Mesoscale Modelling for the Wind Atlas of South Africa (WASA) Project – Phase II

This document reports on the methods used to create and the results of the WRF-based numerical wind atlases developed for the Wind Atlas for South Africa Phase 2 (WASA2) project.
The report is divided into four main parts. In the first part, we document the method used to run the mesoscale simulations and to generalise the WRF model wind climatologies. The second part compares the results from the downscaled numerical wind atlas against the observed wind statistics from the 15 WASA masts. In the third part, the report documents the variability of the 62 m AGL wind speed at the 15 sites in the seasonal and diurnal time scale and compares it with the WRF-simulated winds. In the last part, we document the preliminary results of the ensemble simulations carried out for WASA2 to estimate the uncertainty of the mesoscale simulations.

There have been various updates from the configuration of the WASA simulations in 2014 documented in Hahmann et al. (2015a). Among the most important:

1. Domain covering all South Africa at 3 1/3 km × 3 1/3 km grid spacing.
2. New atmospheric forcing data from latest reanalysis ERA5 at 0.3° × 0.3° spatial resolution; new high-resolution OSTIA SST at a resolution of 1/20° (approx. 5 km).
3. New land cover and land use dataset from ESA/CCI at 300 m resolution.
4. Increased number of vertical levels, from 41 to 61, which results in a much smaller spacing between levels in the PBL.
5. Use of adaptive time step that considerably speeds up the simulations without loss of quality.
6. New PBL and surface layer parameterizations.
7. New translation table from ESA/CCI land cover types to land surface roughness length adapted for South Africa vegetation types.