april 2011 1 jörg trentmann 1, richard müller 1, christine träger- chatterjee 1, rebekka posselt...
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April 2011 1
Jörg Trentmann1, Richard Müller1, Christine Träger-
Chatterjee1, Rebekka Posselt2, Reto Stöckli2
Satellite Application Facility on Climate Monitoring (CM SAF)1Deutscher Wetterdienst, 2MeteoSwiss
Evaluation and applications of a new
satellite-based surface solar radiation data
set for climate analysis
April 2011 2
Motivation
•Surface Solar Irradiance highly relevant:
Climate Monitoring and Climate Analysis
Solar Energy
•Available data sets (ISCCP, GEWEX, ERA-Interim)
agree well on the mean
•Differ substantially in the temporal
evolution
•Coarse spatial resolution
April 2011 3
The CM SAF approach
•Retrieve surface solar irradiance (SIS) from the
geostationary Meteosat satellites (1983 – today)
•Apply a well-established method:
Heliosat (Cano et al.,1986, Hammer et al., 2003)
•Provide the data at high temporal and spatial resolution
•Free and easy accessible
•Validate the data with BSRN surface station data
•Evaluate the data with alternative data sets
April 2011 4
The Heliosat method
Fundamental Assumptions
1. The clear-sky surface radiation can be accurately
calculated (information on the water vapor and aerosol is required)
2. For each satellite pixel and time slot the minimum
reflectance of each months represents clear-sky
conditions (i.e., effect of Rayleigh scattering + surface albedo on the
reflectance)
3. The cloud optical depth is related to the cloud-reflected
solar radiation (= brightness of the visible satellite channel)
4. The degradation of sensor sensitivity can be monitored
with bright cloud targets (maximum reflectance)
April 2011 5
The Heliosat method
•Surface irradiance (global radiation) is
retrieved for each satellite pixel / time
slot (30 min) between 1983 and 2005.
•Average and interpolate to hourly / daily
/ monthly means on a 0.03o-lon-lat-grid.
•Data is freely available in CF-netcdf-format:
www.cmsaf.eu
•Additionally, the direct surface irradiance and the cloud
index are provided.
•Scripts to visualize and analyse the data using open-
source software (cdo, R) are provided.
April 2011 6
Validation
•Monthly means surface irradiance at 14 BSRN stations
April 2011 7
Validation
April 2011 8
Validation
Temporal Stability
a) Hovmöller Diagram
b) Temporal evolution of the bias to BSRN surface stations
c) Apply standard techniques to detect change points (ongoing project, University Frankfurt)
April 2011 9
Evaluation
• CM SAF data about 2-3 Wm-2 higher than alternative data sets
• Temporal evolution of CM SAF consistent with alternative data sets, exception 1991 (Pinatubo??)
April 2011 10
Applications
Close-up Views
Amazon
West Africa
Tanzania
Germany
Mediterranean
April 2011 11
Applications
Surface Irradiance, Mean July, CM SAF Topography
• No validation with DWD surface network available.
• Excellent correspondence with topographic features.
• GEWEX SRB data misses details due to coarse resolution.
GEWEX
April 2011 12
Applications
Close-up Views
Amazon
West Africa
Tanzania
Germany
Mediterranean
April 2011 13
Applications
Mean Surface Radiation, Tanzania
• No surface network available
• Correspondence of surface irradiance with topographic features.
Cooperation with Jörg Bendix, University Marburg
April 2011 14
Applications
Surface Radiation at Mt. Kilimanjaro
• Substantially reduced surface radiation around Kilimanjaro due to enhanced cloud cover
April 2011 15
Applications
Time Series of Surface Radiation
37.6° E, 3.1° S37.5° E, 3.1° S
April 2011 16
Applications
Linear Trend in Surface Radiation(W/m2/dec)
• There are significant trends in the surface radiation at Mt. Kilimanjaro between 1983 and 2005
• Strong increase of surface radiation (more than 15 W/m2/dec) over the summit (> 3000 m)
• Decrease along the northern and southern slopes
April 2011 17
• Heliosat Method is applied to Meteosat Satellites to derive solar surface irradiance from 1983 to 2005
• Validation with BSRN surface measurements shows high quality of the CM SAF satellite-derived data set.
• Temporal (hourly) and spatial (0.03o) resolution of the data set is unique.
• Spatial resolution allows detailed local studies:Strong contrast in temporal trends of surface irradiance at the summit (increase) and the foothills (decrease) of Mt. Kilimanjaro, Tanzania.
• The Data Set is freely available in CF-netcdf-format at www.cmsaf.eu, software tools are also provided
• Processing of the global AVHRR satellite data (1982 – 2009) ongoing (see Poster XY 220, F. Kaspar et al.)
Summary
April 2011 18
April 2011 19
Extra Slides
April 2011 20
The Heliosat method
Reflectivity, 12 UTC, 2 Sept 2008
Min. Reflectivity, Rmin, 12 UTC, Sept 2008
April 2011 21
The Heliosat method
Reflectivity, 12 UTC, 2 Sept 2008Max. reflectance, Rmax:95 % percentile of counts during one month in the reference region
Temporal evolution of Rmax
April 2011
The Heliosat method
The definition of the Cloud Index n:
Cloud Index, 11 UTC, 1 July 2005
April 2011 23
The Heliosat method
• The cloud index, n, is related to the clear-sky index, k.
• The clear-sky index, k, is the ratio between the all-sky surface irradiance, G, and the clear-sky surface irradiance, Gclear
cloud index
clear sky index
April 2011 24
The Heliosat method
• Gclear can be calculated by radiation transfer calculations using
the fast and accurate clear-sky model gnu-MAGIC (Mesoscale Atmospheric Global Irradiance Code, Mueller et al., 2009, http://sourceforge.net/projects/gnu-magic/)
• The cloud index, n, is related to the clear-sky index, k:
• The clear-sky index, k, is the ratio between the all-sky surface irradiance, G, and the clear-sky surface irradiance, Gclear:
G = k * Gclear
k = 1 n
April 2011 25
Validation
• Monthly means SIS from 14 BSRN stations
• Measures: Bias, Variance, Correlation Coefficient of Anomalies, Fraction of months with bias above 15 Wm-2
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