Turbulence in the upper-level outflow of idealized mesoscale convective systems

Convective clouds are known to be an important contributor to aircraft turbulence encounters, both within the clouds and in the clear air surrounding them. Although turbulence within cloud is probably the most intense, turbulence outside of cloud (so-called near-cloud turbulence) is arguably more dangerous because it is invisible and often the cause of unexpected encounters. Observations have demonstrated that the risk of turbulence at cruise altitudes increases with proximity to deep convection. There is anecdotal evidence that the region downstream of convection is more hazardous and it has been suggested that this hazard is analogous to a wake extending downshear. However, the dynamics controlling turbulence around convection is not entirely understood. One important contributor to near-cloud turbulence, recently discussed by Trier et al. (2010, J. Atmos. Sci.), is the destabilization of the upper-level outflow of mesoscale convective systems by enhanced shear and reduced stability.

This work explores the generation of upper-level near-cloud turbulence with idealized three-dimensional simulations of mesoscale convective systems using the Weather Research and Forecasting (WRF) model. The WRF simulations are designed to isolate the key generation processes and explore their sensitivity to environmental conditions like wind shear. Nested domains are used to resolve the turbulence around the convection as well as the broader mesoscale circulations and the convective lifecycles. A number of important turbulence mechanisms are identified. The simulated turbulence extends more than 100 km away from the active convection, over large areas in the clear air as well as regions with low radar reflectivity. The turbulence is dominant in the sectors of preferential upper-level storm outflow and some occurrences are related to enhanced vertical wind shear. The temporal variability of the convective updrafts and the strength of the upper-level convective outflow cause the turbulence to be highly intermittent and localized. The simulations also demonstrate the complexity of the turbulent structures, with coherent features aligned at various directions to the mean flow. These simulation results will be presented as well as their implications for turbulence prediction and avoidance.