Tropical convections role in tropical tropopause cirrus

Thin cirrus layers are frequently observed near the tropical cold-point tropopause. This work shows that tropical convection plays a role in all aspects of the formation of these tropopause cirrus layers. Deep tropical convection is the primary moisture source for tropopause cirrus. It transports moisture from the warm and moist tropical surface into the upper troposphere, where it is detrained into cirrus anvils.;Because tropical convection is capped 2--4 km below the cold-point tropopause in most regions, the in situ formation of tropopause cirrus near the equatorial cold-point tropopause generally requires the transport of moisture into this region. It is shown that an important source of this moisture transport is the meridional circulation that forms in the tropical tropopause transition layer (TTL) in response to eddy momentum flux convergence produced in the tropics by Rossby waves generated by tropical convection. This circulation transports moisture horizontally from the intertropical convergence zone, where it is detrained from convection, to the equator; and vertically from near the base of the TTL to the cold-point tropopause, where tropical tropopause cirrus is observed.;Once moisture is present at the cold-point tropopause, cooling is required to initiate tropopause cirrus formation. Using data from the Nauru99 field experiment, it is shown that stratospheric Kelvin waves excited by tropical convection are an important source of this cooling. In particular, tropopause cirrus forms in the cold phases of Kelvin waves as they descend from the lower stratosphere, across the cold-point tropopause, and into the upper portion of the TTL. The data support the hypothesis that cooling associated with Kelvin waves and the resulting tropopause cirrus play an important role in the dehydration of air ascending into the lower stratosphere.