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The Contribution of Mesoscale Convective Systems to Intense Hourly Precipitation Events during the Warm Seasons over Central East China


doi: 10.1007/s00376-016-6034-x

  • Central East China is an area where both intense hourly precipitation (IHP) events and mesoscale convection systems (MCSs) occur frequently in the warm seasons. Based on mosaics of composite Doppler radar reflectivity and hourly precipitation data during the warm seasons (May to September) from 1 July 2007 to 30 June 2011, the contribution of MCSs to IHP events exceeding 20 mm h-1 over central East China was evaluated. An MCS was defined as a continuous or quasi-continuous band of 40 dBZ reflectivity that extended for at least 100 km in at least one direction and lasted for at least 3 h. It was found that the contribution of MCSs to IHP events was 45% on average over central East China. The largest contribution, more than 80%, was observed along the lower reaches of the Yellow River and in the Yangtze River-Huaihe River valleys. These regions were the source regions of MCSs, or along the frequent tracks of MCSs. There were two daily peaks in the numbers of IHP events: one in the late afternoon and one in the early morning. These peaks were more pronounced in July than in other months. MCSs contributed more to the early-morning IHP event peaks than to the late-afternoon peaks. The contributions of MCSs to IHP events with different intensities exhibited no significant difference, which fluctuated around 50% on average over central East China.
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  • Casati B., G. Ross, and D. B. Stephenson, 2004: A new intensity-scale approach for the verification of spatial precipitation forecasts. Meteorological Applications,11, 141-154, doi: 10.1017/S1350482704001239.10.1017/S13504827040012392b3606fcb0a0c8f2ee8e20301ae13b03http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1017%2FS1350482704001239%2Fabstracthttp://onlinelibrary.wiley.com/doi/10.1017/S1350482704001239/abstractA new intensity-scale method for verifying spatial precipitation forecasts is introduced. The technique provides a way of evaluating the forecast skill as a function of precipitation rate intensity and spatial scale of the error. Six selected case studies of the UK Met Office now-casting system NIMROD are used to illustrate the method.
    Chen J., Y. G. Zheng, X. L. Zhang,X.L. Zhang, and P. J. Zhu, 2013: Distribution and diurnal variation of warm-season short-duration heavy rainfall in relation to the MCSs in China. Acta Meteor. Sinica,27(6), 868-888, doi: 10.1007/s13351-013-0605-x.10.1007/s13351-013-0605-x2cfd5cc1979a2f2650a1129b7a990310http%3A%2F%2Flink.springer.com%2F10.1007%2Fs13351-013-0605-xhttp://d.wanfangdata.com.cn/Periodical/qxxb-e201306008Short-duration heavy rainfall (SDHR) is a type of severe convective weather that often leads to substantial losses of property and life. We derive the spatiotemporal distribution and diurnal variation of SDHR over China during the warm season (April–September) from quality-controlled hourly raingauge data taken at 876 stations for 19 yr (19912-2009), in comparison with the diurnal features of the mesoscale convective systems (MCSs) derived from satellite data. The results are as follows. 1) Spatial distributions of the frequency of SDHR events with hourly rainfall greater than 10–40 mm are very similar to the distribution of heavy rainfall (daily rainfall 82 50 mm) over mainland China. 2) SDHR occurs most frequently in South China such as southern Yunnan, Guizhou, and Jiangxi provinces, the Sichuan basin, and the lower reaches of the Yangtze River, among others. Some SDHR events with hourly rainfall 82 50 mm also occur in northern China, e.g., the western Xinjiang and central-eastern Inner Mongolia. The heaviest hourly rainfall is observed over the Hainan Island with the amount reaching over 180 mm. 3) The frequency of the SDHR events is the highest in July, followed by August. Analysis of pentad variations in SDHR reveals that SDHR events are intermittent, with the fourth pentad of July the most active. The frequency of SDHR over mainland China increases slowly with the advent of the East Asian summer monsoon, but decreases rapidly with its withdrawal. 4) The diurnal peak of the SDHR activity occurs in the later afternoon (1600–1700 Beijing Time (BT)), and the secondary peak occurs after midnight (0100–0200 BT) and in the early morning (0700–0800 BT); whereas the diurnal minimum occurs around late morning till noon (1000–1300 BT). 5) The diurnal variation of SDHR exhibits generally consistent features with that of the MCSs in China, but the active periods and propagation of SDHR and MCSs differ in different regions. The number and duration of local maxima in the diurnal cycles of SDHR and MCSs also vary by region, with single, double, and even multiple peaks in some cases. These variations may be associated with the differences in large-scale atmospheric circulation, surface conditions, and land-sea distribution.
    Chen X. C., K. Zhao, and M. Xue, 2014: Spatial and temporal characteristics of warm season convection over Pearl River Delta region,China, based on 3 years of operational radar data. J. Geophys. Res., 119, 12 447-12 465, doi: 10.1002/ 2014JD021965.10.1002/2014JD02196502feb9f0ccf76057b32dff154651390ehttp%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1002%2F2014JD021965%2Fpdfhttp://onlinelibrary.wiley.com/doi/10.1002/2014JD021965/pdfThis study examines the temporal and spatial characteristics and distributions of convection over the Pearl River Delta region of Guangzhou, China, during the May-September warm season, using, for the first time for such a purpose, 3 years of operational Doppler radar data in the region. Results show that convective features occur most frequently along the southern coast and the windward slope of the eastern mountainous area of Pearl River Delta, with the highest frequency occurring in June and the lowest in September among the 5 months. The spatial frequency distribution pattern also roughly matches the accumulated precipitation pattern. The occurrence of convection in this region also exhibits strong diurnal cycles. During May and June, the diurnal distribution is bimodal, with the maximum frequency occurring in the early afternoon and a secondary peak occurring between midnight and early morning. The secondary peak is much weaker in July, August, and September. Convection near the coast is found to occur preferentially on days when a southerly low-level jet (LLJ) exists, especially during the Meiyu season. Warm, moist, and unstable air is transported from the ocean to land by LLJs on these days, and the lifting along the coast by convergence induced by differential surface friction between the land and ocean is believed to be the primary cause for the high frequency along the coast. In contrast, the high frequency over mountainous area is believed to be due to orographic lifting of generally southerly flows during the warm season.
    Fowle M. A., P. J. Roebber, 2003: Short-range (0-48 h) numerical prediction of convective occurrence,mode, and location. Wea. Forecasting, 18, 782-794, doi: 10.1175/1520-0434 (2003)018<0782:SHNPOC>2.0.CO;2.10.1175/1520-0434(2003)0182.0.CO;251e987de409f529b98dee83521635e6dhttp%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2003WtFor..18..782Fhttp://adsabs.harvard.edu/abs/2003WtFor..18..782FA verification of high-resolution (6-km grid spacing) short-range (0–48 h) numerical model forecasts of warm-season convective occurrence, mode, and location was conducted over the Lake Michigan region. All available days from 5 April through 20 September 1999 were evaluated using 0.5° base reflectivity and accumulated precipitation products from the national radar network and the day-1 (0–24 h) and day-2 (24–48 h) forecasts from a quasi-operational version of the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5). Contingency measures show forecast skill for convective occurrence is high, with day-1 (day 2) equitable threat scores and Kuipers skill scores (KSS) of 0.69 (0.60) and 0.84 (0.75), respectively. Forecast skill in predicting convective mode (defined as linear, multicellular, or isolated) is also high, with KSS of 0.91 (0.86) for day 1 (day 2). Median timing errors for convective initiation/dissipation were within 2.5 h for all modes of convection at both forecast ranges. Forecasts of the areal coverage of the 24-h accumulated precipitation in convective events exhibited skill comparable to the lower-resolution, operational models, with median threat scores at day 1 (day 2) of 0.21 (0.24). When small displacements (less than 85 km) in the precipitation pattern were taken into account, threat scores increased to as high as 0.44 for the most organized convective modes. The implications of these results for the use of mesoscale models in operational forecasting are discussed.
    Frich P., L. V. Alexand er, P. Della-Marta B. Gleason, M. Haylock, A. M. G. K. Tank, and T. Peterson, 2002: Observed coherent changes in climatic extremes during the second half of the twentieth century. Climate Research, 19, 193- 212.10.3354/cr01919381589def43f2db979886ad985d85dfe7http%3A%2F%2Fdx.doi.org%2F10.3354%2Fcr019193http://dx.doi.org/10.3354/cr019193ABSTRACT A new global dataset of derived indicators has been compiled to clarify whether frequency and/or severity of climatic extremes changed during the second half of the 20th century, This period provides the best spatial coverage of homogenous daily series, which can be used for calculating the proportion of global land area exhibiting a significant change in extreme or severe weather. The authors chose 10 indicators of extreme climatic events, defined from a larger selection, that could be applied to a large variety of climates. It was assumed that data producers were more inclined to release derived data in the form of annual indicator time series than releasing their original daily observations. The indicators are based on daily maximum and minimum temperature series, as well as daily totals of precipitation, and represent changes in all seasons of the year. Only time series which had 40 yr or more of almost complete records were used, A total of about 3000 indicator time series were extracted from national climate archives and collated into the unique dataset described here. Global maps showing significant changes from one multi-decadal period to another during the interval from 1946 to 1999 were produced. Coherent spatial patterns of statistically significant changes emerge, particularly an increase in warm summer nights, a decrease in the number of frost days and a decrease in intra-annual extreme temperature range. All but one of the temperature-based indicators show a significant change. Indicators based on daily precipitation data show more mixed patterns of change but significant increases have been seen in the extreme amount derived from wet spells and number of heavy rainfall events. We can conclude that a significant proportion of the global land area was increasingly affected by a significant change in climatic extremes during the second half of the 20th century. These clear signs of change are very robust; however, large areas are still not represented, especially Africa and South America.
    Jirak I. L., W. R. Cotton, and R. L. McAnelly, 2003: Satellite and radar survey of mesoscale convective system development. Mon. Wea. Rev., 131, 2428- 2449.10.1175/1520-0493(2003)131<2428:SARSOM>2.0.CO;2767268647886f416135924604bd27a32http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2003MWRv..131.2428Jhttp://adsabs.harvard.edu/abs/2003MWRv..131.2428JAn investigation of several hundred mesoscale convective systems (MCSs) during the warm seasons (April-August) of 1996-98 is presented. Circular and elongated MCSs on both the large and small scales were classified and analyzed in this study using satellite and radar data. The satellite classification scheme used for this study includes two previously defined categories and two new categories: mesoscale convective complexes (MCCs), persistent elongated convective systems (PECSs), meso- circular convective systems (MCCSs), and meso- elongated convective systems (MECSs). Around two-thirds of the MCSs in the study fell into the larger satellite-defined categories (MCCs and PECSs). These larger systems produced more severe weather, generated much more precipitation, and reached a peak frequency earlier in the convective season than the smaller, meso- systems. Overall, PECSs were found to be the dominant satellite-defined MCS, as they were the largest, most common, most severe, and most prolific precipitation-producing systems. In addition, 2-km national composite radar reflectivity data were used to analyze the development of each of the systems. A three-level radar classification scheme describing MCS development is introduced. The classification scheme is based on the following elements: presence of stratiform precipitation, arrangement of convective cells, and interaction of convective clusters. Considerable differences were found among the systems when categorized by these features. Grouping systems by the interaction of their convective clusters revealed that more than 70% of the MCSs evolved from the merger of multiple convective clusters, which resulted in larger systems than those that developed from a single cluster. The most significant difference occurred when classifying systems by their arrangement of convective cells. In particular, if the initial convection were linearly arranged, the mature MCSs were larger, longer-lived, more severe, and more effective at producing precipitation than MCSs that developed from areally arranged convection.
    Lewis M. W., S. L. Gray, 2010: Categorisation of synoptic environments associated with mesoscale convective systems over the UK. Atmos. Res., 97, 194- 213.10.1016/j.atmosres.2010.04.001c0e45cdeb85f473161aedf79b1a8de8ehttp%3A%2F%2Fwww.sciencedirect.com%2Fscience%2Farticle%2Fpii%2FS0169809510000888http://www.sciencedirect.com/science/article/pii/S0169809510000888Mesoscale convective systems (MCSs) are relatively rare events in the UK but, when they do occur, can be associated with weather that is considered extreme with respect to climatology (as indicated by the number of such events that have been analysed as case studies). These case studies usually associate UK MCSs with a synoptic environment known as the Spanish plume. Here a previously published 17 year climatology of UK MCS events is extended to the present day (from 1998 to 2008) and these events classified according to the synoptic environment in which they form. Three distinct synoptic environments have been identified, here termed the classical Spanish plume, modified Spanish plume, and European easterly plume. Detailed case studies of the two latter, newly defined, environments are presented. Composites produced for each environment further reveal the differences between them. The classical Spanish plume is associated with an eastward propagating baroclinic cyclone that evolves according to idealised life cycle 1. Conditional instability is released from a warm moist plume of air advected northeastwards from Iberia that is capped by warmer, but very dry air, from the Spanish plateau. The modified Spanish plume is associated with a slowly moving mature frontal system associated with a forward tilting trough (and possibly cut-off low) at 500 hPa that evolves according to idealised life cycle 2. As in the classical Spanish plume, conditional instability is released from a warm plume of air advected northwards from Iberia. The less frequent European easterly plume is associated with an omega block centred over Scandinavia at upper levels. Conditional instability is released from a warm plume of air advected westwards across northern continental Europe. Unlike the Spanish plume environments, the European easterly plume is not a warm sector phenomena associated with a baroclinic cyclone. However, in all environments the organisation of convection is associated with the interaction of an upper-level disturbance with a low-level region of warm advection.
    Li J., R. C. Yu, and W. Sun, 2013: Duration and seasonality of hourly extreme rainfall in the central eastern China. Acta Meteor. Sinica,27(6), 799-807, doi: 10.1007/s13351-013-0604-y.10.1007/s13351-013-0604-y63593d9ec521267a1f376d47b2de517dhttp%3A%2F%2Fd.wanfangdata.com.cn%2FPeriodical%2Fqxxb-e201306003http://d.wanfangdata.com.cn/Periodical/qxxb-e201306003Compared with daily rainfall amount, hourly rainfall rate represents rainfall intensity and the rainfall process more accurately, and thus is more suitable for studies of extreme rainfall events. The distribution functions of annual maximum hourly rainfall amount at 321 stations in China are quantified by the Gen-eralized Extreme Value (GEV) distribution, and the threshold values of hourly rainfall intensity for 5-yr return period are estimated. The spatial distributions of the threshold exhibit significant regional differ-ences, with low values in northwestern China and high values in northern China, the mid and lower reaches of the Yangtze River valley, the coastal areas of southern China, and the Sichuan basin. The duration and seasonality of the extreme precipitation with 5-yr return periods are further analyzed. The average duration of extreme precipitation events exceeds 12 h in the coastal regions, Yangtze River valley, and eastern slope of the Tibetan Plateau. The duration in northern China is relatively short. The extreme precipitation events develop more rapidly in mountain regions with large elevation differences than those in the plain areas. There are records of extreme precipitation in as early as April in southern China while extreme rainfall in northern China will not occur until late June. At most stations in China, the latest extreme precipitation happens in August-September. The extreme rainfall later than October can be found only at a small por-tion of stations in the coastal regions, the southern end of the Asian continent, and the southern part of southwestern China.
    Ma Y., X. Wang, and Z. Y. Tao, 1997: Geographic distribution and life cycle of mesoscale convective system in China and its vicinity. Progress in Natural Science, 7, 701- 706. (in Chinese)10.1007/s002690050078fc184ddc8713047fd86c7602bd6c1f3dhttp%3A%2F%2Fwww.cnki.com.cn%2FArticle%2FCJFDTotal-ZKJY199705009.htmhttp://www.cnki.com.cn/Article/CJFDTotal-ZKJY199705009.htm A census of mesoscale convective systems (MCS) has been extended from mesoscale convective complexes (MCCs) to more general meso-a scale convective systems (Ma CSs) and meso-β scale convective systems (Mβ CSs). 234 Ma CSs and 585 Mβ CSs were found in China and its vicinity during the summers of 1993-1995 by the GMS satellite infrared images. The geographic distribution with higher representative shows that the Ma CSs occurred in three favorable zones. One of them, the middle-to-lower reaches basin of the Yellow River and the Yangtze River, were not found in the past researches of MCC census. There are two kinds of life cycles of Ma CS: one is similar to the life cycle of MCC occurring at night and dissipating early in the morning; the other occurs in the afternoon and dissipates in the evening.
    Markowski P., Y. Richardson, 2010: Mesoscale Meteorology in Midlatitudes. John Wiley & Sons,407 pp.10.1002/9780470682104.ch227b74ae651fe9cd00029769552cbd8d8http%3A%2F%2Fci.nii.ac.jp%2Fncid%2FBB02962383http://ci.nii.ac.jp/ncid/BB02962383Mesoscale meteorology in midlatitudes Paul Markowski and Yvette Richardson (Advancing weather and climate science) Wiley-Blackwell, 2010
    Meng Z. Y., D. C. Yan, and Y. J. Zhang, 2013: General features of squall lines in east China. Mon. Wea. Rev.,141, 1629-1647, doi: 10.1175/mwr-d-12-00208.110.1175/MWR-D-12-00208.106aa8679edbba273c85f275458464933http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2013MWRv..141.1629Mhttp://adsabs.harvard.edu/abs/2013MWRv..141.1629MNot Available
    Nesbitt S. W., E. J. Zipser, 2003: The diurnal cycle of rainfall and convective intensity according to three years of TRMM measurements. J. Climate,16, 1456-1475, doi: 10.1175/ 1520-0442-16.10.1456.c9209129c5e80efdc003bf009abbbfedhttp%3A%2F%2Fbioscience.oxfordjournals.org%2Fexternal-ref%3Faccess_num%3D10.1175%2F1520-0442-16.10.1456%26link_type%3DDOIhttp://bioscience.oxfordjournals.org/external-ref?access_num=10.1175/1520-0442-16.10.1456&amp;link_type=DOI
    Newton C. W., 1966: Circulations in large sheared cumulonimbus. Tellus, 18, 699-713, doi: 10.1111/j.2153-3490.1966.tb00291. x.10.1111/j.2153-3490.1966.tb00291.x1e648a1125b060493414df0d00071a70http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1111%2Fj.2153-3490.1966.tb00291.x%2Fabstracthttp://onlinelibrary.wiley.com/doi/10.1111/j.2153-3490.1966.tb00291.x/abstractABSTRACT The structure of a cumulonimbus cloud subjected to vertical shear is interpreted in light of the horizontal forces acting upon it, and of the varying thermodynamic influences in its different parts. In-cloud horizontal velocities depart greatly from those in the environment, and the forms assumed by draft columns (updrafts typically leaning in a sense opposing the vertical shear) vary with the shear, vertical motion, and speed of storm movement. The cumulonimbus is viewed as an ensemble of air elements which have undergone varying degrees of mixing with the environment, penetrating upward to different heights. Some of the air in the updraft rises into stratospheric towers, then descends as a vigorous downdraft which, because of mixing-in of heat and of air having no initial vertical momentum, dies out in the upper troposphere. This air, together with air reaching the upper troposphere in the less buoyant outskirts of the updraft, feeds the expanding anvil plume. A separate downdraft in the lower part of the cloud, originating from middle levels where the wet-bulb potential temperature is low, continually regenerates the updraft though mechanical lifting. Estimates of the air and water budgets of squall-line thunderstorms are given.
    Orlanski I., 1975: A rational subdivision of scales for atmospheric processes. Bull. Amer. Meteor. Soc., 56, 527- 530.102ea44b951efff60a2ad2649803b397http%3A%2F%2Fwww.citeulike.org%2Fgroup%2F17501%2Farticle%2F12086670http://www.citeulike.org/group/17501/article/12086670Search all the public and authenticated articles in CiteULike. Include unauthenticated resultstoo (may include "spam") Enter a search phrase. You can also specify a CiteULike article id(123456),. a DOI (doi:10.1234/12345678). or a PubMed ID (pmid:12345678).
    Parker M. D., R. H. Johnson, 2000: Organizational modes of midlatitude mesoscale convective systems. Mon. Wea. Rev., 128, 3413- 3436.10.1175/1520-0493(2001)1292.0.CO;2bced1c47ac08e60d5a835e4d9b512c6dhttp%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2000mwrv..128.3413phttp://adsabs.harvard.edu/abs/2000mwrv..128.3413pThis paper discusses common modes of mesoscale convective organization. Using 2-km national composite reflectivity data, the authors investigated linear mesoscale convective systems (MCSs) that occurred in the central United States during May 1996 and May 1997. Based upon the radar-observed characteristics of 88 linear MCSs, the authors propose a new taxonomy comprising convective lines with trailing (TS), leading (LS), and parallel (PS) stratiform precipitation. While the TS archetype was found to be the dominant mode of linear MCS organization, the LS and PS archetypes composed nearly 40% of the studied population. In this paper, the authors document the characteristics of each linear MCS class and use operational surface and upper air data to describe their different environments. In particular, wind profiler data reveal that the stratiform precipitation arrangement associated with each class was roughly consistent with the advection of hydrometeors implied by the mean middle- and upper-tropospheric storm-relative winds, which were significantly different among the three MCS modes. Case study examples are presented for both the LS and PS classes, which have received relatively little attention to this point. As well, the authors give a general overview of the synoptic-scale meteorology accompanying linear MCSs in this study, which was generally similar to that observed by previous investigators.
    Raymond D. J., H. Jiang, 1990: A theory for long-lived mesoscale convective systems. J. Atmos. Sci.,47, 3067-3077, doi: 10.1175/1520-0469(1990)047<3067:ATFLLM>2.0.CO; 2.10.1175/1520-0469(1990)0472.0.CO;2b2feb8573f617af33b2422b5e942cb9fhttp%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F1990JAtS...47.3067Rhttp://adsabs.harvard.edu/abs/1990JAtS...47.3067RIt is proposed that certain long-lived mesoscale convective systems maintain themselves through an interaction between quasi-balanced vertical motions and the diabatic effects of moist convection. Latent heat release, evaporation and melting of precipitation, and thermal radiation are all shown to contribute to the creation of a positive potential vorticity anomaly in the lower troposphere. This anomaly can interact with a sheared environment so as to induce further lifting of low-level air and subsequent release of conditional instability.
    Schumacher R. S., R. H. Johnson, 2006: Characteristics of U.S. Extreme Rain Events during 1999-2003. Wea.Forecasting, 21, 69- 85. doi:10.1175/WAF900.110.1175/WAF900.174d72c6ef3d6306ee7d82094430ce882http%3A%2F%2Fci.nii.ac.jp%2Fnaid%2F30022368284http://ci.nii.ac.jp/naid/30022368284This study examines the characteristics of a large number of extreme rain events over the eastern two-thirds of the United States. Over a 5-yr period, 184 events are identified where the 24-h precipitation total at one or more stations exceeds the 50-yr recurrence amount for that location. Over the entire region of study, these events are most common in July. In the northern United States, extreme rain events are confined almost exclusively to the warm season; in the southern part of the country, these events are distributed more evenly throughout the year. National composite radar reflectivity data are used to classify each event as a mesoscale convective system (MCS), a synoptic system, or a tropical system, and then to classify the MCS and synoptic events into subclassifications based on their organizational structures. This analysis shows that 66% of all the events and 74% of the warm-season events are associated with MCSs; nearly all of the cool-season events are caused by storms with strong synoptic forcing. Similarly, nearly all of the extreme rain events in the northern part of the country are caused by MCSs; synoptic and tropical systems play a larger role in the South and East. MCS-related events are found to most commonly begin at around 1800 local standard time (LST), produce their peak rainfall between 2100 and 2300 LST, and dissipate or move out of the affected area by 0300 LST.
    Wang C.-C., J. C.-S. Hsu, G. T.-J. Chen, and D.-I. Lee, 2014: A study of two propagating heavy-rainfall episodes near Taiwan during SoWMEX/TiMREX IOP-8 in June 2008. Part I: Synoptic evolution, episode propagation, and model control simulation. Mon. Wea. Rev., 142, 2619- 2643.10.1175/MWR-D-13-00331.1bd30d2c0c4b448394c4c8984943c38b9http%3A%2F%2Fconnection.ebscohost.com%2Fc%2Farticles%2F97319520%2Fstudy-two-propagating-heavy-rainfall-episodes-near-taiwan-during-sowmex-timrex-iop-8-june-2008-part-i-synoptic-evolution-episode-propagation-model-control-simulationhttp://connection.ebscohost.com/c/articles/97319520/study-two-propagating-heavy-rainfall-episodes-near-taiwan-during-sowmex-timrex-iop-8-june-2008-part-i-synoptic-evolution-episode-propagation-model-control-simulationAbstract This paper is the first of a two-part study to investigate two rain-producing episodes in the longitude–time (Hovm02ller) space upstream from Taiwan during the eighth intensive observing period (IOP-8, 12–17 June 2008) of the Southwest Monsoon Experiment/Terrain-influenced Monsoon Rainfall Experiment (SoWMEX/TiMREX), with a goal to better understand the mechanism and controlling factors for their organization and propagation. Both in a prefrontal environment, the first episode moved eastward and the second was a rare westward-moving event, and each caused heavy rainfall in Taiwan, on 14 and 16 June, respectively. In Part II, the roles played by synoptic conditions and terrain effects are further examined through sensitivity tests. With the aid from a successful simulation with a grid spacing of 2.5 km, the structure and organization of convection embedded in the two episodes are shown to be different. With stronger low-level vertical wind shear in its environment, the first episode consisted of well-organized squall-line-type convective systems and propagated eastward mainly through cold-pool dynamics. However, the convection of the second episode was scattered and less organized with weaker vertical shear, and individual cells traveled with background flow toward the north-northeast. Throughout the 6-day case period, the southwesterly low-level jet (LLJ) is found to have much control over the general region of convection, and thus dictates the overall rainfall pattern in the Hovm02ller space at the regional scale. The rapid development of the mei-yu front and LLJ over southeastern China during 16–17 June, to the west of the previous location of the jet, is found to result in the westward movement of the second episode.
    Yu R. C., J. Li, 2012: Hourly rainfall changes in response to surface air temperature over eastern contiguous China. J.Climate, 25( 19), 6851- 6861.10.1175/JCLI-D-11-00656.1cff43ede691866c68834b8bacd2d889chttp%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2012JCli...25.6851Yhttp://adsabs.harvard.edu/abs/2012JCli...25.6851YNot Available
    Yu R. C., T. J. Zhou, A. Y. Xiong, Y. J. Zhu, and J. M. Li, 2007: Diurnal variations of summer precipitation over contiguous China. Geophys. Res. Lett., 34,L01704, doi: 10.1029/2006 GL028129.10.1029/2006GL0281299dca2266e41acf2c3e6fcd2f9d465f95http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F2006GL028129%2Fabstracthttp://onlinelibrary.wiley.com/doi/10.1029/2006GL028129/abstract[1] Diurnal variations of summer precipitation over contiguous China are studied using hourly rain-gauge data from 588 stations during 1991-2004. It is found that summer precipitation over contiguous China has large diurnal variations with considerable regional features. Over southern inland China and northeastern China summer precipitation peaks in the late afternoon, while over most of the Tibetan Plateau and its east periphery it peaks around midnight. The diurnal phase changes eastward along the Yangtze River Valley, with a midnight maximum in the upper valley, an early morning peak in the middle valley, and a late afternoon maximum in the lower valley. Summer precipitation over the region between the Yangtze and Yellow Rivers has two diurnal peaks: one in the early morning and another in the late afternoon.
    Zhai P. M., R. E. Eskridge, 1997: Atmospheric water vapor over China. J. Climate,10, 2643-2652, doi: 10.1175/1520-0442(1997)010<2643:AWVOC>2.0.CO;2.10.1175/1520-0442(1997)0102.0.CO;2800233c6eab38f9b1ac800521503daa1http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F1997JCli...10.2643Zhttp://adsabs.harvard.edu/abs/1997JCli...10.2643ZAbstract Chinese radiosonde data from 1970 to 1990 are relatively homogeneous in time and are used to examine the climatology, trends, and variability of China’s atmospheric water vapor content. The climatological distribution of precipitable water (PW) is primarily dependent on surface temperature. Influenced by the east Asia monsoon, China’s precipitable water exhibits very large seasonal variations. Station elevation is also a dominant factor affecting water vapor distribution in China. An increase (decrease) in precipitable water over China is associated with an increase (decrease) of precipitation in most regions. Increases in the percentage of PW relative to climatology are greater in winter and spring than in summer and autumn. Interannual variation and trends in precipitable water and surface temperature are closely correlated in China, confirming a positive “greenhouse” feedback. Interannual variations between precipitable water and precipitation are also significantly correlated.
    Zhang H., P. M. Zhai, 2011: Temporal and spatial characteristics of extreme hourly precipitation over eastern China in the warm season. Adv. Atmos. Sci.,28, 1177-1183, doi: 10.1007/ s00376-011-0020-0.10.1007/s00376-011-0020-0e8c4b593a69b036cacf1dbda66a132e5http%3A%2F%2Fwww.cnki.com.cn%2FArticle%2FCJFDTotal-DQJZ201105018.htmhttp://d.wanfangdata.com.cn/Periodical_dqkxjz-e201105017.aspxBased on hourly precipitation data in eastern China in the warm season during 1961 2000,spatial distributions of frequency for 20 mm h-1 and 50 mm h-1 precipitation were analyzed,and the criteria of short-duration rainfall events and severe rainfall events are discussed.Furthermore,the percentile method was used to define local hourly extreme precipitation; based on this,diurnal variations and trends in extreme precipitation were further studied.The results of this study show that,over Yunnan,South China,North China,and Northeast China,the most frequent extreme precipitation events occur most frequently in late afternoon and/or early evening.In the Guizhou Plateau and the Sichuan Basin,the maximum frequency of extreme precipitation events occursin the late night and/or early morning.And in the western Sichuan Plateau,the maximum frequency occursin the middle of the night.The frequency of extreme precipitation (based on hourly rainfall measurements) has increased in mostparts of eastern China,especially in Northeast China and the middle and lower reaches of the Yangtze River,but precipitation has decreased significantly in North China in the past 50 years.In addition,stations inthe Guizhou Plateau and the middle and lower reaches of the Yangtze River exhibit significant increasing trends in hourly precipitation extremes during the nighttime more than during the daytime.
    Zheng Y. G., J. Chen, and P. J. Zhu, 2008: Climatological distribution and diurnal variation of mesoscale convective systems over China and its vicinity during summer. Chin. Sci. Bull.,53(10), 1574-1586, doi: 10.1007/s11434-008-0116-9.10.1007/s11434-008-0116-9acd920c76e21caadef6faf173899cc13http%3A%2F%2Flink.springer.com%2F10.1007%2Fs11434-008-0116-9http://www.cnki.com.cn/Article/CJFDTotal-JXTW200810018.htmThe climatological distribution of mesoscale convective systems (MCSs) over China and its vicinity during summer is statistically analyzed, based on the 10-year (1996―2006, 2004 excluded) June-August infrared TBB (Temperature of black body) dataset. Comparing the results obtained in this paper with the distribution of thunderstorms from surface meteorological stations over China and the distribution of lightning from low-orbit satellites over China and its vicinity in the previous studies, we find that the statistic characteristics of TBB less than -52℃ can better represent the spatiotemporal distribution of MCSs over China and its vicinity during summer.The spreading pattern of the MCSs over this region shows three transmeridional bands of active MCSs, with obvious fluctuation of active MCSs in the band near 30°N. It can be explained by the atmospheric circulation that the three bands of active MCSs are associated with each other by the summer monsoon over East Asia. We focus on the diurnal variations of MCSs over different underlying surfaces, and the result shows that there are two types of MCSs over China and its vicinity during summer. One type of MCSs has only one active period all day long (single-peak MCSs), and the other has multiple active periods (multi-peak MCSs). Single-peak MCSs occur more often over plateaus or mountains, and multi-peak MCSs are more common over plains or basins. Depending on lifetimes and active periods, single-peak MCSs can be classified as Tibetan Plateau MCSs, general mountain MCSs, Ryukyu MCSs, and so on. The diurnal variation of multi-peak MCSs is very similar to that of MCCs (mesoscale convective complexes), and it reveals that multi-peak MCSs has longer life cycle and larger horizontal scale, becomes weaker after sunset, and develops again after midnight. Tibetan Plateau MCSs and general mountain MCSs both usually develop in the afternoon, but Tibetan Plateau MCSs have longer life cycle and more active MαCSs. Ryukyu MCSs generally develop after midnight, last longer time, and also have more active MαCS. The abundant moisture and favorable large-scale environment over Indian monsoon surge areas lead to active MCSs and MαCSs almost at any hour all day during summer. Due to local mountain-valley breeze circulation over the Sichuan Basin, MCSs are developed remarkably more often during the nighttime, and again there are also more active MαCSs. Because of local prominent sea-land breeze circulation over Guangxi and Guangdong, the MCSs over this region propagate from sea to land in the afternoon and from land to sea after midnight. The statistic characteristics of TBB less than -52℃ clearly display the different climatological characteristics of MCSs owing to the thermal difference among water, land and rough terrain. Not only the large-scale atmospheric circulation but also the local atmospheric circulation caused by the thermal difference among water, land and rough terrain, to a great extent, determines the climatological distribution of MCSs over China and its vicinity during summer.
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Manuscript received: 30 January 2016
Manuscript revised: 17 August 2016
Manuscript accepted: 18 September 2016
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The Contribution of Mesoscale Convective Systems to Intense Hourly Precipitation Events during the Warm Seasons over Central East China

  • 1. Laboratory for Climate and Ocean-Atmosphere Studies, Department of Atmospheric and Oceanic Sciences, School of Physics, Peking University, Beijing 100871, China
  • 2. National Meteorological Center, China Meteorological Administration, Beijing 100081, China

Abstract: Central East China is an area where both intense hourly precipitation (IHP) events and mesoscale convection systems (MCSs) occur frequently in the warm seasons. Based on mosaics of composite Doppler radar reflectivity and hourly precipitation data during the warm seasons (May to September) from 1 July 2007 to 30 June 2011, the contribution of MCSs to IHP events exceeding 20 mm h-1 over central East China was evaluated. An MCS was defined as a continuous or quasi-continuous band of 40 dBZ reflectivity that extended for at least 100 km in at least one direction and lasted for at least 3 h. It was found that the contribution of MCSs to IHP events was 45% on average over central East China. The largest contribution, more than 80%, was observed along the lower reaches of the Yellow River and in the Yangtze River-Huaihe River valleys. These regions were the source regions of MCSs, or along the frequent tracks of MCSs. There were two daily peaks in the numbers of IHP events: one in the late afternoon and one in the early morning. These peaks were more pronounced in July than in other months. MCSs contributed more to the early-morning IHP event peaks than to the late-afternoon peaks. The contributions of MCSs to IHP events with different intensities exhibited no significant difference, which fluctuated around 50% on average over central East China.

1. Introduction
  • Short-duration intense precipitation events are responsible for serious disasters, including flash flooding, landslides, mudflows, and urban water logging. Hourly precipitation datasets are very suitable for studying the detailed characteristics of short-duration intense precipitation events, given their high spatial and temporal resolution (Chen et al., 2013; Li et al., 2013). In recent years, changes in the frequency and intensity of intense hourly precipitation (IHP) events have received much attention, and the trends in different regions of China have been found to be different (Zhang and Zhai, 2011; Yu and Li, 2012). These different trends might be associated with many factors, of which the different contribution to IHP events by different kinds of convection may be an important one. IHP events might be caused by either isolated convection or organized convection, such as mesoscale convective systems (MCSs). These two kinds of convection are different mainly in terms of their spatial and temporal scales. Studies have shown that convection of different scales have different spatial and temporal distributions and might be caused by different forcing mechanisms and physical processes (Casati et al., 2004; Chen et al., 2014). Generally, well-organized, long-lived MCSs require stronger synoptic forcing than isolated convection, and suitable deep-layer wind shear plays an important role in the upscale evolution from isolated convection to MCSs, as well as the maintenance of long-lived MCSs (Raymond and Jiang, 1990; Lewis and Gray, 2010; Markowski and Richardson, 2010; Wang et al., 2014). Besides, isolated convection and MCSs have different organizational modes and different precipitation efficiencies, and this may be associated with the differences in their precipitation intensity and contributions to intense precipitation (Newton, 1966; Jirak et al., 2003). Due to these differences between isolated convection and MCSs, it is necessary to study them separately. Thus, the possession of good knowledge regarding their respective contribution to IHP events is important.

    Central East China is known to be an area where IHP events occur frequently (Zhang and Zhai, 2011), and many efforts have been made to investigate the characteristics of IHP events over this region (Chen et al., 2013; Li et al., 2013). However, the nature of the main contributing factors to IHP events in this region remains unclear. Many studies have shown that central East China has a high frequency of MCSs (Ma et al., 1997; Zheng et al., 2008; Meng et al., 2013), but what percentage of IHP events are contributed by MCSs in central East China? To the best of our knowledge, this question has yet to be addressed. And the answer to this question might be helpful in understanding the mechanisms involved in the temporal and spatial variation of IHP events.

    The remainder of this paper is organized as follows: The data and methodology are described in section 2. Section 3 describes the contribution of MCSs to IHP events in terms of spatial and temporal variation, as well as the contributions of MCSs to precipitation events of different intensities. A summary and discussion are given in section 4.

2. Data and methodology
  • Central East China was defined as the region covering (30°-37°N, 110°-112°E), as shown in Fig. 1. The radar data used in this study were digital mosaics of composite Doppler radar reflectivity. The data had a spatial resolution of 4× 4 km, with an interval of 20 min (10 min) before (after) 22 September 2008. We chose a 4-yr period from 1 July 2007 to 30 June 2011 to ensure a relatively large and continuous data record. During this period, the average data coverage was about 85%.

    Figure 1.  The terrain height (color-shaded; units: m), radar locations (light blue dots), and rain gauge station locations (small black dots) in central East China. The names of relevant provinces are marked in the center of each province. The names of the Yellow River, Huaihe River, and Yangtze River are also marked, along each river. The inner black box is central East China (30$^\circ$-37$^\circ$N, 110$^\circ$-112$^\circ$E).

    To incorporate MCSs of various morphologies, the criterion for an MCS in this work, based on radar data, was: a continuous or quasi-continuous band of 40 dBZ reflectivity that extended for at least 100 km in at least one direction and lasted for at least 3 h. This definition is consistent with previous studies (Orlanski, 1975; Parker and Johnson, 2000; Schumacher and Johnson, 2006). Under this criterion, the threshold of 40 dBZ was used to distinguish convective and stratiform echoes (Fowle and Roebber, 2003). The length scale was defined as 100 km so that the Coriolis acceleration was of the same order as other terms in the momentum equations. The appropriate timescale for an MCS was therefore f-1, which yielded 3 h for a typical midlatitude value of the Coriolis parameter f (Parker and Johnson, 2000; Markowski and Richardson, 2010). The MCSs in this study were identified manually. This was because there were serious data quality problems in terms of ground clutter in the radar images. Although it took a lot of work to identify MCSs on a case-by-case, the results may be more reliable compared with automatically detected MCSs. The track of an MCS was defined as the line that joined the location of its formation and dissipation.

    The hourly precipitation dataset used in this work comprised observations from 2420 national rain gauge stations in China during the warm seasons (May to September for most stations) from 1951 to 2012. The data were subjected to strict quality-control procedures by the Chinese National Meteorological Information Center, which is part of the China Meteorological Administration (Yu et al., 2007). In this work, observations from 527 stations (Fig. 1) in central East China during the warm seasons, from 1 July 2007 to 30 June 2011, were used to be consistent with the temporal coverage of the radar data.

    In some previous studies on extreme precipitation, the local percentile threshold was used to define extreme precipitation because climatologies in different regions are different. (Frich et al., 2002; Schumacher and Johnson, 2006). In this study, as we focused on the regional aspects of MCSs and intense rainfall, a fixed threshold was used for simplicity. Another issue concerns how to determine an appropriate hourly precipitation threshold for the region. Based on hourly precipitation data from 1961 to 2000, (Zhang and Zhai, 2011) gave the 95th percentile of the extreme hourly precipitation distribution and deemed 20 mm h-1 as a suitable threshold for eastern China. (Chen et al., 2013) found similar distributional patterns for the thresholds of 10 mm h-1, 20 mm h-1, 30 mm h-1 and 40 mm h-1. When a threshold of 50 mm h-1 was used, however, the distributional pattern was significantly different, and events occurred with a much lower frequency. Based on these results, the threshold of 20 mm h-1 was used in this study, to be consistent with previous research. For any given station, each IHP record exceeding 20 mm h-1 at that station was termed as an IHP event for simplicity.

    For each IHP event, the radar images were examined to determine whether the event was caused by MCSs. During the particular hour with accumulated rainfall exceeding 20 mm at a station, there might be 3-6 radar observations. If the station was affected by convective echoes (>40 dBZ) of an MCS in any of the radar observations, this event was deemed to be caused by an MCS. Otherwise, we deemed it was contributed by isolated convection. The convection that did not reach 100 km before MCS formation or after MCS dissipation was deemed as isolated convection because it was difficult to determine whether the isolated convection was associated with a certain MCS.

3. Results
  • According to the definitions of MCSs and IHP events given above, 302 MCSs and 8162 IHP events were identified during the warm seasons of 1 July 2007 to 30 June 2011 over central East China. For each station, the mean number of IHP events was 3.9 per warm season. Among the 8162 IHP events, 3655 were caused by MCSs, accounting for about 45% of all IHP events. The spatial distribution and diurnal variation of MCS contribution, as well as the contributions of MCSs to precipitation events of different intensities, are given below.

  • Figure 2a shows the distribution of total precipitation over central East China during the warm seasons of June 2007 to July 2011. The total precipitation decreased from the southeast to the northwest. There were several regions where large numbers of IHP events were observed. These regions were distributed along the lower reaches of the Yellow River, the Huaihe River, and the middle and lower reaches of the Yangtze River (Fig. 2b). Most of the regions with large numbers of IHP events were associated with high contributions from MCSs, with the exception of the middle reaches of the Yangtze River where IHP events might mostly have been contributed by scattered or isolated convection rather than organized MCSs.

    Figure 2.  The (a) total precipitation (units: mm), (b) numbers of IHP events, and (c) percentages of IHP events caused by MCSs (units: %), for each station over central East China during the warm seasons of June 2007 to July 2011. The inner black box indicates central East China.

    The contributions of MCSs to IHP events varied significantly among different stations. Figure 2c shows two local maxima where the contribution was greater than 80%. One local maximum was observed along the lower reaches of the Yellow River near the boundary between Henan and Shandong Province. The other was observed in the Yangtze River-Huaihe River valleys near the boundary between Jiangsu and Anhui Province. As indicated in Fig. 3, the maxima were caused by either locally formed MCSs or upstream MCSs propagating downstream.

    There were several regions where MCSs occur very frequently, including the boundary between Anhui and Henan provinces, the boundary between Anhui and Jiangsu provinces, and the southwest of Shandong Province (Fig. 3). These regions were all associated with high contributions of MCSs to IHP events (Fig. 2c). A high contribution of MCSs to IHP events may also have resulted from MCSs propagating from upstream regions. For example, the high contribution of MCSs over the southwest of Shandong Province was partly caused by MCSs propagating northeastward from the source region of MCSs near the boundary between Henan and Shanxi provinces (Fig. 3a).

    Figure 3.  The distribution of MCS tracks during the warm seasons of July 2007 to June 2011 over central East China: MCSs moving (a) northeast, (b) southeast, and (c) in other directions. The color shading represents the numbers of MCSs in the corresponding $1^\circ\times 1^\circ$ box. The dots in each figure represent the locations where MCSs formed. The lines in each figure are the tracks of MCSs from formation to dissipation. The inner black boxes in each subplot indicate central East China.

  • In addition to the spatial distribution of the contribution of MCSs to IHP events, the diurnal variation was also explored. Figure 4 shows the diurnal variation in MCS numbers and the number of IHP events. Peaks in the late afternoon and early morning were observed in the diurnal cycle of MCSs, with a significant peak after midnight in July. In contrast, the diurnal variation in IHP events also exhibited double peaks, but the peak in the early morning was less evident than the peak in the late afternoon, except in July when the early morning peak became nearly comparable with the afternoon peak.

    The number of IHP events contributed by MCSs did not show as strong diurnal variation as the total number of IHP events. However, when calculating the percentage of IHP events caused by MCSs for each 3 h interval, it was found that MCSs contributed more to the early morning peak than to the afternoon peak. For example, the percentage of IHP events contributed by MCSs in July was 59% from 0200 LST (Local Standard Time, UTC+8) to 0500 LST, but the percentage was about 47% from 1700 LST to 2000 LST. This result is consistent with (Nesbitt and Zipser, 2003), who found that nocturnal rain is more often caused by MCSs rather than isolated convection. Therefore, more attention should be paid to MCSs when studying IHP events in the early morning.

    Figure 4.  Diurnal and monthly variation of (a) MCS numbers and (b) IHP events, at 3 h intervals (indicated by the different color bars), during the warm seasons of July 2007 to June 2011 over central East China. The filled parts of the bars in (b) represent events caused by MCSs, while the hatched parts represent IHP events caused by isolated convection. The time reference is LST (UTC+8).

  • As reported above, over central East China, the contribution of MCSs to IHP events exceeding 20 mm h-1 was about 45% on average. But did this percentage vary for IHP events of different intensities? Figure 5 shows the contribution of MCSs to IHP events of different intensities. The number of IHP events decreased as precipitation intensity increased, but the contributions of MCSs to IHP events exhibited no significant difference, which fluctuated near 50% on average over central East China. This suggested that, for IHP events, the contributions of MCSs and isolated convection were similar.

    Figure 5.  Number of IHP events for different precipitation intensities. The filled parts of the bars represent events caused by MCSs, while hatched parts are non-MCS events. The percentages of IHP events contributed by MCSs, for IHP events of different intensities, are marked on top of each bar.

4. Summary and discussion
  • Based on radar and hourly precipitation data, the contribution of MCSs to IHP events was evaluated for the warm seasons of July 2007 to June 2011 in central East China. Using the definitions given in section 2, 302 MCSs and 8162 IHP events were identified. Of all the IHP events, 45% of them were caused by MCSs.

    The spatial distribution and diurnal variation of the contribution of MCSs to IHP events, as well as the contributions of MCSs to precipitation events of different intensities, were also documented. MCSs contributed the most to IHP events in the lower reaches of the Yellow River and the Yangtze River-Huaihe River valleys, where the contribution could reach more than 80%. These regions were found to be the source regions of MCSs, or situated just along the frequent tracks of MCS. The diurnal variation in the number of IHP events showed two peaks: one in the late afternoon and one in the early morning. The early-morning peak was more pronounced in July than in other months. The contribution of MCSs to IHP events in the early morning was larger than that in the late afternoon. The contributions of MCSs to IHP events of different intensity exhibited no significant difference, which fluctuated around 50% on average over central East China.

    Central East China is located in a summer monsoon region, where there is plenty of moisture and a high frequency of IHP events during the warm seasons (Zhai and Eskridge, 1997; Zhang and Zhai, 2011). Intense rainfall events may result from both isolated convection and MCSs. The two kinds of convection should be studied separately due to the different scales involved. Understanding their contribution to intense precipitation could help both forecasting and research communities to focus on key aspects of convection. It was found that organized MCSs and isolated convection may, on average, contribute equally to intense precipitation events in central East China. The contributions depended significantly on spatial and temporal variation. For regions that were frequently affected by MCSs, like the lower reaches of the Yellow River and Yangtze River-Huaihe River valleys, more attention should be paid to MCSs. As MCSs often require strong synoptic forcing compared with isolated convection, studies of convection should concentrate more on synoptic and dynamical forcing. For other regions, such as in the middle reaches of the Yangtze River near the southern boundary of central East China, isolated or scattered convection may play important contrasting roles, and thermal or local effects may be more important for convection. Nocturnal MCSs should also be emphasized when studying intense nocturnal rainfall. These findings are also helpful in understanding the changes in intense precipitation for climate research.

    Whilst it is true that the current criteria used to identify isolated convection and MCSs cannot separate convection of different mechanisms and physical processes well, due to the possible transition between isolated convection and MCSs, the present results nonetheless provide us with an indication of the main mechanisms involved in IHP for different regions, considering MCSs generally require stronger large-scale forcing than isolated convection. It would have been useful and interesting to use different criteria in this study. However, due to the huge amount of work involved in testing different criteria when identifying MCSs and isolated convection manually, we decided against carrying out such an analysis on this occasion. Therefore, further research, using radar data that has been subjected to high levels of quality control, is still needed in the future, to better understand this issue.

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