STRUCTURE OF A MESOSCALE CONVECTIVE SYSTEM EMBEDDED IN TYPHOON ROBYN DURING TCM-93

A mesoscale convective system (MCS) embedded in the circulation of Typ hoon (TY) Robyn was investigated by two aircraft missions during the t ropical cyclone motion (TCM 93) mini field experiment. The MCS initial ly formed north of the typhoon center, bur 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 sl ow drift to the north. The structure of the MCS is investigated to exa mine whether the changes in motion of TY Robyn could be related to a m idtropospheric 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, wit h a shallow surface layer of divergent Row from a weak cold pool. Thes e features are below the MCS stratiform region, which is also forced t o be south of the convective region by the circulation along the weste rn 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 re gion, which was approximately 5 degrees longitude west of the center o f TY Robyn. During the decay stage of the MCS, the strengthening circu lation of TY Robyn results in strong midlevel wind shear that prevents the maintenance of an upright potential vorticity center in the MCS s tratiform region. The track change of Robyn during the period is asses sed relative to a potential Fujiwhara-type interaction with the MCS ve rsus a change in the large-scale steering. Although the TY Robyn circu lation 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 b e responsible for the observed track changes. It is concluded that the changes in speed and direction of the typhoon are caused by a large-s cale circulation pattern that results in a combination of weak net env ironmental flow that is oriented to the north because of Robyn's locat ion at the eastern edge of the western North Pacific monsoon trough.