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In IFS cycle 46r1 the Extreme Forecast Index (EFI) and Shift Of Tails (SOT) fields will be available for Water Vapour Flux (WVF). The WVF parameter is the magnitude of the combined vertical integrals of the eastward and northward WVF components. The average value of each component this integral over a specified forecast period (e.g. 24 hours) is computed using the instantaneous values from each output forecast step, and these are then combined to form the total WVF6-hourly values.  Its units are kg m-1 s-1 and it is available on the same time steps as the precipitation EFI.

The WVF EFI complements the precipitation EFI by highlighting the large-scale water vapour transport in the atmosphere. In so doing, it can provide earlier awareness of extreme precipitation on the west coasts of mid-latitude continents compared to that available from the EFI for precipitation, and additional understanding of the synoptic-scale processes behind an extreme hydro-meteorological event may be obtained. Further details on the parameter can be found in these three publications (Lavers et al., 2016; 2017; 2018). WVF EFI/SOT are available for the same forecast steps as total precipitation EFI/SOT: 24-hourly intervals up to day 7, 72 and 120-hourly intervals up to day 10 and the additional T+240-360h step; 240- and 360-hourly forecasts are available as well.

An example of the WVF EFI is given for an extreme hydro-meteorological event in northern Italy from 27th - 30th October 2018, a time where a few stations in northern Italy reported more than 300 mm in 24 hours. This was the result of a large-scale trough over the Western Mediterranean from which a deep cyclone developed and moved from Sardinia to the north. The EFI maps below show the WVF and precipitation EFI and they are valid on the event on forecast days 7-9. At this time, the precipitation is more skilful in capturing the location of the event, as shown by the co-location of high EFI values and the cyan boxes, which represent the areas with the highest precipitation totals. Although the WVF EFI has a weaker signal in the affected area on this occasion and at this lead time, it does highlight the large-scale nature and atmospheric structure and thus provides a synoptic context for users.

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