A brief explanation of the ATSR cloud screening process
1. Introduction
This is a brief explanation of the basic methods used to
screen the ATSR data for cloudy pixels and identify clear sea pixels: for a
complete description of the ATSR-1/2 clearing process the user is referred to
Závody et al. (2000)
Also as the ATSR Cloud Clearing Scheme is based on the
APOLLO algorithms originally developed by Saunders (1986)
and Saunders and Kriebel (1989)
the user may find these papers useful background to the whole process.
It should be noted that the scheme has been optimised for
use over the ocean and as only certain of the tests are/or can be used over
land, some improvements in the scheme are necessary before the scheme will work
as well as it does over the ocean. The ATSR SST processing software implements
the following sequence of cloud tests to identify the clear pixels in any given
ATSR image.
2. The Cloud Tests
2.1 The Gross Cloud Test
This test is the simplest of the cloud tests and rejects
pixels that are obviously cloudy because they have lower than expected
brightness temperatures (BT). It uses that basic assumption that “clouds are
cold”. The 12µm channel is used in this test in preference to the 11µm channel
because of the larger cloud optical depth at 12µm.
The thresholds for the “expected” BTs are derived for the
case over the ocean from the UKMO Global Ocean and Surface Temperature Atlas
(GOSTA) data set with an allowance for likely atmospheric deficits. The
thresholds for required for ATSR-2 are different to those for ATSR-1 because of
the increased water vapour absorption in the slightly modified ATSR-2 12µm
channel.
Závody, A.M,
C.T. Mutlow and D.T.Llewellyn-Jones, Cloud Clearing over the Ocean in the
Processing of Data from the Along-Track Scanning Radiometer (ATSR), Journal of
Atmospheric and Oceanic Technology, 17, 595-615, 2000.
Saunders,
R.W., An automated scheme for the removal of cloud contamination from AVHRR
radiances over western Europe, Int. J. Remote Sens., 7, 867-888, 1986.
Saunders,
R.W, and K.T. Kriebel, An improved method for detecting clear sky radiances
from AVHRR data, Int. J. Remote Sens., 9, 123-150, 1988; Correction 9,
1393-1394, 1989.
2.2 The Thin Cirrus Test
Large differences between the 11 and 12 µm BTs are
indicators of the presence of medium- and high-level clouds, and especially
thin cirrus. The thresholds on this BT difference test are determined using
radiative transfer calculations based on radiosonde data. The scheme includes 5
different across track threshold bands to allow for the across swath air-mass increase,
and, in addition, separate thresholds are used of the forward and nadir view
cases. The test is formulated with the threshold as a function of the 11µm BT,
and a typical set of thresholds for pixels within ±50km of the satellite track
is shown below.
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Figure 1 Threshold for the 11-12µm test as a function
of the 11µm BT. Curve a) is for the nadir view within ±50km of the
sub-satellite track, and curve b) the same for the corresponding forward view |
2.3 The Fog/Low Stratus Test
This test relies on the fact that stratus and fog have a
higher emissivity at 11µm than at 3.7µm, and hence the 11-3.7µm BT difference
can be used for detecting these types of cloud. This is a well-established test
and is already used by many other researchers, including Saunders and Kriebel.
In our scheme the nadir view threshold is set to 0.9K and the forward view
threshold to 1.1K. This test is one of the key ATSR cloud tests particularly
over land where spatial coherence tests cannot be used because of surface
structure.
2.4 The Medium/High level Test
This test is another of those that has been modified from
those developed by Saunders and Kriebel, and uses the 3.7-12µm BT difference as
a means of identifying semi-transparent and most medium and high level clouds.
In the APOLLO scheme pixels were flagged as cloudy when this difference
exceeded 1.5K. However, this simple single value approach was found too
simplistic for global use so to improve on this ATSR processing uses a variable
threshold scheme based on the 3.7-12µm BT differences tabulated as a function
of the 12µm BT. An example of these computed threshold values is shown in the
figure below.
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Figure 2 Threshold for the 3.7-12µm test as a function
of the 12µm BT for ATSR-1. Curve a) is for the nadir view within ±50km of the
sub-satellite track, and curve b) the same for the corresponding forward view
case. |
The threshold functions are different for ATSR-1 and 2
because of the differences between each of the two instrument’s 12µm channel
spectral responses; the ATSR-2 filter is shifted to longer wavelengths.
2.5 The 11µm spatial coherence cloud test
This test investigates the spatial coherence of the 11µm
BTs, and operates in two parts: 1) spatial scale of a 3km square sub-image, and
2) on the much larger scale of 128km square sub-image.
These two parts of the test are described in the following
sub-sections.
2.5.1 Small-scale spatial coherence
This test relies on that fact that, apart from in active
coastal or frontal regions, the sea surface temperature is relatively unifor
over areas of a few kilometres square and temperature gradients rarely exceed
0.3Kkm-1. The same is not true of clouds, however, and cloud top
temperatures can vary by several, or even tens of, Kelvin in a few kilometres
mainly due to variations in cloud top height.
This difference between the “texture” or “spatial-coherence”
of the ocean and clouds forms the basis of this very powerful cloud test that
can be used both in day- and night-time. The threshold value used for groups of
9 (i.e. 3 x 3 sub-sections) ATSR-1 and 2 pixels is 0.2K for the ATSR-1/2 11µm
channels.
The scheme implemented actually employs a further
enhancement of this method to try to avoid the erroneous flagging of cloud-free
ocean data in the region of temperature gradients. This enhancement relies on
the fact that: a) the 11-12µm BT difference is largely independent of SST, and
b) the atmospheric correction is stable over spatial scales of less that 10km.
In this improved test two passes are made over the data. In the first pass
nine-pixel groups are classified as either clear or cloudy, according to their
standard deviation. On the second pass, those “cloudy” groups that have at
least four of their eight neighbours “clear”, and which are over the ocean, are
checked again. These groups are compared against their “clear” neighbours, and
if the average 11-12µm BT difference is less than 0.1K from the value for its “clear”
neighbours the “cloudy” group is unflagged and marked as “clear”.
2.5.2 Large-scale spatial coherence test
The small-scale spatial coherence test works well for some
types of cloud but is pretty much useless at detecting uniform stratus, and
clouds whose effective temperature is close to that of the sea surface can also
evade detection. This large-scale spatial coherence test was designed to cope
with this by establishing dynamic thresholds for areas greater that 100km by
100km in size and rejecting any pixels colder than this value as cloudy.
For further details
of this test the reader is referred to Závody et al. (2000).
2.6 The 1.6µm reflectance test
Away from sun-glint areas where specular reflection of the
sun can occur (depending on the local wind speed), the visible and
near-infrared reflectivity of water is very low, whereas in contrast clouds scatter
radiation these wavelengths very strongly. Thus, clouds are detected by the
strong scattering signal they return. This test is extremely effective at
detecting most types of clouds and is the reason why all SST sensors include at
least one visible or near-infrared channel specifically for cloud
discrimination (– this, and the fact it is better for discrimination of water
clouds over ice, is why the ATSR instruments all carry a 1.6µm channel).
Over the ocean the test will only work for reliably for identifying
clouds in those regions where the sun-glint signal is relatively small. For
this reason this cloud test is split into 3 separate regions based on the
effect of sun-glint:, there regions are: 1) the glint-free region, 2) the
near-glint region, and 3) the glint region.
These regions are defined by first calculating the angle at
which the ocean surface would have to be tilted at the point considered in
order to produce specular reflection. If vsun and vsat
denote the vectors pointing from a pixel to the Sun and the satellite,
respectively, and the unit vector to the local vertical direction is k then the
tilt angle τ is given by
|
k*(Vsun+Vsat) |
|
|Vsun+Vsat| |
|
|
The 1.6µm reflectance test is split according to sun-glint
regime as determined from the tilt angle above, and ach of these regions is
treated differently in the test as follows, because of the markedly different
effects of sun-glint in each of these parts of the image.
Glint-free region (tilt angle > 23˚). In the
glint-free region the threshold used in the cloud test is determined
dynamically from the bright peak in histograms of the measured 1.6µm
top-of-atmosphere reflectances for 32km x 32km areas.
Near-glint region (tilt angles between 15˚ to
23˚). This test works similarly to the test described above in (a) for
the glint-free case, except that the across-track gradient caused by the
variation in sun-glint needs to be removed before the histograms are formed. If
this de-trending step is omitted then the test can erroneously flag as cloudy
the clear pixels along the east side of the image
Glint
region (tilt angles < 15). In these regions of strong sun-glint a
spatial coherence test is performed such that the standard deviation in
reflectance is estimated for 2 x 4 pixel areas (with its short-side across
track to minimise the effect of the glint gradient) and compared to a
dynamically determined threshold value. When the calculated value exceeds the
dynamic threshold value the area is flagged as cloudy. The determination of the
dynamic threshold value is quite complex and the reader is referred to Závody
et al. However, it should be noted that in the glint region one further test is
performed to mitigate against the theoretical possibility that sun-glint in the
11 and 12µm could contaminate the SST retrievals. Thus, if the 1.6µm
reflectance is greater than 90% the data are flagged as cloudy so any
potentially sun-glint contaminated 11 and 12µm data are not used to derive SST.
There is a further wrinkle in the way tests (a) and
(b) operate which is that if no valid threshold can be determined from the
reflectance histogram, the test reverts to the spatial coherence method
described under item (c) for the glint region.
2.7 Nadir-view/forward-view
tests
These tests are unique to ATSR because there is no analogue
for the forward view in the AVHRR instrument. Each of these tests relies on the
fairly clear relationship that exists, at least for clear pixels over an ocean
surface, between the BTs measured in the two views.
2.7.1 Dual-view test using the 11 and 12µm
BTs
This test works by using the
nadir view 11 and 12µm BTs to “predict” the expected 11µm nadir and forward
view BT difference. The pixel is flagged as cloudy if the absolute value of the
difference between the measured and “predicted” quantities is greater than a
fixed threshold value. These thresholds vary across track to account for the
effects of the increased air-mass with off-nadir viewing.
2.7.2 Dual-view test using the 3.7 and 11µm
BT
This is performed as an
additional test at night when the 3.7µm channel data is available, and is
similar to the previous test except that the 3.7µm BTs are used instead of the
12µm BTs. In this test the nadir view 3.7µm minus forward view 3.7µm BT differences
and the nadir-view 3.7µm and 11µm BT differences are compared, and pixels
flagged as cloudy if they deviate significantly from the expected relationship.
2.8 The infrared
histogram test
This test uses the 11 minus 12µm differences and is only
performed on those pixels that have not been flagged as cloudy by any of the
previous tests.
The test relies on the following.
1. The 11 minus 12µm difference results almost entirely from the
effect of the atmosphere.
2. That over distances of a few 100km in cloud free conditions
over oceans the atmospheric state is similar.
3. That the residual low stratus clouds that have escaped
detection in previous tests affect the BTs differently to the atmospheric
effects produced by the clear atmosphere.
4. If the atmospheric water vapour loading is not uniform over
the image, and the SST is constant then the lowest BTs correspond to the pixels
having the highest water vapour content, and, therefore, the largest 11µm minus
12µm BT differences. This means that the BTs and BT differences are negatively
correlated for clear pixels and positively correlated for the cloudy ones.
The test works
on the principle that the 11µm minus 12µm BT differences are largest for
the clear pixelsan image because of
the effects of the atmosphere, and is qualified by the condition described in
item (4) above. It is only performed on pixels that have been passed as “clear”
by the previous tests. The threshold used is established using a histogram
method.
3. Which Cloud tests are used when?
Each of the cloud tests used in the ATSR cloud screening
processing is discussed in the above sections, however it should be noted that
not all of theses tests are used in every situation for a number of practical
reasons.
The following table summarises the ATSR cloud tests and the
situations where they are used; the tests highlighted in green are the only
tests that operate over land.
[an error occurred while processing this directive]
Test |
Channel(s) |
Day |
Night |
Ocean |
Land |
Gross cloud |
12um |
yes |
yes |
yes |
no |
Thin cirrus |
11/12 |
yes |
yes |
yes |
yes |
Medium/high level |
3.7/12um |
no |
yes |
yes |
yes |
Fog/low status |
3.7/11um |
no |
yes |
yes |
yes |
11um spatial coherence |
11um |
yes |
yes |
yes |
yes |
1.6um histogram |
1.6um |
yes |
no |
yes |
no |
11/12um nadir/forward |
11/12um |
yes |
yes |
yes |
no |
3.7/12um nadir/foward |
3.7/12um |
no |
yes |
yes |
no |
IR histogram test |
11/12 um |
yes |
yes |
yes |
no |
Tests involving the 1.6µm channel can only be used during daylight because the
channel detects scattered sunlight, whereas conversely any tests using the
thermal signals from the 3.7µm channel cannot be used during daylight because
of the risk of them being contaminated by solar scattering.
Also, all of the time for ATSR-1 and most of
the time for ATSR-2, the 3.7µm channel data are not available during the
daytime anyway because of telemetry rate limitations.
Furthermore, while spatial coherence tests work very
effectively over the relatively uniform ocean surfaces they are not at all
reliable over the land surfaces where there is more much more inherent spatial
structure and variance.
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