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 syste m that developed on the trailing front of a primary large-scale baroclinic wave during Intensive Observation Period 17 (IOP17) of FASTEX (the Fronts a nd Atlantic Storm-Track EXperiment). The role of diabatically induced verti cal circulations is investigated with a combination of diagnostic tools, in cluding a generalized omega equation and Keyser's psi-vector technique, app lied to the model outputs. The full-physics simulation shows a good agreeme nt with observed data, particularly the splitting of the upper-level flow i nto a pair of jet streaks. Analysis of the contributions to the vertical ag eostrophic transverse circulations near the jet-streak-pair structure, show s that a positive feedback exists between the storm's dynamical configurati on and the diabatic processes. In the second part of the paper, this interp retation is confirmed by a sensitivity analysis excluding cloud processes. Results suggest that, after initial baroclinic growth, the storm evolves to wards a purely diabatic deepening regime, leading to a 'diabatic Rossby wav e' mechanism, as proposed by recent wave-CISK (Conditional Instability of t he Second Kind) theoretical studies.