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1.
Radar observations of nor’wester (premonsoon thunderstorm) as experienced over the Gangetic West Bengal and the integrated field intensity of atmospherics (IFIA) at 10, 20 and 30 kHz recorded over the tropical station at Calcutta have been analysed. The records of IFIA reveal that, during nor’wester, the enhancement in the signal level occurs in three different stages: (i) gradual rise of atmospherics (GRA), (ii) first sudden enhancement of atmospherics (SEA1) and (iii) second sudden enhancement of atmospherics (SEA2. Their respective occurrences have been found to be prevalent in pre-noon, latenoon and afternoon hours of a day. Of the three stages, GRA and SEA1 have been found to occur before the onset of thunderstorm, while SEA2 may even occur after its onset. A comparative study reveals that the precipitation during premonsoon thunderstorms is associated with the second sudden enhancement, while it is associated with the steady recovery of IFIA during the monsoon and postmonsoon thunderstorms. The characteristics of the first nor’wester of every year have been critically examined with an additional consideration of simultaneous radiosonde records. A probable explanation has been made in terms of the distribution of the effective sources at such times around the observing site.  相似文献   

2.
Applying spectral analysis to the Atlantic and Pacific hurricane time series, we found periodicities that coincide with the main sunspot and magnetic solar cycles. To assess the possibility that these periodicities could be associated with solar activity, we obtain correlations between hurricane occurrence and several solar activity-related phenomena, such as the total solar irradiance, the cosmic ray flux and the Dst index of geomagnetic activity. Our results indicate that the highest significant correlations are found between the Atlantic and Pacific hurricanes and the Dst index. Most importantly, both oceans present the highest hurricane–Dst correlations during the ascending part of odd solar cycles and the descending phase of even solar cycles. This shows not only the existence of a 22 yr cycle but also the nature of such periodicity. Furthermore, we found that the Atlantic hurricanes behave differently from the Pacific hurricanes in relation to the solar activity-related disturbances considered.  相似文献   

3.
In the present study an attempt has been made to understand the variation of surface energy fluxes such as net radiation, sensible, latent and soil heat during different epochs of thunderstorm activity at Kharagpur. The study also focuses in delineating the difference in the surface energy budget from the days of thunderstorm activity to fair weather days in the pre-monsoon months (April and May) which is locally known as thunderstorm season. For this purpose, experimental data obtained from the Severe Thunderstorms- Observations and Regional Modeling (STORM) programme during pre-monsoon months of 2007, 2009 and 2010 at Kharagpur (22°30′N, 87°20′E), West Bengal, India are used. The present study reveals quick response, in the order of a few days, in the variations of transport of energy fluxes at soil-atmosphere interface to the upper atmosphere vis-à-vis to the occurrence of thunderstorm activity. Rise of surface sensible heat flux to the level of surface latent heat flux a day or two before the occurrence of a thunderstorm has been identified as a precursor signal for the thunderstorm occurrence over Kharagpur. Distinguishable differences are found in the partitioning of the surface energy fluxes to that of net radiation between thunderstorm and non-thunderstorm days. The present study reveals more Bowen’s ratio during thunderstorm days to that of nonthunderstorm days. These results are useful in validating mesoscale model simulations of thunderstorm activity.  相似文献   

4.
Tal Ezer 《Ocean Dynamics》2018,68(10):1259-1272
Tropical storms and hurricanes in the western North Atlantic Ocean can impact the US East Coast in several ways. Direct effects include storm surges, winds, waves, and precipitation and indirect effects include changes in ocean dynamics that consequently impact the coast. Hurricane Matthew [October, 2016] was chosen as a case study to demonstrate the interaction between an offshore storm, the Gulf Stream (GS) and coastal sea level. A regional numerical ocean model was used, to conduct sensitivity experiments with different surface forcing, using wind and heat flux data from an operational hurricane-ocean coupled forecast system. An additional experiment used the observed Florida Current (FC) transport during the hurricane as an inflow boundary condition. The experiments show that the hurricane caused a disruption in the GS flow that resulted in large spatial variations in temperatures with cooling of up to ~?4 °C by surface heat loss, but the interaction of the winds with the GS flow also caused some local warming near fronts and eddies (relative to simulations without a hurricane). A considerable weakening of the FC transport (~?30%) has been observed during the hurricane (a reduction of ~?10 Sv in 3 days; 1Sv?=?106 m3 s?1), so the impact of the FC was explored by the model. Unlike the abrupt and large wind-driven storm surge (up to 2 m water level change within 12 h in the South Atlantic Bight), the impact of the weakening GS on sea level is smaller but lasted for several days after the hurricane dissipated, as seen in both the model and altimeter data. These results can explain observations that show minor tidal flooding along long stretches of coasts for several days following passages of hurricanes. Further analysis showed the short-term impact of the hurricane winds on kinetic energy versus the long-term impact of the hurricane-induced mixing on potential energy, whereas several days are needed to reestablish the stratification and rebuild the strength of the GS to its pre-hurricane conditions. Understanding the interaction between storms, the Gulf Stream and coastal sea level can help to improve prediction of sea level rise and coastal flooding.  相似文献   

5.
位涡塔结构及演变对飓风Wilma(2005)强度变化的影响   总被引：2，自引：2，他引：0       下载免费PDF全文
本文利用高分辨率模式输出资料,对飓风Wilma(2005)不同发展阶段内位涡塔(Potential Vorticity Tower:PVT)的结构和演变进行诊断分析,并讨论位涡塔分布特征对飓风快速增强(Rapid Intensification:RI)过程的影响.研究结果表明,内核区域位涡的强度和结构变化主要取决于高层的暖心下传、中层的凝结潜热释放和低层的动力稳定性,而低层切向平均位涡的结构及其稳定性变化,对飓风快速增强的不同发展阶段具有很好的指示意义.飓风Wilma增强过程中,在高层增温、潜热释放和对流垂直混合作用下,PVT的结构出现单极位涡塔(Monopole PVT:MPVT)和中空位涡塔(Hollow PVT:HPVT)的相互转化,也造成了涡旋系统动力稳定性的变化.在实际个例中,重新定义位涡环的结构参数,即相对厚度和中空度,这两个参数能够表征系统的稳定性,将其在RI过程中的变化与飓风Wilma强度变化作相关性分析,表明结构参数能够表征PVT结构的不稳定性,且与飓风系统强度变化参数具有很好的相关性,结构参数与海平面气压变化率的相关性能够通过显著性检验.  相似文献   

6.
The oceanic mixed layer (OML) response to an idealized hurricane with different propagation speeds is investigated using a two-layer reduced gravity ocean model. First, the model performances are examined with respect to available observations relative to Hurricane Frances (2004). Then, 11 idealized simulations are performed with a Holland (Mon Weather Rev 108(8):1212–1218, 1980) symmetric wind profile as surface forcing with storm propagation speeds ranging from 2 to 12 m s−1. By varying this parameter, the phasing between atmospheric and oceanic scales is modified. Consequently, it leads to different momentum exchanges between the hurricane and the OML and to various oceanic responses. The present study determines how OML momentum and heat budgets depend on this parameter. The kinetic energy flux due to surface wind stress is found to strongly depend on the propagation speed and on the cross-track distance from the hurricane center. A resonant regime between surface winds and near-inertial currents is clearly identified. This regime maximizes locally the energy flux into the OML. For fast-moving hurricanes (>6 m s−1), the ratio of kinetic energy converted into turbulence depends only on the wind stress energy input. For slow-moving hurricanes (<6 m s−1), the upwelling induced by current divergence enhances this conversion by shallowing the OML depth. Regarding the thermodynamic response, two regimes are identified with respect to the propagation speed. For slow-moving hurricanes, the upwelling combined with a sharp temperature gradient at the OML base formed in the leading part of the storm maximizes the oceanic heat loss. For fast propagation speeds, the resonance mechanism sets up the cold wake on the right side of the hurricane track. These results suggest that the propagation speed is a parameter as important as the surface wind speed to accurately describe the oceanic response to a moving hurricane.  相似文献   

7.
Attention is paid to the generation of the Katrine hurricane with destructive consequences during the geomagnetic extrastorm of August 24, 2005, at a repeated crossing of the strongly disturbed IMF sector boundary. This fact is discussed in the light of the Eigenson-Usmanov hypothesis that solar activity can affect generation of hurricanes. According to the law of enhancement, solar activity and corresponding geomagnetic disturbances increase baric contrasts in the lower atmosphere over oceans on the Earth’s dayside hemisphere and decrease background pressure at tropical latitudes. It has been assumed that this can be one of the factors facilitating triggering of the positive feedback mechanisms necessary for generation and maintenance of hurricanes according to the Golitsyn [1999] model.  相似文献   

8.
The specific features of a method for radiosounding the lower ionosphere over earthquake epicenters using LF electromagnetic signals of thunderstorm sources (atmospherics) have been considered. The effects of shallow-focus earthquakes with magnitudes larger than 4.0 and their precursors manifest themselves in amplitude characteristics of atmospherics. It has been assumed that variations in the signal characteristics are related to disturbances in the lower ionosphere. According to the results of azimuthal scanning, cross-sectional dimensions of disturbed regions, as a rule, correspond to the dimensions of the first two Fresnel zones for signals at a frequency of 10 kHz. Azimuthal scanning also indicated that the positions of disturbed regions during and before earthquakes could have a certain dynamics and differ from the projection onto the earthquake epicenter. The ratio of the amplitudes of electric and magnetic signal components, in the variations of which seismic effects before earthquakes can also be observed, has been considered. An analysis of the ratio makes it possible to increase the probability of predicting earthquakes when using the characteristics of the electromagnetic signals of lightning discharges as an additional method of complex monitoring of disturbances in the lower ionosphere caused by seismic processes.  相似文献   

9.
The records of VLF atmospherics over Calcutta and then over Kalyani (West Bengal) during the torrential rainfall, caused by violent monsoon and post-monsoon depressions, exhibit distinct long-period fadings both at day and night. Interesting results obtained from an analysis of round-the-clock atmospherics data and associated meteorological parameters are reported in this paper. A possible correlation between the severe meteorological activity with the solar geophysical phenomena was studied. The results are indicative of an interesting sequence of solar-terrestrial events. A tentative conclusion is reached, suggesting an origin of the fading from atmospheric gravity waves generated in the centre of activity of the depressions concerned.Currently at: Department of Physics, Serampore College, Serampore, 712 201 West Bengal  相似文献   

10.
A new approach, combining 15NH4+ isotope dilution and continuous-flow techniques, provided estimates of “actual” and “net” NH4+ flux and sediment NH4+ demand (SAD) at the sediment-water interface (SWI) of sites in the northern Gulf of Mexico (NGOMEX). The sites included a hypoxic site (C6), two sites with intermediate oxygen levels (B7 and F5), and a normoxic site (CT). Control cores without isotope addition and other cores from the same site treated with 15NH4+ labeled overflowing water differentiated between net and actual regeneration flux and actual vs. potential uptake flux of NH4+. Experiments were conducted in 2008 before (July) and after (September) two successive hurricanes (Gustav and Ike) and in January and August, 2009. Actual regeneration was significantly higher than net flux at most sites. Net flux did not differ significantly in most sites/dates, but the actual regeneration, and the actual and potential uptake, showed temporal and spatial variation; the flux at the hypoxic site was more active than non-hypoxic sites. SAD, the difference between potential and actual NH4+ uptake flux, was higher at the hypoxic site than at non-hypoxic sites before and after the hurricanes in 2008 and during the hypoxia season in 2009. SAD related negatively to bottom water DO values. Conclusions: (1) net flux often underestimated actual regeneration, (2) hurricane activity decreased N dynamics, and (3) microbial N limitation status at the hypoxic site related to NH4+ removal processes that were independent of oxygen (e.g., anaerobic heterotrophic uptake or anammox). These results indicate a rather consistent NH4+ demand at the SWI during the hypoxic season and suggest that reduced nitrogen may limit microbial dynamics in the region.  相似文献   

11.
We present the results of a Schumann resonance monitoring campaign held at Lehta observatory, Karelia, Russia during July–August, 1998. Three electromagnetic (EM) field components were recorded simultaneously: the vertical electric and two orthogonal magnetic fields. This paper demonstrates advantages of the Poynting vector (PV) technique when studying the space–time dynamics of the worldwide thunderstorm activity from a single observatory. Analysis of the diurnal PV patterns revealed a night-time peak in African thunderstorm activity. This maximum occurs around 02:00–03:00 UT and reaches 1/3 of regular afternoon level. The Schumann resonance structure was discovered in the spectra of the wave arrival angles.  相似文献   

12.
Narrow positive bipolar pulses (NPBPs), whose origin largely remains unknown as yet, have occasionally been noticed in Sri Lanka. These discharges are found to be opposite in polarity to that of negative return strokes, and are found to occur at the beginning and active stage of thunderstorm activities in Sri Lanka. They are emitted from the thunderstorms that produce other activities also and are relatively narrower, bipolar and isolated in nature. They are neither preceded by leader-type pulses nor succeeded by the subsequent activities and hence, could not be associated with any other known activity. Similar radiations have previously been identified and termed as NPBPs. Their features are indeed unique and different from the other known thunderstorm electrical processes. These events have been found to be accompanied by the HF radiations at 5 and 10 MHz, at the trailing part of the pulse. However, such narrow, bipolar and isolated events have not been observed in Sweden, during a lightning measurement campaign at Uppsala, in 2006. So, it is speculated that the meteorological conditions to be responsible for the thunderstorm activity to produce such pulses. Features of such pulses recorded on different thunderstorm days in 2005 and 2006 have been analyzed and presented in this study.The average rise time (10–90%) (Tr), of the pulses was found to be 2.6 μs, the average zero crossing time (Tz) was found to be 5.85 μs, the average duration of slow front (Ts) was found to be 1.86 μs, and the average ratio of amplitude of overshoot to the corresponding peak amplitude (Os/Pa) of these pulses was found to be 0.39.  相似文献   

13.
Zusammenfassung Der zeitliche Gang der atmosph?tischen St?rungen ist zwei Einflüssen unterworfen: 1) Der zeitlichen und r?umlichen Verteilung der St?rquellen, 2) Den jeweiligen Ausbreitungsbedingungen des zu empfangenden Frequenzbandes. Die bei 5,5 MHz durchgeführten Messungen des atmosph?rischen St?rpegels zeigen den erwarteten Zusammenhang zwischen dem t?glichen Gang und dem Sonnenstand entsprechend den normalen Ausbreitungsbedingungen im 50 m Band. W?hrend M?gel-Dellinger Effekte nicht festgestellt werden konnten, ist die Nordlichtst?rung als Folge einer Korpuskularstrahlung der Sonne klar zu erkennen.
Summary The temporal variation of atmospherics is subjected to two facts: 1) The temporal and local distribution of sources of atmospherics, 2) The present conditions of propagation with regard to the just received short-wave band. Measurements of atmospherics in 50 m-band proved the connection between atmospherics and solar-radiation corresponding to the normal conditions of short-wave propagation, showing a minimum at day and a maximum at night. Whilst the Dellinger-effect did not happen, atmospherics disappeard on account of aurora borealis and strong disturbance of the earth's magnetic field, in consequence of corpusculare-radiation of the sun. Monthly distribution of atmospherics in 50 m-band for 2 years measurements are given.
  相似文献   

14.
A network of interconnected stations was established in the entire area of the Karymskii Volcanic Center and near the active Karymskii Volcano, Kamchatka in 1971–1988 for the purpose of studying ground deformation. Multiple observations by this network yielded quantitative characteristics of the ground deformation related to the following phenomena: the eruption of Karymskii Volcano during the periods 1976–1982 and January 1, 1996, to 2005 (still continuing, written in February 2008); the discharge of basalt on January 2, 1996, in the bottom of Lake Karymskii situated in the caldera of Akademii Nauk Volcano (this volcano had previously been thought to be extinct) and the subsequent phreatomagmatic eruption lasting approximately 24 hours; and the large (M 6.9) earthquake of January 1, 1996, occurring at 21 h 57 min local time in the Karymskii Volcanic Center at a depth of ～10 km. This paper discusses the relationships of ground deformation to volcanic activity and to the abovementioned unique natural occurrences, and their mechanism as deduced from geodetic data.  相似文献   

15.
The results of the experiments, during which we revealed that the flux of neutrons in the atmosphere increases in correlation with electromagnetic pulses of lightning discharges, have been analyzed. The mechanisms of charged particle acceleration and nuclear reactions that can be responsible for neutron generation have been analyzed. It has been indicated that the probability of nuclear synthesis reactions in a lightning channel, which is traditionally considered as the source of neutrons in a thunderstorm atmosphere, is extremely low. The generation of neutrons in thunderstorm electric fields is related to photonuclear reactions in gigantic upward atmospheric discharges caused by relativistic runaway electron bremsstrahlung.  相似文献   

16.
Simultaneous records of the diurnal variations in the quasistatic electric field in the near-Earth atmosphere, fluxes of discrete electromagnetic pulses in the VLF band, source azimuths, narrowband filter output emission intensity at frequencies of 4.65 and 5.3 kHz, and time forms and spectra of VLF pulses have been analyzed. The anomalous behavior of these parameters in October 2002 and August 2004 with different time delay was accompanied by earthquakes near the southeastern coast of Kamchatka at distances of 250–400 km from the registration points. Based on the results of a fine frequency-time analysis of the broadband records of VLF signals, it has been indicated for the first time that discrete electromagnetic pulses observed in anomalous fluxes before earthquakes were signals of local thunderstorm processes.  相似文献   

17.
Effect of variations in the solar-wind parameters on thunderstorm activity   总被引：1，自引：0，他引：1  
Possible correlation between variations of the intensity of lightnings, which are estimated from the flux of thunderstorm-generated VLF-signals, and variations of the solar wind parameters has been investigated. The signals representing the intensity of local thunderstorms in summer and winter are received in Yakutsk (the Eastern Siberia) and in the African World Thunderstorm Center, respectively. The highest correlation coefficient has been obtained between the thunderstorm activity and variations of the solar-wind particle density. This correlation has a season-dependent sign-alternating character. The maximum positive and negative correlation coefficients are observed in August–September and February, respectively; i.e., the manifestation of solar wind density variations in the thunderstorm activity is maximal in the near-equinoctial periods. This may be associated with the peculiarities of the transfer of the magnetospheric electric field “from morning to evening,” which is induced by the solar-wind particle flux, to ionospheric heights.  相似文献   

18.
Hurricanes produce mixing and flow divergences (and convergences) that alter the upper-ocean heat content (OHC), which in turn affects the storm. Ocean observations under a hurricane are rare, making it difficult to validate forecast models. Past research have mainly focused on OHC-changes by vertical mixing and tacitly assumed that horizontal transports are slowly varying. Moreover, effects of coastal boundaries on ocean responses to hurricanes are generally omitted. This work uses satellite data to detect and verify forecast isopycnal motions under hurricane Wilma (Oct/16–26/2005) in the Caribbean Sea and the Gulf of Mexico. The model is then used to show that Wilma-induced convergences in northwestern Caribbean Sea produce increased Yucatan-Channel transport into the Gulf ahead of the storm, and the Yucatan–Loop Current front diverts most of this heat around the Loop. This response is distinct from that of an ocean without the Loop, for which warming is widespread north of the channel. These intricate ocean responses can impact hurricane predictions.  相似文献   

19.
The effects of the solar eclipse of March 29, 2006, in the signals of ULF radio stations, in the intensity of regular radio noise at frequencies of 0.3–10 kHz, and in the number of atmospherics received in Yakutsk mostly from the west have been considered. The observations were performed using a multichannel parallel analyzer-recorder (11 channels in the frequency band 0.47–8.7 kHz), one-point lightning direction and range finder (0.3–100 kHz), narrow-sector radio noise direction-finder (0.3–10 kHz), recorder of signals from VLF radio stations, and broadband radio noise recorder (0.3–100 kHz). A GPS clock was used to synchronize a recorder of signals from VLF radio stations. The effect was observed in radio signals, radio noise, and number of atmospherics from the direction 270° ± 20° counted off clockwise from the north during the last stage of the eclipse (～ 1100–1200 UT), when the lunar shadow approached the line of the nighttime terminator and obscured part of the signal propagation path. The effect was observed as an enhancement of the received signals by a factor of ～1.2, a factor of ～1.4 increase in the number of atmospherics, and a change in the radio station phase values.  相似文献   

20.
A 56-year cyclicity in the occurrence of large Kamchatka earthquakes has been previously detected. This is another manifestation of the tendency for the timing of large Kamchatka earthquakes to be synchronized to the cycles related to the period T o of rotation of the lunar nodes found by V.A. Shirokov in 1974. He identified cycles of 18.6 years = T o and 6.2 years = T o/3, while the period of the 56-year cycle is 3T o. The genuineness of that phenomenon had to be revised in connection with the occurrence of a large (M w = 7.8) earthquake in Kamchatka at the end of 1997, in violation of the 56-year cyclicity. It turned out that, even though the 56-year cycle has become less distinct after the 1997 event, the cyclicity itself has remained statistically significant. A byproduct is an updated forecast of earthquake hazard for Kamchatka. The update is necessary in view of the approaching hazardous period of 2008–2011. It is found that, assuming the validity of these empirical tendencies, the expected rate of large earthquakes off Kamchatka for the period of August 2008 to October 2011 will be four times as high as the long-term mean. We derive the first-ever estimate of future hazard in terms of felt intensity for specified soil conditions (the so-called average soil) at a specified site (the town of Petropavlovsk-Kamchatskii). For these soil conditions, the estimated probability of at least one shock of intensity VII or greater during the period specified above is equal to 0.39 ± 0.15. The expected rate of single events or sets of events with M w ≥ 7.6 in Kamchatka during this period is 0.76 ± 0.25.  相似文献   

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