NCAR ISFF Sonic Anemometer Tilt Correction
Tilt of a sonic anemometer from true vertical can introduce errors
in the measured sensible heat and momentum fluxes.
The tilt error in u* or in a scalar flux
is on the order of 5% per degree of tilt in the vertical plane
aligned with the mean wind direction.
Consequently, it is often necessary to rotate the coordinates of
three-dimensional sonic wind data to correct for this tilt.
NCAR estimates the sonic tilt angles from the archived wind data
using the planar fit technique described in
Wilczak, Oncley, and Stage, 2001, `Sonic anemometer
tilt correction algorithms', Boundary Layer Meteor., 99,
pp. 127-150.
Briefly, this technique assumes that the time-averaged wind field
is confined to a plane surface, nominally parallel to the ground.
It is assumed that the fluxes of interest are those normal to
this wind-field plane and thus
the vertical velocity axis is re-oriented normal to that plane.
The orientation of the wind-field plane relative to the measurement axes
of the sonic anemometer is determined by a least-squares fit of the
wind data to the equation
w = a + bu + cv
where u, v, w are the 5-minute-averaged
horizontal and vertical wind components measured by the sonic anemometer.
The fitted coefficients correspond to
vertical velocity offset = a
sonic lean angle =
atan((b2 + c2)½)
azimuth of sonic lean (relative to u axis) =
atan(c/b)
Confidence in the fit to this model requires data with as wide a range
of wind directions as possible; in practice, 90° or greater.
Thus confidence can best be achieved
with data extending over many hours or days.
If the data are available, this requirement is acceptable
because the sonic tilt angles are expected to be unchanged unless the sonic
anemometer is moved (or the surrounding surface itself changes!).
Thus the tilt angles are calculated for periods delimited by
known physical changes in the sonic orientation, e.g. re-leveling the
mast or swapping the sonic anemometer.
Last modified: Wednesday, 30-Oct-2002
© NCAR/Earth Observing Laboratory
This page was prepared by
Tom Horst,
NCAR ATD Research Technology Facility