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In this study, we propose a methodology for training a deep learning model using historical time series data to predict current values. Using 11,942 past meteorological observations collected over a specific period from the Osan weather station in Gyeonggi-do, we predicted the atmospheric radio refractivity at the current time. As a result of predicting the atmospheric radio refractivity at 00Z on August 4, 2023, and August 5, 2023, at the Osan weather station in Gyeonggi-do, the average MAPE(mean absolute percentage error) and ARIMA(autoregressive integrated moving average) of the proposed model were 2.75 % and 7.61 %, respectively. [ABSTRACT FROM AUTHOR]

In this study, we propose a model for predicting atmospheric refractivity using meteorological data and deep learning. The purpose of this study is to compare the prediction accuracy of traditional interpolation methods and the proposed model, verify whether the deep learning model trained on meteorological data can provide values closer to the true values, and thus demonstrate the potential for utilizing deep learning in predicting atmospheric refractivity. [ABSTRACT FROM AUTHOR]

The quality of services provided via Inter-Terrestrial radio communication links such as GSM networks, Wide Area Network (WAN), Radio and TV broadcasts is largely influenced by some meteorological parameters such as temperature, pressure and humidity. Proper knowledge of these parameters, specifically at microwave antenna heights (about 70m) is important in order to maintain an effective line-of-sight (LOS) link even during the worst weather conditions. The geoclimatic factor is an important quantity that must be considered in the design of terrestrial links for effective wireless communication. This work utilized satellite data from the European Center for Medium-Range Weather Forecasts (ECMWF) to compute the point refractivity gradient and geoclimatic factor for Yenagoa and its environs. The research was necessitated by the paucity of research on this subject matter for Yenegoa. The results of the research show that point refractivity gradient and geoclimatic factor in the study area vary with season. The average point refractivity gradient and geoclimatic factor at 70 m above the ground level are:136.433 N-unit/Km and 6.638633E-05 respectively. This implies that radiowaves propagating in this region at the said altitude is most likely to be super refractive in both rain and clear air atmospheric conditions. Rain or worst condition refers to the period when atmospheric components such as hydrometeor, lithometeor, aerosol have significant effects on propagated radio signals. Clear-air conditions means when maximum possible signal is received such that the most threatening atmospheric components (rain drops) have negligible effects on propagated signal. The results will be useful for radio engineers in the design and configuration of inter-terrestrial microwave links in Yenagoa and Its environs for optimum quality of service. [ABSTRACT FROM AUTHOR]

Dynamic refractive environments within the marine atmospheric boundary layer (MABL) pose difficulties in the prediction of X‐band radar wave propagation due to natural phenomena such as evaporation ducts (ED). This study utilizes a unique data set collected during the Coupled Air‐Sea Processes and Electromagnetic Ducting Research (CASPER)‐East field campaign, including multiple refractivity estimation methods and twelve point‐to‐point (PTP) electromagnetic datasets, to assess the efficacy of PTP inversion techniques for remote sensing of atmospheric refractivity within the MABL. Comparison of refractivity between the inverse and other refractivity methods show reasonable evaporation duct height estimates by the inversion, and inverse‐based propagation predictions are also shown to be more accurate than propagation based on other refractivity prediction methods: numerical weather prediction, theory, and in situ atmospheric measurements. These results propose the effectiveness of a PTP metaheuristic radar inversion to remotely sense refractive environments from radar propagation measurements in stable and unstable atmospheric conditions. Plain Language Summary: Continually varying atmospheric properties in the marine environment, especially at altitudes within 100 m of the surface, cause radar waves to deviate from expected patterns resulting in positioning uncertainties and other adverse effects. Measurements and mathematical models are commonly implemented to understand atmospheric structure, where data from these models or measurements are used within radar simulations to evaluate the impact of such conditions on radar. This study demonstrates the application of a technique that models the atmosphere in a reverse manner‐determining the atmospheric structure based on radar wave propagation measurements that are impacted by this structure. The radar measurements used in this study were collected in conjunction with measured and modeled atmospheric data, which are used to verify the results of the reverse methodology, a unique data set for this application. We find that remote sensing of the atmosphere based on propagation measurements generates adequate representation of the atmospheric structure. The highlighted technique also most accurately predicts the propagation measurements as compared to more common atmospheric measurement and modeling techniques. Thus, this method of determining atmospheric conditions shows merit for sensing of the environment in order to enable corrections of a radar system, paving the way for improvements in associated technologies. Key Points: Inversion employing a genetic algorithm solves for refractivity from point‐to‐point radar propagation measurements in a marine environmentInversion results scrutinized by comparison with other refractivity sources: numerical weather prediction, theory, and in situ measurementsInversion solutions adequately remotely sense atmospheric refractivity in unstable and stable atmospheric regimes [ABSTRACT FROM AUTHOR]

The inner scale and the refractive-index structure parameter C2n are estimated by the intensity scintillations of the laser beam propagated through the turbulent water layer. The laser beam scintillations caused by underwater propagation can be described in the framework of a turbulence model with accounting of the inner scale. An intensity field has been observed at a 2m distance when the laser beam passed through a water convective cell with a characteristic Rayleigh number of about 108. Similar intensity fields were simulated by using the split-step method. Characteristics of the experimentally obtained and numerically simulated intensity fields are compared and analyzed. A simple method of the turbulent parameter estimation is proposed. [ABSTRACT FROM AUTHOR]

Surface radio refractive gradient and k-factor are very imperative in observing propagation conditions, designing and planning of microwave communication links in the lower part of the atmospheric boundary layer. The measurement of weather variables (air temperature, atmospheric pressure, relative humidity and wind speed) were conducted in this study at the School of Science, Emmanuel Alayande College of Education, Oyo (7.83°N, 3.95°E), Oyo State, Nigeria. The research was carried out for a period of one year (January to December, 2020) using self-designed cost effective portable weather monitoring systems. The systems were sited from the ground to a height of 200 m on a 220 m Nigeria Television Authority (NTA) UHF channel 37 tower at Oke-Apitipiti in Oyo Town of Oyo State. The calculated daily and monthly averages data are employed to investigate the vertical surface radio refractivity and its refractivity gradient. The highest value of -1.093E+26 N-units/km was recorded in January, while the month of July experienced the least of about -9.305E+19 N-units/km. The months of January–July verified sub-refractive conditions with propagation conditions having varying degree of occurrence. On the other hand, super-refraction and ducting were recorded mostly between August–December from the study. The results also find applicability in radio engineering for refining VHF/UHF terrestrial links based on clear-air considerations which will support operational planning of terrestrial radio networks in Oyo, South Western Nigeria. [ABSTRACT FROM AUTHOR]

This paper evaluates atmospheric refractivity observations from the COSMIC‐2 (C2) radio occultation (RO) mission by comparing them with radiosonde (RS) observations, the European Center for Medium‐Range Weather Forecasts (ECMWF) ERA5 reanalysis, and the National Centers for Environmental Prediction (NCEP) Global Forecast System (GFS) analysis for the period October 2019 to December 2020. C2 refractivity is overall in good agreement with the RS observations as well as the GFS and ERA5 analyses. The mean and standard deviations of the differences between C2 and RS, ERA5, and GFS refractivity are within ±0.3% and less than 2% respectively, with slightly higher differences (both systematic and random), in the lower levels of the troposphere. Plain Language Summary: After evaluation of C2 refractivities, assimilation and forecast experiments were conducted to determine the potential value of C2 in weather research and forecast (WRF) model during the Indian summer monsoon (June–September) 2020. The model results were validated using RS observations and the ERA5 reanalysis. The use of C2 refractivity observations reduced the model analysis and forecast errors (reduction of root‐mean‐square errors by ∼8–10%) in mid‐upper tropospheric temperature, moisture, and winds. The precipitation prediction skill, particularly of high amounts of precipitation, increased by as much as 20% with the assimilation of C2 refractivity. Finally, a revised refractivity forward operator and its associated tangent and adjoint operators are developed and implemented in the WRF assimilation system. The revised forward operator outperforms the existing operator for refractivity assimilation. Key Points: Evaluated the quality of COSMIC‐2 radio occultation (RO) refractivity with radiosonde observations and two model analysesDeveloped an improved observation operator for the assimilation of RO refractivity in the weather research and forecast (WRF) modelCOSMIC‐2 refractivity observations have a positive impact on a regional numerical weather prediction system [ABSTRACT FROM AUTHOR]

Environmental predictions in the marine atmospheric surface layer (MASL) are imperative to optimize X‐band radar system performance in marine environments. Evaporation ducts (ED) lead to anomalous propagation where characterization of EDs in the MASL occurs primarily through two methods: in‐situ measurements and numerical modeling. This study investigates the differences in co‐located and synchronous refractivity estimations from the CASPER‐East campaign. Propagation predictions are generated for refractive profiles from in‐situ measurements, Monin‐Obukov boundary layer similarity theory, and numerical weather prediction forecasts. Variations in evaporation duct height (EDH) are found to be a primary driver of differences in propagation between the estimated refractivity profiles, where location of the EDH relative to the transmitter changes the sensitivity of propagation predictions to EDH estimates. Differences in propagation are large when EDH estimates span the transmitter height and the lowest EDH across the methods is small, regardless of how much variation there is in EDH estimates. When the lowest EDH is small and EDH estimates span the transmitter height there are differences in physical regimes causing large propagation discrepancies–for example, leakage into versus trapping within the duct. Variation in EDH between the methods is greatest in stable environments. M‐deficit and curvature of the refractive profiles also influence propagation specifically in scenarios when EDH spans the transmitter. When all EDHs are below the transmitter, EDH variance is the primary contributor to propagation variance, but M‐deficit and profile curvature variance play a secondary role. M‐deficits and curvature between the methods agree most often during periods of atmospheric stability. Key Points: Refractivity generated from models and measured data are compared to explore influences on radar wave propagationProximity of duct height to transmitter and its variance have large effects on propagation; typically occurring during stable conditionsM‐deficit and profile curvature affect propagation when duct heights span the transmitter; largest variations occur in unstable conditions [ABSTRACT FROM AUTHOR]

Refractivity-from-clutter (RFC) is a technique for remote sensing of the lowest portion of vertical atmospheric refractivity profile, particularly evaporation and surface-based ducts. It uses the sea-surface backscattered radar signal to estimate the ducting conditions. The Lower Atmospheric Propagation (LATPROP) radar is a software-defined, coherent-on-receive remote sensing system built on a low-cost, commercial off-the-shelf marine radar platform operating at 9.41 GHz. The system is designed to specifically operate as a RFC-capable platform sensitive to low clutter power returns from the sea surface. This paper examines the appropriate radar design, system modifications, and radar signal processing steps, and provides a comparison with existing RFC-capable systems. The LATPROP system was deployed during the Coupled Air-Sea Processes and electromagnetic Ducting Research West Campaign that took place off of the coast of Southern California during October 2017 and was shown to accurately capture the effects of the ducting conditions. [ABSTRACT FROM AUTHOR]

Thirty‐nine years of archived meteorological data measured at two stations located in the northern and southern parts of Quebec, Canada are used to estimate the surface refractivity and its dry and wet components. The results of the comparison of the obtained estimates showed that for all months the values of the dry component are higher in the northern part, whereas the values of the wet component are higher in the southern part. Due to this, for several months of the year, the values of the surface refractivity are higher in the northern part and for the remaining months in the southern part. Moreover, in both parts, August is the month where the highest values of the surface refractivity were recorded. In this particular month, the slope of the surface refractivity trend in the northern part is several times higher than that in the southern part. The obtained results show that the performance of the used direct smoothing forecasting technique depends on the deviation between the values of N in the current year and the previous year. Key Points: The study shows that the dry Nd and wet Nw components of the surface refractivity have different mean monthly variability in north and south of Quebec. For all months, the values of Nd are higher in the northern part, whereas the values of Nw are higher in the southern partSince the water vapor pressure in Montreal undergoes more significant variation than in Kuujjuaq, for the wet component, there is a significant difference between the variations observed in Montreal than in KuujjuaqThe direct smoothing, a forecasting technique which is efficient computationally has been used to estimate the future values of N in Kuujjuaq [ABSTRACT FROM AUTHOR]

In this paper, local meteorological data of one year have been used to calculate the surface atmospheric radio refractivity (N) and estimate the vertical refractivity gradient (dN1) as well as the geoclimatic factor (K) in the lowest atmospheric layer above the ground surface in the station Kuujjuaq (Quebec, Canada). In this region, the climate is arctic, characterized by very long and very cold winters (on average the temperature is below -20◦C for almost 240 days per year). The precipitations are almost nonexistent, and the vegetation is scarce. Average daily, monthly, seasonal, and yearly variations of N, dN1, and K are estimated and analysed. The obtained values of these indices are compared to the corresponding values provided by the ITU. The results show that the more negative values of dN 1 lie in the summer season. This is mainly due to the important variations of the temperature and humidity during this season. However, the estimated values lie in the limits mostly corresponding to standard refraction. [ABSTRACT FROM AUTHOR]

The meteorological elements will affect the propagation of the radio wave. These meteorological elements can be reflected by the atmospheric refractive index, which is derived from the pressure, temperature, relative humidity, and vapor pressure. Using the atmospheric sounding data, a real atmospheric refractive index model for an identified area can be obtained. Combined with the refractive index model, the atmosphere can be divided into layered media with different refractive index, and a numerical model can be established to indicate the wave propagation path. The numerical solution results are verified by the analytical solution and other presented model. Due to its high accuracy and computational efficiency, the proposed numerical model can be used to quickly illustrate the physical path of radio wave atmospheric propagation, providing theoretical guidance for related applications. [ABSTRACT FROM AUTHOR]

The redistribution of thin-film waveguiding emitted light from an organic electroluminescent (EL) layer using high refractive index substrates has been investigated. The internally emitted light can be classified in terms of three modes: external, substrate, and indium-tin-oxide (ITO)/organic modes. According to classical ray optics theory, the ITO/organic mode emission can be completely redistributed to the substrate mode when the refractive index of the substrate becomes larger than that of the emitting layer. The redistributed substrate mode emission should be able to be easily extracted by an uneven surface, such as a microlens array. With this understanding, we prepared organic EL devices in which the substrate refractive index was varied over a range. However, redistribution of the ITO/organic mode emission to the substrate mode was not observed experimentally in our EL devices. In addition, there was no difference in the luminous intensity improvement between glass (n=1.52) substrate and high refractive index (n=1.72) resin substrate using identical microlens array. The improvement in luminous intensity was around a factor of 1.5 in both cases. [ABSTRACT FROM AUTHOR]

A technique for viscosity measurement was developed based on the principle of laser-induced surface deformation. Light incident into liquids increases its momentum due to the difference in refractive index and gives the surface an upward force as a reaction. The plane surface thus swells up and deforms, and the shape is determined so that the force is balanced with the surface tension and the gravity. On sudden laser irradiation, the deformation inevitably accompanies a viscous flow and exhibits a relaxational behavior with a delay time, which gives the viscosity. Theoretical prediction of the step-response function was given that takes surface tension waves excited by the laser into consideration. Nd–yttritium–aluminum–garnet laser with 0.6 W output was focused to ∼200 μm beam waist and used for the pumping. The deformation process was observed sensitively with another probe laser illuminating the activated area. This system was tested with the standard liquids for viscosity ranging from 1 to 106 cSt. The results demonstrated the validity of this technique, though a correction for the inertia effect was needed in the range lower than 10 cSt. Further, effect of the thermal expansion by a slight optical absorption was discussed. This technique is especially useful at high viscosities since the measurement takes only a few seconds even in the specimen with 106 cSt. Besides the rapidity, it has a great advantage of a noncontact feature and is appropriate for measuring the liquids that strongly dislike contamination. It has also potential applications in industries, measurement of liquids isolated in a production line, for instance. [ABSTRACT FROM AUTHOR]

We report on the direct measurement of the electrogyration coefficient of Bi12GeO20 sillenite crystals over the visible spectral range. The coefficient is directly measured on a (111)-cut crystal for which the electro-optic and electrogyratory effects are decoupled. The precision of the measurement method over various parameters including absorption and misorientation is examined. The value of the electrogyration coefficient is found to vary from 0.37±0.03 to 0.05±0.01 pm/V for the 460–760-nm spectrum range, which is within the upper boundaries that have been specified in previous experiments. It is also found that the electrogyration coefficient follows the dispersion law of optical activity and refractive index. [ABSTRACT FROM AUTHOR]

A high-quality smart filter for terahertz range with relative tunability reaching 20% has been demonstrated. The filter is based on a narrow transmission band, which originates from a defect mode that appears due to insertion of a single crystal of KTaO3 into otherwise periodic one-dimensional photonic crystal. Frequencies of defect modes are controlled by the refractive index of the defect: their high tunability is achieved by the strong temperature dependence of the dielectric properties of KTaO3. The low losses of KTaO3 lead to a high peak transmission of the filter. Influence of the defect losses on the properties of the filter is also discussed. [ABSTRACT FROM AUTHOR]

The alignment of nematic liquid crystals in a hybrid cell can be easily reconfigured by applying an external electric field. We exploit this feature to realize a waveguiding structure whose refractive index profile can be dynamically reshaped. The mode structure of the guided light, investigated by m-lines spectroscopy at various electric field strenghts, can be well predicted by a parameter-free model based on the elastic, dielectric, and optical properties of the liquid crystal. According to such a model, the field profiles of the modes supported by hybrid nematic waveguides display very peculiar features. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The authors report on the detection of gamma ray bursts in lightning by the Terrestrial gamma ray flash and Energetic Thunderstorm Rooftop Array (TETRA)-II. They present a description of the (TETRA)-II instrumentation, the correlations with other lightning detection networks, and the connection between the lightning and the detection of gamma rays.

The turbulent nature of the marine atmospheric boundary layer and interactions across the air‐sea interface cause ever‐changing environmental conditions, including atmospheric properties that affect the index of refraction, or atmospheric refractivity. Variations in atmospheric refractivity lead to many types of anomalous propagation phenomena of electromagnetic (EM) signals; thus, improving performance of EM systems requires in situ knowledge of the refractivity. Inversion approaches to estimate refractivity rely on measured EM data; however, despite its importance, few studies have examined the influence of data density on the accuracy of refractivity inversions. This study applies a bistatic radar data inversion process to estimate atmospheric refractivity parameters in evaporative ducting conditions and examines the impacts of the sampling density of radar propagation loss data, and its source location, on accuracy of refractivity inversions. Genetic algorithms and a radar propagation model are used to perform the inversions. Numerical experiments examine various randomly distributed amounts of synthetic data from a 100‐m (altitude) by 60‐km (range) area. Three domains within this area are examined from which data were sourced. A data density of approximately 1% of the prediction domain yielded the smallest errors of refractivity parameters, and root mean square errors of refractivity and propagation loss. These error reductions are attributed to avoidance of nonunique solutions that likely impact lower data densities, supported by their classification as a misleading or difficult inverse problem. Generally, data sourced from long range result in lower refractivity and propagation loss root mean square errors compared to data sourced from other domains. Key Points: The density of data, randomly sampled within different domains, is varied to examine its affect on accuracy of refractivity inversionsRefractivity inversion accuracy is evaluated by errors in refractivity parameters, and root mean square errors of refractivity and propagationData density of approximately 1% of the propagation prediction space yielded the smallest refractivity errors and differences in propagation [ABSTRACT FROM AUTHOR]

In this paper, local meteorological data for a period of 35 years (from 1979 to 2013) from Kuujuaq station have been used to calculate the surface refractivity, N (a link for the data is available in the acknowledgements), and to estimate the vertical refractivity gradient, dN1, in the lowest atmospheric layer above the ground. Monthly and yearly variations of the mean of N and dN1 are provided. The values obtained are compared with the corresponding values from the ITU maps. The long‐term trend of the surface refractivity is also investigated. The data demonstrate that the indices N and dN1 are subject to an evolution that may have significance in the context of climate change. Monthly means of N show an increasing departure from ITU‐R values since 1990. Yearly mean values of the dN1 show a progressive decrease over the period of study. Seasonal means of dN1 show a decrease over time, especially for summer. Such a trend may increase the occurrence of superrefraction. However, currently available ITU‐R recommendations for microwave link design assume a stationary climate, so there is a need for a new modeling approach. Key Points: Surface refractivity and its gradient were calculated using collected local meteorological data for a period of 35 years in the lowest atmospheric layer above the groundThe evolution of the surface refractivity has two cycles; in one cycle, the evolution depends mainly on the inverse of the temperature; in the second cycle, this evolution depends mainly on the water vapor pressure, that is, humidityThe results of the analysis show that the linear trends of the yearly variations of the vertical refractivity gradient over the analyzed period decay from year to year; such a trend may increase the occurrence of superrefraction; note that this observation is based on the estimated values of dN1; in the future, we hope that the measured values of local meteorological data will be available [ABSTRACT FROM AUTHOR]

This paper presents a new method to estimate the height of the atmospheric boundary layer (ABL) by using COSMIC radio occultation bending angle (BA) data. Using the numerical differentiation method combined with the regularization technique, the first derivative of BA profiles is retrieved, and the height at which the first derivative of BA has the global minimum is defined to be the ABL height. To reflect the reliability of estimated ABL heights, the sharpness parameter is introduced, according to the relative minimum of the BA derivative. Then, it is applied to four months of COSMIC BA data (January, April, July, and October in 2008), and the ABL heights estimated are compared with two kinds of ABL heights from COSMIC products and with the heights determined by the finite difference method upon the refractivity data. For sharp ABL tops (large sharpness parameters), there is little difference between the ABL heights determined by different methods, i.e., the uncertainties are small; whereas, for non-sharp ABL tops (small sharpness parameters), big differences exist in the ABL heights obtained by different methods, which means large uncertainties for different methods. In addition, the new method can detect thin ABLs and provide a reference ABL height in the cases eliminated by other methods. Thus, the application of the numerical differentiation method combined with the regularization technique to COSMIC BA data is an appropriate choice and has further application value. [ABSTRACT FROM AUTHOR]

To increase the sensitivity of optical interferometry for diagnostic of gas/plasma jet density with weak refractivity, we realize several multi-pass probing setups with the visualization field. These optical setups are based on a modified Michelson interferometer with multi-pass and multi-angle propagation of the probing beam through the transparent inhomogeneity. In comparison to the standard single-pass Mach-Zehnder setup, the multi-pass interferometer allows for an increase in the measurement sensitivity of up to 8 times. This technique can be applied to a transparent axially symmetrical gas/plasma jet with low refractivity. [ABSTRACT FROM AUTHOR]

In this work, the reconstruction quality of an approach for neutrospheric water vapor tomography based on Slant Wet Delays (SWDs) obtained from Global Navigation Satellite Systems (GNSS) and Interferometric Synthetic Aperture Radar (InSAR) is investigated. The novelties of this approach are (1) the use of both absolute GNSS and absolute InSAR SWDs for tomography and (2) the solution of the tomographic system by means of compressive sensing (CS). The tomographic reconstruction is performed based on (i) a synthetic SWD dataset generated using wet refractivity information from the Weather Research and Forecasting (WRF) model and (ii) a real dataset using GNSS and InSAR SWDs. Thus, the validation of the achieved results focuses (i) on a comparison of the refractivity estimates with the input WRF refractivities and (ii) on radiosonde profiles. In case of the synthetic dataset, the results show that the CS approach yields a more accurate and more precise solution than least squares (LSQ). In addition, the benefit of adding synthetic InSAR SWDs into the tomographic system is analyzed. When applying CS, adding synthetic InSAR SWDs into the tomographic system improves the solution both in magnitude and in scattering. When solving the tomographic system by means of LSQ, no clear behavior is observed. In case of the real dataset, the estimated refractivities of both methodologies show a consistent behavior although the LSQ and CS solution strategies differ. [ABSTRACT FROM AUTHOR]

Reconstruction of interferometric synthetic aperture radar (InSAR) atmospheric delay maps is important for the correction of tropospheric artifacts in differential InSAR (D-InSAR) and for the improvement in persistent scatterer (PS) target identification in PS-InSAR. In this study, we explored the spatial structure of atmospheric delay datasets and assessed its effect on InSAR atmospheric delay correction. Two-dimensional (2D) experimental variogram maps of turbulent mixing components derived from 12 GPS zenith wet delay (ZWD) datasets, 12 MERIS ZWD datasets, and 3 ERS-1/2 tandem interferograms showed that spatial anisotropy is common in these datasets. An anisotropic variogram model was then developed and applied to fit the experimental variograms. The results showed that the proposed 2D anisotropic variogram model is superior to the isotropic model, with average improvements in 31.92 and 33.57% in terms of root-mean-square error and correlation coefficients, respectively. With the proposed anisotropic variogram model, the atmospheric delay maps were reconstructed by kriging interpolation and used to correct the atmospheric artifacts in InSAR interferograms. The results showed that the model considering the anisotropy of atmospheric delay produces better results than that with the isotropy assumption. Finally, the effects of the anisotropy ratio, sampling density, and correlation distance of external water vapor data on the atmospheric delay correction were investigated. The results showed that when the anisotropy ratio is less than 0.3, or the sampling density is less than 1% or more than 60%, the impact of anisotropy on kriging prediction is not obvious. [ABSTRACT FROM AUTHOR]

Gravity wave (GW) activity is analyzed using temperature (T) data retrieved from a Rayleigh light detection and ranging (lidar) at Río Gallegos, Argentina (51.6°S, 69.3°W). GW characteristics are derived from 302 nights of observations providing more than 1,018 hr of high‐resolution lidar data between 20‐ and 56‐km height from August 2005 to December 2015. T measurements are performed by a Differential Absorption Lidar instrument. This lidar was the southernmost outside Antarctica until the end of 2017. Río Gallegos is an exceptional place to observe large amplitude GW. Every lidar measurement is classified according to its relative position to the polar vortex. The lidar measurements are compared with collocated Sounding of the Atmosphere using Broadband Emission Radiometry and Global Positioning System‐Radio Occultation data. The different instruments show different windows of the GW spectrum, providing complementary observations. In general, the geometric mean of the specific GW potential energy (PE) is larger during winter and spring than during summer and autumn. The largest geometric mean of PE is found inside the vortex and decreases monotonically at its edge, outside it and when there is no vortex. The same behavior is observed with satellite data. On average, it can be seen that lidar observations provide larger PE values than limb sounding measurements. From a Morlet continuous wavelet transform analysis, three distinct modes are captured from Sounding of the Atmosphere using Broadband Emission Radiometry and from Global Positioning System‐Radio Occultation data at the upper and lower stratosphere, respectively. In particular, a systematic 3.5‐ to 4‐year oscillation, possibly related to El Niño–Southern Oscillation is observed. Key Points: The GW activity derived from 302 nights of lidar measurements above a subpolar region and from collocated satellite measurements is evaluatedThe GW specific potential energy from lidar and satellite measurements is larger inside the polar vortex than outside it or at its edgeThe specific potential energy is larger during winter and spring [ABSTRACT FROM AUTHOR]

Highlights • We propose a new inversion method, using deep learning (DL) to address the inversion problem of atmospheric refractivity. we established a network mapping model between the sea clutter and the refractivity profile parameters. • Analyzing the factors affecting the accuracy of the DL network model, including training data quantity, the number of iterations and number of hidden layers. • Given the estimated results from three different methods: DL, NN, and GA. • We proved that using DL to solve the inversion problem of atmospheric refractivity estimation can reduce computational time significantly, and yield effective accuracy. Abstract Atmospheric ducts are typical occurrences in marine environments. They can trap electromagnetic waves in ducts layer and extend propagation ranges significantly. Thus, affecting systems such as radar communications. Because the direct measurement of the atmospheric ducts refractivity has a certain degree of difficulty, the radar sea clutter power, which is easily measured, is used to solve the inversion problem of atmospheric refractivity estimation. In this study, based on the refractivity profile of the evaporation duct and the surface based duct, combined with the deep learning, we established a network mapping model between the sea clutter and the refractivity profile parameters. The model is applied to the inversion problem of atmospheric refractivity estimation, and the inversion results are analyzed to verify the feasibility of deep learning in the inversion problem. We herein report the high-precision inversion results of the atmospheric refractivity estimation. [ABSTRACT FROM AUTHOR]

To properly use radar refractivity data quantitatively, good knowledge of its errors is required. The data quality of refractivity critically depends on the phase measurements of ground targets that are used for the refractivity estimation. In this study, the observational error structure of refractivity is first estimated based on quantifying the uncertainties of phase measurements, data processing, and the refractivity estimation method. New correlations between the time series of phase measurements at different elevation angles and between polarizations are developed to assess the bulk phase variability of individual targets. Then, the observational error of refractivity is obtained by simulating the uncertainties of phase measurements through the original refractivity estimation method. Resulting errors in refractivity are found to be smaller than 1 N-unit in areas densely populated with reliable point-like stationary ground targets but grow as the target density becomes sparse. [ABSTRACT FROM AUTHOR]

The fluctuation of radio occultation (RO) signals in the presence of refractivity irregularities in the moist lower troposphere results in uncertainties of retrieved bending angle and refractivity profiles. In this study the local spectral width (LSW) of RO signals, transformed to impact parameter representation, is used for the characterization of the uncertainty (random error) of retrieved bending angle and refractivity profiles. A large LSW has some correlation with the large mean difference (bias) of retrieved refractivity and bending angle from radiosondes and European Centre for Medium-Range Weather Forecasts analyses based on data from 2008 to 2014. An LSW-based quality control (QC) procedure is developed to eliminate low-quality (large random errors and biases) profiles from data assimilation. The LSW-based QC procedure is tested and evaluated in the assimilation of Constellation Observing System for Meteorology, Ionosphere and Climate RO data using the NCAR Data Assimilation Research Testbed and the Weather Research and Forecasting Model. Preliminary results, based on a 2-week data assimilation cycle, show that the LSW-based QC procedure improves water vapor analyses in the moist lower troposphere. [ABSTRACT FROM AUTHOR]

Understanding the hidden dynamics of the atmosphere and its effect on radio wave propagation is paramount. Therefore proper characterization of atmospheric dynamics especially for its annual and seasonal variation is necessary. In this paper, atmospheric weather parameters (pressure, relative humidity and temperature), were used to obtain the radio refractivity at the surface N s level, at 100 m height, N 100 and the differential radio refractivity dN / dH . The extracted radio refractivity parameters were characterized based on chaoticity and dynamical complexity, for Akure meteorological station (7°15′9.22″N, 5°11′35.23″E). The data analysis has been based on the Combined application of Largest Lyapunov Exponent and Tsallis Entropy (CLLETE). The data sets for the year 2011 and 2012 have been used for the computation. It was observed that the CLLETE parameters follow similar trend in most cases, and the Lyapunov exponent is generally positive indicating the presence of chaos for all radio refractivity data sets. It was further revealed that the pattern of the dynamical complexity variations in 2012 follow a similar trend from July to December for N 100 and dN / dH with the values of the dynamical complexity parameters computed for N S having a huge departure from the other two heights with lower complexity values. However, the dynamical complexity parameters computed for N S and N 100 a have similar a trend from the beginning of the year to June. The values of the Chaos and complexity parameters also show a similar variation in 2011 with N S following the same trend between July and December. This indicates the dynamical response of radio refractivity to varying weather conditions. [ABSTRACT FROM AUTHOR]

Two solar proton events in September 2017 had a significant impact on the operation of the Super Dual Auroral Radar Network (SuperDARN), a global network of high‐frequency (HF) radars designed for observing F region ionospheric plasma convection. Strong polar cap absorption caused near‐total loss of radar backscatter, which prevented the primary SuperDARN data products from being determined for a period of several days. During this interval, the high‐latitude and polar cap radars measured unusually low levels of background atmospheric radio noise. We demonstrate that these background noise measurements can be used to observe the spatial and temporal evolution of the polar cap absorption region, using an approach similar to riometry. We find that the temporal evolution of the SuperDARN radar‐derived HF attenuation closely follows that of the cosmic noise absorption measured by a riometer. Attenuation of the atmospheric noise up to 10 dB at 12 MHz is measured within the northern polar cap, and up to 14 dB in the southern polar cap, which is consistent with the observed backscatter loss. Additionally, periods of enhanced attenuation lasting 2–4 hr are detected by the midlatitude radars in response to M‐ and X‐class solar flares. Our results demonstrate that SuperDARN's routine measurements of atmospheric radio noise can be used to monitor 8‐ to 20‐MHz radio attenuation from middle to polar latitudes, which may be used to supplement riometer data and also to investigate the causes of SuperDARN backscatter loss during space weather events. Plain Language Summary: Solar proton events are known to cause widespread disruption to high‐frequency (HF) radio communications in the high‐latitude and polar regions. We demonstrate that SuperDARN HF radars may be used to monitor HF radio wave attenuation during solar proton events using routine measurements of the background radio noise. These background noise measurements are produced as part of the radar data processing, but they are not normally used for science applications. We focus on two solar proton events, which occurred in September 2017, and find that the measured radio attenuation is confined to the polar cap and exhibits temporal and spatial properties that are characteristic of polar cap absorption events. The attenuation measured by the Rankin Inlet SuperDARN radar agrees well with measurements from a nearby riometer, indicating that reasonable estimates of the HF radio attenuation can be obtained from SuperDARN radars despite the high day‐to‐day variability of the atmospheric radio noise. Our technique may also prove useful for determining the reasons for backscatter loss, particularly when riometer data are not available. Key Points: SuperDARN HF radars were strongly affected by polar cap absorption events in September 2017A novel method to estimate HF attenuation using SuperDARN atmospheric noise measurements has been developedUp to 14‐dB attenuation at 12 MHz was measured in the polar caps, resulting in backscatter loss [ABSTRACT FROM AUTHOR]

Inversion of radio occultation profiles usually relies on the Abel transform in a spherically symmetric atmosphere. The main contribution to the refractive index comes from gaseous constituents such as pressure, temperature and water vapor. However, the impact of other particles in the troposphere, neglected in the retrieval, can become important during specific weather events causing the commonly applied assumptions to be invalid. In this study, Global Forecast System model is used to simulate the effect of cloud water on bending angle profiles. The information of solid and liquid fractions was assessed from the cloud mixing ratio based on a temperature-dependent separation. The signal was propagated between GPS satellite and low-Earth orbiter through tangent point placed in the vicinity of cloud water. The assumption of a spherically symmetric distribution for cloud refractivity fields is validated based on 2-dimensional simulations with multiple phase screen method. Using one year of forecast data for 2016, the most statistically probable examples were selected and analyzed as well as the uppermost impact of cloud significance was determined in terms of bending angle errors. Typical refractivity values induced by liquid clouds are within 2 ppm and result in single-spike profile structures. Corresponding fractional errors of the bending angle can reach −4% with respect to retrievals with zero clouds contributions. In the presence of unusual weather phenomena, simulations in a horizontally inhomogeneous atmosphere suggest a large dependency on propagation paths that should be considered in the modeling of cloud impact. Bending angles can be affected by −10% fractional error, which inverted with Abel transform corresponds to refractivity error of −2%. The highest fractional difference induced by the ice water is generally below −1% in the bending angle and −0.5% in the refractivity. The most significant cloud content was observed in the first 6 km above the Earth’s surface, whereas ice clouds can affect radio occultation profiles up to the top height of 16 km. [ABSTRACT FROM AUTHOR]

Precipitable water (PW) inferred from GPS (Global Positioning System) radio occultation (RO) and ground-based (GB) Global Navigation Satellite System (GNSS) observations are compared between years 2007 and 2014. As previous studies were mainly performed over continental areas we now focus over ocean-dominated geographical areas. Our analysis is done in order to find out how the reliability level of RO results over oceanic areas compares to land. As RO soundings usually miss some information close to the ground, we also assess different methods to complete the lacking data. We found 47 terrestrial stations that lie in islands small and far away from continental areas where the weather might be governed by the sea conditions. From comparisons of almost 5000 collocated samples, PW from RO and GB exhibit a global mean difference around 1 mm, root-mean-square deviation about 5 mm and a correlation above 0.9. The 2007–2014 timeseries and the monthly mean RO and GB PW were also compared to reanalyses per hemisphere, latitude regions and oceans. In each zone it was found that PW from RO, GB and reanalyses all exhibit in general consistent seasonal qualitative behavior. However, quantitative differences exist between reanalyses on one side and RO and GB on the other side. It is shown that PW from reanalyses lacks reliability in areas where the island topography is poorly represented by them. We also conclude that RO and GB seem to be more sensitive for the detection of features that depart from the regular annual cycle. [ABSTRACT FROM AUTHOR]

The terrestrial ionosphere over low‐latitude regions presents the unique phenomena of the equatorial ionization anomaly (characterized by global maximum in plasma concentration) and plasma‐depleted regions known as equatorial plasma bubbles and associated smaller‐scale plasma irregularities. Transionospheric radio signals such as those from Global Navigation Satellite Systems constellations, traveling across this ambient, may suffer severe scintillation in amplitude and phase due to these plasma structures. Presently, three civilian signals available for GPS users, at L1 (1575.42 MHz), L2C (1227.60 MHz), and L5 (1176.45 MHz) are used to investigate the propagation effects due to these irregularities. The purpose of the present work is to evaluate statistically the distribution of severe fade events for each of these carrier frequencies based on the nonlinear ionospheric propagation effects as represented by the fading coefficients of α‐μ distribution. The results from the analyses of data sets recorded by stations at different geomagnetic latitude locations in Brazil show that regions closer to the equatorial ionization anomaly crest present higher probability of severe fade events. Additionally, the L5 signals, dedicated for safety‐of‐life applications, revealed more unfavorable results when compared to the L1 and L2C frequencies. The results further showed that for 0.8 ≤ S4 ≤ 1.0 the probabilities of fades deeper than −10 dB were between 8.0% and 6.5% depending on the station position. Considering the case of fades deeper than −20 dB, the results reach values near 1%, which is quite concerning. These results show empirically the fading environment that users of the new civilian signals may experience in low‐latitude region. Additionally, the fading coefficients may help in the comprehension of the distribution of amplitude scintillation and its relation with the frequency used, aiding in the future the development of signal processing algorithms capable to mitigate errors for navigation users. Plain Language Summary: This work shows differences in the statistics of GPS signals at different frequencies. The results warn that new signals will be more affected by the ionosphere in regions of low latitudes. Key Points: GPS signals L2C and L5 are more susceptible to be impacted by ionospheric irregularitiesThe statistics of fading scintillation events verified how degraded are the new signals at L2C and L5 in comparison to the L1 signalExperimental data show that more severe fading are expected to L2C and L5 signals [ABSTRACT FROM AUTHOR]

The characteristics of the flow field around a supersonic aircraft are discussed. It is analyzed that there are interfaces between two adjacent regions with jump change of refractive index in the supersonic flow. The physical mechanism of the Fresnel reflection of an electromagnetic wave at the interfaces mentioned above is analyzed according to the Fresnel reflection phenomenon in the Earth atmosphere. The conditions for the Fresnel reflection in the supersonic flow field are discussed, and the formulae for calculating the Fresnel reflection coefficient are given. According to the process of a radio wave propagating across the interfaces in the supersonic flow field, the other radio blackout phenomenon is proposed for the first time, which is induced by the nonionized atmospheric sheath around a supersonic aircraft, and may be well neglected or mistaken for the plasma‐sheath‐caused radio blackout. The conditions of the nonionized‐atmospheric‐sheath‐induced radio blackout are investigated. The calculation results show that the neglected radio blackout is confined to a rather specific case, a very small range of grazing angles. However, it is significant for identifying the mechanism of radio blackout. Plain Language Summary: It is well known that, when a radio wave propagates from a medium of a given refractive index n1 into a second medium with refractive index n2, both reflection and refraction of the radio wave may occur. Nevertheless, if the reflection coefficient is equal to 1 or the total reflection occurs, the refraction wave disappears; that is, the radio wave cannot pass through the interface between the two media into the second medium. Therefore, for the radio link across two or more media, the radio blackout occurs. However, the radio blackout of a supersonic aircraft is regarded as a plasma sheath‐caused result in general. It is a fact that the mechanism of the nonionized airflow‐induced radio blackout is well founded in physics. In other words, the radio blackout induced by the nonionized flow field around a supersonic aircraft is likely to be neglected or mistaken as the plasma sheath‐caused case. This paper focuses on the neglected radio blackout for the first time. Key Points: The Fresnel reflection mechanism at the interface between two adjacent regions with jump change of refractivity in supersonic nonionized flow is analyzedThe formulae for calculating the Fresnel reflection coefficient are givenThe nonionized supersonic airflow‐induced radio blackout, which may be neglected but is important for exploring radio blackout, is proposed and investigated for the first time [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The sensitivity of GPS radio occultation (GPSRO) bending angle assimilation to vertical resolution was studied within a global three-dimensional variational data assimilation (3DVAR) system. The sensitivity experiments were performed using different vertical resolutions of GPSRO data at 2 km, 1 km, 500 m, and 200 m. The assimilation of the higher vertical resolution GPSRO data showed better consistency in the analysis–forecast cycle in terms of the differences between GPSRO bending angle data and 6-h forecasts (O-F). This resulted in an improved analysis of the temperature, geopotential height, and wind in the mid-/upper-level troposphere by the hydrostatic response and the related model dynamics. It should be noted that the highest vertical resolution of the GPSRO data (200 m in this study) improved the forecasting skill level in terms of the root-mean-square error (against the European Centre for Medium-Range Weather Forecasts analysis) and the anomaly correlation of the geopotential height forecasting at 500 and 200 hPa in both the Northern and Southern Hemispheres. The benefits of assimilating higher vertical resolution GPSRO data were more pronounced in the upper-level troposphere, which was in agreement with previous studies using real GPSRO observations. [ABSTRACT FROM AUTHOR]

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 ±0:5K 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. [ABSTRACT FROM AUTHOR]

Radio occultation (RO) and radiosonde (RS) comparisons provide a means of analyzing errors associated with both observational systems. Since RO and RS observations are not taken at the exact same time or location, temporal and spatial sampling errors resulting from atmospheric variability can be significant and inhibit error analysis of the observational systems. In addition, the vertical resolutions of RO and RS profiles vary and vertical representativeness errors may also affect the comparison. In RO-RS comparisons, RO observations are co-located with RS profiles within a fixed time window and distance, i.e. within 3-6 h and circles of radii ranging between 100 and 500 km. In this study, we first show that vertical filtering of RO and RS profiles to a common vertical resolution reduces representativeness errors. We then test two methods of reducing horizontal sampling errors during RO-RS comparisons: restricting co-location pairs to within ellipses oriented along the direction of wind flow rather than circles and applying a spatial-temporal sampling correction based on model data. Using data from 2011 to 2014, we compare RO and RS differences at four GCOS Reference Upper-Air Network (GRUAN) RS stations in different climatic locations, in which co-location pairs were constrained to a large circle (~666 km radius), small circle (~300 km radius), and ellipse parallel to the wind direction (~666 km semi-major axis, ~133 km semiminor axis). We also apply a spatial-temporal sampling correction using European Centre for Medium-Range Weather Forecasts Interim Reanalysis (ERA-Interim) gridded data. Restricting co-locations to within the ellipse reduces root mean square (RMS) refractivity, temperature, and water vapor pressure differences relative to RMS differences within the large circle and produces differences that are comparable to or less than the RMS differences within circles of similar area. Applying the sampling correction shows the most significant reduction in RMS differences, such that RMS differences are nearly identical to the sampling correction regardless of the geometric constraints. We conclude that implementing the spatial-temporal sampling correction using a reliable model will most effectively reduce sampling errors during RO-RS comparisons; however, if a reliable model is not available, restricting spatial comparisons to within an ellipse parallel to the wind flow will reduce sampling errors caused by horizontal atmospheric variability. [ABSTRACT FROM AUTHOR]

To obtain optimal performance, estimation of radio refractivity is essential in planning and design of radio links/systems. The dependence of radio refractivity on different climatic parameters such as temperature, pressure, humidity and scale height is studied. The method proposed by International Telecommunication Union (ITU) is used in calculation of tropospheric radio refractivity. The main objective of this work is to formulate a mathematical equation/expression which can be used to predict/estimate the value of radio refractivity for an arbitrary day of any year, with a low error of estimation, in a local environment. Sensitivity of radio refractivity to temperature, pressure and humidity has been evaluated for a period of 5 years from 2008 to 2012, in a particular area under consideration. The results presented for tropospheric radio refractivity take into consideration both the location height and scale height parameters. The results are analysed in terms of statistical measures such as the moving averages, probability density function, monthly mean values, and their corresponding standard deviations. Finally, a mathematical model is formulated to calculate the radio refractivity for any day of a whole year. Reliability of error analysis in respect to accuracy is also shown. It was implemented in the industrial enterprise. [ABSTRACT FROM AUTHOR]

Radio refractivity plays a significant role in the development and design of radio systems for attaining the best level of performance. Refractivity in the troposphere is one of the features affecting electromagnetic waves, and hence the communication system interrupts. In this work, a modified artificial neural network (ANN) based model is applied to predict the refractivity. The suggested ANN model comprises three modules: the data preparation module, the feature selection module, and the forecast module. The first module applies pre-processing to make the data compatible for the feature selection module. The second module discards irrelevant and redundant data from the input set. The third module uses ANN for prediction. The ANN model applies a sigmoid activation function and a multi-variate auto regressive model to update the weights during the training process. In this work, the refractivity is predicted and estimated based on ten years (2002–2011) of meteorological data, such as the temperature, pressure, and humidity, obtained from the Pakistan Meteorological Department (PMD), Islamabad. The refractivity is estimated using the method suggested by the International Telecommunication Union (ITU). The refractivity is predicted for the year 2012 using the database of the previous ten years, with the help of ANN. The ANN model is implemented in MATLAB. Next, the estimated and predicted refractivity levels are validated against each other. The predicted and actual values (PMD data) of the atmospheric parameters agree with each other well, and demonstrate the accuracy of the proposed ANN method. It was further found that all parameters have a strong relationship with refractivity, in particular the temperature and humidity. The refractivity values are higher during the rainy season owing to a strong association with the relative humidity. Therefore, it is important to properly cater the signal communication system during hot and humid weather. Based on the results, the proposed ANN method can be used to develop a refractivity database, which is highly important in a radio communication system. [ABSTRACT FROM AUTHOR]

Atmospheric refractivity estimation is an important issue for performance evaluation of communication systems and air surveillance radars. A novel hybrid model based on artificial neural networks (ANNs) and genetic algorithms (GAs) for inversion problem of atmospheric refractivity estimation is introduced. In this paper, inversion problem and clutter model problem of refractivity from clutter (RFC) method are separated and only inversion problem is studied. A problem specific ANN structure is designed and an original GA is developed to fulfill atmospheric refractivity estimations. In hybrid method, ANNs make pre-estimation and GAs use these results as a starting population for post-estimation. When the results obtained from the single solutions of ANNs and GAs are compared to the results obtained from hybrid model, a significant improvement in the accuracy of estimated results is observed. [ABSTRACT FROM AUTHOR]

Linear and non-linear representations of wave fields constitute the basis of modern algorithms for analysis of radio occultation (RO) data. Linear representations are implemented by Fourier Integral Operators, which allow for high-resolution retrieval of bending angles. Non-linear representations include Wigner Distribution Function (WDF), which equals the pseudo-density of energy in the ray space. Representations allow for filtering wave fields by suppressing some areas of the ray space and mapping the field back from the transformed space to the initial one. We apply this technique to the retrieval of reflected rays from RO observations. The use of reflected rays may increase the accuracy of the retrieval of the atmospheric refractivity. Reflected rays can be identified by the visual inspection of WDF or spectrogram plots. Numerous examples from COSMIC data indicate that reflections are mostly observed over oceans or snow, in particular over Antarctica. We introduce the reflection index that characterizes the relative intensity of the reflected ray with respect to the direct ray. The index allows for the automatic identification of events with reflections. We use the radio holographic estimate of the errors of the retrieved bending angle profiles of reflected rays. A comparison of indices evaluated for a large base of events including the visual identification of reflections indicated a good agreement with our definition of reflection index. [ABSTRACT FROM AUTHOR]

The Global Navigation Satellite System (GNSS) Occultation Sounder (GNOS) is one of the new-generation payloads onboard the Chinese FengYun 3 (FY-3) series of operational meteorological satellites for sounding the Earth's neutral atmosphere and ionosphere. The GNOS was designed for acquiring setting and rising radio occultation (RO) data by using GNSS signals from both the Chinese Bei- Dou System (BDS) and the US Global Positioning System (GPS). An ultra-stable oscillator with 1 s stability (Allan deviation) at the level of 10-12 was installed on the FY-3C GNOS, and thus both zero-difference and singledifference excess phase processing methods should be feasible for FY-3C GNOS observations. In this study we focus on evaluating zero-difference processing of BDS RO data vs. single-difference processing, in order to investigate the zero-difference feasibility for this new instrument, which after its launch in September 2013 started to use BDS signals from five geostationary orbit (GEO) satellites, five inclined geosynchronous orbit (IGSO) satellites and four medium Earth orbit (MEO) satellites. We used a 3-month set of GNOS BDS RO data (October to December 2013) for the evaluation and compared atmospheric bending angle and refractivity profiles, derived from single- and zero-difference excess phase data, against co-located profiles from European Centre for Medium-Range Weather Forecasts (ECMWF) analyses. We also compared against co-located refractivity profiles from radiosondes. The statistical evaluation against these reference data shows that the results from single- and zero-difference processing are reasonably consistent in both bias and standard deviation, clearly demonstrating the feasibility of zero differencing for GNOS BDS RO observations. The average bias (and standard deviation) of the bending angle and refractivity profiles were found to be about 0.05 to 0.2% (and 0.7 to 1.6 %) over the upper troposphere and lower stratosphere. Zero differencing was found to perform slightly better, as may be expected from its lower vulnerability to noise. The validation results indicate that GNOS can provide, on top of GPS RO profiles, accurate and precise BDS RO profiles both from single- and zero-difference processing. The GNOS observations by the series of FY-3 satellites are thus expected to provide important contributions to numerical weather prediction and global climate change analysis. [ABSTRACT FROM AUTHOR]

Airborne radio occultation (ARO) measurements collected during a ferry flight at the end of the PREDepression Investigation of Cloud-systems in the Tropics (PREDICT) field campaign from the Virgin Islands to Colorado are analyzed. The large contrast in atmospheric conditions along the flight path from the warm and moist Caribbean Sea to the much drier and cooler continental conditions provides a unique opportunity to address the sensitivity of ARO measurements to the tropospheric temperature and moisture changes. This long flight at nearly constant altitude ( ~13 km) provided an optimal configuration for simultaneous high-quality ARO measurements from two high-gain side-looking antennas, as well as one relatively lower gain zenith (top) antenna. The omnidirectional top antenna has the advantage of tracking robustly more occulting satellites in all direction as compared to the limited-azimuth tracking of the side-looking antennas. Two well-adapted radioholographic bending angle retrieval methods, full-spectrum inversion (FSI) and phase matching (PM), were compared with the standard geometric-optics (GO) retrieval method. Comparison of the ARO retrievals from the top antenna with the near-coincident ECMWF reanalysis-interim (ERAI) profiles shows only a small root-mean-square (RMS) refractivity difference of ~0.3%in the drier upper troposphere from ~5 to ~11.5 km over both land and ocean. Both the FSI and PM methods improve the ARO retrievals in the moist lower troposphere and reduce the negative bias found in the GO retrieval due to atmospheric multipath. In the lowest layer of the troposphere, the ARO refractivity derived using FSI shows a negative bias of about -2 %. The increase of the refractivity bias occurs below 5 km over the ocean and below 3.5 km over land, corresponding to the approximate altitude of large vertical moisture gradients above the ocean and land surface, respectively. In comparisons to radiosondes, the FSI ARO soundings capture well the height of layers with sharp refractivity gradients but display a negative refractivity bias inside the boundary layer. The unique opportunity to make simultaneous independent recordings of occultation events from multiple antennas establishes that high-precision ARO measurements can be achieved corresponding to an RMS difference better than 0.2% in refractivity (or ~0.4 K). The surprisingly good quality of recordings from a very simple zenith antenna increases the feasibility of developing an operational tropospheric sounding system onboard commercial aircraft in the future, which could provide a large number of data for direct assimilation in numerical weather prediction models. [ABSTRACT FROM AUTHOR]

Radio engineers and researchers in conjunction with the International Telecommunication Union (ITU) have established the pivotal role of radio refractivity to the propagation of electromagnetic energy in the troposphere. In particular, the refractivity gradient statistics for the lowest 100 m in the troposphere are used to determine the probability of occurrence of anomalous propagation conditions known as ducting. The major challenge to characterising the propagation condition over any environment is accessing the data of the lowest boundary layer of the atmosphere, which is highly dynamic and turbulent in evolution. High resolution radiosonde data from the Nigerian Meteorological Agency (NiMet) were used for a synoptic study of the rain-harmattan transition phase. The rain-harmattan transition phase marks the onset of the dry season due to the movement of the intertropical convergence zone interplay between (north-easterly and south-westerly) trade winds and monsoonal circulation. The lowest 100 m data were analysed to determine the frequency of ducting per month. Progressive increase in the occurrence of ducting was observed during the rain-harmattan transition phase, which coincides with the West African Monsoon retreat. The results show significant divergence from previous studies, which reported that the tropospheric condition over Lagos (Geo. 6 . 5 ° N, 3 . 3 ° E), Nigeria, is predominantly super-refractive. [ABSTRACT FROM AUTHOR]

Results of 111-MHz monitoring observations carried out on the Big Scanning Antenna of the Pushchino Radio Astronomy Observatory during September 1-28, 2015 are presented. Fifty-four pulsating sources were detected at declinations -9° < Î' < 42°. Forty-seven of these are known pulsars, five are new sources, and two are previously discovered transients. Estimates of the peak flux densities and dispersion measures are presented for all these sources. [ABSTRACT FROM AUTHOR]

Global Navigation Satellite System (GNSS) radio occultation (RO) measurements are promising in sensing the vertical structure of the Earth's planetary boundary layer (PBL). However, large refractivity changes near the top of PBL can cause ducting and lead to a negative bias in the retrieved refractivity within the PBL (below ~ 2 km). To remove the bias, a reconstruction method with assumption of linear structure inside the ducting layer models has been proposed by Xie et al. (2006). While the negative bias can be reduced drastically as demonstrated in the simulation, the lack of high-quality surface refractivity constraint makes its application to real RO data difficult. In this paper, we use the widely available precipitable water (PW) satellite observation as the external constraint for the bias correction. A new framework is proposed to incorporate optimization into the RO reconstruction retrievals in the presence of ducting conditions. The new method uses optimal estimation to select the best refractivity solution whose PW and PBL height best match the externally retrieved PW and the known a priori states, respectively. The near-coincident PW retrievals from AMSR-E microwave radiometer instruments are used as an external observational constraint. This new reconstruction method is tested on both the simulated GNSS-RO profiles and the actual GNSS-RO data. Our results show that the proposed method can greatly reduce the negative refractivity bias when compared to the traditional Abel inversion. [ABSTRACT FROM AUTHOR]