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Robert A. Black, Howard B. Bluestein, and Michael L. Black

Abstract

Unusually strong updrafts and downdrafts in the eyewall of Hurricane Emily (1987) during its rapidly deepening phase are documented by both in situ aircraft measurements and a vertically pointing Doppler radar. Updrafts and downdrafts as strong as 24 and 19 m s−1, respectively, were found. Mean updrafts and downdrafts were approximately twice as strong as those found in other hurricanes. Updrafts had approximately the same width as downdrafts. The most vigorous updrafts were located in the front quadrants of the storm, and most of the strongest downdrafts were found in the rear quadrants. The downdrafts could not be explained in terms of evaporative or melting cooling, or precipitation drag. Evidence is presented that moist symmetric instability initiated by precipitation loading may have been responsible for the strong downdrafts.

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Robert W. Burpee and Michael L. Black

Abstract

The Hurricane Research Division collected radar reflectivity data with a portable recorder attached to National Weather Service (NWS) WSR-57 radar as Hurricanes Alicia of 1983 and Elena of 1985 approached the coastline of the United States. The reflectivity data were used to estimate rain rates for the eyewall region, including the rain-free eye, and the rainbands in the annular area outside the eyewall, but within 75 km of the center of the eye. The rain rates include reflectivity corrections that were based upon the variation of average returned power with range in four hurricanes

This study examines the temporal and spatial variations of rain rates in the cores of Hurricanes Alicia and Elena. In Alicia, variations of area-averaged rain rate (R) in the eyewall region were caused by the growth and decay of mesoscale convective areas. In Elena, the life cycles of individual convective cells also accounted for large changes in the eyewall R. In both hurricanes, the time series of R in the rainband region was less variable than the eyewall R, because the rainband region was larger than the eyewall and contained a smaller percentage of convection.

The distribution of precipitation in the eyewall and rainband regions was asymmetric. For several hours early in the observing period, the maximum rain rates in the eyewall and rainband regions of Alicia occurred in the left-front quadrant relative to the storm motion. Then, the heaviest rain in the eyewall region shifted to the right-front quadrant and that in the rainband region moved to the right of the storm track. In Elena, the maximum rain rates in the eyewall and rainband regions remained in the right-front quadrant throughout the computational period. About 55% of the precipitation in Elena's eyewall region occurred in the right-front quadrant.

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Michael L. Black and Hugh E. Willoughby

Abstract

Hurricane Gilbert of 1988 formed an outer eyewall as it intensified rapidly toward a record minimum pressure of 888 hPa in the western Caribbean. The outer eyewall strengthened and contracted, while the inner eyewall showed some signs of weakening before landfall on the Yucatan Peninsula. Remarkably, both eyewalls survived passage over land, but the storm was much weaker when it entered the Gulf of Mexico. Although the primary cause of weakening was passage over land, the effect of the contracting outer eyewall may have contributed. Later, the outer eyewall completely replaced the inner eyewall. Subsequently, it contracted steadily but slowly as Gilbert maintained nearly constant intensity over the cooler waters of the Gulf before final landfall on the mainland of Mexico.

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Kristen L. Corbosiero, John Molinari, and Michael L. Black

Abstract

One of the most complete aircraft reconnaissance and ground-based radar datasets of a single tropical cyclone was recorded in Hurricane Elena (1985) as it made a slow, 3-day anticyclonic loop in the Gulf of Mexico. Eighty-eight radial legs and 47 vertical incidence scans were collected aboard NOAA WP-3D aircraft, and 1142 ground-based radar scans were made of Elena’s eyewall and inner rainbands as the storm intensified from a disorganized category 2 to an intense category 3 hurricane. This large amount of continuously collected data made it possible to examine changes that occurred in Elena’s inner-core symmetric structure as the storm intensified.

On the first day of study, Elena was under the influence of vertical wind shear from an upper-tropospheric trough to the west. The storm was disorganized, with no discernable eyewall and nearly steady values of tangential wind and relative vorticity. Early on the second day of study, a near superposition and constructive interference occurred between the trough and Elena, coincident with upward vertical velocities and the radial gradient of reflectivity becoming concentrated around the 30-km radius. Once an inner wind maximum and eyewall developed, the radius of maximum winds contracted and a sharp localized vorticity maximum emerged, with much lower values on either side. This potentially unstable vorticity profile was accompanied by a maximum in equivalent potential temperature in the eyewall, deeper and stronger inflow out to 24 km from the eyewall, and mean outflow toward the eyewall from the eye.

Within 6–12 h, intensification came to an end and Elena began to slowly weaken. Vorticity and equivalent potential temperature at 850 hPa showed indications of prior mixing between the eye and eyewall. During the weakening stage, an outflow jet developed at the eyewall radius. A strong 850-hPa updraft accompanied the outflow jet, yet convection was less active aloft than before. This feature appeared to represent a shallow, outward-sloping updraft channel associated with the spindown of the storm.

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Alexander Lowag, Michael L. Black, and Matthew D. Eastin

Abstract

Hurricane Bret underwent a rapid intensification (RI) and subsequent weakening between 1200 UTC 21 August and 1200 UTC 22 August 1999 before it made landfall on the Texas coast 12 h later. Its minimum sea level pressure fell 35 hPa from 979 to 944 hPa within 24 h. During this period, aircraft of the National Oceanic and Atmospheric Administration (NOAA) flew several research missions that sampled the environment and inner core of the storm. These datasets are combined with gridded data from the National Centers for Environmental Prediction (NCEP) Global Model and the NCEP–National Center for Atmospheric Research (NCAR) reanalyses to document Bret’s atmospheric and oceanic environment as well as their relation to the observed structural and intensity changes. Bret’s RI was linked to movement over a warm ocean eddy and high sea surface temperatures (SSTs) in the Gulf of Mexico coupled with a concurrent decrease in vertical wind shear. SSTs at the beginning of the storm’s RI were approximately 29°C and steadily increased to 30°C as it moved to the north. The vertical wind shear relaxed to less than 10 kt during this time. Mean values of oceanic heat content (OHC) beneath the storm were about 20% higher at the beginning of the RI period than 6 h prior. The subsequent weakening was linked to the cooling of near-coastal shelf waters (to between 25° and 26°C) by prestorm mixing combined with an increase in vertical wind shear. The available observations suggest no intrusion of dry air into the circulation core contributed to the intensity evolution. Sensitivity studies with the Statistical Hurricane Intensity Prediction Scheme (SHIPS) model were conducted to quantitatively describe the influence of environmental conditions on the intensity forecast. Four different cases with modified vertical wind shear and/or SSTs were studied. Differences between the four cases were relatively small because of the model design, but the greatest intensity changes resulted for much cooler prescribed SSTs. The results of this study underscore the importance of OHC and vertical wind shear as significant factors during RIs; however, internal dynamical processes appear to play a more critical role when a favorable environment is present.

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Stanley Gedzelman, James Lawrence, John Gamache, Michael Black, Edward Hindman, Robert Black, Jason Dunion, Hugh Willoughby, and Xiaoping Zhang

Abstract

Rain and water vapor were collected during flights in Hurricanes Olivia (1994), Opal (1995), Marilyn (1995), and Hortense (1995) and analyzed for their stable isotopic concentrations, or ratios, H2 18O:H2O and HDO:H2O. The spatial patterns and temporal changes of isotope ratios reflect details of a hurricane's structure, evolution, microphysics, and water budget. At all flight levels over the sea (850–475 hPa) the lowest isotope ratios occur in or near regions of stratiform rains between about 50 and 250 km from the eye. Isotope ratios are higher in the eyewall and were particularly high in the crescent-shaped eyewall of Hurricane Opal at a time when no rain was falling over a large area near the storm center. In Hurricane Olivia, isotope ratios decreased from 24 to 25 September after vertical and radial circulation weakened. A two-layer isotope model of a radially symmetric hurricane simulates these features. The low isotope ratios are caused by fractionation in extensive, thick, precipitating clouds with predominantly convergent low-level flow accompanied by removal of heavy isotopes by falling raindrops. Evaporation and isotope equilibration of sea spray increase isotope ratios of the ambient vapor and produce a deuterium excess or enrichment of D relative to 18O that increases with decreasing relative humidity and increasing wind speed. Model results show that sea spray supplies the eyewall with up to 50% of its water vapor and is largely responsible for its high isotope ratios.

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Frank D. Marks, Peter G. Black, Michael T. Montgomery, and Robert W. Burpee

Abstract

On 15 September 1989, one of NOAA’s WP-3D research aircraft, N42RF [lower aircraft (LA)], penetrated the eyewall of Hurricane Hugo. The aircraft had an engine fail in severe turbulence while passing the radius of maximum wind and before entering the eye at 450-m altitude. After the aircraft returned to controlled flight within the 7-km radius eye, it gained altitude gradually as it orbited in the eye. Observations taken during this period provide an updated model of the inner-core structure of an intense hurricane and suggest that LA penetrated an intense cyclonic vorticity maximum adjacent to the strongest convection in the eyewall [eyewall vorticity maximum (EVM)]. This EVM was distinct from the vortex-scale cyclonic circulation observed to orbit within the eye three times during the 1 h that LA circled in the eye. At the time, Hugo had been deepening rapidly for 12 h. The maximum flight-level tangential wind was 89 m s−1 at a radius of 12.5 km; however, the primary vortex peak tangential wind, derived from a 100-s filter of the flight-level data, was estimated to be 70 m s−1, also at 12.5-km radius. The primary vortex tangential wind was in approximate gradient wind balance, was characterized by a peak in angular velocity just inside the radius of maximum wind, and had an annular vorticity structure slightly interior to the angular velocity maximum. The EVM along the aircraft’s track was roughly 1 km in diameter with a peak cyclonic vorticity of 1.25 × 10−1 s−1. The larger circulation center, with a diameter >15 km, was observed within the eye and exhibited an average orbital period of 19 min. This period is about the same as that of the angular velocity maximum of the axisymmetric mean vortex and is in reasonable agreement with recent theoretical and model predictions of a persistent trochoidal “wobble” of circulation centers in mature hurricane-like vortices. This study is the first with in situ documentation of these vortical entities, which were recently hypothesized to be elements of a lower-tropospheric eye/eyewall mixing mechanism that supports strong storms.

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Kristen L. Corbosiero, John Molinari, Anantha R. Aiyyer, and Michael L. Black

Abstract

A portable data recorder attached to the Weather Surveillance Radar-1957 (WSR-57) in Apalachicola, Florida, collected 313 radar scans of the reflectivity structure within 150 km of the center of Hurricane Elena (in 1985) between 1310 and 2130 UTC 1 September. This high temporal and spatial (750 m) resolution dataset was used to examine the evolution of the symmetric and asymmetric precipitation structure in Elena as the storm rapidly strengthened and attained maximum intensity. Fourier decomposition of the reflectivity data into azimuthal wavenumbers revealed that the power in the symmetric (wavenumber 0) component dominated the reflectivity pattern at all times and all radii by at least a factor of 2. The wavenumber 1 asymmetry accounted for less than 20% of the power in the reflectivity field on average and was found to be forced by the environmental vertical wind shear.

The small-amplitude wavenumber 2 asymmetry in the core was associated with the appearance and rotation of an elliptical eyewall. This structure was visible for nearly 2 h and was noted to rotate cyclonically at a speed equal to half of the local tangential wind. Outside of the eyewall, individual peaks in the power in wavenumber 2 were associated with repeated instances of cyclonically rotating, outward-propagating inner spiral rainbands. Four separate convective bands were identified with an average azimuthal velocity of 25 m s−1, or ∼68% of the local tangential wind speed, and an outward radial velocity of 5.2 m s−1. The azimuthal propagation speeds of the elliptical eyewall and inner spiral rainbands were consistent with vortex Rossby wave theory.

The elliptical eyewall and inner spiral rainbands were seen only in the 6-h period prior to peak intensity, when rapid spinup of the vortex had produced an annular vorticity profile, similar to those that have been shown to support barotropic instability. The appearance of an elliptical eyewall was consistent with the breakdown of eyewall vorticity into mesovortices, asymmetric mixing between the eye and eyewall, and a slowing of the intensification rate. The inner spiral rainbands might have arisen from high eyewall vorticity ejected from the core during the mixing process. Alternatively, because the bands were noted to emanate from the vertical shear-forced deep convection in the northern eyewall, they could have formed through the axisymmetrization of the asymmetric diabatically generated eyewall vorticity.

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Amanda S. Black, James S. Risbey, Christopher C. Chapman, Didier P. Monselesan, Thomas S. Moore II, Michael J. Pook, Doug Richardson, Bernadette M. Sloyan, Dougal T. Squire, and Carly R. Tozer

Abstract

Large-scale cloud features referred to as cloudbands are known to be related to widespread and heavy rain via the transport of tropical heat and moisture to higher latitudes. The Australian northwest cloudband is such a feature that has been identified in simple searches of satellite imagery but with limited investigation of its atmospheric dynamical support. An accurate, longterm climatology of northwest cloudbands is key to robustly assessing these events. A dynamically based search algorithm has been developed that is guided by the presence and orientation of the subtropical jet stream. This jet stream is the large-scale atmospheric feature that determines the development and alignment of a cloudband. Using a new 40-year dataset of cloudband events compiled by this search algorithm, composite atmospheric and ocean surface conditions over the period 1979-2018 have been assessed. Composite cloudband upper level flow revealed a tilted low pressure trough embedded in a Rossby wave train. Composites of vertically integrated water vapor transport centered around the jet maximum during northwest cloudband events reveal a distinct Atmospheric River supplying tropical moisture for cloudband rainfall. Parcel backtracking indicated multiple regions of moisture support for cloudbands. A thermal wind anomaly orientated with respect to enhanced sea surface temperature gradient over the Indian Ocean was also a key composite cloudband feature. 300 years of a freely-coupled control simulation of the ACCESS-D system was assessed for its ability to simulate northwest cloudbands. Composite analysis of model cloudbands compared reasonably well to reanalysis despite some differences in seasonality and frequency of occurrence.

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Amanda S. Black, James S. Risbey, Christopher C. Chapman, Didier P. Monselesan, Thomas S. Moore II, Michael J. Pook, Doug Richardson, Bernadette M. Sloyan, Dougal T. Squire, and Carly R. Tozer

Abstract

Large-scale cloud features referred to as cloudbands are known to be related to widespread and heavy rain via the transport of tropical heat and moisture to higher latitudes. The Australian northwest cloudband is such a feature that has been identified in simple searches of satellite imagery but with limited investigation of its atmospheric dynamical support. An accurate, long-term climatology of northwest cloudbands is key to robustly assessing these events. A dynamically based search algorithm has been developed that is guided by the presence and orientation of the subtropical jet stream. This jet stream is the large-scale atmospheric feature that determines the development and alignment of a cloudband. Using a new 40-yr dataset of cloudband events compiled by this search algorithm, composite atmospheric and ocean surface conditions over the period 1979–2018 have been assessed. Composite cloudband upper-level flow revealed a tilted low pressure trough embedded in a Rossby wave train. Composites of vertically integrated water vapor transport centered around the jet maximum during northwest cloudband events reveal a distinct atmospheric river supplying tropical moisture for cloudband rainfall. Parcel backtracking indicated multiple regions of moisture support for cloudbands. A thermal wind anomaly orientated with respect to an enhanced sea surface temperature gradient over the Indian Ocean was also a key composite cloudband feature. A total of 300 years of a freely coupled control simulation of the ACCESS-D system was assessed for its ability to simulate northwest cloudbands. Composite analysis of model cloudbands compared reasonably well to reanalysis despite some differences in seasonality and frequency of occurrence.

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