Abstract
Abstract. An explicit and detailed treatment of cloud-borne particles allowing for the consideration of aerosol cycling in clouds has been implemented into COSMO-Model, the regional weather forecast and climate model of the Consortium for Small-scale Modeling (COSMO). The effects of aerosol scavenging, cloud microphysical processing and regeneration upon cloud evaporation on the aerosol population and on subsequent cloud formation are investigated. For this, two-dimensional idealized simulations of moist flow over two bell-shaped mountains were carried out varying the treatment of aerosol scavenging and regeneration processes for a warm-phase and a mixed-phase orographic cloud. The results allowed us to identify different aerosol cycling mechanisms. In the simulated non-precipitating warm-phase cloud, aerosol mass is incorporated into cloud droplets by activation scavenging and released back to the atmosphere upon cloud droplet evaporation. In the mixed-phase cloud, a first cycle comprises cloud droplet activation and evaporation via the Wegener–Bergeron–Findeisen (WBF) process. A second cycle includes below-cloud scavenging by precipitating snow particles and snow sublimation and is connected to the first cycle via the riming process which transfers aerosol mass from cloud droplets to snowflakes. In the simulated mixed-phase cloud, only a negligible part of the total aerosol mass is incorporated into ice crystals. Sedimenting snowflakes reaching the surface remove aerosol mass from the atmosphere. The results show that aerosol processing and regeneration lead to a vertical redistribution of aerosol mass and number. Thereby, the processes impact the total aerosol number and mass and additionally alter the shape of the aerosol size distributions by enhancing the internally mixed/soluble Aitken and accumulation mode and generating coarse-mode particles. Concerning subsequent cloud formation at the second mountain, accounting for aerosol processing and regeneration increases the cloud droplet number concentration with possible implications for the ice crystal number concentration.
Highlights
Orography has an important influence on precipitation formation and can be a key factor in hydrology, ecology and climate on the local scale (Smith et al, 2005; Saleeby et al, 2009)
To simulate aerosol processing in clouds, five new aerosol modes corresponding to the five prognostic hydrometeor classes (in-cloud droplet (CD), in-ice crystal (IC), inraindrop (RD), in-snowflake (SF) and in-graupel (GR) aerosol mode; Table 2) are introduced into the model
The effects of warm-phase and mixed-phase orographic clouds on the aerosol population have been evaluated by simulating orographic cloud formation over a 2-D double-bellshaped topography with the regional weather forecast and climate model Consortium for Small-scale Modeling (COSMO)-Model
Summary
Orography has an important influence on precipitation formation and can be a key factor in hydrology, ecology and climate on the local scale (Smith et al, 2005; Saleeby et al, 2009). The WBF process is enhanced under polluted conditions because of the greater surface area exposure of more but smaller cloud droplets, largely compensating for the loss in snow growth by the reduced riming process (Saleeby et al, 2013) Another important characteristic is the ice-nucleation ability of certain aerosol types. Due to the different onset temperatures and efficiencies of the different freezing modes, the ice phase may vary according to the dominant freezing process impacting riming rates and precipitation formation During their residence time in the atmosphere, aerosol particles undergo various modifications which may affect their ability to serve as CCN and IN. The ice-nucleation efficiency of aerosol particles in the different freezing modes depends on their size, composition and mixing state, which may be altered during cloud processing.
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