Sensitivity of Tropical Cyclone Spinup Time and Convection to the Initial Entropy Deficit

The spinup time of a tropical cyclone (TC) is sensitive to the initial moist entropy deficit, a measure of the dryness of the free troposphere. An axisymmetric model is used to investigate the spinup time of idealized TC vortices with varying moist entropy deficits between the boundary layer and free troposphere. The spinup timescale is inversely related to the mean vertical mass flux in the convecting region. Contoured frequency by altitude diagrams demonstrate a decrease in the mean vertical mass flux throughout the troposphere for the higher entropy deficit experiments. Entrainment of low-entropy air into the boundary layer due to Ekman suction and entrainment into the convection itself at midlevels may both reduce the mean vertical mass flux. Convective motions are analyzed in an isentropic framework by considering the conservation of moist entropy (Pauluis and Mrowiec 2013). Isentropic averaging separates higher entropy, upward-moving air parcels from lower entropy, subsiding air parcels, allowing for a systematic way to analyze moist entropy changes along the secondary circulation. The isentropic analysis of the mean vertical mass flux, and the associated streamfunction, show a decrease in the moist entropy within the deep convective updrafts for the higher entropy deficit experiments. This analysis demonstrates the role of dry-air entrainment in reducing the mean upward vertical mass flux of convection. Forward and backward parcel trajectories from the locations of the convective updrafts are utilized to evaluate the relative importance of dry-air entrainment directly into the convection at midlevels versus dry-air entrainment into the boundary layer from Ekman suction. The latter contributes most to decreasing the mean vertical mass flux in the high entropy deficit experiments.