On the Violation of Gradient Wind Balance at the top of Tropical Cyclones

The existence of physical solutions for the gradient wind balance is examined at the top of 12 simulated tropical cyclones. The pressure field at the top of these storms, which depends on the vertically integrated effect of the warm core and the near surface low, is found to violate the gradient wind balance-termed here as a state of nonbalance. Using a toy model, it is shown that slight changes in the relative location and relative widths of the warm core drastically increase the isobaric curvature at the upper level pressure maps leading to nonbalance. While idealized storms return to balance within several days, simulations of real-world tropical cyclones retain a considerable degree of nonbalance throughout the model integration. Comparing mean and maximum values of different storms shows that peak nonbalance correlates with either peak intensity or intensification, implying the possible importance of nonbalance at upper levels for the near surface winds. Plain Language Summary The assumption of a simple force balance (gradient wind balance) has served in idealized models for tropical cyclones in the past century. This assumption decouples the dynamics of the tropical cyclone's upper levels (15 km) from its surface winds. By examining the upper level of tropical cyclones in state of the art computer model, is it shown here that in certain scenarios the gradient wind balance has no physical validity at the top of tropical cyclones. This study motivates detailed upper level observations as well as new idealized theories.