Airborne Radar Observations of a Warm Front during FASTEX

Abstract

An analysis of an oceanic warm front associated with an extratropical cyclone is presented. High-resolution wind syntheses based on data collected by an airborne Doppler radar 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. 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 ? was hypothesized to be a result of the presence of Kelvin?Helmholtz waves. Vertical cross sections through the warm front suggest that it was more clearly defined aloft rather than near the surface. This observation is in contrast with the traditional conceptualization of the vertical structure of surface fronts. Peak values of ? within the frontal zone were also located aloft.