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

==== 11.7.3.2 Observed Trends ====

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Observed trends in severe convective storms or MCSs are not well documented, but the climatology of MCSs has been analysed in specific regions (North America, South America, Europe, Asia; regional aspects of convective storms are separately assessed in Chapter 12). As the definition of severe convective storms varies depending on the literature, it is not straightforward to make a synthesizing view of observed trends in severe convective storms in different regions. However, analysis using satellite observations provides a global view of MCSs ( [[#Kossin--2017|Kossin et al., 2017]] ). The global distribution of thunderstorms is captured ( [[#Zipser--2006|Zipser et al., 2006]] ; [[#Liu--2015|Liu and Zipser, 2015]] ) by using the satellite precipitation measurements by the Tropical Rainfall Measuring Mission (TRMM) and Global Precipitation Mission (GPM) ( [[#Hou--2014|Hou et al., 2014]] ). The climatological characteristics of MCSs are provided by satellite analyses in South America ( [[#Durkee--2010|Durkee and Mote, 2010]] ; [[#Rasmussen--2011|Rasmussen and Houze, 2011]] ; [[#Rehbein--2018|Rehbein et al., 2018]] ) and those of MCCs in the Maritime Continent by [[#Trismidianto%20and%20H.%C2%A0Satyawardhana--2018|Trismidianto and Satyawardhana (2018)]] . Analysis of the environmental conditions favourable for severe convective events indirectly indicates the climatology and trends of severe convective events ( [[#Allen--2018|Allen et al., 2018]] ; [[#Taszarek--2018|Taszarek et al., 2018]] , 2019), though favourable conditions depend on the location, such as the difference for tornadoes associated with ETCs between the USA and Japan ( [[#Tochimoto--2018|Tochimoto and Niino, 2018]] ).

Observed trends in severe convective storms are highly regionally dependent. In the USA, it is indicated that there is no significant increase in convective storms, and hail and severe thunderstorms ( [[#Kunkel--2013|Kunkel et al., 2013]] ; [[#Kossin--2017|Kossin et al., 2017]] ). There is an upward trend in the frequency and intensity of extreme precipitation events in the USA ( ''high confidence'' ) ( [[#Kunkel--2013|Kunkel et al., 2013]] ; Easterling et al., 2017), and MCSs have increased in occurrence and precipitation amounts since 1979 ( ''limited evidence'' ) ( [[#Feng--2016|Feng et al., 2016]] ). Significant interannual variability of hailstone occurrences is found in the Southern Great Plains of the USA ( [[#Jeong--2020|Jeong et al., 2020]] ). The mean annual number of tornadoes has remained relatively constant, but their variability of occurrence has increased since the 1970s,particularly over the 2000s, with a decrease in the number of days per year, but an increase in the number of tornadoes on these days ( [[#Brooks--2014|Brooks et al., 2014]] ; [[#Elsner--2015|Elsner et al., 2015]] , 2019; [[#Kossin--2017|Kossin et al., 2017]] ; [[#Allen--2018|Allen, 2018]] ). There has been a shift in the distribution of tornadoes, with increases in the mid-south of the USA and decreases over the High Plains ( [[#Gensini--2018|Gensini and Brooks, 2018]] ). Trends in MCSs are relatively more visible for particular aspects of MCSs, such as lengthening of active seasons and dependency on duration. MCSs have increased in occurrence and precipitation amounts since 1979 (Easterling et al., 2017). [[#Feng--2016|Feng et al. (2016)]] analysed that the observed increases in spring total and extreme rainfall in the central USA are dominated by MCSs, with increased frequency and intensity of long-lasting MCSs.

Studies on trends in severe convective storms and their ingredients outside of the USA are limited. [[#Westra--2014|Westra et al. (2014)]] found that there is an increase in the intensity of short-duration convective events (minutes to hours) over many regions of the world, except eastern China. In Europe, a climatology of tornadoes shows an increase in detected tornadoes between 1800 and 2014, but this trend might be affected by the density of observations ( [[#Antonescu--2016a|Antonescu et al., 2016a]] , b). An increase in the trend in extreme daily rainfall is found in south-eastern France, where MCSs play a key role in this type of event ( [[#Blanchet--2018|Blanchet et al., 2018]] ; [[#Ribes--2019|Ribes et al., 2019]] ). Trend analysis of the mean annual number of days with thunderstorms since 1979 in Europe indicates an increase over the Alps and central, south-eastern, and eastern Europe, with a decrease over the south-west ( [[#Taszarek--2019|Taszarek et al., 2019]] ). In the Sahelian region, [[#Taylor--2017|Taylor et al. (2017)]] analysed MCSs using satellite observations since 1982 and showed an increase in the frequency of extreme storms. In Bangladesh, the annual number of propagating MCSs decreased significantly during 1998–2015 based on TRMM precipitation data ( [[#Habib--2019|Habib et al., 2019]] ). [[#Prein--2018|Prein and Holland (2018)]] estimated the hail hazard from large-scale environmental conditions using a statistical approach and showed increasing trends in the USA, Europe, and Australia. However, trends in hail on regional scales are difficult to validate because of an insufficient length of observations and inhomogeneous records ( [[#Allen--2018|Allen, 2018]] ). The high spatial variability of hail suggests it is reasonable that there would be local signals of both positive and negative trends, and the trends that are occurring in hail globally are uncertain. In China, the total number of days that have either a thunderstorm or hail have decreased by about 50% from 1961 to 2010, and the reduction in these severe weather occurrences correlates strongly with the weakening of the East Asian summer monsoon (Q. [[#Zhang--2017|]] [[#Zhang--2017|]] [[#Zhang--2017|Zhang et al., 2017]] ). More regional aspects of severe convective storms are detailed in Chapter 12.

In summary, because the definition of severe convective storms varies depending on the literature and the region, it is not straightforward to make a synthesizing view of observed trends in severe convective storms in different regions. In particular, observational trends in tornadoes, hail, and lightning associated with severe convective storms are not robustly detected due to insufficient coverage of the long-term observations. There is ''medium confidence'' that the mean annual number of tornadoes in the USA has remained relatively constant, but their variability of occurrence has increased since the 1970s, particularly over the 2000s, with a decrease in the number of days per year, and an increase in the number of tornadoes on these days ( ''high confidence'' ). Detected tornadoes have also increased in Europe, but the trend depends on the density of observations.

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