High free-tropospheric Aitken-mode aerosol concentrations buffer cloud droplet concentrations in large-eddy simulations of precipitating stratocumulus

Abstract. The effect of 1-D and 3-D thermal radiation on cloud droplet growth in shallow cumulus clouds is investigated using large eddy simulations with size-resolved cloud microphysics. A two-step approach is used for separating microphysical effects from dynamical feedbacks. In step one, an offline parcel model is used to describe the onset of rain. The growth of cloud droplets to raindrops is simulated with bin-resolved microphysics along previously recorded Lagrangian trajectories. It is shown that thermal heating and cooling rates can enhance droplet growth and raindrop production. Droplets grow to larger size bins in the 10–30 µm radius range. The main effect in terms of raindrop production arises from recirculating parcels, where a small number of droplets are exposed to strong thermal cooling at cloud edge. These recirculating parcels, comprising about 6 %–7 % of all parcels investigated, make up 45 % of the rain for the no-radiation simulation and up to 60 % when 3-D radiative effects are considered. The effect of 3-D thermal radiation on rain production is stronger than that of 1-D thermal radiation. Three-dimensional thermal radiation can enhance the rain amount up to 40 % compared to standard droplet growth without radiative effects in this idealized framework. In the second stage, fully coupled large eddy simulations show that dynamical effects are stronger than microphysical effects, as far as the production of rain is concerned. Three-dimensional thermal radiative effects again exceed one-dimensional thermal radiative effects. Small amounts of rain are produced in more clouds (over a larger area of the domain) when thermal radiation is applied to microphysics. The dynamical feedback is shown to be an enhanced cloud circulation with stronger subsiding shells at the cloud edges due to thermal cooling and stronger updraft velocities in the cloud center. It is shown that an evaporation–circulation feedback reduces the amount of rain produced in simulations where 3-D thermal radiation is applied to microphysics and dynamics, in comparison to where 3-D thermal radiation is only applied to dynamics.

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Large-Eddy Simulations of Trade Wind Cumuli Using Particle-Based Microphysics with Monte Carlo Coalescence

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-12-0295.1 ◽

2013 ◽

Vol 70 (9) ◽

pp. 2768-2777 ◽

Cited By ~ 26

Author(s):

Sylwester Arabas ◽

Shin-ichiro Shima

Keyword(s):

Monte Carlo ◽

Droplet Size ◽

Cloud Condensation Nuclei ◽

Cloud Droplet ◽

Large Eddy Simulations ◽

Trade Wind ◽

Type Model ◽

Size Spectrum ◽

Large Eddy ◽

Condensational Growth

Abstract A series of simulations employing the superdroplet method (SDM) for representing aerosol, cloud, and rain microphysics in large-eddy simulations (LES) is discussed. The particle-based formulation treats all particles in the same way, subjecting them to condensational growth and evaporation, transport of the particles by the flow, gravitational settling, and collisional growth. SDM features a Monte Carlo–type numerical scheme for representing the collision and coalescence process. All processes combined cover representation of cloud condensation nuclei (CCN) activation, drizzle formation by autoconversion, accretion of cloud droplets, self-collection of raindrops, and precipitation, including aerosol wet deposition. The model setup used in the study is based on observations from the Rain in Cumulus over the Ocean (RICO) field project. Cloud and rain droplet size spectra obtained in the simulations are discussed in context of previously published analyses of aircraft observations carried out during RICO. The analysis covers height-resolved statistics of simulated cloud microphysical parameters such as droplet number concentration, effective radius, and parameters describing the width of the cloud droplet size spectrum. A reasonable agreement with measurements is found for several of the discussed parameters. The sensitivity of the results to the grid resolution of the LES, as well as to the sampling density of the probabilistic Monte Carlo–type model, is explored.

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Large-eddy simulation of radiation fog with comprehensive two-moment bulk microphysics: Impact of different aerosol activation and condensation parameterizations

10.5194/acp-2018-1139 ◽

2018 ◽

Author(s):

Johannes Schwenkel ◽

Björn Maronga

Keyword(s):

Cloud Droplet ◽

Large Eddy Simulations ◽

Diffusional Growth ◽

Radiation Fog ◽

Eddy Simulation ◽

Large Eddy ◽

Growth Method ◽

Explicit Analysis ◽

Temporal Influence ◽

Comparison Of The Results

Abstract. In this paper we study the influence of the cloud microphysical parameterization on large-eddy simulations of radiation fog. A deep fog case as observed at Cabauw (Netherlands) is investigated using high-resolution large-eddy simulations with different microphysics treatments for activation and diffusional growth. A comparison of the results indicates that the commonly applied assumption of saturation adjustment produces at maximum 6.9 % higher liquid water paths compared to the explicit diffusional growth method but has no significant influence on the general life cycle of the fog layer. Differences are found to be the most pronounced at the top of the fog layer where the highest supersaturations occurs. Furthermore, the effect of different cloud droplet number concentrations is investigated by using a selection of common activation schemes. We find, in line with previous studies, a positive feedback between the cloud droplet number concentration and both the optical thickness and the strength of the fog layer. Furthermore, we perform an explicit analysis of the budgets of microphysical quantities in order to assess which processes have the largest spatial and temporal influence on the development of the fog layer.

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Cloud Droplet Growth in Shallow Cumulus Clouds Considering 1D and 3D Thermal Radiative Effects

10.5194/acp-2018-1204 ◽

2018 ◽

Author(s):

Carolin Klinger ◽

Graham Feingold ◽

Takanobu Yamaguchi

Keyword(s):

Thermal Radiation ◽

Rain Rate ◽

Cloud Droplet ◽

Large Eddy Simulations ◽

Droplet Growth ◽

Radiative Effects ◽

Cumulus Clouds ◽

Shallow Cumulus ◽

Large Eddy ◽

Thermal Cooling

Abstract. The effect of 1D and 3D thermal radiation on cloud droplet growth in shallow cumulus clouds is investigated using large eddy simulations with size resolved cloud microphysics. A two step approach is used for separating microphysical effects from dynamical feedbacks. In step one, an offline parcel model with bin resolved microphysics is used where cloud droplets are grown along previously recorded Lagrangian trajectories. It is shown that thermal heating and cooling rates can enhance droplet growth and rain production. Droplets grow to larger size bins in the 10–30 μm radius range. The main effect in terms of rain production arises from recirculating parcels, where a small number of droplets is exposed to strong thermal cooling at cloud edge. These recirculating parcels, comprising about 6–7 % of all parcels investigated, make up 45 % of the accumulated rain rate for the no radiation simulation and up to 60 % when 3D radiative effects are considered. The effect of 3D thermal radiation on rain production is stronger than that of 1D thermal radiation. 3D thermal radiation can enhance the rain rate up to 40 % compared to standard droplet growth without radiative effects in this idealized framework. In the second stage, fully coupled large eddy simulations show that dynamical effects are stronger than microphysical effects, as far as the production of rain is concerned. 3D thermal radiative effects again exceed 1D thermal radiative effects. Small amounts of rain are produced in more clouds (over a larger area of the domain) when thermal radiation is applied to microphysics. The dynamical feedback is shown to be an enhanced cloud circulation with stronger subsiding shells at the cloud edges due to thermal cooling, and stronger updraft velocities in the cloud center. It is shown that an evaporation-circulation feedback reduces the amount of rain produced in simulations where 3D thermal radiation is applied to microphysics and dynamics, in comparison where 3D thermal radiation is only applied to dynamics.

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Extracting Microphysical Impacts in Large-Eddy Simulations of Shallow Convection

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-14-0231.1 ◽

2014 ◽

Vol 71 (12) ◽

pp. 4493-4499 ◽

Cited By ~ 31

Author(s):

Wojciech W. Grabowski

Keyword(s):

Traditional Approach ◽

Cloud Droplet ◽

Cloud Microphysics ◽

Natural Variability ◽

Large Eddy Simulations ◽

Shallow Convection ◽

Liquid Water Path ◽

Large Eddy ◽

The Mean ◽

The Impact

Abstract A simple methodology is proposed to extract impacts of cloud microphysics on macrophysical cloud-field properties in large-eddy simulations of shallow convection. These impacts are typically difficult to assess because of natural variability of the simulated cloud field. The idea is to use two sets of thermodynamic variables driven by different microphysical schemes or by a single scheme with different parameters as applied here. The first set is coupled to the dynamics as in the standard model, and the second set is applied diagnostically—that is, driven by the flow but without the feedback on the flow dynamics. Having the two schemes operating in the same flow pattern allows for extracting the impact with high confidence. For illustration, the method is applied to simulations of precipitating shallow convection applying a simple bulk representation of warm-rain processes. Because of natural variability, the traditional approach provides an uncertain estimate of the impact of cloud droplet concentration on the mean cloud-field rainfall even with an ensemble of simulations. In contrast, the impact is well constrained while applying the new methodology. The method can even detect minuscule changes of the mean cloud cover and liquid water path despite their large temporal fluctuations and different evolutions within the ensemble.

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