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The European Rainfall-InduCed Hazard Assessment (ERICHA) system for real-time flash flood monitoring and nowcasting (heatherafter the ERICHA system) was implemented to the EFAS flash flood layer within the EC DG-ECHO ERICHA project (2016-2017).

The ERICHA system provides nowcasts of both precipitation and flash flood hazard (by means of a simple traffic-light indicator) in the range 0- 6 hours (a simplified scheme is shown in Fig. 1). The driving variable is the basin-aggregated rainfall estimated from radar-based observations. In EFAS, the European radar composites (with a spatial grid resolution of 2 km) provided by the EUMETNET project OPERA are used to calculate the basin-aggregated rainfall over the drainage network with a grid resolution of 1-km. The output is the ERICHA flash flood hazard indicator for basins up to 5000 km2.


Figure 1: The chain of the updated ERICHA system producing precipitation and flash flood hazard nowcasts

The ERICHA-FF hazard assessment is solely based on radar rainfall inputs, assuming that the basin response will be strongly dominated by the basin-aggregated rainfall. Therefore, the quality of the rainfall inputs is critical to the resulting hazard indicator (Park et al., 2019). A new module has been added to the ERICHA system to compensate for the most systematic biases affecting radar rainfall inputs. The module retrieves daily a multiplicative adjustment factor map estimated by long-term comparison (up to 180 days) between the gauge precipitation records available in EFAS and the collocated radar rainfall estimates (e.g. Fig. 2).


Figure 2: An example of daily adjustment factor map to be applied to each 15-min radar rainfall accumulation during 21 Oct 2019.

The adjustment factor map is updated once a day (after the arrival of the gauge datasets from the previous day) and is applied to the 15-min radar precipitation accumulations. As a result, radar underestimation can be partly improved (e.g., as shown in the 24-h rainfall accumulation of Fig. 3), which has a clear effect on the estimated hazard level and location of flash floods (as shown in Fig. 4, for an event occurred in Northern Italy).

(a)  Before the adjustment

(b)  After the adjustment


Figure 3: 24-h rain accumulation for 21 October 2019 (a) before and (b) after applying the adjustment factor.



(a)  Before the adjustment(b)  After the adjustment

Figure 4: 1-h rain accumulation and estimated hazard level for 21 October 2019 at 1700 UTC (a) before and (b) after applying the adjustment factor. The news (e.g. La Republica) reported significant floods in Castelletto d’Orba (orange FF hazard level in panel b) and Rossiglione (yellow FF hazard level in panel b), where the bottom pictures were taken.

Three ERICHA products are shown in EFAS in real time:

  • ERICHA hourly accumulated precipitation: 1-h precipitation accumulation estimated from the European OPERA radar rainfall composites. This product is displayed in a loop with 15-minute time interval. It shows the estimated precipitation for several hours in the past up to current time and nowcasts (when the loop bar turns red) up to 6 hours. Deterministic precipitation nowcasts are generated by Lagrangian extrapolation. This product was developed in the ERICHA project. Recently, a multiplicative adjustment factor map (retrieved by long-term comparison with rain gauge observations) is applied to compensate for systematic biases in the original OPERA radar rainfall estimates. The gauge-adjustment module has been developed within the SMUFF project.
  • ERICHA FF hazard level: Using the gauge-adjusted precipitation estimates and nowcasts, the flash flood hazard level is estimated by first computing the upstream basin-aggregated rainfall over the drainage network (resolution: 1 km) and then comparing the observed values with a set of reference thresholds to determine the flash flood hazard level (yellow-low, orange-medium, red-high). The ERICHA FF hazard level is displayed in a loop with 15-minute time interval. It shows the estimated flash flood hazard level for several hours in the past up to current time and nowcasts up to 6 hours (when the loop bar turns red). This product was developed in the ERICHA project.
  • ERICHA 24-h precipitation accumulations: Daily gauge-adjusted radar rainfall accumulation is estimated for the last 24 hours finishing at the selected time stamp (0000, 1200 UTC). In EFAS this product is only available for the past 7 days. This product was developed in the ERICHA project.

REFERENCES

Park, S.; M. Berenguer, and D. Sempere-Torres, 2019: Long-term analysis of gauge-adjusted radar rainfall accumulations at European scale. Journal of Hydrology, 573, 768–777.



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