ROM SAF - NRT Notification Detail

NRT Notification Detail

GRAS SLTA Parameter Update - update

DATE: 2009-12-11 15:46
TYPE: NRT-GTS

Description

Dear GRAS SAF GTS User Issued: 2009-12-11 15:45 UTC Due to the changed parameter settings for the GRAS instrument on MetOp, we have experienced that rising occultations processed in NRT generally does not reach below 16 km. About 50% of all rising occultations does not reach below 20 km. Please note that the GRAS SAF marks occultations not reaching below 20 km as non-nominal, but they are still dissiminated via the GTS. Please see the message from Chris Marquardt below for the full story (without his attached figures, though). ---snip--- Dear all, as you know, we changed some parameter settings of the GRAS instrument yesterday, partly in order to cover tropical upper tropospheric altitudes with raw sampling data in rising occultations, and partly to extend the coverage of raw sampling data to signals deep behind the atmosphere. Due to design constraints of the GRAS instrument, this means that the acquisition of L2 signals during rising occultations will also take place later, i.e. at higher altitudes. As a first analysis of the impact of the configuration change on the lowest available L2 data impact altitude, I have attached a figure showing an increase of the lowest L2 impact altitude from previously 10-15 km to currently 18 - 23 km. These numbers are consistent with what we expected. In the current operational bending angle processing at EUMETSAT (and also in the higher level processing of the GRAS SAF), no attempt is made to extrapolate L2 data below altitudes where it has actually been measured, Thus, the penetration depth of GRAS profiles from rising occultations has been also reduced from 10-15 km (previously) to 18 - 23 km (currently). A second potential impact we were anxious about is the number of occultations the instrument provides. As the configuration change increases the length of individual occultations, and because GRAS is limited to tracking not more than two occultations simultaneously at each antenna, it is possible that GRAS is not able to acquire yet another occultation while is still busy with the previous ones. Apparently (see the second figure, showing the number of occultations in a 12 hour window), the impact of the parameter change on the number of occultations is rather moderate. We obviously still have to fully analyze the length and quality of individual occultations to obtain a complete understanding on the amount of occultations really affected. With kind regards, Christian. -- Dr. Christian Marquardt, Polar System Product Expert EUMETSAT, Am Kavalleriesand 31, D-64295 Darmstadt, Germany Tel: +49-6151-807456, Fax: +49-6151-807304 Email: christian.marquardt@eumetsat.int, Web: http://www.eumetsat.int ---snip--- Kind regards The GRAS SAF Team

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