Assessment of GNSS radio occultation refractivity under heavy precipitation

Abstract. A positive bias at heights between 3 and 8 km has been observed when comparing the radio occultation retrieved refractivity with that of meteorological analyses and re-analyses, in cases where heavy precipitation is present. The effect of precipitation in RO retrievals has been investigated as a potential cause of the bias, using precipitation measurements interpolated into the actual three dimensional RO raypaths to calculate the excess phase induced by precipitation. The study consisted in comparing the retrievals when such extra delay is removed from the actual measurement and when it is not. The results show how precipitation itself is not the cause of the positive bias. Instead, we show that the positive bias is linked to high specific humidity conditions regardless of precipitation. This study also shows a regional dependence of the bias. Furthermore, different analyses and re-analyses show a disagreement under high specific humidity conditions and in consequence, heavy precipitation.

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Assessment of global navigation satellite system (GNSS) radio occultation refractivity under heavy precipitation

Atmospheric Chemistry and Physics ◽

10.5194/acp-18-11697-2018 ◽

2018 ◽

Vol 18 (16) ◽

pp. 11697-11708 ◽

Cited By ~ 7

Author(s):

Ramon Padullés ◽

Estel Cardellach ◽

Kuo-Nung Wang ◽

Chi O. Ao ◽

F. Joseph Turk ◽

...

Keyword(s):

Radio Occultation ◽

Three Dimensional ◽

Heavy Precipitation ◽

Satellite System ◽

Specific Humidity ◽

Positive Bias ◽

Actual Measurement ◽

Excess Phase ◽

Global Navigation Satellite ◽

Ray Paths

Abstract. A positive bias at heights between 3 and 8 km has been observed when comparing the radio-occultation (RO)-retrieved refractivity with that of meteorological analyses and reanalyses in cases where heavy precipitation is present. The effect of precipitation in RO retrievals has been investigated as a potential cause of the bias, using precipitation measurements interpolated into the actual three-dimensional RO ray paths to calculate the excess phase induced by precipitation. The study consisted of comparing the retrievals when such extra delay is removed from the actual measurement and when it is not. The results show how precipitation itself is not the cause of the positive bias. Instead, we show that the positive bias is linked to high specific-humidity conditions regardless of precipitation. This study also shows a regional dependence of the bias. Furthermore, different analyses and reanalyses show a disagreement under high specific-humidity conditions and, in consequence, heavy precipitation.

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Observational Error Estimation of FORMOSAT-3/COSMIC GPS Radio Occultation Data

Monthly Weather Review ◽

10.1175/2010mwr3260.1 ◽

2011 ◽

Vol 139 (3) ◽

pp. 853-865 ◽

Cited By ~ 19

Author(s):

Shu-Ya Chen ◽

Ching-Yuang Huang ◽

Ying-Hwa Kuo ◽

Sergey Sokolovskiy

Keyword(s):

Radio Occultation ◽

Specific Humidity ◽

Forecast Errors ◽

Maximum Magnitude ◽

Observational Error ◽

Error Statistics ◽

Gps Radio Occultation ◽

Horizontal Gradients ◽

Excess Phase ◽

Observational Errors

Abstract The Global Positioning System (GPS) radio occultation (RO) technique is becoming a robust global observing system. GPS RO refractivity is typically modeled at the ray perigee point by a “local refractivity operator” in a data assimilation system. Such modeling does not take into account the horizontal gradients that affect the GPS RO refractivity. A new observable (linear excess phase), defined as an integral of the refractivity along some fixed ray path within the model domain, has been developed in earlier studies to account for the effect of horizontal gradients. In this study, the error statistics of both observables (refractivity and linear excess phase) are estimated using the GPS RO data from the Formosa Satellite 3–Constellation Observing System for Meteorology, Ionosphere and Climate (FORMOSAT-3/COSMIC) mission. The National Meteorological Center (NMC) method, which is based on lagged forecast differences, is applied for evaluation of the model forecast errors that are used for estimation of the GPS RO observational errors. Also used are Weather Research and Forecasting (WRF) model forecasts in the East Asia region at 45-km resolution for one winter month (mid-January to mid-February) and one summer month (mid-August to mid-September) in 2007. Fractional standard deviations of the observational errors of refractivity and linear excess phase both show an approximately linear decrease with height in the troposphere and a slight increase above the tropopause; their maximum magnitude is about 2.2% (2.5%) for refractivity and 1.1% (1.3%) for linear excess phase in the lowest 2 km for the winter (summer) month. An increase of both fractional observational errors near the surface in the summer month is attributed mainly to a larger amount of water vapor. The results indicate that the fractional observational error of refractivity is about twice as large as that of linear excess phase, regardless of season. The observational errors of both linear excess phase and refractivity are much less latitude dependent for summer than for winter. This difference is attributed to larger latitudinal variations of the specific humidity in winter.

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Global 3D Features of Error Variances of GPS Radio Occultation and Radiosonde Observations

Remote Sensing ◽

10.3390/rs13010001 ◽

2020 ◽

Vol 13 (1) ◽

pp. 1

Author(s):

Xu Xu ◽

Xiaolei Zou

Keyword(s):

Radio Occultation ◽

Weather Prediction ◽

Lower Troposphere ◽

Error Variance ◽

Specific Humidity ◽

Temperature Error ◽

Observation Error ◽

Bending Angle ◽

Gps Radio Occultation ◽

Low Latitudes

Global Positioning System (GPS) radio occultation (RO) and radiosonde (RS) observations are two major types of observations assimilated in numerical weather prediction (NWP) systems. Observation error variances are required input that determines the weightings given to observations in data assimilation. This study estimates the error variances of global GPS RO refractivity and bending angle and RS temperature and humidity observations at 521 selected RS stations using the three-cornered hat method with additional ERA-Interim reanalysis and Global Forecast System forecast data available from 1 January 2016 to 31 August 2019. The global distributions, of both RO and RS observation error variances, are analyzed in terms of vertical and latitudinal variations. Error variances of RO refractivity and bending angle and RS specific humidity in the lower troposphere, such as at 850 hPa (3.5 km impact height for the bending angle), all increase with decreasing latitude. The error variances of RO refractivity and bending angle and RS specific humidity can reach about 30 N-unit2, 3 × 10−6 rad2, and 2 (g kg−1)2, respectively. There is also a good symmetry of the error variances of both RO refractivity and bending angle with respect to the equator between the Northern and Southern Hemispheres at all vertical levels. In this study, we provide the mean error variances of refractivity and bending angle in every 5°-latitude band between the equator and 60°N, as well as every interval of 10 hPa pressure or 0.2 km impact height. The RS temperature error variance distribution differs from those of refractivity, bending angle, and humidity, which, at low latitudes, are smaller (less than 1 K2) than those in the midlatitudes (more than 3 K2). In the midlatitudes, the RS temperature error variances in North America are larger than those in East Asia and Europe, which may arise from different radiosonde types among the above three regions.

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Parallelization Strategies for the GPS Radio Occultation Data Assimilation with a Nonlocal Operator in the Weather Research and Forecasting Model

Journal of Atmospheric and Oceanic Technology ◽

10.1175/jtech-d-13-00195.1 ◽

2014 ◽

Vol 31 (9) ◽

pp. 2008-2014 ◽

Cited By ~ 2

Author(s):

Xin Zhang ◽

Ying-Hwa Kuo ◽

Shu-Ya Chen ◽

Xiang-Yu Huang ◽

Ling-Feng Hsiao

Keyword(s):

Data Assimilation ◽

Radio Occultation ◽

Weather Prediction ◽

Weather Research And Forecasting ◽

Forecasting Model ◽

Observation Operator ◽

Gps Radio Occultation ◽

Phase Observation ◽

Excess Phase ◽

Weather Research

Abstract The nonlocal excess phase observation operator for assimilating the global positioning system (GPS) radio occultation (RO) sounding data has been proven by some research papers to produce significantly better analyses for numerical weather prediction (NWP) compared to the local refractivity observation operator. However, the high computational cost and the difficulties in parallelization associated with the nonlocal GPS RO operator deter its application in research and operational NWP practices. In this article, two strategies are designed and implemented in the data assimilation system for the Weather Research and Forecasting Model to demonstrate the capability of parallel assimilation of GPS RO profiles with the nonlocal excess phase observation operator. In particular, to solve the parallel load imbalance problem due to the uneven geographic distribution of the GPS RO observations, round-robin scheduling is adopted to distribute GPS RO observations among the processing cores to balance the workload. The wall clock time required to complete a five-iteration minimization on a demonstration Antarctic case with 106 GPS RO observations is reduced from more than 3.5 h with a single processing core to 2.5 min with 106 processing cores. These strategies present the possibility of application of the nonlocal GPS RO excess phase observation operator in operational data assimilation systems with a cutoff time limit.

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The Three-Cornered Hat Method for Estimating Error Variances of Three or More Atmospheric Data Sets – Part II: Evaluating Radio Occultation and Radiosonde Observations, Global Model Forecasts, and Reanalyses

Journal of Atmospheric and Oceanic Technology ◽

10.1175/jtech-d-20-0209.1 ◽

2021 ◽

Author(s):

Therese Rieckh ◽

Jeremiah P. Sjoberg ◽

Richard A. Anthes

Keyword(s):

Radio Occultation ◽

State Of The Art ◽

Specific Humidity ◽

Data Sets ◽

Error Growth ◽

Atmospheric Conditions ◽

Error Statistics ◽

Weather And Climate ◽

Atmospheric Data ◽

The Impact

AbstractWe apply the three-cornered hat (3CH) method to estimate refractivity, bending angle, and specific humidity error variances for a number of data sets widely used in research and/or operations: radiosondes, radio occultation (COSMIC, COSMIC-2), NCEP global forecasts, and nine reanalyses. We use a large number and combinations of data sets to obtain insights into the impact of the error correlations among different data sets that affect 3CH estimates. Error correlations may be caused by actual correlations of errors, representativeness differences, or imperfect co-location of the data sets. We show that the 3CH method discriminates among the data sets and how error statistics of observations compare to state-of-the-art reanalyses and forecasts, as well as reanalyses that do not assimilate satellite data. We explore results for October and November 2006 and 2019 over different latitudinal regions and show error growth of the NCEP forecasts with time. Because of the importance of tropospheric water vapor to weather and climate, we compare error estimates of refractivity for dry and moist atmospheric conditions.

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Sounding Heavy Precipitating Vertical Cloud Structures with Polarimetric Radio Occultations aboard PAZ

10.5194/egusphere-egu21-9405 ◽

2021 ◽

Author(s):

Ramon Padullés ◽

Estel Cardellach ◽

F. Joseph Turk ◽

Chi O. Ao ◽

Kuo Nung Wang ◽

...

Keyword(s):

Radio Occultation ◽

Deep Convection ◽

Heavy Precipitation ◽

Current Data ◽

Data Availability ◽

The Status ◽

Potential Applications ◽

Initial Hypothesis ◽

Forecast Models ◽

Electromagnetic Signals

The Radio Occultation and Heavy Precipitation (ROHP) experiment aboard the Spanish PAZ satellite was activated in May 2018 with the objective to demonstrate the Polarimetric Radio Occultation (PRO) concept for rain detection. This technique enhances standard RO by measuring GNSS signals at two orthogonal linear polarizations (H and V). Owing to hydrometeor asymmetry, electromagnetic signals propagating through regions of heavy precipitation would experience a differential phase delay expected to be measurable by the ROHP experiment. After 2+ years of operations, the initial hypothesis has been verified and the main scientific goals have been achieved. Soon after the activation of the experiment it became clear that PRO observables were sensitive to heavy precipitation, showing positive signatures correlated with the presence and intensity of precipitation. After a thorough on-orbit calibration, it has been demonstrated that the PAZ polarimetric observable can be used as a proxy for heavy precipitation. Furthermore, PRO measurements were shown to be sensitive to the horizontally oriented frozen hydrometeors present throughout the vertical cloud extent, providing valuable information on the vertical structure of precipitating clouds. In addition, PRO can retrieve standard thermodynamic RO products such as temperature, pressure, and water vapor. These products, provided with high vertical resolution, globally distributed and seamlessly over ocean and over land, make PRO observations a unique dataset, with potential applications ranging from the study of deep convection processes to the evaluation and diagnosis of NWP forecast models. In this presentation we will report on the status of the experiment and current data availability. We will also show the results of the sensitivity studies to heavy precipitation and frozen particles, performed using collocated observations between PAZ and GPM-DPR, GPM-GMI, and other radiometers from the GPM constellation, as well as a-priory information from the Cloudsat radar. Finally, we will address potential level-2 products we can expect from PAZ observations.

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Estimation and evaluation of COSMIC radio occultation excess phase using undifferenced measurements

Atmospheric Measurement Techniques ◽

10.5194/amt-10-1813-2017 ◽

2017 ◽

Vol 10 (5) ◽

pp. 1813-1821

Author(s):

Pengfei Xia ◽

Shirong Ye ◽

Kecai Jiang ◽

Dezhong Chen

Keyword(s):

Radio Occultation ◽

Weather Prediction ◽

Lower Troposphere ◽

Double Difference ◽

Cosmic Radio ◽

Processing Strategy ◽

Single Difference ◽

Excess Phase ◽

Mean Square Errors ◽

Better Than

Abstract. In the GPS radio occultation technique, the atmospheric excess phase (AEP) can be used to derive the refractivity, which is an important quantity in numerical weather prediction. The AEP is conventionally estimated based on GPS double-difference or single-difference techniques. These two techniques, however, rely on the reference data in the data processing, increasing the complexity of computation. In this study, an undifferenced (ND) processing strategy is proposed to estimate the AEP. To begin with, we use PANDA (Positioning and Navigation Data Analyst) software to perform the precise orbit determination (POD) for the purpose of acquiring the position and velocity of the mass centre of the COSMIC (The Constellation Observing System for Meteorology, Ionosphere and Climate) satellites and the corresponding receiver clock offset. The bending angles, refractivity and dry temperature profiles are derived from the estimated AEP using Radio Occultation Processing Package (ROPP) software. The ND method is validated by the COSMIC products in typical rising and setting occultation events. Results indicate that rms (root mean square) errors of relative refractivity differences between undifferenced and atmospheric profiles (atmPrf) provided by UCAR/CDAAC (University Corporation for Atmospheric Research/COSMIC Data Analysis and Archive Centre) are better than 4 and 3 % in rising and setting occultation events respectively. In addition, we also compare the relative refractivity bias between ND-derived methods and atmPrf profiles of globally distributed 200 COSMIC occultation events on 12 December 2013. The statistical results indicate that the average rms relative refractivity deviation between ND-derived and COSMIC profiles is better than 2 % in the rising occultation event and better than 1.7 % in the setting occultation event. Moreover, the observed COSMIC refractivity profiles from ND processing strategy are further validated using European Centre for Medium-Range Weather Forecasts (ECMWF) analysis data, and the results indicate that the undifferenced method reduces the noise level on the excess phase paths in the lower troposphere compared to the single-difference processing strategy.

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Atmospheric Factors Governing Banded Orographic Convection

Journal of the Atmospheric Sciences ◽

10.1175/jas3568.1 ◽

2005 ◽

Vol 62 (10) ◽

pp. 3758-3774 ◽

Cited By ~ 36

Author(s):

Daniel J. Kirshbaum ◽

Dale R. Durran

Keyword(s):

Three Dimensional ◽

Heavy Precipitation ◽

Thermal Fluctuations ◽

Dominant Mode ◽

Vertical Shear ◽

Dimensional Structure ◽

Small Scale ◽

Atmospheric Structure ◽

Orographic Convection ◽

Orographic Cloud

Abstract The three-dimensional structure of shallow orographic convection is investigated through simulations performed with a cloud-resolving numerical model. In moist flows that overcome a given topographic barrier to form statically unstable cap clouds, the organization of the convection depends on both the atmospheric structure and the mechanism by which the convection is initiated. Convection initiated by background thermal fluctuations embedded in the flow over a smooth mountain (without any small-scale topographic features) tends to be cellular and disorganized except that shear-parallel bands may form in flows with strong unidirectional vertical shear. The development of well-organized bands is favored when there is weak static instability inside the cloud and when the dry air surrounding the cloud is strongly stable. These bands move with the flow and distribute their cumulative precipitation evenly over the mountain upslope. Similar shear-parallel bands also develop in flows where convection is initiated by small-scale topographic noise superimposed onto the main mountain profile, but in this case stronger circulations are also triggered that create stationary rainbands parallel to the low-level flow. This second dominant mode, which is less sensitive to the atmospheric structure and the strength of forcing, is triggered by lee waves that form over small-scale topographic bumps near the upstream edge of the main orographic cloud. Due to their stationarity, these flow-parallel bands can produce locally heavy precipitation amounts.

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Using Radio Occultation to Detect Clouds in the Middle and Upper Troposphere

Journal of Atmospheric and Oceanic Technology ◽

10.1175/jtech-d-21-0022.1 ◽

2021 ◽

Author(s):

Jeana Mascio ◽

Stephen S. Leroy ◽

Robert P. d’Entremont ◽

Thomas Connor ◽

E. Robert Kursinski

Keyword(s):

Relative Humidity ◽

High Performance ◽

Radio Occultation ◽

Three Dimensional ◽

Roc Curves ◽

Lapse Rate ◽

Cloud Detection ◽

Upper Troposphere ◽

Temperature Lapse Rate ◽

Direct Sensitivity

AbstractRadio occultation (RO) measurements have little direct sensitivity to clouds, but recent studies have shown that they may have an indirect sensitivity to thin, high clouds that are difficult to detect using conventional passive space-based cloud sensors. We implement two RO-based cloud detection (ROCD) algorithms for atmospheric layers in the middle and upper troposphere. The first algorithm is based on the methodology of a previous study, which explored signatures caused by upper tropospheric clouds in RO profiles according to retrieved relative humidity, temperature lapse rate, and gradients in log-refractivity (ROCD-P), and the second is based on inferred relative humidity alone (ROCD-M). In both, atmospheric layers are independently predicted as cloudy or clear based on observational data, including high performance RO retrievals. In a demonstration, we use data from 10 days spanning seven months in 2020 of FORMOSAT-7/COSMIC-2. We use the forecasts of NOAA GFS to aid in the retrieval of relative humidity. The prediction is validated with a cloud truth dataset created from the imagery of the GOES-16 Advanced Baseline Imager (ABI) satellite and the GFS three-dimensional analysis of cloud state conditions. Given these two algorithms for the presence or absence of clouds, confusion matrices and receiver operating characteristic (ROC) curves are used to analyze how well these algorithms perform. The ROCD-M algorithm has a balanced accuracy, which defines the quality of the classification test that considers both the sensitivity and specificity, greater than 70% for all altitudes between 6 and 10.25 km.

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Transparent Object Shape Measurement Based on Deflectometry

Proceedings ◽

10.3390/icem18-05428 ◽

2018 ◽

Vol 2 (8) ◽

pp. 548

Author(s):

Zhichao Hao ◽

Yuankun Liu

Keyword(s):

Data Acquisition ◽

Phase Measurement ◽

Focal Length ◽

Three Dimensional ◽

Normal Direction ◽

Calibration Data ◽

Actual Measurement ◽

Temporal Phase ◽

Convex Lens ◽

Normal Orientation

This paper proposes a method for obtaining surface normal orientation and 3-D shape of plano-convex lens using refraction stereo. We show that two viewpoints are sufficient to solve this problem under the condition that the refractive index of the object is known. What we need to know is that (1) an accurate function that maps each pixel to the refraction point caused by the refraction of the object. (2) light is refracted only once. In the simulation, the actual measurement process is simplified: light is refracted only once; and the accurate one-to-one correspondence between incident ray and refractive ray is realized by known object points. The deformed grating caused by refraction point is also constructed in the process of simulation. A plano-convex lens with a focal length of 242.8571 mm is used for stereo data acquisition, normal direction acquisition, and the judgment of normal direction consistency. Finally, restoring the three-dimensional information of the plano-convex lens by computer simulation. Simulation results suggest that our method is feasibility. In the actual experiment, considering the case of light is refracted more than once, combining the calibration data acquisition based on phase measurement, phase-shifting and temporal phase-unwrapping techniques to complete (1) calibrating the corresponding position relationship between the monitor and the camera (2) matching incident ray and refractive ray.

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