Chemistry Reference
In-Depth Information
of the platform, depth of water above the laser scanner projector, the com-
pass orientation of the platform, and the ambient light intensity.
In field measurements, monitoring of the ambient light is an important
task. Due to the limited laser power (8 mW output, 670 nm wavelength),
the signal intensity deteriorates markedly under bright sunlight although
narrow band-passed filters are used. An analysis of the effect of ambient
light on the slope measurements was presented by Hwang et al. (1996). It
was shown that the ambient light causes an attenuation of the slope signal.
The attenuation factor is a function of the ambient level normalized by the
laser intensity (denoted as
N/S
). Experiments carried out in the laboratory
confirm the analytical calculation. From these results, the data segments
with ambient light intensity
N/S
<2 can be compensated with reasonable
confidence. The ambient intensities of the data selected for analysis pre-
sented in this paper, with
N/S
<0.76, are much less than the critical level.
During the data processing, the ambient light intensity is updated every
10 s.
2.2 Environmental conditions
The SSSB is deployed in the Atlantic Ocean approximately 40 km north-
east of Cape Hatteras (Case 1413) at 35q 26.16' N, 75q 2.90' W and re-
trieved at 35q 24' N, 75q 4' W. The data collection period is from
13:59:29.14 (
t
= 0) to 16:09:22.03 (
t
=7783.15s) UTC. The data for the
first 14 minutes or so are onboard the research vessel and not used. On this
day, two long internal wave bands several kilometres long and approxi-
mately 2 to 4 kilometres apart are spotted on the water surface. The buoy is
released into water at 14:13 UTC. The sky is partly cloudy. At 14:49 (
t
|
3000 s) the ship is in a large dark cloudy area. The buoy records a sharp
drop in the ambient light at
t
>2500 s. The dual surface slick bands are
visible throughout the morning. The buoy obviously crosses one of them at
t
| 3100 s when a dip of 0.1qC water temperatures is recorded (Fig. 1b).
Visual confirmation of this encounter is not available due to the large dis-
tance between the ship and the buoy. However, similar surface temperature
drops (from Jim Edson's onboard met-sensor display) occur when ship
crosses slick bands in a few other occasions.
