September 2012 1
Advertisement: CM SAF
• Data freely available in netcdf-format• User-friendly data access via the Web
User Interface: www.cmsaf.eu/wui • Toolkit (example data + software):
www.cmsaf.eu/tools• CM SAF Community Site available via
EUMETSAT: training.eumetsat.int
Clouds Radiation Water Vapor
• EUMETSAT Satellite Application Facility on Climate Monitoring www.cmsaf.eu
• Provides satellite-derived climate data of geophysical variables
• Regional, up to global coverage• Currently, data available from Jan
1982 to Feb 2012• Spatial resolution from 0.03° to
approx. 1°
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Jörg Trentmann1, Richard Müller1,
Arturo Sanchez-Lorenzo2, Martin Wild2
1Deutscher Wetterdienst, EUMETSAT Satellite
Application Facility on Climate Monitoring (CM SAF)2Eidgenössische Technische Hochschule (ETH), Zürich
Analyzing the Stability of Gridded
Surface Radiation Data Sets
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Motivation
•Gridded Climate Data Sets (e.g., satellite, reanalysis, surface obs)
become increasingly available
•Three aspects of Climate Data Records should be evaluated:
Climatology (mean)
Temporal Variability (anomalies)
Temporal Stability (trend)
•Validation of gridded data sets often does not assess the
temporal stability / homogeneity
Validation of a satellite-derived climate data set of solar
irradiance using surface measurements
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Data Set
CM SAF Meteosat (Posselt et al., 2012)
•1983 – 2005; 0.03 deg; Meteosat Full Disk, monthly/daily/hourly
•6 different satellite instruments (of the
same type) used!Does this data set fulfill the requirements for climate quality?
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Reference Data Set
GEBA:• Global Energy Balance Archive,
hosted at ETH Zürich• > 200 surface stations globally• starting 1930s• Here: ca. 50 European
stations starting 1983
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Data Set Validation
Validation Strategy:
• Climatological measures (mean, variability)
• Evaluation of the stability
1. Comparison of linear trends
2. Temporal evolution of the bias
3. Application of homogeneity test
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Results: Climatology
•Bias in the order of ± 5 W/m2
•Absolute bias: ~ 10 W/m2
•Correlation of anomalies
between 0.8 and 0.9
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Data Set Evaluation
Evaluation Strategy:
• Climatological measures (mean, variability)
• Evaluation of the stability
1. Comparison of linear trends
2. Temporal evolution of the bias
3. Application of homogeneity test
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Positive trends in both time series: GEBA: +5.3 W/m2/decCM SAF: +2.3 W/m2/dec
• Trends agree within their level of significance
Warszawa: CM SAF Meteosat / GEBA
1. Compare Linear Trends
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Mean all stations: CM SAF Meteosat / GEBA
1. Compare Linear Trends
Positive trends in both time series: GEBA: +3.6 W/m2/decCM SAF: +1.8 W/m2/dec
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Data Set Evaluation
Evaluation Strategy:
• Climatological measures (mean, variability)
• Evaluation of the stability
1. Comparison of linear trends
2. Temporal evolution of the bias
3. Application of homogeneity test
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Warszawa: CM SAF Meteosat GEBA
Significant negative trend of 3 W/m2/dec in the bias time series
2. Analyze Bias Time Series
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Mean all stations: CM SAF Meteosat GEBA
2. Analyze Bias Time Series
Significant negative trend of 1.76 W/m2/dec in the mean bias time series
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Data Set Evaluation
Evaluation Strategy:
• Climatological measures (mean, variability)
• Evaluation of the stability
1. Comparison of linear trends
2. Temporal evolution of the bias
3. Application of homogeneity tests
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Warszawa: CM SAF Meteosat GEBA
3. Homogeneity Test
Penalized Maximum T test(PMT, Wang et al., 2008):
• Detects changes of the mean• Model assuming constant
values and (multiple) shifts• Software and documentation
readily available
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3. Homogeneity Test
• No clear shift detected in all time series
• Maximum of (negative) shifts in 1993-1995
• Very few shifts after 1995
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3. Homogeneity Tests
Homogeneity Test (SNHT)
•Shifts detected in 1987 and 1994 (corresponding to changes in satellite)
•No shift detected after 1994, data set is homogeneous between 1994 and 2005
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1. Linear Trends (> 1994)
Compare CM SAF Meteosat / GEBA after 1994
•Consistent linear trends (~ 4.6 W/m2/dec) •Zero trend in the bias time series•High homogeneity of the CM SAF Meteosat data
set after 1994 in Europe
Trend
Bias
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Trend in Surface Radiation
Spatial distribution of linear trend (1994 – 2005) based on CM SAF Meteosat
•Substantial variability (decrease in Mediterranean!)
•Mean Trend: 3.1 W/m2/dec
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• Temporal stability of gridded climate data sets
need to be evaluated
• Linear trends in surface irradiance in Europe
tend to be reproduced by CM SAF data set
• Inhomogeneities have been detected in the
CM SAF data set before 1994
• The trend in surface irradiance in Europe between
1994 and 2005 is spatially inhomogeneous
• Outlook:
Compare with independently-derived
gridded data sets (e.g., surface obs,
reanalysis)
Evaluate the homogeneity of the surface
data using multiple gridded data sets
Conclusions
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