Objective Identification of Occluded Quadrants in Midlatitude Cyclones: Precipitation Processes in Occluded vs. Non-Occluded Cyclones

The leading structural characteristic of occluded mid-latitude cyclones is the thermal ridge that connects the sea-level pressure minimum to the peak of the warm sector. This thermal ridge can be identified in a number of different ways, including as an axis of maximum mid-tropospheric equivalent potential temperature or 1000-500 hPa thickness. As a cyclone progresses through its occluded stage, the length of this thermal ridge increases and it becomes a focus for the cloud and precipitation-producing vertical motions characteristic of the occluded sector. Naud et al. (2023) developed an automated, objective scheme for identifying occluded mid-latitude cyclones from gridded reanalysis data. Utilizing this scheme, we examine differences in the characteristic lengths of occluded thermal ridges in Southern and Northern Hemisphere occlusions. The analysis reveals that Southern Hemisphere occlusions are characterized by shorter thermal ridges. An attempt is made to relate this difference to contrasts in the large-scale environments associated with occluded cyclones in both hemispheres. Finally, cloud and precipitation differences between occluded quadrants in the hemispheres, based upon 11 years of CloudSat/CALIPSO observations, are considered in the context of the quasi-geostrophic dynamics of the occluded sector.