Effects of cloud diabatic heating on the early development of the FASTEX IOP17 cyclone

A set of numerical simulations is used to study the first development phase of ‘The FASTEX cyclone’, a particularly well sampled frontal cyclone system that developed on the trailing front of a primary large‐scale baroclinic wave during Intensive Observation Period 17 (IOP17) of FASTEX (the Fronts and Atlantic Storm‐Track EXperiment). the role of diabatically induced vertical circulations is investigated with a combination of diagnostic tools, including a generalized omega equation and Keyser's psi‐vector technique, applied to the model outputs. the full‐physics simulation shows a good agreement with observed data, particularly the splitting of the upper‐level flow into a pair of jet streaks. Analysis of the contributions to the vertical ageostrophic transverse circulations near the jet‐streak‐pair structure, shows that a positive feedback exists between the storm's dynamical configuration and the diabatic processes. In the second part of the paper, this interpretation is confirmed by a sensitivity analysis excluding cloud processes. Results suggest that, after initial baroclinic growth, the storm evolves towards a purely diabatic deepening regime, leading to a ‘diabatic Rossby wave’ mechanism, as proposed by recent wave‐CISK (Conditional Instability of the Second Kind) theoretical studies.