THE QUASI-BIWEEKLY OSCILLATION OF FLOOD-CAUSING TORRENTIAL RAIN IN XIJIANG RIVER REGION AND ITS ATMOSPHERIC CIRCULATION MODELS
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In order to have better medium-range forecasts of flood-causing torrential rains in the Xijiang river valley,the characteristics of low-frequency oscillations of rainfall in the valley and wind field at 850 hPa are studied,with wavelet transform and Lanczos filter,for 13 cases of such rains since 1961.The effect is studied of low-frequency propagation of 850 hPa wind field on the rains in the region.Using composite analysis,an atmospheric circulation model is constructed of quasi-biweekly oscillation of the rains that are caused by westerly systems.Results show that the rainfall exhibits mainly quasi-biweekly oscillations with periods of 10 to 20 days,which correspond well with the positive phase of 8 to 35 day oscillations of the wind field at 850 hPa.The convergence over the valley between low-frequency zonal winds—either propagating northward from areas south of the valley or travelling southward from areas near 30 oN while strengthening—and those going westward from the northwest Pacific,is one of the possible causes of the quasi-biweekly oscillation of the flood-causing torrential rains in this region.When the rain begins and develops to its maximum intensity gradually,the high ridge east of the Balkhash Lake begins to rise and moves eastwards gradually,an apparent low trough in North China gradually controls an area from Northeast China to the lower reaches of the Yangtze River,while South China is controlled by a deep upper-level trough taking the place of a weak westerly trough east of Qinghai-Tibet Plateau.The subtropical high keeps enhancing and moving westwards,Southwest Monsoon coming from the Bay of Bengal and going through India-China Peninsula at 850 hPa continually strengthens,and a weak cyclonic curvature of streamlines west of a jet stream axis located over South China continually enhances,all at the same time.Sea level pressure appears to be higher in the east and lower in the west and its isobars exhibit significant meridionality.These are beneficial to medium-range forecast of flood-causing torrential rains in the Xijiang river valley.Keywords:
Trough (economics)
Subtropical ridge
Atmospheric Circulation
Yangtze river
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An abrupt torrential rain occurred at the south of Jianghan Plain at the night of 24th May* 2006.Using NCEP analyzed data and automatic meteorological station data* a research is conducted.The results are reached as follows: before the heavy rain* the south border of upper level jet(ULJ) stream retreated from Hubei province to the north* with the occurring and weakening of the rain* the border came back to the upper level over Hubei province again;before the heavy rain* the core of the ULJ moved from the west to the east within the zone from 35°~40°N* and when the heavy rain began* it was at the northeast of Hubei province already.The occurrence of the torrential rain coincided with the strengthening of a meso-scale zone with positive vorticity at boundary layer;water vapor was transported from northwestern South China Sea to Central China at low levels;during the rain there were decrease of inertial stability at high level and maintaining of potential instability at low levels;before the heavy rain* large positive moisture helicity divergence appeared at 950 hPa level.
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Subtropical ridge
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Using daily observed precipitation data from 753 stations of China from 1981 to 2010,NCEP / NCAR daily reanalysis data and real-time weather map,selecting 32 large-scale persistent heavy rainfall processes in the middle-lower Reaches of the Yangtze River,the relationship between the processes and intra-monthly( 10 ~ 30d) low-frequency rainfall and possible forming mechanism of the process are analyzed. The results suggest that:( 1) Rainfall in the middle-lower Reaches of the Yangtze River has significant intra-monthly low-frequency oscillation( LFO). Most large-scale persistent heavy rainfall processes are in its peak stage.( 2) Before the peak phase of LFO rainfall for M eiyu flood period,the west Pacific subtropical high extends northwestward,upper and lower jet streams strengthen. At peak phase of LFO rainfall,upper,middle and lower circulation coordinate with each other very well. While before the peak phase of LFO rainfall for typhoon flood period,with the southeastward retreating of subtropical high,lower jet streams extend southward,cooperating with the upper jet streams,which provide favorable conditions for the heavy rainfall.( 3) At Meiyu flood period,LFO wave train in north Pacific propagates westly and LFO vortex in the northeast Asia propagates southward. They converge at the middle-lower Reaches of the Yangtze River. At the same time,with the northward propagating of high value system of LFO geopotential height in South China Sea,the corresponding distribution of south high north low in LFO geopotential height field contributes to the heavy rainfall process. For typhoon flood period,with the northwestward propagation of the LFO wave train from the tropical west Pacific to the Japan Sea,LFO cyclone at northeast of Philippines with LFO anticyclone on its north moves to the middle-lower reaches of the Yangtze River and Northeast Asia at peak phase of LFO rainfall,which contributes to the severe convergence and results in the heavy rainfall process.( 4) Positive anomalous convection over east of Philippines can be used as previous tropical signals of the heavy rainfall process at M eiyu and typhoon flood period,which is in advance of rainfall peak phase about 10 days.
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Jet stream
Subtropical ridge
Yangtze river
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Geopotential
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We study the circulation features of a torrential rain process at different levels in Guangdong in January: The northwestern part of anticyclone over west Pacific and a strong divergent field at 200hPa in the south side of entrance of southwestly jet stream in central China;The subtropical high pressure moves eastward.The southwestly wind ahead of it is getting stronger when the westerly trough at 500hPa comes up to near Guangdong.At 850hPa,the southwestly wind and southeast wind converge in Guangdong.Thus the convergence became stronger and moisture content increased rapidly at low levels.Moreover we analyzed specifically the case from 23～25 January 2000.
Subtropical ridge
Trough (economics)
Anticyclone
Jet stream
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Trough (economics)
Subtropical ridge
Westerlies
Typhoon
Low-pressure area
Jet stream
Atmospheric Circulation
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Using upper and ground based observations data,by means of Cressman's objective analysis and composite analysis method,the large scale features of persistent rainfall which occurred from 10 to 20 July,2011 over the whole Yangtze-Huaihe region(26-34°N,110-122°E,including the Yangtze River and the Huaihe River valleys)are studied,and the rainstorm occurred in this period from 11 to 13 is dynamically diagnosed. The results are as follows:(1)There are double ridge of high pressure and one trough around 50-70°N in the large scale circulation field,the western one is over the Ural Mountains(at 40°E),the eastern one is over Yakusk(at 130°E),and it is the trough over the Lake Balkhash between them,as well as nontypical double-blocking high type. As the trough is moving eastwardly and deepening,the short-waves trough which split from the bottom of it brings about the heavy rain.The Western Pacific Subtropical Anticyclone is by eastern and its ridge is by northern. A deep trough over the southeast of Lake Baikal extends to the Southern China from Mongolia,the southwestward airflow of the ahead of the trough impacts the Jianghuai Basin steadily.(2)The Yangtze-Huaihe region is located on the right side of the high level southwest jet's entrance area and the nearside of the low level south jet. The dry cold air comes from the midhigh latitudes and the mid-high level,the dry cold air intrusion contributes to the formation and maintenance of the dry leve1. The dry cold air intrusion is one of the important dynamical and thermal causes for the formation and maintenance of moist front during the Meiyu period.(3)The water humidity comes largely from two sources,one is at the Bay of Bengal whose humidity is transferred by westerly flow on the south side of the monsoon depression via the Indo-China peninsula into the Sea and advancing northward into the Yangtze-Huaihe region,and the other is over the South-China Sea which offers water vapor for the rainband mainly through SW monsoon.(4)The cyclonic vorticity and convergence at the low level,the anticyclonic vorticity and divergence at the upper level,deep upward motion plays a main role in the maintenance of sever precipitation occurred in 11-13 July,2011. Based on the moist potential vorticity(MPV)theory,the vertical and horizontal components of MPV,MPV1 and MPV2 are analyzed;symmetric instability is related with convective instability,inertial instability,moist baroclinicity and vertical wind shear. It is favourable to develop symmetric instability when MPV10,MPV20,the symmetric instability intensified the torrential rain. The conditional symmetric instability(MPV 0)provided favorable instability mechanism for the occurrence of rainstorm.
Trough (economics)
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Anticyclone
Jet stream
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An eastward rainstorm process happened in coastal areas of Zhejiang Province during July 24th to 27th,2010.This paper makes diagnostic studies on the circulation background,impact system and thermal and dynamic conditions of this torrential rain process by using objective data.Meanwhile,the coastal terrain influence to the torrential rain is tested by numerical model.The results show:(1) Zhejiang is situated at the low area between the continental high and oceanic subtropical high.This kind of col pressure field offers beneficial background conditions for the heavy rain.The vortex is developed and strengthened again after landing.Southeast jet between the vortex and subtropical anticyclone provides conditions of momentum,water vapor and heat for the rainstorm.(2) Water vapor comes from west Pacific and the South China Sea during the rain course.The convergence of water vapor mainly concentrates in boundary layer.The deeper southerly wind in troposphere extends,the stronger hig-level divergence and mid-level convergence become,and the higher raising of potential equivalent temperature isoline,the heavier rain intensity is.(3) Rainstorm appears in the repeated addition area of negative MPV1 and positive MPV2 isoline concentration belt.The intensity change of positive MPV1 from upper-level and negative MPV1 from lower-level corresponds to rainfall intensity of rainfall center,which makes the development of convection and the increasement of precipitation.(4) Topography plays a remarkable amplification role to heavy rain.Windward slope,which enhances ascending motion by topographic dynamic lifting effect,can increase precipitation by about 40%.
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Anticyclone
Convergence zone
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Yangtze river
River valley
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The circulation characteristics and the influencing systems of the heavy rainfall causing flood in the Pearl River(the Xijiang River) valley in June 2005 are investigated.The heavy rainfall had the outstanding characteristics of multi-scale systems and it occured in warm sector ahead of the front,relating to monsoon activity,the influence of the planetary boundary layer and the dynamic lifting of the topography.The conclusions are as follows:(1) The late onset of the South China Sea monsoon and its continuous impact on the South China,as well as the further south position of the upper-level jet in the westerlies and the subtropical high,which maintained in 18°N,provided the very favorable large-scale circulations for sustaining the heavy rainfalls in South China.(2) There were the heavy rainfalls happening in warm sector ahead of the front in South China during the pre-rainy season.There always existed an approximately east-to-west oriented quasi-stationary front in South China.The calculated results of the frontogenesis function indicate that the weaker cold air played an important role in occurrence of heavy rainfall.The analysis of the vertical cross section of horizontal wind and vertical velocity indicates that there was ascending motion obviously ahead of the front,which was very favorable for the formation and development of the meso-scale convection.The weaker cold air from the north interacted with the warm and wet air coming along the south-west edge of the subtropical high,there is a kind of interaction between middle- and low-latitude systems.(3) The wind speed convergence of south-west wind at low level was the most obvious at 925 hPa.It indicates that the planetary boundary level played an very important role in the formation of the heavy rainfall,the current not only transported sufficient water vapor to the heavy rainfall area from the Sino-Indian Peninsula and the South China Sea,but also contributed to the initiation of the convection.(4) The trumpet-shaped terrain in the Nanling Mountains enhanced the ascending motion of the warm and wet air from the south,which benefited the occurrence of the heavy rainfall.The special terrain in South China,especially in the Xijiang River valley,can be one of the important conditions for the appearance of the continuous heavy rainfalls.(5) The meso-scale convective rainy clusters were the direct influencing systems of the heavy rainfall.There were at least four meso-β-scale rainy clusters existing over the Xijiang River valley(A,B,C,D).Fortunately,A and D rainy clusters were located near Guilin and Wuzhou observation stations,respectively.So the environment of A and D rainy clusters can be carefully studied.The rainfall areas closely related to the convergence center of the low-level wind speed,and the convective available potential energy is cumulated,released and rebuilt again and again.Under such circumstances,the meso-scale convective system and heavy rainy were very active.Furthermore,the convective rainy clusters followed the flood peak,namely moved towards the lower reaches of the river,resulting in the coupling between the heavy rainfall and the flood peak,then brought the very serious disasters.(6) Based on the above-mentioned research,a schematic diagram of the influencing factors for the heavy rainfall in warm sector ahead of the front in South China during the period of pre-rainy season is given.
Rainband
Subtropical ridge
Frontogenesis
Westerlies
Cold front
Jet stream
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Based on precipitation data from 7213 weather stations in Shaanxi province and its surrounding areas,the spatial distributions of precipitation from September 1 to 18,2011 in the West China were analyzed. The results indicate that accumulated precipitation during the period is twice to threefold as much as that during the same period of history. Precipitation is characterized with long duration and serious disaster. This precipitation event can be divided into three processes,i. e. the first process is from September 4 to 6,the second on September 11 and the third from September16 to 18. According to 1° × 1° NCEP reanalysis data,atmospheric circulation characteristics of these three processes suggest that major water vapor transport of lower level in rainstorm region has two branches.One is the southern air flow from eastern Sichuan province to north,and the other is the eastern air flow from southern North China plain to west. Corresponding to these three precipitation processes,three westerly troughs influence the circulation in 500 hPa,and 700 hPa southerly and 850 hPa northerly winds appear in three obvious control periods over the West China. Unstable stratification is formed because southerly airflow in 700 hPa transports a large number of warm-moist airflow to West China in advance. When dry-cold air transported by northern wind enters into the bottom of warm-moist air mass and lifts it,dynamic lift action in the front of the superadded trough and wind field convergence uplift caused by the terrain occur,which makes the convective activity strengthened and potential unstable energy released,so appears heavy rain. Continued unstable stratification and dynamic lifting caused by stable circulation situation( such as blocking high) and specific terrain condition,which benefits to release unstable energy,are the essential reason forming steady and continuous rainstorm in the West China.
Trough (economics)
Warm front
Cold front
Atmospheric Circulation
Synoptic scale meteorology
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Mechanism is compared for two rainstorm processes in different places of the Yangtze River valley in Jul. 21,1998. The torrential rain in Sinan area of Guizhou Province in the upper valleys of the Yangtze River is under the influence of southwest vortex,low-level westerly jet and south Asia high. Rainbelt locates on the left-mid-hand side of the low level jet and the main vapour source is from the Bay of Bengal. But the torrential rain in Wuhan in the mid valleys of the Yangtze River is principally resulted from the coupling of boundary-layer southerly jet,low-level westerly jet and high level westerly jet. Its major vapour source is from South China Sea. Non-thermal wind convective instability at low level and non-thermal wind symmetric instability at high level are the mechanism of torrential rain.
Yangtze river
BENGAL
Westerlies
Jet stream
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