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Use of instability products MUCAPE, MUCAPE-Shear.

CAPE: Convective available potential energy (CAPE) describes the specific potential energy of air in the lower troposphere that potentially could be released in convective storms.  It represents the buoyancy energy of an air parcel freely rising through the atmosphere and depends on atmospheric structure.

MUCAPE: is the highest (most unstable) CAPE value found within the ensemble.

CAPE-Shear:  CAPE-shear is a combination of bulk shear (vector wind shear in the lowest 6km of the atmosphere) and CAPE.  It is used to identify areas of potentially extreme convection.

MUCAPE-Shear is the value of CAPE-Shear using the most unstable CAPE (MUCAPE) found within the ensemble.

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.

CIN: Convective inhibition (CIN) represents the energy needed to lift an air parcel upward to its level of free convection.  CIN does not indicate whether convective instability will be released, but rather provides an indication of the potential for that release.

MUCIN:  This describes the energy required to provide sufficient lift to overcome any capping inversion and to release the most unstable CAPE (MUCAPE).  It does not indicate whether convective instability will be released, but rather provides an indication of the potential for that release. 

None of the parameters give information on the cloud and precipitation.  High CAPE released within a dry atmosphere may give only little amounts of cloud.  For heavy showery rain areas or severe storms it is necessary to have moisture available at the base of convection and preferably throughout the troposphere.  Generally IFS forecasts rain areas prove helpful in giving this information.  However, release of the instability depends on CIN being overcome or on some dynamic uplift higher in the atmosphere. 

The section on Convective cloud processes and precipitation gives more detail.

General rule for forecasting convective storms

As a general rule, heavy showery rain or severe storms, are most likely where areas of high MUCAPE, or more especially MUCAPE-Shear, coincide with areas of forecast precipitation as long as instability is released by some mechanism.  MUCAPE and MUCAPE-Shear alone don't tell the full story.   

Example of parameter use.

Large values of CAPE lie in a zone across the Aegean Sea and parts of mid-Greece.  This is coincident with a belt of strong vertical wind shear resulting in very high values of CAPE-SHEAR (Figs 9.6.4-1 & 9.6.4-2).

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