TRENDS OF DRY SPELLS IN EUROPE 1951 2000
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TRENDS OF DRY SPELLS IN EUROPE (1951 -2000) C. SERRA(1), M. D. MARTÍNEZ(2), X. LANA(1) and A. BURGUEÑO(3) 3 -8 APRIL 2011 Dept. Física i Enginyeria Nuclear, Universitat Politècnica de Catalunya, Av. Diagonal, 647, 08028 Barcelona, Spain (francisco. javier. lana@upc. edu, carina. serra@upc. edu) (2) Dept. Física Aplicada, Universitat Politècnica de Catalunya, Av. Diagonal, 649, 08028 Barcelona, Spain (dolors. martinez@upc. edu) (3) Dept. Astronomia i Meteorologia, Facultat de Física, Universitat de Barcelona, Av. Diagonal, 647, 08028 Barcelona, Spain (1) OBJECTIVE DATABASE Time trend analyses of three dry spells indices: the number of dry spells per year, N, the longest dry spell per year, Lmax, and the average dry spell length per year, L. In addition, spatial patterns across Europe, at annual and seasonal scales, of these three indices are evaluated. Daily precipitation data recorded at 267 European stations for the years 1951– 2000. Most of these series (236) come from the European Climate Assessment and Dataset (ECA&D) and the rest of the series from the Agencia Estatal de Meteorología (AEMET). METHODOLOGY ØComputation of dry spell lengths (DSLs) for 0. 1, 1. 0, 5. 0 and 10. 0 mm/day thresholds for all the stations at annual scale, and for winter (DJF), spring (MAM), summer (JJA) and autumn (SON) seasons. Ø Computation of N, Lmax and L indices. ØSpatial distribution of the average values of N, Lmax and L. ØDetermination of time trends of these indices by the Kendall-tau algorithm and statistical significances, at the 95% confidence level, by the Mann-Kendall test. Fig. 1 European rain gauges 0. 1 mm/day Sign Trend + − 1. 0 mm/day + 5. 0 mm/day − 10. 0 mm/day + − YEAR N 4(5, 0) 52(-2. 5) 5(6, 3) 43(-2. 6) 8(8, 4) 20(-4. 1) 8(8, 3) 34(-4, 9) Lmax 5(8, 4) 24(-5. 1) 5(9, 5) 23(-6, 5) 8(8, 1) 24(-7, 5) 5(8, 9) 22(-7, 4) L 29(4, 8) 16(-5, 2) 16(4, 1) 16(-4, 9) 8(5, 2) 28(-5, 9) 9(7, 5) 47(-6, 4) WINTER (DJF) N 0 56(-3, 8) 0 53(-3, 9) 4(16, 9) 29(-6, 2) 2(12, 9) 17(-8, 1) Lmax 6(12, 3) 27(-6, 4) 4(12, 5) 15(-6, 7) 3(13, 5) 17(-8, 0) 5(9, 7) 14(-5, 6) L 29(8, 1) 9(-8, 2) 8(7, 6) 12(-7, 1) 6(9, 8) 29(-8, 9) 3(10, 1) 29(-6, 1) SPRING (MAM) N 0 49(-3, 3) 3(7, 5) 23(-3, 4) 3(9) 15(-5, 6) 0 13(-7, 0) Lmax 4(11, 5) 41(-6, 7) 0 31(-6, 8) 1(7, 0) 15(-8, 2) 3(7, 2) 23(-6, 3) L 9(7, 4) 20(-6, 6) 2(6, 4) 14(-7, 6) 2(7, 5) 30(-6, 7) 2(11, 7) 47(-4, 5) SUMMER (JJA) N 2(11, 8) 59(-2, 7) 1(14, 1) 67(-3, 5) 1(15, 4) 61(-4, 6) 0 41(-6, 6) Lmax 10(11, 1) 15(-6, 9) 16(10, 9) 13(-5, 8) 21(10, 0) 11(-7, 0) 17(9, 6) 9(-5, 5) L 20(6, 3) 11(-7, 8) 19(6, 2) 9(-7, 4) 9(7, 0) 12(-7, 5) 6(9, 5) 24(-5, 4) AUTUMN (SON) N 0 32(-3, 5) 0 23(-3, 4) 1(10, 3) 14(-4, 6) 0 7(-8) Lmax 0 27(-7, 0) 2(10, 7) 21(-6, 2) 3(11, 2) 16(-7, 1) 4(8, 9) 20(-7, 7) 4(6, 9) 20(-7, 5) 1(8, 0) 15(-7, 0) 2(8, 6) 30(-6, 6) 0 50(-4, 5) L Table 1. Number of stations with significant positive or negative trends (within parentheses, average trend in %/dec). Boxes in orange colour indicate the cases for which the number of stations is greater than 20 RESULTS AND CONCLUSIONS ØMost of significant trends for N are negative for all thresholds and seasons. The largest number of significant negative trends is obtained for winter and summer. For example, for 1. 0 mm/day, 53 stations in winter and 67 in summer. In winter, they are distributed along a fringe extending toward the eastern regions (40ºN - 50ºN and 0ºE 35ºE). In summer, these are mainly located in Western Europe (40ºN – 60ºN ; 10ºW – 20ºE). An average decrease of 3. 5 - 4. 0% per decade in N is detected in both seasons. Fig. 2 Mean values of N, Lmax and L for 0. 1 mm/day (at annual scale and the four seasons) ØPositive trends on L are dominant at annual scale and the winter season for 0. 1 mm/day, and in the summer season for 0. 1 and 1. 0 mm/day. These time trends on L represent an increase of 6. 0 -8. 0% per decade. For 10. 0 mm/day, spring and autumn seasons are mainly characterized by negative trends for L, with average trends of -4. 5% per decade. Stations with negative significant trends are spread across Europe, except for the Mediterranean coast, which is free of negative trends. Ø Lmax presents an area of positive trends in summer for all threshold (45ºN – 55ºN ; 0ºE - 10ºE). These positive trends represent outstanding increases on Lmax close to 10% per decade. Fig. 3 Maps of significant positive (red triangle) and negative (blue triangle) trends for some indices and thresholds REFERENCES - Klein Tank A. M. G. et al. (2002) Daily dataset of 20 -th-century surface air temperature and precipitation series for the European Climate Assessment. Int. J. Climatol. 22, 1441 -1453. - Lana X. , Martínez M. D. , Burgueño A. , Serra C. 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