Point-vortex application to vortex stability, evolution, and statistical equilibrium

June 1995.Also issued as John Persing's thesis (M.S.) -- Colorado State University, 1995.Includes bibliographical references.The nonlinear evolution of an isolated, barotropic vortex in an infinite, frictionless domain is examined with a cloud of like-signed point-vortices. The stability of systems of point­ vortices is reviewed as well as the stability of continuous systems possessing a sign reversal in the radial vorticity gradient like that observed in the inner core of hurricanes. The new result is the application of point-vortices to examine the evolution of a hurricane­ like vortex system. Using a three-region approximation to the radial vorticity profile, the nondimensional problem can be reduced to two parameters. These are the inner radius of the vorticity maximum 8 and the tangential wind speed at this radius Vtan(8). The relaxation time scale is on the order of five circuit times, and the relaxed vorticity profile ranges from near solid-body rotation to highly monopolar profiles. The relaxation time-scale and the monopolicity of the relaxed vorticity profile show some correlation to the strength of the linear instability in the initial system, although a more thorough examination of the parameter space is proposed to obtain a complete understanding of the processes involved in the relaxation.Sponsored by the Office of Naval Research grant ONR-N00014-93-1-0456

Putting to rest WISHE-ful misconceptions for tropical cyclone intensification

The purpose of this article is twofold. The first is to point out and correct several misconceptions about the putative WISHE mechanism of tropical cyclone intensification that currently are being taught to atmospheric science students, to tropical weather forecasters, and to laypeople who seek to understand how tropical cyclones intensify. The mechanism relates to the simplest problem of an initial cyclonic vortex in a quiescent environment. This first part is important because the credibility of tropical cyclone science depends inter alia on being able to articulate a clear and consistent picture of the hypothesized intensification process and its dependencies on key flow parameters. The credibility depends also on being able to test the hypothesized mechanisms using observations, numerical models, or theoretical analyses. The second purpose of the paper is to carry out new numerical experiments using a state-of-the-art numerical model to test a recent hypothesis invoking the WISHE feedback mechanism during the rapid intensification phase of a tropical cyclone. The results obtained herein, in conjunction with prior work, do not support this recent hypothesis and refute the view that the WISHE intensification mechanism is the essential mechanism of tropical cyclone intensification in the idealized problem that historically has been used to underpin the paradigm. This second objective is important because it presents a simple way of testing the hypothesized intensification mechanism and shows that the mechanism is neither essential nor the dominant mode of intensification for the prototype intensification problem. In view of the operational, societal, and scientific interest in the physics of tropical cyclone intensification, we believe this paper will be of broad interest to the atmospheric science community and the findings should be useful in both the classroom setting and frontier research

A Lagrangian trajectory view on transport and mixing processes between the eye, eyewall, and environment using a high resolution simulation of Hurricane Bonnie (1998)

The transport and mixing characteristics of a large sample of air parcels within a mature and vertically sheared hurricane vortex is examined. Data from a high-resolution (2 km grid spacing) numerical simulation of "real-case" Hurricane Bonnie (1998) is used to calculate Lagrangian trajectories of air parcels in various subdomains of the hurricane (namely, the eye, eyewall, and near-environment) to study the degree of interaction (transport and mixing) between these subdomains. It is found that 1) there is transport and mixing from the low-level eye to the eyewall that carries high- Be air which can enhance the efficiency of the hurricane heat engine; 2) a portion of the low-level inflow of the hurricane bypasses the eyewall to enter the eye, that both replaces the mass of the low-level eye and lingers for a sufficient time (order 1 hour) to acquire enhanced entropy characteristics through interaction with the ocean beneath the eye; 3) air in the mid- to upper-level eye is exchanged with the eyewall such that more than half the air of the eye is exchanged in five hours in this case of a sheared hurricane; and 4) that one-fifth of the mass in the eyewall at a height of 5 km has an origin in the mid- to upper-level environment where thet(sub e) is much less than in the eyewall, which ventilates the ensemble average eyewall theta(sub e) by about 1 K. Implications of these findings to the problem of hurricane intensity forecasting are discussed

Cerebral cortex maldevelopment in syndromic craniosynostosis

AIM: To assess the relationship of surface area of the cerebral cortex to intracranial volume (ICV) in syndromic craniosynostosis. METHOD: Records of 140 patients (64 males, 76 females; mean age 8y 6mo [SD 5y 6mo], range 1y 2mo–24y 2mo) with syndromic craniosynostosis were reviewed to include clinical and imaging data. Two hundred and three total magnetic resonance imaging (MRI) scans were evaluated in this study (148 patients with fibroblast growth factor receptor [FGFR], 19 patients with TWIST1, and 36 controls). MRIs were processed via FreeSurfer pipeline to determine total ICV and cortical surface area (CSA). Scaling coefficients were calculated from log‐transformed data via mixed regression to account for multiple measurements, sex, syndrome, and age. Educational outcomes were reported by syndrome. RESULTS: Mean ICV was greater in patients with FGFR (1519cm(3), SD 269cm(3), p=0.016) than in patients with TWIST1 (1304cm(3), SD 145cm(3)) or controls (1405cm(3), SD 158cm(3)). CSA was related to ICV by a scaling law with an exponent of 0.68 (95% confidence interval [CI] 0.61–0.76) in patients with FGFR compared to 0.81 (95% CI 0.50–1.12) in patients with TWIST1 and 0.77 (95% CI 0.61–0.93) in controls. Lobar analysis revealed reduced scaling in the parietal (0.50, 95% CI 0.42–0.59) and occipital (0.67, 95% CI 0.54–0.80) lobes of patients with FGFR compared with controls. Modified learning environments were needed more often in patients with FGFR. INTERPRETATION: Despite adequate ICV in FGFR‐mediated craniosynostosis, CSA development is reduced, indicating maldevelopment, particularly in parietal and occipital lobes. Modified education is also more common in patients with FGFR

Intracranial hypertension and cortical thickness in syndromic craniosynostosis

Aim: To evaluate the impact of risk factors for intracranial hypertension (ICH) on cerebral cortex thickness in syndromic craniosynostosis. Method: ICH risk factors including papilloedema, hydrocephalus, obstructive sleep apnea (OSA), cerebellar tonsillar position, occipitofrontal circumference (OFC) curve deflection, age, and sex were collected from the records of patients with syndromic craniosynostosis (Apert, Crouzon, Pfeiffer, Muenke, Saethre-Chotzen syndromes) and imaging. Magnetic resonance images were analysed and exported for statistical analysis. A linear mixed model was developed to determine correlations with cerebral cortex thickness changes. Results: In total, 171 scans from 107 patients (83 males, 88 females, mean age 8y 10mo, range 1y 1mo–34y, SD 5y 9mo) were evaluated. Mean cortical thickness in this cohort was 2.78mm (SD 0.17). Previous findings of papilloedema (p=0.036) and of hydrocephalus (p=0.007) were independently associated with cortical thinning. Cortical thickness did not vary significantly by sex (p=0.534), syndrome (p=0.896), OSA (p=0.464), OFC (p=0.375), or tonsillar position (p=0.682). Interpretation: Detection of papilloedema or hydrocephalus in syndromic craniosynostosis is associated with significant changes in cortical thickness, supporting the need for preventative rather than reactive treatment strategies

Putting to rest WISHE-ful misconceptions for tropical cyclone intensification

The purpose of this article is twofold. The first is to point out and correct several misconceptions about the putative WISHE mechanism of tropical cyclone intensification that currently are being taught to atmospheric science students, to tropical weather forecasters, and to laypeople who seek to understand how tropical cyclones intensify. The mechanism relates to the simplest problem of an initial cyclonic vortex in a quiescent environment. This first part is important because the credibility of tropical cyclone science depends inter alia on being able to articulate a clear and consistent picture of the hypothesized intensification process and its dependencies on key flow parameters. The credibility depends also on being able to test the hypothesized mechanisms using observations, numerical models, or theoretical analyses. The second purpose of the paper is to carry out new numerical experiments using a state-of-the-art numerical model to test a recent hypothesis invoking the WISHE feedback mechanism during the rapid intensification phase of a tropical cyclone. The results obtained herein, in conjunction with prior work, do not support this recent hypothesis and refute the view that the WISHE intensification mechanism is the essential mechanism of tropical cyclone intensification in the idealized problem that historically has been used to underpin the paradigm. This second objective is important because it presents a simple way of testing the hypothesized intensification mechanism and shows that the mechanism is neither essential nor the dominant mode of intensification for the prototype intensification problem. In view of the operational, societal, and scientific interest in the physics of tropical cyclone intensification, we believe this paper will be of broad interest to the atmospheric science community and the findings should be useful in both the classroom setting and frontier research

Does Balance Dynamics Well Capture the Secondary Circulation and Spinup of a Simulated Hurricane?

The article of record as published may be found at https://doi.org/10.1175/JAS-D-19-0258.1This study investigates a claim made by Heng et al. in an article published in 2017 and intimated soon after in their article published in 2018 that axisymmetric "balanced dynamics can well capture the secondary circulation in the full-physics model" during hurricane spinup. Using output from a new, convection-permitting, three-dimensional numerical simulation of an intensifying hurricane, azimuthally averaged forcings of tangential momentum and heat are diagnosed to force an axisymmetric Eliassen balance model under strict balance conditions. The balance solutions are found, inter alia, to poorly represent the peak inflow velocity in the boundary layer and present a layer of relatively deep inflow extending well above the boundary layer in the high-wind-speed region of the vortex. Such a deep inflow layer, a hallmark of the classical spinup mechanism for tropical cyclones comprising the radial convergence of absolute angular momentum above the boundary layer, is not found in the numerical simulation during the period of peak intensification. These deficiencies are traced to the inability of the balance model to represent the nonlinear boundary layer spinup mechanism. These results are contrasted with a pseudobalance Eliassen formulation that improves the solution in some respects while sacrificing strict thermal wind balance. Overall, the quantitative results refute the Heng et al. claim and implicate the general necessity of the nonlinear boundary layer spinup mechanism to explain the spinup of a hurricane in realistic model configurations and in reality.NSF Grants AGS-1313948 and IAA-1656075, ONR Grant N0001417WX00336, and the U. S. Naval Postgraduate School

On steady-state tropical cyclones

The article of record as published may be located at http://dx.doi.org/10.1002/qj.2329Weexamine the physical constraints thatmust be satisfied to allow for a steady-state tropical cyclone in an isolated environment, paying particular attention to the need to replenish absolute angularmomentum at exactly the rate at which it is consumed and to the vanishing of the spin-up function above the frictional boundary layer. We conclude that it is unlikely that these conditions will bemet simultaneously and question whether globally steady-state tropical cyclone solutions have merit. The implications for previous studies are discussed.MTM acknowledges the support of NSF Grant AGS-0733380