Geoscience Reference
In-Depth Information
will be on the severe weather identification algorithms, the underlying philosophy for its
usage, the level of expertise required, decision-making and the preparation of the warning.
Radar is also used for the precipitation estimation and its application for flash flood
warnings. This is discussed elsewhere (Wilson and Brandes, 1979). Only a few countries
have convective thunderstorm warning services and the target audience for this
contribution are those countries or NHMS' considering developing such a service. The
intent is to provide a broad overview and global survey of radar processing systems for the
provision of severe weather warning services. There is a considerable literature in
convective weather forecasting and warning, this contribution can only explore a few
aspects of this topic (Doswell, 1982: Doswell, 1985; Johns and Doswell, 1992; Wilson et al,
1998).
The forecasting and the warning of severe weather are very briefly described. Then, the
underlying technique for the identification of severe thunderstorms using radar is
presented. This forms the basis for the radar algorithms that identify the severe storm
features. The basic components of the system are then described. Some details and unique
innovations are incorporated in the global survey of operational or near operational use.
This is concluded by a summary.
2. Forecasting/Nowcasting/Severe weather warnings
Severe weather predictions are divided into severe weather watches and severe weather
warnings. In the preceding days, thunderstorm outlooks may be issued. Watches are
predictions of the potential of severe weather. They are strategic in nature and fairly coarse
in spatial and temporal resolution. They are often issued on a schedule or in conjunction
with the public forecast. The expected behaviour is that the public would be aware of the
possibility of severe weather and to listen for future updates. Warnings are predictions of
the occurrence or imminent occurrence (with high certainty) of severe weather. They are
tactical and more specific in location and time. They are also specific in weather element.
They are a call to action and to protect one's property and one's self. They are issued and
updated as necessary. Fig. 1 shows an overview of the process from the Japanese
Meteorological Agency.
Weather advisories are issued if the weather is a concern but not hazardous. Specific types
of warning, such as tornado or hail warnings may then be issued and generally after the
more generic severe thunderstorm warning is issued.
The key difference is that the watch is a forecast or very short range forecast service as
strategic in nature whereas the warning is a nowcast (based on existing data, precise in time,
location and weather element) and tactical in nature.
2.1 Severe weather definition
Severe weather is defined here as heavy rains, hail, strong winds including tornadoes and
lightning. In the production of warnings, thresholds need to be defined. The thresholds are
necessarily locally defined by climatology, local infrastructure and familiarity will dictate
what is extreme. Table 1-4 show the warning criteria for Canada circa 1995. Canada is a very
big country covering many different weather climatologies and therefore is illustrative of
