Characteristics Of Radar Echoes Research Articles

Spatial-Time-Frequency Information Collaborative Learning for Ionosphere Clutter Suppression in HFSWR

Eliminating the ionosphere clutter is particularly important for high frequency surface wave radar (HFSWR). This article fills the gap between the growing interests in the application of deep learning network and the new approach of ionosphere clutter suppression. We present a dynamic collaborative learning strategy to simultaneously learn the spatial-time-frequency (STF) information of multi-component radar echoes for suppressing the ionosphere clutter. For capturing the multi-dimension information, we design a multi-channel time-frequency characteristic learning network (MTF) and a multi-channel spatial characteristic learning network (MS) to sufficiently learn the characteristics of multi-component radar echoes according to the divided regions. The proposed methodology not only suppresses the ionosphere clutter but also preserves the targets from contaminated region after the suppression process. The results presented in the letter allow demonstrating how the proposed methodology outperforms the approaches taken as reference in all the cases under study.

Echoformer: Transformer Architecture Based on Radar Echo Characteristics for UAV Detection

While recent years have witnessed an increasing number of commercial applications of unmanned aerial vehicles (UAVs), an imperative problem people have to face is the rapid growth of malicious use. So, it is imperative for security agencies to develop anti-UAV technology. The introduction of deep learning (DL) has a positive influence on radar signal processing, but DL-based methodologies have yet to be widespread in radar target detection because of the lack of unique architecture based on radar echo characteristics and the annotation method of radar data. In this article, a novel Transformer-based architecture is proposed, which transforms the problem of UAV detection into a binary classification task in each range cell. The complex encoder architecture and the Transformer-based extractor are designed to extract the Doppler frequency shift feature and the micro-Doppler signature (mDS) of a UAV simultaneously. The well-designed architecture based on radar echo characteristics can achieve a combination training of echoes with different coherent processing intervals (CPIs). In addition, we provide an annotation method and a data augmentation skill for our real measured dataset. The results of the experiment demonstrate that the proposed method has better detection performance and measuring accuracy under different SNRs in comparison with traditional radar target detection and other DL-based methods.

Response of Cloud and Precipitation Properties to Seeding at a Supercooled Cloud‐Top Layer

AbstractTo accurately identify ideal operating conditions for scientific cloud seeding and to collect evidence of the effects of seeding, a set of three flights, with figure‐8‐like paths, was designed for supercooled liquid water detection. These flights were conducted near the cloud top before seeding, during seeding, and upon re‐entry. Using both in situ aircraft and satellite remote sensing observations, a comprehensive analysis was conducted to understand the response of cloud properties to seeding within supercooled cloud tops. Satellite observations showed clear icing cloud tracks after seeding with abundant supercooled liquid water near the cloud top. Radar observations confirmed enhanced radar reflectivity echoes in cloud‐seeding regions. For regions cloud seeded with supercooled liquid water, the liquid droplets quickly froze into ice crystals, which further grew into large ice crystals via vapor deposition, riming, and collision‐coalescence processes. With these processes, large ice crystals formed and rapidly fell downward, causing the sinking of cloud tops and precipitation to fall to the ground. By contrast, there were no such phenomena in the regions without supercooled liquid water or in those regions with supercooled liquid, but otherwise without seeding. In addition, radar reflectivity echoes quickly decreased in all regions without seeding. The evolutionary characteristics of radar echoes and in situ observed cloud and precipitation properties suggest that the response of cloud microphysical characteristics to seeding varies with time and location due to various seeding potentials, leading to different formation conditions of both cloud‐top icing and cloud tracks.

Application of a Radar Echo Extrapolation‐Based Deep Learning Method in Strong Convection Nowcasting

AbstractStrong convection nowcasting has been gaining importance in operational weather forecasting. Recently, deep learning methods have been used to meet the increasing requirement for precise and timely nowcasting. One of the promising deep learning models is the convolutional gated recurrent unit (ConvGRU), which has been proven to perform better than traditional methods in strong convection nowcasting. Despite its encouraging performance, ConvGRU tends to produce blurry radar echo images and fails to model radar echo intensities that have multi‐modal and skewed distributions. To overcome these disadvantages, we tested the structural similarity (SSIM) and multiscale structural similarity (MS‐SSIM) indexes as loss functions. The SSIM and MS‐SSIM loss functions are composed of luminance, contrast, and structure and provide more information about the intensity, grade, and shape of the radar echo, which can reduce blurring. Due to multi‐layer downscaling, MS‐SSIM extracted more radar echo characteristics, and its extrapolation was the most realistic and accurate among all of the loss function schemes. Only the MS‐SSIM scheme successfully predicted strong radar echoes after 2 h, especially those at the rainstorm level.

Study on the Strong Radar Echoes at Polar Mesosphere Using a New Dataset Analysis Software

Polar mesosphere echoes and polar mesosphere clouds data extract and analyze software (PMEC_DEAS) was used to deal with the echoes and clouds dataset, then the characteristics of radar echoes were studied in the paper. By analyzing the polar mesosphere summer echoes (PMSE) and polar mesospheric clouds (PMC) data in the short-and long-terms as an example based on the PMEC_DEAS, the correlation between the occurrence rate (OR) of PMSE and PMC is studied. It is found that the correlation between the short-term PMSE OR and PMC is not significant, the long-term double-layer PMSE OR is positively correlated with the PMC. It shows that double-layer PMSE is closely related to PMC. This is consistent with existing conclusions. PMEC_DEAS can effectively adapt to the complex characteristic changes of the events and has good robustness. The data analyzed by PMEC_DEAS was associated with better stability and compatibility. The convenience and utilitarian nature show superiority compared with the current mainstream software.

Hardware-in-the-Loop simulation algorithm for helicopter rotor time-varying echo signals

The inherent micro-Doppler (m-D) feature of helicopter rotor echoes has been widely used in signal detection and identification. To meet the high accuracy and real-time performance requirements of the hardware-in-the-loop (HIL) simulation, this paper proposes a real-time simulation algorithm for the echo signals of helicopter rotor blades. The proposed algorithm can not only simulate the echo signals of different helicopter rotors but can also reflect the m-D characteristics of the helicopter rotor echo signals accurately. Compared to other methods, the simulation algorithm exhibits both high accuracy and superior real-time performance. The experimental results prove the feasibility and effectiveness of this method. The results of this study can have various industrial applications, such as helicopter detection and helicopter identification. This method can ensure the reliability of radar echo characteristics research, while effectively reducing the volume and weight of the system; thus, it can effectively reduce costs.

Advances in Severe Convection Research and Operation in China

This article reviews the advances in severe convection research and operation in China during the past several decades. The favorable synoptic situations for severe convective weather (SCW), the major organization modes of severe convective storms (SCSs), the favorable environmental conditions and characteristics of weather radar echoes and satellite images of SCW and SCSs, and the forecasting and nowcasting techniques of SCW, are emphasized. As a whole, Chinese scientists have achieved a profound understanding of the synoptic patterns, organization, and evolution characteristics of SCW from radar and satellite observations, and the mechanisms of different types of convective weather in China. Specifically, in-depth understanding of the multiple types of convection triggers, along with the environmental conditions, structures and organization modes, and maintenance mechanisms of supercell storms and squall lines, has been obtained. The organization modes and climatological distributions of mesoscale convective systems and different types of SCW, and the multiscale characteristics and formation mechanisms of large hail, tornadoes, downbursts, and damaging convective wind gusts based on radar, satellite, and lightning observations, as well as the related features from damage surveys, are elucidated. In terms of operational applications, different types of identification and mesoanalysis techniques, and various forecasting and nowcasting techniques using methods such as the “ingredients-based” and deep learning algorithms, have been developed. As a result, the performance of operational SCW forecasts in China has been significantly improved.

Radar Simulation of Wake Vortices in Rainy Conditions Based on the Euler-Euler Multiphase Model

The radar monitor of aircraft wake vortex is of great significance to air traffic control. Comparing with the case in clear air, the radar signal of wake vortices in rainy conditions is more easily captured due to the higher levels of signal. To able to analyze radar echo characteristics of evolving wake vortices, the numerical simulation of the evolution of the wake vortices in various rain rates has been performed based on the Euler-Euler frame. The result shows that the rainfall can accelerate the decay of wake vortices. A radar simulator has been represented to process the radar echo signal of raindrops around the wake vortex. The Doppler spectrum of echo signals in radar resolution volumes has been analyzed and the straight vortex and the vortex ring phases show different characteristics of spectrum width. The Doppler width information based on this result of interaction between the wake vortex and rain can be used for assessing the decay phase of wake vortices and monitoring potential hazard for wake vortex encounters.

裕民阿克乔克草原一次暴雨天气雷达回波特征分析

裕民阿克乔克草原一次暴雨天气雷达回波特征分析

Mesoscale characteristics of a rare severe hailstorm event

This study analyzed the environmental field, physical quantify fields, convection parameters and radar echo characteristics of a severe hailstorm that occurred on June 18, 2013, due to a mesoscale convective system. The analysis was performed using ground meteorological data, sounding data and GFS-ANL (gfs 0.5° × 0.5°) reanalysis data in combination with satellite cloud imagery and Doppler radar observations. The results showed that this rare hailstorm event was generated under the influence of a weakening and eastwardly moving low vortex over Lake Balkhash; and a distinct shear line in the middle and lower troposphere over the western South Xinjiang. Strong vertical wind shears, deep conditionally unstable stratification, ample water vapor in the lower troposphere and suitable wet-bulb zero heights are important contributors to the formation of severe hailstorms. An increase in convective available potential energy (CAPE) and a decrease in the lifting index are strong indicators of a severe hailstorm. For the studied hailstorm event, the time corresponding to the maximum value of the normalized CAPE was close to the timing of the actual hailstorm. The severe hailstorm, consisting of multiple supercells, exhibited notable characteristics of hail clouds and supercells. The formation, disappearance and replacement of strong convection cells occurred while the supercells were sustained. The tracking and forecasting results obtained using the Thunderstorm Identification, Tracking, Analysis and Nowcasting system are found to be, to a certain extent, able to indicate of the direction of the supercell storms during this severe hailstorm event.

Characteristics of ultra-wideband radar echoes from a drone

Characteristics of ultra-wideband radar echoes from a drone

Radar Partial Shielding Region Identification Algorithm Based on the Probability Characteristics of Radar Echoes

Partial blockage is one of the most important error sources of radar observation. A new shielding region identification algorithm without any dependence on the terrain information with high resolution is proposed.The algorithm first computes the probability of every grid's probability of emerging specified reflectivity exceeding a threshold which is defined as Z threshold only through checking the continuous operational radar observation data without using any sample database established manually,and then selects adequate probability threshold which is defined as C threshold to delineate the region with different shielding characters. Applying the algorithm with the continuous operational observation dataset during the period betw een May and October in 2012 at Jinahua Quhou and Shangrao,the completely shielding region,partially shielding region and no shielding region on 3 km CAPPI are derived separately. With the comparison of the observation data at April 29 in 2013 in the overlapping region on the 3 km CAPPI betw een Jinhua,Quzhou,Shangrao,Hangzhou and Huangshan,the validation of the identification results of the three radar sites are tested and the parameter sensibility of the Z and C threshold is analyzed,too. The multi-radar mosaic algorithm is optimized applying the partial shielding region results of the radar sites above to enhance the quality of multi-mosaic. The results show that the shielding region with different characters can be identified effectively by selecting adequate Z and C threshold; the selection of the Z and C threshold is the key and moderate Z threshold is beneficial to the further selection of the C threshold; the partial shielding effects can be easily eliminated based on the identification results and the data quality of multi-radar mosaic can be enhanced effectively based on the optimized multi-radar mosaic algorithm.

Narrow-band radar imaging of spinning targets

According to the characteristics of narrow-band radar echoes from spinning targets, this paper applies the complex-valued back-projection method to image formation. Since it accumulates the sinusoidal phase of spinning scatterers effectively, this method can obtain high resolution imaging. The resolution and required radar pulse repetition frequency (PRF) of this method are deduced. For imaging of fast spinning targets, however, spurious peaks appear due to azimuth down sampling with a low PRF. Therefore, a signal model of azimuth down-sampled echoes is created according to the compressed sensing (CS) theory. Then an imaging method based on orthogonal matching pursuit (OMP) is proposed according to the sparsity nature of ISAR echoes. Finally, the effectiveness of the proposed methods is demonstrated using simulated data.

FULL-POLARIMETRIC SPECTRAL CHARACTERISTICS OF CHAFF CLOUDS

Based on the spiral descending model of chaff,the effect of both rotation and translational motion of chaff on radar echo of chaff clouds was studied and the full-polarimetric spectral characteristics of radar echoes from chaff clouds were investigated by using statistical method.Firstly,spiral descending model of chaff was described,and the Sinclair matrix of a dipole with rotation was given.Based on the above analysis,auto correlation and cross-correlation functions of the Sinclair Matrix elements were investigated and further power spectrum and cross-power spectrum were derived.At last,some numerical results of radar echo spectra for randomly oriented and horizontally oriented chaff clouds were presented.

VHF radar echoes in the vicinity of tropopause during the passage of tropical cyclone: First observations from the Gadanki MST radar

First observations on the characteristics of VHF radar echoes in the vicinity of tropopause (VOT) during depression or passage of tropical cyclone made using the Gadanki MST radar are presented. The most significant and new observation is that signal‐to‐noise ratio (SNR) in zenith and off‐zenith beams are nearly equal in the VOT when a cyclone is located close to the radar site. During depression, however, a slight decrease in echo strength with zenith angle was found. Observations made during such disturbed conditions are in contrast to the usual aspect‐sensitive characteristics of the radar echoes. Spectral widths during disturbed atmospheric conditions in general are found to be much larger than that of normal condition. Further, spectral widths observed in the vertical beam during cyclone are considerably larger than that during normal condition. These features, however, have not been noticed at other height regions during such atmospheric conditions. All these observations clearly indicate preferential enhancement in turbulence activity in the VOT. These observations constitute the first experimental evidence of such a zenith angle dependence of VHF radar echo characteristics in the VOT during depression and cyclone. These results are presented and discussed in the light of present understanding of turbulence activity in the VOT.

Identification and Removal of Ground Echoes and Anomalous Propagation Using the Characteristics of Radar Echoes

Abstract This paper explores the removal of normal ground echoes (GREs) and anomalous propagation (AP) in ground-based radars using a fuzzy logic approach. Membership functions and their weights are derived from the characteristics of radar echoes as a function of radar reflectivity. The dependence on echo intensity is shown to significantly improve the proper identification of GRE/AP. In addition, the proposed method has a better performance at lower elevation angles. The overall performance is comparable with that from a polarimetric approach and can thus be easily implemented in operational radars.

Characteristics of Radar Echoes and Meteorological Conditions Related to Tornadoes

Characteristics of Radar Echoes and Meteorological Conditions Related to Tornadoes

Meteorological Value of Ground Target Measurements by Radar

Many characteristics of radar echoes from ground targets vary with time as the properties of the atmosphere in which the radar waves propagate evolve. For example, if the phase of a target varies with time and the target is known to be stationary, that phase variation is related to changes in the refractive index of air between the radar and that target. These changes are themselves caused by variations in the pressure, temperature, and especially the humidity of air. The changing phase and intensity of ground targets are hence records of evolving atmospheric conditions and could therefore in theory be used to retrieve parameters of meteorological value. In this paper, the various ways meteorological conditions can affect radar returns from ground targets are explored and quantified. In particular, the extent with which the changes in the phase of ground targets can be used to extract weather-related information is investigated. While wind, precipitation, and the vertical structure of temperature and humidity all affect ground echo appearance, it is demonstrated that the most promising quantity to be obtained is the refractive index of air near the ground. The process and the uncertainty of refractive index measurements by radar are then described. Comparisons with surface stations indicate that near-surface refractive index can be obtained reasonably accurately by radar in very flat terrain. In more complex topography, sensitivity analyses show that radar-based refractive index measurements will be strongly affected by the vertical profile of refractive index.

Doppler radar sounding of volcanic eruption dynamics at Mount Etna

Besides their common use in atmospheric studies, Doppler radars are promising tools for the active remote sensing of volcanic eruptions but were little applied to this field. We present the observations made with a mid-power UHF Doppler radar (Voldorad) during a 7-h Strombolian eruption at the SE crater of Mount Etna on 11–12 October 1998. Main characteristics of radar echoes are retrieved from analysis of Doppler spectra recorded in the two range gates on either side of the jet axis. From the geometry of the sounding, the contribution of uprising and falling ejecta to each Doppler spectrum can be discriminated. The temporal evolution of total power backscattered by uprising targets is quite similar to the temporal evolution of the volcanic tremor and closely reproduces the overall evolution of the eruption before, during and after its paroxysm. Moreover, during the sharp decrease of eruptive activity following the paroxysm, detailed analysis of video (from camera recording), radar and seismic measurements reveals that radar and video signals start to decrease simultaneously, approximately 2.5 min after the tremor decline. This delay is interpreted as the ascent time through a magma conduit of large gas slugs from a shallow source roughly estimated at about 500 m beneath the SE crater. Detailed analysis of eruptive processes has been also made with Voldorad operating in a high sampling rate mode. Signature of individual outburst is clearly identified on the half part of Doppler spectra corresponding to rising ejecta: temporal variations of the backscattered power exhibit quasi periodic undulations, whereas the maximum velocity measured on each spectrum displays a sharp peak at the onset of each outburst followed by a slow decay with time. Periodicity of power variations (between 3.8 and 5.5 s) is in agreement with the occurrence of explosions visually observed at the SE vent. Maximum vertical velocities of over 160 m s−1 were measured during the paraoxysmal stage and the renewed activity. Finally, by using a simplified model simulating the radar echoes characteristics, we show that when Voldorad is operating in high sampling rate mode, the power and maximum velocity variations are directly related to the difference in size and velocity of particles crossing the antenna beam.

The spectral characteristics of E-region radar echoes co-located with and adjacent to visual auroral arcs

Abstract. Simultaneous all-sky camera and HF radar observations of the visual and E-region radar aurora in the west-ward electrojet suggest a close relationship between a pair of parallel east-west-aligned auroral arcs, separated by ~ 30 km, and a region of strong radar backscatter. Poleward of this a broader region of radar backscatter is observed, though the spectral characteristics of the echoes in these two regions differ considerably. We suggest that the visual aurorae and their radar counterparts are produced in a region of upward field-aligned current (FAC), whereas the backscatter poleward of this is associated with downward FAC. Relatively low electric fields ( ~ 10 mV m-1) are observed in the vicinity of the arc system, suggesting that in this case, two-stream waves are not directly generated through the electrodynamics of the arc. Rather, the generation of irregularities is most probably associated with the gradient drift instability operating within horizontal electron density gradients produced by the filamentary nature of the arc FAC system. The observation of high Doppler shift echoes superimposed on slow background flow within the region of backscatter poleward of the visual aurora is argued to be consistent with previous suggestions that the ion-acoustic instability threshold is reduced in the presence of upwelling thermal electrons carrying downward FAC.Key words. Ionosphere (auroral ionosphere; ionospheric irregularities; particle precipitation)