Revisiting the Boundary Layer Spin Up Mechanism in Tropical Cyclones. Part II: Quantifying Departures from Ekman-like Balance and Searching for Shock-like Structures in Idealized 3D Simulations

In Part I we presented multiple pieces of evidence to strongly refute the purported claim by Heng and Wang (2016) and Heng et al. (2017) that the boundary layer spin up mechanism is generally NOT an important element of simulated or real intensifying tropical cyclones. Here in Part II we further explore this important topic by carefully quantifying the contribution of unbalanced dynamics in the spin up process. We employ the prototype intensification configuration using an idealized quiescent environment as defined by Nguyen et al. (2008) and seek to quantify the important departures from axisymmetric balance theory. Subgrid scale parameters for small-scale turbulent mixing consistent with recent observations are chosen. Axisymmetric balance is defined here either as a strict implementation of the Sawyer-Eliassen gradient balance model throughout the vortex with diagnosed forcing of heat and momentum and eddy effects from the three-dimensional model, or a generalized balance model that relaxes the gradient balance condition in the boundary layer to Ekman-like balance.

We nominally define the hurricane boundary layer as the layer in which the effects of surface friction are associated with significant departures from gradient wind balance. Using this definition, the boundary layer in the intensifying core region of the vortex is found to be strongly nonlinear. At large radii, exterior to the eyewalls, Ekman-like balance as traditionally defined, is found to hold true. Where significant departures from Ekman-like balance are found, the departures are characterized by large departures from gradient wind balance in the boundary layer and large vertical advection of horizontal velocity through the depth of the boundary layer. These effects cannot be described by either of the balance models so defined and the effects are shown to make important contributions to the maximum winds and the primary and secondary circulations. We search also for the existence also of shock-like structures in the azimuthally averaged view of the vortex boundary layer. Implications of our findings will be discussed.

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.

Nguyen SV, Smith RK, Montgomery MT. 2008. Tropical-cyclone intensification and predictability in three dimensions. Q. J. R. Meteorol. Soc. 134: 563–582.