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Krzysztof E. Haman
and
Hanna Pawlowska

Abstract

A model of the dynamically inactive parts of cumulus cloud (i.e., void of bulk, organized vertical drafts) is proposed. It is assumed that such parts of clouds consist of coarse filaments of saturated, droplet-containing air mixed with filaments of air free of liquid water and perhaps even unsaturated. During vertical motions saturated and unsaturated filaments change their buoyancy according to dry- and wet-adiabatic processes, respectively, but being entangled with each other, they cannot develop separate vertical drafts of reasonable scale; the latter can form only if bulk net buoyancy occurs. A preliminary study of evolution of such a mixture due to bulk vertical motion and internal homogenizing mixing (which both affect the latent sensible heat transition) is made by means of a one-dimensional model. Results of a number of runs of the model are presented, confirming the supposition that cloud with such a structure can persist fairly long even in conditionally unstable stratification. Some generalizations of these results and applications to real clouds are briefly discussed.

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Wojciech W. Grabowski
and
Hanna Pawlowska

Abstract

The applicability of the Paluch mixing diagram to studies dealing with cumulus entrainment is reexamined. Theoretical considerations are illustrated with the Paluch method applied to numerically simulated entrainment into a rising thermal. An explanation is given as to why the traditional method of aircraft measurement analysis may result in significant overestimation of the level of origin of the entrained air in the presence of unresolved structures of thermodynamic fields.

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Szymon P. Malinowski
and
Hanna Pawlowska-Mankiewicz

Abstract

The existence small-scale inhomogeneities in cumulus clouds leads to a reinterpretation of the experimental data on total water mixing ratio Q and wet equivalent potential temperature θq. This reinterpretation indicates that the height of the level of entrained air may be sometimes overestimated.

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Dorota Jarecka
,
Wojciech W. Grabowski
, and
Hanna Pawlowska

Abstract

This paper discusses an extension of the approach proposed previously to represent the delay of cloud water evaporation and buoyancy reversal due to the cloud–environment mixing in bulk microphysics large-eddy simulation of clouds. In the original approach, an additional equation for the mean spatial scale of cloudy filaments was introduced to represent the progress toward microscale homogenization of a volume undergoing turbulent cloud–environment mixing, with the evaporation of cloud water allowed only when the filament scale approached the Kolmogorov microscale. Here, it is shown through a posteriori analysis of model simulations that one should also predict the volume fraction of the cloudy air that was diagnosed in the original approach. The resulting model of turbulent mixing and homogenization, referred to as the λβ model, is applied in a series of shallow convection simulations using various spatial resolutions and compared to the traditional bulk model. This work represents an intermediate step in the development of a modeling framework to simulate characteristics of microphysical transformations during entrainment and subgrid-scale turbulent mixing.

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Joanna Slawinska
,
Wojciech W. Grabowski
,
Hanna Pawlowska
, and
Hugh Morrison

Abstract

This paper presents the application of a double-moment bulk warm-rain microphysics scheme to the simulation of a field of shallow convective clouds based on Barbados Oceanographic and Meteorological Experiment (BOMEX) observations. The scheme predicts the supersaturation field and allows secondary in-cloud activation of cloud droplets above the cloud base. Pristine and polluted cloud condensation nuclei (CCN) environments, as well as opposing subgrid-scale mixing scenarios, are contrasted. Numerical simulations show that about 40% of cloud droplets originate from CCN activated above the cloud base. Significant in-cloud activation leads to the mean cloud droplet concentration that is approximately constant with height, in agreement with aircraft observations. The in-cloud activation affects the spatial distribution of the effective radius and the mean albedo of the cloud field. Differences between pristine and polluted conditions are consistent with the authors’ previous study, but the impact of the subgrid-scale mixing is significantly reduced. Possible explanations of the latter involve physical and numerical aspects. The physical aspects include (i) the counteracting impacts of the subgrid-scale mixing and in-cloud activation and (ii) the mean characteristics of the environmental cloud-free air entrained into a cloud. A simple analysis suggests that the entrained cloud-free air is on average close to saturation, which leads to a small difference between various mixing scenarios. The numerical aspect concerns the relatively small role of the parameterized subgrid-scale mixing when compared to mixing and evaporation due to numerical diffusion. Although the results are consistent with aircraft observations, limitations of the numerical model due to low spatial resolution call for higher-resolution simulations where entrainment processes are resolved rather than mostly parameterized as in the current study.

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Gustavo C. Abade
,
Wojciech W. Grabowski
, and
Hanna Pawlowska

Abstract

This paper discusses the effects of cloud turbulence, turbulent entrainment, and entrained cloud condensation nuclei (CCN) activation on the evolution of the cloud droplet size spectrum. We simulate an ensemble of idealized turbulent cloud parcels that are subject to entrainment events modeled as a random process. Entrainment events, subsequent turbulent mixing inside the parcel, supersaturation fluctuations, and the resulting stochastic droplet activation and growth by condensation are simulated using a Monte Carlo scheme. Quantities characterizing the turbulence intensity, entrainment rate, CCN concentration, and the mean fraction of environmental air entrained in an event are all specified as independent external parameters. Cloud microphysics is described by applying Lagrangian particles, the so-called superdroplets. These are either unactivated CCN or cloud droplets that grow from activated CCN. The model accounts for the addition of environmental CCN into the cloud by entraining eddies at the cloud edge. Turbulent mixing of the entrained dry air with cloudy air is described using the classical linear relaxation to the mean model. We show that turbulence plays an important role in aiding entrained CCN to activate, and thus broadening the droplet size distribution. These findings are consistent with previous large-eddy simulations (LESs) that consider the impact of variable droplet growth histories on the droplet size spectra in small cumuli. The scheme developed in this work is ready to be used as a stochastic subgrid-scale scheme in LESs of natural clouds.

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Joanna Slawinska
,
Wojciech W. Grabowski
,
Hanna Pawlowska
, and
Andrzej A. Wyszogrodzki
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Piotr Dziekan
,
Jørgen B. Jensen
,
Wojciech W. Grabowski
, and
Hanna Pawlowska

Abstract

The impact of giant sea salt aerosols released from breaking waves on rain formation in marine boundary layer clouds is studied using large-eddy simulations (LES). We perform simulations of marine cumuli and stratocumuli for various concentrations of cloud condensation nuclei (CCN) and giant CCN (GCCN). Cloud microphysics are modeled with a Lagrangian method that provides key improvements in comparison to previous LES of GCCN that used Eulerian bin microphysics. We find that GCCN significantly increase precipitation in stratocumuli. This effect is strongest for low and moderate CCN concentrations. GCCN are found to have a smaller impact on precipitation formation in cumuli. These conclusions are in agreement with field measurements. We develop a simple parameterization of the effect of GCCN on precipitation, accretion, and autoconversion rates in marine stratocumuli.

Significance Statement

Breaking sea waves release salt particles into the atmosphere. Cloud droplets formed on these salt particles can grow larger than droplets formed on other smaller particles. Therefore, sea salt particles can be important for rain formation over oceans. To investigate this effect, we performed idealized computer simulations of stratocumulus and cumulus clouds. Sea salt particles were modeled with an unprecedented precision thanks to the use of an emerging modeling method. In our simulations sea salt particles significantly enhance rain formation in stratocumuli, but not in cumuli. Our study has implications for climate models, because stratocumuli are important for Earth’s energy budget and for rain enhancement experiments.

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Dorota Jarecka
,
Wojciech W. Grabowski
,
Hugh Morrison
, and
Hanna Pawlowska

Abstract

This paper presents an approach to locally predict homogeneity of the subgrid-scale turbulent mixing in large-eddy simulation of shallow clouds applying double-moment warm-rain microphysics. The homogeneity of subgrid-scale mixing refers to the partitioning of the cloud water evaporation due to parameterized entrainment between changes of the mean droplet radius and changes of the mean droplet concentration. Homogeneous and extremely inhomogeneous mixing represent two limits of possible scenarios, where the droplet concentration and the mean droplet radius remains unchanged during the microphysical adjustment, respectively. To predict the subgrid-scale mixing scenario, the double-moment microphysics scheme is merged with the approach to delay droplet evaporation resulting from entrainment. Details of the new scheme and its application in the Barbados Oceanographic and Meteorological Experiment (BOMEX) shallow convection case are discussed. The simulated homogeneity of mixing varies significantly inside small convective clouds, from close to homogeneous to close to extremely inhomogeneous. The mean mixing characteristics become more homogeneous with height, reflecting increases of the mean droplet size and the mean turbulence intensity, both favoring homogeneous mixing. Model results are consistent with microphysical effects of entrainment and mixing deduced from field observations. Mixing close to homogeneous is predicted in volumes with the highest liquid water content (LWC) and strongest updraft at a given height, whereas mixing in strongly diluted volumes is typically close to extremely inhomogeneous. The simulated homogeneity of mixing has a small impact on mean microphysical characteristics. This result agrees with the previous study applying prescribed mixing scenarios and can be explained by the high humidity of the clear air involved in the subgrid-scale mixing.

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Jean-Louis Brenguier
,
Hanna Pawlowska
,
Lothar Schüller
,
Rene Preusker
,
Jürgen Fischer
, and
Yves Fouquart

Abstract

The plane-parallel model for the parameterization of clouds in global climate models is examined in order to estimate the effects of the vertical profile of the microphysical parameters on radiative transfer calculations for extended boundary layer clouds. The vertically uniform model is thus compared to the adiabatic stratified one. The validation of the adiabatic model is based on simultaneous measurements of cloud microphysical parameters in situ and cloud radiative properties from above the cloud layer with a multispectral radiometer. In particular, the observations demonstrate that the dependency of cloud optical thickness on cloud geometrical thickness is larger than predicted with the vertically uniform model and that it is in agreement with the prediction of the adiabatic one. Numerical simulations of the radiative transfer have been performed to establish the equivalence between the two models in terms of the effective radius. They show that the equivalent effective radius of a vertically uniform model is between 80% and 100% of the effective radius at the top of an adiabatic stratified model. The relationship depends, in fact, upon the cloud geometrical thickness and droplet concentration. Remote sensing measurements of cloud radiances in the visible and near infrared are then examined at the scale of a cloud system for a marine case and the most polluted case sampled during the second Aerosol Characterization Experiment. The distributions of the measured values are significantly different between the two cases. This constitutes observational evidence of the aerosol indirect effect at the scale of a cloud system. Finally, the adiabatic stratified model is used to develop a procedure for the retrieval of cloud geometrical thickness and cloud droplet number concentration from the measurements of cloud radiances. It is applied to the marine and to the polluted cases. The retrieved values of droplet concentration are significantly underestimated with respect to the values measured in situ. Despite this discrepancy the procedure is efficient at distinguishing the difference between the two cases.

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