Mesoscale Convective System (MCS) is an important system that causes heavy rain, hail, thunderstorm, gale, tornado and other disastrous weather.[1]The mesoscale convective system is composed of convective cells, multicell storms and supercell storms in various forms, including squall lines and mesoscale convective convergence, with a horizontal scale of 2-2500 km.
MacGorman et al[2]It is proposed that MCS is a group of storms that interact with the environment and can change the environment, and then produce a larger long-life system than a single storm.Houze[3]usecloud chartThe characteristics describe MCS as a cloud structure containing convective core, which extends about 100 km along a certain direction, forming a general precipitation area.Schumacher[4]useRadar reflectivityThe factor data defines MCS, that is, the area where the reflectivity factor of the convective system is ≥ 40 dBz, the range is>100 km, and the duration is 3~24 h.
Most meteorologists believe that[1]In the life cycle of MCS, there is a structure of convective development to a certain extent, which may be organized linear or quasi circular, or unorganized, but can produce heavy precipitation.Its dynamic characteristics mainly include: 1)Convection zoneAnd layered area;2) The circulation associated with MCS not only affects the small and medium-sized background field, but also affects the large-scale environmental field;3) It is generally found thatGust front, while the gust front andDowndraftIt is related to the cold hills generated.
classification
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According to the organization shape, monomer and system movement characteristics of MCSs, MCSs can be classified into three types. See Table 1 for the results[1]。
Table 1 Classification of Mesoscale Convective Systems (MCSs)
type
to subdivide
features
Monomer MCSs
Do not move single MCSs
Mobile monomer MCSs
Merged monomer MCSs
When mature, there is only one convection core, and there is no other convection development within 60km around
Linear MCSs
Nearby layered areas/unidirectional linear MCSs
Quasi stationary/rear newborn MCSs
MCSs following layered precipitation
MCSs with leading layered precipitation
MCSs of parallel layered precipitation
Broken linear MCSs
MultipleConvection cellThe monomer can be closely arranged or separated at certain intervals, and the direction of arrangement is generally the same as that ofechoThe movement direction of is vertical
Regional MCSs
Regional MCSs without convection embedding
Regional MCSs embedded by convection
Group echo composed of single cluster appears on PPI, or many strong convective cells are embedded in the large area of stratiform cloud echo, and the echo area has the same length and width, without obvious current convection cell zone
Spatiotemporal distribution
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Global
Velasco, Miller, Laing, etc[5]The global survey of MCCs in North and South America, the Western Pacific, Africa and Asia shows that MCCs in different regions (MCCs refer toMesoscale convective complex, only a small part of MCSs) have similar characteristics, generally formed on land, and most likely occur in large mountainsLeeward slope(East side), with an average duration of 9~11.5 h, coverage of (2~3) × 105 km, and propagation towards the low level high potential equivalent temperature airflow;MCC has obvious nocturnal occurrence, that is, it forms at 20-23 local time, reaches the maximum at 03-05 local time, and dissipates at 07-09 local time;The movement direction of MCCs in different regions is different. For example, MCCs in North Africa mostly move to the southwest by west, and move to the northeast and north in southern Africa.
China
Fig. 1 Distribution of M α CS (a) and M β CS (b) in summer of 1993-1995 in China
Ma Yu[6]Using the GMS satellite hourly infrared cloud image data from 1993 to 1995, et al. conducted a comprehensive survey of mesoscale convective systems in China and adjacent areas.MCS in China has obvious geographical distribution characteristics (Figure 1).
It can be seen from Figure 1 that the distribution patterns of M α CS and M β CS in China are basically the same, and the southwest area and its adjacent areas between 100 ° - 110 ° E to the south of 35 ° NVietnam?The north is the area where M α CS occurs most. There is also a concentration area of M α CS in the middle and lower reaches of the Yellow River and the Yangtze River and its eastern coast between 40 ° - 28 ° N to the east of 110 ° E.There are two MCS centers, one is in the Yangtze Huaihe River basin, and the other is on the north bank of the Beibu Gulf. The Yangtze Huaihe River center of M α CS is about 500 km east of M β CS, indicating that the former is moving eastward while developing, and it naturally moves eastward when it reaches the peak of the α scale;In addition,Qinghai Tibet PlateauThere are more M α CS and less M β CS in the east, but the opposite is true near Hunan.
Most M α CSs and M β CSs form in the afternoon (15-17 BST, Beijing time), but M α CS has a more obvious peak at night (21-23 BST), while M β CSs only have a very small peak at night (01 BST), indicating that M α CS is nocturnal.M β CSs can mature within 2h after formation (the area reaches the maximum), while M α CS formed in the afternoon takes 3-4 hours to mature, which is longer than M β CSs.M α CS formed at night will mature after 7-8 hours, and M α CS and M β CSs can generally maintain 2-4 hours after maturity.The life history of M α CS and M β CSs is 7-8 h and 5-6 h respectively, which means that the life history of larger scale MCSs is also longer.
Generate environmental conditions
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MCCs
From the geographical distribution of MCCs, studies at home and abroad show that the most active areas of MCCs are generally located in the lower reaches of large mountains, especially those with high temperature and humidityLow-level jetahead.From the climate background, ChinaYellow RiverandMiddle and lower reaches of the Yangtze RiverSimilar to the mid latitude plain in North America, it has favorable conditions for the occurrence of MCCs.
Linear MCSs
Bluestein et al[7]For many in North AmericaSquall lineThrough statistical analysis of case sounding observations, the most important environmental characteristics of the mid latitude squall line are summarized: first, the troposphere in the large-scale environment is geopotential unstable, that is, there is a large convective effective potential energy;The second is that the maximum vertical shear of the average wind is at the lower level, and the shear vector deviates from the squall line about 45 ° clockwise.
Occurrence instance
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National Climate CenterThe statistical results show that in 1954, 1969, 1980, 1991, 1996, 1998, 1999, 2003 and 2007Heavy rainflood and water-logging, causing heavy losses to the national economy and people's lives and property;Especially in recent 10 years, the average annual economic loss caused by rainstorm and flood in China is more than 100 billion yuan. MCSs are the main impact system in these typical rainstorm events in flood years.[1]Specific examples are as follows:
Figure 2 Precipitation from 08:00 on July 22 to 08:00 on July 23 and 08:00 on July 23 to 08:00 on July 24, 2005
From August 4 to 5, 1993, a MCS process brought heavy rain to Shandong, Hebei, Anhui and Jiangsu provinces,Maximum precipitationUp to 540mm.[8]
From the night of July 21 to 24, 2005, a strong heavy rain weather process occurred in North China.From 08:00 on the 22nd to 08:00 on the 24th (Beijing time, the same below), there were more than 100 counties (cities) in North China with rainfall exceeding 50 mm, of which 33 counties (cities) had rainfall exceeding 100 mm, and 4 counties (cities) had rainfall exceeding 200 mm.HebeiDingzhouThe maximum rainfall is 3292 mm；HebeiRaoyangandNanleThe rainfall is followed by 2261 mm and 2181 mm (Figure 3).The rainstorm process lasted for a long time, affected a wide range, concentrated precipitation, and serious floods occurred in some areas.[9]
