Abstract
Abstract. A large fraction of extreme precipitation and flood events across western Europe are triggered by atmospheric rivers (ARs). The association between ARs and extreme precipitation days over the Iberian Peninsula has been well documented for western river basins. Since ARs are often associated with high impact weather, it is important to study their medium-range predictability. Here we perform such an assessment using the ECMWF ensemble forecasts up to 15 d for events where ARs made landfall in the western Iberian Peninsula during the winters spanning between 2012–2013 and 2015–2016. Vertically integrated horizontal water vapor transport (IVT) and precipitation from the 51 ensemble members of the ECMWF Integrated Forecasting System (IFS) ensemble (ENS) were processed over a domain including western Europe and the contiguous North Atlantic Ocean. Metrics concerning AR location, intensity, and orientation were computed, in order to compare the predictive skill (for different prediction lead times) of IVT and precipitation. We considered several regional boxes over western Iberia, where the presence of ARs is detected in analysis/forecasts, enabling the construction of contingency tables and probabilistic evaluation for further objective verification of forecast accuracy. Our results indicate that the ensemble forecasts have skill in detecting upcoming AR events, which can be particularly useful to better predict potential hydrometeorological extremes. We also characterized how the ENS dispersion and confidence curves change with increasing forecast lead times for each sub-domain. The probabilistic evaluation, using receiver operating characteristic (ROC) analysis, shows that for short lead times precipitation forecasts are more accurate than IVT forecasts, while for longer lead times this reverses (∼10 d). Furthermore, we show that this reversal occurs for shorter lead times in areas where the AR contribution is more relevant for winter precipitation totals (e.g., northwestern Iberia).
Highlights
Extreme precipitation events in the Iberian Peninsula are often due to anomalous vertically integrated water vapor transport (IVT) which are generally associated with an atmospheric river (AR; Ramos et al, 2015, 2018; Eiras-Barca et al, 2016)
The occurrence of extreme precipitation days in different river basins is highly sensitive to the latitudinal location of the AR landfall as shown for the Iberian Peninsula (Ramos et al, 2015)
This is due to ARs being relatively narrow corridors of strong horizontal water vapor transport; their landfall position has influence on the occurrence of a possible extreme precipitation event and its specific location
Summary
Extreme precipitation events in the Iberian Peninsula are often due to anomalous vertically integrated water vapor transport (IVT) which are generally associated with an atmospheric river (AR; Ramos et al, 2015, 2018; Eiras-Barca et al, 2016). The main objective here is twofold: (a) to compare the predictive skill of precipitation and IVT at different lead times during extreme ARs striking western Iberia, using ECMWF ensemble forecasts up to 15 d for the winters between 2012– 2013 and 2015–2016; and (b) to assess the skill (or accuracy) of IVT probabilistic forecasts in terms of location landfall and intensity, through a probabilistic verification procedure, allowing the identification of possible model biases during extreme AR events, and to define simple metrics which may be suitable for operational purposes.
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