Automated Objective Identification of Occluded Sectors in Midlatitude Cyclones: Climatological Applications for the Interrogation of Clouds and Precipitation Distributions

An automated, objective scheme for identifying occluded mid-latitude cyclones from gridded data sets is employed to construct a limited climatology of cloud and precipitation characteristics within occluded sectors. The climatology (2006-2017) applies MERRA-2 reanalysis and CloudSat-CALIPSO observations in a novel compositing technique to reveal differences in the distributions of hydrometeor frequency of occurrence between hemispheres. While hydrometeor towers in both hemispheres tilt poleward with the axis of maximum θ_e characteristic of occluded thermal structures, Northern Hemisphere towers achieve greater vertical growth but with lower hydrometeor frequencies than their Southern Hemisphere counterparts. Hydrometeor frequencies maximize within the lowest layer of the tower associated with the occluded thermal structures, then decrease with height in both hemispheric composites. However, greater frequency characterizes higher elevations in Southern Hemisphere towers and there is a significant drop in hydrometeor occurrences above 5 km in the Northern Hemisphere composite.

The identification scheme accommodates further stratification of occlusions based upon the value of 700 hPa θ_e in their associated thermal ridges. Occlusions with a 700 hPa maximum θ_e in the range of ~294 – 304 K were associated with the highest hydrometeor frequencies of occurrence amongst the global population. The Northern Hemisphere distribution peaks at a 700 hPa value of ~306 K and hydrometeors within Southern Hemisphere towers are most frequent in those occluded structures with a maximum θ_e value of ~295 K at 700 hPa. Each hemisphere’s population is further partitioned into separate terciles, resulting in unique 700 hPa maximum θ_e bins for both. Doing so unveiled a θ_e threshold in the NH that, when surpassed, results in a decline of the hydrometeor maximum within the occluded sector. This phenomenon was not present in the SH composites, despite similar distributions of ω in both hemispheres.