The vorticity budget of developing typhoon Nuri (2008)

Abstract. The formation of west Pacific tropical cyclone Nuri (2008) was observed over four days from easterly wave to typhoon stage by aircraft using scanning Doppler radar and dropsonde data. This disturbance developed rapidly in a significantly sheared environment. In spite of the shear, overlapping closed circulations existed in the frame of reference of the storm in the planetary boundary layer and at 5 km elevation, providing a deep region protected from environmental influences. The rapid spinup of Nuri can be attributed to the strong increase with height at low levels of the vertical mass flux during and after the tropical depression stage, and the correspondingly strong vorticity convergence in the planetary boundary layer. As Nuri developed, convective regions of boundary layer vortex stretching became fewer but more intense, culminating in a single nascent eyewall at the tropical storm stage. A non-developing tropical wave case was also analyzed. This system started with much weaker circulations in the boundary layer and aloft, leaving it unprotected against environmental intrusion. This may explain its failure to develop.