The surface pressure used in the GEO-iSHAI comes from the NWP model. The better resolution the surface pressure field is in the background NWP model, the lower the error will be.
The emissivity is fixed. After decision of NWC SAF Project Team to consider emissivity datasets as common software elements and the incorporation of management functions on the NWC/GEO NWCLIB the NWCSAF precomputed monthly emissivity atlases for each channel of the NWC/GEO satellites are generated from the 2007 IREMIS dataset (U. Wisconsin) RTTOV-11.2 emissivity atlases.
(1) GEO-iSHAI products are available over “clear” FORs only.
(2) Since it is an iterative physical retrieval, computation is relatively expensive and an increase in the width of the FOR could be necessary in case large regions are processed.
(3) The result can exhibit errors over mountainous regions with large difference between the topography and the NWP topography. The same happens in desert pixels, where the distance on skin temperature between the NWP first guess and the actual skin temperature could be high. Similar behaviour is expected on very hot or cold pixels over non-desert land pixels. From our experience, changes in the supply of the background NWP from ECMWF every 6 hours from t+00 to t+24 with 0.5ºx0.5º resolution to every 1 hour from t+00 to t+24 with 0.125ºx0.125º resolution, have improved clearly the quality of the GEO-iSHAI outputs. The GEO-iSHAI HYB mode outputs that uses ECMWF GRIB files on hybrid levels improves the quality. This demonstrates the need to supply the maximum available fixed pressure levels to feed the regular GEO-iSHAI. In general the more accurate the background profile is, the more accurate the retrieval will be. It is strongly recommended to feed GEO-iSHAI with temperature and humidity profiles with as many fixed pressure levels as possible.
(4) In case to use as NWP background the ECMWF model: it is strongly recommended to the GEO-iSHAI users to use ECMWF on hybrid levels as background NWP; although this implies that the user needs to download the GRIB files on hybrid and fixed pressure levels (the latter needed for the Cloud Mask and the other products). The best quality is achieved using as input to GEO-iSHAI GRIB files on hybrid levels and the execution of GEO-iSHAI in HYB mode. As example, at the present time ECMWF GRIB files on fixed pressure levels have available only 25 fixed pressure levels but ECMWF GRIB on hybrid levels files have 137 hybrid levels.
(5) Effect of
emissivity temporal variation is not handled. Precomputed
monthly emissivity atlases for each channel of the NWC/GEO
satellites are generated from the 2007 IREMIS dataset (U.
Wisconsin) RTTOV-11.2 emissivity atlases.