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Convective cloud processes and precipitation

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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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

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.  CAPE can be derived from vertical profiles (measured or modelled) of temperature and humidity throughout the troposphere.  CAPE values lie between zero (no upward buoyancy force) and some positive, and possibly large, value.

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The magnitude of CAPE strongly depends on the choice of the parcel that is lifted.  Terms used are:

Most Unstable CAPE  (MUCAPE)

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 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).

Mixed Layer CAPE  (MLCAPE)

a 50 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 50 hPa above the surface.  CAPE calculated for this parcel is called 50hPa mixed-layer MLCAPE, (or MLCAPE50).

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Processes such as entrainment, detrainment and precipitation load are not considered and so model MUCAPE values are likely to be overestimated.  MUCAPE is available in ecCharts etc.

Convective inhibition (CIN)

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

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The magnitude of CIN depends on the choice of the parcel that is lifted.  Terms used are:

Most unstable CIN  (MUCIN) 

This describes the energy required to provide sufficient lift to overcome any capping inversion and to release the most unstable CAPE (MUCAPE).  It is computed using the model virtual potential temperature. 

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

Mixed-layer CIN. (MLCIN)

Mixed-layer CIN is computed by averaging temperature and humidity in the lowest 50-hPa or 100-hPa layers of the atmosphere. 

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

Image Modified

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

MUCAPE and MUCIN diagrams

MUCAPE and MUCIN values on diagrams are diagnostic.  The diagrams show the general state of the model atmosphere as forecast for that time.  MUCIN 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 MUCAPE after release by the indicated MUCIN.

It is important to assess the likelihood of MUCIN values being overcome during hours following the model profile (e.g. 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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For diagnostic purposes both MUCAPE and MUCAPE-shear should be used together, or alternatively one can examine MUCAPE and wind shear as separate parameters.

MUCAPE and MUCAPE-shear EFI and SOT

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

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

Equilibrium and  non-equilibrium convection

Equilibrium  convection (or quasi-equilibrium convection) considers forcing due to mean advection and to processes other than convection.  It is used by many numerical models and has been found to be valid for synoptic disturbances and for time-scales of the order of one day.  However, deep convection, largely driven by the diurnally varying surface heat flux, generally begins too soon in the morning and ceases too readily in the evening.  This was used in ECMWF IFS before November 2013.

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Night-time convective precipitation remains underestimated.

Importance of available moisture

In convective situations it is important that users do not rely simply upon MUCAPE and MUCAPE-shear charts alone when forecasting rainfall distribution.   MUCAPE and MUCAPE-shear charts signal areas of high probability of deep and active instability but do not give information on the amount of available moisture.   So no information is given on the initiation or even potential existence of moist convection and consequent showery precipitation.  This is especially important when there is a possibility of very heavy or severe instability-related precipitation.

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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 problems.

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.

Charts showing the greatest MUCAPE within the previous six hours are available.  This is to limit data overload from too many single MUCAPE snap-shots; instead now all hourly values are covered with just data for the main forecast times.  This can give the user a much better indication of the potential for active convection.

Inter-model variability of CAPE 

Users should note that evaluation of CAPE differs amongst models at individual forecast centres.  Until the method of computation of CAPE is standardised it is unsafe to compare the magnitude of CAPE derived by different forecast models though of course the changes in magnitude of CAPE derived from each forecast model remain useful.

Changes introduced in Cy49r1

The only CAPE products that are issued are MUCAPE and MUCIN output.  Other CAPE and CIN output have been discontinued.

Additional sources of information

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

• Read more on the Convective Scheme in the atmospheric physics page (scroll down to "Convection") or in "Breakthrough in Forecasting Convection".
• Read more on atmospheric moist convection.

(FUG Associated with Cy49r1)