Severe weather is typically short-lived (<2 hours) and, due to its mesoscale character (<100 km), it affects local/regional areas, necessitating site-specific forecasts. Included in this category are thunderstorms, flash floods, gust fronts, tornadoes, high winds (especially along coasts, over lakes and mountains), heavy snow and freezing precipitation. Mesoscale models with grid cells which can be less than 10 km on a side are used regularly to study such phenomena in detail. The development of radar networks (Box 4.1), new instruments and high-speed communication links has provided a means of issuing warnings of severe weather within the next hour. Several countries have recently developed integrated satellite and radar systems to provide information on the horizontal and vertical extent of thunderstorms, for example. Networks of automatic weather stations (including buoys) that measure wind, temperature and humidity supplement such data. In addition, for detailed boundary layer and lower troposphere data, there is now an array of vertical sounders. These include: acoustic sounders (measuring wind speed and direction from echoes created by thermal eddies), and specialized (Doppler) radar measuring winds in clear air by returns either from insects (3.5 cm wavelength radar) or from variations in the air's refractive index (10 cm wavelength radar). Nowcasting techniques use highly automated computers and image-analysis systems to integrate data rapidly from a variety of sources. Interpretation of the data displays requires skilled personnel and/or extensive software to provide appropriate information. The prompt warning of wind shear and downburst hazards at airports is one example of the importance of nowcasting procedures.
Overall, the greatest benefits from improved forecasting may be expected in aviation and the electric power industry for forecasts less than six hours ahead, in transportation, construction and manufacturing for twelve to twenty-four-hour forecasts and in agriculture for two- to five-day forecasts. In terms of economic losses, the latter category could benefit the most from more reliable and more precise forecasts.
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