A study of orographic effects on mountain-generated precipitation systems under weak synoptic forcing

Summary ?Mountains profoundly impact precipitation systems in Taiwan, particularly in areas occupying roughly two-thirds of the island’s landmass. This study examines the terrain structures possibly affecting the formation of rainfall systems in northern Taiwan by analyzing radar data, surface rainfall data, and simulation results from MM5 (Fifth-Generation NCAR/Penn State Mesoscale Model) under a weak synoptic influence condition. More specifically, this study analyzes precipitation systems formed in three different days with different ambient wind directions (i.e., southwesterly, southerly and south-southeasterly flows) in a low Froude number regime in Mei-Yu (or Baiu) season. The southwesterly (southerly) predominant wind was blocked by CMR (central mountain range) over southwestern (southern) Taiwan. Consequently, the southwesterly (southerly) winds were diverted around southern Taiwan, traveled northward following the terrain contour of CMR and then converged in northeastern (northern) Taiwan to produce a NE-SW (N-S) orientated convergence area. As anabatic flow and onshore flow intensified in northern Taiwan and thus enhanced the existing convergence in the late morning and early afternoon, the precipitation system appeared over slope first and then moved down the slope following the predominant wind direction. Upwards motion persisted in this convergence region, and initiated a new precipitation system. Consequently, rainfall accumulation was orientated in a NE-SW (N-S) direction in northern Taiwan. On the windward side of CMR in central Taiwan, precipitation was first produced in the slope by anabatic flow and was generated in lower land because of the interaction between down slope and onshore flow in the late afternoon. When the flow was predominantly from the south-southeast, the convergence due to the splitting of the predominant over western Taiwan became weaken after onshore flow over west coast developed since the direction of onshore flow was against the splitting predominant flow. Precipitation only appeared in the sloping areas of northwestern and central Taiwan in the relatively dry environment resulting from the anabatic flow. Several sensitivity tests indicated that the lee-side convergence in a low Froude number regime superimposed by anabatic flow and onshore flow is important for producing rainfall in northern Taiwan. The prevailing wind direction determined the orientation of the rainfall accumulation in northern Taiwan. The high relative humidity is important for precipitation to form in lower elevations. Received February 9, 2001; Revised November 23, 2001     

海南2008年秋季持续性暴雨过程的物理机制分析

利用地面常规与加密自动站降水观测、卫星云图、NCEP FNL全球分析等资料对2008年10月12—14日海南持续暴雨天气过程进行诊断分析。分析表明:热带低压、冷空气以及副热带高压三者的相互作用,导致海南低空出现持续的东(东南)风急流,该急流在海南东部沿海形成的向岸风辐合及地形强迫抬升是MCS的重要触发机制,其暖湿输送作用是暴雨区不稳定能量得以循环建立的主要途径。南海东北风冷涌前沿强辐合、潜热加热以及高空强辐散有利于热带扰动的发展与维持。热带低压与冷空气相互作用导致大气斜压性增强,地转强迫作用与惯性振荡机制使得低压北侧东北风急流加强。东北与东南急流之间的形变与辐合导致持续锋生过程,锋生强迫产生的热力直接环流上升支与低空急流左前侧次级环流上升支是对流系统发展的主要动力触发机制,低空强潜热加热也有利于对流向更高层次发展。     

Formation and Development of a Mountain-induced Secondary Center inside Typhoon Morakot(2009)

The structural evolution of Typhoon Morakot(2009) during its passage across Taiwan was investigated with the WRF model. When Morakot approached eastern Taiwan, the low-level center was gradually filled by the Central Mountain Range(CMR), while the outer wind had flowed around the northern tip of the CMR and met the southwesterly monsoon to result in a strong confluent flow over the southern Taiwan Strait. When the confluent flow was blocked by the southern CMR, a secondary center(SC) without a warm core formed over southwestern Taiwan. During the northward movement of the SC along the west slope of the CMR, the warm air produced within the wake flow over the northwestern CMR was continuously advected into the SC, contributing to the generation of a warm core inside the SC. Consequently, a well-defined SC with a warm core, closed circulation and almost symmetric structure was produced over central western Taiwan, and then it coupled with Morakot's mid-level center after crossing the CMR to reestablish a new and vertically stacked typhoon. Therefore, the SC inside Morakot was initially generated by a dynamic interaction among the TC's cyclonic wind, southwesterly wind and orographic effects of the CMR, while the thermodynamic process associated with the downslope adiabatic warming effect documented by previous studies supported its development to be a well-defined SC. In summary, the evolution of the SC in this study is not in contradiction with previous studies, but just a complement, especially in the initial formation stage.     

A study on the structure and precipitation of Morakot (2009) induced by the Central Mountain Range of Taiwan

The three-dimensional structures and ingredients leading to extremely heavy precipitation associated with the passage of Typhoon Morakot (2009) over the Central Mountain Range (CMR) of Taiwan are investigated. Using a numerical model, the track, track deflection, characteristic rainbands, and precipitation patterns and maxima are successfully reproduced after verification against observational data. The high-level outward flow of the secondary circulation around the eyewall is not very clear even during Morakot’s strongest stage. In the control case, the eyewall collapses within 5 h after landfall that is closely associated with limited precipitation along the track after landfall. During the early stage of landfall, the deep convection on the windward (west) side of the CMR helps strengthening the secondary circulation. A quantitative comparison of total precipitable water, translation speed, and orographic lifting among 12 typhoons in recent years causing large accumulated rainfall in Taiwan shows that the abundant water vapor around Taiwan outweighs translation speed and orographic lifting in resulting in the record-breaking precipitation. It is found that the major processes leading to strong upward motion in the extremely heavy precipitation during 0000 UTC 8 August–0000 UTC 9 August are initiated by orographic lifting by CMR.     

INTERDECADAL TRANSITIONS AND TWO EXCEPTIONAL YEARS OF JUNE PRECIPITATION OVER SOUTH CHINA

An analysis of high-resolution precipitation data for 1978-2006 indicates that the precipitation over southern China in June experienced a low-value period in 1980-1989 and a high-value period in 1992-2001.It also reveals that exceptional heavy(light) precipitation occurred in June 2005(2004) since 1951.For these variations on both interdecadal and interannual timescales,fairly uniform anomalies of precipitation appeared over Vietnam,southern China,and southeastern China.Corresponding to positive(negative) precipitation anomalies,anomalous southeasterly(northwesterly) flow at 850 hPa reached Vietnam and anomalous southwesterly(northeasterly) flow expanded to the coastal regions of southern and southeastern China.Precedent to the positive(negative) precipitation anomalies during 1992-2001(1980-1989),positive(negative) anomalies of sea surface temperature appeared over the extratropical northwestern Pacific in the winter and spring seasons,associated with a strong(weak) extension of the warm Kuroshio Current that affects the coastal region of eastern China.The above-normal precipitation in June 2005 was associated with the pseudo-ENSO event in the previous winter,and the below-normal precipitation in June 2004 was associated with negative anomalies of sea surface temperature over the equatorial central Pacific and positive anomalies over the equatorial western and eastern Pacific.     

Diagnosis and numerical simulation of a torrential precipitation event in Catalonia (Spain)

Summary A torrential precipitation event occurred in Catalonia (northeastern part of Spain) during 9 and 10 October 1994. More than 400mm were registered in the south of Catalonia. A diagnostic study shows that most of the ingredients to produce heavy rain (large scale upward vertical motion, instability, high moisture content in all the troposphere) were present over the Spanish coast and western Mediterranean. Mesoscale triggering mechanisms have been associated with the orographic forcing, not only through physical lifting of moist air by the coastal mountains, but also by the redistribution of the surface pressure field induced by the Atlas and Pyrenees ranges. A numerical simulation of the event using a meso- model has been performed. The model forecasts qualitatively well the rainfall distribution but underestimates the maximum rainfalls. The effects of the orography and the evaporation from the sea have been also studied. The simulations have shown that the action of the orography is decisive for the rainfall, pressure and wind distrbutions over the Spanish coast and the western Mediterranean. The isolated action of the evaporation turns to be much less important. However the combined effect of orography and evaporation is the most important factor in the areas where the greatest amount of rainfall occurred.With 26 Figures     

THE SENSITIVITY OF THE NUMERICAL SIMULATION TO OROGRAPHY SPECIFICATION IN THE LOWRESOLUTION SPECTRAL MODEL－PART II: IMPACT OF THE SMOOTHED OROGRAPHY AND RIPPLES ON SIMULATIONS

In order to investigate the impact of the smoothed orography and the spurious orographic ripples on simu-lations in the low-resolution spectral model, three different numerical tests, that is, the unsmoothed orography scheme, the smoothed orography scheme and non-ripples scheme are performed. In this paper, the model used by us is the same as Part I except for orographic specification.The results from simulations indicate that, as far as the climatic simulation is concerned, some aspects of the simulated stationary disturbances, zonal and meridional wind, temperature and precipitation in the low-resolu-tion spectral model with properly smoothed mountains are significantly improved, especially in winter hemis-phere.The deep ripples in the model with the unsmoothed orography produce spurious high pressure regions at the surface with subsidence, and suppress rainfall, causing an unrealistic splitting of the precipitation area in northern winter and summer. Removal of tbe deep ripples by using the special procedure for smoothing topog-raphy allows a strong upward motion in the ripple area with heavy rainfall, eliminating the unrelistic split in the precipitation area.     

地形对门头沟一次大暴雨动力作用的数值研究

2002年6月24—25日,北京门头沟附近发生了一次大暴雨过程。为探讨地形在本次过程中的动力作用,采用美国俄克拉荷马大学风暴分析预测中心开发的ARPS模式,对大暴雨过程进行了数值试验。控制试验采用27、9 km双重单向嵌套网格,网格覆盖范围约为3000 km×3000 km、900 km×900 km。两层网格均采用全物理过程,使用的都是全球30″的地形资料。在控制试验的基础上,进行了3组敏感性试验:第1组试验采用干过程模拟,即不考虑凝结潜热的作用;第2组试验将地形整体向东/西平移1°;第3组试验是将门头沟西部的局地地形抠除一部分。试验结果表明,在不考虑凝结潜热作用时,东南风气流仍然可以爬升到2 km以上,超过了大气的抬升凝结高度,证实了地形的动力作用是本次大暴雨的触发机制;将地形向东/西平移1°后,由于大气的对流稳定度发生了改变,模拟的降水强度和落区也发生了变化,表明山坡和山顶的对流不稳定大气是导致本次大暴雨的必要条件;抠除局地地形后,模拟的降水量也发生了不同程度的改变,再次证明大暴雨是在多尺度地形以及一定的天气系统配置下产生的。     

冷空气和热带低压共同作用下的一次暴雨预报失败原因分析

利用NCEP再分析资料、高空、地面观测资料和雷达资料分三个阶段,从水平、垂直方向分析了2014年8月18—20日冷空气和热带低压共同作用下的浙江暴雨过程中水汽输送、垂直运动过程,以及对流不稳定特征,总结预报失败原因。结果表明:(1)第一阶段浙北的暴雨主要是由冷锋引起的,但南海热带低压的存在加强了西南暖湿气流,使得北方冷空气与西南暖湿气流相遇,产生变形场锋生作用。预报偏小的原因主要是没有注意到热带低压的存在和影响,其次是对降水时间的长度也估计不足。(2)第二阶段浙中南的大暴雨,水汽主要是由热带低压东侧和南侧的偏南或西南气流输送的。热带低压在福建登陆后,与冷空气相遇,移动缓慢,导致低层在浙南有中尺度辐合线长时间维持。两个系统耦合,产生了强烈的上升运动,低层又有不稳定能量释放,最终产生了大暴雨。预报偏小的原因是对热带低压的作用估计不足(包括水汽、动力和不稳定能量方面)。(3)第三阶段预报偏大的原因主要是数值预报误差较大。得出的一些启示:预报暴雨时要注意分析降水时间长度,也应重视分析热力条件,以及对数值预报的订正应用。热带系统和中纬度系统相遇,往往会造成大暴雨,主要原因是两个系统耦合会造成强烈的上升运动,同时两个系统对峙会使强降水区长时间维持在同一个地方,热带系统附带的急流又提供了充足的水汽、能量,所以实际雨量往往会比数值预报更大。     

Assessment of the WRF model in reproducing a flash-flood heavy rainfall event over Korea

This study examines the ability of the cloud-resolving weather research and forecasting (WRF) model to reproduce the convective cells associated with the flash-flooding heavy rainfall near Seoul, South Korea, on 12 July 2006. A triply nested WRF model with the highest resolution of 3-km horizontal grid spacing was integrated with conventional analysis data. The WRF model simulated the initiation of isolated thunderstorms, and the formation of a convective band, cloud cluster, and squall line at nearly the right time. The corresponding precipitation simulation was also reasonably reproduced in its distribution, although the amount was underestimated. A sensitivity experiment that excludes the orography over the peninsula revealed that orographic forcing over the peninsula is responsible for about 20% increase in precipitation over the heavy rainfall region. It was identified that in addition to the up-lifting local orographic forcing to the west of the mountain range in South Korea, anticyclonic circulation due to the presence of the Gaema Heights in North Korea contribute to the confinement of convective activities in the heavy rainfall region.     

影响山东的黄淮气旋暴雨落区分析

应用常规观测资料、NCEP再分析资料,对比分析了山东两次春季黄淮气旋暴雨落区异同点。发现春季影响山东的黄淮气旋暴雨区集中出现在气旋中心北侧的偏东风中,且主要位于东北气流中。暴雨区偏北的程度,与影响系统的后倾程度及我国东北地区是否存在高压有关。当系统明显后倾时,锋面坡度小,暖湿气流沿锋面向北爬升的更远,暴雨区更偏北;当我国东北地区存在高压时,其南侧东北气流经渤海侵入850 hPa低涡后部,与低涡前东南气流在风向上渐近辐合,在低涡北侧产生辐合中心,从而产生暴雨区。此外,地面东北风形成的冷垫,有利于南方暖湿气流向北爬升。实际暴雨落区预报中,需综合分析系统的空间结构、周围系统的影响及温度场的配置等。     

A NUMERICAL STUDY ON THE INFLUENCE OF WIND FIELDS AT UPPER LEVELS IN THE MEIYU PERIOD ON THE DEVELOPMENT OF LOW LEVEL JET AND MESOSCALE SYSTEMS

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.     

A Numerical Study of Precipitation Characteristics over Taiwan Island during the Winter Season

Summary  Two-thirds of the land mass of Taiwan island is mountainous, which affects the airflow and precipitation systems over the island. In this study, we discuss the characteristics of precipitation systems when the prevailing wind direction is from the north-east during winter. Observations indicate that rainfall amounts were higher in northeastern Taiwan (the upstream side of the mountains) and that a rainfall shadow occurred in southwestern Taiwan. Simulation results from a non-hydrostatic model indicate that airflow was deflected in eastern Taiwan, while relatively high (low) pressure areas formed in eastern (western) Taiwan. A higher mixing ratio of rainfall occurred over northeastern Taiwan while lighter rainfall occurred in the eastern, and northwestern areas and the southern tip of Taiwan. This was consistent with the observational data except for the southern tip of Taiwan. Uplift due to the topography near the mountainous areas, as well as low level convergence near the coastal areas (due to the deceleration of an easterly wind in northeastern Taiwan), helped form the mixing ratio of rain. Transportation of the mixing ratio of rainfall, due to low level westward flow and upper level eastward flow, caused it to cover a larger area. The mixing ratio of rainfall formed in the upper mountainous areas in northeastern Taiwan if the upstream moisture content was reduced significantly. A temperature inversion at low levels resulted in a decrease in relative humidity and an increase in stability, requiring that the mixing ratio of rainfall should develop closer to the mountainous areas. If a low level wind blew parallel to the orientation of the mountains (NNE-SSW), a higher mixing ratio of rainfall could occur in the mountainous areas of western Taiwan. Received January 30, 1998 Revised February 19, 1999     

Role of a meso-γ vortex in Meiyu torrential rainfall over the Hangzhou Bay,China: An observational study

A mesoscale torrential rainfall event that occurred over eastern China in June 2013 is analyzed by using observational data. The results show that a mesoscale convergence line and a weak convective cloud line formed over the northern part of the Hangzhou Bay during the onset of the torrential rainfall event. A meso-vortex appeared over the confluence point of northeasterly flow associated with the Yellow-Sea high, easterly flow from rainfall area, and southeasterly flow from the Hangzhou Bay. The meso-vortex with a horizontal scale of 10–20 km lasted for about 1 h for stable surface circulations. The analysis of radar retrieval reveals that the meso-vortex in the boundary layer occurred at the south of strong radar echo. The formation of the meso-vortex turned to enhance convergence and cyclonic vorticity in the lower troposphere, which strengthened updrafts that are tilted into convective clouds and caused torrential rainfall. Thus, the occurrence of the meso-vortex in boundary layer is one of the mechanisms that are responsible for the enhancement of convective development.     

2016年秋季中国南方降水异常的环流特征及海温影响

本文利用中国气象台站观测降水、英国Hadley中心海温和NCEP/NCAR再分析数据集等资料,研究了2016年秋季中国南方降水异常偏多的环流特征及其海温影响。结果表明,2016年秋季东亚副热带西风急流偏强,我国南方地区位于急流入口区的右侧,有利于产生上升运动;同时西太平洋副热带高压强度偏强、面积偏大、位置偏北偏西,对应副高西南侧的东南风将热带太平洋的暖湿气流向我国南方输送,有利于降水偏多。另外,2016年秋季登陆我国的台风异常偏多,频繁活动的台风给我国南方带来了大量降水,也是导致我国南方降水异常偏多的原因之一。进一步研究表明,2016年秋季南方降水异常偏多主要与同期赤道西太平洋和东南太平洋海温异常偏高有关,上述海区的海温异常通过激发向下游传播的遥相关波列或通过Gill响应对东亚环流产生影响,进而有利于中国南方降水增多。通过CAM5.3（Community Atmosphere Model Version 5.3）一系列的敏感性试验,验证了上述的结果。     

A NUMERICAL STUDY ON THE INFLUENCE OF WIND FIELDS AT UPPER LEVELS IN THE MEIYU PERIOD ON THE DEVELOPMENT OF LOW LEVEL JET AND MESOSCALE SYSTEMS*

Using a mesoscale model,a numerical study on a heavy rainfall case occurring in the Changjiang-Huaihe River Basin is made in this paper.The influence of the intensity of northeasterly wind in front of the Qinghai-Xizang high at upper level on the low level wind field and development of mesoscale systems as well as heavy rainfall is investigated.The model well reproduced the heavy rainfall process and the weather systems associated.And it indicates that the strong northeasterly flow around the high at upper troposphere will bring about not only the strengthening of low level southeasterly wind,but also the appearance of shear-line and mesoscale vortex at low level.The coupling of northerly wind at upper level and southerly wind at lower level constructs a vertical indirect circulation which is most favourable for the development of convective motions.Its ascending branch in the shear-line area is very strong and shows a pronounced mesoscale characteristic.     

梅雨期高层流场对低层急流及中尺度系统影响的数值试验

用中尺度模式对一次江淮流域暴雨过程进行了数值试验，并研究了对流层高层青藏高压东侧偏北大风的强弱与低层流场及中尺度系统发生及至降水过程的影响。试验不仅较成功地模拟了本次暴雨过程及相应的系统，而且揭示出较强的高空偏北大风将引起对低空急流的加强。而更为重要的是低层切变线的出现以及其上中尺度涡旋的发生。上下风场所构成的垂直反环流圈大大有利于对流的发展，特别是在切变线地区的上升支，带有明显的中尺度特征。     

A comparison of the effects of interannual Arctic sea ice loss and ENSO on winter haze days: Observational analyses and AGCM simulations

This study compares the impacts of interannual Arctic sea ice loss and ENSO events on winter haze days in mainland China through observational analyses and AGCM sensitivity experiments. The results suggest that (1) Arctic sea ice loss favors an increase in haze days in central–eastern China; (2) the impact of ENSO is overall contained within southern China, with increased (reduced) haze days during La Niña (El Niño) winters; and (3) the impacts from sea ice loss and ENSO are linearly additive. Mechanistically, Arctic sea ice loss causes quasi-barotropic positive height anomalies over the region from northern Europe to the Ural Mountains (Urals in brief) and weak and negative height anomalies over the region from central Asia to northeastern Asia. The former favors intensified frequency of the blocking over the regions from northern Europe to the Urals, whereas the latter favors an even air pressure distribution over Siberia, Mongolia, and East Asia. This large-scale circulation pattern favors more frequent occurrence of calm and steady weather in northern China and, as a consequence, increased occurrence of haze days. In comparison, La Niña (El Niño) exerts its influence along a tropical pathway by inducing a cyclonic (anticyclonic) lower-tropospheric atmospheric circulation response over the subtropical northwestern Pacific. The northeasterly (southwesterly) anomaly at the northwestern rear of the cyclone (anticyclone) causes reduced (intensified) rainfall over southeastern China, which favors increased (reduced) occurrence of haze days through the rain-washing effect.     

Different Asian Monsoon Rainfall Responses to Idealized Orography Sensitivity Experiments in the HadGEM3-GA6 and FGOALS-FAMIL Global Climate Models

Recent work has shown the dominance of the Himalaya in supporting the Indian summer monsoon(ISM),perhaps by surface sensible heating along its southern slope and by mechanical blocking acting to separate moist tropical flow from drier midlatitude air.Previous studies have also shown that Indian summer rainfall is largely unaffected in sensitivity experiments that remove only the Tibetan Plateau.However,given the large biases in simulating the monsoon in CMIP5 models,such results may be model dependent.This study investigates the impact of orographic forcing from the Tibetan Plateau,Himalaya and Iranian Plateau on the ISM and East Asian summer monsoon(EASM) in the UK Met Office's Had GEM3-GA6 and China's Institute of Atmospheric Physics FGOALS-FAMIL global climate models.The models chosen feature oppositesigned biases in their simulation of the ISM rainfall and circulation climatology.The changes to ISM and EASM circulation across the sensitivity experiments are similar in both models and consistent with previous studies.However,considerable differences exist in the rainfall responses over India and China,and in the detailed aspects such as onset and retreat dates.In particular,the models show opposing changes in Indian monsoon rainfall when the Himalaya and Tibetan Plateau orography are removed.Our results show that a multi-model approach,as suggested in the forthcoming Global Monsoon Model Intercomparison Project(GMMIP) associated with CMIP6,is needed to clarify the impact of orographic forcing on the Asian monsoon and to fully understand the implications of model systematic error.     

Elucidating the characteristics of eyewalls and rainbands associated with a severe typhoon influenced by Taiwan��s high orography using Doppler radar analysis

Radar observations of the strong Typhoon Bilis (2000) are unique for investigating the effect of Taiwan high orography on the mesoscale structures of storm system in the vicinity of southeastern Taiwan. Typhoon Bilis was the first tropical storm, which possessed the double eyewall feature observed by Doppler radar over the Taiwan area. The inner eyewall exhibited an approximately circular shape with a diameter of 20?km. Convections associated with the storm were cyclonically and radially outward propagated, with the linear aspect in the right flank of the system and counterclockwise and spiral migration in the left flank, maintaining the development of the outer eyewall. The low-level maximum Doppler winds in the left and right flanks relative to the typhoon movement were comparable, owing to a prominent confluence in the left flank. The prominent confluent zone was constructed by two wind fields, the northwesterly from the inner circulation of the typhoon and the outer circulation in the streamline analysis. The replacement of maximum wind between the inner and outer eyewalls, extending from low levels to middle levels in the left flank of the storm, was a clear model for the examination of the significance of the orographic effect on a severe typhoon. A conceptual model for a case of super typhoon under the influence of Taiwan high terrain was constructed.