

%Satellite Oceanography Coursework         Image %Interpretation

%Notes on Questions - Section 2


\centerline{\bf Answers to Questions --- Section 2}

{\bf Notes on Questions}

\def\degrees{$^\circ$}
\def\minutes{$^{\prime}$}
\def\seconds{$^{\prime\prime}$}
\def\degc{{\degrees}C}
\def\degC{{\degrees}C}
\def\file#1{{\tt#1}}


The lessons provided in the section on image interpretation contain a number of 
questions in the lesson outline instructions.  Many of these have numerical 
answers which can be checked directly by a teacher working through the material 
themselves. 
 There are, however, a number of more open-ended questions which encourage the 
student to think more creatively about the oceanographic interpretation of the 
image data. 
 These do not necessarily have a right or wrong answer, and may not be fully 
answerable without further scientific research of the region in question.  
However, it may be useful to know what the author had in mind as the questions 
were posed.  These brief notes are offered for that purpose.

The numbered paragraphs correspond with the numbered steps in the lesson 
outlines where particular questions are raised.

\bigskip
\centerline{\bf LESSON 1}


{\bf   3.}
  The high temperature part of the histogram corresponds to overland pixels at 
the centre of the image.  These are the Shetland Islands.  The low temperature 
tail of the histogram is the mass of cloud contaminated pixels to the west of 
the image.  In the pre-processing of the data, completely cloudy pixels have 
mostly been returned with a default temperature value of 0 and form a spike at 
the left hand axis of the histogram which can be easily overlooked.

{\bf   5.}
The evidence of the northeastward flow from the Atlantic into the Norwegian Sea 
is the warmer water to be found in the western part of the image.  The cooler 
Norwegian Coastal Current can be found at the eastern edge of the image.

{\bf   7.}
Meanders on the Norwegian coastal current appear to have a typical length scale 
of about 30 pixels, equivalent to a true length scale of 28km.  The temperature 
change across the front is 1.2\degC.

The eddy features have a temperature difference between centre and outside of 
about 0.5\degC\  and a diameter of approximately 22 pixels (20km).

{\bf   11.}
  The temperatures on the image are brightness temperatures based on a single 
channel radiometer measurement (see the definition of the image data).  No 
correction has been applied for the effect of atmospheric contamination of the 
infra-red water-leaving signal.  Generally this would be expected to reduce the 
apparent temperature a few degrees C below the true skin temperature.

\bigskip
\centerline{\bf LESSON 2}


{\bf   3.}
  The twin peaks in the temperature histogram are due to the division of this 
image into two regions, one about 4\degC\  cooler than the other, separated by 
the Iceland-Faroes Front.  The pixels corresponding to the small part of 
Iceland in the top left of the image form a small warm hump in the histogram to 
the right of the two main peaks.  This is an unusual histogram.  Normally such 
a distinct thermal gap between two peaks would indicate the difference between 
sea and land, or sea and cloud temperatures.  The reason why there is not 
simply one broad histogram of sea temperatures in this case is that the front 
is so sharp that very few pixels have temperatures intermediate between the 
two.  The cool peak is smaller because on this scene the cool water occupies a 
smaller area than the warm water to the south.  Given the unusual histogram, 
and the sharpness of the front, it would be easy for an inexperienced analyst 
to interpret the front as a coastal boundary.


{\bf   7.}
To rectify image \file{AB1} so that it represents a true geometry, a 
north-south zoom of magnification $\times2$ should be applied, whilst the 
east-west remains the same ($\times1$).

{\bf   8.}
  A temperature Gradient image generally contains little information.  Most of 
the structure is indistinguishable from noise, except where there is a strong 
thermal gradient.  However in a case like this, it is possible to make the 
frontal position stand out clearly above the noise, and such a display focusses 
the observer's attention on the front which is dynamically the most interesting 
feature in the image.  Operationally it would be possible to use a gradient 
image to note the variation in position of the front between different 
overpasses.

{\bf   9.}
  The quality of the data is degraded by smoothing, since information is lost.  
However, there is sometimes value in smoothing which enables the overall 
structure to be more readily observed because the eye is not so distracted by 
the spottiness and patchiness of a noisy image.  If, as in this case, the image 
radiometric resolution is smaller than is justified by the sensor's 
specification then smoothing does not lose much significant information.  Its 
most degrading effect is found at the frontal boundary, where it weakens the 
horizontal temperature gradient, and at coast or cloud boundaries which it 
smears.

\bigskip
\centerline{\bf LESSON 3}


{\bf   3.}
  The main part of the histogram corresponds to sea pixels.  The low radiance 
pixels are the darkest water.  The high spike corresponds to cloud (the bottom 
right hand corner) or Iceland itself.  Note that over the snow-covered parts of 
Iceland there are very high radiances, but around the coastal region the land 
is darker, but not as dark as the water.  Careful selection of a Look Up Table 
will yield a clear coastline along an appropriate boundary between two colours 
corresponding to the land-sea threshold.

{\bf   5 \& 8.}
  Without water sample analysis it would be unwise to interpret these data 
confidently in terms of water quality, particularly since no aerosol 
atmsopheric corrections have been applied in this case.  However, the signal is 
strong in all channels of the CZCS, suggesting a broadband reflectance due to 
optically scattering material rather than the more subtle green/blue signature 
of an ordinary phytoplankton population.  This is confirmed by the analysis of 
channel ratios (step 6 of the lesson).  The cause could be a coccolithophore 
bloom, but could equally be suspended sediments.  Given the apparent origin of 
the reflective material near the coast of Iceland, the latter seems the most 
likely, with glacial meltwater sediments being washed into the sea from 
Iceland's rivers.

What the patterns of high reflectance tell us which might not readily be 
apparent from conventional local oeanographic measurements is that the plume of 
material is being carried offshore along the line of the Iceland-Faroes Front.

\bigskip
\centerline{\bf LESSON 4}

{\bf   2.}
  Channel 4 should give the best land-sea contrast because it measures light at 
670nm which is more readily absorbed by the sea than shorter wavelengths.  The 
subsurface water reflectance in Channel 4 is thus much less than in Channels 1 
to 3.  The Channel 4 reflectance over land is not necessarily different from 
that in other wavebands, so the land sea contrast in Channel 4 should be better 
than the other bands.

{\bf   4, 5 \& 8.}
  The colour plume appears to be drawn out along the thermal front.  Transects 
across the front should show the highest reflectance at the steepest part of 
the thermal front.  This is to be expected from dynamical considerations, since 
a strong geostrophic flow is to be expected along the front, and this flow must 
be advecting the optically reflective material which it has collected from the 
Iceland coastal area.  The colour imagery thus provides evidence of flow which 
can only be inferred dynamically from the thermal image.  In this respect the 
two types of data are complementary.  The colour is here acting as a tracer of 
water movement.  If it is considered to be a decaying tracer, then the 
reduction in concentration along the front indicates the direction of flow from 
west to east.  The colour tracer identifies very clearly that the flow jet 
associated with the front is meandering to follow the temperature contours.

There is strictly no correlation between the magnitude of the visible 
reflectance and the temperature of corresponding pixels.  A simple statistical 
analysis would reveal little.  Low reflectance is found in high and low 
temperatures.  High reflectance is found at all temperatures, although with a 
greater than average correlation at intermediate temperatures.  However, it is 
when the spatial patterns of the two types of image are compared that the 
interdependence of the interpretation of the two images becomes apparent.

\bigskip
\centerline{\bf LESSON 5}


{\bf   4 \& 6.}
  The night time images appear to be overall a few tenths of a degree C lower 
in temperature than the day time image.  This may be due to the absence of a 
diurnal thermocline at night (some warm patchiness characteristic of afternoon 
surface warming does appear in \file{AD2}).  It may be a skin effect cooling of 
the night images, or even the effect of cooler night atmospheres not being 
adequately handled by the atmospheric correction routine which has been applied 
to these images.

{\bf   8.}
  The warm water discharged by the rivers provides a non-conservative tracer of 
river plume assimilation into the wider North Sea.  There appear to be southward 
trending coastal flows down the north east English coast, but northward flow 
carrying the Rhine discharge along the Dutch coast.  The discharged water 
plumes appear to remain coherent and poorly dispersed, if temperature can be 
relied upon as a representative tracer.

%***********************************************


\bigskip
\centerline{\bf LESSON 6}

{\bf   2.}
 There is evidence from this sequence of images of the development of the 
seasonal stratification west of the Isle of Man, and the associated front 
between stratified and unstratified water (see instructions 6, 7 \& 8).

{\bf   4 \& 5.}
  The warm plumes which appear to start at the headlands and curve gradually 
round into the Atlantic Ocean are probably manifestations of a diurnal 
thermocline effect.  This is only found on image \file{AE2} indicating a 
shallow and ephemeral phenomenon.  The warm streaks appear to line up with the 
wind direction.  Presumably the headlands are creating a wind break behind 
which the wind is sufficiently weakened to permit the diurnal thermocline to 
form, whereas in other regions it is destroyed by wind mixing.





%I S Robinson                March 1989.


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