Experiments with Dry Hurricane Dynamics

This paper presents the development and analysis of a dry hurricane-like vortex. We argue that it is possible to have a hurricane spin up in an environment without any or very little moisture. A dry thermodynamics provides a simplified system for studying the dynamical properties of hurricanes. Dry hurricane dynamics may also be related to other planets, and it proves that we should expect hurricane formation in cold Earth scenarios.

The theoretical framework used here is based on axisymmetric hurricane intensity theory of Emanuel 1986. This model assumes slantwise neutrality, which implies that above the boundary layer, potential temperature is constant along angular momentum lines. The assumption of gradient wind and hydrostatic balance, enables simple analytical relationships between entropy, angular momentum and the tangential wind or central pressure derivation. Emanuel 1986 does not introduce moisture explicitly, but includes its effects in the moist entropy. We demonstrate that the same theoretical considerations are valid for dry thermodynamic system. The maintenance of the tropical storm depends on the energy flux between the ocean and the atmosphere. In a moist environment this energy flux is a combination of latent and sensible heat fluxes. In the dry case only sensible heat flux is present. Compensation for the absence of that latent energy source is possible through enhancement of sensible heat flux.

A dry version of Emanuel's maximum intensity theory is then used for analysing the numerical simulations. In this investigation an axisymmetric, nonhydrostatic version of the ZetaC model, with the GFDL radiation package on a 10 degrees domain has been used to simulate a hurricane-like circulation. The horizontal resolution of the model is 1 km so convection is fully resolved. Simulations were run for about 25 days, producing a steady state. In the series of experiments performed, surface temperature, tropopause temperature, latitude (Coriolis parameter), domain size, resolution, surface heat flux and other parameters were varied.

It is demonstrated that the maximum intensity theory can be studied within a simpler, dry system without moist processes. In the resulting simulations a maximum tangential wind is stronger than the theoretical one by about 10%. It is suggested here, that this is a result of the strong ageostrophic circulation contribution. The analysis of the radius of maximum wind (RMW) dependence on the model's input parameters was also performed in order to attempt an explanation of what controls the RMW. Finally comparison of the dry and moist hurricane simulations was done which allowed to isolate the contribution of moist processes on hurricane dynamics.

Reference: Emanuel, K.A., 1986: An Air-Sea Interaction Theory for Tropical Cyclones. Part I: Steady-State Maintenance. J Atmos.Sci., 43, 585-604