Effects of pre-existing ice crystals on cirrus clouds and comparison between different ice nucleation parameterizations with the Community Atmosphere Model (CAM5)

摘要

In order to improve the treatment of ice nucleation in a more realisticmanner in the Community Atmosphere Model version 5.3 (CAM5.3), the effectsof pre-existing ice crystals on ice nucleation in cirrus clouds areconsidered. In addition, by considering the in-cloud variability in icesaturation ratio, homogeneous nucleation takes place spatially only in aportion of the cirrus cloud rather than in the whole area of the cirrus cloud.Compared to observations, the ice number concentrations and the probabilitydistributions of ice number concentration are both improved with the updatedtreatment. The pre-existing ice crystals significantly reduce ice numberconcentrations in cirrus clouds, especially at mid- to high latitudes in theupper troposphere (by a factor of ~10). Furthermore, thecontribution of heterogeneous ice nucleation to cirrus ice crystal numberincreases considerably.Besides the default ice nucleation parameterization of Liu and Penner (2005,hereafter LP) in CAM5.3, two other ice nucleation parameterizations ofBarahona and Nenes (2009, hereafter BN) and Kärcher et al. (2006,hereafter KL) are implemented in CAM5.3 for the comparison. In-cloud icecrystal number concentration, percentage contribution from heterogeneous icenucleation to total ice crystal number, and pre-existing ice effectssimulated by the three ice nucleation parameterizations have similarpatterns in the simulations with present-day aerosol emissions. However, thechange (present-day minus pre-industrial times) in global annual mean columnice number concentration from the KL parameterization(3.24 × 10 m) is less than that from the LP (8.46 × 10 m)and BN (5.62 × 10 m) parameterizations. As aresult, the experiment using the KL parameterization predicts a much smalleranthropogenic aerosol long-wave indirect forcing (0.24 W m) than thatusing the LP (0.46 W m) and BN (0.39 W m) parameterizations.