Abstract
Abstract. In this study the effect of dust aerosol on upper tropospheric cirrus clouds through heterogeneous ice nucleation is investigated in the Community Atmospheric Model version 5 (CAM5) with two ice nucleation parameterizations. Both parameterizations consider homogeneous and heterogeneous nucleation and the competition between the two mechanisms in cirrus clouds, but differ significantly in the number concentration of heterogeneous ice nuclei (IN) from dust. Heterogeneous nucleation on dust aerosol reduces the occurrence frequency of homogeneous nucleation and thus the ice crystal number concentration in the Northern Hemisphere (NH) cirrus clouds compared to simulations with pure homogeneous nucleation. Global and annual mean shortwave and longwave cloud forcing are reduced by up to 2.0 ± 0.1 W m−2 (1σ uncertainty) and 2.4 ± 0.1 W m−2, respectively due to the presence of dust IN, with the net cloud forcing change of −0.40 ± 0.20 W m−2. Comparison of model simulations with in situ aircraft data obtained in NH mid-latitudes suggests that homogeneous ice nucleation may play an important role in the ice nucleation at these regions with temperatures of 205–230 K. However, simulations overestimate observed ice crystal number concentrations in the tropical tropopause regions with temperatures of 190–205 K, and overestimate the frequency of occurrence of high ice crystal number concentration (> 200 L−1) and underestimate the frequency of low ice crystal number concentration (< 30 L−1) at NH mid-latitudes. These results highlight the importance of quantifying the number concentrations and properties of heterogeneous IN (including dust aerosol) in the upper troposphere from the global perspective.
Highlights
Cirrus clouds composed of ice crystals cover roughly 30 % of the globe (Wang et al, 1996; Wylie and Menzel, 1999), and have been shown to play an important role in modifying the global radiative balance through scattering shortwave radiation and absorbing and emitting longwave (LW) terrestrial radiation (Corti et al, 2005; Kay and Gettelman, 2009; Liou, 1986; Lohmann and Roeckner, 1995; Ramanathan and Collins, 1991)
Using the Liu and Penner (2005) parameterization implemented in the Community Atmospheric Model version 3 (Liu et al, 2007), Liu et al (2009) investigated the impacts of anthropogenic aerosol on ice clouds and found that the effect of anthropogenic black carbon on cirrus clouds through ice nucleation depends critically on its ice nucleation efficiency assumed in the model and the relative importance of homogeneous and heterogeneous ice nucleation
Together with the existing Liu and Penner (2005, hereafter as LP05) ice nucleation parameterization in Community Atmospheric Model version 5 (CAM5), we investigate the sensitivity of cirrus clouds to different formulations of heterogeneous ice nucleation
Summary
Cirrus clouds composed of ice crystals cover roughly 30 % of the globe (Wang et al, 1996; Wylie and Menzel, 1999), and have been shown to play an important role in modifying the global radiative balance through scattering shortwave radiation and absorbing and emitting longwave (LW) terrestrial radiation (Corti et al, 2005; Kay and Gettelman, 2009; Liou, 1986; Lohmann and Roeckner, 1995; Ramanathan and Collins, 1991). Liu and Penner (2005) presented a parameterization for ice number concentration for large-scale models based on parcel model numerical simulations, and considered the competition between homogeneous and heterogeneous nucleation at temperature less than −35 ◦C. Barahona and Nenes (2009a, b) provided a framework for ice nucleation parameterizations by solving analytically the parcel model equations These parameterizations provide expressions for ice crystal number concentration, explicitly considering the effects of aerosol size and number, updraft velocity, and deposition coefficient, and the competition between homogeneous nucleation and heterogeneous nucleation. Using the Liu and Penner (2005) parameterization implemented in the Community Atmospheric Model version 3 (Liu et al, 2007), Liu et al (2009) investigated the impacts of anthropogenic aerosol (black carbon and sulfate) on ice clouds and found that the effect of anthropogenic black carbon on cirrus clouds through ice nucleation depends critically on its ice nucleation efficiency assumed in the model and the relative importance of homogeneous and heterogeneous ice nucleation.
Sign-in/Register to view highlights & summary 
Published Version (
Free)
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call