- Количество слайдов: 25
A Case Study of Two Left-Moving Mesoanticyclonic Supercells on 24 April 2006 Chris Bowman National Weather Service – Wichita, KS
The Situation… Synoptic environment favored supercells. n Tornado Watch 244 already issued. n As the event is unfolding, storms are not taking on classic supercell characteristics. n Two cells and especially one in particular begin to exhibit unusual radar signatures. n
RUC Model 22 Z Forecast Sounding
22 Z RUC Forecast Hodograph
KICT Reflectivity and SRM 0. 5° Evolution 21: 38 Z – 22: 13 Z Animation
KICT Reflectivity and SRM All-Tilts 22: 13 Z 0. 5° -19. 5° Animation
KVNX Reflectivity and SRM 0. 5° Evolution 21: 37 Z – 22: 11 Z Animation
KVNX Reflectivity and SRM 0. 5° Evolution 22: 16 Z – 22: 35 Z Animation
KICT and KVNX Summary Observed Items of Interest Boundary interaction or cell merger (BICM). n Rapid strengthening of storm after BICM. n Persistent and strengthening low-level (0. 5°) rotation. n KICT indicates cyclonic (≈1, 000 ft), KVNX indicates anticyclonic (≈4, 750 ft). n KICT indicates a transition from low-level cyclonic rotation (≈900 – 2, 700 ft) to a convergent signature (≈3, 000 – 10, 000 ft) to a mesoanticyclone (≈12, 000 ft – 25, 000 ft). n
What’s the decision? Tornado? Severe? No warning? You’re in the warning chair under a tornado watch. n Storms have not exhibited typical supercell characteristics. n A storm develops rapidly with strengthening cyclonic rotation corroborated by another radar. n Storm appears to be supercellular but opposite of what was anticipated. n The storm is moving into the Wichita Metro. n
What Actually Happened n n Tornado warning issued at 22: 15 Z for SW Sedgwick and NW Sumner Counties but polygon kept Wichita out of the warning. Two off-duty NWS mets as well as private mets observe a wall cloud with the storm. Only live report came from an off-duty NWS met as the storm and “illdefined wall cloud” moved over his house. No tornado was reported but several severe hail reports were received. The storm, as viewed from KVNX, maintained its supercellular characteristics as it moved over Wichita, producing only hail reports.
KICT and KVNX Summary The Unexplained n n Although the environment favored left-moving, mesoanticyclonic storms (more on this later), there is no clear explanation for KICT’s vertical SRM transition from low-level cyclonic rotation to mid/upper-level anticyclonic rotation The low-level velocity signature from both KICT and KVNX are ≈3 miles north-northeast of the reflectivity signature. Reflectivity aloft is almost on top of low-level SRM signature. Possibly a boundary being ingested and stretched or even thunderstorm outflow.
Storm #2 Actually occurred before the tornado warned storm. n Exhibited similar characteristics (including velocity signature ≈3 miles north-northeast of reflectivity signature) but only observed from KVNX. n Storm not as persistent and not as strong. n
KVNX Storm #2 0. 5° Reflectivity and SRM 21: 27 Z – 21: 51 Z Animation
Storms #1 and #2 Summary Two storms in the same county within several volume scans of each other both develop lowlevel anticyclonic rotation displaced by about 3 miles to the north of the reflectivity signature. n Both have a boundary interaction or cell merger and strengthen rapidly afterward. n Both storms are in the same environment and this is the key. n
Environmental Data n n Parameters derived from surface T/Td of 76/66. Near storm environment was actually ≈ 74/64. Not a huge difference right? In this case it actually makes a big difference. Need to modify sounding as a result.
Environmental Data n n Modified sounding drops CAPE from 2, 038 J/Kg to 1, 324 J/Kg. Increases CIN from -10 J/Kg to -68 J/Kg. Results in an environment that is elevated. As a result, a modified hodograph needs to be used.
Environmental Data n n n Unmodified hodograph with derived parameters. V(obs) SRH values are positive. Storm motion = 255 @ 21 kts.
Environmental Data Modified hodograph with derived parameters using an elevated depth of 1 km and actual storm motion of 200 @ 29 kts. n V(obs) SRH values have become negative. n
Case Summary n n Two storms occurred in the same county and within several volume scans of each other that exhibited unusual radar characteristics including: Persistent and strengthening low-level (0. 5°) rotation. Cycolnic with KICT and anticyclonic with KVNX. The rotation (from KICT and KVNX) is displaced ≈3 miles north-northeast of reflectivity. KICT indicates a transition from low-level cyclonic rotation to a mid/upper-level mesoanticyclone.
Case Summary n n n Model data indicated a surface based environment when the near storm environment was actually elevated. Model forecast storm motion was not correct. Modified hodograph, taking into account the elevated nature and actual storm motion, reveals an environment favorable for left-moving, mesoanticyclonic supercells.
Acknowledgements Matt Bunkers, SOO, WFO Rapid City For his help and expert opinions as well as his hodograph spreadsheet. n Ken Cook, SOO, WFO Wichita n