Research Aircraft Observations of the Mesoscale and Microscale Structure of a Cold Front over the Eastern Pacific Ocean

Abstract

This study describes the structure of an oceanic cold front from research aircraft observations taken during the OCEAN STORMS field experiment. Synoptic and mesoscale analyses compare the structure of an upper-level jet-front system observed slightly downstream from the wind speed maximum (exit region) to its structure in the upstream entrance region. Stratospheric potential vorticity and ozone were found within the frontal zone down to ?800 mb. Microscale analyses of the front near the sea surface were carried out for a portion of the front having the signature of a ?rope? cloud (shallow cumulus line) in satellite imagery. A narrow (<1 km) zone of upward motion (?4 m s?1) and of horizontal shear (?10?2 s?1) characterized the front near the surface. Significant alongfront variability was found, including lateral displacements in the frontal zone where there were weaker updrafts; similar structures have been previously observed along precipitating fronts using Doppler radar. Pressure perturbations measured directly by the aircraft resemble pressure perturbations for a deeper, precipitating front that were derived indirectly from Doppler radar data. Results show that the frontal zone was well defined through the depth of the troposphere, tapering from a width of ?100 km in the upper troposphere to ?1 km at the surface. The collapse of the front to such a narrow scale within the boundary layer occurred in the absence of precipitation, and obviously, in the absence of topographical influences.