Geoscience Reference
In-Depth Information
Watches are generally very broad in spatial nature due to the spatial density of the
observations (soundings and surface observation), and models which are based on the
observations, which is very sparse. The resulting analysis of severe weather potential is
therefore necessarily broad. The situation is also very fluid and there can be many local
factors such a topography or land-water boundaries or rural- urban differences, to name just
a few (King et al, 2003; Wasula et al, 2002; Wilson et al, 2010). What are very difficult to
identify are potential mechanisms to create upward motion (the trigger) or to overcome the
convective inhibition (break the cap). On a synoptic scale, this could be lift generated by
cold or warm fronts but on a smaller scale, they can be created by dry lines, thunderstorm
outflows, lake-land breezes, urban hot spots, etc. Often they are very low level and therefore
hard to observe. So forecasts of severe weather are indications that the potential ingredients
exist. They are therefore very broad and strategic in nature.
2.3 Warnings
Weather warnings are issued when there is very high likelihood of severe weather. A
broadly worded severe weather thunderstorm warning is most often first issued. If
appropriate, it is followed by a more specific warning on a particular thunderstorm and
specific severe weather element. This approach is not universal but is dependent on the
climatology of severe weather and the level of the warning service that can or has been
decided to provide. An important aspect of the detail of the warning is the ability to use the
information by the end-user, which is often the public. The public may not know how to
react. Given the "cry wolf" syndrome, there needs to be an education process (see Fig. 1).
Often, a disaster is needed to get the attention of the public but the significance of the event
can be lost in a few short years. Civil emergency services and hydro utilities can plan their
post- event remediation actions/locations based on the warning areas and products. So,
there can be many variations and underlying philosophies for the provision of warning
services. This partially drives the design of the radar processing, visualization and warning
preparations systems. It is one thing if severe weather is prevalent and there is a dedicated
forecaster for a small area and the public is well attuned to the severity of the weather and
have tornado shelters (Andra et al, 2002). It is another thing if the forecaster has to cover
several radars and dealing with ill informed users (Leduc et al, 2002; Schumacher et al,
2010).
3. Identifying severe thunderstorms
3.1 Lemon technique
The specificity of the severe thunderstorm warning is primarily based on a radar feature
identification technique attributed to Lemon (1977, 1980) and is based on a morphological
approach (Moller et al, 1994). It is beyond the scope of this contribution to present or
describe the various types of thunderstorms (Fig. 3 shows a small sample). As mentioned
earlier, precipitation and precipitation cores form aloft and then descend.
The following features need to be identified:
tilted updraft, and/or weak or bound weak echo region
displaced echo top relative to the low-mid level core
strong reflectivity gradients
