wind.shelter is a function to be used with focal.function to calculate a topographic wind shelter index from a digital elevation model, which is a proxy for snow accumulation on the lee side of topographic obstacles. wind.shelter.prep performs some preparatory calculations to speed up repeated calls to wind.shelter.
wind.shelter(x, prob = NULL, control)
wind.shelter.prep(radius, direction, tolerance, cellsize = 90)NULL, use max (equivalent to prob=1)wind.shelter.prep2*pi, clockwise angles.wind.shelter returns the wind shelter index as described above if a numeric matrix x is provided. If it is missing, it returns the character string "windshelter".wind.shelter.prep returns a list with components mask and dist. Both are square matrices with 2*(ceiling(radius)+1) columns and rows:
FALSE) or not (TRUE)wind.shelter implements a wind shelter index used by Plattner et al. (2004) for modeling snow accumulation patterns on a glacier in the Austrian Alps. It is a modified version of the algorithm of Winstral et al. (2002). The wind shelter index of Plattner et al. (2004) is defined as: Shelter index(S) = arctan( max( (z(x0)-z(x)) / |x0-x| : x in S ) ),
where S = S(x0,a,da,d) is the set of grid nodes within a distance <=d< code=""> from x0, only considering grid nodes in directions between a-da and a+da from x0.
The present implementation generalizes this index by replacing max by the quantile function; the max function is used if prob=NULL, and the same result is obtained for prob=1 using the quantile function.
Winstral, A., Elder, K., Davis, R.E. (2002): Spatial snow modeling of wind-redistributed snow using terrain-based parameters. Journal of Hydrometeorology, 3: 524-538.
focal.function, quantile
# Settings used by Plattner et al. (2004):
ctrl = wind.shelter.prep(6,-pi/4,pi/12,10)
## Not run: focal.function("dem.asc",fun=wind.shelter,control=ctrl,
# radius=6,search.mode="circle")## End(Not run)
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