A global view of severe thunderstorms: Estimating the current distribution and possible future changes

Severe thunderstorms pose a threat to life and property over much of the planet. Determining the current extent of the threat is not straightforward as very few areas have systematic data collection efforts. Some of the resulting problems will be discussed in order to demonstrate the limits of considering reports of severe weather in isolation in an effort to estimate the distribution of events. Another approach to the problem is to consider the relationship between large-scale environmental conditions and severe thunderstorm occurrence. These relationships have been developed in the United States where relatively high-quality databases of severe weather reports exist, initially as part of an effort to improve forecasting of severe thunderstorms. Given that environmental conditions are observed more systematically than severe thunderstorms, this provides the hope of creating distributions of favorable conditions. Further, global reanalysis data provides a source of long-term, relatively high space and time resolution pseudo-observations that can be used to make global pictures. Limitations on the process include the quality of the relationship between the reanalysis and the atmosphere and the possibility that environmental conditions that support severe thunderstorms may exist that are rarely sampled in the US, where the event-environment relationships are developed.

Assuming the relationships are reasonably good, however, the question of possible changes in severe thunderstorms in plausible climate change scenarios is still difficult to answer. Simple considerations of likely changes in mean environmental conditions of convective available potential energy and deep tropospheric shear (the two strongest environmental influences on severe thunderstorms) indicate that it is likely that one will increase while the other is decreasing. The balance between the changes is unknown at this time, as is the distribution of important parameters on individual days. Thus, observational (or reanalysis) data provide a cloudy picture of what to expect in the future.