ORIGINAL PRESENTATION – 11 MAR 2010NBGSA CONFERENCE – SAN DIEGOUPDATED – 5 APR 2013WWW.ROUTE283EAST.COMTHIS IS A SAMPLE PRESENTATION

KINEMATIC STRUCTURE AND FEATURES KINEMATIC STRUCTURE AND FEATURES OF A COOL-SEASON BOW ECHOOF A COOL-SEASON BOW ECHO

Table of Contents

• Intro• System Setup• Storm Overview• Kinematic Structure• Findings• Conclusions

Bow Echo Characteristics

• Bow Echoes are specialized Convective Weather Systems• Identifiable by curving section of thunderstorms on radar• Presence of bow echo indicates strong winds at surface• Can be born from squall lines, supercells or cell merger• 2 typical patterns: Warm Season (Great Plains/Great Lakes) &

Cool Season (Southeast/Atlantic Coast)

Bow Echo near Tuscaloosa, AL

Bow Echoes: Structure & Features

• Strong mid-level winds move through cold dry air at 3-7 km AGL (Johns 1993)

• When they encounter main downdraft, storm is strengthened at surface through increased evaporation and vertical momentum transport (Weisman 2001)

• Combines with the downdraft outflow (gust front) to accelerate system, cause the bowing of line (Smull & Houze 1987b)

• Behind the apex is the Rear Inflow Jet (RIJ) (Przybylinski 1983)

• Bookend vorticies flank and amplify the RIJ (Weisman 1990)

• Long-lived bow echoes can become derechoes (Heinrich 1888)

Bow Echo evolution, from Fujita (1978); note “Bookend Vorticies”

Winds (U vs UR)U-winds (east/west winds) are primarily from the west in the strongest part of the storm.

UR-winds: east/west winds RELATIVE to storm motion.

Stronger winds embedded within the storm flow.

Strong westerly winds alternate with weaker westerly winds.

Mesovortices• Storm motion, downdraft & RIJ account for much damage (especially at the bow apex)• Mesovortices along the line can enhance surface wind along the line• 9 March 2006 had one very strong mesovortex and other weaker ones

• Vortex will ENHANCE ambient flow to the south and INHIBIT flow to the north, possible that wind is STRONGER than at apex• POSITIVE vorticity

Take-Home PointsSquall lines can be as dangerous as more well known weather events (Tornadoes, etc)

Dangerous winds can be from a combination of storm features

Mesovortices are an important part of this storm’s dynamics

More research needs to be done in the areas of:

Cool-Season Severe Weather

Severe Weather in the Southeast US

Damaging windstorms in the Southeast US

Comparison of Derecho Fatalities

153

71

158

254 Derechoes

F0 & F1 Tornadoes

F2 Tornadoes

Hurricanes

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