Abstract
Abstract. Aircraft reconnaissance data collected during the Tropical Cyclone Structure 2008 field campaign are used to examine further kinematical, dynamical, and thermodynamical aspects of the genesis of Typhoon Nuri. Data from the first two missions into the pre-Nuri disturbance 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. In these regions, the spin-up tendency due to the influx of cyclonic absolute vorticity exceeds the frictional spin-down tendency for both Nuri missions. 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. The areal coverage and intensity of these high-vorticity regions increase between the first and second Nuri missions. The findings of this study are consistent in some respects to the Nuri observational analysis carried out by Raymond and López-Carrillo (2011), but differ in their suggested key results and related scientific implications that the pre-Nuri disturbance was spinning down in the planetary boundary layer on the first day of observations. The findings herein strongly support a recent tropical cyclogenesis model positing that the Kelvin cat's eye circulation of the parent wave-like disturbance provides a favorable environment for convective vorticity organization and low-level spin-up on the mesoscale.
Highlights
Advances in the understanding and prediction of some aspects of mature tropical cyclones have improved over recent years (AMS Statement, 2007), there remain significant challenges to fully understand the problem of tropical cyclogenesis
Our analysis suggests that the proposed thermodynamic control presented by Raymond may not be a necessary condition for low-level spin-up in the case of Typhoon Nuri
In this paper we have continued our study of the genesis of Typhoon Nuri (2008)
Summary
Advances in the understanding and prediction of some aspects of mature tropical cyclones have improved over recent years (AMS Statement, 2007), there remain significant challenges to fully understand the problem of tropical cyclogenesis. Two significant hurdles that must be overcome are (i) gaining a complete understanding of the complex multi-scale interactions that occur during the transition of a synoptic-scale precursor to a mesoscale vortex and (ii) addressing the lack of in situ observations in areas where seedling disturbances for tropical cyclones originate and intensify. In their observational study, Dunkerton et al (2009, hereafter DMW09) developed a new tropical cyclogenesis model that outlines the dynamics and thermodynamics of the genesis sequence on the synoptic, meso-, and convective scales. Lussier III et al.: The genesis of Typhoon Nuri as observed during the TCS-08 field experiment
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