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Convective Cloud Processes and Precipitation

The convection scheme does not predict individual convective clouds, 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.  In the current configuration of the convection scheme within IFS, while the effects of convection (changes to the temperature or humidity) drift downwind, any (convective) precipitation that is developed is considered to remain within the column and fall vertically downwards instantaneously (i.e. taking zero time to reach the surface).  This means that showers are not advected with the wind during their life-cycle. The effect is particularly evident in wintertime when the showers developed over the sea do not penetrate beyond the coast while in reality active showers move well inland.  The errors are greater in colder airmasses producing wintry precipitation because snowflakes fall more slowly than raindrops and thus advect further in the wind before reaching the ground.

For a measure of convection intensity the Convective Available Potential Energy (CAPE) is evaluated from the IFS model atmosphere.  At any given grid point the convection scheme inspects the temperature and humidity structure progressively from the surface to 300hPa and if there exists a level of free convection (LFC) it evaluates the CAPE.  Entrainment of surrounding air is not considered and thus the CAPE is likely to be a slight overestimate.  The technique currently in use for estimating CAPE allows for the discovery of elevated instability, even at night, despite low-level stability.  Convective Inhibition (CIN) is assessed from the IFS model atmosphere in a similar way.  CAPE and CIN are computed and provided as (MARS) output parameters in order to help the user assess the likelihood of severe convective storms.  CAPE-shear is a combination of bulk shear (vector wind shear in the lowest 6km of the atmosphere) and CAPE and is used to identify areas of potentially extreme convection. 


Forecast charts:

Some indication of severe precipitation may be deduced from the values of CAPE  and CAPE-shear, coincident with indications of significant rainfall intensity and/or indication of an upper contour pattern favourable for forced broadscale ascent.

Related developments:

An increase in the amount of super-cooled liquid water held by the convection scheme at colder temperatures (down to -38C) was introduced in mid 2017 in Cycle 43R3, to improve the development of convective precipitation. 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.


CIN, CAPE and CAPE-shear

CAPE and CIN are 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.  CAPE and CAPE-shear EFI and SOT computations sample the hourly CAPE and CAPE-shear values during the 24-hour period and the maximum values are what is used.  

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 higher organisation and severity of convection tends to be.  For example supercells can be very long-lived (more than 6 hours in some cases).  They 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).   Therefore for diagnostic purposes both CAPE and CAPE-shear should be used together, or alternatively one can examine together CAPE and wind shear as separate parameters.  The CAPE-shear EFI may be used to anticipate well-organised severe thunderstorms.  For example in the case of very strong wind shear but relatively modest CAPE (e.g. a few hundred J/kg) we can have development of well-organised severe thunderstorms but the EFI for CAPE will give a much weaker signal than the EFI for CAPE-shear.  For extremely severe thunderstorms we need high CAPE and high shear; therefore both EFIs for CAPE and CAPE-shear should show a strong signal.



Fig2.1.23: Rough guidelines on how to use CAPE and CAPE-shear (EFI) values together. Bear in mind also that EFI and SOT are computed relative to reference model climatologies, so "severe" in one region will tend not be at the same level as it is in another.

It is vital to try to diagnose whether or not convection will initiate before giving considering to convective severity, as suggested by CAPE and CAPE-shear. Some broad guidelines, based on the level of Convective INhibition (CIN) can be:

  • small CIN (e.g. <50 J/Kg): diurnal heating and/or local topographic features would be sufficient for triggering
  • moderate CIN (e.g. 50 J/kg to 100 J/Kg): needs more substantial uplift than provided by diurnal heating alone
  • high CIN (e.g. >100 J/kg) needs very substantial uplift (e.g. a well-defined airmass boundary with strong surface convergence), and depending on the CIN level even that may not be enough

These values are not definitive.  It is the users responsibility to assess the impact of local effects (e.g. convergence, changes in the temperature and moisture structure, sea breezes, low. cloud advection etc.) upon the amount of energy required to overcome inhibition. 


With effect from cycle 47R1 in late June 2020 CIN is computed using virtual potential temperature, and is valid for the same most unstable air parcel that is used for the CAPE calculation.  

In cycles up to and including cycle 46R1 CIN computation used equivalent potential temperature, which had many shortcomings, and in addition the most unstable air parcel used for CAPE was identified independently (making the two less comparable).

For more information see Changes in CIN to concur with forecasting practice



Additional Sources of Information

(Note: In older material there may be references to issues that have subsequently been addressed)

Updated/Amended 30/12/19 - Link to issue with CIN

Updated/Amended 02/07/20 - Removed link to issue with CIN. Not applicable with 47R1.

Updated/Amended 24/10/20 - amended chart links to open access.

Updated/Amended 05/11020 - added section on CIN, CAPE and Cape-shear.


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