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