Numerical Analysis of a Spanish Supercell Outbreak

On July 31 2015, a supercell outbreak occurred in Spain, causing significant damage and disruption. More than 20 supercells were responsible for producing multiple large hail, flash floods, and severe wind gusts. The outbreak was driven by a deep shortwave trough over the Iberian Peninsula, bringing with it a strong geopotential height gradient and instability in the Iberian Peninsula. On the front side of the trough axis, positive vorticity advection and divergence helped to promote and strengthen the upper-level forcing favoring thunderstorm episodes.

The event was simulated using the WRF-ARW model, in which several convective variables and instability indices were studied. To track the supercells, a python-based supercell tracking tool was used. This tool identified and tracked every supercell resolved by the model, and these results were verified with the supercell database. Reanalysis and sounding data revealed pre-convective environments favorable for supporting supercells development (i.e., high-level instability coupled with strong deep-layer shear). The results indicate large interaction between topography, convective initiation and supercell life-cycle, inducing their dynamics and the growth of mesocyclones.

The use of the WRF-ARW model and python-based supercell tracking tool allowed for a better understanding of the event and can help improve future forecasting and warning efforts.