Hu Jing-Guo

Trend of extreme precipitation events over China in last 40 years

Using the daily precipitation data of 740 stations in China from 1960 to 2000, the analysis on the variations and distributions of the frequency and the percentage of extreme precipitation to the annual rainfall have been performed in this paper. Results indicate that the percentage of heavy rains (above 25mm/day) in the annual rainfall has increased, while on average the day number of heavy rains has slightly reduced during the past 40 years. In the end of 1970s and the beginning of 1980s, both the number of days with extreme precipitation and the percentage of extreme precipitation abruptly changed over China, especially in the northern China. By moving t test, the abrupt change year of extreme precipitation for each station and its spatial distribution over the whole country are also obtained. The abrupt change years concentrated in 1978–1982 for most regions of northern China while occurred at various stations in southern China in greatly different/diverse years. Besides the abrupt change years of extreme precipitation at part stations of Northwest China happened about 5 years later in comparison with that of the countrys average.

Characteristics of the spatiotemporal distribution of daily extreme temperature events in China: Minimum temperature records in different climate states against the background of the most probable temperature

Based on the skewed function, the most probable temperature is defined and the spatiotemporal distributions of the frequencies and strengths of extreme temperature events in different climate states over China are investigated, where the climate states are referred to as State I, State II and State III, i.e., the daily minimum temperature records of 1961–1990, 1971–2000, and 1981–2009. The results show that in space the frequency of high temperature events in summer decreases clearly in the lower and middle reaches of the Yellow River in State I and that low temperature events decrease in northern China in State II. In the present state, the frequency of high temperature events increases significantly in most areas over China except the north east, while the frequency of low temperature events decreases mainly in north China and the regions between the Yangtze River and the Yellow River. The distributions of frequencies and strengths of extreme temperature events are consistent in space. The analysis of time evolution of extreme events shows that the occurrence of high temperature events become higher with the change in state, while that of low temperature events decreases. High temperature events are becoming stronger as well and deserve to be paid special attention.

The Relationship Between North Pacific Oscillation and Summer Floods/Drougts over North China

The NCEP/NCAR reanalysis data for the period 1948-2011 is used to calculate North Pacific Oscillation Index (NPOI), and then the positive correlation between NPO and summer floods/drougts over North China was discovered through studying the relationship between NPOI and Palmer Drought Severity Index (PDSI). When in positive NPOI years, summer PDSI is higher than normal, and North China has more summer floods; otherwise, summer PDSI is lower than normal, and North China has more summer drougts. Possible mechanism analysis shows that, when in positive(negative) NPOI years, in 850hPa wind anomaly, the common action of anti-cyclonic(cyclonic) in Ural Mountains, the cyclone(anti-cyclonic) of Lake Baikal, and the anti-cyclonic(cyclonic) anomaly circulation in West Pacific, have strengthened(weakened) the convergence of south-west warm and humid air flows of low layer in North China region. And also, when in positive(negative) NPOI years, 500hPa geopotential height anomaly shows’+ - +’(‘- + -‘)wave train, and West Pacific subtropical high is stronger(weaker)/shifting northwest(southeast)than normal years, cold and warm air activities make more (less) rainfall over North China.

Temperature Dependence of Exchange Bias and Coercivity in Ferromagnetic Layer Coupled with Polycrystalline Antiferromagnetic Layer

The temperature dependence of exchange bias and coercivity in a ferromagnetic layer coupled with an antiferromagnetic layer is discussed. In this model, the temperature dependence comes from the thermal instability of the system states and the temperature modulated relative magnetic parameters. Morever, the thermal fluctuation of orientations of easy axes of antiferromagnetic grains at preparing has been considered. From the present model, the experimental results can be illustrated qualitatively for available magnetic parameters. Based on our discussion, we can conclude that soft ferromagnetic layer coupled by hard antiferromagnetic layer may be very applicable to design magnetic devices. In special exchange coupling, we can get high exchange bias and low coercivity almost independent of temperature for proper temperature ranges.

Magnetization Configuration in Magnetic Trilayer System with Out-of-Plane Component Defect*

The equilibrium magnetization configuration, the inducing field and the coercive field in trilayer magnetic materials having an out-of-plane anisotropy defect interlayer between two in-plane anisotropy layers are discussed by both analytical and numerical calculations based on a micromagnet approach. It is shown that the above physical parameters strongly depend on the defect layer such as its thickness and exchange stiffness etc., as well as on the applied fields. It is found that there is a special thickness of defect layer, in which the inducing effect begin to occur, and the critical behavior of inducing field in the vicinity of the special thickness is linearly characterized. Particularly, the magnetic hysteresis shows typical soft hysteresis shape, even though the host material is composed of hard magnets, and the coercivity increases with increasing the thickness of the interlayer.

Nonmonotonic effects of perpendicular magnetic anisotropy on current-driven vortex wall motions in magnetic nanostripes*

In a magnetic nanostripe, the effects of perpendicular magnetic anisotropy (PMA) on the current-driven horizontal motion of vortex wall along the stripe and the vertical motion of the vortex core are studied by micromagnetic simulations. The results show that the horizontal and vertical motion can generally be monotonously enhanced by PMA. However, when the current is small, a nonmonotonic phenomenon for the horizontal motion is found. Namely, the velocity of the horizontal motion firstly decreases and then increases with the increase of the PMA. We find that the reason for this is that the PMA can firstly increase and then decrease the confining force induced by the confining potential energy. In addition, the PMA always enhances the driving force induced by the current.

Topology-like dynamical behavior of magnetization reversal in exchange-bias systems

In an exchange-bias system, the barriers with intrinsic potential energy may be asymmetric due to unidirectional anisotropy. Based on the Stoner—Wohlfarth model, we show that the asymmetric barriers may lead to four kinds of dynamical process underlying the hysteresis-loop measurement. These kinds of dynamical processes are different in a topology-like property, which can be controlled by the orientation of the external field. In our study, a new analysis approach has been proposed to reveal the dynamical behaviors of magnetization reversal. With this approach, coercivity, exchange-bias field, and asymmetry of hysteresis loops can be quantitatively obtained.

Preliminary research on the relationship between long-range correlations and predictability

By establishing the Markov model for a long-range correlated time series (LRCS) and analysing its evolutionary characteristics, this paper defines a physical effective correlation length (ECL) τ, which reflects the predictability of the LRCS. It also finds that the ECL has a better power law relation with the long-range correlated exponent γ of the LRCS: τ = K exp(−γ/0.3) + Y, (0 < γ < 1) — the predictability of the LRCS decays exponentially with the increase of γ. It is then applied to a daily maximum temperature series (DMTS) recorded at 740 stations in China between the years 1960–2005 and calculates the ECL of the DMTS. The results show the remarkable regional distributive feature that the ECL is about 10–14 days in west, northwest and northern China, and about 5–10 days in east, southeast and southern China. Namely, the predictability of the DMTS is higher in central-west China than in east and southeast China. In addition, the ECL is reduced by 1–8 days in most areas of China after subtracting the seasonal oscillation signal of the DMTS from its original DMTS; however, it is only slightly altered when the decadal linear trend is removed from the original DMTS. Therefore, it is shown that seasonal oscillation is a significant component of daily maximum temperature evolution and may provide a basis for predicting daily maximum temperatures. Seasonal oscillation is also significant for guiding general weather predictions, as well as seasonal weather predictions.

Predictability: Beginning from the information entropy

We have established the Markov model for long range correlated time series (LRCS), by analyzing their evolutionary characteristics, then defined a physical effective correlation length (ECL) of the LRCS, which reflects the predictability of the LRCS, and find that the ECL has a better power law relation with the long range correlated exponent (LRCE) of the LRCS. We apply the power law relation between ECL and LRCE to the daily maximum temperature series (DMTS) at 740 stations in China for the period 1960–2005, calculate the ECL of the DMTS, and the results show the remarkable regional distributive features that the ECL is about 10–14 days in west, northwest and northern China and about 5–10 days in east, southeast and southern China. Namely, the predictability of the DMTS is higher in central-west China than in east and southeast China.