Controls on precipitation and cloudiness in simulations of trade-wind cumulus as observed during RICO

The effect of an increase in atmospheric aerosol concentrations on the distribution and radiative properties of Earth's clouds is the most uncertain component of the overall global radiative forcing from preindustrial time. General circulation models (GCMs) are the tool for predicting future climate, but the treatment of aerosols, clouds, and aerosol−cloud radiative effects carries large uncertainties that directly affect GCM predictions, such as climate sensitivity. Predictions are hampered by the large range of scales of interaction between various components that need to be captured. Observation systems (remote sensing, in situ) are increasingly being used to constrain predictions, but significant challenges exist, to some extent because of the large range of scales and the fact that the various measuring systems tend to address different scales. Fine-scale models represent clouds, aerosols, and aerosol−cloud interactions with high fidelity but do not include interactions with the larger scale and are therefore limited from a climatic point of view. We suggest strategies for improving estimates of aerosol−cloud relationships in climate models, for new remote sensing and in situ measurements, and for quantifying and reducing model uncertainty.climate | aerosol−cloud effects | general circulation models | radiative forcing | satellite observations Clouds play a key role in Earth's radiation budget, and aerosols serve as the seeds upon which cloud droplets form. Anthropogenic activity has led to an increase in aerosol particle concentrations globally and an increase in those particles that act as cloud condensation nuclei (CCN) and ice nucleating particles (INP). The effect of an increase in aerosols on cloud optical properties, and associated radiative forcing, is the most uncertain component of historical radiative forcing of Earth's climate caused by greenhouse gases (GHGs) and aerosols. The Intergovernmental Panel on Climate Change (IPCC) AR5 assessment of climate forcing factors (Fig. S1) ascribes "high" confidence to the estimate of direct aerosol radiative forcing (mean

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“…for mixed-and ice-phase clouds, CRM simulations can be quite sensitive to reasonable choices of microphysical parameterization (34).…”

Section: Process-scale Modeling Of Aerosol−cloud Relationshipsmentioning

confidence: 99%

Improving our fundamental understanding of the role of aerosol−cloud interactions in the climate system

Seinfeld

Bretherton

Carslaw

et al. 2016

Proc. Natl. Acad. Sci. U.S.A.

590

474

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“…Moreover, even if the instrument were perfect, the probability of finding a cloud volume that has not been affected by entrainment and mixing 1 km above cloud base is extremely low in real clouds. These limitations notwithstanding, LES of realistic clouds do a reasonable job of reproducing the main features of the droplet spectrum such as the effective radius of the droplets (defined as the ratio of the third moment of the droplet size distribution to its second moment) and the number concentration (see, for example, Jiang et al, 2008;van Zanten et al, 2011). However, the positive tail of the droplet size distribution, which accounts for the largest droplets, is poorly measured with single-particle counters, while it plays a crucial role in the onset of precipitation.…”

Section: Observations Of Cloud Dropletsmentioning

confidence: 99%

Droplet growth in warm turbulent clouds

Devenish

Bartello

Brenguier

et al. 2012

Quart J Royal Meteoro Soc

243

281

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In this survey we consider the impact of turbulence on cloud formation from the cloud scale to the droplet scale. We assess progress in understanding the effect of turbulence on the condensational and collisional growth of droplets and the effect of entrainment and mixing on the droplet spectrum. The increasing power of computers and better experimental and observational techniques allow for a much more detailed study of these processes than was hitherto possible. However, much of the research necessarily remains idealized and we argue that it is those studies which include such fundamental characteristics of clouds as droplet sedimentation and latent heating that are most relevant to clouds. Nevertheless, the large body of research over the last decade is beginning to allow tentative conclusions to be made. For example, it is unlikely that small-scale turbulent eddies (i.e. not the energy-containing eddies) alone are responsible for broadening the droplet size spectrum during the initial stage of droplet growth due to condensation. It is likely, though, that small-scale turbulence plays a significant role in the growth of droplets through collisions and coalescence. Moreover, it has been possible through detailed numerical simulations to assess the relative importance of different processes to the turbulent collision kernel and how this varies in the parameter space that is important to clouds. The focus of research on the role of turbulence in condensational and collisional growth has tended to ignore the effect of entrainment and mixing and it is arguable that they play at least as important a role in the evolution of the droplet spectrum. We consider the role of turbulence in the mixing of dry and cloudy air, methods of quantifying this mixing and the effect that it has on the droplet spectrum. Copyright

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“…In order to achieve this, a probability density function (PDF) of the updraft velocity at cloud base for idealised trade wind Cu based on data from the Rain In Cumulus over the Oceans (RICO) experiment (vanZanten et al, 2011).…”

Section: Application To Model-derived Updraft Pdfsmentioning

confidence: 99%

“…Increasing aerosol concentrations, possibly by anthropogenic activity, affect the radiative properties of stratocumulus (Sc) clouds by increasing the number of cloud drops in them, which increases the scattering of short-wave radiation (Twomey, 1977) and extends the lifetime of the cloud (Albrecht, 1989). This is thought to result in a cooling effect of approximately −0.7 W m −2 globally.…”

Section: Introductionmentioning

confidence: 99%

A parameterisation for the activation of cloud drops including the effects of semi-volatile organics

et al. 2014

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Abstract. We present a parameterisation of aerosol activation, including co-condensation of semi-volatile organics, for warm clouds that has applications in large-scale numerical models. The scheme is based on previously developed parameterisations that are in the literature, but has two main modifications. The first is that the total aerosol mass is modified by the condensation of organic vapours entering cloud base, whereas the second is that this addition of mass acts to modify the median diameter and the geometric standard deviation of the aerosol size distribution. It is found that the scheme is consistent with parcel model calculations of cocondensation under different regimes. Such a parameterisation may find use in evaluating important feedbacks in climate models.

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Controls on precipitation and cloudiness in simulations of trade-wind cumulus as observed during RICO

Cited by 309 publications

References 44 publications

Improving our fundamental understanding of the role of aerosol−cloud interactions in the climate system

Improving our fundamental understanding of the role of aerosol−cloud interactions in the climate system

Droplet growth in warm turbulent clouds

A parameterisation for the activation of cloud drops including the effects of semi-volatile organics

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