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

Abstract

Abstract. In order to improve the treatment of ice nucleation in a more realistic manner in the Community Atmosphere Model version 5.3 (CAM5.3), the effects of pre-existing ice crystals on ice nucleation in cirrus clouds are considered. In addition, by considering the in-cloud variability in ice saturation ratio, homogeneous nucleation takes place spatially only in a portion of the cirrus cloud rather than in the whole area of the cirrus cloud. Compared to observations, the ice number concentrations and the probability distributions of ice number concentration are both improved with the updated treatment. The pre-existing ice crystals significantly reduce ice number concentrations in cirrus clouds, especially at mid-to high latitudes in the upper troposphere (by a factor of ∼ 10). Furthermore, the contribution of heterogeneous ice nucleation to cirrus ice crystal number increases considerably.Besides the default ice nucleation parameterization of Liu and Penner (2005, hereafter LP) in CAM5.3, two other ice nucleation parameterizations of Barahona and Nenes (2009, hereafter BN) and Kärcher et al. (2006, hereafter KL) are implemented in CAM5.3 for the comparison. In-cloud ice crystal number concentration, percentage contribution from heterogeneous ice nucleation to total ice crystal number, and pre-existing ice effects simulated by the three ice nucleation parameterizations have similar patterns in the simulations with present-day aerosol emissions. However, the change (present-day minus pre-industrial times) in global annual mean column ice number concentration from the KL parameterization (3.24 × 10 6 m −2 ) is less than that from the LP (8.46 × 10 6 m −2 ) and BN (5.62 × 10 6 m −2 ) parameterizations. As a result, the experiment using the KL parameterization predicts a much smaller anthropogenic aerosol longwave indirect forcing (0.24 W m −2 ) than that using the LP (0.46 W m −2 ) and BN (0.39 W m −2 ) parameterizations.

Effect of gravity wave temperature fluctuations on homogeneous ice nucleation in the tropical tropopause layer

“…Further, nucleation is calculated only for initially ice-free air parcels. The effect of pre-existing ice on nucleation has been discussed elsewhere (see Shi et al, 2015).…”

The path to CAM6: coupled simulations with CAM5.4 and CAM5.5

“…This bias has been greatly ameliorated due to the new ice nucleation scheme (Wang et al, 2014;Shi et al, 2015) as well as the new prognostic microphysics scheme (Gettelman, 2015). Previous work (Hwang and Frierson, 2013) suggests that an improvement in the Southern Ocean SWCF biases could potentially lead to an improvement in the simulated double Intertropical Convergence Zone (ITCZ) bias, which we will discuss further in this section.…”

“…In addition, at the time of CAM5.4 development, it was unclear if CLUBB would be included into future versions of CAM as the default scheme, as CAM-CLUBB coupled simulations were still being evaluated. Changes from CAM5.3 to CAM5.4 include an upgrade from a diagnostic precipitation scheme (Morrison and Gettelman, 2008) to a prognostic precipitation scheme (Gettelman, 2015), a new ice nucleation scheme (Wang et al, 2014;Shi et al, 2015) to better represent mixed-phase and cirrus ice nucleation, an upgrade from the three-mode Modal Aerosol Module (MAM3) to a four-mode version (MAM4) that includes the treatment of black carbon , the use of an additional two vertical layers near model top for consistent level treatment between CAM and the Whole Atmosphere Community Climate Model (WACCM), an improved treatment of the dust emissions size distributions and dust optical properties (Albani et al, 2014), a fix to the energy formulation in CAM (Williamson et al, 2015), a change to the vertical remapping from energy to temperature in the finite volume dynamical core, and consistent topography files for the finite volume and spectral element dynamical cores.…”

Direct comparisons of ice cloud macro- and microphysical properties simulated by the Community Atmosphere Model version 5 with HIPPO aircraft observations

“…We conduct a sensitivity experiment (referred to as SUL) by removing this size limit (i.e., using all sulfate aerosol particles in the Aitken mode for homogeneous nucleation). Recently, Shi et al (2015) incorporated the effects of pre-existing ice crystals on ice nucleation in CAM5, simultaneously removing the lower limit of sulfate aerosol size and the upper limit of the sub-grid updraft velocity used for the ice nucleation parameterization. Here a sensitivity experiment (referred to as PRE-ICE) with the Shi et al (2015) modifications is conducted (Table 1).…”

“…Ice microphysics parameterizations CTL U , V , T Threshold diameter for autoconversion of cloud ice to snow (D cs ) set to 150 µm DCS75 U , V , T As CTL but with D cs = 75 µm DCS300 U , V , T As CTL but with D cs = 300 µm SUL U , V , T As CTL but without the lower limit (0.1 µm) for sulfate particle diameter for homogeneous freezing PRE-ICE U , V , T As CTL but with the impacts of pre-existing ice crystals on ice nucleation (Shi et al, 2015…”