ATBD - GHG-CCI

ESA Climate Change Initiative (CCI)
Algorithm Theoretical Basis Document
Version 2 (ATBDv2) – UoL-FP
for the Essential Climate Variable (ECV)
Greenhouse Gases (GHG)
Page 23
Version 2 – Draft 1
18 March 2013
3.4 Screening
3.4.1 Pre-Processing Screen
The GOSAT L1B soundings are screened initially to filter out any observations where the
signal to noise (SNR) is < 50, if the sounding is over ocean and if a sounding contains any
invalid L1B data.
3.4.2 Cloud Screen
One major disadvantage of using SWIR observations is that the iFOV must be free of thick
clouds as these will cause irradiation to be reflected towards the satellite from a higher
altitude, that of the cloud, rather than the Earth's surface. Therefore, a cloud screen is
necessary to filter out exposures where thick clouds are within the iFOV. When irradiation
gets reflected towards the satellite from a thick cloud the effective surface is that of the cloud
rather than the Earth's surface. This results in the actual surface pressure being that at the
cloud altitude, which typically is significantly lower than that at the Earth's surface pressure.
An estimate of the actual surface pressure can be obtained by performing O 2 A band
retrievals where the surface pressure is retrieved. We implement a super fast O 2 retrieval by
using a small spectral window (13056 to 13074.8 cm -1 ). If the retrieved surface pressure is
compared to the a priori surface pressure then the difference in surface pressure can be
determined. Since the a priori surface pressure is well constrained, a surface pressure
difference of greater than 30 hPa would normally only occur if a thick cloud is present in the
iFOV, thus the exposures can be cloud screened using this method.
3.4.3 Post-Processing Screen
3.4.3.1 Carbon Dioxide Screen
The CO 2 is post-screened based on the algorithm convergence criteria as above (i.e.
outcome =1 or 2) as well as on the normalised CHI-Squared value for each spectral band.
Currently a threshold of 0 < CHI-Square Band 1 < 1.3, 0.6 < CHI-Square Band 2 < 1.5 and
0.4 < CHI-Square Band 3 < 1.5 is used. In addition to these screens we also presently filter
on a number of other parameters to reduce the
Additionally, these full physics retrievals are subjected to spectral interference from aerosols
and clouds and topographic assumptions of the a priori, therefore we apply a number of
extra screens to reduce these effects as explained in the table below.
Table 3-4. Description of additional parameters used in the Carbon Dioxide post-processing
screen.
Parameter
Minimum
Threshold
Maximum
Threshold
Explanation
Saturation Test Band 2
Polarisation P
0 3.5x10 -5 Instrument saturation Flag determined from L1B
radiance.