| Taihang Mountain is a significant mountain range and geographic dividing line in eastern China,which is of great importance for heavy rainfall events in North China.The Taihang Mountain and its adjacent area to the east are located in central and southern North China.There are significant differences in climatic,topography and underlying conditions between Taihang Mountain and its east and northern North China,which causes obviously difference in the characteristics and mechanisms of heavy rainfall.The single long duration heavy rainfall events,defined as greater than or equal to 7h and 20mm h-1,are a typical type of precipitation in warm season?May-September?.Therefore,synoptic situations of heavy rainfall events in which single LDHR events occur are revealed.On this basis,through the analysis of a heavy rainfall event with wide range and long duration occurred in northern China between18 and 21 July 2016?“7.19”event?,the influence of Taihang Mountain on initiation and maintenance of mesoscale convective system?MCS?is systematically studied.Then,the initiation and maintenance of the MCS in a local burst heavy rainfall event occurring in the valley?113.80-114.60°E,37.75-39.25°N?in the north of Taihang Mountain and nearby between 12 and 13 August 2018?“8.12”event?is investigated.These differences in shape and structure of rain bands that make up the MCS are thoroughly explored and related conceptual models are established.Therefore,we can deeply comprehend,under different synoptic situations,the influence of mountains and cold pools at different scales and morphologies in Taihang Mountain and nearby on heavy rainfall.Also,the study further enrichs research of terrain heavy rainfall and provides a more in-depth theoretical foundation and reference basis for operational forecast of heavy rainfall.The main results are as follows:?1?The results show pronounced variability in the frequency and rainfall amount of LDHR events over Taihang Mountain and its adjacent area to the east.Double peaks of accumulated rainfall are found in western?WHRR?and eastern?EHRR?over Taihang Mountain and its east.The diurnal peaks of rainfall amount,frequency and intensity of LDHR in WHRR and EHRR are observed at night or early morning.The rainfall amount shows a double-peak diurnal variation,which is mainly determined by the rainfall intensity in WHRR,and a single-peak variation is determined by the rainfall frequency in EHRR.Four types of heavy rainfall events,in which single LDHR events occur,are classified according to locations.Each type of event correspond to different flow patterns synoptic systems and moisture transports.The first is heavy rainfall in WHRR with an upper-level jet existing in favor of the development of ascent near Taihang Mountain.Topographic blocking of southerly flow is crucial for the heavy rainfall formation.The second describes heavy rainfall that occurs in EHRR because of the favorable configurations of upper and lower-level systems.Heavy rainfall occurs over both WHRR and EHRR in the third.Two different rainfall situations are found.One situation is a result of the topographic blocking and convergence associated with low-pressure systems,which occurs at different times,or zonal shear lines that lead to simultaneous rainfall in WHRR and EHRR.Another situation is due to mesoscale low vortex or shear concerning topographic effects and positive vorticity advection in the fronts of westerly troughs.Rainfall increases from west to east in the north study area.?2?The rainfall lasts for about 32h within the heavy rainfall zone of Taihang Mountain in“7.19”event with the maximum precipitation of 783.4 mm and the strongest hourly precipitation intensity of 139.7mm observed.The event occurs in a typical situation?ie"East High West Low"?that is related to heavy rainfall in summer in North China.The synoptic low-pressure environmental circulation promotes mesocyclone.The topographic effect in addition to shear zone cyclonic circulation is mainly responsible for the heavy rainfall and convection development,where a strong and moist low-level jet is observed.The interaction between the mesocyclone and mesoscale topography produces two strong,separate precipitation centers,which are clearly the direct results of three MCS.The rainfall in the southern precipitation center is mainly a result of MCS1 and MCS2.Orographic blocking plays a crucial role in the development of MCS.Convection cells are continuously initiated when low-level environmental southeasterly flow encountered topographically blocked northerlies and the MCS1-generated cold outflow,which is the main mechanism for the development of MCS1.Diabatic cooling due to raindrop evaporation in association with MCS1 forms a cold pool on plain east of the central Taihang Mountain area.Convective cell initiation and development in MCS2 is due to the impingement of southeasterly flow near the surface of the hills east of Linzhou basin.Convective cells that are successively generated and shifted along a mesoscale shear line comprise the MCS2 linear distribution and a northwestward rain belt.The northern extreme rainfall center is caused by MCS3 along southern slope of an eastward-opening valley V.Convective cells are triggered along a shear line,which is affected by a combination of topography,a cold pool in the boundary layer and mesocyclone outer flow.As an extension of the topography,cold pool plays a significant effect on MCS3.?3?The“8.12”event occurring in the valley in the north of Taihang Mountain and nearby with unexpected characteristics,more concentrated heavy precipitation and strong local rainfall intensities is mainly caused by a MCS with deep convective cloud clusters?TBB?-60?,echo?50dBZ?.The MCS displays complex shape and structure and goes through linear stage,circularity stage and dumbbell stage successively in 7h.However,the heavy rainfall is generated in the last two stages.It is worth mentioning that the organization of MCS is extremely rare in other regions.Heavy rainfall occurs at the eastern of the area influenced by the favorable configurations of upper and lower-level systems in westerly belt.The southeasterly on the west side of the Subtropical High transports a large amount of water and heat to the valley and nearby.So,convection is easily initiated in the environmental conditions.During linear stage,convection cells are continuously initiated due to upslope lifting occurring in high altitude mountain and the lateral flow confluence induced by deflected flows over isolated hill.Diabatic cooling due to raindrop evaporation in association with the northwest MCS forms a deep cold pool.The outflow from cold pool in all directions is crucial for the formation of circularity stage.Rainbands in east MCS,which are located in the valley,are initiated by lifting easterly flow over the mesoscale outflow boundary to the east of the cold pool.However,rainbands in west MCS,which are on the west side of the valley,are initiated by lifting northerly flow over hills and the mesoscale outflow boundary to the west of the cold pool.Location and intensity of rainbands in west MCS dramatic changed under the influence of trend,altitude,and relative position of isolated hills.During the dumbbell stage,the north outflow caused by cold pool enhances upslope lifting of Taihang Mountain,which becomes a push to increase rainfall in south MCS.Because of blocking of the protruding hill on the northeast side of the valley and cold zone to the west side,the heavy rainfall in north MCS remain in relatively stable region.On the basis of these results we concluded that development of the MCS is limited by terrain in the valley and nearby,which is essential for heavy rainfall.?4?During the circularity stage,the evolution of external shape of the rainband in east MCS in“8.12”event,which can be divided into east rainband and north rainband,is mainly related to cold pool,prevailing airflow,and environmental conditions.The north rainband is characterized with a leading bowing rainband and shows a perfect structure,which is attributed to lifting northeasterly flows over a mesoscale outflow boundary.Furthermore,the north rainband has a large angle with ambient flow.The outflow and ambient flow make north rainband move northeast.The external shape of north rainband is related to the maintenance of cold pool,the merger with rainbands and blocking of Taihang Moutatin.Finally,the north rainband gradually weakens with CAPE decreasing on the moving path.However,the linear east rainband has a small angle with ambient flow,which is favourable for the northeast extension.The speed of cold out flow is comparable to that of the near surface southeasterly from plain,resulting in formation of the quasi-stationary boundary.The unevenly distributed convergence is the decisive factor for scattered structure in east rainband.Convection is inhibited by the dry and cold in mid-high level on the east of east rainband and is initiated in the environmental air with large CAPE on the northeast of east rainband.In addition,because of the reduction of CAPE,The southeastern part in east rainband gradually approaches windward slope of Taihang Moutain.?5?Further research reveals that there are significant differences in internal structure of two rainbands.The north rainband exhibits a convective nature with a sharp horizontal gradient of reflectivity and a significant vertical extent.The front-to-rear flow appears to be lifted upward at and immediately ahead of the leading edge of the low-level rear-to-front flow to form rearward-tilting updrafts.These airflow patterns are similar to those of the convective region of squall lines.The detailed analyses of surface fluctuations during the passage of the north rainband further show an abrupt pressure rise,a temperature drop,and a pronounced deceleration of inflow air coincident with the leading heavy precipitation.In addition to evaporative cooling and water loading effects,the nonhydrostatic pressure generated by dynamic forcings?i.e.,Bernoulli effect?also be important for the north band,which can help explain the significant pressure jump across the leading heaviest precipitation.Furthermore,the proportion of nonhydrostatic pressure in pressure is continues to increase.Based on the evaluation,it is suggested that convectively generated cold pool would be also closely related to the north rainband to strengthen first and then weaken.Moreover,reintensification of north rainband is determined by the blocking of tropography.During early stage,the east rainband has a suitable angle with ambient flow,which causes rainband to tilt outward.Due to transport of low-?e ari originating from higher altitudes by convectively induced entrainment effect,the rainband tilt more outward.As time goes on,the east rainband,undergo a prominent transformation from organized linear rainband to the rainband similar to structure of the MCSs with multiple mesoscale rainbands.New convection is repeatedly triggered in southeasterly flow near a quasi-stationary boundary,resulting in the formation of northeastward“echo training”of convective cells.As the“echo training”of convective cells move northeastward with southwestly flow,“band training”gradually formed.However,the two organizational modes differ in their spatial scales and movement directions.The repeated convective back-building occurring on“echo training”and the“train effect”determined by“echo training”and“band training”account for extreme precipitation and longevity of east rainband. |
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