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 a schematic of model representation of orography and 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 Ensemble control (Ex-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. 

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 .  

Fig9.6.2-1:  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.


Unrealistic extreme convective precipitation focussed near coastlines

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 implausibly large record-breaking near-coast totals (Fig9.6.2-2).

Fig9.6.2-2:  EFI for 24h 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.


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