A modelling study of the impact of cirrus clouds on the moisture budget of the upper troposphere S. Fueglistaler and M. B. Baker University of Washington, Seattle, USA
Abstract. We present a modelling study of the effect of cirrus
on the moisture budget of the layer wherein the cloud formed.
Our framework simplifies
many aspects of cloud microphysics and collapses the problem
of sedimentation onto
a 0-dimensional box model, but retains essential feedbacks between
saturation mixing ratio, particle growth, and water removal
through particle sedimentation. The water budget is
described by two coupled first-order differential equations for
dimensionless particle number density
and saturation point temperature,
where the parameters defining the system
(layer depth, reference temperature, amplitude and time scale
of temperature perturbation
and inital particle number density, which may or may not
be a function of reference temperature and cooling rate)
are encapsulated in a single coefficient.
This allows us to scale the results to a broad range of
atmospheric conditions, and to test sensitivities.
Results of the moisture budget calculations
are presented for a range of
(T: 238–205 K; p: 325–180 hPa) and
a range of time scales τT of the temperature perturbation that
induces the cloud formation.
The cirrus clouds are found to efficiently
remove water for
τT longer than a few hours,
with longer perturbations (τT≳10 h)
required at lower temperatures (T≲210 K).
Conversely, we find that temperature
perturbations of duration order 1 h and less (a typical timescale
for e.g., gravity waves) do not
efficiently dehydrate over most of the upper troposphere.
A consequence is that (for particle densities typical
of current cirrus clouds) the assumption of complete
dehydration to the saturation mixing ratio may yield
valid predictions for upper tropospheric moisture
distributions if it is based on the large scale
temperature field, but this assumption is not necessarily
valid if it is based on smaller scale temperature fields.
Citation: Fueglistaler, S. and Baker, M. B.: A modelling study of the impact of cirrus clouds on the moisture budget of the upper troposphere, Atmos. Chem. Phys., 6, 1425-1434, doi:10.5194/acp-6-1425-2006, 2006.