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Mark D. Powell

The National Weather Service, as a part of its modernization effort, is implementing the Automated Surface Observing System (ASOS). Much discussion has occurred about various aspects of ASOS versus the current system of manual and automated observations. Based upon a study of the ASOS specifications and an informal survey of potential ASOS winddata users, defects of the wind sampling and archival strategy chosen for ASOS are discussed in terms of their impact on various user groups. Limitations include: 1) hourly observation average periods that do not conform to international recommendations for wind reporting made by the World Meteorological Organization, 2) no regular archival of high-resolution data—potentially valuable research data are destroyed if not identified within a 12-h period, and 3) no emergency power for operation in severe weather conditions. An alternative sampling and archiving strategy is recommended that benefits a wider cross section of users, without detracting from aviation and forecast service requirements, at a cost of less than 1 % of the original ASOS portion of the weather service modernization budget.

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Mark D. Powell

Abstract

Numerous aircraft, ship, buoy and land nation data were composited with respect to the center of Hurricane Frederic for two time periods: a 24 h period corresponding to the storm’s position in the open Gulf of Mexico on 12 September 1979, and an 8 h period corresponding to the landfall of Frederic near 0400 GMT on 13 September. Comparison of wind analyses for the two periods indicated a rotation of maximum inflow angles from the southeast to northeast quadrants and a strong frictional decrease of wind speed over land. Them and other features of the landfall analysis were compared with a model landfall study by Moss and Jones (1978). The landfall composite wind field was compared with the Fujita damage vector analysis to determine the damage time interval and mean wind speed range. Damage vector directions were found to be well correlated with the surface streamlines, with the most severe damage being associated with Frederic’s northern eyewall.

Ten-meter-level wind speed data over water (VO) and at coasts stations (VL) were used to formulate approximate relationships of the low-level (500–1500 m) aircraft wind (Va) to the mean coastal wind and peak gust (VLG) in the same position relative to the storm center. It was found that VO = 0.7Va, VL = 0.8V0, VLG = 0.8Va and VL = 0.56Va. These relationships should aid from in their 2 assessments of low-level aircraft reconnaissance wind data for use in issuing warnings.

The vertical shear of the horizontal wind determined from from radiosonde data for two inland stations was compared with shear determined from surface and data data over water. The overland shear was greater than the overwater shear, by a factor of 2, in the same relative part of the storm. The “thermal wind” shear computed in the vicinity of the center was negligible, although the 10 m level air temperature analysis over land indicated a cold core that was probably caused by adiabatic cooling.

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Mark D. Powell

Abstract

Recent aircraft boundary layer measurements in the vicinity of principal hurricane rainbands have confirmed that convective downdrafts are capable of transporting cool, dry, low equivalent potential temperature (θE) air to the surface, where the mixed layer is eliminated. The incorporation of this air into convection near the core of the storm may weaken the storm, depending upon the scale of the disturbance and the processes governing the recovery of the air while it is flowing toward the eyewall. This paper examines the thermodynamic characteristics of the boundary layer in outer convective hurricane rainbands, providing evidence for downdraft modification mechanisms and determining the extent to which disturbed boundary-layer air may be restored on its trajectory to the storm.

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Mark D. Powell

The U.S. Sailing Team competed successfully at the 1991 Pan American Games despite having no previous experience with the sailing conditions off Havana, Cuba. One of the key factors in the team's success was meteorological support in the form of wind climate analysis; application of sea breeze forecasting typical of the south Florida area, modified by tropical weather systems; and effective preregatta briefing.

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Mark D. Powell

Abstract

Results of hurricane boundary layer experiments conducted in outer rainbands of Hurricanes Josephine (1984) and Earl (1986) are presented. Comparisons of precipitation and kinematic structures in these storms and in Hurricane Floyd (1981) indicate that principal rainbands have common characteristic mesoscale and convective-scale features in the boundary layer. The two-dimensional mesoscale structure suggests that these rainbands are made up of a linear aggregate of cellular reflectivity elements (on the inner, upshear side of the band) and stratiform rain (on the outer downshear side). The bands are oriented perpendicular to the shear above the boundary layer and cells move downband at about 85% of the density-weighted mean wind speed of the 0.2–6 km layer. The boundary-layer wind field is strongly influenced by the rainband with alongband and crossband wind maxima located on the outer side of the band axis, and minima 4–8 km to the inner side. Maximum crossband convergence and cyclonic shear vorticity are also found to the inner side of the rainband axis. Updrafts and downdrafts are preferentially located on the inner side of the band axis, with some downdrafts spreading out at the surface. The band-relative positions of the updraft and perturbation pressure minimum suggest that the minimum may be produced by interaction of the wind shear and the updraft. Outer hurricane rainbands show many similarities to tropical squall lines, major differences are associated with propagation and the structure of the leading and trailing edges.

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Mark D. Powell

Abstract

Four diagnostic marine boundary-layer models are evaluated for applicability to the hurricane regime. The goat was to develop an operational method of estimating surface variables with research aircraft flight-level (500 m) data. Evaluation consisted of comparing the four models plus two estimation methods with “ground truth” buoy and ship wind speed data from Hurricanes Eloise and Anita and vertically stacked several-level aircraft data in Eloise and Caroline. Three of the boundary-layer models are capable of estimating wind speed to 10% accuracy. Model results also include 10 m level neutral drag coefficients, which were compared with previous studies.

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Mark D. Powell

Abstract

Aircraft, land station, and buoy data were composited with respect to the center of Hurricane Alicia (1983) for three 8-h periods corresponding to prelandfall in the open Gulf of Mexico, landfall in the Galveston area, and postlandfall in the vicinity of Houston.

Comparison of the wind analyses before, during, and after landfall emphasizes the land-sea frictional asymmetry at landfall. In addition, other asymmetries in the surface wind field and differences between the flight-level and the surface wind fields are revealed. The asymmetric structure of the surface wind field may be interpreted as having resulted from the combined effects of land-sea roughness differences, background environmental flow, and storm translation. The land-sea frictional difference acted to oppose the mean vortex flow over land and reinforce it over water. The southwest background environmental flow acted nearly parallel to the coastline, producing surface inflow on the left side and outflow on the right side, while the effect of the storm translation increased winds on the right and decreased winds on the left. At landfall, the analysis revealed a broad region of high wind speeds and a mesoscale divergence-convergence couplet along the outer rainband axis just offshore on the northeast (right) side of the storm. The outer rainband axis acted as an obstruction to the surface flow, separating the warmer central core of the storm from the environment through which the storm moved. In contrast to recent numerical model studies, surface convergence was also noted on the left side of the storm just offshore, despite outflow at flight level.

Analyses of temperature, dewpoint, and equivalent potential temperature indicate that loss of the oceanic heat and moisture source, combined with advection of drier air on the landward side of the storm, was responsible for cooling and drying of the inflowing boundary layer air. Upon introduction of this air into the core convection and vertical ascent, a decrease in the release of latent heat could then lead to cooling in the middle levels of the storm and a subsequent increase in the central sea-level pressure.

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Mark D. Powell and Sim D. Aberson

About 13% of all Atlantic basin tropical cyclone forecasts issued from 1976 to 2000 are for landfalls along the United States coastline, and 2% more are for storms forecast to make landfall in the United States but that remain at sea. Landfall position and time forecasts are skillful at all forecast time periods and are more skillful than Atlantic basin track forecasts as a whole, but within 30 h of predicted landfall, timing errors demonstrate an early bias of 1.5–2.5 h. Landfall forecasts are most accurate for storms moving at oblique or normal angles to the coastline and slow-moving storms. During the last quarter century, after adjustment for forecast difficulty, no statistically significant improvement or degradation is noted for landfall position forecasts. Time of landfall forecasts indicate no degradation at any period and significant improvement for the 19–30-h period. The early bias and lack of improvement are consistent with a conservative or “least regret” forecast and warning strategy to account for possible storm accelerations. Landfall timing uncertainty is ~11 h at 24 and 36 h, which suggests that hurricane warnings could be disseminated about 12 h earlier (at 36 h, rather than 24 h, before predicted landfall) without substantial loss of lead time accuracy (although warning areas necessarily would be larger). Reconsideration of National Weather Service Strategic Plan and United States Weather Research Program track forecast goals is recommended in light of these results.

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Mark D. Powell and Stephen K. Rinard

Abstract

A team of meteorologists from the United States, Canada, and Australia provided marine weather support to the sailing events of the 1996 Centennial Olympic Games, held in Wassaw Sound near Savannah, Georgia. The team conducted research on the weather and climate and developed a set of forecast products designed to inform athletes, volunteers, and race managers of the wind, tidal current, wave, and weather behavior expected each day during the pre-Olympic and Olympic periods. The Olympic period proved to be a challenge with thunderstorms delaying, abandoning, or postponing races on half of the days. Thunderstorm development and movement was linked to the timing and strength of the sea breeze as well as the direction and speed of the gradient wind. Numerous thunderstorm warnings were issued with the assistance of the WSR-88D radar and the Warning Decision Support System. Frequent lightning was a legitimate safety concern due to the long distances between race courses and lack of suitable shelter; fortunately no one was injured during the lightning episodes. Forecasters benefited from access to a variety of monitoring tools and models including real-time Olympic buoy wind and current time series displays; satellite and radar imagery animation; 2-, 8-, and 10-km resolution mesoscale models; a live video feed of race coverage; and communications with forecasters aboard patrol craft offshore. Official wind forecasts, mesoscale models, and a simple vector addition model performed better than climatology and persistence as defined by mean vector error and rms wind direction error. Climatology was difficult to beat on the basis of wind speed error.

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Mark D. Powell and Timothy A. Reinhold

Tropical cyclone damage potential, as currently defined by the Saffir-Simpson scale and the maximum sustained surface wind speed in the storm, fails to consider the area impact of winds likely to force surge and waves or cause particular levels of damage. Integrated kinetic energy represents a framework that captures the physical process of ocean surface stress forcing waves and surge while also taking into account structural wind loading and the spatial coverage of the wind. Integrated kinetic energy was computed from gridded, objectively analyzed surface wind fields of 23 hurricanes representing large and small storms. A wind destructive potential rating was constructed by weighting wind speed threshold contributions to the integrated kinetic energy, based on observed damage in Hurricanes Andrew, Hugo, and Opal. A combined storm surge and wave destructive potential rating was assigned according to the integrated kinetic energy contributed by winds greater than tropical storm force. The ratings are based on the familiar 1–5 range, with continuous fits to allow for storms as weak as 0.1 or as strong as 5.99.

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