Revisiting the Boundary Layer Spin Up Mechanism in Tropical Cyclones. Part I: Refuting Recent Challenges.

In an azimuthally-averaged view of an intensifying tropical cyclone in three dimensions, the rotating convection paradigm comprises the classical spin up mechanism articulated long ago by Ooyama (1969), a recently articulated boundary layer spin up mechanism and rectified eddy effects associated with the vortical convective plumes and non-axisymmetric vortex waves. It is clear that the classical spin up mechanism contributes positively to enhancing the primary and secondary circulations of an intensifying storm. Recent work has demonstrated that the boundary layer spin up mechanism and the resolved eddy flux effects contribute also to enhancing the primary and secondary circulations of an intensifying storm. Interestingly, the resolved eddy effects during spin up have been shown to exhibit counter-gradient characteristics. Moreover, the boundary layer spin up mechanism has been shown to be essential for explaining why the maximum tangential wind resides in the boundary layer and explaining how the eyewall can be spun up if rising air parcels have an outward-directed velocity component above the boundary layer.

Recent published work by Heng and Wang 2016 and Heng et al. 2017 purport to have shown that the boundary layer spin up mechanism is generally NOT an important dynamical element of simulated or real tropical cyclones. In Part I of this two-part talk, we summarize their purported results and demonstrate the fallacy of their arguments using logic, together with analyses of idealized three-dimensional simulations and observations.

References:

Heng, J. and Y. Wang, 2016: Nonlinear response of a tropical cyclone vortex to prescribed eyewall heating with and without surface friction in TCM4: Implications for tropical cyclone intensification. J. Atmos. Sci., 73, 1315–1333.

Heng, J., Y. Wang, and W. Zhou, 2017: Revisiting the balanced and unbalanced aspects of tropical cyclone intensification. J. Atmos. Sci., 74, 2575–2591.