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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.

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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 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 (CAPE) and most unstable CAPE (MUCAPE)

CAPE represents the buoyancy energy of an air parcel freely rising through the atmosphere.  CAPE lies between zero (no upward buoyancy force) and some positive and possibly large, value that depends on atmospheric structure.  

At any given grid point the convection scheme inspects the temperature structure of the model atmosphere progressively from the surface to 300hPa.  If there exists a level of free convection (LFC) it evaluates the CAPE.  The search for CAPE currently in use allows discovery of elevated instability, even at night when there will often be stability at lower altitudes.  

CAPE, approximated with the equivalent potential temperature, is evaluated for each model level from the surface to 300 hPa pressure level upwards in the atmosphere.  Near-surface based parcels are not evaluated. Instead for each model level in the lowest 60hPa of the atmosphere, 30-hPa mixed-layer parameters are used.  The most unstable parcel is the one with the highest CAPE value.  Once the most unstable parcel is found, MUCAPE is computed using the model virtual potential temperature.  Processes such as entrainment, detrainment and precipitation load are not considered and therefore provided model MUCAPE values are likely to be overestimated.  These values are available in ecCharts etc.

Entrainment of surrounding air is not considered and so CAPE is likely to be a slight overestimate.

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.

Convective Available Potential Energy parameters:

  • CAPE (CAPEθe) is computed using equivalent potential temperature of the parcel (θep), and the environmental saturated equivalent potential temperature (θesat).
  • MUCAPE (CAPEθv) is computed using virtual potential temperature of the parcel (θvp), the virtual potential temperature of the environment (θve), and the environmental saturated equivalent potential temperature (θesat). 

MUCAPE (CAPEθv) has overall higher values than CAPEθe (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. At any given grid point the convection scheme inspects the temperature structure of the model atmosphere progressively from the surface to 300hPa.  If there exists a level of free convection (LFC) it evaluates the CAPE.  The search for CAPE currently in use allows discovery of elevated instability, even at night when there will often be stability at lower altitudes.  

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.

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


Convective inhibition (CIN) and convective inhibition associated with most unstable convection (MUCIN)

CIN represents the energy needed to lift an air parcel upward to its level of free convection (LFC). 

At each grid point the convection scheme inspects the temperature structure of the forecast atmosphere progressively from the surface to 350hPa.  Using the temperatures it works out the value of CIN for parcels rising from each level.  The minimum of these values are stored and are available in ecCharts etc.  

CIN 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 whenever the (minimum) CIN value encountered exceeds a pre-defined very large threshold.   Where 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) then CIN is in effect infinite and a missing value indicator is stored for CIN.

Once the most unstable parcel (MUCAPE) is found then the convective inhibition associated with this parcel (MUCIN) is computed using the model virtual potential temperature. 

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 most unstable CAPE (MUCAPE).

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

At any given grid point the convection scheme inspects the temperature structure of the model atmosphere progressively from the surface upwards.  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.

It is important to assess the likelihood of CIN values being overcome during the following hours (e.g. 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.

Mixed-layer CAPE and mixed-layer CIN

Mixed-layer CAPE and CIN are computed by averaging temperature and humidity in the lowest 50- or 100-hPa layers of the atmosphere. Both are available from ECMWF forecasting system. 

CAPE and CIN Diagrams

CAPE and CIN values on diagrams are diagnostic.  The diagrams show the general state of the model atmosphere as forecast for that time.  CIN does not indicate whether convective instability will be released, but rather provides an indication of the potential for that release.  The box and whisker format gives an indication of probabilities of the value of CAPE after release by the indicated CIN.

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

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 larger wind shear tends to be associated with higher organisation and severity of convection.

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It is vital to view the forecast precipitation fields to locate areas where there is an overlap with the forecast CAPE or CAPE-SHEAR areas. Showers are generally not likely to happen if no forecast precipitation is indicated, no matter how large the values of CAPE or CAPE-SHEAR.  Users should investigate closely all aspects of the forecast model atmosphere in areas of interest.  Of special importance are vertical profiles and indications of an upper contour pattern favourable for forced broadscale ascent.

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 problems.

Forecast charts:

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