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 specify the name of an input slope raster file (created using the Slope tool), an input aspect raster file (created using the Aspect tool), an azimuth value (i.e. a wind direction), and optionally an input maximum upwind slope raster file (created using the Horizon Angle tool). Notice that the specified azimuth represents a regional wind direction. The algorithm works in two modes: 1) exposure is measured to the horizontal component of the wind, and 2) exposure is measured to the sloped wind (i.e. taking into account both the vertical and horizontal vector components). There is no need to specify a horizon angle image when exposure to the horizontal wind component is calculated. 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.
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.
NoData valued grid cells in the input image will be assigned NoData values in the output image. The output raster is of the float data type and continuous data scale. Exposure images are best displayed using the blue-white-red bipolar palette to distinguish between the positive and negative values that are present in the output.
The following is an example of a Python script that uses this tool:
wd = pluginHost.getWorkingDirectory()
slope = wd + "slope.dep"
aspect = wd + aspect.dep"
output = wd + "output.dep"
azimuth = "215.0"
component = "horizontal"
horizonAngle = "not specified"
args = [slope, aspect, output, azimuth, component, horizonAngle]
pluginHost.runPlugin("ExposureTowardsWindFlux", args, False)
This is a Groovy script also using this tool:
def wd = pluginHost.getWorkingDirectory()
def slope = wd + "slope.dep"
def aspect = wd + aspect.dep"
def output = wd + "output.dep"
def azimuth = "215.0"
def component = "sloped"
def horizonAngle = wd + "horizon angle.dep"
String[] args = [slope, aspect, output, azimuth, component, horizonAngle]
pluginHost.runPlugin("ExposureTowardsWindFlux", args, false)