Geoscience Reference
In-Depth Information
but not very precise in terms of location or time. Most, if not all, people would ignore the
warning and take the risk. Another form of the "cry wolf" syndrome is where warnings are
issued indiscriminately for a very precise time and location and with considerable lead time.
However, particularly for rare events (those at the extreme end of a distribution), this is
accompanied by a high false alarm rate. If too many false alarms are issued, then these will
also be ignored. So, for rare extreme hazardous events, high probability of detection is
needed but the false alarms need to be mitigated (Bieringer and Ray, 1996; Black and
Ashley, 2011; Glahn 2005; Hoekstra et al, 2011; Polger et al, 1994).
So the issuance of warnings requires a very fine balance of decision-making that takes into
account lead time, climatology, societal risk behaviour, social-economic infrastructure, warning
service capacity and many other regional, political and societal factors (Baumgart et al, 2008;
Dunn, 1990; Hammer and Schmidlin, 2002; Mercer et al, 2009; Schmeits et al, 2008; Westefeld et
al, 2006; Wilson et al, 2004). Nowcasts in general are user dependent (Baumgart et al, 2008).
Warnings are an extreme kind of nowcasts in which the thresholds apply to a very broad range
of users (the public). However, in the future, one can envision very specific warnings or
nowcasts issued at lower thresholds that may affect specific users requiring tailored
communication techniques and technologies (Keenan et al, 2004; Schumacher et al, 2010).
The wind hazard deserves an extended discussion (Doswell, 2001). There are various kinds
of wind hazards that have distinctive life times and spatial features. Straight line winds can
originate in synoptic systems or typhoons and are ubiquitous, broad in spatial scale (~100+
km) and extended in duration (~hours/days). Derechos
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are also straight line winds
that originate out of mesoscale convective complexes (MCC; Davis et al, 2004; Evans and
Doswell, 2001; Przybylinski, 1995; Weisman, 2001). The damaging portion exists at specific
locations. They are smaller in size and temporal scale than the previous kind of winds. Gust
fronts originate with the downdrafts of MCC's and depending on the nature of the MCC
(isolated thunderstorm, multi- cellular, line echo wave pattern, bow echo, pulse storm); the
gust front can take on many forms but generally emanate outwards from the MCC (Klingle
et al, 1987). They can extend for a long time and there may be extreme winds in portions of
the gust front.
The downdrafts can also generate quasi-circular outward flowing winds called downbursts
(generic term). If the downbursts are over airports, small in diameter (<4km) and intense
(>10 m/s velocity differential) then they are given a very specific term called the microburst
(McCarthy et al, 1982; Wilson et al, 1988; Wilson and Wakimoto 2001). It is arbitrarily
defined this way in order to be very clear to aviators that they are hazardous and should not
be transected. They originate with a descending intense precipitation core and the wind
intensity is enhanced by evaporative cooling (Byko et al, 2009). If evaporation is strong, by
the time the downburst reaches the surface, there may not be any precipitation associated
with it. In this case, the feature is called a dry downburst. If there is precipitation then it is
called a wet downburst or microburst as the case may be.
There are algorithmic radar techniques for the identification of all of these severe weather
features (Dance and Potts, 2002; Donaldson and Desrochers, 1990; Johnson et al, 1998; Joe et
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It is beyond the scope of this contribution to illustrate the various severe hazards in detail -
see references for fourther information.
