Editing IPCC:AR6/WGI/Chapter-11 (section)

==== 11.7.3.1 Mechanisms and Drivers ====

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Severe convective storms are sometimes embedded in synoptic-scale weather systems, such as TCs, ETCs, and fronts ( [[#Kunkel--2013|Kunkel et al., 2013]] ). They are also generated as individual events as mesoscale convective systems (MCSs) and mesoscale convective complexes (MCCs, a special type of a large, organized and long-lived MCS), without being clearly embedded within larger-scale weather systems. In addition to the general vigorousness of precipitation, hail, and winds associated with MCSs, characteristics of MCSs are viewed in new perspectives in recent years, probably because of both the development of dense mesoscale observing networks and advances in high-resolution mesoscale modelling (Sections 11.7.3.2 and 11.7.3.3). The horizontal scale of MCSs is discussed with their organization of the convective structure, and it is examined with a concept of ‘convective aggregation’ in recent years ( [[#Holloway--2017|Holloway et al., 2017]] ). MCSs sometimes take a linear shape and stay almost stationary with successive production of cumulonimbus on the upstream side (back-building type convection), and cause heavy rainfall ( [[#Schumacher--2005|Schumacher and Johnson, 2005]] ). Many of the recent severe rainfall events in Japan are associated with band-shaped precipitation systems ( [[#Kunii--2016|Kunii et al., 2016]] ; [[#Oizumi--2018|Oizumi et al., 2018]] ; [[#Tsuguti--2018|Tsuguti et al., 2018]] ; [[#Kato--2020|Kato, 2020]] ), suggesting common characteristics of severe precipitation, at least in East Asia. The convective modes of severe storms in the USA can be classified into rotating or linear modes and preferable environmental conditions for these modes, such as vertical shear, have been identified ( [[#Trapp--2005|Trapp et al., 2005]] ; [[#Smith--2013|Smith et al., 2013]] ; [[#Allen--2018|Allen, 2018]] ). Cloud microphysics characteristics of MCSs were examined and the roles of warm rain processes on extreme precipitation were emphasized recently ( [[#Sohn--2013|Sohn et al., 2013]] ; [[#Hamada--2015|Hamada et al., 2015]] ; [[#Hamada--2018|Hamada and Takayabu, 2018]] ). Idealized studies also suggest the importance of ice and mixed-phase processes of cloud microphysics on extreme precipitation ( [[#Sandvik--2018|Sandvik et al., 2018]] ; [[#Bao--2019|Bao and Sherwood, 2019]] ). However, it is unknown whether the types of MCS are changing in recent periods or observed ubiquitously all over the world.

Severe convective storms occur under conditions preferable for deep convection, that is, conditionally unstable stratification, sufficient moisture, both in lower and middle levels of the atmosphere, and a strong vertical shear. These large-scale environmental conditions are viewed as necessary conditions for the occurrence of severe convective systems, or the resulting tornadoes and lightning, and the relevance of these factors strongly depends on the region (e.g., [[#Antonescu--2016a|Antonescu et al., 2016a]] ; [[#Allen--2018|Allen, 2018]] ; [[#Tochimoto--2018|Tochimoto and Niino, 2018]] ). Frequently used metrics are atmospheric static stability, moisture content, convective available potential energy (CAPE) and convective inhibition, wind shear or helicity, including storm-relative environmental helicity ( [[#Tochimoto--2018|Tochimoto and Niino, 2018]] ; [[#Elsner--2019|Elsner et al., 2019]] ). These metrics, largely controlled by large-scale atmospheric circulations or synoptic weather systems, such as TCs and ETCs, are then generally used to examine severe convective systems. In particular, there is ''high confidence'' that CAPE in the tropics and the subtropics increases in response to global warming (M.S. [[#Singh--2017|]] [[#Singh--2017|Singh et al., 2017]] ), as supported by theoretical studies ( [[#Singh--2013|Singh and O’Gorman, 2013]] ; [[#Seeley--2015|Seeley and Romps, 2015]] ; [[#Romps--2016|Romps, 2016]] ; [[#Agard--2017|Agard and]] [[#Emanuel--2017|Emanuel, 2017]] ). The uncertainty, however, arises from the balance between factors affecting severe storm occurrence. For example, the warming of mid-tropospheric temperatures leads to an increase in the freezing level, which leads to increased melting of smaller hailstones, while there may be some offset by stronger updrafts driven by increasing CAPE, which would favour the growth of larger hailstones, leading to less melting when falling ( [[#Allen--2018|Allen, 2018]] ; [[#Mahoney--2020|Mahoney, 2020]] ).

There are few studies on relations between changes in severe convective storms and those of the large-scale circulation patterns. Tornado outbreaks in the USA are usually associated with ETCs with their frontal systems and TCs ( [[#Fuhrmann--2014|Fuhrmann et al., 2014]] ; [[#Tochimoto--2016|Tochimoto and Niino, 2016]] ). In early June to late July in East Asia, associated with the Baiu/Changma/Mei-yu, severe precipitation events are frequently caused by MCSs. Severe precipitation events are also caused by remote effects of TCs, known as predecessor rain events ( [[#Galarneau--2010|Galarneau et al., 2010]] ). Atmospheric rivers and other coherent types of enhanced water vapour flux also have the potential to induce severe convective systems ( [[#Kamae--2017a|Kamae et al., 2017a]] ; [[#Waliser--2017|Waliser and Guan, 2017]] ; [[#Ralph--2018|Ralph et al., 2018]] ; see [[IPCC:Wg1:Chapter:Chapter-8#8.3.2.8.2|Section 8.3.2.8.2]] ). Combined with the above drivers, topographic effects also enhance the intensity and duration of severe convective systems and the associated precipitation ( [[#Ducrocq--2008|Ducrocq et al., 2008]] ; [[#Piaget--2015|Piaget et al., 2015]] ). However, the changes in these drivers are not generally significant, so their relations to severe convective storms are unclear.

In summary, severe convective storms are sometimes embedded in synoptic-scale weather systems, such as TCs, ETCs and fronts, and modulated by large-scale atmospheric circulation patterns. The occurrence of severe convective storms and the associated severe events, including tornadoes, hail, and lightning, is affected by environmental conditions of the atmosphere, such as CAPE and vertical shear. The uncertainty, however, arises from the balance between these environmental factors affecting severe storm occurrence.

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