On the Factors Modulating the Stratocumulus to Cumulus Transitions

Sandu, Irina; Stevens, Björn

doi:10.1175/2011jas3614.1

Cited by 150 publications

(252 citation statements)

References 26 publications

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“…Because meteorology and aerosol typically covary in somewhat predictable ways, neither of the methods is a realistic sampling of what the atmosphere presents (30) is used to simulate a transition case in the presence of (absorbing) smoke aerosol residing some distance above, and later entrained into cloud. Forcings, including a gradual increase in sea surface temperature, are applied (30). For this transition case, the SW heating associated with the aerosol is also coupled to dynamics (31).…”

Section: Resultsmentioning

confidence: 99%

New approaches to quantifying aerosol influence on the cloud radiative effect

Feingold

McComiskey

Yamaguchi

et al. 2016

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

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The topic of cloud radiative forcing associated with the atmospheric aerosol has been the focus of intense scrutiny for decades. The enormity of the problem is reflected in the need to understand aspects such as aerosol composition, optical properties, cloud condensation, and ice nucleation potential, along with the global distribution of these properties, controlled by emissions, transport, transformation, and sinks. Equally daunting is that clouds themselves are complex, turbulent, microphysical entities and, by their very nature, ephemeral and hard to predict. Atmospheric general circulation models represent aerosol−cloud interactions at everincreasing levels of detail, but these models lack the resolution to represent clouds and aerosol−cloud interactions adequately. There is a dearth of observational constraints on aerosol−cloud interactions. We develop a conceptual approach to systematically constrain the aerosol−cloud radiative effect in shallow clouds through a combination of routine process modeling and satellite and surfacebased shortwave radiation measurements. We heed the call to merge Darwinian and Newtonian strategies by balancing microphysical detail with scaling and emergent properties of the aerosol−cloud radiation system.T he climate system, with its couplings between land surface, vegetation, ocean, cryosphere, and atmosphere, is an extraordinarily complex system that is under intensive scrutiny for the purposes of climate analysis and prediction. The atmospheric aerosol and its interaction with clouds is a poorly quantified component of the climate system and is the focus of the current study. The aerosol comprises suspended particles that derive from the oceans, land surface, volcanoes, and anthropogenic activities. The difficulty in quantifying climate forcing by the aerosol emanates partly from the complexity in the aerosol itself, and partly from the fact that its influence on clouds requires detailed understanding of clouds and cloud feedbacks at a range of spatiotemporal scales. Untangling the multiple cloud responses that occur as a result of aerosol perturbations is particularly difficult (1). As one example, consider the influence of the aerosol on clouds and precipitation. Assuming no change in condensed water, the aerosol, by acting as nucleation sites for droplets, might generate smaller droplets, more reflective clouds (2), and reduced precipitation (3). However, through a multitude of complex and contingent pathways, aerosol-perturbed clouds sometimes appear to have similar reflectance because brightening is offset by reductions in cloud water, a fundamental property controlling cloud reflectance. On short timescales (hours), the aerosol tends to reduce precipitation in shallow, liquid-only clouds, but this may be offset over longer periods (multiple days) (4). Deep, mixed-phase convective clouds present even more complex pathways for generation of precipitation, and even more contingencies. The aerosol appears to change the distribution and intensity of surface rain from deep c...

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Section: Resultsmentioning

confidence: 99%

New approaches to quantifying aerosol influence on the cloud radiative effect

Feingold

McComiskey

Yamaguchi

et al. 2016

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

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“…This provides a framework for systematically comparing LES responses with those of climate models in which the above forcings change in model-dependent and location-dependent ways as climate warms. It could also help integrate results from a small but growing set of LES studies investigating the response of different types of cloudtopped boundary layers to different types of forcing perturbations possibly relevant to climate change [e.g., Blossey et al, 2009;Lock, 2009;Xu et al, 2010;Sandu and Stevens, 2011;Nuijens and Stevens, 2012;Rieck et al, 2012].…”

Section: Introductionmentioning

confidence: 99%

Mechanisms of marine low cloud sensitivity to idealized climate perturbations: A single‐LES exploration extending the CGILS cases

Bretherton

Blossey

Jones

2013

J Adv Model Earth Syst

200

358

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[1] Climate change sensitivities of subtropical cloud-topped marine boundary layers are analyzed using large-eddy simulation (LES) of three CGILS cases of well-mixed stratocumulus, cumulus under stratocumulus, and shallow cumulus cloud regimes, respectively. For each case, a steadily forced control simulation on a small horizontally doubly periodic domain is run 10-20 days into quasi-steady state. The LES is rerun to steady state with forcings perturbed by changes in temperature, free-tropospheric relative humidity (RH), CO 2 concentration, subsidence, inversion stability, and wind speed; cloud responses to combined forcings superpose approximately linearly. For all three cloud regimes and 23 CO 2 forcing perturbations estimated from the CMIP3 multimodel mean, the LES predicts positive shortwave cloud feedback, like most CMIP3 global climate models. At both stratocumulus locations, the cloud remains overcast but thins in the warmer, moister, CO 2 -enhanced climate, due to the combined effects of an increased lower-tropospheric vertical humidity gradient and an enhanced free-tropospheric greenhouse effect that reduces the radiative driving of turbulence. Reduced subsidence due to weakening of tropical overturning circulations partly counteracts these two factors by raising the inversion and allowing the cloud layer to deepen. These compensating mechanisms may explain the large scatter in low cloud feedbacks predicted by climate models. CMIP3-predicted changes in wind speed, inversion stability, and free-tropospheric RH have lesser impacts on the cloud thickness. In the shallow cumulus regime, precipitation regulates the simulated boundary-layer depth and vertical structure. Cloud-droplet (aerosol) concentration limits the boundary-layer depth and affects the simulated cloud feedbacks.Citation: Bretherton, C. S., P. N. Blossey, and C. R. Jones (2013), Mechanisms of marine low cloud sensitivity to idealized climate perturbations: A single-LES exploration extending the CGILS cases, J. Adv. Model. Earth Syst., 5, 316-337,

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“…Myers and Norris (2013) corroborated this finding by showing from observations that low cloud amount in the subtropics tends to decrease as subsidence becomes stronger. Moreover, Sandu and Stevens (2011) performed several LESs of stratocumulus transition cases and found that although the entrainment rate increased in the sensitivity run with reduced subsidence, the larger entrainment drying and warming trend of the boundary layer did apparently not lead to a more rapid cloud break up. To shed some light on this finding, a budget equation for the tendency of the LWP of the stratocumulus layer as derived by Van der Dussen et al (2014) is used to analyze results of idealized LESs in order to determine the role of each individual physical process during stratocumulus transitions.…”

Section: Introductionmentioning

confidence: 99%

How large-scale subsidence affects stratocumulus transitions

2016

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Abstract. Some climate modeling results suggest that the Hadley circulation might weaken in a future climate, causing a subsequent reduction in the large-scale subsidence velocity in the subtropics. In this study we analyze the cloud liquid water path (LWP) budget from large-eddy simulation (LES) results of three idealized stratocumulus transition cases, each with a different subsidence rate. As shown in previous studies a reduced subsidence is found to lead to a deeper stratocumulus-topped boundary layer, an enhanced cloud-top entrainment rate and a delay in the transition of stratocumulus clouds into shallow cumulus clouds during its equatorwards advection by the prevailing trade winds. The effect of a reduction of the subsidence rate can be summarized as follows. The initial deepening of the stratocumulus layer is partly counteracted by an enhanced absorption of solar radiation. After some hours the deepening of the boundary layer is accelerated by an enhancement of the entrainment rate. Because this is accompanied by a change in the cloudbase turbulent fluxes of moisture and heat, the net change in the LWP due to changes in the turbulent flux profiles is negligibly small.

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On the Factors Modulating the Stratocumulus to Cumulus Transitions

Cited by 150 publications

References 26 publications

New approaches to quantifying aerosol influence on the cloud radiative effect

New approaches to quantifying aerosol influence on the cloud radiative effect

Mechanisms of marine low cloud sensitivity to idealized climate perturbations: A single‐LES exploration extending the CGILS cases

How large-scale subsidence affects stratocumulus transitions

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