CSAT3B Three-Dimensional Sonic Anemometer
47
Liu et al., 2001, recommend that CSAT3B sonic temperature
variances and sonic sensible heat flux are corrected for the effects
of cross wind. The CSAT3B, however, performs an online
correction. Additional correction of CSAT3B data for cross-wind
effects will cause errors in the measured fluxes.
8.1.3 Sonic Transducer Shadow Correction
The CSAT3, CSAT3A, and CSAT3B suffer minimal wind distortion compared
to many other sonic anemometer designs because of a very low ratio of
transducer diameter to sonic path length and since the non-orthogonal
geometry keeps the sonic paths at angles closer to perpendicular with respect to
a mean horizontal wind flow. However, there are cases, such as when
oncoming wind has high angles of attack (for example, low frequency eddies
on a tall tower), that will lead to more shadowing. Accordingly, an
optional
wind shadow correction specific to the CSAT3-series transducer diameter and
sonic path length has been implemented in the CSAT3B OS versions 1.11 and
newer.
This shadow correction, known as the Kaimal correction (Kaimal, 1979 and
Horst et al, 2015), is applied according to:
𝑈𝑈
𝑖𝑖
𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚
=
𝑈𝑈
𝑖𝑖
�
0.84 + 0.16
𝑠𝑠𝑠𝑠𝑠𝑠
(
𝜃𝜃
)
�
Where:
U
i
= the magnitude of the wind vector parallel to the sonic path without
shadowing errors
i
= a placeholder index for
a
,
b
, or
c
, representing the three sonic paths,
and
θ
i
= the angle between the three-dimensional wind vector and the wind
vector component along the a-, b-, or c-sonic path (see FIGURE
The CSAT3B embedded code improves the estimates of
θ
– and therefore the
accuracy of the correction – by iteratively applying the above correction three
times for each measurement of each sonic path. Since there continues to be
some debate on the appropriateness of this and other shadow corrections in
turbulent versus laminar flows, the default of this setting is
Disabled
.
NOTE