WAVE-MEAN FLOW INTERACTION AND FORMATION OF BLOCKING——PERSISTENT ANOMALOUS WEATHER IN CHINA IN THE SUMMER OF 1980

In the summer of 1980,serious persistent abnormal weather occurred over vast areas in China.While record-break-ing cold and flood were observed in the reaches of the Changjiang and Huaihe Rivers,severe hot wave and drought dom-inated the entire northern China.The long-lasting disastrous weather is mainly due to the stable development and main-tenance of blocking anticyclone over the northeastern Asia.This study aims at the understanding of the roles oftime-varying weather system transport in the formation of the blocking.It was shown that during this period,there appeared continuous generation of synoptic-scale perturbations alongthe strong baroclinic zone over Europe and the western Asia.While such perturbations propagated eastward,energyconversion occurred.At equivalent barotropic layer,with weak dissipation,such energy conversion was subjected to theso-called bi-directional principle:while the energy of the synoptic-scale system cascaded to smaller scale system,amuch larger portion was transferred to the blocking system with larger scale.Potential vorticity diagnoses also revealedthat the transient weather systems played the roles of maintaining the mean anticyclonic vorticity to the south,and meancyclonic vorticity to the north,of the westerly jet,and exciting strong anticyclonic vorticity growth and correspondinggeopotential height increase in high latitude area downstream of the westerly diffluence region.The research also showed that,the intensity of the forcing of the blocking formation via wave-mean flowinteraction in this Asian case was much stronger than that occurring in the western Europe in the summer of 1976.It wastherefore concluded that when persistent abnormal weather in the northern China was studied,in addition to thesubtropical weather systems,attention should also be drawn to the development of baroclinic zone over Europe and thewestern Asia,and the propagation and transfer properties of the synoptic systems embedded in the baroclinic zone.     

EFFECTS OF ANOMALOUS HEATING ON SUMMER MONSOON OVER ASIA*

Based on a zonally non-uniform mean circulation in summer simulated by numerical modelling,perturbation heatings ever South Asia and a perturbation cooling over Northwest Australia were incorporated in a numerical model to discuss their effects on summer monsoon over Asia and the structure of flow disturbance.     

STUDY OF THE NONLINEAR MECHANISM ABOUT THE FORMATION OF BLOCKING HIGH*

In this paper,a nonlinear dynamical system considering the interactions among ultra-long wave,long wave and zonal flow in a baroelinic atmosphere was found.In this system,thermal forcing and frictional dissipation are regarded as the external parameters.The procedure,in which the maps of orbit in phase space,contour of stream function and section of Poincaré mapping are combined with each other,is used to determine the distributions of system states and circulation patterns and to draw the corresponding diagrams.The results show that by using the Galerkin method,there is no essential influence of the increasing of orders of truncated-spectrum in a certain range on the distributions of system states and circulation patterns,but it makes them somewhat complexities.By contrast,the changing of distribution of thermal forcing exerts a great influence on the system.Blocking high links closely with the oscillating of the atmospheric circulation and the meridional growing intensely,while the development of synoptic wave can promote the oscillating.     

CHARACTERISTICS OF TURBULENCE SPECTRA IN DIFFERENT WEATHER PROCESSES UNDER UNSTABLE CONDITIONS*

In this paper,the data chosen from measurements by supersonic anemometers in Tongxian County of Beijing in 1990 are used to study characteristics of turbulence spectra in precipitation weather.Some turbulence parameters such as turbulence intensity,heat and momentum fluxes,friction velocity,M-O length and stability parameter are calculated and their dependence on turbulence spectra in different stability conditions are analyzed.Some encouraging results are obtained and compared with others' results.     

西太平洋副热带高压东西位置异常与华北夏季酷暑

文中研究了副热带高压 (副高 )东西位置异常与中国东部地区夏季气温的关系。发现两者之间有很高的正相关 ,当西太平洋副高偏西时 ,大片的北方地区气温会降低 ;而当副高偏东时 ,该地区的气温将偏高。西太平洋副高东西位置的异常对应着亚洲太平洋地区的长波位置和强度的很大变化 ,从而影响到中国北方地区的气温。更重要的是 ,副高西端位移所产生的扰动还会以波列的形式向极地和北美方向传播 ,从而从更大的范围内影响西风带环流。对 2 0 0 2年夏季中国北方出现的持续高温天气进行了分析。该年夏季副高持续偏东。西风带大槽也处于偏东的位置。西部的大陆副高东移扩展 ,与槽后的高压脊打通 ,造成该地区的持续高温。副高的东西位置决定了季风气流 (也就是水汽的主要通道 )的北向转折的路径。当副高偏东时 ,向北折向的位置随之东移 ,东亚大陆的水汽供应变为负距平 ,特别是中国北方地区 ,变得十分干燥。引起中国北方高温的直接系统是大陆高压中的极强的下沉气流 ,副高的偏东位置为大陆副高的东进和加强提供了大尺度的背景条件     

EXTRA-SEASONAL PREDICTIONS OF SUMMER RAINFALL IN CHINA AND ENSO IN 2001 BY CLIMATE MODELS*

China is a monsoon country.The most rainfalls in China concentrate on the summer seasons.More frequent floods or droughts occur in some parts of China.Therefore,the prediction of summer rainfall in China is a significant issue.As we know,the obvious impacts of the sea surface temperature anomalies(SSTA)on the summer rainfall over China have been noticed.The predictions of the SSTA have been involved in the research.The key project on short-term climate modeling prediction system has been finished in 2000.The system included an atmospheric general circulation model named AGCM95,a coupled atmospheric-oceanic general circulation model named AOGCM95,a regional climate model over China named RegCM95,a high-resolution Indian-Pacific OGCM named IPOGCM95,and a simplified atmosphere-ocean dynamic model system named SAOMS95.They became the operational prediction models of National Climate Center(NCC).Extra-seasonal predictions in 2001 have been conducted by several climate models,which were the AGCM95,AOGCM95,RegCM95,IPOGCM95,AIPOGCM95,OSU/NCC,SAOMS95,IAP APOGCM and CAMS/ZS.All of those models predicted the summer precipitation over China and/or the annual SSTA over the tropical Pacific Ocean in the Modeling Prediction Workshop held in March 2001.The assessments have shown that the most models predicted the distributions of main rain belt over Huanan and parts of Jiangnan and droughts over Huabei-Hetao and Huaihe River Valley reasonably.The most models predicted successfully that a weaker cold phase of the SSTA over the central and eastern tropical Pacific Ocean would continue in 2001.The evaluations of extra-seasonal predictions have also indicated that the models had a certain capability of predicting the SSTA over the tropical Pacific Ocean and the summer rainfall over China.The assessment also showed that multi-model ensemble(super ensembles)predictions provided the better forecasts for both SSTA and summer rainfall in 2001,compared with the single model.It is a preliminary assessment for the extra-seasonal predictions by the climate models.The further investigations will be carried out.The model system should be developed and improved.     

最敏感扰动的演变与夏季乌拉尔地区的持续性异常环流

通过数值实验对初始扰动不稳定发展建立异常环流型的过程进行了分析。选择了东亚夏季风异常的１９９１年和１９８５年两个典型年份的平均环流作为基流,以共轭敏感性分析得到的最敏感扰动作为初始扰动,分析其在不同的环流背景下的发展情况。结果说明,１９９１年欧亚地区的基流不稳定性较强,且初始扰动结构有利扰动发展,因而通过能量频散在乌拉尔地区激发扰动并强烈发展,建立起阻塞形势。而１９８５年的扰动在乌拉尔地区形成正涡度距平,不利于阻高的建立。从而导致１９９１年和１９８５年夏季乌拉尔地区几乎相反的异常环流。一系列对比实验说明,异常环流型的建立,不仅依靠能量的频散,更依靠扰动通过正压不稳定过程从基流吸收能量而发展。它既有赖于基流自身的不稳定,又取决于初始扰动的结构以及相对于基流的位置。基流选择了具有特定结构的初始扰动型,只要在有利位置上给予扰动,就能够激发出扰动波列,建立强的持续异常环流型。不论初始波列的位相在一定范围内如何改变,扰动总是倾向于在基流的特定不稳定区域发展。     

EFFECTS OF OROGRAPHY AND AIR FLOW IN EKMAN BOUNDARY LAYER*

It is convenient to use σ-coordinates to discuss the dynamic effect of orography and the flow in Ekman boundary layer.In this paper,the theory of mixing length is generalized to the σ-coordinate system.Then the governing equations,describing the motion in the boundary layer over the mountain regions are derived.The features of flow in the boundary layer,especially the effects of Ekman pumping,are discussed indetails.It is pointed out that there are three factors affecting the vertical motion at the top of the boundary layer:(1) vorticity distribution in the boundary layer,which is directly related to the divergence and convergence of air flow caused by friction,(2) the upslope or downslope motion of flow over the mountain slopes,and(3) the mutual effect of orography and friction induced by the ageostrophic component climbing upward or downward in the boundary layer over mountain regions.     

SIMULATION OF HEAVY RAINFALL IN THE SUMMER OF 1998 OVER CHINA WITH REGIONAL CLIMATE MODEL*

The heavy rainfall in the summer of 1998 over China has been simulated with the NCC Regional Climate Model(RegCM_NCC).It was successful for RegCM_NCC to reproduce the location and seasonal shift of the seasonal rain belt in the summer of 1998 over China.The rainy season in the summer of 1998 over China can be divided into 7 episodes,including the pre-summer rainy season in South China.the Meiyu onset over the Yangtze-Huaihe River Basin,short appearance of North China rain season and the retreat of seasonal rain belt,the second Meiyu season over the Yangtze River Valley,the rainy period over the Yellow and Huaihe River Valley and the seasonal retreat of rain belt over North China.The shortcoming of the RegCM_NCC is over-estimation of precipitation amounts.The regions with large latent heat flux,upper soil moisture and total runoff are located in the rainy area and move with the simulated rain belt during the different episodes.On the contrary,the regions with small sensible heat flux are located in the simulated rainy area and move with the simulated rain belt during the different episodes.     

THE ROLE OF CLOUDE/RADIATION IN LONG-RANGE NUMERICAL WEATHER PREDICTION*

We investigate the role of clouds and radiation in the general circulation of the atmosphere using a model designed for 30-day predictions.Comprehensive verifications of 30-day predictions for the 500 hPa geopotential height field have been carried out,using the data from ECMWF objective analyses that cover the period from May 5 to June 3,1982.We perform three model simulations,including experiments with interactive cloud formation,without clouds,and without radiative heating.The latter two experiments allow us to study the effects of cloud/radiation interactions and feedbacks on the predicted vertical velocity,and the meridional and zonal wind profiles,averaged over a 30-day period.We demonstrate that the Hadley circulation is maintained by the presence of clouds.The radiative cooling in the atmosphere intensifies the vertical motion in low latitudes and,to some extent,also strengthens the overall meridional circulation.The meridional winds are correctly reproduced in the model if clouds are incorporated.The zonal winds are significantly affected by clouds and radiative cooling.Without an appropriate incorporation of these physical elements,the model results would deviate significantly from observations.The presence of clouds strengthens the westerlies in middle and high levels.In May,the northerly movemen of the jet stream over eastern Asia is,in part,associated with the presence of clouds.     

EFFECT OF LOCAL BASIC FLOW STRUCTURE PATTERN IN FORMATION OF BLOCKING HIGHS*

The effect of local basic flow structure pattern (BFSP) on a blocking high formation is investigated within the framework of forced dissipation KDV dynamics.The zonal and meridional positions of the high's center excited by a heat source depend on the BFSP selected.     

南北方人工造雾的气象可行性研究

本文分析了造雾剂的成雾原理和影响成雾效果的温度、相对湿度和风速等环境因素,对典型南方城市(长沙)和北方城市(北京)的相关气象资料进行统计分析表明,南方的气候除14时外非常适合造雾,而北方02时和08时造雾完全可行,20时也存在很大的可能性。外场试验表明,由于开放空间的特点,造雾剂成雾所需的相对湿度降低,同时也由于大面积人造雾的集团效应和外场空间较高的凝水效率,使人工造雾的抗风能力加强,外场人工造雾的可行性很大。     

THE DECADAL CLIMATE VARIABILITY AND THE ANOMALOUS ENSO DEVELOPMENTS IN 1990S*

By using the wavelet transform method,the ENSO (2-7 a) signal and the decadal variability (8-20 a) are filtered out from the long-term SST data sets in order to investigate characteristics of the decadal variability and its impact on the ENSO.It is found that there are two different kinds of decadal SSTA modes-horseshoe and horse saddle patterns in the tropical Pacific.The horseshoe pattern represents that the decadal SSTA variability in the central Pacific is in phase with that in the eastern Pacific.The horse saddle pattern is named that they are out of phase.The former constituted the decadal variability before 1990s and the latter mainly prevailed during 1990s.As the response of atmosphere to the ocean,two decadal wind patterns appear in association with the SST decadal modes.One is characterized by anomalous development of the zonal wind,the other by anomalous development of the meridional wind.These two kinds of modes can also be regarded as different phases of the decadal oscillation.Further studies have shown that the influences of the two kinds of modes on the ENSO are different.The horse saddle mode has a stronger impact on the ENSO than the horseshoe mode.A possible mechanism for the influence of the decadal variability on the ENSO signal is presented.The central part of the thermocline along the equatorial Pacific moves up or down simultaneously with its eastern part while the decadal variability bears the horseshoe pattern.But the two segments of the thermocline in the central and eastern Pacific act oppositely while the decadal variability shows the horse saddle pattern.In this case it has an-influence on the individual ENSO'events by the superposition of the decadal variability.     

MOVEMENT OF THE RIDGE LINE OF SUMMER WEST PACIFIC SUBTROPICAL HIGH AND ITS POSSIBLE MECHANISM*

The analysis of the 40-year averaged daily data in this paper suggests that the vertical structure and movement of subtropical high (SH) ridge in summer evidently differ in different areas,which is close related with spatially nonuniform heating.The SH ridge over the West Pacific tilts northwards with height,while one over the Central Pacific tilts southwards.The northward movements of the Central/West Pacific SH ridges both show distinct low frequency oscillations of 10-20 days,and the movement over the East Asian monsoon area shows obvious oscillations of quasi-40-days as well.The analysis shows that the solar radiation drives the seasonal meridional movement of SH,while the spatially nonuniform heating modifies its movement speed and intensity,thus resulting in its anomalous motion.     

ADVANCE AND RETREAT OF THE SUMMER MONSOON IN CHINA

By using the daily-14 year(1983—1996)NCEP/NCAR 2.5°×2.5° reanalysis data,wecarefully study in each pentad the advance and retreat of the summer monsoon in China and givento it a new definition.This definition considered the intensity of southwesterly winds at 850 hPatogether with its degree in temperature and moisture.The result revealed that:(1)The advance of the summer monsoon in China shows three abrupt northward shifts andfour relatively stationary stays.The four stable stages correspond to the peak of the pro-summerrainy period in South China,the“Meiyu”season in the Changjiang(Yangtze)-Huaihe RiverValleys.the rainy season in the downstream of the Huanghe(Yellow)Riyer Valleys and the rainyseason in northern China.The retreat of the summer monsoon is so fast that it totally retreatsfrom the mainland at about the mid-August.(2)The northward advance of summer monsoon in China is basically controlled by theseasonal variation latitudinally of the upper level planetary westerlies.It is in roughly accord withthe temporal variation in the position of 15 m s~(-1) isotaeh at 200 hPa.The fast retreat of thesummer monsoon is mainly due to the blocking effect of the Tibetan Plateau.(3)The advance of 500 hPa subtropical high of the western Pacific is also in aecordanee withthe advance of the summer monsoon in China.During the advancement of the summer monsoon,the eastward movement of the subtropical high shows great meaning that it creates the essentialcondition for the convergence of southward intrusion cold airs with the warm and humidsouthwesterly winds,which result in precipitation.There are three manifest eastward movementsof the subtropical high during its northward advancement.They coincide correspondingly to thebeginning of the peak of the pre-summer rainy period in South China,the“Meiyu”season in theChangjiang(Yangtze)-Huaihe River Valleys and finally the rainy season in northern China.Thewestern part of the subtropical high moves eastward to the region of Japan in late July and thebeginning of August.It then stays there for quite a long time which results in the straightmovement of cold airs intruding from the north to the east of Tibetan Plateau,i.e.the easternregion of China.This provides good condition for the fast southward retreat of the summermonsoon.(4)The intensifieation and development of the Tibetan high at 200 hPa are closely related to the eastward movement of the subtropical high,they often occur simultaneously.     

A STUDY OF CIRCULATION CHARACTERISTICS AND INDEX OF SOUTH CHINA SEA SUMMER MONSOON*

Based on height and wind data of NCEP/NCAR and OLR data,patterns of upper air circulation from April to October have been analyzed,and the South China Sea (SCS) Summer Monsoon Onset (SMO) and retreat have been defined.The empirical formula fitting to the onset index of the SCS SMO has been established,and the onset and ending time of monsoon,together with the intensity index sequence during 1953-1999 are given by the analysis of characteristics of thermodynamic and dynamic factors during the process of SMO.The emergence and development of symmetric vortex pair at both sides of the equator in tropical East Indian Ocean,which may excite the SCS SMO,can be taken as a short-term prediction indicator of SMO.     

A FURTHER STUDY ON INTERACTION BETWEEN ANOMALOUS WINTER MONSOON IN EAST ASIA AND EL NINO*

The interaction between anomalous winter monsoon in East Asia and El Nino is further studied in this paper.The new results still more proved a previous conclusion:there are clear interactions between El Nino and winter monsoon in East Asia.The continual westerly burst andstronger cumulus convection over the equatorial central-western Pacific caused by stronger wintermonsoon in East Asia can respectively excite anomalous oceanic Kelvin wave and stronger atmospheric intraseasonal oscillation in the tropics,then excite the El Nino event through air-sea interction.In El Nino winter,there are warmer and weaker winter monsoons in East Asia.The El Nino will still reduce the intensity of intraseasonal oscillation and leads it to be barotropic structure.     

WAVE-FLOW INTERACTIONS IN THE ATMOSPHERIC LOW-FREQUENCY VARIATION OF BLOCKING PATTERN*

Through a simple review of low-frequency variation of blocking pattern and diagnoses of observational data and combining some experience in forecast practice,we put forward some thought about the mechanism of formation and maintenance of the blocking flow pattern from the synoptic/climatological point of view,It is emphasized that eastward moving and deepening of troughs in the upstream of the blocking high,the SST anomalies and topography effect are the main factors of impelling the variation of the blocking high.During the maintenance of Ural blocking high wave-flow interactions are studied through disposition of baroclinic trough(transient wave)and the blocking flow.This study will offer a primary basis for the further theoretical study on the formation mechanism of the blocking high.     

HEAVY PRECIPITATION IN HONG KONG, 1951-2000*

Daily precipitation amounts greater than 50 mm from 1951 to 2000 in Hong Kong were analyzed to determine their seasonal and annual frequencies.A total of 627 extreme rainfall days happened and 334 (53.27%) days occurred in summer.Non significant increasing trends for annual precipitation and number of heavy rain days were detected.Daily surface weather maps were used to identify the synoptic weather types producing the heavy rainfall.It was discovered that tropical cyclone and low pressure induced most of the heavy precipitation.Seven out of twelve years with counts of heavy rain days over fifteen coincided with E1 Nino events.     

STUDY ON THE AIR-SEA INTERACTION ON THE INTERANNUAL TIME SCALE IN THE SOUTH CHINA SEA*

In this paper we document the correlationship between sea surface temperature(SST) and low level-winds such as sea level wind and 850 hPa wind in the South China Sea(SCS) based on COADS(1958-1987) and ECMWF objective analysis data(1973-1986).Further statistical analyses tell us that there is a fixed SCS basin mode for variations both of SST and low-level winds in the region on the interannual time scale due to air-sea interactions.A simplified,coupled model that is designed following the McCreary and Anderson's(1985) model and includes the feedback between the upper ocean and the circulation of East Asian monsoon demonstrates an interannual oscillation in the coupled air-sea system,which is similar to the observations in the SCS.