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Bow Echoes

Bow Echoes. Bow Echo: radar-observed features. mesovortex. mesovortex. weak echo notch. apex of bow echo. bookend vortex. mid-level overhang. Houze et al. (BAMS 1989). Front-to-Rear Flow. All flows are system-relative. Ascending front-to-rear flow:

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Bow Echoes

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  1. Bow Echoes

  2. Bow Echo: radar-observed features mesovortex mesovortex weak echo notch apex of bow echo bookend vortex mid-level overhang

  3. Houze et al. (BAMS 1989) Front-to-Rear Flow All flows are system-relative • Ascending front-to-rear flow: • Instrumental in creating stratiform rain shield • Forced by a horizontal pressure gradient associated with mid-levelmesolow in the stratiform rain area; mesolow marks the hydrostatic responseto the net latent heat release by the convective cells

  4. - - + + + Phenomena associated with horizontal vorticity h • Updraft 2. Vertical Shear 3. Cold Pool

  5. Interactions of Vorticity Regions Mismatched vorticity regions of opposite sense Matching vorticity regions of opposite sense

  6. Life Cycle • Downshear tilt due toambient shear • Balance of cold pool and low-level shear • Cold pool overwhelms low-level shear; formation of a rear inflowjet “RKW Theory” (Weisman andRotunno; JAS 2004)

  7. Houze et al. (BAMS 1989) Rear Inflow Jet • Rear Inflow Jet (RIJ): • Generated by a vertical gradient in horizontal buoyancy contrasts • The circulation associated with latent heat release aloft reinforces the cold pool circulation • In other words, RIJ accelerates into mid-level L

  8. Houze et al. (BAMS 1989) Rear Inflow Jet • Rear Inflow Jet (RIJ): • RIJ descends towards the front of the squall line where as • -- jet cools due to melting of ice • -- rain falls into its dry air cooling it through evaporation • RIJ helps maintain strong rising motion near the leading edge of the cold pool

  9. Strong Squall Lines • Strong environmental shear • Gust Front stays close to precipitation • Storm remains upright near deep gust front

  10. Vorticity Interaction: Cold Pool Lift 2.3: Matching cold pool + shear LFC = + Deep Lift shear counteracts the cold pool’s tendency to sweep environmental air over the top of the cold pool.

  11. Strong Squall Lines • Storm-Relative Velocity shows gust front at leading edge of reflectivity core • Gust front is also vertically-stacked and deep

  12. Bow Echoes • 20-200 km long curved line of cells usually associated with long swaths of damaging surface winds • Develop some hours into MCS lifecycle + can persist for several hours • Forms in environments similar to supercells (CAPE > 2000 J/kg; strong vertical shear of 30 kts+ over lowest 2.5-5 km) except for mechanisms promoting linear, rather than cellular, organisation (high T/Td spread etc.) • Rear Inflow Jet associated with a pair of cyclonic and anti-cyclonic “bookend” vortices • mid-level vortices(~ 3-7 km above ground), one at each ends of line • primarily produced by tilting of horizontal vorticity at downdraft edge • cyclonic poleward vortex usually becomes dominant over time due toCoriolis  comma-shaped system appearance

  13. Areas of particularly severe winds within bow echoes • strong straight-line winds in apex of bow • cyclonic bookend vortex on poleward end • anticyclonic bookend vortex on equatorward end • low-level mesovortices near or poleward of apex of any bowing segments

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