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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.  

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, 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

CAPE and CIN are computed in order to help the user assess the likelihood of severe convective storms.

CAPE

Convective Available Potential Energy (CAPE) gives information on the the convective energy and the availability of low-level moisture.  CAPE can be a guide to the intensity of convection, but only if convection triggers.  It is only one measure of the potential for severe convection and thunderstorms.  Instability indices also give some indication.

CAPE is 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.  At any given grid point the convection scheme inspects the temperature structure of the model atmosphere 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 evaluated CAPE is likely to be a slight overestimate.  The search for CAPE currently in use allows discovery of elevated instability, even at night when there will often be low-level stability.  

CAPE (or CAPEθe to distinguish it from MUCAPE) is computed assuming a pseudo-adiabatic parcel ascent (all condensate is removed as soon as it forms) using the equivalent potential temperature (θep), and the environmental saturated equivalent potential temperature (θesat). 

CIN

Convective Inhibition (CIN) gives information on the energy required to provide sufficient lift to overcome any capping inversion and to release the CAPE.  CIN is assessed from the model atmosphere in a similar way to the process to identify CAPE.  At any given grid point the convection scheme inspects the temperature structure of the model atmosphere progressively from the surface upwards and 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.   

MUCAPE

Most Unstable CAPE (MUCAPE) is also evaluated.  The only difference from the CAPEθe is that MUCAPE is computed using virtual potential temperature of the parcel (θvp) and virtual potential temperature of the environment (θve).  This change provides an estimate of CAPE for the most unstable parcel which is more in line with parcel theory, and with what forecasters should diagnose from vertical profiles of the atmosphere.

MUCAPE has overall higher values than CAPEθe.  

Broadly, values >1000 J kg-1 indicate potential for development of moderate thunderstorms and values >2000 J kg-1 indicate a potential for severe ones.   MUCAPE of 3000 to 4000 J kg-1 or even higher is usually a signal of a very volatile atmosphere that could produce severe storms if other environmental parameters are in place.

MUCIN

Convective inhibition of the most unstable parcel (MUCIN) which corresponds to MUCAPE is the identical to CIN as this already uses the virtual temperature correction.

CAPE-shear

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.  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).  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.

CAPE and CAPE-shear EFI and SOT

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


Fig2.1.5.4-1: 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 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 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 it is important that users do not rely simply upon CAPE and CAPE-SHEAR charts alone when forecasting rainfall distribution.   CAPE and CAPE-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 CAPE or CAPE-SHEAR areas. Showers are generally not likely to happen if no forecast precipitation is indicated, no matter how large the values of CAPE or CAPE-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.

Forecast charts:

Charts showing the greatest CAPE within the previous six hours are available.  This is to limit data overload from too many single CAPE 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.

Additional Sources of Information

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


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