FORECASTING TURBULENCE
NEAR THE SURFACE
Over land during nighttime hours there is very little
turbulence near the surface. The only exception is that
of high wind speeds over rough terrain. This type of
turbulence decreases with increasing height.
During the daylight hours, turbulence near the
surface depends on the radiation intensity, the lapse rate,
and the wind speed. Turbulence intensity tends to
increase with height throughout the unstable and neutral
layers above the surface to the first inversion, or stable
layer. Similar turbulence occurs in fresh polar
outbreaks over warm waters.
Vertical gustiness increases with height more
rapidly than horizontal gustiness.
Situations for violent turbulence near the surface
occur shortly after a cold frontal passage, especially
over rough terrain. Other examples of turbulence occur
over deserts on hot days and during thunderstorms.
Peak gusts at the surface can be estimated to be
essentially equal to the wind speeds at the gradient level,
except in thunderstorms.
FORECASTING TURBULENCE IN
CONVECTIVE CLOUDS
In the absence of any dynamic influences that might
serve to drastically modify the vertical temperature and
moisture distribution, on-hand rawinsonde data maybe
used to evaluate the turbulence potential of convective
clouds or thunderstorms for periods up to 24 hours. The
following is one procedure for predicting turbulence in
such clouds. This method is often referred to as the
Eastern Airlines Method:
The technique is as follows:
. Determine the CCL.
. From the CCL, proceed along the moist adiabat
to the 400-hPa level. This curve is referred to as the
updraft curve.
. Compare the departure of the updraft curve with
the free air temperature (T) curve at the 400-hPa level.
For positive values, the updraft curve should be warmer
than the (T) curve. That is, the updraft curve would be
to the right of the (T) curve. The value of the maximum
positive departure obtained in this step is referred to as
AT.
Table 5-3 is based on several years relating AT
values to commercial pilot reports of thunderstorm
turbulence, and can be used to predict the degree of
turbulence in air mass thunderstorms.
This method of forecasting thunderstorm
turbulence is almost exclusively confined to the warmer
months when frontal cyclonic activity is at a minimum.
During the cooler months, frontal and cyclonic
influences may cause rapid changes in the vertical
distribution of temperatures and moisture, and some
other methods have to be used.
FORECASTING CLEAR AIR
TURBULENCE (CAT)
Clear air turbulence is one of the more common
in-flight hazards encountered by high-altitude,
high-performance aircraft.
Not all high-level turbulence occurs in clear air.
However, a rough, bumpy ride may occur in clear air,
without visual warning. This turbulence may be violent
enough to disrupt tactical operations, and possibly cause
serious airframe stress and/or damage.
Most cases of CAT at high altitudes can be attributed
to the jetstream, or more specifically, the abrupt vertical
wind shear associated with the jetstream. CAT is
experienced most frequently during the winter months
when the jetstream winds are the strongest.
The association of CAT with recognizable synoptic
features has become better understood over the last few
years. The following are general areas where CAT may
occur:
. In general, in any region along the jetstream axis
where wind shear appears to be strong horizontally,
vertically, or both.
. In the vicinity of traveling jet maxima,
particularly on the cyclonic side.
l In the jetstream below and to the south of the
core.
l Near 35,000 feet in cold, deep troughs.
The instruction Atmospheric Turbulence and Icing
Criteria, NAVMETOCCOMINST 3140.4, sets forth
associated phenomena, as well as a common set of
criteria for the reporting of turbulence.
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