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The convection scheme does not predict individual convective clouds, but only their physical effect on the surrounding atmosphere in terms of latent heat release, precipitation and the associated transport of moisture and momentum.  The scheme differentiates between deep, shallow and mid-level convection but only one type of convection can occur at any given grid point at any one time.  Super-cooled liquid water is held by the convection scheme and even at colder temperatures (down to -38C) aids the development of convective precipitation.    Convective precipitation produced by IFS is in the form of convective rain or convective snow.  Hail is not forecast.

The effects of the model convection (changes to the temperature or humidity) drift downwind with model winds.   However, any convective precipitation that is developed by the model is considered to remain within the grid box column and fall vertically downwards instantaneously (i.e. taking zero time to reach the surface).   Thus

Thus, model showers are not advected with the wind during their life-cycle.  In particular, any showers that the model develops over the sea do not penetrate beyond the coast.

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New ways of forecasting the degree of sub-grid variability in precipitation totals have also been developed (Point Rainfall).  Future updates to the IFS may allow some of the convective precipitation (mainly as snow) to be advected downstream into adjoining grid boxes.

CAPE and CIN

CAPE and CIN These parameters are computed in order to help the user assess the likelihood of severe convective storms.

CAPE

Convective Available Potential Energy (CAPE) gives  They give information on the the convective energy and the availability of low-level moisture.  CAPE can be a guide to the intensity of convection, but only if convection triggers.  It is only one measure of the potential for severe convection and thunderstorms.  Instability indices also give some indication. 

CAPE and MUCIN, and CIN and MUCIN are parameters that can be derived from vertical profiles of temperature and humidity throughout the troposphere that have either been measured or modelled.  The parameters are widely used in the prediction of convective storms, as they describe the specific potential energy of air in the lower troposphere that is potentially released in convective storms.  

The parameters are physical quantities with a direct physical interpretation, which set them apart from Instability indices that only relate to the physics of convection in an indirect way.  

Convective Available Potential Energy parameters:

• CAPE (CAPE_θe) is computed using equivalent potential temperature of the parcel (θ_ep), and the environmental saturated equivalent potential temperature (θ_esat).
• MUCAPE (CAPE_θv) is computed using virtual potential temperature of the parcel (θ_vp), the virtual potential temperature of the environment (θ_ve), and the environmental saturated equivalent potential temperature (θ_esat). 

  
  

MUCAPE (CAPE_θv) has overall higher values than CAPE_θe (and indeed what forecasters would diagnose from vertical profiles of the atmosphere).

CAPE and MUCAPE are computed according to parcel theory.   Both assume:

• a pseudo-adiabatic parcel ascent.
• all condensate removed as soon as it forms.
• no entrainment of surrounding air (evaluated CAPE or MUCAPE is likely to be a slight overestimate).

This is exactly similar to a forecaster analysis of the tephigram.

CAPE is computed in the IFS according to parcel theory, assuming pseudoadiabatic ascent and no entrainment.   This is exactly what one should see if one analyses the tephigram.  At any given grid point the convection scheme inspects the temperature structure of the model atmosphere progressively from the surface to 300hPa and if .  If there exists a level of free convection (LFC) it evaluates the CAPE.   Entrainment of surrounding air is not considered and thus the evaluated CAPE is likely to be a slight overestimate.  The  The search for CAPE currently in use allows discovery of elevated instability, even at night when there will often be low-level stability at lower altitudes.  

CAPE (or CAPE_θe to distinguish it from MUCAPE) is computed assuming a pseudo-adiabatic parcel ascent (all condensate is removed as soon as it forms) using the equivalent potential temperature (θ_ep), and the environmental saturated equivalent potential temperature (θ_esat). 

CIN

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As a guide MUCAPE values:

• greater than 1000 J kg^-1 indicate potential for development of moderate thunderstorms
• greater than 2000 J kg^-1 indicate a potential for severe thunderstorms.  
• 3000 to 4000 J kg^-1 or even higher usually signify a very volatile atmosphere that could produce severe storms if other environmental parameters are in place.

CAPE and MUCAPE can be a guide to the intensity of convection, but only if convection triggers.

Convective Inhibition parameters:

• CIN describes the energy required to provide sufficient lift to overcome any capping inversion and to release the CAPE.
• MUCIN describes the energy required to provide sufficient lift to overcome any capping inversion and to release the CAPE.

MUCIN is identical to CIN as both use virtual temperature during evaluation.

At  CIN is assessed from the model atmosphere in a similar way to the process to identify CAPE.  At any given grid point the convection scheme inspects the temperature structure of the model atmosphere progressively from the surface upwards and if .  If there exists a level of free convection (LFC) it evaluates the energy required for a rising parcel to overcome the inhibiting effect of the underlying temperature structure.   

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CAPE

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MUCAPE has overall higher values than CAPE_θe.  

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MUCIN

Convective inhibition of the most unstable parcel (MUCIN) which corresponds to MUCAPE is the identical to CIN as this already uses the virtual temperature correction.

CAPE-shear

CAPE-shear is a combination of bulk shear (vector wind shear in the lowest 6km of the atmosphere) and CAPE or MUCAPE and is used to identify areas of potentially extreme convection.  Vertical wind shear tends to promote thunderstorm organisation, although excessive wind shear can be detrimental to convective initiation by increasing entrainment of environmental air into the storm.  But if active convection is indeed established, then the larger the wind shear the tends to be associated with higher organisation and severity of convection tends to be.  For

For example supercells can be very long-lived (more than 6 hours in some cases).  They : supercells produce the majority of strong to violent tornadoes and very large hail (more than 5cm in diameter) and they tend to occur in environments with strong wind shear (0-6 km shear > 20 ms^-1).    Supercells can be very long-lived (more than 6 hours in some cases).

For diagnostic purposes both CAPE/MUCAPE and CAPE-shear should be used together, or alternatively one can examine together CAPE and wind shear as separate parameters.

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The CAPE-shear EFI may be used to anticipate well-organised severe thunderstorms.   Strong wind shear but relatively modest CAPE (e.g. a few hundred J kg^-1) well-organised severe thunderstorms can develop but EFI for CAPE will give a much weaker signal than the EFI for CAPE-shear.  Extremely severe thunderstorms show high CAPE and high shear; therefore both EFIs for CAPE EFI and CAPE-shear EFI should show a strong signal.

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• available on ecCharts and web open charts:
  • MUCAPE and MUCIN (from CONTROL-10/HRES)
  • CAPE Extreme Forecast Index and CAPE-shear Extreme Forecast Index. 
  • probability of CAPE and probability of CAPE-shear above or below a user-defined threshold.
  • 24h 4-value-maximum CAPE and CAPE-shear from M-Climate at various user-defined percentiles.

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