The generation and evolution of roll vortices in the hurricane boundary layer: A numerical study

Roll vortices are counter-rotating vortices existing in the atmospheric boundary layer, with axes approximately parallel to the mean wind. Recent observations indicate roll vortices are prevalent in hurricane boundary layer (HBL). These features can redistribute the momentum, mass, and energy throughout the HBL and therefore play an important role in determining the structure of the HBL. However, the roll vortices are not explicitly represented in existing hurricane models because of their small spatial scales. In order to parameterize the effect of roll vortices in hurricane models we need to better understand their formation mechanisms and their interactions with the large-scale flow.

In this study, we coupled a two-dimensional LES (2D-LES) model with an axisymmetric hurricane Boundary Layer (AHBL) model to simulate the generation and evolution of roll vortices, as well as the interactions between rolls and the large-scale wind in the HBL. The AHBL model is initialized from a NOAA/HRD observed multi-storm composite dataset. It is capable of producing a realistic structure of the HBL velocity field. The 2D-LES model is non-hydrostatic and can explicitly simulate large eddies in the HBL, like roll vortices. It is placed at various distances from the storm center. The two models are coupled and explicitly solve the two-way interaction between the large-scale HBL flow and roll vortices.

The model results reveal the roll vortices can be generated by the inflection point instability of the HBL flow, with the major energy source coming from the vertical shear of the cross-roll mean wind. The roll vortices are tilted near surface to extract kinetic energy from the mean flow in the exponentially growing phase. The stable stratification can limit or even completely suppress the growth of roll vortices because of negative work done by buoyancy. The change of characteristics of roll vortices with the distance from the storm center and their effects on the large-scale HBL flow are also analyzed.