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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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Francis J. Mercerent and Peter G. Black

Abstract

No abstract available.

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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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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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Peter G. Black and Greg J. Holland

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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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Eric W. Uhlhorn and Peter G. Black

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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, HARRY V. SENN, and CHARLES L. COURTRIGHT

Abstract

Project Stormfury radar precipitation data gathered before, during, and after the multiple seedings of the eyewall region of hurricane Debbie on Aug. 18 and 20, 1969, are used to study changes in the eye configuration, the characteristics of the radar bright band, and the precipitation tilt. Increases in the echo-free area within the eye followed each of the five seedings on the 18th, but followed only one seeding on the 20th. Changes in major axis orientation followed only one seeding on the 18th, but followed each seeding on the 20th. Similar studies conducted recently on unmodified storms suggest that such changes do not occur naturally. However, the studies do not exclude this possibility. Changes in the radius of maximum winds follow closely the changes in eyewall radius. It is suggested that the different results on the 2 days might be attributable to seeding beyond the radius of maximum winds on the 18th and inside the outer radius of maximum winds on the 20th.

The bright band is found in all quadrants of the storm within 100 n.mi. of the eye, sloping slightly upward near the eyewall. The inferred shears are directed outward and slightly down band with height in both layers studied. The hurricane Debbie bright band and precipitation tilt data compared favorably with those gathered in Betsy of 1965 and Beulah and Heidi of 1967.

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Joseph J. Cione, Peter G. Black, and Samuel H. Houston

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Composite analyses of marine surface observations from 37 hurricanes between 1975 and 1998 show that the difference between the sea surface temperature and the surface air temperature significantly increases just outside the hurricane inner core. This increase in the sea–air contrast is primarily due to a reduction in surface air temperature and is more likely to occur when sea temperatures are at least 27°C. Results show that 90% of the observed cooling occurs 3.25°–1.25° latitude from the hurricane center, well outside the region of strongest surface winds. Since surface pressure only decreases 3 mb over this interval, the ∼2°C drop in air temperature is not a result of adiabatic expansion.

For the subset of observations that contained moisture measurements, surface specific humidity decreased 1.2 g kg−1 4.5°–1.75° latitude from the storm center. This finding suggests that the observed reduction in surface air temperature is not simply a result of near-surface evaporation from sea spray or precipitation. An alternate explanation may be that outside the hurricane inner core, unsaturated convective downdrafts act to dry and evaporatively cool the near-surface environment.

Between 3.25° and 1.25° radius, composite analyses show that low-level inflow is not isothermal, surface moisture is not constant, and the near-surface environment is not in thermodynamic equilibrium with the sea. Calculations based on these observations show that θ e decreases between 4.0° and 1.25° radius and then quickly rises near the inner core as surface pressures fall and specific humidity increases. Surface fluxes of heat and moisture are also observed to significantly increase near the inner core. The largest increase in surface sensible heat flux occurs radially inward of 1.5°, where surface winds are strong and sea–air temperature contrasts are greatest. As a result, the average Bowen ratio is 0.20∼0.5° radius from the composite storm center. This increase in sensible heat flux (in conjunction with near-saturated conditions at low to midlevels) may help explain why average surface air temperatures inside 1.25° radius remain relatively constant, despite the potential for additional cooling from evaporation and adiabatic expansion within the high wind inner core.

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

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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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