Upper-Tropospheric Low Richardson Number in Tropical Cyclones: Sensitivity to Cyclone Intensity and the Diurnal Cycle

High-vertical-resolution rawinsondes were used to document the existence of low–bulk Richardson number (Rb) layers in tropical cyclones. The largest frequency of low Rb existed in the inner 200 km at the 13.5-km level. This peak extended more than 1000 km from the storm center and sloped downward with radius. The presence of an extensive upper-tropospheric low-Rb layer supports the assumption of Richardson number criticality in tropical cyclone outflow by Emanuel and Rotunno. The low-Rb layers were found to be more common in hurricanes than in tropical depressions and tropical storms. This sensitivity to intensity was attributed to a reduction of upper-tropospheric static stability as tropical cyclones intensify. The causes of this destabilization include upper-level cooling that is related to an elevation of the tropopause in hurricanes and greater longwave radiative warming in the well-developed hurricane cirrus canopy. Decreasedmean static stability makes the production of lowRb by gravity waves and other perturbations easier to attain. Themean static stability and vertical wind shear do not exhibit diurnal variability. There is some indication, however, that low Richardson numbers are more common in the early morning than in the early evening, especially near the 200–300-km radius. The location and timing of this diurnal variability is consistent with previous studies that found a diurnal cycle of infrared brightness temperature and rainfall in tropical cyclones.