On Steady-State Tropical Cyclones Continued

A recent study by the authors' (“On steady-state tropical cyclones”, QJRMS-accepted and presented in this session) concludes inter alia that it is unlikely that absolute angular momentum would be replenished in a large-scale domain containing an isolated hurricane necessary to balance the loss of AAM due to friction in the region of cyclonic surface flow. Some studies, however, report quasi-steady intensity behavior for significant periods of time, e.g. for 400 days in Hakim (2012), where the introduction of a more complicated atmospheric radiation scheme is cited for promoting the long-period steady state. We will investigate this hypothesis here by conducting experiments with the CM1 model, in both three-dimensional and in axisymmetric configurations. Results find a prolonged, quasi-steady intensity in an axisymmetric simulation with a detailed water microphysics scheme but without radiation versus a similar simulation using a simple fall-speed microphysics scheme (Rotunno and Emanuel 1987). Other short quasi-steady intensity periods are found for our other simulations. However, in every case where a quasi-steady intensity period is found, the environment of the hurricane is far from a steady state, with continuing evolution of the secondary circulation and absolute angular momentum field. An eventual inability to supply cyclonic relative angular momentum to the vicinity of the hurricane is associated with the long-term weakening of the storm. The role of the choice of microphysics scheme to forestall this weakening is explored. The implications of our findings will be discussed.