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Peter. G. Black

Complicated internal-wave patterns are revealed in Apollo-Soyuz Test Project (ASTP) photographs of marine fog patches. Evidence for the reflection of atmospheric wave packets at water-land boundaries is contained in these patterns. Photographs of “arc,” or “ring,” clouds show that these clouds are associated with decaying maritime cumulonimbus clouds. The cloud-free inner ring delineates cold air that has been transported from aloft by thunderstorm downdrafts, thus inhibiting cumulus cloud development. There is no evidence to indicate that this clear inner ring is a result of cold-core eddies in the ocean. Other interesting cloud patterns revealed by ASTP photographs include 1) linear trails created in low stratocumulus cloud decks by passing ships, and 2) dome-shaped anvil clouds created by vigorous thunderstorms.

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David Atlas and Peter G. Black

SEASAT synthetic aperture radar (SAR) echoes from the sea have previously been shown to be the result of rain and winds produced by convective storms; rain damps the surface waves and causes echo-free holes, while the diverging winds associated with the downdraft generate waves and associated echoes surrounding the holes. Gust fronts are also evident. Such a snapshot from 8 July 1978 has been examined in conjunction with ground-based radar. This leads to the conclusion that the SAR storm footprints resulted from storm processes that occurred up to an hour or more prior to the snapshot. A sequence of events is discerned from the SAR imagery in which new cell growth is triggered in between the converging outflows of two preexisting cells. In turn, the new cell generates a mini–squall line along its expanding gust front. While such phenomena are well known over land, the spaceborne SAR now allows important inferences to be made about the nature and frequency of convective storms over the oceans. The storm effects on the sea have significant implications for spaceborne wind scatterometry and rainfall measurements. Some of the findings herein remain speculative because of the great distance to the Miami weather radar—the only source of corroborative data.

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Peter G. Black and Greg J. Holland

Abstract

The boundary layer structure of Tropical Cyclone Kerry (1979) is investigated using composite analysis of research aircraft, surface ship, and automatic weather station observations. The boundary layer was moist, convective, and strongly confluent to the east of the tropical cyclone center but was dry, subsident, and diffluent to the west. The vertical momentum transport in the eastern convective sector of Kerry was around two to three times the surface frictional dissipation. In contrast, the stable boundary layer in the western sector consisted of a shallow mixed layer capped by an equivalent potential temperature minimum and a low-level jet, which underwent a marked diurnal oscillation. Three mechanisms appear to have contributed to the observed asymmetry: 1) a general, zonal distortion arose from cyclonic rotation across a gradient of earth vorticity; 2) a westerly environmental vertical shear produced forced ascent on the east side of the storm and subsidence on the west side throughout the lower and midtroposphere; and 3) the western sector boundary layer was modified by an upstream cold tongue generated by the tropical cyclone passage. The authors present evidence that substantial drying also resulted from shear-induced mixing of the subsident environmental air in the region of the low-level jet.

Thermal boundary layer budgets are derived using both a general mixing theory approach and direct flux calculations from aircraft reconnaissance data. Use of actual sea surface temperature fields are essential. The surface flux estimates of latent heat are near the average of previous studies, but the sensible heat fluxes are downward into the ocean. Since horizontal advection also cooled the boundary layer, the thermal structure was maintained by downward fluxes of sensible heat from the top of the boundary layer of around 100 W m−2. We conclude that the pattern of oceanic cooling directly determines the pattern of vertical air-sea and advective sensible heat fluxes and indirectly determines the pattern of latent heat fluxes through forcing of PBL drying at the downwind end of the SST cold pool. It further enhances the inward penetration and negative feedback resulting from an easterly trade wind surge associated with a mobile trough in the westerlies.

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Francis J. Mercerent and Peter G. Black

Abstract

No abstract available.

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Roger M. Wakimoto and Peter G. Black

A damage map documenting Hurricane Andrew's destructive landfall over southern Florida is presented. Vectors that represent the direction of winds causing damage to trees and structures are shown along with an F-scale rating in order to assess the strength of the near-surface winds. It is hypothesized that increased surface roughness once the hurricane made landfall may have contributed to a surface wind enhancement resulting in the strongest winds ever estimated (F3) for a landfall hurricane. This intense damage occurred primarily during the “second” period of strong winds associated with the east side of the eyewall. For the first time, a well-defined circulation inthe damage pattern by the second wind was documented. A superposition of radar data from Miami and Key West on top of the damage map provides the first detailed examination of the relationship between the eyewall and the surface flow field as estimated from the damage vectors.

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Eric W. Uhlhorn and Peter G. Black

Abstract

Surface winds in hurricanes have been estimated remotely using the Stepped-Frequency Microwave Radiometer (SFMR) from the NOAA WP-3D aircraft for the past 15 years. Since the use of the GPS dropwindsonde system in hurricanes was first initiated in 1997, routine collocated SFMR and GPS surface wind estimates have been made. During the 1998, 1999, and 2001 hurricane seasons, a total of 249 paired samples were acquired and compared. The SFMR equivalent 1-min mean, 10-m level neutral stability winds were found to be biased high by 2.3 m s−1 relative to the 10-m GPS winds computed from an estimate of the mean boundary layer wind. Across the range of wind speeds from 10 to 60 m s−1, the rmse was 3.3 m s−1. The bias was found to be dependent on storm quadrant and independent of wind speed, a result that suggests a possible relationship between microwave brightness temperatures and surface wave properties. Tests of retrieved winds' sensitivities to sea surface temperature, salinity, atmospheric thermodynamic variability, and surface wind direction indicate wind speed errors of less than 1 m s−1 above 15 m s−1.

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Peter G. Black and Richard A. Anthes

Abstract

ATS-III satellite data and conventional aerological data are used to construct detailed wind analyses of the outflow layer for four hurricanes and one tropical storm. Harmonic analysis of these data, and of the data for a mean Atlantic hurricane and a mean Pacific typhoon, shows that wave numbers 1 and 2 around the circumference of the storm account for most of the variance of momentum and kinetic energy. Subtraction of the symmetric part of the vortex circulation from the total flow to yield the “asymmetric wind” reveals two eddies located in preferred quadrants of the storm. An anticyclonic eddy is found to the right and a cyclonic eddy to the left of the storm motion. These eddies transport absolute vorticity inward, opposing the outward transport by the mean circulation. They also transport a significant amount of negative relative angular momentum outward.

The presence of inertial (or dynamic) instability is investigated. Although substantial areas of negative absolute vorticity and anomalous anticyclonic winds exist in all cases, these areas are correlated so well that the regions of dynamic instability are small.

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Matthew D. Eastin, William M. Gray, and Peter G. Black

Abstract

The buoyancy of hurricane convective vertical motions is studied using aircraft data from 175 radial legs collected in 14 intense hurricanes at four altitudes ranging from 1.5 to 5.5 km. The data of each leg are initially filtered to separate convective-scale features from background mesoscale structure. Convective vertical motion events, called cores, are identified using the criteria that the convective-scale vertical velocity must exceed 1.0 m s−1 for at least 0.5 km. A total of 620 updraft cores and 570 downdraft cores are included in the dataset. Total buoyancy is calculated from convective-scale virtual potential temperature, pressure, and liquid water content using the mesoscale structure as the reference state.

Core properties are summarized for the eyewall and rainband regions at each altitude. Characteristics of core average convective vertical velocity, maximum convective vertical velocity, and diameter are consistent with previous studies of hurricane convection. Most cores are superimposed upon relatively weak mesoscale ascent. The mean eyewall (rainband) updraft core exhibits small, but statistically significant, positive total buoyancy below 4 km (between 2 and 5 km) and a modest increase in vertical velocity with altitude. The mean downdraft core not superimposed upon stronger mesoscale ascent also exhibits positive total buoyancy and a slight decrease in downward vertical velocity with decreasing altitude. Buoyant updraft cores cover less than 5% of the total area in each region but accomplish ∼40% of the total upward transport.

A one-dimensional updraft model is used to elucidate the relative roles played by buoyancy, vertical perturbation pressure gradient forces, water loading, and entrainment in the vertical acceleration of ordinary updraft cores. Small positive total buoyancy values are found to be more than adequate to explain the vertical accelerations observed in updraft core strength, which implies that ordinary vertical perturbation pressure gradient forces are directed downward, opposing the positive buoyancy forces. Entrainment and water loading are also found to limit updraft magnitudes.

The observations support some aspects of both the hot tower hypothesis and symmetric moist neutral ascent, but neither concept appears dominant. Buoyant convective updrafts, however, are integral components of the hurricane’s transverse circulation.

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Matthew D. Eastin, Peter G. Black, and William M. Gray

Abstract

The implications of flight-level instrument wetting error removal upon the mean thermodynamic structure across the eyewall, buoyancy of rainband vertical motions, and vertical energy fluxes near the top of the inflow layer are studied. Thermodynamic quantities across the mean eyewall are found to increase at all levels. As a result, maximum radial gradients of each quantity are shifted from the center of the eyewall cloud toward the outer edge. The increase in equivalent potential temperature lifts eyewall values to comparable magnitudes observed in the eye. The mean virtual potential temperature deviation of rainband updrafts increases from slightly negative to slightly positive. This increase and shift in sign are more pronounced in stronger updrafts. The mean deviation in rainband downdrafts decreases slightly toward neutral conditions. Vertical sensible heat fluxes near the top of the inflow layer are found to shift from downward to upward. Upward latent heat fluxes increase. Implications of these results upon hurricane structure and evolution are discussed.

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Matthew D. Eastin, William M. Gray, and Peter G. Black

Abstract

This is the second of two papers on the buoyancy of convective vertical motions in the inner core of intense hurricanes. This paper uses extensive airborne radar, dropwindsonde, and flight-level observations in Hurricanes Guillermo (1997) and Georges (1998) to illustrate typical azimuthal distribution of buoyant convection and demonstrate that the low-level eye can be an important source region for buoyant eyewall convection.

In both hurricanes, eyewall vertical velocity and radar reflectivity are asymmetric and exhibit persistent relationships with the direction of the environmental vertical wind shear. Mesoscale vertical motions exhibit a wavenumber-1 structure with maximum ascent downshear and weak descent upshear. The mesoscale reflectivity maxima are located left-of-shear. Buoyant eyewall updraft cores and transient convective-scale reflectivity cells are predominantly downshear and left-of-shear. Most eyewall downdraft cores that transport significant mass downward are located upshear. Negative buoyancy was most common in left-of-shear downdrafts, with positive buoyancy dominant in upshear downdrafts. Inward-spiraling rainbands located outside the eyewall exhibit upband/downband asymmetries. Upband segments contain more convective reflectivity cells and buoyant updraft cores than the more stratiform downband segments. Equal numbers of downdraft cores are found upband and downband, but the majority exhibit negative buoyancy.

Several buoyant updraft cores encountered in the midlevel eyewall exhibit equivalent potential temperatures (θe) much higher than the θe observed in the low-level eyewall, but equivalent to the θe observed in the low-level eye. Asymmetric low-wavenumber circulations appear responsible for exporting the high-θe eye air into the relatively low-θe eyewall and generating the locally buoyant updraft cores.

Implications of these results upon conceptual models of hurricane structure are discussed. Three mechanisms, whereby an ensemble of asymmetric buoyant convection could contribute to hurricane evolution, are also discussed.

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