DESCRIPTION OF COEFFICIENTS FOR GEO-iSHAI

The GEO-iSHAI coefficients files are described here together with a brief summary of some characteristics that the user must know and take into account for proper operation of the GEO-iSHAI. The scientific aspects about how these coefficients files were obtained can be seen in the iSHAI ATBD.

The coefficients of version 2018, 2018.1 and 2021 have been updated and calculated with profiles from 2017 year of the iSHAI validation and training dataset. The 1 out of 2 observations of 2017 not used to build the training dataset have been used for training. The other half of data have been used for writing a Validation Report that it is available in the NWCSAF web.

Bias correction coefficients.

The real satellite measurements must be bias-adjusted to account for the bias between the satellite observation and the synthetic radiative transfer model BTs.

In NWC/GEO package, BT bias correction coefficients are considered as common coefficients available for all the products and specific coefficient files and functions in the NWCLIB to access them are available.

GEO-iSHAI use this common BT bias correction coefficients and functions to get the optimal BT bias corrections and then applies them to the real satellite BTs before they are used in the First-Guess regression and/or Physical retrieval steps.

In a best effort basis the BT bias correction coefficients are being updated monthly for MSG and the ECMWF model in the NWCSAF web; in this link http://www.nwcsaf.org/web/guest/bias-bt-correction-coefficients. Similar process is being established for AHI and ABI.

GEO-iSHAI First Guess regression coefficients files

These files contain the non-linear regression coefficients used to generate the First Guess from the Background NWP fields and the bias corrected satellite IR BTs (see iSHAI ATBD).

The names of the files can be configured editing in the GEO-iSHAI configuration file the keywords FG_reg_*_filename. As example, in release 2021 the default values of FG coefficients file for MSG are:

The FG coefficients files must be located at $SAFNWC/import/Aux_data/iSHAI

The files contain regression coefficients for every zenith degree between 0º to 75º. There is a regression for each one of the output variables and every satellite zenith angle from [0º, 75º]. Since the number of RTTOV-11.2 RTTOV pressure levels is 54 then the number of regressions inside the T, logq and logozone coefficients file is 76x54.

In the case of FG_reg_SKT_filename there are just 76 regressions every satellite zenith angle from [0º, 75º].

In the case of FG_reg_logOzone_filename the number of predictors is different because is used also as inputs an IR channel in the 9.7 μm region and the logarithm of the background NWP ozone profile.

The predictors for each regression can be seen in Eq.1 and Eq.2 of the iSHAI ATBD.

The FG regressions coefficients are the same for P and HYB mode.

GEO-iSHAI Empirical Orthogonal functions (EOF) for T and q

As it’s explained in the iSHAI ATBD,in the GEO-iSHAI algorithm the minimisation of the error between the real satellite BTs and the synthetic RTTOV BTs is made in the Empirical Orthogonal Functions (EOF) or Principal Components space. The name of the files that contain the EOFs coefficients for temperature and specific humidity appear in the GEO-iSHAI configuration file. In the GEO-iSHAI 2021 and 2018.1 versions the default name is:

The EOF files are located in $SAFNWC/import/Aux_data/iSHAI directory.

Each EOF file is a float (4 bytes) array of (2*54+1) x (2*54+1) dimensions (47524 bytes).

The order is 2*54+1 elements of first EOF, then the 2*54+1 elements of the second EOF, and so on. In versions 2021 and 2018.1 in the default distributed EOF coefficients the order on the EOFS has been reorder in order to have first the 3 main EOFs for T, second the 3 main EOFs for Log(q) and then the 1 EOF for SKT.

GEO-iSHAI background NWP inverse covariance matrix B^-1

The name of the file that contains the B^-1 matrix appears in the GEO-iSHAI configuration file in the keyword Binverse_covariance_matrix_filename

The default B^-1 file is Binv_iSHAI_v2018_date_sep2018.bin

The file is a binary file of 109 x 109 dimensions of floats (4 bytes).

The file with the inverse covariance matrix is located in $SAFNWC/import/Aux_data/iSHAI directory. Its size is 47524 bytes (54*2+1)x(54*2+1)x4

GEO-iSHAI observation error inverse covariance matrix E^-1

As it’s explained in the iSHAI ATBD, the errors of the real observed brightness temperatures and the errors of the radiative transfer model are represented by the matrix E in the GEO-iSHAI algorithm. In GEO-iSHAI 2021 and 2018.1 versions, E^-1 matrix has been computed with the available GEO-iSHAI training and validation dataset with ECMWF model outputs.

The name of the file that contains the E^-1  inverse covariance matrix appears in the GEO-iSHAI configuration file in the keyword Einverse_covariance_matrix_filename

Since the number of IR channels used in GEO-iSHAI depends on the satellite class, there are default E^-1 coefficient files for each satellite class. Although the users can supply their own E^-1 coefficient files and specify any valid file name, the default name in versions 2021 and 2018.1 are:

Since the generation of GEO-iSHAI training validation datasets from AHI and ABI instruments is being made the initial E^-1 matrix in GEO-iSHAI versions 2021 and 2018.1 has been calculated with a limited period but it will be improved and distributed later on the NWC SAF web.

The file with the inverse covariance matrix is located in $SAFNWC/import/Aux_data/iSHAI directory. 

Surface IR emissivity atlas files.

In NWC/GEO package versions 2021 and 2018, monthly emissivity atlases are considered as common coefficients available for all the products and specific files and functions in the NWCLIB to access to them are available.

GEO-iSHAI uses these common emissivity atlases and functions to get the emissivity values on land pixels. Precomputed monthly emissivity atlases for each channel of the NWC/GEO satellites are generated from the 2007 IREMIS dataset (See http://cimss.ssec.wisc.edu/iremis/) RTTOV-11.2 emissivity atlases. There are monthly emissivity atlas for every IR channel, supported GEO satellite position (example in case MSG: 0º for operational satellite, 9.5º E for Rapid Scan satellite and 41.5º E for MSG-1 in IODC mission, Himawari, etc) and for every month.

RTTOV-11.2 coefficients

GEO-iSHAI software strongly relies on a clear sky fast and accurate radiative transfer model. In our case, RTTOV-11.2 software from EUMETSAT NWPSAF is used. RTTOV-11.2 is part of the COTS software installed in the NWC/GEO package installation.

Ancillary files sets.

The elevation mask remapped onto geostationary satellite image projections is an ancillary file needed to run GEO-iSHAI. The elevation data are used to distinguish between land and sea pixels (sea pixels are those whose elevation value is equal to missing code). Then, in the case of land pixels the emissivity values are read from the emissivity files.

Atlas and sea/land mask datasets covering the whole satellite disk in the default satellite projection at full satellite IR horizontal resolution are available within the NWC/GEO software package. They are extracted on the processed region by the GEO-iSHAI software using NWCLIB functions. There are elevation data for every supported GEO satellite position (example in case MSG: 0º for operational satellite, 9.5º E for Rapid Scan satellite and 41.5º E for MSG-1 in IODC mission).