Michael L. Black

GPS Dropwindsonde Wind Profiles in Hurricanes and Their Operational Implications

Abstract The recent development of the global positioning system (GPS) dropwindsonde has allowed the wind and thermodynamic structure of the hurricane eyewall to be documented with unprecedented accuracy and resolution. In an attempt to assist operational hurricane forecasters in their duties, dropwindsonde data have been used in this study to document, for the first time, the mean vertical profile of wind speed in the hurricane inner core from the surface to the 700-hPa level, the level typically flown by reconnaissance aircraft. The dropwindsonde-derived mean eyewall wind profile is characterized by a broad maximum centered 500 m above the surface. In the frictional boundary layer below this broad maximum, the wind decreases nearly linearly with the logarithm of the altitude. Above the maximum, the winds decrease because of the hurricanes warm core. These two effects combine to give a surface wind that is, on average, about 90% of the 700-hPa value. The dropwindsonde observations largely confirm recent...

Eastern Pacific Hurricanes Jimena of 1991 and Olivia of 1994: The Effect of Vertical Shear on Structure and Intensity

Abstract Shear is a key inhibitor of tropical cyclone intensification. Although its signature is readily recognized in satellite imagery and theoretical or modeling studies provide some insight, detailed observations have been limited. Airborne radar and in situ observations in Hurricanes Jimena of 1991 and Olivia of 1994 are a step toward better understanding. Each storm was observed on two consecutive days. Initially, both had small eyes, 16–18-km radius, and maximum winds of ∼57 m s−1 over sea surface temperatures (SST) >28°C in easterly environmental shear. Jimena maintained constant intensity or weakened gradually for 2 days in 13–20 m s−1 easterly shear. Olivia intensified in 8 m s−1 shear on the first day. Overnight, the shear diminished to reverse and became westerly. On the second day, Olivia weakened as the shear increased to >15 m s−1 from the west, the storm moved over cooler SST, and became surrounded by dryer air. As convection weakened and the outer rainbands ceased to be effective barriers...

Vertical Motion Characteristics of Tropical Cyclones Determined with Airborne Doppler Radial Velocities

Abstract Vertical motions in seven Atlantic hurricanes are determined from data recorded by Doppler radars on research aircraft. The database consists of Doppler velocities and reflectivities from vertically pointing radar rays collected along radial flight legs through the hurricane centers. The vertical motions are estimated throughout the depth of the troposphere from the Doppler velocities and bulk estimates of particle fallspeeds. Portions of the flight tracks are subjectively divided into eyewall, rainband, stratiform, and “other” regions. Characteristics of the vertical velocity and radar structure are described as a function of altitude for the entire dataset and each of the four regions. In all of the regions, more than 70% of the vertical velocities range from −2 to 2 m s−1. The broadest distribution of vertical motion is in the eyewall region where ∼5% of the vertical motions are >5 m s−1. Averaged over the entire dataset, the mean vertical velocity is upward at all altitudes. Mean downward mot...

The Concentric Eyewall Cycle of Hurricane Gilbert

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.

The landfall of Hurricane Hugo in the Carolinas : surface wind distribution

Abstract Hurricane Hugo struck Charleston, South Carolina, on 22 September 1989 as the most intense hurricane to affect the United States since Camille in 1969. The northeastern eyewall, which contained the maximum winds measured by reconnaissance aircraft shortly before landfall, moved inland over a relatively unpopulated area and there were few fatalities. However, no observations were available to document the surface wind distribution in this part of the storm as it continued inland. To improve specification of surface winds in Hugo, empirically adjusted aircraft winds were combined with coastal, offshore, and inland surface observations and were input to the Ooyama objective analysis algorithm. The wind analysis at landfall was then compared with subsequent analyses at 3 and 6 h after landfall. Reconstruction of the surface wind field at landfall suggests that the maximum (∼13 min mean) surface wind at the coast was 50 m s−1 in the Bulls Bay region, ∼40 km northeast of Charleston. Surface roughness o...

Unusually Strong Vertical Motions in a Caribbean Hurricane

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.

Probing Hurricanes with Stable Isotopes of Rain and Water Vapor

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, H218O:H2O and HDO:H2O. The spatial patterns and temporal changes of isotope ratios reflect details of a hurricanes 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 cl...

Temporal and Spatial Variations of Rainfall Near the Centers of Two Tropical Cyclones

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...

THIRTY YEARS OF TROPICAL CYCLONE RESEARCH WITH THE NOAA P-3 AIRCRAFT

In 1976 and 1977, the National Oceanic and Atmospheric Administration purchased two customized WP-3D (P-3) aircraft to conduct tropical cyclone (TC) research. During their first 30 years, the P-3s have proved to be invaluable research platforms, obtaining data at the micro- to synoptic scale, with missions conducted in 134 TCs in the Atlantic and eastern Pacific Oceans and near Australia. Analyses of the observations led to many new insights about TC structure, dynamics, thermodynamics, and environmental interactions. The real-time use of the information by the National Hurricane and Environmental Modeling Centers of the National Centers for Environmental Prediction (NCEP), as well as later research, has helped to increase the accuracy of wind, flood, and storm surge forecasts and severe weather warnings and has resulted in significant improvements to operational numerical model guidance for TC-track forecasts. In commemoration of the first 30 years of research with these aircraft, this manuscript present...

NOAA'S Hurricane Intensity Forecasting Experiment: A Progress Report

An update of the progress achieved as part of the NOAA Intensity Forecasting Experiment (IFEX) is provided. Included is a brief summary of the noteworthy aircraft missions flown in the years since 2005, the first year IFEX flights occurred, as well as a description of the research and development activities that directly address the three primary IFEX goals: 1) collect observations that span the tropical cyclone (TC) life cycle in a variety of environments for model initialization and evaluation; 2) develop and refine measurement strategies and technologies that provide improved real-time monitoring of TC intensity, structure, and environment; and 3) improve the understanding of physical processes important in intensity change for a TC at all stages of its life cycle. Such activities include the real-time analysis and transmission of Doppler radar measurements; numerical model and data assimilation advancements; characterization of tropical cyclone composite structure across multiple scales, from vortex s...