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Abstract
“Best tracks” are National Hurricane Center (NHC) poststorm analyses of the intensity, central pressure, position, and size of Atlantic and eastern North Pacific basin tropical and subtropical cyclones. This paper estimates the uncertainty (average error) for Atlantic basin best track parameters through a survey of the NHC Hurricane Specialists who maintain and update the Atlantic hurricane database. A comparison is then made with a survey conducted over a decade ago to qualitatively assess changes in the uncertainties. Finally, the implications of the uncertainty estimates for NHC analysis and forecast products as well as for the prediction goals of the Hurricane Forecast Improvement Program are discussed.
Abstract
“Best tracks” are National Hurricane Center (NHC) poststorm analyses of the intensity, central pressure, position, and size of Atlantic and eastern North Pacific basin tropical and subtropical cyclones. This paper estimates the uncertainty (average error) for Atlantic basin best track parameters through a survey of the NHC Hurricane Specialists who maintain and update the Atlantic hurricane database. A comparison is then made with a survey conducted over a decade ago to qualitatively assess changes in the uncertainties. Finally, the implications of the uncertainty estimates for NHC analysis and forecast products as well as for the prediction goals of the Hurricane Forecast Improvement Program are discussed.
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 hurricane's 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 operational practices at the National Hurricane Center for the interpretation of flight-level data. Hurricane wind profiles outside of the eyewall region are characterized by a higher level of maximum wind, near 1 km, and a more constant wind speed between 700 hPa and the top of the boundary layer. Two factors that likely affect the eyewall profile structure are wind speed and vertical motion. A minimum in surface wind adjustment factor (i.e., relatively low surface wind speeds) was found when the wind near the top of the boundary layer was between 40 and 60 m s−1. At higher wind speeds, the fraction of the boundary layer wind speed found at the surface increased, contrary to expectation. Low-level downdrafts, and enhanced vertical motion generally, were also associated with higher relative surface winds. These results may be of interest to engineers concerned with building codes, to emergency managers who may be tempted to use high-rise buildings as a “refuge of last resort” in coastal areas, and to those people on locally elevated terrain. The top of a 25-story coastal high-rise in the hurricane eyewall will experience a mean wind that is about 17% higher (or one Saffir–Simpson hurricane-scale category) than the surface or advisory value. For this reason, residents who must take refuge in coastal high-rises should generally do so at the lowest levels necessary to avoid storm surge.
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 hurricane's 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 operational practices at the National Hurricane Center for the interpretation of flight-level data. Hurricane wind profiles outside of the eyewall region are characterized by a higher level of maximum wind, near 1 km, and a more constant wind speed between 700 hPa and the top of the boundary layer. Two factors that likely affect the eyewall profile structure are wind speed and vertical motion. A minimum in surface wind adjustment factor (i.e., relatively low surface wind speeds) was found when the wind near the top of the boundary layer was between 40 and 60 m s−1. At higher wind speeds, the fraction of the boundary layer wind speed found at the surface increased, contrary to expectation. Low-level downdrafts, and enhanced vertical motion generally, were also associated with higher relative surface winds. These results may be of interest to engineers concerned with building codes, to emergency managers who may be tempted to use high-rise buildings as a “refuge of last resort” in coastal areas, and to those people on locally elevated terrain. The top of a 25-story coastal high-rise in the hurricane eyewall will experience a mean wind that is about 17% higher (or one Saffir–Simpson hurricane-scale category) than the surface or advisory value. For this reason, residents who must take refuge in coastal high-rises should generally do so at the lowest levels necessary to avoid storm surge.
Abstract
A one-dimensional local spline smoothing technique is applied to Omega navigational signals for the purpose of windfinding. Wind profiles so produced depend largely on two parameters of the smoothing procedure: the nodal spacing, which determines the smallest resolvable scale, and a filtering wavelength, which produces the necessary smoothing of the phase data, and prevents representational distortion of any power from the unresolved scales. Phase “noise” from stationary test sondes is superimposed on synthetic Omega signals to compare wind profiles obtained with this new procedure with profiles computed using other techniques.
Is it shown that the effect of aircraft maneuvers on Omega wind accuracy is not completely removed by the normal practice of evaluating all phase derivatives at a common time. Additional improvements in accuracy of 2–3 m s−1 can be obtained by a “rate-aiding” technique using aircraft navigational data.
Abstract
A one-dimensional local spline smoothing technique is applied to Omega navigational signals for the purpose of windfinding. Wind profiles so produced depend largely on two parameters of the smoothing procedure: the nodal spacing, which determines the smallest resolvable scale, and a filtering wavelength, which produces the necessary smoothing of the phase data, and prevents representational distortion of any power from the unresolved scales. Phase “noise” from stationary test sondes is superimposed on synthetic Omega signals to compare wind profiles obtained with this new procedure with profiles computed using other techniques.
Is it shown that the effect of aircraft maneuvers on Omega wind accuracy is not completely removed by the normal practice of evaluating all phase derivatives at a common time. Additional improvements in accuracy of 2–3 m s−1 can be obtained by a “rate-aiding” technique using aircraft navigational data.
Previous studies have identified statistically significant long-term improvements in forecasts issued by the National Hurricane Center (NHC) for Atlantic basin tropical cyclones. Recently, however, attention has been focused on the forecast accuracy of landfall location and timing, and the long-term improvement trends for this relatively small sample of forecasts were mixed. These results may lead some to conclude that the accuracy of NHC forecasts close to the United States has not improved over time.
A statistically robust dataset can be obtained by considering “landfall-threatening” storms, defined as one for which tropical cyclone watches or warnings are in effect for some portion of the continental United States. In this study, long-term trends in accuracy are determined for NHC forecasts issued during these periods of threat and compared to trends for the Atlantic basin overall. A second set of trends are determined for forecasts verifying during the periods of threat.
The analysis shows that NHC forecasts for land-threatening tropical cyclones are improving. These improvement trends are statistically significant, although the improvement rates for the land-threatening storms are smaller than those for the basin overall. Over the period 1970–2001, forecasts issued during the watch/warning stage improved at annual average rates of 0.7%, 1.6%, and 1.9% at 24,48, and 72 h, respectively.
Previous studies have identified statistically significant long-term improvements in forecasts issued by the National Hurricane Center (NHC) for Atlantic basin tropical cyclones. Recently, however, attention has been focused on the forecast accuracy of landfall location and timing, and the long-term improvement trends for this relatively small sample of forecasts were mixed. These results may lead some to conclude that the accuracy of NHC forecasts close to the United States has not improved over time.
A statistically robust dataset can be obtained by considering “landfall-threatening” storms, defined as one for which tropical cyclone watches or warnings are in effect for some portion of the continental United States. In this study, long-term trends in accuracy are determined for NHC forecasts issued during these periods of threat and compared to trends for the Atlantic basin overall. A second set of trends are determined for forecasts verifying during the periods of threat.
The analysis shows that NHC forecasts for land-threatening tropical cyclones are improving. These improvement trends are statistically significant, although the improvement rates for the land-threatening storms are smaller than those for the basin overall. Over the period 1970–2001, forecasts issued during the watch/warning stage improved at annual average rates of 0.7%, 1.6%, and 1.9% at 24,48, and 72 h, respectively.
Abstract
One hundred and thirty Omega dropwindsondes deployed within 500-km radius of the eye of six North Atlantic hurricanes are used to determine the magnitudes and trends in convective available potential energy, and 10–1500-m and 0–6-km shear of the horizontal wind as a function of radius, quadrant, and hurricane intensity.
The moist convective instability found at large radii (400–500 km) decreases to near neutral stability by 75 km from the eyewall. Vertical shears increase as radius decreases, but maximum shear values are only one-half of those found over land. Scatter for both the conditional instability and the shear is influenced chiefly by hurricane intensity, but proximity to reflectivity features does modulate the pattern. The ratio of the conditional instability to the shear (bulk Richardson number) indicates that supercell formation is favored within 250 km of the circulation center, but helicity values are below the threshold to support strong waterspouts.
The difference between these oceanic observations and those made over land by other researchers is evidence for significant modification of the vertical profile of the horizontal wind in a hurricane at landfall.
Abstract
One hundred and thirty Omega dropwindsondes deployed within 500-km radius of the eye of six North Atlantic hurricanes are used to determine the magnitudes and trends in convective available potential energy, and 10–1500-m and 0–6-km shear of the horizontal wind as a function of radius, quadrant, and hurricane intensity.
The moist convective instability found at large radii (400–500 km) decreases to near neutral stability by 75 km from the eyewall. Vertical shears increase as radius decreases, but maximum shear values are only one-half of those found over land. Scatter for both the conditional instability and the shear is influenced chiefly by hurricane intensity, but proximity to reflectivity features does modulate the pattern. The ratio of the conditional instability to the shear (bulk Richardson number) indicates that supercell formation is favored within 250 km of the circulation center, but helicity values are below the threshold to support strong waterspouts.
The difference between these oceanic observations and those made over land by other researchers is evidence for significant modification of the vertical profile of the horizontal wind in a hurricane at landfall.
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
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.
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.
