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1.
Numerical simulation of a heavy rainfall case in Henan area during 16-17 July 2004 is performed using the LASG (State Key Laboratory of Numerical Modelling for Atmospheric Sciences and Geophysical Fluid Dynamics) mesoscale model AREM (Advanced Regional Eta Model) developed by Yu (1989) and Yu et al. (1994). The results are shown: the air in the middle part of troposphere within the horizontal range of meso-βscale convective system is heated by condensation latent heat. The isobaric surface in the middle and upper part of troposphere is rising, and thus meso-βscale high is formed; the isobaric surface in the lower part of troposphere is depressed, and thus meso-βscale low is formed. The interaction between the high and low layer flow promotes the strong development of the vertical motion. While the rising motion is developing strongly, obvious compensation sinking motion appears around it. In the south of rising motion region, the divergence current in the upper part of troposphere back lows towards south, which leads to the vertical circulation appearing in the upper part of troposphere. The sinking branch of the circulation integrates in the compensation sinking air current in the south of rising motion region and takes the horizontal momentum of upper air to the lower part of troposphere and forms a new meso-βscale jet. In the north of the rising motion region, a mesoscale vertical circulation develops in the low layer of troposphere. The divergence current of the sinking branch of the circulation, which flows southward, converges with warm and humid air current in the low layer of troposphere which flows from southwest, and forms a meso-βscale convergence line. Then it strengthens the convergence over the low level of heavy rain area. In the east of the rising motion region, a mesoscale vertical circulation also develops in low layer of troposphere. The divergence current of the sinking branch of the circulation, which flows westward, causes originally more consistent southwest air current in this region to the east deflection, and thus forms the cyclone curve in the southwest air current. The convergence is further strengthened in the meso-βscale convergence line.The strong development of ageostrophic vorticity in the lower part of troposphere is the important factor of the formation of the meso-βscale cyclone. At last the three-dimensional structure chart of development of heavy rain meso-βscale stream filed is given.  相似文献   

2.
By means of the Penn State-NCAR Mesoscale Model Version 5(MM5)with a horizontal resolution of 20 km.the maintainable mechanism of Meso-β scale Convective System(MβCS)has been investigated on the basis of simulation of the temporal and spatial thermodynamics structure of the MβCS which occurred in Wuhan and its surroundings on 21 July 1998.The occurrence of the significant warm-core in the center of the MβCS happened in Changjiang River Basin between the Mufu Mountain and the Dabie Mountain.To the southern side of the MβCS,there exist the southwest low-level jet(LLJ)and the vertical secondary circulation in the low and middle troposphere respectively.In respect to the northern of the MβCS,the northwest jet emerges in the upper troposphere,accompanied with cold and dry atmosphere downdrafts,resulting in another secondary circulation.The foregoing mentioned vertical wind shear provides a favorable dynamical environment for the intensification and maintenance of the MβCS.Equally important,the latent heat release associated with the MβCS produces the warm center in the middle troposphere and the pressure falls.The pressure drop then accelerates air parcels toward the low leading to strong convergence as well as the intensified convection,establishing a positive feedback between the convection and the latent heat release,which is the thermodynamic mechanism of the development and maintenance of the MβCS.  相似文献   

3.
Based on intensive automatic weather station data, satellite cloud imagery, NCEP reanalyzed data, and the simulation results from mesoscale numerical models, this study analyzes the characteristics and formation mechanisms of the mesoscale convection system (MCS) during the extreme precipitation event that was triggered by a weakened low-pressure inverted trough of Typhoon Haikui on August 10/2012. The results of this study show that cold air at the rear of a northeastern cold vortex creates thermodynamic conditions favorable to the development of extreme precipitation. The main body of the cold air is northward located so that the cold air invades only the middle layer of the periphery of the inverted trough. Thus, the cold air minimally affects the lower layer, which results in a vertically distributed structure of the temperature advection that augments the formation and development of convective instability stratification. In the middle troposphere, the cold air encounters the convergent, ascending, warm moist air from the low-pressure inverted trough, leading to frontogenesis. The frontogenesis enhances wind convergence which, in turn, further enhances the frontogenesis, and the positive feedback between these two forces augments the development of meso- and small-scale convection systems in the rainstorm region and its vicinity, which strengthens the upward transportation of water vapor from low layers and thickening of water vapor convergence and results in local heavy rains.  相似文献   

4.
The evolution of a mesoscale convective system (MCS) that caused strong precipitation in the northern area of Dabie Mountain during 21-22 June 2008 is analyzed, along with the evolution of the associated meso-β-scale convective vortex (MCV). The mesoscale reanalysis data generated by the Local Analysis and Prediction System (LAPS) at a 3-km horizontal resolution and a 1-h time resolution during the South China Heavy Rainfall Experiment (SCHeREX) were utilized. The results show that two processes played key roles in the enhancement of convective instability. First, the mesoscale low-level jet strengthened and shifted eastward, leading to the convergence of warm-wet airflow and increasing convective instability at middle and low levels. Second, the warm-wet airflow interacted with the cold airflow from the north, causing increased vertical vorticity in the vicinity of steeply sloping moist isentropic surfaces. The combined action of these two processes caused the MCS to shift progressively eastward. Condensation associated with the MCS released latent heat and formed a layer of large diabatic heating in the middle troposphere, increasing the potential vorticity below this layer. This increase in potential vorticity created favorable conditions for the development of a low-level vortex circulation. The vertical motion associated with this low-level vortex further promoted the development of convection, creating a positive feedback between the deep convection and the low-level vortex circulation. This feedback mechanism not only promoted the maturation of the MCS, but also played the primary role in the evolution of the MCV. The MCV formed and developed due to the enhancement of the positive feedback that accompanied the coming together of the center of the vortex and the center of the convection. The positive feedback peaked and the MCV matured when these two centers converged. The positive feedback weakened and the MCV began to decay as the two centers separated and diverged.  相似文献   

5.
A heavy rainfall event that occurred in Shandong Province in 26 28 August 2004 was caused mainly by Typhoon Acre and cold air activities related to a westerly trough. The event was triggered by an inverted typhoon trough, which was closely associated with the intensification of the low-level southeasterly flow and the northward transport of heat and momentum in the periphery of the typhoon low. A numerical simulation of this event is performed using the nonhydrostatic mesoscale model MM5 with two-way interactive and triply-nested grids, and the structure of the inverted typhoon trough is studied. Furthermore, the formation and development mechanism of the inverted typhoon trough and a mesoscale vortex are discussed through a vorticity budget analysis. The results show that the heavy rainfall was induced by the strong convergence between the strong and weak winds within the inverted typhoon trough. Dynamic effects of the low-level jet and the diabatic heating of precipitation played an important role in the development of the inverted typhoon trough and the formation of the mesoscale vortex. The vorticity budget analysis suggests that the divergence term in the low troposphere, the horizontal advection term, and the convection term in the middle troposphere were main contributors to positive vorticity. Nonetheless, at the same pressure level, the effect of the divergence term and that of the adveetion term were opposite to each other. In the middle troposphere, the vertical transport term made a positive contribution while the tilting term made a negative contribution, and the total vorticity tendency was the net result of their counteractions. It is found that the change tendency of the relative vorticity was not uniform horizontally. A strong positive vorticity tendency occurred in the southeast of the mesoscale vortex, which is why the heavy rainfall was concentrated there. The increase of positive vorticity in the low （upper） troposphere was caused by horizontal convergence （upward transport of vorticity from the lower troposphere）. Therefore, the development of the inverted typhoon trough and the formation of the mesoscale vortex were mainly attributed to the vorticity generated in the low troposphere, and also the vertical transport of vorticity from the low and middle troposphere.  相似文献   

6.
A heavy rainfall associated with the deepening of a monsoon depression happened in the summer of 2005. This process was first diagnostically analyzed and the 3D structure of the monsoon depression was discussed~ then this structure was compared with those of the monsoon depression in South Asia and the low vortex in the Meiyu front. The results showed that the heavy rainfall directly resulted from a monsoon depression in South China~ and the large-scale environment provided a favorable background for the deepening of the monsoon depression. The 3D structure of the monsoon depression was as follows. In the horizontal direction, there existed a convective cloud band to the south of the monsoon depression, which lay in a convectively instable area, with a relatively strong ascending motion in the mid and low levels of the troposphere, and the ascending motion matched well with a moist tongue, a convergence area, and a band of positive vorticity in the mid and low levels of the troposphere. In the vertical direction, the depression had an obviously cyclonic circulation in the mid and low levels of the troposphere, but no circulation from above 300 hPa. The monsoon depression corresponded to convergence and positive vorticity in the low levels, but to divergence and negative vorticity in the upper levels. The upward draft of the depression could reach the upper levels of the troposphere in the west of the depression, while the descending motion lay in the east. There was a low-level jet to the south of the depression, while the upper-level jet was not obvious. The depression was vertically warm in the upper levels and cold in the low levels, and the axis of the depression tilted southeastward with height, whose characteristics were different not only from the monsoon depression in South Asia but also from the low vortex in the Meiyu front.  相似文献   

7.
Atmospheric Infrared Sounder (AIRS) data show that the Saharan air layer (SAL) is a dry, warm, and well-mixed layer between 950 and 500 hPa over the tropical Atlantic, extending westward from the African coast to the Caribbean Sea. The formations of both Hurricane Isabel and Tropical Depression 14 (TD14) were accompanied with outbreaks of SAL air during the period 1-12 September 2003, although TD14 failed to develop into a named tropical cyclone. The influence of the SAL on their formations is investigated by examining data from satellite observations and numerical simulations, in which AIRS data are incorporated into the MM5 model through the nudging technique. Analyses of the AIRS and simulation data suggest that the SAL may have played two roles in the formation of tropical cyclones during the period 1-12 September 2003. First, the outbreaks of SAL air on 3 and 8 September enhanced the transverse-vertical circulation with the rising motion along the southern edge of the SAL and the sinking motion inside the SAL, triggering the development of two tropical disturbances associated with Hurricane Isabel and TD14. Second, in addition to the reduced environmental humidity and enhanced static stability in the lower troposphere, the SAL dry air intruded into the inner region of these tropical disturbances as their cyclonic ?ows became strong. This effect may have slowed down the formation of Isabel and inhibited TD14 becoming a named tropical cyclone, while the enhanced vertical shear contributed little to tropical cyclone formation during this period. The 48-h trajectory calculations confirm that the parcels from the SAL can be transported into the inner region of an incipient tropical cyclone.  相似文献   

8.
The external source/sink of potential vorticity (PV) is the original driving force for the atmospheric circulation. The relationship between surface PV generation and surface PV density forcing is discussed in detail in this paper. Moreover, a case study of the extreme winter freezing rain/snow storm over South China in January 2008 is performed, and the surface PV density forcing over the eastern flank of the Tibetan Plateau (TP) has been found to significantly affect the precipitation over South China in this case. The TP generated PV propagated eastward in the middle troposphere. The associated zonal advection of positive absolute vorticity resulted in the increasing of cyclo-nic relative vorticity in the downstream region of the TP. Ascending air and convergence in the lower troposphere developed, which gave rise to the development of the southerly wind. This favored the increasing of negative meridio-nal absolute vorticity advection in the lower troposphere, which provided a large-scale circulation background conducive to ascending motion such that the absolute vorticity advection increased with height. Consequently, the ascending air further strengthened the southerly wind and the vertical gradient of absolute vorticity advection between the lower and middle troposphere in turn. Under such a situation, the enhanced ascending, together with the moist air transported by the southerly wind, formed the extreme winter precipitation in January 2008 over South China.  相似文献   

9.
During 8-9 July 2007,several successively developed rainstorms along the Meiyu front produced heavy rainfall in the Huaihe River Valley,which led to the most catastrophic flooding in this region since 1954.Through mesoscale analysis of both conventional and intensive observations from upper air and surface stations,automatic weather stations,Doppler radars,and the FY-2C satellite,the current study examines the developing style and environmental conditions of the mesoscale convective systems(MCSs)that led to the development of the rainstorms.Our analysis showed that this event went through three phases.The first phase of the heavy rainfall(Phase Ⅰ)was caused by a meso-α-scale wind shear in the lower troposphere during 0200-1700 BT(Beijing Time)8 July.Phase Ⅱ was characterized by a reduction in rain rate and the formation of a low-level vortex between 1700 BT 8 and 0200 BT 9 July.In Phase Ⅲ,the well-organized mature meso-α-scale low-level vortex brought about intensified rains during 0200-0800 BT 9 July.Satellite and raclar observations showed a backward development of MCSs(new convective cells were generated at the back of the system)in PhaseⅡ,a forward development in Phase Ⅲ,and a spiral organization of the convective lines in Phase Ⅱ.The heavy rainstorm systems were initiated continuously along a surface mesoscale dew-point front with a horizontal scale of～300 km(as part of the Meiyu front)in the upper reaches of the Huaihe River Valley near Fuyang City,Anhui Province and then gradually decayed in the middle and lower reaches.It is hypothesized that lifting by strong low-level convergence is sufficient to trigger convection in the high CAPE(convective available potential energy)environment.  相似文献   

10.
NCEP/NCAR data are utilized to analyze an extreme flood year （1998） and an extreme dry year （2006） in the Sichuan-Chongqing region （SCR） and the results are as follows. The positive divergence of South Asia High （SAH） is stronger in the flood year; the position of the ridge line of SAH is southward compared with the annual average; Western Pacific Subtropical High （WPSH） extends westward and its ridge line is southward. In the drought year, the positive divergence of SAH is weaker, its ridge line is northward, and the position of WPSH is also northward. As shown in the dynamics, in drought （flood） years, negative （positive） vorticity advection in the upper atmosphere can cause the atmosphere to ascend （descend）, and anomalous circulation of SAH displays divergence （convergence）, and anomalous circulation of the lower atmosphere shows convergence （divergence）. Thermal structure of the atmosphere shows that there is warm （cold） temperature advection in the lower atmosphere, and the vertical distribution of diabetic heating causes SAH＇s local circulation to display convergence （divergence） and affects vertical motion of the lower atmosphere circulation eventually. To some extent, the two extreme years in the SCR is closely related to the vertical motion of atmosphere circulation and the variation of such vertical motion is caused by differences of interactions between SAH and lower atmosphere circulations.  相似文献   

11.
This paper reviews the main theoretical progress of mesoscale weather dynamics since 2003, including： （1） The dynamic mechanisms of balanced and unbalanced flow are applied to study the genesis and development problems of mesoscale circulation. The symmetric instability and transverse-wave instability are analyzed in line and vortex atmosphere convection, and further research has been done on nonlinear convective symmetric instability. The interaction between forced convection and unstable convection and the wave characteristics of mesoscale motion are also discussed. （2） Intermediate atmosphere dynamic boundary layer models are developed. The complicated nonlinear interaction is analyzed theoretically between the atmospheric boundary layer and the free atmosphere. The structure of the topography boundary layer, atmospheric frontogenesis, the structure and circulation of the low-level front and other boundary layer dynamic problems are discussed. （3） The formation and development of meso-β-scale rainstorms under the background of the East-Asia atmosphere circulation are diagnosed with the variation of MPV （moist potential vorticity） anomalies. And some physical vectors are modified and applied in the moist atmosphere.  相似文献   

12.
In this study, a three-dimensional mesoscale model was used to numerically simulate the well-known "98.7" heavy rainfall event that affected the Yangtze Valley in July 1998. Two experiments were conducted to analyze the impact of moist processes on the development of meso-β scale vortices(MβV) and their triggering by mesoscale wind perturbation(MWP). In the experiment in which the latent heat feedback(LHF) scheme was switched off, a stable low-level col field(i.e., saddle field—a region between two lows and two highs in the isobaric surface) formed, and the MWP triggered a weak MβV. However, when the LHF scheme was switched on as the MWP was introduced into the model, the MβV developed quickly and intense rainfall and a mesoscale low-level jet(mLLJ) were generated. The thickness of the air column and average temperature between 400 and 700 hPa decreased without the feedback of latent heat, whereas they increased quickly when the LHF scheme was switched on, with the air pressure falling at low levels but rising at upper levels. A schematic representation of the positive feedbacks among the mesoscale vortex, rainfall, and mLLJ shows that in the initial stage of the MβV, the MWP triggers light rainfall and the latent heat occurs at low levels, which leads to weak convergence and ageostrophic winds. In the mature stage of the MβV, convection extends to the middle-to-upper levels, resulting in an increase in the average temperature and a stretching of the air column. A low-level cyclonic circulation forms under the effect of Coriolis torque, and the m LLJ forms to the southeast of the MβV.  相似文献   

13.
Under the influence of the synoptic-scale environmental flows, the implicit nonthermal windvorticity imbalance with ξ_T>0 may arise when the change in the vertical shear vorticity of the flows,which is caused by the change of vorticity advection with altitude, is larger than that in the thermalwind vorticity of temperature fields, which is caused by the horizontal inhomogeneity of temperatureadvection. This kind of implicit imbalance will cause ascending motion to develop in the adjustmentprocess. If the static stratification is horizontally inhomogeneous with the presence of its mesoscalehigh-value regions or the conditionally unstable region where moisture can be fully provided, thedevelopment of mesoscale disturbance with scale L_m may result from the coupling of the ascendingmotion on synoptic scale L_1 with the parameter field of static stratification on mesoscale L_2. The disturbance possesses such a vertical structure as ascending motion in the center, horizontaldivergence in the upper and convergence in the lower atmosphere. This developmental process of insta-bility may be one of the most important processes responsible for the growing of heavy rains in summerhalf year in China.  相似文献   

14.
Employing the mesoscale WRF (Weather Research and Forecast) model, Super Typhoon Saomai (2006) is simulated. The variation of track and intensity and its offshore rapid intensification process are well demonstrated by the model, and the temperature and humidity patterns associated with the dry cold air activity and their impact on and mechanism of the offshore rapid intensification of Saomai are mainly studied in this paper. The results indicate that high-resolution water vapor imagery can visually reveal the development, evolution, interaction as well as the mutual complementation of the dry cold air activity accompanied with the development of Saomai. The offshore rapid intensification phenomenon of Saomai is closely related to the dry cold air which originates from the upper- and mid- troposphere. Besides, the dry cold air from the upper troposphere is stronger than that from the mid-troposphere. Saomai intensifies as the dry cold air from the northwest moves toward its circulation but weakens when the dry cold air from the southwest is drawn into the storm. Dry cold airflows and their cold advection effect caused by the downward motion across the isentropic surface are favorable to the development of Saomai. The dry cold air always moves along an isentropic surface from the upper troposphere to the mid-troposphere around the typhoon circulation and contributes to Saomai’s abrupt intensity change.  相似文献   

15.
Warm-sector heavy rainfalls along the south China coast from April to June during 2009–2014 can be divided into two main types based on their low-level circulations. Type I is the southerly pattern with meridional convergence line at the west of the Pearl River estuary, which is formed by the convergence of southeasterly, southerly, and southwesterly flows. Type II is the southwesterly pattern with a latitudinal convergence line at the east of the Pearl River estuary, which is formed by the convergence of westerly and southwesterly flows. Statistics on 6-hourly heavy rainfall events indicates that, during the afore-mentioned 6 years, there were on average 73.2 occurrences of the southerly pattern and 50.3 occurrences of the southwesterly pattern per year. After the onset of summer monsoon in the South China Sea, the occurrence frequencies of both patterns increase remarkably. The synthetic diagnosis of pattern circulation shows that, at 500 hPa, for the southerly pattern, there is a broad warm high ridge, and a temperature ridge is behind the high ridge, which causes an obvious warm advection at the high ridge area. There is no frontal region. For the southwesterly pattern, the circulation is a weak trough with a temperature trough behind it. The position of the frontal region is near Yangzi River, and the south China coast is in the warm-sector of the frontal region. At the vertical cross-section of each of the two categories of heavy rainfall, there is a strong vertical motion center stretching to 400 hPa, where the convergence layer in the rainfall region is deep and with several vertical convergence centers overlapping one another. Both types of heavy rainfalls are with abundant water vapor, accompanied with deep convective instability energy layers, and with strong release of latent heat caused by condensation of water vapor. The release of latent heat leads to the warming-up and stretching of the air column, thus strengthens deep convergence and vertical velocity upward. There is a stronger latent heat-release in the southwesterly pattern than in the southerly pattern, while in the southerly pattern, the warm advection at middle and upper levels is stronger than the latent head release. To study the thermo-dynamic development mechanisms, weather research and forecasting model (WRF) numerical simulations are made and the results show that, in the two rainstorm regions, latent heat release warms up the air column, hence significantly increase the depth and strength of the vertical velocity. Moreover, the release of latent heat strengthens convergent circulation at lower levels and weakens divergent circulation at middle levels, whose influence can be as strong as 30%–50% of the wind circulation strength of the two types of the warm-sector heavy rainfall over the south China coast, and further enhances deep convection, promoting warm-sector storm development.  相似文献   

16.
Using a mesoscale model,a numerical study on a heavy rainfall case occurring in theChangjiang-Huaihe River Basin is made in this paper.The influence of the intensity ofnortheasterly wind in front of the Qinghai-Xizang high at upper level on the low level wind fieldand development of mesoscale systems as well as heavy rainfall is investigated.The model wellreproduced the heavy rainfall process and the weather systems associated.And it indicates that thestrong northeasterly flow around the high at upper troposphere will bring about not only thestrengthening of low level southeasterly wind,but also the appearance of shear-line and mesoscalevortex at low level.The coupling of northerly wind at upper level and southerly wind at lowerlevel constructs a vertical indirect circulation which is most favourable for the development ofconvective motions.Its ascending branch in the shear-line area is very strong and shows apronounced mesoscale characteristic.  相似文献   

17.
By means of the Penn State-NCAR Mesoscale Model Version 5(MM5)with a horizontalresolution of 20 km.the maintainable mechanism of Meso-β scale Convective System(Mβ CS)hasbeen investigated on the basis of simulation of the temporal and spatial thermodynamics structureof the MβCS which occurred in Wuhan and its surroundings on 21 July 1998.The occurrence of thesignificant warm-core in the center of the MβCS happened in Changjiang River Basin between theMufu Mountain and the Dabie Mountain.To the southern side of the MβCS,there exist thesouthwest low-level jet(LLJ)and the vertical secondary circulation in the low and middletroposphere respectively.In respect to the northern of the MβCS,the northwest jet emerges in theupper troposphere,accompanied with cold and dry atmosphere downdrafts,resulting in anothersecondary circulation.The foregoing mentioned vertical wind shear provides a favorable dynamicalenvironment for the intensification and maintenance of the MβCS.Equally important,the latentheat release associated with the MβCS produces the warm center in the middle troposphere and thepressure falls.The pressure drop then accelerates air parcels toward the low leading to strongconvergence as well as the intensified convection,establishing a positive feedback between theconvection and the latent heat release,which is the thermodynamic mechanism of the developmentand maintenance of the MβCS.  相似文献   

18.
Northwest China （NWC） is a typical arid and semi-arid region. In this study, the main summer climate features over NWC are presented and the performance of an atmospheric general circulation model （NCEP GCM/SSiB） over this region is evaluated. Satellite-derived vegetation products are applied in the model. Based on comparison with observational data and Reanalysis II data, the model generally captures major features of the NWC summer energy balance and circulation. These features include： a high surface tem- perature center dominating the planetary boundary layer; widespread descending motion; an anticyclone （cyclone） located in the lower and middle （upper） troposphere, covering most parts of central NWC; and the precipitation located mainly in the high elevation areas surrounding NWC.
The sensitivity of the summer energy balance and circulation over NWC and surrounding regions to land surface processes is assessed with specified land cover change. In the sensitivity experiment, the degradation over most parts of NWC, except the Taklimakan desert, decreases the surface-absorbed radiation and leads to weaker surface thermal effects. In northern Xinjiang and surrounding regions, less latent heating causes stronger anomalous lower-level anticyclonic circulation and upper-level cyclonic circulation, leading to less summer precipitation and higher surface temperature. Meanwhile, the dry conditions in the Hexi Corridor produce less change in the latent heat flux. The circulation change to the north of this area plays a domi- nant role in indirectly changing lower-level cyclonic conditions, producing more convergence, weaker vertical descending motion, and thus an increase in the precipitation over this region.  相似文献   

19.
In this study, regional persistent haze events(RPHEs) in the Beijing–Tianjin–Hebei(BTH) region were identified based on the Objective Identification Technique for Regional Extreme Events for the period 1980–2013. The formation mechanisms of the severe RPHEs were investigated with focus on the atmospheric circulation and dynamic mechanisms. Results indicated that:(1) 49 RPHEs occurred during the past 34 years.(2) The severe RPHEs could be categorized into two types according to the large-scale circulation, i.e. the zonal westerly airflow(ZWA) type and the high-pressure ridge(HPR) type. When the ZWA-type RPHEs occurred, the BTH region was controlled by near zonal westerly airflow in the mid–upper troposphere.Southwesterly winds prevailed in the lower troposphere, and near-surface wind speeds were only 1–2 ms~(-1). Warm and humid air originating from the northwestern Pacific was transported into the region, where the relative humidity was 70% to 80%, creating favorable moisture conditions. When the HPR-type RPHEs appeared, northwesterly airflow in the mid–upper troposphere controlled the region. Westerly winds prevailed in the lower troposphere and the moisture conditions were relatively weak.(3) Descending motion in the mid-lower troposphere caused by the above two circulation types provided a crucial dynamic mechanism for the formation of the two types of RPHEs. The descending motion contributed to a reduction in the height of the planetary boundary layer(PBL), which generated an inversion in the lower troposphere. This inversion trapped the abundant pollution and moisture in the lower PBL, leading to high concentrations of pollutants.  相似文献   

20.
The dynamic effects of small topography (in the sense of the characteristic height of the topography as compared with the vertical thickness of the system of motion) and the Ekman pumping caused by the frictional convergence in the bounary layer on the motion of a typhoon have been qualitatively discussed in this part based on the governing equation of typhoon motion derived in part I of this paper. The results show that a topographical ridge tends to attract the typhoon approaching it and this explains at least partially the phenomenon that the typhoon over the western Pacific tends to accelerate just before their making land fall over the coastal areas. It is also shown that the Ekman pumping at the top of the boundary layer favors the typhoon acceleration along the local steering current.  相似文献   

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