Sources and Sinks of Available Potential Energy in a Moist Atmosphere

Ríos-Berríos

Alland

et al. 2016

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.

Section: November 2016 T a N G E T A Lmentioning

confidence: 99%

Sensitivity of Axisymmetric Tropical Cyclone Spinup Time to Dry Air Aloft

Ríos-Berríos

Alland

et al. 2016

“…The power lost is composed of the direct contribution of ventilation via (4.68) and the model's turbulence parameterization. Both represent a sink of available potential energy, which can be estimated by integrating the product of the divergence of the turbulent entropy flux and the difference between the parcels' temperature and reference temperature (Pauluis, 2007): As the ventilation increases, the power loss due to diffusion of entropy increases steadily and becomes comparable to the power dissipated by friction. In the A10 experiment, the power loss due to turbulent entropy mixing is about twice that of the control experiment.…”

Section: Ventilation Amplitudementioning

confidence: 99%

Midlevel Ventilation’s Constraint on Tropical Cyclone Intensity

Emanuel

2010

252

190

Midlevel ventilation, or the flux of low-entropy air into the inner core of a tropical cyclone (TC), is a hypothesized mechanism by which environmental vertical wind shear can constrain a TC's intensity. An idealized framework is developed to assess how ventilation affects TC intensity via two pathways: downdrafts outside the eyewall and eddy fluxes directly into the eyewall. Three key aspects are found: ventilation has a detrimental effect on TC intensity by decreasing the maximum steady state intensity, imposing a minimum intensity below which a TC will unconditionally decay, and providing an upper ventilation bound beyond which no steady TC can exist.Based on the idealized framework, a ventilation index is derived that is equal to the environmental vertical wind shear times the midlevel entropy deficit divided by the potential intensity. The ventilation index has a strong influence on the presentday climatology of tropical cyclogenesis and the distribution of TC intensification. Additionally, changes in the ventilation index are also examined in general circulation models (GCMs) between the late 20th century and the late 22nd century. Individual GCMs indicate potential regional shifts in preferred locations of tropical cyclogenesis and changes in TC intensity statistics due to shifts in the seasonal ventilation index, but a statistically significant projection cannot be given. The GCMs do show a robust increase in the midlevel entropy deficit and potential intensity nearly everywhere in the tropics.Lastly, an axisymmetric model with parameterized ventilation is used to examine the sensitivity of TC intensity to the strength and location of the ventilation and to examine the findings of the idealized framework. Increasing the strength of the ventilation and placing the ventilation at lower to middle levels results in a greater decrease in the quasi-steady intensity, whereas upper-level ventilation has little effect on the intensity. For strong ventilation, an oscillatory intensity regime materializes and is tied to transient convective bursts and strong downdrafts into the boundary layer. The sensitivity of TC intensity to ventilation can be viewed in the context of a modified thermal wind relation or the fractional Carnot efficiency of the inner-core.

“…The power loss is equal to the integral of the product of the divergence of the entropy flux, including both the turbulence and ventilation parameterization terms, and the difference between the parcel temperature and reference temperature (Pauluis 2007):…”

Section: A Ventilation Amplitudementioning

confidence: 99%

Sensitivity of Tropical Cyclone Intensity to Ventilation in an Axisymmetric Model

Emanuel

2012

103

The sensitivity of tropical cyclone intensity to ventilation of cooler, drier air into the inner core is examined using an axisymmetric tropical cyclone model with parameterized ventilation. Sufficiently strong ventilation induces cooling of the upper-level warm core, a shift in the secondary circulation radially outward, and a decrease in the simulated intensity. Increasing the strength of the ventilation and placing the ventilation at middle to lower levels results in a greater decrease in the quasi-steady intensity, whereas upper-level ventilation has little effect on the intensity. For strong ventilation, an oscillatory intensity regime materializes and is tied to transient convective bursts and strong downdrafts into the boundary layer.The sensitivity of tropical cyclone intensity to ventilation can be viewed in the context of the mechanical efficiency of the inner core or a modified thermal wind relation. In the former, ventilation decreases the mechanical efficiency, as the generation of available potential energy is wasted by entropy mixing above the boundary layer. In the latter, ventilation weakens the eyewall entropy front, resulting in a decrease in the intensity by thermal wind arguments.The experiments also support the existence of a threshold ventilation beyond which a tropical cyclone cannot be maintained. Downdrafts overwhelm surface fluxes, leading to a precipitous drop in intensity and a severe degradation of structure in such a scenario. For a given amount of ventilation below the threshold, there exists a minimum initial intensity necessary for intensification to the quasi-steady intensity.