Tornadoes - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 402

to studies on the return on their investment to tornado research. To be able to
formulate an estimate of the return on investments in research, one needs to esti-
mate the reduction in loss of life, injuries, damage, etc. Fatalities occur most often
in mobile homes. Most fatalities, injuries, and damage come from a small fraction
of all tornadoes. Tornado fatalities in the U. S. have fallen from about 4 per
250,000 people from 1900 to 1930, to about 1 per 250,000 people from the mid-
1980s until now, in spite of an increase in population, especially outside urban
areas. In 2011, however, the number rose dramatically, owing to an unusually
large number of outbreaks. It is thought that our improved understanding of
tornadoes, improved forecasts, and nowcasts, in part as a result of the national
network of Doppler radars (NEXRAD) in the U. S. and knowledge passed to
forecasters from researchers (''technology transfer''), have contributed to the
lower fatality rate. In addition, heightened media awareness is also probably
responsible for improved dissemination of warnings.
Warning lead times have increased over the years, but there is a tradeoff
between the probability of detection and the false alarm rate. If warnings were
issued for all possible tornado-producing storms, fewer tornadoes would be
missed, but the false alarm rate would increase and the public would be less likely
to respond to warnings; if fewer warnings were issued for tornado-producing
storms so as to reduce the false alarm rate, some tornadoes would be missed and
the public's confidence in warnings would also decrease.
In addition to direct tornado damage, which in the U. S. equals about $1
billion per year (inflation-adjusted to 2007), estimated measures such as the ''value
of a statistical life'' (VSL) and the ''value of a statistical injury'' (VSI) have been
devised by economists. The VSL has been estimated to be in the range of
$1 million to $10 million or greater. The VSL has been estimated based on
factors such as wages lost, among other things. Structures that can survive even
the most powerful tornadoes can be built, but at a cost. Efforts by the National
Weather Service to reduce the fatality rate in tornadoes have made tornado
shelters less cost effective. Until recently, the overall cost of responding to warn-
ings (and disrupting normal activities) was greater than the costs inflicted by
tornadoes.

6.8 UNRESOLVED PROBLEMS AND CHALLENGES FOR FUTURE
RESEARCH, WITH SUGGESTIONS FOR IMPROVED
MEASUREMENT CAPABILITIES

Much of the theory of tornado boundary layers and tornado structure involves
axisymmetric, steady-state flow in an incompressible atmosphere that is homo-
geneous. In supercells the flow is far from axisymmetric and homogeneous; the
real atmosphere is compressible and since tornadoes form near the forward-flank
downdraft and the rear-flank downdraft, there can be significant horizontal