Supplementary material to "How Well do Models Represent the Development of Extra-Tropical Cyclones? Evaluation of Two General Circulation Models Against NAWDEX IOP 6 Observations"

Abstract. The dynamical and microphysical properties of a well-observed cyclone from the North Atlantic Waveguide and Downstream impact Experiment (NAWDEX), called the Stalactite cyclone and corresponding to Intensive Observation Period 6, is examined using two atmospheric global circulation models: CNRM-CM6-1 and IPSL-CM6A. The hindcasts are performed in weather forecast mode , run at CMIP~6 resolution (LR) and c.~0.5° (HR) and initialized during the initiation stage of the cyclone. Cyclogenesis results from the merging of two relative vorticity maxima at low levels: one is associated with a Diabatic Rossby Vortex (DRV) propagating from the subtropics and the other is initiated by baroclinic development and interaction with a pre-existing upper-level PV cut-off. All hindcasts produce (to some extent) a DRV. However, the second vorticity maximum is almost absent in LR hindcasts because of an underestimated upper-level PV cut-off. The evolution of the cyclone is examined via the quasi-geostrophic ω equation, which separates the diabatic heating component from the dynamical one at each given time. In contrast with some previous studies, there is no change in the relative importance of diabatic heating with increased resolution. The analysis also shows that IPSL-CM6A produces a more active cyclone compared to the CNRM-CM6-1 due to stronger diabatism. To examine this further, hindcasts initialized during the mature stage of the cyclone are compared with airborne remote-sensing measurements. There is generally an underestimation of the ice water content in the model compared to the one retrieved from radar-lidar measurements, even when the liquid water content is added. Consistent with the increased diabatism in IPSL-CM6A compared to CNRM-CM6-1, the sum of liquid and ice water contents is higher in IPSL-CM6A than CNRM-CM6-1 and, in that sense, IPSL-CM6A is closer to the observations. However, ISPL-CM6A strongly overestimates the fraction of super-cooled liquid compared to the observations by a factor of approximately 50.