Chemistry Reference
In-Depth Information
In order to alleviate these problems, several solutions are available. Spa-
tial scanning techniques can be applied to the design of laser slope gauge
to eliminate the problem associated with Doppler shift and still maintain
high frequency response (e.g., Hwang et al. 1993, 1996, Li et al. 1993,
Bock and Hara 1995). In order to reduce flow disturbance, Hwang et al.
(1996) initiated the design of free-drifting deployment. The instruments
are mounted on an open-frame structure, which also serves as a free drift-
ing wave-following buoy such that the relative velocity between flow and
instruments can be reduced to minimize disturbance to the sensing area.
The scanning slope sensor buoy (SSSB) participated in the HiRes II
Field Experiment onboard R/V Columbus Iselin in June 1993. The average
wave number spectra are reported by Hwang et al. (1996). The results in-
dicate that (1) a pronounced peak at the wave number
k
= 9 rad cm
-1
is
evident in the curvature spectra for wind speeds below 6 m s
-1
; (2) the
slopes of the curvature spectra are 1 and -1 on the two sides of the spectral
peak; (3) the spectral density and mean square roughness properties in-
crease linearly with wind speed; and (4) these observations suggest a spec-
tral function of the form F()=
-2
-4
mm
which is proportional to
u
*
k
-3
in the short gravity wave region and
u
*
k
-5
in the capillary wave re-
gion, where
u
*
is the wind friction velocity,
c
m
the minimum phase velo-
city of surface waves, and
k
m
the corresponding wave number. In this pa-
per, the variability of spectral intensity under natural field conditions and
the modulation due to internal waves are studied. Section 2 describes the
field experiment and the measurements. Section 3 presents the analysis
procedure and the results of spectral variation. Section 4 is a summary.
k ucckk
,
*
2 Field experiment
2.1 Measurements
The key instrument for CG wave measurements is the scanning slope sen-
sor. In the present design, a laser beam scans a two-dimensional pattern of
eight linear segments spaced 0.5 cm apart. Each linear segment, 10 cm in
length, is sampled at 50 positions with an equal spacing of 0.2 cm. The du-
ration to sample the 50 positions is 2.5 ms, and the mechanical flyback
time of the scanning mirror facets to generate two consecutive scan lines is
4.3 ms. In addition to the scanning slope sensor, other data collected from
the wave-following SSSB include wind speed and direction at 1.32 m
above mean water level (MWL), air temperature (0.97 m above MWL),
water temperature (0.47 m below MWL), long wave induced acceleration
