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1
Averbuch, Gil, Jelle D. Assink e Läslo G. Evers. "Long-range atmospheric infrasound propagation from subsurface sources". Journal of the Acoustical Society of America 147, n. 2 (febbraio 2020): 1264–74. http://dx.doi.org/10.1121/10.0000792.
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Gibson, Robert G., e David E. Norris. "Long‐range infrasound propagation modeling using updated atmospheric characterizations". Journal of the Acoustical Society of America 112, n. 5 (novembre 2002): 2380. http://dx.doi.org/10.1121/1.4779677.
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Hart, Carl R., D. Keith Wilson, Chris L. Pettit e Edward T. Nykaza. "Machine-learning of long-range sound propagation through simulated atmospheric turbulence". Journal of the Acoustical Society of America 149, n. 6 (giugno 2021): 4384–95. http://dx.doi.org/10.1121/10.0005280.
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4
Eisenmann, Shmuel, Einat Louzon, Yiftach Katzir, Tala Palchan, Arie Zigler, Yonatan Sivan e Gadi Fibich. "Control of the filamentation distance and pattern in long-range atmospheric propagation". Optics Express 15, n. 6 (2007): 2779. http://dx.doi.org/10.1364/oe.15.002779.
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Lim, Tea Heung, Minho Go, Chulhun Seo e Hosung Choo. "Analysis of the Target Detection Performance of Air-to-Air Airborne Radar Using Long-Range Propagation Simulation in Abnormal Atmospheric Conditions". Applied Sciences 10, n. 18 (16 settembre 2020): 6440. http://dx.doi.org/10.3390/app10186440.
Testo completoAbstract (sommario):
In this paper, we propose the analysis of the target detection performance of air-to-air airborne radars using long-range propagation simulations with a novel quad-linear refractivity model under abnormal atmospheric conditions. The radar propagation characteristics and the target detection performance are simulated using the Advanced Refractive Effects Prediction System (AREPS) software, where the refractivity along the altitude, array antenna pattern, and digital terrain elevation data are considered as inputs to obtain the path loss of the wave propagation. The quad-linear model is used to approximate the actual refractivity data, which are compared to the data derived using the conventional trilinear refractivity model. On the basis of the propagation simulations, we propose a detection performance metric in terms of the atmosphere (DPMA) for intuitively examining the long-range propagation characteristics of airborne radars in air-to-air situations. To confirm the feasibility of using the DPMA map in various duct scenarios, we employ two actual refractive indices to observe the DPMA results in relation to the height of the airborne radar.
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Drob, D. P., D. Broutman, M. A. Hedlin, N. W. Winslow e R. G. Gibson. "A method for specifying atmospheric gravity wavefields for long-range infrasound propagation calculations". Journal of Geophysical Research: Atmospheres 118, n. 10 (20 maggio 2013): 3933–43. http://dx.doi.org/10.1029/2012jd018077.
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Rajendran, K., e A. Kitoh. "Modulation of Tropical Intraseasonal Oscillations by Ocean–Atmosphere Coupling". Journal of Climate 19, n. 3 (1 febbraio 2006): 366–91. http://dx.doi.org/10.1175/jcli3638.1.
Testo completoAbstract (sommario):
Abstract The impact of ocean–atmosphere coupling on the structure and propagation characteristics of 30–60-day tropical intraseasonal oscillations (TISOs) is investigated by analyzing long-term simulations of the Meteorological Research Institute coupled general circulation model (CGCM) and its stand-alone atmospheric general circulation model (AGCM) version forced with SSTs derived from the CGCM and comparing them with recent observation datasets [Global Precipitation Climatology Project (GPCP) precipitation, 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40), and Reynolds SST]. Composite events of (i) eastward propagating Madden–Julian oscillations (MJOs) during boreal winter and (ii) northward propagating intraseasonal oscillations (NPISOs) during boreal summer, constructed based on objective criteria, show that the three-dimensional structure, amplitude, and speed of propagation, and the phase relationship among surface fluxes, SST, and convection, are markedly improved in the CGCM simulation. Consistent with the frictional wave conditional instability of the second kind mechanism, successive development of low-level convergence to the east (north) of deep convection was found to be important for eastward (northward) propagation of MJO (NPISO). Complex interaction between large-scale dynamics and convection reveals the importance of atmospheric dynamics and suggests that they are intrinsic modes in the atmosphere where coupling is not essential for their existence. However, as in observations, realistic coupling in the CGCM is found to result in the evolution of TISOs as coupled modes through a coherent coupled feedback process. This acts as an amplifying mechanism for the existing propagating convective anomalies and plays an important modifying role toward a more realistic simulation of TISOs. In contrast, the simulated TISOs in its atmosphere-alone component lack many of the important features associated with their amplitude, phase, and life cycle. Thus, a realistic representation of the interaction between sea surface and the atmospheric boundary layer is crucial for a better simulation of TISOs.
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Tahira, Makoto. "A Study of the Long Range Propagation of Infrasonic Waves in the Atmosphere". Journal of the Meteorological Society of Japan. Ser. II 66, n. 1 (1988): 17–26. http://dx.doi.org/10.2151/jmsj1965.66.1_17.
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Hussain, Hammad, e Guillaume Dutilleux. "A parametric study of long-range atmospheric sound propagation using Bellhop Ray-tracing Model". Journal of the Acoustical Society of America 148, n. 4 (ottobre 2020): 2562. http://dx.doi.org/10.1121/1.5147110.
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Waxler, Roger, Claus H. Hetzer, Jelle D. Assink e Philip Blom. "A two-dimensional effective sound speed parabolic equation model for infrasound propagation with ground topography". Journal of the Acoustical Society of America 152, n. 6 (dicembre 2022): 3659–69. http://dx.doi.org/10.1121/10.0016558.
Testo completoAbstract (sommario):
A terrain capable parabolic equation (PE) propagation algorithm for long range infrasound propagation modeling has been implemented using Padé approximations for the various operator valued functions that arise in PE algorithms. In this work, the influence of the winds are captured by the effective sound speed approximation and propagation is restricted to the range-altitude plane. The ground topography is included by the addition of an impenetrable fluid below the ground surface. The impedance condition at the ground is handled explicitly, including both vertical and radial components. It is found that including terrain can have a large influence on long range propagation. In particular, reflections from a sufficiently steep slope can change the inclination angle enough to move the propagation path from one atmospheric duct to another.
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Arrowsmith, Stephen, Petru Negraru e Greg Johnson. "Bolide Energetics and Infrasound Propagation: Exploring the 18 December 2018 Bering Sea Event to Identify Limitations of Empirical and Numerical Models". Seismic Record 1, n. 3 (1 ottobre 2021): 164–71. http://dx.doi.org/10.1785/0320210034.
Testo completoAbstract (sommario):
Abstract Infrasound observations are an important tool in assessing the energetics of bolides and can help quantify the flux of meteoroids through Earth’s atmosphere. Bolides are also important atmospheric sources for assessing long-range infrasound propagation models and can be used as benchmark events for validating the International Monitoring System (IMS) infrasound network, which is designed to detect nuclear tests in the atmosphere. This article exploits unique infrasound observations from a large bolide recorded on IMS infrasound arrays and high-density infrasound deployments in the United States to assess limitations in infrasound source scaling relationships. The observations provide an unprecedented sampling of infrasound propagation along a transect at an azimuth of 60° from the source to a distance of ∼8000 km. Widely used empirical laws for assessing bolide energetics and state-of-the-art numerical models for simulating infrasound propagation are assessed to quantify important discrepancies with the observations. In particular, empirical laws for equivalent yield, which are based on signal period and are assumed to be relatively unaffected by propagation effects, can be heavily contaminated by site noise. In addition, by modeling infrasound propagation over a range of ∼8000 km, we show that state-of-the-art models do not reproduce the observed amplitude decay over this long range (which decays by a rate of at least 2 higher than can be modeled).
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Hazra, Abheera, e V. Krishnamurthy. "Space–Time Structure of Diabatic Heating in Monsoon Intraseasonal Oscillation". Journal of Climate 28, n. 6 (13 marzo 2015): 2234–55. http://dx.doi.org/10.1175/jcli-d-14-00280.1.
Testo completoAbstract (sommario):
Abstract The space–time structure of the leading monsoon intraseasonal oscillation (MISO) in three-dimensional diabatic heating is studied. Using the ERA-Interim data of the European Centre for Medium-Range Weather Forecasts, the diabatic heating data were constructed by the residual method of the thermodynamic equation. The MISO was extracted by applying multichannel singular spectrum analysis on the daily anomalies of three-dimensional diabatic heating over the South Asian monsoon region for the period 1979–2011.The diabatic heating MISO has a period of 45 days, and exhibits eastward propagation in the equatorial Indian and Pacific Oceans and northward propagation over the entire monsoon region. The horizontal structure shows a long tilted band of heating anomalies propagating northeastward. The period, horizontal pattern, and propagation properties of the diabatic heating MISO are similar to those found in precipitation, outgoing longwave radiation, and circulation in earlier studies. The vertical structure of the diabatic heating MISO indicates deep columns, with maximum values at about 450 hPa, propagating northeastward. The vertical structure of the heating anomalies has good correspondence with that of the moisture anomalies but with a phase difference. The moisture anomalies lead the heating anomalies and may provide a preconditioning process for the propagation mechanism. The temperature anomalies also show oscillatory behavior corresponding to the diabatic heating MISO but the phase difference between the two varies from region to region.
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Laing, Arlene G., Richard E. Carbone e Vincenzo Levizzani. "Cycles and Propagation of Deep Convection over Equatorial Africa". Monthly Weather Review 139, n. 9 (settembre 2011): 2832–53. http://dx.doi.org/10.1175/2011mwr3500.1.
Testo completoAbstract (sommario):
Long-term statistics of organized convection are vital to improved understanding of the hydrologic cycle at various scales. Satellite observations are used to understand the timing, duration, and frequency of deep convection in equatorial Africa, a region with some of the most intense thunderstorms. Yet little has been published about the propagation characteristics of mesoscale convection in that region. Diurnal, subseasonal, and seasonal cycles of cold cloud (proxy for convective precipitation) are examined on a continental scale. Organized deep convection consists of coherent structures that are characteristic of systems propagating under a broad range of atmospheric conditions. Convection is triggered by heating of elevated terrain, sea/land breezes, and lake breezes. Coherent episodes of convection result from regeneration of convection through multiple diurnal cycles while propagating westward. They have an average 17.6-h duration and 673-km span; most have zonal phase speeds of 8–16 m s−1. Propagating convection occurs in the presence of moderate low-level shear that is associated with the southwesterly monsoonal flow and midlevel easterly jets. Convection is also modulated by eastward-moving equatorially trapped Kelvin waves, which have phase speeds of 12–22 m s−1 over equatorial Africa. Westward propagation of mesoscale convection is interrupted by the dry phase of convectively coupled Kelvin waves. During the wet phase, daily initiation and westward propagation continues within the Kelvin wave and the cold cloud shields are larger. Mesoscale convection is more widespread during the active phase of the Madden–Julian oscillation (MJO) but with limited westward propagation. The study highlights multiscale interaction as a major source of variability in convective precipitation during the critical rainy seasons in equatorial Africa.
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Vecchiotti, Andrea, Hannah Blackburn, Kyle Kirian, Joseph Vignola, Diego Turo, Jeff Foeller e Teresa J. Ryan. "Scanning Doppler LIDAR wind profiles to inform near shore atmospheric acoustic propagation modeling". Journal of the Acoustical Society of America 152, n. 4 (ottobre 2022): A57. http://dx.doi.org/10.1121/10.0015531.
Testo completoAbstract (sommario):
This work presents a comprehensive experimental system to measure concurrent atmospheric acoustic transmission loss and meteorological conditions. A three-dimensional scanning Doppler lidar wind profiler captures real-time wind speed gradients at many locations along the acoustic propagation path of a simple pitch catch style study. A long-range acoustic device on an anchored pontoon sends known chirp sequences to a seven-channel receiver array at the water’s edge at ranges up to approximately one kilometer. Additional synchronized meteorological observations include temperature, humidity, and wind measured with anemometers. The meteorological data stream is used to inform the sound speed gradient implemented in a parabolic equation based numerical model of atmospheric acoustic propagation. The model can account for sea surface roughness and accommodate a sound speed profile that changes along the propagation range. Model predictions are compared to measured transmission losses. An assessment of the value of the computational cost of incorporating the varying sound speed profiles in the model is presented.
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Bennett, A. J., C. Gaffard, J. Nash, G. Callaghan e N. C. Atkinson. "The Effect of Modal Interference on VLF Long-Range Lightning Location Networks Using the Waveform Correlation Technique". Journal of Atmospheric and Oceanic Technology 28, n. 8 (1 agosto 2011): 993–1006. http://dx.doi.org/10.1175/2011jtecha1527.1.
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Abstract The arrival time difference (ATD) long-range lightning location network (ATDnet) is the long-range very low frequency (VLF) lightning location network owned and operated by the Met Office, locating lightning using a waveform correlation technique. Pronounced differences in the waveform shape with distance to lightning have been observed and attributed to interference between different propagation modes within the earth–ionosphere waveguide. During the day, waveform correlations were significantly degraded at a distance of 450 km from the sensor, with the main degradations occurring during the night centered at propagation distances of 650 and 2150 km. The observation and simple modeling of modal interference spacing allowed the effective ionospheric height during summer over Europe to be estimated for day and night as 69 and 88.5 km, respectively, for 13.7 kHz. Wider distribution of sensor sites, lowering of the receiver frequency, and adaptation of the reference waveform selection criteria have been suggested to mitigate the effect of this interference on network performance.
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Kulichkov, S. N., I. P. Chunchuzov e O. I. Popov. "Simulating the influence of an atmospheric fine inhomogeneous structure on long-range propagation of pulsed acoustic signals". Izvestiya, Atmospheric and Oceanic Physics 46, n. 1 (febbraio 2010): 60–68. http://dx.doi.org/10.1134/s0001433810010093.
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Zhang, Tianshu, Steven A. Miller, Mariel T. Ojeda e Kurtis Gurley. "Prediction of long-range infrasound propagation from tornadoes based on new atmospheric boundary layer wind tunnel experiments". Journal of the Acoustical Society of America 148, n. 4 (ottobre 2020): 2493. http://dx.doi.org/10.1121/1.5146910.
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Parihar, N., e A. Taori. "An investigation of long-distance propagation of gravity waves under CAWSES India Phase II Programme". Annales Geophysicae 33, n. 5 (18 maggio 2015): 547–60. http://dx.doi.org/10.5194/angeo-33-547-2015.
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Abstract. Coordinated measurements of airglow features from the mesosphere–lower thermosphere (MLT) region were performed at Allahabad (25.5° N, 81.9° E) and Gadanki (13.5° N, 79.2° E), India to study the propagation of gravity waves in 13–27° N latitude range during the period June 2009 to May 2010 under CAWSES (Climate And Weather of Sun Earth System) India Phase II Programme. At Allahabad, imaging observations of OH broadband emissions and OI 557.7 nm emission were made using an all-sky imager, while at Gadanki photometric measurements of OH (6, 2) Meinel band and O2 (0, 1) Atmospheric band emissions were carried out. On many occasions, the nightly observations reveal the presence of similar waves at both locations. Typically, the period of observed similar waves lay in the 2.2–4.5 h range, had large phase speeds (~ 77–331 m s−1) and large wavelengths (~ 1194–2746 km). The images of outgoing long-wave radiation activity of the National Oceanic and Atmospheric Administration (NOAA) and the high-resolution infrared images of KALPANA-1 satellite suggest that such waves possibly originated from some nearby convective sources. An analysis of their propagation characteristics in conjunction with SABER/TIMED temperature profiles and Horizontal Wind Model (HWM 2007) wind estimates suggest that the waves propagated over long distances (~ 1200–2000 km) in atmospheric ducts.
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Zeng, Qingwei, Lei Liu, Kejin Zhang, Shuai Hu, Taichang Gao, Chensi Weng e Ming Chen. "Numerical Investigation on the Influence of Water Vapor Ionization on the Dynamic and Energy Deposition of Femtosecond Ultraviolet Laser Filamentation in Air". Applied Sciences 9, n. 20 (9 ottobre 2019): 4201. http://dx.doi.org/10.3390/app9204201.
Testo completoAbstract (sommario):
The effects of water vapor ionization on the nonlinear propagation of femtosecond laser pulses with a 248 nm wavelength are numerically investigated in this paper. It is found that ionization of H2O molecules plays a significant role in air ionization, which seriously affects the dynamic and energy deposition of filamentation. The propagation of femtosecond pulses in air with different humidity levels are compared. The total number of electrons and total deposited pulse energy increase with the humidity increases. However, they tend to be saturated in high humidity conditions. Results presented here are conducive to characterizing the long-range propagation of filaments under atmospheric conditions.
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LaBelle, J. "High-latitude propagation studies using a meridional chain of LF/MF/HF receivers". Annales Geophysicae 22, n. 5 (8 aprile 2004): 1705–18. http://dx.doi.org/10.5194/angeo-22-1705-2004.
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Abstract. For over a decade, Dartmouth College has operated programmable radio receivers at multiple high-latitude sites covering the frequency range 100-5000kHz with about a 1-s resolution. Besides detecting radio emissions of auroral origin, these receivers record characteristics of the ionospheric propagation of natural and man-made signals, documenting well-known effects, such as the diurnal variation in the propagation characteristics of short and long waves, and also revealing more subtle effects. For example, at auroral zone sites in equinoctial conditions, the amplitudes of distant transmissions on MF/HF frequencies are often enhanced by a few dB just before they fade away at dawn. The polarization and/or direction of the arrival of ionospherically propagating signals in the lower HF range (3-5MHz) show a consistent variation between pre-midnight, post-midnight, and pre-dawn conditions. As is well known, magnetic storms and substorms dramatically affect ionospheric propagation; data from multiple stations spanning the invariant latitude range 67-79° reveal spatial patterns of propagation characteristics associated with magnetic storms and substorms. For example, in the hours preceding many isolated substorms, favorable propagation conditions occur at progressively lower latitudes as a function of time preceding the substorm onset. For some of these effects, explanations follow readily from elementary ionospheric physics, but understanding others requires further investigation.Key words. Magnetospheric physics (annual phenomena) – Radio science (ionosphere propagation; radio-wave propagation)6
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Rapaport, Liat, Gad A. Pinhasi e Yosef Pinhasi. "Millimeter Wave Propagation in Long Corridors and Tunnels—Theoretical Model and Experimental Verification". Electronics 9, n. 5 (26 aprile 2020): 707. http://dx.doi.org/10.3390/electronics9050707.
Testo completoAbstract (sommario):
The development of the Fifth-Generation (5G) of cellular communications considers bands in millimeter waves (MMW) for indoor, short-range links. The propagation of MMW is affected by atmospheric and weather conditions, specular reflections from surfaces, and the directivity of the antennas. The short wavelength enables utilization of a quasi-optical propagation model for the description of indoor multi-path scenarios. A study of MMW propagation in tunnels, long corridors, or canyons is carried out using ray-tracing to evaluate the link budget and group delay. The analysis considers radiation patterns of both transmitting and receiving antennas, deriving a criterion for the number of dominating rays. Error analysis demonstrates the convergence of the method, while using a finite number of reflected rays. Experiments in a small-scale tunnel model demonstrate the accuracy of the analysis.
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Menke, Robert, Nikola Vasiljević, Kurt S. Hansen, Andrea N. Hahmann e Jakob Mann. "Does the wind turbine wake follow the topography? A multi-lidar study in complex terrain". Wind Energy Science 3, n. 2 (10 ottobre 2018): 681–91. http://dx.doi.org/10.5194/wes-3-681-2018.
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Abstract. The wake of a single wind turbine in complex terrain is analysed using measurements from lidars. A particular focus of this analysis is the wake deficit and propagation. Six scanning lidars (three short-range and three long-range WindScanners) were deployed during the Perdigão 2015 measurement campaign, which took place at a double-ridge site in Portugal. Several scanning scenarios, including triple- and dual-Doppler scans, were designed to capture the wind turbine wake of a 2 MW turbine located on one of the ridges. Different wake displacements are categorized according to the time of the day. The results show a strong dependence of the vertical wake propagation on the atmospheric stability. When an atmospheric wave is observed under stable conditions, the wake follows the terrain down the ridge with a maximum inclination of -28∘. During unstable conditions, the wake is advected upwards by up to 29∘ above the horizontal plane.
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Ko, Ken-Chung, e Huang-Hsiung Hsu. "Downstream Development of the Summertime Tropical Cyclone/Submonthly Wave Pattern in the Extratropical North Pacific". Journal of Climate 23, n. 8 (15 aprile 2010): 2223–29. http://dx.doi.org/10.1175/2009jcli3248.1.
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Abstract The impact of tropical perturbation on the extratropical wave activity in the North Pacific in the submonthly time scale is demonstrated here. Previous studies identified a tropical cyclone (TC)/submonthly wave pattern, which propagated north-northwestward in the Philippine Sea and recurved in the oceanic region between Japan and Taiwan. This study found that, after the arrival of the TC/submonthly wave pattern at the recurving region, the eastward-propagating wave activity in the extratropical North Pacific was significantly enhanced. It is suggested that the TC/submonthly wave pattern, which is originated in the tropical western North Pacific, enhances the eastward energy propagation of Rossby wave–like perturbation in the extratropical North Pacific and may have an impact on the long-range weather predictability in the eastern North Pacific and western North America.
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SUBRAMANIAN, S. K., e U. R. JOSHI. "Usability of the low frequency mode in southwest monsoon circulation for long range prediction of rainfall activity over central India". MAUSAM 45, n. 1 (1 gennaio 2022): 43–48. http://dx.doi.org/10.54302/mausam.v45i1.1874.
Testo completoAbstract (sommario):
pro pagation have been ctudicd on the hasi, of upper air data of a few sta tions,The frequency of occurrence Ill' significant periodicity in th is mode i.. rchuively high for Visakhapatnam andMadras. 1 here appears a large inter-annual variability of the maximum amplitudes of rhe filtered series with nospecial preference to any latitudinal bell. Northward propagation of this mode also slums large inn.....-annunlvarisbility. In some ~ears the propaga tion " as totally absent. The phusc changes in the filtered ser ies o fVisakbapnma rn match ed with the cha nges in weekly ra infall activity over central India and thi.. may, pcrhups.be used to foreshadow the activ ity of the monsoon over central India ,
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Bramberger, Martina, Andreas Dörnbrack, Henrike Wilms, Steffen Gemsa, Kevin Raynor e Robert Sharman. "Vertically Propagating Mountain Waves—A Hazard for High-Flying Aircraft?" Journal of Applied Meteorology and Climatology 57, n. 9 (settembre 2018): 1957–75. http://dx.doi.org/10.1175/jamc-d-17-0340.1.
Testo completoAbstract (sommario):
AbstractStall warnings at flight level 410 (12.5 km) occurred unexpectedly during a research flight of the High Altitude and Long Range Research Aircraft (HALO) over Italy on 12 January 2016. The dangerous flight situation was mitigated by pilot intervention. At the incident location, the stratosphere was characterized by large horizontal variations in the along-track wind speed and temperature. On this particular day, strong northwesterly winds in the lower troposphere in concert with an aligned polar front jet favored the excitation and vertical propagation of large-amplitude mountain waves at and above the Apennines in Italy. These mountain waves carried large vertical energy fluxes of 8 W m−2 and propagated without significant dissipation from the troposphere into the stratosphere. While turbulence is a well-acknowledged hazard to aviation, this case study reveals that nonbreaking, vertically propagating mountain waves also pose a potential hazard, especially to high-flying aircraft. It is the wave-induced modulation of the ambient along-track wind speed that may decrease the aircraft speed toward the minimum needed stall speed.
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Ma, Qian, e Hengkai Zhao. "Capacity of a Radio Vortex Communication System Using a Partial Angular Aperture Receiving Scheme under the Horizontal Non-Kolmogorov Model". Sensors 21, n. 5 (4 marzo 2021): 1778. http://dx.doi.org/10.3390/s21051778.
Testo completoAbstract (sommario):
A partial receiving scheme based on limited angular aperture multi-beam receiving and demultiplexing can solve the difficulty caused by the divergence of the vortex beam in the conventional whole beam receiving scheme and realize the long-distance transmission of the vortex wave. The propagation of the radio vortex beam in atmospheric turbulence is of significant importance in theoretical study and practical applications. In this paper, the influence of atmospheric turbulence on the performance of a radio vortex (RV) communication system based on a partial angular aperture receiving (PAAR) scheme under the horizontal non-Kolmogorov channel model is studied. The spiral spectrum of the PAAR scheme and the channel capacity of the RV communication system using the PAAR scheme are derived. Simulation results demonstrate that the selected transmission frequency range has a great influence on the RV communication system based on the PAAR scheme, and the choice of the orbital angular momentum (OAM) mode number L has an influence on the propagation distance. The capacity of RV communication systems based on the PAAR scheme increases with the increase of the transmission frequency in the selected transmission frequency range of 10 GHz–60 GHz. When the number of orbital angular momentum (OAM) modes L is small, we can improve the signal-to-noise ratio (SNR) to obtain a larger capacity of the RV communication system based on the PAAR scheme over a longer propagation distance.
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Huang, Li-Feng, Cheng-Guo Liu, Hong-Guang Wang, Qing-Lin Zhu, Li-Jun Zhang, Jie Han, Yu-Sheng Zhang e Qian-Nan Wang. "Experimental Analysis of Atmospheric Ducts and Navigation Radar Over-the-Horizon Detection". Remote Sensing 14, n. 11 (27 maggio 2022): 2588. http://dx.doi.org/10.3390/rs14112588.
Testo completoAbstract (sommario):
Since the height of sea detection radar antenna and ship targets is relatively low, it is generally believed that its over-the-horizon detection is mainly caused by the evaporation duct at sea. To fully understand the influence of atmospheric ducts on radar over-the-horizon detection, a shore-based navigation radar was used to carry out over-the-horizon detection experiments; radiosondes were used to measure the atmospheric profile and evaporation duct monitoring equipment was used to measure the evaporation duct. Based on experimental data and model simulation, a comparative analysis of a navigation radar’s over-the-horizon detection, the evaporation duct, and the lower atmospheric duct is presented in this study. The results show that the atmospheric duct can affect the signal propagation of the navigation radar, thus resulting in over-the-horizon detection. The long-range over-the-horizon detection of the navigation radar is caused by the strong lower atmospheric duct, while the evaporation duct can generally only form weak over-the-horizon detection, which is different from the general cognition.
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Portele, Tanja C., Andreas Dörnbrack, Johannes S. Wagner, Sonja Gisinger, Benedikt Ehard, Pierre-Dominique Pautet e Markus Rapp. "Mountain-Wave Propagation under Transient Tropospheric Forcing: A DEEPWAVE Case Study". Monthly Weather Review 146, n. 6 (giugno 2018): 1861–88. http://dx.doi.org/10.1175/mwr-d-17-0080.1.
Testo completoAbstract (sommario):
The impact of transient tropospheric forcing on the deep vertical mountain-wave propagation is investigated by a unique combination of in situ and remote sensing observations and numerical modeling. The temporal evolution of the upstream low-level wind follows approximately a [Formula: see text] shape and was controlled by a migrating trough and connected fronts. Our case study reveals the importance of the time-varying propagation conditions in the upper troposphere and lower stratosphere (UTLS). Upper-tropospheric stability, the wind profile, and the tropopause strength affected the observed and simulated wave response in the UTLS. Leg-integrated along-track momentum fluxes ([Formula: see text]) and amplitudes of vertical displacements of air parcels in the UTLS reached up to 130 kN m−1 and 1500 m, respectively. Their maxima were phase shifted to the maximum low-level forcing by ≈8 h. Small-scale waves ([Formula: see text] km) were continuously forced, and their flux values depended on wave attenuation by breaking and reflection in the UTLS region. Only maximum flow over the envelope of the mountain range favored the excitation of longer waves that propagated deeply into the mesosphere. Their long propagation time caused a retarded enhancement of observed mesospheric gravity wave activity about 12–15 h after their observation in the UTLS. For the UTLS, we further compared observed and simulated [Formula: see text] with fluxes of 2D quasi-steady runs. UTLS momentum fluxes seem to be reproducible by individual quasi-steady 2D runs, except for the flux enhancement during the early decelerating forcing phase.
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An, Xiadong, Lifang Sheng, Qian Liu, Chun Li, Yang Gao e Jianping Li. "The combined effect of two westerly jet waveguides on heavy haze in the North China Plain in November and December 2015". Atmospheric Chemistry and Physics 20, n. 8 (21 aprile 2020): 4667–80. http://dx.doi.org/10.5194/acp-20-4667-2020.
Testo completoAbstract (sommario):
Abstract. Severe haze occurred in the North China Plain (NCP) from November to December 2015, with a wide spatial range and long duration. In this paper, the combined effect of the anomalous stationary Rossby waves within two westerly jet waveguides on this haze event in the NCP is investigated based on observational visibility data and NCEP/NCAR reanalysis data. The results show that circulation anomalies in Eurasia caused by the propagation of anomalous stationary Rossby wave energy along two waveguides within the westerly jet originating from the Mediterranean were responsible for haze formation in the NCP. The Rossby waves propagated eastward along the subtropical westerly jet and the polar front jet, causing an anomalous anticyclone over the Sea of Japan and anticyclonic wind shear at 850 hPa over the NCP, which enhanced the anomalous descent in the middle and lower troposphere and subsequently resulted in a stable lower atmosphere. Furthermore, the anomalous stationary Rossby waves propagating along the polar front jet weakened the East Asia trough and Ural ridge and strengthened the anomalous southerly wind at 850 hPa over the coastal areas of eastern China, decelerating the East Asia winter monsoon. The above meteorological conditions modulated haze accumulation in November and December 2015. Meanwhile, continuous rainfall related to ascending motion due to Rossby wave propagation along the waveguide provided by the subtropical westerly jet occurred in a large area of southern China. The associated latent heat release acted as a heat source, intensifying the ascending motion over southern China so that the descending motion over the NCP was strengthened, favoring the maintenance of severe haze. This study elucidates the formation and maintenance mechanism of large-scale haze in the NCP in late fall and boreal winter.
30
Smirnov, Alexandr, Marine De Carlo, Alexis Le Pichon, Nikolai M. Shapiro e Sergey Kulichkov. "Characterizing the oceanic ambient noise as recorded by the dense seismo-acoustic Kazakh network". Solid Earth 12, n. 2 (25 febbraio 2021): 503–20. http://dx.doi.org/10.5194/se-12-503-2021.
Testo completoAbstract (sommario):
Abstract. In this study, the dense seismo-acoustic network of the Institute of Geophysical Research (IGR), National Nuclear Centre of the Republic of Kazakhstan, is used to characterize the global ocean ambient noise. As the monitoring facilities are collocated, this allows for a joint seismo-acoustic analysis of oceanic ambient noise. Infrasonic and seismic data are processed using a correlation-based method to characterize the temporal variability of microbarom and microseism signals from 2014 to 2017. The measurements are compared with microbarom and microseism source model output that are distributed by the French Research Institute for Exploitation of the Sea (IFREMER). The microbarom attenuation is calculated using a semi-empirical propagation law in a range-independent atmosphere. The attenuation of microseisms is calculated taking into account seismic attenuation and bathymetry effect. Comparisons between the observed and predicted infrasonic and seismic signals confirm a common source mechanism for both microbaroms and microseisms. Multi-year and intra-seasonal parameter variations are analyzed, revealing the strong influence of long-range atmospheric propagation on microbarom predictions. In winter, dominating sources of microbaroms are located in the North Atlantic and in the North Pacific during sudden stratospheric warming events, while signals observed in summer could originate from sources located in the Southern Hemisphere; however, additional analyses are required to consolidate this hypothesis. These results reveal the strengths and weaknesses of seismic and acoustic methods and lead to the conclusion that a fusion of two techniques brought the investigation to a new level of findings. Summarized findings also provide a perspective for a better description of the source (localization, intensity, spectral distribution) and bonding mechanisms of the ocean–atmosphere–land interfaces.
31
Wang, Chung-Chieh, George Tai-Jen Chen, Hsiao-Ling Huang, Richard E. Carbone e Sau-Wa Chang. "Synoptic Conditions Associated with Propagating and Nonpropagating Cloud/Rainfall Episodes during the Warm Season over the East Asian Continent". Monthly Weather Review 140, n. 3 (1 marzo 2012): 721–47. http://dx.doi.org/10.1175/mwr-d-11-00067.1.
Testo completoAbstract (sommario):
Abstract Using European Centre for Medium-Range Weather Forecasts (ECMWF) analyses, this study identifies common synoptic flow patterns associated with propagating (long lived) and nonpropagating (short lived) precipitation/cloud episodes in the warm season over East Asia (25°–40°N, 95°–125°E). Among 123 initial cases during May–July 1997–2003, 86 were classified into three categories (propagating cases with strong and weak forcing, and nonpropagating ones with strong inhibition) consisting of 10 types based on their 500-hPa flow patterns. For each type, composites at various levels when episodes moved across 100°, 105°, 110°, 115°, and 120°E were made and discussed to elucidate the influence of synoptic conditions. For propagating episodes with strong 500-hPa forcing (46 cases), four types were identified. In their composites, favorable positive vorticity advection forcing from an approaching trough/short wave exists aloft, and concurrent development of southwest vortex and front/wind-shift line at low levels leads to enhanced southwesterly flow to provide vertical shear and moisture. For nonpropagating episodes with strong inhibition (25 cases), three types dominated by synoptic-scale subsidence from either negative vorticity advection or the subtropical high were identified. Thus, episodes weaken shortly after they move out from the Tibetan Plateau. There are also propagating episodes but with weak forcing (15 cases), for which another three types were found. The differences in their composites from nonpropagating types are mainly at low levels, where the southerly flow is stronger and can penetrate deeper inland. As stability conditions are similar for all type of episodes, the results suggest an important role of stronger low-level southwesterly winds in providing moisture and vertical wind shear to sustain episodes and their propagation.
32
SCHILLING, BRADLEY W., STEPHEN R. CHINN, BRIAN THOMAS e TIMOTHY J. SCHOLZ. "EYESAFE ACTIVE IMAGING OF HARD TARGETS: AN OVERVIEW OF TECHNIQUES UNDER INVESTIGATION BY NVESD". International Journal of High Speed Electronics and Systems 18, n. 02 (giugno 2008): 375–91. http://dx.doi.org/10.1142/s0129156408005412.
Testo completoAbstract (sommario):
The U.S. Army is actively pursuing 3D active imaging techniques using laser sources emitting at 1.5 μm. This eyesafe short wave infrared (SWIR) waveband is advantageous due to both the improved eye safety and atmospheric propagation through obscurants. NVESD has several active programs in this area, which will be reviewed in this paper. These are: 1) single-pixel scanned imaging laser radar, 2) 2D gated SWIR imaging, and 3) 3D-flash laser radar. These systems are being evaluated for various targeting scenarios, including as potential payloads on unmanned air-vehicles, ground vehicles and other sensor suites. Applications include low-cost long-range target identification, identification of heavily obscured targets, obstacle avoidance, and high resolution imaging.
33
Ji, Hanjie, Bo Yin, Jinpeng Zhang, Yushi Zhang, Qingliang Li e Chunzhi Hou. "Multiscale Decomposition Prediction of Propagation Loss for EM Waves in Marine Evaporation Duct Using Deep Learning". Journal of Marine Science and Engineering 11, n. 1 (29 dicembre 2022): 51. http://dx.doi.org/10.3390/jmse11010051.
Testo completoAbstract (sommario):
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.
34
Jones, Charles, Abheera Hazra e Leila M. V. Carvalho. "The Madden–Julian Oscillation and Boreal Winter Forecast Skill: An Analysis of NCEP CFSv2 Reforecasts". Journal of Climate 28, n. 15 (30 luglio 2015): 6297–307. http://dx.doi.org/10.1175/jcli-d-15-0149.1.
Testo completoAbstract (sommario):
Abstract The Madden–Julian oscillation (MJO) is the main mode of tropical intraseasonal variations and bridges weather and climate. Because the MJO has a slow eastward propagation and longer time scale relative to synoptic variability, significant interest exists in exploring the predictability of the MJO and its influence on extended-range weather forecasts (i.e., 2–4-week lead times). This study investigates the impact of the MJO on the forecast skill in Northern Hemisphere extratropics during boreal winter. Several 45-day forecasts of geopotential height (500 hPa) from NCEP Climate Forecast System version 2 (CFSv2) reforecasts are used (1 November–31 March 1999–2010). The variability of the MJO expressed as different amplitudes, durations, and recurrence (i.e., primary and successive events) and their influence on forecast skill is analyzed and compared against inactive periods (i.e., null cases). In general, forecast skill during enhanced MJO convection over the western Pacific is systematically higher than in inactive days. When the enhanced MJO convection is over the Maritime Continent, forecasts are lower than in null cases, suggesting potential model deficiencies in accurately forecasting the eastward propagation of the MJO over that region and the associated extratropical response. In contrast, forecasts are more skillful than null cases when the enhanced convection is over the western Pacific and during long, intense, and successive MJO events. These results underscore the importance of the MJO as a potential source of predictability on 2–4-week lead times.
35
Kocańda, Dorota, e Janusz Mierzyński. "The Effect of a Complex Stress State on Fatigue Crack Propagation and the Orientation of the Cracking Plane in VT3-1 Aeronautical Titanium Alloy". Fatigue of Aircraft Structures 2009, n. 1 (26 luglio 2009): 116–30. http://dx.doi.org/10.2478/v10164-010-0011-0.
Testo completoAbstract (sommario):
The Effect of a Complex Stress State on Fatigue Crack Propagation and the Orientation of the Cracking Plane in VT3-1 Aeronautical Titanium AlloyThe subject of the paper is the investigations of fatigue crack imitation and propagation in notched specimens made of the VT3-1 aeronautical russian titanium alloy under combined bending - torsion loading. The presence of short cracks was revealed at various ratios of bending to torsion. Experimental courses of short and long crack growth rates have been proved by the SEM and TEM micrographs which illustrated the changes in the mechanism of cracking in the examined specimens. The attempt was undertaken in order to explain partly brittle fracture that was observed in the range of fatigue short crack growth in the VT3-1 titanium alloy specimens. The results of the study of atmospheric hydrogen absorption capability and its ability for penetration inside the faces of nucleated and propagated microcracks in the surface layer allowed for suggestion that the cleavage mechanism of fracture found in the regime of short crack growth in the VT3-1 titanium alloy specimens was induced by hydrogen.
36
Shcherbina, Andrey Y., Daniel L. Rudnick e Lynne D. Talley. "Ice-Draft Profiling from Bottom-Mounted ADCP Data". Journal of Atmospheric and Oceanic Technology 22, n. 8 (1 agosto 2005): 1249–66. http://dx.doi.org/10.1175/jtech1776.1.
Testo completoAbstract (sommario):
Abstract The feasibility of ice-draft profiling using an upward-looking bottom-mounted acoustic Doppler current profiler (ADCP) is demonstrated. Ice draft is determined as the difference between the instrument depth, derived from high-accuracy pressure data, and the distance to the lower ice surface, determined by the ADCP echo travel time. Algorithms for the surface range estimate from the water-track echo intensity profiles, data quality control, and correction procedures have been developed. Sources of error in using an ADCP as an ice profiler were investigated using the models of sound signal propagation and reflection. The effects of atmospheric pressure changes, sound speed variation, finite instrument beamwidth, hardware signal processing, instrument tilt, beam misalignment, and vertical sensor offset are quantified. The developed algorithms are tested using the data from the winter-long ADCP deployment on the northwestern shelf of the Okhotsk Sea.
37
Füllekrug, M., C. Hanuise e M. Parrot. "Experimental simulation of satellite observations of 100 kHz radio waves from relativistic electron beams above thunderclouds". Atmospheric Chemistry and Physics 11, n. 2 (24 gennaio 2011): 667–73. http://dx.doi.org/10.5194/acp-11-667-2011.
Testo completoAbstract (sommario):
Abstract. Relativistic electron beams above thunderclouds emit 100 kHz radio waves which illuminate the Earth's atmosphere and near-Earth space. This contribution aims to clarify the physical processes which are relevant for the spatial spreading of the radio wave energy below and above the ionosphere and thereby enables an experimental simulation of satellite observations of 100 kHz radio waves from relativistic electron beams above thunderclouds. The simulation uses the DEMETER satellite which observes 100 kHz radio waves from fifty terrestrial Long Range Aid to Navigation (LORAN) transmitters. Their mean luminosity patch in the plasmasphere is a circular area with a radius of 300 km and a power density of 22 μW/Hz as observed at 660 km height above the ground. The luminosity patches exhibit a southward displacement of 450 km with respect to the locations of the LORAN transmitters. The displacement is reduced to 150 km when an upward propagation of the radio waves along the geomagnetic field line is assumed. This residual displacement indicates that the radio waves undergo 150 km sub-ionospheric propagation prior to entering a magnetospheric duct and escaping into near-Earth space. The residual displacement at low (L < 2.14) and high (L > 2.14) geomagnetic latitudes ranges from 100 km to 200 km which suggests that the smaller inclination of the geomagnetic field lines at low latitudes helps to trap the radio waves and to keep them in the magnetospheric duct. Diffuse luminosity areas are observed northward of the magnetic conjugate locations of LORAN transmitters at extremely low geomagnetic latitudes (L < 1.36) in Southeast Asia. This result suggests that the propagation along the geomagnetic field lines results in a spatial spreading of the radio wave energy over distances of 1 Mm. The summative assessment of the electric field intensities measured in space show that nadir observations of terrestrial 100 kHz radio waves, e.g., from relativistic electron beams above thunderclouds, are attenuated by at least 50 dB when taking into account a transionospheric attenuation of 40 dB.
38
Jameson, A. R. "Statistical Reliability of Neighboring Range Bin Estimates of Coherent Fractional Contributions to Radar Backscattered Power". Journal of Applied Meteorology and Climatology 50, n. 3 (1 marzo 2011): 745–49. http://dx.doi.org/10.1175/2010jamc2649.1.
Testo completoAbstract (sommario):
Abstract It was recently demonstrated that magnitudes of the power-normalized cross-correlation functions of complex amplitudes in neighboring range bins are identical to the fractional contributions made by radar coherent backscatter in the direction of propagation to the total backscattered power in rain and snow. Here, a theoretical framework is presented for calculating the noise associated with estimates of these normalized cross correlations. This noise is identical to the statistical uncertainties in . Radar signals consist of two components: the usual incoherent backscatter often modeled by a Gaussian process and a coherent component modeled for the purposes of these calculations by a phasor C of fixed magnitude that rotates at a constant angular velocity ωC. Using the representation of the cross-correlation function as the average over the real part of the phasor dot products, it is found that the noise in this function comes from the dot products of C with the incoherent-scatter phasors in each range bin as well as the dot product between the two incoherent phasors. Furthermore, as long as ωC ≠ 0 and the number of statistically independent realizations (samples) k is sufficiently large, the noise is represented well by a normal distribution with mean 0 and with a variance that goes as 1/(2k). It is then shown that as the magnitude of C increases it acts to suppress the variance of . A formula is derived that gives the standard deviation of as a function of the number of statistically independent samples in the observation and the observed value of . Two examples, one in rain and the other in snow, are also presented.
39
Boeckenstedt, Alexander, Jack McCrae, Santasri Bose-Pillai, Benjamin Wilson e Steven Fiorino. "Improving on Atmospheric Turbulence Profiles Derived from Dual Beacon Hartmann Turbulence Sensor Measurements". Applied Sciences 12, n. 12 (8 giugno 2022): 5822. http://dx.doi.org/10.3390/app12125822.
Testo completoAbstract (sommario):
Atmospheric turbulence is an inevitable source of wavefront distortion in all fields of long range laser propagation and sensing. However, the distorting effects of turbulence can be corrected using wavefront sensors contained in adaptive optics systems. Such systems also provide deeper insight into surface layer turbulence, which is not well understood. A unique method of profile generation by a dual source Hartmann Turbulence Sensor (HTS) technique is introduced here. Measurements of optical turbulence along a horizontal path were taken to create Cn2 profiles. Two helium-neon laser beams were directed over an inhomogeneous horizontal path and captured by the HTS. The measured differential tilt variances imposed on the laser wavefronts were used in conjunction with a set of computed weighting functions to profile the turbulence over the sensing path. The weighting function matrix is inherently ill-conditioned, therefore, Tikhonov regularization was applied to produce accurate Cn2 profiles. A distribution of sonic anemometers and a co-located boundary layer scintillometer (BLS) collected independent Cn2 measurements to add confidence to the HTS profiles. The Cn2 profiles generated by this approach agree very well with the auxiliary anemometer and scintillometer measurements. This method of producing turbulence profiles may be useful in future multi-conjugate adaptive optics applications.
40
Preusse, P., M. Ern, P. Bechtold, S. D. Eckermann, S. Kalisch, Q. T. Trinh e M. Riese. "Characteristics of gravity waves resolved by ECMWF". Atmospheric Chemistry and Physics 14, n. 19 (2 ottobre 2014): 10483–508. http://dx.doi.org/10.5194/acp-14-10483-2014.
Testo completoAbstract (sommario):
Abstract. Global model data from the European Centre for Medium-Range Weather Forecasts (ECMWF) are analyzed for resolved gravity waves (GWs). Based on fitted 3-D wave vectors of individual waves and using the ECMWF global scale background fields, backward ray tracing from 25 km altitude is performed. Different sources such as orography, convection and winter storms are identified. It is found that due to oblique propagation waves spread widely from narrow source regions. Gravity waves which originate from regions of strong convection are frequently excited around the tropopause and have in the ECMWF model low phase and group velocities as well as very long horizontal wavelengths compared to other models and to measurements. While the total amount of momentum flux for convective GWs changes little over season, GWs generated by storms and mountain waves show large day-to-day variability, which has a strong influence also on total hemispheric fluxes; from one day to the next the total hemispheric flux may increase by a factor of 3. Implications of these results for using the ECMWF model in predicting, analyzing and interpreting global GW distributions as well as implications for seamless climate prediction are discussed.
41
Zhou, Changyan, Ping Zhao e Junming Chen. "The Interdecadal Change of Summer Water Vapor over the Tibetan Plateau and Associated Mechanisms". Journal of Climate 32, n. 13 (18 giugno 2019): 4103–19. http://dx.doi.org/10.1175/jcli-d-18-0364.1.
Testo completoAbstract (sommario):
Abstract In recent decades, long-term changes of the Tibetan Plateau (TP) water vapor and the associated mechanisms have not been investigated fully. This study aims to examine the interdecadal change of summer TP water vapor using the monthly mean European Centre for Medium-Range Weather Forecasts interim reanalysis during 1979–2014. The results show a drier phase in the TP during 1979–94, with a subsequent wetter phase, which suggests an interdecadal variation of summer TP water vapor around the middle of the 1990s. This interdecadal variation is mainly due to a significant change of the water vapor export on the eastern boundary of the TP, which is closely associated with a summer atmospheric circulation anomaly near Lake Baikal. When a cyclonic (an anticyclonic) anomaly occurs near Lake Baikal, there is less (more) water vapor over the TP. On the interdecadal scale, the atmospheric circulation anomaly near Lake Baikal is related to an extratropical large-scale anomalous wave train over the northwestern Atlantic–East Asian region, with an eastward propagation of the anomalous wave energy from the Atlantic to East Asia. Climate model simulations further demonstrate an impact of sea surface temperature (SST) anomalies in the northwestern Atlantic on the anomalous wave train. Both the extratropical tropospheric anomalous wave train and the anomalous atmospheric circulation near Lake Baikal are successfully simulated by changing the summer northwestern Atlantic SST. Therefore the warming northwestern Atlantic is an important factor contributing to the wetting TP in recent decades.
42
Cotrim, Camila de Sa, Alvaro Semedo e Gil Lemos. "Brazil Wave Climate from a High-Resolution Wave Hindcast". Climate 10, n. 4 (31 marzo 2022): 53. http://dx.doi.org/10.3390/cli10040053.
Testo completoAbstract (sommario):
A detailed climatology of ocean wind waves in the South Atlantic Ocean, based on ERA-5 reanalysis and in a higher-resolution wave hindcast (ERA-5H), both developed by the European Centre for Medium-Range Weather Forecasts, is presented. The higher resolution of the wave fields in the ERA-5H (22 km) allowed for a better description of the wind sea and swell features compared to previous global and regional studies along the Brazilian coast. Overall, it is shown that swell waves are more prevalent and carry more energy in the offshore area of the study area, while wind sea waves dominate the nearshore regions, especially along the northern coast of Brazil. The influence of different climate indices on the significant wave heights patterns is also presented, with two behavioral groups showing opposite correlations to the North Atlantic Oscillation and Southern Annular Mode than to the Southern Oscillation Index. The analysis of the decadal trends of wind sea and swell heights during the ERA-5H period (1979–2020) shows that the long-term trends of the total significant wave height in the South Atlantic Ocean are mostly due to swell events and the wave propagation effect from Southern Ocean storms.
43
Elgered, Gunnar, Tong Ning, Peter Forkman e Rüdiger Haas. "On the information content in linear horizontal delay gradients estimated from space geodesy observations". Atmospheric Measurement Techniques 12, n. 7 (12 luglio 2019): 3805–23. http://dx.doi.org/10.5194/amt-12-3805-2019.
Testo completoAbstract (sommario):
Abstract. We have studied linear horizontal gradients in the atmospheric propagation delay above ground-based stations receiving signals from the Global Positioning System (GPS). Gradients were estimated from 11 years of observations from five sites in Sweden. Comparing these gradients with the corresponding ones from the European Centre for Medium-Range Weather Forecasts (ECMWF) analyses shows that GPS gradients detect effects over different timescales caused by the hydrostatic and the wet components. The two stations equipped with microwave-absorbing material below the antenna, in general, show higher correlation coefficients with the ECMWF gradients compared to the other three stations. We also estimated gradients using 4 years of GPS data from two co-located antenna installations at the Onsala Space Observatory. Correlation coefficients for the east and the north wet gradients, estimated with a temporal resolution of 15 min from GPS data, can reach up to 0.8 for specific months when compared to simultaneously estimated wet gradients from microwave radiometry. The best agreement is obtained when an elevation cut-off angle of 3∘ is applied in the GPS data processing, in spite of the fact that the radiometer does not observe below 20∘. We also note a strong seasonal dependence in the correlation coefficients, from 0.3 during months with smaller gradients to 0.8 during months with larger gradients, typically during the warmer and more humid part of the year. Finally, a case study using a 15 d long continuous very-long-baseline interferometry (VLBI) campaign was carried out. The comparison of the gradients estimated from VLBI and GPS data indicates that a homogeneous and frequent sampling of the sky is a critical parameter.
44
Ermakova, Tatiana S., Andrey V. Koval, Sergei P. Smyshlyaev, Ksenia A. Didenko, Olga G. Aniskina, Elena N. Savenkova e Ekaterina V. Vinokurova. "Manifestations of Different El Niño Types in the Dynamics of the Extratropical Stratosphere". Atmosphere 13, n. 12 (16 dicembre 2022): 2111. http://dx.doi.org/10.3390/atmos13122111.
Testo completoAbstract (sommario):
The behavior of planetary waves and their influence on the global circulation of the Northern Hemisphere during different El Niño types is studied. Three sets of five boreal winters were chosen for each El Niño type: Modoki I and II and canonical El Niño. Based on data of the Japanese 55-year Reanalysis and the Modern-Era Retrospective Analysis for Research and Applications, the spatio-temporal structure of planetary waves and the residual mean circulation were analyzed. The results show that the canonical El Niño type is characterized by the weakest wave activity in March. It is also demonstrated that warming of the polar stratosphere, accompanied by maximizing wave activity and weakening of the zonal wind, may lead to earlier stratospheric polar vortex collapse and the early spring transition under Modoki I conditions. This study is the next step in understanding of the so-called long-range teleconnections, consisting of the propagation of a signal from the tropical El Niño Southern Oscillation source into the polar stratosphere.
45
Kalashnik, Maxim V., e Otto Chkhetiani. "Generation of Gravity Waves by Singular Potential Vorticity Disturbances in Shear Flows". Journal of the Atmospheric Sciences 74, n. 1 (1 gennaio 2017): 293–308. http://dx.doi.org/10.1175/jas-d-16-0134.1.
Testo completoAbstract (sommario):
Abstract The linear mechanism of generation of gravity waves by potential vorticity (PV) disturbances in flows with constant horizontal and vertical shears is studied. The case of the initial singular distribution of PV, in which the PV is localized in one coordinate and is periodic with respect to other coordinates, is considered. In a stratified rotating medium, such a distribution induces a vortex wave (continuous mode), the propagation of which is accompanied by the emission of gravity waves. To find the emission characteristics, a linearized system of dynamical equations is reduced to wave equations with sources that are proportional to the initial distributions of PV. The asymptotic solutions of the equations are constructed for small Rossby numbers (horizontal shear) and large Richardson numbers (vertical shear). When passing through the inertial levels symmetrically located with respect to a vortex source, the behavior of the solutions for wave amplitudes radically changes. Directly in the vicinity of the source, the solutions are of monotonic character, corresponding to a quasigeostrophic vortex wave. At long distances from the source, the solutions oscillate. The horizontal momentum flux and the Eliassen–Palm flux are estimated using asymptotic solutions. It is found that, within the indicated range of both Rossby and Richardson numbers, these fluxes are exponentially small: that is, the emission of waves is weak.
46
Strube, Cornelia, Peter Preusse, Manfred Ern e Martin Riese. "Propagation paths and source distributions of resolved gravity waves in ECMWF-IFS analysis fields around the southern polar night jet". Atmospheric Chemistry and Physics 21, n. 24 (22 dicembre 2021): 18641–68. http://dx.doi.org/10.5194/acp-21-18641-2021.
Testo completoAbstract (sommario):
Abstract. In the southern winter polar stratosphere, the distribution of gravity wave momentum flux in many state-of-the-art climate simulations is inconsistent with long-time satellite and superpressure balloon observations around 60∘ S. Recent studies hint that a lateral shift between prominent gravity wave sources in the tropospheric mid-latitudes and the location where gravity wave activity is present in the stratosphere causes at least part of the discrepancy. This lateral shift cannot be represented by the column-based gravity wave drag parameterisations used in most general circulation models. However, recent high-resolution analysis and re-analysis products of the European Centre for Medium-Range Weather Forecasts Integrated Forecast System (ECMWF-IFS) show good agreement with the observations and allow for a detailed investigation of resolved gravity waves, their sources, and propagation paths. In this paper, we identify resolved gravity waves in the ECMWF-IFS analyses for a case of high gravity wave activity in the lower stratosphere using small-volume sinusoidal fits to characterise these gravity waves. The 3D wave vector together with perturbation amplitudes, wave frequency, and a fully described background atmosphere are then used to initialise the Gravity Wave Regional or Global Ray Tracer (GROGRAT) gravity wave ray tracer and follow the gravity waves backwards from the stratosphere. Finally, we check for the indication of source processes on the path of each ray and, thus, quantitatively attribute gravity waves to sources that are represented within the model. We find that stratospheric gravity waves are indeed subject to far (>1000 km) lateral displacement from their sources, which already take place at low altitudes (<20 km). Various source processes can be linked to waves within stratospheric gravity wave (GW) patterns, such as the orography equatorward of 50∘ S and non-orographic sources above the Southern Ocean. These findings may explain why superpressure balloons observe enhanced gravity wave momentum fluxes in the lower stratosphere over the Southern Ocean despite an apparent lack of sources at this latitude. Our results also support the need to improve gravity wave parameterisations to account for meridional propagation.
47
Zou, Yufei, Yuhang Wang, Zuowei Xie, Hailong Wang e Philip J. Rasch. "Atmospheric teleconnection processes linking winter air stagnation and haze extremes in China with regional Arctic sea ice decline". Atmospheric Chemistry and Physics 20, n. 8 (28 aprile 2020): 4999–5017. http://dx.doi.org/10.5194/acp-20-4999-2020.
Testo completoAbstract (sommario):
Abstract. Recent studies suggested significant impacts of boreal cryosphere changes on wintertime air stagnation and haze pollution extremes in China. However, the underlying mechanisms of such a teleconnection relationship remains unclear. Here we use the Whole Atmosphere Community Climate Model (WACCM) to investigate dynamic processes leading to atmospheric circulation and air stagnation responses to Arctic sea ice changes. We conduct four climate sensitivity experiments by perturbing sea ice concentrations (SIC) and corresponding sea surface temperature (SST) in autumn and early winter over the whole Arctic and three subregions in the climate model. The results indicate distinct responses in circulation patterns and regional ventilation to the region-specific Arctic changes, with the largest increase of both the probability (by 132 %) and the intensity (by 30 %) of monthly air stagnation extremes being found in the experiment driven by SIC and SST changes over the Pacific sector of the Arctic (the East Siberian and Chukchi seas). The increased air stagnation extremes are mainly driven by an amplified planetary-scale atmospheric teleconnection pattern that resembles the negative phase of the Eurasian (EU) pattern. Dynamical diagnostics suggest that convergence of transient eddy forcing in the vicinity of Scandinavia in winter is largely responsible for the amplification of the teleconnection pattern. Transient eddy vorticity fluxes dominate the transient eddy forcing and produce a barotropic anticyclonic anomaly near Scandinavia and wave train propagation across Eurasia to the downstream regions in East Asia. The piecewise potential vorticity inversion analysis reveals that this long-range atmospheric teleconnection of Arctic origin takes place primarily via the middle and upper troposphere. The anomalous ridge over East Asia in the middle and upper troposphere worsens regional ventilation conditions by weakening monsoon northwesterlies and enhancing temperature inversions near the surface, leading to more and stronger air stagnation and pollution extremes over eastern China in winter. Ensemble projections based on state-of-the-art climate models in the Coupled Model Intercomparison Project Phase 6 (CMIP6) corroborate this teleconnection relationship between high-latitude environmental changes and midlatitude weather extremes, though the tendency and magnitude vary considerably among each participating model.
48
Reinoso-Rondinel, Ricardo, Christine Unal e Herman Russchenberg. "Improved Estimation of the Specific Attenuation and Backscatter Differential Phase over Short Rain Paths". Journal of Atmospheric and Oceanic Technology 35, n. 12 (dicembre 2018): 2359–81. http://dx.doi.org/10.1175/jtech-d-17-0219.1.
Testo completoAbstract (sommario):
AbstractIn radar polarimetry, the differential phase consists of the propagation differential phase and the backscatter differential phase . While is commonly used for attenuation correction (i.e., estimation of the specific attenuation A and specific differential phase ), recent studies have demonstrated that can provide information concerning the dominant size of raindrops. However, the estimation of and is not straightforward given their coupled nature and the noisy behavior of , especially over short paths. In this work, the impacts of estimating on the estimation of A over short paths, using the extended version of the ZPHI method, are examined. Special attention is given to the optimization of the parameter α that connects and A. In addition, an improved technique is proposed to compute from and in rain. For these purposes, diverse storm events observed by a polarimetric X-band radar in the Netherlands are used. Statistical analysis based on the minimum errors associated with the optimization of α and the consistency between and A showed that more accurate and stable α and A are obtained if is estimated at range resolution, which is not possible by conventional range filtering techniques. Accurate estimates were able to depict the spatial variability of dominant raindrop size in the observed storms. By following the presented study, the ZPHI method and its variations can be employed without the need for considering long paths, leading to localized and accurate estimation of A and .
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Cho, John Y. N., e Edward S. Chornoboy. "Multi-PRI Signal Processing for the Terminal Doppler Weather Radar. Part I: Clutter Filtering". Journal of Atmospheric and Oceanic Technology 22, n. 5 (1 maggio 2005): 575–82. http://dx.doi.org/10.1175/jtech1730.1.
Testo completoAbstract (sommario):
Abstract Multiple pulse repetition interval (multi-PRI) transmission is part of an adaptive signal transmission and processing algorithm being developed to aggressively combat range–velocity ambiguity in weather radars. In the past, operational use of multi-PRI pulse trains has been hampered due to the difficulty in clutter filtering. This paper presents finite impulse response clutter filter designs for multi-PRI signals with excellent magnitude and phase responses. These filters provide strong suppression for use on low-elevation scans and yield low biases of velocity estimates so that accurate velocity dealiasing is possible. Specifically, the filters are designed for use in the Terminal Doppler Weather Radar (TDWR) and are shown to meet base data bias requirements equivalent to the Federal Aviation Administration’s specifications for the current TDWR clutter filters. Also an adaptive filter selection algorithm is proposed that bases its decision on clutter power estimated during an initial long-PRI surveillance scan. Simulations show that this adaptive algorithm yields satisfactory biases for reflectivity, velocity, and spectral width. Implementation of such a scheme would enable automatic elimination of anomalous propagation signals and constant adjustment to evolving ground clutter conditions, an improvement over the current TDWR clutter filtering system.
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Grubor, D. P., D. M. Šulić e V. Žigman. "Classification of X-ray solar flares regarding their effects on the lower ionosphere electron density profile". Annales Geophysicae 26, n. 7 (18 giugno 2008): 1731–40. http://dx.doi.org/10.5194/angeo-26-1731-2008.
Testo completoAbstract (sommario):
Abstract. The classification of X-ray solar flares is performed regarding their effects on the Very Low Frequency (VLF) wave propagation along the Earth-ionosphere waveguide. The changes in propagation are detected from an observed VLF signal phase and amplitude perturbations, taking place during X-ray solar flares. All flare effects chosen for the analysis are recorded by the Absolute Phase and Amplitude Logger (AbsPal), during the summer months of 2004–2007, on the single trace, Skelton (54.72 N, 2.88 W) to Belgrade (44.85 N, 20.38 E) with a distance along the Great Circle Path (GCP) D≈2000 km in length. The observed VLF amplitude and phase perturbations are simulated by the computer program Long-Wavelength Propagation Capability (LWPC), using Wait's model of the lower ionosphere, as determined by two parameters: the sharpness (β in 1/km) and reflection height (H' in km). By varying the values of β and H' so as to match the observed amplitude and phase perturbations, the variation of the D-region electron density height profile Ne(z) was reconstructed, throughout flare duration. The procedure is illustrated as applied to a series of flares, from class C to M5 (5×10−5 W/m2 at 0.1–0.8 nm), each giving rise to a different time development of signal perturbation. The corresponding change in electron density from the unperturbed value at the unperturbed reflection height, i.e. Ne(74 km)=2.16×108 m−3 to the value induced by an M5 class flare, up to Ne(74 km)=4×1010 m−3 is obtained. The β parameter is found to range from 0.30–0.49 1/km and the reflection height H' to vary from 74–63 km. The changes in Ne(z) during the flares, within height range z=60 to 90 km are determined, as well.