Factors influencing the generation of turbulence above deep convection

Deep moist convection generates turbulence in the clear-air above and around developing clouds, penetrating convective updrafts, and mature thunderstorms. This turbulence can be due to shearing instabilities caused by the strong flow deformation near the cloud boundary, and also due to breaking gravity waves. Cloud-induced turbulence can lead to mixing and redistribution of atmospheric constituent species, and may play a role in stratosphere-troposphere exchange. Also, turbulence above and around deep convection is an important safety issue for aviation, and improved understanding of the conditions that lead to out-of-cloud turbulence formation may result in better turbulence avoidance guidelines or forecasting capabilities.

A series of high-resolution two- and three-dimensional model simulations of a severe thunderstorm are conducted to examine the sensitivity of above-cloud turbulence to a variety of background flow conditions, in particular the above-cloud wind shear and stability. Shortly after the initial convective overshoot, the above-cloud turbulence and mixing is dominated by mixing near the cloud boundary. At later times, when the convection is more mature, gravity wave breaking further aloft dominates the turbulence generation. This wave breaking is caused by critical level interactions, where the height of the critical level is controlled by the above-cloud wind shear. Furthermore, the strength of the above-cloud wind shear has a strong influence on the occurrence and intensity of above-cloud turbulence, with intermediate shears generating the largest volume of turbulent air, and strong shear conditions producing the most intense turbulence. Also, as expected, more stable situations are less prone to turbulence than less stable situations. These simulations will be presented, along with explanations of the mechanisms controlling the sensitivities, and a detailed comparison of the differences between the two- and three-dimensional results.