Subtropical cyclones (STCs) are characterized by a hybrid structure sharing tropical and extratropical features. The impacts of this kind of cyclones are similar to the generated by tropical storms or even hurricanes, leading to widespread social damage and great economic losses. Carrying out accurate simulations becomes key for a forecast improvement of these extreme events. In this study, several STCs that hit the northeastern Atlantic coasts and experimented a transition into tropical storms (Delta 2005) or even hurricanes (Vince 2005 and Ophelia 2017) are assessed by means of simulations using the high-resolution HARMONIE-AROME model. This model is developed and operated at 2.5 km resolution through the collaboration of the 10 European National Meteorological Services (NMS) that are part of the international research program HIRLAM together with the 16 countries that comprise the ALADIN consortium. The HARMONIE-AROME model has a convection-permitting configuration and uses a non-hydrostatic spectral dynamical core with a semi-Lagrangian and semi-implicit discretization of the equations. In this way, more realistic results are obtained, which allows providing an added value to the study of tropical transitions.

Turbulence and icing represent a danger for the aircrafts. In fact, turbulence is the leading cause of aircraft accidents related to meteorology. Both turbulence and icing weather hazards can occur in mountain waves simultaneously. For this reason, it is important to study these weather phenomena. Here, the HARMONIE-AROME model is used in order to analyze mountain wave events in the Guadarrama and the Cantabrian Mountain Ranges (Spain). This short-range mesoscale numerical weather prediction model is used for operational weather forecasts in many European meteorological organizations with a horizontal resolution of 2.5 km. In this study, a spatial resolution of 1 x 1 km is selected. In the analyzed areas, mountain waves are formed on the leeward side when strong winds, perpendicular to the mountains (usually north or northwest winds), are forced to ascend. Simulated total cloud cover and z-wind component have been analyzed and compared with satellite images. The validation of these mountain wave episodes between the HARMONIE-AROME simulations and satellite observations were satisfactory. The most notable differences were observed in the hours of the mountain wave formation/dissipation.