QUASI STEADY-STATE HURRICANES REVISITED

We revisit the theoretical possibility of long-term, sustained tropical cyclone solutions using a state-of-the-art numerical model that incorporates the most recent observational guidance for subgrid scale parameters and airsea exchange coefficients of heat and momentum. Emphasis is placed on the realism of such solutions and the sources of cyclonic relative angular momentum (RAM) that are necessary to replenish that lost by friction at the surface. For simplicity, we confine our attention to strictly axisymmetric numerical experiments. We are able to replicate Hakim's long-term simulation of a quasi-steady state cyclone in a 1500 km radial domain. The structure of the wind field is found to be somewhat realistic compared to observations, but sustained by unrealistic processes. Artificial sources of cyclonic RAM are quantified and the lateral damping of the anticyclonic wind near the outer boundary is found to make the largest contribution to the source of cyclonic RAM. When the domain size is extended to 9,000 km radius and lateral damping is removed altogether, a quasi-steady vortex emerges, but the structure of this vortex has many unrealistic features. In this solution, the remaining upper- level Rayleigh damping contributes a major portion of the needed source of cyclonic RAM. In a simulation in which the upper-level damping is removed also, the solution is found to be neither quasi-steady nor realistic. These findings call into question the realism of long-term, sustained tropical cyclone simulations, which require a sufficiently large source of cyclonic RAM to facilitate the existence of a quasi-steady state.