For the new release 5 of PVGIS we have included two solar radiation databases that are derived from Climate Reanalysis data. These data have been calculated using Numerical Weather Prediction models that have then been corrected with data from meteorological stations, in order to make estimates of weather parameters for time periods in the past. Some of the existing Reanalysis data sets also contain data for solar radiation and these have been used in PVGIS to provide data for areas of Northern Europe that are not covered by the satellite-based solar radiation data sets. Here we will present a brief comparison between the two reanalysis-based data sets and the satellite-based data (where present), as well as a comparison with the PVGIS-CLASSIC data set that has been available in PVGIS up to version 4, but is not included in PVGIS version 5.
Choice of reanalysis-based solar radiation data
There exist a number of reanalysis data sets with either regional or global data coverage. We have investigated the following data sets:
- MERRA-2 MERRA and MERRA-2 have been developed by NASA's Global Modelling and Assimilation Office. Data have worldwide coverage at hourly time resolution and a spatial resolution of 0.5x0.625° lat/lon. (Rienecker et al., 2011)
- ERA-Interim has been developed by the European Centre for Medium-range Weather Forecast (ECMWF). This data set has worldwide coverage at 3-hourly time resolution and a spatial resolution of 0.75° lat/lon. (Dee et al., 2011)
- ERA-5 This is the new reanalysis product from ECMWF, at the time of writing, the data period is limited to the years 2010-2016. Coverage is worldwide at hourly time resolution and a spatial resolution of 0.28° lat/lon.
- COSMO-REA is a regional reanalysis product, covering Europe at hourly time resolution and a spatial resolution of about 6km (we use 3' lat/lon in PVGIS). The data set is available here. See Bollmeyer et al., 2015 for more information.
Of these data sets, MERRA-2 and ERA-Interim only provide global horizontal irradiance, while ERA-5 and COSMO-REA also provide direct or diffuse irradiance.
Validation of the different reanalysis data sets
The different reanalysis-based solar radiation sets have been validated using ground station measurements. The ground stations used for the validation are those also used in Urraca et al., 2017. The results are shown in the next figure, which also contains a comparison with the SARAH solar radiation data set used as the PVGIS-SARAH database in PVGIS. The results are shown as the annual Mean Bias Deviation in the yearly average global horizontal irradiance, between the different products and the ground station measurements. The values are shown in W/m2.
The results clearly show that for most of Europe both MERRA2 and ERA-Interim show large positive bias. COSMO-REA has large negative bias in Southern Europe, while results are much better in Central and Northern Europe. ERA-5 also shows much less bias than MERRA-2 and ERA-Interim, except in some stations near the coastline. Generally, ERA-5 tends to show a positive bias in Northern Europe, while COSMO-REA shows the opposite tendency.
Based on these results we have chosen to include both ERA-5 and COSMO-REA as options for the choice of solar radiation database in Europe.
Comparison of ERA-5 and COSMO-REA against satellite data and PVGIS-CLASSIC
We have performed a number of calculations showing the difference in annual average global horizontal irradiance between the two chosen reanalysis data sets, and the SARAH solar radiation dataset, as well as a comparison with the PVGIS-CLASSIC data set that has been present in PVGIS up to version 4 of PVGIS, but which will not be available in PVGIS 5.
the next figure shows the relative difference in annual average global horizontal irradiance between the two reanalysis products (COSMO-REA and ERA-5) and the SARAH data, for the area where SARAH dat are available in PVGIS (up to 62°30' N). As in the validation results, we see also here that COSMO-REA tends to show lower values than SARAH in most of Europe, with the exception of mountain areas. The differences are highest in Southern Europe. In contrast, ERA-5 shows higher values than SARAH in some areas (especially Northern Europe and mountain areas) while the two data sets have very similar values in Central Europe. In the Norwegian mountains, both reanalysis data sets have values that exceed the SARAH data by more than 30% (and are therefore shown as white).
The next figure shows the relative difference in annual average global horizontal irradiance between COSMO-REA and ERA-5, again in percentage points. COSMO-REA shows much lower values than ERA-5 in Southern Europe, and moderately lower values in most of the rest of the area, with the exception of Northern Scandinavia and most of Iceland.
Finally, next three figures show the relative difference to the PVGIS-CLASSIC solar radiation database in terms of annual average global horizontal irradiation. Here we compare both the two reanalysis data sets and SARAH against PVGIS-CLASSIC.
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- Dee, D. P., Uppala, S. M., Simmons, A. J., Berrisford, P., Poli, P., Kobayashi, S., Andrae, U., Balmaseda, M. A., Balsamo, G., Bauer, P., Bechtold, P., Beljaars, A. C. M., van de Berg, L., Bidlot, J.,d Bormann, N., Delsol, C., Dragani, R., Fuentes, M., Geer, A. J., Haimberger, L., Healy, S. B., Hersbach, H., Holm, E. V.,d Isaksen, L, Kållberg, P., Köhler, M., Matricardi, M., McNally, A. P, Monge-Sanz, B. M., Morcrette, J.-J., Park, B.-K, Peubey, C., de Rosnay, P., Tavolato, C., Thépaut, J.-N. and Vitart, F.: The ERA-Interim reanalysis: configuration and performance of the data assimilation system. Quarterly Journal of the Royal Meteorological Society, 2011, 137, 553-597.
- Bollmeyer, C., Keller, J.D., Ohlwein, C., Wahl, S., Crewell, S., Friederichs, P., Hense, A., Keune, J., Kneifel, S., Pscheidt, I., Redl, S. , Steinke, S.: Towards a high-resolution regional reanalysis for the European CORDEX system Quarterly Journal of the Royal Meteorological Society2015, 141, 1-15
- Urraca, R., Gracia-Amillo, A.M., Koubli, E. and Huld, T., Trentmann, J., Riihelä, A., Lindfors, A., Palmer, D., Gottschalg, R., Antonanzas-Torres, F.: Extensive validation of CM SAF surface radiation products over Europe. Remote Sensing of Environment, 2017, 199, 171-186