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Comparison of precipitation totals as forecast by IFS and as observed by radar illustrates systematic downstream precipitation biases in the CONTROL-10/HRES forecast (Fig9.6.4A & B).  This relates to the parametrisation of convection.  Convective precipitation produced in the IFS is considered to fall out immediately at the grid point and is not advected laterally irrespective of the winds experienced during descent of the precipitation.  This means that precipitation reaching the ground can extend further inland than CONTROL-10/HRES shows when:

• snow is falling (because in reality the snow flake drifts further in the wind than does rain), and/or
• there are strong steering winds (because in reality rain can be advected laterally during descent).  

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Fig9.6.4B: Comparison of 12hr precipitation totals from maritime convection in a cold (sub 528dam thickness) westerly flow to 09UTC on 7 Jan 2022.

• Forecast CONTROL-10/HRES precipitation (brown land, smooth outlines of precipitation areas)
• Radar precipitation  (white land/no precipitation, striated precipitation areas)

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The release of convection is strongly dependent upon correct analysis and forecasting of boundary layer humidity and land surface characteristics.  This can result in a mismatch, mainly in arid coastal regions, between the location and/or severity of forecasts of active convection and the corresponding observations.  Showers may be forecast in the wrong location or not forecast at all.  Users should consider the possible effects of more moist air feeding into the boundary layer, perhaps by considering the potential for moist marine air to spread inland in a more pronounced way than CONTROL-10/HRES forecasts suggest.  Users should consider the possibility of an influx of low level air that is dissimilar to forecast values - i.e. moist air across coastal areas that might allow release of convection, or the converse if an influx from drier areas occurs.  Daytime heating in upland locations and/or upslope flow over the mountains can also cause destabilisation that may not be captured by the forecast models.

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Considerations related to extreme convection 

CONTROL-10/HRES tends to over-forecast extreme convection, especially in the maritime tropics.  Spurious quasi-circular waves (sometimes rings) in convective precipitation fields can emanate from the forecast rapid uplift that is associated. These spread outwards. The gravity waves can even move well upwind, giving a false impression of an eastward-moving trough.  The source of these false features should be recognised and their effects discarded from forecasts. Changes to the IFS moist physics in 2019 have mitigated but not entirely removed this effect. 

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Fig 9.6.11: 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.

Fig9.6.12: CONTROL-10/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. 

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