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

Orographic and Rainshadow effects

Orographic and rain shadow effects can be strong in unstable onshore air flow, particularly when an unstable marine airmass meets coastal mountains.   However, precipitation forecasts can be incorrectly represented.

This is because the forced uplift triggers immediate development of parametrised showers, with precipitation (snow in the illustrated case) falling immediately and vertically to the ground.  

In reality the showers take time to grow while also being driven downwind.  The snow that falls from them also drifts downwind as it falls.  However, neither "drift" mechanism is represented in the IFS and the net effect is that the snow in reality is spread across a much larger distance downwind than in the raw model output .  

Image Added

Fig9.6.1-5A:  The diagram shows an area of NW Scandinavia with snow accumulation indicated by colours (large accumulations blues, small amounts, green).  Topographically the area is complex, but the key feature are strong upslopes near the exposed NW coast of Norway, with a line of mountains reaching about 2000m but interspersed with lower lying gaps.  

The top left diagram shows accumulated snow derived over a 15 day period ending 00UTC 1 May 2023.  Forecast accumulation of snowfall is predominantly on the exposed NW-facing mountainous coastal areas but a strong indication of little or no snow accumulation to the lee of the mountain ranges (shown by dashed line).

The central diagram shows the ECMWF analysed accumulation of snow at 00UTC 1 May 2023 which uses observations supplied by the relevant meteorological service but also uses the predicted accumulations given in the top diagram.  Thus there remains a bias towards the clearer area near the dotted line despite the observations.

The bottom diagram is the snow depth analysis by the Swedish meteorological service.  There are more observations than are shown plotted, and the rain shadow effect is not as well marked as suggested by model forecast or analyses.  

Convective Severity - CAPE and CAPE-shear

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