Fastem-4 vs Fastem-5/6 Nadir issue

[Anonymous]

I’ve been having a small issue with the Fastem surface emissivity results at nadir for MWHS (also other cross track scanning satellites with frequencies 183 GHz). I’ve just started using fastem so I can do some work related to the change in surface emissivity due to different instrument channel polarizations (such as with the MWHS quasi-horizontal and quasi-vertical polarizations channels at 150 GHz).

I compared the brightness temperatures at nadir over water in the simulation and had approximately a 3 K difference which varied depending on the atmospheric profile (about 1-2 kg/m2 water vapour column). I was expecting there to be a minimal difference. When I changed to Fastem-4 from Fastem-5 or 6 the difference did minimize. I was reading the notes in the rttov user guides which stated the reflection fitting should force different polarizations to be the same at nadir for fastem-5 or newer but I ended up seeing the opposite. I was wondering if anyone ran across this or maybe someone may have had this happen and know if it’s an incorrect setting in the model or something like that.

Thanks

[James Hocking]

Hi Chris,

FASTEM-5 was intended to reduce the discrepancies between H- and V-pol emissivities at nadir that were observed in FASTEM-4, but it does appear that nevertheless the FASTEM-5 differences can be larger than those for FASTEM-4. In this respect FASTEM-5 and 6 are similar so FASTEM-6 also suffers from the same issue.

There are a number of problems with FASTEM that are not easy to fix. Our plans include development of a completely new MW sea surface emissivity model for RTTOV, although it is not clear when this will be done – and I appreciate that this is not much help to you right now.

Sorry I can’t be of more help. The only other thing I can suggest is that if you have access to an alternative emissivity model you could use this to provide the emissivity values to RTTOV instead of using FASTEM.

Best regards,
James

[Anonymous]

Hi James,

Thank you for the reply. I just wanted to make sure that I was not doing something incorrect in RTTOV. For what I’m doing at the moment, the Fastem-4 results will be perfectly fine.

Cheers,
Chris

[Author]

Posts