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Abstract
The ground-based velocity track display (GBVTD) was developed to deduce a three-dimensional primary circulation of landfalling tropical cyclones from single-Doppler radar data. However, the cross-beam component of the mean wind 
Abstract
The ground-based velocity track display (GBVTD) was developed to deduce a three-dimensional primary circulation of landfalling tropical cyclones from single-Doppler radar data. However, the cross-beam component of the mean wind
Abstract
Two sets of equations are derived to 1) map airborne Doppler radar data from an aircraft-relative coordinate system to an earth-relative coordinate system, and 2) remove the platform motion from the observed Doppler velocities. These equations can be applied to data collected by the National Oceanic and Atmospheric Administration WP-3D system, the National Center for Atmospheric Research ELDORA system, and other airborne radar systems.
Abstract
Two sets of equations are derived to 1) map airborne Doppler radar data from an aircraft-relative coordinate system to an earth-relative coordinate system, and 2) remove the platform motion from the observed Doppler velocities. These equations can be applied to data collected by the National Oceanic and Atmospheric Administration WP-3D system, the National Center for Atmospheric Research ELDORA system, and other airborne radar systems.
Abstract
The ground-based velocity track display (GBVTD) technique was developed to estimate the primary circulations of landfalling tropical cyclones (TCs) from single-Doppler radar data. However, GBVTD cannot process aliased Doppler velocities, which are often encountered in intense TCs. This study presents a new gradient velocity track display (GrVTD) algorithm that is essentially immune to the Doppler velocity aliasing. GrVTD applies the concept of gradient velocity–azimuth display (GVAD) to the GBVTD method. A GrVTD-simplex algorithm is also developed to accompany GrVTD as a self-sufficient algorithm suite.
The results from idealized experiments demonstrate that the circulation center and winds retrieved from GrVTD with aliased velocity and GBVTD with dealiased velocity are in good agreement, but GrVTD is more sensitive to random observation errors. GrVTD was applied to Hurricane Charley (2004) where the majority of the Doppler velocities of the inner-core region were aliased. The GrVTD-retrieved circulation pattern and magnitude are nearly identical to those retrieved in GBVTD with manually dealiased velocities. Overall, the performance of GrVTD is comparable but is more sensitive to the data distribution than that of the original GBVTD using dealiased velocity. GrVTD can be used as a preprocessor for dealiasing velocity in TCs before the data are used in GBVTD or other algorithms.
Abstract
The ground-based velocity track display (GBVTD) technique was developed to estimate the primary circulations of landfalling tropical cyclones (TCs) from single-Doppler radar data. However, GBVTD cannot process aliased Doppler velocities, which are often encountered in intense TCs. This study presents a new gradient velocity track display (GrVTD) algorithm that is essentially immune to the Doppler velocity aliasing. GrVTD applies the concept of gradient velocity–azimuth display (GVAD) to the GBVTD method. A GrVTD-simplex algorithm is also developed to accompany GrVTD as a self-sufficient algorithm suite.
The results from idealized experiments demonstrate that the circulation center and winds retrieved from GrVTD with aliased velocity and GBVTD with dealiased velocity are in good agreement, but GrVTD is more sensitive to random observation errors. GrVTD was applied to Hurricane Charley (2004) where the majority of the Doppler velocities of the inner-core region were aliased. The GrVTD-retrieved circulation pattern and magnitude are nearly identical to those retrieved in GBVTD with manually dealiased velocities. Overall, the performance of GrVTD is comparable but is more sensitive to the data distribution than that of the original GBVTD using dealiased velocity. GrVTD can be used as a preprocessor for dealiasing velocity in TCs before the data are used in GBVTD or other algorithms.
Airborne radar platforms have played an increasingly important role in advancing our understanding of storms that are either too remote or occur too infrequently for ground-based radars. Recognizing this potential, the National Center for Atmospheric Research and the Centre de Recherche en Physique de l'Environnment Terrestre et Planetaire have designed and developed ELDORA (Electra Doppler Radar), a tail Doppler radar with the following unique capabilities: 1) increased accuracy and sensitivity by averaging more independent samples in the radar pulse volume, 2) higher spatial resolution in the along-track direction by using a faster rotation rate of the antenna, and 3) a large unambiguous velocity measuring interval by means of a dual pulse repetition frequency. Although the first field deployment of ELDORA occurred in 1993, it was not until VORTEX (Verification of the Origins of Rotation in Tornadoes Experiment) that the system was operating up to its full capabilities. Examples of the radar's ability to detect clear-air phenomena are presented along with high-resolution images near severe local storms and tornadoes.
Airborne radar platforms have played an increasingly important role in advancing our understanding of storms that are either too remote or occur too infrequently for ground-based radars. Recognizing this potential, the National Center for Atmospheric Research and the Centre de Recherche en Physique de l'Environnment Terrestre et Planetaire have designed and developed ELDORA (Electra Doppler Radar), a tail Doppler radar with the following unique capabilities: 1) increased accuracy and sensitivity by averaging more independent samples in the radar pulse volume, 2) higher spatial resolution in the along-track direction by using a faster rotation rate of the antenna, and 3) a large unambiguous velocity measuring interval by means of a dual pulse repetition frequency. Although the first field deployment of ELDORA occurred in 1993, it was not until VORTEX (Verification of the Origins of Rotation in Tornadoes Experiment) that the system was operating up to its full capabilities. Examples of the radar's ability to detect clear-air phenomena are presented along with high-resolution images near severe local storms and tornadoes.
Abstract
Airborne Doppler radar captured the inner core of Hurricane Earl during the early stages of secondary eyewall formation (SEF), providing needed insight into the SEF dynamics. An organized rainband complex outside of the primary eyewall transitioned into an axisymmetric secondary eyewall containing a low-level tangential wind maximum. During this transition, the downshear-left quadrant of the storm exhibited several notable features. A mesoscale descending inflow (MDI) jet persistently occurred across broad stretches of stratiform precipitation in a pattern similar to previous studies. This negatively buoyant jet traveled radially inward and descended into the boundary layer. Farther inward, enhanced low-level inflow and intense updrafts appeared. The updraft adjacent to the MDI was likely triggered by a region of convergence and upward acceleration (induced by the negatively buoyant MDI) entering the high-θe boundary layer. This updraft and the MDI in the downshear-left quadrant accelerated the tangential winds in a radial range where the axisymmetric wind maximum of the secondary eyewall soon developed. This same quadrant eventually exhibited the strongest overturning circulation and wind maximum of the forming secondary eyewall. Given these features occurring in succession in the downshear-left quadrant, we hypothesize that the MDI plays a significant dynamical role in SEF. The MDI within a mature rainband complex persistently perturbs the boundary layer, which locally forces enhanced convection and tangential winds. These perturbations provide steady low-level forcing that projects strongly onto the axisymmetric field, and forges the way for secondary eyewall development via one of several SEF theories that invoke axisymmetric dynamical processes.
Abstract
Airborne Doppler radar captured the inner core of Hurricane Earl during the early stages of secondary eyewall formation (SEF), providing needed insight into the SEF dynamics. An organized rainband complex outside of the primary eyewall transitioned into an axisymmetric secondary eyewall containing a low-level tangential wind maximum. During this transition, the downshear-left quadrant of the storm exhibited several notable features. A mesoscale descending inflow (MDI) jet persistently occurred across broad stretches of stratiform precipitation in a pattern similar to previous studies. This negatively buoyant jet traveled radially inward and descended into the boundary layer. Farther inward, enhanced low-level inflow and intense updrafts appeared. The updraft adjacent to the MDI was likely triggered by a region of convergence and upward acceleration (induced by the negatively buoyant MDI) entering the high-θe boundary layer. This updraft and the MDI in the downshear-left quadrant accelerated the tangential winds in a radial range where the axisymmetric wind maximum of the secondary eyewall soon developed. This same quadrant eventually exhibited the strongest overturning circulation and wind maximum of the forming secondary eyewall. Given these features occurring in succession in the downshear-left quadrant, we hypothesize that the MDI plays a significant dynamical role in SEF. The MDI within a mature rainband complex persistently perturbs the boundary layer, which locally forces enhanced convection and tangential winds. These perturbations provide steady low-level forcing that projects strongly onto the axisymmetric field, and forges the way for secondary eyewall development via one of several SEF theories that invoke axisymmetric dynamical processes.
Abstract
Strong tropical cyclones often undergo eyewall replacement cycles that are accompanied by concentric eyewalls and/or rapid intensity changes while the secondary eyewall contracts radially inward and eventually replaces the inner eyewall. To the best of our knowledge, the only documented partial/incomplete tertiary eyewall has been mostly inferred from two-dimensional satellite images or one-dimensional aircraft flight-level measurements that can be regarded as indirect and tangential. This study presents the first high spatial and temporal resolution Doppler radar observations of a tertiary eyewall formation event in Typhoon Usagi (2013) over a 14-h time period before it makes landfall. The primary (tangential) and secondary (radial) circulations of Usagi deduced from the Ground-Based Velocity Track Display (GBVTD) methodology clearly portrayed three distinct axisymmetric maxima of radar reflectivity, tangential wind, vertical velocity, and vertical vorticity. Usagi’s central pressure steadily deepened during the contraction of the secondary and tertiary eyewalls until the tertiary eyewall hit the coast of southeast China, which erminated the intensification of the storm.
Abstract
Strong tropical cyclones often undergo eyewall replacement cycles that are accompanied by concentric eyewalls and/or rapid intensity changes while the secondary eyewall contracts radially inward and eventually replaces the inner eyewall. To the best of our knowledge, the only documented partial/incomplete tertiary eyewall has been mostly inferred from two-dimensional satellite images or one-dimensional aircraft flight-level measurements that can be regarded as indirect and tangential. This study presents the first high spatial and temporal resolution Doppler radar observations of a tertiary eyewall formation event in Typhoon Usagi (2013) over a 14-h time period before it makes landfall. The primary (tangential) and secondary (radial) circulations of Usagi deduced from the Ground-Based Velocity Track Display (GBVTD) methodology clearly portrayed three distinct axisymmetric maxima of radar reflectivity, tangential wind, vertical velocity, and vertical vorticity. Usagi’s central pressure steadily deepened during the contraction of the secondary and tertiary eyewalls until the tertiary eyewall hit the coast of southeast China, which erminated the intensification of the storm.
Abstract
A ground-based velocity track display (GBVTD) analysis of the LaGrange, Wyoming, tornado on 5 June 2009 during the Second Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX2) is photogrammetrically combined with a series of pictures of the funnel cloud. This analysis reveals the relationship between the vertical velocity, radial and tangential velocities, perturbation pressure, vertical vorticity, and angular momentum with the visual features of the tornado. An intense axial downdraft was evident and was supported by a downward-directed perturbation pressure gradient. The radial inflow at low levels was weak and difficult to retrieve owing to a combination of centrifuging of hydrometeors/debris in the intense circulation and the inability of the radar beam to fully resolve the flow. The tornado was weakening during the analysis period, which was supported by angular momentum being advected out of the tornado.
The availability of a dual-Doppler wind synthesis for this tornadic event provided a unique opportunity to assess the assumptions in the GBVTD methodology. The analysis suggests that the simplified GBVTD equations that have been applied in past studies of tornadoes are not appropriate in the present case. The most accurate retrieval of the radial velocities requires that a higher-order term that is typically neglected be retained. A quantitative assessment of the impact of centrifuging of hydrometeors on the synthesized wind field was attempted. The results suggest that the radial and vertical velocity profile near and within the tornado core can be significantly altered for tornadoes (EF2) that are accompanied by a small radius of maximum wind and relatively weaker low-level inflow.
Abstract
A ground-based velocity track display (GBVTD) analysis of the LaGrange, Wyoming, tornado on 5 June 2009 during the Second Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX2) is photogrammetrically combined with a series of pictures of the funnel cloud. This analysis reveals the relationship between the vertical velocity, radial and tangential velocities, perturbation pressure, vertical vorticity, and angular momentum with the visual features of the tornado. An intense axial downdraft was evident and was supported by a downward-directed perturbation pressure gradient. The radial inflow at low levels was weak and difficult to retrieve owing to a combination of centrifuging of hydrometeors/debris in the intense circulation and the inability of the radar beam to fully resolve the flow. The tornado was weakening during the analysis period, which was supported by angular momentum being advected out of the tornado.
The availability of a dual-Doppler wind synthesis for this tornadic event provided a unique opportunity to assess the assumptions in the GBVTD methodology. The analysis suggests that the simplified GBVTD equations that have been applied in past studies of tornadoes are not appropriate in the present case. The most accurate retrieval of the radial velocities requires that a higher-order term that is typically neglected be retained. A quantitative assessment of the impact of centrifuging of hydrometeors on the synthesized wind field was attempted. The results suggest that the radial and vertical velocity profile near and within the tornado core can be significantly altered for tornadoes (EF2) that are accompanied by a small radius of maximum wind and relatively weaker low-level inflow.
Abstract
On 15 May 1999, a storm intercept team from the University of Oklahoma collected high-resolution, W-band Doppler radar data in a tornado near Stockton, Kansas. Thirty-five sector scans were obtained over a period of approximately 10 min, capturing the tornado life cycle from just after tornadogenesis to the decay stage. A low-reflectivity “eye”—whose diameter fluctuated during the period of observation—was present in the reflectivity scans. A ground-based velocity track display (GBVTD) analysis of the W-band Doppler radar data of the Stockton tornado was conducted; results and interpretations are presented and discussed. It was found from the analysis that the axisymmetric component of the azimuthal wind profile of the tornado was suggestive of a Burgers–Rott vortex during the most intense phase of the life cycle of the tornado. The temporal evolution of the axisymmetric components of azimuthal and radial wind, as well as the wavenumber-1, -2, and -3 angular harmonics of the azimuthal wind, are also presented. A quasi-stationary wavenumber-2 feature of the azimuthal wind was analyzed from 25 of the 35 scans. It is shown, via simulated radar data collection in an idealized Burgers–Rott vortex, that this wavenumber-2 feature may be caused by the translational distortion of the vortex during the radar scans. From the GBVTD analysis, it can be seen that the maximum azimuthally averaged azimuthal wind speed increased while the radius of maximum wind (RMW) decreased slightly during the intensification phase of the Stockton tornado. In addition, the maximum azimuthally averaged azimuthal wind speed, the RMW, and the circulation about the vortex center all decreased simultaneously as the tornado decayed.
Abstract
On 15 May 1999, a storm intercept team from the University of Oklahoma collected high-resolution, W-band Doppler radar data in a tornado near Stockton, Kansas. Thirty-five sector scans were obtained over a period of approximately 10 min, capturing the tornado life cycle from just after tornadogenesis to the decay stage. A low-reflectivity “eye”—whose diameter fluctuated during the period of observation—was present in the reflectivity scans. A ground-based velocity track display (GBVTD) analysis of the W-band Doppler radar data of the Stockton tornado was conducted; results and interpretations are presented and discussed. It was found from the analysis that the axisymmetric component of the azimuthal wind profile of the tornado was suggestive of a Burgers–Rott vortex during the most intense phase of the life cycle of the tornado. The temporal evolution of the axisymmetric components of azimuthal and radial wind, as well as the wavenumber-1, -2, and -3 angular harmonics of the azimuthal wind, are also presented. A quasi-stationary wavenumber-2 feature of the azimuthal wind was analyzed from 25 of the 35 scans. It is shown, via simulated radar data collection in an idealized Burgers–Rott vortex, that this wavenumber-2 feature may be caused by the translational distortion of the vortex during the radar scans. From the GBVTD analysis, it can be seen that the maximum azimuthally averaged azimuthal wind speed increased while the radius of maximum wind (RMW) decreased slightly during the intensification phase of the Stockton tornado. In addition, the maximum azimuthally averaged azimuthal wind speed, the RMW, and the circulation about the vortex center all decreased simultaneously as the tornado decayed.
Abstract
In this study, the structure and evolution of a coastal barrier jet (CBJ) along the east coast of Taiwan is documented using operational Doppler radars. The formation of the CBJ was controlled by the flow regime associated with the approaching Typhoon Haitang (2005). The CBJ persisted for 6 h and was approximately 140 km long and 25 km wide. The northern branch of the CBJ had stronger winds with maximum wind speed 49–52 m s−1, a greater vertical extent with jet core between 1.0 and 2.5 km in height, and a more persistent jet signal than the southern branch with maximum wind speed 43–46 m s−1 and jet core between 1.0 and 2.0 km. We investigated the terrain blocking effect leading to the CBJ formation using an idealized simulation. A vortex resembling Haitang is constructed based on circulation retrieved from generalized velocity track display (GVTD) technique. The result of a no-terrain simulation reveals wind speed 10–22 m s−1 lower than the observed Doppler velocity. The difference suggests the enhanced wind speed along the coast was most likely due to the terrain blocking effect.
Abstract
In this study, the structure and evolution of a coastal barrier jet (CBJ) along the east coast of Taiwan is documented using operational Doppler radars. The formation of the CBJ was controlled by the flow regime associated with the approaching Typhoon Haitang (2005). The CBJ persisted for 6 h and was approximately 140 km long and 25 km wide. The northern branch of the CBJ had stronger winds with maximum wind speed 49–52 m s−1, a greater vertical extent with jet core between 1.0 and 2.5 km in height, and a more persistent jet signal than the southern branch with maximum wind speed 43–46 m s−1 and jet core between 1.0 and 2.0 km. We investigated the terrain blocking effect leading to the CBJ formation using an idealized simulation. A vortex resembling Haitang is constructed based on circulation retrieved from generalized velocity track display (GVTD) technique. The result of a no-terrain simulation reveals wind speed 10–22 m s−1 lower than the observed Doppler velocity. The difference suggests the enhanced wind speed along the coast was most likely due to the terrain blocking effect.
Abstract
The National Center for Atmospheric Research (NCAR) and Montana State University jointly developed water vapor micropulse differential absorption lidars (MPDs) that are a significant advance in eye-safe, unattended, lidar-based water vapor remote sensing. MPD is designed to provide continuous vertical water vapor profiles with high vertical (150 m) and temporal resolution (5 min) in the lower troposphere. This study aims to investigate MPD observation impacts and the scientific significance of MPDs for convective weather analyses and predictions using observation system simulation experiments (OSSEs). In this study, the Data Assimilation Research Testbed (DART) and the Advanced Research version of the Weather Research and Forecasting (WRF-ARW) Model are used to conduct OSSEs for a case study of a mesoscale convective system (MCS) observed during the Plains Elevated Convection At Night (PECAN) experiment. A poor-performing control simulation that was drawn from a 40-member ensemble at 3-km resolution is markedly improved by assimilation of simulated observations drawn from a more skillful simulation that served as the nature run at 1-km resolution. In particular, assimilating surface observations corrected surface warm front structure errors, while MPD observations remedied errors in low- to midlevel moisture ahead of the MCS. Collectively, these analyses changes led to markedly improved short-term predictions of convection initiation, evolution, and precipitation of the MCS in the simulations on 15 July 2015. For this case study, the OSSE results indicate that a more dense MPD network results in better prediction performance for convective precipitation while degrading light precipitation prediction performance due to an imbalance of the analysis at large scales.
Abstract
The National Center for Atmospheric Research (NCAR) and Montana State University jointly developed water vapor micropulse differential absorption lidars (MPDs) that are a significant advance in eye-safe, unattended, lidar-based water vapor remote sensing. MPD is designed to provide continuous vertical water vapor profiles with high vertical (150 m) and temporal resolution (5 min) in the lower troposphere. This study aims to investigate MPD observation impacts and the scientific significance of MPDs for convective weather analyses and predictions using observation system simulation experiments (OSSEs). In this study, the Data Assimilation Research Testbed (DART) and the Advanced Research version of the Weather Research and Forecasting (WRF-ARW) Model are used to conduct OSSEs for a case study of a mesoscale convective system (MCS) observed during the Plains Elevated Convection At Night (PECAN) experiment. A poor-performing control simulation that was drawn from a 40-member ensemble at 3-km resolution is markedly improved by assimilation of simulated observations drawn from a more skillful simulation that served as the nature run at 1-km resolution. In particular, assimilating surface observations corrected surface warm front structure errors, while MPD observations remedied errors in low- to midlevel moisture ahead of the MCS. Collectively, these analyses changes led to markedly improved short-term predictions of convection initiation, evolution, and precipitation of the MCS in the simulations on 15 July 2015. For this case study, the OSSE results indicate that a more dense MPD network results in better prediction performance for convective precipitation while degrading light precipitation prediction performance due to an imbalance of the analysis at large scales.
