Observations and fine-grid simulations of an orographically-initiated convective event in northeastern Spain

Ordinary convective cells develop almost every afternoon during summer over the elevated terrain of the Pyrenees and Iberic System in northeastern Spain (see figure). With favorable synoptic conditions, as with cold air aloft, short-wave troughs moving over the region at upper levels, or under the influence of the extreme of a higher-latitude cold front, convection can be more intense and extensive, attain higher organization and lifetime and move well into the adjacent lowlands. One of such events (7 August 1996) is presented based on synoptic map composites, radar and radiosounding information, surface observations and numerical simulations.

Most of the convection was organized in two main mesoscale convective systems (MCSs), basically mobile, but isolated convective cells attached to the slopes of the Pyrenees mountains persisted during several hours and one of these cells produced rainfalls in excess of 200 mm in 3 h and severe flash floods in the Biescas area (interior rectangle in the figure). Radar information indicates that the end of the convection in the Biescas area occurred after its interaction with one of the MCSs that approached from the southwest through a propagation effect.

A high-resolution (4 km grid length) simulation of the event using the PSU-NCAR model reproduces the observed timing and interactions of the convective systems, as well as the general rainfall pattern. A comparison of the simulation against observations suggests that the success of the model for triggering the convection in the proper locations and times results from a correct prediction of mesoscale features of the low-level flow pattern, such as a mesolow in the Ebro valley, deformation zone, upslope wind systems and the pushing of a cold front in the upper portion of the Ebro valley. After the onset of the first convection, the life cycle and movement of the simulated convective systems appears largely influenced by the convectively-generated outflow boundaries.

Finally, the work examines the effects of the diurnal forcing; the explosive character of convection after noon and its initial focusing in uplands and slopes suggest that such factor could have played an important role. It is found, after performing one simulation without solar radiative forcing, that the diurnal forcing was indeed critical for the triggering and evolution of the convection.