Assessing the threat of severe convective storms over Europe

Project supported by the AXA Research Fund at the University of Manchester (United Kingdom)

1. Introduction

A short description of my research project supported by AXA during AXA Pop Days, a science popularization workshop for grantees of the AXA Research Fund held in Paris from 18 to 20 June 2014.

The frequencies of severe convective storms – hail, convective wind gusts, tornadoes, and lightning – are generally lower over Europe compared to the United States. For example, approximately 300 tornadoes are reported each year in Europe, while approximately 1000 tornadoes are observed each year in United States. Although the threat is smaller in Europe, the true magnitude of the problem is not known because of the lack of assembled datasets. These datasets are essential for a better understanding of the spatial and temporal distribution of severe convective storms over Europe. Knowing the distribution of severe convective storms will improve the forecasting of these high impact events, will help better quantify the risk and year-to-year variability that insurance companies face, and will serve as a baseline for understanding the possible influence of climate change.

The forecasting of severe convective storms remains one of the most important challenges facing operational meteorology. Although the forecasting of the occurrence and intensity of such storms is improving due to the advancements in global and mesoscale modeling, the impacts of severe convective storms cannot be explicitly forecast within these models. Keys to addressing this problem are the spatial and temporal distributions, or climatologies of severe weather events. The climatologies of severe weather events for Europe are limited by inconsistencies in observational networks and reporting practices. These generally vary over Europe and also vary with the population density with each European country. Unfortunately, there have been very few efforts addressing this problem, which resulted in a lack of pan-European data sets.

"Too many tornadoes in Europe?" an interview for Sciences et Avenir on my research with AXA Research Fund on tornadoes in Europe for the special feature "UNDERSTANDING AND BETTER GRASPING CLIMATE RISKS" (Cahier partenaire FONDS AXA /SCIENCES ET AVENIR n° 825 novembre 2015). (French version)

2. Research plan

Recently, new pan-European datasets have become available that will allow a step-change in our ability to observe and understand damaging convective storms on a European scale. This proposal seeks to exploit these datasets for the first time to (1) create spatial and temporal distributions of severe weather events, (2) better understand the factors controlling the distribution of severe weather events over Europe, (3) examine the synoptic-scale flow pattern associated with convective storms producing severe weather events, and (4) develop new physically-based conceptual models for convective storms that will benefit our understanding and forecasting.

(1) To create climatologies of severe weather events on a European scale, we will use new pan-European severe weather and lightning datasets. We will use the European Severe Weather Database (ESWD), a unique database of severe-weather maintained by European Severe Storm Laboratory. The ESWD is a joint effort between National Meteorological and Hydrological Services and voluntary observers. Also, the public can contribute and retrieve observations. The quality of the collected data is assessed and flagged. Two lightning datasets: the European Cooperation for Lightning Detection (EUCLID) and Met Office Arrival Time Difference Network (ATDnet) will be used to study the spatial and temporal distribution of lightning strikes over Europe. The EUCLID network provides cloud-to-ground lightning data from cooperating countries around Europe, whereas the Met Office’s ATDnet uses a surface-based network of 11 VHF radio receivers.

(2) To better understand the factors controlling the distribution of severe weather events over Europe we will be the first to exploit a new pan-European weather radar dataset. Recent studies have shown that certain storm morphologies (e.g., isolated storms, multicellular storms, supercells) favor the producing of one or more types of severe weather events. To study the morphologies of the storms associated with the severe weather events included in the ESWD we will exploit the European-scale radar dataset collected by OPERA, the European Weather Radar Network, one of the programs within EUMETNET, the Network of European Meteorological Services, and a joint effort of 30 countries. OPERA processes data from 120 operational radars to produce the only unified and homogeneous radar mosaic over Europe, making it ideally suited for this project. An automated method based on the OPERA database will be developed to define and track the convective storms associated with severe weather events, and also to extract their morphologies.

(3) To examine the synoptic-scale flow pattern associated with convective storms producing severe weather events. Satellite, surface, and upper-air data (available from British Atmospheric Data Center) will also be employed to understand the environments in which these storms form. We will also use European Centre for Medium-Range Weather Forecasts reanalyses to categorize the synoptic-scale patterns associated with each storm. Thus, we will know the prevalence of deep convection, their environments, and the synoptic-scale patterns (e.g., troughs and ridges in the jet stream, jet exits and entrances, fronts, tropopause-level potential vorticity). This will allow us to examine also long-cherished assumptions about the association between the synoptic-scale flow pattern and convective storms producing severe weather events, in particular the role of synoptic-scale velocity in initiating convection. Despite conventional wisdom stating that synoptic-scale vertical velocities are insufficient to initiate convection, this conventional wisdom remains untested. Furthermore, the factors that explain the association of convective storms with preferred regions of synoptic-scale phenomena are unknown.

(4) Finally, synthesizing the results from these first three objectives – knowing the climatology of severe weather, the factors that affect storms, and the synoptic-scale flow pattern in which these storms occur – will enable us to develop new physically-based conceptual models for convective storms. These conceptual models will benefit our understanding of how such storms occur and our ability to forecast them. These models will differ from those in the United States, where most of the research has been done to date. Such work will be useful for determining the baseline for storm occurrence and intensity in a changing climate.