Synoptic Analysis and Simulation of the High-Shear, Low-CAPE (HSLC) F4-Tornado in Hautmont, France from August 03, 2008 using ERA5 data and Cloud Model 1

On August 03, 2008, an F4 tornado struck the city of Hautmont, France, causing extensive damage across seven municipalities and injuring numerous individuals during its 14-minute lifespan. The tornado developed from a pre-frontal convective system within a high-shear, low-CAPE (HSLC) environment - a setting in which the occurrence of strong tornadoes is considered atypical. This study aims to analyze the synoptic-scale situation with a particular focus on tornado-favorable and convectively relevant parameters, utilizing ERA5 reanalysis data. A reconstructed atmospheric sounding and hodograph were used as initial conditions for idealized simulations with the Cloud Model 1 (CM1), incorporating various convective initiation triggers. The objective was to explore potential polarimetric signatures indicative of supercellular structures or bow echoes.

The analysis indicates that a jet coupling event triggered quasi-geostrophic cyclogenesis, which led to the development of a surface low and the formation of a low-level jet. Together with high moisture content near the surface and within the atmospheric boundary layer, these factors were identified as key contributors to tornadogenesis in this case.

Simulation results highlight limitations in representing soundings with high boundary layer moisture within CM1. Among the tested initiation triggers, updraft nudging proved to be the most effective and, in fact, the only one capable of producing a tornado-like vortex. This suggests that updraft nudging can enhance or even partially compensate for the inherently weak convective updrafts in HSLC environments, potentially enabling the formation of short-lived supercells following convection initiation. Based on these findings, it is proposed to conceptually differentiate between two categories of convective triggers: those that initiate convection and those that support and maintain it under marginal conditions.

As an outlook first attempts of creating a series of synthetic idealizied soundings for the use of numerical simulations were attemptend with the goal of testing the limits and boundaries of various numerical convective simulation models.