Abstract
Abstract. The salient features of mixed-phase and ice clouds in a GCM cloud scheme are examined using the ice nucleation parameterizations of Liu and Penner (LP) and Barahona and Nenes (BN). The performance of both parameterizations was assessed in the GEOS-5 AGCM using the McRAS-AC cloud microphysics framework in single column mode. Four dimensional assimilated data from the intensive observation period of ARM TWP-ICE campaign was used to drive the fluxes and lateral forcing. Simulation experiments were established to test the impact of each parameterization in the resulting cloud fields. Three commonly used IN spectra were utilized in the BN parameterization to describe the availability of IN for heterogeneous ice nucleation. The results showed large similarities in the cirrus cloud regime between all the schemes tested, in which ice crystal concentrations were within a factor of 10 regardless of the parameterization used. In mixed-phase clouds there were some persistent differences in cloud particle number concentration and size, as well as in cloud fraction, ice water mixing ratio, and ice water path. Contact freezing in the simulated mixed-phase clouds contributed to the effective transfer of liquid to ice, so that on average, the clouds were fully glaciated at T 260 K, irrespective of the ice nucleation parameterization used. Comparison of simulated ice water path to available satellite derived observations were also performed, finding that all the schemes tested with the BN parameterization predicted average values of IWP within ±15% of the observations.
Highlights
The role of atmospheric aerosols in modulating the atmospheric radiative balance, by directly scattering solar radiation, or indirectly, by modifying cloud optical and microphysical properties, has received considerable attention during the last couple of decades
We report the implementation of the Barahona and Nenes (BN) ice nucleation scheme into the Microphysics of Clouds with Relaxed Arakawa-Schubert and AerosolCloud interaction (McRAS-AC) (Sud and Lee, 2007) driven by the Goddard Earth Observing System Model, version 5 (GEOS-5)
Nc,nuc calculated with the BNPDA08 and BN-MY92 is systematically lower for the mixedphase cloud regime, as compared to Liu and Penner (LP)-CTRL
Summary
The role of atmospheric aerosols in modulating the atmospheric radiative balance, by directly scattering solar radiation, or indirectly, by modifying cloud optical and microphysical properties, has received considerable attention during the last couple of decades. This approach has been progressively replaced by a less empirical and more physically-based representation, in which the deposition growth of cloud ice at the expense of the liquid water, the Bergeron-Findeisen process (Pruppacher and Klett, 1997), is taken into account (e.g., Rotstayn et al, 2000) This prognostic approach for condensate partitioning which includes explicit dependence of the deposition rate on microphysical variables such as ice content qi, and ice crystal concentration, Nc, has been adopted by a variety of GCMs (Sud and Lee, 2007; Liu et al, 2007; Salzmann et al, 2010). The simulations were forced with data collected during the Tropical Warm Pool International Cloud Experiment (TWP-ICE) intensive observation period (IOP) of the ARM program (May et al, 2008), that took place around Darwin, Australia in early 2006
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