Abstract
Abstract. This study presents a comprehensive ice cloud formation parameterization that computes the ice crystal number, size distribution, and maximum supersaturation from precursor aerosol and ice nuclei. The parameterization provides an analytical solution of the cloud parcel model equations and accounts for the competition effects between homogeneous and heterogeneous freezing, and, between heterogeneous freezing in different modes. The diversity of heterogeneous nuclei is described through a nucleation spectrum function which is allowed to follow any form (i.e., derived from classical nucleation theory or from observations). The parameterization reproduces the predictions of a detailed numerical parcel model over a wide range of conditions, and several expressions for the nucleation spectrum. The average error in ice crystal number concentration was −2.0±8.5% for conditions of pure heterogeneous freezing, and, 4.7±21% when both homogeneous and heterogeneous freezing were active. The formulation presented is fast and free from requirements of numerical integration.
Highlights
Ice clouds play a key role in rain production (e.g., Lau and Wu, 2003), heterogeneous chemistry (Peter, 1997), stratospheric water vapor circulation (Hartmann et al, 2001), and the radiative balance of the Earth (Liou, 1986)
At any height during the parcel ascent, supersaturation with respect to ice, si, develops and the ice crystal size distribution is determined by heterogeneous freezing of IN, homogeneous freezing of droplets, and growth of existing ice crystals
We present an ice cloud formation parameterization that calculates Nc and smax explicitly considering the competition between homogeneous and heterogeneous freezing from a polydisperse aerosol population
Summary
Ice clouds form by homogeneous freezing of liquid droplets or heterogeneous freezing upon ice nuclei, (IN) (e.g., Pruppacher and Klett, 1997). Karcher et al (2006) proposed a physically based approach to parameterize cirrus cloud formation combining solutions for the pure homogeneous freezing (Karcher and Lohmann, 2002b), and heterogeneous freezing (Karcher and Lohmann, 2003) into a numerical scheme This approach includes all known relevant factors that determine Nc, it may be computationally intensive; its application is limited to cases where IN can be characterized by a few, well defined, freezing thresholds. The new parameterization builds upon the frameworks of Barahona and Nenes (2008, 2009) that combine homogeneous and heterogeneous mechanisms of ice formation, and explicitly resolves the dependency of Nc on conditions of cloud formation (i.e., T , p, V ), aerosol number and size, and the freezing characteristics of the IN
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