INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 28: 833–842 (2008) Published online 13 February 2008 in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/joc.1672 Short Communication United Kingdom daily precipitation intensity: improved early data, error estimates and an update from 2000 to 2006 D. Maraun,* T. J. Osborn and N. P. Gillett Climatic Research Unit, School of Environmental Sciences, University of East Anglia, Norwich, UK ABSTRACT: This paper updates the analysis by Osborn et al. of trends in the contribution of heavy events to precipitation in the UK. We spatially extended the previous analysis of 110 rain gauges to a set of 689 rain gauges covering almost the whole UK, and updated the results to November 2006. For each station and season, we calculated ten time series of the contribution of ten precipitation amount categories to the total seasonal precipitation. A principal component analysis of post-1961 trends of all categories and stations is consistent with earlier results, namely, widespread shifts towards greater contribution from heavier precipitation categories during winter, and towards light and moderate categories during summer. Regional and UK average time series of the contribution from the category consisting of the heaviest events indicate that the increased winter intensity was sustained during the most recent ten years, but the trend did not continue at the rate reported previously for 1961–1995. For summer, the decreasing contribution from the heaviest rainfall category reported for 1961–1995 underwent a reversal during the most recent decade, returning towards the 1961–1995 reference level of intensity. Confidence intervals for these regional and UK average time series were estimated by a bootstrap approach and indicate that the sparser observations from the first half of the 20th century are still sufficient to estimate UK average change. These longer records support the existence of a long-term increase in winter precipitation intensity, and similar trends have now also become evident in spring and (to a lesser extent) autumn. The summer rainfall intensity has exhibited changes that are more consistent with inter-decadal variability than any overall trend. Copyright  2008 Royal Meteorological Society KEY WORDS observed climate; daily precipitation; precipitation intensity trends; climate change; decadal variability; UK Received 6 July 2007; Revised 2 November 2007; Accepted 30 November 2007 1. Introduction Since the mid-19th century, a considerable increase in global, hemispheric and regional average surface tem- peratures has been observed (Brohan et al., 2006; Tren- berth et al., 2007) and future warming owing to anthro- pogenic greenhouse gas emissions is very likely to hap- pen (Meehl et al., 2007). It is well understood that higher temperatures affect the hydrological cycle, leading, at the global scale, to higher atmospheric moisture con- tent and increased evapotranspiration (Trenberth, 2005; Meehl et al., 2007). Regionally varying contributions of these thermodynamic factors in relation to dynamical factors, i.e. changes in the advection and convergence of moisture, lead to a geographically complex response of mean precipitation to global warming. Most Atmo- sphere–Ocean General Circulation Models (AOGCMs) predict an increase in annual mean precipitation in the equatorial region and at high to mid-latitudes, but a decrease from the sub-tropics towards the mid-latitudes * Correspondence to: D. Maraun, Climatic Research Unit, School of Environmental Sciences, University of East Anglia, Norwich, UK. E-mail: d.maraun@uea.ac.uk (Meehl et al., 2007). Regional studies for Europe (Chris- tensen et al., 2007) simulate a decrease in annual mean precipitation for southern Europe, and an increase in northern Europe, with increases in winter over central and northern Europe, and decreases in summer over central and southern Europe. As surface run-off depends strongly on the rainfall intensity and frequency, changes in intense precipitation events rather than mean precipitation will impact more strongly on floods (and also agriculture, via, e.g. soil ero- sion). A multi-model study for the mid-latitudes predicts an increase in precipitation intensity, partly due to an increased atmospheric content of water vapour and partly due to atmospheric circulation; for northern Europe, the latter advective effects dominate (Emori and Brown, 2005; Meehl et al., 2005). Voss et al. (2002) found trends in simulated precipitation extremes and Watterson and Dix (2003) detected an increase of the gamma scale parameter for monthly precipitation extremes in the IPCC SRES scenario A2. These trends in extreme precipitation are predicted to accelerate in the period 2000–2099 rela- tive to the 1900–1999 period (Semenov and Bengtsson, 2002). Copyright  2008 Royal Meteorological Society