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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 .  

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.  

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Fig9.6.1-15: An example of the effect of under-representation of low-level temperature and dew point by the IFS near an urban heat island.  Forecast values of near-surface temperature and dew point are about 3C cooler than observed.  Modifying the forecast vertical profile using observed values results in significantly greater CAPE than forecast which, together with the forecast shear, would indicate much more active convection.  Flash flooding and a tornado were observed near the urban heat island.

Precipitation over mountainous coasts and islands

Too much precipitation can be forecast over mountainous coasts and islands.  At a location on a mountain the model height can be significantly lower than the true height of the of the land surface.  See Tenerife as an example of a mountainous island.  Thus temperatures at the location can be forecast to be too warm.  This can then result in in a reduction of CIN from true values and the release of convection, possibly with large MUCAPE, over the mountains.

Higher forecast temperatures inland are likely to induce more onshore flow of moist air from nearby sea or lake.  This can substantially alter the structure of the forecast airmass and possibly encourage release of convection.It is possible there is excess convergence in HRES and the ensemble.  This would result in greater vertical velocity  and moisture convergence that could lead to errors in forecast precipitation over steep or poorly resolved orography.

It is for the forecaster to assess critically the expected and changing structure and evolution of the airmass at a given upland location.  Forecasters should not rely on a single model forecast, but view the ensemble of forecasts and a whole.  Further, although precipitation could be heavy over the mountains, it might not be widespread nor extend to adjacent low-lying areas.   Equally, dry zone underlying the altitude of the mountain convection may reduce precipitation penetrating to lower levels. 

Medium level instability in drier areas

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