The Seasonal Climate and Low Frequency Oscillation in the Simulated Mid-Holocene Megathermal Climate

The mid—Holocene climate about 6000 years ago was simulated by using the atmospheric general circulation model. The orbital parameters for 6 ka BP (before present) were prescribed and other forcing factors were set in the modern conditions. Results show that the large—scale climate change in the African—Asian monsoon areas during the summer—time is strongly compared to the present climate, while the changes in other seasons and regions are generally weak. The results also revealed the change of the low frequency oscillation in the atmosphere. Key words Seasonal climate - Low frequency oscillation - Mid—Holocene This research was jointly supported by the National Natural Science Foundation of China (NSFC) key project “ Variation of Paleo—environment over East Asia and Its Dynamic Relation with the Global Change” under contract 49894170 and the NSFC project “ Coupling of the AGCM with an Biome Model and the Simulation on the Mid-Holocene Climate” under contract 49975018.

Dust Storms in North China in 2002: A Case Study of the Low Frequency Oscillation

The low frequency oscillation in both hemispheres and its possible role in the dust weather storm events over North China in 2002 are analyzed as a case study. Results show that the Aleutian Low is linked with the Circumpolar Vortex in the Southern Hemisphere on a 30-60-day oscillation, with a weak Circumpolar Vortex tending to deepen the Aleutian Low which may be helpful for the generation of dust storm events. The possible mechanism behind this is the inter-hemispheric interaction of the mean meridional circulation, with the major variability over East Asia. The zonal mean westerly wind at high latitudes of the Southern Hemisphere in the upper level troposphere may lead that of the Northern Hemisphere, which then impacts the local circulation in the Northern Hemisphere. Thus, the low frequency oscillation teleconnection is one possible linkage in the coupling between the Southern Hemisphere circulation and dust events over North China. However, the interannual variation of the low frequency oscillation is unclear.

Impacts of SST and SST Anomalies on Low-Frequency Oscillation in the Tropical Atmosphere

Considering the multiscale character of LFO effects of SST on LFO in the tropical atmosphere （low-frequency oscillation） in the tropical atmosphere, the are discussed by using an absolute ageostrophic, baroclinic model. Here, SST effects include sea surface heating and forcing of SST anomalies （SSTAs）. Studies of the influences of sea surface heating on LFO frequency and stability show that sea surface heating can slow the speed of waves and lower their frequency when SST is comparatively low; while higher SST leads to unstable waves and less periods of LFO. Since the impact of a SSTA on ultra-long waves is more evident than that on kilometer-scale waves, long-wave approximation is used when we continue to study the effect of SSTAs. Results indicate that SSTAs can lead to a longer period of LFO, and make waves unstable. In other words, positive （negative） SSTAs can make waves decay （grow）.

CLIMATOLOGICAL LOW-FREQUENCY OSCILLATION OF OLR OVER THE MARITIME CONTINENT WITH ITS POSSIBLE LINKAGE TO SUMMER PRECIPITATION IN CHINA

Using the 1979-2009 NCEP/NACR reanalysis data and precipitation records in East China, research is performed of the climatological features of low-frequency oscillation(LFO) in OLR over the Maritime Continent(MC) as well as their associations with precipitation disturbance in the eastern part of China. Results suggest that in the MC there is significant climatological low-frequency oscillation(CLFO) in outgoing long-wave radiation(OLR), with the intraseasonal oscillation(30-60 days) being the strongest for April-September, and the MC acting as a high-value region of percentage contributions of low-frequency OLR variance. On the low-frequency time scale there occur four events of more intense active OLR during this time interval. In the January-April(May-August) phase, MC convection is relatively weak(vigorous). The CLFO makes pronounced eastward displacement at tropics, with phase propagation seen longitudinally, too. There occur low-frequency disturbance circulations similar to the EAP wavetrain or P-J teleconnection,starting from the MC via the South China Sea and the Philippines to the Yangtze valley of China. At different phases,the variation in the low-frequency circulations and heating fields shows that the rainfall disturbance in eastern China is likely to be under possible effects of the CLFO from the MC in April-September, and the low-frequency heating variation exhibits a meridional pattern as an EAP wavetrain or P-J teleconnection. As the OLR CLFO is in a peak(valley)phase the low-level divergence or convergence with the reversal at high levels over the MC is related to relatively feeble(robust) low frequency convection, thereby exciting an EAP or P-J wavetrain from the MC to the Sea of Japan. At the higher levels, the South-Asian high is eastward(westward) of normal due to effects of low-frequency cyclones(anticyclones), resulting in less(more) rainfall in the Jiangnan(areas in the middle and lower reaches of Yangtze and to the south of the river) and Hetao(the Great Bend of Yellow River) areas, and increased(decreased) rainfall in SW China,Qinghai Plateau and Gansu. At the conversion phases, low-frequency convection becomes more active in parts of the MC, consequently exciting low-frequency wavetrain of cyclones-anticyclones-cyclones at low levels, making the South-Asian high southward of the mean, so that strong convergent zones emerge in the upper and middle Yangtze basins and Jilin of NE China, responsible for plentiful precipitation there in sharp contrast to the rainfall over the band between the Yellow and Huaihe Rivers and the Yunnan-Guizhou Plateau. These results help understand in depth the climatological LFO characteristics and the phase-locked feature, thereby further improving our understanding of the causes of rainfall disturbances in different parts of the country.

Low-frequency oscillations in Hall thrusters

In this paper, we summarize the research development of low-frequency oscillations in the last few decades. The findings of physical mechanism, characteristics and stabilizing methods of low-frequency oscillations are discussed. It shows that it is unreasonable and incomplete to model an ionization region separately to analyze the physical mechanism of low-frequency oscillations. Electro-dynamics as well as the formation conditions of ionization distribution play an important role in characteristics and stabilizing of low-frequency oscillations. Understanding the physical mechanism and characteristics of low- frequency oscillations thoroughly and developing a feasible method stabilizing this instability are still important research subjects.

A STUDY ON THE CHARACTERISTICS AND EFFECT OF THE LOW-FREQUENCY OSCILLATION OF THE ATMOSPHERIC HEAT SOURCE OVER THE EASTERN TIBETAN PLATEAU

There has been a lot of discussion about the atmospheric heat source over the Tibetan Plateau(TP)and the low-frequency oscillation of atmospheric circulation.However,the research on low-frequency oscillation of heat source over TP and its impact on atmospheric circulation are not fully carried out.By using the vertically integrated apparent heat source which is calculated by the derivation method,main oscillation periods and propagation features of the summer apparent heat source over the eastern TP(Q1ETP)are diagnosed and analyzed from 1981 to 2000.The results are as follows:(1)Summer Q1ETP has two significant oscillation periods:one is 10-20d(BWO,Quasi-Biweekly Oscillation)and the other is 30-60d(LFO,Low-frequency Oscillation).(2)A significant correlation is found between Q1ETP and rainfall over the eastern TP in 1985 and 1992,showing that the low-frequency oscillation of heat source is likely to be stimulated by oscillation of latent heat.(3)The oscillation of heat source on the plateau mainly generates locally but sometimes originates from elsewhere.The BWO of Q1ETP mainly exhibits stationary wave,sometimes moves out(mainly eastward),and has a close relationship with the BWO from the Bay of Bengal.Showing the same characteristics as BWO,the LFO mainly shows local oscillation,occasionally propagates(mainly westward),and connects with the LFO from East China.In summary,more attention should be paid to the study on BWO of Q1ETP.

THE CHARACTERISTICS OF LOW-FREQUENCY OSCILLATIONS OVER THE SOUTH CHINA SEA

Using 1975-1993 (with 1978 missing) data of the outgoing longwave radiation (OLR), characteristics of seasonal variation of low-frequency oscillations in the South China Sea and its relation to the establishment and activity of the summer monsoon there are studied. As is shown in the result, the low-frequency oscillation in the South China Sea is much stronger in the period of summer monsoon than in that of winter monsoon and the summer monsoon there usually begins to set up in a negative phase of the first significant low-frequency oscillation for the early summer. The study also reveals that the circulation for the low-frequency oscillation during the summer monsoon in the Sea is embodied as north-south fluctuations of the ITCZ and east-west shifts of western ridge point of the West Pacific subtropical high, suggesting close correlation between the low-frequency oscillation and the active and break (decay) of the South China Sea monsoon. In the meantime. the work illustrates how the low-frequency oscillation in the South China Sea are superimposed with the seasonal variation of the general circulation. so that the summer inonsoon covers the establishment of the I st, intensification of the 2nd and 3rd the low-frequency oscillations and decay of the 4th oscillation.

Effect of low-frequency oscillation on performance of Hall thrusters

In this paper, a direct connection between the discharge current amplitude and the thruster performance is established by varying solely the capacitance of the filter unit of the Hall thrusters. To be precise, the variation characteristics of ion current, propellant utilization efficiency, and divergence angle of plume at different low-frequency oscillation amplitudes are measured. The findings demonstrate that in the case of the propellant in the discharge channel just meets or falls below the full ionization condition, the increase of low-frequency oscillation amplitude can significantly enhance the ionization degree of the neutral gas in the channel and increase the thrust and anode efficiency of thruster. On the contrary, the increase in the amplitude of low-frequency oscillation will lead to increase the loss of plume divergence, therefore the thrust and anode efficiency of thruster decrease.

NUMERICAL EXPERIMENTS OF EFFECT OF SSTA OVER THE INDIAN OCEAN ON ATMOSPHERIC LOW-FREQUENCY OSCILLATION IN THE EXTRATROPICAL LATITUDE

Numerical experiments on forcing dissipation and heating response of dipole (unipole) are carried out using global spectral models with quasi-geostrophic barotropic vorticity equations. For each experiment model integration is run for 90 days on the condition of three-wave quasi-resonance. The results are given as follows: Under the effects of dipole (unipole) forcing source and basic flow intensity, there exist strong interactions among the three planetary waves and quasi-biweekly and intraseasonal oscillation of the three planetary waves. In the meantime, the changes in the intensity of dipole or unipole forcing source and basic flow have different frequency modulation effects on LFO in the middle and higher latitudes. The results of the stream function field of three quasi-resonant waves evolving with time confirm that the low-frequency oscillation exists in extratropical latitude.

Novel Adaptive Neural Controller Design Based on HVDC Transmission System to Damp Low Frequency Oscillations and Sub Synchronous Resonance

This paper presents the effect of the high voltage direct current (HVDC) transmission system based on voltage source converter (VSC) on the sub synchronous resonance (SSR) and low frequency oscillations (LFO) in power system. Also, a novel adaptive neural controller based on neural identifier is proposed for the HVDC which is capable of damping out LFO and sub synchronous oscillations (SSO). For comparison purposes, results of system based damping neural controller are compared with a lead-lag controller based on quantum particle swarm optimization (QPSO). It is shown that implementing adaptive damping controller not only improves the stability of power system but also can overcome drawbacks of conventional compensators with fixed parameters. In order to determine the most effective input of HVDC system to apply supplementary controller signal, analysis based on singular value decomposition is performed. To evaluate the performance of the proposed controller, transient simulations of detailed nonlinear system are considered.

The low frequency oscillations of the sea surface temperature in the Equatorial Eastern Pacific and El Nino formation

-In this paper the variations of the sea surface temperature anomalies (SSTA) in the Equatorial Eastern Pacific are analysed. The results show that there are two peaks in the spectrum. One is the low frequency oscillation with a period of 3 - 5 years, and the other is the quasi-biennial oscillation. The former shows a westward migration in the warm episode of SSTA and the latter has the opposite trend. The El Nino events will be formed while the two frquency bands are in phase in the warming stage of SSTA in the Equatorial Eastern Pacific

The Low-Frequency Oscillation Restrain Method Research of Converter-Fed Multi-Phase Induction Motor Propulsion system

The inverter-fed induction motor drive system may become unstable at low frequencies and light load, and phase current and speed of the induction motor may oscillate periodically, which will threaten safety and reliability of the system. This paper chooses nine-phase induction motor simulated propulsion system as the research object, small disturbance model of three-phase induction motor is built, and average equivalent model of the converter is built by introducing switch function. On the basis above, small disturbance mathematic model of the whole system is obtained. As for the limitation of parameters adjustment method of restrain low-frequency oscillation, the restrain method combining current close-loop with dead-time compensation is put forward. Finally, the proposed restrain method is verified respectively on the built simulation and experimental analogue platform. And the simulation and experimental results indicate that the proposed method can not only satisfy the requirement of low-frequency oscillation restraining, but also be expanded widely, and the stability of the system can get improved greatly.

A New Method for Visual Real-Time Monitoring of Low Frequency Oscillation

Visual real-time monitoring is the premise of low frequency oscillation control in power grids. This paper showed a visual method for the control center of power grids to monitor low frequency oscillation. It processed the PMU real-time data with incomplete S-transform, and converted the waveforms to two-dimensional time-frequency figures which showed the initial time, frequency and amplitude of each low frequency oscillation mode directly. GPU was used to show figures and calculate FFT with the purpose of improving calculation efficiency. The results of practical cases show that the real-time characters of low frequency oscillation can be identified availably by this visualization real-time monitoring method which is helpful and suitable for practical application.

ANALYSIS OF LOW-FREQUENCY OSCILLATIONS FOR THE SOUTH CHINA SEA SUMMER MONSOON IN 1998

With NCEP/NCAR reanalysis daily data and SST for 1998, the paper investigates the features of summer monsoon low-frequency oscillation (LFO) over the South China Sea (SCS). Results show that SCS summer monsoon onset is enhanced because of its LFO. Low-frequency (LF) low-level convergence (divergence) region of SCS is in the LF positive (negative) rainfall area. LFO of the SCS region migrates from south to north in the meridian and from west to east in zonal direction. LF divergence of SCS is vertically compensating to each other between high and low level.

SST THERMAL FORCING -A DISCUSSION ON THE MECHANISM OF ATMOSPHERIC LOW-FREQUENCY OSCILLATION

Dynamic and numerical methods are used to discuss the atmospheric response to SST thermal forcing. The results show that for planetary scale systems, the standing SST thermal forcing can quickly excite a stable atmospheric equilibrium state response, which is characterized by obvious large-scale teleconnection oscillation in east-west and south-north directions. For synoptic scale systems, the SST thermal forcing mainly excites the atmospheric low-frequency oscillation. Some basic relation and dynamic processes between SST thermal forcing and atmospheric response pattern are revealed and some new viewpoints are presented.

Low Frequency Oscillations of the Heat Distribution in the Global Upper Ocean Layers

在海洋的上面的层的热分布被使用各种各样的典型因素学习了，一些重要低频率摆动(LFO ) 已经被发现并且确定。在这篇论文，“一个海区域的热中心”与一个简单方法被定义。然后全球海洋的上面的层的温度数据集(从在 to400m 下面的表面， 1955-2003 ) 被分析检测可能的 LFO。不仅一些带的 LFO，早被报导，而且垂直、南方的热分发的一些强壮的 LFO 被检测。类似的垂直摆动模式能在太平洋，大西洋和印度洋被发现，这应该被注意。从一些初步的研究的结果证明垂直 LFO 可能被太阳的发光异例引起。这研究可以帮助在全球海洋揭示一些未知动态过程并且可以也有益于另外的相关研究。

Numerical study of low-frequency discharge oscillations in a 5kW Hall thruster

A two-dimensional particle-in-cell plasma model is built in the R-Z plane to investigate the low- frequency plasma oscillations in the discharge channel of a 5 kW LHT-140 Hall thruster. In addition to the elastic, excitation, and ionization collisions between neutral atoms and electrons, the Coulomb collisions between electrons and electrons and between electrons and ions are analyzed. The sheath characteristic distortion is also corrected. Simulation results indicate the capability of the built model to reproduce the low-frequency oscillation with high accuracy. The oscillations of the discharge current and ion density produced by the model are consistent with the existing conclusions. The model predicts a frequency that is consistent with that calculated by the zero-dimensional theoretical model.

Characteristics of low-frequency oscillations in course of SCS summer monsoon onset

Some of the low-frequency oscillation (LFO) characteristics of summer moan onset over the South China Sea (SCS) are examined based on NCEP/NCAR reanalysis data and OLR from NOAA in the period of 1979-1996. It is found that the SCS summer monsoon is characterized by predominance of low-frequency activities with significant interannual variability closely related with onset time. The LFOs over the SCS strengthen after onset. Composite analyses for LFO components of OLR and kinetic energy indicate that the eastward propagation of LFO over the equatorial Indian Ocean and LFO coming from the western Pacific strongly affect the physical processes responsible for the SCS summer monsoon onset. The SCS is the place where the Indian Ocean LFO links with low-frequency activities over the western pacific during summer monsoon onset.

NUMERICAL STUDY OF OROGRAPHIC INFLUENCE ON LOW FREQUENCY OSCILLATION

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The Propagation of Disturbances Excited by Low-Frequency Oscillations in the Tropics

The propagation of disturbances excited by low-frequency oscillations in the tropics is investigated by applying the theory of wave packet dynamics. For simplicity, a linearized barotropic model is adopted and the zonal circulation is taken as basic current. Suppose that the disturbances or waves are superimposed on jet-like westerly basic cur-rent and excited by the forcing in the tropics. We have (1) only the eastward propagating (m>0, n>0 and σ>0) low-frequency disturbances and the stationary (σ = 0) waves can propagate into the middle and high latitudes in the Northern Hemisphere; the others, such as the westward propagating low-frequency wave (m>0, n<0, σ<0) and the high-frequency waves, are restricted only in the vicinity of source region; (2) a stationary wave (σ = 0) reaches a given latitude even more quickly than some low-frequency ones (σ>0) due to the fact that the group velocity of stationary wave is larger; (3) there is a whole wave train excited by the forcing in the tropics and extended into the middle and high latitudes, if the amplitude of the source is independent on time, especially, the low-frequency wave (σ > 0) is of travelling type propagating along the ray; (4) if the source lasts only for an interval of time, namely, its amplitude also has the character of low-frequency oscillation, the excited wave train is not always a whole one, but is restricted in the vicinity of source region in the beginning, extended from the source region to the middle and high latitudes in its saturated stage, after that it gradually becomes weaker and weaker and is detectable only in some area at high latitude, and eventually disappears. Undoubtedly, case (4) is closer to the reality, even though case (3) gives a more impressive wavy pattern.