P. Geissbuhler et al., Eddy covariance measurements on mountain slopes: The advantage of surface-normal sensor orientation over a vertical set-up, BOUND-LAY M, 96(3), 2000, pp. 371-392
The measurement of scalar fluxes employing the eddy covariance method is a
widely used experimental approach, for which the flow distortion due to obs
tacles (e.g., sensor mounts and mast) is a well-known but not fully solved
problem. In order to reduce flow distortion we installed a sonic anemometer
in a surface-normal orientation relative to the terrain slope, and a secon
d instrument in a vertical position at a horizontal distance of 1.54 m from
the first instrument
We found a significant reduction in the rotation angle necessary for the co
ordinate rotation procedure in the x-z plane when computing 30-minute flux
averages with the surface-normal orientation. In 91% of all cases this rota
tion angle remained within the angle of incidence of +/- 10 degrees recomme
nded by the manufacturer. In contrast, only 24% of the measurements taken w
ith the vertically mounted anemometer were obtained at an angle of incidenc
e within +/- 10 degrees, and 3% were outside the +/- 30 degrees range speci
fied for an acceptable operation.
A data quality test based on the variance of vertical wind speed normalized
with friction velocity (sigma(w)/u(*)) revealed problems for application u
nder stable conditions due to large uncertainties in the determination of t
he Monin-Obukhov stability parameter z/L. An alternative test using the bul
k drag coefficient C-D revealed other problems related to the dependence of
C-D on z/z(0), the measuring height normalized by the roughness length, wh
ich do not appear to be constant in complex terrain. With both tests, a ten
dency for a slightly improved data quality was found for the surface normal
set-up, which, however, proved statistically insignificant.
It is concluded that the surface-normal set-up of a sonic anemometer signif
icantly reduces flow distortion by the sensor head. Although the surface-no
rmal mounting position therefore appears to be the preferred one, with decr
eased flow distortion and a slightly improved data quality, no significant
differences in turbulent quantities were found between the two set-up posit
ions. Hence, the consequences for short-term measurements of mass and energ
y fluxes with a surface-normal set-up in complex terrain appear to be relev
ant only if single flux events are to be inspected, while for long-term mea
surements of integrated fluxes both the surface-normal and vertical install
ation of the sonic anemometer are adequate, indicating that eddy covariance
measurements in complex terrain are less delicate than expected.