Search Results

You are looking at 1 - 10 of 22 items for

  • Author or Editor: Michael M. French x
  • Refine by Access: All Content x
Clear All Modify Search
Michael M. French and Darrel M. Kingfield

Abstract

Weather Surveillance Radar–1988 Doppler (WSR-88D) data from 36 tornadic supercell cases from 2012 to 2016 are investigated to identify common tornadic vortex signature (TVS) behaviors prior to tornado dissipation. Based on the results of past case studies, four characteristics of TVSs associated with tornado dissipation were identified: weak or decreasing TVS intensity, rearward storm-relative motion of the TVS, large or increasing TVS vertical tilt, and large or increasing TVS horizontal displacement from the main storm updraft. Only cases in which a TVS was within 60 km of a WSR-88D site in at least four consecutive volumes at the end of the tornado life cycle were examined. The space and time restrictions on case selection ensured that the aforementioned quantities could be determined within ~500 m of the surface at several time periods despite the relatively coarse spatiotemporal resolution of WSR-88D systems. It is found that prior to dissipation, TVSs become increasingly less intense, tend to move rearward in a storm-relative framework, and become increasingly more separated from the approximate location of the main storm updraft. There is no clear signal in the relationship between tornado tilt, as measured in inclination angle, and TVS dissipation. The frequency of combinations of TVS dissipation behaviors, the impact of increased low-level WSR-88D scanning on dissipation detection, and prospects for future nowcasting of tornado life cycles also are discussed.

Full access
Darrel M. Kingfield and Michael M. French

Abstract

The Weather Surveillance Radar - 1988 Doppler (WSR-88D) network has undergone several improvements in the last decade with the upgrade to dual-polarization capabilities and the ability for forecasters to re-scan the lowest levels of the atmosphere more frequently through the use of Supplemental Adaptive Intra-volume Scanning (SAILS). SAILS reduces the revisit period for scanning the lowest 1 km of the atmosphere but comes at the cost of a longer delay between scans at higher altitudes. This study quantifies how often radar Volume Coverage Patterns (VCPs) and all available SAILS options are used during the issuance of 148,882 severe thunderstorm and 18,263 tornado warnings, and near 10,474 tornado, 58,934 hail, and 127,575 wind reports in the dual-polarization radar era.

A large majority of warnings and storm reports were measured with a VCP providing denser low-level sampling coverage. More frequent low-level updates were employed near tornado warnings and reports compared to severe thunderstorm warnings and hail or wind hazards. Warnings issued near a radar providing three extra low-level scans (SAILSx3) were more likely to be verified by a hazard with a positive lead time than warnings with fewer low-level scans. However, extra low-level scans were more frequently used in environments supporting organized convection as shown using watches issued by the Storm Prediction Center. Recently, the number of mid-level radar elevation scans is declining per hour, which can adversely affect the tracking of convective polarimetric signatures, like ZDR columns, which were found above the 0.5° elevation angle in over 99% of cases examined.

Restricted access
Michael M. French and Darrel M. Kingfield

Abstract

A sample of 198 supercells are investigated to determine if a radar proxy for the area of the storm midlevel updraft may be a skillful predictor of imminent tornado formation and/or peak tornado intensity. A novel algorithm, a modified version of the Thunderstorm Risk Estimation from Nowcasting Development via Size Sorting (TRENDSS) algorithm is used to estimate the area of the enhanced differential radar reflectivity factor (Z DR) column in Weather Surveillance Radar–1988 Doppler data; the Z DR column area is used as a proxy for the area of the midlevel updraft. The areas of Z DR columns are compared for 154 tornadic supercells and 44 nontornadic supercells, including 30+ supercells with tornadoes rated EF1, EF2, and EF3; 8 supercells with EF4+ tornadoes also are analyzed. It is found that (i) at the time of their peak 0–1-km azimuthal shear, nontornadic supercells have consistently small (<20 km2) Z DR column areas, while tornadic cases exhibit much greater variability in areas; and (ii) at the time of tornadogenesis, EF3+ tornadic cases have larger Z DR column areas than tornadic cases rated EF1/2. In addition, all eight violent tornadoes sampled have Z DR column areas > 30 km2 at the time of tornadogenesis. However, only weak positive correlation is found between Z DR column area and both radar-estimated peak tornado intensity and maximum tornado path width. Planned future work that focuses on mechanisms linking updraft size and tornado formation and intensity is summarized and the use of the modified TRENDSS algorithm, which is immune to Z DR bias and thus ideal for real-time operational use, is emphasized.

Restricted access
Jacob H. Segall, Michael M. French, Darrel M. Kingfield, Scott D. Loeffler, and Matthew R. Kumjian

Abstract

Polarimetric radar data from the WSR-88D network are used to examine the evolution of various polarimetric precursor signatures to tornado dissipation within a sample of 36 supercell storms. These signatures include an increase in bulk hook echo median raindrop size; a decrease in midlevel differential radar reflectivity factor (ZDR) column area; a decrease in the magnitude of the ZDR arc; an increase in the area of low-level large hail; and a decrease in the orientation angle of the vector separating low-level ZDR and KDP maxima. Only supercells that produced “long-duration” tornadoes (with at least four consecutive volumes of WSR-88D data) are investigated, so that signatures can be sufficiently tracked in time, and novel algorithms are used to isolate each storm-scale process. During the time leading up to tornado dissipation, we find that hook echo median drop size (D0) and median ZDR remain relatively constant, but hook echo median KDP and estimated number concentration (NT) increase. ZDR arc maximum magnitude and ZDR-KDP separation orientation angles are observed to decrease in most dissipation cases. Neither the area of large hail nor the ZDR column area exhibit strong signals leading up to tornado dissipation. Finally, combinations of storm-scale behaviors and TVS behaviors occur most frequently just prior to tornado dissipation, but also are common 15-20 min prior to dissipation. The results from this study provide evidence that nowcasting tornado dissipation using dual-polarization radar may be possible when combined with TVS monitoring, subject to important caveats.

Restricted access
Kristofer S. Tuftedal, Michael M. French, Darrel M. Kingfield, and Jeffrey C. Snyder

Abstract

The time preceding supercell tornadogenesis and tornadogenesis “failure” has been studied extensively to identify differing attributes related to tornado production or lack thereof. Studies from the Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX) found that air in the rear-flank downdraft (RFD) regions of non- and weakly tornadic supercells had different near-surface thermodynamic characteristics than that in strongly tornadic supercells. Subsequently, it was proposed that microphysical processes are likely to have an impact on the resulting thermodynamics of the near-surface RFD region. One way to view proxies to microphysical features, namely, drop size distributions (DSDs), is through use of polarimetric radar data. Studies from the second VORTEX used data from dual-polarization radars to provide evidence of different DSDs in the hook echoes of tornadic and nontornadic supercells. However, radar-based studies during these projects were limited to a small number of cases preventing result generalizations. This study compiles 68 tornadic and 62 nontornadic supercells using Weather Surveillance Radar–1988 Doppler (WSR-88D) data to analyze changes in polarimetric radar variables leading up to, and at, tornadogenesis and tornadogenesis failure. Case types generally did not show notable hook echo differences in variables between sets, but did show spatial hook echo quadrant DSD differences. Consistent with past studies, differential radar reflectivity factor (Z DR) generally decreased leading up to tornadogenesis and tornadogenesis failure; in both sets, estimated total number concentration increased during the same times. Relationships between DSDs and the near-storm environment, and implications of results for nowcasting tornadogenesis, also are discussed.

Restricted access
Howard B. Bluestein, Michael M. French, Ivan PopStefanija, Robert T. Bluth, and Jeffrey B. Knorr

A mobile X-band, phased-array Doppler radar was acquired from the U.S. Army by the Center for Interdisciplinary Remotely Piloted Aircraft Studies (CIRPAS) at the Naval Postgraduate School and adapted for meteorological use by ProSensing, Inc. The radar was used during field experiments conducted in the Southern Plains by faculty and students from the School of Meteorology at the University of Oklahoma during the spring storm seasons of 2007 and 2008. During these field experiments, storm-scale, rapid-scan, volumetric, Doppler-radar observations were obtained in tornadic and nontornadic supercells, quasilinear mesoscale convective systems, and in both boundary layer–based and elevated ordinary convective cells. A case is made for the use of the radar for studies of convective weather systems and other weather phenomena that evolve on time scales as short as tens of seconds.

Full access
Michael M. French, Howard B. Bluestein, Louis J. Wicker, David C. Dowell, and Matthew R. Kramar

Abstract

On 16 May 2003, two ground-based, mobile, Doppler radars scanned a potentially tornadic supercell in the Texas Panhandle intermittently from ∼0200 to 0330 UTC. The storm likely was tornadic, but because it was dark, visual confirmation of any tornadoes was not possible. A damage survey was completed after the storm moved through the area. The final conclusion of the damage survey prior to this analysis was that there were two tornadoes near Shamrock, Texas: one that formed prior to 0300 UTC and one that formed at or after 0300 UTC. High-resolution, mobile, Doppler radar data of the supercell were compared with the damage survey information at different times. The location of the first tornado damage path was not consistent with the locations of the low-level circulations in the supercell identified through the mobile, Doppler radar data. The damage within the first path, which consisted mostly of downed trees, may have been caused by straight-line winds in a squall line that moved through the area after the passage of the supercell. The mobile, Doppler radar data did not provide any supporting evidence for the first tornado, but the data did support the existence of the second tornado in Wheeler County on the evening of 15 May 2003. Ground-based, mobile, Doppler radar data may be used as an important tool to help to confirm (or deny) tornado damage reports in situations in which a damage survey cannot be completed or in which the survey does not provide clear evidence as to what phenomenon caused the damage.

Full access
Howard B. Bluestein, Michael M. French, Jeffrey C. Snyder, and Jana B. Houser

Abstract

Supercells dominated by mesocyclones, which tend to propagate to the right of the tropospheric pressure-weighted mean wind, on rare occasions produce anticyclonic tornadoes at the trailing end of the rear-flank gust front. More frequently, mesoanticyclones are found at this location, most of which do not spawn any tornadoes. In this paper, four cases are discussed in which the formation of anticyclonic tornadoes was documented in the plains by mobile or fixed-site Doppler radars. These brief case studies include the analysis of Doppler radar data for tornadoes at the following dates and locations: 1) 24 April 2006, near El Reno, Oklahoma; 2) 23 May 2008, near Ellis, Kansas; 3) 18 March 2012, near Willow, Oklahoma; and 4) 31 May 2013, near El Reno, Oklahoma. Three of these tornadoes were also documented photographically. In all of these cases, a strong mesocyclone (i.e., vortex signature characterized by azimuthal shear in excess of ~5 × 10−3 s−1 or a 20 m s−1 change in Doppler velocity over 5 km) or tornado was observed ~10 km away from the anticyclonic tornado. In three of these cases, the evolution of the tornadic vortex signature in time and height is described. Other features common to all cases are noted and possible mechanisms for anticyclonic tornadogenesis are identified. In addition, a set of estimated environmental parameters for these and other similar cases are discussed.

Full access
Michael M. French, Howard B. Bluestein, Ivan PopStefanija, Chad A. Baldi, and Robert T. Bluth

Abstract

Observations from a hybrid phased-array Doppler radar, the Mobile Weather Radar, 2005 X-band, Phased Array (MWR-05XP), were used to investigate the vertical development of tornadic vortex signatures (TVSs) during supercell tornadogenesis. Data with volumetric update times of ∼10 s, an order of magnitude better than that of most other mobile Doppler radars, were obtained up to storm midlevels during the formation of three tornadoes. It is found that TVSs formed upward with time during tornadogenesis for two cases. In a third case, missing low-level data prevented a complete time–height analysis of TVS development; however, TVS formation occurred first near the ground and then at storm midlevels several minutes later. These results are consistent with the small number of volumetric mobile Doppler radar tornadogenesis cases from the past ∼10 years, but counter to studies prior to that, in which a descending TVS was observed in roughly half of tornado cases utilizing Weather Surveillance Radar-1988 Doppler (WSR-88D) data. A comparative example is used to examine the possible effects relatively long WSR-88D volumetric update times have on determining the mode of tornadogenesis.

Full access
Michael M. French, Howard B. Bluestein, Ivan PopStefanija, Chad A. Baldi, and Robert T. Bluth

Abstract

A mobile, phased-array Doppler radar, the Mobile Weather Radar, 2005 X-band, Phased Array (MWR-05XP), has been used since 2007 to obtain data in supercells and tornadoes. Rapidly updating, volumetric data of tornadic vortex signatures (TVSs) associated with four tornadoes are used to investigate the time–height evolution of TVS intensity, position, and dissipation up through storm midlevels. Both TVS intensity and position were highly variable in time and height even during tornado mature phases. In one case, a TVS associated with a tornado dissipated aloft and a second TVS formed shortly thereafter while there was one continuous TVS near the ground. In a second case, the TVS associated with a long-lived, violent tornado merged with a second TVS (likely a second cyclonic tornado) causing the original TVS to strengthen. TVS dissipation occurred first at a height of ~1.5 km AGL and then at progressively higher levels in two cases; TVS dissipation occurred last in the lowest 1 km in three cases examined. Possible explanations are provided for the unsteady nature of TVS intensity and a conceptual model is presented for the initial dissipation of TVSs at ~1.5 km AGL.

Full access