Analysis and Simulation of a Dry Hurricane

In this work we analyze the maintenance of a dry hurricane-like vortex. We show that dry thermodynamics provides a simplified system for studying the dynamical properties of hurricanes. We also show that, given the energy sources to sustain hurricane-like circulation, it is possible to achieve a steady state in a dry vortex simulation. To simulate a hurricane-like circulation we used an axisymmetric, nonhydrostatic, cloud resolving numerical model on a 1000 km horizontal domain. Experiments were run until a steady state is produced. In the series of simulations performed, surface temperature, tropopause temperature, latitude (Coriolis parameter), resolution, surface heat flux and other parameters were varied. Our dry thermodynamic setup is similar to what can be observed in polar lows and to some extent in dust devils.

From the theoretical point of view, it is argued that axisymmetric hurricane intensity theory (Maximum Potential Intensity, or MPI) of Emanuel (1986) is well suited for the analysis of a dry hurricane. This model does not introduce moisture explicitly, but includes its effects in the moist entropy. Emanuel (1986) assumes slantwise neutrality, which implies that above the boundary layer, potential temperature is constant along angular momentum lines. We show here that the same theoretical considerations are valid for a 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. Compensation for the absence of that latent energy source in the dry hurricane is possible through enhancement of sensible heat flux.

One of the fundamental assumptions of the MPI theory is that a hurricane vortex is in gradient wind balance. However, the observational as well as numerical modeling evidence reveals that unbalanced flow exists in the vicinity of the hurricane eyewall, thus causing the tangential wind to be stronger than that predicted by MPI. The excess winds are referred to as “supergradient”. While the presence of the supergradient flow is well documented (Gray and Shea, 1973), and shown not to be a measurement or averaging artifact (Zhang et al., 2001), there is still no satisfactory explanation for it. We suggest that the supergradient winds we observe are a result of the strong ageostrophic circulation contribution. There needs to be additional explanation for what factors control the entropy and angular momentum distribution, and surface fluxes alone may not be sufficient.