Category-5 hurricanes are the most devastating from the standpoint of human and economic losses. The occurrence of this kind of hurricane is believed to be of quasi-random nature, so it is very difficult to predict th...Category-5 hurricanes are the most devastating from the standpoint of human and economic losses. The occurrence of this kind of hurricane is believed to be of quasi-random nature, so it is very difficult to predict them well in advance. Warnings at this regard are generally given in the course of their development. We propose here, that there are some inherently periodicities of the phenomena that allow to predict category-5 hurricanes, even with some years of anticipation. For our study, we consider the North Atlantic category-5 hurricanes since 1920. We consider in this study data of the SST （sea surface temperature） of the North Atlantic Ocean as a representative parameter of hurricane activity. Then, by means of the wavelet analysis, we determine the dominant oscillation periods and establish correspondence rules using fuzzy logic. The wavelet power spectrum yields the following dominant periodicities： 0.5, 1, 3, 11, 22 and 32 years. The fuzzy logic searches for associations between the hurricanes occurrence and the behavior of the harmonics. Such correspondence rules lead us to restrict dates of possible hurricane occurrence. Interpolation of the periodic behavior allows for a good reconstruction of past hurricanes dates since 1920, as well as extrapolation to predict dates of occurrence in the future. We conclude that the conditions for the formation of the next category-5 hurricane in the North Atlantic may occur during the seasons of 2015-2017 with the highest probability in 2017.展开更多
Compared with Gaussian wind loads, there is a higher probability of strong suction fluctuations occurrence for non-Gaussian wind pressures. These instantaneous and intermittent fluctuations are the initial cause of lo...Compared with Gaussian wind loads, there is a higher probability of strong suction fluctuations occurrence for non-Gaussian wind pressures. These instantaneous and intermittent fluctuations are the initial cause of local damage to roof structures, par- ticularly at the edges and comers of long-span roofs. Thus, comparative errors would occur if a Gaussian model is used to de- scribe a non-Gaussian wind load, and structural security would not be guaranteed. This paper presents a simplified method based on the inverse fast Fourier transform （IFFT）, in which the amplitude spectrum is established via a target power spectrum. Also, the phase spectrum is constructed by introducing the exponential peak generation （EPG） model. Finally, a random pro- cess can be generated via IFFT that meets the specified power spectral density （PSD）, skewness and kurtosis. In contrast to a wind tunnel experiment, this method can avoid the coupled relation between the non-Gaussian and the power spectrum char- acteristics, and lead to the desired computational efficiency. Its fitting accuracy is not affected by phase spectrum. Moreover, the fitting precision of the kurtosis and PSD parameters can be guaranteed. In a few cases, the fitting precision of the skewness parameter is fairly poor, but kurtosis is more important than skewness in the description of the non-Gaussian characteristics. Above all, this algorithm is simple and stable and would be an effective method to simulate a non-Gaussian signal.展开更多
On 24 August 2005, an impulse of solar wind dynamic pressure(Psw) hit the magnetosphere. Using the high resolution geomagnetic field data from 15 ground stations and the data from Geotail and TC-1, we studied the geom...On 24 August 2005, an impulse of solar wind dynamic pressure(Psw) hit the magnetosphere. Using the high resolution geomagnetic field data from 15 ground stations and the data from Geotail and TC-1, we studied the geomagnetic pulsations at auroral latitudes driven by the sharp decrease of Psw at the trailing edge of the impulse. The results show that the sharp decrease of Psw can excite a global pulsation in the frequency range 4.3–11.6 m Hz. The pulsation has a reversal of polarization between two auroral latitude stations, a larger power spectral density(PSD) close to resonant latitude and increasing frequency with decreasing latitude. All these features indicate that the pulsations are associated with field line resonance(FLR). The fundamental resonant frequency(the peak frequency of PSD between 4.3 and 5.8 m Hz) is dependent on magnetic local time and is largest around magnetic local noon. This feature is due to the fact that the size of magnetospheric cavity is dependent on local time and smallest at noon. A second harmonic wave at about 10 m Hz is also observed, which is strongest in the daytime sector, and becomes heavily attenuated in the night sector. The comparison of the PSDs of the pulsations driven by sharp increase and sharp decrease of Psw shows that the frequency of pulsations is negatively proportional to the size of magnetopause. Since the FLR is excited by compressional cavity/waveguide waves, the above results indicate that the resonant frequency in the magnetospheric cavity/waveguide is controlled not only by solar wind parameters but also by magnetic local time of observation point.展开更多
文摘Category-5 hurricanes are the most devastating from the standpoint of human and economic losses. The occurrence of this kind of hurricane is believed to be of quasi-random nature, so it is very difficult to predict them well in advance. Warnings at this regard are generally given in the course of their development. We propose here, that there are some inherently periodicities of the phenomena that allow to predict category-5 hurricanes, even with some years of anticipation. For our study, we consider the North Atlantic category-5 hurricanes since 1920. We consider in this study data of the SST （sea surface temperature） of the North Atlantic Ocean as a representative parameter of hurricane activity. Then, by means of the wavelet analysis, we determine the dominant oscillation periods and establish correspondence rules using fuzzy logic. The wavelet power spectrum yields the following dominant periodicities： 0.5, 1, 3, 11, 22 and 32 years. The fuzzy logic searches for associations between the hurricanes occurrence and the behavior of the harmonics. Such correspondence rules lead us to restrict dates of possible hurricane occurrence. Interpolation of the periodic behavior allows for a good reconstruction of past hurricanes dates since 1920, as well as extrapolation to predict dates of occurrence in the future. We conclude that the conditions for the formation of the next category-5 hurricane in the North Atlantic may occur during the seasons of 2015-2017 with the highest probability in 2017.
基金supported by the National Natural Science Fund for Distinguished Young Scholars (Grant No. 51125031)
文摘Compared with Gaussian wind loads, there is a higher probability of strong suction fluctuations occurrence for non-Gaussian wind pressures. These instantaneous and intermittent fluctuations are the initial cause of local damage to roof structures, par- ticularly at the edges and comers of long-span roofs. Thus, comparative errors would occur if a Gaussian model is used to de- scribe a non-Gaussian wind load, and structural security would not be guaranteed. This paper presents a simplified method based on the inverse fast Fourier transform （IFFT）, in which the amplitude spectrum is established via a target power spectrum. Also, the phase spectrum is constructed by introducing the exponential peak generation （EPG） model. Finally, a random pro- cess can be generated via IFFT that meets the specified power spectral density （PSD）, skewness and kurtosis. In contrast to a wind tunnel experiment, this method can avoid the coupled relation between the non-Gaussian and the power spectrum char- acteristics, and lead to the desired computational efficiency. Its fitting accuracy is not affected by phase spectrum. Moreover, the fitting precision of the kurtosis and PSD parameters can be guaranteed. In a few cases, the fitting precision of the skewness parameter is fairly poor, but kurtosis is more important than skewness in the description of the non-Gaussian characteristics. Above all, this algorithm is simple and stable and would be an effective method to simulate a non-Gaussian signal.
基金supported by the Fundamental Research Funds for the Central Universities(Grant No.NS2015089)
文摘On 24 August 2005, an impulse of solar wind dynamic pressure(Psw) hit the magnetosphere. Using the high resolution geomagnetic field data from 15 ground stations and the data from Geotail and TC-1, we studied the geomagnetic pulsations at auroral latitudes driven by the sharp decrease of Psw at the trailing edge of the impulse. The results show that the sharp decrease of Psw can excite a global pulsation in the frequency range 4.3–11.6 m Hz. The pulsation has a reversal of polarization between two auroral latitude stations, a larger power spectral density(PSD) close to resonant latitude and increasing frequency with decreasing latitude. All these features indicate that the pulsations are associated with field line resonance(FLR). The fundamental resonant frequency(the peak frequency of PSD between 4.3 and 5.8 m Hz) is dependent on magnetic local time and is largest around magnetic local noon. This feature is due to the fact that the size of magnetospheric cavity is dependent on local time and smallest at noon. A second harmonic wave at about 10 m Hz is also observed, which is strongest in the daytime sector, and becomes heavily attenuated in the night sector. The comparison of the PSDs of the pulsations driven by sharp increase and sharp decrease of Psw shows that the frequency of pulsations is negatively proportional to the size of magnetopause. Since the FLR is excited by compressional cavity/waveguide waves, the above results indicate that the resonant frequency in the magnetospheric cavity/waveguide is controlled not only by solar wind parameters but also by magnetic local time of observation point.
