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Robert E. Hart

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.

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Clark Evans and Robert E. Hart

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.

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Benjamin A. Schenkel and Robert E. Hart

Abstract

The present study examines the environmental moisture anomalies present during western North Pacific tropical cyclone (TC) passage using storm-relative composites. Composited precipitable water anomalies reveal asymmetric anomalies with dry anomalies to the northwest and southwest of the TC and moist anomalies to the east of the TC. Precipitable water anomalies filtered in space and time suggest that the moisture anomalies in the northwest, southwest, and east regions (NWR, SWR, and ER, respectively) are partially due to the TC, while the anomalies in the SWR are also forced by a convectively suppressed Madden–Julian oscillation (MJO) and equatorial Rossby wave (ERW). Composited vertically integrated moisture budgets and backward parcel trajectories reveal that the moisture anomalies in the NWR, SWR, and ER are primarily due to the convergence of climatological mean moisture by the anomalous meridional wind. This convergence is induced by the secondary circulation of the TC in the NWR and ER and by inertial instability induced by the TC, MJO, and ERW in the SWR and ER as also suggested by prior work. Dry anomalies in the NWR are also forced by the advection of moisture by lower-tropospheric northerly wind anomalies associated with the primary circulation of the TC. Together with prior work, these results suggest that TCs can have significant impacts on their large-scale atmospheric environment extending well beyond the spatiotemporal scales of the lower-tropospheric cyclonic circulation of the TC.

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Andrew T. Hazelton and Robert E. Hart

Abstract

Understanding and predicting the evolution of the tropical cyclone (TC) inner core continues to be a major research focus in tropical meteorology. Eyewall slope and its relationship to intensity and intensity change is one example that has been insufficiently studied. Accordingly, in this study, radar reflectivity data are used to quantify and analyze the azimuthal average and variance of eyewall slopes from 124 flight legs among 15 Atlantic TCs from 2004 to 2011. The slopes from each flight leg are averaged into 6-h increments around the best-track times to allow for a comparison of slope and best-track intensity. A statistically significant relationship is found between both the azimuthal mean slope and pressure and between slope and wind. In addition, several individual TCs show higher correlation between slope and intensity, and TCs with both relatively high and low correlations are examined in case studies. In addition, a correlation is found between slope and radar-based eye size at 2 km, but size shows little correlation with intensity. There is also a tendency for the eyewall to tilt downshear by an average of approximately 10°. In addition, the upper eyewall slopes more sharply than the lower eyewall in about three-quarters of the cases. Analysis of case studies discusses the potential effects on eyewall slope of both inner-core and environmental processes, such as vertical shear, ocean heat content, and eyewall replacement cycles. These results indicate that eyewall slope is an important measure of TC inner-core structure, and may prove useful for future study of the processes that drive changes in the TC core.

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David R. Ryglicki and Robert E. Hart

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.

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Robert E. Hart and Gregory S. Forbes

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This paper presents results from pilot studies of the use of model-generated hourly soundings to forecast nonconvectively produced strong wind gusts. Model soundings from the operational Eta and Meso Eta Models were used for a period of 14 months in 1996 and 1997. Skill does exist in forecasting strong to damaging surface wind gusts, although the forecasts are at the mercy of the model-based boundary layer stability forecast. The wind gust forecasts are more accurate during the daytime, when the boundary layer depth and stability is more accurately forecasted and also more conducive to vertical mixing of boundary layer winds. The results of this preliminary evaluation show that the model sounding–based forecasts provide a reasonable prediction tool for nonconvective strong wind gusts. Additionally, the results warrant more complete evaluations once the dataset has grown to sufficient size.

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Charles N. Helms and Robert E. Hart

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.

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Robert E. Hart and Richard H. Grumm

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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.

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Benjamin A. Schenkel and Robert E. Hart

Abstract

The present study examines the tropospheric thermodynamic anomalies induced by western North Pacific tropical cyclone (TC) passage using storm-relative composites. Negative moist static energy (MSE) anomalies containing embedded westward-propagating anomalies generally occur only following larger TCs for two months following TC passage in a region extending from the domain center to ~3000 km to its west. Larger TCs force negative MSE anomalies likely because of feedbacks from stronger, broader TC-induced negative sea surface temperature (SST) anomalies and the excitation of TC-induced Rossby waves to the southeast of the TC. The negative MSE anomalies are composed of lower- and midtropospheric negative latent energy anomalies with smaller contributions from boundary layer and upper-tropospheric negative sensible heat anomalies. The lower- and midtropospheric negative MSE anomalies are forced by the TC, whereas the upper-tropospheric negative MSE anomalies are forced by the Madden–Julian oscillation. Vertically integrated MSE budgets at the domain center reveal negative MSE tendencies that are primarily forced by surface latent heat flux anomalies resulting from the TC-induced negative SST anomalies. Smaller negative MSE tendencies are due to 1) zonal and meridional advection of MSE anomalies by the Rossby waves and 2) enhanced top-of-the-atmosphere longwave radiative flux anomalies potentially associated with a reduction in the greenhouse gas effect of water vapor. The budget analysis in the west region is generally similar except that all terms are comparable in magnitude and relatively weaker. These results conservatively suggest that larger TCs can anomalously cool and dry their synoptic-scale environment for ~40 days following TC passage.

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Benjamin A. Schenkel and Robert E. Hart

Abstract

The following study examines the position and intensity differences of tropical cyclones (TCs) among the Best-Track and five atmospheric reanalysis datasets to evaluate the degree to which reanalyses are appropriate for studying TCs. While significant differences are found in both reanalysis TC intensity and position, the representation of TC intensity within reanalyses is found to be most problematic owing to its underestimation beyond what can be attributed solely to the coarse grid resolution. Moreover, the mean life cycle of normalized TC intensity within reanalyses reveals an underestimation of both prepeak intensification rates as well as a delay in peak intensity relative to the Best-Track. These discrepancies between Best-Track and reanalysis TC intensity and position can further be described through correlations with such parameters as Best-Track TC age, Best-Track TC intensity, Best-Track TC location, and the extended Best-Track TC size. Specifically, TC position differences within the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40), ECMWF Interim Re-Analysis (ERA-I), and Modern Era Retrospective-Analysis for Research and Applications (MERRA) exhibit statistically significant correlations (0.27 ≤ R ≤ 0.38) with the proximity of TCs to observation dense areas in the North Atlantic (NATL) and western North Pacific (WPAC). Reanalysis TC intensity is found to be most strongly correlated with Best-Track TC size (0.53 ≤ R ≤ 0.70 for maximum 10-m wind speed; −0.71 ≤ R ≤ −0.53 for minimum mean sea level pressure) while exhibiting smaller, yet significant, correlations with Best-Track TC age, Best-Track TC intensity, and Best-Track TC latitude. Of the three basins examined, the eastern North Pacific (EPAC) has the largest reanalysis TC position differences and weakest intensities possibly due to a relative dearth of observations, the strong nearby terrain gradient, and the movement of TCs away from the most observation dense portion of the basin over time. The smaller mean Best-Track size and shorter mean lifespan of Best-Track EPAC TCs may also yield weaker reanalysis TC intensities. Of the five reanalyses, the smaller position differences and stronger intensities found in the Climate Forecast System Reanalysis (CFSR) and Japanese 25-year Reanalysis (JRA-25) are attributed to the use of vortex relocation and TC wind profile retrievals, respectively. The discrepancies in TC position between the Best-Track and reanalyses combined with the muted magnitude of TC intensity and its partially nonphysical life cycle within reanalyses suggests that caution should be exercised when utilizing these datasets for studies that rely either on TC intensity (raw or normalized) or track. Finally, several cases of nonphysical TC structure also argue that further work is needed to improve TC representation while implying that studies focusing solely on TC intensity and track do not necessarily extend to other aspects of TC representation.

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