Monday, 3 August 2015
Back Bay Ballroom (Sheraton Boston )
On 19 October 2014, a squall line with an embedded 'comma-shaped' bow echo (inverted comma in Southern Hemisphere) impacted Southern Brazil, causing material damage due to severe straight-line winds. The damaging-wind path and duration suggest this event was a derecho. Storm morphology was analyzed using a S-band radar in the city of Santiago, Southern Brazil. Synoptic and thermodynamic environments were analyzed through surface stations and sounding data, as well as the CFSR reanalysis fields preceding the event. Several intense storms formed along an outflow boundary over Northern Argentina, where a tornado was reported, and then moved east toward Brazil organized as a squall line. Most severe wind gusts were associated with embedded bow echoes, and the most intense measured gust, of 60 kt, occurred near the head of the comma-shaped bow echo. Moreover, the damage registered in some locations lead to estimates of 80-100 kt wind gusts. The squall line formed under weak forcing, in the left entrance of an upper-level jet streak and ahead of a surface cold front. A sounding launched just before the squall line passage showed intense low-level vertical wind shear (17 m/s 0-1-km and 15 m/s 0-3-km line-perpendicular shear), favoring bowing segments within the squall line. Below approximately 2.5 km above ground level the shear was mostly directional, becoming predominantly a speed shear above this height. Most unstable CAPE was 2100 J/kg, indicating moderate thermodynamic instability, but precipitable water was more than 50 mm, which favors heavy precipitation and strong cold pools. 250-hPa winds of more than 85 kt were observed, coherent with a leading-line propagation at approximately 50 kt. The comma-shaped bow echo formed in response to strengthening of the cyclonic bookend vortex associated with the bow echo. The high-precipitation supercell responsible for the tornado in Argentina possibly provided cyclonic vorticity to the bookend vortex. The supercell formed before the squall-line organization and may have played an important role in creating the cold pool responsible for driving the system. Evolution of the comma-shaped system follows theoretical models for the Northern Hemisphere. The bowing segment moved faster than other sectors of the squall line, which might have been caused by acceleration of the Rear-Inflow Jet. After bookend vortex formation, the northern flank of the squall line caused wind gusts of more than 50 kt in other locations, while the southern flank weakened, causing wind gusts of less than 25 kt. This study describes a classic case of a strong comma-shaped bow echo embedded in a squall line, and it is the first of this kind realized in Southern Brazil. Future work on numerical simulations of bow echoes is required to elucidate the behavior of these systems in this region.
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