A Red Herring? In Search of the Cold Occluded Cyclone

Questions arise from time to time regarding the relative frequency of cold-front-type (CFT) versus warm-front-type (WFT) occluded fronts, in particular: “Which occurs most often?” In order to satisfy this question, North American weather maps were examined for the eight more active months (September – April) of the year over three seasons (2001-2002, 2002-2003, 2003-2004). Those occluded frontal zones analyzed at 1200 UTC on the Daily Weather Map series of maps were the focus of this investigation, provided that adequate data were available both upstream and downstream of the front's location. Surface potential temperatures were employed for the pertinent stations (1° latitude downstream versus 1° latitude upstream) in order to eliminate adiabatic differences due to elevation. First, occluded fronts were categorized by temperature change only. Over the three years studied, significant cold-front-type (25) occluded fronts occurred with about the same frequency as warm-front-type (23) systems. Neutral fronts of significant length (with a downstream-upstream temperature difference of less than 3K) were few (2) and were outnumbered by the other categories. Additionally, no category seemed to exhibit stronger downstream-upstream temperature changes than another. Next, the occluded fronts were typed according to the static stability and slope of the occluded frontal zone. Of the 45 fronts for which adequate data were available, more than half (24) were warm-front-type occluded fronts, while 20 were neutral (having no discernable slope), and only one was a cold-front-type occlusion. Lastly, of the occluded frontal zones examined for static stability and slope, 32 had active precipitation areas close by; 15 of these had warm-front-type precipitation (along and ahead of the surface occluded front location), while only two featured cold-front-type precipitation (along and behind the surface front), and the remaining 15 had precipitation on either side of the surface occluded front.

From this analysis, we identified one cyclone that appeared to meet both the thermal change criterion as well as the stability gradient criterion to be classified a cold occluded cyclone. Even so, this cyclone occurred just east of the Rocky Mountains, so there may have been significant terrain influences. Our case study of that cyclone is currently underway.