TRENDS OF DRY SPELLS IN EUROPE (1951-2000)
C. SERRA(1), M.D. MARTÍNEZ(2), X. LANA(1) and A. BURGUEÑO(3)
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)
3-8 APRIL 2011
OBJECTIVE
80
DATABASE
80
80
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.
Latitude
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).
Computation 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.
60
50
40
0.1 mm/day
ANNUAL <Lmax>
30
0.1 mm/day
100 70
60
90
55 60
80
-20
-10
0
50
40
30
-10
0
10
20
WINTER <N>
30
40
30
40
-10
0
0.1 mm/day
10
20
WINTER <Lmax>
30
40
-10
0
10
20
WINTER <L>
−
30
40
4(5,0)
52(-2.5)
5(6,3)
11
9
11
-10
0
10
20
SPRING <N>
30
40
40
8
7
-10
0
0.1 mm/day
10
20
SPRING <Lmax>
70
30
-10
0
10
20
SPRING <L>
0.1 mm/day
30
40
0.1 mm/day
25
45
40
40
35
35
-15
-10
-5
0
5
10
15
20
25
30
35
40
45
50
55
30
-20
60
7
50
50
10
0
10
20
SUMMER <N>
30
40
8
-10
0
0.1 mm/day
10
20
SUMMER <Lmax>
30
40
0
10
20
SUMMER <L>
30
40
1
0.1 mm/day
70
50
55
14 60
60
-10
0.1 mm/day
16
45
10
30
35
50
50
8
20
25
6
40
4
-10
0
10
20
AUTUMN <N>
30
40
15
40
2
-10
0
0.1 mm/day
10
20
AUTUMN <Lmax>
70
30
40
10
40
5
-10
0
0.1 mm/day
70
17
10
20
AUTUMN <L>
33
8(8,4)
20(-4.1)
8(8,3)
30
40
0
0.1 mm/day
70
11
15 60
25
13
50
60
9
21
50
17
11
7
50
5
13
9 40
40
-10
0
10
20
30
40
7
9
-10
0
10
20
30
40
5
15
20
25
30
35
40
45
50
55
60
60
70
70
65
65
60
60
55
55
50
50
45
45
40
40
35
35
30
-20
30
-20
-15
-10
-5
0
5
10
15
20
25
30
35
40
45
50
55
60
N WINTER
50
55
60
-15
-10
-5
0
5
10
15
20
25
30
35
40
45
80
80
Lmax
0.1 mm/day
10 mm/day
SPRING
L SPRING
75
70
70
65
65
34(-4,9)
23(-6,5)
8(8,1)
24(-7,5)
5(8,9)
22(-7,4)
60
60
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)
55
55
50
50
N
0
56(-3,8)
0
53(-3,9)
4(16,9)
29(-6,2)
2(12,9)
17(-8,1)
45
45
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)
40
40
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)
35
35
SPRING (MAM)
30
-20
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)
-15
-10
-5
0
5
10
15
20
25
30
35
40
45
50
55
60
30
-20
-15
-10
-5
0
5
10
15
20
25
30
35
40
45
50
55
60
50
55
60
55
60
80
80
N SUMMER
1 mm/day
75
75
70
70
65
65
N
2(11,8)
59(-2,7)
1(14,1)
67(-3,5)
1(15,4)
61(-4,6)
0
41(-6,6)
60
60
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)
55
55
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)
50
50
45
45
AUTUMN (SON)
N
0
32(-3,5)
0
23(-3,4)
1(10,3)
14(-4,6)
0
7(-8)
40
40
Lmax
0
27(-7,0)
2(10,7)
21(-6,2)
3(11,2)
16(-7,1)
4(8,9)
20(-7,7)
35
35
4(6,9)
20(-7,5)
1(8,0)
15(-7,0)
2(8,6)
30(-6,6)
0
50(-4,5)
30
-20
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
Most 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.
29
60
10
L
0.1 mm/day
SUMMER
-15
-10
-5
0
5
10
15
20
25
30
35
40
45
50
55
60
30
-20
-15
-10
-5
0
5
10
15
20
25
30
35
40
45
80
10 mm/day
N AUTUMN
0.1 mm/day
75
L AUTUMN
75
70
70
65
65
60
60
55
55
50
50
45
45
40
40
35
35
40
60
12
50
5
3
40
5
18 70
70
0
75
80
-10
-5
1 mm/day
N WINTER
0.1 mm/day
5(9,5)
L
10
-10
5
15
40
−
50
-15
80
24(-5.1)
9
60
20
40
45
5(8,4)
14
12
50
11
30
60
50
50
Lmax
1
70
16
+
3
40
3
40
70
60
55
50
5
40
−
10.0
mm/day
WINTER (DJF)
19
15
+
43(-2.6)
0.1 mm/day
60
50
5.0
mm/day
−
+
7
10
55
2
70
14
40
4
40
23
12
60
YEAR
N
0.1 mm/day
60
+
1.0
mm/day
6
16
50
60
75
27
60
30
Fig.1 European rain gauges
50
10
70
Sign
Trend
8
20
25
70
10
60
40
35
20
Longitude
0.1
mm/day
12
60
50
10
0.1 mm/day
70
50
40
ANNUAL <L>
65 70
45
65
75
Determination of time trends of these indices by the Kendall-tau algorithm and statistical significances, at the 95% confidence level,
by the Mann-Kendall test.
50
65
80
Spatial distribution of the average values of N, Lmax and L.
60
70
30
-20
 Computation of N, Lmax and L indices.
70
70
L ANNUAL
0.1 mm/day
75
70
METHODOLOGY
ANNUAL <N>
N ANNUAL
0.1 mm/day
75
40
3
-10
0
10
20
Fig.2 Mean values of N, Lmax and L for 0.1 mm/day (at annual scale and the four seasons)
30
40
1
Positive 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.
 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.
30
-20
-15
-10
-5
0
5
10
15
20
25
30
35
40
45
50
55
60
30
-20
-15
-10
-5
0
5
10
15
20
25
30
35
40
45
50
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.
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based on Weibull distribution. Hydrol. Science Journal, 53(1) 48-64.
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lengths in the Iberian Peninsula for the second half of the twentieth century. Theor. Appl. Climatol. 91, 99-116.
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537-555.
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