Abstract
Abstract. Anthropogenic aerosols impact cirrus clouds through ice nucleation, thereby changing the Earth's radiation budget. However, the magnitude and sign of anthropogenic forcing in cirrus clouds is still very uncertain depending on the treatments for ice-nucleating particles (INPs), the treatments for haze particle freezing, and the ice nucleation scheme. In this study, a new ice nucleation scheme (hereafter the HYBRID scheme) is developed to combine the best features of two previous ice nucleation schemes, so that global models are able to calculate the ice number concentration in both updrafts and downdrafts associated with gravity waves, and it has a robust sensitivity to the change of aerosol number. The scheme is applied in a box model, and the ice number concentrations (9.52±2.08 L−1) are somewhat overestimated but are in reasonable agreement with those from an adiabatic parcel model (9.40±2.31 L−1). Then, the forcing and cloud changes associated with changes in aircraft soot, sulfur emission, and all anthropogenic emissions between the preindustrial (PI) period and the present day (PD) are examined using the CESM/IMPACT global model with the HYBRID scheme. Aircraft soot emissions decrease the global average ice number concentration (Ni) by -1.0±2.4×107 m−2 (−1 %) (over the entire column) due to the inhibition of homogeneous nucleation and lead to a radiative forcing of -0.14±0.07 W m−2, while the increase in sulfur emissions increases the global average Ni by 7.3±2.9×107 m−2 (5 %) due to the increase in homogeneous nucleation and leads to a radiative forcing of -0.02±0.06 W m−2. The possible effects of aerosol and cloud feedbacks to the meteorological state in remote regions partly contribute to reduce the forcing and the change in Ni due to anthropogenic emissions. The radiative forcing due to all increased anthropogenic emissions from PI to PD is estimated to be -0.20±0.05 W m−2. If newly formed secondary organic aerosols (SOAs) act as INPs and inhibit homogeneous nucleation, the Ni formed from heterogeneous nucleation is increased. As a result, the inclusion of INPs from SOA increases the change in Ni to 12.0±2.3×107 m−2 (9 %) and increases (makes less negative) the anthropogenic forcing on cirrus clouds to -0.04±0.08 W m−2 from PI to PD.
Highlights
Atmospheric aerosol loading has increased significantly since the preindustrial (PI) period, mainly due to anthropogenic emissions associated with the burning of fossil fuels and biomass
In the HYBRID scheme, the supersaturation (Si) in the cloud parcel is calculated explicitly using the KL scheme so that ice particles are able to grow or decay throughout the time variations in the updrafts and downdrafts associated with gravity waves
The comparison of ice number concentration between our model and observation has not improved significantly compared to that shown by Penner et al (2018), the new nucleation scheme improves the ability of nucleation to occur on small-sized particles, since it avoids the calculation of ice nucleation chronologically from large sizes to small sizes used in the KL scheme, which results in an underestimation of ice crystals formed from small-sized particles
Summary
Atmospheric aerosol loading has increased significantly since the preindustrial (PI) period, mainly due to anthropogenic emissions associated with the burning of fossil fuels and biomass. Most studies to date have focused on how the increase in anthropogenic aerosols impacts climate via warm clouds, thereby exerting a net cooling effect (Wang and Penner, 2009; Zhu et al, 2019; Gordon et al, 2016; IPCC, 2013). There has been much less attention paid to anthropogenic forcing as a result of changes to cirrus clouds, which is one of the least understood processes in the climate system (Fan et al, 2016). Ice particles in cirrus clouds are nucleated on aerosol particles, so that changes to the aerosol composition and loading may alter cirrus clouds by altering cloud microphysics, resulting in a cirrus cloud radiative forcing
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