John Persing

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 are examined. Data from a high-resolution (2-km horizontal grid spacing) numerical simulation of real-case Hurricane Bonnie (1998) are 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 air possessing relatively high values of equivalent potential temperature (e), 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, and this air both replaces the mass of the low-level eye and lingers for a sufficient time (order 1 h) 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 i n5hi nthis 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 e is much less than in the eyewall, which ventilates the ensemble average eyewall e by about 1 K. Implications of these findings for the problem of hurricane intensity forecasting are briefly discussed.

Environmental interactions in the GFDL Hurricane model for Hurricane Opal

Hurricane Opal (1995) crossed the Gulf of Mexico rapidly intensifying to a 130-kt storm, then fortunately weakening before landfall on the Florida panhandle. This intensification was underforecast by the National Hurricane Center. Forecast fields from the 1997 version of the Geophysical Fluid Dynamics Laboratory Hurricane Prediction System (GFDL model) for Hurricane Opal are used to diagnose the rapid intensification of the tropical cyclone. While falling short of the realized peak intensity, the simulation did capture the phase of intensification. This study presents the first step toward diagnosing the mechanisms for intensification within a moderate resolution (;15 km) hydrostatic model and testing the extant hypotheses in the literature. Using a mean tangential wind budget, and the Eliassen balanced vortex model, positive eddy vorticity fluxes aloft are identified in the vicinity (;600 km) of Opal, but are not found to aid intensification. A detailed examination of each of the terms of the budget (mean and eddy vorticity flux, mean and eddy vertical advection, and ‘‘friction’’) shows for the most rapidly intensifying episodes a greater forcing for mean tangential winds near the center of the storm, particularly from the mean vertical advection and mean vorticity flux terms. Variations in these mean terms can be primarily attributed to variations in the heating rate. Upper-level divergence exhibits significant vertical structure, such that single-level or layer-average analysis techniques do not capture the divergence signature aloft. Far from the storm (

Is Environmental CAPE Important in the Determination of Maximum Possible Hurricane Intensity

400 km), divergence features near 200 mb are significantly influenced by convective events over land that are, perhaps, only indirectly influenced by the hurricane. While there is a trough interaction simulated within the model, we suggest that the hurricane develops strongly without an important interaction with the trough. A synthetic removal of specific potential vorticity features attributed to the trough is proposed to test this hypothesis. Imposed shear is proposed to weaken the storm at later times, which is at odds with other recent ‘‘nontrough’’ theories for the behavior of Opal.

Putting to rest WISHE-ful misconceptions for tropical cyclone intensification

Abstract In numerical simulations using an axisymmetric, cloud-resolving hurricane model, hurricane intensity shows quasi-steady-state behavior. This quasi-steady intensity is interpreted as the maximum possible intensity (MPI) of the model. Within the literature, numerical demonstrations have confirmed theoretically anticipated influences on hurricane intensity such as sea surface temperature, outflow temperature, and surface exchange coefficients of momentum and enthalpy. Here these investigations are extended by considering the role of environmental convective available potential energy (CAPE) on hurricane intensity. It is found that environmental CAPE (independent of changes to the outflow level) has no significant influence on numerically simulated maximum hurricane intensity. Within this framework, MPI theories that are sensitive to environmental CAPE should be discarded.

Infrasound Emitted by Tornado-Like Vortices: Basic Theory and a Numerical Comparison to the Acoustic Radiation of a Single-Cell Thunderstorm

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.

Do tropical cyclones intensify by WISHE

This paper addresses the physics and numerical simulation of the adiabatic generation of infrasound by tornadoes. Classical analytical results regarding the production of infrasound by vortex Rossby waves and by corotating “suction vortices” are reviewed. Conditions are derived for which critical layers damp vortex Rossby waves that would otherwise grow and continually produce acoustic radiation. These conditions are similar to those that theoretically suppress gravity wave radiation from larger mesoscale cyclones, such as hurricanes. To gain perspective, the Regional Atmospheric Modeling System (RAMS) is used to simulate the infrasound that radiates from a single-cell thunderstorm in a shear-free environment. In this simulation, the dominant infrasound in the 0.1–10-Hz frequency band appears to radiate from the vicinity of the melting level, where diabatic processes involving hail are active. It is shown that the 3D Rossby waves of a tornado-like vortex (simulated with RAMS) can generate stronger infrasound if the maximum wind speed of the vortex exceeds a modest threshold. Technical issues regarding the numerical simulation of tornado infrasound are also addressed. Most importantly, it is shown that simulating tornado infrasound likely requires a spatial resolution that is an order of magnitude finer than the current practical limit (10-m grid spacing) for modeling thunderstorms.