Abstract. Marine low-level clouds are key to the Earth's energy budget due to their expansive coverage over global oceans and their high reflectance of incoming solar radiation. Their responses to anthropogenic aerosol perturbations remain the largest source of uncertainty in estimating the anthropogenic radiative forcing of climate. A major challenge is the quantification of the cloud water response to aerosol perturbations. In particular, the presence of feedbacks through microphysical, dynamical, and thermodynamical pathways at various spatial and temporal scales could augment or weaken the response. Central to this problem is the temporal evolution in cloud adjustment, governed by entangled feedback mechanisms. We apply an innovative conditional Monte Carlo subsampling approach to a large ensemble of diurnal large-eddy simulation of non-precipitating marine stratocumulus to study the role of solar heating in governing the evolution in the relationship between droplet number and cloud water. We find a persistent negative trend in this relationship at night, confirming that the role of microphysically enhanced cloud-top entrainment. After sunrise, the evolution in this relationship appears buffered and converges to ∼-0.2 in the late afternoon. This buffering effect is attributed to a strong dependence of cloud-layer shortwave absorption on cloud liquid water path. These diurnal cycle characteristics further demonstrate a tight connection between cloud brightening potential and the relationship between cloud water and droplet number at sunrise, which has implications for the impact of the timing of advertent aerosol perturbations.
Abstract The net shortwave radiative impact of aerosol on simulations of two shallow marine cloud cases is investigated using a Monte Carlo radiative transfer model. For a shallow cumulus case, increased aerosol concentrations are associated not only with smaller droplet sizes but also reduced cloud fractions and cloud dimensions, a result of evaporation-induced mixing and a lack of precipitation. Three-dimensional radiative transfer (3DRT) effects alter the fluxes by 10%–20% from values calculated using the independent column approximation for these simulations. The first (Twomey) aerosol indirect effect is dominant but the decreased cloud fraction reduces the magnitude of the shortwave cloud forcing substantially. The 3DRT effects slightly decrease the sensitivity of the cloud albedo to changes in droplet size under an overhead sun for the two ranges of cloud liquid water paths examined, but not strongly so. A popular two-stream radiative transfer approximation to the cloud susceptibility overestimates the more directly calculated values for the low liquid-water-path clouds within pristine aerosol conditions by a factor of 2 despite performing well otherwise, suggesting caution in its application to the cloud albedos within broken cloud fields. An evaluation of the influence of cloud susceptibility and cloud fraction changes to a “domain” area-weighted cloud susceptibility found that the domain cloud albedo is more likely to increase under aerosol loading at intermediate aerosol concentrations than under the most pristine conditions, contrary to traditional expectations. The second simulation (cumulus penetrating into stratus) is characterized by higher cloud fractions and more precipitation. This case has two regimes: a clean, precipitating regime where cloud fraction increases with increasing aerosol, and a more polluted regime where cloud fraction decreases with increasing aerosol. For this case the domain-mean cloud albedo increases steadily with aerosol loading under clean conditions, but increases only slightly after the cloud coverage decreases. Three-dimensional radiative transfer effects are mostly negligible for this case. Both sets of simulations suggest that aerosol-induced cloud fraction changes must be considered in tandem with the Twomey effect for clouds of small dimensions when assessing the net radiative impact, because both effects are drop size dependent and radiatively significant.
The extent to which the rain rate from shallow, liquid‐phase clouds is microphysically influenced by aerosol, and therefore drop concentration N d perturbations, is addressed through analysis of the precipitation susceptibility, S o . Previously published work, based on both models and observations, disagrees on the qualitative behavior of S o with respect to variables such as liquid water path L or the ratio between accretion and autoconversion rates. Two primary responses have emerged: (i) S o decreases monotonically with increasing L and (ii) S o increases with L , reaches a maximum, and decreases thereafter. Here we use a variety of modeling frameworks ranging from box models of (size‐resolved) collision‐coalescence, to trajectory ensembles based on large eddy simulation to explore the role of time available for collision‐coalescence t c in determining the S o response. The analysis shows that an increase in t c shifts the balance of rain production from autoconversion (a N d ‐dependent process) to accretion (roughly independent of N d ), all else (e.g., L ) equal. Thus, with increasing cloud contact time, warm rain production becomes progressively less sensitive to aerosol, all else equal. When the time available for collision‐coalescence is a limiting factor, S o increases with increasing L whereas when there is ample time available, S o decreases with increasing L . The analysis therefore explains the differences between extant studies in terms of an important precipitation‐controlling parameter, namely the integrated liquid water history over the course of an air parcel's contact with a cloud.
The effect of nitric acid (HNO 3 ) on cloud microphysical and radiative properties is studied using an adiabatic cloud parcel model for a range of aerosol size distributions, different water vapor mass accommodation coefficients, and HNO 3 concentrations. Results show that HNO 3 not only increases cloud drop number concentration N d , but also leads to significantly broader droplet size spectra at both the small‐ and large‐size ends. The broader spectra are generally the result of competition for H 2 O and HNO 3 among the polydisperse droplets. The increase in the number of activated cloud droplets in the presence of HNO 3 , and the deactivation of some of the small cloud droplets due to the outgasing of HNO 3 , lead to spectral broadening at the small‐size end. At the large‐size end the broadening is caused by an increase in the driving force for growth. For small drops the driving force tends to be decreased by the presence of HNO 3 . Although N d increases with increasing HNO 3 concentration, the increases in cloud optical depth and albedo due to HNO 3 cannot necessarily be predicted by the commonly used relationships for cloud optical properties. The dependence of the cloud optical depth on N d to the one‐third power is shown to be an overestimate because droplet spectra are significantly broadened by HNO 3 . We show that broadening effects due to HNO 3 and other chemical or microphysical factors need to be considered when estimating cloud optical properties and their effect on climate.
Abstract. The interaction between marine boundary layer cellular cloudiness and surface fluxes of sensible and latent heat is investigated. The investigation focuses on the non-precipitating closed-cell state and the precipitating open-cell state at low geostrophic wind speed. The Advanced Research WRF (Weather Research and Forecasting) model is used to conduct cloud system-resolving simulations with interactive surface fluxes of sensible heat, latent heat, and of sea salt aerosol, and with a detailed representation of the interaction between aerosol particles and clouds. The mechanisms responsible for the temporal evolution and spatial distribution of the surface heat fluxes in the closed- and open-cell state are investigated and explained. It is found that the closed-cell state imposes its horizontal spatial structure on surface air temperature and water vapor, and, to a lesser degree, on the surface sensible and latent heat flux. The responsible mechanism is the entrainment of dry, free tropospheric air into the boundary layer. The open-cell state is associated with oscillations in surface air temperature, water vapor, and in the surface fluxes of sensible heat, latent heat, and of sea salt aerosol. Here, the responsible mechanism is the periodic formation of clouds, rain, and of cold and moist pools with elevated wind speed. Open-cell cloud formation, cloud optical depth and liquid water path, and cloud and rain water path are identified as good predictors of the horizontal spatial structure of surface air temperature and sensible heat flux, but not of surface water vapor and latent heat flux. It is shown that the open-cell state creates conditions conducive to its maintenance by enhancing the surface sensible heat flux. The open-cell state also enhances the sea salt flux relative to the closed-cell state. While the open-cell state under consideration is not depleted in aerosol and is insensitive to variations in sea salt fluxes, in aerosol-depleted conditions, the enhancement of the sea salt flux may replenish the aerosol needed for cloud formation and hence contribute to the maintenance of the open-cell state. Spatial homogenization of the surface fluxes is found to have only a small effect on cloud properties in the investigated cases.
Earth and Space Science Open Archive PosterOpen AccessYou are viewing the latest version by default [v1]Bounding aerosol radiative forcing of climate changeAuthors Nicolas Bellouin iD Johannes Quaas iD Ed Gryspeerdt Stefan Kinne Philip Stier iD Duncan Watson-Parris iD Olivier Boucher Ken Carslaw Matt Christensen iD Anne-Laure Daniau iD Jean-Louis Dufresne Graham Feingold iD Stephanie Fiedler Piers Forster Andrew Gettelman iD Jim Haywood Florent Malavelle Ulrike Lohmann Thorsten Mauritsen iD Daniel McCoy Gunnar Myhre Johannes Muelmenstaedt David Neubauer iD Anna Possner Maria Rugenstein Yousuke Sato Michael Schulz iD Stephen Schwartz iD Odran Sourdeval Trude Storelvmo Velle Toll iD David Winker iD Bjorn Stevens iDSee all authors Nicolas BellouiniDCorresponding AuthorDepartment of Meteorology, University of ReadingiDhttps://orcid.org/0000-0003-2109-9559view email addressThe email was not providedcopy email addressJohannes QuaasiDUniversity of LeipzigiDhttps://orcid.org/0000-0001-7057-194Xview email addressThe email was not providedcopy email addressEd GryspeerdtImperial College Londonview email addressThe email was not providedcopy email addressStefan KinneMax Planck Institute for Meteorology, Hamburgview email addressThe email was not providedcopy email addressPhilip StieriDUniversity of OxfordiDhttps://orcid.org/0000-0002-1191-0128view email addressThe email was not providedcopy email addressDuncan Watson-ParrisiDUniversity of OxfordiDhttps://orcid.org/0000-0002-5312-4950view email addressThe email was not providedcopy email addressOlivier BoucherLaboratoire de Meteorologie Dynamiqueview email addressThe email was not providedcopy email addressKen CarslawUniversity of Leedsview email addressThe email was not providedcopy email addressMatt ChristenseniDUniversity of OxfordiDhttps://orcid.org/0000-0002-4273-6644view email addressThe email was not providedcopy email addressAnne-Laure DaniauiDEPOC UMR5805, CNRS, University of BordeauxiDhttps://orcid.org/0000-0002-1621-3911view email addressThe email was not providedcopy email addressJean-Louis DufresneLaboratoire de Meteorologie Dynamiqueview email addressThe email was not providedcopy email addressGraham FeingoldiDNOAA CSDiDhttps://orcid.org/0000-0002-0774-2926view email addressThe email was not providedcopy email addressStephanie FiedlerMax Planck Institute for Meteorology, Hamburgview email addressThe email was not providedcopy email addressPiers ForsterUniversity of Leedsview email addressThe email was not providedcopy email addressAndrew GettelmaniDNCARiDhttps://orcid.org/0000-0002-8284-2599view email addressThe email was not providedcopy email addressJim HaywoodUniversity of Exeter, UK Met Officeview email addressThe email was not providedcopy email addressFlorent MalavelleUniversity of Exeterview email addressThe email was not providedcopy email addressUlrike LohmannETH Zurichview email addressThe email was not providedcopy email addressThorsten MauritseniDStockholm UniversityiDhttps://orcid.org/0000-0003-1418-4077view email addressThe email was not providedcopy email addressDaniel McCoyUniversity of Leedsview email addressThe email was not providedcopy email addressGunnar MyhreCenter for International Climate and Environmental Research Osloview email addressThe email was not providedcopy email addressJohannes MuelmenstaedtUniversity of Leipzigview email addressThe email was not providedcopy email addressDavid NeubaueriDETH Swiss Federal Institute of Technology ZurichiDhttps://orcid.org/0000-0002-9869-3946view email addressThe email was not providedcopy email addressAnna PossnerUniversity of Frankfurtview email addressThe email was not providedcopy email addressMaria RugensteinMax Planck Institute for Meteorology, Hamburgview email addressThe email was not providedcopy email addressYousuke SatoHokudai Universityview email addressThe email was not providedcopy email addressMichael SchulziDNorwegian Meteorological InstituteiDhttps://orcid.org/0000-0003-4493-4158view email addressThe email was not providedcopy email addressStephen SchwartziDBrookhaven National LaboratoryiDhttps://orcid.org/0000-0001-6288-310Xview email addressThe email was not providedcopy email addressOdran SourdevalUniversite de Lilleview email addressThe email was not providedcopy email addressTrude StorelvmoUniversity of Osloview email addressThe email was not providedcopy email addressVelle TolliDUniversity of TartuiDhttps://orcid.org/0000-0002-8760-7803view email addressThe email was not providedcopy email addressDavid WinkeriDNASA Langley Research CenteriDhttps://orcid.org/0000-0002-3919-2244view email addressThe email was not providedcopy email addressBjorn StevensiDMax Planck Institute for Meteorology, HamburgiDhttps://orcid.org/0000-0003-3795-0475view email addressThe email was not providedcopy email address
We present a new large eddy simulation model that comprises coupled components representing size‐resolved aerosol and cloud microphysics, radiative properties of aerosol and clouds, dynamics, and a surface soil and vegetation model. The model is used to investigate the effect of increases in aerosol on liquid water path LWP, cloud fraction, optical depth, and precipitation formation in warm, continental cumulus clouds. Sets of simulations that either neglect, or include the radiative properties of a partially absorbing aerosol are performed. In the absence of aerosol radiative effects, an increase in aerosol loading results in a reduction in precipitation. However, the clouds do not experience significant changes in LWP, cloud fraction and cloud depth; aerosol effects on LWP and cloud fraction are small compared to the dynamical variability of the clouds at any given aerosol concentration. Reasons for this response are discussed. When aerosol radiative effects are included, the modification in atmospheric heating profiles, and the reduction in surface latent and sensible heat fluxes resulting from the presence of these particles, have a significant effect on cloud parameters and boundary layer evolution. For the case considered, there is a significant reduction in the strength of convection, LWP, cloud fraction and cloud depth. Cloud optical depth responds non‐monotonically to the increase in aerosol. These results indicate that in continental regions surface processes must be included in calculations of aerosol‐cloud‐precipitation interactions. Neglect of these surface processes may result in an overestimate of the second aerosol indirect effect.
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 ever-increasing 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 surface-based 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.
