Ordinary-cell convective storms - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 118

when the airflow is opposite that of the aircraft motion, followed by a brief period
when the airflow is in the same direction as that of the aircraft motion. Thus, the
aircraft experiences a brief period of enhanced lift, followed by a period of dimin-
ished lift. Too much overcompensation for the period of enhanced lift can result
in stalling and crashes as the aircraft pulls away from the center of the microburst.
Microbursts may be strong enough to cause ''straight-line'' wind damage at the
surface, especially if the downdraft is intense and narrow or if there is a strong
enough component of the ambient wind near the ground ( Figure 3.23 ).
Damage from microbursts is typically associated with diuent patterns in ground
debris.
The foundation of our knowledge about microbursts is the early investigations
of aircraft accidents by Ted Fujita. The crash of Eastern Airlines Flight 66 on
June 24, 1975 at John F. Kennedy Airport in New York City, in which 112
people were killed and 12 injured, was the seminal disaster that spurred research
into microbursts. Fujita originally termed the event a downburst, subdividing the
downburst into microbursts when they are 4 km or less across and macrobursts
when they are wider than 4 km. At issue and a source of controversy was whether
they caused airplanes to crash owing to strong downdrafts or otherwise, since ver-
tical velocity weakens to zero at the ground. Three major field experiments were
conducted to study microbursts: NIMROD (Northern Illinois Meteorological
Research on Downbursts) held in the spring and summer of 1978, during which
an event (at Yorkville, Illinois) was captured by Doppler radar, JAWS (Joint
Airport Weather Studies project) held in the Denver area during the summer of
1982, and MIST (Microburst and Severe Thunderstorm Project) held near Hunts-
ville, Alabama during the summer of 1986. In the latter, dual-polarization radar
was used for the first time to probe microbursts and an event at Monrovia,
Alabama was studied intensively.
Microbursts have been classified as being ''dry'' or ''wet''. Dry microbursts
occur over relatively arid terrain when the cloud base is relatively high ( Figure
3.24 ), around 3 km AGL (i.e., when the lapse rate is nearly dry adiabatic and the
boundary layer relatively dry). Thus, the potential for evaporative cooling is great
as water drops and droplets fall through unsaturated air for a relatively long time
and may evaporate completely producing ''virga''. Even light precipitation can
produce strong downdrafts. On the other hand, wet microbursts occur when the
atmosphere is relatively moist and the cloud base is relatively low, so that the
potential for evaporative cooling at low levels is relatively low. In this case,
negative buoyancy is created mainly from water loading in intense precipitation
( Figures 3.22 and 3.25 ) or evaporative cooling aloft. Cooling from the melting of
ice particles (graupel) on the way down in a region of precipitation may enhance
negative buoyancy in both dry and wet microbursts.
Mark Hjelmfelt found in JAWS that microbursts were associated with
descending reflectivity cores seen by radar, as would be expected when new cells
form and precipitation falls out. An area of convegence at mid-levels in a con-
vective storm, near the region of minimum equivalent potential temperature, is
observed in microburst-producing storms. Results from early microphysical