The Role of Boundary Layer Dynamics on Tropical Cyclone Intensity

In this study, idealized simulations are conducted in the MM5 model to investigate the sensitivity of tropical cyclone (TC) structure, intensification and steady state intensity to the surface drag coefficient (CD), and also to examine the intensification mechanisms. Employing the bulk aerodynamic boundary layer scheme, three experiments are performed with standard CD (1CD, control run), half (0.5CD) and double (2CD) of the standard, respectively. Results of the three experiments indicate that changing the surface drag coefficients leads to a small variation of intensity (within 10%) in terms of the maximum surface wind and a clear change of TC structure. The 2CD experiment shows the smallest radius of maximum tangential wind, strongest azimuthally-averaged inflow and the weakest tangential wind speed. The gradient wind balance analyses demonstrate that, for both intensifying and steady state TCs, the gradient imbalance arises in the boundary layer and the accompanied development of supergradient wind is a salient feature in the inner-core region. The imbalance of tangential wind assessed by the gradient wind balance relationship becomes greater and the supergradient flow is more evident in the experiment with increased CD. It is inferred that the gradient wind imbalance and the development of supergradient wind speed are responses of the free atmosphere to the boundary layer surface drag force.

The tangential wind momentum budget analyses include the contribution from the axisymmetric-mean and eddy terms. The analyses show that the actual location of the tangential wind speed increase should be regions where the remaining contribution of the two dominant terms (friction and axisymmetric-mean radial advection) and therefore the eddy terms are important to tangential wind increase. This reveals that the competition between the gain from advection by inflow and the loss to surface friction is the key for the evolution of TC intensity, rather than the sole impact of radial advection. Particular noteworthy is that friction, on one hand, provides the subgradient force near the surface to enhance inflow and thus the radial advection conducive to TC intensification, while on the other, it dissipates the momentum and inhibits TCs from intensifying. Our analyses highlight the dual roles of friction in the boundary layer in the intensification of TCs.