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New ROM SAF Report: An investigation into the impacts of the vertical smoothing of GNSS-RO bending angle observations on Met Office NWP forecasts

DATE: 2025-06-30 15:07
TYPE: NRT-GTS

Description

Dear ROM SAF GTS User

Issued: 2025-06-30 15:04 UTC

We are pleased to release a ROM SAF report which contains an investigation into the impacts of the vertical smoothing of GNSS-RO bending angle observations on Met Office NWP forecasts: https://rom-saf.eumetsat.int/general-documents/rsr/rsr_44.pdf

Abstract:

Motivated by apparent forecast improvements in the Met Office system with Spire-processed GNSS radio occultation bending angle observations, various experiments have been run to test the effect of increased vertical smoothing on forecast quality.

The initial experiments were run with additional smoothing applied to Spire's observations as part of the EUMETSAT Secretariat processing. In these experiments it was seen that increasing the smoothing decreased the standard deviation of the observation departures, but also increased the vertical correlation length-scales. These observations with additional smoothing were then ingested within a low-resolution version of the Met Office NWP system, and the forecast quality was seen to be improved with the observations using additional smoothing compared with the observations using the operational processing.

A second set of experiments were run which applied additional smoothing as a pre-processing step within the Met Office system. The smoothing is thus applied to the low-resolution BUFR observations which are normally assimilated operationally. This method has the advantage that it is applied to the whole observations dataset, but a disadvantage that it is applied to the low-resolution observations which posed some technical challenges. It also meant that it was possible to make the smoothing length-scale proportional to the spacing between vertical levels in the Met Office model. Tests with applying the additional smoothing in this way demonstrated improved forecast performance over a wide range of variables. However, using a large smoothing length-scale produced degraded results, and the degradation was seen first in the tropical region, suggesting that less smoothing is beneficial there.

Further experimentation is planned which would demonstrate the impact of additional smoothing on a second NWP system. If these experiments show that additional, or different, smoothing is optimal in that system then it would demonstrate that this method provides an opportunity for all EUMETSAT members that assimilate bending angles to improve their forecasting systems. To achieve this would require data processing centres, such as EUMETSAT, make their observations available at high-resolution. It would also need tools to be made available for NWP centres to process these high-resolution observations in a manner which is best suited to their system.

Kind regards
The ROM SAF Team

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