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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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Jason C. Knievel, David S. Nolan, and James P. Kossin

Abstract

The authors examine the degree of hydrostatic and gradient balances in a mesoscale convective vortex (MCV) in the stratiform region of a mesoscale convective system (MCS) that crossed Oklahoma on 1 August 1996. Results indicate that the MCV was partially unbalanced because the cool layer at the base of its core was too cool and too shallow to balance the tangential winds about the MCV's axis. The apparent imbalance may have been due to strong, unsteady forcing on the vortex; insufficient or unrepresentative data; approximations used in the analysis; or reasons that are unknown.

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James P. Kossin, Wayne H. Schubert, and Michael T. Montgomery

Abstract

Intense tropical cyclones often exhibit concentric eyewall patterns in their radar reflectivity. Deep convection within the inner, or primary, eyewall is surrounded by a nearly echo-free moat, which in turn is surrounded by an outer, or secondary ring of deep convection. Both convective regions typically contain well-defined tangential wind maxima. The primary wind maximum is associated with large vorticity just inside the radius of maximum wind, while the secondary wind maximum is usually associated with relatively enhanced vorticity embedded in the outer ring. In contrast, the moat is a region of low vorticity. If the vorticity profile across the eye and inner eyewall is approximated as monotonic, the resulting radial profile of vorticity still satisfies the Rayleigh necessary condition for instability as the radial gradient twice changes sign.

Here the authors investigate the stability of such structures and, in the case of instability, simulate the nonlinear evolution into a more stable structure using a nondivergent barotropic model. Because the radial gradient of vorticity changes sign twice, two types of instability and vorticity rearrangement are identified: 1) instability across the outer ring of enhanced vorticity, and 2) instability across the moat. Type 1 instability occurs when the outer ring of enhanced vorticity is sufficiently narrow and when the circulation of the central vortex is sufficiently weak (compared to the outer ring) that it does not induce enough differential rotation across the outer ring to stabilize it. The nonlinear mixing associated with type 1 instability results in a broader and weaker vorticity ring but still maintains a significant secondary wind maximum. The central vortex induces strong differential rotation (and associated enstrophy cascade) in the moat region, which then acts as a barrier to inward mixing of small (but finite) amplitude asymmetric vorticity disturbances. Type 2 instability occurs when the radial extent of the moat is sufficiently narrow so that unstable interactions may occur between the central vortex and the inner edge of the ring. Because the vortex-induced differential rotation across the ring is large when the ring is close to the vortex, type 2 instability typically precludes type 1 instability except in the case of very thin rings. The nonlinear mixing from type 2 instability perturbs the vortex into a variety of shapes. In the case of contracting rings of enhanced vorticity, the vortex and moat typically evolve into a nearly steady tripole structure, thereby offering a mechanism for the formation and persistence of elliptical eyewalls.

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Stephanie C. Herring, Martin P. Hoerling, James P. Kossin, Thomas C. Peterson, and Peter A. Stott

Editors note: For easy download the posted pdf of the Explaining Extreme Events of 2014 is a very low-resolution file. A high-resolution copy of the report is available by clicking here. Please be patient as it may take a few minutes for the high-resolution file to download.

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Stephanie C. Herring, Martin P. Hoerling, James P. Kossin, Thomas C. Peterson, and Peter A. Stott
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Stephanie C. Herring, Martin P. Hoerling, James P. Kossin, Thomas C. Peterson, and Peter A. Stott
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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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Kimberly J. Mueller, Mark DeMaria, John Knaff, James P. Kossin, and Thomas H. Vonder Haar

Abstract

Geostationary infrared (IR) satellite data are used to provide estimates of the symmetric and total low-level wind fields in tropical cyclones, constructed from estimations of an azimuthally averaged radius of maximum wind (RMAX), a symmetric tangential wind speed at a radius of 182 km (V182), a storm motion vector, and the maximum intensity (VMAX). The algorithm is derived using geostationary IR data from 405 cases from 87 tropical systems in the Atlantic and east Pacific Ocean basins during the 1995–2003 hurricane seasons that had corresponding aircraft data available. The algorithm is tested on 50 cases from seven tropical storms and hurricanes during the 2004 season. Aircraft-reconnaissance-measured RMAX and V182 are used as dependent variables in a multiple linear regression technique, and VMAX and the storm motion vector are estimated using conventional methods. Estimates of RMAX and V182 exhibit mean absolute errors (MAEs) of 27.3 km and 6.5 kt, respectively, for the dependent samples. A modified combined Rankine vortex model is used to estimate the one-dimensional symmetric tangential wind field from VMAX, RMAX, and V182. Next, the storm motion vector is added to the symmetric wind to produce estimates of the total wind field. The MAE of the IR total wind retrievals is 10.4 kt, and the variance explained is 53%, when compared with the two-dimensional wind fields from the aircraft data for the independent cases.

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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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Carl J. Schreck III, Lei Shi, James P. Kossin, and John J. Bates

Abstract

The Madden–Julian oscillation (MJO) and convectively coupled equatorial waves are the dominant modes of synoptic-to-subseasonal variability in the tropics. These systems have frequently been examined with proxies for convection such as outgoing longwave radiation (OLR). However, upper-tropospheric water vapor (UTWV) gives a more complete picture of tropical circulations because it is more sensitive to the drying and warming associated with subsidence. Previous studies examined tropical variability using relatively short (3–7 yr) UTWV datasets. Intersatellite calibration of data from the High Resolution Infrared Radiation Sounder (HIRS) has recently produced a homogeneous 32-yr climate data record of UTWV for 200–500 hPa. This study explores the utility of HIRS UTWV for identifying the MJO and equatorial waves.

Spectral analysis shows that the MJO and equatorial waves stand out above the low-frequency background in UTWV, similar to previous findings with OLR. The fraction of variance associated with the MJO and equatorial Rossby waves is actually greater in UTWV than in OLR. Kelvin waves, on the other hand, are overshadowed in UTWV by horizontal advection from extratropical Rossby waves.

For the MJO, UTWV identifies subsidence drying in the subtropics, poleward of the convection. These dry anomalies are associated with the MJO’s subtropical Rossby gyres. MJO events with dry anomalies over the central North Pacific Ocean also amplify the 200-hPa flow pattern over North America 7 days later. These events cannot be identified using equatorial OLR alone, which demonstrates that UTWV is a useful supplement for identifying the MJO, equatorial waves.

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