single-Doppler Radar Data Research Articles

Quantification of QLCS Tornadogenesis, Associated Characteristics, and Environments across a Large Sample

Abstract The skillful anticipation of tornadoes produced by quasi-linear convective systems (QLCSs) is a well-known forecasting challenge. This study was motivated by the possibility that warning accuracy of QLCS tornadoes depends on the processes leading to tornadogenesis, namely, one that is dominated by an apparent release of horizontal shearing instability [shearing instability dominant (SID)] and one by a pre-tornadic mesocyclone [pre-tornadic mesocyclone dominant (PMD)] and its associated generative mechanisms. The manual classification of the genesis of 530 QLCS tornadoes as either SID or PMD was performed using heuristic, yet process-driven criteria based on single-Doppler radar (WSR-88D) data. This included 214, 213, and 103 tornadoes that occurred during 2019, 2017, and 2016, respectively. As a function of tornadogenesis process, 36% were classified as SID, and 60% were classified as PMD; the remaining 4% could not be classified. Approximately 30% of the SID cases were operationally warned prior to tornadogenesis, compared to 44% of the PMD cases. PMD tornadoes were also more common during the warm season and displayed a diurnal, midafternoon peak in frequency. Finally, SID cases were more likely to be associated with QLCS tornado outbreaks but tended to be slightly shorter lived. A complementary effort to investigate environmental characteristics of QLCS tornadogenesis revealed differences between SID and PMD cases. MLCAPE was relatively larger for warm-season SID cases, and 0–3-km SRH was relatively larger in warm-season PMD cases. Additionally, pre-tornadic frontogenesis was more prominent for cool-season SID cases, suggestive of a more significant role of the larger-scale meteorological forcing in vertical vorticity that fosters tornadogenesis through SID processes.

A Variational Method for Filling in Missing Data in Doppler Velocity Fields

AbstractSingle-Doppler radar data are often missing in important regions of a severe storm due to low return power, low signal-to-noise ratio, ground clutter associated with normal and anomalous propagation, and missing radials associated with partial or total beam blockage. Missing data impact the ability of WSR-88D algorithms to detect severe weather. To aid the algorithms, we develop a variational technique that fills in Doppler velocity data voids smoothly by minimizing Doppler velocity gradients while not modifying good data. This method provides estimates of the analysed variable in data voids without creating extrema.Actual single-Doppler radar data of four tornadoes are used to demonstrate the variational algorithm. In two cases, data are missing in the original data, and in the other two, data are voided artificially. The filled-in data match the voided data well in smoothly varying Doppler velocity fields. Near singularities such as tornadic vortex signatures, the match is poor as anticipated. The algorithm does not create any velocity peaks in the former data voids, thus preventing false triggering of tornado warnings. Doppler circulation is used herein as a far-field tornado detection and advance-warning parameter. In almost all cases, the measured circulation is quite insensitive to the data that have been voided and then filled. The tornado threat is still apparent.

An Automatic Recognition Method for Airflow Field Structures of Convective Systems Based on Single Doppler Radar Data

Airflow structures within convective systems are important predictors of damaging convective disasters. To automatically recognize different kinds of airflow structures (the convergence, divergence, cyclonic rotation, and anticyclonic rotation) within convective systems, an airflow structure recognition method is proposed, in this paper, based on a regular hexagonal template. On the basis of single Doppler radar data, the template is designed according to the appearance model of airflows in radial velocity maps. The proposed method is able to output types and intensities of airflow structures within convective systems. In addition, the outputs of the proposed method are integrated into a projection map of the airflow field structure types and intensities (PMAFSTI), which is developed in this work to visualize three-dimensional airflow structures within convective cells. The proposed airflow structure automatic recognition method and the PMAFSTI were tested using three typical cases. Results of the tests suggest the following: (1) At different evolution stages of the convective systems, e.g., growth, split, and dissipation, the three-dimensional distribution of the airflow fields within convective systems could be clearly observed through the PMAFSTI and (2) on the basis of recognizing the structures of the airflow field, the complex airflow field, such as a squall line, could be further divided into several small parts making the analysis of convective systems more scientific and elaborate.

The Improvement to the Environmental Wind and Tropical Cyclone Circulation Retrievals with the Modified GBVTD (MGBVTD) Technique

AbstractThe 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 cannot be resolved and is consequently aliased into the retrieved axisymmetric tangential wind . Recently, the development of the hurricane volume velocity processing method (HVVP) enabled the independent estimation of ; however, HVVP is potentially limited by the unknown accuracy of empirical assumptions used to deduce the modified Rankine-combined vortex exponent . By combing the GBVTD with HVVP techniques, this study proposes a modified GBVTD method (MGBVTD) to objectively deduce from the GBVTD technique and provide a more accurate estimation of and via an iterative procedure to reach converged and cross-beam component of solutions. MGBVTD retains the strength of both algorithms but avoids their weaknesses. The results from idealized experiments demonstrate that the MGBVTD-retrieved cross-beam component of is within 2 m s−1 of reality. MGBVTD was applied to Hurricane Bret (1999) whose inner core was captured simultaneously by two Weather Surveillance Radar-1988 Doppler (WSR-88D) instruments. The MGBVTD-retrieved cross-beam component of from single-Doppler radar data is very close to that from dual-Doppler radar synthesis using extended GBVTD (EGBVTD); their difference is less than 2 m s−1. The mean difference in the MGBVTD-retrieved from the two radars is ~2 m s−1, which is significantly smaller than that resolved in GBVTD retrievals (~5 m s−1).

The Gradient Velocity Track Display (GrVTD) Technique for Retrieving Tropical Cyclone Primary Circulation from Aliased Velocities Measured by Single-Doppler Radar

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.

Objective Tropical Cyclone Center Tracking Using Single-Doppler Radar

AbstractThis study presents an extension of the ground-based velocity track display (GBVTD)-simplex tropical cyclone (TC) circulation center–finding algorithm to further improve the accuracy and consistency of TC center estimates from single-Doppler radar data. The improved center-finding method determines a TC track that ensures spatial and temporal continuities of four primary characteristics: the radius of maximum wind, the maximum axisymmetric tangential wind, and the latitude and longitude of the TC circulation center. A statistical analysis improves the consistency of the TC centers over time and makes it possible to automate the GBVTD-simplex algorithm for tracking of landfalling TCs. The characteristics and performance of this objective statistical center-finding method are evaluated using datasets from Hurricane Danny (1997) and Bret (1999) over 5-h periods during which both storms were simultaneously observed by two coastal Weather Surveillance Radar-1988 Doppler (WSR-88D) units. Independent single-Doppler and dual-Doppler centers are determined and used to assess the absolute accuracy of the algorithm. Reductions of 50% and 10% in the average distance between independent center estimates are found for Danny and Bret, respectively, over the original GBVTD-simplex method. The average center uncertainties are estimated to be less than 2 km, yielding estimated errors of less than 5% in the retrieved radius of maximum wind and wavenumber-0 axisymmetric tangential wind, and ~30% error in the wavenumber-1 asymmetric tangential wind. The objective statistical center-finding method can be run on a time scale comparable to that of a WSR-88D volume scan, thus making it a viable tool for both research and operational use.

The LaGrange Tornado during VORTEX2. Part I: Photogrammetric Analysis of the Tornado Combined with Single-Doppler Radar Data

Abstract This study presents a single-Doppler radar analysis combined with cloud photography of the LaGrange, Wyoming, tornado on 5 June 2009 in an attempt to relate the radar-observed hook echo, weak-echo hole (WEH), and rotational couplet to the visual characteristics of the tornado. The tornado was rated EF2. The circulation at low levels went through two intensification periods based on azimuthal shear measurements. The first intensification was followed by the appearance of a brief funnel cloud. The second intensification was coincident with the appearance of a second funnel cloud that remained in contact with the ground until the tornado dissipated. A deep WEH rapidly formed within the hook echo after damaging wind was identified at the ground and before the appearance of a funnel cloud. The echo pattern through the hook echo on 5 June undergoes a dramatic evolution. Initially, the minimum radar reflectivities are near the surface (<15 dBZ) and the WEH does not suggest a tapered structure near the ground. Subsequently, higher reflectivities appear at low levels when the funnel cloud makes contact with the ground. During one analysis time, the increase of the echo within the WEH at low levels results in a couplet of high/low radar reflectivity in the vertical. This increase in echo at low levels is believed to be associated with lofted debris although none was visibly apparent until the last analysis time. The WEH was nominally wider than the visible funnel cloud. The dataset provides the first detailed analysis of the double-ring structure within a hook echo that has been reported in several studies. The inner high-reflectivity region is believed to be a result of lofted debris. At higher-elevation angles, a small secondary WEH formed within the first WEH when debris was lofted and centrifuged. A feature noted in past studies showing high-resolution vertical cross sections of single-Doppler velocity normal to the radar beam is an intense rotational couplet of negative and positive values in the lowest few hundred meters. This couplet was also evident in the analysis of the LaGrange tornado. The couplet was asymmetric with stronger negative velocities owing to the motion of the tornado toward the radar. The damaging wind observed by radar extended well beyond the condensation funnel in the lowest few hundred meters. However, another couplet indicating strong rotation was also noted aloft in a number of volume scans. The decrease in rotational velocities between the low-and upper-level couplets may be related to air being forced radially outward from the tornado center at a location above the intense inflow.

Theoretical analyses and numerical experiments for two-dimensional wind retrievals from single-doppler data

Based on variational adjoint methods, theoretical analyses and numerical experiments are performed for retrievals of two-dimensional winds and related parameters from single-Doppler radar data in polar coordinates. In the method, the reflectivity conservation equation is used as the governing equation and the mass continuity equation as a weak constraint. At the same time, the stable functional of regularization and background field are also introduced in the cost functional. Based on the ideal experiments with artificial data, real two-dimensional wind fields are retrieved with low-altitude data from the Nanjing Doppler Radar Station, and the results are encouraging and promising.

Observations of dual slantwise circulations above a cool undercurrent in a mesoscale convective system

AbstractA series of mesoscale convective systems (MCSs) was observed in the UK during one of the Intensive Observation Periods (IOP 3) in the Convective Storm Initiation Project (CSIP). A detailed case‐study of one of the MCSs, involving some novel interpretation of single Doppler radar data, reveals the evolution of stacked slantwise circulations associated with elevated upright convection. A feature of this occasion was the existence near the surface of a strong undercurrent of cool air flowing against the direction of travel of the storm. The air feeding both the upright and slantwise ascent originated not from near the surface but, rather, from layers between 1 and 3 km. The upright convection was intense at first and was accompanied by a region of dry subsidence surrounding it, together with the alternating layers of slantwise ascent and descent within the storm's precipitation area. When the elevated upright convection weakened, vigorous slantwise motions continued for some time, but their slope decreased from the initial 1 in 4 to 1 in 9. The lowermost layer of slantwise descent corresponded to a moderately intense rear‐inflow jet. The rear‐inflow jet did not penetrate the cool undercurrent and reach the surface; instead, beneath where it impacted the warm air capping the undercurrent, the undercurrent took on the structure of a gravity wave without stagnation. The raised head of the undercurrent lifted overlying air of high wet‐bulb potential temperature sufficiently to overcome the convective inhibition. Copyright © 2010 Royal Meteorological Society

Real-Time Implementation of Single-Doppler Radar Analysis Methods for Tropical Cyclones: Algorithm Improvements and Use with WSR-88D Display Data

Abstract The NOAA/NWS/NCEP/Tropical Prediction Center/National Hurricane Center has sought techniques that use single-Doppler radar data to estimate the tropical cyclone wind field. A cooperative effort with NOAA/Atlantic Oceanographic and Meteorological Laboratory/Hurricane Research Division and NCAR has resulted in significant progress in developing a method whereby radar display data are used as a proxy for a full-resolution base data and in improving and implementing existing wind retrieval and center-finding techniques. These techniques include the ground-based velocity track display (GBVTD), tracking radar echoes by correlation (TREC), GBVTD- simplex, and the principal component analysis (PCA) methods. The GBVTD and TREC algorithms are successfully applied to the Weather Surveillance Radar-1988 Doppler (WSR-88D) display data of Hurricane Bret (1999) and Tropical Storm Barry (2001). GBVTD analyses utilized circulation center estimates provided by the GBVTD-simplex and PCA methods, whereas TREC analys...

Short-Term Numerical Forecasting of a Shallow Storms Complex Using Bistatic and Single-Doppler Radar Data

Abstract A new method based on four-dimensional variational radar data assimilation into a cloud-resolving model has been developed for nowcasting purposes. This method allows for the retrieval of the model prognostic variables that compose the initial state of the simulation. The echo-free regions are filled by a 3D wind analysis from single-Doppler data based on linearity of the horizontal wind components in a moving reference frame, which provides a realistic mesoscale flow that is in better agreement with the air circulation retrieved from dual-Doppler observations within the precipitating regions. Furthermore, the near-ground refractivity index of air derived from ground targets is used to diagnose a high-resolution and two-dimensional distribution of relative humidity in the mixed layer. Two experiments are performed: one uses multiple-Doppler information coming from McGill University's bistatic radar network and the second considers only single-Doppler observations. This updated algorithm has been ...

3D Wind Field Retrieval Applied to Snow Events Using Doppler Radar

Abstract The variational analysis method of Laroche and Zawadzki for the retrieval of the 3D wind field from single–Doppler radar data is adapted to analyze stratiform snow events. Modifications include prefiltering of noisy radial velocity data using a velocity azimuth display (VAD) analysis, the use of snow particle fall speeds derived from extended VAD analyses, and the usage of high-elevation angle data. Arctic cases were collected during the Beaufort and Arctic Storms Experiment with X-band radars located at Inuvik and Tuktoyaktuk, Northwest Territories, Canada. These were compared with C-band radar data acquired by the King City Doppler radar in southern Ontario, Canada, a midlatitude site. Retrieved vertical airspeeds were verified using data from a research aircraft. Mean horizontal speeds were verified using VAD analyses, and horizontal wind speed anomalies were checked using Mesonet station data and VAD residual analyses. The results indicate that retrieved vertical velocities are in relative ag...

Three-Dimensional Simple Adjoint Velocity Retrievals from Single-Doppler Radar

In this paper, the original simple adjoint method of Qiu and Xu is improved and tested for retrieving the 3D storm-scale wind field from single-Doppler radar data. The method incorporates single-radar data and background fields, along with a dynamical constraint, smoothness, and anelastic mass continuity constraint, in a cost function. The minimization of the cost function, with the desired fit to these constraints, is obtained in an iterative procedure. The current method is tested on simulated datasets of supercell storms. It is shown that the circulation inside and around the storms, including the strong updraft and associated downdraft, are well retrieved. Furthermore, the method is robust in the presence of data error.

Tropical Cyclone Kinematic Structure Retrieved from Single-Doppler Radar Observations. Part II: The GBVTD-Simplex Center Finding Algorithm

Abstract This paper is the second of a series and focuses on developing an algorithm to objectively identify tropical cyclone (TC) vorticity centers using single-Doppler radar data. The first paper dealt with the formulation of a single-Doppler radar TC wind retrieval technique, the ground-based velocity-track-display (GBVTD), and the results are verified using analytical TCs. It has been acknowledged that the quality of the GBVTD-retrieved TC circulation strongly depends on accurately knowing its center position. However, existing single-Doppler radar center finding algorithms are limited to estimate centers for axisymmetric TCs. The proposed algorithm uses a simplex method to objectively estimate the TC vorticity center by maximizing GBVTD-retrieved mean tangential wind. When tested with a number of axisymmetric and asymmetric analytical TCs, the accuracy of the TC centers estimated by the GBVTD-simplex algorithm is ≈340 m from the true center. When adding 5 m s−1 random noise to the Doppler velocities,...

Analysis of the Gal-Chen–Zhang Single-Doppler Velocity Retrieval

Abstract The authors present herein an analysis of a single-Doppler velocity retrieval (SDVR) technique whereby the unobserved wind components are determined from single-Doppler radar data. The analysis is designed to provide information about the behavior and/or sensitivity of the SDVR scheme as a function of various internal and external parameters as well as about observational errors and weights. Results presented for retrieval of both the mean and local flow indicate that the SDVR breaks down if the reflectivity gradient vanishes or if a reflectivity isoline is locally perpendicular to the radar beam. In the absence of reflectivity or radial velocity errors, the mean flow solution is independent of the integration area, the radar location, the signal wavenumber, and the weights. Given perfect radial wind information, error in the reflectivity field degrades the solution. Contrary to the error-free solution, the solution with error depends on the integration area. Error statistics indicate that radial...

Vertical motion, diabatic heating, and rainfall characteristics in north Australia convective systems

Very-high-frequency wind-profiler data are used to study the vertical draught structure within 13 tropical Mesoscale Convective Systems (MCSs) near Darwin, Australia during the 1989-90 and 1990-91 wet seasons. These studies are supported by single-Doppler radar, soundings, and surface rainfall data to correlate radar reflectivity, thermal buoyancy, and surface rainfall patterns with vertical air-motion structures. Because of Darwin's unique location at the southern tip of the Maritime Continent, vertical draughts in both the monsoon (maritime) and monsoon break (continental) convective regimes can be observed. The break-regime MCSs (six in total) were all squall lines, characterized by a leading line of convection with heavy precipitation and trailing stratiform rainfall containing a characteristic radar bright band. These MCSs exhibited a pronounced life-cycle pattern and were all sampled by the profiler in the mature to dissipating stages. In contrast, the monsoon systems (seven in total) were composed of regions of stratiform cloud with embedded convective bands which moved on-shore in the monsoonal flow. Results from the Darwin rain-gauge network indicated that the majority of the total rainfall in each MCS (break and monsoon) was associated with the convective portion of the system. The break-regime MCSs were all characterized by a low-level (4 km) updraught peak associated with convective cells on the leading edge of each squall line, trailed by deeper convective updraughts in the middle and upper troposphere. For the monsoon cases, the lower-troposphere convective updraughts were typically less than those in the squall lines, yet were stronger in the upper troposphere. The low-level differences in the convective updraughts were consistent with the smaller virtual-temperature excess in the monsoon soundings, as well as the larger vertical radar-reflectivity gradients observed in monsoon convection. Consistent with the differences in vertical air-motion patterns, diabatic heating and moistening profiles for the monsoon MCSs were characterized by a higher-level heating and drying peak compared with the break MCSs. The results have important implications for cumulus parametrizations in numerical models since the large-scale circulation is sensitive to the vertical distribution of diabatic heating in tropical MCSs.

The Front Range Blizzard of 1990. Part II: Melting Effects in a Convective Band

Abstract A north–south-oriented line of convection, associated with a melting-induced mesoscale front, formed south of the Mile High Radar (MHR) during the early hours of the Front Range blizzard of 6–7 March 1990. The kinematic structure of the mesoscale front, that is, precipitation band, was examined by analyzing single-Doppler radar data from MHR with two analysis techniques. The results from the analyses were augmented and compared with observations from three surface stations within the Winter Icing and Storms Project (WISP) area. Because of the extraction of the latent heat of fusion from the atmosphere as ice particles melted, the temperature dropped rapidly as the rainband passed. This resulted in a large pool of stable, 0°C air behind the mesoscale front. As the melting continued, this pool expanded toward the east where it was not blocked by terrain. It expanded into a convectively unstable air mass with strong southeasterly winds. A band-parallel low-level jet developed east of the band and re...

Least Squares Retrieval of Microburst Winds from Single-Doppler Radar Data

Abstract A least squares (LS) method is developed for retrieving low-altitude winds from single-Doppler radar scans. The method is tested with Denver airport microburst data and the results compared with the previously developed simple adjoint (SA) method. It is found that the LS method is slightly superior to the SA method for the microburst data obtained with fast radar scans (Δτ ≈ 60 s) but will become inferior to the SA method if the radar scans are twice as long (Δτ ≈ 120 s). Four previously developed detailed techniques for the SA method are used to improve the LS retrievals, and these include (i) using multiple-time-level data, (ii) imposing the weak divergence and weak vorticity constraints, (iii) retrieving the eddy coefficient and time-mean forcing term, and (iv) using the, observed time-mean radial wind as a weak constraint. Because the control equation is used as a weak constraint in a finite-difference form, the LS method depends more on the smoothness constraints but is less sensitive to equ...

A Variational Analysis Method for Retrieval of Three-Dimensional Wind Field from Single-Doppler Radar Data

A variational method for retrieval of the three-dimensional wind field from single-Doppler radar observations is developed and tested. The method uses the conservation equation for reflectivity and the continuity equation as a constraining model. Weak and strong constraint formalisms in data analysis am reviewed and compared using a one-dimensional advection equation for reflectivity considered as a passive tracer. The authors show that a model equation should be used as a weak constraint when the model does not predict exactly the evolution of the observations (such as the conservation equation for reflectivity). Consequently, the variational method presented here combines both formalisms: the conservation equation for reflectivity is used as a weak constraint, while the continuity equation is used as a strong constraint. The method is applied to retrieve detailed three-dimensional wind field of a microburst observed by two C-band Doppler radars during the Phoenix II Convective Boundary Layer Experiment. Retrieved wind fields are compared with dual-Doppler wind analysis. Results of experiments show that the cost function has multiple minima, and consequently retrievals are sensitive to the initial guess. To find the true minimum the retrieval is performed from large to small scale. The results are very encouraging.

Fitting a Cartesian Prediction Model to Radial Velocity Data from Single-Doppler Radar

Previous experiments with the adjoint technique for determining the three-dimensional wind and thermodynamic fields from single-Doppler radar data have assumed that the radar observes one component of the velocity in Cartesian coordinates. This technique is generalized to radial velocity observations by fitting a Cartesian prediction model to interpolated radial velocity data in Cartesian coordinates. The three-dimensional wind and temperature are determined by minimizing the difference between simulated single-Doppler observations of radial velocity and reflectivity in Cartesian coordinates and their predictions from a numerical model. An application of this technique to a simulation of dry convection gives successful results.