LONG-TERM CLIMATE MONITORING AND EXTREME EVENTS

Problems with long-term monitoring of various extreme meteorological e vents (including tropical and extratropical cyclones, extreme winds, t emperatures and precipitation, and mesoscale events) are examined. For many types of extreme events, the maintenance of long-term homogeneit y of observations is more difficult than is the case for means of vari ables. In some cases, however, a strategy of using more than a single variable to define an event, along with the careful elimination of bia ses in the data, can provide quantitative information about trends. Sp ecial care needs to be taken with extreme events deduced from meteorol ogical analyses, because changes in analysis and observation systems a re certain to have affected extremes. Also, compositing of observation s from more than one station, using differences in means (of temperatu re for instance) to produce a single long-term site, may not remove th e biases in the extremes. These problems, along with ambiguities in de fining extreme events, and difficulties in combining different analyse s from different sites, complicate (and perhaps invalidate) attempts t o determine whether extreme weather is becoming more frequent. The bes t that is likely to be achieved, even with increased emphasis on attai ning the high-level of homogeneity necessary in the observations, is t o monitor long-term variations in certain important extreme events, in select locations with high-quality data. Regional indices of importan t extreme events, selected on the basis of their damage potential and capable of adequate monitoring, may be established. If, in the future, we are to answer the question ''Are extreme weather events becoming m ore frequent?'', we must establish and protect high-quality stations c apable of monitoring the most important extreme events (perhaps with s uch regional indices), and ensure that changes affecting the recording of extreme events (e.g., changes in exposure) are meticulously docume nted.