Airborne Radar Observations of a Warm Front

Observational and numerical studies on cold fronts have dominated the literature largely as a result of its well-defined structure, often manifested as abrupt windshifts and temperature discontinuities, and its frequent association with severe convective weather. In comparison, there have been relatively few studies on warm fronts leading to a less complete conceptualization of its structure. The dearth of observational analyses on the warm front is, in part, owing to the weakness of the associated surface discontinuity and general absence of severe convective activity. The warm front can often be difficult to locate on a surface map and is of limited horizontal extent compared to the cold front.

An analysis of an oceanic warm front associated with an extratropical cyclone that developed during the Fronts and Atlantic Storm Track Experiment (FASTEX) is presented. High resolution wind syntheses based on data collected by an airborne Doppler radar (ELDORA) combined with numerous dropsondes from a high altitude aircraft provide an unprecedented view of the frontal structure. Two regions of the front, separated by a horizontal distance of ~130 km, were sampled. The warm front near the surface was not identified as a strong discontinuity in wind speed, temperature, or moisture. The frontal zone intersected the ocean surface in the region closest to the central low pressure (referred to as the western sector) but remained aloft in the region farther to the east (referred to as the eastern sector). Only a weak horizontal temperature gradient and wind shift were noted near the surface in the eastern sector.

Vertical cross sections through the warm front suggest that it was more clearly defined aloft rather than near the surface. Indeed, the horizontal temperature gradient in the direction perpendicular to the frontal boundary was 4 times greater at a height of 4 km than it was at the surface. This observation is in contrast with the traditional conceptualization of the vertical structure of surface fronts. Peak values of vertical vorticity within the frontal zone were also located aloft. The vertical structure of the frontal zone had disparate dimensions in the horizontal (50-60 km) and vertical (1-2 km). Trajectories of numerous air parcels illustrate that there is strong flow of air through the front.

A portion of the frontal zone near the surface during the western penetration by the aircraft revealed a discontinuous and elongated structure of the positive vertical vorticity. This pattern of vertical vorticity was hypothesized to be a result of the presence of Kelvin-Helmholtz waves.