Objective classification of extratropical cyclogenesis

Abstract

Extratropical cyclones experience vastly different genesis conditions at the first point of their tracks. A novel method is introduced to characterize this variability and classify genesis events by computing 30 diagnostic variables that describe the synoptic‐scale environment of 16 029 genesis events in the Northern Hemisphere extratropics, using ERA‐Interim reanalyses from 2000–2011. These variables are referred to as precursors and include parameters characterizing upper‐level forcing, low‐level baroclinicity, thermodynamic stability, surface fluxes and moist processes. The genesis events spread over a large portion of the 30‐dimensional precursor phase space and no obvious clusters occur, which highlights the high variability of cyclogenesis processes and indicates that they form in a continuum rather than a few distinct categories. A projection of the genesis events to the first two principle components (PC) of the precursor phase space allows reduction of the dimensionality and introduction of a meaningful segmentation of the genesis events in five classes. The first two PCs are characterized by upper‐level forcing (e.g. the amplitude of the upper‐level potential vorticity (PV) anomaly) and low‐tropospheric diabatic processes (e.g. precipitation and diabatically produced low‐level PV), respectively. The first of the five classes identified constitutes the centre of the PC1–PC2 phase space and represents average conditions. Composites reveal that the four classes of events characterized by large positive or negative scores of PC1 and PC2 occur in distinct and strongly differing flow regimes, characterized by the strength of the upper‐level forcing, the structure of the upper‐level jet and the amplitude of low‐level moist processes and baroclinicity. The four classes also have clearly differing geographical distributions. Many well‐known cyclogenesis events fall within classes characterized by strong low‐level moist processes with or without strong upper‐level forcing. Also discussed are the robustness of the method and the linkage to classical concepts of cyclone classifications.