Mean annual solar radiation data layer used in the creation of Land Environments of New Zealand (LENZ) classification. The classification layers have been made publicly available by the Ministry for the Environment (see koordinates.com/#/layers/?q=LENZ for to access these layers).
Development of surfaces for annual and monthly solar radiation required substantially more data preparation than for the other climate surfaces, reflecting the small number of stations at which solar radiation has traditionally been measured.
Monthly estimates of average daily solar radiation to 1980 were available for 22 meteorological stations, but measurements of sunshine hours were available for a total of 98 stations, including 18 of the stations for which solar radiation measurements were available. To extract as much information as possible from these data, a surface was fitted first that predicted for each month the ratio of solar radiation reaching the earth's surface to that reaching the top of the atmosphere, with the latter calculated from solar geometry equations. In fitting this surface, only the 18 data points where measurements were made of both solar radiation and sunshine hours were used.
In addition to NZMG coordinates, it used as an additional predictor the ratio of measured sunshine hours for each month to the maximum possible sunshine hours given no cloud. This surface was then used to estimate the monthly solar radiation received at each of the 80 sites for which measurements of sunshine hours alone were available. Using a total of 98 sites for which solar radiation data were either measured directly or estimated from sunshine hours, surfaces predicting annual and monthly solar radiation were then fitted. Data describing monthly humidity was used as a surrogate measure of cloudiness to improve the fit of the surface to the underlying data. This also increases the local accuracy of the surface predictions, as the number of meteorological stations used to fit the humidity surface is more than three times greater than the number of sites used to fit the solar radiation surface.
The units for this layer are in MJ/m2/day, higher values signifiy areas that have higher levels of solar radiation. This layer has been multiplied by a factor of 10 (i.e. converted into an integer grid) to save space and make the grids more responsive. A value of 123 is actually 12.3 MJ/m2/day.
Additional details such as the climate station locations used in the creation of the layer and error maps are defined in the attached LENZ Technical Guide.