Vertical Wind Shear near Airports as an Aviation Hazard

Wind shear has been recognized as an aviation hazard since the beginning of human flight. Before the early 1980's, gust fronts were considered to be the primary hazard. To detect wind shear from gust fronts and warn pilots, the original Low Level Windshear Alert System (LLWAS) was developed. Better understanding of the phenomenology of wind shear in the 1980's led to the understanding that microbursts were the primary wind shear hazard in aviation. With further field testing of airport wind measurement systems, it is becoming clear that vertical shear in the wind is also a significant aviation hazard.

The MIT/LL Integrated Terminal Weather System prototypes are running a gridded wind analyses in Dallas/Ft. Worth, New York City, and Memphis. In addition, Lincoln is operating a multi-sensor high-resolution (50 m or finer) wind profiling system to support the NASA Aircraft Vortex Spacing System (AVOSS) prototype at the Dallas/Ft. Worth airport. In both New York and Dallas wind shears of 20 knots/1000 ft (10 m/s / 300 m) near the ground are relatively common. Aircraft on approach for landing have a 3o glide slope. A shear of 20 knots per thousand feet of descent gives an aircraft on final approach a shear of 20 knots per 4 km of ground distance, which is 5 knots above the threshold for a wind shear warning. The total headwind decrease between the outer marker and threshold (assuming 5 nmi.) is greater than the minimum microburst alert of 30 knots. The strong vertical shear events in Dallas/Ft. Worth and New York are characterized, and a candidate algorithm for providing significant vertical wind shear warnings is discussed.