Extreme daily temperature and precipitation events in north western Europe and the role of atmospheric circulation

Abstract

This thesis examines changes in the extremes of daily temperature using 4 long-term daily series from northwest Europe, and regional daily precipitation series for the UK. Extremes are defined by a range of indices, principally percentile-based thresholds which allow the study of relative extremes.

The gamma distribution is used to calculate temperature percentiles and is here applied to both minimum and maximum temperature distributions. Different patterns of change are identified for minima and maxima, and for the warm and cold tails of each distribution. Increased warming rates in the last 40 years are associated with more frequent warm extremes whilst the decrease in cold extremes occurred earlier in the 20^th century. Indices reflecting changes in spells of extreme temperature are examined and compared with changes in mean temperature and total extreme frequencies.

Precipitation extremes are examined using a percentile threshold calculated using a method based on the ranks of the data and the issue of the relative change in extremes compared to the change in total precipitation is examined. Fewer significant trends are identified in extremes than for temperature with the greatest change occurring since 1970 with considerable spatial variation in trends across the UK.

The daily atmospheric circulation is represented by 3 air flow indices derived from mean sea level pressure. The influence of each index on daily minimum and maximum temperature is observed to be different to those previously identified with mean temperature and their influence on the occurrence of daily temperature and precipitation extremes is also studied. The contribution of the atmospheric circulation to identified changes of extremes is examined using a method previously employed for the analysis of mean temperature anomalies and precipitation totals and indicates that changes in circulation are unable to explain most of the long-term trends, particularly in extreme temperature spells.

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