BSRN products and the New Zealand Climate Network

Transcription

1 BSRN products and the New Zealand Climate Network Ben Liley National Institute of Water & Atmospheric Research Presented to Thirteenth BSRN Scientific Review and Workshop ISAC-CNR, Bologna, Italy 9-12 September 2014

2 Antipodes

3 Horizontal Latitude Tilt Direct Tracking

4 Solarview Average daily solar energy for any location in New Zealand, calculated for diffuse and direct irradiance inferred from measured global horizontal irradiance i at the nearest of over 100 climate stations with at least 10 years of hourly h data.

5 Lauder Kaitaia Auckland Hamilton Paraparaumu Wellington Christchurch The NIWA Climate Database holds solar radiant energy data for 150 stations, for varying periods. The data are largely from Licor photodiode sensors, and from Eppley PSPs at some sites. There are good multi-year data for about 70 sites, including all major population centres. The best solar radiation data for New Zealand are from the BSRN station at Lauder, in Central Otago. There are also hourly measurements of global, diffuse, and direct radiation with Eppley sensors at Kaitaia, Paraparaumu, and Invercargill since Invercargill Dunedin

6 Climate Network Irradiance Data From an error when the climate database was established, the data a were stored at degraded precision. There are also questions about the calibration stability, as apparent in these data from the Climate Network (not BSRN) station at Lauder.

7 Clear Sky Fits Hourly The two-parameter global irradiance fit produces data for a 87 closer NIWA fit, sites but were some examined. cross-talk (high b => low I b ), and worse extrapolation to zenith sun. The mean value of b = 1.15 was used to refit These data were fitted with a simple parametric curve. all data, (which can then be done by linear regression).

8 Clear Sky Fits The two-parameter fit produces a closer fit, but some cross-talk (high b => low I b ), and worse extrapolation to zenith sun. The mean value of b = 1.15 was used to refit all data, (which can then be done by linear regression).

9 Clear Sky Fits About 30% of the data are retained here, for Χ 2 < 0.01 I 1.152, and I 1.15 > 850 W m - 2. Thus, many partially cloudy days are retained, but the highest points p are nearly cloud-free. The large changes in clear sky irradiance are clearly unphysical.

10 Radiation Sensor Records To proceed further we need the history of sensor calibrations and d deployments, as recorded in a spreadsheet,, unfortunately with many errors and ambiguities. An improved system is in development, but the spreadsheet data illustrated below are used in the analysis here.

11 Site Sensor Histories Both Eppley pyranometers and Licor silicon photosensors have been used, but most sites have deployed just one type over the last decade. Sensors have been cycled, with regular return to the MetService calibration facility at Paraparaumu.

12 Comparison with BSRN Data The Climate Network data at Lauder can be compared with BSRN data.. With time resolution of 1 minute for the latter, better discrimination of clear days is possible, and only the 12% clearest days are shown.

13 Comparison with BSRN Data Although 1-minute resolution greatly improves clear sky detection, it has a smaller effect on the fitted peak. Aggregating to 10-minute and to hourly totals still shows the same stability in computed zenith irradiance.

14 Comparison with BSRN Data There is no trend in the BSRN data, and even the low values from 1999 to 2002 are probably incorrect. The Diffuse + downward Direct values show no such pattern.

15 Sensor History We can use the deployment database to follow the history of a sensor as it is installed at different sites and returned for calibration.

16 Sensor History Dividing out the applied calibration, and applying just its mean value, reduces the variability for some sensors.

17 Sensor History Dividing out the applied calibration, and applying just its mean value, reduces the variability for some sensors.

18 Sensor History Dividing out the applied calibration, and applying just its mean value, reduces the variability for some sensors. It seems that at least some of the variation is induced by changing the applied calibration when there has been no real change.

19 Lauder Comparison Looking again at the Lauder Climate Network data, the uncalibrated uncalibrated values show less change over time. The now more closely resemble the BSRN data, albeit biased somewhat low.

20 Lauder Comparison Looking again at the Lauder Climate Network data, the uncalibrated uncalibrated values show less change over time. They now more closely resemble the BSRN data,, albeit with some low bias.

21 Conclusions Solar radiation sensors belonging to NIWA, NZ MetService,, and others are deployed around New Zealand, and reported in the NIWA Climate Database. All sensors have previously been calibrated regularly at Paraparaumu by MetService.. NIWA s s sensors are now being calibrated at Lauder. Review of past data shows more variation than expected in clear sky values extrapolated to zenith sun. At least some of this variation appears to result from updating sensor calibration when there has in fact been little change in sensitivity.