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 22 Version 2 – Draft 1 18 March 2013 The state vector of our retrieval consists of a 20-level profile for CH 4 and CO 2 VMRs, scaling factors for H 2 O VMR and temperature profiles, surface albedo and a spectral shift/stretch. Table 3-3. State Vector for CH 4 retrievals. Description Parameters Number of Elements CH 4 20 levels 20 Albedo 4 4 Dispersion 2 2 Temperature Scalar 1 Water Vapour Scalar 1 CO 2 Scalar 1 Total 29 The following sub-sections describe the a priori value and corresponding uncertainties of each state vector quantity. 184.108.40.206 Profile of Methane Concentration The a priori profile of CH 4 is acquired from a TM3 model run (Sander Houweling, personnel communication) and is interpolated to the desired latitude, longitude and time. 220.127.116.11 Profile of Carbon Dioxide Concentration Same as Section 18.104.22.168 22.214.171.124 Surface Pressure Same as Section 126.96.36.199 188.8.131.52 Temperature and Water Vapour Same as section 184.108.40.206 220.127.116.11 Aerosol Profiles The aerosol profile is based upon the GEMS/MACC aerosol model, interpolated to the time/location of the GOSAT sounding. 18.104.22.168 Surface Properties A 3 rd order polynomial is retrieved for the albedo to allow for a spectrally dependent albedo. 22.214.171.124 Instrument Properties Same as Section 126.96.36.199
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 188.8.131.52 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.