mesovortex

mesovortex

apexof bow echo

Bow Echo: radar-observed features

mid-level overhang

weak echonotch

bookendvortex

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Front-to-Rear Flow

Houze et al. (BAMS 1989)

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

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Phenomena associated with horizontal vorticity Phenomena associated with horizontal vorticity

+-

1. Updraft 2. Vertical Shear 3. Cold Pool

+

+ -

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Interactions of Vorticity Regions Interactions of Vorticity Regions

Matching vorticity regions of opposite sense

Mismatched vorticity regions of opposite sense

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Life Cycle Life Cycle

(a) Downshear tilt due toambient shear

(b) Balance of cold pool and low-level shear

(c) Cold pool overwhelms low-level shear; formation of a rear inflowjet

“RKW Theory” (Weisman andRotunno; JAS 2004)

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Rear Inflow Jet

Houze et al. (BAMS 1989)

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

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Houze et al. (BAMS 1989)

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

Rear Inflow Jet

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Strong Squall LinesStrong Squall Lines

Strong environmental shear

Gust Front stays close to precipitation

Storm remains upright near deep gust front

Strong environmental shear

Gust Front stays close to precipitation

Storm remains upright near deep gust front

1-4 August 2007 WMO Workshop on Radar Meteorology & Nowcasting1-4 August 2007 WMO Workshop on Radar Meteorology & Nowcasting

Vorticity Interaction: Cold Pool LiftVorticity Interaction: Cold Pool Lift

+ =

2.3: Matching cold pool + shear

LFC

DeepLift

shear counteracts the cold pool’s tendency to sweep environmental air over the top of the cold pool.

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Strong Squall LinesStrong Squall Lines

Storm-Relative Velocity shows gust front at leading edge of reflectivity core

Gust front is also vertically-stacked and deep

Storm-Relative Velocity shows gust front at leading edge of reflectivity core

Gust front is also vertically-stacked and deep

1-4 August 2007 WMO Workshop on Radar Meteorology & Nowcasting1-4 August 2007 WMO Workshop on Radar Meteorology & Nowcasting

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 to

Coriolis comma-shaped system appearance

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 to

Coriolis comma-shaped system appearance

Bow EchoesBow Echoes

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Areas of particularly severe winds within bow echoesAreas 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