Difference between domain average wind and weather forecast wind in a single grid

[myyesterdays]

Hi WindNinja team, I am asking about the difference between weather forecast wind and domain average wind. Are they having the same physical meaning?

An imaginary scenario to elaborate my question:

1. let's assume the weather forecast data resolution is 5 kM and only one grid there, i.e. a grid of 5 by 5 kM, and the wind speed for this grid is 30m/s.
2. There's another domain which is also 5 by 5 kM with the same elevation (and same location) as the previous grid, and I will apply a wind of 30 m/s (same direction but domain average wind) to this domain.

Will I get the same results from 1 and 2? Thanks

[nwagenbrenner]

@myyesterdays For the example you gave where the weather model initialization has data from only one weather model grid point, yes, the domain average initialization and the weather model initialization will give you the same output. Frequently though, the weather model will have several grid points within the WindNinja domain, in this case each weather model grid point influences the simulation. The way we ingest the weather model data is different between our two solvers (just due to the way the solvers are formulated and how they work), but here is a general description of what happens:

1. For the mass solver, each weather model grid point within the WindNinja domain influences the simulation. The data from each grid point are interpolated to fill the initial WindNinja 3-D wind field. This field is then run through the solver to conserve mass. The mass solver produces a solution that satisfies conservation of mass with a minimal change to the initial wind field.

2. For the mass and momentum solver, the weather model grid points that fall within the WindNinja domain are averaged to get an average input wind speed and direction which is applied (using a log profile to fill vertically) at the inlet boundaries of the WindNinja domain. This is because the mass and momentum solver is driven by the boundary conditions and not the internal initial conditions like the mass solver.

[myyesterdays]

@nwagenbrenner Great, thanks for this, very helpful. It makes me more curious about how the wind inputs working. What if I add a new scenario, say applying a point wind (same wind speed and direction, and the point wind has influence radius covering the entire grid) at one corner or edge of this grid. Will it give the same outputs as the above two scenarios?

[nwagenbrenner]

@myyesterdays Yes, if there is just one weather station with the same wind data as in the above then it will give the same answer.

[myyesterdays]

@nwagenbrenner Thanks! Are outputs also dependent on what types of wind as inputs? Say if the inputs are gust wind speed, then outputs will also be gust winds speed? Similarly, if inputs are sustained (mean) wind speed, then outputs will be sustained wind speeds spatially distributed over the study area?

Also found that the output wind speed is in mile per hour. Is this always the case? even though the input wind speed is set to m/s

[nwagenbrenner]

@myyesterdays The output wind field is an average wind field based on the input wind you give it. WindNinja doesn't simulate gust speeds, per se, but if you feed it a "gust speed and direction" as an input it will treat it the same as if you had fed it a 10-min average speed and direction.

The output speed units are user-customizable. Choices include mph, mps, and kph. You can find a lot of other details about the model and how to use it in our tutorials: https://github.com/firelab/windninja/tree/master/doc/tutorials.

[myyesterdays]

ah I see, thank you very much