An Observational Study of Fronts and Frontal Mergers over the Continental United States

Abstract

The Stormscale Operational and Research Meteorology-Fronts Experiment Systems Tests (STORM-FEST) field program was carried out over the central United States to investigate the structure and dynamics of fronts and their attendant precipitation systems adjacent to complex topography and to evaluate new observing networks and analysis techniques for mesoscale research and weather forecasting. This study presents the analysis of data taken between 8 and 10 March 1992, which was the most active period of STORM-FEST. This three-day observing period was characterized by the passage of an extratropical cyclone and related fronts. The synoptic-scale conditions related to the cyclone and frontal evolutions are described. Multiple frontal zones (i.e., Pacific and arctic cold fronts, quasi-stationary polar front, dryline, maritime warm front) and associated regions of precipitation are documented, and their origins and interactions are explored. These features, both well defined and subtle, were observed with unique clarity on the mesoscale by the temporally and spatially enhanced experimental observing networks that included an unprecedented combination of aircraft, rawinsondes, and radar wind profilers. The 8?10 March period was dominated by two synoptic-scale cyclonic waves of Pacific and arctic origin that phased over the STORM-FEST domain. As the Pacific wave moved northeastward over the central United States it became elongated, while beneath it the arctic air spread southward and eastward, yielding a set of three tropopause folds, including one associated with a subtropical jet. Each wave included a cold front (i.e., the Pacific and arctic) at its leading edge. Owing to the relatively large mean density of the cold arctic air mass, the arctic front was clearly seen at the surface as it undercut all other weather systems and its westward movement was blocked by the Rocky Mountains. The downward penetration of the Pacific cold front to the surface was interrupted east of the dryline by the shallow (<150 mb deep), potentially more dense Gulf of Mexico air mass. Sensible heating and adiabatic descent from the elevated Mexican Plateau likely contributed to this density contrast. In effect, the top of the gulf air mass acted as the proxy ground level for the Pacific front, creating a cold front aloft. Detailed observations of the relationship between a prefrontal squall line and the Pacific cold front are provided. A preexisting quasi-stationary polar front situated over the central United States was involved in two frontal mergers, first with an overrunning maritime warm front whose warm-sector air mass was modified by the Gulf of Mexico, and then with the undercutting arctic front. The structural characteristics of these frontal zones became joined into a single ?hybrid? or merged zone following the merger process. Unlike occlusions, for which the fronts slope in generally opposite directions, a frontal merger is characterized by fronts that slope in roughly the same direction. These observations and analyses illustrate the complexity of wintertime frontal systems and interactions that occur regularly over the central United States.