Tropical cyclone outflow layer structure and dynamics using G-IV dropsondes

We are making use of dropsondes from the NOAA G-IV aircraft to examine the structure, thermodynamics, and dynamics of the tropical cyclone outflow layer. We are examining the vertical and radial structure of the central dense overcast (CDO) and the extended outflow beyond it. Preliminary work has determined the frequency and spatial distribution of Richardson number < 0.25, which is sufficiently low to result in turbulence. We have found three primary types of such events: beneath cirrus base and consistent with low stability beneath a layer of sublimation of precipitation; above cirrus base that appears to relate to cloud-top cooling; and at the base of the outflow layer outside the CDO in association with large vertical wind shear. We will examine the relationship between these low Richardson number layers and the presence of inertial and symmetric instability. Also of interest is the sharp transition at the base of the outflow layer, indicated by a rapid shift from weak inflow to strong outflow in the presence of a strong vertical gradient of absolute momentum. Special emphasis will be placed on major hurricanes in which large numbers of dropsondes were released.