Large-Eddy Simulation of Roll Vortices in a Hurricane Boundary Layer

Abstract

For the last decade, horizontal roll vortices have been often observed in hurricane boundary layers (HBLs). In this study, a large-eddy simulation is performed to explore the formation mechanism of the horizontal roll vortices and their significance in a near-neutrally stratified HBL at 40 km (R40) and 100 km (RlOO) from the center of the hurricane. Results are examined through turbulence statistics and empirical orthogonal function (EOF) analysis. The EOF analysis and budgets of turbulent kinetic energy demonstrate that an inflectionpoint instability in the radial velocity profile is responsible for the roll vortices with horizontal wavelengths of 1.5-2.4 km in the HBL both for R40 and RlOO. The roll vortices for R40 are nearly aligned with the gradient wind, while those for RlOO are oriented slightly to the left of that wind. Also the horizontal distributions of velocity fluctuations suggest the presence of streaklike structures at horizontal intervals of several hundred meters near the ground surface. Internal gravity waves, Kelvin-Helmholtz waves, and entrainments occur above the HBL and are partly coupled with the roll vortices in the HBL. implying an enhancement of vertical transports of momentum and other quantities between the HBL and the free atmosphere.

A Simple Method for Simulating Wind Profiles in the Boundary Layer of Tropical Cyclones

“…Of note, Nakanishi and Niino (2012) and Green and Zhang (2015) considered centrifugal acceleration terms in their model equations, as does our new method, but we show herein that inclusion of radial advection is needed to produce mean wind profiles that are similar to observed profiles in tropical cyclones.…”

“…Previous methods for simulating the boundary layer of tropical cyclones were evaluated alongside the new method, including a classic Ekman-type method that adds only a large-scale (geostrophic) pressure gradient acceleration to the horizontal velocity equations, and two recently developed methods that also account for centrifugal-accelerationlike terms (Nakanishi and Niino 2012;Green and Zhang 2015). The new method, which also accounts for large-scale radial (i.e., horizontal) advection, has clear advantages: it produces realistic mean wind-speed profiles compared with composites of dropsonde observations in tropical cyclones.…”

“…The variables V and ∂ V /∂ R may be functions of height, or could be considered constant with height for shallow domains of a few km depth. Several recent studies (Nakanishi and Niino (2012), hereafter NN12; Green and Zhang (2015), hereafter GZ15) have included centrifugal-like terms in their model equations, similar to the second term of (4). However, inclusion of an advection term (first term) makes this approach clearly different from these recent studies.…”

On the Generation of Roll Vortices due to the Inflection Point Instability of the Hurricane Boundary Layer Flow

“…He found roll-like perturbations existed in a statically stable boundary layer environment, suggesting these features were generated by the inflection point instability rather than the thermal instability. Nakanishi and Niino (2012) conducted an LES study at two different locations in the idealized HBL. By applying the EOF analysis to the model results, they identified rolls generated by the inflection point instability with spatial scales similar to those found by Foster (2005).…”

“…These previous studies investigated the rolls in neutral or near-neutral stratification (Foster 2005;Nolan 2005) or the rolls at specific locations within the HBL (Zhu 2008;Nakanishi and Niino 2012), but they did not systematically examine the conditions favorable for the formation of rolls by the inflection point instability and the impacts of the mean wind and stratification on the characteristics of rolls. In this study, we aim to gain a more comprehensive understanding of the formation of rolls under various HBL conditions and identify the important mean-flow factors affecting the characteristics of rolls.…”