Abstract

Trends in monthly rainfall during the period from 1950 to 2012 and the variability thereof in spaceand time are investigated. The time series analysed are from 120 stations and cover mainly thecontinental part of Greece. To estimate their trends, linear regression is used for each time seriesseparately for: (a) the entire record length (1950 - 2012) and (b) for each half of the period (1950 -1981 and 1982 - 2012). A spatially aggregated time series of rainfall over Greece is also producedand its correlations with climatic features of the northern hemisphere are explored.

1. Introduction - motivation

Monthly rainfall slopes, at the period (1950 - 2010) depict the spatialand temporal variability of the examined variable. The entire periodwas divided in various subperiods (1950-1980, 1981-2010), in order toexamine whether the behaviour of the time series in a longer periodfollows the slopes of the shorter ones. Additionally, the seasonality ofslopes and long time series (70-150 years) were analysed to furtherexplore the variability of annual rainfall. Furthermore, theircorrelation to the entire northern hemisphere sea-level pressuregradient was processed, as well as the air and sea surface temperaturein order to disclose any relevant relationships among them. The abovementioned data analysis is motivated by the need to assess theuncertainty in temporal and spatial rainfall variability. The overallconclusions show that the changes in the rainfall are small anddepend on the time scale.

2. Study area and dataR software was used for our statistical analysis. 136 stations of monthly rainfallwere derived from Hellenic National Meteorological service (HNMS) and fromSpecial Secretariat of Water (SSW), numbered 29 and 107, respectively(http://www.hydroscope.gr/). Stations, elevation clustering and missing valuesare shown in the next figures.

33%

16%18%

19%

14%0-100

100-200

200-500

500-750

750-1000

1%

24%

39%

15%

11%

7%

3%

1%

5%

10%

15%

20%

25%

>30%

3. Spatial variability of rainfall over Greece

Rainfall observed values were spatially integrated in order to disclose grouped areas, with similar rainfall characteristics. The most rainy areas, are located at the western part of Greece. This agrees with the general atmospheric circulation pattern, where the fronts move eastwards above the Mediterranean Sea. Moving further across the mainland, the humid air masses are blocked by the Pindus ridge, and hence central and northern Greece experiences less rainfall. Rainfall rises again at the easternmost part of Greece, and the islands of the south, due to the effect of convective storms and the cyclonic centers in the eastern Mediterranean (between Cyprus and Turkey).

4.Slopes of rainfall during the period 1950 - 2010

Rainfall slopes were estimated for the overall observed period (1950-2010) at monthly time scale, following linear regression of the stations data. Results of the above mentioned process revealed areas with positive and negative tendencies. In the majority of the reporting area were captured negative slopes at the range of (-20 to -2 mm/y). A few exceptions were found in the south-western continental Greece (3 stations), in the north-northeastern Greece (2 stations) and in the central Greece (3 stations), where the positive slopes were ranged from (2 to 20 mm/y). Finally, at many stations at the central and northern Greece, both positive and negative slopes were found at the range of (-2 to 2 mm/y).

5. Seasonality of slopes

Slopes were also estimated at seasonal basis due to the high variability of rainfall through out the year. Each and every season has different estimation slope ranges (see figure below), with the greater ones to be occurred during the winter season (-8 to 4 mm/y) and the smaller in the summer season (-2 to 2 mm/y), as expected. Thus, we see that the over decrease of rainfall during the last 60 years can be linked to the decrease of the winter rainfall.

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

-8 -6 -4 -2 0 2 4 6

Fre

qu

en

cy (

%)

Slope (mm/y)

Autumn

Winter

Spring

Summer

Again, the largest slope are located at the western part of Greece, depicted at the map above.

6. Slopes of rainfall of spatially integrated time series

y = -3.596x + 1030.

0

500

1000

1500

2000

1951 1961 1971 1981 1991 2001 2011

An

nu

al

P (

mm

)

Year

Western

y = -3.332x + 658.6

0

250

500

750

1000

1951 1961 1971 1981 1991 2001 2011

An

nu

al

P (

mm

)

Year

Northern

y = 1.178x + 521.1

0

250

500

750

1000

1951 1961 1971 1981 1991 2001 2011

An

nu

al P

(m

m)

Year

Central

y = -1.631x + 585.7

0

500

1000

1951 1961 1971 1981 1991 2001 2011

An

nu

al P

(m

m)

Year

Southern

y = -1.040x + 453.8

0

250

500

750

1951 1961 1971 1981 1991 2001 2011

An

nu

al P

(m

m)

Year

Southern [islands]

y = -1.797x + 655.8

0

250

500

750

1000

1951 1961 1971 1981 1991 2001 2011

An

nu

al P

(m

m)

Year

Eastern

The estimation of the slopes was also implemented at the spatially integrated time series of 6 regions with high rainfall corellations, which where aggregated over hydrological year (October – September). The figures on the right show the slopes of the annual precipitation (mm) at different regions of Greece, where all the results show a decrease except the central area (mainly Thessalia region). The highest negative slopes are found in western and northern Greece.

7. The effect of time window in the estimation of slopes

-5

-2.5

0

2.5

5

7.5

10

12.5

W N E C S SS

Slo

pe

(mm

)

Region

1950-2010

1960-2010

1970-2010

1980-2010

250

500

750

1000

1250

1500

1950 1960 1970 1980 1990 2000 2010

An

nu

al

P (

mm

)

Year

The rainfall slopes were also estimated at different time intervals of 60, 50, 40 and 30 years. The results show that slopes are highly sensitive to the time period taken, which is shown in the figures below. On the left, the integrated time series of western Greece is taken and the change in the slope (from negative to positive) as the time interval decreases is evident. On the right, we can see that these changes are different and come with varying patterns for each region, i.e. they are monotonous for western, eastern and southern Greece, but not for the other parts of Greece. the show different behaviour of rainfall slopes, estimated for different regions.

8.Estimation of slopes in larger scales

y = 0.056x + 290.5

0

200

400

600

800

1840 1860 1880 1900 1920 1940 1960 1980 2000 2020

An

nu

al P

(m

m)

Year

Athens [observatory]

y = -2.223x + 766.1

0

200

400

600

800

1000

1200

1400

1908 1918 1928 1938 1948 1958 1968 1978 1988 1998

An

nu

al P

(m

m)

Year

Aliartos

y = 0.092x + 587.6

0

200

400

600

800

1000

1200

1933 1943 1953 1963 1973 1983 1993 2003

An

nu

al P

(m

m)

Year

Marathonas

y = -0.228x + 495.6

0

200

400

600

800

1000

1931 1941 1951 1961 1971 1981 1991 2001

An

nu

al P

(m

m)

Year

Heraklion

Four stations with longer records (70-150 y) were also examined to further investigate the effect of time window in the estimation of the slopes. Although the sample size is extremely small the analysis was conducted for exemplary purposes, as at these scales the overall slopes become milder or zero. Furthermore, if we split the Athens (observatory), which is the largest instrumental time series available in Greece, time series in 30-year intervals and estimate the individual slopes, we see that there are three periods with strongly positive slopes. However, the overall slope is very close to zero!

9. Mean differences of rainfall between the periods of 1950 – 1980 and 1981 – 2010

The overall picture that the slopes give suggest that there was a humid period in the first 30 years of our records with a succeeding dry one in the last 30. To validate this we estimated the difference of the means between the two periods for each station as a percentage of the means of the 1950 – 1980 period. As expected the majority of the examined area were negative (map on the right). The largest differences were found at a) westernmost regions, b) easternmost regions and c) high elevation stations (i.e. Pindus ridge). This implies that there could be some links with the general atmospheric circulation, especially for a), as suggested also by Moschou et al. (2013).

10. Seasonal correlations between rainfall at W. Greece and Northern Hemisphere climatic features

Oct – Dec Jan – Mar Ap – Jun Jul – Sep

Sea

Su

rfac

e T

emp

erat

ure

[Had

ISS

T1

reco

nst

ruct

ion

]S

ea L

evel

Pre

ssu

re

[Tre

nb

erth

's N

H]

Lan

d A

ir T

emp

erat

ure

[Ber

kle

y 1

º]

KNMI explorer (http://climexp.knmi.nl/s) was used for the analysis and the field data sets.

Cor. Coef.

Western Northern EasternSouthern [Islands]

11. Correlations between Jan-Mar rainfall at different regions of Greece and Northern Hemisphere climatic features

Sea

Su

rfac

e T

emp

erat

ure

[Had

ISS

T1

reco

nst

ruct

ion

]

Sea

Lev

el P

ress

ure

[Tre

nb

erth

's N

H]

Lan

d A

ir T

emp

erat

ure

[Ber

kle

y 1

º]

KNMI explorer (http://climexp.knmi.nl/s) was used for the analysis and the field data sets.

Cor. Coef.

12. Conclusions

• There is a decrease in rainfall in most regions of Greece, near 2.5 mm/y for the overall estimated period, 1950-2010, which is in good agreement with the results of other works for smaller rainfall records (Feidas et al., 2006) or adjacent regions (Türkeş et al, 2008).

• The rainfall slope s decrease is observed mainly during the winter and is higher in western Greece. Central Greece is the only region presenting a positive slope.

• Examining the slopes in the 30-year intervals, we observe that 1950-1980 was awetter period than 1980-2010 showing a negative trend. On the contrary, duringthe last 30 years there was a rise in rainfall in all regions except central Greeceand the islands at the south.

• The slopes of longer records (70-150 years) show milder, nearly neutral slopes .

• Notably, splitting the slopes in 30 year intervals may create the impression of steeper slopes, which can be misleading.

• Significant correlations were found between regional rainfall (especially for western Greece) with land air temperature, sea level pressure, and sea surface temperature. These patterns show seasonal variability with highest correlations during winter and nearly non-existent in spring.

References

Feidas, H., Ch. Noulopoulou, T. Makrogiannis, E. Bora-Senta, Trend analysis of precipitation time series in Greece and their relationship with circulation using surface and satellite data: 1955–2001, Theoretical and Applied Climatology, 87: 155-177.

Moschou, E. C., S. C. Batelis, Y. Dimakos, I. Fountoulakis, Y. Markonis, S.M. Papalexiou, N. Mamassis, and D. Koutsoyiannis, Spatial and temporal rainfall variability over Greece, 5th EGU Leonardo Conference – Hydrofractals 2013 – STAHY , . Türkeş, M., Koç, T. and Sariş, F. (2009), Spatiotemporal variability of precipitation total series over Turkey. Int. J. Climatol., 29: 1056–1074.

European Geosciences Union – General Assembly 2014 Vienna, Austria, 27 April – 02 May 2014 Session HS7.4: Change in climate, hydrology and society

Yiannis Dimakos, Efie Christina Moschou, Stamatis Christos Batelis, Yiannis Markonis, and Demetris Koutsoyiannis Department of Water Resources and Environmental Engineering National Technical University of Athens

Monthly rainfall trends in Greece (1950 – 2012)