It uses the nearest grid point from reanalysis data to obtain the climatological mean temperature, pressure and humidity, but the user can also specify their reference measurements to get the air density. The effect of humidity was previously ignored in WAsP but is now taken into account.
The paper that documents all these changes is published online. It provides extensive validation of the methods, and it is shown that using the nearest reanalysis grid point performs as well as nearby long-term measurements to estimate the air density. The measurements were on average located 40 km away from the sites where the new models were evaluated. It has been typical for WAsP users to forget to apply density specific power curves for each turbine in the wind farm and the process involved manual work. Now the process is fully automated, leading to a much-simplified workflow and more accurate air density estimations. In a future release of WAsP, we will release the ERA5 data, which includes a local temperature lapse rate. This leads to even more accurate air density estimations. You can consult the paper for more details about the new methods and the impact on AEP calculations for a hypothetical wind farm.
Floors, Rogier, and Morten Nielsen. “Estimating Air Density Using Observations and Re-Analysis Outputs for Wind Energy Purposes.” Energies (May 2019).
Find publication on https://doi.org/10.3390/en12112038.