Airborne radar observations of a cold front during FASTEX

A detailed analysis using airborne Doppler radars of an oceanic cold front associated with precipitation core and gap regions is presented. The precip itation cores appear to form as a result of the combined effects of horizon tal shearing instability and the advection of hydrometeors by the core-rela tive winds. In contrast to previous schematic models, it is shown that a st rong surface discontinuity does not exist along the entire length of the pr ecipitation core. The southern section of the core can be accompanied by an abrupt discontinuity and lighter precipitation while the northern section can be associated with more slowly changing variables but heavier precipita tion. The peak updrafts at the leading edge of the front appeared to be pri marily driven by frictional convergence and the acceleration of the vertica l vorticity in the boundary layer. The overall motion of the cold front was not well predicted using density current theory even though the kinematic structure of the front resembled classical studies of these types of flows. Local regions of the cold front in the vicinity of the precipitation cores , however, did appear to propagate as a density current in a direction perp endicular to the major axis of the cores. A large gap region (>10 km) withi n a narrow cold frontal rainband is examined. While small gaps are generate d by shearing instability, the large gap is created by differential movemen t of two segments of the front.