ROM SAF - NRT Notification Detail

NRT Notification Detail

Possible change to the Metop-A and B NRT refractivity products

DATE: 2013-11-30 17:08
TYPE: NRT-GTS

Description

Dear ROM SAF GTS User

Issued 2013-11-30 17:08 UTC

We would like to ask your opinion about a future possible change to the ROM SAF NRT refractivity products (GRM-01 & GRM-40). As you may recall, EUMETSAT CF uploaded (in the spring and early summer 2013) new configuration parameters to the GRAS instruments on both Metop-A and Metop-B, and enabled the so-called L2 extrapolation. The consequences of these changes to the refractivity products were previously described in a message from June 18th, 2013: http://www.romsaf.org/view_notification.php?id=429

On June 20th, the ROM SAF consequently implemented an enhanced QC to accommodate the new data and to flag a few bad outliers in the L2-extrapolation as having non-nominal refractivity. But this flagging is only present in the ROM SAF BUFR, not in the EUMETSAT BUFR. Scientists and engineers at EUMETSAT has since then given priority to implement a change to their L2-extrapolation to avoid these outliers. EUMETSAT is considering putting this change into operation in the near future, pending further analyses. Together with EUMETSAT CF, the ROM SAF have analyzed the test data currently being produced at EUMETSAT (the so-called GS2 data stream; only internally available to the ROM SAF). Whereas there is no significant difference between rising and setting occultations in bending angle standard deviation when comparing against ECMWF forecasts, there is a noticeable increase in the refractivity standard deviation for rising occultations. The reason is not yet fully understood. Two figures summarising the analyses so far can be found at the following links:

http://web.dmi.dk/pub/grassaf/UNS/refrac_stdv_M02_stage.png
This shows the refractivity standard deviations for Metop-A (figures for Metop-B are similar; not shown) for a four day period as it looks currently (Metop-A NRT; the data that you are currently receiving over GTS with planned ROM SAF QC applied - see http://www.romsaf.org/view_notification.php?id=445)

http://web.dmi.dk/pub/grassaf/UNS/refrac_stdv_M02_test.png
This shows the same when the refractivity is based on the GS2 test data from EUMETSAT CF (Metop-A GS2; same QC applied).

As you see there would be an increase in the standard deviation of the refractivity for rising occultations of 30-40% between 10 and 20 km (compared to the current values), if the changes prepared at EUMETSAT are put into operations. Please note that the ROM SAF (and EUMETSAT CF) recommends not using the current GRAS data below about 8 km.

It is not yet decided to upgrade the operational system at EUMETSAT CF, and as mentioned, further analyses are ongoing. But we would like to hear if you think this increase in refractivity standard deviation would be critical to your assimilation system. As mentioned, there is no significant difference between the setting and rising bending angle standard deviation against ECMWF forecasts, only for the refractivity standard deviation. The increase in standard deviation is accompanied by a change in vertical correlations when compared to ECMWF forecasts (figures not shown). Please let us know of any concern that you might have of this possible change.

With kind regards
The ROM SAF Team

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