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Convective cloud processes and precipitation

The convection scheme does not predict individual convective clouds.  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.

The effects of the model convection (changes to the temperature or humidity) drift downwind with model winds.   However, any convective precipitation that is developed by the model is considered to remain within the grid box column and fall vertically downwards instantaneously (i.e. taking zero time to reach the surface).

Thus, model showers are not advected with the wind during their life-cycle.  In particular, any showers that the model develops over the sea do not penetrate beyond the coast.

In reality, showers normally advect with the wind during their life-cycle.  Users should allow for:

  • 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 advect further inland before reaching the ground.

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.

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.  

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) the scheme 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 is computed according to parcel theory.   It assumes:

  • 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.  


In IFS, CAPE is approximated using the equivalent potential temperature.  It is evaluated using each model level from rather above the surface upwards in the atmosphere to 300 hPa pressure level.  CAPE (CAPEθe) is computed using equivalent potential temperature of the parcel (θep), and the environmental saturated equivalent potential temperature (θesat).

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 temperature and identified as the most unstable (MUCAPE) value.   MUCAPE (CAPEθv) is computed using:

  • the 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).

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).

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 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.


Note:

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

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.

At each grid point the convection scheme inspects the temperature structure of the forecast atmosphere progressively from the surface to 350hPa.  At each level, 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.

The minimum of these values are available in ecCharts etc.  


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 either:

  • the (minimum) CIN value encountered exceeds a pre-defined very large threshold.
  • 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).


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. 


Note

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.  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.

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.  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.

For example: supercells produce the majority of strong to violent tornadoes and very large hail (more than 5cm in diameter) and tend to occur in environments with strong wind shear (0-6 km shear > 20 ms-1).  Supercells can be very long-lived (more than 6 hours in some cases).

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.


Fig2.1.5.4-1: Rough guidelines on how to use MUCAPE and MUCAPE-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 MUCAPE and MUCAPE-shear.

Some broad guidelines, based on CIN can be:

  • small CIN (e.g. <50 J kg-1): diurnal heating and/or local topographic features would be sufficient for triggering.
  • moderate CIN (e.g. 50 J kg-1 to 100 J kg-1): needs more substantial uplift than provided by diurnal heating alone.
  • high CIN (e.g. >100 J kg-1) 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.  The user should 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. 

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.

Non-equilibrium convection considers forcing varying on time scales of a few hours rather than diurnal changes.  It takes into account that not all boundary layer heating is available for conversion into deep convection, but only a fraction that varies through the day.  During the morning and noon, most of the heating induces dry and shallow non-precipitating convection.  Only later does it release deeper, more active convection as convective inhibition is overcome.  This is currently used in ECMWF IFS.

The intrinsically slower convective adjustment in non-equilibrium convection produces:

  • a somewhat more realistic diurnal cycle of convection over land.
  • better temporal and spatial distribution and local intensity of showers.
  • an improved diurnal cycle in coastal regions.
  • a slightly more realistic penetration of convective precipitation inland from coasts concurrent with a reduction in unrealistically heavy precipitation at the coast itself.

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.

It is vital to view the forecast precipitation fields to locate areas where there is an overlap with the forecast MUCAPE or MUCAPE-shear areas. Showers are generally not likely to happen if no forecast precipitation is indicated, no matter how large the values of MUCAPE or MUCAPE-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 the sections regarding convective precipitation considerations.

Forecast charts:

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 

Evaluation of CAPE has not yet been standardised and differs amongst models at individual forecast centres.  It is unsafe to compare the magnitude of CAPE derived by different forecast models.   However, 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)



(FUG Associated with Cy49r1)