A Refractive Index Mapping Operator for Assimilation of Occultation Data

Syndergaard, Stig; Kursinski, E. R.; Herman, Benjamin M.; Lane, Emily M.; Flittner, D. E.

doi:10.1175/mwr3001.1

Cited by 41 publications

(35 citation statements)

References 56 publications

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“…Foelsche et al: Errors in GNSS radio occultation data study. Later works by Sokolovskiy et al (2005) and Syndergaard et al (2005) developed so-called non-local observation operators and showed that these representativeness errors can be significantly reduced in future data assimilation systems by interpreting the retrieved profiles as complicated weighted averages of the three-dimensional atmospheric field. Since the contribution is focused around the tangent point of each ray (the point of closest approach), RO measurements can alternatively be regarded as profiles that follow the tangent point trajectory (TPT), but only in case of spherical symmetry this profile would exactly equal a profile of point measurements along the TPT.…”

Section: Introductionmentioning

confidence: 99%

Errors in GNSS radio occultation data: relevance of the measurement geometry and obliquity of profiles

et al. 2011

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Abstract. Atmospheric profiles retrieved from GNSS (Global Navigation Satellite System) radio occultation (RO) measurements are increasingly used to validate other measurement data. For this purpose it is important to be aware of the characteristics of RO measurements. RO data are frequently compared with vertical reference profiles, but the RO method does not provide vertical scans through the atmosphere. The average elevation angle of the tangent point trajectory (which would be 90 • for a vertical scan) is about 40 • at altitudes above 70 km, decreasing to about 25 • at 20 km and to less than 5 • below 3 km. In an atmosphere with high horizontal variability we can thus expect noticeable representativeness errors if the retrieved profiles are compared with vertical reference profiles. We have performed an end-to-end simulation study using high-resolution analysis fields (T799L91) from the European Centre for MediumRange Weather Forecasts (ECMWF) to simulate a representative ensemble of RO profiles via high-precision 3-D ray tracing. Thereby we focused on the dependence of systematic and random errors on the measurement geometry, specifically on the incidence angle of the RO measurement rays with respect to the orbit plane of the receiving satellite, also termed azimuth angle, which determines the obliquity of RO profiles. We analyzed by how much errors are reduced if the reference profile is not taken vertical at the mean tangent point but along the retrieved tangent point trajectory (TPT) of the RO profile. The exact TPT can only be determined by performing ray tracing, but our results confirm that the retrieved TPT -calculated from observed impact parametersis a very good approximation to the "true" one. Systematic and random errors in RO data increase with increasing azCorrespondence to: U. Foelsche (ulrich.foelsche@uni-graz.at) imuth angle, less if the TPT is properly taken in to account, since the increasing obliquity of the RO profiles leads to an increasing sensitivity to departures from horizontal symmetry. Up to an azimuth angle of 30 • , however, this effect is small, even if the RO profiles are assumed to be vertical. For applications requiring highest accuracy and precision it is advisable to exclude RO profiles with ray incidence angles beyond an azimuth of 50 • . Errors in retrieved atmospheric profiles decrease significantly, by up to a factor of 2, if the RO data are exploited along the retrieved TPT. The tangent point trajectory of RO profiles should therefore be exploited whenever this is possible.

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Section: Introductionmentioning

confidence: 99%

Errors in GNSS radio occultation data: relevance of the measurement geometry and obliquity of profiles

et al. 2011

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“…It is not obvious whether the ray tangent height error should be called an observation or forward model error, but we use the combined observation and forward model errors to weight the observations in the assimilation so the distinction is academic. Syndergaard et al (2005) recently presented a 'nonlocal' (i.e. 2D) refractivity operator.…”

Section: A Review Of 2d Gpsro Operator Researchmentioning

confidence: 99%

Assimilating GPS radio occultation measurements with two‐dimensional bending angle observation operators

Healy

Eyre

Hamrud

et al. 2007

Quart J Royal Meteoro Soc

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Four-dimensional variational (4D-Var) assimilation forecast impact experiments were conducted with CHAMP radio occultation measurements from 1st June to 31st July 2004. The CHAMP bending angles were assimilated with one-dimensional (1D) and two-dimensional (2D) bending angle observation operators, enabling the potential benefits of the latter to be investigated. Two computationally affordable 2D bending angle observation operators were considered. The first can be viewed as a straightforward generalisation of the 1D operator. The second is based on a fourth order Runge-Kutta solution of the differential equations defining the ray-path through the refracting atmosphere.It is demonstrated that the computational cost of the 2D bending angle operators is not prohibitive and that they improve the root-mean-square of the observed minus background (O-B) bending angle departures by around 5% in the lowertroposphere of the northern and southern hemispheres. The O-B distributions in the tropical lower-troposphere are biased low, but the first-guess departure quality control procedure removes ∼2-3% of the observations and this reduces the bias by around 20%. The assimilation of the CHAMP measurements with both the 1D and 2D operators improves the analysis and forecast fit to radiosonde temperature measurements in the southern hemisphere on the 300, 200, 100 and 50 hPa levels, over the day-1 to day-5 forecast range. The forecast scores in the lower-troposphere for geopotential height and relative humidity are generally neutral, but there is a small improvement in the anomaly correlation of the 500 hPa geopotential height field in the southern hemisphere, over the day-1 to day-2 forecast range. However, there are no clear, statistically significant differences between the 1D and 2D experiments for this two-month period.

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“…The limitation of computational resources and the timing desired for issuing forecasts encourage a more economical approach, especially after more radio occultation data become available from the COSMIC project. As a more efficient alternative to the assimilation of bending angles, Syndergaard et al (2005) and Sokolovskiy et al (2005) recently developed a new methodology, using a linearized observational operator (non-local refractivity operator) to assimilate the observed refractivity. The fractional errors of these new observables are one order of magnitude smaller than those of local refractivity.…”

Section: Introductionmentioning

confidence: 99%

“…Different methodologies have been applied to estimate observational errors (Xu et al 2001;Xu and Li 2001;Healy 2001;Zou 2002;Seemann et al 2003;Kuo et al 2004;Poli and Joiner 2004;Sokolovskiy et al 2005;Syndergaard et al 2005). Innovation vectors (i.e., observation-background) can be used to estimate the observation errors if: (i) the forecast errors are known; (ii) observation errors are independent of forecast errors; and (iii) the observations are spatially isotropic and uncor-related.…”

Section: Introductionmentioning

confidence: 99%

“…Moreover, there are not many conventional observations available over oceans. Therefore, it has been relatively difficult to study error characteristics of radio occultation observables using other observations, and simulation studies have been often used (Zou et al 1999;Healy 2001;Zou et al 2002;Poli and Joiner 2004;Sokolovskiy et al 2005;Syndergaard et al 2005). Zou et al (1999) developed a ray-tracing code, which was applied to estimate errors of local refractivity, using coarse-resolution National Centers for Environmental Prediction (NCEP) 62-wave (approximately 1:9 Â 1:9 ) global reanalysis.…”

Section: Introductionmentioning

confidence: 99%

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Error Characteristics of GPS Retrieved Refractivity Using a Simulation Study

Chen

Vandenberghe

Huang

2006

Journal of the Meteorological Society of Japan

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A simulation study was designed to investigate the error characteristics of retrieved local refractivity arising only from the assumption of spherical symmetry. The convergence of error estimation with respect to different horizontal resolutions was examined, and a resolution higher than 30 km was found to be reasonable for this simulation study.Two cases, one in summer 1997 and the other in winter 1998, were used to study errors of retrieved local refractivity. The error can reach 10 units in the lower troposphere under the assumption of spherical symmetry. Error decreases with height linearly, until about 3 km and then dramatically above 3 km. A local maximum error occurs at approximately 6 km in the 1997 case because there were large refractivity disturbances on the lee side of the Rocky Mountains around that level.The level of the maximum error over land (@2-3 km) was higher than over ocean (@1 km) due to complex topography, as well as a deeper boundary layer over land. As a result, the average root mean square error below 3 km presents a meandering feature over the entire domain of interest. Moreover, the error close to the surface over ocean was slightly higher than that over land. However, in the 2-4 km range the error was slightly lower over ocean than over land. Note that the altitude of maximum error over ocean can be shifted upward when strong disturbances, such as severe weather, exist over the region. The error during summer was larger than that during winter, and it was greater at lower latitudes than at higher latitudes, as reported in previous studies.When modeling retrieved local refractivity, the observational error is a function of the model horizontal resolution. The error was reduced as model resolution was decreased, but there was a lower limit for an optimal resolution. This optimal resolution in the lower atmosphere was higher (@190 km) than that in the middle and upper atmosphere (@250 km) in this study, due to a stronger vertical gradient of refractivity and higher vertical resolution in the lower troposphere.

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A Refractive Index Mapping Operator for Assimilation of Occultation Data

Cited by 41 publications

References 56 publications

Errors in GNSS radio occultation data: relevance of the measurement geometry and obliquity of profiles

Errors in GNSS radio occultation data: relevance of the measurement geometry and obliquity of profiles

Assimilating GPS radio occultation measurements with two‐dimensional bending angle observation operators

Error Characteristics of GPS Retrieved Refractivity Using a Simulation Study

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