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June 1, 2010  

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Wind shear - A case of benign-looking convective environments   Wet microburst example

     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.  
Most 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 most aircraft.

     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.   

Too 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:

1.  Concentrated rain shaft or virga shaft of about 1 kilolmeter in width
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.

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