6533b82efe1ef96bd1293f2c
RESEARCH PRODUCT
Assessing the predictability of Medicanes in ECMWF ensemble forecasts using an object-based approach
Michael Maier-gerberAndreas H. FinkEnrico Di MuzioEnrico Di MuzioMichael Riemersubject
Atmospheric ScienceDynamic time warping010504 meteorology & atmospheric sciencesMeteorologyFOS: Physical sciencesStorm01 natural sciences010305 fluids & plasmasPhysics - Atmospheric and Oceanic Physics0103 physical sciencesAtmospheric and Oceanic Physics (physics.ao-ph)Data analysisTrajectoryJumpCycloneEnvironmental sciencePredictabilityLead timePhysics::Atmospheric and Oceanic Physics0105 earth and related environmental sciencesdescription
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.
year | journal | country | edition | language |
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2018-12-16 |