Chemistry Reference
In-Depth Information
temperature. For example, under a cold sky a radiometric measurement of
surface temperature is quite sensitive to the assumed emissivity of the sur-
face, and this is invariably quite uncertain due to the sensitivity of emissi-
vity to surface films and surface roughness. Thus, it is not always clear if
the warmest water at the sea surface is at precisely the temperature of the
bulk water (as implied by absolute renewal). Also, our field measurements
are very restricted in resolution. We have resorted to small-scale laboratory
experiments in which we can resolve even millimetre-scale fluctuations by
viewing at short distances, and are able to diminish errors in temperature
measurement greatly. These were simple experiments using tap water, with
no effort to remove the inevitable surfactant loading of the water. Mea-
surements were made at low-incidence angles (usually 30
o
) in order to de-
crease reflection, and in most cases with a background temperature close
to the water temperature (this greatly reduces the error associated with un-
certainty or variability in emissivity). We have conducted a large variety of
experiments in free and forced convection conditions. A general obser-
vation is that water of bulk temperature (within experimental uncertainty)
is only observed to outcrop at the surface in the more energetic cases.
Here, we feature a single experiment designed to investigate the effect of
bubble plumes on surface renewal.
It is thought that large enhancements of air-sea gas exchange are likely
as a result of “whitecapping” breaking waves (Woolf 1997). Transfer will
be enhanced by exchange across the surface of bubbles and by additional
exchange across the sea surface, notably in the wake of the breaking
waves. The simplest description of mass, heat and momentum exchange in
the wake zone simply considers an enhanced level of turbulence. Within
these patches, bubbles contribute buoyancy and alter significantly the be-
haviour of the turbulent plumes. In addition, when bubbles surface and
burst, they will disrupt the surface microlayer.
A basic appreciation of the effect of bubble plumes on the sea surface
can be reached through simple laboratory measurements. An open con-
tainer of water at approximately room temperature will usually be cooled
at the surface by vapour loss to the laboratory, resulting in a surface “skin
temperature” up to ~1K cooler than the bulk of the water. We have con-
ducted experiments in a temperature-controlled laboratory using a con-
tainer with a thermally insulated base and walls, but open at the top and
filled with tap water. The effect on surface temperature of surfacing bub-
bles was measured with a thermal camera, calibrated
via
reference targets
in the field of view. Bubbles were generated by pumping air from the labo-
ratory through a peristaltic pump to a simple aquarium "air stone" in the
base of the container.
