The structure and evolution of a wintertime occluded front

Abstract

A study, making use of serial radiosonde soundings, weather radar measurements and aircraft reports, has been made of the structure of a deeply occluded wintertime front. The sounding interval during the frontal passage was exceptionally short. The synoptic-scale front-relative flow could be described using the concepts of cyclone warm and cold conveyor belt flows (WCB and CCB) only in its initial stages. During the final stages of the occlusion process, the WCB became diffuse, the CCB weakened and penetrated below the occluded front, and a third relatively dry, cool, and diffluent airflow of mid-tropospheric origin was established above the occluded front. The subsynoptic structure of the occlusion was characterized by a warm frontal zone and, lying above it, a series of “warm tongues” and “cold surges” with a wavelength of about 100 km. The related circulations were also responsible for the formation of the observed banded structures in the snowfall, and agreed with the observed distributions of wind, humidity and temperature. As a whole, the warm tongues and the cold surges together with their related circulations transferred warm moist air upwards and northwards as required in the occlusion process. The observed wavelength, as well as the thermal and kinematic features of the circulations agreed well with the theory of conditional symmetric instability (CSI), though in this case, only small areas with pure CSI were found. The most significant area with CSI was found behind the occluded surface front at the top of the moist layer, while the other areas of CSI, found at higher elevations both above and on the occluded frontal surface were clearly smaller. This was considered to be related to the time history and the life-cycle of the roll circulations probably produced by the CSI. DOI: 10.1034/j.1600-0870.1990.00012.x