Upper-level extratropical disturbances over the low-latitude North Atlantic: Generation mechanisms and predictability

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Upper-level extratropical disturbances penetrating to low latitudes are a frequent feature of the synoptic-scale circulation over the North Atlantic and can be involved in significant weather events such as heavy precipitation or Saharan dust outbreaks. Linear Rossby-wave theory successfully explains many climatological characteristics, but the factors controlling the evolution of individual disturbances are still not well understood. Prior work on this matter is based on statistical analyses of observational data or case studies with numerical models. The proposed project is the first that systematically explores the huge dynamical information content of long-term operational ensemble prediction data. Since every ensemble member is a physically realistic realisation of a possible synoptic evolution, this approach allows a direct assessment of the relative importance of different factors such as baroclinic energy conversion or latent heat release. Other objectives are a probabilistic forecast validation, the estimation of predictability from the ensemble spread and the assessment of implications for high-impact weather forecasts. The results of this project will foster a better physical understanding of extratropical influences on the Tropics over the North Atlantic and unveil the ability of state-of-the-art operational systems to successfully predict low-latitude disturbances and their impacts.