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Sim D. Aberson

The important role of the correct use of statistics in the atmnospheric sciences literature is once again emphasized. Despite previous work on this topic, statistical techniques, even very simple ones, continue to be misused or altogether neglected, with the inevitable result of misleading or erroneous conclusions. An example concerning the impact of global climate change and hurricane activity is presented.

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Sim D. Aberson

The suite of tropical cyclone track forecast models in the Atlantic basin from the 1976 to 2000 hurricane seasons are treated as a forecast ensemble. The 12-h ensemble mean forecast, adjusted for forecast difficulty, has improved at a rate of just under 1% per year, and the improvement rate increases to almost 2.4% per year for the 72-h forecasts. The average size of the 72-h (48-h) error in 1976 is less than the average size of the 48-h (36-h) error in 2000. The average 36-h forecast error in 2000 is comparable to the 24-h forecast error in 1976. The ensemble currently spans the true path of the tropical cyclone in the cross-track direction more than 90% of the time and in the alongtrack direction between 60% and 90% of the time depending on the forecast lead time. The ensemble spread is unable to provide estimates of individual forecast reliability, likely making probabilistic landfall forecasts from this ensemble unreliable. The reliability of the spread in the cross-track direction suggests the possibility of limiting hurricane watch and warning regions depending upon the ensemble spread at landfall.

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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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Sim D. Aberson and James L. Franklin

In 1997, the Tropical Prediction Center (TPC) began operational Gulfstream-IV jet aircraft missions to improve the numerical guidance for hurricanes threatening the continental United States, Puerto Rico, and the Virgin Islands. During these missions, the new generation of Global Positioning System dropwindsondes were released from the aircraft at 150–200-km intervals along the flight track in the environment of the tropical cyclone to obtain profiles of wind, temperature, and humidity from flight level to the surface. The observations were ingested into the global model at the National Centers for Environmental Prediction, which subsequently serves as initial and boundary conditions to other numerical tropical cyclone models. Because of a lack of tropical cyclone activity in the Atlantic basin, only five such missions were conducted during the inaugural 1997 hurricane season.

Due to logistical constraints, sampling in all quadrants of the storm environment was accomplished in only one of the five cases during 1997. Nonetheless, the dropwindsonde observations improved mean track forecasts from the Geophysical Fluid Dynamics Laboratory hurricane model by as much as 32%, and the intensity forecasts by as much as 20% during the hurricane watch period (within 48 h of projected landfall). Forecasts from another dynamical tropical cyclone model (VICBAR) also showed modest improvements with the dropwindsonde observations. These improvements, if confirmed by a larger sample, represent a large step toward the forecast accuracy goals of TPC. The forecast track improvements are as large as those accumulated over the past 20–25 years, and those for forecast intensity provide further evidence that better synoptic-scale data can lead to more skillful dynamical tropical cyclone intensity forecasts.

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Sim D. Aberson, Michael T. Montgomery, Michael Bell, and Michael Black

An unprecedented dataset of category-5 Hurricane Isabel was collected on 12–14 September 2003. This two-part series focuses on novel dynamical and thermodynamical aspects of Isabel's innercore structure on 13 September. In Part I, using a composite of dropwindsonde and in situ aircraft data, the authors suggested that the axisymmetric structure of Isabel showed that the storm was superintense. Mesocyclones seen clearly in satellite imagery within the eye of Hurricane Isabel are hypothesized to mix high-entropy air at low levels in the eye into the eyewall, stimulating explosive convective development and a concomitant local horizontal wind acceleration.

Part II focuses on a unique set of observations into an extraordinary small- (miso) scale cyclonic feature inside of the inner edge of the eyewall of Hurricane Isabel. A dropwindsonde released into this feature measured the strongest known horizontal wind in a tropical cyclone. This particular observation is discussed in the context of concurrent observations from airborne Doppler radar and other airborne instruments. These observations show wind even stronger than the system-scale superintense wind suggested in Part I. Speculation on the frequency of occurrence of these “little whirls” and their potentially catastrophic impacts are presented.

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Michael T. Montgomery, Michael M. Bell, Sim D. Aberson, and Michael L. Black

This study is an observational analysis of the inner-core structure, sea surface temperature, outflow layer, and atmospheric boundary layer of an intense tropical cyclone whose intensity and structure is consistent with recent numerical and theoretical predictions of superintense storms. The findings suggest new scientific challenges for the current understanding of hurricanes.

Unprecedented observations of the category-5 Hurricane Isabel (2003) were collected during 12–14 September. This two-part article reports novel dynamic and thermodynamic aspects of the inner-core structure of Isabel on 13 September that were made possible by analysis of these data. Here, a composite of the axisymmetric structure of the inner core and environment of Isabel is estimated using global positioning system dropwindsondes and in situ aircraft data. In Part II, an extreme wind speed observation on the same day is discussed in the context of this work.

The axisymmetric data composite suggests a reservoir of high-entropy air inside the low-level eye and significant penetration of inflowing near-surface air from outside. The analysis suggests that the low-level air penetrating the eye is enhanced thermodynamically by acquiring additional entropy through interaction with the ocean and replaces air mixed out of the eye. The results support the hypothesis that this high-entropy eye air “turboboosts” the hurricane engine upon its injection into the eyewall clouds. Recent estimates of the ratio of sea-to-air enthalpy and momentum exchange at high wind speeds are used to suggest that Isabel utilized this extra power to exceed the previously assumed intensity upper bound by 10–35 m s−1 for the given environmental conditions. Additional study with other datasets is encouraged to further test the superintensity hypothesis.

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Robert W. Burpee, James L. Franklin, Stephen J. Lord, Robert E. Tuleya, and Sim D. Aberson

Since 1982, the Hurricane Research Division (HRD) has conducted a series of experiments with research aircraft to enhance the number of observations in the environment and the core of hurricanes threatening the United States. During these experiments, the National Oceanic and Atmospheric Administration WP-3D aircraft crews release Omega dropwindsondes (ODWs) at 15–20-min intervals along the flight track to obtain profiles of wind, temperature, and humidity between flight level and the sea surface. Data from the ODWs are transmitted back to the aircraft and then sent via satellite to the Tropical Prediction Center and the National Centers for Environmental Prediction (NCEP), where the observations become part of the operational database.

This paper tests the hypothesis that additional observations improve the objective track forecast models that provide operational guidance to the hurricane forecasters. The testing evaluates differences in forecast tracks from models run with and without the ODW data in a research mode at HRD, NCEP, and the Geophysical Fluid Dynamics Laboratory. The middle- and lower-tropospheric ODW data produce statistically significant reductions in 12–60-h mean forecast errors. The error reductions, which range from 16% to 30%, are at least as large as the accumulated improvement in operational forecasts achieved over the last 20–25 years. This breakthrough provides strong experimental evidence that more comprehensive observations in the hurricane environment and core will lead to immediate improvements in operational forecast guidance.

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Sim D. Aberson, Michael L. Black, Robert A. Black, Robert W. Burpee, Joseph J. Cione, Christopher W. Landsea, and Frank D. Marks Jr.

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 presents a brief overview of the instrumentation aboard the aircraft and the major research findings during this period.

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Robert F. Rogers, Sim Aberson, Michael M. Bell, Daniel J. Cecil, James D. Doyle, Todd B. Kimberlain, Josh Morgerman, Lynn K. Shay, and Christopher Velden

Abstract

Hurricane Patricia was a historic tropical cyclone that broke many records, such as intensification rate, peak intensity, and overwater weakening rate, during its brief 4-day lifetime in late October 2015 in the eastern Pacific basin. Patricia confounded all of the intensity forecast guidance owing to its rapid intensity changes. Fortunately, the hurricane-penetrating National Oceanic and Atmospheric Administration WP-3D and U.S. Air Force C-130 aircraft and the National Aeronautics and Space Administration WB-57 high-altitude jet, under support of the Office of Naval Research, conducted missions through and over Patricia prior to and during its extreme intensity changes on all 4 days, while an extensive array of pressure sensors sampled Patricia after landfall. The observations collected from these missions include traditional data sources such as airborne Doppler radar and flight-level instruments as well as new data sources like a high-density array of dropsondes released from high-altitude and wide-swath radiometer. The combination of data from these sources and from satellites provides an excellent opportunity to investigate the physical processes responsible for Patricia’s structure and evolution and offers the potential to improve forecasts of tropical cyclone rapid intensity changes. This paper provides an overview of Patricia as well as the data collected during the aircraft missions.

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Robert W. Burpee, Sim D. Aberson, Peter G. Black, Mark DeMaria, James L. Franklin, Joseph S. Griffin, Samuel H. Houston, John Kaplan, Stephen J. Lord, Frank D. Marks Jr., Mark D. Powell, and Hugh E. Willoughby

The Hurricane Research Division (HRD) is NOAA's primary component for research on tropical cyclones. In accomplishing research goals, many staff members have developed analysis procedures and forecast models that not only help improve the understanding of hurricane structure, motion, and intensity change, but also provide operational support for forecasters at the National Hurricane Center (NHC). During the 1993 hurricane season, HRD demonstrated three important real-time capabilities for the first time. These achievements included the successful transmission of a series of color radar reflectivity images from the NOAA research aircraft to NHC, the operational availability of objective mesoscale streamline and isotach analyses of a hurricane surface wind field, and the transition of the experimental dropwindsonde program on the periphery of hurricanes to a technology capable of supporting operational requirements. Examples of these and other real-time capabilities are presented for Hurricane Emily.

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