Observational Evidence of Hurricane Outflow Layer Richardson Number Criticality

While tropical cyclones are powered by enthalpy fluxes from the ocean, they can be strongly influenced by their outflow boundary condition. We previously proposed that this important outflow boundary condition is a specific relationship between angular momentum, moist entropy, and temperature arising from an assumption that the Richardson Number is near its critical value for the onset of Kelvin-Helmholtz instability. Imposition of such a condition yields solutions for the intensity, rate of intensification, and storm structure that are consistent with observations. We also showed that the Richardson Number is indeed near its critical value in the outflow of storms simulated with an axisymmetric, nonhydrostatic model. Here we calculate Richardson Numbers in high-level outflow observed during the course of ONR's Tropical Cyclone Intensity (TCI) experiment. We will present results of these calculations supporting or refuting the hypothesis that Richardson Number criticality is the appropriate outflow boundary condition.