On the Equilibrium-State Roll Vortices and Their Effects in the Hurricane Boundary Layer

Abstract

In this study, the authors numerically simulate roll vortices (rolls) generated by the inflection-point instability in the hurricane boundary layer (HBL). The approach is based on embedding a two-dimensional high-resolution single-grid roll-resolving model (SRM) at selected horizontal grid points of an axisymmetric HBL model. The results from a set of idealized experiments indicate that the mixed-layer height is an important factor affecting the magnitude of the roll velocities and the structure of the internal waves triggered in the stably stratified layer above. This study reveals the important difference between the roll-induced cross-roll (nearly radial) and alongroll (nearly azimuthal) momentum fluxes: while the cross-roll momentum flux is well correlated to the cross-roll mean wind shear, the along-roll momentum flux is typically not correlated with the along-roll mean wind shear. Therefore, the commonly used K theory in the boundary layer parameterizations cannot reasonably capture the vertical distribution of the roll-induced along-roll momentum flux. Moreover, the authors find that the rolls induce more significant changes in the mean radial wind profile than in the mean azimuthal wind profile. Specifically, rolls reduce the inflow near surface, enhance the inflow at upper levels, and increase the inflow-layer height. Based on a linear dynamical HBL model, the authors find that the impact of rolls on the mean radial wind profile is essentially due to their redistribution effect on the mean azimuthal momentum in the HBL.

Effect of Boundary Layer Roll Vortices on the Development of an Axisymmetric Tropical Cyclone

“…8)]. Studies focusing on the formation mechanism of rolls in the TC BL based on linearized models (Foster 2005;Gao and Ginis 2014) suggest that inflection points in the basicstate radial wind profiles cause the instability and lead to the generation of rolls. Since this type of instability is related to inflection points in the boundary layer wind profile, it is commonly referred to as the inflection point instability.…”

“…Based on the spectral analysis of LES-resolved turbulent fluxes, Wang and Jiang (2017) found that the roll-scale fluxes constituted a large portion (up to 40%) of the total turbulent fluxes in the TC BL. Using a roll-resolving BL model Gao and Ginis (2016) found that the along-and cross-roll components of the roll-induced momentum fluxes are fundamentally different: while the roll-induced cross-roll (approximately radial) momentum flux is mostly downgradient, the roll-induced along-roll (approximately tangential) momentum flux is largely countergradient. Therefore, the commonly used BL parameterizations based on K theory 1 cannot reasonably capture the vertical fluxes caused by rolls.…”

“…Therefore, the commonly used BL parameterizations based on K theory 1 cannot reasonably capture the vertical fluxes caused by rolls. By considering the roll-induced momentum fluxes in a TC BL model, Gao and Ginis (2016) found that rolls could significantly affect the magnitude and structure of the radial inflow in the TC BL.…”

“…Even though the potential importance of rolls in the vertical transport in the TC BL is well recognized, there is still limited understanding of their role in the evolution of the entire TC. In the modeling system used by Gao and Ginis (2016), the upper TC vortex is fixed with time, and therefore the effect of rolls on the entire TC cannot be explored. In this study, we aim to implement the roll modeling approach proposed by Gao andGinis (2014, 2016) into a three-dimensional full-physics TC model and investigate the response of the entire TC vortex to the roll-induced vertical fluxes.…”

“…In the modeling system used by Gao and Ginis (2016), the upper TC vortex is fixed with time, and therefore the effect of rolls on the entire TC cannot be explored. In this study, we aim to implement the roll modeling approach proposed by Gao andGinis (2014, 2016) into a three-dimensional full-physics TC model and investigate the response of the entire TC vortex to the roll-induced vertical fluxes. Specifically, we focus on the impact of rolls on an axisymmetric TC vortex in a quiescent environment.…”