Diagnosis of a high-impact secondary cyclone during HyMeX-SOP1 IOP18

Abstract

Multiple high-impact weather events occurred during HyMeX-SOP1, which was intensively monitored by a largenumber of ordinary and extraordinary observations. The availability of special observations offers an un-precedented opportunity to explore these events in depth and assess the capabilities of current numericalweather prediction tools. In this case, a small-scale secondary cyclone formed within a prominent cyclone thatintensified in the north-western part of the western Mediterranean during IOP18 on October 31, 2012. The smallsecondary system formed near Catalonia, where heavy rain was observed, and then moved to the northern partof the island of Minorca, producing very strong winds. Finally, the secondary cyclone moved northeast whilemerging with the main cyclone and evolving as a cyclonic perturbation towards the Gulf of Genoa, bringingheavy precipitation to some Italian regions.This work aims at providing a detailed diagnosis of the genesis and evolution of the secondary cyclone, usinghigh-resolution numerical tools. Furthermore, with the main objective of identifying the main physical me-chanisms involved in the genesis and evolution of the small-scale secondary cyclone, sensitivity experimentswere performed taking into account three main factors: latent heat release, upper-level dynamical forcing andtopographical effects. Results show that in terms of individual cyclogenetic contributions, the upper level PVanomaly contribution dominated the initial phase and the diabatic heating from condensation contributed to thefurther deepening during the later stages of the secondary cyclone. The initial dynamical effect from the upper-levels forcing was amplified by the local topographic features, becoming a key synergistic factor for the for-mation of the damaging secondary cyclonic system.