202011 - Rainfall - Sardinia

 Status: Ongoing analysis Material from: Linus, Fatima, Tim H. 

1. Impact

Some extraordinary flash floods hit several parts of Sardinia on 28 November, which for a widespread event was probably only eclipsed in the last decade, on that island, by the events of 18 November 2013. In the 2013 event the peak event rainfall was about 450mm, in the recent event it was closer to 350mm.They were in fact remarkably similar, in synoptic terms, with an 'upside down' cold front moving south to north across the island, over about a day, on the eastern flank of a deep Mediterranean cyclone, and with warm moist air on the NE flank of the cold front, feeding up from a marine source that starts near the N coast of Libya. This airflow delivered periods of heavy rain to the eastern side of the island in particular, where sharply rising mountains can also lead to significant orographic enhancement. Widespread lightning strikes over land and sea, together with IFS model output (profiles), together attest to the highly convective nature of the 2020 event.

2. Description of the event

Animation of synoptic charts.

Rainfall observation breakdown:

6-hourly totals:

12-hourly totals (third plot, with smaller spots, shows data from the second plot augmented by data from another Sardinian network, for 23-11UTC):

24-hourly totals:

The plots below show analyses of MSLP and 6-hour forecasts of precipitation from 27 November 00UTC to 29 November 00UTC, every 12th hour.

3. Predictability

3.1 Data assimilation

3.2 HRES

The plots below show 24-hour precipitation on 28 November in observations (first plot) and short-range HRES forecasts.

3.3 ENS

The plots below show EFI for 24-hour precipitation valid 28 November in forecasts with different initial times.

The plot below shows the evolution of the forecasts for 24-hour precipitation on 28 November in the box outlined in the plots above. The plot includes HRES(red), ENS control (purple), ENS distribution (blue), model climate distribution (red) and mean of observations (green).

3.4 Monthly forecasts

3.5 Comparison with other centres

4. Experience from general performance/other cases

5. Good and bad aspects of the forecasts for the event

6. Additional material

This was also a case for the MISTRAL initiative (an EU project in which Italian partners and ECMWF have collaborated). In that project we have the "Italy flash flood use case", spearheaded by ECMWF, in which post-processing, of different types, is applied to the ECMWF ENSemble, and also a 2.2km Italy-centred limited area COSMO ensemble. The post-processed outputs are blended together, with lead-time-dependant weighting, to make the final product, which aims to give better probabilistic rainfall forecasts, in particular with a view to providing improved early warnings of flash flood risk (via the association with extreme short period rainfall). Users are encouraged to focus on the higher percentiles (or probabilities of exceeding high thresholds) in the MISTRAL products, to gauge the potential for localised extremes. However the plots below illustrate how the ensemble mean is handled in the raw model and post-processed output, with differences between the two (for a given system) indicating the nature of any bias-correction being applied on the model grid-scale.