2.2.2. Numerical Weather Model Data

The second data source used in this study comes from one of the NWP models belonging to the NOAA. The GDAS sflux model, which assimilates scattered observational measurements into a regular grid of data used to initialise the GFS sflux forecast model, was chosen. The GDAS sflux, like its GFS counterpart, has been regularly evolving since its creation, improving its resolution and its physics and numerical components. The version used in this study applies a hybrid Eulerian grid (technical identifier T574) with 23 km of horizontal resolution and 64 vertical levels. Like all GFS and GDAS products, it is solved four times each day, with cycles starting at 00, 06, 12 and 18 h UTC. Each cycle, it generates data outputs with three-hourly resolution up to 9 hours. Only the most recent forecast hours (00 and 03) of each cycle are used, as weather conditions from the farther hours (06 and 09) can be more accurately described by the first hours of the next cycle. The output of this product, starting from 13th February 2012, is available from the AIRS online repository. Thus, the same time span of the monitoring dataset can be covered with GDAS data.

The nearest neighbour point of the GDAS sflux horizontal grid is located at coordinates 40◦22 29"21 N, 18◦0 1"56 E. The horizontal distance between said point and the location of the modelled PV system is roughly 9.9 km. This distance is considered small enough to directly use the data from the GDAS point, as in a nearest neighbour strategy, instead of applying more complex spatial interpolation algorithms.

The output files generated by the different GDAS models contain results for multiple weather variables for all the points of their horizontal grids and vertical levels. For temperature, different variables are available for instantaneous, average, maximum, and minimum values at soil-, distance-, and pressure-based vertical levels. For solar irradiation, two fluxes (short- and long-wave) are available with two propagation directions (downwards and upwards) at the local surface level, along with other levels. The instantaneous air temperature predicted at 2 m over the local surface and the instantaneous downward short-wave radiation flux at local surface level are the chosen variables, as they are most analogous to air temperature and global solar irradiation as measured by a weather station [25]. Those two variables compose the GDAS dataset, the second one used in this study.
