Surface Duct Research Articles

Environmentally adaptive acoustic communication and navigation for underice autonomous operation

The Arctic acoustic environment has changed dramatically, with multi-year ice being less abundant and the influx of warmer Pacific water, the so-called “Beaufort Lens,” creating a local maximum in the sound speed at a depth of 50–80 m. In contrast to the classic Arctic sound speed profile which had a monotonic increase in sound speed yielding a surface duct with strong ice interaction, the warm water lens creates a lower duct supporting extremely efficient propagation to long ranges. On the other hand, this double duct environment is decremental to short range communication and navigation of underwater vehicles. Thus, for a transmitter in one channel, the lens creates distinct shadow zones in the other channel at ranges between 1 and 4 km, which are the typical operational ranges of small AUVs, severely affecting navigation and communication performance. In addition, the temporal variability of is significant due to internal waves. During ICEX20 MIT and WHOI demonstrated an integrated communication and navigation concept where a network of ice-moored modem buoys with GPS tracking were used to track an under-ice AUV using regular CTD updates of optimal ranging, which in combination with a novel dynamic model constrained navigation fusion engine demonstrating GPS-grade navigation accuracy over several hours of operation. Using an EOF framework for updating the onboard environmental awareness on the AUV, the concept supports autonomous depth selection for optimal communication connectivity. [Work supported by ONR and UWDC/ASL.]

Statistical Analysis of the Spatiotemporal Distribution of Lower Atmospheric Ducts over the Seas Adjacent to China, Based on the ECMWF Reanalysis Dataset

On the basis of 12 years of the European Centre for Mesoscale Weather Forecasts (ECMWF) reanalysis dataset, we statistically analyzed the spatiotemporal distribution of lower atmospheric ducts over the seas around China, and we investigated the possible generation mechanisms. The results show that the ducts’ occurrence had obvious seasonal and regional variations. Ducting events were more likely to occur in spring and summer, and the maximum occurrence rate reached 45.6%, which was closely related to the East Asian monsoon. The ducts’ altitude in continental coastal areas was lower than that far from the coast due to the dominance of surface ducts. The ducts’ thickness varied between 50 m and 450 m, and the thicker ducts were mainly concentrated in the South China Sea and the Pacific Ocean on the east side of the East China Sea near the Philippines and Taiwan. Except for a few areas, the ducts’ intensity was less than 10 M-units (an M-unit is the unit of atmospheric modified refractivity) and the diurnal variations were less pronounced. The duct formation in the lower atmosphere was related to factors such as monsoons, tropical cyclones, ocean currents, radiative cooling, and sea–land breezes.

Full-Coupled Convolutional Transformer for Surface-Based Duct Refractivity Inversion

A surface-based duct (SBD) is an abnormal atmospheric structure with a low probability of occurrence buta strong ability to trap electromagnetic waves. However, the existing research is based on the assumption that the range direction of the surface duct is homogeneous, which will lead to low productivity and large errors when applied in a real-marine environment. To alleviate these issues, we propose a framework for the inversion of inhomogeneous SBD M-profile based on a full-coupled convolutional Transformer (FCCT) deep learning network. We first designed a one-dimensional residual dilated causal convolution autoencoder to extract the feature representations from a high-dimension range direction inhomogeneous M-profile. Second, to improve efficiency and precision, we proposed a full-coupled convolutional Transformer (FCCT) that incorporated dilated causal convolutional layers to gain exponentially receptive field growth of the M-profile and help Transformer-like models improve the receptive field of each range direction inhomogeneous SBD M-profile information. We tested our proposed method performance on two sets of simulated sea clutter power data where the inversion of the simulated data reached 96.99% and 97.69%, which outperformed the existing baseline methods.

Surface atmospheric duct over Svalbard, Arctic, related to atmospheric and ocean conditions in winter

ABSTRACT Atmospheric ducts could disturb radar detection and radio communication in the Arctic and risk increasing human activities under Arctic warming. It is an urgent need to understand atmospheric ducts in the Arctic, with distinct atmospheric systems and environment. This study attempts to analyze the surface atmospheric ducts over Svalbard, as a critical region in the Arctic Ocean, and discuss the relation to atmospheric and oceanic conditions, based on observation and reanalysis data. The results show an occurrence of 12.6 percent, mean strength of 1.30 MU, and mean depth of 13 m for the surface duct in Svalbard winter from December 2018 to February 2020. The surface duct is characterized as two types: T-type ducts mainly constructed by temperature inversion and H-type ducts constructed by a humidity gradient. The T-type duct dominates the surface duct in Svalbard winter, with an appearance percentage of 80 percent, indicating an importance of temperature inversion in surface duct construction. The surface duct is closely related to atmospheric and oceanic conditions in Svalbard winter, because the T-type duct occurs in colder, drier, and northeasterly weather and the H-type duct occurs in warmer, wetter, and southeasterly weather with higher sea surface temperature.

Mixed layers and surface ducts

The upper ocean is strongly effected by the sun and the wind. In an idealized view, this leads to a warm mixed layer with a nearly constant temperature and salinity. Then, the increase in pressure with depth causes an increase of about 1.7 m/s in sound speed for every 100 m in depth (i.e., a gradient of 0.017/s). This gradient forms an acoustic surface duct, which is a nearly universal feature of the world’s oceans. They can be just tens of meters thick or several hundred meters in the Southern Ocean or the North Atlantic during periods of winter storms. Not surprisingly, the surface duct is very important in terms of sound propagation; However, as we shall describe, the surface duct has some peculiar acoustic properties and can produce vast shadow zones inside the duct. This simplified description is a good beginning; however, “mixed” layers are generally not well-mixed. It is more precise to speak of the upper ocean as a zone where there may be periods of mixing and heating driven by changes in winds and cloud cover. Upper ocean models predict this evolution and often lead to much more complicated sound speed structures. This, in turn, has lead to a wide variety of ways of characterizing the features of the surface duct. This talk will touch on the interesting acoustics of surface ducts and how variations in different (oceanographic) upper ocean models effect the sound propagation.

Sensitivity of mixed layer duct propagation to deterministic ocean features: The interaction parameter

Surface duct propagation can be affected by deterministic ocean features such as fronts, eddies, filaments, and/or density compensated temperature and salinity anomalies (spice). Using the concept of duct modal structure and the Dyson series solution to the wave equation, we present a non-dimensional parameter Γmn, termed the interaction parameter, that quantifies the strength of mode coupling between a surface duct mode m and any other mode n as a function of ocean feature scales and acoustic frequency. Weak, first order coupling is given by Γmn < 1 where as strong, higher order coupling has Γmn > 1. Importantly, there is a frequency dependent resonance condition associated with the range width of the perturbations, Δ, such that Γmn goes to zero as Δ approaches 0 and infinite. The utility of the interaction parameter in estimating the stability of mixed layer propagation is demonstrated using numerical examples of a cold eddy and of a hot and salty spice feature.

Inversion for Inhomogeneous Surface Duct without a Base Layer Based on Ocean-Scattered Low-Elevation BDS Signals

The anomalous propagation conditions, particularly the tropospheric ducts, severely impact the regular operation and performance evaluation of radio devices in the atmospheric boundary layer. Therefore, it is necessary to provide the regional distribution of tropospheric ducts for utilizing or avoiding these abnormal propagation phenomena. As significant uncooperative signal sources, the global navigation satellite systems (GNSS) have been widely applied in the remote sensing of the ocean and atmosphere due to the greater convenience and lower cost. With the completed deployment of the BeiDou Navigation Satellite System (BDS) in 2020, an additional source can be chosen in the relevant studies. Taking the BDS as an example, since the scattered signals from the ocean surface at low satellite elevation angles can be effectively trapped by tropospheric ducts, we propose a method to invert for the regional distribution of tropospheric ducts using the received power of ocean-scattered signals in this paper. Firstly, the propagation model was built to calculate the received power, and a suite of simulations was made in various atmospheric environments. The results suggested that the received power is more sensitive to the surface duct without a base layer. Then, we made a preliminary estimation of the tropospheric ducts on the ocean nearby Qingdao utilizing the Weather Research and Forecasting (WRF) model as well as the echo data measured by a Doppler weather radar. Before the inversion, the actual satellite azimuth and elevation angles should be obtained to evaluate the bistatic scattering coefficients and the received powers of the selected satellite signals. Finally, we presented an inversion example using the proposed method. In absence of the actual measurements, the received powers pre-estimated at different SNRs served as the inputs of the inversion process and the estimated duct parameters were used to verify the validity of the proposed inversion method. For both the received power and modified refractivity profile, the fitness between the values pre-estimated using the estimated duct parameters and calculated by the inverted duct parameters gets better as the elevation angle decreases and the SNR increases. The variation of the fitness between the estimated and inverted values is slightly different for each duct parameter. Moreover, the calculation of inversion errors further explained the above behaviors, including the mean absolute error (MAE) and the root mean square error (RMSE). Despite some certain errors, the inversion results maintain the overall tendencies and most characteristics of the estimated values, thus proving the validity of the inversion method.

Duct climatology over the South China Sea based on European Center for Medium Range Weather Forecast reanalysis data

Duct climatology over the South China Sea (SCS) was statistically analyzed in the present study based on 10-year ECMWF reanalysis data. The results show that the spatial distributions of the ducting events over the SCS exhibit obvious monthly variation. May is a key time point that separates the spatial distribution pattern of duct characteristics over the SCS. Ducting events often occur in the first half year and higher ducting probabilities are the broadest area of the SCS in March and April. There are two higher ducting probability regions including the northeast and west of the SCS since June. According to seasonal variations, the duct probability, the duct strength and thickness reach their maximum in spring, followed by winter, autumn, and summer. The atmospheric ducts occur more often in the north than south of the SCS and more ducts especially the surface ducts are present off the mainland. The wind field over the SCS plays a very important role in ducting events and the winter monsoon over the SCS allows ducts to form readily in the middle and north SCS until the onset of the summer monsoon. By contrast, the summer monsoon makes it difficult for ducts to form over the SCS. In the monsoon reversal periods, the higher duct occurrence area decreases and less ducts form in most of the SCS.

Sensitivity of mixed layer duct propagation to deterministic ocean features.

Here, the problem of mode coupling in a mixed layer (ML) surface duct is considered where the coupling is induced by deterministic upper ocean features such as eddies, filaments, and/or density compensated temperature and salinity anomalies (spice). The single scatter Dyson series solution for mode energy is used to define a non-dimensional mode interaction parameter Γmn that quantifies the strength of coupling between modes m and n as a function of environmental factors and frequency. Direct coupled mode simulations at 400 and 1000 Hz show weak, first order coupling and small ML transmission loss (TL) variability when Γmn<1, while for Γmn>1, there is strong, higher order coupling with large changes in ML TL. Importantly, there is a frequency dependent resonance condition associated with the range width of the perturbations, Δ, such that Γmn→0 as Δ→0 and ∞.

Effects of swells on sound propagation in surface duct environment in shallow water

Surface duct is a common duct due to strong sea winds and sea-atmosphere interactions in winter and it is an excellent waveguide in which energy may propagate a long distance. However, the rough interface formed by sea surface waves will seriously damage this excellent performance. In this study, the experimental data of sound propagation over the continental slope in the South China Sea are used to analyze the characteristics of sound propagation in a surface duct. Modeling analyses based on the parabolic equation model RAM and ray trace theory BELLHOP are used to examine these characteristics. The parameters of sea bottom, source depth, wind-driven sea surface, and swell-containing sea surface are taken into consideration in the model. The results show that when the source is located in the surface duct, the parameters of the sea bottom have little influence on sound propagation, while the change of source depth exerts some effects on the sound propagation. By combining the Pierson Moscowitz (PM) spectrum with Monte Carlo method, the rough sea surface is investigated. Since the PM spectrum is related only to wind speed, the wind-driven sea surface is generated by using the actual wind speed measured by the shipborne anemometer. The swell-containing sea surface is defined as a superposition of a sinusoidal pressure-release surface and the wind-driven sea surface. By comparing the effects of two sea surfaces on sound propagation, it is found that when the wind speed is small, swells play an important role in the surface-duct propagation. Experimental data show that for the acoustic signal with a center frequency of &lt;inline-formula&gt;&lt;tex-math id="M3"&gt;\begin{document}$1000\;{\rm{Hz}}$\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20201549_M3.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20201549_M3.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt;, the swell-containing sea surface brings around &lt;inline-formula&gt;&lt;tex-math id="M4"&gt;\begin{document}$10 \;{\rm{dB}}$\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20201549_M4.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20201549_M4.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt; loss to a distance of &lt;inline-formula&gt;&lt;tex-math id="M5"&gt;\begin{document}$70 \;{\rm{km}}$\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20201549_M5.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20201549_M5.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt;. For the two kinds of rough sea surfaces, rays at launch angles of &lt;inline-formula&gt;&lt;tex-math id="M6"&gt;\begin{document}$\pm 1^{\circ}, 0^{\circ}$\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20201549_M6.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20201549_M6.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt; are plotted to examine their effects on sound propagation. The results indicate that the swell-containing sea surface which has greater roughness makes rays go toward the sea bottom, thus resulting in larger loss. Therefore, in order to investigate the characteristics of the sound field in the northern South China Sea in winter, especially with high frequency sound signals, the influences of not only winds and waves, but also the swells from the surrounding sea should be taken into consideration. It is important to study the characteristics of sound propagation with swells for improving the performance of sonar equipment in poor sea conditions.

Numerical and Physical Simulation of Heat Transfer Enhancement Using Oval Dimple Vortex Generators—Review and Recommendations

Vortex generation and flow disruption in heat exchanger passages by means of surface modification is a widely used passive heat transfer augmentation technique. The present paper contains the results of numerical and experimental studies of the hydraulic resistance and heat transfer in the rectangle duct with oval-trench- and oval-arc-shaped dimples applied to the heat transfer surface. For the turbulent flow in the duct (Pr = 0.71, Red = 3200–9 × 104—for heat transfer determination and Red = 500–104—for the friction factor measurements), rational geometrical parameters of the oval-trench dimple were determined: relative elongation of dimple l/b = 5.57–6.78 and relative depth l/b = 5.57–6.78, while the value of the attack angle to the mean flow was fixed φ = (45–60)°. The comparison of the experimental and numerical modeling for the flow in the narrow duct over the surface with a single- and multi-row dimple arrangement has revealed a good agreement. It was found that the average heat transfer coefficient magnitudes in such ducts could be increased 1.5–2.5 times by means of single and multi-row dimple application on the heat transfer surface. The heat transfer augmentation for the surfaces with the oval-arched dimples was found to be 10% greater than the one for the oval-trench dimples. The corresponding friction factor augmentation was found to be 125–300% in comparison to the smooth surface duct. The obtained experimental data were used for the data generalization. Derived generalized equation allows for predicting the friction factor and heat transfer coefficient values for the flow over the single-row oval-trench simple arrangement. The maximal deviation of the experimental data from the proposed equations was found to be 20%. The application of the artificial neural networks for predicting the hydraulic resistance and heat transfer augmentation in such ducts was presented.

Operation improvement measures of dry type air-core reactor in severe cold environment

In severe cold environment, the surface of dry type air-core reactor would easily be cracked, once natural contamination accumulated on the cracks, surface flashover would have more possibility to happen, leading to more serious reactor faults. The most commonly used methods to avoid reactor surface discharge includes RTV coating and surface decontamination using water-based cleaning agent, yet the effects of these methods, as well as possible impact on reactor performance have not been fully studied. In this paper, RTV coating and decontamination tests were carried out to evaluate the effect of the methods, also the impact of these two methods on reactor electrical performance were analyzed. The research result showed that: (1)Water-based cleaning agent was effective in removing contamination on insulation paint surface, but less effective in removing contamination on RTV coated surface; However, the water-based cleaning agent would cause damping between the winding strands, and damping recovery degree was not easy to control. Therefore, water-based cleaning agent was not suitable to solve surface flashover problem of dry type air-core reactors. (2) The overall coating for reactor surface and air duct could be realized by special spraying equipment. The adhesion test showed that RTV was of good covering and adhesion performance, and had little impact on reactor temperature rise, indicating that RTV could be an effective measure to improve operation quality of dry type air-core reactor in extreme cold environment.

Inversion Method of Regional Range-Dependent Surface Ducts with a Base Layer by Doppler Weather Radar Echoes Based on WRF Model

Ground clutter caused by variations of atmospheric refraction environment can occur when the weather radar is observing precipitation systems, especially in the presence of a tropospheric duct. Therefore, the acquisition of duct parameters is very important for evaluating radar performance and improving data quality. Based on the measured echo data of a Doppler weather radar located at Qingdao and the numerical simulation results of modified refractivity profiles from the Weather Research and Forecasting (WRF) model, an inversion method for regional range-dependent tropospheric duct parameters over the sea area is proposed in this paper. Due to the higher antenna height of up to 169 m, the transmission environment is assumed to be a surface duct with a base layer for locating the antenna in the trapping layer. The Principal Component Analysis (PCA) and Parabolic Equation (PE) methods were used to characterize the horizontal inhomogeneity of duct parameters and the propagation of electromagnetic waves in the tropospheric duct. In the inversion model, duct parameters extracted from WRF outputs were used as the initial values. Additionally, multithread parallel processing was adopted in order to reduce the inversion time based on the characteristics of the optimization algorithm. The overall variation tendencies of the WRF simulation results in the regional distribution of duct parameters were well consistent with the inversion results, but were relatively lower in terms of specific values. Due to the influence of precipitation targets on measured echo data, the inversed echo data had different agreements with the measurements in space, and the absolute error values were less than 5 dB in about 90% of the region of interest.

Spatiotemporal Distribution of Atmospheric Ducts in Alaska and Its Relationship with the Arctic Vortex

InORCID account, please go to the Account Update page (http://mts.hindawi.com/update/) in our Manuscript Tracking System and after you have logged in click on the ORCID link at the top of the page. This link will take you to the ORCID website where you will be able to create an account for yourself. Once you have done so, your new ORCID will be saved in our Manuscript Tracking System automatically."?>this paper, global position system high-resolution sounding data from 1998 to 2008 were used to statistically analyze the spatiotemporal distribution and determine the probability, thickness, and intensity of atmospheric ducts at 12 stations in Alaska. In addition, the singular value decomposition (SVD) was used to examine the relationship between the Arctic vortex and atmospheric ducts. The annual average probability of atmospheric ducts, primarily surface and elevation ducts, was approximately 30% in Alaska. The probability of elevation ducts was greater than that of surface ducts. The Arctic vortex area and intensity index of each subarea were significantly negatively correlated with the occurrence of atmospheric ducts. Thus, when the area of the Arctic vortex increased and the intensity index of each subarea strengthened, the probability of atmospheric ducts decreased and their characteristics weakened.

A Novel PE/FDTD Hybrid Model for Predicting Echo Signals of Radar Targets in Large-Scale Complex Environments

The prediction of radar target echo signal in a large-scale complex environment is of great significance in target detection, radar design and other applications. In this paper, a novel PE/FDTD hybrid model is proposed to predict monostatic radar target echo signals in large-scale complex environments. The target echo signal can be regarded as the output response of the transmitted signal which passes through a linear time invariant system composed of complex environment and target. The transport function of the complex environment is computed by the parabolic equation (PE) method and the scattering characteristics of the target are calculated by finite difference time domain method (FDTD). The combination of PE and FDTD is realized through the system response function. In combination with the “stop-go” method, the prediction of moving target echo signal is realized. In addition, the error of combining PE with FDTD is analyzed, and the result shows that the error is less than 0.1% when the target distance is more than 10km. Additional numerical examples are given to demonstrate the correctness of the method in semi-space, rain, fog and atmospheric duct environments. The calculated results are compared with those of theoretical method, time-domain shooting and bouncing ray (TDSBR), multi-level fast multi-pole method (MLFMM) and waveguide mode theory, and good agreement among them is observed. Finally, the simulation analysis of the missile echo signal in the mixed sea-land environment with surface duct is carried out. The simulation results show that this model can be used to predict the echo signal of airborne targets in a large-scale complex environment. It is a promising option for multi-scale computing involving radar target and large-scale environment.

Analysis of spherical propagation models for multipath-propagation predictions

Multipath fading leads to a limitation in the availability and/or reliability of microwave links. In this paper is study the propagation mechanism under fading conditions propagation models, based on ray theory, above a spherical earth have been developed and compared to the well-known planar propagation model above a “flattened eartha” The models are shown, including an accuracy analysis, and the use of spherical propagation models is illustrated for surface duct layers above water.

Inversion of the surface duct from radar sea clutter using the improved whale optimization algorithm

ABSTRACTAn improved whale optimization algorithm (IWOA) is presented to estimate the surface duct. First, the iteration and selection mechanism in whale optimization algorithm (WOA) are modified to avoid local optimum. Then, orthogonal crossover is embedded into WOA to improve its exploration ability. Finally, the IWOA is applied to benchmark functions and the optimization problem of surface duct to test the performance of IWOA, and the results are compared with those of genetic algorithm (GA), WOA and variant WOA. The comparison results show that IWOA has best accuracy and stability for almost all the benchmark functions and the inversion of the surface duct.

The effect of atmospheric ducts on the propagation of AIS signals

ABSTRACT Electromagnetic wave propagates abnormally under atmospheric duct conditions. It is very important to study the influence of atmospheric duct on the radio propagation of the shipborne automatic identification system (AIS). AIS is a maritime navigation safety communication system that operates in the very high frequency (VHF) maritime mobile band and was developed primarily for collision avoidance. When AIS signals propagating in the atmospheric duct, the trapping effect can extend the signal range beyond the line-of-sight. In this work, the transmission of AIS signals under atmospheric duct conditions is modelled using the parabolic equation method. The simulation results demonstrate that both standard surface duct and surface-based duct can extend the detection range of AIS, whereas surface-based duct shows higher trapping capabilities over the standard surface duct. The results can be implemented to improve maritime situation awareness.

Effect of eddy on acoustic propagation from the surface duct perspective

Eddy-acoustical effects were investigated from the surface duct perspective. The presence of the eddy would influence sound propagation in surface duct by modifying the mix layer depth (MLD). Altimetry data along with the Argo profiles were used to obtain statistics of the effects of eddy on MLD. Results suggest that in the Northwestern Pacific Ocean, the mean MLD anomaly brought by warm eddy was 26.7 m and the value was 8.6 m for the cold eddy. Acoustic data from an acoustic experiment conducted in a warm eddy in South China Sea (SCS) were analyzed to study the effects of eddy on surface duct energy leakage (SDEL) phenomenon. Numerical simulation was carried out to study the sensitivity of SDEL on MLD variation. Results suggest that the SDEL phenomenon was highly sensitive to the MLD variation when the frequency was lower than the duct-thickness-related critical frequency. The signal attenuation rate of the SDEL signal received out of the duct related with MLD and frequency were also calculated. Acoustic data collected in the experiment agrees well with the simulation results. Given that the warm eddy would increase the MLD for almost 26.7 m on an average level, it is favorable for SDEL phenomenon.

Influence of surface duct on the vertical spatial characteristics of wind-generated noise in deep ocean

Vertical spatial characteristics of the wind-generated noise include the noise vertical directionality and the noise vertical coherence, which seriously affect the performance of sonar devices and play an important role in ocean environment parameters inversion. In this paper, we investigate the influence of surface duct on the noise vertical spatial characteristics at the depths below the duct. The Kuperman-Ingenito (K/I) model is employed to describe the distribution of the noise sources, and Pekeris-branch-cut-based normal modes are used to represent the Green's functions between the noise sources and the receivers. Both the noise vertical directionality and the noise vertical coherence are expressed as a function of the normal modes, so that we can investigate the physical reason for the variance of the noise vertical spatial characteristics by analyzing the variance of the normal modes. The numerical simulations on the noise vertical spatial characteristics show that the influence of surface duct above the critical depth is different from that below the critical depth. Above the critical depth, there exists a peak in the noise vertical directionality at the edge of the horizontal notch close to the bottom side. In the presence of surface duct, this peak significantly rises up, and the noise vertical coherence deviates from that in the absence of surface duct and tends to be perfect positive coherence and perfect negative coherence periodically as the vertical distance between the two receivers increases. On the other hand, below the critical depth, the noise power comes from the horizontal direction becomes stronger and the noise vertical coherence tends to be perfect positive coherence in the presence of surface duct as compared with the case in the absence of surface duct. Moreover, the influence will become severer as the thickness of the surface duct increases, while almost keep unchanged when the sound speed gradient in the surface duct varies. The modal analysis indicates that the noise source excites more refracted normal modes with stronger modal intensity in the presence of surface duct, and the excited refracted normal modes become more and stronger if the surface duct is thicker. As the result, the increase of the refracted mode number and the enhancement of their modal intensity cause the vertical spatial characteristics of noise to change.