Comparison study of COSMIC RO dry-air climatologies based on average profile inversion

Abstract. Global Navigation Satellite System (GNSS) radio occultation (RO) data enable the retrieval of near-vertical profiles of atmospheric parameters like bending angle, refractivity, pressure, and temperature. The retrieval step from bending angle to refractivity, however, involves an Abel integral with an upper limit of infinity. RO data are practically limited to altitudes below about 80 km and the observed bending angle profiles show decreasing signal-to-noise ratio with increasing altitude. Some kind of high-altitude background data are therefore needed in order to perform this retrieval step (this approach is known as high-altitude initialization). Any bias in the background data will affect all RO data products beyond bending angle. A reduction of the influence of the background is therefore desirable – in particular for climate applications. Recently a new approach for the production of GNSS radio occultation climatologies has been proposed. The idea is to perform the averaging of individual profiles in bending angle space and then propagate the mean bending angle profiles through the Abel transform. Climatological products of refractivity, density, pressure, and temperature are directly retrieved from the mean bending angles. The averaging of a large number of profiles suppresses noise in the data, enabling observed bending angle data to be used up to 80 km without the need for a priori information. Some background information for the Abel integral is still necessary above 80 km. This work is a follow-up study, having the focus on the comparison of the average profile inversion climatologies (API) from the two processing centers WEGC and DMI, which study monthly COSMIC (Constellation Observing System for Meteorology, Ionosphere, and Climate) data from January to March 2011. The impact of different backgrounds above 80 km is tested, and different implementations of the Abel integral are investigated. Results are compared for the climatological products with ECMWF analyses, MIPAS, and SABER data. It is shown that different implementations of the Abel integral have little impact on the API climatologies. On the other hand, different extrapolations of the bending angle profile above 80 km play a key role in the resulting monthly mean refractivities above 35 km in altitude. Below that respective altitude the API climatologies show a good agreement between the two processing centers WEGC and DMI. Due to the downward propagation within the retrieval, effects of the high-altitude initialization lead to differences in dry-temperature climatologies down to 20 km in altitude. When applying an exponential extrapolation to the bending angles above 80 km at both centers, the dry-temperature climatologies agree among WEGC, DMI, ECMWF analysis, and MIPAS up to 35 km in altitude within ±0.5 K and up to 40 km in altitude within ±1 K. We conclude that the API retrieval is a valid approach up to the lower stratosphere. It is a computationally efficient alternative method for producing dry atmospheric RO climatologies.

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Comparison study of COSMIC RO dry air climatologies based on average profile inversion

10.5194/amt-2018-29 ◽

2018 ◽

Author(s):

Julia Danzer ◽

Marc Schwärz ◽

Veronika Proschek ◽

Ulrich Foelsche ◽

Hans Gleisner

Keyword(s):

A Priori ◽

Computation Time ◽

Background Information ◽

Bending Angle ◽

Angle Space ◽

Average Profile ◽

The Mean ◽

Profile Inversion ◽

The Impact ◽

Bending Angles

Abstract. Recently a new approach for the production of GNSS radio occultation climatologies has been proposed. The idea is to perform the averaging of individual profiles already in bending angle space and propagating the mean bending angle profiles through the Abel transform. Climatological products of refractivity, density, pressure, and temperature are directly retrieved from the mean bending angles. The averaging of a large number of profiles suppresses noise in the data, enabling observed bending angle data to be used up to 80 km without the need of a priori information. Above that altitude some background information for the Abel integral is still necessary. This work is a follow up study, having the focus on the comparison of the average profile inversion climatologies (API) from the two processing centers WEGC and DMI, studying monthly COSMIC data from January to March 2011. The impact of different backgrounds above 80 km is tested, and different implementations of the Abel integral are investigated. Results are compared for the climatological products against ECMWF analysis, MIPAS, and SABER data. It is shown that different implementations of the Abel integral have only little impact on the average profile inversion climatologies. On the other hand, different expansions of the bending angle profile above 80 km play a key role on the resulting monthly mean refractivities above 35 km altitude. Below that respective altitude the API climatologies show a good agreement between the two processing centers WEGC and DMI. Due to the downward propagation within the retrieval, effects of the upper initialization lead to differences in dry temperature climatologies already at 20 km altitude. Applying at both centers an exponential extrapolation to the bending angles above 80 km, dry temperature climatologies agree between WEGC, DMI, ECMWF analysis, and MIPAS up to 35 km altitude within ± 0.5 K, and up to 40 km altitude within ± 1 K. We conclude that up to the lower stratosphere the average profile inversion is a valid – and in computation time much faster – alternative for the production of dry atmospheric RO climatologies.

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Ionospheric correction of GPS radio occultation data in the troposphere

Atmospheric Measurement Techniques Discussions ◽

10.5194/amtd-8-7781-2015 ◽

2015 ◽

Vol 8 (7) ◽

pp. 7781-7803

Author(s):

Z. Zeng ◽

S. Sokolovskiy ◽

W. Schreiner ◽

D. Hunt ◽

J. Lin ◽

...

Keyword(s):

Radio Occultation ◽

Neutral Atmosphere ◽

Ionospheric Correction ◽

Bending Angle ◽

Gps Radio Occultation ◽

Occultation Data ◽

L1 And L2 ◽

The Impact ◽

Radio Occultation Data ◽

Bending Angles

Abstract. For inversions of the GPS radio occultation (RO) data in the neutral atmosphere, this study investigates an optimal transition height for replacing the standard ionospheric correction by the linear combination of the L1 and L2 bending angles with the correction of the L1 bending angle by the L1-L2 bending angle extrapolated from above. The optimal transition height depends on the RO mission (i.e., the receiver and firmware) and is different between rising and setting occultations and between L2P and L2C GPS signals. This height is within the range approximately 10–20 km. One fixed transition height, which can be used for the processing of currently available GPS RO data, can be set to 20 km. Analysis of the L1CA and the L2C bending angles in the presence of a sharp top of the boundary layer reveals differences that can be explained by shifts in the impact parameter. The ionosphere-induced vertical shifts of the bending angle profiles require further investigation.

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Improved model for correcting the ionospheric impact on bending angle in radio occultation measurements

10.5194/amt-2017-162 ◽

2017 ◽

Cited By ~ 1

Author(s):

Matthew J. Angling ◽

Sean Elvidge ◽

Sean B. Healy

Keyword(s):

Zenith Angle ◽

Solar Zenith Angle ◽

Radio Occultation ◽

Neutral Atmosphere ◽

Night Time ◽

Bending Angle ◽

Height Range ◽

Improved Model ◽

The Impact ◽

Bending Angles

Abstract. The standard approach to remove the effects of the ionosphere from neutral atmosphere GPS radio occultation measurements is to estimate a corrected bending angle from a combination of the L1 and L2 bending angles. This approach is known to result in systematic errors and an extension has been proposed to the standard ionospheric correction that is dependent on the squared L1/L2 bending angle difference and a scaling term (κ). The variation of κ with height, time, season, location and solar activity (i.e. the f10.7 flux) has been investigated by applying a 1D bending angle operator to electron density profiles provided by a monthly median ionospheric climatology model. As expected, the residual bending angle is well correlated (negatively) with the vertical TEC. However, κ is more strongly dependent on the solar zenith angle. Furthermore, over the height range of interest (40–80 km) κ is approximately linear with height. A simple κ model has also been developed. It is independent of ionospheric measurements, but incorporates geophysical dependencies (i.e. solar zenith angle, solar flux, altitude). The global mean error (i.e. bias) and the standard deviation of the residual errors are reduced to −2.2 × 10−10 rad and 2.0 × 10−9 rad respectively. Although a fixed scalar κ also reduces bias for the global average the selected value of κ (14 rad−1) is only appropriate for a small band of locations around the solar terminator. In the day time, the scalar κ is consistently too high and this results in an over correction of the bending angles and a positive bending angle bias. Similarly, in the night time, the scalar κ is too low. However, in this case, the bending angles are already small and the impact of the choice of κ is less pronounced.

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Retrieval of temperature and water vapor profiles from radio occultation refractivity and bending angle measurements using an Optimal Estimation approach: a simulation study

Atmospheric Chemistry and Physics ◽

10.5194/acp-5-1665-2005 ◽

2005 ◽

Vol 5 (6) ◽

pp. 1665-1677 ◽

Cited By ~ 5

Author(s):

A. von Engeln ◽

G. Nedoluha

Keyword(s):

Water Vapor ◽

Radio Occultation ◽

A Priori ◽

Estimation Method ◽

Lower Troposphere ◽

Optimal Estimation ◽

Pronounced Difference ◽

Bending Angle ◽

Advantages And Disadvantages ◽

The Impact

Abstract. The Optimal Estimation Method is used to retrieve temperature and water vapor profiles from simulated radio occultation measurements in order to assess how different retrieval schemes may affect the assimilation of this data. High resolution ECMWF global fields are used by a state-of-the-art radio occultation simulator to provide quasi-realistic bending angle and refractivity profiles. Both types of profiles are used in the retrieval process to assess their advantages and disadvantages. The impact of the GPS measurement is expressed as an improvement over the a priori knowledge (taken from a 24h old analysis). Large improvements are found for temperature in the upper troposphere and lower stratosphere. Only very small improvements are found in the lower troposphere, where water vapor is present. Water vapor improvements are only significant between about 1 km to 7 km. No pronounced difference is found between retrievals based upon bending angles or refractivity. Results are compared to idealized retrievals, where the atmosphere is spherically symmetric and instrument noise is not included. Comparing idealized to quasi-realistic calculations shows that the main impact of a ray tracing algorithm can be expected for low latitude water vapor, where the horizontal variability is high. We also address the effect of altitude correlations in the temperature and water vapor. Overall, we find that water vapor and temperature retrievals using bending angle profiles are more CPU intensive than refractivity profiles, but that they do not provide significantly better results.

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Analysis of ionospheric structure influences on residual ionospheric errors in GNSS radio occultation bending angles based on ray tracing simulations

Atmospheric Measurement Techniques ◽

10.5194/amt-11-2427-2018 ◽

2018 ◽

Vol 11 (4) ◽

pp. 2427-2440 ◽

Cited By ~ 3

Author(s):

Congliang Liu ◽

Gottfried Kirchengast ◽

Yueqiang Sun ◽

Kefei Zhang ◽

Robert Norman ◽

...

Keyword(s):

Impact Parameter ◽

Radio Occultation ◽

Global Climate ◽

Weather Prediction ◽

Signal Propagation ◽

Satellite System ◽

Primary Role ◽

Dual Frequency ◽

Bending Angle ◽

Bending Angles

Abstract. The Global Navigation Satellite System (GNSS) radio occultation (RO) technique is widely used to observe the atmosphere for applications such as numerical weather prediction and global climate monitoring. The ionosphere is a major error source to RO at upper stratospheric altitudes, and a linear dual-frequency bending angle correction is commonly used to remove the first-order ionospheric effect. However, the higher-order residual ionospheric error (RIE) can still be significant, so it needs to be further mitigated for high-accuracy applications, especially from 35 km altitude upward, where the RIE is most relevant compared to the decreasing magnitude of the atmospheric bending angle. In a previous study we quantified RIEs using an ensemble of about 700 quasi-realistic end-to-end simulated RO events, finding typical RIEs at the 0.1 to 0.5 µrad noise level, but were left with 26 exceptional events with anomalous RIEs at the 1 to 10 µrad level that remained unexplained. In this study, we focused on investigating the causes of the high RIE of these exceptional events, employing detailed along-ray-path analyses of atmospheric and ionospheric refractivities, impact parameter changes, and bending angles and RIEs under asymmetric and symmetric ionospheric structures. We found that the main causes of the high RIEs are a combination of physics-based effects – where asymmetric ionospheric conditions play the primary role, more than the ionization level driven by solar activity – and technical ray tracer effects due to occasions of imperfect smoothness in ionospheric refractivity model derivatives. We also found that along-ray impact parameter variations of more than 10 to 20 m are possible due to ionospheric asymmetries and, depending on prevailing horizontal refractivity gradients, are positive or negative relative to the initial impact parameter at the GNSS transmitter. Furthermore, mesospheric RIEs are found generally higher than upper-stratospheric ones, likely due to being closer in tangent point heights to the ionospheric E layer peaking near 105 km, which increases RIE vulnerability. In the future we will further improve the along-ray modeling system to fully isolate technical from physics-based effects and to use it beyond this work for additional GNSS RO signal propagation studies.

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A modification to the standard ionospheric correction method used in GPS radio occultation

Atmospheric Measurement Techniques ◽

10.5194/amt-8-3385-2015 ◽

2015 ◽

Vol 8 (8) ◽

pp. 3385-3393 ◽

Cited By ~ 20

Author(s):

S. B. Healy ◽

I. D. Culverwell

Keyword(s):

Impact Parameter ◽

Radio Occultation ◽

Order Term ◽

Weather Prediction ◽

Implicit Assumption ◽

Ionospheric Correction ◽

Gps Radio Occultation ◽

Uncertainty Estimates ◽

The Impact ◽

Bending Angles

Abstract. A modification to the standard bending-angle correction used in GPS radio occultation (GPS-RO) is proposed. The modified approach should reduce systematic residual ionospheric errors in GPS radio occultation climatologies. A new second-order term is introduced in order to account for a known source of systematic error, which is generally neglected. The new term has the form κ(a) × (αL1(a)-αL2(a))2, where a is the impact parameter and (αL1, αL2) are the L1 and L2 bending angles, respectively. The variable κ is a weak function of the impact parameter, a, but it does depend on a priori ionospheric information. The theoretical basis of the new term is examined. The sensitivity of κ to the assumed ionospheric parameters is investigated in one-dimensional simulations, and it is shown that κ &simeq; 10–20 rad−1. We note that the current implicit assumption is κ=0, and this is probably adequate for numerical weather prediction applications. However, the uncertainty in κ should be included in the uncertainty estimates for the geophysical climatologies produced from GPS-RO measurements. The limitations of the new ionospheric correction when applied to CHAMP (Challenging Minisatellite Payload) measurements are noted. These arise because of the assumption that the refractive index is unity at the satellite, made when deriving bending angles from the Doppler shift values.

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An Impact Study of GPS Radio Occultation Observations on Frontal Rainfall Prediction with a Local Bending Angle Operator

Weather and Forecasting ◽

10.1175/waf-d-15-0085.1 ◽

2016 ◽

Vol 31 (1) ◽

pp. 129-150 ◽

Cited By ~ 6

Author(s):

Ching-Yuang Huang ◽

Shu-Ya Chen ◽

S. K. A. V. Prasad Rao Anisetty ◽

Shu-Chih Yang ◽

Ling-Feng Hsiao

Keyword(s):

Radio Occultation ◽

Positive Impact ◽

Three Dimensional ◽

Wrf Model ◽

Satellite Mission ◽

Bending Angle ◽

Nonlocal Operators ◽

Gps Radio Occultation ◽

Rainfall Prediction ◽

The Impact

Abstract The impact of global positioning system (GPS) radio occultation (RO) soundings on the prediction of severe mei-yu frontal rainfall near Taiwan in June 2012 was investigated in this study using a developed local bending angle (LBA) operator. Two operators for local refractivity (REF) and nonlocal refractivity [excess phase (EPH)] were also used for comparisons. The devised LBA simplifies the calculation of the Abel transform in inverting model local refractivity without a loss of accuracy. These operators have been implemented into the three-dimensional variational data assimilation system of the Weather Research and Forecasting (WRF) Model to assimilate GPS RO soundings available from the Formosa Satellite Mission 3/Constellation Observing Systems for Meteorology, Ionosphere and Climate (FORMOSAT-3/COSMIC). The RO data are found to be beneficial to the WRF forecast of local severe rainfall in Taiwan. Characteristics of assimilation performance and innovation for the three operators are discussed. Both of the local operators performing assimilation at observation levels appear to produce mostly larger positive moisture increments than do the current nonlocal operators performing assimilation on the mean height of each model vertical level. As the information of the initial increments has propagated farther south with the frontal flow, the simulation for LBA shows better prediction of rainfall peaks in Taiwan on the second day than both REF and EPH, with a maximum improvement of about 25%. The positive impact of the RO data results partially from several RO observations near Mongolia and north China. This study provides an intercomparison among the three RO operators, and shows the feasibility of regional assimilation with LBA.

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Integrating uncertainty propagation in GNSS radio occultation retrieval: from excess phase to atmospheric bending angle profiles

10.5194/amt-2017-159 ◽

2017 ◽

Cited By ~ 2

Author(s):

Jakob Schwarz ◽

Gottfried Kirchengast ◽

Marc Schwaerz

Keyword(s):

Monte Carlo ◽

Radio Occultation ◽

Uncertainty Propagation ◽

Processing System ◽

Background Information ◽

Climate Variables ◽

Free Atmosphere ◽

Bending Angle ◽

Covariance Propagation ◽

Excess Phase

Abstract. Global Navigation Satellite System (GNSS) radio occultation (RO) observations are highly accurate, long-term stable data sets, and are globally available as a continuous record since 2001. Essential climate variables for the thermodynamic state of the free atmosphere, such as pressure, temperature and tropospheric water vapor profiles (involving background information), can be derived from these records, which therefore have the potential to serve as climate benchmark data. However, to exploit this potential, atmospheric profile retrievals need to be very accurate and the remaining uncertainties quantified and traced throughout the retrieval chain from raw observations to essential climate variables. The new Reference Occultation Processing System (rOPS) at the Wegener Center aims to deliver such an accurate RO retrieval chain with integrated uncertainty propagation. Here we introduce and demonstrate the algorithms implemented in the rOPS for uncertainty propagation from excess phase to atmospheric bending angle profiles, for estimated systematic and random uncertainties, including vertical error correlations and resolution estimates. We estimated systematic uncertainty profiles with the same operators as used for the basic state profiles retrieval. The random uncertainty is traced through covariance propagation and validated using Monte-Carlo ensemble methods. The algorithm performance is demonstrated using test-day ensembles of simulated data as well as real RO event data from the satellite missions CHAMP and COSMIC. The results of the Monte-Carlo validation show that our covariance propagation delivers correct uncertainty quantification from excess phase to bending angle profiles. The results from the real RO event ensembles demonstrate that the new uncertainty estimation chain performs robustly. Together with the other parts of the rOPS processing chain this part is thus ready to provide integrated uncertainty propagation through the whole RO retrieval chain for the benefit of climate monitoring and other applications

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Simultaneous Radio Occultation Predictions for Inter-Satellite Comparison of Bending Angle Profiles from COSMIC-2 and GeoOptics

Remote Sensing ◽

10.3390/rs13183644 ◽

2021 ◽

Vol 13 (18) ◽

pp. 3644

Author(s):

Yong Chen ◽

Xi Shao ◽

Changyong Cao ◽

Shu-peng Ho

Keyword(s):

Radio Occultation ◽

International System ◽

Weather Prediction ◽

Prediction Method ◽

Low Earth Orbit ◽

Weather Data ◽

Bending Angle ◽

System Of Units ◽

Leo Satellites ◽

Bending Angles

The Global Navigation Satellite System (GNSS) radio occultation (RO) is a remote sensing technique that uses International System of Units (SI) traceable GNSS signals for atmospheric limb soundings. The RO bending angle/sounding profiles are needed for assimilation in Numerical Weather Prediction (NWP) models, weather, climate, and space weather applications. Evaluating these RO data to ensure the high data quality for these applications is becoming more and more critical. This study presents a method for predicting radio occultation events, from which simultaneous radio occultation (SRO) for a pair of low-Earth-orbit (LEO) satellites on the limb to the same GNSS satellite can be obtained. The SRO method complements the Simultaneous Nadir Overpass (SNO) method (for nadir viewing satellite instruments), which has been widely used to inter-calibrate LEO to LEO and LEO to geosynchronous-equatorial-orbit (GEO) satellites. Unlike the SNO method, the SRO method involves three satellites: a GNSS and two LEO satellites with RO receivers. The SRO method allows for the direct comparison of bending angles when the simultaneous RO measurements for two LEO satellites receiving the same GNSS signal pass through approximately the same atmosphere within minutes in time and within less than 200 km of distance from each other. The prediction method can also be applied to radiosonde overpass prediction, and coordinate radiosonde launches for inter-comparisons between RO and radiosonde profiles. The main advantage of the SRO comparisons of bending angles is the significantly reduced uncertainties due to the much shorter time and smaller atmospheric path differences than traditional RO comparisons. To demonstrate the usefulness of this method, we present a comparison of the Constellation Observing System for Meteorology, Ionosphere, and Climate-2 (COSMIC-2) and GeoOpitcs RO profiles using SRO data for two time periods: Commercial Weather Data (CWD) data delivery order-1 (DO-1): 15 December 2020–15 January 2021 and CWD DO-2: 17 March 2021–31 August 2021. The results show good agreement in the bending angles between the COSMIC-2 RO measurements and those from GeoOptics, although systematic biases are also found in the inter-comparisons. Instrument and processing algorithm performances for the signal-to-noise ratio (SNR), penetration height, and bending angle retrieval uncertainty are also characterized. Given the efficiency of this method and the many RO measurements that are publicly and commercially available as well as the expansion of receiver capabilities to all GNSS systems, it is expected that this method can be used to validate/inter-calibrate GNSS RO measurements from different missions.

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Detection of reflections in GNSS radio occultation measurements using phase matching

10.5194/amt-2017-216 ◽

2017 ◽

Author(s):

Thomas Sievert ◽

Joel Rasch ◽

Anders Carlström ◽

Mats I. Pettersson

Keyword(s):

Radio Occultation ◽

Complex Function ◽

Lower Troposphere ◽

Phase Matching ◽

Matching Method ◽

Bending Angle ◽

Abel Transform ◽

Lower Region ◽

Additional Information ◽

The Impact

Abstract. It is well-known that in the presence of super-refractive (SR) layers in the lower troposphere inversion of GNSS radio occultation (RO) measurements using the Abel transform yields biased refractivity profiles. As such it is problematic to reconstruct the true refractivity from the RO signal. Additional information about this lower region of the atmosphere might be embedded in reflected parts of the signal. To retrieve the bending angle, the phase matching operator can be used. This operator produces a complex function of the impact parameter, and from its phase we can calculate the bending angle. Instead of looking at the phase, in this paper we focus on the function's amplitude. The results in this paper show that the signatures of surface reflections in GNSS RO measurements can be significantly enhanced when using the phase matching method by processing only an appropriately selected segment of the received signal. We can then identify reflection signatures even in cases where they are normally obscured by the direct signal's influence on the phase matching amplitude. This signature enhancement is demonstrated by simulations and confirmed with real MetOp-A data.

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