Chemistry Reference
In-Depth Information
ing the bulk temperature to be captured). The radiation reaching the radi-
ometer is modified also by the atmosphere between the instrument and the
target. Each of the mechanisms described above can contribute errors in
the estimated surface temperature at each point of ~ 100mK, which is a
significant obstacle to this methodology.
Another factor that is difficult to eliminate is the direct effect of varying
air-sea fluxes on surface temperature fluctuations. It is clear that a sus-
tained horizontal variation in these fluxes (at the sea surface in the Lagran-
gian frame of the surface elements) will modify the surface temperature
distribution. It may not always be possible to determine if a feature is es-
sentially “oceanographic” (i.e., forced from below by surface renewal;
which ultimately, but not directly, depends on atmospheric forcing), or
“meteorological” (i.e., directly associated with variability in atmospheric
fluxes).
In spite of the limitations described above, we can be certain that if a
subsurface eddy is apparent in a thermal image of the sea surface, then that
eddy has some finite influence on exchange across the microlayer. Thus,
imagery at least must have a role in identifying significant processes. It
should also be possible in some cases to demonstrate that a process does
not absolutely renew the surface. The observations reported in the follow-
ing sections address these issues and are largely qualitative, but we hope
they will be useful in constructing more complete quantitative models of
sea surface renewal.
3 Field observations
Images were taken from a fixed platform, Meetpost Noordwijk, 9 km off
the Dutch Coast in May and June 1996. The region is influenced by the
Rhine river plume, and surface slicks and other features associated with
surfactants and freshwater were often observed. We report the analysis of
data from late (night time) on 3rd of June. The instrument used was a
scanning thermal imager sensitive at longwave (8-12 Pm) set up at an ele-
vation of 16.5 m on an overhanging platform on the North side of the plat-
form. The camera was pointed at a cardinal angle of 030
o
and at 45
o
to the
vertical. Heat flux components and standard meteorological and ocea-
nographic variables were measured (and recorded in most cases as ten-
minute averages). The ten-metre-elevation wind was 6-8 m s
-1
from the
Southwest (unfortunately wind and waves will have been distorted by the
North West corner of the platform). The current was carrying water to-
wards the imager at up to 0.5 m s
-1
. The sky was clear as far as can be es-
tablished. The resulting net infrared heat loss (30-90 Wm
-2
within the data
