The impact of cirrus clouds on tropical troposphere-to-stratosphere transport

Although it is well known that air enters the stratosphere preferentially through upwelling in the tropics, the exact mechanisms of troposphere-to- stratosphere transport (TST) are still unknown. Previously proposed mechanisms have been found either to be too slow (e.g., clear sky up-welling) to provide agreement with in situ tracer measurements, or to be insufficient in mass flux to act as a major supply for the Brewer-Dobson circulation (e.g., convective overshooting). In this study we evaluate whether the lofting of air via cirrus cloud-radiation interaction might offer an alternative path for TST, which is responsible for a significant fraction of the observed air mass transport. We find that a combination of deep convection and subsequent upwelling associated with cirrus clouds and clear sky can explain the supply of air for the Brewer-Dobson circulation. Thus, up-welling associated with cirrus clouds offers a mechanism for the missing second stage, which links the first stage of TST, deep convection, to the third stage, the Brewer-Dobson circulation.