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Abstract
While qualitative information from meteorological satellites has long been recognized as critical for monitoring tropical cyclone activity, quantitative data are required to improve the objective analysis and numerical weather prediction of these events. In this paper, results are presented that show that the inclusion of high-density, multispectral, satellite-derived information into the analysis of tropical cyclone environmental wind fields can effectively reduce the error of objective track forecasts. Two independent analysis and barotropic track-forecast systems are utilized in order to examine the consistency of the results. Both systems yield a 10%–23% reduction in middle- to long-range track-forecast errors with the inclusion of the satellite wind observations.
Abstract
While qualitative information from meteorological satellites has long been recognized as critical for monitoring tropical cyclone activity, quantitative data are required to improve the objective analysis and numerical weather prediction of these events. In this paper, results are presented that show that the inclusion of high-density, multispectral, satellite-derived information into the analysis of tropical cyclone environmental wind fields can effectively reduce the error of objective track forecasts. Two independent analysis and barotropic track-forecast systems are utilized in order to examine the consistency of the results. Both systems yield a 10%–23% reduction in middle- to long-range track-forecast errors with the inclusion of the satellite wind observations.
Abstract
The 2008 Atlantic hurricane season is summarized and the year’s tropical cyclones are described. Sixteen named storms formed in 2008. Of these, eight became hurricanes with five of them strengthening into major hurricanes (category 3 or higher on the Saffir–Simpson hurricane scale). There was also one tropical depression that did not attain tropical storm strength. These totals are above the long-term means of 11 named storms, 6 hurricanes, and 2 major hurricanes. The 2008 Atlantic basin tropical cyclones produced significant impacts from the Greater Antilles to the Turks and Caicos Islands as well as along portions of the U.S. Gulf Coast. Hurricanes Gustav, Ike, and Paloma hit Cuba, as did Tropical Storm Fay. Haiti was hit by Gustav and adversely affected by heavy rains from Fay, Ike, and Hanna. Paloma struck the Cayman Islands as a major hurricane, while Omar was a major hurricane when it passed near the northern Leeward Islands. Six consecutive cyclones hit the United States, including Hurricanes Dolly, Gustav, and Ike. The death toll from the Atlantic tropical cyclones is approximately 750.
A verification of National Hurricane Center official forecasts during 2008 is also presented. Official track forecasts set records for accuracy at all lead times from 12 to 120 h, and forecast skill was also at record levels for all lead times. Official intensity forecast errors in 2008 were below the previous 5-yr mean errors and set records at 72–120 h.
Abstract
The 2008 Atlantic hurricane season is summarized and the year’s tropical cyclones are described. Sixteen named storms formed in 2008. Of these, eight became hurricanes with five of them strengthening into major hurricanes (category 3 or higher on the Saffir–Simpson hurricane scale). There was also one tropical depression that did not attain tropical storm strength. These totals are above the long-term means of 11 named storms, 6 hurricanes, and 2 major hurricanes. The 2008 Atlantic basin tropical cyclones produced significant impacts from the Greater Antilles to the Turks and Caicos Islands as well as along portions of the U.S. Gulf Coast. Hurricanes Gustav, Ike, and Paloma hit Cuba, as did Tropical Storm Fay. Haiti was hit by Gustav and adversely affected by heavy rains from Fay, Ike, and Hanna. Paloma struck the Cayman Islands as a major hurricane, while Omar was a major hurricane when it passed near the northern Leeward Islands. Six consecutive cyclones hit the United States, including Hurricanes Dolly, Gustav, and Ike. The death toll from the Atlantic tropical cyclones is approximately 750.
A verification of National Hurricane Center official forecasts during 2008 is also presented. Official track forecasts set records for accuracy at all lead times from 12 to 120 h, and forecast skill was also at record levels for all lead times. Official intensity forecast errors in 2008 were below the previous 5-yr mean errors and set records at 72–120 h.
Abstract
In 1982, the National Oceanic and Atmospheric Administration's Hurricane Research Division began a series of experiments to collect Omega dropwindsonde (ODW) observations within about 1000 km of the center of tropical cyclones. By 1992, 16 ODW datasets had been collected in 10 Atlantic basin hurricanes and tropical storms. Objective wind analyses for each dataset 10 levels from 100 mb to the surface, have been produced using a consistent set of analysis parameters. The objective analyses, which resolve synoptic-scale features in the storm environment with an accuracy and confidence unattainable from routine operational analyses, have been used to examine relationships between a tropical cyclone's motion and its surrounding synoptic-scale flow.
Tropical cyclone motion is found to be consistent with barotropic steering of the vortex by the surrounding flow within 3° latitude (333 km) of the cyclone center. At this radius, the surrounding deep-layer-mean flow explains over 90% of the variance in vortex motion. The analyses show vorticity asymmetries that strongly resemble the β gyres common to barotropic models, although other synoptic features in the environment make isolation of these gyres from the wind fields difficult. A reasonably strong relationship is found between the motion of the vortex (relative to its large scale surrounding flow) and the absolute vorticity gradient of the vortex environment.
Abstract
In 1982, the National Oceanic and Atmospheric Administration's Hurricane Research Division began a series of experiments to collect Omega dropwindsonde (ODW) observations within about 1000 km of the center of tropical cyclones. By 1992, 16 ODW datasets had been collected in 10 Atlantic basin hurricanes and tropical storms. Objective wind analyses for each dataset 10 levels from 100 mb to the surface, have been produced using a consistent set of analysis parameters. The objective analyses, which resolve synoptic-scale features in the storm environment with an accuracy and confidence unattainable from routine operational analyses, have been used to examine relationships between a tropical cyclone's motion and its surrounding synoptic-scale flow.
Tropical cyclone motion is found to be consistent with barotropic steering of the vortex by the surrounding flow within 3° latitude (333 km) of the cyclone center. At this radius, the surrounding deep-layer-mean flow explains over 90% of the variance in vortex motion. The analyses show vorticity asymmetries that strongly resemble the β gyres common to barotropic models, although other synoptic features in the environment make isolation of these gyres from the wind fields difficult. A reasonably strong relationship is found between the motion of the vortex (relative to its large scale surrounding flow) and the absolute vorticity gradient of the vortex environment.
Abstract
A set of three-dimensional, filtered, multiply nested objective analyses has been completed for the wind field of Hurricane Gloria for 0000 UTC 25 September 1985. At this time Gloria was one of the most intense hurricanes ever observed in the Atlantic basin, with a minimum sea level pressure of 919 mb. The nested analyses, based on observations from airborne Doppler radar and Omega dropwindsondes, simultaneously describe eyewall and synoptic-scale features, and are the most comprehensive analyses of a single hurricane constructed to date. The analyses have been used to document the multiscale kinematic structure of Gloria and to investigate the relationship between the kinematic fields and the motion of the vortex.
The analyses indicate that the vortex was unusually barotropic. The radius of maximum wind (RMW) was nearly vertical below 500 mb, with a slight inward slope with height between 750 and 550 mb. The strongest azimuthal mean tangential winds were found well above the boundary layer, near 550 mb, where the RMW was smallest. We speculate that this unusual structure was associated with a concentric eye cycle. A persistent asymmetry in the distribution of eyewall convection was associated with the vertical shear of the environmental flow.
The vortex moved approximately 2.5 m s−1 faster than the deep layer mean flow averaged at 667-km radius from the center. Barotropic models have predicted a relationship between the relative motion of the vortex and the gradients of absolute vorticity in the cyclone's environment; however, the predicted relationship was not found for Gloria. The vortex also did not move with the mean flow in the immediate vicinity of the center; the motion of the hurricane was most consistent with the 300–850-mb layer mean flow well outside the eyewall, at a radius of 65 km. The analyses suggest that the environmental flow near the center had been distorted by eyewall convection, with the scale of the distortion determined by the local Rossby radius of deformation.
Abstract
A set of three-dimensional, filtered, multiply nested objective analyses has been completed for the wind field of Hurricane Gloria for 0000 UTC 25 September 1985. At this time Gloria was one of the most intense hurricanes ever observed in the Atlantic basin, with a minimum sea level pressure of 919 mb. The nested analyses, based on observations from airborne Doppler radar and Omega dropwindsondes, simultaneously describe eyewall and synoptic-scale features, and are the most comprehensive analyses of a single hurricane constructed to date. The analyses have been used to document the multiscale kinematic structure of Gloria and to investigate the relationship between the kinematic fields and the motion of the vortex.
The analyses indicate that the vortex was unusually barotropic. The radius of maximum wind (RMW) was nearly vertical below 500 mb, with a slight inward slope with height between 750 and 550 mb. The strongest azimuthal mean tangential winds were found well above the boundary layer, near 550 mb, where the RMW was smallest. We speculate that this unusual structure was associated with a concentric eye cycle. A persistent asymmetry in the distribution of eyewall convection was associated with the vertical shear of the environmental flow.
The vortex moved approximately 2.5 m s−1 faster than the deep layer mean flow averaged at 667-km radius from the center. Barotropic models have predicted a relationship between the relative motion of the vortex and the gradients of absolute vorticity in the cyclone's environment; however, the predicted relationship was not found for Gloria. The vortex also did not move with the mean flow in the immediate vicinity of the center; the motion of the hurricane was most consistent with the 300–850-mb layer mean flow well outside the eyewall, at a radius of 65 km. The analyses suggest that the environmental flow near the center had been distorted by eyewall convection, with the scale of the distortion determined by the local Rossby radius of deformation.
Abstract
Omega dropwindsonde and other in situ (INS) data collected during the NOAA/Hurricane Research Division's (HRD) Hurricane Field Program are used as a ground truth dataset for the evaluation of VISSR Atmospheric Sounder (VAS) soundings over the subtropical Atlantic. The experiments were coordinated with the Cooperative Institute for Meteorological Satellite Services at the University of Wisconsin. The focus of this study is to determine whether soundings derived from VAS radiances are an improvement over the first-guess data used as a starting point in the sounding retrieval process. First guess inputs for this study are provided by NMCs Regional Analysis and Forecast System (RAFS) nested grid model (NGM).
In a case study, an objective algorithm is used to analyze the INS, VAS, and first-guess data at and below 500 mb from an HRD experiment on 1–2 September 1988. The case study is supplemented by a statistical investigation of data composited from other HRD experiments. In particular, we examine VAS estimates of horizontal temperature and moisture gradients to see if they represent improvements over the first guess.
The temperature and moisture descriptions in the vicinity of a 500 mb cold low were improved by the VAS in the case study; however, VAS temperature gradients were found to be generally less accurate than those of the first guess. Temperature gradients from the VAS were also consistently stronger than INS or first-guess gradients. The composite study found that large-scale VAS moisture gradients were better than those of the first guess. Other results indicate a preferred mode for VAS modifications to the guess: the primary impact of the VAS radiances on the first guess was to improve the description of the phasing of horizontal features. The VAS representation of the amplitude of features, however, was not consistently an improvement. This suggests that in tropical applications, VAS data may be most suitable for subjective forecasting uses; if VAS data are to be used in numerical weather prediction, strongest weight should be given to the representation of the location of weather features (troughs, ridges, etc.), and relatively weak weight should be given to the representation of the strength of these features.
Abstract
Omega dropwindsonde and other in situ (INS) data collected during the NOAA/Hurricane Research Division's (HRD) Hurricane Field Program are used as a ground truth dataset for the evaluation of VISSR Atmospheric Sounder (VAS) soundings over the subtropical Atlantic. The experiments were coordinated with the Cooperative Institute for Meteorological Satellite Services at the University of Wisconsin. The focus of this study is to determine whether soundings derived from VAS radiances are an improvement over the first-guess data used as a starting point in the sounding retrieval process. First guess inputs for this study are provided by NMCs Regional Analysis and Forecast System (RAFS) nested grid model (NGM).
In a case study, an objective algorithm is used to analyze the INS, VAS, and first-guess data at and below 500 mb from an HRD experiment on 1–2 September 1988. The case study is supplemented by a statistical investigation of data composited from other HRD experiments. In particular, we examine VAS estimates of horizontal temperature and moisture gradients to see if they represent improvements over the first guess.
The temperature and moisture descriptions in the vicinity of a 500 mb cold low were improved by the VAS in the case study; however, VAS temperature gradients were found to be generally less accurate than those of the first guess. Temperature gradients from the VAS were also consistently stronger than INS or first-guess gradients. The composite study found that large-scale VAS moisture gradients were better than those of the first guess. Other results indicate a preferred mode for VAS modifications to the guess: the primary impact of the VAS radiances on the first guess was to improve the description of the phasing of horizontal features. The VAS representation of the amplitude of features, however, was not consistently an improvement. This suggests that in tropical applications, VAS data may be most suitable for subjective forecasting uses; if VAS data are to be used in numerical weather prediction, strongest weight should be given to the representation of the location of weather features (troughs, ridges, etc.), and relatively weak weight should be given to the representation of the strength of these features.
Abstract
The Hurricane Research Division has developed a technique for real-time airborne analysis of aircraft data from reconnaissance and research flights in tropical cyclones. The technique uses an onboard workstation that analyzes flight-level observations, radar reflectivity patterns, radial Doppler velocities, and vertical soundings from Omega dropwindsondes (ODWs).
Many of the workstation analyses are in storm-relative coordinates that depend upon interactive identification of the cyclone center from the radar reflectivity data. Displays of the lower fuselage radar reflectivity, composited for 1–2 h, provide an overall perspective of the horizontal patterns of precipitation and a framework for interpretation of thermodynamic and kinematic observations. The workstation runs algorithms for estimation of the horizontal wind field in the hurricane core using radial velocities measured by the airborne Doppler radar during one or more penetrations of the storm center. Interactive software also supports real-time processing of ODW wind and thermodynamic data, objective editing of bad data, and automatic dissemination of mandatory and significant-level data in the standard dropwindsonde code. Similarly processed ODWs have been consistently shown to reduce forecast errors of hurricane track in several objective models used by the forecasters at NHC.
Plans for the 1992 hurricane season include the transmission of subsets of the data to the National Hurricane Center (NHC) through the Geostationary Operational Environmental Satellite (GOES) communications system and the display of the aircraft analyses for the forecasters at NHC. With the implementation of these plans, NHC will receive two-dimensional analyses of the mesoscale precipitation and wind structure of the storm core and more frequent estimates of the location and recent motion of tropical cyclones. The information will enable forecasters to take advantage of recent advances in the understanding of hurricane-intensity change.
Abstract
The Hurricane Research Division has developed a technique for real-time airborne analysis of aircraft data from reconnaissance and research flights in tropical cyclones. The technique uses an onboard workstation that analyzes flight-level observations, radar reflectivity patterns, radial Doppler velocities, and vertical soundings from Omega dropwindsondes (ODWs).
Many of the workstation analyses are in storm-relative coordinates that depend upon interactive identification of the cyclone center from the radar reflectivity data. Displays of the lower fuselage radar reflectivity, composited for 1–2 h, provide an overall perspective of the horizontal patterns of precipitation and a framework for interpretation of thermodynamic and kinematic observations. The workstation runs algorithms for estimation of the horizontal wind field in the hurricane core using radial velocities measured by the airborne Doppler radar during one or more penetrations of the storm center. Interactive software also supports real-time processing of ODW wind and thermodynamic data, objective editing of bad data, and automatic dissemination of mandatory and significant-level data in the standard dropwindsonde code. Similarly processed ODWs have been consistently shown to reduce forecast errors of hurricane track in several objective models used by the forecasters at NHC.
Plans for the 1992 hurricane season include the transmission of subsets of the data to the National Hurricane Center (NHC) through the Geostationary Operational Environmental Satellite (GOES) communications system and the display of the aircraft analyses for the forecasters at NHC. With the implementation of these plans, NHC will receive two-dimensional analyses of the mesoscale precipitation and wind structure of the storm core and more frequent estimates of the location and recent motion of tropical cyclones. The information will enable forecasters to take advantage of recent advances in the understanding of hurricane-intensity change.
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.
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.
The Joint Hurricane Testbed (JHT) is reviewed at the completion of its first decade. Views of the program by hurricane forecasters at the National Hurricane Center, the test bed's impact on forecast accuracy, and highlights of the top-rated projects are presented. Key concerns encountered by the test bed are identified as possible “lessons learned” for future research-to-operations efforts. The paper concludes with thoughts on the potential changing role of the JHT.
The Joint Hurricane Testbed (JHT) is reviewed at the completion of its first decade. Views of the program by hurricane forecasters at the National Hurricane Center, the test bed's impact on forecast accuracy, and highlights of the top-rated projects are presented. Key concerns encountered by the test bed are identified as possible “lessons learned” for future research-to-operations efforts. The paper concludes with thoughts on the potential changing role of the JHT.
Abstract
Consensus forecasts (forecasts created by combining output from individual forecasts) have become an integral part of operational tropical cyclone track forecasting. Consensus aids, which generally have lower average errors than individual models, benefit from the skill and independence of the consensus members, both of which are present in track forecasting, but are limited in intensity forecasting. This study conducts experiments with intensity forecast aids on 4 yr of data (2003–06). First, the skill of the models is assessed; then simple consensus computations are constructed for the Atlantic, eastern North Pacific, and western North Pacific basins. A simple (i.e., equally weighted) consensus of three top-performing intensity forecast models is found to generally outperform the individual members in both the Atlantic and eastern North Pacific, and a simple consensus of two top-performing intensity forecast models is found to generally outperform the individual members in the western North Pacific.
An experiment using an ensemble of dynamical model track forecasts and a selection of model fields as input in a statistical–dynamical intensity forecast model to produce intensity consensus members is conducted for the western North Pacific only. Consensus member skill at 72 h is low (−0.4% to 14.2%), and there is little independence among the members. This experiment demonstrates that a consensus of these highly dependent members yields an aid that performs as well as the most skillful member. Finally, adding a less skillful, but more independent, dynamical model-based forecast aid to the consensus yields an 11-member consensus with mixed yet promising performance compared with the 10-model consensus.
Based on these findings, the simple three-member consensus model could be used as a standard of comparison for other deterministic ensemble methods for the Atlantic and eastern North Pacific. Both the two- and three-member consensus forecasts may also provide useful guidance for operational forecasters. Likewise, in the western North Pacific, the 10- and 11-member consensus could be used as operational forecast aids and standards of comparison for other ensemble intensity forecast methods.
Abstract
Consensus forecasts (forecasts created by combining output from individual forecasts) have become an integral part of operational tropical cyclone track forecasting. Consensus aids, which generally have lower average errors than individual models, benefit from the skill and independence of the consensus members, both of which are present in track forecasting, but are limited in intensity forecasting. This study conducts experiments with intensity forecast aids on 4 yr of data (2003–06). First, the skill of the models is assessed; then simple consensus computations are constructed for the Atlantic, eastern North Pacific, and western North Pacific basins. A simple (i.e., equally weighted) consensus of three top-performing intensity forecast models is found to generally outperform the individual members in both the Atlantic and eastern North Pacific, and a simple consensus of two top-performing intensity forecast models is found to generally outperform the individual members in the western North Pacific.
An experiment using an ensemble of dynamical model track forecasts and a selection of model fields as input in a statistical–dynamical intensity forecast model to produce intensity consensus members is conducted for the western North Pacific only. Consensus member skill at 72 h is low (−0.4% to 14.2%), and there is little independence among the members. This experiment demonstrates that a consensus of these highly dependent members yields an aid that performs as well as the most skillful member. Finally, adding a less skillful, but more independent, dynamical model-based forecast aid to the consensus yields an 11-member consensus with mixed yet promising performance compared with the 10-model consensus.
Based on these findings, the simple three-member consensus model could be used as a standard of comparison for other deterministic ensemble methods for the Atlantic and eastern North Pacific. Both the two- and three-member consensus forecasts may also provide useful guidance for operational forecasters. Likewise, in the western North Pacific, the 10- and 11-member consensus could be used as operational forecast aids and standards of comparison for other ensemble intensity forecast methods.
Abstract
The 1999 Atlantic basin hurricane season produced 4 tropical storms and 8 hurricanes for a total of 12 named tropical cyclones. Seven of these affected land. Hurricane Floyd—the deadliest U.S. hurricane since Agnes in 1972—caused a disastrous flood event over the U.S. mid-Atlantic and northeastern coastal states, resulting in 56 U.S. deaths and 1 death in the Bahamas. Heavy rain from a tropical depression contributed to some 400 inland flood deaths in Mexico.
Abstract
The 1999 Atlantic basin hurricane season produced 4 tropical storms and 8 hurricanes for a total of 12 named tropical cyclones. Seven of these affected land. Hurricane Floyd—the deadliest U.S. hurricane since Agnes in 1972—caused a disastrous flood event over the U.S. mid-Atlantic and northeastern coastal states, resulting in 56 U.S. deaths and 1 death in the Bahamas. Heavy rain from a tropical depression contributed to some 400 inland flood deaths in Mexico.
