GAW2005 Workshop, Geneva, March 2005 Christoph Wehrli Physikalisch-Meteorologisches Observatorium Dorfstrasse 33, 7260 Davos Dorf, Switzerland WORCC tasks assigned in 1995: Development of a radiometric reference for Sunphotometry Development of new instrumentation, algorithms and QC/QA Implementation of a global demonstration network  Summary of global AOD networks World Optical depth Research and Calibration Center, WORCC

GAW2005 Workshop, Geneva, March 2005 Radiometric Calibration of WORCC reference standards Spectral radiance SI scale based on the cryogenic radiometer of PTB (Berlin), transfer via calibrated trap detector Radiometric response of PFR deter- mined every 6 months by spectral comparator facility.  Reference instrument N01 assumed to be radiometrically stable to <±0.5% Top_of_Atmosphere calibration V 0 could be determined in laboratory, if an accurate (<1%) extraterrestrial solar spectrum was available.

GAW2005 Workshop, Geneva, March 2005 Precision Filter Radiometer Network Specifications  Classic sun-pointed filter radiometer, custom built  4 simultaneous channels at 368, 412, 500 and 862nm  Thermostatic operation, minimized filter exposition  Internal Sun-pointing and pressure sensors for QC  1 minute sampling rate for hourly/daily statistics  Weekly/monthly data transmission, central processing  Cloud screening algorithms  Data products: AOD, Ångström  (no inversions)  Continuous in-situ and/or (annual) centralized calibration based on high altitude reference, linear interpolation in time

GAW2005 Workshop, Geneva, March 2005 GAWPFR: global extent 2004: 9 GAW stations operational, 4 stations pending deployment, 5 additional PFRs operated by FMI, DWD, ETHZ

GAW2005 Workshop, Geneva, March 2005 GAWPFR: temporal extent 6 stations with  4 years at >50% coverage 4 stations with  20% cloud free samples Data available at WDCA up to 2003

GAW2005 Workshop, Geneva, March 2005 GAWPFR: results 2003 Forest fires SiteAnnual MLO0.022 IZN0.024 JFJ0.030 ASP0.049 NAS0.089 HPB0.104 MHD0.116 BLO0.119 RYO0.208

GAW2005 Workshop, Geneva, March 2005 AOD Comparison Results  FRC-I Davos, 2 days, fall radiometers at 7, given calibration2  <0.016  Mauna Loa, 1 year 2000 AERONET – GAWPFR at 4, in-situ cal.U95  0.007,  (  )<0.95  Alice Springs, summer radiometers, 2-4, in-situ calibrationU )  Bratt’s Lake, summer radiometers, 4 networks, 5, given cal.2  <0.010, U95(  )<0.25  WMO goal of AOD uncertainty <0.02 OD is achieved in routine network operations.

GAW2005 Workshop, Geneva, March 2005 Ground based global AOD Networks “are currently mostly un-coordinated” Latitudinal distribution Polar regions: 4 Midlatitude North:50 Tropics:26 Midlatitude South:10 Total 90 Major data gaps Africa, Asia, India, Polar region and Oceans International: AERONET, BSRN, GAWPFR, SKYNET National: Australia, China, Finland, Germany, Japan, Netherlands, Russia, USA(4)

GAW2005 Workshop, Geneva, March 2005