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James P. Kossin

Abstract

Using the National Environmental Satellite, Data, and Information Service–Cooperative Institute for Research in the Atmosphere (NESDIS–CIRA) tropical cyclone infrared (IR) imagery archive, combined with best track storm center fix information, a coherent depiction of the temporal and azimuthally averaged spatial structure of hurricane cloudiness is demonstrated. The diurnal oscillation of areal extent of the hurricane cirrus canopy, as documented in a number of previous studies, is clearly identified but often found to vanish near the convective region of the hurricane eyewall. While a significant diurnal oscillation is generally absent near the storm center, a powerful and highly significant semidiurnal oscillation is sometimes revealed in that region. This result intimates that convection near the center of tropical storms and hurricanes may not be diurnally forced, but might, at times, be semidiurnally forced. A highly significant semidiurnal oscillation is also often found in the near environment beyond the edge of the hurricane cirrus canopy. The phase of the semidiurnal oscillations in both the central convective region and the region beyond the canopy remains relatively fixed during the lifetime of each storm and is not found to vary much between individual storms. This fixed phase near the central convective region insinuates a mechanistic link between hurricane central convection and the semidiurnal atmospheric thermal tide S 2.

Two hypotheses are constructed. The first is offered to explain the diurnal oscillation of the canopy in the absence of a diurnal oscillation of the convective regions. The hypothesized mechanism is based on the radial variation of nighttime net radiational cooling and subsidence. The second hypothesis is offered to explain the semidiurnal oscillation near the central convective region, and is based on the possible presence of a semidiurnal oscillation of local lapse rates associated with S 2.

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James P. Kossin and Matthew Sitkowski

Abstract

Hurricanes, and particularly major hurricanes, will often organize a secondary eyewall at some distance around the primary eyewall. These events have been associated with marked changes in the intensity and structure of the inner core, such as large and rapid deviations of the maximum wind and significant broadening of the surface wind field. While the consequences of rapidly fluctuating peak wind speeds are of great importance, the broadening of the overall wind field also has particularly dangerous consequences in terms of increased storm surge and wind damage extent during landfall events. Despite the importance of secondary eyewall formation in hurricane forecasting, there is presently no objective guidance to diagnose or forecast these events. Here a new empirical model is introduced that will provide forecasters with a probability of imminent secondary eyewall formation. The model is based on environmental and geostationary satellite features applied to a naïve Bayes probabilistic model and classification scheme. In independent testing, the algorithm performs skillfully against a defined climatology.

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James P. Kossin and Christopher S. Velden

Abstract

A pronounced and highly significant bias is uncovered in tropical cyclone minimum sea level pressure (MSLP) estimates calculated using the Dvorak technique. The bias is present in operational estimates from each of the primary Atlantic tropical analysis centers (TACs). The bias can be approximated as a linear function of latitude and is caused by the dependence of tropopause temperature on latitude. On average, MSLP estimates from each TAC are consistently too high (compared to aircraft reconnaissance measurements) at higher latitudes and too low at lower latitudes. The latitude of zero bias is near 23°N. Because the relationship between tropopause temperature and latitude is fairly robust among the global ocean basins, the latitude-dependent bias that exists in Dvorak technique MSLP estimates of Atlantic basin tropical cyclones should extend to Dvorak technique estimates in all ocean basins.

A simple linear fit is constructed between the Dvorak technique MSLP estimate errors and latitude, and this is applied as a latitude-dependent bias correction to the MSLP estimates. The correction has a significant effect on the error statistics of the samples from each TAC. Root-mean-square error is reduced by roughly 11%, 9%, and 10%, respectively, in the Tropical Analysis and Forecast Branch (TAFB), Satellite Analysis Branch (SAB), and Air Force Global Weather Center (AFGWC) samples.

Using available wind data, it is shown that a much weaker latitude-dependent bias exists in Dvorak technique estimates of near-surface wind (V max). This is consistent with a recent study that used aircraft-based data from Atlantic tropical cyclones (TCs) to demonstrate that for a given MSLP, the associated measured V max tends to be weaker at higher latitudes. The empirical relationship between MSLP and V max used in the Dvorak technique has no dependence on latitude, which indirectly introduces a bias in the estimated wind that counteracts the bias in the MSLP estimates. This suggests that historical best-track data formed using Dvorak technique estimates contain a systematic latitude-dependent MSLP bias and a systematic inconsistency in the relationship between MSLP and V max. Correction of the MSLP bias in past tropical cyclones that were estimated using the Dvorak technique may have measurable effects on the present tropical cyclone climatology.

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Christopher Velden, Timothy Olander, Derrick Herndon, and James P. Kossin

Abstract

In recent years, a number of extremely powerful tropical cyclones have revived community debate on methodologies used to estimate the lifetime maximum intensity (LMI) of these events. And how do these storms rank historically? In this study, the most updated version of an objective satellite-based intensity estimation algorithm [advanced Dvorak technique (ADT)] is employed and applied to the highest-resolution (spatial and temporal) geostationary satellite data available for extreme-intensity tropical cyclones that occurred during the era of these satellites (1979–present). Cases with reconnaissance aircraft observations are examined and used to calibrate the ADT at extreme intensities. Bias corrections for observing properties such as satellite viewing angle and image spatiotemporal resolution, and storm characteristics such as small eye size are also considered.

The results of these intensity estimates (maximum sustained 1-min wind) show that eastern North Pacific Hurricane Patricia (2015) ranks as the strongest storm in any basin (182 kt), followed by western North Pacific Typhoons Haiyan (2013), Tip (1979), and Gay (1992). The following are the strongest classifications in other basins—Atlantic: Gilbert (1988), north Indian Ocean basin: Paradip (1999), south Indian Ocean: Gafilo (2004), Australian region: Monica (2006), and southeast Pacific basin: Pam (2015). In addition, ADT LMI estimates for four storms exceed the maximum allowable limit imposed by the operational Dvorak technique. This upper bound on intensity may be an unnatural constraint, especially if tropical cyclones get stronger in a warmer biosphere as some theorize. This argues for the need of an extension to the Dvorak scale to allow higher intensity estimates.

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Matthew Sitkowski, James P. Kossin, and Christopher M. Rozoff

Abstract

A flight-level aircraft dataset consisting of 79 Atlantic basin hurricanes from 1977 to 2007 was used to develop an unprecedented climatology of inner-core intensity and structure changes associated with eyewall replacement cycles (ERCs). During an ERC, the inner-core structure was found to undergo dramatic changes that result in an intensity oscillation and rapid broadening of the wind field. Concentrated temporal sampling by reconnaissance aircraft in 14 of the 79 hurricanes captured virtually the entire evolution of 24 ERC events. The analysis of this large dataset extends the phenomenological paradigm of ERCs described in previous observational case studies by identifying and exploring three distinct phases of ERCs: intensification, weakening, and reintensification. In general, hurricanes intensify, sometimes rapidly, when outer wind maxima are first encountered by aircraft. The mean locations of the inner and outer wind maximum at the start of an ERC are 35 and 106 km from storm center, respectively. The intensification rate of the inner wind maximum begins to slow and the storm ultimately weakens as the inner-core structure begins to organize into concentric rings. On average, the inner wind maximum weakens 10 m s−1 before the outer wind maximum surpasses the inner wind maximum as it continues to intensify. This reintensification can be quite dramatic and often brings the storm to its maximum lifetime intensity. The entire ERC lasts 36 h on average.

Comparison of flight-level data and microwave imagery reveals that the first appearance of an outer wind maximum, often associated with a spiral rainband, typically precedes the weakening of the storm by roughly 9 h, but the weakening is already well under way by the time a secondary convective ring with a well-defined moat appears in microwave imagery. The data also show that winds beyond the outer wind maximum remain elevated even after the outer wind maximum contracts inward. Additionally, the contraction of the outer wind maximum usually ceases at a radius larger than the location of the inner wind maximum at the start of the ERC. The combination of a larger primary eyewall and expanded outer wind field increase the integrated kinetic energy by an average of 28% over the course of a complete ERC despite little change in the maximum intensity between the times of onset and completion of the event.

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Carl J. Schreck III, Kenneth R. Knapp, and James P. Kossin

Abstract

Using the International Best Track Archive for Climate Stewardship (IBTrACS), the climatology of tropical cyclones is compared between two global best track datasets: 1) the World Meteorological Organization (WMO) subset of IBTrACS (IBTrACS-WMO) and 2) a combination of data from the National Hurricane Center and the Joint Typhoon Warning Center (NHC+JTWC). Comparing the climatologies between IBTrACS-WMO and NHC+JTWC highlights some of the heterogeneities inherent in these datasets for the period of global satellite coverage 1981–2010. The results demonstrate the sensitivity of these climatologies to the choice of best track dataset. Previous studies have examined best track heterogeneities in individual regions, usually the North Atlantic and west Pacific. This study puts those regional issues into their global context. The differences between NHC+JTWC and IBTrACS-WMO are greatest in the west Pacific, where the strongest storms are substantially weaker in IBTrACS-WMO. These disparities strongly affect the global measures of tropical cyclone activity because 30% of the world’s tropical cyclones form in the west Pacific. Because JTWC employs similar procedures throughout most of the globe, the comparisons in this study highlight differences between WMO agencies. For example, NHC+JTWC has more 96-kt (~49 m s−1) storms than IBTrACS-WMO in the west Pacific but fewer in the Australian region. This discrepancy probably points to differing operational procedures between the WMO agencies in the two regions. Without better documentation of historical analysis procedures, the only way to remedy these heterogeneities will be through systematic reanalysis.

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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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Erin M. Dougherty, John Molinari, Robert F. Rogers, Jun A. Zhang, and James P. Kossin

Abstract

Hurricane Bonnie (1998) was an unusually resilient hurricane that maintained a steady-state intensity while experiencing strong (12–16 m s−1) vertical wind shear and an eyewall replacement cycle. This remarkable behavior was examined using observations from flight-level data, microwave imagery, radar, and dropsondes over the 2-day period encompassing these events. Similar to other observed eyewall replacement cycles, Bonnie exhibited the development, strengthening, and dominance of a secondary eyewall while a primary eyewall decayed. However, Bonnie’s structure was highly asymmetric because of the large vertical wind shear, in contrast to the more symmetric structures observed in other hurricanes undergoing eyewall replacement cycles. It is hypothesized that the unusual nature of Bonnie’s evolution arose as a result of an increase in vertical wind shear from 2 to 12 m s−1 even as the storm intensified to a major hurricane in the presence of high ambient sea surface temperatures. These circumstances allowed for the development of outer rainbands with intense convection downshear, where the formation of the outer eyewall commenced. In addition, the circulation broadened considerably during this time. The secondary eyewall developed within a well-defined beta skirt in the radial velocity profile, consistent with an earlier theory. Despite the large ambient vertical wind shear, the outer eyewall steadily extended upshear, supported by 35% larger surface wind speed upshear than downshear. The larger radius of maximum winds during and after the eyewall replacement cycle might have aided Bonnie’s resiliency directly, but also increased the likelihood that diabatic heating would fall inside the radius of maximum winds.

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Matthew Sitkowski, James P. Kossin, Christopher M. Rozoff, and John A. Knaff

Abstract

Flight-level aircraft data are used to examine inner-core thermodynamic changes during eyewall replacement cycles (ERCs) and the role of the relict inner eyewall circulation on the evolution of a hurricane during and following an ERC. Near the end of an ERC, the eye comprises two thermodynamically and kinematically distinct air masses separated by a relict wind maximum, inside of which high inertial stability restricts radial motion creating a “containment vessel” that confines the old-eye air mass. Restricted radial flow aloft also reduces subsidence within this confined region. Subsidence-induced warming is thus focused along the outer periphery of the developing post-ERC eye, which leads to a flattening of the pressure profile within the eye and a steepening of the gradient at the eyewall. This then causes a local intensification of the winds in the eyewall. The cessation of active convection and subsidence near the storm center, which has been occurring over the course of the ERC, leads to an increase in minimum pressure. The increase in minimum pressure concurrent with the increase of winds in the developing eyewall can create a highly anomalous pressure–wind relationship. When the relict inner eyewall circulation dissipates, the air masses are free to mix and subsidence can resume more uniformly over the entire eye.

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Kenneth R. Knapp, Jessica L. Matthews, James P. Kossin, and Christopher C. Hennon

Abstract

The Cyclone Center project maintains a website that allows visitors to answer questions based on tropical cyclone satellite imagery. The goal is to provide a reanalysis of satellite-derived tropical cyclone characteristics from a homogeneous historical database composed of satellite imagery with a common spatial resolution for use in long-term, global analyses. The determination of the cyclone “type” (curved band, eye, shear, etc.) is a starting point for this process. This analysis shows how multiple classifications of a single image are combined to provide probabilities of a particular image’s type using an expectation–maximization (EM) algorithm. Analysis suggests that the project needs about 10 classifications of an image to adequately determine the storm type. The algorithm is capable of characterizing classifiers with varying levels of expertise, though the project needs about 200 classifications to quantify an individual’s precision. The EM classifications are compared with an objective algorithm, satellite fix data, and the classifications of a known classifier. The EM classifications compare well, with best agreement for eye and embedded center storm types and less agreement for shear and when convection is too weak (termed no-storm images). Both the EM algorithm and the known classifier showed similar tendencies when compared against an objective algorithm. The EM algorithm also fared well when compared to tropical cyclone fix datasets, having higher agreement with embedded centers and less agreement for eye images. The results were used to show the distribution of storm types versus wind speed during a storm’s lifetime.

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