Impact of upstream flow conditions on the initiation of moist convection over the island of Corsica

Abstract

The influence of upstream flow on the initiation of moist convection over Corsica is investigated using semi-idealized simulations. The simulations, performed with the Consortium for Small-scale Modeling (COSMO) model, are initialized with one vertical profile at all grid points and under constant boundary conditions. The upstream wind velocity and wind direction, as well as the saturation deficit and stability, are systematically varied to estimate the impact of the individual factors on the initiation and strength of moist convection. Also, the influence of radiation is evaluated. It is found that deep convection can only develop with high solar radiation. With lower radiation, thermal wind systems are weak, and only shallow convection is triggered. Results show that with strong solar radiation, a strong upstream wind velocity is less efficient in triggering deep convection than a weaker one. This is due to the dynamic effects on the flow field. A low upstream wind velocity enables thermal wind systems to become dominant and the preconvective conditions for deep convection are more favorable. This leads to more events with deep convection and precipitation. The wind direction can be the determining factor for the initiation of deep convection and also controls the location of precipitation. A change of just 15° can decide on whether deep convection occurs or not. With a high upstream wind velocity, the location is always on the lee side of the island. With a lower upstream wind velocity, two hot spots for convection initiation, one in the northern central part and one in the western part of the island, are identified. It is further found that the impact of saturation deficit is higher than the impact of stability. With increased instability, the atmospheric conditions are more favorable for the development of convection, but the high saturation deficit in the middle troposphere prevents the formation of deep convection. Through reduction of the saturation deficit, multiple convective cells can develop in the more humid atmosphere. Additionally, the integrated water vapor is larger, which means that more precipitable water is available allowing larger amounts of precipitation.