Lagrangian condensation microphysics with Twomey CCN activation
We report the development of a novel Lagrangian microphysics methodology for simulations of warm ice-free clouds. The approach applies the traditional Eulerian method for the momentum and continuous thermodynamic fields such as the temperature and water vapor mixing ratio, and uses Lagrangian <...
| Main Authors: | , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2018-01-01
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| Series: | Geoscientific Model Development |
| Online Access: | https://www.geosci-model-dev.net/11/103/2018/gmd-11-103-2018.pdf |
| Summary: | We report the development of a novel Lagrangian microphysics
methodology for simulations of warm ice-free clouds. The approach applies the traditional Eulerian method for the momentum and continuous thermodynamic fields such as the temperature and water vapor mixing ratio, and uses Lagrangian
<q>super-droplets</q> to represent condensed phase such as cloud droplets and
drizzle or rain drops. In other applications of the Lagrangian warm-rain
microphysics, the super-droplets outside clouds represent unactivated cloud
condensation nuclei (CCN) that become activated upon entering a cloud and can
further grow through diffusional and collisional processes. The original
methodology allows for the detailed study of not only effects of CCN on cloud
microphysics and dynamics, but also CCN processing by a cloud. However, when
cloud processing is not of interest, a simpler and computationally more
efficient approach can be used with super-droplets forming only when CCN is
activated and no super-droplet existing outside a cloud. This is possible by
applying the Twomey activation scheme where the local supersaturation
dictates the concentration of cloud droplets that need to be present inside a
cloudy volume, as typically used in Eulerian bin microphysics schemes. Since
a cloud volume is a small fraction of the computational domain volume, the
Twomey super-droplets provide significant computational advantage when
compared to the original super-droplet methodology. Additional advantage
comes from significantly longer time steps that can be used when modeling of
CCN deliquescence is avoided. Moreover, other formulation of the droplet
activation can be applied in case of low vertical resolution of the host
model, for instance, linking the concentration of activated cloud droplets to
the local updraft speed.
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This paper discusses the development and testing of the Twomey super-droplet
methodology, focusing on the activation and diffusional growth. Details of the
activation implementation, transport of super-droplets in the physical space, and the
coupling between super-droplets and the Eulerian temperature and water vapor
field are discussed in detail. Some of these are relevant to the original
super-droplet methodology as well and to the ice phase modeling using the
Lagrangian approach. As a computational example, the scheme is applied to an
idealized moist thermal rising in a stratified environment, with the original
super-droplet methodology providing a benchmark to which the new scheme is
compared. |
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| ISSN: | 1991-959X 1991-9603 |