Idealized mesoscale numerical study of Mediterranean heavy precipitating convective systems

Summary

The western Mediterranean mountainous areas are prone to heavy precipitating events during the fall season. The ingredients that favour these systems are well known but it is still difficult to understand why a precipitating system can become paroxysmal or to forecast the accurate location of the system. To investigate these predictability issues, an idealized framework to simulate quasi-stationary mesoscale convective systems is set up and serves as a basis for studying the sensitivity of the location and intensity of precipitating systems to the characteristics of the low-level upstream flow over the Mediterranean Sea. High resolution simulations are performed with the non-hydrostatic MESO-NH model for an idealized moist unstable flow but using the real topography. Low-level humidity distribution, convective available potential energy and speed of the flow are varied. It is found that various lifting mechanisms are involved to explain the specific location (orographic lifting, cold pool dynamics, low-level convergence due to deflection of the flow by the Alps). When the speed of the upstream flow is increased (decreased) compared to the CTRL run, the area of precipitation moves downstream (upstream). When the mixing ratio of the environment outside of the jet is less (more) than the CRTL run, the system is located more upstream (downstream). When the instability of the upstream flow is increased (decreased) compared to the CTRL run, the convective system moves upstream (downstream). The cold pool strength increases with slower flow and/or more instability. The maximum of rainfall is obtained when the convective system is over the relief with a strong low-level flow or a weak CAPE. The area covered by heavy precipitation is maximum when CAPE is high or low-level flow is strong.