This tool creates a new raster in which each grid cell is assigned the exposure of the land-surface to a hypothetical wind flux. It can be conceptualized as the angle between a plane orthogonal to the wind and a plane that represents the local topography at a grid cell (Bohner and Antonic, 2007). The user must input a digital elevation model (`dem`), as well as the dominant wind azimuth (`azimuth`) and a maximum search distance (`max_dist`) used to calclate the horizon angle. Notice that the specified azimuth represents a regional average wind direction.

Exposure towards the sloped wind flux essentially combines the relative terrain aspect and the maximum upwind slope (i.e. horizon angle). This terrain attribute accounts for land-surface orientation, relative to the wind, and shadowing effects of distant topographic features but does not account for deflection of the wind by topography. This tool should not be used on very extensive areas over which Earth's curvature must be taken into account. DEMs in projected coordinate systems are preferred.

Algorithm Description:

Exposure is measured based on the equation presented in Antonic and Legovic (1999):

> cos(E) = cos(S) sin(H) + sin(S) cos(H) cos(Az - A)

Where, E is angle between a plane defining the local terrain and a plane orthogonal to the wind flux, S is the terrain slope, A is the terrain aspect, Az is the azimuth of the wind flux, and H is the horizon angle of the wind flux, which is zero when only the horizontal component of the wind flux is accounted for.

Exposure images are best displayed using a greyscale or bipolar palette to distinguish between the positive and negative values that are present in the output.

## References

Antonić, O., & Legović, T. 1999. Estimating the direction of an unknown air pollution source using a digital elevation model and a sample of deposition. Ecological modelling, 124(1), 85-95.

Böhner, J., & Antonić, O. 2009. Land-surface parameters specific to topo-climatology. Developments in Soil Science, 33, 195-226.

## See Also

relative_aspect

## Function Signature

`def exposure_towards_wind_flux(self, dem: Raster, azimuth: float = 0.0, max_dist: float = float('inf'), z_factor: float = 1.0) -> Raster: ...`

## Project Links

WbW Homepage User Manual Support WbW