Wind shear - A case of
benign-looking convective environments
Most pilots equate wind shear to turbulence or
convection. Certainly some forms of wind shear are indeed
turbulent and convection can also induce dangerous wind shear.
If the wind shear occurs close to the surface when the
aircraft is landing or departing, it may be difficult for the pilot to
recover resulting in an accident that is often fatal.
Wind shear is simply a change of wind
direction and/or wind speed over a given distance. There's
always some kind of wind shear present in the atmosphere on any given
day or time. But, it is the gradient of wind shear (how
rapidly the wind direction and/or speed changes over a specified
distance and/or time) that's the most important.
Shear is evident when there's a change
of wind direction or wind speed in the horizontal and/or vertical.
We often see shear created when the wind is gusty.
As the wind gusts, the speed increases suddenly (horizontal
speed shear) and the wind may even shift direction during a gust.
Perhaps the most dangerous wind shear
occurs in a downburst where downdrafts in thunderstorms have been estimated
to be greater than 100 miles per hour. These high-speed winds
strike the surface and spread out creating a gust front that can also
be very dangerous especially when landing or departing. When that downburst occurs in a
very small area spatially it is referred to as a microburst as can be
seen in the image above. Click here to view a larger image.
microbursts are ephemeral lasting no more than five minutes
(although there is evidence that some do last longer).
A microburst was defined by Dr. Ted
Fujita in 1985 as a downburst with the spatial scale on the order of a
runway length. That would involve no more than 4 kilometers
(2.2 nautical miles) over the surface. The smaller spatial
scale of a microburst converts into tighter wind shear gradients that
are experienced by penetrating aircraft as more rapid changes in wind
direction and/or wind speed. For many microbursts, this high
wind shear gradient will be in excess of the inertial capabilities of
Microbursts generally occur in the
extreme cases where the atmosphere is very moist or very dry
(but still moist enough to produce convection). These are generally
categorized as wet and dry microbursts, respectively. However, it is
still possible to have a microburst even if the environment is
somewhere in between. The mechanisms that create a dry
microburst are very well understood. On the other hand, much
less is known how wet microbursts arise which makes them much more
difficult to forecast.
At this point in time, there have been
several large turbojet aircraft succumb to the forces associated with a
microburst. The one that is especially memorable is Delta
Airlines Flight 191 that encountered a microburst on approach
to the Dallas Fort Worth International Airport (KDFW).
Delta Flight 191 didn't fly through or under an
intense supercell thunderstorm. In fact, a thunderstorm with
a base of nearly 10,000 feet was the culprit. High-base convection with
heavy rain signature should be of particular concern to pilots since
they signal a deep mixed layer with a high lapse rate and plenty of
precipitation to fuel a strong downdraft. The issue is that
high-based convection does not seem very threatening (especially to
pilots flying large turbojet aircraft) which makes pilots more likely
to stumble into the path of a downburst.
Here's a quote from a paper written by Captain William W. Melvin entitled Windshear Revisited, Air Line Pilot Magazine, Nov 1994.
many windshear accidents have been analyzed with emphasis on pilot
error without attempting to understand why the errors were made.
In most cases, the analyses were flawed, and no substantial pilot error
existed. This has caused considerable misunderstanding of serious
aspects of windshear hazards that still exist in pilot training
literature. These misunderstandings pose human factor problems for
pilots when they have to deal with windshear. Many
pilots have been trained to avoid large supercell-type thunderstorms in
the belief that this will prevent encounters with microbursts. Yet no
evidence exists that any of the known microburst encounters have
occurred in supercell storms. Dr. Ted Fujita and Dr. Fernando
Caracena recognized authorities in this field have repeatedly
emphasized that microbursts are frequently generated from
benign-appearing cells. Many "experts" who disagree with Drs. Fujita
and Caracena have emphasized the supercell storms with warnings of
dangers of gust fronts. These so-called experts are leading pilots down
the primrose path for microburst encounters.
In a high-based thunderstorm with an
extremely dry environment between the cloud base and the surface,
little or no rain may reach the surface initially. As the
rain falls out of the cloud into this very dry atmosphere it evaporates
quickly which causes a cooling effect relative to the air around the
precipitation. Such cooling makes the air much denser, and
therefore negatively buoyant, effectively creating a very intense
downburst with winds that may exceed hurricane force, even approaching
the speeds found in a weak to moderate tornado (EF1).
It is very important to pay close
attention to what you see below the cloud deck if you choose to fly
below the convective cloud bases. Look for the following:
Concentrated rain shaft or virga shaft of about 1 kilolmeter
2. Precipitation (or dust) curl that is carried by the wind
back up toward cloud base.
3. Horizontal bulging near the surface in a precipitation
shaft, forming a foot-shaped prominence.