Abstract Data with high temporal and spatial resolution from Hurricanes Isabel (2003) and Frances (2004) were analyzed to provide a detailed study of near-surface linear structures with subkilometer wavelengths of the hurricane boundary layer (HBL). The analysis showed that the features were omnipresent throughout the data collection, displayed a horizontal and vertical coherency, and maintained an average orientation of 7° left of the low-level wind. A unique objective wavelength analysis was conducted, where wavelength was defined as the distance between two wind maxima or minima perpendicular to the features’ long axis, and revealed that although wavelengths as large as 1400 m were observed, the majority of the features had wavelengths between 200 and 650 m. The assessed wavelengths differ from those documented in a recent observational study. To evaluate the correlation between the features and the underlying near-surface wind field, time and spectral analyses were completed and ground-relative frequency distributions of the features were retrieved. High-energy regions of the power spectral density (PSD) determined from near-surface data were collocated with the features’ ground-relative frequency, illustrating that the features have an influence on the near-surface wind field. The additional energy found in the low-frequency range of the PSDs was previously identified as characteristic of the hurricane surface flow, suggesting that the features are an integral component of the HBL flow.
Hurricane Andrew of 1992 caused unprecedented economic devastation along its path through the Bahamas, southeastern Florida, and Louisiana. Damage in the United States was estimated to be $26 billion (in 1992 dollars), making Andrew one of the most expensive natural disasters in U.S. history. This hurricane struck southeastern Florida with maximum 1-min surface winds estimated in a 1992 poststorm analysis at 125 kt (64 m s−1). This original assessment was primarily based on an adjustment of aircraft reconnaissance flight-level winds to the surface. Based on recent advancements in the understanding of the eyewall wind structure of major hurricanes, the official intensity of Andrew was adjusted upward for five days during its track across the Atlantic Ocean and Gulf of Mexico by the National Hurricane Center Best Track Change Committee. In particular, Andrew is now assessed by the National Hurricane Center to be a Saffir–Simpson Hurricane Scale category-5 hurricane (the highest intensity category possible) at its landfall in southeastern Florida, with maximum 1-min winds of 145 kt (75 m s−1). This makes Andrew only the third category-5 hurricane to strike the United States since at least 1900. Implications for how this change impacts society's planning for such extreme events are discussed.
The RADARSAT synthetic aperture radar (SAR) acquired C-band HH polarization images over four 1998 hurricanes: Bonnie, Danielle, Georges, and Mitch. The authors present the SAR images and discuss their quantitative use in understanding hurricane morphology. The SAR provides a complimentary "view from below" that is most beneficial when considered in the context of more conventional hurricane observations.
Radarsat-1 ScanSAR wide mode images of Atlantic basin hurricanes have been acquired over two hurricane seasons through the Canadian Space Agency's Disaster Watch program. These images provide a high-resolution view of these intense storms and new insights into their morphology. A new result is recognition of the widespread presence of boundary layer rolls in these storms. In 2001, "Hurricane Watch" will focus on obtaining contemporaneous aircraft-based turbulence measurements at Radarsat-1 pass times.
Radar reflectivity and raingage data obtained during six springtimes indicate the types of mesoscale organization that occur in association with major rain events in Oklahoma (at least 25 mm of rain in 24 h over an area exceeding 12 500 km2). In these storms the primary rain area is found to be a contiguous region of precipitation 10s to 100s of km in scale that consists partly of deep convection and partly of stratiform rain. The patterns of rain formed by the convective and stratiform areas comprise a continuous spectrum of mesoscale structures. About two-thirds of the cases examined exhibited variations on the type of organization in which convective cells arranged in a moving line are followed by a region of stratiform rain. Storm organization was graded according to the degree to which it matched an idealized model of this "leading-line/trailing-stratiform" structure. The precipitation pattern was further graded according to whether its structure was relatively symmetric with respect to an axis normal to and passing through the midpoint of the line, or asymmetric, in which case the storm was biased toward having stronger, more discrete convective structure at the upwind (south or southwestern) end of the line and/or the most extensive stratiform precipitation behind the downwind (north to northeastern) end of the line. About one-third of the cases examined displayed much more chaotic, unclassifiable arrangements of convective and stratiform areas. Among the cases with leading-line/trailing-stratiform structure, severe weather was most frequent in systems with (i) a strong degree of leading-line/trailing-stratiform structure, in which a solid, relatively uniform, are-shaped line had stratiform rain centered symmetrically behind it, and (ii) a weaker degree of leading-line/trailing-stratiform structure in which a southwest-northeast line was biased toward having narrow, intensely convective, irregularly spaced cell structure at its southwestern (upwind) end and stratiform rain confined to the region behind the broader northeastern (downwind) portion of the line. Although all mesoscale organization types were characterized by all types of severe weather, the type (ii) cases were the most prolific category in terms of tornado and hail production, while type (i) cases were prone to be associated with flooding. The chaotic, unclassifiable cases, which exhibited no line organization, had just as much severe weather as the cases with line organization, but were more likely to produce hail and somewhat less likely to produce tornadoes and flooding than the systems with line structure. Major rain events occurred whenever a mesoscale convective complex (MCC) was passing over the study area, unless the MCC was dissipating or merely skirting the area. However, 75% of the major rain events occurred under cloud shields that failed to meet the MCC criteria explicitly, although they often resembled MCCs qualitatively. No particular type of mesoscale radar-echo organization was favored when cloud shields meeting the MCC criteria were observed. A slight preference for the more chaotic type of organization was suggested; however, the data sample is not large enough for this finding to be regarded as conclusive. Mean soundings and hodographs generally show no sign of a low-level jet in environments associated with chaotically arranged rain areas that lacked any line structure. On the other hand, a low-level jet and resulting curved hodograph were typically associated with cases in which line organization was evident. The wind shear in the low-to-mid troposphere, the bulk Richardson number and other familiar parameters characterizing squall fine environments are consistent with results from recent modeling studies. When leading-line/trailing-stratiform structure was present, the cross-line shear in the environment was of a magnitude associated with model simulations in which a rearward sloping updraft circulation favorable to trailing-stratiform anvil formation quickly develops. The along-line component of shear was greater when the squall system structure was of the asymmetric type and the degree of leading-line/trailing-stratiform structure was not as strong, i.e. in those mesoscale systems favoring tornado occurrence.
Corresponding author address: Dr. Steven Businger, Department of Meteorology, University of Hawaii at Manoa, 2525 Correa Road, Honolulu, HI 96822. Email: businger@hawaii.edu
A National Oceanic and Atmospheric Administration aircraft recorded the first Doppler radar data in a tropical cyclone with a minimum sea level pressure (MSLP) <900 mb during a reconnaissance mission in Hurricane Gilbert on 14 September 1988, when its MSLP was ∼895 mb. A previous mission had found an MSLP of 888 mb, making Gilbert the most intense tropical cyclone yet observed in the Atlantic basin. Radar reflectivity identified the hurricane eye, inner and outer eyewalls, a stratiform region between the eyewalls, and an area outside the outer eyewall that contained a few rainbands but that had mostly stratiform rain. Pseudo–dual Doppler analyses depict the three-dimensional kinematic structure of the inner eyewall and a portion of the outer eyewall. The vertical profiles of tangential wind and reflectivity maxima in the inner eyewall are more erect than in weaker storms, and winds >50 m s−1 extended to 12 km, higher than has been reported in previous hurricanes. The inner eyewall contained weak inflow throughout most of its depth. In contrast, the portion of the outer eyewall described here had shallow inflow and a broad region of outflow. The stratiform region between the two eyewalls had lower reflectivities and was the only region where the vertically incident Doppler radar data seemed to show downward motion below the freezing level. Gilbert's structure is compared with other intense Atlantic and eastern North Pacific hurricanes with MSLP >900 mb. Storms with lower MSLP have higher wind speeds in both inner and outer eyewalls, and wind speeds >50 m s−1 extend higher in storms with lower MSLP. Hurricanes Gilbert and Gloria (1985), the strongest Atlantic hurricanes yet analyzed by the Hurricane Research Division, had different outer eyewall structures. Gloria's outer eyewall had a deep region of inflow, while Gilbert's inflow layer was shallow. This may explain differences in the subsequent evolution of the two storms.
In 2005, NOAA's Hurricane Research Division (HRD), part of the Atlantic Oceanographic and Meteorological Laboratory, began a multiyear experiment called the Intensity Forecasting Experiment (IFEX). By emphasizing a partnership among NOAA's HRD, Environmental Modeling Center (EMC), National Hurricane Center (NHC), Aircraft Operations Center (AOC), and National Environmental Satellite Data Information Service (NESDIS), IFEX represents a new approach for conducting hurricane field program operations. IFEX is intended to improve the prediction of tropical cyclone (TC) intensity change by 1) collecting observations that span the TC life cycle in a variety of environments; 2) developing and refining measurement technologies that provide improved real-time monitoring of TC intensity, structure, and environment; and 3) improving the understanding of the physical processes important in intensity change for a TC at all stages of its life cycle. This paper presents a summary of the accomplishments of IFEX during the 2005 hurricane season. New and refined technologies for measuring such fields as surface and three-dimensional wind fields, and the use of unmanned aerial vehicles, were achieved in a variety of field experiments that spanned the life cycle of several tropical cyclones, from formation and early organization to peak intensity and subsequent landfall or extratropical transition. Partnerships with other experiments during 2005 also expanded the spatial and temporal coverage of the data collected in 2005. A brief discussion of the plans for IFEX in 2006 is also provided.
Abstract The downshear reformation of Tropical Storm Gabrielle (2001) was investigated using radar reflectivity and lightning data that were nearly continuous in time, as well as frequent aircraft reconnaissance flights. Initially the storm was a marginal tropical storm in an environment with strong 850–200-hPa vertical wind shear of 12–13 m s−1 and an approaching upper tropospheric trough. Both the observed outflow and an adiabatic balance model calculation showed that the radial-vertical circulation increased with time as the trough approached. Convection was highly asymmetric, with almost all radar return located in one quadrant left of downshear in the storm. Reconnaissance data show that an intense mesovortex formed downshear of the original center. This vortex was located just south of, rather than within, a strong downshear-left lightning outbreak, consistent with tilting of the horizontal vorticity associated with the vertical wind shear. The downshear mesovortex contained a 972-hPa minimum central pressure, 20 hPa lower than minimum pressure in the original vortex just 3 h earlier. The mesovortex became the new center of the storm, but weakened somewhat prior to landfall. It is argued that dry air carried around the storm from the region of upshear subsidence, as well as the direct effects of the shear, prevented the reformed vortex from continuing to intensify. Despite the subsequent weakening of the reformed center, it reached land with greater intensity than the original center. It is argued that this intensification process was set into motion by the vertical wind shear in the presence of an environment with upward motion forced by the upper tropospheric trough. In addition, the new center formed much closer to the coast and made landfall much earlier than predicted. Such vertical-shear-induced intensity and track fluctuations are important to understand, especially in storms approaching the coast.
