Slab- and height-resolving models of the tropical cyclone boundary layer. Part I: Comparing the simulations

Abstract

Diagnostic models of the tropical cyclone boundary layer have important practical uses, including engineering design and climatological risk-assessment studies and as components of tropical cyclone potential-intensity models. A widely used class of such models has been slab models, in which the governing equations are depthaveraged. Here, a slab model is compared with one that fully resolves height, and it is shown that the vertical averaging leads to substantial differences in the simulations. The slab model produces excessively strong inflow and too great a departure of the boundary-layer mean winds from gradient balance. In moving storms, the slab models produce much too strong a motion-induced asymmetry, and can support an asymmetric analogue of the quasi-inertial oscillation previously noted in axisymmetric slab models. Given the considerable impact of the vertical averaging in slab models on the simulated flow in the tropical cyclone boundary layer, it is difficult to recommend their further use for applications where quantitative accuracy is important. Other applications will require care to ensure that the results are not unduly affected by the depth averaging. Part II of this article presents a detailed analysis of the reasons for the differences between depth-averaged and height-resolving simulations of the tropical cyclone boundary layer.

Why Do Model Tropical Cyclones Intensify More Rapidly at Low Latitudes?

“…As discussed by Smith and Montgomery (2010), there are serious mathematical issues with the more sophisticated model in that it requires a specification of both the radial and tangential wind components at the top of the boundary layer where the flow exits the boundary layer. Despite the claims by Kepert (2010Kepert ( , p. 1689) that he did not apply such a boundary condition but rather zero vertical gradient condition, it was shown by Smith and Montgomery (2010) that the zero-gradient boundary condition is equivalent to the imposition of a prescribed flow in gradient wind balance with effectively zero radial inflow. For this reason, we consider such comparisons to be problematic.…”

“…The inner-core breakdown of the slab model is of no consequence for these arguments. Kepert (2010) wrote a useful summary of the potential inaccuracies of the slab boundary layer, comparing the predictions of such models with a more sophisticated boundary layer model that goes some way to resolving the vertical structure of the boundary layer. As discussed by Smith and Montgomery (2010), there are serious mathematical issues with the more sophisticated model in that it requires a specification of both the radial and tangential wind components at the top of the boundary layer where the flow exits the boundary layer.…”

Dynamical system properties of an axisymmetric convective tropical cyclone model

“…Notwithstanding, it is probably much more realistic to calculate the turbulent surface exchange [eq. (11)] on the basis of 10 m-wind speeds rather than that at the much higher first model level, which would possibly result in an excessive inflow and, consequently, a stronger updraught (Kepert, 2010). The effect of radiation is parameterised in a most simplistic manner by application of a so-called 'Newtonian cooling' (Rotunno and Emanuel, 1987), given by…”

Hurricane risk analysis

“…The other approach, developed by Emanuel et al (2006 and, applies a deterministic, coupled ocean-atmosphere TC model to simulate storm development driven by largescale environmental conditions whose statistics are derived from reanalysis or global climate model data. This statistical/deterministic TC risk model does not rely on the limited historical TC data but generates large samples of synthetic storms that are in statistical agreement with observations (Emanuel et al 2006), and compares well with other methods used to study the effects of climate change on TCs (Emanuel et al 2008 and2010;Bender et al 2010;Knutson et al 2010). This approach has been used to investigate TC wind and surge risk (e.g.…”

“…A boundary layer model (e.g., Thompson and Cardone, 1996;Vickery et al, 2009;Kepert, 2010) may be applied to more accurately calculate the surface wind from the gradient condition, but it is nonparametric and more computationally demanding. A number of gradient wind profiles (e.g., Holland, 1980;Jelesnianski et al, 1992;Emanuel, 2004;Emanuel and Rotunno, 2011) have been used in wind and surge analysis; it may be difficult at this point to identify the "best" wind profile, as each profile has its own strengths and limitations.…”