Automated identification of occluded sectors in midlatitude cyclones: Method and some climatological applications

Abstract

AbstractA novel automated scheme for identifying occluded mid‐latitude cyclones from gridded datasets is described and employed to construct a limited climatology of such storms as well as composites of their thermodynamic and kinematic structures. The climatology (2006–2017) is derived from the MERRA‐2 reanalysis and reveals differences in the distribution of occlusions between the hemispheres. Northern Hemisphere occlusions are most frequent in winter (DJF) and are found poleward of the mean tropopause‐level jets in both the Atlantic and Pacific basins. In the Southern Hemisphere, however, occlusions are most frequent during autumn (MAM) and are almost never found equatorward of . Using the identification scheme, wintertime occlusions are stratified based upon the value of 700 hPa (equivalent potential temperature) in their characteristic thickness ridges. Composites of six groups of occlusions, based upon this distinction, are constructed for each hemisphere. The composites reveal notable differences in the thermodynamic structures among these six groups with more poleward (lower or “colder”) storms exhibiting shallower, less developed thermal structures as compared to their lower‐latitude (higher or “warmer”) counterparts. These differences are attended by contrasts in the intensity of upward vertical motions in the occluded sectors of the various composite storms implying that “warm” storms are associated with greater latent heat release than “colder” storms. It is suggested that these coincident differences between “cold” and “warm” storms provide further evidence of the fundamental importance of latent heat release to the development of occluded thermal structures.