Formative Stage of a Long-Lived Mesoscale Vortex Observed by Airborne Doppler Radar

Abstract

The formative stage of a long-lived mesoscale cyclonic vortex was captured by the NOAA P-3 aircraft as it investigated a developing mesoscale convective system (MCS) near the southeastern coast of Taiwan on 16 June 1987 during the Taiwan Area Mesoscale Experiment. The supporting environment of the mesovortex was characterized by an exceptionally moist atmosphere and moderate ambient vertical shear through a deep layer from the near surface to ?6 km, with much weaker shear and winds aloft. In addition, a pronounced low-level mesoscale shear/convergence zone, which resulted from the interaction of southeasterly flow with northeasterly flow confined to the near-coast region, existed in the vicinity of the observed mesovortex. Composite three-dimensional wind fields derived via pseudo-dual-Doppler synthesis show the vortex had a horizontal diameter expanding from ?40 km to ?70 km in the lower to midtroposphere, respectively, and exhibited considerable tilt through this layer. Contrary to previously documented mesovortices, which have generally been fully developed and observed in the stratiform region of mature-to-decaying MCSs, the present vortex was intimately coupled to convective precipitation within this developing MCS. This study provides unique observational evidence that under appropriate environmental conditions a long-lasting mesovortex may originate in the convective region of an MCS.