# Assessing the predictability of Medicanes in ECMWF ensemble forecasts using an object-based approach

## TL;DR

This study evaluates the predictability of Medicanes using ECMWF ensemble forecasts and an object-based approach, revealing forecast accuracy improves at short lead times and identifying predictability barriers and forecast jumps.

## Contribution

It introduces an object-based method to assess storm features and demonstrates the limited lead time for accurate Medicane predictions with ECMWF models.

## Key findings

- Forecast accuracy improves at short lead times.
- Compactness and symmetry are underpredicted, especially at long lead times.
- Rapid forecast jumps occur between 5 and 7 days lead time.

## Abstract

The predictability of eight southern European tropical-like cyclones, seven of which Medicanes, is studied evaluating ECMWF operational ensemble forecasts against operational analysis data. Forecast cyclone trajectories are compared to the cyclone trajectory in the analysis by means of a dynamic time warping technique, which allows to find a match in terms of their overall spatio-temporal similarity. Each storm is treated as an object and its forecasts are analysed using metrics that describe intensity, symmetry, compactness, and upper-level thermal structure. This object-based approach allows to focus on specific storm features, while tolerating their shifts in time and space to some extent. The compactness and symmetry of the storms are generally underpredicted, especially at long lead times. However, forecast accuracy tends to strongly improve at short lead times, indicating that the ECMWF ensemble forecast model can adequately reproduce Medicanes, albeit only few days in advance. In particular, late forecasts which have been initialised when the cyclone has already developed are distinctly more accurate than earlier forecasts in predicting its kinematic and thermal structure, confirming previous findings of high sensitivity of Medicane simulations to initial conditions. Findings reveal a markedly non-gradual evolution of ensemble forecasts with lead time. Specifically, a rapid increase in the probability of cyclone occurrence ("forecast jump") is seen in most cases, generally between 5 and 7 days lead time. Jumps are also found for ensemble median and/or spread for storm thermal structure forecasts. This behaviour is compatible with the existence of predictability barriers. On the other hand, storm position forecasts often exhibit a consistent spatial distribution of storm position uncertainty and bias between consecutive forecasts.