Status:Ongoing analysis Material from: Linus, Calum
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Discussed in the following Daily reports: |
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The plots below show 16 ensemble members from 6 August 00z for 24-hour precipitation (6 August 06z - 7 August 06z).
ECMWF
COSMO-LEPS
3.6 EFAS
Calum had a look at the EFAS forecast for the Macedonia floods, specifically at the 2016080512 forecast. Firstly the soil moistures were quite dry, in NW Macedonia the values were ~20% in all the ensemble members (this is calculated by: (forecast soil moisture - minimum climatology soil moisture)/(maximum climatology soil moisture - minimum climatology soil moisture)). Secondly in EFAS the flash flood warnings are generated from the surface runoff rather than the precipitation, so the dry soils mean that not much of the rain was translated into runoff (less than 20%). We generate flash flood warnings when there is a high probability of exceeding the mean annual maximum climatology runoff value, in the attached map the smaller dark-red pixels show the probability of exceeding this threshold, typically they were very low ~25%. The system has automatically generated 4 warning points (3 in Macedonia and 1 in Serbia) but as you can see from the graphs the probabilities of exceeding any of the return period levels is quite low. EFAS only display points on the website which have >20% probability of exceeding the 5 year return period, because this condition wasn't met at these locations they therefore weren't displayed on the website.
This case might raise an issue about how we generate runoff during an extreme rainfall event, in the flash floods we're using an empirical relationship between soil moisture and the runoff coefficient but the parameters are derived from the LISFLOOD climatology run so the same problem might emerge from the original model results. LISFLOOD uses the Xinanjiang model (aka VIC/ARNO) to generate surface runoff, maybe we should test how suitable it is for extreme flash flood events.
4. Experience from general performance/other cases
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5. Good and bad aspects of the forecasts for the event
- The extreme rainfall was missed over Skopje, but present further to the north in the forecasts
- Negative influence of very dry soil in the EFAS forecast?