
%lesson 5



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%\centerline{
%\underbar{ Practical Lessons in Satellite Oceanography} }

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\lessons

\centerline{ \bf  OCEANOGRAPHIC INTERPRETATION OF IMAGES} 

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\centerline{\bf  5 : SHELF SEAS THERMAL STRUCTURE }
\centerline{\bf 1 - NORTH SEA, EARLY SUMMER} 

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%\centerline{\bf \underbar{Aim of Lesson}}
\aims

To provide experience in the analysis and interpretation of
sea-surface temperature images of a typical shallow sea when
coastal discharges have a contrasting temperature.


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%\centerline{ \bf\underbar{Specific Objectives}}
\objectives

After completing this unit, students should have learned:-

1) to use the image processing toolkit to enhance thermal images
of shelf seas in order to clarify the sea-surface temperature
structure;

2) to generate  a LUT to provide a temperature contour map which
can be used for comparing the temperatures on different images;

3) to detect the effect of river and estuarine discharge on the
temperature of a shallow sea;

4) to suggest hypotheses about shelf sea dispersion processes from
analysis of temperature images.

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%\centerline{\bf   \underbar{Background Information} }  
\background

{\bf  The Bilko image-processing software }


No prior knowledge of the BILKO system is required.  The online
 HELP  facility, accessed by typing  \type{H}, explains the operation of
the current command and  \key{escape}   returns to the main menu. This
lesson will illustrate the application of some of the facilities
provided by the toolkit, and guidance will be given in using them
to best effect.

 {\bf Other equipment} 

In order to work through this exercise, you will probably find it
helpful to have a calculator, graph paper for plots, and coloured
pencils if you want to sketch some of the features observed.  To
relate the images to the underlying oceanographic processes, it
will be useful to refer to a hydrographic chart covering the
region under study.

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 {\bf The images used for this lesson }

Three images are provided for this lesson.  They are sea-surface
temperature (SST) images of the North Sea derived using the \lq split
window\rq\ algorithm from channels 4 and 5 of the AVHRR/2 sensor on
the NOAA-7 satellite.  The data have been recieved at Dundee
university satellite receiving station and processed using
Southampton University's MIDOCEAN Software.  

The split window algorithm is one derived at Rutherford Appleton
Laboratory, based on atmospheric conditions appropriate for the
North Atlantic.  It is normally possible to obtain an absolute
accuracy of sea-surface temperature calibration better than 1
\degc .  However, because conditions over the North Sea may
sometimes be those of a continental airmass rather than an oceanic
atmosphere, the algorithm can on occasions be in error by much
more than 1\degc .  Relative accuracy (i.e. the ability to detect
temperature gradients on an image) should be about 0.2\degc .  The
derived SST values have been coded in 8-bit form with 0.1\degc\ 
resolution, so that  the number range 0 to 255 corresponds to 0 to
25.5\degc . 

These images have been approximately geolocated, and geometrically
rectified, with a pixel size of 1\minutes\ latitude by 1.75\minutes\ longitude,
which approximately represents square ground cells.  However, in
an enclosed sea surrounded by a coastline with characteristic
landmarks it is easy to locate the same relative positions on
different images, and no attempt has been made to exactly overlay
the different images.

The image definitions are as follows:-

\def\burp{
height2pt&\omit&&\omit&&\omit&&\omit&&\omit&&\omit&&\omit&&\omit&\cr}
$$\vbox{\offinterlineskip
 \hrule
 \halign {&\vrule#&\strut \kern1mm \  \hfil#\hfil\kern1mm \  \cr
 height2pt&\omit&&\multispan3&&\multispan3&&\multispan5&\cr
 &Image&&\multispan3 \hfil Satellite \hfil&& 
 \multispan3 \hfil Overpass data \hfil &&
 \multispan5 \hfil Data Type \hfil&\cr
 &\omit&\multispan{14}\hrulefill&\cr
 &Filename&&Name&&Orbit&&Date &&Time&&Sensor&&Chan&&Product&\cr
\noalign{\hrule}
\burp
height10pt&  AD1&&      NOAA-7&&   14622&&    24-4-84&&  03.50&&   AVHRR/2&&  4\&5&&   SST(2)&\cr
&  AD2&&      NOAA-7&&   14629&&    24-4-84&&  15.25&&   AVHRR/2&&  4\&5&&   SST(2)&\cr
depth 5pt&  AD3&&      NOAA-7&&   14679&&    28-4-84&&  04.42&&   AVHRR/2&&  4\&5&&   SST(2)&\cr
}\hrule}$$


[ Product SST(2) from the MIDOCEAN software is the sea surface
temperature atmospherically corrected using the RAL split window
algorithm based on channel 4 and 5 brightness temperatures.]

The geometric resampling details of the images are:-

$$\vbox{\offinterlineskip\hrule
\halign{&\vrule#&\strut\quad\hfil#\hfil\quad\cr
%first line
height10pt&  &&\multispan3\hfil    Pixel dimension \hfil &&
\multispan3\hfil \   Approximate image location \ \hfil&\cr
%second line
&\omit&&\multispan3&&\multispan3\hrulefill&\cr
%third line
depth4.5pt&  Image &&\multispan3\hfil (arc minutes)\hfil&&         left margin&&    top margin&\cr
%fourth line
&\omit&&\multispan3\hrulefill&&\omit&&\omit&\cr
%fifth line
&Filename&&Long&&    Lat&&    longitude&&      latitude&\cr
\noalign{\hrule}
%data here
height10pt&  AD1&&  1.75\minutes\  &&  1.0\minutes\  &&          2\degrees 45\minutes W&&
   55\degrees 15\minutes N&\cr
&  AD2&&  1.75\minutes\ &&  1.0\minutes\  &&
  2\degrees 45\minutes W&&
   55\degrees 30\minutes N&\cr
depth5pt&  AD3&& 1.75\minutes\  && 1.0\minutes\  &&          3\degrees 40\minutes W&&
  54\degrees 50\minutes N&\cr}
\hrule}$$

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{\bf  The Thermal structure of the North Sea }


The North Sea is a shallow sea less than 200m deep, bounded by the
United Kingdom to the west and the continent of Europe to the
 south and east.  The dynamical processes of the sea are dominated by the
tides, and in the southern region under study the tidal streams
are generally sufficiently vigorous to ensure that the sea is
vertically well-mixed throughout the year.  Thus the sea-surface
temperature is characteristic of the whole water column, and can
act as a tracer of the different water masses.  This is
particuarly the case when rivers and estuaries are discharging
water which has a different temperature to the main sea.  In late
April, early May, solar heatng is sufficient to ensure that
nearshore water and estuarine discharge is warmer than the sea,
and thus the plumes of discharged water are readily apparent. 

Some stratification does occur in summer in the Northern part of
the Sea, and when properly established the boundary between
stratified and unstratified water is located off the Yorkshire
coast - the Flamborough front.  (Flamborough Head is the prominent
headland just north of the Humber estuary).  On sunny days when the
winds are light, solar heating may produce a diurnal thermocline
which can raise the sea surface temperature within the top metre
by up to one or two degrees C.

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%\centerline{{\bf  OUTLINE of LESSON }}
\outline


 1.  Display image AD1.  Familiarise yorself with this image, select
a suitable look up table (LUT) to distinguish between land, sea
and cloud.  Note that on this nightime image the land is cooler
than the sea.  Identify coastal landmarks on a map or chart.
Note that there is cloud and partial cloud contamination in the
north east of the image.

 2.  Set up a LUT using the linear stretch which will provide a
calibrated contour map of sea surface temperature.  By setting the
extremities of the stretch at 52 and 94, the 14 intervening levels
represent a range of 4.2\degc , i.e. each colour spans a range of
0.3\degc .  Set this stretch up and check that your contouring is
correct by using the Cursor function.

 3.  Sketch the thermal structure, and make a note of the spatial
extent and the size of the thermal signature (i.e the temperature
difference compared with its surroundings) of any prominent
features.  For example you might detect the following:- 


\begingroup
\parindent=26pt
\parskip=0pt

\item{a)} A warm patch off the North Yorkshire coast, possibly the
onset of stratification,
 \item{b)} The plume of water discharging from the River Thames.
 \item{c)} The fringe of warm coastal water at many locations,
particularly along the Belgian coast,
 \item{d)} The discharge from the River Rhine,
\item{e)} The water in the inland seas of Holland and between the
Dutch coast and the offshore islands (The Wadden Sea).
\item{f)} The water from the Wash and the Humber Estuary.
\endgroup

 4.  Load image AD2 and display using the same LUT produced in step
2.  Comment on the overall temperature level compared with image
AD1.  Bearing in mind that these are from the same satellite
within 12 hours of each other, suggest possible reasons other than
random error why AD2 is generally warmer than AD1.  Note that
whilst sound reasons may be suggested to account for the
temperature discrepancy, there is no objective way of validating
these without in situ measurements.

 5.  Select a more appropriate LUT to highlight the thermal
structure of AD2.  Which features persist from the earlier image? 
Compare the spatial extent and thermal magnitude of the features
on the two images.  Note that you should compare the temperatures
of features relative to their surroundings, rather than their
absolute temperatures which depend on the overall temperature
levels considered in 4.

 6.  Load image AD3.  Note that this extends slightly further
south than the others and reveals part of the English Channel. 
There is a large region of cloud in the central northern part of
the image.  Using a suitable LUT compare the overall temperature
of this image with the previous two.  To which does it conform more
closely?  Why might this be?

 7.  Compare the features on this image with those recorded in
steps 3 and 5.  Record any changes and any similarities, and try
to account for them in terms of shallow sea physical processes.
You may need to return to the earlier images to aid the
comparison.  

Note that by comparing several images over a few days it is more
apparent which features on an image are due to cloud contamination
and therefore move significantly between images, which are diurnal
effects and therefore disappear from nightime images, and which
are genuine aspects of the sea's thermal structure, varying only
slowly.  Note any features which could be misinterpreted if only a
single image were consulted instead of using information from all
three.

 8.  Using the thermal structure of the North Sea detected from
the images, suggest possible residual flow patterns occuring in
the sea at the time, and consider what insights may be gained from
the images about the dispersal of material in the region.

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\byline{I S Robinson,   April 1988.}

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