Perturbation and uncertainty growth along the jet stream: the role of tropical cyclones, jet stream dynamics, and sensitivity to resolution

We investigate the development of ensemble forecast uncertianty associated with jet stream perturbations and dynamics. We partition uncertainty growth into diabatic and dynamic processes. A case study focusses on the recent Fujiwara-style interaction of Hurricanes Humberto and Imelda , and their subsequent interactions with the jet stream. These are seen to be able to perturb the jet and inject considerable uncertainty via diabatic processes. Later, dynamical processes along the jet (such as the development of cut-of features) act to further magnify uncertainty. The result for Europe was Storm Amy, which caused significant damage and some loss of life, but which was not well predicted. Through further experimentation, we try to understand the key diabatic and dynamical processes, how they combine to govern operational predictive skill, and their sensitivity to model resolution.