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Montgomery, Michael T.; Kilroy, Gerard; Smith, Roger K.; Crnivec, Nina (2020): Contribution of mean and eddy momentum processes to tropical cyclone intensification. In: Quarterly Journal of the Royal Meteorological Society, Vol. 146, No. 732: pp. 3101-3117
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An idealized, three-dimensional, 1 km horizontal grid spacing numerical simulation of a rapidly intensifying tropical cyclone is used to extend basic knowledge on the role of mean and eddy momentum transfer on the dynamics of the intensification process. Examination of terms in the tangential and radial velocity tendency equations provides an improved quantitative understanding of the dynamics of the spin-up process within the inner-core boundary layer and eyewall regions of the system-scale vortex. Unbalanced and non-axisymmetric processes are prominent features of the rapid spin-up process. In particular, the wind asymmetries, associated in part with the asymmetric deep convection, make a substantive contribution (similar to 30%) to the maximum wind speed inside the radius of this maximum. The analysis provides a novel explanation for inflow jets sandwiching the upper-tropospheric outflow layer which are frequently found in numerical model simulations. In addition, it provides an opportunity to assess the applicability of generalized Ekman balance during rapid vortex spin-up. The maximum tangential wind occurs within and near the top of the frictional inflow layer and as much as 10 km inside the maximum gradient wind. Spin-up in the friction layer is accompanied by supergradient winds that exceed the gradient wind by up to 20%. Overall, the results affirm prior work pointing to significant limitations of a purely axisymmetric balance description, for example, gradient balance/Ekman balance, when applied to a rapidly intensifying tropical cyclone.