
%lesson 6 

%\centerline{
%\underbar{ Practical Lessons in Satellite Oceanography }}
%\vskip 0.5cm
\lessons

\centerline{\bf  OCEANOGRAPHIC INTERPRETATION OF IMAGES }
\vskip 0.5cm


\centerline{\bf  6 : SHELF SEAS THERMAL STRUCTURE }
\centerline{\bf 2 - EARLY SUMMER STRATIFICATION IN THE IRISH SEA}

%\vskip 1cm

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

To provide experience in the analysis and interpretation of
sea-surface temperature images of a shelf sea during the onset of
thermal stratification in late spring / early summer.

%\vskip 1cm

 %\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 compare and contrast the thermal structure of several images
of the same area spaced a few hours or days apart;

3) to distinguish between diurnal sea surface temperature changes
and those which are more permanent;

4) to detect the onset of patchy thermal stratification in a shelf
sea by identifying the frontal boundary separating stratified from
well-mixed water;

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

%\vskip 1cm

%\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.

 {\bf The images used for this lesson }

Four images are provided for this lesson.  They are sea-surface
temperature (SST) images of the Irish Sea and West Atlantic off
the coast of Ireland, 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 scenes span a period of four
days.  There are a day and a night image for the first day, a
daytime  image two days later and a night image two days after
that.

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 deg C.  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  {\bf 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}
$$\vbox{\offinterlineskip
 \hrule
 \halign {&\vrule#&\strut \kern0.4mm \  \hfil#\hfil\kern0.4mm \  \cr
 height2pt&\omit&&\multispan3&&\multispan3&&\multispan5&\cr
 &Image&&\multispan3 \hfil Satellite \hfil&& 
 \multispan3 \hfil Overpass data \hfil &&
 \multispan5 \hfil Data Type \hfil&\cr
 &\omit&\multispan{14}\hrulefill&\cr
 &Filename&&Name&&Orbit&&Date &&Time&&Sensor&&Chan&&Product&\cr
\noalign{\hrule}
\burp
height10pt&  AE1&&  NOAA-7&&   14622&&    24-4-84&&  03.50&&   AVHRR/2&&  4\&5&&   SST(2)&\cr
 & AE2&&      NOAA-7&&   14629&&    24-4-84&&  15.25&&   AVHRR/2&&  4\&5&&   SST(2)&\cr
  & AE3&&      NOAA-7&&   14657&&    26-4-84&&  15.00&&   AVHRR/2&&  4\&5&&   SST(2)&\cr
 depth5pt & AE4&&      NOAA-7&&   14679&&    28-4-84&&  04.42&&   AVHRR/2&&  4\&5&&   SST(2)&\cr}
\hrule}$$


[ 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:-

$$\vbox{\offinterlineskip\hrule
\halign{&\vrule#&\strut\quad\hfil#\hfil\quad\cr
%first line
height10pt&  &&\multispan3\hfil    Pixel dimension \hfil &&
\multispan3\hfil \   Approximate image location \ \hfil&\cr
%second line
&\omit&&\multispan3&&\multispan3\hrulefill&\cr
%third line
depth4.5pt&  Image &&\multispan3\hfil (arc minutes)\hfil&&         left margin&&    top margin&\cr
%fourth line
&\omit&&\multispan3\hrulefill&&\omit&&\omit&\cr
%fifth line
&Filename&&Long&&    Lat&&    longitude&&      latitude&\cr
\noalign{\hrule}
%data here
height10pt&  AE1&&  1.75\minutes\ &&   1.0\minutes\ &&             13\degrees 50\minutes W&&
     55\degrees 50\minutes N&\cr
&  AE2&&   1.75\minutes\ &&  1.0\minutes\ && 
  15\degrees 00\minutes W&&
      55\degrees 15\minutes N&\cr
&  AE3&&        1.75\minutes\   && 1.0\minutes\ &&            14\degrees 10\minutes W&&
      55\degrees 00\minutes N&\cr
depth5pt&  AE4&&   1.75\minutes\ &&  1.0\minutes\ &&            13\degrees 00\minutes W&&
      55\degrees 20\minutes N&\cr
}\hrule}$$


 Factors affecting the thermal structure of the Irish Sea 

The Irish Sea is a shallow sea (typically less than 100m deep)
semi-enclosed by Great Britain and Ireland, but connected to the
rest of the N.W. European continental shelf by straits at the
north and the south.  Tidal ranges are up to 8m , and the tidal
streams are vigorous, except between Ireland and the Isle of Man. 
In this region stratification develops in summer but elsewhere the
sea is vertically well mixed.  Summer stratification also develops
in the Celtic Sea, south of Ireland.  The onset of stratification
is to be expected in late April and early May.  Once established
it is a stable phenomenon. but initially its formation will be
promoted by a spell of calm sunny weather.  

There is considerable coastal influence around the margins of the
Irish sea , particularly in the north east where estuaries such as
the Dee, Mersey and Solway discharge into the Irish Sea.  Also
shown on most of the images is the region to the west of Ireland. 
This is a typical continental shelf at the margin of the Atlantic
Ocean, and its physical oceanography should be controlled by local
coastal inputs and by those oceanic processes which are able to
penetrate across the continental slope region and onto the shelf. 

%\vskip 1cm
%\centerline{\bf  OUTLINE of LESSON }
\outline


 1.  Load image \file{AE1} and familiarise yourelf with the location. 
Use a suitable stretch to determine which are areas of land, sea
and cloud.  Following the procedure learned in lesson 5, establish
a LUT which can provide a colour contour map of surface
temperature.  Select appropriate range limits to span the
temperature range encountered within the sea on this image.  

 2.   Describe the overall temperature structure in the Irish Sea
apparent in this image.  Note on a sketch map the spatial shape
and size, and thermal magnitude of any prominent features. 
Suggest physical explanations for these (e.g dispersal of coastal
discharge, inflow of Atlantic or Celtic Sea water).  Is there any
evidence of thermal stratification west of the Isle of Man which
would lead to higher surface temperatures separated by a front
from the surrounding cooler well mixed water?  

 3.   Load image \file{AE2}.   Familiarise yourself with this image.  Why
is the LUT used for \file{AE1} ineffective for this image?  

[ NOTE: This is a daytime image recorded less than 12 hours after
image \file{AE1} and it is unlikely that the thermal structure of the
water column would have changed significantly in this time.  Any
differences between the images are probably due to either
inadequacies of the atmospheric correction procedure to cope with
the changes of air temperature which occur between day and night,
or the formation of a temporary warm surface layer during the day
if there is solar heating without very much wind mixing (the
diurnal thermocline).  The former effect is expected to occur over
large areas, whilst the latter may occur in relatively small
patches. ]

What is the typical temperature difference between the land on
this and the previous image?  What is the range of sea
temperatures encountered in this image?  Create a LUT to span this
range.  Compare with the sea temperature range in image \file{AE1}.

 4.   Identify any new thermal features which have appeared on
this image compared with \file{AE1}.  Sketch their structure and measure
the magnitude of their thermal signature.  Note particularly the
plumes off the headlands on the Irish Atlantic coast.

 5.   Records indicate that there was a light wind (force 2 to 4)
blowing towards the north west.  Does this information help you to
suggest a hypothesis which accounts for the \lq hot spots\rq\ associated
with the coastal headlands.

 6.   Load image \file{AE3}.  Establish a suitable LUT to display the
thermal structure.  Note the overall sea temperature range in this
daytime image compared with the previous images.  Note a frontal
region which appears to have formed along (295,50) to (330,50),
and off the Irish coast at (285,90).  [(X,Y) are the pixel
co-ordinates defined in the cursor function.]  Sketch the exact
location of these fronts, and the magnitude of their thermal
signatures.

 7.   Load image \file{AE4}.  Note the overall temperature changes from
the previous images.  Establish a suitable LUT to display the
thermal structure.  Has the apparent frontal region noted in \file{AE3}
persisted or disappeared?  

If the latter then it was probably a diurnal \lq hot spot\rq\ on the
daytime image \file{AE3}.  If the former, then its persistence on a
nightime image implies that it is a more robust feature, probably
the onset of seasonal stratification and a tidal mixing front
which is expected in this region.

  8.   In the previous instruction you should have noted that the
front has shifted to lie approximately between (245,125) and
(295,75).  Sketch the location of the front, including any
meanders, and note the magnitude of the thermal signature.  How has
the stratified region (the warm side of the front) grown spatially
and in thermal magnitude compared with image \file{AE3}?

 9.   So far attention has been concentrated on the expected
growth of the stratified region west of the Isle of Man, and on
the diurnal effects evident on \file{AE2}.  Now review all four images,
identifying other features, noting which persist between day and
night and over the four day period, and if any movement, growth or
contraction is observed.  These are the features which are most
significant for understanding the physical oceanography of the
region.  Suggest oceanographic explanations of these features.
\vskip 1cm
\hrule

\byline{I S Robinson,  April 1988.}
\vfil
\eject

