Geoscience Reference
In-Depth Information
the two beams as well. Together with the knowledge of the wind direction, thus
the horizontal wind speed can be derived. Furthermore, if the surface roughness
length is available, the surface layer similarity equations for the wind speed and
C
n
2
can be solved iteratively to yield two prime turbulence variables: the friction
velocity
u
∗
and the surface flux of sensible heat. A sensitivity analysis suggests that,
in optimum conditions, a crosswind scintillometer should be capable of providing
u
∗
and the turbulent heat flux with uncertainties of 10-15% and 20-30%, respectively
(Andreas
2000
).
3.10 Tomography
Tomographic methods are able to deliver cross-sections through an air volume.
It is often also called computed tomography (CT). The optical method is based
on integrated optical measurements along a net of intersecting paths covering the
area or volume of interest and a subsequent inverse computation of the absorption
along the cross-sections. The optical emitters and receivers can be installed either
along the perimeter of the area of interest (if the area to be probed should not be
accessed) or the light source can also be erected inside the area with all the detec-
tors at the perimeter. The latter mode of installation can be realised with less effort
(Hashmonay et al.
1999
;Wuetal.
1999
). The original idea for this technique dates
back to Byer and Shepp (
1979
). Tomographic methods are also applied in several
other disciplines such as medicine, geophysics, or oceanography.
The acoustic travel time tomography is based on an analogue principle (Wilson
and Thomson
1994
; Raabe et al.
2001
; Tetzlaff et al.
2002
). Here, acoustic emitters
and receivers are mounted on the perimeter of an area (Fig.
3.17
). From the travel
Fig. 3.17
Schematic design
of the arrangement of sound
sources (full symbols) and
receivers (open symbols) for
acoustic travel-time
tomography of wind and
temperature fields in a
rectangular area
Search WWH ::
Custom Search