Convective Cloud Processes and Precipitation

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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. 

See warning regarding ongoing issues with the CIN parameter.

Forecast charts:

• available on ecCharts are:
  • CAPE (from HRES),
  • CAPE Extreme Forecast Index and CAPE-shear Extreme Forecast Index. 
  • probability of CAPE and CAPE-shear above or below a user-defined threshold.
  • 24h 4-value-maximum CAPE and CAPE-shear from M-Climate at various user-defined percentiles.
• available on the standard web plots are:
  • CAPE Extreme Forecast Index and CAPE-shear Extreme Forecast Index.

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.

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.
• Watch a comprehensive lecture on model physics (30sec delay before video begins).  The convection parameterisation scheme is considered in the lecture between 36min45sec and 40min45sec.  Penetration inland of showers is considered in the lecture between 45min20sec and 46min50sec.
• Watch a comprehensive lecture on model clouds and precipitation (25sec delay before video begins).

_{Updated/Amended 30/12/19 Link to issue with CIN}