A new scheme for stratiform cloud microphysics has been implemented in the
ECHAM6-HAM2 general circulation model. It features a widely used description
of cloud water with two categories for cloud droplets and raindrops. The
unique aspect of the new scheme is the break with the traditional approach to
describe cloud ice analogously. Here we parameterize cloud ice by a single
category that predicts bulk particle properties (P3). This method has already
been applied in a regional model and most recently also in the Community
Atmosphere Model 5 (CAM5). A single cloud ice category does not rely on
heuristic conversion rates from one category to another. Therefore, it is
conceptually easier and closer to first principles.
This work shows that a single category is a viable approach to describe cloud
ice in climate models. Prognostic representation of sedimentation is achieved
by a nested approach for sub-stepping the cloud microphysics scheme. This
yields good results in terms of accuracy and performance as compared to
simulations with high temporal resolution. Furthermore, the new scheme allows
for a competition between various cloud processes and is thus able to
unbiasedly represent the ice formation pathway from nucleation to growth by
vapor deposition and collisions to sedimentation.
Specific aspects of the P3 method are evaluated. We could not produce a
purely stratiform cloud where rime growth dominates growth by vapor
deposition and conclude that the lack of appropriate conditions renders the
prognostic parameters associated with the rime properties unnecessary.
Limitations inherent in a single category are examined
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