Observations of Low Richardson Number in the Tropical Cyclone Outflow Layer

The tropical cyclone (TC) outflow layer is a primary pathway through which a TC communicates with its surrounding environment. Despite its importance, surprisingly little observational work has been done to document the 3-dimensional structure of TC outflow and how it may affect TC evolution. This study assesses the vertical structure of the TC outflow layer, with a focus on turbulence, using high-resolution radiosonde observations and dropsondes from the NASA Global Hawk aircraft.

A composite vertical and radial structure of the TC outflow layer is constructed from all sondes released within 1000 km of storm center. Layers of bulk Richardson number (RB) less that 0.25 – indicative of turbulence in the layer – are located. Nearby pilot reports are used to link layers with RB less than 0.25 to observed aircraft-scale turbulence, providing confirmation that turbulence is actually occurring in these low RB layers.

Also of interest is the vertical profile of striated cirrus outflow clouds known as “transverse bands.” These cirrus striations seem to be intimately tied to the TC diurnal cycle, appearing as the diurnal convective maximum weakens and spreads radially in the early afternoon. The primary mechanism that causes striated cirrus and low RB near them is also investigated here, with the help of flight-level data from aircraft reconnaissance.