Sensitivities of a Flash Flood Event over Catalonia: A Numerical Analysis

Abstract

On 9 and 10 June 2000, the northeastern part of the Iberian Peninsula was affected by heavy rains that produced severe floods over densely populated areas. The zones most affected were the provinces of Tarragona and Barcelona, located in the region of Catalonia. Five people were killed, 500 were evacuated, and the property losses were estimated to exceed 65 million euros. The episode was characterized by the entrance of an Atlantic low-level cold front and an upper-level trough that contributed to the generation of a mesoscale cyclone in the Mediterranean Sea east of mainland Spain. The circulation associated with this mesoscale cyclone advected warm and moist air toward Catalonia from the Mediterranean Sea. The convergence zone between the easterly flow and the Atlantic front, as well as the complex orography of the region, are shown to be involved in the triggering and organization of the convective systems. Radar shows the development of two long-lived mesoscale convective systems that merged and remained quasi-stationary nearby the city of Barcelona for nearly 2 h.

The fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5) short-range numerical simulations of the episode succeed reasonably well in capturing the accumulated rainfall patterns and important mesoscale aspects of the event. The role of the orography and latent heat release in generating and sustaining the quasi-stationary precipitating systems is assessed through numerical sensitivity analysis. A piecewise potential vorticity inversion technique, assisted by sensitivity fields derived from an adjoint model, provides a test bed to investigate the predictability of the damaging rains given uncertainties in the initial fields. The adjoint results highlight the importance of the details associated with the upper-level precursor trough. Therefore, the effects of small perturbations to the trough intensity and location are also investigated. Interestingly, despite the orography’s active role in the generation of the Mediterranean mesoscale cyclone and, hence, on the location of the precipitation maxima, the predictability of the spatial and temporal distribution and amounts of precipitation is shorter than might be expected. This reduced predictability of the rainfall field is attributed to the high sensitivity of the location and depth of the mesoscale cyclone identified in the Mediterranean Sea east of Catalonia to perturbations; that is, to the location and intensity of the precursor upper-level trough, compatible with realistic analysis errors.