Abstract:Rainstorms frequently occur in China,resulting in significant loss of economic assets.For this reason,meteorologists throughout the country have performed great amounts of research regarding rainstorm prediction.Although the synoptic-scale evolution of the typical mid-latitude weather system is relatively well forecasted,numerical weather prediction models still face many difficulties in forecasting the storm-scale details.The storm-scale numerical weather predictions have significant limitations,e.g.the development process of medium-scale and micro-scale systems is sensitive to the physical and boundary layer processes,which are responsible for forecast uncertainty;and the chaotic characteristics and nonlinear action of the atmosphere result in predictability limitations.Considering both the problems described above and the successful application of an ensemble forecast technique for global medium-scale forecasts,it is reasonable to choose an ensemble forecast.In comparison with a single control forecast,an ensemble forecast can provide a more accurate estimate of the first moment of the probability density function of future atmospheric states,and can also provide higher-order moment estimations,such as the forecast error variance.How to obtain fast-growth errors,which is comparable to the actual forecast growth error,is an important problem involved in ensemble forecast.In order to obtain the actual fast-growth errors,the Breeding of Growth Mode (BGM) has been used in this paper.The method,proposed by Toth and Kalnay(1997),which has been used to generate perturbations for medium-scale ensemble forecast at NCEP,is a reasonable choice for capturing growing errors modes,especially with extreme weather.There are some problems to be solved when generating initial perturbation with BGM,such as the number of ensemble members,breeding cycles,and time span.It has been demonstrated that the number of BGM ensemble members should be no less than eight,thus in this paper nine was chosen as the number.The breeding cycles were designed for six hours,the forecasts ran four times per day,and the time span was designed for three days.In this paper I conducted some research,due to the uncertainty of the initial field.Based on the concept of breeding growth modes,I conducted some rainstorm ensemble prediction examples which had occurred around Henan Province(located in northern China) in September 2010,by using the Weather Research Forecast model(WRF),a non-hydrostatic medium-scale model with a full physics package.The model domain was centered at 33°N,119°E,and covered Henan.In this paper,three methods based on BGM have been made to improve the prediction results.Through the improvement of the breeding growth method,I conducted both the scroll breeding growth method and regional breeding growth method.The scroll breeding growth method combines the breeding growth and time lag methods.The purpose of this method is to take advantage of background fields,thereby making the prediction results more accurate.The regional breeding growth method takes both time and space factors into consideration.It enhances the strength of convection by multiplying different factors in different regions.From the three groups of experiments with different perturbation types,the following conclusions are obtained:(1) Both the scroll breeding growth method and regional breeding growth method have positive contributions to the storm-scale precipitation forecast.The accuracies of prediction of rain location and rain intensity are also improved.(2) The scroll breeding growth method and regional breeding growth method also make a significant improvement on the perspective of forecast error.Compared with BGM,they take the information of growing errors into account,and improve the defects of ensemble dispersion.(3) In terms of different intensities and different stages of rainstorms,the regional breeding growth method maintains a certain degree of stability from a variety of rates.

Abstract:As an important component of the East Asian winter monsoon,the Siberian High(SH) can cause low temperatures,snowy weather and strong wind,which has a substantial influence on the winter climate of China.In order to mitigate the effects of the SH,it is important to study in depth the climate characteristics of the SH and its impact factors.Using NCEP/NCAR reanalysis monthly data and sea surface temperature(SST) data from NOAA and other data for the period 1951-2014,this study investigates the variational characteristics of the SH and SST in the North Atlantic,employing the methods of empirical orthogonal function(EOF) analysis,conventional synthesis analysis,Gaussian filtering,and the t-test of statistical significance.Moreover,their connection and its temporal and spatial changes are also revealed,based on correlation analysis and regression analysis.The results are as follows:(1) The intensity of the SH in winter changed from strong to weak in the mid to late 1960s,but enhanced slightly again after 2003.The SH expands outward significantly in strong anomaly periods,whereas it shrinks obviously in weak anomaly periods;and the swing amplitude of its east and north edge is bigger than that of the west and south edge.(2) The North Atlantic SST index(normalized anomaly value of the whole region's average SST) decreased in all seasons in the mid-1960s,but rose again from the late 1990s.The time in which the North Atlantic SST tripole converted from positive to negative phase came relatively later in the early 1970s(spring and winter) than in the early 1960s(summer and autumn),but then the tripole of all four seasons turned into a positive phase again in the mid-1990s.The phase variation of the North Atlantic SST tripole commonly induces the change of the North Atlantic SST index.(3) An anomalously high(low) North Atlantic SST index and positive(negative) phase of the North Atlantic SST tripole in all seasons can together make the winter SH strengthen(weaken).Furthermore,by comparison,the relationship between the former and the SH intensity is more outstanding,and their correlation gradually decreases to a minimum from spring to summer,meaning the period is the relationship conversion period,and then gradually increases to a maximum from autumn to winter.In all seasons the northern part and southwest part of the North Atlantic are both key regions having a positive impact on SH intensity.The SH's response range to the SST anomaly of the north part is widest in winter and smaller in summer and autumn;whereas for the SST anomaly of the southwest part,the SH's response range is maximum in summer and minimum in winter.(4) When the North Atlantic SST index is anomalously high in winter,the winter 500 hPa height above its northern waters is abnormally high owing to the heating of the underlying surface,and a Rossby wave train is excited in downstream Eurasia,making the Ural Mountains' high-pressure ridge enhance prominently,and resulting in a larger negative vorticity being delivered to the upper levels of the SH by the northwest airstream in front of the Urals ridge.As a result,negative relative vorticity advection over the SH increases and,along with the reinforcement of high-level convergence and low-level divergence,the downdraft in the whole troposphere also grows significantly.The above situation causes the enhancement of SH intensity,and vice versa.

Abstract:Recently,a new functional analysis tool was developed,the multiscale window transform(MWT),to realistically represent and decompose the complex nonlinear multiscale processes within atmospheric and oceanic circulations.In this study,two types of idealized series were constructed to test the effect of the MWT versus the traditional and most widely used band-pass Butterworth filter.It was found that the MWT produced fairly accurate reconstructions,while the Butterworth filter tended to yield series with smaller amplitude.Besides,the boundary effect was successfully suppressed in the MWT reconstructions,in contrast to the Butterworth results.With these observations in mind,we reconstructed the MJO using these two methods.The results showed little difference in the meridional propagation pattern.However,with respect to the zonal distribution,both the strength and propagation patterns were quite different,especially in the western Pacific Ocean.This was particularly significant in terms of the annual mean distribution.It was found that,from January through June,the MJO wave packet propagated northward;but from June through December,the propagation reversed its direction,becoming southward.This propagation reversion is quite different from the scenarios revealed in previous studies.The zonal propagation in the Indian Ocean and Pacific Ocean was different.The results showed that,in the Indian Ocean area,the oscillations in the Northern Hemisphere had a northward trend in spring and summer with the Butterworth filter;while in the case of MWT,the oscillations in spring and summer were less significant than the Butterworth case.In the western Pacific,the zonal propagation of the MJO showed a significant difference between the Butterworth filter and MWT filter.In the Butterworth case,the northward trend of the MJO in the western Pacific Ocean was still evident in summer;while in the case of MWT,the oscillation showed a significant southward trend.It was apparent that the southward trend of zonal propagation was mainly caused by the zonal propagation in the western Pacific Ocean.The MJO distribution was different in all seasons.The strongest oscillation occurred in winter and spring,which,in spring,was obvious in the western Pacific Ocean,whereas in winter it occurred obviously in the Indian Ocean.

Abstract:The air-moisture ratio is one of the products of the Chinese FY-3A satellite.The product enables atmospheric moisture conditions to be described at fine scale.By overlapping the air-moisture ratio field of FY-3A and the wind streamline field at low level together,the quantitative diagnostic and pre-estimate on precipitation over local area can be done.Due to the fact that the satellite data and reanalysis data come from different digital sources,and have their own coordinate systems,it is needed to build the same coordinate as a base for both datasets.The satellite coordinate changes with time as its covering area is different to any given position on Earth,so it needs to transfer the satellite coordinate into the coordinate of the wind streamline fields.The whole data preprocessing includes the following steps:interpolation of the vertical standard pressure levels from the satellite's own pressure levels;switching the satellite orbit coordinate into the traditional latitude-longitude coordinate;deleting outliers via the Bi-weight Check method,smoothing the data field and removing the data "noise" by the nine smooth method;Testing preprocessing results via cross-correlation testing,to check that the fine-scale features of satellite data are still keep satisfactory.For analysis of the wind streamline fields,key systems of heavy rainfall need to be determined at first.According to 5 year (2009-2013)statistics,the shallow low vortex and convergence line are two key system types for heavy rainfall over the Jianghui Valley during the Meiyu season.Three of five low vortices are active in the downstream area of Changjiang River,and two of three convergence lines are active in the Huaihe Basin.Synthetic analysis of these typical systems according to their different types shows that the convergence line is associated with a moisture front that has a stronger latitude directional,whereas the low vortex is associated with a longitudinally directed wet tongue and more blending of the wet and dry air masses.Furthermore,by carrying out the combinational analysis of the FY-3A air-moisture ratio distribution and wind streamline field of heavy rainfall at the same moment,the results show that the large-value areas of FY-3A air-moisture ratio corresponding to the cyclonic convergence areas in the streamline fields.The area can indicate the locations of heavy rainfall at fine scale.The atmospheric precipitable water amount at low levels (1000-850 hPa)can be calculated according to the FY-3A satellite air-moisture ratio,The precipitable water amount and the high energy area of the pseudo-equivalent potential temperature,especially the high-energy fronts in the rainfall area are all proportional to the precipitation intensity.To apply the satellite moisture-dynamic diagnostic method over a near-shore area,the precipitation location and intensity at the sea area can be estimated,which compensates efficiently for the shortage of observations at sea.

Abstract:Meiyu refers to a special season of rainy weather that usually happens in early summer in China's Yangtze River region.The intensity,range and other characteristics of Meiyu precipitation,including trends of change,have important influences on the economic development,agricultural production,and lives of the people residing in this region.In recent years,an abnormal enhancement in rainstorm intensity,an increase in associated damage,and other atypical features have become more frequent.In other words,the Meiyu has begun to show new characteristics against the background of global warming.Choosing the moisture source during the Meiyu season as the starting point,the vapor transport characteristics over China's Yangtze River region were studied and discussed in this paper.The spatiotemporal characteristics of June-July Asian monsoon moisture transport and its relationship with precipitation over the Yangtze-Huaihe River Basin were investigated using ERA reanalysis data of wind and pressure,along with APHRODITE daily precipitation data for East Asia,over the period 1958-2007.Results showed that different moisture paths connected with different regional rainfall in the Yangtze-Huaihe River Basin.By calculating the water vapor flux intensity,three main water vapor paths that influence Meiyu precipitation in the Yangtze-Huaihe River Basin were partitioned out.The strongest transmission intensity belonged to the southwest path,but the greater contribution to Meiyu precipitation was from the South China Sea path and the southeast path.Meridional water vapor transport of the southwest path enhanced local precipitation in the western part of the Yangtze-Huaihe River Basin,while zonal transport reduced precipitation in the northern part;The influence of the South China Sea path on precipitation over the Yangtze-Huaihe River Basin featured reverse characteristics in the north and south.The precipitation in the area north of the basin was weakened by the direction of the meridional transport,while most areas south of the basin experienced an increase in precipitation.When water vapor transport was anomalously strong,there was less rain in the northern part of the basin and rain in the southeast;The zonal transport of the southeast path featured a significant increase in precipitation in the southwest and central to northern parts of the Yangtze-Huaihe River Basin.When the water vapor transport path was strong,a corresponding increase in precipitation in the central eastern Yangtze-Huaihe river basin was found.The characteristics of moisture transport during the Mei-yu season were found to exhibit interannual variation.Furthermore,precipitation intensity over the Yangtze-Huaihe River Basin was closely related to the strength of moisture transport.Using correlation analysis to identify the most significant areas of water vapor transport,nd combined with the synthesis of difference analysis of anomalous years of water vapor transport,he impact of water vapor transport intensity on the strength of Meiyu precipitation in the Yangtze-Huaihe River Basin was studied.The results of the correlation analysis and the synthesis of difference demonstrated that precipitation during the Mei-yu period over the Yangtze-Huaihe River Basin was mainly influenced by the southeast path of moisture transport,when the moisture transport derived from the West Pacific.The western Pacific Ocean contributed much more than the Indian Ocean in terms of vapor transfer into the Yangtze-Huaihe River Basin region.Against the background of global warming,the correlation and range of influence between the precipitation of the Meiyu season over the Yangtze-Huaihe River Basin and the intensity of the main water vapor transport sources has changed.The effect of western Pacific transportation into the Yangtze-Huaihe River Basin region on the enhancement of rainfall has weakened,while the scale of the decreased influence of Indian Ocean transportation on rainfall has significantly increased.Since the 1980s,the slowly increasing trend of precipitation over the Yangtze-Huaihe River Basin has been related to the moisture transportation anomalies caused by the easing of Asian circulation anomalies as a result of global warming.

Abstract:In the present study,based on the NCEP/NCAR reanalyses dataset and best track data of the typhoons provided by China Meteorological Administration(CMA),the influence of the Australian cold air activity on the genesis of tropical cyclones(TCs) over the western North Pacific was investigated.It was found that,off the northeastern coast of Australia,the occurrence frequency of the meridional wind larger than 6 m/s was the largest(50 days annually).Therefore,the Solomon Sea was determined as the key area of influence of the Australian cold air activity on the low latitudes in both hemispheres,and a cold air intensity index over Australia was defined by the low level meridional wind over the Solomon Sea(SSV).The SSV was shown to be closely related to the cross-equatorial flows and westerly over the Equator,as well as the SOI.As the SOI becomes weaker(stronger),the SSV becomes stronger(weaker).The location of the tropical cyclogenesis(TCG) over the western North Pacific coincides with the variation of the monsoon trough during the years of strong and weak cold air activities over Australia.However,the total TC number has no significant correlation with the SSV.The monsoon trough over the western North Pacific is stronger and located more eastward than normal,and the location of the TCG is more eastward and southward than normal during years of strong cold air activity.Meanwhile,the monsoon trough is weaker and located more westward than normal,and the location of the TCG is more westward and northward than normal during years of weak cold air activity.The distribution of vorticity in the lower troposphere,the moisture transport,the vertical shear of horizontal wind and the convections in low latitudes all indicate that the locations of the TCs over the western North Pacific are more eastward and southward(westward and northward) than normal during strong(weak) years of cold air activities over Australia.

Abstract:Tropical cyclones(TCs) cause substantial adverse impacts on economies and human life.Forecasting the landfall of TCs is an important part in the prediction of TC tracks.Previous studies have focused mainly on the moving paths of TCs,especially the moving direction before landfall.However,not only the location but the timing of landfall is crucial.A successful prediction emerges only if these two aspects are combined.After landing,slow-moving TCs induce large amounts of precipitation locally,causing serious problems to the people living in the area.This study focuses on the changes in TC moving speed in the TC tracks,and analyzes the moving speed of TCs along with their temporal and spatial distribution changes in the hope to help improve TC track predictions.The study is based on the TC best track dataset for the western North Pacific over the period 1970-2009 provided by the National Meteorological Bureau and Shanghai Typhoon Institute(CMA-STI),along with TC track prediction error data for 2005-2010,and applies a percentile method to determine the anomaly index of the TC moving speed.The characteristics of the temporal and spatial distributions of anomalous TC moving speed and its change during the last 40 years over the western North Pacific are analyzed.The effect of TC moving speed prediction error on track prediction error and the relationship between large-scale steering flow and the error for TC moving speed prediction are studied.The results show that:(1) The cumulative probability at the 95%(5%) quantile thresholds for TC moving speed and its variation of anomalies in the western North Pacific are 10.8 m·s^-1(1.43 m·s^-1) and 2.42 m·s^-1(-1.72 m·s^-1),respectively;(2) Fast-moving or accelerated-moving TCs mostly appear in the area of the Japan Sea,while slow-moving or decelerated-moving TCs occur mainly in the region of the South China Sea;(3) The seasonal variation of anomalous-moving TC represents that the frequency of fast-moving(slow-moving) TCs is highest in May(October) and that of accelerated-moving TCs tends to be highest in June;(4) In the last 6 years (2006-2011),the prediction error of TC moving speed contributes on average about 41.6% of TC track prediction error;(5)In the case study of a TC with large track prediction error that is deduced more by moving speed prediction error,the results show that the weak large-scale environmental steering flow makes the TC move slowly.If the forecasted large-scale environmental steering flow is strong,the predicted TC moving speed will be faster,which may lead to large track prediction error.

Abstract:On May 27 2008,the first satellite in the new Chinese Fengyun polar-orbiting satellite series,FY-3A,was successfully launched into a circular,sun-synchronous,near-polar and morning-configured(10:00 LST,mean equator crossing local solar time of the descending node) orbit,with an altitude of 836 km above the Earth's surface,and an inclination angle of 98.75°.The Microwave Temperature Sounder(MWTS) on board the FY-3A has four channels in the oxygen band,at frequencies ranging from 50.3 to 57.3 GHz,for temperature sounding from the surface,to about 1 hPa,due to the absorption and emission of microwave radiation by atmospheric oxygen.MWTS provides a total of only 15 field of views(FOVs) along each scan line.While the calibration accuracy of the MWTS's brightness temperature was well characterized during its prelaunch period using a two-point calibration algorithm,which converts the Earth scene,warm target and cold target counts into Earth scene radiances,the MWTS's on-orbit performance remains important after the launch of each satellite.This is a relative calibration method,and satisfies weather forecasting application,but it is not sufficiently suitable for climate application,as the requirements on satellite data calibration for climate studies are different from weather forecasting applications.Issues such as variable calibration accuracy associated with each satellite instrument and bias changes with respect to time due to the satellite orbital drift,warm target calibration problems and pass-band frequency shifts must be resolved,as they may be mistakenly interpreted as climate influences.In this study,Global Position System Radio Occultation(GPS RO) data are used to absolutely calibrate the MWTS data throughout all of 2010,based on two methods.The first method is to calculate the scan-dependent global mean differences between the MWTS upper air sounding channels 2-4 measurements and GPS RO simulations over ocean under clear-sky conditions.This can be expressed as a scan-dependent function μ₁^o(α)=T_b,MWTS^obs-T_b,1^sim,where α is the scan angle,T_b,MWTS^obs represents the MWTS brightness temperature,and T_b,1^sim represents the CRTM brightness temperature simulations with input GPS RO profiles.The second method is similar to the first,except for the fact that the simulated brightness temperature T_b,2^sim is derived through the linear regression of MWTS measurements and GPS RO simulations at each scan angle.The absolute calibration of the MWTS data is to subtract either μ₁^o(α) or μ₂^o(α).The effects of the above two calibration methods are examined by comparing the biases of the MWTS observations with respect to the National Centers for Environmental Prediction(NCEP) Global Forecast System(GFS) simulations,with and without calibration.A unique feature of a cross-track scanning radiometer instrument is that the impacts of side-lobe effects and spacecraft radiation on the MWTS observations are higher at larger scan angles,and these effects may be difficult to take into a full account in the calibration process.In short,the biases are channel dependent,and all three of the channels 2-4 are of negative biases.It can also be seen that the bias at channel 3 is the largest.Moreover,the biases are all seasonal dependent.It is also found that the MWTS global biases and standard deviations are both reduced.The channel and time dependences of the MWTS biases are eliminated as well.It is well know that a single satellite has a limited life span,varying from 5 to 13 years.However,long-term climate microwave temperature sounder data records may be obtained if the measurements from different satellites are linked together.The absolute calibration using GPS RO data is definitely one of the more effective methods.This paper also represents a preliminary study toward establishing a refined satellite microwave temperature sounding instruments fundamental climate data records (FCDR).Further work includes diurnal correction using GPS RO observations to remove the impact of orbital drift.The impact of the first guess and the vertical error covariance matrix on the quality of the temperature and water vapor profiles retrieved by a 1D-Var approach require assessment.

Abstract:Studying the impact of land-use and hydrological change on regional climate can provide theoretical guidance for socioeconomic development.The impact of land-use change associated with the development of a hydropower station on the Daduhe River,located in Sichuan Province,on the regional climate of the region,was examined through comparison of two simulations-a control experiment and a land-use change(sensitivity) experiment-using the WRF(Weather Research and Forecasting) model.Two study areas-Daduhe River and Pubugou Hydropower Station-were selected.According to precipitation characteristics,representative years(dry/wet/normal) were chosen,and the normal year of 2011 was selected as the simulation year.In addition,the months of January,April,July and October were taken as spring,summer,autumn and winter,respectively.First,the temporal variability and accumulated frequency of precipitation were studied by applying statistical analysis methods to daily precipitation data for the period 1970-2012 at 12 observation stations along the Daduhe River.Then,the control experiment and sensitivity experiment were separately conducted for the Daduhe River and Pubugou Hydropower Station study areas.The results indicated that the climatic effects of the land-use change associated with the hydropower station in the above two areas were weak on the local scale.The annual mean surface temperature decreased,ranging from 0.2℃ to 0.4℃.In addition,the temperature increased weakly in winter and decreased in the other seasons.No significant influence on precipitation in winter,spring and autumn was found,but a weak decrease in summer was detected.The relative humidity increased weakly surrounding the water body,and the wind speed showed almost no change.These meteorological variables only changed in the vicinity of the water body.The farther from the reservoir,the weaker the influence.

Abstract:Based on agrometeorological station soil moisture data from 1981 to 2007 in China,along with 10 days of surface meteorological observation data,East Asian summer monsoon intensity index data,and NCEP reanalysis-Ⅱ data,this paper uses diagnostic analysis to discuss the relationship between the soil moisture anomaly of the Wei River Basin(an area sensitive to soil moisture change) and the country's climate background,in order to explore the feedback mechanisms of land surface soil moisture to the short-term climate.Composite analysis shows that the soil is drier,the rainfall amount is lower,the pan evaporation is stronger,and the atmospheric relative humidity is smaller,in most parts of central-eastern North China during abnormally dry years of soil moisture in the Wei River Basin.The opposite is the case during abnormally wet years.Atmospheric circulation is also opposite:The anomaly is positive in the west and negative in the east over middle and high latitudes of Asia at the 500 hPa geopotential height during abnormally dry years,but negative in the west and positive in the east during abnormally wet years.Lag correlation analysis shows that a change in soil moisture can cause a change in the 500 hPa atmospheric geopotential height field so the standing wave change in the location and intensity and which led to precipitation field change.The correlation coefficients show an atmospheric teleconnection type of Europe-Asia and the Pacific between 13-16 ten days' soil moisture of Wei River Basin and 17-20 ten days' 500 hPa geopotential height atmospheric circulation that indicate the impact of soil moisture on atmosphere spread to other regions by the atmospheric teleconnection.Soil moisture contributes positively to the low-level atmospheric water vapor flux,but this kind of contribution is secondary for water vapor to produce precipitation.The previous soil moisture's influence on summer precipitation is small,but has an obvious impact on autumn precipitation in East China.(3)Soil moistue of Wei River Basin of fall of last year and spring of the year is positive correlated significantly with the east asian summer monsoon index.It valid that the soil moisture conditions of land surface of fall and spring fed back positively to the east asian summer monsoon's intensity.At 850 hPa geopotential height,it is east wind anomaly from India Peninsula to the South China Sea in June to August after the soil moisture dry year in autumn and spring of Weihe River basin and it maintain anticyclonic circulation anomaly in South China Sea to the Western Pacific region-such as weak Asian Monsoon.It is contrary in wet years.(4)This paper disscuss preliminarily the relation of the soil moisture of Wei River Basin and several climate background in China by the observation data analysis.If it is positive(negative) in west and negative(positive) in east at middle and high altitudes in Asia in 500 hPa geopotential height anomaly distribution than the rainfall less(more) in most part of central-eastern North China and soil moisture of Wei River Basin is drier(wetter).Soil moisture anomaly of Wei River Basin's drier(wetter) can means the land surface's drier(wetter) of the most part of central-eastern North China. Drier(wetter) soil moisture can make the standing wave's location and intensity change at the atmosphere geopotential height field through the exchange of energy and moisture in the process of land surface and it can result further the change of precipitation field.The influence of soil dry(wet) exist atmosphere teleconnection in 500 hPa geopotential height.This is in agreement with the results of numerical experiments.Therefore the soil moisture anomaly of Wei River Basin can be tried as a signal of short-term climate prediction in central-eastern China.

Abstract:Atmospheric electric field instruments can record the entire process of thunderstorms for the creation,development and extinction,based on the principle of conductor induced charges in the electric field.As a result,the instrument scan continuously measure the strength and polarity of atmospheric electric fields for a long period time.In addition,the thunderstorm process is a complex nonlinear chaotic system,which leads to the measured electric field values having strongly nonlinear oscillation characteristics.At the same time,Empirical Mode Decomposition is a new type of nonlinear and nonstationary signal processing method which does not have a pre-determined basis function,and is able to smoothly process the signal based on its own characteristics,which leads to a new concept for ground electric field research.However,atmospheric electric field instruments are susceptible to the effects of the surrounding environment,the observation data quality are severely affected by noise,and model aliasing problems often exist.Fortunately,the Ensemble Empirical Mode Decomposition(EEMD) method has been developed,and is able to resolve these problems by enhancing signal continuity to weaken the mode aliasing effect,and by better reconstructing the amplitude and frequency characteristics of the signal component,based on the Gaussian white noise statistical characteristics.Therefore,this paper analyzes the atmospheric electric field data in the Nanjing area from June to August,2010 and 2011 by means of EEMD,and respectively discusses the oscillation characteristics of the atmospheric electric field in fair,weak and severe thunderstorm weather.In addition,the energy of thunderstorm is found to be far higher than the fair weather,which mainly contains low frequency components.Meanwhile,with the further development of thunderstorm,the electric field energy is mostly concentrated in the high frequency parts.Clearly,the intrinsic mode function(IMF) component variances are significantly different in the three types of weather conditions,and show some dynamic characteristics,which may be a result of the thunder cloud charge accumulation,charge and discharge,and the complex oscillation of internal charge.Although one station observation is influenced by the distance and location of the thunderstorm,the IMF component simply expresses the oscillation characteristic of the thunderstorm interior,which means that the observation data can intuitively reflect the thunderstorm development stages and ignore the position relation between the station and thunder cloud.Therefore,the variance characteristics of the IMF components can be studied to provide the basis for thunderstorm forecast.Moreover,according to the analysis results of fair weather and two different thunderstorms,some energy dynamic changing characteristics of fair weather,weak and strong thunderstorm may also exist.Thus,we take fair weather as the background with only one station,and establish the two-level thunderstorm nowcasting model,based on the dynamic characteristic between the maximum variance related layers and total number of decomposition layers of IMF.As for the weak thunderstorm,the related IMF layers exhibit obvious oscillation before the actual thunderstorm time above 1 hour.Furthermore,the severe thunderstorm development can be divided into the three sections of unrelated,energy storage and discharge.In the energy storage section,the decline of related IMF layers indicates rapid accumulation of electric charge before the thunderstorm arrives,and the four consecutive decline points are taken as the forecast standard,which can effectively increase the forecast time to more than 1 hour.Finally,the independent samples are selected to forecast and verify the proposed method.The results show that energy is concentrated in the low-frequency component in fair weather,and the high-frequency component in thunderstorm.The maximum variance related layers of IMF change stably before weak thunderstorm weather,and fiercely before severe thunderstorm.According to these characteristics,92 independent samples are tested,and the results show that the detection probability of warning is 73.3%,and the false alarm rate is 14.5%.This proves that the established thunderstorm nowcasting model meets the forecast basic requirements,and has a certain degree of application value.

Abstract:Lightning is a type of strong discharge phenomenon in nature which can radiate intensive electro magnetism pulses.The electro magnetism fields may couple into the overhead transmission lines or power installations lying underground and cause significant damage to power systems,so it is important to research the overvoltage caused by lightning.It is well known that there are three different yet equivalent field-to-transmission line coupling models,namely the model of Taylor,the model of Agrawal,and the model of Rachidi.Based on these models,the determination of induced currents and voltages requires the evaluation of specific field components depending on the employed coupling model.Among them,the accuracy of Agrawal coupling model has been proven by artificial triggering lightning experiments.In the model of Agrawal,the horizontal electric field component along the line is required.Considering the inherent difficulty in measuring lightning horizontal electric fields,namely for the observation points to be located on the ground or a few meters above the ground,the vertical electric field magnitude is much larger than that of the horizontal component(typically two orders of magnitude),thus even a small tilt in the measurement sensor could render the measured horizontal field useless,and therefore it is necessary to create an accurate and efficient algorithm to obtain the results.The exact solution of the lightning horizontal field requires calculation of the Sommerfeld integrals for several elementary dipoles at many frequencies,which is very time consuming.The finite different time domain(FDTD) method is a numerical computation method which can be used to calculate the lightning-radiated horizontally electric field over different types of ground;however,the use of numerical calculations in this method will affect thememory,as well as time consumption.Therefore,it is necessary to develop a method to calculate the lightning-radiated horizontal electric field with high accuracy and rapid computation.The C-R formula is one approximation method by which to calculate the lightning-radiated horizontal electric field over homogeneous conductivity soil and horizontally stratified ground.In this paper we have extended the C-R formula to calculate the lightning-radiated horizontally electric field over vertically stratified soil conductivity based on the MTLL return stroke model and double-Herdler function base current,and have examined its accuracy over different types of ground by using the FDTD method.The results show that,when the distance between the lighting stroke point and observation point ranges from 100 m to 1 000 m,and the soil conductivity ranges from 0.001 S/m to 0.01 S/m,the error of the C-R formula is less than about 10%.For the homogeneous conductivity soil,the C-R formula achieves a more accurate prediction result than that for other soil conductivity cases;for the horizontally stratified soil,the accuracy of the C-R formula is better for the upper layer with less conductivity than that for the lower ground with higher conductivity;and for the vertically stratified ground,the accuracy of the C-R formula is better when the soil conductivity near the observation point is less than that near the strike point.Finally it can be concluded that the C-R formula can achieve a more accurate prediction result for the first return strokes than that for the subsequent return strokes.

Abstract:Given the increasing importance of the power system to society,as well as the increasing sophistication of electronic equipment,studying lightning-induced voltage changes on overhead lines is of great practical significance.Due to the low probability of overhead lines being struck directly by lightning,accounting for about 10% of all lightning strikes,lightning is the main factor causing power transmission line faults.A number of analyses were carried out in this study,including the establishment of the lightning return stroke channel,the development of an Earth and overhead wires integrated model,the simulation of overhead line coupling of the lightning current tests,and comparative analysis and simulation.The simulated lightning current impulse generator,and the metal rods at both ends of the metal rod,with a height of 1.2 m and diameter of 20 mm,and the 8/20 μs electromagnetic wave transmitted in a simulated lightning channel,are applied in the simulation of the lightning.The electric dipole is applied at both ends of the metal bar from a lightning impulse of 5 kA to 29 kA and current step size of 2 kA.When the over-voltage amplitude of the current in the lightning return stroke channel is from 5kA to 29 kA,the amplitude of over-voltage in the process of the overhead conductors coupled with lightning electromagnetic increases linearly with the lightning current;coupling energy shows a good power function with the lightning current.Overhead conductors lightning electromagnetic coupling is indicated not only by the over-voltage amplitude and magnitude of the lightning current,but also by the overhead wire length and related cross-sectional area.The longer the overhead wire,the smaller the over-voltage and power energy.And the greater the cross-sectional area of the lightning electromagnetic coupling formed,the larger the over-voltage and energy.

Abstract:Based on a large dataset of ion drift velocity measurements onboard the DMSP F13 satellite and interplanetary magnetic field measurements onboard the Wind satellite,a study is made of the plasma bulk upward flowing ions in the topside polar ionosphere,with an emphasis on the quiet-time and storm-time changes in the distribution of upward occurrence and the impact of different interplanetary magnetic fields and seasons on the upward occurrence.The results show that the storm-time occurrence possibility of upward events is higher than that during quiet times in both the Southern Hemisphere and Northern Hemisphere.In the Southern Hemisphere and north of the interplanetary magnetic field,the possibility of upward events is higher than south of the interplanetary magnetic field;and in the Northern Hemisphere and south of the interplanetary magnetic field,the possibility of upward events is higher than north of the interplanetary magnetic field.These conclusions are more obvious in the magnetic storm period than the quiet period.Seasonally,in the Southern Hemisphere,upward events in winter are twice as likely as in summer,and this is the case during quiet times and north of the interplanetary magnetic field as well as during storm times and south of the interplanetary magnetic field.Mean while,in the Northern Hemisphere,the possibility of upward events in summer is greater than in winter,and this is true for quiet times north and south of the interplanetary magnetic field.Large amounts of energetic particles are propagated by the dayside soft precipitating particles (1 keV for ions and 100 eV for electrons) and night side auroral sub-storm particle in jections.This causes the moment transfer and heating of the cold plasma in the polar cap area by fractional heating and the ambipolar diffusion topside the F region of the ionosphere.At the same time,plasma waves,such as extremely low-frequency broadband waves,and ion cyclotron and lower hybrid waves,are involved by the convective velocity shear and plasma instability.Therefore,the cold upflows could be heated into thermal upflows at first,to reach a higher altitude,where they can be accelerated by the plasma waves into super thermal ions.In this paper,the occurrence frequency of upflow events during magnetically quiet periods is less than that during magnetically disturbed periods.This phenomenon can be explained by this mechanism.The seasonal variations of upward ions are discussed in this paper,in that the occurrence frequency in local winter is higher than in local summer in the Northern Hemisphere.However,the opposite is true in the Southern Hemisphere.This conclusion is consistent with previous studies in which it was reported that seasonal variations in ionospheric upflow do indeed exist and that the occurrences are closely related to solar activity and the interplanetary magnetic field.Under conditions of solar illumination,the enhanced solar extreme ultraviolet radiation ionizes the neutral gas.The conductivity of the ionosphere increases,while at the same time the cross-polar cap potential difference decreases.Therefore,the occurrence frequency of upflows is small in local summer compared with local winter.

Abstract:The randomized statistical experiments of precipitation enhancement performed in the Gutian Reservoir for 12 years (1975-1986) has gained achievements for scientific research,and have had international influence in the world.For the continuous study of precipitation enhancement in the Gutian Reservoir,the cloud echo characteristics and operational conditions of precipitation enhancement in the Gutian experiment area are analyzed based on the weather analysis and data of Doppler weather radar,sounding,ground rainfall and other observations from April to June of 2008-2012.The cloud echo characteristics in this study include echo intensity(Z),echo area(S),echo top(H),vertical integrated liquid water(VIL) and echo moving direction by Doppler radar,located in Jianyang,on the northwest side of the Gutian experiment area.Combining the sounding data and echo characteristics,the depth of the negative temperature layer is calculated,and the cloud echo types are also confirmed by the echo characteristics.The results show that the weather systems influencing the Gutian experiment area include the low vortex shear,warm-sector convergence,upper trough,continental high,subtropical high pressure,and typhoon or intertropical convergence zones.There are evident differences between the cloud echo characteristics and precipitation features in the different weather systems.Under low vortex shear,the stratus-cumulus mixed clouds are more numerous than cumulus,the rainfall intensity is greater,and the cloud cover time is longer.The convection clouds are more active under warm-sector convergence,upper trough and continental high than the others,indicating local precipitation and short lifecycle.Unlike warm-sector convergence and upper trough,the rainfall intensity and cloud cover time of stratus-cumulus mixed clouds are nearly half the size as under continental high.The weather systems in the above-mentioned influenced the Gutian experiment area mainly from April to June of 2008-2012.Then the subtropical high pressure zone and typhoon or intertropical convergence zone occurred rarely in the Gutian experiment area,but they all had much higher rainfall intensity and more cloud cover time than the others.Overall,the precipitation cloud systems in the Gutian experiment area are mainly stratus-cumulus mixed clouds,followed by cumulus.The echo intensity of the cumulus clouds are higher than the stratus-cumulus mixed clouds,whose center intensity are mostly higher than 40 dBz,presented as block structures with vigorously vertical development.The horizontal scales of the cumulus clouds in the Gutian experiment area are nearly equal to the vertical scales.The stratus represents many small cumulus clouds embedded in the large area of stratus.The echo center intensity of the stratus can also be higher than 40 dBz partially.The structure of the stratus-cumulus mixed clouds is favorable to rainfall.Its strong echo area S_{>25 dBZ} (S_{>25 dBZ} referring to the area where the echo intensity is larger than 25 dBz) is is clearly larger than that of the cumuliform clouds,while its average echo height and echo top are both lower than those of the cumuliform clouds.The vertical integrated liquid water in the cumuliform clouds is much greater than that in the stratus-cumulus mixed clouds,and the depth of the negative temperature layer of both types of clouds exceeds 2 km.The maximum echo intensity,strong echo area S_{>25 dBz} and depth of negative temperature layer of the stratus-cumulus mixed clouds all correspond well to the regional mean daily rainfall.The operational indexes of the stratus-cumulus mixed clouds in the Gutian experiment area are as follows:the echo intensity should be larger than 25 dBz,S_{>25 dBz} larger than 400 km²,echo top higher than 5.5 km,depth of negative temperature layer larger than 1.5 km,and vertical integrated liquid water more than 1 kg/m².