[세미나] Dr. Fabian Hoffmann

November 15, 2018

Entrainment and Mixing in Warm Boundary Layer Clouds: Development and Results of an Explicit Subgrid-Scale Scheme for Large-Eddy Simulations with Particle-Based Microphysics


Dr. Fabian Hoffmann

2018년 11월 15일 (목) 16:00

과학관 553호


Abstract

Entrainment and Mixing in Warm Boundary Layer Clouds: Development and Results of an Explicit Subgrid-Scale Scheme for Large-Eddy Simulations with Particle-Based Microphysics

Entrainment and Mixing in Warm Boundary Layer Clouds: Development and Results of an Explicit Subgrid-Scale Scheme for Large-Eddy Simulations with Particle-Based Microphysics To fully understand the microphysical composition of clouds, their radiative properties, and their ability to precipitate, lengthscales of multiple orders of magnitude need to be considered. During entrainment, for example, the switch from predominantly inhomogeneous to homogenous mixing (and their distinct effects on cloud microphysics) takes place at the centimeter-scale, but standard large-eddy simulations (LES) with grid spacings on the order of decameters exert erroneous homogeneous mixing over the entire subgrid-scale. On the other

hand, ultra-high resolution direct numerical simulation (DNS) captures the physics of small- scale mixing correctly, but does not represent the large-scale dynamics of the cloud responsible

for entrainment. The challenge is to represent this range of scales with a single model. In this talk, a novel modeling approach will be presented in which the LES subgrid-scale is represented by the ‘linear eddy model’, an economical, one-dimensional model that resolves turbulent compression, folding, and molecular diffusion in each grid box of the LES explicitly. Results from test cases of shallow cumuli and stratocumuli are presented, and first applications for mixed-phase clouds are discussed. Generally, clouds susceptible to inhomogeneous mixing show a reduction in the droplet number concentration and stronger droplet growth, in agreement with theory. Stratocumulus entrainment rates tend to be lower in the new approach compared to simulations without it. All in all, the simulations presented can be seen as a first step to bridge the gap between DNS and LES, allowing an appropriate representation of small-scale mixing processes, but also the consideration of the large-scale cloud system.