Representation of line convection by IFS

Line convection on very active cold frontscan be difficult for the models to resolve.  Typically the line convection is quite narrow while the resolution of IFS is 9km.

In Cy49 (and earlier) convection scheme, the convective precipitation falls to the surface immediately within the grid square.  Thus there can be some cells showing high precipitation rates and totals.  However, there is some under-indication of maximum precipitation rates.

In Cy50 the convection scheme transfers about 40% of forecast convective precipitation to the broad scale scheme.  This scheme advects the precipitation while allowing it to fall driven by the underlying winds.  In this way the precipitation is effectively smeared downwind and the forecast precipitations appear reduced.

In the case shown in Fig9.6.8-1, the peak rates which exceeded 50mm in reality are not shown, as they were too small scale for a 9km resolution model to pick up.  There are hints in both Cy49 and Cy50 model runs of local intense precipitation in the rain bands associated with two cold fronts but not definitive in intensity nor location. Also precipitation is smeared into rather more uniform bands but with the loss of detail of heavier precipitation on the line convection itself.

This effect should be taken into account where users anticipate line convection, and modifications made to output when advising customers.


Example comparing output from Cy49 and Cy50

Fig9.6.8-1: Example of a very active cold front on 13 March 2026 compared with T+24 forecasts from Cy49 and its replacement Cy50 DT 00UTC 12 Mar 2026, VT 00UTC 13 March 2026.  Radar imagery shows line convection crossing England.  The diagrams show precipitation rates.  Naturally the peak rates which exceeded 50mm in reality are not identified, as they were too small scale for a 9km resolution model to pick up.  Convective precipitation in Cy49 falls immediately within the grid square.   Some of the convective precipitation in Cy50 is transferred to the broad scale scheme and advects with the ambient winds while falling to the surface.  Thus the convective precipitation is spread over several grid squares and therefore is shown as less intense.  There are hints in both model runs of more intense precipitation band associated with two cold fronts but not definitive in intensity nor location. 


Example of IFS capture of line convection

Example of line convection crossing southern Britain 06UTC 4 Sep 2026.  In the southwest convection depth limited to ~2000m, in the northeast convection depth 4000m-6000m (see vertical profiles).  Line convection is well below the IFS resolution and so cannot be expected to exactly capture the intensity and location. Nevertheless, IFS forecast of convective precipitation rate gives a good indication of the position, movement and relative activity of the line convection.  However it does not capture the intensity well.  The radar chart shows more active sections of the line convection near:

  • The Wash (radar 16-32mm/hr, forecast local max 3-6mm/hr).
  • south Midlands (radar 16-32mm/hr, forecast local max 1-3mm/hr).
  • Bristol (radar small area 16-32mm/hr, forecast local max 0.2-0.3mm/hr).

Users should consider the relative maxima of forecast intensities and the corresponding potential for convection released by convergence at the front. In cases of line convection, precipitation rates can be much greater than IFS indicates.   Vertical profiles can give useful insights.


Fig9.6.8-2: Example of line convection crossing southern Britain. The chart shows precipitation rates taken from the Met Office radar network.  DT 06UTC 4 Sep 2026. Line convection is clearly shown with local areas of higher intensity (red: 16-32mm/hr).

 

Fig9.6.8-3: Example of line convection crossing southern Britain. The chart shows convective precipitation rate DT18UTC 3 Sep 2026, VT 06UTC 4 Sep 2026.  IFS forecast gives a good indication of the position, movement and relative activity of the line convection.  However it does not capture the intensity well. There is strong shear, convergence and forced uplift near the surface front. The vertical profiles show:

  • in the southwest convection depth limited to ~2000m with minimal MUCAPE, but the air is saturated and lies on the saturated adiabatic lapse rate (SALR).
  • in the northeast convection depth 4000m-6000m with moderate MUCAPE (say ~100J/Kg),  but the air is saturated with scope for convective overturning.

 

Fig9.6.8-4: Example of line convection crossing southern Britain. The chart shows total precipitation rate (convective + stratiform) DT18UTC 3 Sep 2026, VT 06UTC 4 Sep 2026.  Stratiform precipitation includes a proportion of the convective precipitation (~40%) which is advected with the winds.  Thus the line convection area becomes rather broadened and convective detail is lost.  Almost completely stratiform precipitation is within the large area over south Wales and central Midlands.



 

 

 

(FUG associated with Cy50r1)

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