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The convection scheme does not predict individual convective clouds, but only .  It does predict 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.

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• possible advection of any showers developed by the convection scheme. 
• penetration of maritime showers inland from windward coasts, .  This is especially important in winter or with wintry precipitation because snowflakes fall more slowly than raindrops and thus advect further inland before reaching the ground.

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CAPE is widely used in the prediction of convective storms.  It is a physical quantity with a direct physical interpretation.  This sets it apart from Instability indices that only relate to the physics of convection in an indirect way.  

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The parcel that yields the highest CAPE is found from the ensemble.   CAPE for this parcel is then re-computed using the model virtual potential temperatureand identified as the most unstable (MUCAPE) value.   MUCAPE (CAPE_θv) is computed using:

• the virtual potential temperature of the parcel (θ_vp)

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• the virtual potential temperature of the environment (θ_ve)

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• 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).

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a 100 hPa mixed-layer parcel, which is lifted from the surface, having the potential temperature and the water vapour mixing ratio of the air in the lowest 100 hPa above the surface.  CAPE calculated for this parcel is called 100hPa mixed-layer MLCAPE, (or MLCAPE100).

As a guide 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.

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CIN describes the energy required to provide sufficient lift to overcome any capping inversion and to release the CAPE.  It must always either be zero (no extra energy required) or a positive value (additional energy needed to overcome underlying stability).  Negative CIN is meaningless.   A missing value indicator is stored for CIN whenevereither:

• the (minimum) CIN value encountered exceeds a pre-defined very large threshold, or.
• the parcel curve (from any of the levels tested) never even reaches the environment curve (i.e. the parcel curve lies always to the left of the environment curve).

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CIN does not indicate whether convective instability will be released, but rather provides an indication of the potential for that release.  It  It is important to assess the likelihood of CIN values being overcome during hours following the model profile (e.g. .  This might be by diurnal heating, by dynamically induced uplift of the airmass, or by mechanical uplift caused by flow over mountains etc.). 

Fig: Vertical profile showing the derivation of CAPE and CIN.

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It is important to assess the likelihood of MUCIN values being overcome during hours following the model profile (e.g. .  This might be by diurnal heating, by dynamically induced uplift of the airmass, or by mechanical uplift caused by flow over mountains etc.). 

MUCAPE-shear

MUCAPE-shear is a combination of bulk shear (vector wind shear in the lowest 6km of the atmosphere) and MUCAPE.  It 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 larger wind shear tends to be associated with higher organisation and severity of convection.

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Low level moisture is important for triggering convection yet may be imprecisely predicted by the models.  Users should review the moisture content within the low-level inflow to areas with potential for significant convective development.  This can be done by comparing forecast values with available observations upstream (e.g. by comparing upstream dew points or vertical profiles).  Users should review the location of convective release and consider whether there is a possibility of deeper, more active convection.  See also examples of convection problemsthe sections regarding convective precipitation considerations.

Forecast charts:

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

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Inter-model variability of CAPE 

Users should note that evaluation Evaluation of CAPE differs has not yet been standardised and differs amongst models at individual forecast centres.  Until the method of computation of CAPE is standardised it   It is unsafe to compare the magnitude of CAPE derived by different forecast models though of course .   However, the changes in magnitude of CAPE derived from each forecast model remain useful.

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