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Unorganized airmass convection

Organized Convection

Atmospheric ingredients and forecast parameters/thresholds defining environments favorable for dynamic squall lines, bow-echoes and supercells

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Atmospheric Ingredients

Ingredients necessary for organized thunderstorms : 

 

These include the ingredients necessary for ordinary unorganized airmass thunderstorms plus the organizing parameter known as "vertical wind shear". Vertical wind shear (of the horizontal wind) allows a storm's updraft to remain separate from it's downdraft thereby helping strengthen both of them and aids in lengthening the storm's lifetime. This aids in organizing the convection and shifting the convective modes towards dynamic squall lines, bow echoes and/or supercells which in turn tends to increase the risk of large hail production, severe straight-line winds and sometimes tornadoes.

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  • Moisture - most important at low-levels (PBL) to increase dewpoints and mixing ratios (the deeper and more pronounced the low-level moisture, the more potentially unstable the airmass becomes and the lower the Level of Free Convection - LFC, which facilitates convective initiation

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  • Instability - The higher the instability, the more energy available for organized storms

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  • Lifting mechanism - necessary at low-levels (front, topography, diurnal heating) to reach the Level of Free Convection (LFC) and at mid/upper-levels (large-scale lift) to cool the air aloft and increase instability in order to erode the capping inversion (convective inhibition - CIN)

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  • Vertical wind Shear - It's the "organizational" ingredient... the higher the vertical wind shear, the more organized and long-lived the convection can become.  

Forecast Parameters

Forecast parameters per ingredient useful in determining whether pre-convective environments are favorable for organized convection :

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  • For Instability :

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Forecast Parameter Thresholds

Moisture

Mixing Ratios

< 3 g/kg : very dry

​3 - 5 g/kg : dry 

5-7 g/kg : slightly humid/moist

7-12 g/kg :  moderately humid/moist

12-15 g/kg : very humid/moist

> 15 g/kg : extremely humid/moist

Dew Points

< -3 °C : very dry

​-3 to +5 °C : dry 

5 to 10 °C : slightly humid/moist

10 to 17 °C :  moderately humid/moist

17 to 20 °C : very humid/moist

> 20 °C : extremely humid/moist

Equivalent Potential Temperatures (850 hPa)

< 5 °C : very dry

​5 - 18 °C : dry 

18 - 30 °C : slightly humid/moist

30 - 45 °C :  moderately humid/moist

45 - 60 °C : very hot & humid (très lourd)

> 60 °C : extremely hot & humid (ext. lourd)

Instability

CAPE

​0 : stable

0-700 J/kg : weakly unstable

700-1500 J/kg :  moderately unstable

1500-3000 J/kg : very unstable

> 3000 J/kg : extremely unstable

Lifted Indices

>+2°C : stable

​+2 to 0 °C : stable/neutral

0 to -2 °C : weakly unstable

 -2 to -4 °C :  moderately unstable

-4 to -6 °C : very unstable

< -6 °C : extremely unstable

Temperature Lapse Rates

​<  5.5 - 6.0 °C/km : stable

6.0 - 7.0 °C/km : slightly unstable/steep

7.0 - 8.0 °C/km  :  moderately unstable/steep

8.0 - 9.0 °C/km : very unstable/steep

> 9.0 °C/km : extremely unstable/steep

Lift

Level of Free Convection (LFC)

​< 1000 m : very low (convective initiation very easy) 

1000 - 1500 m : moderately low (conv. initiation easy)

1500 - 2000 m :  average height (conv. initiation probable if low-level lift or convective temperature reached)

2000-3000 m : moderately high (conv. initiation more difficult and isolated unless strong lift)

> 3000 m : very high (conv. initiation very difficult and unlikely unless strong lift)

Convective Inhibition (CIN)

​0 to -50 J/kg  : weak CIN (cap easily broken)

-50 to - 100 J/kg : moderate CIN (cap regionally broken if moderate lift) 

-100 to -150 J/kg :  strong CIN (cap locally broken if moderate lift)

< -150  J/kg : very strong CIN (strong large-scale lift needed to erode cap)

Temperature Advection (850 & 700 hPa)

​> -5 °C/hr : strong CAA / strong subsidence

-5 to -2 °C/hr : moderate CAA / moderate subsidence

-2 to 0 °C/hr :  weak CAA / weak subsidence

0 to +2 °C/hr : weak WAA / weak ascent

+2 to +5 °C/hr : moderate WAA / moderate ascent

> +5 °C/hr : strong WAA / strong ascent

CAA = Cold Air Advection

WAA = Warm Air Advection

Also use square method : the smaller the geopotential/isotherm square, the stronger the temperature advection. 

Vorticity Advection (500 hPa)

​> -30 x10-5/s : strong NVA / strong subsidence

-30 to -15 x10-5/s : moderate NVA / moderate subsidence

-15 to 0 x10-5/s :  weak NVA / weak subsidence

0 to 15 x10-5/s : weak PVA / weak ascent

15 to 30 x10-5/s : moderate PVA / moderate ascent

> 30 x10-5/s : strong PVA / strong ascent

NVA = Negative Vorticity Advection

PVA = Positive Vorticity Advection

Also use square method : the smaller the geopotential/vorticity isopleth square, the stronger the vorticity advection.

Q-Vectors

Divergence of Q-vectors : sinking motion (the stronger the Q-vector divergence (blue contours) , the stronger the sinking motion)​

Convergence of Q-vectors : rising motion (the stronger the Q-vector convergence (red contours) , the stronger the rising motion)

Potential Vorticity (PV)

Advection of High PV : airmass ascent along isentropes

Advection of Low PV : airmass subsidence along isentropes

Convection can produce high PV areas due to diabatic heating

Dynamic Tropopause

Advection of low tropopause Theta air : airmass ascent

Advection of high tropopause Theta air : airmass subsidence

Advection of low tropopause pressure : airmass ascent

Advection of high tropopause pressure : airmass subsidence

Vertical Wind Shear

0-6 km Bulk Shear

0 - 15 kts : weak shear (favors ordinary convection - airmass/pulse thunderstorms)

15 - 35 kts : moderate shear (favors multicellular convection - multicell clusters/squall lines)

> 35 kts : strong shear (favors organized convection - isolated supercells, supercells embedded in lines, bow-echoes)

0-3 km Bulk Shear

0 - 10 kts : weak shear (favors gust fronts clearly outrunning the convection)

10 - 30 kts : moderate shear (favors gust-fronts closer to the convection)

> 30 kts : strong shear (favors deep cold pools with gust fronts on leading edge of convection)

Contact

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01 23 45 67 89

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