An observational analysis of the genesis of Typhoon Nuri during the Tropical Cyclone Structure 2008 (TCS-08) field campaign in the western North Pacific sector is presented. On the synoptic-scale we use analysis and observational data to show that the surrounding base state flow was an easterly trade wind flow and the precursor disturbance to Typhoon Nuri was an easterly wave. The easterly wave critical layer protected the proto-vortex and allowed it to gestate until it reached a favorable environment, consistent with a recently proposed tropical cyclogenesis sequence. Within this protective Kelvin cat's eye flow, located within the wave's critical layer, existed a "sweet spot", defined as the intersection between the wave trough and critical latitude, which was the preferred location for tropical cyclogenesis.
On the mesoscale, we use data from the first two aircraft missions into the pre-Nuri disturbance to document the transition from a tropical wave to a tropical depression. Dropwindsonde-derived tangential wind profiles at several radii from the low-level circulation center indicate that the magnitude of low-level circulation increases and that the corresponding tangential velocity maximum moves inward from the first to second reconnaissance mission. To compliment these findings, a three-dimensional variational analysis incorporating both dropwindsonde and aircraft Doppler radar data is conducted. These data are used to perform circulation tendency calculations at multiple distances from the low-level circulation center. The results demonstrate a net spin-up of the system-scale circulation in the low-levels near the center and in the outer regions of the recirculating Kelvin cat's eye circulation. The system-scale spin-up is found to be accompanied by areas of low-level vorticity concentration through vortex-tube stretching associated with cumulus convection. Despite initial asymmetries in the spatial distribution of convection, convection becomes more frequent and organizes around the sweet spot location during the genesis sequence.