Abstract

An objectively defined three-dimensional cyclone phase space is proposed and explored. Cyclone phase is described using the parameters of storm-motion-relative thickness asymmetry (symmetric/nonfrontal versus asymmetric/frontal) and vertical derivative of horizontal height gradient (cold- versus warm-core structure via the thermal wind relationship). A cyclone's life cycle can be analyzed within this phase space, providing substantial insight into the cyclone structural evolution. An objective classification of cyclone phase is possible, unifying the basic structural description of tropical, extratropical, and hybrid cyclones into a continuum.

Stereotypical symmetric warm-core (tropical cyclone) and asymmetric cold-core (extratropical cyclone) life cycles are illustrated using 1° Navy Operational Global Atmospheric Prediction System (NOGAPS) operational analyses and 2.5° NCEP–NCAR reanalyses. The transitions between cyclone phases are clearly illustrated within the phase space, including extratropical transition, subtropical and tropical transition, and the development of warm seclusions within extratropical cyclones. The planet's northwestern hemisphere inhabitance of the proposed phase space between 1980 and 1999 is examined using NCEP–NCAR 2.5° reanalyses. Despite the inability to adequately resolve tropical cyclones at the coarse 2.5° resolution, warm-core cyclones (primarily warm-seclusion extratropical cyclones) have a mean intensity that is 10 hPa lower than that of cold-core cyclones. Warm-core cyclones also have a much larger variability for intensity distribution, with an increased occurrence of lower MSLP. Further, at 2.5° resolution the lowest analyzed MSLP for a warm-core cyclone was 14 hPa lower than that for a cyclone that remains cold core. These results suggest that cyclones that maintain solely a cold-core structure (no warm-seclusion or tropical development) may be associated with a significantly weaker minimum observed intensity at 2.5° resolution, although further examination using higher-resolution data is required to refine this.

Phase diagrams are being produced in real time to improve the forecasting of cyclone phase evolution and phase transitions, and to provide measures of phase predictability through ensembling of multiple models. The likelihood of warm-core development in cyclones can be anticipated by applying the diagnostics to various model forecasts, illuminating the potential for large intensity changes when the explicit model intensity forecasts may be insufficient.

Abstract

An objective algorithm is developed for identifying jets in 200-hPa flow and applied to reanalysis data within 2000 km of Atlantic tropical cyclones (TCs) during 1979–2015. The resulting set of 16 512 jets is analyzed both qualitatively and quantitatively to describe the climatology of TC–jet configurations and jet behavior near TCs. Jets occur most commonly poleward of TCs within the 500–1000-km annulus, where TC outflow amplifies the background potential vorticity gradient. A rigorous clustering analysis is performed, resulting in statistically distinct clusters of jet traces that correspond to common configurations of large-scale flow near Atlantic TCs. The speed structure of westerly jets poleward of TCs is found to vary with location in the Atlantic basin, but acceleration of jets downstream of their closest approach to the TC due to interaction with the TC’s diabatic outflow is a consistent feature of these structures. In addition to the climatology developed here, this objectively constructed dataset of upper-tropospheric jets opens unique avenues for exploring TC–environment interactions and utilizing jets to quantitatively describe large-scale flow.

Abstract

The processes by which tropical cyclones evolve from loosely organized convective clusters are still poorly understood. Because of the data-sparse regions in which tropical cyclones form, observational studies of tropical cyclogenesis are often more difficult than studies of land-based convective phenomena. As a result, many studies of tropical cyclogenesis are limited to either a few case studies or rely on simulations. The 2010 PREDICT and GRIP field experiments have provided a new opportunity to gain insight into these processes using unusually dense observations in both time and space.

The present study aims at using these recent datasets to perform a detailed analysis of the three-dimensional evolution of both kinematic and thermodynamic fields in both developing and nondeveloping tropical convective systems in the western Atlantic. Five tropical convective systems are analyzed in this study: two nondeveloping, two developing, and one dissipating system. Although the analysis necessarily includes only a very limited number of cases, the results suggest that the convectively active nondeveloping systems and developing systems examined here have similar kinematic structures. The most notable difference is the distribution of humidity and the impacts this distribution has on the thermodynamics of the system. Displacements between the upper-level warm anomaly, responsible for midlevel vorticity generation, and the midlevel vorticity maximum are observed in both developing and nondeveloping cases. In the nondeveloping case the displacement appears to be caused by mid- and upper-level dry air. Further work is needed to fully understand the cause of these displacements and their relation to tropical cyclogenesis.

Abstract

Extratropical transition brings about a number of environmentally induced structural changes within a transitioning tropical cyclone. Of particular interest among these changes is the acceleration of the wind field away from the cyclone’s center of circulation along with the outward movement of the radial wind maximum, together termed wind field expansion. Previous informal hypotheses aimed at understanding this evolution do not entirely capture the observed expansion, while a review of the literature shows no formal work done upon the topic beyond analyzing its occurrence. This study seeks to analyze the physical and dynamical mechanisms behind the wind field expansion using model simulations of a representative transition case, North Atlantic Tropical Cyclone Bonnie of 1998. The acceleration of the wind field along the outer periphery of the cyclone is found to be a function of the net import of absolute angular momentum within the cyclone’s environment along inflowing trajectories. This evolution is shown to be a natural outgrowth of the development of isentropic conveyor belts and asymmetries associated with extratropical cyclones. Asymmetries in the outer-core wind field manifest themselves via the tightening and development of height and temperature gradients within the cyclone’s environment. Outward movement of the radial wind maximum occurs coincident with integrated net cooling found inside the radius of maximum winds. Tests using a secondary circulation balance model show the radial wind maximum evolution to be similar yet opposite to the response noted for intensifying tropical cyclones with contracting eyewalls.

Abstract

A method for ranking synoptic-scale events objectively is presented. NCEP 12-h reanalysis fields from 1948 to 2000 are compared to a 30-yr (1961–90) reanalysis climatology. The rarity of an event is the number of standard deviations 1000–200-hPa height, temperature, wind, and moisture fields depart from this climatology. The top 20 synoptic-scale events from 1948 to 2000 for the eastern United States, southeast Canada, and adjacent coastal waters are presented. These events include the “The Great Atlantic Low” of 1956 (ranked 1st), the “superstorm” of 1993 (ranked 3d), the historic New England/Quebec ice storm of 1998 (ranked 5th), extratropical storm Hazel of 1954 (ranked 9th), a catastrophic Florida freeze and snow in 1977 (ranked 11th), and the great Northeast snowmelt and flood of 1996 (ranked 12th).

During the 53-yr analysis period, only 33 events had a total normalized anomaly (M _TOTAL) of 4 standard deviations or more. An M _TOTAL of 5 or more standard deviations has not been observed during the 53-yr period. An M _TOTAL of 3 or more was observed, on average, once or twice a month. October through January are the months when a rare anomaly (M _TOTAL ≥ 4 standard deviations) is most likely, with April through September the least likely period. The 1960s and 1970s observed the fewest number of monthly top 10 events, with the 1950s, 1980s, and 1990s having the greatest number. A comparison of the evolution of M _TOTAL to various climate indices reveals that only 5% of the observed variance of M _TOTAL can be explained by ENSO, North Atlantic oscillations, or Pacific–North American indices. Therefore, extreme synoptic-scale departures from climatology occur regardless of the magnitude of conventional climate indices, a consequence of a necessary mismatch of temporal and spatial scale representation between the M _TOTAL and climate index measurements.

Abstract

Forty-six percent of Atlantic tropical storms undergo a process of extratropical transition (ET) in which the storm evolves from a tropical cyclone to a baroclinic system. In this paper, the structural evolution of a base set of 61 Atlantic tropical cyclones that underwent extratropical transition between 1979 and 1993 is examined. Objective indicators for the onset and completion of transition are empirically determined using National Hurricane Center (NHC) best-track data, ECMWF 1.125° × 1.125° reanalyses, and operational NCEP Aviation Model (AVN) and U.S. Navy Operational Global Atmospheric Prediction System (NOGAPS) numerical analyses. An independent set of storms from 1998 to 2001 are used to provide a preliminary evaluation of the proposed onset and completion diagnostics.

Extratropical transition onset is declared when the storm becomes consistently asymmetric, as measured by the 900–600-hPa thickness asymmetry centered on the storm track. Completion of the ET process is identified using a measure of the thermal wind over the same layer. These diagnostics are consistent with the definitions of tropical and baroclinic cyclones and are readily calculable using operational analyses. Comparisons of these objective measures of ET timing with more detailed three-dimensional analyses and NHC classifications show good agreement.

Abstract

For over a century it has been known that each vortex in a multiple vortex configuration will move in response to the other vortices. However, despite advances since that time, the complexities of multiple vortex scenarios when sheared environments are present are still not completely understood. The interaction of binary vortices within horizontal environmental shear is explored here through shallow water simulations on a β plane. Due to nonlinear feedbacks, the combination of environmental vorticity (or vorticity gradient) and shear, as well as the multiple vortex situation, results in a more complicated track than for a storm experiencing any individual component. Despite the complexity of these vortex–environment interactions, the use of previous single-vortex studies greatly aids interpretation. Centroid-relative motion of the individual vortices is considered, as well as the propagation of the vortex pair centroid, to understand motion effects of the different vortex–environment combinations.

As the vortices interact, vortex Rossby waves are generated through distortion of the symmetric vorticity field by the opposing vortex. Initially, the high-frequency waves have an insignificant effect upon vortex intensity or propagation, and β-induced wavenumber one asymmetry dominates as expected. However, as the waves approach a critical radius (ζ = 0), wave potential vorticity filamentation and stretching by the circulation of the adjacent vortex leads to a coupling of the two vortices. This vortex coupling results in enhanced propagation speeds of the two vortices proportional to the effective size of the dual-vortex system.

The sign of vorticity of the environmental flow can act to enhance or negate β-drift such that single- or dual-vortex propagation is altered. Further, when environmental vorticity is present, the rate of mutual orbit from Fujiwhara rotation is altered. When the environmental flow is cyclonic, the cyclonic mutual rotation of the vortices is accelerated. Conversely, when the environmental flow is anticyclonic, the mutual rotation of the vortices is substantially decelerated, but remains cyclonic.

Abstract

A variety of tropical-cyclone (TC) center-finding methods aggregated from previous works of mesoscale modeling and operational analysis are compared. The previous methods used can be divided into three classes: local extreme, weighted grid point, and minimization of azimuthal variance. To analyze these methods, four representative separate TC forecasts from three operational models—the Coupled Ocean–Atmosphere Mesoscale Prediction System Tropical Cyclone version, a Geophysical Fluid Dynamics Laboratory model, and the Hurricane Weather Research and Forecasting Model—are examined. It is found that for this dataset the spread of the derived TC centers is fairly small between 1000 and 600 hPa but begins to increase rapidly at higher levels. All models exhibit increased center spread at upper levels when the TCs’ strengths fall below approximately hurricane strength. On a given pressure level, tangential wind differences calculated from different centers are generally small and localized, whereas radial wind differences are often much larger in both space and relative magnitude. Center-finding techniques that use mass fields to calculate centers exhibit the smallest vertical tilts for hurricane-strength TCs. Conversely, potential vorticity centroids with large weighting areas produce the largest tilts. Given the potential sensitivity of center determination and implied tilt for various other measures of TC structure (radius of maximum winds), these results may have large repercussions on both past and future analyses.

Abstract

Traditional observational analysis of derivative-based variables (e.g., vorticity) usually relies on interpolating observations and evaluating spatial derivatives either on a Cartesian grid or on a spherical grid. Great care must be taken in selecting the domain and the interpolation scheme to properly represent the features. There exist a number of alternative methods of calculating such variables by evaluating line integrals on triangular regions according to Green’s theorem. Since these methods rely on only three observations to perform calculations, they are overly sensitive to observations dominated by local phenomena as well as instrument noise. A few studies have attempted to minimize the impact of nonrepresentative or noisy observations by using higher-order polygons, but they have been limited to fitting regular polygons to near-regularly gridded data. The current study describes a new approach to calculating these fields by constructing higher-order polygons from a triangle tessellation and then applying Green’s theorem. Since the polygons are constructed using an automated triangle tessellation, the polygon construction process can proceed without the need for uniformly spaced data. The triangle tessellation employed here is unique for a given set of points, generating easily reproducible results. In addition, this method reduces the impact of noise associated with individual observations with only a minor loss in the length of the resolvable scale. An error analysis of the proposed method shows a large decrease in errors in comparison with purely triangle-based calculations. These improvements are present with a variety of data distributions (random and along research aircraft flight paths) and kinematic variables (vorticity, divergence, and deformation).

As part of the American Meteorological Society's 30th Conference on Hurricanes and Tropical Meteorology in Ponte Vedra Beach, Florida, in April 2012, an academic lineage (“family tree”) of that community was presented to document the history of contributors to the field on the anniversary. For every self-identified or colleague-identified tropical meteorology scientist, the year of the person's most senior degree, major professor or mentors of that degree, and institution of that degree were documented and graphically presented. This information was supplemented through mining of websites, libraries, news and journal articles, obituaries, and other various historical archives. This manuscript documents the genesis of the family tree, the overall history represented by it, some statistics represented by the current incarnation, colorful personal stories that have come forward during its development, and plans for its expansion to the broader meteorology community.