Radiative forcing of anthropogenic aerosols on cirrus clouds using a hybrid ice nucleation scheme

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.