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Table of Contents

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.  An increase in the amount of super Super-cooled liquid water is held by the convection scheme and even at colder temperatures (down to -38C) improves aids 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.      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 (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).

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

Users 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, especially in winter or with wintry precipitation because snowflakes fall more slowly than raindrops and thus 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 and CIN

Convective Available Potential Energy (CAPE) gives These parameters are computed in order to help the user assess the likelihood of severe convective storms.  They give information on the the convective energy and the availability of low-level moisture.   It is only one measure of the potential for severe convection and thunderstorms.

Convective Inhibition (CIN) gives information on the energy required to provide sufficient lift to overcome any capping inversion and to release the CAPE.

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 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 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. 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. 

At any given grid point the convection scheme inspects the temperature and humidity structure of the model atmosphere progressively from the surface to 300hPa and if .  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  The search for CAPE 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 in order to help the user assess the likelihood of severe convective storms.  when there will often be stability at lower altitudes.  

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.

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


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 CAPE.

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.

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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.  

CAPE-shear

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 tends to be associated with higher organisation and severity of convection tends to be.  For

For example supercells can be very long-lived (more than 6 hours in some cases).  They : supercells 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 ).  Supercells can be very long-lived (more than 6 hours in some cases).

For diagnostic purposes both CAPE/MUCAPE and CAPE-shear should be used together, or alternatively one can examine together CAPE and wind shear as separate parameters.

CAPE and CAPE-shear EFI and SOT

The  The CAPE-shear EFI may be used to anticipate well-organised severe thunderstorms.  For example in the case of very strong   Strong wind shear but relatively modest CAPE (e.g. a few hundred J / kg-1) we can have development of well-organised severe thunderstorms can develop but the EFI for CAPE will give a much weaker signal than the EFI for CAPE-shear.  For extremely  Extremely severe thunderstorms we need show high CAPE and high shear; therefore both EFIs for CAPE EFI and CAPE-shear EFI should show a strong signal.


Fig2.1.5.234-1: 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-1): diurnal heating and/or local topographic features would be sufficient for triggering.
  • moderate CIN (e.g. 50 J kg-1 to 100 J/KgJ 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.  It is the users responsibility to  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. 

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.

Considerations in interpretation of CAPE charts

EFI and SOT computations of CAPE and CAPE-shear sample the hourly CAPE and CAPE-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 When deducing the forecast rainfall distribution, in convective situations, it is important that users do not rely simply upon CAPE and CAPE-SHEAR charts alone . At first sight these charts appear to when forecasting rainfall distribution.   CAPE and CAPE-shear charts signal areas of high probability of deep and active instability .  However, they but do not give information on the amount of available moisture and hence .   So no information is given on the initiation or even potential existence of moist convection and consequent showery precipitation. This  This is especially important when there is a possibility of very heavy or severe instability-related precipitation.  Users should investigate closely all aspects of the forecast model atmosphere.  In particular the charts of forecast precipitation should be viewed to identify

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 . Showers are generally not likely to happen if no forecast precipitation is indicated, no matter how large the values of CAPE or CAPE-SHEAR.  It is vital to view the precipitation fields and vertical profiles in connection with CAPE and CAPE-SHEAR fields. 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.

Forecast charts:

Charts showing CAPE6 charts have been introduced to show the greatest CAPE within the previous six hours are available. This  This is in order to limit data overload from too many single CAPE snap-shots; instead now we can cover 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.

An example - Northern Greece, 11 July 2019

Large values of CAPE lie in a zone across the Aegean Sea and parts of mid-Greece coincident with a belt of strong vertical wind shear resulting in very high values of CAPE-SHEAR (Figs 2.1.24 & 2.1.25).  In particular, high forecast values of CAPE and CAPE-SHEAR are indicated at Pilio while much lower forecast values are shown at Kavala.  This might suggest at first sight that any instability that is released in the region of Pilio would be very active with the possibility of severe storms and rainfall.  At the same time much less showery activity might be expected at Kavala on the northern flank of the CAPE and CAPE-SHEAR zone.  Such a snap assessment would be incorrect.

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Fig2.1.24a: Forecast CAPE (Blue high, Red low) and Fig2.1.24b: Bulk Wind Shear (Orange high, Yellow low).  T+24 VT00UTC 11 July 2019, DT00UTC 10 July 2019.  

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Fig2.1.25a: Forecast CAPE-SHEAR (Purple high, Blue low).  Fig2.1.25b: Max CAPE-SHEAR (Red high, Blue low). T+24 VT00UTC 11 July 2019, DT00UTC 10 July 2019.  Very high values are indicated in the vicinity of Pilio.  More modest values are indicated in the vicinity of Kavala on the CAPE-SHEAR chart but note that the maximum CAPE-SHEAR chart shows there have been much higher values during the previous 6hrs.

The forecast precipitation field (Fig2.1.26) shows a belt of rainfall across North Greece, Albania and Bulgaria.  This indicates that, as a minimum, in this area there is sufficient moisture in the forecast atmosphere to provide precipitation.  The area of forecast precipitation intersects the northern flank of the forecast CAPE and CAPE-SHEAR areas and thus it is this area that is more likely to see release of deep and active convection with availability of plenty of moisture.   Little or no precipitation is indicated in mid-Greece but nevertheless these lie within the areas of very high CAPE and isolated but local very heavy showers are possible and, bearing in mind the high bulk shear and CAPE-SHEAR values, local storms cannot be ruled out. 

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Fig2.1.26: Forecast precipitation (12hr). T+24 VT00UTC 11 July 2019, DT00UTC 10 July 2019.

The corresponding diagnostic charts for the probability of high rainfall (>40mm/24hr) and Extreme Forecast Index (EFI) for precipitation (Fig2.1.27) identify the areas at greatest risk of a major precipitation event.     

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Fig2.1.27a: Probability of total precipitation >40mm (24hr).  Green shading represents 35-65% probability.  Fig2.1.27b: Precipitation extreme forecast index (EFI).  Red shading represents EFI>0.8, Dark red >0.9 EFI. T+24 VT00UTC 11 July 2019, DT00UTC 10 July 2019.

Forecast vertical profiles are very helpful in assessing the potential for severe events.  The forecast vertical profile at Pilio shows large CAPE but with relatively dry convection, possibly released by high surface daytime temperatures.  Very little moisture is indicated and precipitation looks very unlikely.  However some moisture is available locally over mid-Greece (Fig 2.1.26) mainly at medium levels producing possible local showery outbreaks given some form of dynamic uplift.  The relevant wind shear to consider for this is probably between medium and upper tropospheric levels rather than between lower and medium levels (the bulk shear).  Inspection of the hodograph suggests the upper tropospheric shear is not great, so shower organisation/activity would lack this element of support.  Note, however, that heavy medium level showers can penetrate downwards through underlying dry layers more than IFS forecasts tend to suggest, even reaching down to the surface.  HRES* and some ENS members do show a very humid boundary layer at Pilio, but it would require large energy input at the surface (2m temperatures above about 35°C) to overcome the large CIN and to lift the low level moisture to release moist convective cells.

The forecast vertical profile at Kavala shows rather less CAPE and CAPE-SHEAR but with an almost saturated atmosphere and absolute instability at around 750hPa.  Recall also that the max CAPE-SHEAR over the previous 6hrs was higher.  So very active moist convection is extremely likely in the northern Greece region.  Inspection of the hodograph suggests significant shear throughout lower and medium layers allowing separation of up draughts and down draughts with persistent active precipitation cells.  

Violent storms with local hail swept across northern Greece overnight 10/11 July 2019 causing seven deaths and widespread damage.

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Fig2.1.28: Forecast vertical profiles for Kavala and Pilio, Greece. T+24 VT00UTC 11 July 2019, DT00UTC 10 July 2019.

A sequence of forecast EFI charts gives early indication of forthcoming severe weather potential (Fig2.1.29), and some idea of the confidence that may be placed on the forecast event.  In this case, northern Greece is identified as being at moderately high risk of an extreme event (EFI ~ 0.6) four days before, rising steadily to a very high risk of an extreme event (EFI ~ 0.9) two days before the occurrence of the severe weather.  Note how there is consistent indication of a very high risk of an extreme event (EFI ~ 0.9) over the Balkan states through the sequence of forecast runs.  The consistency in the areas shown at risk leads to a higher confidence in forecasts of severe weather.  Users should inspect forecast fields using ecCharts and vertical profiles as outlined above to assess forecast details, and also add in the influence of additional factors using local knowledge (e.g. regarding topographic influences) wherever possible.   

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Fig2.1.29: Sequence of EFI precipitation charts from four EFI runs at 24hr intervals (DT 12UTC on 6, 7, 8, 9 July 2019). Increasingly high EFI precipitation values identify the areas at greatest risk.

Points to note regarding IFS forecasts of convection and convective precipitation.

The IFS currently shows:

  • a bias towards:
    • insufficient convective precipitation in arid regions (e.g. parts of West Africa, the Middle East, and central Australia).
    • excessive convective rainfall near orography.
  • a tendency to:
    • under-forecast precipitation amounts from large scale convection (e.g. MCSs),
    • over-forecast convective precipitation amounts otherwise.
  • imprecision in the diurnal cycle of convection.  In particular:
    • the convective inhibition (CIN) tends break too easily.  CIN is not currently well evaluated and consequently there can be unreliable timing of release convection – generally too soon. 
    • there is too rapid increase to a peak in convection (by about 3-4hrs, i.e. around local noon rather than in the afternoon).  
    • there is too rapid decay in convection.  CAPE is destroyed too quickly and showers die away too soon.  With active convection some showers may be expected to persist much longer and linger into the night.  In general, showers die out:
      • 2-3hrs too early in west Europe,
      • 1-2hrs too early in east Europe,
      • about right in USSR.

Considerations regarding medium level instability in drier areas

It is important that moist medium level instability is modelled sufficiently as even relatively small CAPE can produce precipitation.  Users should check forecast vertical profiles against local observations and profiles.

Heavy precipitation developed aloft from medium level instability can have drop sizes sufficiently large that they will penetrate through dry air to reach the ground.  IFS tends to evaporate precipitation from medium levels too much during descent (in part due to limitations of assumed drop size distribution), and consequently insufficient rain is forecast to reach the ground.  

Lightning associated with medium level instability is often indicated on forecast charts although no precipitation is forecast at the surface. Whilst lightning activity tends to be over-predicted (sometimes considerably) it can be a reasonable indicator of the potential for active medium level instability.  Forecast lightning activity often covers a greater area than does forecast surface precipitation. 

Convective Available Potential Energy (CAPE) is very sensitive to the humidity in the boundary layer.   A slight change in dewpoint, particularly within the boundary layer will leads to a significant change in CAPE.  Thus any medium level showers that do penetrate to the surface can locally increase boundary layer moisture - observed surface dew points can become several ºC higher than forecast T2m dew points (up to ~12ºC difference has been observed).  This leads to a local, possibly major, reduction in CIN and an increase in CAPE.   Further instability may then be released inducing further showery activity.   Forecast charts of surface precipitation are not likely to capture all such details.

Where medium level instability is forecast above a dry lower atmosphere, users should use forecast lightning charts and forecast vertical profiles to extend and improve precipitation forecasts.   Where medium level instability is forecast (even with only moderate CAPE), some additional showers should be forecast within the areas of forecast lightning.  Owing to resolution issues, forecast intensity of lightning strikes gives only a rough idea of regions where there is more active medium level instability but it does not reliably indicate that showers will penetrate to the surface, nor their intensity if they do so.  However, probability of precipitation should be increased. 

An example - Central Australia, 17 January 2019

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Fig2.1.30: Forecast IFS data for central and northwest Australia 17 Jan 2019.  Local time is about 10hrs ahead of European time zones. The circled triangle locates Alice Springs.

  • Fig2.1.30a: Total 6hr precipitation from 9km resolution model DT 12UTC 16 Jan 2019,  T+21 VT 09UTC 17 Jan 2019.
  • Fig2.1.30b: Lightning density in 6hr (flashes 100km-2hr-1) DT 12UTC 16 Jan 2019, T+21 VT 09UTC 17 Jan 2019.
  • Fig2.1.30c: ENS probability of total precipitation >1mm: DT 12UTC 16 Jan 2019, T+12 VT 00UTC 17 Jan 19 to T+36 00UTC 18 Jan 2019.
  • Fig2.1.30d: Total 6hr precipitation from 9km resolution model DT 12UTC 16 Jan 2019, T+21 VT 09UTC 17 Jan 2019, and Observed lightning flashes VT 09UTC 17 Jan 2019.
  • Fig2.1.30e: Forecast vertical profile at Alice Springs DT 12UTC 16 Jan 19, T+18 VT 06UTC 17 Jan 2019.

Medium level thunderstorms developed and extended well into central parts of Australia (Fig2.1.30d, observed lightning) but no underlying surface rainfall is forecast (Figs2.1.30a & 2.1.30d), nor any probability of rain (Fig2.1.30c).  Forecast lightning flashes (Fig2.1.30b) is overly extensive in northwest Australia but although there is some indication in central parts it is under-indicated (compare with Fig2.1.30d). 

The model boundary layer was generally dry in central Australia but observations showed much higher dew points where showers have occurred.  Near Alice Springs the model T2m dewpoint was 4.2C lower than the observed dew point, and at a location to the northwest the error was 11.8ºC.  Both discrepancies were probably due to storms that the model didn't represent.

The forecast vertical profile for Alice Springs (Fig2.1.30e) shows possible (surface-based) medium level instability with just moderate CAPE (e.g. cyan line construction).  Note that some ensemble members have higher low-level dew points which means a lower CIN to initiate medium level convection with greater CAPE (e.g. red dashed line construction).  Further, if the boundary layer is moistened after any medium level showers penetrate to the surface then there is a higher likelihood of more energetic convection being released afterwards with much greater CAPE (e.g. black dashed line construction).

In Central Australia, no precipitation is indicated; any precipitation in the model is being evaporated before reaching the ground.  However, the lightning activity chart suggests that, though the deep moist convection isn't very well-organised, scattered thunderstorms appear likely.  This was bourne out by observations.  Note also that the model greatly over-predicted lightning activity over northwest Australia.

Additional Sources of Information

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

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