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act as an excellent reflector for the radar’s microwave energy. Under
most circumstances, some radar energy will also penetrate rain targets
in order to detect weather that lies behind the initial target.
Dry snow and ice crystals are very poor reflectors of radar energy. The
ice crystal’s lattice structure prevents the bipolar water molecules from
aligning to reflect radar energy.
Wet hail provides the strongest reflection of radar energy. The size of the
target (hail) combined with the ability of the bipolar water molecules on
its surface to align to reflect radar energy ensures maximum reflectivity
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AVIATION WEATHER COLLINS
Reflectivity MultiScan™ Radar
levels. In many cases radar energy is unable to penetrate beyond this
type of target and weather behind the initial target is masked (hidden).
Dry hail reflects some radar energy simply due to its size. However, the
crystal structure of the dry hail will fail to reflect significant amounts
of radar energy. This situation can cause the radar to underestimate
an area of severe weather.
BRIGHT BAND
“Bright Band” is associated with stratiform rain and occurs at or within
3,000 feet below the freezing level. In this region, ice crystals begin
to melt and are coated with a layer of water. Similar to the wet hail
described above (but on a smaller scale), this results in very strong
radar returns. If the radar beam is directed into this region it may cause
the entire weather picture to turn red (red out) due to the fact that the
stratiform rain clouds may cover a large geographical region. N NOTE
If the aircraft encounters stratiform rain conditions and red out
occurs at or near the freezing level, changing the tilt so that the
radar beam is either above or below the area of bright band may
improve the radar picture. In addition, turning the gain below the
CAL (calibrated) position should allow the flight crew to detect any
“hot spots” or areas of severe precipitation.
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COLLINS AVIATION WEATHER
MultiScan™ Radar Reflectivity
THUNDERSTORM REFLECTIVITY
A thunderstorm is composed of three parts, each with different weather
radar reflectivity characteristics as shown in figure 5-3.
Figure 5-3 Anatomy of Thunderstorm Weather Radar Reflectivity
The bottom portion of the storm, below the freezing level, is composed
entirely of liquid precipitation (i.e., rain) and is the most reflective portion
of the storm. Raindrops serve as excellent reflective surfaces for the
10,000 MHz radar energy produced by today’s X-band airborne weather
radars.
The middle portion of a thunderstorm occurs above the freezing level
(0° C) and up to the altitude where the outside air temperature drops
below –40° C. This section of the storm is composed of a combination
of ice crystals and supercooled water. The supercooled water provides
moderate reflectivity, but some reflective energy will be lost due to the
presence of the ice crystals. The top of this section of the storm is often
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AVIATION WEATHER COLLINS
Reflectivity MultiScan™ Radar
referred to as the wet top or radar top of the thunderstorm because the
radar detects very little of the thunderstorm above this point.
WARNING
Significant vertical thunderstorm development with severe
turbulence and dry hail may exist above the radar top.
The top of a thunderstorm is composed entirely of ice crystals and
reflects very little radar energy. At temperatures less than –40° C, liquid
water no longer exists and only ice crystals are present. The altitude at
which this temperature occurs varies depending on the time of day, time
of year and is based on latitude and longitude. The top of this section of
the storm is referred to as the actual or visible top.
Thunderstorms can grow as rapidly as 6,000 feet per minute. Building
thunderstorms have a turbulence bow wave that may extend several
thousand feet above the visible top of the storm. The bow wave may
cause severe turbulence but is completely invisible to radar.
It is important to note that significant turbulence may exist well above
the radar top of the thunderstorm. Serious injury and even death have
occurred due to inadvertent penetration of thunderstorm tops or the
turbulence bow wave that radar failed to detect.
Figure 5-4 shows an observed thunderstorm and the corresponding
radar displays at six different tilt angle settings. Note the change in the
weather and ground clutter reflectivity as the radar beam moves up and
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Collins Weather Radar operator’s guide(28)
