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weather are examined.
1st Edition, 1st Revision
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AVIATION WEATHER COLLINS
Thunderstorms MultiScan™ Radar
Figure 5-12 Thunderstorm Vaulting
OCEANIC WEATHER CELLS
Oceanic weather cells tend to have less mass and significantly
less reflectivity than continental thunderstorms of equivalent height.
Compare the oceanic thunderstorm shown in figure 5-13 with the
equivalent height land mass thunderstorm shown in figure 5-5.
Interestingly, the reduced mass and lower reflectivity levels found in
oceanic weather cells occur due to the fact that they have, on average,
1st Edition, 1st Revision
5-16 18 Sep 03
COLLINS AVIATION WEATHER
MultiScan™ Radar Thunderstorms
less water content than do thunderstorms that form over land. As a
result, a higher gain setting and/or a lower tilt setting may be required to
adequately detect thunderstorm threats at higher cruise altitudes.
It is not unusual for oceanic weather cells to be very “skinny” but still
have significant vertical development, especially in equatorial regions
(figure 5-14). This type of thunderstorm has very little moisture content
and is extremely difficult for radar to see. The anvil tops on these type
storms can extend for several mile (figure 5-15) and may be almost
invisible to radar. At night it may be necessary to look for an area
where stars are blocked by the thunderstorm cell to prevent inadvertent
thunderstorm top penetration.
N NOTE
During automatic MultiScan operation, the radar automatically
adjusts gain and tilt in oceanic regions to more accurately depict
oceanic weather cells (♦page 4-43).
Figure 5-13 Oceanic Weather Cell
1st Edition, 1st Revision
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AVIATION WEATHER COLLINS
Thunderstorms MultiScan™ Radar
Figure 5-14 “Skinny” Oceanic Weather Cell
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COLLINS AVIATION WEATHER
MultiScan™ Radar Thunderstorms
Figure 5-15 “Anvil Top” Oceanic Weather Cell
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AVIATION WEATHER COLLINS
Squall Lines MultiScan™ Radar
SQUALL LINES
Squall lines are organized lines of thunderstorms that form in both
mid-latitudes and the tropics and may extend for as long as 500 NM.
Mid-latitude squall lines are classified as either “ordinary” or “prefrontal”.
Prefrontal squall lines (figure 5-17) form in front of cold fronts. They may
form right in front of the cold front boundary, but the most severe lines
precede the cold front by as much as 150 NM. Research is ongoing as
to how these kind of lines form, but one theory holds that the cold front
may cause air at higher altitudes to form “waves” (called gravity waves)
that then cause the thunderstorms to form ahead of the advancing front.
It is known that the thunderstorms in this type of squall line are massive
and can contain strong winds, hail and sometimes tornadoes. Ordinary
squall lines normally exhibit weaker updrafts and down drafts, have
shorter life spans, and have less severe thunderstorms than prefrontal
squall lines. Squall lines in the tropics tend to have a structure that is
similar to ordinary mid-latitude squall lines.
Figure 5-16 Prefontal Squall Lines
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COLLINS AVIATION WEATHER
MultiScan™ Radar Squall Lines
Figure 5-17 Prefontal Squall Lines Weather Radar Display
1st Edition, 1st Revision
18 Sep 03 5-21
AVIATION WEATHER COLLINS
Microbursts And Windshear MultiScan™ Radar
MICROBURSTS AND WINDSHEAR
MICROBURSTS
Microbursts can be an extreme hazard to aircraft during the landing
and take off phases of flight. Microbursts normally cover an area
approximately 2 NM in diameter, although the area of resulting high
winds created when the microburst hits the ground can be much higher.
Winds of up to 130 knots may result when a microburst is formed.
However, it should be remembered that on occasion microbursts may
produce windshear events that are small enough to pass unnoticed
between the low-level windshear sensors that are installed at some
airports.
Microburst typically occur in the vicinity of thunderstorms when the
surrounding air is dry. In general, the dry air causes the rain from the
thunderstorm to evaporate. The process of evaporation cools the dry
air which then becomes heavier than the surrounding air. This cooler,
heavier air then plunges to the surface (figure 5-18).
Figure 5-18 Microburst Formation
When the microburst reaches the surface, it spreads out from the
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Collins Weather Radar operator’s guide(31)
