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

Abstract 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 foundational studies of TCs and persists in both the peer-reviewed and popular literature in numerous diagrams and cartoons. Updrafts in rainbands are nearly always depicted as not supplying 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 airmass 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. These findings are consistent with the recently developed appreciation of the diurnally forced rainband complexes that have been shown to expand the cirrus shield. The simulations show that moist air and condensate can be lifted into the outflow in either narrow convective towers or in mesoscale ascending updrafts, and these features can be found in airborne Doppler radar observations. These findings update our understanding of the physical significance of changes in size and thickness of the cirrus shield. Significance Statement Tropical cyclones are recognized from satellite images of their high clouds that spiral outward from the storm center. The size and evolution of this outflow are used by experts and algorithms to estimate the intensity and future behavior of these storms. Conventional wisdom holds that the overwhelming source of these high-altitude clouds is the upward transport of moisture in thunderstorms around the calm center. Computer simulations of tropical cyclones and radar observations taken by aircraft show that in fact most of these clouds come from thunderstorms in the surrounding rainbands. These findings highlight the importance of the rainband convection in controlling the size and thickness of the outflow clouds, which in turn inform our estimates of storm intensity.