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

Abstract

A method to simulate characteristics of wind speed in the boundary layer of tropical cyclones in an idealized manner is developed and evaluated. The method can be used in a single-column modelling set-up with a planetary boundary-layer parametrization, or within large-eddy simulations (LES). The key step is to include terms in the horizontal velocity equations representing advection and centrifugal acceleration in tropical cyclones that occurs on scales larger than the domain size. Compared to other recently developed methods, which require two input parameters (a reference wind speed, and radius from the centre of a tropical cyclone) this new method also requires a third input parameter: the radial gradient of reference wind speed. With the new method, simulated wind profiles are similar to composite profiles from dropsonde observations; in contrast, a classic Ekman-type method tends to overpredict inflow-layer depth and magnitude, and two recently developed methods for tropical cyclone environments tend to overpredict near-surface wind speed. When used in LES, the new technique produces vertical profiles of total turbulent stress and estimated eddy viscosity that are similar to values determined from low-level aircraft flights in tropical cyclones. Temporal spectra from LES produce an inertial subrange for frequencies 0.1 Hz, but only when the horizontal grid spacing 20 m.

Using Large-Eddy Simulations to Define Spectral and Coherence Characteristics of the Hurricane Boundary Layer for Wind-Energy Applications

“…A prognostic subgrid-scale turbulent kinetic energy (TKE) model (i.e., Deardorff-type TKE scheme) is used to parametrize unresolved turbulence (Stevens et al 1999;Bryan et al 2017). For all of the simulations, surface ocean waves are not modelled explicitly; rather, the surface roughness is varied as a function of the time-averaged 10-m wind speed Ū , as described in Bryan et al (2017).…”

“…A prognostic subgrid-scale turbulent kinetic energy (TKE) model (i.e., Deardorff-type TKE scheme) is used to parametrize unresolved turbulence (Stevens et al 1999;Bryan et al 2017). For all of the simulations, surface ocean waves are not modelled explicitly; rather, the surface roughness is varied as a function of the time-averaged 10-m wind speed Ū , as described in Bryan et al (2017). Herein, forŪ ≥ 25 m s −1 , the roughness length is held constant at 0.0028 m. This parametrization yields a surface friction velocity u * that varies roughly linear withŪ , as suggested by recent observational studies (e.g., Andreas et al 2012).…”

“…2) and accounts for the large-scale tendencies associated with a hurricane (e.g., pressure-gradient and centrifugal accelerations) via mesoscale tendency terms, as formulated by Bryan et al (2017). The Simple set-up uses a LES configuration, so the planetary boundary-layer (PBL) parametrization in the model is removed and the subgrid turbulence model described in the Appendix of Bryan et al (2017) is employed. In addition, to simplify initial conditions and decrease computational expense, the Simple set-up does not include moisture and therefore employs no microphysical scheme, unlike the Complex set-up.…”

“…In addition, to simplify initial conditions and decrease computational expense, the Simple set-up does not include moisture and therefore employs no microphysical scheme, unlike the Complex set-up. Surface heat fluxes are also neglected (see Bryan et al 2017) and thus the PBL in these simulations is neutral. As will be shown, wind profiles from the Simple set-up compare well to those from the Complex set-up (Sect.…”

“…2), and the radial gradient of wind speed above the boundary layer dV /dR. The mesoscale tendency terms, which use these parameters, are described in detail in Bryan et al (2017). For the present study, we include subsidence terms (see Siebesma et al 2003) except that the minimum vertical velocity w min , has a larger amplitude (stated below) and is located at 1 km a.s.l.…”