The genesis of Typhoon Nuri as observed during the Tropical Cyclone Structure 2008 (TCS-08) field experiment

An observational and real-time model forecast study of the genesis of Typhoon Nuri during the Tropical Cyclone Structure 2008 (TCS-08) field campaign in the western North Pacific sector is presented. Analysis and observational data show that the surrounding base state flow was an easterly trade wind flow and the precursor disturbance to Typhoon Nuri was an easterly wave that originated in the ITCZ in the Central Pacific and can be tracked more than 10 days prior to tropical storm formation. An overview of the field data is presented here using a newly proposed dynamical framework for tropical cyclone formation within the critical layer of an easterly wave. Despite propagating through a hostile environment ripe with strong vertical wind shear and relatively dry air, the easterly wave critical layer protected the proto-vortex and allowed it to gestate until it reached a more favorable environment. Within this protective "Kelvin's 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. Global Forecast System Final Analyses and IR satellite imagery, which shows convective bands wrapping around the sweet spot as genesis nears, confirm that this sweet spot is the location where Typhoon Nuri's dominant low-level circulation emerges. 

United States Air Force C130 and Naval Research Laboratory P3 research flights on 16 and 17 August collected flight-level, dropwindsonde, and Doppler radar data that allowed an evaluation of the dynamic and thermodynamic processes within the cat's eye. The dropwindsonde analyses identified the precursor easterly wave disturbance on 16 August and identified an area of weak low-level cyclonic circulation on 17 August.

On the sub-synoptic scales, the ELDORA derived wind field on 16 and 17 August confirmed the presence of a low-level cyclonic circulations, likely evidence of the Kelvin's cat's eye circulation. Additionally, ELDORA observations identified scattered precipitation within the Kelvin's cat's eye and a convective tower near the sweet spot on 16 August. During the 17 August flight, the ELDORA observations indicated a greater areal coverage of precipitation when compared to the previous flight, and a larger number of convective towers within the Kelvin's cat's eye circulation as the pre-Nuri disturbance intensified into a tropical depression. These observations along with Guam Doppler radar observations and microwave imagery are used to examine the diabatic activation within the pre-Nuri disturbance.

Finally, "real-time" forecasts were produced during the TCS-08 experiment using operational global prediction model data to support scientific missions. These forecasts were found to be useful in flight planning discussions and predicted Nuri's genesis latitude within 1.5° 72 h in advance.