Abstract
Abstract. A common deficiency of many cloud-physics parameterizations including the NASA's microphysics of clouds with aerosol-cloud interactions (hereafter called McRAS-AC) is that they simulate lesser (larger) than the observed ice cloud particle number (size). A single column model (SCM) of McRAS-AC physics of the GEOS4 Global Circulation Model (GCM) together with an adiabatic parcel model (APM) for ice-cloud nucleation (IN) of aerosols were used to systematically examine the influence of introducing ammonium sulfate (NH4)2SO4 aerosols in McRAS-AC and its influence on the optical properties of both liquid and ice clouds. First an (NH4)2SO4 parameterization was included in the APM to assess its effect on clouds vis-à-vis that of the other aerosols. Subsequently, several evaluation tests were conducted over the ARM Southern Great Plain (SGP) and thirteen other locations (sorted into pristine and polluted conditions) distributed over marine and continental sites with the SCM. The statistics of the simulated cloud climatology were evaluated against the available ground and satellite data. The results showed that inclusion of (NH4)2SO4 into McRAS-AC of the SCM made a remarkable improvement in the simulated effective radius of ice cloud particulates. However, the corresponding ice-cloud optical thickness increased even more than the observed. This can be caused by lack of horizontal cloud advection not performed in the SCM. Adjusting the other tunable parameters such as precipitation efficiency can mitigate this deficiency. Inclusion of ice cloud particle splintering invoked empirically further reduced simulation biases. Overall, these changes make a substantial improvement in simulated cloud optical properties and cloud distribution particularly over the Intertropical Convergence Zone (ITCZ) in the GCM.
Highlights
Scores of papers have documented a substantial increase in anthropogenic aerosol loading over Asia and its possible impact on the environment, on clouds and rainfall over many regions of the world in the industrially developing regions of India and East Asia through the aerosol direct (ADE) and indirect effect (AIE) (e.g., Rosenfeld, 2000; Nakajima et al, 2001; Menon, 2002; Chylek et al, 2006; Krishnamurti et al, 2009)
AIE parameterizations in the present-day atmospheric general circulation models (GCM) allow aerosols to be activated as cloud condensation nuclei (CCN) for liquid clouds and ice nuclei (IN) for ice clouds
The parameterization modules comprise of: Fountoukis and Nenes scheme (2005) aerosol activation for liquid cloud particles, Seifert and Beheng (2001, 2006) precipitation microphysics modified for a coarse resolution GCM (Sud and Lee, 2007), Khvorostyanov and Curry (1999) scheme for inferring effective radius and optical properties of clouds, together with Liu and Penner (2005) parameterization for ice and mixed phase clouds drawn from Liu et al (2007)
Summary
Scores of papers have documented a substantial increase in anthropogenic aerosol loading over Asia and its possible impact on the environment, on clouds and rainfall over many regions of the world in the industrially developing regions of India and East Asia through the aerosol direct (ADE) and indirect effect (AIE) (e.g., Rosenfeld, 2000; Nakajima et al, 2001; Menon, 2002; Chylek et al, 2006; Krishnamurti et al, 2009). NASA GSFC single column model (SCM) was used to examine the aerosol-cloud interactions over several regions to isolate the causes of aforementioned biases These biases are closely linked to the precipitation microphysics affecting aerosol-cloud-radiation interactions whereby we can expect to significantly ameliorate, if not entirely mitigate, these biases. Talbot et al (1998) found presence of NH+4 in the upper troposphere and Tabazadeh and Toon (1998) have shown that ammonium sulfate ((NH4)2SO4) aerosols can influence the cirrus clouds These studies showed potential impact of ammonia and ammonium sulfate on ice clouds and that motivated the present investigation. The Wang et al (2008) dataset was used to prescribe the (NH4)2SO4 and assess its influence on the ice clouds simulated in McRASAC
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