Reexamining the cold conveyor belt

Despite the popularity of the conveyor-belt model for portraying the airflo w through midlatitude cyclones, questions arise as to the path of the cold conveyor belt, the lower-tropospheric airflow poleward of and underneath th e warm front. Some studies, beginning with Carlson's analysis of the easter n U.S. cyclone of 5 December 1977, depict the cold conveyor belt moving wes tward, reaching the northwest quadrant of the storm, turning abruptly antic yclonically, rising to jet level, and departing the cyclone downstream (her eafter, the anticyclonic path). Other studies depict the cold conveyor belt reaching the northwest quadrant, turning cyclonically around the low cente r, and remaining in the lower troposphere (the cyclonic path). To clarify t he path of the cold conveyor belt, the present study reexamines Carlson's a nalysis of the cold conveyor belt using an observational and mesoscale nume rical modeling study of the 5 December 1977 cyclone. This reexamination raises several previously unappreciated and underappreci ated issues. First, airflow in the vicinity of the warm front is shown to b e composed of three different airstreams: air-parcel trajectories belonging to the ascending warm conveyor belt, air-parcel trajectories belonging to the cyclonic path of the cold conveyor belt that originate from the lower t roposphere, and air-parcel trajectories belonging to the anticyclonic path of the cold conveyor belt that originate within the midtroposphere. Thus, t he 5 December 1977 storm consists of a cold conveyor belt with both cycloni c and anticyclonic paths. Second, the anticyclonic path represents a transi tion between the warm conveyor belt and the cyclonic path of the cold conve yor belt, which widens with height. Third, the anticyclonic path of the col d conveyor belt is related to the depth of the closed circulation associate d with the cyclone, which increases as the cyclone deepens and evolves. Whe n the closed circulation is strong and deep, the anticyclonic path of the c old conveyor belt is not apparent and the cyclonic path of the cold conveyo r belt dominates. Fourth, Carlson's analysis of the anticyclonic path of th e cold conveyor belt was fortuitous because his selection of isentropic sur face occurred within the transition zone, whereas, if a slightly colder ise ntropic surface were selected, the much broader lower-tropospheric cyclonic path would have been evident in his analysis instead. Finally, whereas Car lson concludes that the clouds and precipitation in the cloud head were ass ociated with the anticyclonic path of the cold conveyor belt, results from the model simulation suggest that the clouds and precipitation originated w ithin the ascending warm conveyor belt. As a consequence of the reexamination of the 5 December 1977 storm using ai r-parcel trajectories, this paper clarifies the structure of and terminolog y associated with the cold conveyor belt. It is speculated that cyclones wi th well-defined warm fronts will have a sharp demarcation between the cyclo nic and anticyclonic paths of the cold conveyor belt. In contrast, cyclones with weaker warm fronts will have a broad transition zone between the two paths. Finally, the implications of this research for forecasting warm-fron tal precipitation amount and type are discussed.