Document

Report
Roger Lhermitte, Pavlos Kollias, and Bruce Albrecht
RSMAS/MPO, Univ. of Miami, FL, USA
94-GHz Doppler radars are about the shortest wavelength radars that
are typically assembled for meteorological use. Their sensitivity results
from the proportionality of the backscattering cross section in the
Rayleigh regime (D « ) to 1/4 (Lhermitte, 1987). Millimeter radars are
capable of detecting very small droplets with diameters of tens of
microns. In addition to their high sensitivity, millimeter radars can be
configured to have excellent temporal and spatial resolution and can
operate with antennas that have a very narrow beamwidth. These
factors result in sampling volumes that are very small compared with
those of longer wavelength radars. This reduced sampling volume
decreases the effects of the Doppler spectrum broadening due to
turbulence. The capability of 94 GHz radars for cloud detection and their
portability make them a good tool for studying cloud microphysics and
dynamics of boundary layer clouds and cirrus. Because of the deep Mie
backscattering oscillations occurring in the raindrop particle size range
(Lhermitte, 1988), the 94-GHz radar is also an attractive choice for
vertical air motion and drop size distribution measurements, particularly
when used in conjunction with an S-band or an X-band radar.
In the early eighties, a very short wavelength Doppler radar ( =3.2 mm
or 94 GHz , W-band) was introduced to radar meteorology by Lhermitte
(1981, 1987), as a new tool for cloud physics research, primary for the
observation of low radar reflectivity clouds. Similar systems were
developed later (e.g. Clothiaux et al., 1994) and used in various
meteorological projects. Starting with the operation of the first cloud
radar, a large number of research papers (Danne et al., 1999; French
et al., 2000; Hogan et al., 2000; Sekelsky et al., 1999) have been
published on the performance and advantages of millimeter wave
radars and the results acquired through their use in various research
projects. This poster provides a brief description of the University of
Miami 94-GHz Doppler radar and its development and demonstrates
some major applications of 94-GHz Doppler radars to cloud and
precipitation studies using a series of examples. Other groups working
on similar developments and applications are listed.
Although centimeter wavelength radars may have the sensitivity required
to detect small cloud droplets provided these radars have sufficient
power and a large antenna, the short range performance of such radars
is limited by ground clutter. Since the backscattering from small targets
such as cloud particles increases proportionally to -4 , a relatively low
power radar operating at a shorter wavelength constitutes a better
alternative.

(cm)
Pf
(dB(w))
Antenna
size
dB(m)
Rec. NF -4 gain
figure
dB
(dB)
10
0 (500)
0 (10)
0(1)
5.6
-2 (300)
-10 (3.6)
3.2
-3 (250)
0.85
Beam
width
degree
Far field
km
Total
dB
0
0.7
2.0
0
-1(2)
10
1.0
0.23
-3
-10 (3.0)
-2(3)
20
0.7
0.28
5
-13 (25)
-14 (2.0)
-4(5)
42
0.3
0.47
11
0.32
-20 (5)
-20 (1.0)
-4(5)
60
0.2
0.31
16
0.14
-30 (0.5)
-34 (0.2)
-7(8)
73
0.3
0.03
2
Babb, D.M., J. Verlinde, and B.A. Albrecht, 1999: Retrieval of cloud
microphysical parameters from 94-GHz radar Doppler power spectra. J.
Atmos.
Oceanic
Technol.,
16,
489-503.
Clothiaux, E.E., M.A. Miller, B.A. Albrecht, T.P. Ackermann, J. Verlinde, D.M.
Babb, R.M. Peters, and W.J. Syrett, 1994: An evaluation of a 94 GHz radar
for remote sensing of cloud properties. J. Atmos. Oceanic Technol., 12 ,
201-229.
Danne, O., M. Quante, D. Milferstädt, H. Lemke, and E. Raschke, 1999:
Relationships between Doppler spectral moments within large-scale cirroand altostratus cloud fields observed by a ground-based 95 GHz cloud
radar.
J.
Appl.
Meteorol.,
38
(2),
175-189.
French, J.F., G. Vali, and R.D. Kelly, 2000: Observations of microphysics
pertaining the development of drizzle in warm, shallow cumulus clouds.
Quart.
J.
Royal.
Meteor.
Soc.,
126
(563),
415-443.
Hogan, R. J., A. J. Illingworth and H. Sauvageot, 2000: Measuring crystal
size in cirrus using 35- and 94-GHz radars J. Atmos. Oceanic Tech., 17(1),
27-37.
Table I, which covers radar wavelengths from 10 to 0.14 cm, shows that
despite large variations of some of the individual terms, there is only a
relatively small change in the overall radar sensitivity for cloud
observations. Note that the above data relates to typical meteorological
radar characteristics that do not necessarily represent state-of-the art radar
development.
Besides the far field condition, which for antennae producing the same
beamwidth, is more favorable at shorter wavelengths, the most important
consideration for close range detection of weak atmospheric targets is not
only the receiver noise level, but also ground echoes leaking through the
antenna sidelobes. This is especially true for high-power long-wavelength
radars. Assuming the same sidelobe structure for all antennae regardless of
the wavelength, the effect of ground echo interference in the measurement
of the weak backscattering produced by a small cloud at close range can
be characterized by the contrast between returns from the ground and from
clouds. Ground echoes are caused by a variety of large size targets such
as buildings, trees, terrain, etc. The variation as a function of radar
wavelength of ground clutter radar cross section per unit area ° is not well
known and depends on the targets, but it seems to increase only very
slowly with a decrease of the wavelength. On the other hand, cloud echo
intensity increases proportionally to 1/ 4 , or a dramatic increase of 60 dB
from 10 to 0.32 cm wavelength. Therefore, even with a possible 10 dB
increase of ° with the shorter wavelength, the ground clutter-cloud return
contrast will improve by at least 50 dB from a 10 to 0.3 cm wavelength. It is
our experience that even surrounded with buildings, a 94 GHZ radar,
providing a theoretical 15 dB (no clutter) cloud detection improvement over
an S-band radar, does not show any measurable (less than 20 dB below
receiver noise) ground clutter signal at short ranges (200 m) for elevation
angles above approximately 5°. Therefore, a millimeter-wave radar
operated in a vertically pointing mode (or slightly off vertical if three –
dimensional scanning is required) is an attractive solution for the
observation of low altitude low reflectivity clouds such as fair-weather
cumuli.
Fig. 1 Examples of time–height mapping of radar
reflectivity (top) and mean Doppler velocity in a
marine stratus. The data demonstrate the excellent
sensitivity of the radar and illustrate updraft and
downdraft structures within these non-drizzling
stratus.
Lhermitte, R. M., 1981. Satellite borne dual millimetric wavelength Radar.
Precipitation Measurements from Space, David Atlas and Otto W. Thiele
(eds.), Workshop report, October 1981, NASA/GODDARD Space Flight
Center, Greenbelt , MD, D-277-D-282.
Lhermitte, R. M., 1981. Millimeter Wave Doppler Radar , Proc. 20th
Conference on Radar Meteor., Am. Meteor. Soc., 744-748.
Using short wavelengths, narrow beamwidths are produced by
relatively small size antenna. Also the radar reflectivity of weak
targets (e.g. clouds) increases in proportion to -4, thereby reducing
the need for high transmitter power. Short wavelengths thus
represent an attractive choice for the probing of motion fields in
precipitation systems by a Doppler radar installed aboard a satellite
where physical space and power supply is limited. The increased
signal attenuation by cloud and precipitation at shorter wavelengths
is, however, a critical issue.
Lhermitte (1981;1989) first proposed the use of a space-borne 94
GHz Doppler radar for the detection of clouds and precipitation. 15
years later the CloudSat mission is under way. CloudSat is a multisatellite, multi-sensor experiment designed to measure cloud
properties from space. The primary CloudSat instrument is a nonDoppler 94 GHz cloud profiling radar. This space-borne radar is
designed to have a sensitivity of – 29 dBZ and 500 m vertical
resolution. The radar measurements will be supported by an optical
imager
and
a
lidar.
• University of Reading radar group (UK)
• GKSS 95 GHz cloud radar (Germany)
Lhermitte, R. M., 1987. A 94 GHz Doppler Radar for Cloud Observations. J.
Atmos. Ocean. Tech., 4(1), 36-48.
Lhermitte, R. M., 1988. Observations of rain at vertical incidence with a 94
GHz Doppler Radar: an insight of Mie scattering. Geophys. Res. Lett., 15,
1125-1128.
• Pennsylvania State University (Department of Meteorology)
• University of Massachusetts (Microwave Remote Sensing Laboratory)
• University of Wyoming (Department of Atmospheric Science)
• University of Miami (Meteorology and Physical Oceanography)
Lhermitte, R. M., 1989. Satelliteborne Millimeter Wave Doppler Radar. URSI
Commision F. Open Symposium Proceedings, La Londe-les Maures,
France, Sept. 11-15, 1989.
Sekelsky, S.M., W.L. Ecklund, J.M. Firda, K.S. Gage, and R.E. McIntosh,
1999: Particle size estimation in ice-phase clouds using multifrequency
radar reflectivity at 95, 33, and 2.8 GHz. J. Appl. Meteor., 38, 5-28.
http://cloudsat.atmos.colostate.edu/mission.html
Fig. 2 Examples of time–height mapping of mean
Doppler velocity in fair-weather cumuli. The top panel
shows a small cumulus observed in 1987 (Lhermitte,
1987) and the bottom panel shows a fair-weather
cumulus observed in 2001. There are remarkable
similarities in the observed mean Doppler velocity
field with a well-defined updraft core and surrounding
downdrafts.
Lhermitte (1988) first used the observed Mie
backscattering oscillations at 94 GHz to
calculate the vertical air motion and raindrop
size distributions in stratiform rain. Fig. 3.
Right panel: An example of Doppler spectrum
observed in stratiform rain using vertically
pointing 94 GHz Doppler radar.
Fig. 4. Top panel: 915-MHz radar reflectivity
mapping of a convective core. The reflectivity
exceeds 50 dBZ at levels between 1.5 and 4 km.
The box defined by the white lines indicates the
area where the millimeter radar data were used
for microphysics and air motion retrievals. Bottom
panel: Vertical air motion field retrieved using the
shifting of the Mie minima from millimeter radar
Doppler spectra.
Fig. 5. Top panel: Cross section of radar reflectivity
observed with the UM 915 MHz wind profiler (black)
at 1.3 km altitude and the corresponding rainfall rate
at the surface (red). Bottom panel: Retrieved vertical
air motion within the convective core (black) using
the shifting of the first Mie minima and the median
volume diameter (red) retrieved from the Doppler
spectra.
Fig. 6. Doppler moments from a cirrus anvil: power
(top), mean Doppler velocity (middle) and spectrum
width (bottom) calculated from the recorded FFTs.
The spectra were recorded within a variable range
window (6 km layer) with a gate spacing of 60 m.

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