m ^ <(. -Zc'.^.^Z l INTERNATIONAL COUNCIL OF WORLD METEOROLOGICAL SCIENTIFIC UNIONS ORGANIZATION RADIATION AND CLIMATE SECOND WORKSHOP ON IMPLEMENTATION OF THE BASELINE SURFACE RADIATION NETWORK (Davos, Switzerland, 6-9 August 1991) NOVEMBER 1991 WCRP-64 WMO/TD-No. 453 ) i t 'I The World Climate Programme launched by the World Meteorological Organization (WMO) includes four components: The World Climate Data and Monitoring Programme The World Climate Applications and Services Programme The World Climate Impact Assessment and Response Strategies Programme The World Climate Research Programme The World Climate Research Programme is jointly sponsored by the WMO and the International Council of Scientific Unions. I NOTE The designations employed and the presentation of material in this publication do not imply the expression of any opinion whatsoever on the part of the Secretariat of the World Meteorological Organizaüon concerning the legal status of any country, territory, city or area, or of its authoriues, or concerning the delimitation of its frontiers or boundaries. This report has been produced without editorial revision by the WMO Secretariat. It is not an official publication and its distribution in this form does not imply endorsement by the Organization of the ideas expressed. R. t; TABLE OF CONTENTS Page No. l. OBJECTIVES OF WORKSHOP l 2. REPORTS ON INSTRUMENTATION AND MEASUREMENT UNCERTAINTY STUDIES 2 3. RECOMMENDATIONS FOR DETERMINATION OF BASIC BSRN PARAMETERS AND BSKN OPERATIONAL PROCEDURES 8 '9^ 3.l Instruments and methods 8 •i:'K.'"- 3.2 Calibration procedures 11 3.3 Data acquisition 12 3.4 Measurement uncertainty techniques 13 3.5 Preparation of operations manual for the BSRN 13 4. BSKN DATA MANAGEMENT 15 4.1 Data management plan 15 4.2 The BSRN data set 15 4.3 Station data collection and transfer to WBMC 16 4.4 Data archiving and distribution 17 4.5 Scientific Evaluation Panel 17 4.6 BSBN status reports 17 4.7 Suninary of BSRN elements and responsibilities 18 4.8 Data system test 18 5. REPORTS ON BSRN STATION IMPLEMENTATION 19 6. PROGRESS IN SURFACE RADIATION BUDGET ESTIMATIONS FROM SATELLITE DATA AND SURFACE MEASUREMENTS 24 7. FUTURE ACTIVITIES 25 8. CLOSURE OF WORKSHOP 26 APPENDIX A» LIST OF PARTICIPANTS APPENDIX B» MEASUREMENTS OF D3NGWAVE DOWNWARD IRRADIANCE USING A "PIR" PYRGEOMETER ] i l. OBJECTIVES OF WORKSHOP 1.1 The second workshop on the implementation of the WCKP Baseline Surface Radiation Network (BSRN) was opened by Dr. C. Fröhlich at 0900 hours on 6 August 1991 at the World Radiation Center, Physikalisch - Meteorologisches Observatorium, Davos (WRC/PMOD). Participants in the workshop are listed in Appendix A to this report. 1.2 Dr' Fröhlich welcomed participants and hoped that, as well as discussions at the workshop, there would also be time to enjoy the magnificent mountain scenery offered by Davos. On behalf of participants. Dr. J. DeLuisi (BSBN Project Manager), expressed gratitude to Dr. Fröhlich for hosting this I workshop on the implementation of the BSRN and to his staff for the excellent arrangements made. 1.3 Dr. J. DeLuisi reminded the workshop of the need to strengthen the conunitment to uniform global observations of surface radiation components and to assure the highest possible quality for the data gathered in order to support climate studies in an effective manner. Workshops of this type provided an avenue for interaction and co-operation between radiation laboratories and BSRN participants. It was also necessary to consider how to obtain the resources to support the BSRN and, in particular, finance for the development of radiation instrumentation that was now so pressingly required. 1.4 More specifically, the present workshop had, as agreed at the earlier meeting on the implementation of the BSRN (Washington, December 1990), been convened to review progress in studies of instrumental uncertainties, proposals for calibration and instrument traceability procedures, the contents of a draft operations manual and data management plan. 1.5 In an initial general review, i-t was noted that, in addition to planned BSRN measurements and other existing observations (e.g. from solar radiation networks), measurements may also be made by other groups not closely linked to the radiation climatological community (e.g. measurements by the Solar Energy Research Institute, now the National Renewable Energy Laboratory). It was recommended that strong efforts be made (e.g. through the International GEWEX Project Office and WCKP Radiation Projects Office) to make as widely known as possible the standards, methods of calibration and accuracy targets of the BSRN, and to encourage co-operation and exchange of views with these groups. In particular, in field experiments such as FIRE Phase II and, later, in the GEWEX Continental-scale International Project (GCIP), the importance of aiming for BSRN standards and instrument recommendations ( see section 3) should be emphasized in order to achieve as much compatibility and comparability as possible. 1.6 Another item of particular importance was the question of the absolute accuracy of existing techniques for determining surface longwave irradiances, as being used for ground truth for the Surface Radiation Budget Climatology Project. This information was now urgently needed to be able to validate methodologies for obtaining estimates of the global distribution of the surface longwave flux from satellite data (see also paragraph 6.3). »" 't •-•• 2 - a 2. REPORTS ON INSTRUMENTATION AND MEASUREMENT UNCERTAINTY STUDIES ':i Var^i^bi_l^.ty^ ln_a_PPPlllat_ion °f_tw.e^ve. iiormal iiic_id^n^e pyrhe^l^pme_ter^s .^ (J. DeLuisi) a 2.1 Changes in measurements with time from twelve normal incidence I pyrheliometers (mounted on separate trackers) after initial calibration with ^ an absolute radiometer were examined. Individual instrument values were compared with the average from all twelve instruments for each minute of b'.11 operation during cloud-free viewing conditions for periods of several days. Typical results for a one-day sample for solar zenith angles varying from 30° to 85° showed that departures from the mean for the normal incidence pyrheliometers were in the range +0.8%. Further work is being undertaken to determine differences in cold periods and for extended time operations. Also, all instruments will be mounted on a single solar tracker in order to assure their exact alignment. A coirparison of 21 mean minute irradiances from two normal incidence pyrhelipmet_ers (B. For ga n) 2.2 The use of one or more normal incidence pyrheliometers to measure the direct mean solar irradiance over periods of a minute in the BSRN is dependent on their conforming to the uncertainty specification (initial BSRN requirement is 15 W/m2 for a one minute exposure). A long series of data collected under operational conditions was available from two Eppley normal incidence pyrheliometers at Cape Grim, and these data have been analyzed to see whether BSRN accuracies can be met in the field. The basic data set for the analysis was derived by calculating 21 minute sample statistics. The long term average values suggest that the relative calibration of the two instruments remained stable within 0.3% - hence the uncertainty in the long term record is less than 0.5%. The mean ratio of the 21 minute averages during the period of the conparison is within 0.1% of the expected value, but this is significant at the 95% confidence level. The uncertainty of any ratio at this level is expected to be approximately 2.4%, representing a difference of 21 W/m2 for the mean irradiance in the study. For one minute exposure, the uncertainty would be larger. Furthermore, given that the comparison with absolute radiometers shows that the 21 minute average can be in error by 2%, reaching the BSRN accuracy of 15 W/m2 for a minute sample under clear sky conditions is unlikely for a single normal incidence pyrheliometer, or even for an average from more than one such instrument. 2.3 These results imply that an absolute radiometer should be used as the primary direct solar irradiance monitoring instrument. Given that the primary instrument will be off-line during calibration or during the shaded part of the measurement cycle, a thermopile pyrheliometer (e.g., normal incidence or i siinilar continuously measuring device) should be employed to observe solar irradiance, with the thermopile reference temperature being monitored. The primary radiometer signal together with the thermopile pyrheliometer data can then be used to produce valid irradiance measurements and/or minute exposure data. In such a procedure, the absolute radiometer is effectively providing continuous calibration for the thermopile instrument. 3 - Calibration °f_a_JloJ:^L^_ ^nci.dence^ pyrhel^pmeter_ (J. Olivieri) 2.4 Dr- Olivieri demonstrated the importance of turbidity on the accuracy of the calibration of a normal incidence pyrheliometer (in the same way that the temperature can influence the calibration factor), and how, using a seauential operation, it was possible to obtain measurements of irradiance closer to values from an absolute radiometer. After measuring the signal, the calibration factor at the ambient air temperature a substitute for the pyrheliometer temperature) can be estimated and a first approximation to the direct normal solar irradiance obtained. The turbidity factor can then be estimated, which is used, in turn, to correct the originally measured pyrheliometer signal and hence to provide a more accurate value for the direct normal solar irradiance. Using this approach, values closer to those from an absolute radiometer have been obtained. Error caa_sed_by^ circ'^spl^ar^ xa.^i^.3X)r^ ^n_mea^sur^in^ ^l^ial radiatTon^ a's—asun—of direct and diffuse components (G. Major) 2.5 As well as direct solar irradiance, pyrheliometers also measure some circumsoldr sky radiation. On the other hand, diffusometers exclude a proportion of the circumsolar sky radiation together with the direct solar irradiance. Based on calculations in mean conditions and using a number of assunptions.