A28-1146-102-00
Federal Aviation Administration (FAA) Advisory Circulars
A-11
National Severe Storms Laboratory (NSSL)
Thunderstorm Research
The NSSL has, since 1964, been the focal point of our thunderstorm
research. In-flightconditions obtained from thunderstorm penetration
by controlled, especially equipped high performance aircraft are
compared by the NSSL with National Weather Service (NWS) type
ground-based radar and with newly developed doppler radar. The
following comments are based on NSSL’s interpretation of information
and experience from this research.
RELATIONSHIP BETWEEN TURBULENCE AND REFLECTIVITY
Weather radar reflects precipitation such as rain and hail, turbulence.
It has been found, however, that the intensity level of the precipitation
reflection does correlate with the degree of turbulence in a
thunderstorm. The most severe turbulence is not necessarily found at
the same place that gives the greatest radar reflection.
RELATIONSHIP BETWEEN TURBULENCE AND ALTITUDE
TheNSSLstudiesofthunderstormsextendingto60,000feetshowlittle
variation of turbulence intensity with altitude.
TURBULENCE AND ECHO INTENSITY ON NWS RADAR (WSR-57)
The frequency and severity of turbulence increases with radar reflectivity,
a measure of the intensity of echoes from storm targets at a standard
range. Derived gustvelocities exceeding2,100feetperminute(classified
as severe turbulence) are commonly encountered in level 3 storms. In
level2storms, gustsof intensitybetween1,200and 2,100feet perminute
(classified as moderate turbulence) are encountered approximately once
for each 10 nautical miles of thunderstorm flight.
TURBULENCE IN RELATION TO DISTANCE FROM STORM CORE
NSSL data indicates that the frequency and severity of turbulence
encountersdecreaseslowly with distancefromstormcores. Significantly,
the data indicates that within 20 miles from the center of severe storm
cores,moderatetosevereturbulenceisencountered atanyaltitudeabout
one-fifth as often as in the cores of Level 3 or greater thunderstorms.
Further,thedataindicatesthatmoderateturbulenceisencounteredatany
altitudeup to 10 milesfrom the center oflevel 2thunderstorms. SEVERE
TURBULENCE IS OFTEN FOUND IN TENUOUS ANVIL CLOUDS 15
TO 20 MILES DOWNWIND FROM SEVERE STORM CORES. Our
findings agree with meteorological reasoning that THE STORM CLOUD
IS ONLY THE VISIBLE PORTION OF A TURBULENT SYSTEM
WHOSE UPDRAFTS AND DOWN-DRAFTS OFTEN EXTEND
OUTSIDE OF THE STORM PROPER.
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