Minimal conceptual models for tropical cyclone intensification

We examine a hierarchy of minimal conceptual models for tropical cyclone intensification.These models are framed mostly in terms of axisymmetric balance dynamics.In the first set of models,the heating rate is prescribed in such a way to mimic a deep overturning circulation with convergence in the lower troposphere and divergence in the upper troposphere,characteristic of a region of deep moist convection.In the second set,the heating rate is related explicitly to the latent heat release of ascending air parcels.The release of latent heat markedly reduces the local static stability of ascending air,raising two possibilities in the balance framework.The first possibility is that the effective static stability and the related discriminant in the Eliassen equation for the overturning circulation in saturated air,although small,remains positive so the Eliassen equation is globally elliptic.The second possibility,the more likely one during vortex intensification,is that the effective static stability in saturated air is negative and the Eliassen equation becomes locally hyperbolic.These models help to understand the differences between the early Ooyama models of 1968 and 1969,the Emanuel,1989 model,and the later Emanuel models of 1995,1997 and 2012.They provide insight also into the popular explanation of the WISHE feedback mechanism for tropical cyclone intensification.Some implications for recent work are discussed.

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.