

%lesson2


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

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\lessons

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

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\centerline{{\bf 2 : THERMAL IMAGES OF OCEAN FRONTS }}

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%\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, demonstrating the oceanographic opportunities and
limitations of such data for the study of ocen fronts.

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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 image data in
order to reveal ocean frontal structures;

2) to identify eddies and instabilities on an ocean front;

3) to measure the sea-surface temperature gradient across ocean
fronts and other features; 

4) to use the \menu{transects} function to reveal the temperature
profile across a front and how it varies along the front;

5) to use the \menu{gradients} function of the image processing toolkit
to generate images denoting the location of frontal zones; and

6) to use the \menu{zoom} and \menu{smooth} functions to improve the
geometric properties of a distorted image.

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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.


{\bf The Images used for this lesson} 



Two versions of a single image dataset 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 NOAA-6 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 instruments 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 C 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\degrees C}   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 either 0.5\minutes\   or 1\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 pixels based on 1\minutes \  x 1\minutes\   represent a rectangle 1.85 km high
north-to-south and (at 60 degN) 0.93km wide east-to-west.

The image definitions are as follows:-

\def\burp{
height2pt&\omit&&\omit&&\omit&&\omit&&\omit&&\omit&&\omit&&\omit&\cr}
$$\vbox{\offinterlineskip
 \hrule
 \halign {&\vrule#&\strut \kern1.3mm \  \hfil#\hfil\kern1.3mm \  \cr
 height2pt&\omit&&\multispan3&&\multispan3&&\multispan5&\cr
 &Image&&\multispan3 \hfil Satellite \hfil&& 
 \multispan3 \hfil Overpass data \hfil &&
 \multispan5 \hfil Data Type \hfil&\cr
 &\omit&\multispan{14}\hrulefill&\cr
 &Filename&&Name&&Orbit&&Date &&Time&&Sensor&&Chan&&Product&\cr
\noalign{\hrule}
\burp
 & AB1&& NOAA-6&& 4388 &&  1-5-80 && 08.00 &&   AVHRR &&  4 &&  BT4&\cr
 &AC1&&\multispan{13} \quad\dotfill\quad as image AB1 \quad\dotfill\quad&\cr
\burp}
\hrule}$$

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


The geometric resampling and location coordinates of the images
are:-
$$\vbox{\offinterlineskip\hrule
\halign{&\vrule#&\strut\quad\hfil#\hfil\quad\cr
 height10pt&   &&\multispan3\hfil  Pixel dimension\hfil && 
   \multispan7\hfil  Image location coordinates\hfil&\cr
&\omit&&\multispan3&&\multispan7\hrulefill&\cr
depth4.5pt& Image&&\multispan3\hfil (minutes of arc)\hfil &&
 \multispan3\hfil Top left\hfil &&
 \multispan3\hfil Bottom right\hfil&\cr
&\omit&&\multispan{11}\hrulefill&\cr
& Filename && Long &&Lat && Long&&Lat&&  Long&&  Lat&\cr
\noalign{\hrule}
height10pt& AB1&& 1.0\minutes&& 1.0\minutes&&
 15\degrees 00\minutes W&&
  66\degrees 48\minutes N&&
     6\degrees 28\minutes W&&
62\degrees 32\minutes N&\cr
depth5pt& AC1&&        1.0\minutes &&      0.5\minutes &&
     15\degrees 00\minutes W&&
  65\degrees 00\minutes N&&
     6\degrees 28\minutes W&& 
62\degrees 52\minutes N&\cr}
\hrule}$$


These details can be checked in the $\langle$Filename$\rangle$.HDR file by using
the TYPE command in MS-DOS. 

Additional files supplied with the image data are a palette
 \file{TEMP1.PAL} appropriate to temperature representation, and two
look-up-tables (LUTs) \file{AB.STR}   and \file{AC1.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  east of Iceland ( see Fig. 1 ).  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.  Topographically, these seas are
separated by a series of ridges which run from Iceland to the
Faroe Islands and across to the UK Shelf by the Shetland Isles. 
These ridges act as a sill to restrict the cool bottom water of
the Norwegian Sea from flowing into the North Atlantic except
during episodic periods of \lq overflowing\rq .  The overflowing water
contributes to the densest layer of deep waters in the Atlantic. 
For this reason oceanographers have studied the area extensively
with conventional ship and buoy observations.  

The overflow occurs at depth and does not necessarily affect the
conditions at the surface, but there are some near surface
phenomena with obvious satellite IR signatures.  East of Iceland,
at about 64\degrees N, part of the Arctic Front is found, which forms a
boundary between the surface wind-driven Sub-arctic Gyre to the
south and the Arctic Ocean to the north. 

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

 1.   Load image AC1 and display.   This subimage was selected
from close to the western extremity of the AVHRR swath, where the
viewing angle is very oblique.  As well as introducing larger
atmospheric effects, this also reduces the spatial resolution. 
This accounts for the \lq pixellated\rq\ effects which are particularly
noticeable at the west of the image.  The image therefore provides
an example of poorer quality AVHRR data.

 2.   If the default palette is presently loaded, it may be helpful
to use the function \menu{Modify Palette} to provide a new palette
representative of temperature.  Alternatively the file \file{TEMP1.PAL} 
may be loaded using function \menu{Load palette} .  

This may not appear to improve the image if the default LUT, or
another inappropriate LUT is currently operating.  Before
proceeding with step 3 use the function \menu{Cursor} to find out the
temperature range across the image, and to locate the position of
Iceland on the image.  Note that Iceland is in the top left hand
corner of the image only and does not extend further east than
about 50 pixels.

 3.   Use function \menu{Modify LUT} to enhance the image into
a form in which it readily yields oceanographic information.

Note the twin peaks in the histogram.  What is the cause of these?
Using the information obtained in step 2 determine the
appropriate high and low limits of the stretch in order to enhance
the SST patterns at the expense of variability over the land.

If in doubt, use function \menu{Load LUT} to implement the LUT \file{AB.STR} 
which is provided.  Use this to understand the content of the
image, and then try again yourself to recreate a suitable LUT
using \menu{Modify LUT} .

 4.   Describe the temperature field depicted by the image, and
relate it to the known physical oceanography of the region.  How
large is the temperature difference between the water masses to
the North and South of the image?

On a sketch map, plot out the position of the Iceland-Faroes
Front, and the meanders and eddies created from it towards the
east of the image. 

Use the \menu{Cursor} function to estimate the length scale of
meanders and variability along the front.

 5.   Use the \menu{Cursor function} to plot the temperature along
north-south transects crossing the front, every 30\minutes\    of longitude. 
Measure the steepness (i.e. the maximum temperature gradient) of
the front, and describe how the steepness varies with longitude
along the front. 

Note that you can use the \menu{Transect} function to display the
temperature profile across the front and to check your plots
approximately, although it is not possible to obtain accurate
measurements using this function.  It is possible to display
several transects at once using the Bilko software, enabling
comparisons to be made of the profile variations along the front.
This is done by pressing \key{Return} instead of \key{Escape} after
displaying a transect.  Select a new site for the transect and
when it is displayed it will show in a new colour, the previous
transect remaining in its original colour.  It is most helpful to
use this operation by selecting a north-south transect and then
shifting  the transect a  fixed number of pixels east or west, building up a
clear picture of how the frontal profile changes from east to
west. 

 6.   Load image AB1 and display.  Note that this is the same
image of the front, but has pixels with dimensions 1\minutes\   latitude x
1\minutes\   longitude.  If necessary change the LUT to obtain a clear
presentation of the temperature structure.  Note that in this
distorted form, the eddies on the front sre severely flattened. 
This image has been included to demonstrate the problems of
interpretation which can be encountered if geometric distortion is
present

 7.    Use the \menu{zoom} option of the \menu{Modify Image} function to
remove the distortion introduced by the fact that square pixels on
the screen represent rectangles on the ground twice as high
(north-south) as they are wide (east-west).  

Which zoom factors should be used if the width of the image is to
be unaffected?  Note that if you select the correct ratio, the
Front, and the shape of Iceland, will be stretched out in the
North-South direction, and eddies will adopt a circular shape. 
The result should look very similar to image AC1.  Will the
North-South resolution be the same on each of the images?

 8.   Return to the original image AC1.  Use the Gradients
option of the \menu{Modify image} function to highlight the position of
the front and that of other regions of thermal gradients.  (Note
that a new LUT will need to be used for displaying the gradient
image.)  Consider the operational oceanographic value of a
gradient image.

 9.   To return to the original image it must be reloaded and the
appropriate LUT recreated or reloaded.  Enter the Smooth Option of
the \menu{Modify image} function.  Does this improve the look of the
image by removing the pixellations?  Does it improve the quality
of the data or is it simply a cosmetic exercise?

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\hrule

\byline{I S Robinson, November 1987.} 

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