Unorganized Airmass Convection

Ingredients, parameters and thresholds defining environments favorable for airmass thunderstorms and pulse storms

Ingredients

Ingredients necessary for airmass thunderstorms :

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).

Instability - The higher the instability, the stronger the "pulse storms"

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 and increase instability in order to erode the capping inversion (convective inhibition - CIN)

Parameters

Parameters useful in determining whether pre-convective environments are favorable for airmass thunderstorms :

For Moisture :
- Mixing ratio [g/kg] / Specific Humidity [g/kg] :
  - 0-500 m mixing ratio (ESSL Model Maps - GFS)
  - 0-1 km mixing ratio (Lightning Wizard - GFS)
  - 500 m specific humidity (ESSL Weather Data Displayer - ICON-EU)
- Dew points [°C] :
  - 2m AGL dew points (Pivotal Weather - ICON) , 2m AGL dew points (Météociel - WRF-NMM)
  - 925 hPa dew points (Pivotal Weather - ICON)
  - 850 hPa dew points (Pivotal Weather - ICON)
- Equivalent Potential Temperature [°C] :
  - 850 hPa Theta-e (Météociel - GFS) , 850 hPa Theta-e (Wetter Online - ECMWF)
  - 700 hPa Theta-e (Wetter Online - ECMWF)

For Instability :
- CAPE [J/kg] :
  - MLCAPE (Pivotal Weather - ICON)
  - MUCAPE (Météociel - AROME) , MUCAPE (Pivotal Weather - GFS)
- Lifted Index [°C]
  - SBLI (Pivotal Weather - GFS)
  - MULI (WRF-NMM 2km)
- Temperature Lapse Rates [°C/km]
  - 700-500 hPa lapse rates (Pivotal Weather - ICON)
  - 850-500 hPa lapse rates (ESSL Weather Data Displayer - ICON-EU)

For Lifting Mechanisms :
- Level of Free Convection (LFC) [m]
  - LFC (ESSL Weather Data Displayer - C-LAEF)
- Convective Inhibition (CIN) [ J/kg ]
  - MLCIN (ESSL Weather Data Displayer - ICON-D2)
- Low-Level Lift :
  - 850 hPa Frontogenesis (Pivotal Weather - GFS)
  - 850 hPa Temperature Advection (Pivotal Weather - GFS)
  - 850 hPa temperatures (Pivotal Weather - ICON)
  - For Convective Condensation Level (CCL) and Convective Temperatures (Tc) -> check soundings
- Mid/Upper-Level Lift :
  - 500 hPa relative vorticity (Pivotal Weather - ICON) , 500 hPa relative vorticity (WXCharts - GFS)
  - Q-Vectors : 700 hPa , 500 hPa , 300 hPa (ESSL Weather Data Displayer - ICON-EU)
  - 700 hPa Temperature Advection (Pivotal Weather - GFS)
  - Potential Vorticity on 320K isentropic surface (ESSL Model Maps - GFS)
  - Dynamic Tropopause : 2 PVU Theta , 2 PVU Pressure (Pivotal Weather - GFS)

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

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