Adjust wind speed from one measurement height to another

Buoy and shore anemometers commonly sit 2–3 m above the surface, whereas reanalyses (ERA5, ERA5-Land) and the lake-model convention report wind at 10 m. This rescales a wind-speed series to a different height, assuming a neutrally-stratified surface layer.

Usage

wind_at_height(
  u,
  from,
  to = 10,
  z0 = 2e-04,
  method = c("log", "power"),
  exponent = 0.11
)

Arguments

u

wind speed at height from, m/s (vector).

from

measurement height of u, m.

to

target height, m (default 10).

z0

roughness length in m for method = "log" (default 2e-4), or the string "charnock" for a wind-speed-dependent open-water roughness.

method

"log" (default) or "power".

exponent

power-law exponent for method = "power" (default 0.11).

Value

wind speed at height to, m/s, same length as u.

Details

method = "log"

(default) the logarithmic wind profile, u(z) = (u* / kappa) log(z / z0), so u(to) = u(from) * log(to / z0) / log(from / z0). z0 is the aerodynamic roughness length; the default 2e-4 m is a typical open-water value. z0 = "charnock" instead solves the Charnock relation z0 = a u*^2 / g (with a = 0.013) iteratively, so the adjustment grows with wind speed as a real sea/lake surface roughens.

method = "power"

the power law u(to) = u(from) (to/from)^p with p = exponent (default 0.11, the open-water value; use about 1/7 over land).

For 2 m -> 10 m over water all three give a factor of roughly 1.15–1.20. The neutral-stability assumption is good for the moderate-to-strong winds that matter for lake mixing; it over-corrects a little in very light, stable conditions.

See also

met_wind_at_height() to apply this to a MET_* data frame.

Examples

wind_at_height(5, from = 2)              # 2 m -> 10 m, ~5.9 m/s
#> [1] 5.873713
wind_at_height(c(2, 6, 12), from = 3, to = 10)
#> [1]  2.250415  6.751246 13.502492
wind_at_height(10, from = 2, z0 = "charnock")
#> [1] 11.8164