Sensitivity of axisymmetric tropical cyclone spinup time to dry air aloft

Larger initial entropy deficits correspond to less moisture above the initial lifting condensation level of a subcloud-layer parcel. The spinup time is quantified in terms of thresholds of integrated horizontal kinetic energy within a radius of 300 km and below a height of 1.5 km.

The spinup time increases sublinearly with increasing entropy deficit, indicating the greatest sensitivity lies with initial moisture profiles closer to saturation. As the moisture profile approaches saturation, there is a large increase in the low-level, area-averaged, vertical mass flux over the spinup period because of the predominance of deep convection. Higher entropy deficit experiments have a greater amount of cumulus congestus and reduced vertical mass flux over a longer duration.