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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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Presentation Transcript
slide2

Bow Echo: radar-observed features

mesovortex

mesovortex

weak echo

notch

apex

of bow echo

bookend

vortex

mid-level overhang

slide3

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
phenomena associated with horizontal vorticity h

-

-

+

+

+

Phenomena associated with horizontal vorticity h
  • Updraft

2. Vertical Shear

3. Cold Pool

interactions of vorticity regions
Interactions of Vorticity Regions

Mismatched vorticity regions

of opposite sense

Matching vorticity regions

of opposite sense

life cycle
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)

slide7

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

slide8

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
strong squall lines
Strong Squall Lines
  • Strong environmental shear
  • Gust Front stays close to precipitation
  • Storm remains upright near deep gust front
vorticity interaction cold pool lift
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.

strong squall lines1
Strong Squall Lines
  • Storm-Relative Velocity shows gust front at leading edge of reflectivity core
  • Gust front is also vertically-stacked and deep
bow echoes
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
areas of particularly severe winds within bow echoes
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