The impact of gradient wind imbalance on tropical cyclone intensification

The validity of the gradient wind balance approximation for tropical cyclone intensification has not been assessed sufficiently. To address this issue, the sensitivity to the balance approximation is examined in Ooyama's three-layer model by various modifications. The first modification is the suspension of the balance approximation in the free-atmosphere, the second one includes unbalanced dynamics in the boundary layer and the third one imposes a rigid lid. The tropical cyclone intensifies more rapidly with an unbalanced boundary layer than in the cases in which the boundary layer tangential wind is in gradient wind balance. Contraction of the wind maximum and intensification is induced by a convective ring in the experiments with unbalanced boundary layer dynamics. The balanced boundary layer experiments also reveal single convective rings but the contraction it induces is much weaker. In all experiments the rigid lid and the balance approximation for the free-atmosphere have no crucial impact on the intensification phase and a linear stability analysis cannot explain the found sensitivity in intensification. Most likely, momentum advection in the boundary layer plays an important role and reinforces the importance of nonlinear terms. A diagnostic equation relating middle-layer radial wind to various source terms can be derived and it is found that the source term for latent heat provides the largest contribution to intensification. Furthermore, it turns out that the position of the convective ring inside or outside of the radius of maximum gradient wind is of vital importance for intensification and can explain the large impact of gradient wind imbalance in the boundary layer.