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The water vapour flux (WVF) is a vertical integral combining eastwards and northwards water vapour fluxes. This is integrated over all model levels and the average value over a specified forecast period (e.g. 24 hours) is computed using the instantaneous 6-hourly values. Its The units are kg m-1 s-1. Note:
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The Extreme Forecast Index (EFI) for water vapour flux compares the ENS ensemble forecast probability distribution of an event to the M-climate distribution for the chosen location, time of year and forecast lead time. The water vapour flux EFI can provide an understanding of the synoptic-scale processes behind an extreme rainfall events.
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- Water vapour flux EFI tends to show up a signal earlier in the forecast than the total precipitation EFI.
- Extreme water vapour flux EFI can enable earlier awareness of extreme precipitation, and in particular early identification of the risk of flooding.
- Consideration of other factors such as orography can help interpretation of the forecasts (e.g. west coasts of continents or enhancement of orographic precipitation on windward slopes in regions of high water vapour flux EFI).
- It can be especially helpful where complex topographic details are not well captured at ENS ensemble resolution and forecast sub-synoptic flow can result in underestimation of precipitation values and EFI on the upslope side (and overestimation on the downslope side). The broader scale flow implicit in evaluation of water vapour flux EFI gives better indication of risk areas.
- Heaviest precipitation often occurs at the edge of the area with large water vapour flux EFI.
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