On the Hypothesized Outflow Control of Tropical Cyclone Intensification

We present a series of idealized, three-dimensional, convection-permitting numerical experiments to evaluate the revised theory of tropical cyclone intensification proposed by Emanuel (2012). In this revised theory, small-scale turbulence in the upper tropospheric outflow layer is hypothesized to determine the thermal stratification of the outflow and, in turn, an amplification of the system-scale tangential wind field above the boundary layer. The aim is to determine whether parameterized small-scale turbulence in the outflow region of the developing storm is an essential process in the spin up of the maximum tangential winds.

Compared to the control experiment in which the small-scale, shear-stratified turbulence is parameterized in the usual way based on a Richardson number criterion, the vortex in a calculation without a representation of vertical diffusion above the boundary layer evolves in a similar way with no significant difference in upper-level outflow layer temperature or intensification rate with time. Richardson number near-criticality is found mainly in the outflow outside the eyewall; in contrast, the rearrangement of moist entropy is found in the eyewall itself. The identified eddy processes in the eyewall are largely realizations of asymmetric deep convection in the model and are not obviously governed by any Richardson number-based criterion. These three-dimensional calculations do not support the new hypothesis on which the new theory is based. The results appear to have ramifications for recent studies that invoke the new theory.

References:

Emanuel, K. A., 2012: Self-stratification of tropical cyclone outflow. Part II: Implications for storm intensification. J. Atmos. Sci., 69, 988–996.