Subject: RE: RTTOV Query Ref: 10987/10988 Date: Mon, 8 Dec 2014 17:18:15 +0000 From: Hocking, James To: 'patrick.eriksson@chalmers.se' RTTOV query Hi Patrick, FASTEM is not available as a separate module so you will have to extract it from the RTTOV code unfortunately. It is also unfortunately the case that the FASTEM code as it stands is not well documented: there are now 6 versions of the model which have involved incremental changes and additions to the code. I recommend looking at Table 14 (p42) in the RTTOV v11 user guide which lists the 6 versions and gives references which provide information on the science behind the code and should help decipher what is going on. FASTEM-6 is now the recommended version because it fixes a problem with the azimuthal dependence which has existed since at least FASTEM-4. There are some caveats regarding the use of FASTEM-6 in Table 14. The coefficients used by FASTEM 1-3 are in src/main/mod_rttov_fastem3_coef.F90. The coefficients used by FASTEM 4-6 are in src/main/mod_rttov_fastem5_coef.F90. FASTEM 4-6 are implemented in rttov_fastem5.F90. This subroutine is called by rttov_calcemis_mw.F90 and this latter subroutine also implements FASTEM 1-3. There shouldn’t be any problems if you want to implement FASTEM in ARTS in respect of licenses. However we would like to ensure that if FASTEM-X is implemented in ARTS then it is the same FASTEM-X as is implemented in RTTOV. FASTEM has also been included in the CRTM (a fast model similar to RTTOV) for a long time, but it recently became apparent that there are significant differences in the implementations of FASTEM-5 between RTTOV and CRTM. This is a situation we would like to avoid in the future. Our plans for RTTOV v12 (due for release end of 2016) include consolidating the FASTEM versions into a single version. We hope to also make this available as a stand-alone module/subroutine which other RTM developers can use. This will be based on FASTEM-6. Regarding your specific questions: -Frequency is in GHz -You are right: emissivity(1:4) corresponds to eqn 13a-d in the FASTEM-4 paper. -In RTTOV the surface-reflected downwelling atmospheric emission is treated according to the assumption of specular reflection when the RT equation is integrated. FASTEM modifies the surface emissivity in such a way to account for the anisotropic downwelling radiation. “Transmittance” is the top-of-atmosphere to surface transmittance. This is described in http://nwpsaf.eu/deliverables/rtm/evalfastems.pdf. I’d assume for ARTS that you would not want to include this since ARTS is a fully 3-D RT model (if I remember rightly!) Best regards, James