Environmental Conditions Prior to Formation of a Midget Tropical Cyclone during TCM-93

Abstract

During a 10-day period in the Tropical Cyclone Motion (TCM-93) field experiment over the tropical western North Pacific, tropical cyclone formation occurred in association with persistent deep convection that was observed over low-level, north-oriented confluent flow between a large monsoon gyre to the west of a strong subtropical ridge. The convection was also modulated by a strong diurnal cycle with a convective maximum just before dawn and a convective minimum during the late afternoon. Observations from two aircraft observing periods (A0Ps) during two consecutive daytime periods identified three distinct mesoscale convective vortices (MCVS) in the persistent deep convection. During the initial AOP (AOP-1A), a well-defined mesoscale circulation at 500 mb was located directly above the strong low-level, south-southwesterly confluent flow. However, reduction in convection and associated midlevel forcing during the convective minimum period contributed to the decay of the MCV before it could penetrate downward through the strong low-level flow to tap ocean surface energy sources. During the second AOP (AOP-1B), which was approximately 24 h after AOP-1A, two MCVs were identified by aircraft observations. A northern MCV, which dissipated shortly after the AOP, had a structure similar to the observed MCV in AOP-1A and was also located directly above the strong low-level north-oriented flow. A second midtropospheric MCV over the southern portion of the aircraft operating area extended down to 850 mb and was located in the cyclonic shear of the low-level flow. Although convection over the large area was decreasing during the diurnal minimum, several convective cells formed and grew in association with local low-level confluence between the low-level MCV circulation and the large-scale flow. In contrast to AOP-1A, this convection persisted and acquired a rotation as part of a northward-moving circulation that can he traced to a small low-level mesoscale circulation in satellite visible imagery approximately 10 h after the AOP as the same circulation observed over the southern region of AOP-1B. Satellite visible imagery documents the explosive convective development associated with the low-level circulation that led to the formation of Tropical Storm Ofelia. It is concluded that the southern MCV in AOP-1B was able to persist because of its extension to low levels, which was linked to its location on the cyclonic shear side of the strong low-level flow.