Appendix B. Re-analyses and General Circulation Models Limitations of Re-analyses Products
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t is important to realize that despite being very widely used (in excess of 7,000 citations between the two considered here), re-analyses have limitations. Such limitations are detectable by looking at the differences between variables in two re-analyses. Some of these differences can be substantial. In the case of annual global solar radiation, for instance, these are as large as 100 W m-2 over areas such as Central Africa (Figure B1, right), therefore almost as big as the signal itself (Figure B1, left). The situation is further complicated by the fact that while photovoltaic panels (PV) respond to global radiation, CSP devices require mainly the direct beam component of the global radiation. The assessment of the global radiation components is a current research topic and products available are at a preliminary stage. It is clear therefore that the planning of solar power plants would carry a high level of estimated output uncertainty in such circumstances. In principle it would clearly be preferable to use direct measurements for this purpose, but in developing countries these may not be available and re-analyses (or other model simulation) thus become the only or principal tool available for assessing solar power output. Indeed, it is likely that the paucity of observations, which represents a strong constraint to the output of the re-analyses, is responsible for the marked discrepancy in the re-analyses in such poorly observed regions. Similarly to solar radiation, the average 2-meter temperatures for DJF and, to a lesser extent, JJA are also affected by sizeable differences between the two re-analyses (Figure B2). Note that again the larger differences are present over the World Bank regions; a sign that even for this comparatively well-observed variable, re-analyses can diverge over less-well-observed, and possibly poorer, regions.
Figure B1. Global Solar Radiation (Left) Average of the Two ERA-40 and NCEP/ NCAR Re-analyses and (Right) Their Difference
Source: Generated by authors. Note: Large differences are present over most of Africa, South America, and Asia.
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