Abstract:Based on the methods of multivariate EOF(MVE-OF) and lead-lag correlation and regression analysis,the spatiotemporal variations of the 10 m horizontal wind over the tropical Pacific(10°S-10°N,100°E-60°W) and their associations with East Asian atmospheric circulation were investigated using the monthly global NCEP/NCAR reanalysis data for the period 1958 to 2001. The MV-EOF results show that there are two leading modes of the 10 m horizontal wind anomalies over the tropical Pacific.The first leading mode captures the well-known wind anomaly pattern associated with El Niño(La Niña),and is also characterized by the equatorially quasi-symmetric westerly(easterly) wind anomalies over the central Pacific and convergence(divergence) onto the equator in the east.The corresponding PC time series(PC1) is highly correlated(R=0.74) with the time series of the Niño3.4 index,suggesting that PC1 captures the main ENSO mode and that the mode corresponds to the low-level horizontal wind anomalies during the warm(cold) phase of ENSO.The second leading mode,however,is highly antisymmetric about the equator and exhibits a pattern of easterly(westerly) anomalies north of the equator and westerly(easterly) anomalies south of the equator,along with a strong low-level anticyclonic(cyclonic) circulation anomaly over the western North Pacific.The westerly(easterly) anomalies south of the equator,at about 10°S,in the central Pacific,correspond to the southward displacement of zonal wind anomalies during the decaying spring season of El Niño(La Niña),which is in accordance with the wind anomalies during the transition phase of ENSO.The corresponding PC2 usually emerges rapidly several months after the PC1 peak.The two corresponding time series both feature significant interannual varibility and seasonal phase-locking characteristics,but their dominant oscillation periods and relationships with the ENSO cycle vary prominently. The lead-lag regression analysis reveals that persistent significant anomalous positive(negative) geopotential height at 500 hPa and remarkable anticyclonic(cyclonic) atmospheric circulations at 850 hPa are set up to the south of Lake Baikal(SLB) when the two leading modes are in their positive(negative) phases.Further examination of the corresponding velocity potential quantities show that the connection pathways between the two modes and the anomalous SLB circulations differ from one another.The first mode,which is characterized by abnormal dipole Walker Circulation forced by the warm water in the eastern tropical Pacific,may be affected by the anomalous SLB circulation through the northerly winds over the East Asian coast and the South China Sea trough during its developing period.In addition,the anomalous SLB circulation also plays an important role in the maintenance of the mode after it matures.The circulation is linked with large-scale vertical motion over the central eastern tropical Pacific,and also the abnormal convergence motion over Southeast China.The opposite situation occurs when the first mode is in its negative phase.The second mode features abnormal quadrupole Walker Circulation forced by the warm water over the regions from the eastern tropical Indian Ocean to the western tropical Pacific,as well as the Atlantic Ocean.Thus the corresponding anomalous SLB circulation may be mainly related to the strong meridional Hadley Circulation over the the region from the eastern tropical Indian Ocean to the western tropical Pacific,and vice versa.

Abstract:The variability of the thermal state over the western Pacific warm pool(WP) and the characteristics of its adjacent convective activities were studied using the monthly subsurface sea temperature data of GODAS from 1981 to 2011,together with a variety of statistical methods.The heat content(HC) was defined as the subsurface vertical temperature averaged from 5 to 366 m.The results demonstrate that:(1)The WP area is the place where the HC has its largest variability,and its maximum value can reach 1.2 ℃ which is equivalent to the variation in SST.The depth layer from 100 to 200 m is the largest in terms of the change in subsurface temperature,usually changing from -4.0 ℃ to 4.0 ℃ Its seasonal variation is highly consistent with SST,and its interannual variability is much greater than that of SST.There is a certain degree of contribution to the HC from the SST of the WP.The thermal condition of the WP is closely related with ENSO events,as the largest cold(warm) SST anomalies correspond to El Niño(La Niña) events in the ENSO cycle.(2)The dominant mode of HC is one of positive anomalies,and its corresponding time coefficient(PC1) shows evident annual variability in the sub-layer heat condition over the WP.A heat content index(HCI) was constructed by using the HC averaged over the WP region.Then,five colder years(HCI smaller than -1) and six warmer years(HCI larger than 1) were identified from the total 31 years,with the total abnormal years accounting for 1/3 of the whole period.(3)There is a close relationship between the convective activities nearby the WP and the abnormal thermal state there in summer.The area of(16-26°N,126-146°E) around the Philippines was chosen as the key area to reflect the characteristics of convection of the adjacent areas of the WP,revealing that the convective activities over this critical region significantly enhance(weaken) when the WP is warmer(colder).The fact that convective activities are affected by the HC can be traced back to the July prior to a particular year,and through to the September of that current year,demonstrating the strongly persistent effects of the HC anomalies on the convective activities.(4)HC anomalies of the WP cause abnormal convection around the Philippines,and thus stimulate the East Asia-Pacific teleconnection pattern,with a positive-negative-positive pattern of centers in the 500 hPa height fields from the low to the high latitudes over East Asia in summer.The region with positive correlation in the low latitudes corresponds to the western Pacific subtropical high;the negative correlation in the midlatitudes corresponds to the Mei-yu front;and the other positive correlation in the high latitudes represents the Okhotsk high.This circulation situation indicates that when the convective activities around the Philippines are enhanced,it is conducive to the western Pacific subtropical high controlling the Yangtze River region,with its intensity weakened.As a result,the precipitation and convective activities over the Yangtze River in summer will be affected,due to the weakened southward movement of cold air,which is the result of the weakened Okhotsk high.In order to study the responses of the atmospheric circulation in East Asia to the thermal states of the WP,the anomalous wind fields in the lower troposphere were composited between the significant warmer and colder years.The results reveal that there is an abnormal anticyclonic(cyclonic) circulation bias in the lower troposphere over the western Pacific Ocean to the east of the Philippines when the WP is warmer(colder),which has an important influence on the East Asian monsoon and the precipitation over eastern China.

Abstract:Based on the daily winter minimum temperature data of 201 Chinese meteorological stations,without the influence of station relocation,for the period 1960-2009,along with NCEP/NCAR monthly mean reanalysis data and NOAA reconstructed monthly mean sea surface temperature data,the spatial and temporal variation in the frequency of warm winter nights(FWWN) and the relationship with sea surface temperature(SST) and atmospheric circulation was studied.First,we ruled out the influence of the homogeneity generated by meteorological station relocation by using the t-test and multiple regression methods.Then,the FWWNs at the 201 stations in China were calculated.During 1960-2009,FWWN presents an increasing trend over most parts of China.The maximum values of the standard deviation and trend coefficient were located in Northwest China,while the minimum values lay in Southwest China.The first EOF mode's spatial vector of FWWN produced an identical signal over the whole of China,indicating that there is an identical high winter warm night frequency abnormal or low frequency abnormal characteristic.The first EOF mode's time coefficient indicates that FWWN increased during the 50-yr study period,and the mutation year of the first EOF time coefficient is 1988.In addition,FWWN shows obvious regional features,and can be divided into five consistent regions via the rotated EOF (REOF) method:(1)South China,Southeast China and Southwest China;(2)Central China and the east part of Southwest China;(3)Northeast China;(4)The east part of Northwest China,North China and East China;(5)The Xin Jiang area.Most of the regions' FWWN showed an increasing trend,with the trend coefficients being statistically significant at the 0.05 level of reliability.Next,the relationship between FWWN and SST was analyzed.The Pacific Warm Pool SST index(I_PW),Kuroshio SST index(I>_Ks),North Atlantic SST index(I_NA) and South Pacific index(I_SP) showed significant positive correlation with FWWN,with their correlation coefficients passing the significance test over most parts of China.To examine the reason behind the relationship,the synthetic analysis method was used.The results showed that when the four SST indices were abnormal,the East Asian Trough intensity and Siberian High intensity were also abnormal.As the anomaly of the East Asian Trough intensity and Siberian High intensity can influence the cold air intensity,the four SST indexes therefore influenced the FWWN through their close relationship with the East Asian Trough intensity and Siberian High intensity.

Abstract:The characteristics of low-frequency oscillation during heavy rainfall in the Yangtze-Huaihe River valley(YHRV) in summer 2007 and their relationships with Rossby wave packets in the midlatitudes were investigated in the present studyusing NCEP/NCAR reanalysis data along with precipitation data from 740 weather stations in China.Based on other studies,300 hPa meridional wind variations were chosen to facilitate the analysis of the characteristics of the wave packets.Wave packet envelopes were also defined by performing Hilbert demodulation on the wind data.The relationship between wave packets and precipitation is mainly illustrated using time-lagged one-point correlation maps produced for v'(the wind disturbance of 10-30 d filtering after deducting the seasonal average value of the meridional) and wave packet envelopes.The results demonstrate that the total precipitation was more than 800 mm in summer 2007,and the low-frequency component of precipitation playedan important rolein precipitation disturbances.The intraseasonal variation of precipitation wasmainly dominated by 10-30-day oscillation.The time series of the 10-30-day precipitation component wassignificantly and strongly correlated with the daily rainfall anomaly, with correlation coefficient of 0.44.Eight phases were defined for describing the 10-30-day low-frequency oscillation of circulation.It was found that most regions in the YHRV were affected by the southwest warm air of the northwest side of the anticyclone near the southeast coastal area of mainland China in the active precipitation phase in the lower troposphere(850 hPa),which was able to provide adequate water vapor for heavy rainfall events.On the other hand,it was found that there was a strong low-frequency anticyclone in the upper troposphere(200 hPa), with vigorous upward movement at 500 hPa,which was also favorable for the precipitation.The circulation variations with 10-30-day periods were baroclinic in the vertical direction over East China,according to the composite analysis of low-frequency circulation and vertical velocity.Related to the 10-30-day variations of precipitation in the YHRV,the low-frequency perturbations in circulations showed wave-train structures in both the upper and lower troposphere.At 300 hPa,the dominant waves were found to be those with zonal wave-numbers 4-9.So,the characteristics of the Rossby wave packets during these events were further investigated.The low-frequency wave packets propagated slowly eastward at a phase speed of approximately 2°-3° zonally per day, averaged over 30-60°N.However,the wave trains propagated toward the downstream region in the westerly at a much higher speed;the zonal group velocity was found to be about 14° per day in regions west of 120°E,indicating that the 10-30-day wave packets exhibited the characteristics of downstream development.Five days before each of the heavy rainfall events in the YHRV,the wave packets of low-frequency oscillation originated from the Urals,and then propagated along a path that extended from the Urals to the YHRV.Specifically, the wave packets moved from the Urals,via the Tianshan Mountains,then across Qinghai Lake,and finally into the YHRV.The differences inthe envelope function of low-frequency disturbances inphase 3(the active phase of precipitation) from that inphase 1(the incipient phase before precipitation) indicatethat the wave energy may have accumulated over the YHRV before the occurrence of heavy rain.This wave energy accumulation before heavy rain was a result of wave energy dispersion from the Urals to the YHRV.Therefore,by examining the low-frequency components of circulations along with wave packets using time-lagged one-point correlation techniques,we conclude that 10-30-day low-frequency oscillations played an important role in precipitation variations in the YHRV in the summer of 2007.The Rossby wave packets of 10-30-day oscillation, as denoted by the meridional wind, propagated eastward from the Urals into the YHRV,facilitating the occurrence there of severe rainfall events.These results are beneficial for finding clues to better forecasting and an improved understanding ofheavy rainfall episodesin the YHRV.

Abstract:In order to improve the accuracy of tropical cyclogenesis forecasting,it is vitally important to study how Tropical Cloud Clusters(TCCs) develop into Tropical Cyclones(TCs),and the related climatic characteristics.TC genesis is poorly understood,since most previous studies have focused on the time after genesis.In the present study,global tropical cloud cluster data,NCEP/NCAR reanalysis data,Hadley Centre sea surface temperature data,and Genesis Productivity(GP) data (representing the ratio of TCCs that develop into TCs) were used to analyze the different impacts of two El Niño types on the evolution of TCCs in the Western North Pacific (WNP) from 1982 to 2009.The results indicate that GP is significantly and positively correlated with the Niño3 index in the southeastern sub-region of the WNP during summer.During fall,GP is significantly and positively correlated with the Niño3 index in the southwestern and southeastern sub-regions.The El Niño Modoki Index(EMI) has a markedly positive(negative) correlation with GP in the southeastern(South China Sea) sub-region during summer(fall).All these results are consistent with the large-scale environmental flow that affects TCC activity.Eastern Pacific(EP) El Niño events are associated with a significant increase in 850 hPa relative vorticity,200 hPa divergence and 700 hPa relative humidity in the southeastern sub-region during summer.The increase of GP in the southwestern sub-region during fall occurs because the relative vorticity and divergence are favorable for TCCs in EP El Niño years,whereas the opposite situation occurs during La Niña years.Central Pacific(CP) El Niño events enhance(suppress) GP in the southeastern sub-region(South China Sea) during summer(fall) because relative vorticity,divergence,relative humidity and vertical zonal wind shear are favorable(unfavorable) for the development of TCCs.In summary,the large-scale environmental flow anomalies,caused by El Niño events,create distinct effects on TCC activity in the WNP.Positive feedback is produced by CP El Niño events and EP El Niño events in the southeastern sub-region,but the effects of the two El Niño types are completely different in the southwestern sub-region and South China Sea.After comparing the different impacts of the two El Niño types on TCC activity,it was found that the changes in large-scale environmental factors produce different feedback effects on the development of TCCs in the WNP.Specifically,it is in the eastern part of the WNP that EP El Niño events affect TCC activity,but for CP El Niño events the impacts on the evolution of TCCs spread to the South China Sea.And it is in the South China Sea that the influences of the two El Niño types on TCCs are opposite.

Abstract:Based on the tropical cyclone(TC) datasets provided by the Shanghai Typhoon Institute of China Meteorological Administration,the characteristics of the low-frequency oscillation over the South China Sea-western North Pacific (SCS-WNP) and its modulation of the clustering characteristics of TCs are studied.The results show that there are two convection regions located in the SCS-WNP and southeast Bay of Bengal.The wavelet analysis and power spectrum analysis results show that the SCS-WNP convective activities exhibit an intraseasonal oscillation with a period of 10-20 days.In most years,the variance contribution of the quasi-biweekly oscillation(QBWO) accounts for more than 20% of the original sequence,showing that the QBWO in the SCS-WNP plays an important role in atmospheric low-frequency oscillation.The strength of the QBWO has obvious interannual variability,with the strength of change greater before the 1980s and smaller after the 1980s.There were two significant oscillation periods in the SCS-WNP from June to September in 1990:15-30 days and 8-14 days.TCs were prone to occur in the wet phase,and when the wet phases of the two convections overlapped,the frequency of TCs was highest.According to predefined standards of TCs clustering,there were four TC clusters from June to September in 1990,and they showed a close relationship with the evolution of the low-frequency circulation.There was a complete wet phase from 24 July to 2 August,during which time the development of TCs was strong.The period 4-9 August was a complete dry phase,and in this time TCs gradually weakened and disappeared.The results show that TCs are prone to occur and develop in the wet phase.During the wet phase,there is low-frequency cyclonic circulation in the low-level atmosphere,and the strong positive vorticity is conducive to the formation and development of the initial vortex disturbance,which is favorable for TCs clustering.With the westward propagation of the low-frequency oscillation,the Northwest Pacific gradually enters into the dry phase,which is not conducive to TC clustering.The monsoon trough has a significant impact on the clustering of TCs.During the wet phase,the SCS monsoon trough and the West Pacific monsoon trough are active,and the intensity of the eastward extension of the monsoon trough is obviously enhanced.Whereas,in the dry phase,the monsoon trough is interrupted,and the intensity is very weak.According to the defined monsoon trough intensity index,there is a good corresponding relationship between TC clustering and the monsoon trough intensity.From May to October in 1990,the monsoon trough intensity index had four enhancement processes,which matched with the TC clustering.An active monsoon trough turns the convection into the wet phase,while providing favorable positive vorticity,which is advantageous for TC clustering.

Abstract:Cloud is an important internal factor of the climate system,especially in the earth-atmosphere system.The formation of clouds and their characteristics mainly result from both dynamic and thermodynamic processes of the surface and the atmosphere.An accurate grasp of the distribution of cloud and its variability can contribute greatly when attempting to assess the role of cloud in climate change.And related to this,a calculation scheme that is effective at describing cloudiness is a key part of improving the ability to simulate cloud in numerical models.In order to identify a satisfactory cloudiness calculation scheme,the present study employed NCEP reanalysis data to calculate cloudiness and relative humidity,based on three schemes(Slingo,NCAR,and Qian),during a Mei-yu rainfall process in the Yangtze River basin in 2010.Comparative analysis between the calculation results and Local Analysis and Prediction System(LAPS) reanalysis data,considered as the cloud observation,was conducted in terms of cloud distribution and cloudiness values at upper and lower levels,represented by 400 hPa and 850 hPa,respectively.Based on the comparison results,as well as statistical analysis involving anomaly correlation coefficients(ACCs) and RMSE,it was found that the three cloudiness calculation schemes all managed to successfully simulate the cloud central positions,but each had their own advantages and particular characteristics when it came to cloudiness values.The results calculated using the NCAR scheme matched the LAPS outputs very well at large-value centers of cloud,but the regions containing these values were always too large.To a certain extent,although it was found that the Slingo scheme could also describe the cloud well,it showed a slightly lower capacity than the NCAR scheme in terms of its cloudiness calculation.Additionally,the Qian scheme demonstrated fairly limited ability to calculate the cloudiness values,but always presented the cloud profile and its distribution accurately.Based on the statistical analysis,at 850 hPa,the NCAR scheme produced its maximum ACC and minimum RMSE,indicating its superiority over the other schemes at calculating the cloudiness at that height.However,the Qian scheme yielded the best statistical results at 400 hPa,possibly due to the close correspondence of its cloudiness distribution results with the observation.Of importance here is that,to a certain extent,the Qian scheme takes into account the cloud formation mechanism and the influence of atmospheric vertical motion on cloud formation when calculating the cloudiness.Overall,based on this comprehensive comparison of relevant factors,we conclude that the NCAR scheme is superior to the others,particularly at the lower level(850 hPa).

Abstract:Valley fog in China has barely been studied.To improve our understanding of its macro and microphysics in the upper and middle reaches of the Yangtze River,a comprehensive fog experiment was conducted in Xuanen during 6-31 December 2010 with a tethered balloon system,fog droplet spectrometer,and visibility meter.Based on the data of the boundary layer profile,fog droplet spectrum,and visibility over the Xuanen Mountains of Hubei Province,six fog events are discussed in terms of their boundary layer structures and development processes.The results show that fog events in the Xuanen Mountains are mainly formed by radiation cooling at night and the visibility is usually more than 200 m.The fog-layer top is relatively uniform at 400-600 m above ground level.Xuanen Station is located at the bottom of the valley near the south hill,and the surface wind at night in Xuanen is mainly controlled by downslope wind with a southeasterly direction.The surface wind speed is no more than 0.5 m·s^-1 in the early stage of fog formation,and increases to 2.0 m·s^-1 in the dissipation stage.A C shape of the temperature profile in the boundary layer is observed before fog,and the rate of temperature decrease reaches 0.3-1.0 ℃/(100 m) in the middle and low levels.Upper-level inversion is formed due to the transportation of cold air from the surface by upward flow of the mountain wind cycle,but anticyclonic subsidence can also lead to its formation.With dew-point temperature inversion and an approaching isothermal temperature distribution at low levels,the air at a height of 200 m reaches saturation first.Dew deposition is observed on the grass in the early evening on clear nights with low wind speed,and this creates the low-level dew-point inversion.The bottom of low-level cloud is raised by upward flow in the early evening.When low cloud appears,it will absorb the upward longwave radiation from the surface and launch downward longwave radiation,with upward soil heat flux and upward latent heat flux by dew deposition,and the layer beneath the low cloud develops an unstable lapse rate.The evaporation of large droplets falling from the low cloud,and surface dew,which provides water vapor,causes fog droplets to form at ground level.The relative humidity remains high at low levels during the dissipation stage,which will form rising fog.Fog lifting is also found at midnight,when it cannot be caused by solar radiation.A steeper than moist adiabatic lapse rate beneath the persisting part of the fog is found during these fog events.Upward flow due to the unstable layer near the surface and raised upslope wind(northeasterly) may lead to fog aloft.Finally,it is important to note that,at 1-2 h before fog occurrence,the surface temperature and vegetation temperature rise significantly,even 1.0 ℃ h^-1 in some cases,with saturated relative humidity at the surface,which are quite different conditions to those of other regions,and thus has positive implications for fog forecasting in this region.

Abstract:With the continuous development of China's economy,haze pollution is a serious issue that urgently requires in-depth investigation.To date,very few haze pollution studies have focused on the formation and causal variation from the point of view of climate change.Against the background of a rapidly increasing number of haze days and global warming,we studied the long-term variation in haze days and discussed the reasons for the change from the climate perspective in the city of Nanjing.The study is vitally important for a deeper understanding of the formation and variation of haze.Based on daily meteorological data from the climate reference station of the China Meteorological Bureau in Nanjing from 1954 to 2012,the long-term variation in haze days was calculated using two methods:haze days identified according to the standards of meteorological industry,and haze records from the observations of meteorological observers.The number of haze days showed a clear increasing trend,from 40 d yr^-1 in the 1950s to 230 d·yr^-1 in the 21st century.The variation in haze days according to the two methods had a similar change trend,indicating favorable applicability of the standards of meteorological industry in Nanjing.During the study period,the number of fog-haze days increased and then decreased in Nanjing,accompanied by a continuing decrease in visibility,which reduced by around 4 km,as well as a decrease in relative humidity,which reduced by around 4%,changing from 89.5% in the 1950s to 85.5% currently.The change in relative humidity may be a key factor for the reduction in fog-haze days and the increase of haze days,indicating a shift from the former to the latter.The visibility in Nanjing gradually decreased by 8.4 km in the last 30 years.The annual average visibility of haze days also decreased and the proportion of severe haze increased.The correlation coefficient between haze days and visibility reached -0.91,with haze days increasing almost linearly with the decrease in visibility.The annual average relative humidity in Nanjing declined by a large margin from 1985,changing from around 80% to 68% by the end of the study period.The change in the average relative humidity on haze days was very similar to the annual change.The correlation coefficient between haze days and relative humidity was -0.72.The number of haze days increased as the humidity decreased.The annual average temperature in Nanjing increased obviously by approximately 1.8 ℃rom 1985.The warming trend was greatest in winter,increasing by approximately 2.1 ℃and smallest in summer(approximately 0.7 ℃.The annual average temperature was positively correlated with haze days and negatively correlated with relative humidity.The rise in temperature caused a decrease in relative humidity,which then resulted in more haze days.The annual average wind velocity decreased gradually from 1978,reducing by approximately 1.5 m·s^-1 by the end of the 20th century.The wind speed presented a significant negative correlation with the number of haze days.With a reduction in wind speed,pollutants diffuse less easily and accumulate in the atmosphere,which results in an increase in particulate matter and the frequency of haze days.In summary,against the background of global warming,the long-term variation in haze days in Nanjing is influenced by many climatic factors,among which the visibility,relative humidity,temperature and wind velocity are all particularly important.

Abstract:Recently,the continuously high incidence of fog and haze events has been a topic of considerable concern among the public,since such events can bring serious harm in terms of the health and daily lives of the masses.A rare fog and haze event,which was wide-ranging,long-lasting and heavily polluting,occurred during 12-17 January 2013 in the Huai'an area of Jiangsu Province.In particular,the visibility of most of the area during 13-14 January was lower than 0.1 km,which is extremely rare.The event caused considerable disruption to expressways,air transport and sea transportation.In this study,in order to reveal the formation mechanism of regional and successional fog and haze,and to provide scientific evidence for forecasting and prediction and air pollution abatement,this event was chosen as a case of typical heavy fog and haze for the analysis of pollution variation and its characteristics.The variation characteristics of PM₁₀ and PM_2.5,the relationship between visibility and meteorological variables,the circulation features at middle-low levels,and the pollution sources were explored based on the pollutant particle concentration,FNL data,conventional meteorological data,and Global Data Assimilation System data.The results indicated that the minimum value of hourly average PM₁₀ and PM_2.5 concentrations during this fog and haze event occurred at 0500-0700 BST and 1300-1700 BST,while the maximum value occurred at 2100-2300.For PM₁₀ and PM_2.5,the maximum concentrations appeared at different times.The increase inrelative humidity and the increase of PM_2.5 and PM₁₀ concentrations in total pollutant particles were the leading cause of visibility variation and persistent pollution.Several conditions,such as the slight variations inpressure gradient,sudden increase inrelative humidity,relatively low wind speed,and the increase in PM₁₀ and PM_2.5,were the main reasons for the rapid development of the fog and haze event.A good co-relationship between visibility and pressure,relative humidity,and PM_2.5concentration was observed.The overall variation trend of visibility could be well fitted by the regression equation.The representativeness of these results was analyzed and verified based on long-termsample data.The stable upper-air circulation situation,the warm advection at middle-low levels,and the stable ground pressure field provided abeneficial circulation situation for the development of the fog and haze event.During the event,varying levels of the thermal inversion layer werefound at lower layers.The deepthermal inversion layer and east wind near the surface layer wereconducivetothe low visibility and its wide distribution.The stable stratification structure,east-northeast air mass at the inversion layer and middle-low levels,local-source pollution and serious air pollution weremainly attributed to the relatively low visibility and longevity of the haze.The air quality was dependent on the concentration of pollutants.Meteorologists should identify meteorological indicesthatareadverse to the diffusion and dilution of pollutants when meteorological conditions that are adverse to pollutant diffusion appear.Early forecasting and timely warningsareessential for the prevention and treatment of air pollution.

Abstract:Rainstorms in Zhejiang mainly occur in the flood season(February-October),during which heavy rainfall is common.The focus of the present study was two flood season rainstorm processes that occurred in Zhejiang in 2013,one belonging to the heavy spring rains hereafter referred to as the 4.29 rainstorm,based on its date of occurrence(29 April 2013) and the other being a typical Mei-yu rainstorm(hereafter referred to as the 6.6 rainstorm,using the same notation as previously mentioned).Based on NCEP analysis data,the circulation,thermal,dynamic and water vapor conditions of these rainstorm processes were analyzed.Through comparative analysis,the mechanistic similarities and differences were studied,providing a theoretical basis for further work on local rainstorm forecasting.The results can be summarized as follows:(1)During the 4.29 rainstorm,the high latitude circulation at the height of 500 hPa in East Asia was characterized by a western high and eastern low pattern.The main system of impact was the westerly trough at high latitudes together with a lower-level vortex,with stronger cold air,and a more southerly northward position of warm air,which induced southerly rain.A double block was present in the high-latitude circulation of the 6.6 rainstorm,and a low vortex was the main influence from the upper to lower levels,with weak cold air,and southwesterly flow further north,thus resulting in a northerly storm.(2)During the 4.29 process,an upper-level strong westerly jet existed,with a positive vertical circulation and low-level convergence corresponding to high-level divergence.During the 6.6 rainstorm,the high-level westerly wind was weak,with a more complex divergence field of a divergence zone during the early stage of precipitation from the near-surface to the lower levels,and a convergence zone in the middle levels,which transferred to weak convergence until the later stage of precipitation near the ground and with no apparent vertical circulation.(3)The source of water vapor for the two rainstorm processes was different:the water vapor for the 4.29 rainstorm process was mainly provided by the southwest jet stream,with the main source regions being the Bay of Bengal,the South China Sea and the western Pacific.The divergence distribution of water vapor flux was convergence in the lower layers,corresponding to upper-level scattered divergence.In the 6.6 rainstorm,the water vapor come from the southwest and southeast,with the main source regions being the Bay of Bengal,the South China Sea,the western Pacific,and the East China Sea.There was low-level divergence in the early stages of the divergence distribution of water vapor flux and convergence in the middle stages.In the later stages,however,there was consistent convergence under the middle level.The strengthened vapor flux convergence above the near-surface level played an obvious role in the rainfall intensity increasing.(4)An east-west oriented front with a strong cold air intrusion existed in the 4.29 process,while a northwest-southeast front with a weak cold air intrusion was apparent in the 6.6 process,both of which were advantageous for the release of baroclinic potential energy,and could promote convection development.

Abstract:A comparative analysis of two thunderstorms that occurred in the Nanjing area was conducted by using NCEP,sounding,lightning location and Doppler radar data,in Nanjing and Changzhou,with the aim being to study the differences among the key factors causing thunderstorms in spring and summer.One of the thunderstorms took place on 22 February 2012,and the other was on 10 August 2011.The results of the analysis of the atmospheric energy structure show that the wind-potential temperature plots can reveal the dynamic,thermodynamic and water vapor characteristics before the occurrence of thunderstorms.Specifically,the tropopause height increases,providing sufficient space for the development of the thunderstorm.The "ultra-low" temperature phenomenon of the tropopause is a sign of thermal instability,while the clockwise rotation of the wind direction along with an increase in the height provides favorable dynamic conditions for the thunderstorm.Meanwhile,large quantities of water vapor are transported by the moisture gradient in the vertical direction.Comparison of the physical diagnosis reveals that the dynamical lifting effect of the spring thunderstorm is stronger than that in summer,with the latter mainly caused by heat convection;the dynamical suction effect in the upper troposphere is not predominant.Comparison of the characteristics of Cloud-to-Ground(CG) lightning evolution indicates that the CG lightning frequency of the spring thunderstorm is low,but the percentage of positive CG lightning is high.The situation for the summer storm is just the opposite,showing multiple peaks.The percentage of negative CG lightning is high.In the mature stage,the radar echo top height has a negative correlation with the CG flash rate,and it reaches its peak before the occurrence of CG flashes.Comparison of the radar data shows that the echo of the spring thunderstorm appears similar to a stratocumulus cloud precipitation echo.There is obvious wind shear,strong warm advection,and convergence activities in the storm.The echo of the summer thunderstorm shows it to be a convective precipitation process.There is wind shear and convergence activities in the summer thunderstorm,and a clear vortex tube can also be found.It shows the features of a local thunderstorm.Analysis of the matching of relationships between the radar echo intensity and the CG distribution reveals that,tor both the weak spring thunderstorm and the strong summer strong thunderstorm,the locations of CG flashes correspond well to the convergence zone and radar reflectivity.Most of the CG lightning of the spring thunderstorm happens around the regions where the echo intensity is above 40 dBz,or in the regions where the echo intensity is approximately 30 dBz.The CG lightning of the summer thunderstorm mainly occurs in the regions where the echo intensity is above 40 dBz.In developing stage of the strong thunderstorm cloud,CG flashes are mainly distributed on the side of the stretching direction of the storm,and they can be the guidance of the radar echo.Most of the CG flashes happen in the vertical wind shear area.

Abstract:Thunderstorms are a type of convective weather phenomenon that happen frequently in nature with high intensity.The United Nations Committee for Disaster Reduction has referred to lightning as "one of the ten worst natural disasters".Jiangsu Province is in the transition zone between the subtropical and warm temperate zones,and is one of the provinces in China most frequently affected by lightning.As a relatively wealthy province in economic terms,the electrical infrastructure(power grid,electrical equipment etc.) of Jiangsu is also well-developed.Therefore,when lightning-related accidents occur,incalculable harm can be caused not only to personal safety but also socioeconomically.To develop a deeper understanding of the laws governing lightning activities in Jiangsu Province,as well as improve lightning protection and disaster reduction services,the present study set about analyzing the lightning characteristics of the region(interannual variation,monthly variation and daily variation),based on data from two sources:the Advanced Time of Arrival and Direction System(ADTD) and the Worldwide Lightning Location Network(WWLLN).The ADTD data were provided by Jiangsu Meteorological Department,whose time zone is Beijing standard time(BST).Because the time zone of the WWLLN is Greenwich Mean Time(GMT),we converted GMT to Beijing time.The study spanned a period of four years,from 2006 to 2009,throughout contiguous Jiangsu Province.When carrying out the study,the detection efficiency of ADTD was assumed to be 100% because the ADTD data were considered to be the "ground truth",and a time-matching window of 0.5 ms was chosen for the lightning location data of WWLLN/ADTD(i.e.,when lightning observed by WWLLN occurred 0.5 ms before or after that of ADTD,the flash was considered to have been captured by WWLLN).The WWLLN and ADTD data were then processed and analyzed for the study period.The analysis confirmed that WWLLN favors high peak current return stroke lightning discharges.Meanwhile,WWLLN could not distinguish the lightning polarity.For example,it was unable to distinguish a positive flash from a negative flash.The polarity could,however,be determined by simultaneous observation with ADTD.WWLLN showed good correspondence with ADTD in terms of the temporal and spatial distribution of lightning activities.Specifically:the frequency difference between day and night was not large;the number of lightning flashes occurring during the day was slightly higher than at night;and the lightning mostly occurred in the period from June to August,especially midsummer,which was clearly the most active season.The amount of lightning detected by WWLLN from June to August was about 83.09% of the total amount in one year,while the amount of lightning detected by ADTD was 88.48%.During a day,the highest peak of lightning appeared at roughly 04:00 PM Lightning mainly occurred in the period between 13:00 BST and 20:00 BST,while low-level activity occurred between 02:00 BST and 11:00 BST on the second day.The lightning flash density in Jiangsu Province was higher in western regions than in eastern regions,and also higher in southern regions than in northern regions.Besides,those regions with frequent occurrence of lightning activity roughly coincided with the most economically developed parts of Jiangsu Province.The cloud-to-ground detection efficiency of WWLLN was shown to improve greatly from the first to the fourth year of the study,with an overall detection efficiency of cloud-to-ground flashes increasing from 3.95% in 2006 to 11.58% in 2009,possibly due to the increased number of stations and upgrading of the algorithm.The detection accuracy of WWLLN is now approximately 100 km,but its ultimate goal is an accuracy of 10 km.On the whole,the lightning density determined by WWLLN was much lower than that of ADTD,Compared to ADTD,WWLLN failed to detect the high incidence of lightning in the central area of Jiangsu,The difference in the detection accuracy is related to the characteristics of WWLLN itself;WWLLN is a large-scale(worldwide) lightning location system whose detection accuracy is often hundreds or even thousands of kilometers,The detection efficiency was related to the polarity and strength of the return stroke,The detection efficiency of positive ground flashes was higher than that of negative ground flashes,Meanwhile,the greater the peak current of the return stroke was,the more accurate the detection efficiency of WWLLN would be.

Abstract:In order to reduce the limitations and bias of the temperature and humidity profiles extrapolation scheme of the GRAPES(Global/Regional Assimilation and Prediction System) 3-DVar systems for the upper-atmosphere background,this paper proposes to use climatic data to reconstruct upper-atmosphere vertical profiles of temperature and humidity.First,a series of simulation experiments were conducted by a one-dimensional variational data assimilation system to analyze the bias of the current extrapolation program and its effects on inversion results.Then,vertical profiles were constructed using upper-atmosphere climatic data,and assimilation deviation was also discussed.Finally,assimilation experiments were performed and analyzed through the GRAPES global analysis and forecast system.The one-dimensional variational system experiments showed that there was deviation between the structure of the current upper background field of temperature and humidity profiles extrapolation scheme and the actual conditions of the upper atmosphere.Its inversion results suggested a greater negative impact of the stratosphere and troposphere.The application of climatic data to correct the upper atmospheric background field(temperature and humidity profiles) could reduce the deviation by over 50%,which further proved the feasibility of using upper climatic data to replace the upper background field produced by the temperature and humidity profiles interpolation scheme in the current GRAPES model assimilation system.The results from the GRAPES global 3DVar assimilation experiment indicated that using climatic data to replace the temperature and humidity profile structure of the upper atmosphere can significantly improve the inversion results of the stratosphere and upper troposphere.The upper atmosphere climate profile corrections for the background field can improve the analysis results of the GRAPES-3DVar system,reboot the upper atmosphere satellite channel and optimize the assimilation and forecast result of the GRAPES model.The final results showed that the maximum of the upper background temperature and humidity profiles extrapolation scheme error in the one-dimensional experiments could reach tens of degrees or more around the 1 hPa level,not only affecting the stratosphere but also having an impact on the troposphere.Using climate data to correct the GRAPES upper temperature and humidity data,the temperature deviations can be reduced by over 50%,which fully proves the feasibility of replacing the interpolation assimilation system in the current GRAPES model with the top-level climate data scheme.The global GRAPES-3DVar assimilation test results showed that the new scheme not only significantly improves the quality of the analysis of the stratosphere,but also the upper troposphere.This paper has preliminarily improved the upper atmospheric temperature and moisture profiles construction scheme for the GRAPES model.However,there is still room for optimizing the GRAPES-3DVar model upper-atmosphere assimilation.Channel selection schemes for different kinds of data and error correction techniques need to be further studied.The data used in the assimilation cycle experiment were conventional observations and microwave thermometer data(AMSU-A) from NOAA-15/16.The NCEP reanalysis data were used as testing data.The control group(CTRL) used the original upper-atmosphere background field produced by the temperature and humidity profile extrapolation scheme;the experiment group(TEST) used the climate profile correction scheme.During the experiments,the control group and the experimental group both used the same data and satellite channel selection method.The radiative transfer model was RTTOV7 in the GRAPES system.