Prognostic precipitation with three liquid water classes in the ECHAM5-HAM GCM
Abstract. In order to improve the global representation of rain formation in marine stratiform clouds a new parameterization with three prognostic liquid water classes was implemented into the general circulation model ECHAM5 with the aerosol module HAM. The additionally introduced drizzle class improves the physical representation of the droplet spectrum and more importantly, improves the microphysical processes relevant for precipitation formation compared to the standard parameterization. In order to avoid a mismatch of the liquid and ice phase, the prognostic treatment of snow has been introduced too. This has a significant effect on the amount and altitude of ice clouds, which in turn does not only affect in- and outgoing radiation, but also the parameterized collection rates. With the introduction of a prognostic precipitation scheme a more realistic representation of both liquid and ice phase large-scale precipitation is achieved compared to a diagnostic treatment. An encouraging finding is that the sensitivity of the liquid water path to the anthropogenic aerosol forcing with the prognostic treatment is reduced by about 25%. Although the total net radiative forcing is increased from 1.4±0.4 to 1.6±0.4 W m−2 from the control to the prognostic model version, the difference is within the interannual variability. Altogether the results suggest that the treatment of precipitation in global circulation models has a significant influence on the phase and lifetime of clouds, but also hints towards the uncertainties related to a prognostic precipitation scheme.