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Brian Tang and Kerry Emanuel

Abstract

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.

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Brian Tang and Kerry Emanuel

An important environmental control of both tropical cyclone intensity and genesis is vertical wind shear. One hypothesized pathway by which vertical shear affects tropical cyclones is midlevel ventilation—or the flux of low-entropy air into the center of the tropical cyclone. Based on a theoretical framework, a ventilation index is introduced that is equal to the environmental vertical wind shear multiplied by the nondimensional midlevel entropy deficit divided by the potential intensity. The ventilation index has a strong influence on tropical cyclone climatology. Tropical cyclogenesis preferentially occurs when and where the ventilation index is anomalously low. Both the ventilation index and the tropical cyclone's normalized intensity, or the intensity divided by the potential intensity, constrain the distribution of tropical cyclone intensification. The most rapidly intensifying storms are characterized by low ventilation indices and intermediate normalized intensities, while the most rapidly weakening storms are characterized by high ventilation indices and high normalized intensities. Since the ventilation index can be derived from large-scale fields, it can serve as a simple and useful metric for operational forecasts of tropical cyclones and diagnosis of model errors.

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Brian H. Tang

Abstract

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.

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Brian H. Tang

Abstract

A diagnostic framework to investigate the role of processes around and during tropical cyclogenesis is presented. The key framework metric is the ratio of bulk differences of moist entropy over differences of angular momentum between an inner and outer region of a tropical disturbance or cyclone. This ratio is hypothesized to decrease and become negative as both the high-entropy core and low-level vortex in the inner region amplify during tropical cyclogenesis. The time tendency of this ratio can be split into two forcings: a moist entropy forcing and an angular momentum forcing. Each forcing can be further divided into components comprising differences in net advective fluxes and nonadvective boundary fluxes of moist entropy or angular momentum between each region. The framework provides a comprehensive way to compare the relative importance of processes leading to tropical cyclogenesis in a tractable, consistent manner. Suggestions on how to apply the framework to numerical model output are given.

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Brian Tang and Kerry Emanuel

Abstract

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 tropical cyclone’s intensity. An idealized framework based on steadiness, axisymmetry, and slantwise neutrality is developed to assess how ventilation affects tropical cyclone intensity via two possible pathways: the first through downdrafts outside the eyewall and the second through eddy fluxes directly into the eyewall. For both pathways, ventilation has a detrimental effect on tropical cyclone intensity by decreasing the maximum steady-state intensity significantly below the potential intensity, imposing a minimum intensity below which a TC will unconditionally decay, and providing an upper-ventilation bound beyond which no steady tropical cyclone can exist. Ventilation also decreases the thermodynamic efficiency as the eyewall becomes less buoyant relative to the environment, which compounds the effects of ventilation alone. Finally, the formulation presented in this study is shown to be invariant across a range of thermodynamic environments after a suitable normalization and shows little sensitivity to external parameters.

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Rosimar Rios-Berrios, Tomislava Vukicevic, and Brian Tang

Abstract

Quantifying and reducing the uncertainty of model parameterizations using observations is evaluated for tropical cyclone (TC) intensity prediction. This is accomplished using a nonlinear inverse modeling technique that produces a joint probability density function (PDF) for a set of parameters. The dependence of estimated parameter values and associated uncertainty on two types of observable quantities is analyzed using an axisymmetric hurricane model. When the observation is only the maximum tangential wind speed, the joint PDF of parameter estimates has large variance and is multimodal. When the full kinematic field within the inner core of the TC is used for the observations, however, the joint parameter estimates are well constrained. These results suggest that model parameterizations may not be optimized using the maximum wind speed. Instead, the optimization should be based on observations of the TC structure to improve the intensity forecasts.

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Brian H. Tang and Nick P. Bassill

Abstract

A statistical downscaling algorithm is introduced to forecast surface wind speed at a location. The downscaling algorithm consists of resolved and unresolved components to yield a time series of synthetic wind speeds at high time resolution. The resolved component is a bias-corrected numerical weather prediction model forecast of the 10-m wind speed at the location. The unresolved component is a simulated time series of the high-frequency component of the wind speed that is trained to match the variance and power spectral density of wind observations at the location. Because of the stochastic nature of the unresolved wind speed, the downscaling algorithm may be repeated to yield an ensemble of synthetic wind speeds. The ensemble may be used to generate probabilistic predictions of the sustained wind speed or wind gusts. Verification of the synthetic winds produced by the downscaling algorithm indicates that it can accurately predict various features of the observed wind, such as the probability distribution function of wind speeds, the power spectral density, daily maximum wind gust, and daily maximum sustained wind speed. Thus, the downscaling algorithm may be broadly applicable to any application that requires a computationally efficient, accurate way of generating probabilistic forecasts of wind speed at various time averages or forecast horizons.

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Michael S. Fischer, Brian H. Tang, and Kristen L. Corbosiero

Abstract

The role of upper-tropospheric troughs on the intensification rate of newly formed tropical cyclones (TCs) is analyzed. This study focuses on TCs forming in the presence of upper-tropospheric troughs in the North Atlantic basin between 1980 and 2014. TCs were binned into three groups based upon the 24-h intensification rate starting at the time of genesis: rapid TC genesis (RTCG), slow TC genesis (STCG), and neutral TC genesis (NTCG). Composite analysis shows RTCG events are characterized by amplified upper-tropospheric flow with the largest upshear displacement between the TC and trough of the three groups. RTCG events are associated with greater quasigeostrophic (QG) ascent in upshear quadrants of the TC, forced by differential vorticity advection by the thermal wind, especially around the time of genesis. This pattern of QG ascent closely matches the RTCG composite of infrared brightness temperatures.

Conversely, NTCG events are associated with an upper-tropospheric trough that is closest to the TC center. The distribution of QG ascent in NTCG events becomes increasingly asymmetric around the time of genesis, with a maximum that shifts downshear of the TC center, consistent with infrared brightness temperatures. It is hypothesized that the TC intensification rate after tropical cyclogenesis, in environments of upper-tropospheric troughs, is closely linked to the structure and temporal evolution of the upper-level trough. The TC–trough configurations that provide greater QG ascent to the left of, and upshear of, the TC center feature more symmetric convection and faster TC intensification rates.

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Joshua J. Alland, Brian H. Tang, and Kristen L. Corbosiero

Abstract

Idealized experiments conducted with an axisymmetric tropical cyclone (TC) model are used to assess the effects of midlevel dry air on the axisymmetric TC secondary circulation. Moist entropy diagnostics of convective parcels are used to determine how midlevel dry air affects the distribution and strength of convection. Analyzing upward and downward motions in the Eulerian radius–height coordinate system shows that the moistest simulation has stronger vertical motions and a wider overturning circulation compared to drier simulations. A Lagrangian entropy framework further analyzes convective motions by separating upward higher-entropy streams from downward lower-entropy streams. Results show that the driest simulation has a weaker mean overturning circulation with updrafts characterized by lower mean entropy compared to moister simulations. Turbulent entrainment of dry air into deep convection at midlevels is small, suggesting that the influence of midlevel dry air on convective strength and the structure of the secondary circulation are through modification of the inflow layer. Backward trajectories show low-entropy air subsiding into the subcloud layer from low to midlevels of the atmosphere between radii of 200 and 400 km. Surface fluxes increase the entropy of these parcels before they rise in convective updrafts, but the increased recovery time, combined with descending motion closer to the inner core, decreases the width of the TC secondary circulation in the driest simulation.

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Yong Ming Tang, Brian Sanderson, Greg Holland, and Roger Grimshaw

Abstract

A two-dimensional numerical model of the shallow-water equations, with a modified Orlanski-type radiation boundary condition, is applied to study storm surges and tides on the North Queensland coast. The numerical simulations show that with the tides included in the storm surge model the sea level elevation is generally lower than if we simply add the astronomical tides to the surge. This has been previously observed and has been commonly explained as a nonlinear interaction between the storm surge and the tides. The authors demonstrate that this effect is due to the quadratic bottom friction law. Analysis of the important dynamical processes yields a simple rule to estimate the total sea level due to the combined effects of a storm surge and tide.

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