Evaporation Duct Research Articles

Inversions of the Evaporation Duct Heights Utilizing Opportunistic Signals of Navigation Radar

Inversions of the Evaporation Duct Heights Utilizing Opportunistic Signals of Navigation Radar

Research on a Multimodel Fusion Diagnosis Method for Evaporation Ducts in the East China Sea

Evaporation ducts are abnormal states of the atmosphere in the air–sea boundary layer that directly affect the propagation trajectory of electromagnetic (EM) waves. Therefore, an accurate diagnosis of the evaporation duct height (EDH) is important for studying the propagation trajectory of EM waves in evaporation ducts. Most evaporation duct models (EDMs) based on the Monin–Obukhov similarity theory are empirical methods. Different EDMs have different levels of environmental adaptability. Evaporation duct diagnosis methods based on machine learning methods only consider the mathematical relationship between data and do not explore the physical mechanism of evaporation ducts. To solve the above problems, this study observed the meteorological and hydrological parameters of the five layers of the low-altitude atmosphere in the East China Sea on board the research vessel Xiangyanghong 18 in April 2021 and obtained the atmospheric refractivity profile. An evaporation duct multimodel fusion diagnosis method (MMF) based on a library for support vector machines (LIBSVM) is proposed. First, based on the observed meteorological and hydrological data, the differences between the EDH diagnosis results of different EDMs and MMF were analyzed. When ASTD ≥ 0, the average errors of the diagnostic results of BYC, NPS, NWA, NRL, LKB, and MMF are 2.57 m, 2.92 m, 2.67 m, 3.27 m, 2.57 m, and 0.24 m, respectively. When ASTD < 0, the average errors are 2.95 m, 2.94 m, 2.98 m, 2.99 m, 2.97 m, and 0.41 m, respectively. Then, the EM wave path loss accuracy analysis was performed on the EDH diagnosis results of the NPS model and the MMF. When ASTD ≥ 0, the average path loss errors of the NPS model and MMF are 5.44 dB and 2.74 dB, respectively. When ASTD < 0, the average errors are 5.21 dB and 3.46 dB, respectively. The results show that the MMF is suitable for EDH diagnosis, and the diagnosis accuracy is higher than other models.

A Comparative Study of Rough Sea Surface and Evaporation Duct Models on Radio Wave Propagation

Microwave and millimeter-wave propagations near the sea surface are greatly affected by the vertical atmospheric refractivity profile and the sea surface. To accurately simulate or predict the performance of ship radar systems, the evaporation duct model and rough sea surface models are usually incorporated into the parabolic equation to model electromagnetic wave propagation. The Paulus-Jeske (PJ) model and Naval Postgraduate School (NPS) model are the most widely used to make the vertical atmospheric refractivity profile, and the roughness sea surface is modeled with the effective reflection coefficient of the rough surface or the generation of the rough sea surface. There are four widely used rough sea surface approximations called Ament, Miller-Brown, Shadowed, and Hybrid for rough sea surface modeling. In this study, numerical simulation is first used to compare the different rough sea surface modeling methods. Then, evaporation duct measurement data and C-band propagation experimental data are used to compare the evaporation duct models and the four rough sea surface models that are combined with evaporation duct models for propagation loss prediction. The simulation results show that the shadowing of radio waves by the rough sea surface and sea surface structure are important factors affecting the transmission of radio waves in evaporation ducts, and the difference in the path loss predicted by different rough sea surface approximations may exceed 10 dB at the same location. In particular, the frequency and wind speed will increase. In addition, the experimental data show that the evaporation duct can be better diagnosed using different evaporation duct models at different atmospheric stabilities. When the air-sea temperature difference is positive, the PJ evaporation duct is recommended; otherwise, the NPS model is better, and the results of the Shadowed and Hybrid rough sea surface models are in better agreement with the observations.

Atmospheric Ducts and Their Electromagnetic Propagation Characteristics in the Northwestern South China Sea

The propagation of electromagnetic waves beyond the line of sight can be caused by atmospheric ducts, which are significant concerns in the fields of radar and communication. This paper utilizes data from seven automatic weather stations and five radio-sounding stations to statistically analyze the characteristics of the atmospheric ducts in the northwest region of the South China Sea (SCS). After verifying the practicality of numerical analysis data from NCEP CFSv2 and ECMWF in studying atmospheric ducts using measured data, we analyzed the spatial–temporal distribution characteristics of the height of the regional evaporation duct and the bottom height of the elevated duct. The study found that the NCEP CFSv2 data accurately capture the evaporation duct height and duct occurrence rate in the study area, and the elevated duct bottom height calculated from ERA5 and the measured data have good consistency. The occurrence rate and height of the evaporation duct in coastal stations in the northwest of the SCS vary significantly by month, demonstrating clear monthly distribution patterns; conversely, changes in the Xisha station are minimal, indicating good temporal uniformity. For lower atmospheric ducts, the difference in occurrence rates between 00:00 and 12:00 (UTC) is negligible. The occurrence probability of elevated ducts in the Beibu Gulf area is relatively high, mainly concentrated from January to April, and the Xisha area is dominated by surface ducts without foundation layers, mainly concentrated from June to August. Monsoons play a critical role in the generation and evolution of atmospheric ducts in the northwest of the SCS, with the height of the evaporation duct increasing and the bottom height of the elevated duct decreasing after the onset of the summer monsoon. In the end, we simulated electromagnetic propagation loss under different frequencies and radiation elevation angles in various duct environments within a typical atmospheric duct structure.

Observed Over-the-Horizon Propagation Characteristics and Evaporation Duct Inversion During the Entire Process of Tropical Cyclone Mulan (202207)

An evaporation duct, which is often occurs in the air–sea boundary layer, have a considerable impact on the remote sensing detection capability of electromagnetic (EM) systems. Tropical cyclones (TCs) substantially increase the transfer of water vapor and heat in the air–sea boundary and inevitably affect EM propagation environments. However, observations of duct environments and their effects on EM propagation during a TC process have seldom been reported. In this paper, X-band EM propagation systems that can operate continuously in the TC environment were specially developed, and observations of multiple over-the-horizon propagation links during TC Mulan (202207) were presented. The path loss (PL) of five links, and the surface variations both in Mulan’s area of high wind speed (WS) and eye were recorded. The average PL of a 53-km link was approximately 155 dB in the high WS area, while a 30 dB increase in PL was detected in Mulan’s eye. The inversion and simulation results show that the evaporation duct height (EDH) was high in the high WS area, which led to low PL in EM propagation; conversely, EDH was low inside Mulan’s eye, resulting in high PL.

Comparison of Evaporation Duct Height Statistics Based on Surface Bulk Measurements Over Yongxing Island

Six-year high-precision surface bulk measurements are used to analyze the evaporation duct height (EDH) distributions over Yongxing Island. Yearly statistics of the EDH computed by the PJ model and the NPS model are compared with an existed historical climatology results used in the AREPS tool. The EDH occurrence frequency distributions (OFD) of the PJ and NPS estimations in different atmospheric regimes are also presented. The findings are outlined as follows: (1) The mean EDHs for the PJ model and the NPS model are 14.3 m and 11.9 m, respectively. (2) The yearly EDH distributions of the PJ estimations are similar to, but with approximately 2 m lower than that of the AREPS climatology. However, the distributions of the NPS estimations are different, with the highest EDHs occurred in Apr., May, and Jun. of an average height of 14.2 m, compared to that of the PJ estimations occurred in Oct., Nov., and Dec. of an average height of 16.3 m. (3) For all the partitioned atmospheric regimes, there exists a shifted threshold for the OFD comparisons. Above the threshold, the OFDs of the PJ estimations are higher than that of the NPS estimations, and an opposite situation arises below the threshold.

Impact of Evaporation Duct on Line-of-sight Propagation of Electromagnetic Waves

Generated by the evaporation of seawater, evaporation duct (ED) is an air-sea interaction that occurs over the sea surface. The electromagnetic (EM) waves will be trapped in ED when propagating beyond line-of-sight. However, the impact of ED on line-of-sight propagation is rarely reported. The path loss (PL) oscillates within the first few kilometers when the EM wave propagating in line-of-sight and then greatly reduced. Simulation results shown that the PL in ED layer can be reduced by more than 40 dB on line-of-sight propagation. The experiment PL selected from the South China Sea in March 2022 is 28.2 dB lower than the simulation PL in standard atmosphere, which verifies the conclusions. The conclusions contribute to the design and use of marine radar and communication systems.

The Comparison of Long Short-Term Memory Neural Network and Deep Forest for the Evaporation Duct Height Prediction

An evaporation duct is a type of atmospheric stratification that affects radio systems. Atmospheric duct prediction is helpful for radar detection. In this paper, we used the deep forest, which is different from a deep learning framework, to predict the atmospheric duct height. At the same time, the long short-term memory neural network and other machine learning algorithms, such as the logistic regression, random forest, Bayes, and support vector regression algorithms, were adopted to predict the evaporation duct height. The predicted results with filled and unfilled missing data show that an accurate prediction of the evaporation duct height can be achieved using the deep forest.

Prediction of Over-the-Horizon Electromagnetic Wave Propagation in Evaporation Ducts Based on the Gated Recurrent Unit Network Model

Evaporation ducts have a significant impact on electromagnetic (EM) wave propagation. The accurate prediction of EM propagation loss (PL) is important for the practical application of EM systems. However, the random weather processes and interaction of air-sea variables will cause complex spatial and temporal fluctuations in PL, especially for an over-the-horizon link. The physics prediction models show some limitations in practice. This article proposes a data-driven PL prediction method based on the gated recurrent unit network (GRU-PL). An X-band EM propagation system was developed and deployed for one month to collect PL of a 53 km link in the South China Sea for model training and testing. The GRU-PL model uses the air-sea variables as inputs and outputs the predicted PL. The root-mean-square error (RMSE) of the GRU-PL model is 7.04 dB, decreasing by 57.2% compared with the classical method. Furthermore, a method, which uses the inversion evaporation duct heights (EDHs) as the output to train the GRU network and then uses the parabolic equation model to predict the PL based on the GRU predicted EDHs, is also discussed. The RMSE is 7.3 dB. This method shows good potential in PL prediction of dynamically changing EM propagation links.

Statistical Analysis of Hybrid Atmospheric Ducts over the Northern South China Sea and Their Influence on Over-the-Horizon Electromagnetic Wave Propagation

Atmospheric ducts are special super-refractive atmospheric structures that can cause over-the-horizon propagation of electromagnetic waves. Different types of atmospheric ducts have different influences on electromagnetic wave propagation. Owing to the complex marine atmospheric environment, different types of atmospheric ducts often occur together. When evaluating the performance of an electromagnetic system near the sea surface, the combined influence of various atmospheric ducts should be considered comprehensively. In this paper, the statistical distribution of atmospheric ducts over the northern South China Sea is analyzed using sounding data and reanalysis data. This paper uses the parabolic equation model to analyze the propagation characteristics of microwaves near the sea surface in the presence of both surface and evaporation ducts. It is found that compared with cases where only one type of atmospheric duct is considered, a hybrid atmospheric duct structure can capture more microwave energy at a lower receiving height. At an antenna height of 5 m, the path loss begins to fluctuate beyond a propagation distance of 50 km, with the maximum fluctuation reaching about 15 dB. Microwave propagation characteristics at different microwave frequencies and antenna heights are also simulated and analyzed.

Long-term over-the-horizon microwave channel measurements and statistical analysis in evaporation ducts over the Yellow Sea

Maritime high-speed over-the-horizon wireless communication is realizable through evaporation ducts. Detailed measurement, analysis, and modeling of duct channels are essential for application of this communication technique. In this paper, X-band electromagnetic (EM) wave propagation systems were developed and deployed for a 133-km over-the-horizon microwave link in coastal areas of the Yellow Sea. The propagation length was 7.7 times the line-of-sight length. Measurement results including the path loss (PL) and meteorological data were obtained during a 54-day period in autumn 2021. The long-term channel results were analyzed on the basis of statistical analysis and model simulations. Results showed that our measurement system, with a maximum measurable power loss of 200 dB, had connected with a probability of 56.2% during the measurement period. Model simulation showed that evaporation duct environments are not ideal in autumn, with an average evaporation duct height (EDH) of 10.6 m. The land breeze in autumn introduced dry and cold air to the link, which could promote evaporation of seawater and reduce PL by approximately 40 dB. Annual spatiotemporal characteristics of EDH showed that evaporation ducts are most suitable for over-the-horizon communication in spring, especially May.

Analysis of the accuracy of using ERA5 reanalysis data for diagnosis of evaporation ducts in the East China Sea

Evaporation ducts are a phenomenon that occurs with extremely high frequency at the boundary between the atmosphere and the ocean. Because they directly affect the propagation of electromagnetic waves, it is necessary to study their various characteristics. Since it is difficult to conduct large-scale observations at sea, many researchers use reanalysis data for this task instead of observation data. However, there have been no studies verifying accuracy of this analysis method for the diagnosis of evaporation ducts. Therefore, in this work, observations of the low-altitude atmospheric refractivity profile were carried out over the East China Sea on board the research vessel Xiangyanghong 18 in April 2021. First, the differences between different evaporation duct models were examined based on the meteorological and hydrological data obtained at different heights. It was concluded that the diagnostic accuracy of the evaporation duct model is low in stable conditions and a low-wind-speed environment. Under the same conditions, the Naval Postgraduate School (NPS) model showed high diagnostic accuracy when compared with other models. Second, Taylor plots were used to verify the accuracy of the reanalysis data and the observation data. It was concluded that the single-parameter precision of the reanalysis data is relatively high, and there were strong correlations with the observation data. Finally, the observation and reanalysis data were used to compare and analyze the false-report rate, the missing-report rate, and the accuracy of the diagnosed evaporation duct height using the NPS model. The false-report rate and missing-report rate were found to be 1.93% and 1.52%, respectively. The average diagnosis deviation was 3.34 m. The Pearson correlation coefficient was found to be close to 1. The results indicate that it is basically feasible to analyze the characteristics of evaporation ducts based on the NPS model using reanalysis data.

NWPP-EDH: Numerical weather prediction products evaporation duct regional prediction model

The evaporation duct is a special atmospheric stratification that can affect the propagation path of electromagnetic waves at sea, thus it is crucial for the ability of the radio communication systems. The traditional theoretical models of the evaporation duct often have limited accuracy. The actual observational data from voyages and stations are insufficient, and the existing data-driven evaporation duct height prediction models can only predict a particular point or route but cannot reproduce the regional distribution of the evaporation duct. To address these issues, we propose NWPP-EDH model in this work. The fitting ability of the NWPP-EDH model was tested. Its accuracy was compared with that of the Babin–Young–Carton model, Musson–Gauthier–Bruth model, and the classical Naval Postgraduate School model; compared to these models, the root mean square error (RMSE) of NWPP-EDH model was reduced by 71.8%, 87%, and 60.9%, respectively. Thus, we find that the model shows a better performance than the existing theoretical models.

Observed Diurnal Variations of Over-the-horizon Propagation Loss in Evaporation Duct Over the South China Sea

Observed Diurnal Variations of Over-the-horizon Propagation Loss in Evaporation Duct Over the South China Sea

Multiscale Decomposition Prediction of Propagation Loss for EM Waves in Marine Evaporation Duct Using Deep Learning

A tropospheric duct (TD) is an anomalous atmospheric refraction structure in marine environments that seriously interferes with the propagation path and range of electromagnetic (EM) waves, resulting in serious influence on the normal operation of radar. Since the propagation loss (PL) can reflect the propagation characteristics of EM waves inside the duct layer, it is important to obtain an accurate cognition of the PL of EM waves in marine TDs. However, the PL is strongly non−linear with propagation range due to the trapped propagation effect inside duct layer, which makes accurate prediction of PL more difficult. To resolve this problem, a novel multiscale decomposition prediction method (VMD−PSO−LSTM) based on the long short−term memory (LSTM) network, variational mode decomposition (VMD) method and particle swarm optimization (PSO) algorithm is proposed in this study. Firstly, VMD is used to decompose PL into several smooth subsequences with different frequency scales. Then, a LSTM−based model for each subsequence is built to predict the corresponding subsequence. In addition, PSO is used to optimize the hyperparameters of each LSTM prediction model. Finally, the predicted subsequences are reconstructed to obtain the final PL prediction results. The performance of the VMD−PSO−LSTM method is verified by combining the measured PL. The minimum RMSE and MAE indicators for the VMD−PSO−PSTM method are 0.368 and 0.276, respectively. The percentage improvement of prediction performance compared to other prediction methods can reach at most 72.46 and 77.61% in RMSE and MAE, respectively, showing that the VMD−PSO−LSTM method has the advantages of high accuracy and outperforms other comparison methods.

Deep Learning Method for Evaporation Duct Inversion Based on GPS Signal

Accurate evaporation duct prediction is one of the critical technologies for realizing the over-the-horizon impact of marine communication, ship radar, and other systems. Using GPS signals to invert evaporation ducts provides more benefits in terms of method realization and ease. In order to invert the evaporation duct from GPS-received power data, a deep learning technique based on Bayesian optimization is proposed to increase the prediction accuracy of evaporation ducts. The evaporation duct propagation mechanism of the GPS signal is explored. The GPS-received power is estimated using the two-parameter evaporation duct model, and a better neural network structure is built using Bayesian optimization. The study results show that the Bayesian optimization model has a smaller root mean square error (RMSE) than the human empirical model, which allows for rapid and accurate inversion of duct parameters even in noisy interference.

Analysis and Research on Chaotic Dynamics of Evaporation Duct Height Time Series with Multiple Time Scales

The evaporation duct is a particular type of atmospheric structure that always appears on the open ocean. Predicting the evaporation duct height (EDH) accurately and in a timely manner is of great significance for the practical application of marine wireless communication equipment. Understanding the characteristics of EDH time series is an essential prerequisite for establishing an appropriate prediction model. Moreover, the sampling timescales of EDH data may influence the dynamic characteristics of the EDH time series as well. In this study, EDH time series datasets at three timescales, hourly, daily, and monthly, were constructed as the case study. Statistical methods, namely the augmented Dickey–Fuller test and Ljung–Box test, were adopted to verify the stationary and white noise characteristics of the EDH time series. Then, rescaled range analysis was applied to calculate the Hurst exponent to study the fractal characteristics of the EDH time series. An extensive analysis and discussion of the chaotic dynamics of the EDH time series are provided. From the perspective of nonlinear dynamics, the phase space was constructed from the time delay τ and embedding dimension m, which were calculated from the mutual information method and the Grassberger–Procaccia algorithm, respectively. The maximum Lyapunov exponent was also calculated by the small data volume method to explore the existence of chaos in the EDH time series. According to our analysis, the EDH time series are stationary and have a non-white noise characteristic. The Hurst exponents for all three timescales were greater than 0.5, indicating the predictability of the EDH time series. The phase space diagrams exhibited strange attractors in a well-defined region for all the timescales, suggesting that the evolution of the EDH time series can possibly be explained by deterministic chaos. All of the maximum Lyapunov exponents were positive, confirming the chaos in the EDH time series. Further, stronger chaotic characteristics were found for the finer-resolution time series than the coarser-resolution time series. This study provides a new perspective for scholars to understand the fluctuation principles of the evaporation duct at different timescales. The findings from this study also lay a theoretical and scientific foundation for the future application of chaotic prediction methods in the research on the evaporation duct.

A Model for Calculating Electromagnetic Scattering From Target in Evaporation Duct

In this letter, we proposed a new model to solve the scattering problem from target in evaporation duct. First, the traditional two-dimensional (2-D) subsection ray tracing (SRT) method is extended to 3-D case. Then the 3D SRT is used to obtain the propagation direction and attenuation of the transmitted electromagnetic wave from antenna. Second, shooting and bouncing rays (SBR) is used in the simulation of complex target scattering. Different from traditional SBR, the launched rays are not planed wave. A virtual transmitting plane is established to obtain the amplitude and direction of these rays in evaporation duct in front of the target. The transmitting plane in evaporation duct can be regard as the incident field of SBR method. What's more, the new model based on SBR is accelerated by CUDA and OptiX from Nvidia to deal with the tremendous number of rays. Finally, the scattering EM field and power distributions in evaporation duct environment are calculated and discussed in the last section.

Electromagnetic Ducting in the Near‐Shore Marine Environment: Results From the CASPER‐East Field Experiment

AbstractWe used the observations from the east coast of the USA and model simulations to elicit marine atmospheric surface layer (MASL) processes that controlled the spatial variability of electromagnetic ducting conditions in the near‐shore (60 km) marine environment during CASPER‐East field campaign. The estimated evaporation duct height (EDH) showed an average decrease of ∼2.5 m over every 10 km distance from the shore. The surface layer stability conditions chiefly determined the duct heights. While the mean duct height remained ∼10 m under unstable conditions, deeper (>20 m) evaporation ducts were frequently evident (∼40% of the time) during stable MASL conditions, which were responsible for the observed near‐shore EDH variability. Episodes of warm offshore flow and advection of hot and humid air from the Gulfstream region created ideal environmental conditions for stable MASL development and thus deeper ducts over the coastal ocean. A detailed case study of warm and humid Gulfstream air advection over the cooler coastal ocean is presented to elucidate the temporal evolution of the surface layer stability and associated changes in the spatial distribution of the evaporation duct height.

Performance analysis of code division multiplexing communication under evaporation duct environment

The evaporation duct is an effective means for realizing non-line-of-sight (NLOS) wireless transmission over the sea. However, the effects of marine weather conditions on electromagnetic propagation have rarely been studied. In this study, the influence of the marine atmospheric environment on electromagnetic propagation was analyzed through numerical simulation. Additionally, the impacts of antenna height, transmission distance, and electromagnetic wave frequency on path loss were studied. Finally, the link capacity of the code division multiplexing (CDM) communication system in the evaporation duct environment was studied via numerical analysis and simulations. Simulation results demonstrated that CDM communication technology can improve the link capacity under an evaporation duct compared with that of the spread-spectrum communication technology.