roughness.parameters {bigleaf}

Roughness Parameters

Description

A simple approximation of the two roughness parameters displacement height (d) and roughness length for momentum (z0m).

Usage

roughness.parameters(
  method = c("canopy_height", "canopy_height&LAI", "wind_profile"),
  zh,
  frac_d = 0.7,
  frac_z0m = 0.1,
  LAI,
  zr,
  cd = 0.2,
  hs = 0.01,
  data,
  Tair = "Tair",
  pressure = "pressure",
  wind = "wind",
  ustar = "ustar",
  H = "H",
  d = NULL,
  z0m = NULL,
  stab_roughness = TRUE,
  stab_formulation = c("Dyer_1970", "Businger_1971"),
  constants = bigleaf.constants()
)

Arguments

method

Method to use, one of "canopy_height","canopy_height&LAI","wind_profile"
NOTE: if method = "canopy_height", only the following three arguments are used. If method = "canopy_height&LAI", only zh, LAI, cd, and hs are required.

zh

Vegetation height (m)

frac_d

Fraction of displacement height on canopy height (-)

frac_z0m

Fraction of roughness length on canopy height (-)

LAI

Leaf area index (-)

zr

Instrument (reference) height (m)

cd

Mean drag coefficient for individual leaves. Defaults to 0.2. Only needed if method = "canopy_height&LAI".

hs

roughness length of the soil surface (m). Only needed if method = "canopy_height&LAI" The following arguments are only needed if method = "wind_profile"!

data

Data.frame or matrix containing all required variables

Tair

Air temperature (deg C)

pressure

Atmospheric pressure (kPa)

wind

Wind speed at height zr (m s-1)

ustar

Friction velocity (m s-1)

H

Sensible heat flux (W m-2)

d

Zero-plane displacement height (m); optional

z0m

Roughness length for momentum (m); optional

stab_roughness

Should stability correction be considered? Default is TRUE.

stab_formulation

Stability correction function used (If stab_correction = TRUE). Either "Dyer_1970" or "Businger_1971".

constants

k - von-Karman constant (-)
Kelvin - conversion degree Celsius to Kelvin
cp - specific heat of air for constant pressure (J K-1 kg-1)
g - gravitational acceleration (m s-2)
se_median - conversion standard error (SE) of the mean to SE of the median

Details

The two main roughness parameters, the displacement height (d) and the roughness length for momentum (z0m) can be estimated from simple empirical relationships with canopy height (zh). If method = "canopy_height", the following formulas are used:

d = frac_d * zh

z0m = frac_z0m * zh

where frac_d defaults to 0.7 and frac_z0m to 0.1.

Alternatively, d and z0m can be estimated from both canopy height and LAI (If method = "canopy_height&LAI"). Based on data from Shaw & Pereira 1982, Choudhury & Monteith 1988 proposed the following semi-empirical relations:

X = cd * LAI

d = 1.1 * zh * ln(1 + X^(1/4))

z0m = hs + 0.3 * zh * X^(1/2) for 0 <= X <= 0.2

z0m = hs * zh * (1 - d/zh) for 0.2 < X

If method = "wind_profile", z0m is estimated by solving the wind speed profile for z0m:

z0m = median((zr - d) * exp(-k*wind / ustar - psi_m)

By default, d in this equation is fixed to 0.7*zh, but can be set to any other value. psi_m is 0 if stab_roughness = FALSE.

Value

a data.frame with the following columns:

d

Zero-plane displacement height (m)

z0m

Roughness length for momentum (m)

z0m_se

Only if method = wind_profile: Standard Error of the median for z0m (m)

References

Choudhury, B. J., Monteith J.L., 1988: A four-layer model for the heat budget of homogeneous land surfaces. Q. J. R. Meteorol. Soc. 114, 373-398.

Shaw, R. H., Pereira, A., 1982: Aerodynamic roughness of a plant canopy: a numerical experiment. Agricultural Meteorology, 26, 51-65.

See Also

wind.profile

Examples

# estimate d and z0m from canopy height for a dense (LAI=5) and open (LAI=2) canopy
roughness.parameters(method="canopy_height&LAI",zh=25,LAI=5)
roughness.parameters(method="canopy_height&LAI",zh=25,LAI=2)   
   
# fix d to 0.7*zh and estimate z0m from the wind profile
df <- data.frame(Tair=c(25,25,25),pressure=100,wind=c(3,4,5),ustar=c(0.5,0.6,0.65),H=200)
roughness.parameters(method="wind_profile",zh=25,zr=40,frac_d=0.7,data=df)

# assume d = 0.8*zh
roughness.parameters(method="wind_profile",zh=25,zr=40,frac_d=0.8,data=df)