Cloud ice particle nucleation and atmospheric ice supersaturation in numerical weather prediction models

Cirrus cloud genesis is a multiscale problem. This makes the
parameterization in numerical weather prediction models a
challenging task. In order to improve the prediction of cirrus
clouds and ice supersaturation formation in the German Weather
Service (DWD) model chain, the controlling physical processes are
investigated and parameterised in a new cloud ice microphysics
scheme. Scale dependencies of the ice microphysical scheme were
assessed by conducting simulations with an idealised and realistic
regional Consortium for Small-Scale Modeling (COSMO) model setup
and a global model (GME). The developed two-moment two-mode cloud
ice scheme includes state-of-the-art parameterisations for the two
main ice creating processes, homogeneous and heterogeneous
nucleation. Homogeneous freezing of supercooled liquid aerosols is
triggered in regions with high atmospheric ice supersaturations
(145-160 %) and high cooling rates. Heterogeneous nucleation
depends mostly on the existence of sufficient ice nuclei in the
atmosphere and occurs at lower ice supersaturations. The larger
heterogeneously nucleated ice crystals can deplete ice
supersaturation and inhibit subsequent homogenenous freezing. In
order to avoid an overestimation of heterogeneous nucleation, cloud
ice sedimentation and a prognostic budget variable for activated
ice nuclei are introduced. A consistent treatment of the
depositional growth of the two ice particle modes and the larger
snowflakes using a relaxation timescale method was applied which
ensures a physical representation for depleting ice
supersaturation. Comparisons between the operational and the new
cloud ice microphysics scheme in the GME revealed that the location
of cirrus clouds is dominated by the model dynamics whereas the
cirrus cloud structures strongly differed for the different
schemes. Especially a reduction in the ice water content between 9
and 11 km was observed when using the new cloud ice scheme. This
change is an improvement as demonstrated by a comparison with the
Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations
(CALIPSO) ice water content product. Further comparisons of the GME
with the Integrated Forecast System (IFS) model of the European
Centre for Medium-Range Weather Forecasts (ECMWF) show a clear
improvement of the ice supersaturation distribution with the new
two-moment cloud ice scheme. In-cloud ice supersaturation is
correctly captured, which is compliant with in-situ measurements.
This is a more physical description then in the IFS model, where
in-cloud ice saturation is assumed.