The Violation of the Gradient Wind Balance in the Top of Tropical Cyclones

In order for the gradient wind balance to have physically valid solutions around an anticyclone the relationship between the Coriolis force (fv), centrifugal force (v²/r) and pressure gradient force (∂φ/∂r=fv_g) are constrained. This constraint may be represented by a nondimensional number Ro_g=v_g/fr which cannot be smaller than -1/4. In this work we analyze the pressure maps at the top of 12 WRF-simulated tropical cyclones in different setups and calculate the values of Ro_g. The violation of the gradient wind balance (Ro_g<-1/4) is found in both idealized (f-plane) and real-case (sphere) simulations and it is accompanied by changes in both maximum wind and radius of maximum wind. In the idealized simulations this violation exists only in the first phase of intensification which is accompanied by rapid reduction in the radius of maximum wind, but does not occur in later phases of intensification, which take place with nearly constant radius of maximum wind, or in the steady state. The storm's lifetime-mean violation of this balance correlates with the storm’s lifetime-mean intensity (maximum wind) implying on the possible importance of this process to storm intensification.