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  1. Abstract

    The formation of a plausible secondary eyewall is examined with two principal simulation experiments that differ only in the fixed value of rain fall speed, one with a value of 70 m s−1(approaching the pseudo-adiabatic limit) that simulates a secondary eyewall, and one with a value of 7 m s−1that does not simulate a secondary eyewall. Key differences are sought between these idealized three-dimensional simulations. A notable expansion of the lower-tropospheric tangential wind field to approximately 400-km radius is found associated with the precursor period of the secondary eyewall. The wind field expansion is traced to an enhanced vertical mass flux across the 5.25-km height level, which leads, in turn, to enhanced radial inflow in the lower troposphere and above the boundary layer. The inflow spins up the tangential wind outside the primary eyewall via the conventional spinup mechanism. This amplified tangential wind field is linked to a broad region of outwardly directed agradient force in the upper boundary layer. Whereas scattered convection is found outside the primary eyewall in both simulations, the agradient force is shown to promote a ring-like organization of this convection when boundary layer convergence occurs in a persistent, localized region of supergradient winds. The results support prior work highlighting a new model of secondary eyewall formation emphasizing a boundary layer control pathway for initiating the outer eyewall as part of the rotating convection paradigm of tropical cyclone evolution.

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  2. We present idealized, three‐dimensional, convection‐permitting numerical experiments to evaluate the premise of the revised theory of tropical cyclone intensification proposed by Emanuel, . The premise is that small‐scale turbulence in the upper tropospheric outflow layer determines the thermal stratification of the outflow and, in turn, an amplification of the system‐scale tangential wind field above the boundary layer. The aim of our article is to test whether parametrized small‐scale turbulence in the outflow region of a developing storm is an essential process in the spin‐up of the maximum tangential winds.

    Compared with the control experiment, in which the small‐scale, shear‐stratified turbulence is parametrized in the usual way based on a Richardson number criterion, the vortex in a calculation without a parametrized representation of vertical mixing above the boundary layer has similar evolution of intensity. Richardson number near‐criticality is found mainly in the upper‐level outflow. However, the present solutions indicate that eddy processes in the eyewall play a significant role in determining the structure of moist entropy surfaces in the upper‐level outflow. In the three‐dimensional model, these eddy processes are largely realizations of asymmetric deep convection and are not obviously governed by any Richardson‐number‐based criterion. The experiments do not support the premise on which the new theory is based. The results would appear to have ramifications for recent studies that invoke the new theory.

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