GetWind: Get the average wind speed at center of canopy layer

In VEZY/DynACof: A Process-Based Model to Study Growth, Yield and Ecosystem Services of Coffee Agroforestry Systems

Description

Calculate the wind speed decrease in two steps:

1. Decrease the measured wind speed from measurement height until top of the canopy using the formulation of Van de Griend and Van Boxel (1989)

2. Decrease wind speed further with increasing canopy depth using an exponential extinction coefficient and a cumulated LAI above the target point.

Usage

GetWind(
  Wind,
  LAI_lay,
  LAI_abv,
  extwind = 0,
  Z_top,
  ZHT,
  Z0 = Z_top * 0.1,
  ZPD = Z_top * 0.75,
  alpha = 1.5,
  ZW = ZPD + alpha * (Z_top - ZPD),
  vonkarman = Constants()$vonkarman
)

Arguments

Wind	Above canopy wind speed (m s-1)
LAI_lay	Leaf area index of the layer (m2 leaves m-2 soil)
LAI_abv	Cumulated leaf area index above the layer (m2 leaves m-2 soil)
extwind	Extinction coefficient. Default: 0, no extinction.
Z_top	Average canopy height of the taller crop (m)
ZHT	Wind measurement height (m)
Z0	Roughness length (m). Default: 0.1*Z_top
ZPD	Zero-plane displacement (m), Default: 0.75*Z_top
alpha	Constant for diffusivity at top canopy. Default: 1.5 following Van de Griend et al (1989).
ZW	Top height of the roughness sublayer (m). Default: ZPD+alpha*(Z2-ZPD)
vonkarman	Von Karman constant, default to Constants()$vonkarman, 0.41.

Details

The function computes the average wind speed at the center of the canopy layer. It is considered that the leaf distibution is homogeneous in the layer, so the LAI_lay parameter is used to add half of the target layer to the cumulated LAI above:

WindLay= Wh*e(-extwind*(LAI_abv+LAI_lay/2)

with Wh the wind speed at top of the canopy. Note: the alpha parameter can also be computed as:

alpha= (zw-d)/(Z2-d)

Value

WindLay

The winspeed at the center of the layer (m s-1)

References

Van de Griend, A.A. and J.H. Van Boxel, Water and surface energy balance model with a multilayer canopy representation for remote sensing purposes. Water Resources Research, 1989. 25(5): p. 949-971. Part of the code is taken from the MAESPA model.

Examples

# Windspeed in a coffee layer managed in agroforestry system
GetWind(Wind=3,LAI_lay=4,LAI_abv=0.3,extwind= 0.58,Z_top = 24,ZHT = 25)

VEZY/DynACof documentation built on Feb. 3, 2021, 8:52 p.m.