Observations and Fine-Grid Simulations of a Convective Outbreak in Northeastern Spain: Importance of Diurnal Forcing and Convective Cold Pools

Abstract

The life cycle and interactions of a series of convective systems that developed in northeastern Spain during the afternoon of 7 August 1996 are examined based on remote sensing products, surface observations, and numerical simulations. Most of the convection was organized in two mesoscale convective systems (MCSs) and a line of storms attached to the Pyrenees Mountains. One of these storms produced rainfalls in excess of 200 mm in 3 h and severe flash floods in the Biescas area. The end of the convection in the Biescas area occurred after merger of the storm with one of the MCSs that approached from the southwest. A high-resolution (4-km grid length) simulation of the event reproduces the observed timing and interactions of the convective systems, as well as the general rainfall pattern. The highly localized rainfall core at Biescas is well located, although the peak rainfall amount is underestimated. The success of the model at triggering the convection at the proper locations and time results from a reasonably accurate prediction of mesoscale features of the low-level flow pattern, such as a thermal mesolow in the Ebro valley, deformation zone, upslope wind systems, and the pushing of a cold front in the upper portion of the valley. After the onset of the initial convection, the role of the convectively induced cold pools and outflows for the propagation of the convective systems is shown to be very important. In particular, the MCS interacting with the Biescas convection was driven by strong mesoscale ascent established between the convectively induced outflows, the upvalley southeasterly winds sustained by the mesolow, and the downvalley northwesterly winds associated with the cold front. After the interaction, the convection in Biescas ceased because of the interruption of the southerly upslope flow caused by the convective cold pools. Finally, the explosive character of convection after noon and its initial focusing in uplands and slopes suggest that diurnal forcing could have played a decisive role. This idea is validated by means of simulations.