Maintenance of tropical tropopause layer cirrus

A two-dimensional cloud resolving model with explicit bin microphysics is used to study the maintenance of tropical tropopause layer (TTL) cirrus. Numerical simulations using this model show that a TTL cirrus with a maximum radiative heating rate of 3 K/day is able to self-maintain for as long as 2 days if it contains ice crystals whose initial mean radius is smaller than about 5 mm. The key to the maintenance of the cloud is the circulation thermally forced by the cloud radiative heating. When the cloud layer is at ice saturation and temperature decreases with height, advection of water vapor by the thermally forced circulation results in water vapor flux convergence in the cloud. This leads to growth of ice crystals despite the diabatic warming produced by the radiative heating. The source of water vapor for the growth of ice crystals is outside the cloud lateral edge, which is outside the vertical column that contains the initial cloud. The conversion of water vapor into ice in the simulated TTL cirrus indicates its potential to dehydrate the surrounding environment. This dehydration mechanism does not involve adiabatic cooling associated with external large-scale uplift.