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Convective Initiation by Topographically Induced Convergence Forcing over the Dabie Mountains on 24 June 2010


doi: 10.1007/s00376-016-6024-z

  • The initiation of convective cells in the late morning of 24 June 2010 along the eastward extending ridge of the Dabie Mountains in the Anhui region, China, is studied through numerical simulations that include local data assimilation. A primary convergence line is found over the ridge of the Dabie Mountains, and along the ridge line several locally enhanced convergence centers preferentially initiate convection. Three processes responsible for creating the overall convergence pattern are identified. First, thermally-driven upslope winds induce convergence zones over the main mountain peaks along the ridge, which are shifted slightly downwind in location by the moderate low-level easterly flow found on the north side of a Mei-yu front. Second, flows around the main mountain peaks along the ridge create further convergence on the lee side of the peaks. Third, upslope winds develop along the roughly north-south oriented valleys on both sides of the ridge due to thermal and dynamic channeling effects, and create additional convergence between the peaks along the ridge. The superposition of the above convergence features creates the primary convergence line along the ridge line of the Dabie Mountains. Locally enhanced convergence centers on the primary line cause the initiation of the first convection cells along the ridge. These conclusions are supported by two sensitivity experiments in which the environmental wind (dynamic forcing) or radiative and land surface thermal forcing are removed, respectively. Overall, the thermal forcing effects are stronger than dynamic forcing given the relatively weak environmental flow.
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  • Bao X. H., F. Q. Zhang, and J. H. Sun, 2011: Diurnal variations of warm-season precipitation east of the Tibetan Plateau over China. Mon. Wea. Rev., 139, 2790- 2810.10.1175/MWR-D-11-00006.11b99e9cda4d909ca3dfe3fc1fa5e6c96http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2011MWRv..139.2790Bhttp://adsabs.harvard.edu/abs/2011MWRv..139.2790BNot Available
    Bennett, L. J., Coauthors, 2011: Initiation of convection over the Black Forest mountains during COPS IOP15a. Quart. J. Roy. Meteor. Soc., 137, 176- 189.10.1002/qj.760088e027e29fa24a1b87aa39d97486d52http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1002%2Fqj.760%2Ffullhttp://onlinelibrary.wiley.com/doi/10.1002/qj.760/fullAbstract Top of page Abstract 1.Introduction 2.Details of the experiment 3.Model results 4.Summary and conclusions Acknowledgements References Doppler-On-Wheels radar observations made during the Convective and Orographically-induced Precipitation Study (COPS) on 12 August 2007 showed that precipitating clouds only developed between the north outh orientated Murg and Nagold Valleys of the northern Black Forest. The clouds produced moderate precipitation. The Weather Research and Forecasting (WRF) model run at 700 m horizontal resolution in the inner domain was able to simulate the location of the precipitation. Insight is therefore gained into the physical mechanisms responsible for the initiation and development of the convection. Convergence lines resulting from thermally driven flows constrained the initial location of the convection within warm and moist cores produced by heating on elevated surfaces. The heaviest precipitation was later produced by secondary convection, which was initiated within the cores at the boundary between cold-pool outflows and thermal flows. Copyright 2011 Royal Meteorological Society
    Bosart L. F., 2003: Whither the weather analysis and forecasting process? Wea.Forecasting, 18, 520- 529.10.1175/1520-0434(2003)182.0.CO;2f205df0305c68fcb8870fe8e5851aef6http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2003WtFor..18..520Bhttp://adsabs.harvard.edu/abs/2003WtFor..18..520BAbstract An argument is made that if human forecasters are to continue to maintain a skill advantage over steadily improving model and guidance forecasts, then ways have to be found to prevent the deterioration of forecaster skills through disuse. The argument is extended to suggest that the absence of real-time, high quality mesoscale surface analyses is a significant roadblock to forecaster ability to detect, track, diagnose, and predict important mesoscale circulation features associated with a rich variety of weather of interest to the general public.
    Chen S.-J., Y.-H. Kuo, W. Wang, Z.-Y. Tao, and B. Cui, 1998: A modeling case study of heavy rainstorms along the Mei-yu front. Mon. Wea. Rev., 126, 2330- 2351.10.1175/1520-0493(1998)126<2330:AMCSOH>2.0.CO;2277897a823bd6cb321ba17de71bfa296http%3A%2F%2Fci.nii.ac.jp%2Fnaid%2F10013125854%2Fhttp://ci.nii.ac.jp/naid/10013125854/A modeling case study of heavy rain storm along the Mei-Yu front CHEN S. -J. Mon. Wea. Rev. 126, 2330-2351, 1998
    Chow F. K., S. F. J. De Wekker, and B. J. Snyder, 2013: Mountain Weather Research and Forecasting: Recent Progress and Current Challenges. Springer,750 pp.10.1007/978-94-007-4098-3d85f1f3cb21556cc894c680e88264f21http%3A%2F%2Fwww.springer.com%2F978-94-007-4097-6http://www.springer.com/978-94-007-4097-6Recent Progress and Current Challenges
    Crook N. A., D. F. Tucker, 2005: Flow over heated terrain. Part I: Linear theory and idealized numerical simulations. Mon. Wea. Rev., 133, 2552- 2564.c063d061a412f1f1abed0d07db3d6e35http%3A%2F%2Fadsabs.harvard.edu%2Fcgi-bin%2Fnph-data_query%3Fbibcode%3D2005MWRv..133.2552C%26db_key%3DPHY%26link_type%3DEJOURNALhttp://xueshu.baidu.com/s?wd=paperuri%3A%28d8ddd90546c71d5f0a1e44b93664ca0c%29&filter=sc_long_sign&tn=SE_xueshusource_2kduw22v&sc_vurl=http%3A%2F%2Fadsabs.harvard.edu%2Fcgi-bin%2Fnph-data_query%3Fbibcode%3D2005MWRv..133.2552C%26db_key%3DPHY%26link_type%3DEJOURNAL&ie=utf-8&sc_us=9635967640261028722
    Demko J. C., B. Geerts, 2010: A numerical study of the evolving convective boundary layer and orographic circulation around the Santa Catalina mountains in Arizona. Part II: Interaction with deep convection. Mon. Wea. Rev., 138, 3603- 3622.10.1175/2010MWR3318.10f93711dae029958fb511af2213df66ahttp%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2010mwrv..138.3603dhttp://adsabs.harvard.edu/abs/2010mwrv..138.3603dAbstract This is the second part of a study that examines the daytime evolution of the thermally forced boundary layer (BL) circulation over a relatively isolated mountain, about 30 km in diameter and 2 km high, and its interaction with locally initiated deep convection by means of numerical simulations validated with data collected in the 2006 Cumulus Photogrammetric, In Situ, and Doppler Observations (CuPIDO) field campaign in southeastern Arizona. Part I examined the BL circulation in cases with, at most, rather shallow orographic cumulus (Cu) convection; the present part addresses deep convection. The results are based on output from version 3 of the Weather Research and Forecasting model run at a horizontal resolution of 1 km. The model output verifies well against CuPIDO observations. In the absence of Cu convection, the thermally forced (solenoidal) circulation is largely contained within the BL over the mountain. Thunderstorm development deepens this BL circulation with inflow over the depth of th...
    Ding Y. H., 1992: Summer monsoon rainfalls in China. J. Meteor. Soc.Japan, 70, 337- 396.cce449cfd51cb2024f70cfc3af8e54echttp%3A%2F%2Fci.nii.ac.jp%2Fnaid%2F10013125892%2Fhttp://ci.nii.ac.jp/naid/10013125892/Summer monsoon rainfalls in China DING Y.-H. J. Meteor. Soc. Japan 70, 337-396, 1992
    Ding Y.-H., J.-J. Liu, Y. Sun, Y.-J. Liu, J.-H. He, and Y.-F. Song, 2007: A study of the synoptic-climatology of the Meiyu system in East Asia. Chinese J. Atmos. Sci., 31, 1082- 1101. (in Chinese)10.1002/jrs.1570fd65846059f31333001265d3e45dc264http%3A%2F%2Fen.cnki.com.cn%2FArticle_en%2FCJFDTOTAL-DQXK200706006.htmhttp://en.cnki.com.cn/Article_en/CJFDTOTAL-DQXK200706006.htmBy using NCEP reanalysis datasets and daily surface observations at 740 stations in China,the climatological aspect of the East Asian rainy season(Meiyu) has been studied.The results have shown that this unique East Asian rainy season in early summer in the Yangtze River basin has an average duration of 21 days,starting from June 17 and ending on July 8.Its concentrated and intensive rainfall of 200-300 mm accounts for about 45% of total summer rainfall amount.The East Asian rainy season occurs when the East Asian summer monsoon abruptly jumps northward from South China to the Yangtze River basin in mid-June and at the same time the Indian summer monsoon sets in over the Indian Peninsula.The moisture supply coming from the South China Sea and the Bay of Bengal is greatly enhanced,thus providing a very favorable condition for Meiyu rainfall.The structure of the climatological Meiyu front is characterized by strong convective precipitation embeded in nearly zonal wide cloud zone,highly moist air column ahead of the Meiyu front of high 胃se,relatively lower temperature field with very weak or nearly dissipating horizontal temperature gradient across the Meiyu front at low level,strong low-level jet to the south of the Meiyu zone and wind shear line accompanying the Meiyu front,and strong upward motion in the Meiyu zone,with a monsoon type or indirect vertical circulation dominating the region equatorward from the Meiyu zone.In many ways,the Meiyu front in East China is quite different from the Baiu front in Japan and the Changma front in Korea,which are basically of mid-latitude baroclinic frontal zone.The East Asian summer monsoon has a more significant effect on the Meiyu in China than the Baiu and Changma.A comprehensive intercomparison of regional differences of the Meiyu system has been made.
    Fu S. M., F. Yu, D. H. Wang, and R. D. Xia, 2012: A comparison of two kinds of eastward-moving mesoscale vortices during the mei-yu period of 2010. Science China Earth Sciences, 56, 282- 300.10.1007/s11430-012-4420-51e877d13cfb8d8240fd210d975791d9chttp%3A%2F%2Fwww.cnki.com.cn%2FArticle%2FCJFDTotal-JDXG201302011.htmhttp://www.cnki.com.cn/Article/CJFDTotal-JDXG201302011.htmDuring the mei-yu period,the east edge of the Tibetan Plateau and the Dabie Mountain are two main sources of eastward-moving mesoscale vortices along the mei-yu front(MYF).In this study,an eastward-moving southwest vortex(SWV) and an eastward-moving Dabie vortex(DBV) during the mei-yu period of 2010 have been investigated to clarify the main similarities and differences between them.The synoptic analyses reveal that the SWV and DBV were both located at the lower troposphere;however,the SWV developed in a "from top down" trend,whereas the DBV developed in an opposite way.There were obvious surface closed low centers corresponding to the DBV during its life span,whereas for the SWV,the closed low center only appeared at the mature stage.Cold and warm air intersected intensely after the formation of both the vortices,and the cold advection in the SWV case was stronger than that in the DBV case,whereas the warm advection in the DBV case was more intense than that in the SWV case.The Bay of Bengal and the South China Sea were main moisture sources for the SWV,whereas for the DBV,in addition to the above two moisture sources,the East China Sea was also an important moisture source.The vorticity budget indicates that the convergence was the most important common factor conducive to the formation,development,and maintenance of the SWV and DBV,whereas the conversion from the vertical vorticity to the horizontal one(tilting) was the most important common factor caused the dissipation of both of the vortices.The kinetic energy(KE) budget reveals that the KE generation by the rotational wind was the dominant factor for the enhancement of KE associated with the SWV,whereas for the DBV,the KE transport by the rotational wind was more important than the KE generation.The KE associated with the SWV and the DBV weakened with different mechanisms during the decaying stage.Furthermore,the characteristics of baroclinic and barotropic energy conversions during the life spans of both vortices indicate that the SWV and DBV both belong to the kind of subtropical mesoscale vortices.
    Gao J. D., M. Xue, K. Brewster, and K. K. Droegemeier, 2004: A three-dimensional variational data analysis method with recursive filter for Doppler radars. J. Atmos. Oceanic Technol., 21, 457- 469.e485d4c3dc07366bc3f77e8a55f22a67http%3A%2F%2Fadsabs.harvard.edu%2Fcgi-bin%2Fnph-data_query%3Fbibcode%3D2004JAtOT..21..457G%26db_key%3DPHY%26link_type%3DABSTRACThttp://xueshu.baidu.com/s?wd=paperuri%3A%28bea7369f63db7495437d458da988726a%29&filter=sc_long_sign&tn=SE_xueshusource_2kduw22v&sc_vurl=http%3A%2F%2Fadsabs.harvard.edu%2Fcgi-bin%2Fnph-data_query%3Fbibcode%3D2004JAtOT..21..457G%26db_key%3DPHY%26link_type%3DABSTRACT&ie=utf-8&sc_us=13188604239151258641
    Guo R., C. S. Miao, and N. Zhang, 2013: Sensitivity experiments of effects of Dabie mountains terrain on Meiyu front rainstorm over Huaihe River basin. Transactions of Atmospheric Sciences, 36, 626- 634. (in Chinese)
    Hagen M., J. van Baelen, and E. Richard, 2011: Influence of the wind profile on the initiation of convection in mountainous terrain. Quart. J. Roy. Meteor. Soc., 137, 224- 235.10.1002/qj.78427228b121548f50bce26a956c9a0e2f5http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1002%2Fqj.784%2Ffullhttp://onlinelibrary.wiley.com/doi/10.1002/qj.784/fullAbstract Top of page Abstract 1.Introduction 2.The COPS field campaign 3.Observations of isolated cells 4.Results 5.Discussion and conclusions Acknowledgements References A number of days with small precipitating convective cells were investigated using weather radars during the COPS (Convective and Orographically-induced Precipitation Study) field campaign in the region of the Vosges and the Rhine Valley in Central Europe. Depending on the weather situation, two distinct mechanisms could be identified for the initiation of convection. On some days, cells were initiated over the ridge of the Vosges, whereas on other days cells were initiated in the lee of the Vosges. The initiation of convection appeared to be concentrated in a few favourable locations. Using the Froude number, it was possible to describe the two distinct mechanisms. When the Froude number was low, the flow was diverted around the Vosges and thermally driven convergence at the ridge initiated convection, whereas when the Froude number was high, the flow passed through mountain gaps and then converged on the lee side with the flow in the Rhine Valley. The convergence on the lee side was enhanced at locations where the outflows through valleys converged. Low Froude numbers were accompanied by weak winds varying with height, whereas high Froude numbers were observed during situations with stronger southwesterly winds increasing with height. Copyright 2011 Royal Meteorological Society
    Horel J., Coauthors, 2002: Mesowest: Cooperative mesonets in the western United States. Bull. Amer. Meteor. Soc., 83, 211- 226.10.1175/1520-0477(2002)0832.3.CO;2aaf84a3b937ac23c3025884745a7ff2ehttp%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2002BAMS...83..211Hhttp://adsabs.harvard.edu/abs/2002BAMS...83..211HAbstract Meteorological data from over 2800 automated environmental monitoring stations in the western United States are collected, processed, archived, integrated, and disseminated as part of the MesoWest program. MesoWest depends upon voluntary access to provisional observations from environmental monitoring stations installed and maintained byfederal, state, and local agencies and commercial firms. In many cases, collection and transmission of these observations are facilitated by NWS forecast offices, government laboratories, and universities. MesoWest augments the Automated Surface Observing System (ASOS) network maintained by the NWS, Federal Aviation Administration, and Department of Defense. MesoWest increases the coverage of observations in remote locations and helps capture many of the localand mesoscale weather phenomena that impact the public. The primary goal of MesoWest is to improve timely access to automated observations for NWS forecasters at offices throughout the western United States. ...
    Houze R. A., Jr., 2012: Orographic effects on precipitating clouds. Rev. Geophys.,50, RG1001.10.1029/2011RG0003652d5a36d8b60acb5514b86f811ef8ca53http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F2011RG000365%2Ffullhttp://onlinelibrary.wiley.com/doi/10.1029/2011RG000365/fullPrecipitation over and near mountains is not caused by topography but, rather, occurs when storms of a type that can occur anywhere (deep convection, fronts, tropical cyclones) form near or move over complex terrain. Deep convective systems occurring near mountains are affected by channeling of airflow near mountains, capping of moist boundary layers by flow subsiding from higher terrain, and triggering to break the cap when low-level flow encounters hills near the bases of major mountain ranges. Mesoscale convective systems are triggered by nocturnal downslope flows and by diurnally triggered disturbances propagating away from mountain ranges. The stratiform regions of mesoscale convective systems are enhanced by upslope flow when they move over mountains. In frontal cloud systems, the poleward flow of warm-sector air ahead of the system may rise easily over terrain, and a maximum of precipitating cloud occurs over the first rise of terrain, and rainfall is maximum on ridges and minimum in valleys. If the low-level air ahead of the system is stable, blocking or damming occurs. Shear between a blocked layer and unblocked moist air above favors turbulent overturning, which can accelerate precipitation fallout. In tropical cyclones, the tangential winds encountering a mountain range produce a gravity wave response and greatly enhanced upslope flow. Depending on the height of the mountain, the maximum rain may occur on either the windward or leeward side. When the capped boundary layer of the eye of a tropical cyclone passes over a mountain, the cap may be broken with intense convection resulting.
    Kain, J. S., Coauthors, 2013: A feasibility study for probabilistic convection initiation forecasts based on explicit numerical guidance. Bull. Amer. Meteor. Soc., 94, 1213- 1225.10.1175/BAMS-D-11-00264.1229287dd5dc7d7a777ce1f8080625911http%3A%2F%2Fonlinelibrary.wiley.com%2Fresolve%2Freference%2FXREF%3Fid%3D10.1175%2FBAMS-D-11-00264.1http://onlinelibrary.wiley.com/resolve/reference/XREF?id=10.1175/BAMS-D-11-00264.1The 2011 Spring Forecasting Experiment in the NOAA Hazardous Weather Testbed (HWT) featured a significant component on convection initiation (CI). As in previous HWT experiments, the CI study was a collaborative effort between forecasters and researchers, with equal emphasis on experimental forecasting strategies and evaluation of prototype model guidance products. The overarching goal of the CI effort was to identify the primary challenges of the CI forecasting problem and to establish a framework for additional studies and possible routine forecasting of CI. This study confirms that convection-allowing models with grid spacing ~4 km represent many aspects of the formation and development of deep convection clouds explicitly and with predictive utility. Further, it shows that automated algorithms can skillfully identify the CI process during model integration. However, it also reveals that automated detection of individual convection cells, by itself, provides inadequate guidance for the disruptive poten...
    Luo Y. L., H. Wang, R. H. Zhang, W. M. Qian, and Z. Z. Luo, 2013: Comparison of rainfall characteristics and convective properties of monsoon precipitation systems over south China and the Yangtze and Huai River basin. J.Climate, 26, 110- 132.10.1175/JCLI-D-12-00100.117935b62-5319-4556-aab1-6a3be0f97dbe7706938239b9c8b56b1e8c89e7ca376dhttp%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2013JCli...26..110Lrefpaperuri:(24c8edaaa02d048e8bbd8d9e0698d6f3)http://adsabs.harvard.edu/abs/2013JCli...26..110LNot Available
    Miao C. S., W. X. Liu, J. H. Wang, M. Wu, and T. Li, 2014: Comparison between two rainstorm meso-scale vortices circumambulated respectively from two sides of Mountain Dabieshan during Meiyu season. Plateau Meteorology, 33, 394- 406. (in Chinese)b86496c32bee94c6b94496827d266d1ehttp%3A%2F%2Fen.cnki.com.cn%2FArticle_en%2FCJFDTotal-GYQX201402010.htmhttp://en.cnki.com.cn/Article_en/CJFDTotal-GYQX201402010.htmA statistic analysis of rainstorm vortices during Meiyu seasons from 2007 to 2011 at Yangtze uaihe River basin shows that the low vortex rainstorm is up to 41%in all rainstorm day during the statistical period,and most of the rainstorm vortices are shallow vortices(under 700 hPa),so they are easy to be impacted by the Dabieshan mountain(about 1500 m height).Two rainstorm vortices are analyzed when they circumambulate and climb the Dabieshan mountain from south along Yangtze River Basin and from north along Huaihe River Basin,respectively under both influences of mountain and steering current of trough foreside at high level.The shallow vortices go round the mountain from two sides with north stronger than south one,and cause their rain zone positions difference along Huaihe River basin and along Yangtze River basin respectively.Meantime the wind shear structure of high level jet and the low level jet together increase the cyclone vorticity of the shallow vortices and indicate the eastward paths and positions of low vortices.Meanwhile,the low level jet also responses to the mountain hindering,the weak low level jet is weaken and shows a circumambulated state to the mountain by blocking of the south part of Mountain Dabieshan.The circulation state causes a weak growing of both vortex and rainstorm intensities at south end than strong growing at north end of Mountain Dabieshan.The distribution of the moisture potential vorticity of vortex rainstorm shows that at vertical-longitudinal cross-section the baroclinic and barotropy gradient areas overlap each other and make a strong rainfall environment,the moisture potential vorticity is proportional to the intensity of rainfall,and the Huaihe River Basin vortex has stronger moisture potential vorticity as there is stronger vertical wind shear at north part of Mountain Dabieshan.The numerical simulation shows that the Dabieshan mountain topography remarkably impacts on the low vortex circumambulated paths from south or north,the low vortex intensity increases at leeward or decrease at windward,and moisture flux convergence strong or weak with vortex intensity.The topography effect causes the center position of low vortex,moisture convergence and rainfall easily close or overlap at two key areas(face-wind side of the mountain south part and the lee-ward arc area of mountain north part),where the air vertical stretched,moisture convergence strengthen and the rainfall of the vortex rainstorm are special heavy.Due to the south part of the mountain is larger and higher than the north part,the topography impact on the south vortex along Yangtze River Basin is more obvious.
    Rotunno R., R. A. Houze, 2007: Lessons on orographic precipitation from the Mesoscale Alpine Programme. Quart. J. Roy. Meteor. Soc., 133, 811- 830.10.1002/qj.6776f09f714bcf6d72087e1b7888be577bhttp%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1002%2Fqj.67%2Ffullhttp://onlinelibrary.wiley.com/doi/10.1002/qj.67/fullAbstract Although moisture-laden airflow towards a mountain is a necessary ingredient, the results from the Mesoscale Alpine Programme (MAP) demonstrate that detailed knowledge of the orographically modified flow is crucial for predicting the intensity, location and duration of orographic precipitation. Understanding the orographically modified flow as it occurs in the Alps is difficult since it depends on the static stability of the flow at low levels, which is heavily influenced by synoptic conditions, the complex effects of latent heating, and the mountain shape, which has important and complicated variations on scales ranging from a few to hundreds of kilometres. Central themes in all of the precipitation-related MAP studies are the ways in which the complex Alpine orography influences the moist, stratified airflow to produce the observed precipitation patterns, by determining the location and rate of upward air motion and triggering fine-scale motions and microphysical processes that locally enhance the growth and fallout of precipitation. In this paper we review the major findings from the MAP observations and describe some new research directions that have been stimulated by MAP results. Copyright 漏 2007 Royal Meteorological Society
    Schmidli J., R. Rotunno, 2010: Mechanisms of along-valley winds and heat exchange over mountainous terrain. J. Atmos. Sci., 67, 3033- 3047.10.1175/2010JAS3473.1f7adc6e1addad072dc95f51e29ee377chttp%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2010JAtS...67.3033Shttp://adsabs.harvard.edu/abs/2010JAtS...67.3033SThe physical mechanisms leading to the formation of diurnal along-valley winds are investigated over idealized three-dimensional topography. The topography used in this study consists of a valley with a horizontal floor enclosed by two isolated mountain ridges on a horizontal plain. A diagnostic equation for the along-valley pressure gradient is developed and used in combination with numerical model simulations to clarify the relative role of various forcing mechanisms such as the valley volume effect, subsidence heating, and surface sensible heat flux effects. The full diurnal cycle is simulated using comprehensive model physics including radiation transfer, land surface processes, and dynamic surface-atmosphere interactions. The authors find that the basic assumption of the valley volume argument of no heat exchange with the free atmosphere seldom holds. Typically, advective and turbulent heat transport reduce the heating of the valley during the day and the cooling of the valley during the night. In addition, dynamically induced valley-plain contrasts in the surface sensible heat flux can play an important role. Nevertheless, the present analysis confirms the importance of the valley volume effect for the formation of the diurnal along-valley winds but also clarifies the role of subsidence heating and the limitations of the valley volume effect argument. In summary, the analysis brings together different ideas of the valley wind into a unified picture.
    Soderholm B., B. Ronalds, and D. J. Kirshbaum, 2014: The evolution of convective storms initiated by an isolated mountain ridge. Mon. Wea. Rev., 142, 1430- 1451.10.1175/MWR-D-13-00280.1fb8f3e49e8a0cdb44630ef0e19cf354bhttp%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2014MWRv..142.1430Shttp://adsabs.harvard.edu/abs/2014MWRv..142.1430SNot Available
    Sun J. H., F. Q. Zhang, 2012: Impacts of mountain-plains solenoid on diurnal variations of rainfalls along the Mei-yu front over the East China Plains. Mon. Wea. Rev., 140, 379- 397.10.1175/MWR-D-11-00041.169a9a6de3f24d388ec880b69ef800abdhttp%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2012MWRv..140..379Shttp://adsabs.harvard.edu/abs/2012MWRv..140..379SNot Available
    Sun J. H., X. L. Zhang, L. L. Qi, and S. X. Zhao, 2005: An analysis of a meso-尾 system in a mei-yu front using the intensive observation data during CHeRES 2002. Adv. Atmos. Sci.,22, 278-289, doi: 10.1007/BF02918517.10.1007/BF029185176718f9b4cae838773dd36ce1757a514ahttp%3A%2F%2Fd.wanfangdata.com.cn%2FPeriodical_dqkxjz-e200502012.aspxhttp://d.wanfangdata.com.cn/Periodical_dqkxjz-e200502012.aspxThe conventional and intensive observational data of the China Heavy Rain Experiment and Study (CHeRES) are used to specially analyze the heavy rainfall process in the mei-yu front that occurred during 20-21 June 2002, focusing on the meso-β system. A mesoscale convective system (MCS) formed in the warm-moist southwesterly to the south of the shear line over the Dabie Mountains and over the gorge between the Dabie and Jiuhua Mountains. The mei-yu front and shear line provide a favorable synoptic condition for the development of convection. The GPS observation indicates that the precipitable water increased obviously about 2 3 h earlier than the occurrence of rainfall and decreased after that. The abundant moisture transportation by southwesterly wind was favorable to the maintenance of convective instability and the accumulation of convective available potential energy (CAPE). Radar detection reveals that meso-β and -γ systems were very active in the Mα CS. Several convection lines developed during the evolution of the MαCS, and these are associated with surface convergence lines. The boundary outflow of the convection line may have triggered another convection line. The convection line moved with the mesoscale surface convergence line, but the convective cells embedded in the convergence line propagated along the line. On the basis of the analyses of the intensive observation data, a multi-scale conceptual model of heavy rainfall in the mei-yu front for this particular case is proposed.
    Tao S. Y., 1980: Heavy Rain in China. Science Press,225 pp.
    Trier S. B., G. S. Romine, D. A. Ahijevych, R. J. Trapp, R. S. Schumacher, M. C. Coniglio, and D. J. Stensrud, 2015: Mesoscale thermodynamic influences on convection initiation near a surface dryline in a convection-permitting ensemble. Mon. Wea. Rev., 143, 3726- 3753.10.1175/MWR-D-15-0133.1a5881efd9c2d795d8a989cd7dccc7948http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2015MWRv..143.3726Thttp://adsabs.harvard.edu/abs/2015MWRv..143.3726TAbstract In this study, the authors examine initiation of severe convection along a daytime surface dryline in a 10-member ensemble of convection-permitting simulations. Results indicate that the minimum buoyancy B min of PBL air parcels must be small ( B min > 鈭0.5掳C) for successful deep convection initiation (CI) to occur along the dryline. Comparing different ensemble members reveals that CAPE magnitudes (allowing for entrainment) and the width of the zone of negligible B min extending eastward from the dryline act together to influence CI. Since PBL updrafts that initiate along the dryline move rapidly northeast in the vertically sheared flow as they grow into the free troposphere, a wider zone of negligible B min helps ensure adequate time for incipient storms to mature, which, itself, is hastened by larger CAPE. Local B min budget calculations and trajectory analysis are used to quantify physical processes responsible for the reduction of negative buoyancy prior to CI. Here, the grid-resolved forcing and forcing from temperature and moisture tendencies in the PBL scheme (arising from surface fluxes) contribute about equally in ensemble composites. However, greater spatial variability in grid-resolved forcing focuses the location of the greatest net forcing along the dryline. The grid-resolved forcing is influenced by a thermally direct vertical circulation, where time-averaged ascent at the east edge of the dryline results in locally deeper moisture and cooler conditions near the PBL top. Horizontal temperature advection spreads the cooler air eastward above higher equivalent potential temperature air at source levels of convecting air parcels, resulting in a wider zone of negligible B min that facilitates sustained CI.
    Tucker D. F., N. A. Crook, 2005: Flow over heated terrain. Part II: Generation of convective precipitation. Mon. Wea. Rev., 133, 2565- 2582.10.1175/MWR2965.1e913cd83d3d70c89f3fd1b11d06e8526http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2005MWRv..133.2565Thttp://adsabs.harvard.edu/abs/2005MWRv..133.2565TAbstract Previous studies have shown that thunderstorms in the Rocky Mountain region have preferred areas in which to form. There has been some indication that these areas depend on the midtropospheric wind direction. A nonhydrostatic model with a terrain-following horizontal grid is employed to investigate the initiation of precipitating convection over heated topography. Horizontally homogeneous meteorological conditions with no directional shear in the vertical wind profile are used. The numerical simulations indicate that precipitating convection was more likely to be generated downwind of ridges than upwind of them. Initiation of these storms was more likely downwind of ridges with their long axis parallel to the wind direction than downwind of ridges with their long axis perpendicular to the wind direction. In Part I of this study it was shown that heating-induced convergence is larger downwind of a ridge with its longer axis parallel to the wind direction. For the orographic configuration of the Ro...
    Wang Q. W., Z. M. Tan, 2006: Flow regimes for major topographic obstacles of China. Chinese Journal of Geophysics, 49, 971- 982.10.1002/cjg2.906c8adf41915daf7a101005233f705b7f4http%3A%2F%2Fen.cnki.com.cn%2FArticle_en%2FCJFDTOTAL-DQWX200604006.htmhttp://en.cnki.com.cn/Article_en/CJFDTOTAL-DQWX200604006.htmMajor topographic obstacles in China are taken into account as a whole and approximate scales of these topographies are dynamically idealized according to the theoretical results of mountain flow dynamics.For the typical west-east and north-south upstream flows,the questions are focused on the different flow regimes,such as flow-over/flow-around and quasi-geostrophic balanced or not.The results show that main topographies in China can be classified as three types,one is over-flow dominated and quasi-geostrophic balanced,the second is over-flow dominated but quasi-geostrophic unbalanced,and the third is around-flow dominated and quasi-geostrophic unbalanced.In fact,the primary flow characteristics are determined by not only the scales of the topographies but also their shapes and the upstream flow direction.The rationality of the regimes is validated through qualitative analysis of the flow over the Dabie Mountains and Taiwan Mountains which are numerically simulated using a mesoscale model.
    Wang Q. W., Z.-M. Tan, 2009: Idealized numerical simulation study of the potential vorticity banners over a mesoscale mountain: Dry adiabatic process. Adv. Atmos. Sci.,26, 906-922, doi: 10.1007/s00376-009-8004-z.10.1007/s00376-009-8004-zd894f89b1df5ef2d550d96d79070d12fhttp%3A%2F%2Fwww.cnki.com.cn%2FArticle%2FCJFDTotal-DQJZ200905008.htmhttp://d.wanfangdata.com.cn/Periodical_dqkxjz-e200905008.aspxTopography-induced potential vorticity (PV) banners over a mesoscale topography (Dabie Mountain, hereafter DM) in eastern China, under an idealized dry adiabatic flow, are studied with a mesoscale num
    Wang Q. W., M. Xue, 2012: Convective initiation on 19 June 2002 during IHOP: High-resolution simulations and analysis of the mesoscale structures and convection initiation. J. Geophys. Res., 117, D12107.10.1029/2012JD017552a279002dbdb497a064b14297e2558db7http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F2012JD017552%2Fcitedbyhttp://onlinelibrary.wiley.com/doi/10.1029/2012JD017552/citedbyThe 19 June 2002 convective initiation (CI) case from the International HO Project (IHOP) is simulated using the ARPS at 1 km grid spacing. It involves three distinct CI groups, CI-A, CI-B, and CI-C, associated with a cold front-dryline system. Initial condition at 1800 UTC, 19 June 2002 was created using the ARPS 3DVAR, including standard as well as special observations. The simulation captured the three groups of CIs rather well, with small timing and spatial errors. CI-A is most typical of dryline initiation. Vertical cross sections through the initiation locations show typical dryline structures with an upward moisture bulge forced by lifting along the dryline convergence. For CI-B, the model-predicted mesoscale structures agree with special IHOP aircraft dropsonde observations closely. Strong low-level convergence from opposing cold front and dryline circulations produces deep symmetric upwelling of moist air, and the convection is initiated at the crest of the moisture bulge. CI-C is more typical of frontal convection where deep convection is first initiated at the front edge of the cold air. For CI-B and CI-C, north-south moisture bands that have dryline characteristics (and can be considered multiple drylines) and contain enhanced vertical motion and upward moisture bulges intercept the dryline and cold front, respectively, providing the most favored locations on the dryline or cold front for initiation. Furthermore, the air parcels feeding the initial cells at CI-B and CI-C have trajectories along the moisture bands and have already experienced uplifting when they reach the dryline and cold front locations.
    Worthington R. M., 2015: Organisation of orographic convection by mountain waves above Cross Fell and Wales. Weather, 70, 186- 188.10.1002/wea.2475b3cba728bed054a9f6c403886812f48ahttp%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1002%2Fwea.2475%2Fpdfhttp://onlinelibrary.wiley.com/doi/10.1002/wea.2475/pdfNot Available
    Wulfmeyer, V., Coauthors, 2008: RESEARCH CAMPAIGN: The convective and orographically induced precipitation study. Bull. Amer. Meteor. Soc., 89, 1477- 1486.10.1175/2008BAMS2367.14997b7d9149b019ed85c8f38251f97cehttp%3A%2F%2Fieeexplore.ieee.org%2Fxpls%2Ficp.jsp%3Farnumber%3D4619008http://ieeexplore.ieee.org/xpls/icp.jsp?arnumber=4619008ABSTRACT The international field campaign called the Convective and Orographically-induced Precipitation Study (COPS) took place from June to August 2007 in southwestern Germany/eastern France. The overarching goal of COPS is to advance the quality of forecasts of orographically-induced convective precipitation by four-dimensional observations and modeling of its life cycle. COPS was endorsed as one of the Research and Development Projects of the World Weather Research Program (WWRP), and combines the efforts of institutions and scientists from eight countries. A strong collaboration between instrument principal investigators and experts on mesoscale modeling has been established within COPS. In order to study the relative importance of large-scale and small-scale forcing leading to convection initiation in low mountains, COPS is coordinated with a one-year General Observations Period in central Europe, the WWRP Forecast Demonstration Project MAP D-PHASE, and the first summertime European THORPEX Regional Campaign. Furthermore, the Atmospheric Radiation Measurement program Mobile Facility operated in the central COPS observing region for nine months in 2007. The article describes the scientific preparation of this project and the design of the observation systems. COPS will rest on three pillars: A unique synergy of observing systems, the next-generation high-resolution mesoscale models with improved model physics, and advanced data assimilation and ensemble prediction systems. These tools will be used to separate and to quantify errors in quantitative precipitation forecasting as well as to study the predictability of convective precipitation.
    Xue, M., Coauthors, 2001: The Advanced Regional Prediction System (ARPS)multi-scale nonhydrostatic atmospheric simulation and prediction tool. Part II: Model physics and applications. Meteor. Atmos. Phys., 76, 143- 165.
    Xue M., W. J. Martin, 2006a: A high-resolution modeling study of the 24 May 2002 dryline case during IHOP. Part I: Numerical simulation and general evolution of the dryline and convection. Mon. Wea. Rev., 134, 149- 171.10.1175/MWR3071.1b7f05f7d004f2909e44fd15ebde264eahttp%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2006MWRv..134..149Xhttp://adsabs.harvard.edu/abs/2006MWRv..134..149XResults from a high-resolution numerical simulation of the 24 May 2002 dryline convective initiation (CI) case are presented. The simulation uses a 400 km 01— 700 km domain with a 1-km horizontal resolution grid nested inside a 3-km domain and starts from an assimilated initial condition at 1800 UTC. Routine as well as special upper-air and surface observations collected during the International H2O Project (IHOP_2002) are assimilated into the initial condition. The initiation of convective storms at around 2015 UTC along a section of the dryline south of the Texas panhandle is correctly predicted, as is the noninitiation of convection at a cold-front09“dryline intersection (triple point) located farther north. The timing and location of predicted CI are accurate to within 20 min and 25 km, respectively. The general evolution of the predicted convective line up to 6 h of model time also verifies well. Mesoscale convergence associated with the confluent flow around the dryline is shown to produce an upward moisture bulge, while surface heating and boundary layer mixing are responsible for the general deepening of the boundary layer. These processes produce favorable conditions for convection but the actual triggering of deep moist convection at specific locations along the dryline depends on localized forcing. Interaction of the primary dryline convergence boundary with horizontal convective rolls on its west side provides such localized forcing, while convective eddies on the immediate east side are suppressed by a downward mesoscale dryline circulation. A companion paper analyzes in detail the exact processes of convective initiation along this dryline.
    Xue M., W. J. Martin, 2006b: A high-resolution modeling study of the 24 May 2002 dryline case during IHOP. Part II: Horizontal convective rolls and convective initiation. Mon. Wea. Rev., 134, 172- 191.10.1175/MWR3072.17c500383-38e7-4616-8f26-4d84315121cac297f0dfb5e64a747511b7cc1e4778bfhttp%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2006MWRv..134..172Xrefpaperuri:(a53ec02959c753c198fed9a3c440bf0e)http://adsabs.harvard.edu/abs/2006MWRv..134..172XNot Available
    Xue M., K. K. Droegemeier, and V. Wong, 2000: The Advanced Regional Prediction System (ARPS)multi-scale nonhydrostatic atmospheric simulation and prediction model. Part I: Model dynamics and verification. Meteor. Atmos. Phys., 75, 161- 193.
    Xue M., D. H. Wang, J. D. Gao, K. Brewster, and K. K. Droegemeier, 2003: The Advanced Regional Prediction System (ARPS), storm-scale numerical weather prediction and data assimilation. Meteor. Atmos. Phys., 82, 139- 170.10.1007/s00703-001-0595-64cab6cb2342b233c1b67deb658994032http%3A%2F%2Fwww.springerlink.com%2Findex%2FCG08L17MKCFAHA14.pdfhttp://www.springerlink.com/index/CG08L17MKCFAHA14.pdfIn this paper, we first describe the current status of the Advanced Regional Prediction System of the Center for Analysis and Prediction of Storms at the University of Oklahoma. A brief outline of future plans is also given. Two rather successful cases of explicit prediction of tornadic thunderstorms are then presented. In the first case, a series of supercell storms that produced a historical number of tornadoes was successfully predicted more than 8 hours in advance, to within tens of kilometers in space with initiation timing errors of less than 2 hours. The general behavior and evolution of the predicted thunderstorms agree very well with radar observations. In the second case, reflectivity and radial velocity observations from Doppler radars were assimilated into the model at 15-minute intervals. The ensuing forecast, covering a period of several hours, accurately reproduced the intensification and evolution of a tornadic supercell that in reality spawned two tornadoes over a major metropolitan area. These results make us optimistic that a model system such as the ARPS will be able to deterministically predict future severe convective events with significant lead time. The paper also includes a brief description of a new 3DVAR system developed in the ARPS framework. The goal is to combine several steps of Doppler radar retrieval with the analysis of other data types into a single 3-D variational framework and later to incorporate the ARPS adjoint to establish a true 4DVAR data assimilation system that is suitable for directly assimilating a wide variety of observations for flows ranging from synoptic down to the small nonhydrostatic scales.
    Yang Y. M., W. L. Gu, R. L. Zhao, and J. Liu, 2010: The statistical analysis of low vortex during Meiyu season in the lower reaches of the Yangtze. Journal of Applied Meteorological Science 21, 11- 18. (in Chinese)10.14236/jhi.v10i4.26488095862d0fbe9d444fb5806bc36c216http%3A%2F%2Fen.cnki.com.cn%2FArticle_en%2FCJFDTOTAL-YYQX201001002.htmhttp://en.cnki.com.cn/Article_en/CJFDTOTAL-YYQX201001002.htmLocal-generated mesoscale vortexes(LMVs) in the lower reaches of the Yangtze is one key factor to improve rainstorm forecast accuracy since they can efficiently trigger and organize mesoscale convective systems(MCSs).There are many case studies focusing on the MCSs caused by LMVs,but in order to improve operational weather forecast ability during Meiyu season,the study about LMVs activities,structures and formative environment are necessary.Based on conventional weather data,satellite cloud images and numerical weather predictive outputs during Meiyu season of 1998鈥2005,the activities and structures of LMVs related to rainstorms in the lower reaches of the Yangtze are statistically analyzed,including the vortex generation,movement,life cycle,spatial range and related convective activities etc.Large scale environments and physical conditions are also synthetically analyzed.The result shows that LMVs in the lower reaches of Yangtze mainly generated around the Dabie Mountain area in Anhui Province.Typically,their horizontal scale changes from 100 km to 400 km,and most of them vary from 200 km to 300 km.Their vertical developing height usually changes from 1000 hPa to 700 hPa.LMVs moves mainly in two directions,one is northeast to as far as the Yellow Sea through north of Shandong Province,ant the other is southeastward towards the East China Sea through south of Jiangsu Province or north of Zhejiang Province.The life cycle of LMVs is less than 48 hours and it has no clear relations with their spatial scales.About 70% LMVs trigger single MCS or series MCSs and can lead to rainstorms.Rainstorms mainly happen to the south or southeast of LMVs,where there are enough warm and moisture flow.LMVs generated at the bottom of upper level trough of 500 hPa are especially possible to cause rainstorms.Analysis of large scale environments and physical conditions show more than 90% of LMVs develops at the bottom or in front of upper level trough at 500 hPa.The positive vorticity advection in front of upper level trough is necessary to LMVs generation and development.While the other conditions such as low level and upper level jet,middle level turbulence,low level moisture transfer and topographic conditions are all important to LMVs generation and development.Further studies such as mechanisms about LMV generation,development and the mechanisms about LMV trigger and organize MCSs are necessary.
    Zhang J., Z. M. Tan, 2009: A simulation study of the mesoscale convective systems associated with a Meiyu frontal heavy rain event. Acta Meteor.Sinica, 23, 438- 454. (in Chinese)10.1016/j.agrformet.2009.02.014276d18b3d65a927a881ef5133c3a5e02http%3A%2F%2Fd.wanfangdata.com.cn%2FPeriodical%2Fqxxb-e200904005http://d.wanfangdata.com.cn/Periodical/qxxb-e200904005In this study, evolution of the mesoscale convective systems (MCSs) within a Meiyu front during a partic- ularly heavy rainfall event on 22 June 1999 in East China was simulated by using a nonhydrostatic numerical model ARPS (Advanced Regional Prediction System). Investigations were conducted with emphasis on the impact of the interaction among multi-scale weather systems (MWSs) on the development of MCSs in the Meiyu frontal environment.For this case, the development of MCSs experienced three different stages. (1) The convections associ- ated with MCSs were firstly triggered by the eastward-moving Southwest Vortex (SWV) from the Sichuan Basin, accompanying the intensification of the upper-level jet (ULJ) and the low-level jet (LLJ) that were approaching the Meiyu front. (2) Next, a low-level shear line (LSL) formed, which strengthened and orga- nized the MCSs after the SWV decayed. Meanwhile, the ULJ and LLJ enhanced and produced favorable conditions for the MCSs development. (3) Finally, as the MCSs got intensified, a mesoscale convective vortex (MCV), a mesoscale LLJ and a mesoscale ULJ were established. Then a coupled-development of MWSs was achieved through the vertical frontal circulations, which further enhanced the MCV and resulted in the heavy rainfall. This is a new physical mechanism for the formation of Meiyu heavy rainfall related to the SWV during the warm season in East China.In the three stages of the heavy rainfall, the vertical frontal circulations exhibited distinguished structures and played a dynamic role, and they enhanced the interaction among the MWSs. A further examination on the formation and evolution of the MCV showed that the MCV was mainly caused by the latent heat release of the MCSs, and the positive feedback between the MCSs and MCV was a key characteristic of the scale interaction in this case.
    Zhao Y. C., 2015: A study on the heavy-rain-producing mesoscale convective system associated with diurnal variation of radiation and topography in the eastern slope of the western Sichuan plateau. Meteor. Atmos. Phys., 127 123- 146.10.1007/s00703-014-0356-y3ddf1e81a5981a11b9d85e45ca4a4c05http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2014map...tmp...43zhttp://adsabs.harvard.edu/abs/2014map...tmp...43zOn 24鈥25 July 2010, a Plateau Vortex system forming to the north of Tibetan Plateau dramatically changed its moving direction to westward after several days of eastward movement. Observational analysis showed that, during its westward movement, a low-level southeasterly or easterly wind developed over the Sichuan basin. The large-scale forcing became favorable for the convection development. The low-level warm advection was more favorable for convection development than the differential vorticity advection. The daytime scattered convections were organized into a mesoscale convective system (MCS) after sunset, which produced extremely heavy rainfall in the eastern slope of the Western Sichuan Plateau. The observational evidences and numerical simulations have indicated that the topographically induced dynamical lifting over the lower topography and the convergence caused by the topographical blocking provided strong support for the convection initiation. The cold outflows caused by surface evaporative cooling of rain steered the MCS to move away from its original place, while the convergence between the cold outflows and the environmental southeasterly flow apparently helped the maintenance of the MCS. The intensification of the low-level flow, which was associated with the diurnal variation of radiative forcing, contributed to the organization and intensification of the MCS. The results of sensitivity experiments further confirmed the impact of topography in the convection initiation, and the influences of cold outflows caused by surface evaporative cooling of rain on the movement and maintenance of the MCS. The effects of the diurnal variation of radiative forcing on the organization of the MCS are also well represented in the model results.
    Zhao Y. C., X. F. Xu, and C. G. Cui, 2012: Case study of the impact of mesoscale topography on the Meiyu frontal rainstorm. Plateau Meteorology, 31, 1268- 1282. (in Chinese)10.1016/0032-3861(96)00343-6592815ae8f6c1b62f0747755d1467415http%3A%2F%2Fen.cnki.com.cn%2FArticle_en%2FCJFDTOTAL-GYQX201205010.htmhttp://en.cnki.com.cn/Article_en/CJFDTOTAL-GYQX201205010.htmThe mechanism and the formation behaviour of banded texture in non-isothermal processes have been studied for two aromatic main-chain liquid cyrstalline polyesters designated as P(2,8) and PTDT-Br which contain the X-shaped and linear rod-like mesogens along the polymer backbones, respectively. For P(2,8) regular and perfect banded textures were observed within its oriented films which were prepared by shearing in mesomorphic state and subsequent cooling down to room temperature under various conditions. Bandwidth was dependent sensitively on the cooling conditions, about 8 μm for rapid and 2 μm for slow coolings. During the cooling of an oriented film the bands were first generated around 170°C, and their regularity was improved with lowering temperature. The bandwidth as well as the extinction angle of the bands were changed drastically. In such a cooling process the birefringence Δn , the difference between the refractive indices along the shear and lateral directions, of the oriented film was decreased gradually from about 0.06 to 0.03. In the case of PTDT-Br, clear and perfect banded texture could be observed only for the rapid cooling case. The banded formation behaviour was discussed on the basis of a contraction mechanism and it could be explained as the result of zigzag rearrangement of straightforward oriented fibrils under certain contraction effects. The origins of these effects were considered to be the elastic energy stored in the specimens during shearing and the thermal contraction during cooling. The latter was more evident in the rapid cooling case. The orientational relaxation of fibrils or the formation of banded texture occurs during cooling in a small temperature range after an ‘induction stage’, while the relaxation due to free thermal motion of individual molecules may proceed in the whole temperature region before the solidification of specimens.
    Zhong S. Y., J. Fast, 2003: An evaluation of the MM5, RAMS, and Meso-Eta models at subkilometer resolution using VTMX field campaign data in the Salt Lake valley. Mon. Wea. Rev., 131, 1301- 1322.741dc894a08f84a15c2a25c3c095f4abhttp%3A%2F%2Fadsabs.harvard.edu%2Fcgi-bin%2Fnph-data_query%3Fbibcode%3D2003MWRv..131.1301Z%26db_key%3DPHY%26link_type%3DABSTRACT%26high%3D18446http://xueshu.baidu.com/s?wd=paperuri%3A%288ff8a48568d11812f91f3ed92d52b6ae%29&filter=sc_long_sign&tn=SE_xueshusource_2kduw22v&sc_vurl=http%3A%2F%2Fadsabs.harvard.edu%2Fcgi-bin%2Fnph-data_query%3Fbibcode%3D2003MWRv..131.1301Z%26db_key%3DPHY%26link_type%3DABSTRACT%26high%3D18446&ie=utf-8&sc_us=468934838933603939
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Manuscript received: 11 March 2016
Manuscript revised: 13 June 2016
Manuscript accepted: 13 June 2016
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Convective Initiation by Topographically Induced Convergence Forcing over the Dabie Mountains on 24 June 2010

  • 1. Key Laboratory of Mesoscale Severe Weather and School of Atmospheric Sciences, Nanjing University, Nanjing 210023, China
  • 2. Center for Analysis and Prediction of Storms and School of Meteorology, University of Oklahoma, Norman, Oklahoma 72072, USA

Abstract: The initiation of convective cells in the late morning of 24 June 2010 along the eastward extending ridge of the Dabie Mountains in the Anhui region, China, is studied through numerical simulations that include local data assimilation. A primary convergence line is found over the ridge of the Dabie Mountains, and along the ridge line several locally enhanced convergence centers preferentially initiate convection. Three processes responsible for creating the overall convergence pattern are identified. First, thermally-driven upslope winds induce convergence zones over the main mountain peaks along the ridge, which are shifted slightly downwind in location by the moderate low-level easterly flow found on the north side of a Mei-yu front. Second, flows around the main mountain peaks along the ridge create further convergence on the lee side of the peaks. Third, upslope winds develop along the roughly north-south oriented valleys on both sides of the ridge due to thermal and dynamic channeling effects, and create additional convergence between the peaks along the ridge. The superposition of the above convergence features creates the primary convergence line along the ridge line of the Dabie Mountains. Locally enhanced convergence centers on the primary line cause the initiation of the first convection cells along the ridge. These conclusions are supported by two sensitivity experiments in which the environmental wind (dynamic forcing) or radiative and land surface thermal forcing are removed, respectively. Overall, the thermal forcing effects are stronger than dynamic forcing given the relatively weak environmental flow.

1. Introduction
  • Complex terrain plays important roles in the initiation and organization of convection (e.g., Rotunno and Houze, 2007; Wulfmeyer et al., 2008; Houze, 2012; Kain et al., 2013; Trier et al., 2015; Worthington, 2015; Zhao, 2015). For strong airflows encountering complex terrain, dynamic forcing can be strong enough to cause convective initiation (CI). For weak environmental airflows, the dynamic effects tend to be weaker, meaning CI is often caused mainly by thermally-driven mountain wind systems. For moderate environmental airflows, both dynamic and thermal effects associated with the complex terrain can be important. The dynamic and thermal effects of mountains on CI and precipitation have been investigated in a number of studies for mountains in the United States and Europe, as well as for idealized mountains (e.g., Crook and Tucker, 2005; Tucker and Crook, 2005;Demko and Geerts, 2010; Schmidli and Rotunno, 2010; Bennett et al., 2011; Hagen et al., 2011; Soderholm et al., 2014). Low-level convergence forcing induced by dynamically and/or thermally-driven local wind systems have been found to be of importance for CI over mountainous terrain. Most current operational models are, however, still unable to successfully represent the detailed flow interactions and associated CI over complex terrain, due to inadequate model resolution, model physics errors, and a lack of observations providing accurate enough initial conditions (e.g., Horel et al., 2002; Bosart, 2003; Zhong and Fast, 2003; Chow et al., 2013).

    Figure 1.  Geopotential height (blue contours, units: gpm) and wind fields (vectors, units: m s$^-1$) from the NCEP operational GFS 0.5$^\circ$ global analysis at 0000 UTC 24 June 2010 at (a) 500 hPa and (b) 850 hPa. The black body temperature (color-shaded; units: $^\circ$C) is superimposed to show regions of clouds or convection associated with the Mei-yu front that features a shear line at 850 hPa (bold red curved line). The red box outlines the Dabie Mountains region shown in Fig. 2, and is also shown in Fig. 4. The location of the Anqing sounding is marked by a red dot in (a). The country/province borders are drawn in bold green. The bold black lines in (b) indicate where the 850 hPa pressure level intersects the terrain.

    Figure 2.  Observed composite radar reflectivity (color-shaded; units: dB$Z$) from Hefei Doppler weather radar at (a) 0000 UTC, (b) 0400 UTC, (c) 0430 UTC, (d) 0500 UTC, (e) 0530 UTC, (f) 0600 UTC, (g) 0630 UTC, and (h) 0700 UTC 24 June 2010. Also shown in (a) are the 10 m wind vectors and 2 m specific humidity (green contours) from NCEP GFS analysis, and locations of the automatic weather stations (plus signs) for rain gauge measurements. The Hefei radar and Anqing sounding locations are also shown as red filled circles with labels in (a). Hourly precipitation (in 0.1 mm) at 0500 UTC, 0600 UTC and 0700 UTC from the automatic weather stations are shown in (c), (e) and (g), respectively. The terrain elevation is shaded in gray and the plotted region corresponds to the red box shown in Fig. 1. The important mountain peaks are labelled P1 and P2 in (a). The three cells initiated in the hour before 0600 UTC along a convergence line are labelled as C1, C2 and C3 in (f).

    Climatological studies over the middle and lower reaches of the Yangtze River in eastern China (e.g., Bao et al., 2011; Luo et al., 2013) suggest that convection and associated precipitation are to some extent linked to terrain in the region. The Dabie Mountains is the primary mesoscale mountain range located in the region, which has a horizontal width of about 200 km and a height of about 1 km (Wang and Tan, 2006). A number of studies (e.g., Tao, 1980; Sun et al., 2005; Yang et al., 2010; Fu et al., 2012; Sun and Zhang, 2012; Zhao et al., 2012; Guo et al., 2013; Miao et al., 2014) have investigated the general influences of the Dabie Mountains on convection and precipitation along the Mei-yu fronta(aA Mei-yu front is a quasi-stationary or slow-moving frontal system over East Asia, and is also called “Baiyu” in Japan and “Changma” in Korea (e.g.,Ding, 1992; Chen et al., 1998; Zhang and Tan, 2009).), which is a prominent precipitating system that produces persistent rainfall along the Yangtze River in early summer. In spite of these studies, the role of the Dabie Mountains in initiating convection during the Mei-yu season has received little attention. During the Mei-yu season, especially in the Mei-yu frontal zone, the environmental airflows are usually relative weak, and the relative roles of dynamic versus thermodynamic forcing of the Dabie Mountains in producing and supporting convection are not well understood. Documenting and understanding the CI processes associated with the Dabie Mountains is important for improving the prediction of convection and precipitation in this region, especially given that convection can develop and propagate eastward into the even more heavily populated coastal regions and impose considerable socioeconomic impacts. This paper studies one such event.

    Figure 3.  The sounding averaged from the GFS analysis data at 0000 UTC 24 June 2010 over the Dabie Mountains region within the box in Fig. 1 (black lines and wind barbs). The observed Anqing sounding near the Dabie Mountains at the same time is shown for comparison (gray lines and wind barbs).

    During 24 June 2010, a moderate low-level easterly airflow north of a Mei-yu front occupied the Dabie Mountains region. At 0000 UTC 24 June 2010 (Fig. 1), a cloud band associated with the Mei-yu front was situated on the south side of the Dabie Mountains region (red box in Fig. 1). Climatologically, this type of synoptic situation tends to persist for around one week during the Mei-yu season, which generally lasts approximately 20 to 25 days from late June to early July. The precipitation band associated with a Mei-yu front is generally situated south of 31°N (Ding et al., 2007), which is the latitude of the center of the Dabie Mountains. In the present case, westerly flow at the 500 hPa level associated with a cut-off low dominated over the Dabie Mountains region (Fig. 1a), while an easterly flow of approximately 5 m s-1 was situated on the north side of the Mei-yu front (indicated by the shear line) at the 850 hPa level (Fig. 1b). The easterly flow still extended to the surface and was slightly weaker (Fig. 2a). The near-surface specific humidity was around 17 g kg-1 (Fig. 2a)——a very moist environmental condition favorable for convection.

    Observations show that convective cells initiated along a line over the Dabie Mountains. The first weak reflectivity echoes occurred on the eastern edge of the Dabie Mountains (Fig. 2b) and between mountain peak P1 and P2 (Fig. 2c). A line of weak echoes or convective cells along the eastward extending ridge of the Dabie Mountains could be identified by 0500 UTC (Fig. 2d) and 0530 UTC (Fig. 2e), respectively. The convective cells along the line produced a line of precipitation echoes within an hour (Fig. 2e), which later evolved into three larger convective cell groups, labelled C1, C2 and C3 in Fig. 2f. The three cell groups changed only slightly in location and produced considerable precipitation of more than 10 mm in the following hour (Figs. 2g and h). More than 30 mm of rainfall within 3 h between 0500 and 0800 UTC was observed at some of the automatic weather stations (figure not shown), which indicates that the CIs led to a significant weather event with short-duration heavy precipitation.

    The environment for CI over the Dabie Mountains is indicated by a sounding (Fig. 3) averaged from the GFS analysis data at 0000 UTC over the box region shown in Fig. 1. Both the GFS average sounding and the observed sounding at Anqing (see Fig. 2a for its location) near the Dabie Mountains feature relatively weak low-level easterly flows. The temperature difference between the two soundings is small, except near the surface due to local modifications of the flow by surrounding mountains at Anqing. The GFS average and Anqing sounding moisture profiles show large differences, with the latter showing much higher relative humidity in a deep layer. This is believed to be because Anqing was at that time in a moister and more unstable environment close to the Mei-yu front and associated clouds. The average GFS sounding is believed to better represent the thermodynamic and wind structures of the background flow over the Dabie Mountains region. The background flow provides an environment of moderate CAPE (approximately 924 J kg-1) and weak convective inhibition (approximately -26 J kg-1).

    Figure 4.  The 3 km model domain with terrain elevation shaded. The filled square marks the Hefei radar location, with the large circle indicating the maximum radar range. The triangles and small filled circles indicate the stations from conventional radiosonde and surface networks, respectively. The Anqing sounding site is labelled. The dashed box denotes the plotting region in later figures. The solid gray box indicates the red box region in Fig. 1.

    The line of convection was unlikely to be a direct result of mountain-flow interactions, since such interactions typically trigger upslope or leeside convection instead of convection on both sides of mountain peaks at the same time, as seen here. Besides, the environmental flows appear to have been too weak to have directly forced convection. Nor does it appear to have been purely a result of thermally-driven circulations, which typically trigger convection over mountain peaks. Both the environmental flow-mountain interactions and thermally-driven circulations and their interactions with several peaks along the east-west oriented ridge of the Dabie Mountains may have played a role in triggering the line of convection. The purpose of this paper is to investigate the roles of the dynamic and thermal effects of the Dabie Mountains in producing the low-level convergence forcing for CI, and this is achieved by producing and analyzing realistic numerical simulations of the 24 June 2010 case with the aid of local data assimilation.

    The rest of this paper is organized as follows: In section 2, the numerical experiments are introduced, including the numerical model and its configurations, as well as the observational datasets used. The results are analyzed and discussed in detail in section 3. Section 4 presents a summary and conclusions.

2. Numerical model, data and experiment design
  • Version 5.3.3 of ARPS (Xue et al., 2000, 2001, 2003) is used to perform the simulations in this study. ARPS is a non-hydrostatic atmospheric model suitable for mesoscale and convective-scale simulation and prediction. The model domain has 259× 259 horizontal grid points and a 3-km grid spacing (Fig. 4). In the vertical direction, 53 stretched levels are defined on a generalized terrain-following coordinate, with the vertical grid spacing increasing from about 20 m near the surface to about 800 m near the model top at a height of about 20 km. ARPS is used in its full physics mode with the same configurations as used in (Wang and Xue, 2012).

    Figure 5.  Observed (black contours) and CNTL experiment simulated (red contours) composite radar reflectivity at 0330 UTC, 0430 UTC and 0530 UTC 24 June 2010. Both the black and the red contours are at intervals of 20 dB$Z$ and start from 20 dB$Z$ level. The terrain elevation is shaded.

    NCEP operational GFS 0.5° global analysis data are used to provide the lateral boundary conditions at 6 h intervals, and a 6-h forecast is first produced using the 1800 UTC 23 June 2010 GFS analysis as the initial condition background. Using the 6-h forecast as the background, the ARPS three-dimensional variational (3DVAR) system (Gao et al., 2004) is then used to assimilate conventional radiosonde and surface (not radar) data (see station sites in Fig. 4) at 0000 UTC 24 June. This final analysis is used as the initial condition by the control (CNTL) experiment, and by experiment NoHEAT (Table 1), which is the same as CNTL except that the radiation physics and surface sensible and latent heat fluxes in ARPS are turned off to isolate the thermal effects associated with the Dabie Mountains. In another sensitivity experiment, NoWIND, the model is initialized at 0000 UTC 24 June from a sounding instead of the three-dimensional fields from ARPS 3DVAR. This sounding is created by averaging the 0000 UTC 24 June GFS analysis in the Dabie Mountains region but with the wind set to zero; the sounding is shown in Fig. 3. As an idealized experiment, this simulation uses the open (radiation) lateral boundary condition instead of GFS analyses as in CNTL and NoHEAT. This experiment is designed to remove the dynamic forcing associated with environmental flow-mountain interactions. In other words, it helps isolate the dynamic effects from thermodynamic effects.

3. Results
  • The simulated initiation and evolution of convection in a line pattern in CNTL are compared with radar observations in terms of the composite (column maximum) reflectivity in Fig. 5. Three convective cells first initiate at 0330 UTC in the simulation (Fig. 5a), with their locations close to the mountain peaks on the eastward-extending ridge line of the Dabie Mountains. The observed composite radar reflectivity at 0430 UTC first shows three areas of convection with locations close to the three convective cells in the simulation (Fig. 5b), and even closer to the location of the simulated cells in Fig. 5a one hour earlier. This indicates reasonably good agreement between the simulated and observed CIs, albeit the simulated CIs occur slightly too early (by nearly one hour). Despite the timing error of the initial CI, the simulated convective line is more comparable to the observation by 0530 UTC (Fig. 5c), due to the quick development of the convective cells and organization into a convective line. The exact reason for the CI timing error is difficult to ascertain. Both model and initial condition errors can cause such an error, but we suspect excessive mountain thermal forcing as a cause. In general, predicting the initiation and evolution of individual convective cells is more difficult to achieve than for mesoscale convective systems, as pointed out by (Kain et al., 2013). Because the main goal of this study is to investigate the physical processes responsible for the CI, rather than to obtain the most accurate forecast, we feel that the timing error, while not desirable, does not greatly affect our analysis of the physical processes. In fact, similar timing errors were seen in CIs in the study of (Wang and Xue, 2012), and they too compared model simulations and observations despite the time shift, as is the case here. Overall, the observed convective line is reasonably well simulated in CNTL. The timing and other differences in the detail do not prevent efforts to understand the processes associated with CI, as were attempted, with good successes, for example, in Xue and Martin (2006a, 2006b) and (Wang and Xue, 2012).

    Figure 6.  Composite radar reflectivity of 20 dB$Z$ (bold red contours), near-surface (about 20 m above ground level) wind vectors and divergence [contours (negative, blue; positive, pink)] from CNTL at (a) 0000 UTC, (b) 0130 UTC, (c) 0230 UTC, (d) 0330 UTC, (e) 0430 UTC, and (f) 0530 UTC 24 June 2010. The contour levels for divergence are $-2\times 10^-3$, $-0.5\times 10^-3$, $0.5\times 10^-3$ and $2\times 10^-3$ s$^-1$. The bold blue and red arrows in (c) schematically outline the dynamically and thermally driven flows, respectively. In (d), line L0 indicates the position of the vertical cross sections shown in Fig. 8 and Fig. 10. Vertical cross sections through line L1 and L2-L5 in (d) are shown in in Fig. 11 and Fig. 12, respectively. The terrain elevation is shaded.

  • The CIs and subsequent convection evolution from CNTL are shown in Fig. 6. A low-level convergence line actually forms before the convection line formation. At 0000 UTC 24 June 2010 (Fig. 6a), the dynamic effects of the Dabie Mountains in a weak near-surface easterly flow seem to play the main role. The near-surface flows generally go around the main mountain peaks, producing divergent flows on the windward side and convergence on the lee side. The thermal effects of the mountains due to radiation heating become active one and a half hours later at 0130 UTC (Fig. 6b) and patches of convergence and divergence are now found above the mountain peaks. At this time, both dynamic and thermal effects are present, while the thermal effects tend to become more dominant into the late morning hours (Figs. 6c and d). Given the moderate easterly background flow, the dynamically-driven flows (schematically denoted as bold red arrows in Fig. 6c) clearly induce leeside convergence zones; the thermally-driven upslope winds are expected to converge from both (south and north) sides of the eastward extending ridge line of the mountains (schematically denoted as bold blue arrows in Fig. 6c), which is roughly along the line through mountain peaks P1 and P2, as denoted in Fig. 2a.

    The thermally-forced convergence zones along the ridge combined with the dynamically-induced leeside convergence zones establish a somewhat organized line of convergence along the ridge by 0230 UTC (Fig. 6c), after which this line becomes more organized and the convergence zones are more or less connected by 0330 UTC (Fig. 6d). At this time, the thermally-forced convergence areas over the main mountain peaks can be recognized with locations slightly biased towards the downwind side of the peaks due to the weak near-surface easterly environmental flows. Such convergence areas merge with and enhance the dynamically-driven leeside convergence areas (e.g., the convergence features over mountain peak P1 in Fig. 6d). By 0330 UTC (Fig. 6d), three convective cells, as indicated by the bold 20 dBZ contours, have already been initiated over the convergence line, with one (the westernmost cell) to the leeside of mountain peak P1, the second between P1 and P2, and the third on the windward side of P2 (the easternmost cell). The three convective cells continue to develop along the convergence line over the next couple of hours and organize through mergers (Fig. 6e) until a full line of convection forms by 0530 UTC (Fig. 6f).

    The dynamic and/or thermal convergence forcing along the convergence line that contributes to the initiation of the three initial convective cells can be more clearly identified at the 900 m MSL level in Fig. 7. At this level, the dynamic blocking effects of the main mountain peaks are less affected by the thermal effects that gradually develop from below due to mountain surface heating. At 0130 (Fig. 7a), convergence on the lee side of the main mountain peaks are induced by the around-peak flows, which are characterized by pairs of positive and negative vorticity centers attached to the mountain peaks (Fig. 7b), as discussed in (Wang and Tan, 2009). By 0230 UTC (Fig. 7c), the convergence features at the 900 m level have been affected by the thermally-driven convergence developing from the surface (Fig. 6b, c), with enhanced convergence found between mountain peaks P1 and P2 and new convergence patches on the north and east sides of mountain peak P2 (Figs. 7a and c). A slight enhancement of vertical vorticity at the 900 m level is also found by 0230 UTC (Figs. 7b and d), which indicates the modification of the around-peak flows by the thermal effects of the Dabie Mountains. By 0330 UTC (Fig. 7e), the convergence field tends to be dominated by the thermally-driven convergence, although the around-peak flows and the associated leeside convergence also act to enhance the overall convergence (Fig. 7f). The locations of the three convective cells relative to the convergence zones at this level (Fig. 7e) and near the surface (Fig. 6d) suggest that the dynamically- and thermally-driven convergence forcing both contribute to the CIs of the first and second (the westernmost and middle) cells, while the initiation of the third, easternmost, cell is mainly associated with thermal forcing effects.

    The CI processes associated with the low-level convergence forcing are more clearly seen in Fig. 8 in vertical cross sections along the primary convergence line (L0 in Fig. 6d). The dynamic leeside convergence is clear at 0130 UTC when the thermal effects are weak, while the associated lifting forcing at this time is not strong and only induces shallow clouds (Fig. 8a). As the thermal effects become stronger, the leeside convergence is also enhanced, with the convergence between peaks P1 and P2 becoming connected with the area of relatively weak thermally-driven convergence on the windward side of P2 (Fig. 8b). The forcing over the enhanced leeside convergence has induced deeper clouds on the leeside of mountain peaks P1 and P2 at 0230 UTC (Fig. 8b), which subsequently develop into the first and second convective cells at 0330 UTC (Fig. 8c). The relatively weak, mostly thermally forced convergence upwind of P2 at 0230 UTC only induces shallower clouds (Fig. 8b), but it develops quickly and generally exceeds the dynamic leeside convergence, resulting in the more intense third (easternmost) convective cell (as indicated by the bold red contours of reflectivity) by 0330 UTC (Fig. 8c).

    At 0330 UTC, intense forcing associated with the much stronger low-level convergence of greater than 2× 10-3 s-1 supports the initiation of the second (middle) and third (easternmost) convective cells (Figs. 6d, 7e and 8c). The strong convergence between mountain peaks P1 and P2 does not appear to be due to the thermally enhanced leeside convergence only, as the enhanced convergence on the lee side of peak P1 is similar but the overall convergence is not as strong. Also, the strong convergence on the windward side of mountain peak P2 cannot be explained by the thermal effects only, because the thermally-driven convergence should be near the mountain peaks and shifted slightly downwind by the easterly background flow, as with that over mountain peak P1. The Dabie Mountains feature southwest-northeast oriented (northwest-southeast oriented) mountain valleys (denoted as lines L2 through L5 in Fig. 6d) on the north (south) sides of the eastward extending main ridge line. In an easterly flow, these valleys can enhance up-valley winds toward the ridge line (e.g., the stronger wind vectors pointing along the valley in Figs. 6c-e), roughly where the areas of stronger convergence are located (Figs. 6d, 7e and 8c). This suggests that up-valley winds can make further contributions to produce even stronger convergence that forces the initiation of the second and the third convective cells.

    For the first three initiations along the primary convergence line, it is not clear if dynamic leeside convergence forcing only is enough for the initiation of the first and second cells. It is also unclear how the leeside convergence is enhanced by the thermal effects, and how the thermal convergence forcing is dynamically modified. Moreover, the additional up-valley wind convergence is suggested to promote the initiation of the second and third cells, but how it works needs to be further examined. The roles of the dynamically-driven, thermally-driven, and valley-enhanced convergence for the three initial CIs along the primary convergence line over the ridge are further examined via sensitivity experiments in the following subsections.

    Figure 7.  Wind fields (vectors) and (a, c, e) divergence [contours (negative, blue; positive, pink], or (b, d, f) relative vertical vorticity [black contours (negative, dashed; positive, solid] at 900 m MSL from CNTL at (a, b) 0130 UTC, (c, d) 0230 UTC and (e, f) 0330 UTC 24 June 2010. The contour levels for divergence in (a, c, e) are $-2\times 10^-3$, $-0.5\times 10^-3$, $0.5\times 10^-3$ and $2\times 10^-3$ s$^-1$, and for vorticity in (b, d, f) are $-7.5\times 10^-4$, $-2.5\times 10^-4$, $2.5\times 10^-4$ and $7.5\times 10^-4$ s$^-1$. The bold red contours are for the 20 dB$Z$ composite radar reflectivity. The terrain elevation is shaded. The 900 m terrain elevation is outlined in white contours.

    Figure 8.  Wind fields (wind vectors projected to the plane of the vertical cross section), convergence (blue contours, s$^-1$), equivalent potential temperature (thin black contours, K), total water mixing ratio (dashed green contours, g kg$^-1$), and radar reflectivity (bold red contours at 20 dB$Z$ intervals starting from 20 dB$Z$ level) in the vertical cross section through line L0 in Fig. 6d, at (a) 0130 UTC, (b) 0230 UTC and (c) 0330 UTC 24 June 2010. The convergence is shown at the levels of 0.5 and $2\times 10^-3$ s$^-1$, with strong convergence greater than $2\times 10^-3$ s$^-1$ shaded in gray. The total water mixing ratio of 0.01 g kg$^-1$ outlines the clouds. The mountain peaks P1 and P2 are denoted in (a).

  • Figures 9 and 10 show the results of experiments NoWIND and NoHEAT, designed to examine the contributions of thermal and dynamic effects, respectively. They show that the thermally-driven convergence alone can induce forcing strong enough to trigger convection (left panels of Figs. 9 and 10), while the dynamically-driven convergence alone cannot (right panels of Figs. 9 and 10). By the time of CI at 0330 UTC, the near-surface maximum thermally-driven convergence right above the mountain peaks can reach a value greater than 2× 10-3 s-1 (Fig. 9a), while the dynamically-driven convergence is only around 0.5× 10-3 s-1 and is generally attached to the lee sides of mountain peaks or ridges (Fig. 9b).

    Figure 9.  Horizontal cross sections from (a, c, e) NoWIND and (b, d, f) NoHEAT at 0330 UTC 24 June 2010. Fields shown in (a, b), (c, d) and (e, f) are the same as in Figs. 6a-c and Figs. 7b-f, respectively. The terrain elevation is shaded. The bold red contours are for 20 dB$Z$ composite reflectivity.

    Figure 10.  As in Fig. 8, but for (a, c, e) NoWIND and (b, d, f) NoHEAT.

    Figure 11.  Vertical cross sections from (a, d) NoWIND, (b, e) NoHEAT and (c, f) CNTL through line L1 in Fig. 6d at 0330 UTC 24 June 2010. Fields shown in (a-c) are the same as in Fig. 8, except horizontal wind speed (color-shaded) along the cross section is additionally shown. Horizontal wind speed perpendicular to the cross section is shown in (d-f).

    At the 900 m level, there are areas of convergence extending away from the 900 m contour of terrain elevation in NoWIND, which indicates that the thermally-driven convergence can reach this level (Fig. 9c). The position of the convergence extending along mountain peaks P1 and P2 in NoWIND is generally consistent with that in CNTL, but the convergence in NoWIND is rather weak (Figs. 9c and 7e). The dynamically-driven leeside convergence in NoHEAT (Fig. 9d) is also consistent with that in CNTL, which is more clearly shown earlier at 0130 UTC in Fig. 7a. Apparently, the dynamically-driven leeside convergence in NoHEAT is also weaker than that in CNTL, when comparing the associated around-peak flow and the vertical vorticity pairs (Figs. 9f and 7f).

    In the vertical cross sections through the primary convergence line (line L0 in Fig. 6d), the thermally-driven convergence in NoWIND is right over individual mountain peaks, and the associated forcing is strong enough to trigger convection (left panels of Fig. 10). The dynamically-driven convergence in NoHEAT roughly maintains a quasi-steady pattern similar to that in Fig. 8a, which is weak and only induces shallow clouds (right panels of Fig. 10). The thermally-forced convective cells over mountain peaks P1 and P2 correspond to those on the lee side of P1 and P2 in CNTL (Fig. 8), which are linked to the forcing of both thermally-driven and dynamically-driven convergence. Note that the clouds mainly form on the lee side of the lower mountain peak P2 in NoHEAT (Fig. 10f). The convergence below the clouds is weaker than that on the lee side of the higher mountain peak P1 at 0130 UTC (Fig. 10b) but is stronger later (Fig. 10f). This suggests that the valley-enhanced convergence through channeling plays a role for the thicker clouds forming between P1 and P2. The clouds upwind of mountain peaks P1 and P2 are very shallow, suggesting that dynamic upslope lifting is weak given the weak near-surface easterly flow.

    The different features between the sensitivity experiments and CNTL suggest that complex interactions between the thermal and dynamic effects exist in CNTL. The interactions are illustrated in the vertical cross sections through line L1 shown in Fig. 6d (Fig. 11), which is roughly perpendicular to the background flow and through mountain peak P2. The thermally-driven convergence is evidently modified by the dynamic effects in CNTL. The thermally-driven convergence is the strongest at mountain peak P2 in NoWIND, being enhanced by the convergence between strong upslope winds on both the south and north sides of the ridge (Fig. 11a). But the thermally-driven convergence in CNTL is weaker, mainly due to the much weaker upslope winds on the south side of mountain peak P2 (on the right side of Figs. 11a and c). The weaker upslope winds on the south side in CNTL (Fig. 11c) are due to the expected strong upslope winds being advected downstream by the easterly background flow (Fig. 11f) and are therefore not found in this vertical cross section. The upslope winds on the north side of mountain peak P2 (left side of Fig. 11c) are also affected by the flow advection but the difference is less in the direction of background winds because the terrain on the north side of mountain peak P2 is almost uniform in the direction of the easterly flow. The upslope winds on the north side of mountain peak P2 (left side of Fig. 11a) in NoWIND are comparable to that in CNTL (left side of Fig. 11c), suggesting that the sounding used to initialize NoWIND captures the main thermodynamic characteristics of the background flow over the Dabie Mountains in CNTL. The advection of the thermally-driven upslope convergence winds by the background flow results in a downwind shift of the convergence in CNTL (Fig. 8c) relative to that in NoWIND above mountain peaks P1 and P2 (Fig. 10e).

    On the other hand, the dynamically-driven convergence is also modified by the thermal effects in CNTL. One important role of the thermal effects is to enhance the around-peak flows. The thermal effects also induce easterly winds (Fig. 11d) because the overall terrain slopes upward toward the center of the Dabie Mountains. The thermally-induced easterly winds enhance the dynamically-driven around-peak flows (Fig. 11e) and result in stronger around-peak flows in CNTL (Fig. 11f). The enhanced around-peak flows are associated with the vertical vorticity attached to the mountain peaks in CNTL (Fig. 7f), which is stronger than that in NoHEAT (Fig. 9f).

    It is expected that we can obtain low-level convergence structures similar to those in CNTL (Fig. 8c) by adding the thermally-driven (Fig. 10e) and dynamically-driven convergence (Fig. 10f) and taking into account the thermal and dynamic interactions discussed above. Upon doing so, it is found that the much stronger convergence on the windward side of P2 and between P1 and P2 (shaded in Fig. 8c) still cannot be fully explained. As discussed in section 3.1, the strong convergence is believed to be associated with the additional convergence enhancement by valleys, which is discussed next.

    Figure 12.  Wind vectors and horizontal wind speed (color-shaded) along vertical cross sections from (a, d, g, j) NoWIND, (b, e, h, k) NoHEAT and (c, f, i, l) CNTL through lines (a-c) L2, (d-f) L3, (g-i) L4 and (j-l) L5 in Fig. 6d at 0330 UTC 24 June 2010.

    Figure 13.  Conceptual model for the formation of low-level convergence regions and the primary convergence line over the eastward-extending Dabie Mountains ridge in a moderate low-level easterly flow (indicated by the wind barbs). The thermally-driven upslope winds in the late morning (shown by red curved arrows with feather tails) induce convergence zones (red ellipses) over the main mountain peaks along the ridge. Additional convergence zones are induced on the leeside of the main mountain peaks on the ridge (blue ellipses) by around-peak flows (shown as blue curved arrows with rhomboid tails). Further, upslope winds (green short arrows) along the roughly north-south oriented valleys (thick dashed lines), due to thermal and channeling effects, create additional convergence zones (green ellipses) between the peaks along the ridge. The superposition of the flow convergence due to the above processes creates a primary convergence line (bold solid line) along the eastward-extending ridge line of the Dabie Mountains, with locally enhanced convergence centers (three small white-filled black circles labeled C1 through C3) that tend to initialize convection the quickest. The terrain elevation is shaded. See text for further detail.

  • Figure 12 shows the winds along the valleys, in vertical cross sections along lines L2, L3, L4, and L5 in Fig. 6d. The thermal effects alone in NoWIND can induce up-valley winds greater than 2.5 m s-1 (Figs. 12a, d, g and j), which rise along the slopes and meet at the ridge line to force upward motion. The up-valley winds induced purely by dynamic channeling effects in NoHEAT are also clear along the valleys (Figs. 12b, e, h and k). In CNTL, the up-valley winds include both thermally-driven and dynamically-channeled winds along the valleys and are therefore stronger, with values greater than 5 m s-1 (Figs. 12c, f, i and l). In addition, the wind speed (momentum) at approximately 4 km is slightly larger than that between this level and the near-surface layer (Figs. 12c, f, i and l), and vertical circulation induced by thermal forcing will help transfer momentum from above, resulting in acceleration of momentum near the surface layer. This process is similar to the winds associated with low-level open convective cells discussed in (Xue and Martin, 2006b). The stronger up-valley winds in CNTL reach the upper valley ends (Figs. 12c, f, i and l), and converge to produce the strongest upward motion at the ridge top among the three experiments, confirming the additional contributions made by the valley thermal and dynamic channeling effects on the windward slope of P2 and between P1 and P2.

4. Summary and conclusions
  • Convective initiation by topographically induced low-level convergence forcing over the Dabie Mountains during 24 June 2010 is studied through numerical simulations with sensitivity experiments using the ARPS model, run at a 3 km horizontal grid spacing. The synoptic background over the Dabie Mountains is characterized by a moderate low-level easterly flow on the north side of a Mei-yu front. The initiation of the three earliest convective cells along a primary low-level convergence line aligned along the eastward-extending Dabie Mountains ridge is reasonably well simulated.

    The formation of the primary low-level convergence line along the ridge and the locally enhanced convergence regions along the line that produce the initial CIs involve dynamic, thermal, and topographic effects associated with the Dabie Mountains and their interactions with the weak environmental flow. The key processes involved are summarized in a conceptual model illustrated in Fig. 13. In a moderate low-level easterly flow that roughly parallels the eastward-extending ridge line of the Dabie Mountains, the convergence zones induced by the thermally-driven upslope winds during late morning are more or less aligned along the ridge line. Instead of being right over mountain peaks, the thermally-driven convergence zones shift slightly downwind from the mountain peaks on the ridge in the moderate easterly environmental flow. The dynamically-driven around-peak flows create convergence zones on the lee side of individual mountain peaks. In addition, valley-enhanced upslope winds due to thermal and dynamic channeling converge at the ridge top from the north and south sides of the ridge and produce another form of convergence zones along the ridge. In the easterly background flow, the three forms of convergence zones are organized along the eastward-extending ridge line of the Dabie Mountains to form a primary low-level convergence line. Locally enhanced convergence centers are found along the primary convergence line due to superposition of two or three forms of the convergence zones. The locally enhanced convergence centers on the primary convergence line provide the strongest forcing that initiates the earliest convective cells, which eventually organize into a line of convection.

    Sensitivity experiments with the surface heating and the environmental flow removed, respectively, show that the thermal effects play a dominant role in the CIs along the primary convergence line. In addition to directly inducing thermally-driven convergence, the thermal effects also strengthen the dynamically-driven leeside convergence by enhancing the around-peak flows through the upslope effect. The thermally-driven vertical circulations can additionally transfer larger momentum from above to the near surface layer to further enhance the up-valley winds, which are both thermally-induced and dynamically-forced through the channeling effect. Knowledge of the dynamic, thermal, and topographic effects of the Dabie Mountains and their interactions with the environmental winds are helpful for the understanding and prediction of precipitation events in the Dabie Mountains region.

Reference

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