GRAS SAF Reports

The GRAS SAF Report series provides additional information related to scientific investigations and the underlying algorithms used in our radio occultation processing code and the ROPP software package.

These documents are Copyright material.

GRAS SAF Report 13            H. Lewis            10 Jun 2010

The Radio Occultation Processing Package (ROPP) software includes functionality for users to process radio occultation data to derive atmospheric bending angle and refractivity profiles. This report presents refractivity and bending angle profiles processed using the ROPP (v4.1) tool ropp_pp_occ_tool. This serves to illustrate the use of ROPP for occultation processing and validate its results. Note that results presented here referring to ROPP-4 (v4.1) also apply to the ROPP-5 (v5.0) user release distribution.

GRAS SAF Report 12            P. Poli, S. B. Healy, and D. P. Dee   2 Feb 2012

This paper presents results of the assimilation of Global Positioning System (GPS) Radio Occultation (RO) bending angle data from CHAMP, FORMOSAT-3/COSMIC, and MetOp-A GRAS in the ECMWF global re-analysis ERA-Interim.
We find that GPSRO data present the highest daily assimilation percentage rate among all the various types of observations, suggesting that these data are readily usable by today's re-analysis systems.
Over time, except when additional GPSRO data are introduced, the ERA-Interim short-term forecasts (background) are found to be stable as compared to GPSRO data, and so are the ERA-Interim temperatures as compared to radiosondes. This suggests that the GPSRO data are potentially as stable as the verification used here, or at least to the extent that the possible quality variations in GPSRO data assimilated in ERA-Interim appear invisible when compared to radiosondes.
We observe very good consistency between all six COSMIC receiver data. Small differences between COSMIC and CHAMP (GRAS) data are observed in the lower troposphere (stratosphere). The mean effect of adding GPSRO data in ERA-Interim is to reduce temperature biases with respect to radiosondes in the ERA-Interim background by warming the upper troposphere and lower stratosphere by about 0.1--0.2~K in all hemispheres. We also find hints of a drying effect in the mean water vapor content in the tropics when GPSRO data are introduced. The fit to upper tropospheric wind observations in the Southern Hemisphere and the tropics is also improved when GPSRO data are present.
Overall the GPSRO data act as references via the variational bias correction to correct satellite radiances. Removing the GPSRO data from the ERA-Interim system leaves the latter more prone to fitting the warmly biased aircraft data.
All these effects are mostly apparent when large amounts of GPSRO data started being assimilated, end 2006, with the introduction of COSMIC data.

An updated verision of this report has been published see
(Poli, P., Healy, S. B. and Dee, D. P. (2010), Assimilation of Global Positioning System radio occultation data in the ECMWF ERA Interim reanalysis, Quarterly Journal of the Royal Meteorological Society, 136: 1972-1990. doi: 10.1002/qj.722).

GRAS SAF Report 11            H Lewis             19 Apr 2010

The ROPP 1dVar processing is used to perform retrievals using one day of GRAS refractivity observations and ECMWF background data. Results illustrate the ROPP functionality and the output diagnostics generated. Tests using different configuration parameter settings and the error descriptions for optimal solutions are discussed. Results processed using COSMIC occulations and Met Office forecast model background data are also presented.

GRAS SAF Report 10            H Gleisner          10 Apr 2011

When forming an area-weighted mean within a latitude grid box from data given on a regular latitude-longitude grid, cosine weighting is applied in order to compensate for the meridian convergence toward higher latitudes. In the scientific literature on climate applications of RO data one can also find examples of cosine weighting being used to form grid-box means from irregularly distributed data. In this report, we point out that cosine weighting assumes that the data to be weighted have a distribution that is uniform per degree of latitude. If the data are randomly, or quasi-randomly, drawn from other distributions, an error is made that may introduce a bias. For actual RO climate data, errors due to under-sampling of longitudinal and temporal variability act to hide any biases caused by the use of cosine weighting. Hence, the resulting effect of using an alternative weighting strategy is mostly small. Nevertheless, the use of cosine weighting of data appears not to be appropriate for the irregularly distributed RO data. An alternative spatial averaging method is devised that provides a better approximation to the area-weighting integral with fewer assumptions concerning the latitudinal distribution of observations.

GRAS SAF Report 09            S Healy             5 Oct 2009

The sensitivity of ECMWF numerical weather prediction analyses to the empirical refractivity coefficients and the introduction of non-ideal gas effects in the bending angle operator have been investigated. A review of the literature on the refractivity coefficients suggests that the uncertainty in the values is probably larger than has been recognised. The results reinforce the need for new measurements of the refractivity coefficients at radio frequencies.

GRAS SAF Report 08            H Lewis             22 Jun 2009

The ropp_io module within ROPP contains routines to enable thinning of Level 1b (bending angle), Level 2a (refractivity) and Level 2b (meteorological variables) data. Thinning aims to reduce the amount of data without reducing the information content. GRAS SAF Report 01 presented some possible methods for thinning 1-dimensional (profile) data. It was discussed that a typical bending angle profile does not contain more than about 250 pieces of information, and it is therefore desirable to reduce the data volume to about this number from up to 5000 measurements obtained during an occultation. A list of 247 set impact heights were defined for use in the GRAS SAF thinned BUFR productsto meet a strong user requirement for NRT data on set levels. These heights are based on the criteria of having a maximum of four points per Fresnel diameter. It is possible to perform thinning using the ROPP software for any pre-defined set of impact heights. This report documents the thinning algortitm as implemented in ROPP.

GRAS SAF Report 07            H Lewis             1 Oct 2008

The ROPP pre-processor module ropp_pp includes routines to perform an Abel transform to derive a bending angle profile from model refractivity data and invert a corrected bending angle profile to derive refractivity. This report describes the Abel inversion and Abel transform calculations included within ROPP. The sensitivity of ropp_pp and ropp_fm results to the choice of algorithm used is discussed.

GRAS SAF Report 06            H Lewis             4 Feb 2008

The ROPP 1dVar retrieval has been tested using the Levenberg-Marquardt method (LevMarq) to solve the 1dVar minimisation problem. Results are compared with the performance of the ROPP-specific minimiser minROPP. Tests are conducted using synthetic profiles based on ECMWF background data and real observed profiles using Met Office background data. In some cases LevMarq is found to produce 1dVar solutions with smaller cost function values and which deviate more from the background profiles than minROPP for a given set of convergence criteria. The minROPP routine is typically three times faster than LevMarq however. The most suitable convergence criteria for minROPP are discussed. It is recommended that both minimisers are supported in the ROPP software, with users able to select the most appropriate algorithm for their own particular need.

GRAS SAF Report 05            H Lewis             23 Jan 2008

A summary of the refractivity calculation used in the ROPP forward model is provided. Several expressions are available for calculating a refractivity profile from background pressure, humidity and temperature data. Results using an established two-term expression (Smith and Weintraub 1956) and a more recent three-term expression (Rueger 2002) are presented. It is recommended that the Rueger (2002) expression is implemented by users for GPSRO processing when provided in future versions of the ROPP software.

GRAS SAF Report 04            H Lewis             23 Nov 2007

The error function erf is used in the Abel transform as part of the ROPP bending angle operator. It has been suggested that this is computed using a polynomial approximation to avoid the cost incurred by calling an external DCDFLIB library function many times while performing the 1dVar processing. This document describes the polynomial expression and the corresponding tangent linear and adjoint codes. Test results demonstrate that the approximation is a suitable replacement for the DCDFLIB function. It is strongly recommended that the polynomial approximation is implemented by users for GPSRO processing when provided in future versions of the ROPP software.

GRAS SAF Report 03            H Lewis             20 Nov 2007

A new ROPP-specific minimiser minROPP has been developed as part of the GRAS-SAF for implementation in the 1dVar retrieval processing. A new minimiser routine ropp_1dvar_minropp} replicates the current third-party minimiser M1QN3 to minimise the cost function using a quasi-Newton method with diagonal scaling. This document summarises the minimisation processing and outlines the new ROPminimiser. Test results demonstrate that minROPP is a suitable replacement for M1QN3. It is strongly recommended that minROPP is implemented by users for GPSRO processing when provided in future versions of the ROPP software.

GRAS SAF Report 02            H Lewis             24 Oct 2007

A summary of the geodetic calculations used in ROPP to relate the geometric and geopotential height scales is provided. This document describes the most appropriate functions to use to compute height scales relative to the WGS-84 reference ellipsoid. The sensitivity of results to the choice of algorithm is shown.

GRAS SAF Report 01            C Buontempo         1 Mar 2007

Radio occultation data tends to be oversampled in respect of the ac tual resolution and information content of the dataset. Thinning aims to reduce the amount of data without reducing the information content. A number of 1-dimensional thinning algorithms are presented and their strengths and weaknesses discussed.