Introduction

The ECMWF operational ensemble forecasts for the western Mediterranean region exhibited high uncertainty while Hurricane Nadine was slowly moving over the eastern North Atlantic in September 2012. Interaction with an Atlantic cut-off low produced a bifurcation in the ensemble and significant spread, influencing both the track of Hurricane Nadine and the synoptic conditions downstream.

The HyMEX (Hydrological cycle in Mediterranean eXperiment) field campaign was also underway and forecast uncertainty was a major issue for planning observations during the first special observations period of the campaign.

This interesting case study examines the forecasts in the context of the interaction between Nadine and the Atlantic cut-off low in the context of ensemble forecasting. It will explore the scientific rationale for using ensemble forecasts, why they are necessary and how they can be interpreted, particularly in a "real world" situation of forecasting for an observational field campaign.

In this case study

In the exercises for this interesting case study we will:

• Study the development of Hurricane Nadine and the interaction with the Atlantic cut-off low using the ECMWF analyses.
• Study the performance of the ECMWF high resolution (HRES) deterministic forecast of the time.
• Use the operational ensemble forecast to look at the forecast spread and understand the uncertainty downstream of the interaction.
• Compare a reforecast using the May/2016 ECMWF operational ensemble with the 2012 ensemble forecasts.
• Use manual clustering to characterize the behaviour of the ensembles and compare the results with clustering based on principal component analysis (PCA; see Pantillon et al.).
• Study the performance of the ECMWF ensemble forecasts trough RMSE curves.