Tropical‐cyclone intensification and predictability in three dimensions

Abstract

ABSTRACT:We present numerical-model experiments to investigate the dynamics of tropical-cyclone amplification and its predictability in three dimensions. For the prototype amplification problem beginning with a weak-tropical-storm-strength vortex, the emergent flow becomes highly asymmetric and dominated by deep convective vortex structures, even though the problem as posed is essentially axisymmetric. The asymmetries that develop are highly sensitive to the boundarylayer moisture distribution. When a small random moisture perturbation is added in the boundary layer at the initial time, the pattern of evolution of the flow asymmetries is changed dramatically, and a non-negligible spread in the local and azimuthally-averaged intensity results. We conclude, first, that the flow on the convective scales exhibits a degree of randomness, and only those asymmetric features that survive in an ensemble average of many realizations can be regarded as robust; and secondly, that there is an intrinsic uncertainty in the prediction of maximum intensity using either maximumwind or minimum-surface-pressure metrics. There are clear implications for the possibility of deterministic forecasts of the mesoscale structure of tropical cyclones, which may have a major impact on the intensity and on rapid intensity changes.Some other aspects of vortex structure are addressed also, including vortex-size parameters, and sensitivity to the inclusion of different physical processes or higher spatial resolution. We investigate also the analogous problem on a β-plane, a prototype problem for tropical-cyclone motion. A new perspective on the putative role of the wind-evaporation feedback process for tropical-cyclone intensification is offered also.The results provide new insight into the fluid dynamics of the intensification process in three dimensions, and at the same time suggest limitations of deterministic prediction for the mesoscale structure. Larger-scale characteristics, such as the radius of gale-force winds and β-gyres, are found to be less variable than their mesoscale counterparts.

Simulation and Interpretation of the Genesis of Tropical Storm Gert (2005) as Part of the NASA Tropical Cloud Systems and Processes Experiment

“…The evolution of the low-level potential vorticity field in the Gert simulations is qualitatively similar to that seen in idealized simulations by Van Sang et al (2008). In their experiments, storm intensification begins with the development of a ring of convection that produces intense small-scale vorticity dipoles, with strong cyclonic vorticity and weak anticyclonic vorticity.…”

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

“…However, as in previous studies (e.g., Nguyen et al 2008;Fang and Zhang 2011;Gopalakrishnan et al 2011;Persing et al 2013), the flow evolution during the process of intensification is distinctly nonaxisymmetric, with rotating convective structures and their progressive aggregation being a dominant feature. In fact, during the intensification phase, the azimuthally averaged fields of vertical velocity, vertical vorticity, and diabatic heating rate are dominated by these local features.…”

The effect of latent cooling processes in tropical cyclone simulations

“…This process is in agreement with the axisymmetrization of convectively forced PV anomalies in a barotropic vortex as found by Montgomery and Enagonio (1998) in a quasigeostrophic model. Furthermore, a very similar picture of axisymmetrization arises in a nonhydrostatic three-dimensional cyclogenesis simulation by Van Sang et al (2008) who also neglected latent cooling. Finally, at t = 42 h the cyclone is quite axisymmetric.…”