federal departement of home affairs fdha federal office of meteorology and climatology meteoswiss...

Federal Departement of Home Affairs FDHAFederal Office of Meteorology and Climatology MeteoSwiss

Revisiting Swiss temperature trends 1959-2008*

Paulo Ceppi, Simon C. Scherrer, Andreas M. Fischer, Christof Appenzeller

Federal of Office of Meteorology and Climatology MeteoSwiss

16 July 201011th International Meeting on Statistical Climatology, Edinburgh UK

* submitted to the International Journal of Climatology

2 Revisiting Swiss temperature trends | 11th IMSC, 16 July 2010, Edinburgh [email protected]

MotivationTemperature trends: The hallmark of climate change

IPCC 2007, WG1

seasonal differences different processes on different spatial scales

local climate change = large scale + local processes

new Swiss grid


Outline

Geographical setting, input data and gridding

What are the linear temperature trends (yearly / seasonal)?

Can regional climate models explain the observed trends?

What is the trend contribution of

… large scale circulation?

… local factors?

Conclusions


Geographical setting and input data

91 homogeneous station series, 1959-2008 station altitude range: 203 to 3580 m asl ~2 km x 2 km gridded data set

Edinburgh

Switzerland


Monthly anomaly griddingclimatological distance λ

spatial interpolation of altitude corrected residuals influence of topography determined independently every month

C. Frei, MeteoSwiss

valley setting

λ = 0.01 λ = 0.05 λ = 0.1

summit setting


Gridded temperature anomaliesExample December 2009

good representation of local/altitude effects!

C. Frei, MeteoSwiss

topography


Yearly temperature trends1959-2008 (°C/10yrs)

average OLS trend: +0.35°C/10yrs

stronger trends than in global mean (+0.13°C/10yrs)

highly significant trends everywhere (p < 0.0005)

small spatial variability

no altitude dependence


Seasonal temperature trends1959-2008 (°C/10yrs)

°C/10yrs trends positive, (almost) everywhere significant

large seasonal differences of the trends [°C/10yrs] autumn weak (+0.02-0.38), summer strong (+0.34-0.62)

clearly larger than NH-land (exception autumn)

altitude depences in autumn (stronger for low altitudes)

NH-land

trend [°C/10yrs]

dens

ity

SONJJA

MAM

DJF


Can climate models reproduce warming?Northern Switzerland

ENSEMBLES models (smoothed)

observed change (smoothed)

ano

mal

y [°

C]

wrt

196

1-19

90

20yr Gaussian smoothing

but: models are NOT forced with observed circulation! Can observed circulation changes explain the differences?


)(tTOBS = + )(tε)(tTCIRC

Different warming by circulation changes? Linear regression model

Quantify effect of circulationon T via regression model:

• principal components (PCs) of geopotential height at 500 hPa over N-Atlantic/Europe

• model calibration: 1959-83 / validation: 1984-2008• choice of PCs by „stepwise selection“

observed temperatures

“modell error”other effects

)(10

1

tPCi

ii∑=

β

atmospheric circulation


Circulation effects on temperatureSwiss mean 1959-2008

trend lines 1959-2008observed / modelled trend (°C/10yrs)

observed / modelled temperatures

bright: calibration dark: validation

0.35 0.13

0.39 0.21

0.46 0.07

0.39 0.08

0.17 0.02


Summary temperature trend analysisSwitzerland 1959-2008

[°C/10yrs]DJF MAM JJA SON YEAR

observed 0.40 0.39 0.46 0.17 0.35

circulation 0.21 (54%) 0.08 (21%) 0.07 (15%) 0.02 (12%) 0.13 (37%)

residual 0.19 0.31 0.39 0.15 0.22

Can regional climate models account for the residual (circulation corrected) trends?


Summary temperature trend analysisSwitzerland 1959-2008

[°C/10yrs]DJF MAM JJA SON YEAR

observed 0.40 0.39 0.46 0.17 0.35

circulation 0.21 (54%) 0.08 (21%) 0.07 (15%) 0.02 (12%) 0.13 (37%)

residual 0.19 0.31 0.39 0.15 0.22

RCM 0.16 0.18 0.16 0.24 0.18


trend (°C/10yrs)

Altitude dependencegridded temperature trends 1959-2008

negative

anomaly

°C/10yrs

altitude [m

asl]

positive

anomaly

0.6

0.4

0.2

0

-0.2


Swiss temperature trend anomalies1959-2008 [°C/10yrs] month-to-month evolution

altitude [m

asl]

month of the year

more trend than all altitude average

less trend than all altitude average

3000

2500

2000

1500

1000

500

Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

changes in fog/visibility?


Swiss temperature trend anomalies1959-2008 [°C/10yrs] incl. mean temperature evolution [°C]

altitude [m

asl]

month of the year



3000

2500

2000

1500

1000

500



altitude [m

asl]

month of the year

Swiss temperature trend anomalies1959-2008 [°C/10yrs] … and mean snow pack [cm]



100

30

5

100

30

5

snow-albedo effect?3000

2500

2000

1500

1000

500



Conclusions

Swiss temperature trends stronger than NH-land trend (1.6)

Large seasonal differences, altitude dependence in autumn

~50% of the trend in winter due to changes in circulation – much less in other seasons

Regional climate models underestimate circulation corrected trends in spring & summer

Local processes important for local trends, especially in autumn (fog), spring (snow-albedo effect) and summer

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