Model output associated with extreme convection 

Ensemble Control Forecast (ex-HRES) tends to over-forecast extreme convection, especially in the maritime tropics.  Forecast rapid vertical motion can result in spurious quasi-circular waves (sometimes rings) in convective precipitation fields. These gravity waves gradually spread outwards.  They can even move well upwind, giving a false impression of an eastward-moving trough.  The source of these false features should be identified and the effects discarded from forecasts.  Changes to the IFS moist physics in 2019 have reduced but not entirely removed this effect. 

Fig 9.6.6-1: An example of a forecast precipitation chart showing a very active convective area in the mid-Indian Ocean. The surrounding ring of forecast precipitation is associated with a spurious gravity wave (ripple effect) moving outward from the initial convective cell.  These rings of precipitation are incorrect and should be ignored.


Fig9.6.6-2: Ensemble control (Ex-HRES) forecast of precipitation associated with gravity waves propagating outward across central Africa from a pulse of very strong convection formed over the Gulf of Guinea (just to the southwest of the charts).  Model precipitation rate are shown for one such feature at T+120, T+132, T+144, data time 00Z 3 Sept 2018. The apparent trough is propagating anomalously against the flow and the precipitation is not correct.



IFS can forecast spurious squall lines, driven by the convection parameterisation. The convection scheme can produce low-level cooling.  This creates a density current with ascent on the leading edge in the lower troposphere. The ascent destabilises the model atmosphere and enables the convection scheme to trigger again at the edge of the cold pool and the process repeats.

Before accepting forecast squall lines, users should assess whether the line of convection, with or without precipitation, is realistic. Satellite and radar imagery will show any bands of cloud or rain that are forming and indicate the time and location. This should be compared with model output.  The features should be viewed with suspicion when rainfall totals are similar or greater than all-time 24hr total M-Climate extreme.

Squall-line features in the shape of bow waves do occur in the real world, but probably they are more common in models than in reality. 

This is a known problem in IFS - See Item P7 in Known Model Issues.


Fig9.6.6-3: An example of spurious squall lines over the Arabian Sea 7 and 8 May 2025. FigA: IR imagery shows a convective area off Western India so some squally activity is possible.  FigB: Simulated IR imagers shows ensemble control (Ex-HIRES) has identified a similar feature but more active and too far west.  FigC: simulated IR imagery and associated active convection squall line has continued to progress southwestwards with a second band formed formed near Western India.   Errors in forecast position of such an active feature and with precipitation values near or above M-climate suggest the systems are spurious.



(FUG Associated with Cy49r1)




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