Observations of a non-developing tropical disturbance in the western North Pacific during TCS-08

During the Tropical Cyclone Structure-2008 (TCS-08) field program in the western North Pacific, a tropical system (TCS025) was closely monitored for potential development from 24 August until 3 September 2008. Three reconnaissance flights were conducted with the USAF WC-130J, and two flights were conducted with the Navy Research Lab (NRL) P3. Both aircraft deployed GPS dropwindsondes during each flight, and the NRL-P3 was able to observe the three-dimensional cloud and precipitation structure of TCS025 with the tail-mounted dual-Doppler ELDORA radar. The low-level portion of TCS025 existed as one closed circulation that defined a broad trough oriented northwest to southeast. The upper-level flow was dominated by two cells within the tropical upper-tropospheric trough (TUTT) that were located to the north of TCS025. Northerly vertical wind shear of moderate strength persisted over the low-level circulation. During the evolution of TCS025, multiple mesoscale convective systems (MCSs) developed and moved southward (down-shear). While deep convection generally began near the time of maximum diurnal forcing, each convective episode failed to persist through diurnal convective minima. A set of 3DVAR analyses that incorporates flight level, dropwindsonde and ELDORA radar data are used to examine the dynamical and thermodynamical structure of TCS025 over a 48-h period. Results indicate a complex environment of a broad closed low-level circulation and mid-level vortices that are related to individual MCSs. Based on several theories of tropical cyclone formation, the overall environment appears to be favorable for development into a tropical cyclone. However, it is hypothesized that the failure of low- and mid-level circulations to become favorably aligned prevents the development of TCS025 into a tropical storm. Several mechanisms related to the vortex misalignment in the vertical are examined and the sensitivity to the large-scale conditions is examined and discussed.