A flexible very high frequency (VHF) stratosphere-troposphere (ST) radar configured for meteorological research is to be constructed near Urbana, Illinois. Measurement of small vertical velocities associated with synoptic-scale meteorology can be performed. A large Doppler microwave radar (CHILL) is located a few km from the site of the proposed ST radar. Since the microwave radar can measure the location and velocity of hydrometeors and the VHF ST radar can measure clear (or cloudy) air velocities, simultaneous observations by these two radars of stratiform or convective weather systems would provide valuable meteorological information

Gage, K. S.

Green, J. L.

Nastrom, G. D.

Vanzandt, T. E.

English

NASA Technical Reports Server

N8T- 1 0482
6.2 THE PROPOSED FLATLAND RADAR
J. L. Green, K. S. Gage, T. E. VanZandt,
Aeronomy Laboratory
Boulder, CO 80303
and
G. D. Nastrom
Control Data Corp
Minneapolis, MN 55440
INTRODUCTION
The Aeronomy Laboratory of NOAA has proposed to the National Science
Foundation to construct a VHF ST radar near Urbana, Illinois for meteorological
research. For the reasons given below, this site has been selected because it
is located in flat terrain far from mountains.
An effort will be made to involve faculty members from university
departments of meteorology and atmospheric sciences in experiments with this
radar. It is believed that the combination of the Aeronomy Laboratory's
experience with this type of radar, the meteorological expertise of the univer-
sity faculty members and the meteorological consulting experience of Control
Data Corp will be very productive in the further development of this technique
as a meteorological tool. A university, private industry, and government
effort is envisioned.
A wide range of experimental studies can be made at this radar as can be
seen in the diverse subject matter covered in this Workshop on the Technical
and Scientific Aspects of MST Radar. However, only three of the proposed
experiments are emphasized here as examples.
DESCRIPTION OF RADAR
The Flatland radar will be a flexible ST radar configured for
meteorological research. Its characteristics are summarized in Table i.
Continuous, unattended operation is anticipated. As this is a fourth or fifth
generation radar, many features that have been found to be desirable in
previous research will be incorporated in its construction. For example, its
receiving system is designed to have a dynamic range of I00 dB to maintain
linearity over the wide range of reflectivities encountered by an ST radar, and
will be calibrated against a standard. The array antenna of the Flatland radar
will be electronically steerable in the east-west and north-south vertical
planes. It is anticipated that initial operation will be with five antenna
beam positions as shown in Figure i. This configuration has been tested at the
Sunset radar and found to have many advantages. Velocities measured with
redundant beam positions can be compared as in CLARK et al. (1983) or GRE_ et
al. (1986). The use of redundant beam positions has been found to be crucial
to the interpretation of ST radar data near convective storms and will provide
a better spatial average of both reflectivity and velocity. The rapid
electronic steering of the radar antenna will allow i0 s (one Doppler spectrum
per range gate) of data to be accumulated on each of the five beam positions
each minute. These spectra will be saved in the on-line computer memory and at
the end of a five-minute period, median spectra for each range gate and each
direction will be recorded on magnetic tape (RASTOGI, 1984; GRE_, 1986). This
rapid scan will prevent the aliasing of most gravity-wave modulation of radial
velocity since the Nyquist period will be 2.5 min. This data-acquisition
scheme is shown in Figure 2.
https://ntrs.nasa.gov/search.jsp?R=19870001049 2020-03-23T14:38:39+00:00Z
327
Sampled
Volume
h
11////
Antenna
_t --
Nor th//
_-- East
Figure I. Location of bean positions. The relative location of
the radar volumes at an arbitrary altitude corresponding to
five radar antenna bean positions.
TI_
Antenna, Pos 0 1 2 3 4 5 minutes
Vertical x x x x x x calculate
15 ° North x x x x x x median
15 ° South x x x x x x spectra
15 ° East x x x x x x and
15 ° West x x x x x x record
Figure 2. Aquisition of data (one cycle) at the end of the observation
cycle the median spectra for each range gate of every antenna
position is individually calculated.
i328
THE MEASUR]_4ENT OF SYNOPTIC-SCALE VERTICAL Vk_.OCITIES
One of the many experiments that can be performed with the Flatland radar
is the measurement of the small vertical velocities associated with
synoptic-scale meteorology. This velocity is the atmospheric variable most
closely linked with weather, except perhaps moisture, yet it has been almost
impossible to measure it directly. The Flatland radar, sited in very flat
terrain, offers an opportunity to measure vertical velocity directly and
continuously.
In a recent study (NASTROM, 1984), it was found that the time-averaged
vertical motion over the Platteville, Colorado ST radar compared favorably with
the computed synoptic-scale vertical velocity under certain synoptic
conditions: when the prevailing winds were from the west, across the Rocky
Mountains, the meteorological noise was too large to prevent computing a mean
vertical velocity with sufficiently small statistical uncertainty, or else
standing lee waves made the radar site unrepresentative of the large
geographical area of the synoptic-scale system. In either case, it was a
signal-to-noise problem induced by the mountains. A similar situation was
found in the ALPEX data from Southern France (CARTER et al., 1984), i.e., when
the winds were off the sea, the measured and computed vertical velocities
agreed favorably, but when the winds were across the mountains, the agreement
was poor or uncertain. Thus, we can conclude that at ST radar stations near
the mountains, the synoptic-scale vertical velocity can be measured only under
restricted conditions. These statistics suggest that it is probable that such
a measurement can be made at the proposed Flatland radar which would be
situated in very flat terrain far from mountains. As mentioned above, the
proposed radar will also be capable of measuring the horizontal components of
wind in the troposphere and lower stratosphere which can be used to sense the
synoptic-scale systems.
COMPARISONS WITH THE URBANA RADAR
The Aeronomy Laboratory of the University of Illinois operates the Urbana
radar. This radar, with its large power-aperture product, has made many
observations in the stratosphere and mesosphere (ROYRVIK and GOSS, 1983). It
is anticipated that the specialized meteorological measurements by the proposed
Flatland would be compared to those of the Urbana radar in a study of
troposphere-stratosphere-mesosphere coupling as in BOWHILL and GNANALINGAM
(1986).
EXPERI_NTS IN COOPERATION WITH THE CHILL RADAR
A large Doppler microwave radar, the CHILL radar is located a few km from
the site of the proposed Flatland radar (MUELLER and SILHA, 1978). This radar
is operated by the Illinois State Water Survey. Since this microwave radar can
measure the location and velocity of hydrometeors and the VMF ST radar can
measure clear (or cloudy) air velocities, simultaneous observations by these
two radars of stratiform or convective weather systems would provide unique and
valuable meteorological information.
ADDITIONAL STUDIES
The radar data set recorded for the synoptic-scale vertical velocity
experiment in a geographical region free from orographic effects would also be
useful for studies of clear-air turbulence (VANZANDT, 1983; NASTROM, 1984),
gravity waves (VANZANDT, 1986), transport of momentum by gravity waves (NASTROM
and GRE_2q, 1986) tropopause morphology (VANZANDT and VINCENT, 1983; GAGE et
329
al., 1984)andtropopauseheight (GAGE and GRE_, 1982). This continuous data
set could be used for the compositing of synoptic and mesoscale systems under
various synoptic conditions. There is an intriguing possibility of estimating
the thermal structure of the a_nosphere from a combination of radar wind
velocity and satellite radiance measurements (GAL-CHEN, 1986).
An effort will be made to attract experimenters with a wide range of
meteorological sensors to special observational periods held at least once
a year. Many of the additional capabilities inherent in the design of the
proposed radar (variable altitude resolution, additional antenna positions, and
real-time access to profiles of wind velocity and tropopause altitude) can be
used, as required, in support of these experiments.
TABLE 1
Characteristics of the Flatland Radar
Location
Frequency
Wavelength
Ant enna
Type
Size
Feed
Steerability (initial)
Transmitter
Peak power
Average power
Range Resolution
Operation
South of Urbana, IL
40.475 MHz
7.41 m
Two collocated arrays of
coaxial, collinear dipoles
60 m x 60 m
Tapered to reduce sidelobes
Five preset beam positions
Vertical, 15 ° from the
vertical to the north, south,
east and west
100 kW
2kW
Variable 150 m to 2.4 km
Continuously sequence through 5
antenna positions.
REFER_q CES
Bowhill, S. A., and S. Gnanalingam (1986), Gravity waves in severe weather,
this volume.
Carter, D. A., B. B. Balsley, W. L. Ecklund, M. Crochet, A. C. Riddle, and R.
Garello (1984), Tropospheric gravity waves observed by three closely spaced
ST radars, Handbook for MAP, 14, SOOSTEP Secretariat, Dep. Elec.
Computer Eng., Univ. IL, Urbana, IL, 219-228.
Clark, W. L., J. L. Green, and J. M. Warnock (1983), Estimating unbiased veloc-
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Gage, K. S., W. L. Ecklund, and B. B. Balsley (1984), A modified scattering
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of tropopause height from VHF radar observations, J. Appl. Meteorol., 21,
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Gal-Chen, T. (1986), A theory for the retrievals of virtual temperature from
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Green, J. L. (1986), An example of scaling MST Doppler spectra using median
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limitations, Handbook for MAP, 9, SCOSTEP Secretariat, Dep. Elec.
Computer Eng., Univ. IL, Urbana, IL, 346-356.
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Elec. Eng., Univ. IL, Urbana, IL, 78-80.


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