The National Weather Service's WSR-88D Doppler
between drop size and backscatter complex and diffi-
radar system is S band; this is the most common wave-
cult to predict (Battan 1973, Toomay 1982, Rinehart
length for land-based weather radars because of its abil-
1997). In addition, backscatter from ice particles vs.
liquid drops reverses due to differences in the real part
ity to detect precipitation-size particles (Crum and
of the complex index of refraction between water and
Alberty 1993, Houze 1993). C-band radar is a com-
ice, with ice particles providing about 10 times the back-
mon shipboard radar, and X band is the most common
scatter energy of liquid drops of a given size (Battan
airborne weather radar. Bands with shorter wavelengths
1962).
than X band suffer from precipitation attenuation, but
Doppler radar measures the motion of drops or ice
the shorter wavelengths may also allow detail to be
crystals along the axis of the radar beam (Battan 1973).
retrieved about precipitation. Most cloud microphysi-
cal radar work is currently in the X, Ka, and W bands,
The fall speed of water drops is related to their size, and
but disadvantages of the millimeter wavelengths include
the fall speed of ice crystals is related to their size and
increased absorption by water vapor and oxygen and
shape (Rogers and Yau 1989). A vertically oriented radar
strong extinction by cloud droplets, drizzle, and rain
can distinguish drop sizes, and distinguish drops from
(Klugmann and Judaschke 1996).
ice crystals, by their fall speed. Shorter-wavelength
Radar detects droplets and ice crystals in the atmo-
radars have a greater ability to distinguish between fall
speeds and thus are better able to resolve drop sizes.
sphere because of signal backscatter from droplets.
Because of the large volume a radar beam senses, and
Backscatter occurs as either Rayleigh or Mie scattering.
the large number of drops within a given volume of air,
Rayleigh scattering occurs when droplet diameters are
a spread of the Doppler spectrum typically occurs, which
significantly smaller than the radar wavelength (Battan
makes the Doppler signal difficult to interpret. Causes
1962). After some absorption by the droplets, the
of spread include spread in terminal velocities of the
amount of energy backscattered to the radar is propor-
drops or ice crystals within a sensing volume, turbu-
tional to the sixth power of the drop diameter (Battan
lence, and wind shear across the radar beam (Battan
1962). Backscatter is also inversely proportional to the
1973). Although Doppler radar is useful for determining
fourth power of the wavelength. As drop diameter
drop-size spectra from ground-based radar, it is likely
approaches wavelength in size, backscatter increases,
that Doppler techniques will be difficult from aircraft-
so the strongest backscatter from small droplets occurs
mounted radars because most drops will be falling ortho-
from the shortest radar wavelengths (Battan 1962). At
wavelengths greater than 3 cm, droplets of 2-mm diam-
gonally to the radar beam, making a Doppler shift less
eter and smaller are Rayleigh scatterers, and at wave-
detectable. However, turbulence can cause a Doppler
lengths of 10 cm, nearly all drops are Rayleigh scatterers
shift because small drops, which are more influenced
(Battan 1973). Small frozen drops and small ice crystals
by turbulence than large drops, can be carried toward or
backscatter about 20% as strongly as liquid drops of
away from the aircraft, allowing their identification.
the same size (Battan 1962). In all cases, the tempera-
The polarization of transmitted and received radar
ture of droplets must be determined to evaluate fully
energy can be used to determine the mean values and dis-
the amount of water contributing to the backscatter,
tributions of particle size, shape, and spatial orientation
because attenuation from droplets also has a tempera-
and to determine their phase (Houze 1993, Zrnic 1996).
ture dependency (Battan 1973, Rinehart 1997).
Radar signals may be linearly or circularly polarized.
Attenuation by scattering also increases as wave-
Linearly polarized radars transmit and receive energy
length decreases, because more energy is scattered by
in horizontal and vertical planes, principally because
intervening precipitation and cloud droplets. These are
falling drops typically shorten in the vertical axis and
conflicting factors to consider, because as wavelength
lengthen in their horizontal axis (Houze 1993). Differ-
is decreased to detect cloud drops, attenuation increases,
ential reflectivity and the linear depolarization ratio are
which limits range in high drop concentrations and high
computed from horizontal and vertical polarization.
liquid-water contents (Battan 1962, Rinehart 1997).
Differential reflectivity, the ratio of the horizontal trans-
As drop diameter approaches radar wavelength or
mitted-to-received energy to the vertical transmitted-to-
becomes larger, Rayleigh scattering no longer applies.
received energy, typically indicates the oblateness of
Instead, Mie scattering occurs, which produces a less
falling drops--a measure of drop size. Drops smaller
than 300 m are typically spherical, but as size increas-
predictable backscatter due to complex interactions
between energy reflected within the droplet and energy
es, oblateness and differential reflectivity both increase.
waves traveling along the droplet surface (Toomay
Ice crystals typically show no differential reflectivity.
1982). Depending upon the exact ratio of the drop size
The linear depolarization ratio, the ratio of horizontally
to the wavelength, these reflections and traveling waves
transmitted to vertically received energy, indicates how
may be additive or subtractive, making the relationship
much of the transmitted signal is depolarized. Wet ice
32
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