Coupled Dynamic–Thermodynamic Forcings during Tropical Cyclogenesis. Part II: Axisymmetric Experiments

An ensemble of axisymmetric model experiments with simplified physics is used to evaluate the diagnostic framework presented in Part I. The central piece of the framework is understanding what causes decreases in the ratio of bulk differences of moist entropy over differences of angular momentum between two defined regions, the boundary between the two demarcating the approximate location of the emergence of the radius of maximum wind of the developing meso-beta-scale protovortex. Within a day before tropical cyclogenesis, the moist entropy forcing results in a decrease of this ratio. Net advective fluxes act to export moist entropy from the outer region and import moist entropy into the inner region, resulting in a positive radial gradient in gross moist stability that is maximized around the time of genesis. While surface moist entropy fluxes are needed for genesis to occur, they act synergistically with the net advective fluxes to decrease the ratio before and during genesis. Within a day after tropical cyclogenesis, surface moist entropy fluxes directly amplify the positive difference in moist entropy between the inner and outer regions, and radial fluxes of angular momentum reduce the magnitude of the negative difference in angular momentum between the inner and outer regions. Both of these processes act to reduce the ratio further. The framework highlights differences in processes occurring before, during, and after genesis as the meso-beta-scale protovortex develops and intensifies.