Rapport Klima 2, 2003

Rasmus E Benestad:

Past and future trends in the occurrence of wet and dry periods (RegClim)

Analysis of climatic events may encompass extreme value analysis as well as the duration of the events (also referred to as "runs") and their timing. This study looks at the duration of continuously wet and dry episodes in terms of monthly values and the time interval between each event from global climate models. The analysis is applied both to monthly observational precipitation records as well as the results from empirical downscaling of monthly precipitation.

The analysis suggests that long-lasting wet runs tend to have longer duration than corresponding dry runs. There are no clear trends in either the duration of these runs or in the time interval between each event. The timing of the long-lasting runs is clustered in time, possibly exhibiting a fractal structure. The timing of these events does not follow a Poisson distribution.

Return values for extreme events were estimated though fits to General Pareto and General Extreme Value distribution. The extreme value analysis for the downscaled results suggests that the return values may increase in the future at some but not all locations in Norway.

Keywords: Duration of precipitation anomalies Timing of precipitation anomalies Empirical downscaling Extreme value analysis

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