Introduction of prognostic rain in ECHAM5: design and single column model simulations R. Posselt and U. Lohmann Institute for Atmospheric and Climate Science, ETH Zurich, Universitaetsstrasse 16, 8092 Zurich, Switzerland
Abstract. Prognostic equations for the rain mass mixing ratio and the rain
drop number concentration are introduced into the large-scale cloud
microphysics parameterization of the ECHAM5 general circulation
model (ECHAM5-PROG). To this end, a rain flux from one level to the
next with the appropriate fall speed is introduced. This maintains
rain water in the atmosphere to be available for the next time step.
Rain formation in ECHAM5-PROG is, therefore, less dependent on the
autoconversion rate than the standard ECHAM5 but shifts the emphasis
towards the accretion rates in accordance with observations.
ECHAM5-PROG is tested and evaluated with Single Column Model (SCM)
simulations for two cases: the marine stratocumulus study EPIC
(October 2001) and the continental mid-latitude ARM Cloud IOP
(shallow frontal cloud case – March 2000). In case of heavy
precipitation events, the prognostic equations for rain hardly
affect the amount and timing of precipitation at the surface in
different SCM simulations because heavy rain depends mainly on the
large-scale forcing. In case of thin, drizzling clouds (i.e.,
stratocumulus), surface precipitation is sensitive to the number of
sub-time steps used in the prognostic rain scheme. Cloud
microphysical quantities, such as cloud liquid and rain water within
the atmosphere, are sensitive to the number of sub-time steps in
both considered cases. This results from the decreasing
autoconversion rate and increasing accretion rate.
Citation: Posselt, R. and Lohmann, U.: Introduction of prognostic rain in ECHAM5: design and single column model simulations, Atmos. Chem. Phys., 8, 2949-2963, doi:10.5194/acp-8-2949-2008, 2008.