Brian D. McNoldy

Rapid Filamentation Zones in Intense Tropical Cyclones

Intense tropical cyclones often possess relatively little convection around their cores. In radar composites, this surrounding region is usually echo-free or contains light stratiform precipitation. While subsidence is typically quite pronounced in this region, it is not the only mechanism suppressing convection. Another possible mechanism leading to weak-echo moats is presented in this paper. The basic idea is that the strain-dominated flow surrounding an intense vortex core creates an unfavorable environment for sustained deep, moist convection. Strain-dominated regions of a tropical cyclone can be distinguished from rotationdominated regions by the sign of S 2 S 2 2 , where S1 ux y and S2 x uy are the rates of strain and x uy is the relative vorticity. Within the radius of maximum tangential wind, the flow tends to be rotation-dominated ( 2 S 2 S 2), so that coherent structures, such as mesovortices, can survive for long periods of time. Outside the radius of maximum tangential wind, the flow tends to be strain-dominated (S 2 S 2 2 ), resulting in filaments of anomalous vorticity. In the regions of strain-dominated flow the filamentation time is defined as fil 2(S 2 S 2 2 ) 1/2 . In a tropical cyclone, an approximately 30-kmwide annular region can exist just outside the radius of maximum tangential wind, where fil is less than 30 min and even as small as 5 min. This region is defined as the rapid filamentation zone. Since the time scale for deep moist convective overturning is approximately 30 min, deep convection can be significantly distorted and even suppressed in the rapid filamentation zone. A nondivergent barotropic model illustrates the effects of rapid filamentation zones in category 1–5 hurricanes and demonstrates the evolution of such zones during binary vortex interaction and mesovortex formation from a thin annular ring of enhanced vorticity.

Multiscale Variability of the Flow during the North American Monsoon Experiment

Abstract The 2004 North American Monsoon Experiment (NAME) provided an unprecedented observing network for studying the structure and evolution of the North American monsoon. This paper focuses on multiscale characteristics of the flow during NAME from the large scale to the mesoscale using atmospheric sounding data from the enhanced observing network. The onset of the 2004 summer monsoon over the NAME region accompanied the typical northward shift of the upper-level anticyclone or monsoon high over northern Mexico into the southwestern United States, but in 2004 this shift occurred slightly later than normal and the monsoon high did not extend as far north as usual. Consequently, precipitation over the southwestern United States was slightly below normal, although increased troughiness over the Great Plains contributed to increased rainfall over eastern New Mexico and western Texas. The first major pulse of moisture into the Southwest occurred around 13 July in association with a strong Gulf of Californi...

Vortical Swirls in Hurricane Eye Clouds

Abstract A collection of images depicting various swirling patterns within low-level cloud decks in hurricane eyes is presented and described. A possible causal mechanism for the presence of these cloud patterns is suggested by comparison of the observed cloud patterns with the evolution of passive tracers in a simple 2D barotropic model. The model is initialized with a barotropically unstable flow field that imitates the observed flows in hurricanes, and numerical integration of this field simulates vigorous mixing between eye and eyewall. During the mixing process, passive tracers initially embedded in the flow form swirling patterns in the eye that are strikingly similar to cloud patterns often observed in the eyes of hurricanes.

The low-level circulation of the North American Monsoon as revealed by QuikSCAT

Five years (1999–2003) of near-surface QuikSCAT ocean winds over the Gulf of California and northeast Pacific Ocean are used to characterize the changes in the low-level circulation associated with the North American Monsoon. Our analysis shows that the onset of the summer season is accompanied by a seasonal reversal of the flow along the Gulf of California, with the establishment of a time-mean southerly wind throughout the gulf. This reversal, not evident in the global reanalysis products, occurs in late spring and precedes the onset of the monsoonal rains. In the core of the monsoon, the time-mean flow is found to be modulated by transient events, namely gulf surges, detected in the near-surface wind field as periods of enhanced southerly flow which typically originate at the southern end of the gulf and propagate northward. The histogram of the summertime along-shore winds identifies these surges as a distinct population of events, readily distinguishable from the background flow.

A Classification of Binary Tropical Cyclone–Like Vortex Interactions*

Abstract The interaction between two tropical cyclones with different core vorticities and different sizes is studied with the aid of a nondivergent barotropic model, on both the f plane and the sphere. A classification of a wide range of cases is presented, using the Dritschel–Waugh scheme, which subdivides vortex interactions into five types: elastic interaction, partial straining out, complete straining out, partial merger, and complete merger. The type of interaction for a vortex pair on the f plane, and the same pair on the sphere, was the same for 77 out of 80 cases studied. The primary difference between the results on the f plane and those on the sphere is that the vorticity centroid of the pair is fixed on the f plane but can drift a considerable distance poleward and westward on the sphere. In the spherical case, the interaction between the cyclone pair and the associated β-induced cyclonic and anticyclonic circulations can play an important role. The “partial merger” regime is studied in detail...

Observed Inner-Core Structural Variability in Hurricane Dolly (2008)*

AbstractHurricane Dolly (2008) exhibited dramatic inner-core structural variability during a 6-h rapid intensification and deepening event just prior to making landfall in southern Texas at 1800 UTC 23 July. In particular, the eyewall was highly asymmetric from 0634–1243 UTC, with azimuthal wavenumber m = 4–7 patterns in the eyewall radar reflectivity and prominent mesovortex and polygonal eyewall signatures. Evidence is presented that the most likely cause of the high-wavenumber asymmetries is a convectively modified form of barotropic instability of the thin eyewall potential vorticity ring. The rapid intensification and deepening event occurred while Dolly was in a favorable environment with weak deep-layer vertical wind shear and warm sea surface temperatures; however, the environmental conditions were becoming less favorable during the period of rapid intensification. Therefore, it is plausible that the internal vortex dynamics were dominant contributors to the rapid intensification and deepening.

A cognitive-affective scale for hurricane risk perception

The aim of this study was to develop a reliable and valid measure of hurricane risk perception. The utility of such a measure lies in the need to understand how people make decisions when facing an evacuation order. This study included participants located within a 15-mile buffer of the Gulf and southeast Atlantic U.S. coasts. The study was executed as a three-wave panel with mail surveys in 2010-2012 (T0 baseline N = 629, 56%; T1 retention N = 427, 75%; T2 retention N = 350, 89%). An inventory based on the psychometric model was developed to discriminate cognitive and affective perceptions of hurricane risk, and included open-ended responses to solicit additional concepts in the T0 survey. Analysis of the T0 data modified the inventory and this revised item set was fielded at T1 and then replicated at T2 . The resulting scales were assessed for validity against existing measures for perception of hurricane risk, dispositional optimism, and locus of control. A measure of evacuation expectation was also examined as a dependent variable, which was significantly predicted by the new measures. The resulting scale was found to be reliable, stable, and largely valid against the comparison measures. Despite limitations involving sample size, bias, and the strength of some reliabilities, it was concluded that the measure has potential to inform approaches to hurricane preparedness efforts and advance planning for evacuation messages, and that the measure has good promise to generalize to other contexts in natural hazards as well as other domains of risk.

Triple Eyewall in Hurricane Juliette

|urricane eyewalls are one of the more enigmatic phenomena in the atmosphere. Even more mysterious are concentric eyewall cycles: the development of a ring of deep convection within a larger ring of deep convection, with a “moat” between them. On radar, the moat would appear as a nearly echofree annulus, while an eyewall would appear as an annulus with radar echoes typically greater than 35 dBZ. From the first days of aircraft reconnaissance into hurricanes during the 1940s, these “double eyes” were occasionally observed in strong storms, and the advent of satellite meteorology in the 1960s has provided additional cases that we otherwise would not have known about. In particular, passive microwave imagery is the most ideal and prominent tool available for monitoring the internal precipitation structure, concentric eyewalls, and eyewall replacement cycles (Hawkins and Helveston 2004). Concentric eyewalls are ephemeral; once formed, they typically are not maintained for much longer than 12 h. As the new outer eyewall forms, the original inner eyewall usually lacks the necessary inflow to maintain itself, and it gradually dissipates. In time, dynamic processes cause the outer eyewall to contract, and the process can repeat itself; this is called an eyewall replacement cycle (Black and Willoughby 1992). Multiple eyewalls are more commonly observed in intense tropical cyclones (i.e., Category 3, 4, and 5 on the Saffir–Simpson scale; Simpson 1974). From a study of western North Pacific tropical cyclones (TCs) during 1969–71, Willoughby et al. (1982) estimated that approximately 53% of intense TCs (winds greater than 65 m s −1 ) exhibit concentric eyewalls, while only 14% of weaker TCs do.

Design and construction of an affordable rotating table for classroom Demonstrations of geophysical fluid dynamics principles

Rotating tables have been in use for many years because of their ability to demonstrate fluid dynamical phenomena, shedding insight on the sometimes complicated or esoteric mathematics used to describe such processes. A small team of students at the Colorado State University (CSU) Department of Atmospheric Science constructed a rotating table, or “spin tank,” assembly that is simple and affordable, yet instructive. The apparatus is designed to be easy to maintain and operate. The number of moving parts is kept at a minimum, and the electrical components chosen are of high quality. With the aid of a brief instruction manual or tutorial, students and faculty can operate the rotating table and easily perform many demonstrations, with the freedom to vary fluid depth, rotation rate, and acceleration. The entire design and construction process was conducted on a limited budget of

A Study of the HWRF Analysis and Forecast Impact of Realistically Simulated CYGNSS Observations Assimilated as Scalar Wind Speeds and as VAM Wind Vectors

3,000. A spin tank such as this has practical applications for the qualitative study of fluid dynamics. Fundamental concepts in rota...