Sensitivity of Axisymmetric Tropical Cyclone Spinup Time to Dry Air Aloft

• Published in: Journal of the atmospheric sciences 2016-11, Vol.73 (11), p.4269-4287
• Main Authors: Tang, Brian H., Rios-Berrios, Rosimar, Alland, Joshua J., Berman, Jeremy D., Corbosiero, Kristen L.
• Format: Article
• Language: English
• Subjects: Aerodynamics; Angular momentum; Boundary layer; Circulation; Clouds; Computational fluid dynamics; Condensation; Convection; Cumulus congestus; Cyclones; Dry air; Duration; Enthalpy; Entropy; Environmental science; Experiments; Flux; Gravitational waves; Hoisting; Hurricanes; Kinetic energy; Levels; Lifting; Lifting condensation level; Mass flux; Moisture; Moisture profiles; Momentum; Precipitation; Pressure gradients; Profiles; Saturation; Sensitivity; Studies; Temperature; Thresholds; Tropical climate; Tropical cyclones; Troposphere
• ISSN: 0022-4928; 1520-0469
• DOI: 10.1175/JAS-D-16-0068.1

The sensitivity of tropical cyclone spinup time to the initial entropy deficit of the troposphere is examined in an axisymmetric hurricane model. 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. Consequently, the secondary circulation takes longer to build upward, and the radial influx of angular momentum is reduced. There is also a reduction in the conversion of potential available enthalpy to horizontal kinetic energy, as a result of reduced flow down the radial pressure gradient early in the spinup period. Later in the spinup period, the low-level vortex spins up relatively quickly near the nascent radius of maximum wind in the high-entropy deficit experiments, whereas the low-level vortex spins up over a wider area in the low-entropy deficit experiments.