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The guidelines in Section 1.1.6 hold for most cyclones, but very small cyclones pose extra difficulties. They may be beyond the capabilities of the IFS horizontal resolution. And even very small cyclones can have a sting jet associated. Accordingly forecasters need to treat ensemble output with particular care when there is potential for very small cyclones to develop - say with lateral dimensions of order 200km or less.
1.1.7 Local winds due due to orography
Detail of winds to the lee of and around isolated mountains inland or mountainous islands are not likely to be captured. This is because:
- horizontal resolution (~9km) may not capture the geographic features sufficiently.
- smoothing of the orography is likely to underestimate the mountain heights (possibly by factor of 4 in isolated mountainous island).
However, the IFS ensemble can give useful guidance on some local winds. The effect can be further enhanced where the flow is trapped beneath an inversion and the flow confined within favourably aligned valleys.
The detail of the forecast wind strength depends upon vertical and lateral constraints in the flow. The model orography defines lateral limitation to the flow but valleys may not be identified well. The strength and height of an inversion governs the vertical constriction of the flow. Ensemble members usually give a range of forecasts of inversion heights. A low inversion implies winds are enhanced, particularly where the inversion is below model mountain heights. A higher inversion implies winds that are less modified from gradient values.
In such conditions, it is better to use ensemble forecasts to assess the probability of extreme winds. Ensemble members can give an idea of the range of inversion height. CDF plots of forecast wind gusts can show probability of high or extreme values and the variation in the values over recent forecasts. EFI and SOT can be evaluated from the CDF.
Extreme winds or gusts can be readily identified.
A case of a strong Bora winds.
The vertical profile shows a large spread in the inversion layer suggesting different boundary layer heights and consequently also wind profiles.
The CDF diagram shows the range of wind gusts at Zadar, on the Croatian coast. The red line shows CDF from latest ensemble (DT 00UTC 12 Jan 2025; VT 00UTC t0 2359 UTC 13 Jan 2025). The values are all greater than the M-climate with a large positive EFI (shaded) and SOT about 1.3. Further, all previous forecasts valid for the same date are similar in shape and shape.
The latest ensemble shows a median around the 13 m s-1 with the tail beyond hurricane force (> 32 m s-1). The Control run forecast has a max gust of about 13 m s-1 (also near the median). However, the high EFI and SOT values and the consistency over recent forecasts allows a good deal of confidence in forecasting the possibility of very strong gusts. The user should assess the probable structure of the boundary layer and modify the ensemble products if necessary. The height of the inversion should be compared with the actual mountain heights, rather than the smoothed model orography. Hydraulic jumps and rotor effects might also occur.
Fig9.3-7: Forecast vertical profile at Zadar on the Croatian coast showing a large spread in the inversion layer height. Winds from the ensemble control are between 35-45 m s-1 constrained beneath the inversion. (DT: 00UTC 12 Jan 2025; VT 06UTC 13 Jan 2025).
Fig9.3-8: CDF diagram for Zadar showing forecast of wind gusts consistently above M-climate over the last 6 forecasts. The gust values ofthe extreme members of the latest ensemble forecast (37 m s-1) are well in excess of the M-climate (33 m s-1). Shaded area shows EFI of the latest ensemble forecast. High SOT value is derived from the diagram. (DT: 00UTC 12 Jan 2025; VT: 13 Jan 2025).
Fig9.3-9: Max 10m gust in 6 hours to 06UTC 13 Jan 2025. Ensemble control (Ex-HRES). (DT: 00UTC 12 Jan 2025; VT 06UTC 13 Jan 2025).
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