Storm intensification driven by soil moisture gradients in global hotspot regions

Organised thunderstorm clusters known as Mesoscale Convective Systems (MCSs) can bring high impact hazards such as flash floods, lighting and destructive winds. It is crucial for the forecasting and mitigation of these hazards to understand the processes that influence the characteristics of storms and thereby contribute to extreme events. Soil moisture is known to influence the initiation of MCSs in several regions of the world, but the influence of soil moisture on the later stages of MCS lifecycles is less well understood. Work in West Africa has revealed that dry soil moisture structures on scales > 200 km can increase the scale and longevity of propagating, mature afternoon MCSs, but this has not been investigated for other regions. In the current work we simultaneously analyse seven global MCS hotspot regions where storms may be sensitive to soil moisture, the US Great Plains, China, India, West Africa, Australia, South Africa and South America, to gain a more global perspective of the impact of soil moisture conditions on mature MCS characteristics. Using a combination of global datasets, storm tracks, satellite data, reanalysis data and CMIP6 simulations, we reveal that large-scale soil moisture gradients (100s of km) can intensify storms by driving favourable shear conditions through the strengthening of low-level atmospheric temperature gradients. By separating storms by soil moisture conditions, we show an increase in precipitation feature area and rainfall production on days with favourable gradients compared to days with unfavourable gradients. This is a newly identified mechanism through which soil moisture can influence storm hazards globally, which has implications for the forecasting and future projection of extreme events under climate change.