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NRT Notification Detail

Publication of ROM SAF VS41 Report on Intercomparison of GNSS RO quality control methods in NWP

DATE: 2024-11-05 17:24
TYPE: DATA

Description

Dear ROM SAF User

Issued: 2024-11-05 17:22 UTC

The ROM SAF is pleased to release Visiting Scientist report No. 41 (https://www.romsaf.org/Publications/reports/romsaf_vs41_rep_v10.pdf) by Dr. Hailing Zhang, UCAR, Boulder, CO, USA

Abstract

This project looked at the quality control procedures that NWP centres use for assimilating GNSS-RO bending angle observations. The super-refraction checks were examined in detail, and large differences were found between the methods that various centres use.

While understanding that quality control (QC) processes crucially affect the statistics of the observations compared to short-range NWP forecasts and the subsequent analyses, we also realize the fact that many centres implement entirely different QC methods from each other in operations. The overall objective of this visiting scientist project is to catalogue and compare the QC methods of GNSS RO observations used for data assimilation (DA) by NWP centres within EUMETSAT member states and in other major NWP centres. This should provide guidance to members of the ROM SAF and EUMETSAT communities about the differences between the methods, and potentially improve the exploitation of this data.

This study was conducted with the JEDI framework that contains unified generic QC filters and facilitates easy implementation of new QC methods. The inter-comparison of RO QC methods is based on the same framework, i.e., the Met Office forecast model and bending angle forward operator, and a common RO observation dataset. This project is composed of three major steps of work. The first step is the collection of the QC methods from the major NWP centres around the world. The second effort is the implementation of the QC methods as filters in JEDI. The third effort is the evaluation and inter-comparison of the implemented QC methods through conducting a series of month-long experiments.

The inter-comparison demonstrates that the different QC methods for super refraction behave very differently. The methods used by NRL (U.S. Naval Research Laboratory) and Météo France remove the most observations, those used by Met Office and NCEP (U.S. National Centers for Environmental Prediction) remove the next most, and a method based on differences in impact parameter removed the least. There are subtle differences in the number of observations removed when comparing the different RO missions, with a higher proportion of COSMIC-2 observations being identified as having super refractions, presumably due to the fact that these observations penetrate deeper into the lower troposphere. Comparisons of the vertical refractivity gradients from the observations and the model highlight systematic differences, with sharp refractivity gradients being largely absent from the observations.

Kind regards
The ROM SAF Team

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