On the quasi-steady vorticity balance in the mature stage of hurricane Irma (2017)

Abstract. Tropical cyclone (TC) intensification is a process depending on many factors related to the thermodynamical state and environmental influences. It remains a challenge to accurately model TC intensity due to the role of unsteady features like deep convective bursts, boundary layer dynamics and eddy processes. The impermeability theorem for potential vorticity substance, PVS, on isentropic surfaces provides a way to analyze the absolute vorticity structure and tendency in TCs. We will examine this theorem in a numerical simulation of hurricane Irma (2017) near lifetime-peak intensity. Hurricane Irma was a very intense hurricane that persisted as a category five hurricane on the Saffir-Simpson intensity scale for three consecutive days, the longest for any Atlantic hurricane since satellite observations. During this period the intensity of Irma was remarkably constant. According to the impermeability theorem, the radially outward vorticity flux due to divergence above the atmospheric boundary layer must be compensated by an equally strong radially inward vorticity flux due to the effect of diabatic heating in the presence of vertical wind shear. The model results agree with this theorem and we find a strong anticorrelation between the advective and diabatic components of the radial vorticity flux. The impact of parametrized turbulence on the vorticity balance is found to be weak and does not explain the residual flux that would otherwise close the vorticity balance.