Mobile, phased-array, X-band Doppler radar observations of tornadogenesis in the central U. S.

Transcription

1 Mobile, phased-array, X-band Doppler radar observations of tornadogenesis in the central U. S. Howard B. Bluestein 1, Michael M. French 2, Ivan PopStefanija 3 and Robert T. Bluth 4 Howard (Howie Cb ) B. Bluestein 1,2 School of Meteorology, University of Oklahoma, Norman, Oklahoma, U. S., hblue@ou.edu, mfrench@ou.edu 3ProSensing, Inc., Amherst, Massachusetts, U. S., popstefanija@prosensing.com 4Naval Postgraduate School, Monterey, California, U. S., rtbluth@nps.edu 1. Introduction Since tornadoes evolve on very short time scales (~ 1 10 s), it is necessary to probe them at rapid rates. Since 2007, a group from the University of Oklahoma (OU) in collaboration with the Naval Postgraduate School and ProSensing, Inc., has been using a truck-mounted, X-band, phased-array Doppler radar (originally designed for military applications) to probe tornadoes and severe convective storms with the main objective of documenting tornadogenesis on the storm and sub-storm scale. Details on the radar, the MWR-05XP (Meteorological Weather Radar 2005, X-band, Phased array) and its use in the field are found in Bluestein et al. (2010). The most significant properties of the radar are as follows: The antenna scans very rapidly: mechanically in azimuth and electronically in elevation; the elevation angle increases in increments of The antenna also electronically backscans ~ so that the beam remains nearly fixed in spaced long enough to obtain enough independent samples to get accurate estimates of Doppler velocity and negate beam smearing. Electronic scanning in elevation angle and azimuth angle is accomplished through changes in phase delay and changes in frequency, respectively. Range resolution is 150 m and data are oversampled every 75 m. The angular resolution of the antenna is in azimuth angle and 2 0 in elevation angle, which is less than the resolution of most other mobile, X-band, Doppler radars, which have half-power beamwidths of ~ 1 0 ; however, the resolution of the radar is adequate for resolving storm and sub-storm scale vortices, but not most sub-tornado scale motions unless the tornado is large and at close range. In most cases, volume sector scans with oversampling in the vertical are possible up to 20 0 elevation angle every 6 7 seconds. Storm top may be reached and exceeded with elevation angle extended up to when the storm is far enough away to obtain full volume scans ~ every seconds. 2. Examples of data collected 2.1 The 2008 storm season No tornadoes were sampled during the first year the radar was used (spring, 2007). During the 2008 spring season, tornadogenesis was captured with an update time of s (frequency agility had not yet been implemented) on 23 May in Kansas and described in Bluestein et al. (2010). More recent analysis of the data from this case is shown in Fig. 1 (in this and subsequent figures, the resolution may not be good enough to resolve the scales; however, the overall patterns should be apparent), which depicts the temporal evolution of the parent vortex. It is seen that initially there is a relatively broad mesocyclone signature (lateral shear of Doppler velocity) centered at 3 4 km AGL, whose tilt with height above is opposite in direction (in the height azimuth plane) to the tilt below. A smaller-scale vortex formed ~ 2 km AGL and then intensified both above and below, extending from the surface to ~ 3.5 km when the vortex was

2 mature. This process proceeded very rapidly and would not have been resolved if the volume update time were a minute or longer, as in operational WSR-88D data collection. Another cyclonic tornado was resolved, but its formation was not captured, so it is not discussed here. In addition, a strong anticyclonic vortex paired with a companion cyclonic vortex/tornado was noted. The formation of a lone anticyclonic tornado along the southern edge of a rear-flank gust front is depicted in Fig. 2. It is seen that unlike the cyclonic tornado documented in Fig. 1, the anticyclonic tornado began at the ground and built upward, reaching 3 km AGL about a minute later. The formation of the latter is akin to the formation of a landspout, or non-supercell tornadogenesis. FIG. 1. Vertical cross sections [height above radar level in km up to 6 km ( elevation angle) vs. azimuth in degrees] of edited ground-relative Doppler velocity in m s -1 across a developing tornado and its parent cyclone at 17 km range, near Hog Back, Kansas from 0203: :48 UTC 24 May 2008, at approximately second intervals.

3 2.2 VORTEX-2, year 1 (spring, 2009) In 2009, frequency agility was implemented in the MWR-05XP for the first time, so that the update time was reduced by factor of two over what it had been in 2007 and The 2009 field program was embedded within VORTEX-2 (Verification of the Origins of Rotation in Tornadoes Experiment), conducted in the Plains of the U. S. and involved many other mobile Doppler radars. While many supercells were sampled, only one tornado was documented, but its entire life, from tornadogenesis to tornado dissipation was documented (Figs. 3 5). FIG. 2. As in Fig. 1, but for an anticyclonic tornado. The range to the tornado is km and panels shown are at 0205:17, 0205:45, and 0206:13 UTC. (Only every other scan is shown.) FIG. 3. (left): MWR-05XP scanning a tornado in southeastern Wyoming on 5 June (Photograph courtesy of C. Baldi.) FIG. 4. (right): Radar reflectivity factor (dbz) of the tornado and parent storm seen in Fig. 2 at 2247:19 UTC, at elevation angle, at 5.5 km range. A weak-echo hole and spiral bands are evident.

4 FIG. 5. Ground-relative, edited Doppler velocity (m s -1 ) of the tornado and its parent cyclone seen in Fig. 2 from 2205:05 to 2205:30 UTC at approximately 6 7 second intervals, at 1 0 elevation angle and 10.5 km range FIG. 6. (part 1).

5 2.3 VORTEX-2, year 2 (2010) During the second year of VORTEX-2 (spring, 2010), tornadogenesis was captured in a supercell on 10 May during a major tornado outbreak in central Oklahoma (Fig. 6). Tornadogenesis was also captured near the Colorado Kansas border in a supercell (Fig. 7). Other tornadoes may have been detected in other supercells (e. g., on 19 May in Oklahoma), but tornadogenesis was not captured. Finally, many non-tornadic supercells or supercells that produced tornadoes at other times were sampled (e. g., Fig. 8). FIG. 6. (part 2): Radar reflectivity factor in dbz (left panels) and unedited, ground-relative Doppler velocity (right panels) in m s -1 at elevation angle on 10 May 2010 in east-central Oklahoma from 2348: :04 UTC, at approximately 8 9 second intervals, while a tornado was forming at a range of ~ 14 km. Data were collected up to 20 0 elevation angle.

6 FIG. 7. Tornado probed by the MWR-05XP on 25 May Tornadogenesis was captured for this brief tornado near Towner, CO and Tribune, KS; at the time of this writing processed data are not ready yet for analysis. (Photograph by H. Bluestein) FIG. 8. MWR-05XP probing a non-tornadic supercell on 10 June 2010 in eastern Colorado (northwest of Last Chance). (Photograph by H. Bluestein). 3. Summary and concluding discussion The MWR-05XP has been used successfully to document the formation of four tornadoes in supercells, some tornadoes that had already formed in others (an exact number is not known yet), and many other non-tornadic supercells or supercells that had produced tornadoes earlier or later. Analysis of processed data is underway, while at the time of this writing other datasets from the recent, 2010 field program (VORTEX-2, year 2) have yet to be fully processed. Scientific conclusions regarding tornadogenesis will therefore be delayed pending the completion of analysis of all the datasets collected in the past several years. Differences in the structure and evolution between the tornadic and nonsupercell tornadic storms will be sought. One of the advantages of using this radar is that the structure and evolution on short time scales of at least the bottom half of the parent storm are sampled, not just the lower portion, and in some instances the entire storm is sampled. Doing so allows us to test hypotheses regarding tornado formation that involve features in the middle and upper reaches of the parent storm (in distinction with higher-resolution, rapid-scan radars that peer mainly into the boundary layer). It is emphasized that owing to the relatively coarse spatial resolution, this radar is most suited for following the evolution of storm-scale and sub-storm-scale vortices, not features within tornadoes themselves. To extend the usefulness of the radar facility when there are insufficient scatters in the boundary layer in the vicinity of the tornado and/or the area in which it forms, a pulsed Doppler lidar was installed in the radar truck and tested during the second year of VORTEX-2. The lidar data, which complement the radar data, are being processed and will be analyzed and discussed in future presentations.

7 Acknowledgments Chad Baldi (ProSensing, Inc.) was a major participant each year in the field programs involving the MWR-05XP. Bethany Seeger (ProSensing, Inc.) provided software support for both the field phases and post-processing. Paul Buczynski (Naval Postgraduate School) also provided support for the field programs. The first two authors are appreciative of support from NSF grants ATM and AGS for funding the data analysis phase of the projects and some of the field costs. VORTEX-2 field coordinators provided expert guidance in 2009 and References Bluestein, Howard B., Michael M. French, Ivan PopStefanija, Robert T. Bluth, and Jeffrey P. Knorr, 2010: A mobile, phased-array Doppler radar for the study of severe convective storms. Bull. Amer. Meteor. Soc., 91,