engleski

Dynamical downscaling of ECMWF EPS forecasts applied to cases of severe weather in Croatia

Dynamical downscaling has been applied to global ensemble forecasts to assess its potential impact during cases of severe weather (precipitation and wind) over various parts of Croatia. It was based on the Croatian 12.2-km version of the Aladin limited area model, nested in the ECMWF TL255 (approximately 80 km) global Ensemble Prediction System (EPS). The 3-hourly EPS output was used to force the Aladin model over the central European/northern Mediterranean domain. Four synoptic cases are considered, for which both global EPS and regional Aladin 51-member ensembles were run. From synoptic cases studied in more detail, downscaling brings improvement in one case where a well-resolved small-scale orography was important. In the second case, no improvement in the downscaled ensemble is seen. This was the case with severe weather defined at very small scales over an area of Croatia with no major orographic differences between the global and regional models. In the third case, severe weather covered a large area over the Adriatic Sea and the coastal Croatia and it was well captured by the global model. For this case, the limited area model yielded only a relatively little improvement in both synoptic development and in terms of probabilistic forecasts for precipitation and wind. Our results indicate that downscaling can have a large impact on clustering: when applied to both global and downscaled sets of ensembles the same clustering algorithm may yield differing results. For example, even in the case when clusters in both global and regional models are made of the same individual members, cluster means could differ considerably. This implies that downscaling may affect dynamical and physical properties of the global ensemble forecasts. It has been argued that this is due to explicitly resolved small spatial scales in the downscaled ensemble, in particular to those related to orography. Therefore, in the process of downscaling, it may not always be feasible to make a selection (or a subset) of global ensemble members that might be representative of all possible evolution scenarios. This notion is supported by a larger spread in the Aladin than in the EPS ensembles for the fields closely related to small-scale variations in orography (for example, wind and vertical velocity). Although this paper discusses too few cases for any statistically significant conclusions to be drawn, it provides useful indications on the accuracy of the ECMWF EPS and on the Aladin ensembles in cases of severe weather. Since from the cases considered no clear-cut difference between the global and the downscaled ensembles was detected, additional work with more synoptic cases and more detailed analysis is required to be able to draw some comprehensive conclusions.

downscaling; ensembles; severe weather

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