ROM SAF Reports

The ROM SAF (previously 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.

ROM SAF Report 43             Sean Healy            11 Jul 2023

A one-dimensional variational (1D-Var) retrieval approach for ionospheric GNSS radio occultation (GNSS-RO) measurements is now included in version 11 of the Radio Occultation Processing Package (ROPP-11: see https://rom-saf.eumetsat.int/ropp/). This 1D-Var code is applied to Metop extension data, where the bending angles extend up to 600 km. It is shown that the 1D-Var code can process the Metop data, but around 15 % of the retrievals either converge to a high cost at convergence value, or fail to converge in 50 iterations. Various problematic cases are presented. Missing bending angles and inconsistent bending angles at the L1 and L2 frequencies can cause retrieval problems. In some cases, the retrieval produces ionospheric layers peaking well below 100 km. A number of additional quality control (QC) checks are suggested. It is found that single frequency retrievals can be performed with simple source code changes.

ROM SAF Report 42             Sean Healy and Ian Culverwell  18 Oct 2021

A new one-dimensional variational (1D-Var) retrieval approach for ionospheric GNSS radio occultation (GNSS-RO) measurements is described. The approach maps a one-dimensional ionospheric electron density profile, modeled with multiple “VaryChap” layers, to bending angle space. It is demonstrated that gradient based minimisation techniques can be applied to this retrieval problem. The use of ionospheric bending angles is discussed. This approach potentially circumvents the need to for a Differential Code Bias (DCB) estimate when using the measurements. This new, general retrieval method is applicable to both standard GNSS-RO retrieval problems, and the truncated geometry of EUMETSAT’s Metop Second Generation (Metop-SG), which will provide GNSS-RO measurements up to ~ 500 km above the surface. It has been tested with 143 COSMIC measurements, and compared with other retrieval approaches. We find that the solution converges in 135 of the cases, but around 25 cases have high “cost at convergence” values. It is argued that 1D-Var diagnostics like cost at convergence would be useful for quality control (QC) in an operational system. In general, a two layer VaryChap solution is better than one layer. The two layer retrieval problem is well-posed when bending angles in the interval from 120 km to 500 km are available. However, the bending angles up to 750 km improve the solution. The climatological a priori information used in the 1D-Var is effectively a starting point for the 1D-Var minimisation, rather than a constraint on the final solution. The use of bending angles here may be relevant for how Metop-SG and other GNSS-RO measurements are used in ionospheric data assimilation systems in the future.

An ionospheric 1D-Var retrieval will be included in ROPP-11, which is scheduled for release in early 2022. A more detailed statistical analysis of the performance of the 1D-Var retrieval is given by Elvidge (2021) in ROM SAF Visiting Scientist Report 39 (VS39).

ROM SAF Report 41             J. K. Nielsen         6 Jul 2021

A series of experiments based on the ROM SAF Offline v1.1 processessing chain were performed to test the 1D-Var sensitivity to various choices of observation uncertainty and error correlation. The version of 1D-Var used as baseline was 1DV v4.2, the version used for production of ROM SAF Offline v1.1 data set. The test data set chosen was a series of Metop and COSMIC GNSS radio occultations, collocated with nighttime GRUAN RS92 radiosondes. Six perturbations of the refractivity error covariance matrix were probed, to test sensitivity to both uncertainty magnitude and correlation structure. Results were evaluated in terms of changes in bias, Desroziers diagnostics and averaging kernels, broken down on mission and latitude bands.

Generally 1D-Var temperature is negatively biased in the troposphere and positively biased in the stratosphere. 1D-Var specific humidity is dry biased in most of the troposphere. Deflation of the refractivity error covariance enhances these biases. Suppression of refractivity error correlations also increases 1D-Var temperature bias structures in stratosphere, but has little effect on tropospheric temperature retrievals. The negative specific humidity bias in middle and upper troposphere is enhanced by suppression of refractivity error correlations. Desroziers diagnostics suggests that a substantial deflation of tropospheric refractivity error covariance, with a factor of 2 or 3, can be justified.

By comparing Desroziers estimates of refractivity variance, with assumed variance, and relate that to the analytical expression for Desroziers matrices it can be inferred that the current ROM SAF background uncertainties are overestimated.

ROM SAF Report 40             Ian Culverwell        15 Jun 2021

This report briefly discusses the ‘anomalous’ occultations — those for which the L1 bending angle exceeds the L2 bending angle — that are observed surprisingly often (10–15% of the time) by the GRAS instruments on the Metop satellites. Their distribution in space and time is examined. They are found to be more abundant than average in winter, at night and in a patch in the southern hemisphere. In the first two cases the background electron density is smaller than average; in the third various localised ionospheric features may be at work.

ROM SAF Report 39             Hans Gleisner         14 Apr 2021

The ROM SAF CDR v1.0 COSMIC and Metop data records during 2007–2016 are characterised by a relatively constant offset between the two missions. The magnitude of the offset vary with latitude and altitude. During the same time period the weight of Metop in a combined RO data set increase from contributing less than 20% to about 70% of data. These two factors in combination (constant offset and changing data numbers) are expected to introduce a spurious trend in the multi-mission data record. We have investigated differences in bending angle, refractivity, and dry temperature trends amongst 10-year time series (2007–2016) of RO data based on Metop, COSMIC, and the multi-mission data set, all of them part of the ROM SAF monthly-mean gridded CDR v1.0.

ROM SAF Report 38             N E Bowler            19 Aug 2020

The COSMIC-2 / FORMOSAT-7 constellation of six satellites were launched in June 2019 into low-inclination orbits. These provide radio-occultation observations that are principally focussed on the tropical region. Each satellite carries a TGRS receiver that can make observations from both the GPS and GLONASS GNSS networks.

The constellation provides a substantial increase in the total amount of radio occultation data available, with between 5000 and 6000 per day being expected to be provided. This is more than a 100% increase on the current operational network. The observations are generally of good quality and comparable to observations from the Metop satellites. There are some differences between the COSMIC-2 and Metop constellations, with some indications that COSMIC-2 is performing better in the troposphere. The design of the TGRS receiver gives a higher signal-to-noise ratio compared to other receivers with the hope that this will improve the performance in lower regions of the atmosphere.

Assimilation of these observations indicates that there are substantial benefits from assimilating these data, particularly to NWP forecast performance in the tropics. This study has been focussed on offline data that have been provided. These observations have recently become available in near-real time on the Global Telecommunications System (GTS). Therefore operational assimilation of these data is something that is being actively investigated.

ROM SAF Report 37             Sean B. Healy and Kent B. Lauritsen  10 Feb 2020

The ROM SAF took the opportunity of the international GNSS radio occultation (GNSS-RO) community meeting for the joint 6th ROM SAF User Workshop and the 7th IROWG Workshop (held jointly as ROM SAF – IROWG 2019) to formulate a set of recommendations to inform priorities for CDOP-4. The four IROWG working groups were provided with a set of questions and discussion points, and asked to return written recommendations to the ROM SAF. These working group recommendations are summarised in this report.

In general, there remains strong support for continuing the work of the ROM SAF. In the context of NWP, this includes continuing the near-real-time (NRT) provision of refractivity, further development of the ROPP software package, and maintaining the NRT monitoring system. The climate recommendations include more work on the retrieval of tropospheric humidity information, improving the retrieval of temperature information in the upper stratosphere, and the provision of robust uncertainty information with all climate data records. The Space Weather group support the ROM SAF developing ionospheric products from EPS-SG. They also support the development of 1D-Var retrieval techniques, if the ultimate aim is to include horizontal gradient information in order to reduce retrieval errors. The Receiver Technology and Innovative Occultation Techniques group recommend that the ROM SAF starts developing forward models appropriate for the assimilation of PAZ and new LEO-LEO observations. The latter is required for Observing System Simulation Experiments (OSSEs).

ROM SAF Report 36             N E Bowler            23 Jun 2020

The PAZ satellite was launched on 22nd February 2018 into a sun-synchronous polar orbit. This satellite carries a polarimetric instrument for taking radio-occultation measurements of the atmosphere. The instrument, known as ROHPP (Radio Occultations and Heavy Precipitation with PAZ) is a variant of an earlier IGOR (Integrated GPS Occultation Receiver) instrument which has been adapted to measure horizontally and vertically polarised signals. Only one radio-occultation instrument is carried by the satellite, and therefore it is only able to measure setting occultations. Due to the dual-polarisation nature of the instrument, the calculation of bending angles follows a slightly different path from conventional instruments.

Overall the performance of the bending angle data is similar to that from other operational instruments. There are differences for the bending angle data in the troposphere, but this is most likely to be due to differences between processing centres, rather than instrumental differences. The refractivity data above 35 km is more accurate than similar data from FY-3C, even though the bending angle accuracy is similar at these levels. This is believed to be due to differences in the choice of climatology used in the processing.

The data have recently been made available in near-real-time. Due to problems with connection between the satellite to the ground infrastructure, the data is experiencing long delays. Improvements have been made recently, but more work in this area is still needed.

ROM SAF Report 35             N E Bowler            11 Oct 2019

The Feng-Yun 3D (FY-3D) satellite was launched on 14th November 2017 into a sun-synchronous polar orbit. After considerable work the data were made available via the GTS on 11th January 2019. Just under 500 occultations are provided each day, so the satellite represents an important component of the (Global Navigation Satellite System - Radio Occultation) GNSS-RO observing system. Both bending angle and refractivity measurements are made available.

Overall the quality of this data is similar to that from FY-3C. However there are two issues where the data from FY-3D is noticeably worse than that from FY-3C: a bias in setting occultations above 40km and a reduction in the number of observations below 20km. Work is being actively undertaken at the China Meteorological Administration (CMA) to address these issues. Tests have been run to assimilate bending angles from FY-3D into the Met Office system, but excluding setting occultations above 40km. These demonstrate a small but statistically significant improvement from using the data. Therefore, it is planned to include data from FY-3D into the Met Office’s operational system in early 2020.

ROM SAF Report 34             N E Bowler            21 Mar 2019

The Metop-C satellite was launched on 7th November 2018 into a sun-synchronous polar orbit. The first radio occultations from the GRAS instrument were received less than a week after this. Level 1 data (including bending angles) have been made available from EUMETSAT Secretariat from the very beginning. The ROM SAF have processed these data and calculated refractivity profiles - these profiles have been calculated for all the bending angle profiles, including the very earliest measurements. The Level 1 and Level 2 data are being disseminated via the EUMETCast system and GTS. Operational dissemination began on the 7th March 2019.

Overall the quality of this data is very similar to that from Metop-A and B, as might be expected from systems which use the same hardware and software. Very slight differences between the satellites exist, and these will be highlighted.

ROM SAF Report 33             Sean Healy            14 Sep 2018

ROPP-9.1 is a minor release of the ROM SAF’s Radio Occultation Processing Package (ROPP). This report summarises some of the main science changes in ROPP 9.1. These include 1) modifications for GNOS processing, 2) improvements in the wave optics propagator and 3) changes to the L1/L2 bending angle forward models. Possible developments for future ROPP releases are discussed briefly.

ROM SAF Report 32             N E Bowler            28 Jun 2018

The KOMPSAT-5 satellite was launched in 2013 into a sun-synchronous polar orbit. Recently UCAR have started processing the RO data which was being produced by this satellite. These data are made available via FTP from the CDAAC website. The observations are processed twice per day, with a delay of a few hours, which means that they are not currently suitable for operational implementation.

Overall the quality of this data is excellent - approximately as good as from the COSMIC-1 satellites. There is no evidence of systematic biases in the data. The standard deviation of differences between the observation and NWP model background is generally comparable to that of other satellites, except in the lower-stratosphere where they are slightly larger. Due to the orbital period the satellite produces occultations in a regular pattern across the globe. In the one-month period considered we received 12901 observations (approx 440 per day) which compares with 16874 for Metop-A and 7956 for COSMIC-1 FM1.

ROM SAF Report 31             Ian Culverwell        7 May 2019

This report contains brief notes on the bending angles and slantwise total electron contents that result from the use of a single, spherically symmetric ‘VaryChap’ ionosphere. This is a simple but realistic model of the ionospheric electron density distribution whose shape can be easily altered. It is used to give some insight into the sensitivity of radio occultation results to the shape of the electron density profile.

ROM SAF Report 30             Ian Culverwell and Sean Healy  27 Mar 2018

The report describes the results of some numerical experiments designed to investigate the feasibility of carrying out 1D variational retrievals using dual or single frequency radio occultation bending angles, rather than the usual ionospherically corrected ‘neutral’ bending angles. A detailed analysis of the results of carrying out these experiments on two example radio occultation profiles is followed by a statistical analysis of the results of doing so on 3500 consecutive profiles observed by the GRAS instrument on Metop-A. The retrieved ionospheric parameters are briefly reviewed. Further work is suggested.

ROM SAF Report 29             Sean Healy and Andras Horanyi  4 Apr 2017

The ROM SAF plans to run two long GPS radio occultation (GPS-RO) reanalyses for the period 2007-2015. Temporal averaging of the gridded, three-dimensional reanalysis fields is an alternative way of producing level-3 climatologies based on GPS-RO data. In preparation for this activity, a series of experiments have been performed using the ERA5 reanalysis system. The experiments assimilate reprocessed GPS-RO datasets from both UCAR and the ROM SAF, and the operational GPS-RO data from the period. The impact of a subset of other measurement types has also been tested. We have found that the mean bending departure differences between the GPS-RO datasets in the vertical interval between ∼ 10-30 km are around 0.1 %, which is about a tenth of the assumed measurement/forward model uncertainty. Consequently, the zonally averaged temperatures computed from the reanalyses are reasonably consistent below 5 hPa. We have also found that assimilating AMSU-A channel 14 radiances without bias correction is crucial for correcting large model biases around 2 hPa. Furthermore, these model biases appear to be in the GPS-RO null-space. Therefore, we conclude that AMSU-A channel 14 should be included in both of the proposed ROM SAF reanalyses.

It is noted that one internal (non public) ROM SAF dataset has poorer bending angle departure statistics, because quality control flags are not set correctly, but this is resolved in the later dataset. The ROM SAF data numbers tend to be lower than the operational GRAS data from the period. The reasons are understood and they will be resolved in the official version 1 (V1.0) of the ROM SAF Climate Data Record (CDR V1.0) reprocessing.

ROM SAF Report 28             Sean Healy            26 Mar 2018

The physical basis of the wave optics simulation code included in ROPP-9 is described. The ROPP wave optics simulation code is based on a multiple phase screen (MPS) approach which is well known, and so it is only reviewed briefly. A new method for computing the phase and amplitude at the LEO satellite, given the phase and amplitude at the final phase screen, is described in detail. More specifically, it is shown that the solution at the LEO can be computed efficiently in terms of standard Fresnel integrals. The simulations are tested by inverting the phase and amplitudes with a Full Spectrum Inversion (FSI) approach to produce bending angle profiles, and then comparing these with a one-dimensional geometrical optics forward model. Some errors for large bending angle cases in the lowest 2 km are identified, and improvements for implementation in ROPP-9.1 are presented.

ROM SAF Report 27             Sean Healy            11 Nov 2016

The results from a recent set of forecast impact experiments with GPS radio occultation GPS-RO) measurements are summarised. Although the main impact of GPS-RO is in the upper-troposphere and stratosphere, it is shown that the data has some positive impact on tropospheric humidity and winds, particularly in the southern hemisphere. Spatial maps of GPS-RO bending angle departure statistics for the lowest 2 km are presented, and it is suggested that some bias features may be related to ECMWF humidity forecast bias, rather than just observation or forward model errors. The GPS-RO impact does not appear to be sensitive to increasing and reducing the assumed errors statistics in the troposphere, but increasing the assumed errors statistics in the stratosphere does have a positive impact. The relevance of these results to ongoing ROM SAF activities is discussed.

ROM SAF Report 26             Sean Healy            23 Aug 2016

The ECMWF numerical weather prediction (NWP) system has been used to estimate the GNSS radio occultation bending angle error statistics from Metop GRAS and COSMIC measurements. These statistics have been mapped to refractivity space using a linear Abel transform. We have found that the broader vertical bending angle error correlations for rising GRAS measurements account for most of the difference between the rising and setting refractivity profile statistics. The senstivity of the refractivity results to bending angle error correlations is demonstrated and discussed. The importance of modelling the refractivity error correlations correctly is emphasised. The results also suggest that the refractivity error model used in the ROM SAF 1D-Var should be revisited.

ROM SAF Report 25             Estel Cardellach, Kent B. Lauritsen, Riccardo Notarpietro, Axel von Engeln  20 Feb 2017

This document presents the outcome of a survey that was conducted by the ROM SAF in cooperation with EUMETSAT to determine user requirements for ionospheric Radio Occultation (RO) products; thus establishing a general guideline for the potential delivery of GNSS RO observables and products at ionospheric altitudes. The main focus is on possible EUMETSAT Polar System-Second Generation (EPS-SG) RO products, but the outcome is also relevant for other RO instruments. The document compiles the answers of the users and their concerns. It is however clearly noted that the RO instruments on EPS-SG focus on the neutral atmosphere, and ionospheric sounding capabilities are an add-on that do not drive the instrument or programme. Nor should suggested requirements be seen as a commitment by EUMETSAT or the ROM SAF to meet these requirements.

ROM SAF Report 24             Ian Culverwell        20 Jun 2016

This report concerns the Planetary Boundary Layer Height (PBLH) diagnostics that have been implemented in the ROM SAF’s Radio Occultation Processing Package, ROPP. Algorithms for calculating the PBLH are briefly described. PBLHs calculated from six different profile variables — three ‘observation fields’ and three ‘model fields’ — are compared and contrasted, both by profile-by-profile scrutiny of selected examples, and by inspection of the average of a month of occultations. PBLHs derived from forward modelled background fields are also briefly considered. Tentative conclusions are drawn before possible future work is considered.

ROM SAF Report 23             Estel Cardellach and Santiago Oliveras  23 Jan 2016

This document presents the implementation of a technique intended to identify the presence of reflected signals in GNSS radio-occultation (RO) data. Typically, the reflection occurs at the Earth surface level, or off atmospheric layers very close to the surface. The technique is based on supervised learning methods, in particular on Support Vector Machines (SVM). The report briefly describes the SVM approach, and how it is implemented for detection of reflected signals in RO data. Other aspects described in this document are the validation of the technique and computing requirements to run it. The document includes a preliminary discussion about the na ture of these reflection events, based on the statistics of occurrence over land and ocean, seasonal features, analysis of correlation with certain atmospheric or RO conditions, and comparative performance of the ECMWF model when reflections are captured or not.

ROM SAF Report 22             Sean Healy            27 Aug 2015

A COSMIC dataset has been modified by IEEC and the Met Office to include a reflection flag. The flagged measurements are composed of a direct and reflected signal. They have been detected automatically using a technique developed by IEEC. The error statistics of the COSMIC data subsets with and without the flag activated have been computed in data assimilation experiments. It has been confirmed that the bending angle departure statistics of measurements in the troposphere with the flag activated tend to fit ECMWF forecasts more closely. However, the variability of the observations in the flagged subset is much lower, and the flag is more probable for colder, drier tropospheres. The improvement in the departure statistics varies spatially, indicating that a simple scaling of the bending error statistics used to assimilate the data when the flag is activated, is probably not appropriate. This will complicate any use of a single on/off flag in NWP applications.

ROM SAF Report 21             Sean B. Healy and Andras Horanyi  27 Feb 2015

ECMWF hosted EUMETSAT’s Fifth ROM SAF User Workshop (UW5) on Applications of GPS radio occultation measurements, on June 16-18, 2014. The workshop was attended by around 30 international scientists, with expertise in both GPS radio occultation and climate applications. It followed the standard format of a series of presentations, followed by a day in NWP and climate working groups, and then a plenary session. The presentations and program from the 5th user workshop are available at the ECMWF workshop page. The value and continued importance of GPS-RO in both operational NWP and reanalysis applications was emphasised at the plenary session. Overall, this was a useful and enjoyable workshop, which benefited greatly from the diversity of the participant’s research interests.

ROM SAF Report 20             Hans Gleisner         6 Feb 2015

During the development of the ROM SAF Level 3 gridded climate data processing software, a pro-nounced wave pattern artefact was frequently found in plots of the standard deviations of ECMWF model data. It was exclusively a problem for model data, and did not affect the observed RO data. The wave pattern was most easily discerned at low latitudes and above an altitude of 20 kilometers, and there was a seasonal variation with a tendency for the wave pattern to become stronger in the summer hemisphere. It is here shown that the wave pattern artefact is caused by the reduction of variance at the interpolation points as a result of the weighted averaging prescribed by the interpolation from model levels to intermediate points. It does not depend on the detailed choice of interpolation function. It is also shown that the dependence of the wave pattern on latitude, altitude, and season is governed by the mixing of model levels at fixed altitudes along zonal latitude bands. We propose to use log-spline interpolation of refractivity as a remedy for this problem. This intro-duces an additional source of variability at the interpolation points, which is enough to substantially reduce the problem even though it is not fundamentally resolved. The interpolated refractivity profile is also used as input to the Abel transform, at a higher vertical resolution compared to the standard bending-angle forward model. Spline interpolation is also used to interpolate ECMWF model temperature, humidity, and geopotential heights from model levels to intermediate altitudes. The solution described in this report has been implemented in the ROM SAF Level 3 processing software, romclim-1.1, used in the generation of data products GRM-17,…,23, as well as plots at the ROM SAF climate monitoring web pages. The artefact described in this report, and the chosen solu-tion, only concerns the ECMWF model data used as a reference, not the observed RO data.

ROM SAF Report 19             Sean Healy            30 Apr 2014

The Radio Occultation Processing Package (ROPP) includes a two-dimensional (2D) bending angle operator. This has been tested in the ECMWF numerical weather prediction system, with a view to operational implementation possibly during 2014. This report outlines how the 2D operator is implemented at ECMWF. Issues related to parallel computing architectures are discussed. We explain why problems associated with the 2D “occultation plane” spanning more than processor (or core) do not arise at ECMWF. This is because the observations are split into “pools” containing roughly equal numbers of each observation type for load-balancing, and the forward modelling is decomposed into distinct horizontal and vertical interpolation tasks. Recent results with the 2D operator are presented showing an improvement in the bending angle departure statistics with respect to observations, indicating that the forward model errors are reduced. However, the additional computational cost during the 4D-Var minimization is large. New ideas to reduce the computational cost of the ROPP 2D operator are discussed, and a new approach is suggested based on the incremental formulation of 4D-Var.

ROM SAF Report 18             Sean Healy            3 Mar 2014

The impact of single frequency processing of GPS radio occultation (GPS-RO) measurements on numerical weather prediction has been investigated. Metop-B GRAS measurements have been degraded by adding 6 microradian and 12 microradian random Gaussian noise. The additional noise is most significant in the stratosphere, because to first order bending angle values fall exponentially with height. The degraded data have been assimilated in a system where all other GPS-RO measurements are restricted to a latitude band between 40 North and 40 South. The impact of noisy data has been compared with experiments where the weight of the Metop-B GRAS measurements in the stratosphere has been increased by reducing the assumed standard deviation of the observation errors to 1.5 microradians. The main forecast impact is on temperature biases in the stratosphere. The results show that the increase in noise levels will have a noticeable impact on the mean temperature analysis, particularly in polar regions and above 5 hPa. The bias corrections applied to satellite radiances are also affected. For example, the bias corrections applied to channel 13 of Metop-B AMSU-A differ by  0.15 K. Increasing the noise levels degrades the mean of the short-range forecast departures with respect to radiosonde temperature measurements above 200 hPa. The root-mean-square (RMS) temperature errors are degraded by  1% at 30 hPa for the day-1 to day-5 forecast range because of the additional noise. However, there is no degradation in RMS at 100 hPa. The introduction of vertically correlated errors does not affect the main results.

ROM SAF Report 17             I D Culverwell and S B Healy  21 Dec 2015

This report describes the modelling of the L1 and L2 bending angles by means of a simple idealised ionosphere consisting of a single Chapman layer. There are three main reasons for doing this: - The desire for an improved interpolation/extrapolation of the L1 – L2 bending angle difference where either signal (but especially L2) drops out; - The wish to make retrievals using observational data that are closer to what is observed, rather than the ionospherically corrected neutral bending angles that are usually used; - It allows the sensitivity of the observed bending angles with respect to the unknown ionospheric parameters to be assessed. Sensitivities to horizontal gradients and to the shape and size of the model ionosphere are briefly examined before calculating the bending angles induced by a single Chapman layer. The feasibility of making retrievals directly with L1 and L2, and with L1 alone, are studied in a simple ‘toy’ system, and the results are compared against retrievals made in the same system with the usual ionospherically corrected bending angles. All GRAS occultations within a five day period have their L1 – L2 bending angle differences modelled by this theory, and the derived ionospheric parameters are examined. Areas for further study are suggested.

ROM SAF Report 16             Dave Offiler          3 Aug 2016

This report reviews the calculation of undulation – the difference between the (WGS-84) ellipsoid and the (EGM96) geoid – in ROPP. The undulation value at the nominal RO profile location is subtracted from the Impact Height to obtain the height with respect to the geoid of any point in the profile. For practical purposes, the resulting geoid height can be considered to be the same as height above mean sea level. The current method of calculating undulations (or geoid heights in ROPP) uses a full spherical harmonic expansion which is both complex and cpu-intensive. An alternative grid interpolation method has been developed which is shown to be significantly simpler in code terms and so substantially quicker to run, and without any practical loss of accuracy when compared to the present full calculation method. The interpolation method is proposed to replace the current undulation code in ROPP.

ROM SAF Report 15             C. Burrows, S. Healy, I. Culverwell        13 May 2013

The current bending angle operator in the ROPP package (ROPP_FM module) is based on Healy & Thepaut (2006). This operator is also used operationally at the Met Office, ECMWF and other NWP centres. The Met Office bending angle innovation statistics have shown features which can be attributed to shortcomings in the assumption of exponentially varying refractivity between model levels when the spacing is large. However, when the level spacing is small, the exponential assumption is acceptable. Similar features can be observed in the refractivity statistics. This report aims to address these issues by proposing a more physical approximation to the refractivity as a function of height between model levels which can be used in the Abel integral to forward-model bending angles. Improvements in both the refractivity and bending angle biases are demonstrated, along with several approaches for implementing an improved bending angle calculation.

ROM SAF Report 14             Ian Culverwell        16 Oct 2013

This report reviews the geodesy calculations in ROPP, in response to concerns raised by the ROPP2.0 beta reviewer in 2008. Various expressions for the surface gravity, effective radius, radius of curvature and geopotential height are compared, and the impact of the differences in RO applications is assessed. Supporting sensitivity studies are reported. A simple model is developed that sheds light on some of the findings.

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.