%lesson 1 



% 17/11/88 tex document version of UNESC.BBC
% unesco oceanography.......second section
%lesson 1
% revised version 21/2/88

%\centerline{
%\underbar{Practical Lessons in Satellite Oceanography}}
 
%\vskip .5cm
\lessons


\centerline{{\bf  OCEANOGRAPHIC INTERPRETATION OF IMAGES}}

\vskip .5cm

 

\centerline{{ \bf 1 : IMAGES OF OPEN OCEAN SEA-SURFACE TEMPERATURE} }

%\vskip 1cm


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

\aims

   

To provide experience in the interpretation of sea-surface
temperature images based on infra-red data from the NOAA AVHRR
sensor, and to demonstrate the oceanographic opportunities and
limitations of such data for the study of the thermal  structure of the open
ocean.



%\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 image data in
order to reveal the spatial patterns of sea-surface temperature
structure;

2) to identify various mesoscale dynamical features (e.g. ocean
eddies, fronts and frontal instabilities), and to relate the
satellite view of them to conventional observations;

3) to identify the temperature at a particular point specified by
latitude and longitude; 

4) to plot a transect of sea-surface temperature along a given
track;

5) to measure the length scale of dynamical features (e.g.
diameter of eddies, wavelength of instabilities);

%\vskip 1cm
%\centerline{ \underbar{\bf   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 }


Three image datasets are provided for this lesson.  They are based
on infra-red data received at The University of Dundee's Satellite
Receiving Station from an overpass of the AVHRR instrument on the
TIROS-N polar-orbitting meteorological satellite.  These data have
been processed using the Southampton University  MIDOCEAN  
Software to convert the raw data into resampled, geolocated
temperature images.   
 [ MIDOCEAN = Medium-resolution Image Data - Oceanographic
Calibration, Enhancement And Navigation]

Because there is no split window for the 10-12  $\mu$m waveband on
the AVHRR instrument used, the temperature product is the
brightness temperature of channel 4, uncorrected for atmospheric
effects.  This is typically a few degrees C lower than the true
sea surface temperature, but it represents the temperature
gradients accurately, with a sensitivity of 0.2\degc . Temperatures
below 0\degc\   have been assumed to be cloud and set to zero.   {\bf Digital
Values 0--255 in the processed image correspond to temperatures of
0 -- 25.5\degc}.   It should be noted that the apparent temperature
resolution of 0.1\degc\  is used for convenience, and is not
justified by the instrument sensitivity of about 0.2\degc .

The geolocation is expected to be accurate to within 3 km.  The
images have been resampled onto a Latitude/Longitude grid with
cell size 0.5\minutes\  in latitude and 1\minutes\  in longitude.

It should be recalled that at latitude 60\degrees N a pixel which is
0.5\minutes\  high in latitude and 1.0\minutes\  longitude wide represents a square
of side 0.93km.  At other latitudes, such a pixel represents a
rectangle on the ground whose width varies as cos(Latitude),
increasing southwards,  but whose height remains constant
throughout the image.  This is a higher resolution than is
justified by the sensor which has a nadir resolution of 1.1km. 

The image definitions are as follows:-

\def\burp{
height2pt&\omit&&\omit&&\omit&&\omit&&\omit&&\omit&&\omit&&\omit&\cr}
$$\vbox{\offinterlineskip
 \hrule
 \halign {&\vrule#&\strut \kern1.3mm \  \hfil#\hfil\kern1.3mm \  \cr
 height2pt&\omit&&\multispan3&&\multispan3&&\multispan5&\cr
 &Image&&\multispan3 \hfil Satellite \hfil&& 
 \multispan3 \hfil Overpass data \hfil &&
 \multispan5 \hfil Data Type \hfil&\cr
 &\omit&\multispan{14}\hrulefill&\cr
 &Filename&&Name&&Orbit&&Date &&Time&&Sensor&&Chan&&Product&\cr
\noalign{\hrule}
\burp
&AA1&&   TIROS-N&&  8155&&  13-5-80&&  14.22&&  AVHRR&& 4&& BT4&\cr 
&AA2&&   TIROS-N&&  8155&&  13-5-80&&  14.22&&  AVHRR&& 4&& BT4&\cr
&AA3&&   TIROS-N&&  8155&&  13-5-80&&  14.22&&  AVHRR&& 4&& BT4&\cr
\burp}
\hrule}$$


[ Product BT4 from the MIDOCEAN software is the brightness
temperature recorded by channel 4.]


The geometric resampling and location coordinates of the images
are:-


$$\vbox{\offinterlineskip\hrule
\halign{&\vrule#&\strut\quad\hfil#\hfil\quad\cr
 height10pt&   &&\multispan3\hfil  Pixel dimension\hfil && 
   \multispan7\hfil  Image location coordinates\hfil&\cr
&\omit&&\multispan3&&\multispan7\hrulefill&\cr
depth4.5pt& Image&&\multispan3\hfil (minutes of arc)\hfil &&
 \multispan3\hfil Top left\hfil &&
 \multispan3\hfil Bottom right\hfil&\cr
&\omit&&\multispan{11}\hrulefill&\cr
& Filename && Long &&Lat && Long&&Lat&&  Long&&  Lat&\cr
\noalign{\hrule}
height10pt&AA1 &&  1.0\minutes &&  0.5\minutes && 
    4\degrees 45\minutes W&&
   61\degrees 40\minutes N&& 
   3\degrees 46\minutes E&&
   59\degrees 33\minutes N&\cr
&AA2 &&  1.0\minutes && 0.5\minutes&&
  4\degrees 45\minutes W&&
  63\degrees 48\minutes N&&
   3\degrees 46\minutes E&& 
  61\degrees 41\minutes N&\cr 
depth5pt&AA3 && 1.0\minutes &&     0.5\minutes&& 
  4\degrees 27\minutes W&&
  65\degrees 56\minutes N&&
     4\degrees 04\minutes E&&
  63\degrees 49\minutes N&\cr
}\hrule}$$





These details can be checked in the $\langle$Filename$\rangle$.HDR file by using
the TYPE command in MS-DOS.  The approximate location of the
images, and their relation to one another, is shown in figure 1.

Additional files supplied with the image data are a palette
 \file{TEMP1.PAL} appropriate to temperature representation, and a
look-up-table (LUT) \file{AA.STR} .


{\bf  The Thermal Structure of the North-East Atlantic Ocean}

The images have been selected to cover the North East Atlantic
Ocean, in the region between Norway and Scotland.  This is an
important area in regard to the water masses and circulation of
the North Atlantic since it is here that water is exchanged
between the Norwegian Sea in the north and the main part of the
North Atlantic to the South West.  It is also at the northern
boundary of the North Sea, where the shallow shelf sea, typically
200m deep, meets the ocean at the shelf break, which runs
approximately south-west to north east.  The shelf region is
intersected by a deep canyon off the Norwegian coast.  The
Norwegian Coastal Current flows above this, from the Baltic,
northwards along the coast of Norway.

The waters at these high latitudes are not strongly stratified. 
In May only a weak thermocline will have developed, and therefore
sea surface temperature (SST) images should reveal something of
the dynamical structure of the upper ocean.  However, in daytime
images there is always the possibility that a diurnal thermocline
may be present which may mask the structure of SST as it would be
measured by instruments a few metres below the surface.

%\vskip 1cm

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


The lesson is likely to take at least an hour.

 1.   Enter the Bilko software, load image AA1 and display.

 2.   You may wish to use function \menu{Modify palette} to produce a
colour scale to your own taste which seems more representative of
temperature than that provided as default on entering Bilko. 
Alternatively use the function \menu{Load palette} to enter a palette
provided in file \file{TEMP1.PAL} .  This contains a palette ranging
from white at the low (cold) end through blue, green, yellow and
red to brown at the high (warm) end.  Note however that the very
lowest value is black.


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

%put figure 1 in here...leave a blank page


\null
\vfil
\centerline{Figure 1.}
\eject


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

  

 3.   Use function \menu{Modify LUT} to improve the image until it
reveals patterns of SST structure.   Note when using \menu{Modify LUT} 
that the linear stretch is most useful.  (N.B. Answer \type{No} when
prompted to choose whether or not to overlay the stretch)

There is a large range of temperatures above about 10\degc\  which
are rarely found on these images.  These can all be set to the
maximum of the Look-Up-Table.  Take care at the low temperature
end that you do not let the stretch limits encroach on the
sea-temperature range, and only relegate cloudy areas to zero on
the display.  

Identify on the image the areas which have higher or lower
temperatures than the main peak of the histogram, and account for
their different temperatures.  What is the land that can be seen
in the image? Which parts of the image are cloudy?

If your stretch does not achieve the desired enhancement of SST
variability, make sure you are enhancing the correct part of the
histogram.  It is a simple matter to re-enter the \menu{Modify LUT} 
function and try again, because the pixel values in the computer's
image store are not changed by the procedure.



 4.   If in doubt about the results of your attempts to modify the
LUT, you can use the function \menu{Load LUT} and request a file named
 \file{AA} , which has been prepared for use with images AA1, AA2, AA3. 
However, it may not reveal all the spatial features as clearly as
it might, and experimentation is recommended.

 5.   Using the enhanced display, give a general description of the
overall distribution and trend of temperature across the image,
ignoring the westernmost quarter which is cloud contaminated.  

What evidence is there of the Norwegian Current which flows
northeastwards from the Atlantic, between the Faroe Islands and
Scotland into the Norwegian Sea?

Can you identify the Norwegian Coastal current which has its
origins in the Baltic and which will be cooler than North Sea
water at this time of the year?

 6.   Identify any mesoscale features present on the image (i.e.
dynamical features with length scales between 10 and 100km.

 7.   Enter function \menu{Cursor} and familiarise yourself with its
operation.  Use the (x,y) pixel count, and the geolocation
information provided with the image, to confirm the accuracy of
the location of the Shetland Isles.

What is the length scale of the meanders on the Norwegian coastal
current? What is the temperature change across the Norwegian
Coastal Current (e.g. between (450,85) and (470,70) ?

What is the spatial length scale and the magnitude of the
temperature signature (i.e. the size of temperature variation) of
the eddy-like features centred at (344,128) and also at (164,48) ?

 8.   Load image AA2.  The palette need not be changed, and since it
is a northward continuation of image AA1 the use of the same LUT
will help to reveal the extension of the thermal patterns from the
top of the previous image.  However, it may be possible to improve
the SST pattern enhancement with a different LUT.

 9.   Repeat steps 5 and 6 for this image.

 10.   Load image AA3 and repeat steps 8 and 9.  Draw a sketch map
of the thermal structure of the whole region covered by image AA1,
AA2 and AA3.

 11.   Using images AA1, AA2 and AA3, obtain the value of SST at the
following locations:-
$$\vbox{ \halign{&\ \hfil#\hfil\ \cr
&R&[&0\degrees 00\minutes E;&60\degrees 00\minutes N&],\cr
&S&[&2\degrees 30\minutes E;&64\degrees 30\minutes N&],\cr
&T&[&1\degrees 20\minutes W;&62\degrees 50\minutes N&].\cr
}
}$$


Why would you expect these values to be lower than the temperature
recorded at the same time as the overpass by  in situ  measurements?

 12. [Optional]   A research vessel steams due north along the
Greenwich Meridian from 60\degrees N to 65 \degrees N.  Plot out the satellite
temperature along this transect at 5\minutes\   intervals and comment on
whether this is sufficiently high spatial resolution for
comparison with in situ measurements.  You could use the
 \menu{Transect} function to obtain a quick overview of parts of the
transect within a single image, but this does not provide enough
temperature resolution for accurate plotting.

\vskip 1cm
\hrule 
\vskip 1cm

\centerline{{\bf Bibliography }}

Physical Oceanography of the North-east Atlantic:-

Chapter 5 of  {\bf Tchernia P, 1980.}
\underbar{Descriptive Regional Oceanography} , Pergamon Press, 253pp.
 
Chapter 7 of  {\bf Pickard G L  and Emery W J, 1982.}
\underbar{Descriptive Physical}\hfil\break  
\underbar{Oceanography} ,
 4th Edition, Pergamon Press, 249pp. 

SST measurement using satellite infra-red readiometry:-

{\bf  Maull G A, 1985.}   \underbar{ Introduction to Satellite Oceanography}
 , Martinus Nijhof Publishers, 606pp.
 
Chapter 7 of  {\bf Robinson I S , 1985.}  \underbar{  Satellite Oceanography}
 ,  Ellis
Horwood Publishers, 455pp.

\vskip 1cm
\hrule

\byline{I S Robinson, November 1987.} 


\vfill
\eject


