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Airborne Radar Observations of a Cold Front during FASTEX

Roger M. WakimotoDepartment of Atmospheric Sciences, University of California, Los Angeles, Los Angeles, California

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Brian L. BosartDepartment of Atmospheric Sciences, University of California, Los Angeles, Los Angeles, California

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Abstract

A detailed analysis using airborne Doppler radars of an oceanic cold front associated with precipitation core and gap regions is presented. The precipitation cores appear to form as a result of the combined effects of horizontal shearing instability and the advection of hydrometeors by the core-relative winds. In contrast to previous schematic models, it is shown that a strong surface discontinuity does not exist along the entire length of the precipitation 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 precipitation. The peak updrafts at the leading edge of the front appeared to be primarily driven by frictional convergence and the acceleration of the vertical 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 perpendicular to the major axis of the cores. A large gap region (>10 km) within a narrow cold frontal rainband is examined. While small gaps are generated by shearing instability, the large gap is created by differential movement of two segments of the front.

Corresponding author address: Prof. Roger M. Wakimoto, Dept. of Atmospheric Sciences, University of California, Los Angeles, 405 Hilgard Ave., Los Angeles, CA 90095-1565.

Email: roger@atmos.ucla.edu

Abstract

A detailed analysis using airborne Doppler radars of an oceanic cold front associated with precipitation core and gap regions is presented. The precipitation cores appear to form as a result of the combined effects of horizontal shearing instability and the advection of hydrometeors by the core-relative winds. In contrast to previous schematic models, it is shown that a strong surface discontinuity does not exist along the entire length of the precipitation 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 precipitation. The peak updrafts at the leading edge of the front appeared to be primarily driven by frictional convergence and the acceleration of the vertical 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 perpendicular to the major axis of the cores. A large gap region (>10 km) within a narrow cold frontal rainband is examined. While small gaps are generated by shearing instability, the large gap is created by differential movement of two segments of the front.

Corresponding author address: Prof. Roger M. Wakimoto, Dept. of Atmospheric Sciences, University of California, Los Angeles, 405 Hilgard Ave., Los Angeles, CA 90095-1565.

Email: roger@atmos.ucla.edu

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