202309 - Rainfall - Daniel (Greece, Bulgaria)

Status:Ongoing analysis Material from: Linus, Ivan, Tim

1. Impact

On 4-5 September the low-pressure system named Storm Daniel produced extreme rainfall over south-eastern Europe, notably Greece where the one station measured more than 1000 mm/24h. The rainfall led to widespread flooding. Also in Bulgaria and Turkey the system caused damages and fatalities.

This event has also been discussed in two ECMWF Newsletter articles: here and here.

https://en.wikipedia.org/wiki/Storm_Daniel

From MedCyclones: https://medcyclones.eu/wp-content/uploads/2023/09/MedCyclone_Daniel_v2.pdf

2. Description of the event

The plots below show analyses of MSLP and 6 hour rainfall from 2 September 00UTC to 7 September 00UTC, every 12th hour.

The plots below show analyses of z500 and T850 from 1 September to 7 September 00UTC, every 24 hour.

Station precipitation from Greece: https://www.meteo.gr/UploadedFiles/articlePhotos/SEP23/compare_kakoκairies_ianos_daniel_meteogr.jpg , and some graphics below (unclear to which accumulation period it refers).

3. Predictability

3.1 Data assimilation

3.2 HRES

The plot below show precipitation totals 4 Sept 00UTC to 6 September 00UTC from observations and HRES.

The plots below show 72-hour precipitation (4 September 00UTC - 7 September 00UTC) in observations (first plot) concatenated short forecasts (second plot) and ENS control forecasts with different lead times.Note that sea points have been masked out. The square marks a 0.5x0.5 degree box around Volos in Greece, which got more than 600 mm.

The plot below shows the same as above but for 4.4km DestinE forecasts.

3.3 ENS

The plots below shows EFI and SOT for 3-day precipitation (4 September 00UTC - 7 September 00UTC) from different initial dates. The signal for extreme precipitation appeared 4 days before the event (31 August).

The plots below show the same as above but with SOT included. Also in SOT, there is not much of a signal from before 31 August.

The plot below shows the forecast evolution plot for 72-hour precipitation (4 September 00UTC - 7 September 00UTC) for land points inside a 0.5x0.5 box centred on Volos in Greece. Concatenated 6-hour forecasts - green dot, ENS control –red, DestinE4.4 - purple, ENS blue box-and-whisker, Model climate – cyan box-and-whisker. Ensemble mean as black diamonds. Triangle marks the maximum in the model climate based on 1800 forecasts. Also here the step change from 30 to 31 August is evident in the ensemble.

The plots below show z500 ensemble mean forecast (black), verifying analysis (red) and error (shading) for the forecast from 30 August 00z for different time-steps (shortest forecast first), which resulted in a large-scale forecast bust over Europe. Large errors appear at step 24 (first plot) east of Canada, related to a rapid development of a surface cyclone. At the same time TC Franklin and TC Idalia were present in the western Atlantic. The error propagated to north-east and probably caused the error in the omega blocking that had storm Daniel forming on its south-eastern side day 6.