How is Convection Initiation Sensitive to Hodograph Characteristics?

The initiation of thunderstorms in environments characterized by strong vertical wind shear presents a forecasting challenge due to the complexities of the interaction between growing cumulus clouds and wind shear. Previous research has shown that wake entrainment is an important entrainment pathway in deepening sheared cumulus clouds, and that wake entrainment can result in the failure of sustained convection initiation in narrow clouds. However, it is not yet known how strong wind shear must be for the detrimental effects of wake entrainment on convection initiation to become significant. Moreover, it is unclear if hodograph curvature has any impact on the effectiveness of wake entrainment. The goal of this research is to examine these two uncertainties using large-eddy simulations.

The results from two sets of simulations conducted using CM1 will be presented. In the first set of simulations, cumulus clouds are forced to develop in environments characterized by a linear hodograph and variable wind shear magnitude in the 1–6-km layer. The results of these simulations indicate that wake entrainment is strongly sensitive to the wind shear magnitude. When the bulk wind difference in the 1–6-km layer is greater than approximately 10–15 m s^-1, the effects of wake entrainment become substantial. This result is insensitive to whether shallow warm bubbles or surface sensible heat fluxes are used to force convection initiation in the simulations. In the second set of simulations, the hodograph length is kept constant, but the amount of hodograph curvature is varied. The results from these simulations demonstrate that wake entrainment develops in deepening clouds even in cases with strong hodograph curvature in the 1–6-km layer. Theoretical arguments further support the conclusion that wake entrainment is insensitive to hodograph curvature. Together, these simulations show that cumulus cloud structure and convection initiation are particularly sensitive to the wind shear magnitude and not to hodograph curvature.