Inland penetration of wintertime maritime showers

Wintertime maritime convection can penetrate further inland than indicated by IFS forecasts.

Comparing Ensemble Control Forecast (ex-HRES) precipitation totals with radar observations shows a systematic bias towards too little inland penetration of showers.  This is due in part to the parametrisation of convection.  Convective precipitation produced by IFS is considered to fall out immediately at the grid point.  It is not advected laterally irrespective of the winds experienced during descent of the precipitation.

In strong winds, maritime showers can extend further inland than Ensemble Control Forecast (ex-HRES) shows because in reality:

• active maritime convective cells can persist for an hour or so.
• rain can be advected laterally during descent.  
• snow flakes drifts further than rain because of slower fall speed.

See also section on convective cloud processes and precipitation.

Fig9.6.1-1: An Illustration of observed showery precipitation over NW England on 28 February 2017.

A line of active wintry showers formed over the Irish Sea. The convective cells moved towards the ESE steered by the 700hPa wind.  Local rainfall totals were observed >10mm/12hr.  Higher probability of heavier showers is confined to coastal grid points.  Moderate probability of lighter showers (2-4mm/12hr) penetrating inland.  The red outline (top left frame) highlights where errors were particularly acute.  

• Top left frame. Observed total 12hr rainfall. Scale shows totals in mm/12h). 
• Top right frame. HRES (identical to current ensemble control) total 12hr rainfall. Scale shows totals in mm/12h.
• Lower left frame.  Ensemble probability of rainfall >2mm in 12hr. Scale shows percentage probabilities.
• Lower right frame. Radar rainfall imagery. Scale shows instantaneous rates in mm/hr).  Thi

Fig9.6.1-2: Comparison of precipitation totals from maritime convection in a cold (sub 528dam thickness) westerly flow.  Totals are for 12hr ending 09UTC on 7 Jan 2022.

• Forecast  Ensemble Control Forecast (ex-HRES) precipitation (brown land, smooth outlines of precipitation areas).
• Radar precipitation  (white land/no precipitation, striated precipitation areas).

Fig9.6.1-3: An example of non-equilibrium convection from  Ensemble Control Forecast (ex-HRES) VT 06UTC 01 Jan 2025, DT00UTC 01 Jan 2025.  Cold north westerly winds (yellow arrows, 850hPa temperature about -7C) blowing over a relatively warm lake water (temperature around +3C).  Significant blizzards affected many towns on the southeastern coasts of the lakes.

Fig9.6.1-2: An earlier example using non-equilibrium convection.  Showers developed over the relatively warm Great Lakes in very cold air in a westerly airflow.  Significant showers over the lakes where strong heating from below. Only very small precipitation totals down-wind little or no heating over the cold land.  In reality the showers developing over the Great Lakes persisted long enough to be blown well inland as active convection. Note in particular the difference in precipitation east of Lake Michigan.

Effect of onshore wind strength

Any showers that IFS develops over the sea do not penetrate beyond the coast.  IFS develops inland showers using the local model atmospheric structure that has drifted in from upwind.  Therefore shower precipitation is biased rather too much in coastal parts, rather too little inland.  The areas affected are extended with stronger flow.

Fig9.6.1-5: Schematic illustration of systematic precipitation biases in onshore maritime convection.  Too much precipitation is forecast for windward coastal zones.  Too little precipitation is forecast for areas leeward of high ground.  These areas expand and move downwind with stronger winds.