Reconsideration of the Mass and Condensate Sources for the Tropical Cyclone Outflow

The widely accepted view of the secondary circulation of a mature tropical cyclone (TC) consists of boundary layer inflow that turns upward through the eyewall and then turns outward to form the outflow layer and the cirrus shield. This view can be traced to schematics shown in several seminal studies of TCs, and it persists in both the peer-reviewed and popular literature in numerous diagrams and cartoons. Updrafts in rainbands are nearly always depicted as not reaching the primary outflow. However, examination of the mass and moisture budgets of the cirrus outflow shield - i.e., the outflow layer from about 100 to 300 km radius - in mesoscale model simulations of hurricanes reveals a different picture. A significant fraction of the dry air mass flux (varying widely but around 50%) and even larger fraction of the condensate in the outflow comes from rainbands. The mass flux from the eyewall is limited by its small size, and condensate is falling out rapidly. Instead, the condensate shield and outflow mass flux are significantly supplied by deep convection in the surrounding rainbands. This result is also consistent with our recently found appreciation of the diurnally forced rainband complexes that have been shown to expand the cirrus shield. The modeling results are supported by airborne Doppler radar observations that show convective cells in rainbands reaching and even overshooting above the outflow layer. These findings may change our perceptions of the physical significance of changes in size and thickness of the cirrus shield.