scholarly journals Global anthropogenic aerosol effects on convective clouds in ECHAM5-HAM

2007 ◽  
Vol 7 (5) ◽  
pp. 14639-14674 ◽  
Author(s):  
U. Lohmann
Keyword(s):  
Liquid Water ◽  
General Circulation ◽  
Hydrological Cycle ◽  
Cloud Microphysics ◽  
Liquid Water Path ◽  
Anthropogenic Aerosol ◽  
Water Path ◽  
Convective Clouds ◽  
Stratiform Clouds ◽  
Aerosol Effects

Abstract. Aerosols affect the climate system by changing cloud characteristics in many ways. They act as cloud condensation and ice nuclei and may have an influence on the hydrological cycle. Here we investigate aerosol effects on convective clouds by extending the double moment cloud microphysics scheme developed for stratiform clouds to convective clouds in the ECHAM5 general circulation model. This increases the liquid water path in the tropics and reduces the sensitivity of the liquid water path with increasing aerosol optical depth in better agreement with observations and large-eddy simulation studies. In simulations in which greenhouse gases and aerosols emissions are increased since pre-industrial times, accounting for microphysics in convective clouds matches most closely the observed increase in precipitation. The total anthropogenic aerosol effect since pre-industrial time is slightly reduced from −1.6 to −1.9 W m−2 when microphysics are only included in stratiform clouds to −1.5 W m−2 when microphysics are included both in stratiform and convective clouds.

scholarly journals Global anthropogenic aerosol effects on convective clouds in ECHAM5-HAM

2008 ◽  
Vol 8 (7) ◽  
pp. 2115-2131 ◽  
Author(s):  
U. Lohmann
Keyword(s):  
General Circulation ◽  
Large Scale ◽  
Hydrological Cycle ◽  
Circulation Model ◽  
Cloud Microphysics ◽  
Convective Precipitation ◽  
Anthropogenic Aerosol ◽  
Convective Clouds ◽  
Double Moment ◽  
Aerosol Effects

Abstract. Aerosols affect the climate system by changing cloud characteristics in many ways. They act as cloud condensation and ice nuclei and may have an influence on the hydrological cycle. Here we investigate aerosol effects on convective clouds by extending the double-moment cloud microphysics scheme developed for stratiform clouds, which is coupled to the HAM double-moment aerosol scheme, to convective clouds in the ECHAM5 general circulation model. This enables us to investigate whether more, and smaller cloud droplets suppress the warm rain formation in the lower parts of convective clouds and thus release more latent heat upon freezing, which would then result in more vigorous convection and more precipitation. In ECHAM5, including aerosol effects in large-scale and convective clouds (simulation ECHAM5-conv) reduces the sensitivity of the liquid water path increase with increasing aerosol optical depth in better agreement with observations and large-eddy simulation studies. In simulation ECHAM5-conv with increases in greenhouse gas and aerosol emissions since pre-industrial times, the geographical distribution of the changes in precipitation better matches the observed increase in precipitation than neglecting microphysics in convective clouds. In this simulation the convective precipitation increases the most suggesting that the convection has indeed become more vigorous.

A Characterization of Tropical Transient Activity in the CAM3 Atmospheric Hydrologic Cycle

2006 ◽  
Vol 19 (11) ◽  
pp. 2222-2242 ◽  
Author(s):  
Philip J. Rasch ◽  
Mark J. Stevens ◽  
Lucrezia Ricciardulli ◽  
Aiguo Dai ◽  
Andrew Negri ◽  
...  
Keyword(s):  
Brightness Temperature ◽  
Time Scales ◽  
Liquid Water ◽  
General Circulation ◽  
Hydrological Cycle ◽  
Hydrologic Cycle ◽  
Liquid Water Path ◽  
Lower Boundary Condition ◽  
Water Path ◽  
Model Version

Abstract The Community Atmosphere Model version 3 (CAM3) is the latest generation of a long lineage of general circulation models produced by a collaboration between the National Center for Atmospheric Research (NCAR) and the scientific research community. Many aspects of the hydrological cycle have been changed relative to earlier versions of the model. It is the goal of this paper to document some aspects of the tropical variability of clouds and the hydrologic cycle in CAM3 on time scales shorter than 30 days and to discuss the differences compared to the observed atmosphere and earlier model versions, with a focus on cloud-top brightness temperature, precipitation, and cloud liquid water path. The transient behavior of the model in response to changes in resolution to various numerical methods used to solve the equations for atmospheric dynamics and transport and to the underlying lower boundary condition of sea surface temperature and surface fluxes has been explored. The ratio of stratiform to convective rainfall is much too low in CAM3, compared to observational estimates. It is much higher in CAM3 (10%) than the Community Climate Model version 3 (CCM3; order 1%–2%) but is still a factor of 4–5 too low compared to observational estimates. Some aspects of the model transients are sensitive to resolution. Higher-resolution versions of CAM3 show too much variability (both in amplitude and spatial extent) in brightness temperature on time scales of 2–10 days compared to observational estimates. Precipitation variance is underestimated on time scales from a few hours to 10 days, compared to observations over ocean, although again the biases are reduced compared to previous generations of the model. The diurnal cycle over tropical landmasses is somewhat too large, and there is not enough precipitation during evening hours. The model tends to produce maxima in precipitation and liquid water path that are a few hours earlier than that seen in the observations over both oceans and land.

scholarly journals Cloud optical thickness and liquid water path. Does the <i>k</i> coefficient vary with droplet concentration?

2011 ◽  
Vol 11 (2) ◽  
pp. 5173-5215
Author(s):  
J.-L. Brenguier ◽  
F. Burnet ◽  
O. Geoffroy
Keyword(s):  
Correction Factor ◽  
Optical Thickness ◽  
Liquid Water ◽  
General Circulation ◽  
Field Experiments ◽  
General Circulation Models ◽  
Light Extinction ◽  
Liquid Water Path ◽  
Water Path ◽  
Droplet Concentration

Abstract. Cloud radiative transfer calculations in general circulation models involve a link between cloud microphysical and optical properties. Indeed, the liquid water content expresses as a function of the mean volume droplet radius, while the light extinction is a function of their mean surface radius. There is a small difference between these two parameters because of the droplet spectrum width. This issue has been addressed by introducing an empirical multiplying correction factor to the droplet concentration. Analysis of in situ sampled data, however, revealed that the correction factor decreases when the concentration increases, hence partially mitigating the aerosol indirect effect. Five field experiments are reanalyzed here, in which standard and upgraded versions of the droplet spectrometer were used to document shallow cumulus and stratocumulus topped boundary layers. They suggest that the standard probe noticeably underestimates the correction factor compared to the upgraded versions. The analysis is further refined to demonstrate that the value of the correction factor derived by averaging values calculated locally along the flight path overestimates the value derived from liquid water path and optical thickness of a cloudy column, and that there is no detectable correlation between the correction factor and the droplet concentration. It is also shown that the droplet concentration dilution by entrainment-mixing after CCN activation is significantly stronger in shallow cumuli than in stratocumulus layers. These various effects are finally combined to produce the best estimate of the correction factor to use in general circulation models.

scholarly journals The role of droplet sedimentation in the evolution of low-level clouds over southern West Africa

2018 ◽  
Vol 18 (19) ◽  
pp. 14253-14269 ◽  
Author(s):  
Christopher Dearden ◽  
Adrian Hill ◽  
Hugh Coe ◽  
Tom Choularton
Keyword(s):  
West Africa ◽  
Liquid Water ◽  
Large Scale ◽  
Monsoon Season ◽  
Cloud Microphysics ◽  
Cloud Base ◽  
Liquid Water Path ◽  
Cloud Droplets ◽  
Water Path ◽  
Low Level

Abstract. Large-eddy simulations are performed to investigate the influence of cloud microphysics on the evolution of low-level clouds that form over southern West Africa during the monsoon season. We find that, even in clouds that are not precipitating, the size of cloud droplets has a non-negligible effect on liquid water path. This is explained through the effects of droplet sedimentation, which acts to remove liquid water from the entrainment zone close to cloud top, increasing the liquid water path. Sedimentation also produces a more heterogeneous cloud structure and lowers cloud base height. Our results imply that an appropriate parameterization of the effects of sedimentation is required to improve the representation of the diurnal cycle of the atmospheric boundary layer over southern West Africa in large-scale models.

scholarly journals Untangling causality in midlatitude aerosol–cloud adjustments

2020 ◽  
Vol 20 (7) ◽  
pp. 4085-4103 ◽  
Author(s):  
Daniel T. McCoy ◽  
Paul Field ◽  
Hamish Gordon ◽  
Gregory S. Elsaesser ◽  
Daniel P. Grosvenor
Keyword(s):  
Liquid Water ◽  
Radiative Forcing ◽  
Sensible Heat Flux ◽  
Cloud Droplet ◽  
Causal Link ◽  
Cloud Microphysics ◽  
Liquid Water Path ◽  
Water Path ◽  
Aerosol Cloud ◽  
Causal Pathways

Abstract. Aerosol–cloud interactions represent the leading uncertainty in our ability to infer climate sensitivity from the observational record. The forcing from changes in cloud albedo driven by increases in cloud droplet number (Nd) (the first indirect effect) is confidently negative and has narrowed its probable range in the last decade, but the sign and strength of forcing associated with changes in cloud macrophysics in response to aerosol (aerosol–cloud adjustments) remain uncertain. This uncertainty reflects our inability to accurately quantify variability not associated with a causal link flowing from the cloud microphysical state to the cloud macrophysical state. Once variability associated with meteorology has been removed, covariance between the liquid water path (LWP) averaged across cloudy and clear regions (here characterizing the macrophysical state) and Nd (characterizing the microphysical) is the sum of two causal pathways linking Nd to LWP: Nd altering LWP (adjustments) and precipitation scavenging aerosol and thus depleting Nd. Only the former term is relevant to constraining adjustments, but disentangling these terms in observations is challenging. We hypothesize that the diversity of constraints on aerosol–cloud adjustments in the literature may be partly due to not explicitly characterizing covariance flowing from cloud to aerosol and aerosol to cloud. Here, we restrict our analysis to the regime of extratropical clouds outside of low-pressure centers associated with cyclonic activity. Observations from MAC-LWP (Multisensor Advanced Climatology of Liquid Water Path) and MODIS are compared to simulations in the Met Office Unified Model (UM) GA7.1 (the atmosphere model of HadGEM3-GC3.1 and UKESM1). The meteorological predictors of LWP are found to be similar between the model and observations. There is also agreement with previous literature on cloud-controlling factors finding that increasing stability, moisture, and sensible heat flux enhance LWP, while increasing subsidence and sea surface temperature decrease it. A simulation where cloud microphysics are insensitive to changes in Nd is used to characterize covariance between Nd and LWP that is induced by factors other than aerosol–cloud adjustments. By removing variability associated with meteorology and scavenging, we infer the sensitivity of LWP to changes in Nd. Application of this technique to UM GA7.1 simulations reproduces the true model adjustment strength. Observational constraints developed using simulated covariability not induced by adjustments and observed covariability between Nd and LWP predict a 25 %–30 % overestimate by the UM GA7.1 in LWP change and a 30 %–35 % overestimate in associated radiative forcing.

scholarly journals Radiative–Dynamical Feedbacks in Low Liquid Water Path Stratiform Clouds

2012 ◽  
Vol 69 (5) ◽  
pp. 1498-1512 ◽  
Author(s):  
Jonathan L. Petters ◽  
Jerry Y. Harrington ◽  
Eugene E. Clothiaux
Keyword(s):  
Liquid Water ◽  
Cloud Layer ◽  
Flux Divergence ◽  
Liquid Water Path ◽  
Stratiform Cloud ◽  
Water Path ◽  
Droplet Concentration ◽  
Cloud Dynamics ◽  
Stratiform Clouds ◽  
Cloud Evolution

Abstract When stratiform-cloud-integrated radiative flux divergence (heating) is dependent on liquid water path (LWP) and droplet concentration Nd, feedbacks between cloud dynamics and this heating can exist. These feedbacks can be particularly strong for low LWP stratiform clouds, in which cloud-integrated longwave cooling is sensitive to LWP and Nd. Large-eddy simulations reveal that these radiative–dynamical feedbacks can substantially modify low LWP stratiform cloud evolution when Nd is perturbed. At night, more rapid initial evaporation of the cloud layer occurs when Nd is high, leading to more cloud breaks and lower LWP values that both result in less total cloud longwave cooling. Weakened circulations result from this reduced longwave cooling and entrainment drying is able to counteract cloud growth. When Nd is low, the cloud layer is better maintained because cloud longwave cooling is still relatively strong. During the day, the addition of shortwave warming leads to reduced LWP for all values of Nd and, consequently, further reduced longwave cooling and weakened circulations. For high Nd, these reductions are such that the cloud layer cannot be maintained. For lower Nd, the reductions are smaller and the cloud layer thins but does not dissipate. These results suggest that low LWP cloud layers are more tenuous when Nd is high and are more prone to dissipating during the day. Comparison with other studies suggests the modeled low LWP cloud response may be sensitive to the initial thermodynamic profile and model configuration.

scholarly journals Cloud optical thickness and liquid water path – does the <i>k</i> coefficient vary with droplet concentration?

2011 ◽  
Vol 11 (18) ◽  
pp. 9771-9786 ◽  
Author(s):  
J.-L. Brenguier ◽  
F. Burnet ◽  
O. Geoffroy
Keyword(s):  
Correction Factor ◽  
Optical Thickness ◽  
Liquid Water ◽  
General Circulation ◽  
Field Experiments ◽  
General Circulation Models ◽  
Light Extinction ◽  
Liquid Water Path ◽  
Water Path ◽  
Droplet Concentration

Abstract. Cloud radiative transfer calculations in general circulation models involve a link between cloud microphysical and optical properties. Indeed, the liquid water content expresses as a function of the mean volume droplet radius, while the light extinction is a function of their mean surface radius. There is a small difference between these two parameters because of the droplet spectrum width. This issue has been addressed by introducing an empirical multiplying correction factor to the droplet concentration. Analysis of in situ sampled data, however, revealed that the correction factor decreases when the concentration increases, hence partially mitigating the aerosol indirect effect. Five field experiments are reanalyzed here, in which standard and upgraded versions of the droplet spectrometer were used to document shallow cumulus and stratocumulus topped boundary layers. They suggest that the standard probe noticeably underestimates the correction factor compared to the upgraded versions. The analysis is further refined to demonstrate that the value of the correction factor derived by averaging values calculated locally along the flight path overestimates the value derived from liquid water path and optical thickness of a cloudy column, and that there is no detectable relationship between the correction factor and the droplet concentration. It is also shown that the droplet concentration dilution by entrainment-mixing after CCN activation is significantly stronger in shallow cumuli than in stratocumulus layers. These various effects are finally combined to produce the today best estimate of the correction factor to use in general circulation models.

scholarly journals Aerosol Impacts on the Diurnal Cycle of Marine Stratocumulus

2008 ◽  
Vol 65 (8) ◽  
pp. 2705-2718 ◽  
Author(s):  
Irina Sandu ◽  
Jean-Louis Brenguier ◽  
Olivier Geoffroy ◽  
Odile Thouron ◽  
Valery Masson
Keyword(s):  
Boundary Layer ◽  
Diurnal Cycle ◽  
Liquid Water ◽  
Large Scale ◽  
Cloud Microphysics ◽  
Cloud Base ◽  
Liquid Water Path ◽  
Marine Stratocumulus ◽  
Water Path ◽  
The Impact

Abstract Recent large-eddy simulation (LES) studies of the impact of aerosol on the dynamics of nocturnal marine stratocumulus revealed that, depending on the large-scale forcings, an aerosol-induced increase of the droplet concentration can lead to either an increase or a decrease of the liquid water path, hence contrasting with the cloud thickening that is expected from a reduction of the precipitation efficiency. In this study, the aerosol impacts on cloud microphysics are examined in the context of the boundary-layer diurnal cycle using 36-h LES simulations of pristine and polluted clouds. These simulations corroborate previous findings that during nighttime aerosol-induced liquid water path changes are sensitive to the large-scale forcings via enhancement of cloud-top entrainment such that, ultimately, the liquid water path may be reduced when the free-tropospheric-entrained air is drier. During the day, however, enhanced entrainment, inhibition of drizzle evaporation below cloud base, and reduced sensible heat flux from the surface lead to a more pronounced decoupling of the boundary layer, which significantly amplifies the liquid water path reduction of the polluted clouds. At night the sign of the liquid water path difference between pristine and polluted clouds depends upon large-scale forcings, while during the day the liquid water path of polluted clouds is always smaller than the one of the pristine clouds. Suggestions are made on how observational studies could be designed for validation of these simulations.

scholarly journals Model intercomparison of indirect aerosol effects

2006 ◽  
Vol 6 (1) ◽  
pp. 1579-1617 ◽  
Author(s):  
J. E. Penner ◽  
J. Quaas ◽  
T. Storelvmo ◽  
T. Takemura ◽  
O. Boucher ◽  
...  
Keyword(s):  
Liquid Water ◽  
Indirect Effect ◽  
Radiative Forcing ◽  
Cloud Fraction ◽  
Liquid Water Path ◽  
Aerosol Indirect Effect ◽  
Water Path ◽  
Series Of Experiments ◽  
Aerosol Effects ◽  
Indirect Forcing

Abstract. Modeled differences in predicted effects are increasingly used to help quantify the uncertainty of these effects. Here, we examine modeled differences in the aerosol indirect effect in a series of experiments that help to quantify how and why model-predicted aerosol indirect forcing varies between models. The experiments start with an experiment in which aerosol concentrations, the parameterization of droplet concentrations and the autoconversion scheme are all specified and end with an experiment that examines the predicted aerosol indirect forcing when only aerosol sources are specified. Although there are large differences in the predicted liquid water path among the models, the predicted aerosol indirect effect for the first experiment is rather similar. Changes to the autoconversion scheme can lead to large changes in the liquid water path of the models and to the response of the liquid water path to changes in aerosols. Nevertheless, these changes do not necessarily lead to large changes in the radiative forcing. The parameterization of cloud fraction within models is not sensitive to the aerosol concentration, and, therefore, the response of the modeled cloud fraction within the present models appears to be smaller than that which would be associated with model ''noise''. The prediction of aerosol concentrations, given a fixed set of sources, leads to some of the largest differences in the predicted aerosol indirect radiative forcing among the models. Thus, this aspect of modeling requires significant improvement in order to improve the prediction of aerosol indirect effects.

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