WMO Experts Workshop Davos, March 2004 Christoph Wehrli Physikalisch-Meteorologisches Observatorium Dorfstrasse 33, 7260 Davos Dorf, Switzerland What is measured and how Network extent Calibration scheme and Quality Control Results from selected stations Comparison with other networks Sustainability and perspective GAWPFR Network for Aerosol Optical Depth

WMO Experts Workshop Davos, March 2004 A World Optical depth Research and Calibration Center was proposed by GAW QA/SAC and established in 1996 at Davos through sponsoring by MeteoSwiss after BAPMoN turbidity program was abandoned in WORCC mandate Summary of WORCC Tasks Development of new instrumentation, radiometric calibration, data quality control and protocols for aerosol optical depth determination. Implementation of a trial network at 12 Global Observatories to test new instruments and calibration methods. Train station operators in sun photometer measurements. Participating GAW stations are proposed by SAG/Aerosols in collaboration with WMO; WORCC supplies instrumentation donated by MeteoSwiss, while host stations are to provide solar tracking facility and limited manpower.

WMO Experts Workshop Davos, March 2004 What is measured and how? Classic extinction measurements at 4 WMO wavelengths 368, 415, 500 and 862nm  Optical depth, Ångström exponent. Identical, precision filter radiometers PFR (manufactured by PMOD/WRC). Continuous sampling at 1or 2 minute cadence by automated system. Monthly data transfer to WORCC for centralized evaluation. Quality Control flags cover solar pointing, temperature control, cloud screening, … Hourly means archived at WDCA, Ispra; high frequency data available from WORCC upon request.

WMO Experts Workshop Davos, March 2004 GAWPFR network sites

WMO Experts Workshop Davos, March 2004 GAWPFR Station List LocationsinceAltitude Davos, Switzerland (PMOD)10/ Hohenpeissenberg, Germany (DWD co-loc.)06/ Mace Head, Ireland07/ Mauna Loa, Hawaii (AERONET co-loc.)11/ Jungfraujoch, Switzerland (PMOD, Lidar)03/ Bratt‘s Lake, Canada (MSC, BSRN co-loc.)04/ Izaña, Tenerife06/ Ryori, Japan (JMA co-location, Lidar)06/ Alice Springs, Australia (BoM, BSRN co-loc.)07/ Ny Ålesund, Spitzbergen05/ Summit, Greenland (ETH Zürich, campaign) 2001/02/033150

WMO Experts Workshop Davos, March 2004 Precision Filter Radiometer PFR specifications Automated, solar spectral radiometer 4 simultaneous channels at 862, 500, 412, 368nm using IAD interference filters Field of View: ±2.5°, slope angle 0.7° Dimensions: Ø90 x L300mm, Mass 3 kg Continuous, high cadence measurements Sensor at 20±0.1°C in range –25 ÷ 35°C; internal shutter; airtight N 2 flushed housing built-in pointing and barometric sensor Data logger with 30 day storage capacity PFR N21 at Riory, Japan

WMO Experts Workshop Davos, March 2004 Cloud Filtering 3 different cloud flags are determined for individual samples: 1.Harrison & Michalsky algorithm, modified for air masses <2 2.Triplet algorithm applied as moving filter on continuous samples 3.Optical thick (OD>3) clouds

WMO Experts Workshop Davos, March 2004 Data Exchange Format %STATION =MaunaLoa %LATITUDE = %North %LONGITUDE = %West %ALTITUDE = %above sea level %ZONE = 0%UTC=logger time - Zone %STATIDENT = 11 %DATE = %date containing solar noon %AIRPRESS = 666.7%average Sun above horizon %AIRTEMP = 23.8%instrument body %OZONE = 277.3%climatological or actual DU %INSTRUMENT=PFR-N27 %INSTIDENT = 27 %NCHANNELS = 4 %NREADINGS =1439%actual number of records %WAVELENGTH= %nominal or measured %CALDATE = %recalibrated at MLO %CALIBRAT = %Extraterrestrial signal %SLOPE = %mV per day since CALDATE %O3ABS = %Gueymard, filter averaged %RAYLSCATT = %Bodhain at WAVELENGTH %SUNEARTH = % %DECLINAT = %EQOFTIME = %NOON = %LOGFILE =H:\WORCC\PFR\MaunaLoa\mlo_n27.LOG %VERSION = 2.80 %COMMENT =for column description, see PFRLEV2.HLP %END % UTC SunElv Barom Tsens Tbody PntgV PntgH Flg Stations to WORCC: Self-contained daily ASCII files. Meta Data Header with keyword = parameter set. Data Body containing standardized measure- ments with quality flags and solar elevation added. Generated at stations by standardized software. Submission to WDCA in NARSTO format

WMO Experts Workshop Davos, March 2004 WORCC Calibration Hierarchy Standard instruments are calibrated at high altitude stations (Jungfraujoch, Mauna Loa) by atmospheric extrapolation methods. Stability of standard instruments is monitored by spectral comparison with a metrologically traceable absolute detector. Field instruments are calibrated by comparison with WORCC standards at Davos or Langley calibration at Jungfraujoch. Station instruments are linked to WORCC standards by travelling standards, exchange of sensors or re-calibration at Davos. Performance is tested through intercomparison of field instruments with other, co-located networks. Launch of SIMBA98 experi- ment in Aire s Adours, F

WMO Experts Workshop Davos, March 2004 High Altitude Calibration Sphinx Research Station at Jungfraujoch Altitude 3580m Latitude46°32’55” N Longitude 7°59’11” E Automated solar dome of MCH Objective, daily Langley calibrations Robust, statistical analysis for V 0 Annual drift <1±0.15%/y

WMO Experts Workshop Davos, March 2004 Radiometric Calibration Spectral irradiance scale based on a cryogenic radiometer of PTB (Berlin), transferred via calibrated trap detector Radiometric response determined every 6 months by spectral comparator facility.  Reference instrument N01 is assumed to be radiometrically stable to <±0.5%  Top_of_Atmosphere reading V 0 could be determined by laboratory calibration, if an accurate (<1%) extraterrestrial solar spectrum was available.

WMO Experts Workshop Davos, March 2004 Polar Stations

WMO Experts Workshop Davos, March 2004 Inter-annual Variations Baikal Fires Canadian Fires ?

WMO Experts Workshop Davos, March 2004 MLO Comparison 2000: Methodology Mauna Loa co-location site for Aeronet and GAW/PFR AERONET/CIMEL and GAW/PFR data compared at individual samples level Aeronet Level2 data (380, 440, 500, 870nm) as published on internet † GAW/PFR with interpolated calibration (drift <0.25%/year)  CIMEL channels are interpolated to WMO wavelengths of PFR by Ångström’s law using instantaneously derived wavelength exponents.  PFR one minute samples are interpolated in time to CIMEL observation scheme † with kind permission of Dr. Brent Holben, Aeronet PI, NASA/GSFC

WMO Experts Workshop Davos, March 2004 MLO Comparison 2000: Optical Depths N = 4705 cloud free samples compared Wavelength: Mean slope: Mean bias : RMS diff. : Mean AOD :  Excellent agreement within OD at calibration site!

WMO Experts Workshop Davos, March 2004 MLO Comparison 2000: Ångström Averages Alpha(Cimel): 1.81±0.5 Alpha(PFR): 1.63±0.26; Aeronet(2000) : 1.83±0.30 Aeronet(climat) : 1.69±0.24  Marked differences between both radiometers, although they agree well in optical depth.  Ångström exponent  is a less reliable parameter.

WMO Experts Workshop Davos, March 2004 BLO Comparison 2001 Five instruments operating under 4 network protocols co-located at Bratt‘s Lake were compared during 3 months in summer 2001: Cimel (Aeronet); PFR(GAW); SP01 (MSC), 2 MFRSR (MSC and USDA)  3 direct pointed radiometers agreed per observation within ±0.01 (2  )  Significant improvements were obtained through cloud screening algorithms  Differences between 2 MFRSR were almost as large as optical depths  Significant differences in derived Ångström  parameters were found McArthur, B.L.J., Halliwell, D.H., Neibergall, O.J. O'Neill, N.T., Slusser, J.R. and Wehrli, C.; Field comparison of network sunphotometers JGR 108, (D19), 4596, doi: /2002JD002964, 2003

WMO Experts Workshop Davos, March 2004 Sustainability GAWPFR network is in transition from trial phase to operational status. Long-term aspect requires stable structures and resources. WORCC’s additional role as central facility for data collection, evaluation and quality assurance is under revision; resources are limited. All 8 current stations are willing to continue measurements as in trial phase. Distributed processing: most station would process data on-site, when supplied with automated software; some have difficulties providing additional manpower.

WMO Experts Workshop Davos, March 2004 Future Perspectives and Questions WORCC should focus on providing calibration services, including different instrumentation and wavelengths. Archiving of QC measurements besides averaged results. Network Expansion: filling gaps in global coverage. Additional GAW stations using alternative instruments? Lacking solar trackers. Network Collaboration: with BSRN for centralized processing? With other networks: subject of this workshop.

WMO Experts Workshop Davos, March 2004