Dehydration in cirrus clouds in the tropical tropopause layer: Eulerian and Lagrangian perspectives

Numerical simulations of cirrus clouds in the tropical tropopause layer have been carried out in 2D. The degree of dehydration following the occurrence of the clouds in the simulations is computed from the changes in water vapor over the domain of simulation (Eulerian perspective) and as following air parcels (Lagrangian perspective). Interpretations regarding the transport of water vapor into the stratosphere are specific with respect to the domain or to the air parcels. For instance, downward transport of water vapor in the domain may effectively lead to dehydration of the stratosphere in the Eulerian perspective. However, this is not equivalent to dehydration of all air parcels that pass through the clouds.

The commonly used assumption of dehydration to the saturation vapor mixing ratio most likely results in a dry bias. The cloudy air may be supersaturated because microphysical processes cannot instantaneously reduce water vapor to saturation. The dry bias increases markedly in the presence of the cloud radiative heating, in which case both dehydrated and hydrated air parcels are transported (diabatically) upward. The radiatively induced dynamics leads to upward advection of the cloudy air, mixing between the cloudy and clear air, and between dehydrated and hydrated air parcels. Consequently, for both the Eulerian and Lagrangian perspectives, the degree of dehydration deviate considerably from calculations considering microphysical processes alone. These dynamical processes typically cannot be solved explicitly by parcel models which are often used to predict transport of water into the stratosphere.