INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 20: 347–364 (2000) OBSERVED TRENDS IN THE DAILY INTENSITY OF UNITED KINGDOM PRECIPITATION TIMOTHY J. OSBORN a, *, MIKE HULME a , PHILIP D. JONES a and TRACY A. BASNETT b a Climatic Research Unit, School of Enironmental Sciences, Uniersity of East Anglia, Norwich, UK b Hadley Centre for Climate Prediction and Research, UK Meteorological Office, Bracknell, UK Receied 8 February 1999 Reised 7 July 1999 Accepted 8 July 1999 ABSTRACT The intensity distribution of daily precipitation amounts in the UK has changed over the period 1961 – 1995, becoming on average more intense in winter and less intense in summer. This result is based on an analysis of 110 UK station records. In winter, and in terms of their relative contributions to total winter precipitation, there has been a decline in light and medium events and an increase in the heaviest events. This change is fairly uniform across the whole country and is apparent even when longer records (with reduced spatial coverage/detail) are analysed back to 1931 or 1908. The reverse is found in summer: over 1961–1995 there has been a decline in the proportion of the seasonal total being provided by the heaviest events. In the longer term context, however, the summer changes appear to be a return to earlier levels after a period in the 1960s when heavy summer rainfall made a greater than normal contribution. More complex changes have occurred in the intensity distribution of spring and autumn precipitation, with opposite changes in different regions of the UK. Copyright © 2000 Royal Meteorological Society. KEY WORDS: observed climate; daily precipitation; precipitation intensity trends; climate change; UK 1. INTRODUCTION Global warming induced by anthropogenic emissions of greenhouse gases is likely to impact upon the hydrological cycle at all scales from the global (Chahine, 1992) to the local (Frei et al., 1998). Changes in the intensity of precipitation events will have important implications for water resources and flood risk/control (Arnell et al., 1997; but see Chagnon, 1998), soil degradation (Boardman and Favis- Mortlock, 1993) and crop yields (Riha et al., 1996). One possible cause of a change in intensity would be a shift in the relative contributions of precipitation originating from frontal, orographic and convective mechanisms. Under increased greenhouse gas concentrations, some global climate models (GCMs) exhibit enhanced mid-latitude precipitation intensity and shortened return periods of extreme events (Fowler and Hennessy, 1995; Hennessy et al., 1997; McGuffie et al., 1999), driven in some cases by a shift from large-scale (frontal and orographic) to convective mechanisms (Gregory and Mitchell, 1995). Given the unproven reliability of GCMs at small scales (e.g. Mearns et al., 1995), especially at simulating precipitation (due to: the complexity and scale of the physics involved, Boyle, 1998; their failure to resolve all orographic variation, Crane and Hewitson, 1998; Sevruk, 1997; and the difficulties in relating the area–mean GCM output to the point or station scale, Osborn, 1997; Osborn and Hulme, 1997), it is desirable to search for signals of precipitation change in the observational record. The aim of this study, therefore, is to analyse detailed observational records for the UK to identify changes through time in the daily precipitation intensity distribution. It is the changes themselves that are important for impact sectors, regardless of whether the reason is a change in the relative contribu- tions from different precipitation mechanisms. The empirical results might be indicative of changing * Correspondence to: Climatic Research Unit, University of East Anglia, Norwich, NR4 7TJ, UK. Tel.: +44 1603 592089; fax: +44 1603 507784; e-mail: t.osborn@uea.ac.uk Copyright © 2000 Royal Meteorological Society