ENVIRONMENTAL-CONDITIONS PRIOR TO FORMATION OF A MIDGET TROPICAL CYCLONE DURING TCM-93

During a 10-day period in the Tropical Cyclone Motion (TCM-93) field e xperiment over the tropical western North Pacific, tropical cyclone fo rmation occurred in association with persistent deep convection that w as observed over low-level, north-oriented confluent flow between a la rge monsoon gyre to the west of a strong subtropical ridge. The convec tion was also modulated by a strong diurnal cycle with a convective ma ximum just before dawn and a convective minimum during the late aftern oon. Observations from two aircraft observing periods (AOPs) during tw o consecutive daytime periods identified three distinct mesoscale conv ective vortices (MCVs) in the persistent deep convection. During the i nitial AOP (AOP-1A), a well-defined mesoscale circulation at 500 mb wa s located directly above the strong low level, south-southwesterly con fluent 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-l evel flow to tap ocean surface energy sources. During the second AOP(A OP-1B), which was approximately 24 h after AOP-1A, two MCVs were ident ified by aircraft observations. A northern MCV, which dissipated short ly after the AOP, had a structure similar to the observed MCV in AOP-1 A and was also located directly above the strong low-level north-orien ted flow. A second midtropospheric MCV over the southern portion of th e aircraft operating area extended down to 850 mb and was located in t he cyclonic shear of the low-level flow. Although convection over the large area was decreasing during the diurnal minimum, several convecti ve cells formed and grew in association with local low-level confluenc e between the low-level MCV circulation and the large-scale flow. In c ontrast to AOP-1A, this convection persisted and acquired a rotation a s part of a northward-moving circulation that can be traced to a small low-level mesoscale circulation in satellite visible imagery approxim ately 10 h after the AOP as the same circulation observed over the sou thern region of AOP-1B. Satellite visible imagery documents the explos ive convective development associated with the low level circulation t hat led to the formation of Tropical Storm Ofelia. It is concluded tha t the southern MCV in AOP-1B was able to persist because of its extens ion to low levels, which was linked to its location on the cyclonic sh ear side of the strong low-level flow.