R/z0_fun.R

In tom-locatelli/fgr: R Version of the ForestGALES wind risk model

#' Calculates the length (m) of the canopy surface roughness.
#' @title Canopy Roughness Function.
#' @param cr_width Width of the canopy in windless conditions (m).
#' @param cr_depth Length of the canopy in windless conditions (m).
#' @param spacing Mean distance between trees (m).
#' @param uguess Critical wind speed at canopy top calculated with the roughness or single-tree method (m s-1).
#' @param n_drag N parameter of the drag coefficient formula (dimensionless).
#' @param c_drag C parameter of the drag coefficient formula (dimensionless).
#' @param drag_upper_limit Maximum wind speed used during the experiments from which \code{n_drag} and \code{c_drag} were derived (m*s-1).
#' @param ht Height of the tree. In the 'roughness' method, this is stand mean height. In the TMC method, this is 'equivalent mean height' (m).
#' @return \code{z0}, the length (m) of the surface roughness of the tree canopy.
z0_fun <- function(cr_width, cr_depth, spacing, uguess, n_drag, c_drag, drag_upper_limit, ht, fgr_constants) {
  z0 <- (ht - zpd_fun(cr_width, cr_depth, spacing, uguess, n_drag, c_drag, drag_upper_limit, ht, fgr_constants)) *
    exp((-fgr_constants$k * gammasolved_fun(cr_width, cr_depth, spacing, uguess, n_drag, c_drag, drag_upper_limit, fgr_constants)) + (log(fgr_constants$cw) -1 + fgr_constants$cw^(-1)))
  return(z0)
}
#Calculation of Zero-plane Displacement (d) and Aerodynamic Roughness (z0) from "Simplified Expressions for vegetation roughness and
# zero-plane displacement as functions of canopy height and area index" by M.R. Raupach (1994)