CR 31:35-49 (2006)  -  doi:10.3354/cr031035

Defining dry/wet spells for point observations, observed area averages, and regional climate model gridboxes in Europe

Lars Bärring1,2,*, Tom Holt3, Maj-Lena Linderson1, Maciej Radziejewski4,5, Marco Moriondo6, Jean P. Palutikof7

1Department of Physical Geography and Ecosystems Analysis, GeoBiosphere Science Centre, Lund University, Sölvegatan 12, 223 62 Lund, Sweden
2Rossby Centre, Swedish Meteorological and Hydrological Institute, 601 76 Norrköping, Sweden
3Climatic Research Unit, University of East Anglia, Norwich NR4 7TJ, UK
4Faculty of Mathematics and Computer Science, Adam Mickiewicz University, ul. Matejki 48/49, 60-769 Poznan, Poland
5Research Centre for Agricultural and Forest Environment, Polish Academy of Sciences, Poznan, Poland
6Department of Agronomy and Land Management, University of Florence, Piazza S. Marco, 4-50121 Florence, Italy
7Hadley Centre, Met Office, Fitzroy Road, Exeter EX1 3PB, UK

ABSTRACT: A new method for optimising threshold values of dry/wet spells is evaluated. A set of indices is used to find the best threshold giving good correspondence between the frequency of dry/wet spells in Hadley Centre regional model (HadRM3) output, reference observations with predetermined thresholds, and area-averaged observations. The analyses focus on selected model gridboxes in 3 different European climate regimes (Sweden, UK, Italy), where station data are available from several locations. In addition, a pan-European analysis using the European Climate Assessment (ECA) dataset is carried out. Generally, there is good agreement between point observations and the corresponding area average using the common thresholds of 0.1 or 1.0 mm with observational data. Applying the optimal thresholds on the model output is important, as it typically results in substantially better agreement between the simulated and observed series of dry/wet days. The fitted optimal pan-European dry/wet threshold is (1) 0.47 or 0.15 mm, depending on model version, for the observed point data threshold of 0.1 mm, and (2) 1.2 or 0.56 mm, depending on model version, for the threshold of 1.0 mm.

KEY WORDS: Dry spell · Wet spell · Regional climate model · Precipitation · Areal precipitation · Europe

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