Development of pre-depression Hagupit observed during TCS08

Observations collected during two consecutive aircraft missions into pre-depression Hagupit reveal a developing low-level circulation (LLC) four days prior to the Joint Typhoon Warning Center tropical cyclone formation alert. Model analyses and satellite imagery suggest that the nascent circulation was part of a westward propagating disturbance at 18 N latitude, well-displaced from the ITCZ and any southwesterly monsoonal flow. At 00Z 14 September 2008 the tropical disturbance exhibited a classical 'open-wave' structure at the flight level of the NRL P3 (~650 hPa), and was under the influence of deep-layer northwesterly shear from a nearby upper-level trough. A weak LLC was clearly evident in satellite imagery and dropsonde observations and this has been confirmed by ELDORA radar-derived winds. The two-plane mission on the next day revealed that the LLC was persistent as the disturbance moved away from the upper-level trough, but with a complex vertical shear signature still evident in the convective structures. Hagupit later developed into a major typhoon causing over 1 billion dollars in damage and 67 deaths. ELDORA radar data was collected in both stratiform and deep convective regions enabling one to document the structure and evolution of the pre-depression disturbance. A new finite-element based variational technique called SAMURAI (Spline Analysis at Mesoscale Utilizing Radar and Aircraft Instrumentation) has been developed in order to integrate the numerous aircraft observations into comprehensive mesoscale composites. Analysis and interpretation of these observations and composites will be presented.