Abstract
Abstract. A new anelastic large-eddy simulation (LES) model with an Eulerian dynamical core and Lagrangian particle-based microphysics is presented. The dynamical core uses the multidimensional positive-definite advection transport algorithm (MPDATA) advection scheme and the generalized conjugate residual pressure solver, whereas the microphysics scheme is based on the super-droplet method. Algorithms for coupling of Lagrangian microphysics with Eulerian dynamics are presented, including spatial and temporal discretizations and a condensation substepping algorithm. The model is free of numerical diffusion in the droplet size spectrum. Activation of droplets is modeled explicitly, making the model less sensitive to local supersaturation maxima than models in which activation is parameterized. Simulations of a drizzling marine stratocumulus give results in agreement with other LES models. It is shown that in the super-droplet method a relatively low number of computational particles is sufficient to obtain correct averaged properties of a cloud, but condensation and collision–coalescence have to be modeled with a time step of the order of 0.1 s. Such short time steps are achieved by substepping, as the model time step is typically around 1 s. Simulations with and without an explicit subgrid-scale turbulence model are compared. Effects of modeling subgrid-scale motion of super-droplets are investigated. The model achieves high computational performance by using graphics processing unit (GPU) accelerators.
Highlights
Over the last decade, Lagrangian particle-based cloud microphysics schemes have been drawing increasing attention
The coalescence algorithm used in the superdroplet method (SDM) of Shima et al (2009) seems to be the most promising, as it was found to be the most accurate of the coalescence algorithms used in various Lagrangian microphysics schemes (Unterstrasser et al, 2017, where it is called the “all-or-nothing” algorithm)
Liquid water is resolved by the SDM and ql could be diagnosed from the super-droplet size spectrum each time it is needed in the buoyancy term in Eq (3) or radiative term in Eq (4)
Summary
Lagrangian particle-based cloud microphysics schemes have been drawing increasing attention. We document the development of a new large-eddy simulation (LES) model called the University of Warsaw Lagrangian Cloud Model (UWLCM). It is an anelastic model with a finite-difference Eulerian dynamical core and a Lagrangian microphysics scheme. Sensitivity of UWLCM to the parameters of the microphysics scheme and to the description of the subgrid-scale (SGS) turbulence is studied It is of particular interest how much drizzle a LES model with Lagrangian microphysics produces, compared to models with bin or bulk microphysics that took part in the intercomparison. A list of symbols used and their definitions are given in Appendix A, Appendix B compares two substepping algorithms for condensation and Appendix C contains a brief description of the software implementation of the model
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