An official website of the United States government
Abstract Increases in aerosol concentration are well known to influence the microphysical processes and radiative properties of clouds. By reducing droplet size, an increase in aerosol can lessen collision efficiency and increase liquid water path (LWP) in precipitating clouds or enhance evaporation rate and decrease LWP in non‐precipitating clouds. We utilize large eddy simulations to further investigate these aerosol indirect effects in Arctic mixed‐phase clouds and find, in agreement with previous studies, precipitating clouds to experience an increase in LWP and non‐precipitating clouds a decrease in LWP. Most importantly however, our results reveal a different explanation for why such an LWP decrease occurs in decoupled, non‐precipitating clouds. We find enhanced evaporation near cloud top to be driven primarily by a strengthening of maximum radiative cooling rate with aerosol concentration which drives stronger entrainment, an effect that holds true even in clouds that are optically thick.
more »
« less
The Impact of Resolving Subkilometer Processes on Aerosol‐Cloud Interactions of Low‐Level Clouds in Global Model Simulations
Abstract Subkilometer processes are critical to the physics of aerosol‐cloud interaction (ACI) but have been dependent on parameterizations in global model simulations. We thus report the strength of ACI in the Ultra‐Parameterized Community Atmosphere Model (UPCAM), a multiscale climate model that uses coarse exterior resolution to embed explicit cloud‐resolving models with enough resolution (250 m horizontal, 20 m vertical) to quasi‐resolve subkilometer eddies. To investigate the impact on ACIs, UPCAM's simulations are compared to a coarser multiscale model with 4 km horizontal resolution. UPCAM produces cloud droplet number concentrations (Nd) and cloud liquid water path (LWP) values that are higher than the coarser model but equally plausible compared to observations. Our analysis focuses on the Northern Hemisphere (20–50°N) oceans, where historical aerosol increases have been largest. We find similarities in the overall radiative forcing from ACIs in the two models, but this belies fundamental underlying differences. The radiative forcing from increases in LWP is weaker in UPCAM, whereas the forcing from increases inNdis larger. Surprisingly, the weaker LWP increase is not due to a weaker increase in LWP in raining clouds, but a combination of weaker increase in LWP in nonraining clouds and a smaller fraction of raining clouds in UPCAM. The implication is that as global modeling moves toward finer than storm‐resolving grids, nuanced model validation of ACI statistics conditioned on the existence of precipitation and good observational constraints on the baseline probability of precipitation will become key for tighter constraints and better conceptual understanding.
more »
« less
- PAR ID:
- 10452371
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Advances in Modeling Earth Systems
- Volume:
- 12
- Issue:
- 11
- ISSN:
- 1942-2466
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Aerosol-cloud interactions (ACIs) are vital for regulating Earth’s climate by influencing energy and water cycles. Yet, effects of ACI bear large uncertainties, evidenced by systematic discrepancies between observed and modeled estimates. This study quantifies a major bias in ACI determinations, stemming from conventional surface or space measurements that fail to capture aerosol at the cloud level unless the cloud is coupled with land surface. We introduce an advanced approach to determine radiative forcing of ACI by accounting for cloud-surface coupling. By integrating field observations, satellite data, and model simulations, this approach reveals a drastic alteration in aerosol vertical transport and ACI effects caused by cloud coupling. In coupled regimes, aerosols enhance cloud droplet number concentration across the boundary layer more homogeneously than in decoupled conditions, under which aerosols from the free atmosphere predominantly affect cloud properties, leading to marked cooling effects. Our findings spotlight cloud-surface coupling as a key factor for ACI quantification, hinting at potential underassessments in traditional estimates.more » « less
-
Abstract. Aerosol–cloud interactions (ACIs) are considered to be the most uncertaindriver of present-day radiative forcing due to human activities. Thenonlinearity of cloud-state changes to aerosol perturbations make itchallenging to attribute causality in observed relationships of aerosolradiative forcing. Using correlations to infer causality can be challengingwhen meteorological variability also drives both aerosol and cloud changesindependently. Natural and anthropogenic aerosol perturbations from well-defined sources provide “opportunistic experiments” (also known as natural experiments) to investigate ACI in cases where causality may be more confidently inferred. These perturbations cover a wide range of locations and spatiotemporal scales, including point sources such as volcanic eruptions or industrial sources, plumes from biomass burning or forest fires, and tracks from individual ships or shipping corridors. We review the different experimental conditions and conduct a synthesis of the available satellite datasets and field campaigns to place these opportunistic experiments on a common footing, facilitating new insights and a clearer understanding of key uncertainties in aerosol radiative forcing. Cloud albedo perturbations are strongly sensitive to background meteorological conditions. Strong liquid water path increases due to aerosol perturbations are largely ruled out by averaging across experiments. Opportunistic experiments have significantly improved process-level understanding of ACI, but it remains unclear how reliably the relationships found can be scaled to the global level, thus demonstrating a need for deeper investigation in order to improve assessments of aerosol radiative forcing and climate change.more » « less
-
Abstract Aerosol‐cloud interactions (ACI) in warm clouds are the primary source of uncertainty in effective radiative forcing (ERF) during the historical period and, by extension, inferred climate sensitivity. The ERF due to ACI (ERFaci) is composed of the radiative forcing due to changes in cloud microphysics and cloud adjustments to microphysics. Here, we examine the processes that drive ERFaci using a perturbed parameter ensemble (PPE) hosted in CAM6. Observational constraints on the PPE result in substantial constraints in the response of cloud microphysics and macrophysics to anthropogenic aerosol, but only minimal constraint on ERFaci. Examination of cloud and radiation processes in the PPE reveal buffering of ERFaci by the interaction of precipitation efficiency and radiative susceptibility.more » « less
-
Abstract. Twelve months of measurements collected during the Two-ColumnAerosol Project field campaign at Cape Cod, Massachusetts, which started inthe summer of 2012, were used to investigate aerosol physical, optical, andchemical properties and their influences on the dependence of clouddevelopment on thermodynamic (i.e., lower tropospheric stability, LTS)conditions. Relationships between aerosol loading and cloud properties underdifferent dominant air-mass conditions and the magnitude of the firstindirect effect (FIE), as well as the sensitivity of the FIE to differentaerosol compositions, are examined. The seasonal variation in aerosol numberconcentration (Na) was not consistent with variations in aerosoloptical properties (i.e., scattering coefficient, σs, andcolumnar aerosol optical depth). Organics were found to have a largecontribution to small particle sizes. This contribution decreased during theparticle growth period. Under low-aerosol-loading conditions, the liquidwater path (LWP) and droplet effective radius (DER) significantly increasedwith increasing LTS, but, under high-aerosol-loading conditions, LWP and DERchanged little, indicating that aerosols significantly weakened thedependence of cloud development on LTS. The reduction in LWP and DER fromlow- to high-aerosol-loading conditions was greater in stable environments,suggesting that clouds under stable conditions are more susceptible toaerosol perturbations than those under more unstable conditions. Highaerosol loading weakened the increase in DER as LWP increased andstrengthened the increase in cloud optical depth (COD) with increasing LWP,resulting in changes in the interdependence of cloud properties. Under bothcontinental and marine air-mass conditions, high aerosol loading cansignificantly increase COD and decrease LWP and DER, narrowing theirdistributions. Magnitudes of the FIE estimated under continental air-massconditions ranged from 0.07±0.03 to 0.26±0.09 with a meanvalue of 0.16±0.03 and showed an increasing trend as LWP increased.The calculated FIE values for aerosols with a low fraction of organics aregreater than those for aerosols with a high fraction of organics. Thisimplies that clouds over regions dominated by aerosol particles containingmostly inorganics are more susceptible to aerosol perturbations, resultingin larger climate forcing, than clouds over regions dominated by organicaerosol particles.more » « less
