Structure of a Mesoscale Convective System Embedded in Typhoon Robyn during TCM-93

Abstract

A mesoscale convective system (MCS) embedded in the circulation of Typhoon (TY) Robyn was investigated by two aircraft missions during the tropical cyclone motion (TCM-93) mini field experiment. The MCS initially formed north of the typhoon center, but was rapidly advected to the west by the strong midlevel easterlies located between Robyn and the subtropical ridge to the north. Coincident with the occurrence of the MCS, the motion of the typhoon changed from west-northwestward to a slow drift to the north. The structure of the MCS is investigated to examine whether the changes in motion of TY Robyn could be related to a midtropospheric vortex circulation in the MCS. During the mature stage, the MCS has a convective region and an extensive stratiform region. A vigorous updraft in the convective region is tilted southward by the primary circulation around TY Robyn. Below the tilted updraft, descent in a mesoscale downdraft contributes to drying in the low levels, with a shallow surface layer of divergent flow from a weak cold pool. These features are below the MCS stratiform region, which is also forced to be south of the convective region by the circulation along the western side of TY Robyn. A potential vorticity maximum near 500 mb extends downward to 800 mb at the very southern edge of the MCS stratiform region, which was approximately 5° longitude west of the center of TY Robyn. During the decay stage of the MCS, the strengthening circulation of TY Robyn results in strong midlevel wind shear that prevents the maintenance of an upright potential vorticity center in the MCS stratiform region. The track change of Robyn during the period is assessed relative to a potential Fujiwhara-type interaction with the MCS versus a change in the large-scale steering. Although the TY Robyn circulation clearly had an effect on the MCS, the MCS circulation is judged to be too weak, too shallow, and separated too far from TY Robyn to be responsible for the observed track changes. It is concluded that the changes in speed and direction of the typhoon arc caused by a large-scale circulation pattern that results in a combination of weak net environmental flow that is oriented to the north because of Robyn's location at the eastern edge of the western North Pacific monsoon trough.