that results to date indicate that there can be some advantages of the spaced antenna method over the Doppler method, especially when small-antenna and low-power VHF systems are being considered. We describe the technique and summarize the diverse results that relate to a comparison between the Doppler and spaced antenna methods. The expected advantages of the spaced antenna method for small radar systems are also described.

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C. R. Cornish
and
M. F. Larsen

Abstract

Observations of the horizontal wind in the subtropical upper troposphere and lower stratosphere were made with the 430-MHz radar located at Arecibo, Puerto Rico (18.4°N), in May 1982 and April 1983. Both sets of observations displayed a slowly varying, anticyclonically rotating, persistent structure in the wind field just above the tropopause, similar to what would be expected if the oscillations are associated with quasi-inertia period waves. A spectral analysis of the May 1982 data revealed that a wave with a period in the earth-fixed earth-fixed reference frame close to 26 h was present. Our calculations show that the intrinsic wave periods varied from ∼2.5 times the inertial frequency to zero at a critical level near 18-km altitude. The changes in phase between perturbation velocities perpendicular and parallel to the propagation direction were systematic below the height where the intrinsic frequency was equal to the inertial frequency but showed a complicated behavior between that height and the critical level. Hines has recently proposed an explanation of similar observations as being due to orographic waves with intrinsic periods that are short compared to the inertial period. Our analysis indicates discrepancies with his simple explanation, although the data show that the waves may have an orographic source but with the earth-fixed period near 24 h determined by the very regular diurnal fluctuation in the surface winds over Puerto Rico. Thus, the observed waves may be orographically generated but with intrinsic frequencies, determined by the 24-h earth-fixed period, small enough to require the use of the dispersion relations for inertia-gravity waves with Coriolis effect included in order to explain the characteristics of the observations.

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M. F. Larsen
and
J. Röttger

Abstract

No abstract available

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M. F. Larsen
and
J. Röttger

Applications of VHF and UHF Doppler radars to research in synoptic meteorology are reviewed. We find that these radars show great potential for studies of large scales, but the area of research where the instruments really excel is in studying the interaction between the synoptic scale and the mesoscale. Several examples of results in both these areas are presented. Finally, the potential for operational use of the radar systems is discussed.

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M. F. Larsen
,
J. Rottger
, and
T. S. Dennis

Abstract

We Present a comparison of vertical velocity data obtained with the SOUSY-VHF-Radar operating near Bad Lauterberg in West Germany and the vertical velocities produced by the operational analysis of the European Centre for Medium-range Weather Forecasting. Three cases are presented including a 3-day period in March 1981. an 11-day period in November 1981, and a 5-day period in April 1984. The synoptic conditions were characterized by wintertime flows, and a number of warm- and cold-frontal passages occurred. We find that the magnitude of the vertical velocity variations are comparable in the observations and in the analysis data in the troposphere, but the radar shows much larger fluctuations in the stratosphere. Although the same trends in ascent and subsidence are present in both datasets, there is, on the whole, no one-to-one correspondence between individual features in the radar and the analysis vertical velocities over time scales of a day or two.

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J. G. Yoe
,
M. F. Larsen
, and
E. J. Zipser

Abstract

Very high frequency (VHF) Doppler radar measurements of the horizontal and vertical winds are used to examine three procedures to extract mean profiles of horizontal and vertical winds. These are 1) time averaging of first-moment estimates of radial velocity from the high time resolution Doppler spectra; 2) time averaging of radial velocities estimated from a least-squares fitting of either one or two Gaussians to the spectra in order to account for the double peaks corresponding to turbulent and precipitation scattering that appear in the spectra during heavy rain; and 3) consensus averaging of the least-square-fitted radial velocities. Horizontal winds produced by these procedures were compared to each other and to those from two 5-cm radars operating nearby. Least-squares fitting yielded the best wind estimates, although a slight relaxation of the consensus criterion was sometimes found to be necessary in order to avoid the failure to find a consensus. The simple first-moment method produced comparable results, except below the melting level, where it performed more poorly. Vertical winds from the fitted VHF spectra were compared with those derived from the 5-cm-radar data using the extended velocity-azimuth display (VAD) technique. Reasonable agreement was found at heights above the freezing level.

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M. F. Larsen
,
J. Rutger
, and
D. N. Holden

Abstract

We present power spectra of vertical velocities measured with the SOUSY-VHF-Radax wind profiler.over a 5-day period in October and November 1981. Most of the data consist of hourly vertical velocity profiles based on 12-rain averages, but, for somc periods, the radar was operated continuously, providing a profile once every 12 minutes. The frequency spectra of both the 12-rain and hourly data show slopes close to -1 for the log power versus log frequency curves in the lower troposphere, decreasing toward a slope near or around the tropopause and in the lower stratosphere. The frequency range covered by the spectra corresponds to the rangeof periods from 24 minutes to 1:5 days. Thus, the slope span the time scales characteristic of the mesoscale and synoptic scales. The 300-m height resolution of the data above 3-kin altitude allowed meaningful vertical wave-number spectra to be calculnted and showed a slope close to -1.5 for the height range from 3 to IS kin.

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