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Eric A. Hendricks, Brian D. McNoldy, and Wayne H. Schubert

Abstract

Hurricane 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.

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James P. Kossin, Brian D. McNoldy, and Wayne H. Schubert

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.

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David S. Nolan, Brian D. McNoldy, and Jimmy Yunge

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Although global and regional dynamical models are used to predict the tracks and intensities of hurricanes over the ocean, these models are not currently used to predict the wind field and other impacts over land. This two-part study performs detailed evaluations of the near-surface, overland wind fields produced in simulations of Hurricane Wilma (2005) as it traveled across South Florida. This first part describes the production of two high-resolution simulations using the Weather Research and Forecasting (WRF) Model, using different boundary layer parameterizations available in WRF: the Mellor–Yamada–Janjić (MYJ) scheme and the Yonsei University (YSU) scheme. Initial conditions from the Global Forecasting System are manipulated with a vortex-bogusing technique to modify the initial intensity, size, and location of the cyclone. It is found possible through trial and error to successfully produce simulations using both the YSU and MYJ schemes that closely reproduce the track, intensity, and size of Wilma at landfall. For both schemes the storm size and structure also show good agreement with the wind fields diagnosed by H*WIND and the Tropical Cyclone Surface Wind Analysis. Both over water and over land, the YSU scheme has stronger winds over larger areas than does the MYJ, but the surface winds are more reduced in areas of greater surface roughness, particularly in urban areas. Both schemes produced very similar inflow angles over land and water. The overland wind fields are examined in more detail in the second part of this study.

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Richard H. Johnson, Paul E. Ciesielski, Brian D. McNoldy, Peter J. Rogers, and Richard K. Taft

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 California surge. This surge was linked to the westward passages of Tropical Storm Blas to the south and an upper-level inverted trough over northern Texas. The development of Blas appeared to be favored as an easterly wave moved into the eastern Pacific during the active phase of a Madden–Julian oscillation.

On the regional scale, sounding data reveal a prominent sea breeze along the east shore of the Gulf of California, with a deep return flow as a consequence of the elevated Sierra Madre Occidental (SMO) immediately to the east. Subsidence produced a dry layer over the gulf, whereas a deep moist layer existed over the west slopes of the SMO. A prominent nocturnal low-level jet was present on most days over the northern gulf. The diurnal cycle of heating and moistening (Q 1 and Q 2) over the SMO was characterized by deep convective profiles in the mid- to upper troposphere at 1800 LT, followed by stratiform-like profiles at midnight, consistent with the observed diurnal evolution of precipitation over this coastal mountainous region. The analyses in the core NAME domain are based on a gridded dataset derived from atmospheric soundings only and, therefore, should prove useful in validating reanalyses and regional models.

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Christopher M. Rozoff, Wayne H. Schubert, Brian D. McNoldy, and James P. Kossin

Abstract

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 rotation-dominated regions by the sign of S 2 1 + S 2 2ζ 2, where S 1 = uxυy and S 2 = υx + uy are the rates of strain and ζ = υxuy is the relative vorticity. Within the radius of maximum tangential wind, the flow tends to be rotation-dominated (ζ 2 > S 2 1 + S 2 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 1 + S 2 2 > ζ 2), resulting in filaments of anomalous vorticity. In the regions of strain-dominated flow the filamentation time is defined as τ fil = 2(S 2 1 + S 2 2ζ 2)−1/2. In a tropical cyclone, an approximately 30-km-wide annular region can exist just outside the radius of maximum tangential wind, where τ 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.

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Jason A. Otkin, William E. Lewis, Allen J. Lenzen, Brian D. McNoldy, and Sharanya J. Majumdar

Abstract

In this study, cycled forecast experiments were performed to assess the ability of different cloud microphysics and cumulus parameterization schemes in the Hurricane Weather Research and Forecasting (HWRF) Model to accurately simulate the evolution of the cloud and moisture fields during the entire life cycle of Hurricane Edouard (2014). The forecast accuracy for each model configuration was evaluated through comparison of observed and simulated Geostationary Operational Environmental Satellite-13 (GOES-13) infrared brightness temperatures and satellite-derived tropical cyclone intensity estimates computed using the advanced Dvorak technique (ADT). Overall, the analysis revealed a large moist bias in the mid- and upper troposphere during the entire forecast period that was at least partially due to a moist bias in the initialization datasets but was also affected by the microphysics and cumulus parameterization schemes. Large differences occurred in the azimuthal brightness temperature distributions, with two of the microphysics schemes producing hurricane eyes that were much larger and clearer than observed, especially for later forecast hours. Comparisons to the forecast 10-m wind speeds showed reasonable agreement (correlations between 0.58 and 0.74) between the surface-based intensities and the ADT intensity estimates inferred via cloud patterns in the upper troposphere. It was also found that model configurations that had the smallest differences between the ADT and surface-based intensities had the most accurate track and intensity forecasts. Last, the cloud microphysics schemes had the largest impact on the forecast accuracy.

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Ricardo Prieto, Brian D. McNoldy, Scott R. Fulton, and Wayne H. Schubert

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. In this regime the interaction between vortices can lead to episodic exchanges of vorticity, with both vortices surviving and entering a stage of continued but weaker interaction. With the aid of passive tracers, it is found that the exchange of vorticity is restricted to the vortex periphery even when the vorticity field within each vortex is flat, so that the vortex core is the last region to be eroded. It is hypothesized that some observed interacting tropical cyclones actually do undergo this partial-merger process.

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David S. Nolan, Brian D. McNoldy, Jimmy Yunge, Forrest J. Masters, and Ian M. Giammanco

Abstract

This is the second of a two-part study that explores the capabilities of a mesoscale atmospheric model to reproduce the near-surface wind fields in hurricanes over land. The Weather Research and Forecasting (WRF) Model is used with two planetary boundary layer parameterizations: the Yonsei University (YSU) and the Mellor–Yamada–Janjić (MYJ) schemes. The first part presented the modeling framework and initial conditions used to produce simulations of Hurricane Wilma (2005) that closely reproduced the track, intensity, and size of its wind field as it passed over South Florida. This part explores how well these simulations can reproduce the winds at fixed points over land by making comparisons with observations from airports and research weather stations. The results show that peak wind speeds are remarkably well reproduced at several locations. Wind directions are evaluated in terms of the inflow angle relative to the storm center, and the simulated inflow angles are generally smaller than observed. Localized peak wind events are associated with vertical vorticity maxima in the boundary layer with horizontal scales of 5–10 km. The boundary layer winds are compared with wind profiles obtained by velocity–azimuth display (VAD) analyses from National Weather Service Doppler radars at Miami and Key West, Florida; results from these comparisons are mixed. Nonetheless the comparisons with surface observations suggest that when short-term hurricane forecasts can sufficiently predict storm track, intensity, and size, they will also be able to provide useful information on extreme winds at locations of interest.

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Brian D. McNoldy, Anning Cheng, Zachary A. Eitzen, Richard W. Moore, John Persing, Kevin Schaefer, and Wayne H. Schubert

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 $3,000.

A spin tank such as this has practical applications for the qualitative study of fluid dynamics. Fundamental concepts in rotating flow dynamics can be demonstrated to supplement the more rigorous mathematical treatment typically given in oceanography or atmospheric physics graduate-level courses. Topics that have been explored thus far are Ekman pumping, Taylor columns, and barotropic instability, but could be broadened to include subjects such as Rossby waves, baroclinic instability, vortex merger, and thermal convection.

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