Chemistry Reference
In-Depth Information
so that associated temperature fluctuations are restricted to below the sur-
face. By contrast, an absolute renewal model implies a distribution of sur-
face temperatures with the bulk water temperature at one extreme. Partial
renewal will produce less extreme variations in surface temperature than
absolute renewal. In general, the distribution of surface temperature fluctu-
ations is a function of both the frequency and character (i.e., absolute or
partial) of renewing eddies.
Thermal imagery offers the possibility of directly measuring tempera-
ture fluctuations at the sea surface, and thereby inferring the character and
intensity of surface renewal. The utility of thermal imagery is limited by its
departure from an “ideal surface thermometer” and the extent to which ap-
parent temperature fluctuations are forced by extraneous factors. We might
wish that an imaging radiometer would solely measure heat radiated from
the surface that it is focussed upon, but reality intervenes. Three factors
that slightly degrade real radiometric measurements are surface re-
flectivity, penetration of thermal radiation through water, and atmospheric
absorption and emission. The sea surface behaves approximately as a black
body at low incidence angles, but a small fraction (~ 1%, depending on in-
cidence angle and wavelength) of radiation received by an imager will
have been reflected at the sea surface rather than emitted there. It is possi-
ble to see reflections of objects in thermal images of the water surface, and
these can contribute to measured temperature fluctuations. Similarly,
variations in the temperature of the sky reflected to the camera will con-
tribute. Variations of the emissivity of the surface can be particularly im-
portant especially for relatively high incidence angles, and when the sky
temperature is very low. Waves, whose slope alters the incidence angle,
can cause significant variability in emissivity across an image. Some sur-
factants can significantly alter the intrinsic emissivity of the surface water.
The penetration of thermal infrared radiation through sea water is wave-
length dependent; it is only an approximation to assume that air-sea infra-
red radiation is radiated and absorbed at the absolute sea surface (this also
has implications for how we model heat transport through the microlayer).
Typically, a longwave (8-12 Pm) thermal imager will receive radiation
from approximately the top 20 Pm of the sea. (It might be noted here also,
that the assumption in most models of the thermal skin that heat transfer by
radiation is insignificant below the absolute surface is also only an ap-
proximation. One notable consequence of finite radiative transfer within
the surface skin is that the rate of change of surface temperature immedi-
ately following an absolute renewal event is also finite; thus while a prac-
tical measurement cannot normally capture surface temperatures exactly
equal to the bulk temperature below, the rate of cooling following an abso-
lute renewal should be sufficiently low for temperatures closely approach-
