Airborne radar observations of a warm front during FASTEX

An analysis of an oceanic warm front associated with an extratropical cyclo ne is presented. High-resolution wind syntheses based on data collected by an airborne Doppler radar combined with numerous dropsondes from a high-alt itude aircraft provide an unprecedented view of the frontal structure. Two regions of the front, separated by a horizontal distance of similar to 130 km, were sampled. The warm front near the surface was not identified as a s trong discontinuity in wind speed, temperature, or moisture. The frontal zo ne intersected the ocean surface in the region closest to the central low p ressure (referred to as the western sector) but remained aloft in the regio n farther to the east (referred to as the eastern sector). Only a weak hori zontal temperature gradient and wind shift were noted near the surface in t he eastern sector. A portion of the frontal zone near the surface during the western penetrati on by the aircraft revealed a discontinuous and elongated structure of the positive vertical vorticity (zeta). This pattern of zeta was hypothesized t o be a result of the presence of Kelvin-Helmholtz waves. Vertical cross sec tions through the warm front suggest that it was more clearly defined aloft rather than near the surface. This observation is in contrast with the tra ditional conceptualization of the vertical structure of surface fronts. Pea k values of zeta within the frontal zone were also located aloft.