Vertical redistribution of moisture and aerosol in orographic mixed-phase clouds
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Published:2020-07-10
Issue:13
Volume:20
Page:7979-8001
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ISSN:1680-7324
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Container-title:Atmospheric Chemistry and Physics
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language:en
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Short-container-title:Atmos. Chem. Phys.
Author:
Miltenberger Annette K.ORCID, Field Paul R., Hill Adrian H., Heymsfield Andrew J.ORCID
Abstract
Abstract. Orographic wave clouds offer a natural laboratory to investigate cloud microphysical processes and their representation
in atmospheric models. Wave clouds impact the larger-scale flow by the vertical redistribution of moisture and aerosol.
Here we use detailed cloud microphysical observations from the Ice in Clouds Experiment – Layer Clouds (ICE-L) campaign to evaluate the recently developed Cloud
Aerosol Interacting Microphysics (CASIM) module in the Met Office Unified Model (UM) with a particular focus on
different parameterizations for heterogeneous freezing. Modelled and observed thermodynamic and microphysical properties
agree very well (deviation of air temperature <1 K; specific humidity <0.2 g kg−1;
vertical velocity <1 m s−1; cloud droplet number concentration <40 cm−3), with the
exception of an overestimated total condensate content and too long a sedimentation tail. The accurate reproduction of
the environmental thermodynamic and dynamical wave structure enables the model to reproduce the right cloud in the right
place and at the right time. All heterogeneous freezing parameterizations except Atkinson et al. (2013) perform reasonably
well, with the best agreement in terms of the temperature dependency of ice crystal number concentrations for the
parameterizations of DeMott et al. (2010) and Tobo et al. (2013). The novel capabilities of CASIM allowed testing of the
impact of assuming different soluble fractions of dust particles on immersion freezing, but this is found to only have a
minor impact on hydrometeor mass and number concentrations. The simulations were further used to quantify the modification of moisture and aerosol profiles by the wave cloud. The
changes in both variables are on order of 15 % of their upstream values, but the modifications have very
different vertical structures for the two variables. Using a large number of idealized simulations we investigate how
the induced changes depend on the wave period (100–1800 s), cloud top temperature (−15 to
−50 ∘C), and cloud thickness (1–5 km) and propose a conceptual model to describe these
dependencies.
Publisher
Copernicus GmbH
Subject
Atmospheric Science
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