The ATSR Project

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.

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.

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 v_sun and v_sat 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

Page last modified : Tuesday, 04-Dec-07