Page History

...

This effect can occur with onshore cyclonic flow of maritime air that is marginally unstable to sea surface temperatures.  At certain times SSTs can be higher close to coastlines than offshore.   In these cases the inshore sea surface temperature can be high enough for the convection scheme in IFS to trigger release of convection with high CAPE values.  Just upstream the lower sea surface temperatures offshore cannot overcome convective inhibition at low or mid-tropospheric levels.    The IFS convective scheme triggers Instantaneous shower development at each step but does not advect the showers down wind.  This results in repeated convective rainfall over the same inshore locations which can add up to large or very implausibly large implausibly record-breaking near-coast totals (Fig9.6.1-6).

Fig9.6.1-6:  EFI for 24h ppn precipitation for period 00UTC 6 Nov 2023 to 00UTC 7 Nov 2023, DT 00UTC 6 Nov 2023.  Unrealistically large or extreme precipitation totals on coasts exposed to the northwest are caused by instantaneous shower development by the convection scheme over inshore waters that are slightly warmer than offshore.

...

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 severity of forecasts of active convection and verifying 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.

...