Importance of cold pools and associated outflows to the evolution of a convective outbreak in northeastern Spain

The study investigates the mesoscale mechanisms responsible for the initiation, life cycle and interactions of a series of convective systems that affected northeastern Spain during the afternoon of 7 August 1996. For that purpose, fine-grid (4 km grid length) simulations of the event are performed using the PSU-NCAR numerical model.

A full physics simulation of the event reproduces reasonably well the observed timing and evolution of the convective systems. A comparison of the model-predicted fields against available observations suggests that the success of the model for triggering the convection in the proper locations and time results from a correct prediction of mesoscale features of the low-level flow pattern, such as a thermal mesolow in the Ebro valley, deformation zone, diurnally-forced upslope wind systems and the pushing of a cold front in the upper portion of the valley. The full simulation also indicates that evaporatively induced cold pools and their associated outflows, continuously emerging from areas of convection, become eventually a dominant aspect of the flow and are largely responsible for the propagation of the convective systems. In order to better investigate these actions, another simulation is designed in which the cooling due to evaporation of rain falling through subsaturated layers is not incorporated in the model temperature prognostic equation.

Results of this simulation indicate that the convectively-generated cold pools and outflows are indeed very influential for the evolution of the simulated convection. The cold pools naturally act to stabilize the environment, and the accompanying outflows help to propagate the convection quite in the same way as indicated by the observations, avoiding unrealistic, excessive precipitation in the areas dominated by the thermal mesolow.