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A Vortex-Based Doppler Velocity Dealiasing Algorithm for Tropical CyclonesAbstract
In this study, a vortex-based Doppler velocity dealiasing (VDVD) algorithm for tropical cyclones (TCs) is proposed. The algorithm uses a Rankine combined vortex model as a reference field for dealiasing based on an inner–outer iterative procedure. The structure of the reference vortex is adjusted in an inner iterative procedure of VDVD that applies the ground-based velocity track display (GBVTD) technique. The outer loop of the VDVD based on the GBVTD-simplex algorithm is used for center correction. The VDVD is able to recover not only the aliased Doppler velocities from a simulated symmetric vortex but also those superimposed with wavenumber-1 asymmetry, radial wind, or mean flow. For real cases, the VDVD provides dealiased Doppler velocity with 99.4% accuracy for all pixels, based on 472 elevation sweeps from a typhoon without landfall. It is suggested that the VDVD algorithm can improve the quality of downstream applications such as Doppler wind retrievals and radar data assimilations of TCs and other storms, such as tornadoes and mesocyclones, with vortex signatures.
Abstract
In this study, a vortex-based Doppler velocity dealiasing (VDVD) algorithm for tropical cyclones (TCs) is proposed. The algorithm uses a Rankine combined vortex model as a reference field for dealiasing based on an inner–outer iterative procedure. The structure of the reference vortex is adjusted in an inner iterative procedure of VDVD that applies the ground-based velocity track display (GBVTD) technique. The outer loop of the VDVD based on the GBVTD-simplex algorithm is used for center correction. The VDVD is able to recover not only the aliased Doppler velocities from a simulated symmetric vortex but also those superimposed with wavenumber-1 asymmetry, radial wind, or mean flow. For real cases, the VDVD provides dealiased Doppler velocity with 99.4% accuracy for all pixels, based on 472 elevation sweeps from a typhoon without landfall. It is suggested that the VDVD algorithm can improve the quality of downstream applications such as Doppler wind retrievals and radar data assimilations of TCs and other storms, such as tornadoes and mesocyclones, with vortex signatures.
Abstract
As Typhoon Goni (2015) passed over Ishigaki Island, a maximum gust speed of 71 m s−1 was observed by a surface weather station. During Typhoon Goni’s passage, mountaintop radar recorded antenna elevation angle oscillations, with a maximum amplitude of ~0.2° at an elevation angle of 0.2°. This oscillation phenomenon was reflected in the reflectivity and Doppler velocity fields as Typhoon Goni’s eyewall encompassed Ishigaki Island. The main antenna oscillation period was approximately 0.21–0.38 s under an antenna rotational speed of ~4 rpm. The estimated fundamental vibration period of the radar tower is approximately 0.25–0.44 s, which is comparable to the predominant antenna oscillation period and agrees with the expected wind-induced vibrations of buildings. The reflectivity field at the 0.2° elevation angle exhibited a phase shift signature and a negative correlation of −0.5 with the antenna oscillation, associated with the negative vertical gradient of reflectivity. FFT analysis revealed two antenna oscillation periods at 0955–1205 and 1335–1445 UTC 23 August 2015. The oscillation phenomenon ceased between these two periods because Typhoon Goni’s eye moved over the radar site. The VAD analysis-estimated wind speeds at a range of 1 km for these two antenna oscillation periods exceeded 45 m s−1, with a maximum value of approximately 70 m s−1. A bandpass filter QC procedure is proposed to filter out the predominant wavenumbers (between 40 and 70) for the reflectivity and Doppler velocity fields. The proposed QC procedure is indicated to be capable of mitigating the major signals resulting from antenna oscillations.
Abstract
As Typhoon Goni (2015) passed over Ishigaki Island, a maximum gust speed of 71 m s−1 was observed by a surface weather station. During Typhoon Goni’s passage, mountaintop radar recorded antenna elevation angle oscillations, with a maximum amplitude of ~0.2° at an elevation angle of 0.2°. This oscillation phenomenon was reflected in the reflectivity and Doppler velocity fields as Typhoon Goni’s eyewall encompassed Ishigaki Island. The main antenna oscillation period was approximately 0.21–0.38 s under an antenna rotational speed of ~4 rpm. The estimated fundamental vibration period of the radar tower is approximately 0.25–0.44 s, which is comparable to the predominant antenna oscillation period and agrees with the expected wind-induced vibrations of buildings. The reflectivity field at the 0.2° elevation angle exhibited a phase shift signature and a negative correlation of −0.5 with the antenna oscillation, associated with the negative vertical gradient of reflectivity. FFT analysis revealed two antenna oscillation periods at 0955–1205 and 1335–1445 UTC 23 August 2015. The oscillation phenomenon ceased between these two periods because Typhoon Goni’s eye moved over the radar site. The VAD analysis-estimated wind speeds at a range of 1 km for these two antenna oscillation periods exceeded 45 m s−1, with a maximum value of approximately 70 m s−1. A bandpass filter QC procedure is proposed to filter out the predominant wavenumbers (between 40 and 70) for the reflectivity and Doppler velocity fields. The proposed QC procedure is indicated to be capable of mitigating the major signals resulting from antenna oscillations.
Abstract
A tropical cyclone (TC) size parameter, which is defined here as the radius of 15 m s−1 near-surface wind speed (R15), is calculated for 145 TCs in the western North Pacific during 2000–05 based on QuikSCAT oceanic winds. For the 73 TCs that intensified to typhoon intensity during their lifetimes, the 33% and 67% respective percentiles of R15 at tropical storm intensity and at typhoon intensity are used to categorize small, medium, and large TCs. Whereas many of the small TCs form from an easterly wave synoptic pattern, the monsoon-related formation patterns are favorable for forming medium to large TCs. Most of these 73 TCs stay in the same size category during intensification, which implies specific physical mechanisms for maintaining TC size in the basin. The 18 persistently large TCs from the tropical storm to the typhoon stage mostly have northwestward or north-northwestward tracks, while the 16 persistently small TCs either move westward–northwestward in lower latitudes or develop at higher latitudes with various track types. For the large TCs, strong low-level southwesterly winds exist in the outer core region south of the TC center throughout the intensification period. The small TCs are more influenced by the subtropical high during intensification. The conclusion is that it is the low-level environment that determines the difference between large and small size storms during the early intensification period in the western North Pacific.
Abstract
A tropical cyclone (TC) size parameter, which is defined here as the radius of 15 m s−1 near-surface wind speed (R15), is calculated for 145 TCs in the western North Pacific during 2000–05 based on QuikSCAT oceanic winds. For the 73 TCs that intensified to typhoon intensity during their lifetimes, the 33% and 67% respective percentiles of R15 at tropical storm intensity and at typhoon intensity are used to categorize small, medium, and large TCs. Whereas many of the small TCs form from an easterly wave synoptic pattern, the monsoon-related formation patterns are favorable for forming medium to large TCs. Most of these 73 TCs stay in the same size category during intensification, which implies specific physical mechanisms for maintaining TC size in the basin. The 18 persistently large TCs from the tropical storm to the typhoon stage mostly have northwestward or north-northwestward tracks, while the 16 persistently small TCs either move westward–northwestward in lower latitudes or develop at higher latitudes with various track types. For the large TCs, strong low-level southwesterly winds exist in the outer core region south of the TC center throughout the intensification period. The small TCs are more influenced by the subtropical high during intensification. The conclusion is that it is the low-level environment that determines the difference between large and small size storms during the early intensification period in the western North Pacific.
