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

==== 11.7.3.5 Projections ====

<div id="h3-43-siblings" class="h3-siblings"></div>

Future projections of severe convective storms are usually studied either by analysing the environmental conditions simulated by climate models, or by a time-slice approach with higher-resolution convection-permitting models by comparing simulations downscaled with climate model results under historical conditions and those under hypothesized future conditions ( [[#Kendon--2017|Kendon et al., 2017]] ; [[#Allen--2018|Allen, 2018]] ). Up to now, individual studies using convection-permitting models gave projections of extreme events associated with severe convective storms in local regions, and it is not generally possible to obtain global or general views of projected changes of severe convective storms. [[#Prein--2017|Prein et al. (2017)]] investigated future projections of North American MCS simulations and showed an increase in MCS frequency and an increase in total MCS precipitation volume by the combined effect of increases in maximum precipitation rates associated with MCSs and increases in their size. [[#Rasmussen--2020|Rasmussen et al. (2020)]] investigated future changes in the diurnal cycle of precipitation by capturing organized and propagating convection and showed that weak-to-moderate convection will decrease, and strong convection will increase in frequency in the future. [[#Ban--2015|Ban et al. (2015)]] found that the day-long and hour-long precipitation events in summer intensify in the European region covering the Alps. [[#Kendon--2019|Kendon et al. (2019)]] showed future increases in extreme three-hourly precipitation in Africa. [[#Murata--2015|Murata et al. (2015)]] investigated future projections of precipitation around Japan and showed a decrease in monthly mean precipitation in the eastern Japan Sea region in December, suggesting that convective clouds become shallower in the future in the winter over the Japan Sea.

The other approach is the projection of the environmental conditions that control characteristics of severe convective storms by analysing climate model results. There is ''high confidence'' that CAPE, particularly summer mean CAPE and high percentiles of the CAPE in the tropics and subtropics, increases in response to global warming in an ensemble of climate models including those of CMIP5, mainly from increased low-level specific humidity ( [[#Sobel--2011|Sobel and Camargo, 2011]] ; M.S. [[#Singh--2017|]] [[#Singh--2017|Singh et al., 2017]] ; J. [[#Chen--2020|]] [[#Chen--2020|Chen et al., 2020]] a). Convective inhibition becomes stronger over most land areas under global warming, resulting mainly from reduced low-level relative humidity over land (J. [[#Chen--2020|]] [[#Chen--2020|Chen et al., 2020]] a). However, there are large differences within the CMIP5 ensemble for environmental conditions, which contribute to some degree of uncertainty ( [[#Allen--2018|Allen, 2018]] ). Because the relation between simulated environments in models and the occurrence of severe convective storms are, in general, insufficiently validated, there is generally ''low confidence'' in the projection of severe convective storms with the approach of the environmental conditions.

In the USA, projected changes in the environmental conditions show an increase in CAPE and no changes or decreases in the vertical wind shear, suggesting favourable conditions for an increase in severe convective storms in the future, but the interpretation of how tornadoes or hail will change is an open question because of the strong dependence on shear ( [[#Brooks--2013|Brooks, 2013]] ). [[#Diffenbaugh--2013|Diffenbaugh et al. (2013)]] showed robust increases in the occurrence of the favourable environments for severe convective storms with increased CAPE and stronger low-level wind shear in response to future global warming. A downscaling approach showed that the variability of the occurrence of severe convective storms increases in spring in late 21st-century simulations ( [[#Gensini--2015|Gensini and Mote, 2015]] ). Future changes in hail occurrence in the USA examined through convection-permitting dynamical downscaling suggested that the hail season may begin earlier in the year and exhibit more interannual variability, with increases in the frequency of large hail in broad areas over the USA ( [[#Trapp--2019|Trapp et al., 2019]] ). There is ''medium confidence'' that the frequency and variability of the favourable environments for severe convective storms will increase in spring, and ''low confidence'' for summer and autumn ( [[#Diffenbaugh--2013|Diffenbaugh et al., 2013]] ; [[#Gensini--2015|Gensini and Mote, 2015]] ; [[#Hoogewind--2017|Hoogewind et al., 2017]] ). The occurrence of hail events in Colorado in the USA was examined by comparing both present-day and projected future climates using high-resolution model simulations capable of resolving hailstorms ( [[#Mahoney--2012|Mahoney et al., 2012]] ), which showed that hail is almost eliminated at the surface in the future in most of the simulations, despite more intense future storms and significantly larger amounts of hail generated in-cloud.

Future changes in severe convection environments show enhancement of instability with less robust changes in the frequency of strong vertical wind shear in Europe ( [[#Púčik--2017|Púčik et al., 2017]] ) and in Japan ( [[#Muramatsu--2016|Muramatsu et al., 2016]] ). In Japan, the frequency of conditions favourable for strong tornadoes increases in spring, and partly in summer.

In summary, the average and maximum rain rates associated with severe convective storms increase in a warming world in some regions, including the USA ( ''high confidence'' ). There is ''high confidence'' from climate models that CAPE increases in response to global warming in the tropics and subtropics, suggesting more favourable environments for severe convective storms. The frequency of severe convective storms in spring is projected to increase in the USA, leading to a lengthening of the severe convective storm season ( ''medium confidence'' ); evidence in other regions is limited. There is significant uncertainty about projected regional changes in tornadoes, hail, and lightning due to limited analysis of simulations using convection-permitting models ( ''hi'' ''gh confidence'' ).

<div id="11.7.4" class="h2-container"></div>

<span id="extreme-winds"></span>