scholarly journals Aerosol indirect effect on warm clouds over South-East Atlantic, from co-located MODIS and CALIPSO observations

2013 ◽  
Vol 13 (1) ◽  
pp. 69-88 ◽  
Author(s):  
L. Costantino ◽  
F.-M. Bréon
Keyword(s):  
Indirect Effect ◽  
Air Entrainment ◽  
Cloud Microphysics ◽  
Aerosol Concentration ◽  
Physical Interaction ◽  
East Atlantic ◽  
Aerosol Indirect Effect ◽  
Dry Air ◽  
Cloud Properties ◽  
Low Cloud

Abstract. In this study, we provide a comprehensive analysis of aerosol interaction with warm boundary layer clouds over the South-East Atlantic. We use aerosol and cloud parameters derived from MODIS observations, together with co-located CALIPSO estimates of the layer altitudes, to derive statistical relationships between aerosol concentration and cloud properties. The CALIPSO products are used to differentiate between cases of mixed cloud-aerosol layers from cases where the aerosol is located well-above the cloud top. This technique allows us to obtain more reliable estimates of the aerosol indirect effect than from simple relationships based on vertically integrated measurements of aerosol and cloud properties. Indeed, it permits us to somewhat distinguish the effects of aerosol and meteorology on the clouds, although it is not possible to fully ascertain the relative contribution of each on the derived statistics. Consistently with the results from previous studies, our statistics clearly show that aerosol affects cloud microphysics, decreasing the Cloud Droplet Radius (CDR). The same data indicate a concomitant strong decrease in cloud Liquid Water Path (LWP), which is inconsistent with the hypothesis of aerosol inhibition of precipitation (Albrecht, 1989). We hypothesise that the observed reduction in LWP is the consequence of dry air entrainment at cloud top. The combined effect of CDR decrease and LWP decrease leads to rather small sensitivity of the Cloud Optical Thickness (COT) to an increase in aerosol concentration. The analysis of MODIS-CALIPSO coincidences also evidences an aerosol enhancement of low cloud cover. Surprisingly, the Cloud Fraction (CLF) response to aerosol invigoration is much stronger when (absorbing) particles are located above cloud top than in cases of physical interaction. This result suggests a relevant aerosol radiative effect on low cloud occurrence: absorbing particles above the cloud top may heat the corresponding atmosphere layer, decrease the vertical temperature gradient, increase the low tropospheric stability and provide favourable conditions for low cloud formation. We also analyse the impact of anthropogenic aerosols on precipitation, through the statistical analysis of CDR-COT co-variations. A COT value of 10 is found to be the threshold beyond which precipitation is mostly formed, in both clean and polluted environments. For larger COT, polluted clouds show evidence of precipitation suppression. Results suggest the presence of two competing mechanisms governing LWP response to aerosol invigoration: a drying effect due to aerosol enhanced entrainment of dry air at cloud top (predominant for optically thin clouds) and a moistening effect due to aerosol inhibition of precipitation (predominant for optically thick clouds).

scholarly journals Aerosol indirect effect on warm clouds over South-East Atlantic, from co-located MODIS and CALIPSO observations

2012 ◽  
Vol 12 (6) ◽  
pp. 14197-14246 ◽  
Author(s):  
L. Costantino ◽  
F.-M. Bréon
Keyword(s):  
Cloud Microphysics ◽  
Aerosol Concentration ◽  
Droplet Radius ◽  
Physical Interaction ◽  
Aerosol Layer ◽  
Anthropogenic Aerosols ◽  
East Atlantic ◽  
Dry Air ◽  
Low Cloud ◽  
The Impact

Abstract. In this study, we provide a comprehensive analysis of aerosol interaction with warm boundary layer clouds, over South-East Atlantic. We use MODIS retrievals to derive statistical relationships between aerosol concentration and cloud properties, together with co-located CALIPSO estimates of cloud and aerosol layer altitudes. The latter are used to differentiate between cases of mixed and interacting cloud-aerosol layers from cases where the aerosol is located well-above the cloud top. This strategy allows, to a certain extent, to isolate real aerosol-induced effect from meteorology. Similar to previous studies, statistics clearly show that aerosol affects cloud microphysics, decreasing the Cloud Droplet Radius (CDR). The same data indicate a concomitant strong decrease in cloud Liquid Water Path (LWP), in evident contrast with the hypothesis of aerosol inhibition of precipitation (Albrecht, 1989). Because of this water loss, probably due to the entrainment of dry air at cloud top, Cloud Optical Thickness (COT) is found to be almost insensitive to changes in aerosol concentration. The analysis of MODIS-CALIPSO coincidences also evidenced an aerosol enhancement of low cloud cover. Surprising, the Cloud Fraction (CLF) response to aerosol invigoration is much stronger when (absorbing) particles are located above cloud top, than in cases of physical interaction, This result suggests a relevant aerosol radiative effect on low cloud occurrence. Heating the atmosphere above the inversion, absorbing particles above cloud top may decrease the vertical temperature gradient, increase the low tropospheric stability and provide favorable conditions for low cloud formation. We also focus on the impact of anthropogenic aerosols on precipitation, through the statistical analysis of CDR-COT co-variations. A COT value of 10 is found to be the threshold beyond which precipitation mostly forms, in both clean and polluted environments. For larger COT, polluted clouds showed evidence of precipitation suppression. Results suggest the presence of two competing mechanisms governing LWP response to aerosol invigoration: a drying effect due to aerosol enhanced entrainment of dry air at cloud top (predominant for optically thin clouds) and a moistening effect due to aerosol inhibition of precipitation (predominant for optically thick clouds).

scholarly journals Large-Eddy Simulations of Trade Wind Cumuli: Investigation of Aerosol Indirect Effects

2006 ◽  
Vol 63 (6) ◽  
pp. 1605-1622 ◽  
Author(s):  
Huiwen Xue ◽  
Graham Feingold
Keyword(s):  
Indirect Effect ◽  
Cloud Microphysics ◽  
Cloud Fraction ◽  
Aerosol Concentration ◽  
Trade Wind ◽  
Liquid Water Path ◽  
Cloud Optical Depth ◽  
Aerosol Indirect Effect ◽  
Cloud Parameters ◽  
Large Eddy

Abstract The effects of aerosol on warm trade cumulus clouds are investigated using a large-eddy simulation with size-resolved cloud microphysics. It is shown that, as expected, increases in aerosols cause a reduction in precipitation and an increase in the cloud-averaged liquid water path (LWP). However, for the case under study, cloud fraction, cloud size, cloud-top height, and depth decrease in response to increasing aerosol concentration, contrary to accepted hypotheses associated with the second aerosol indirect effect. It is found that the complex responses of clouds to aerosols are determined by competing effects of precipitation and droplet evaporation associated with entrainment. As aerosol concentration increases, precipitation suppression tends to maintain the clouds and lead to higher cloud LWP, whereas cloud droplets become smaller and evaporate more readily, which tends to dissipate the clouds and leads to lower cloud fraction, cloud size, and depth. An additional set of experiments with higher surface latent heat flux, and hence higher LWP and drizzle rate, was also performed. Changes in cloud properties due to aerosols have the same trends as in the base runs, although the magnitudes of the changes are larger. Evidence for significant stabilization (or destabilization) of the subcloud layer due to drizzle is not found, mainly because drizzling clouds cover only a small fraction of the domain. It is suggested that cloud fraction may only increase with increasing aerosol loading for larger clouds that are less susceptible to entrainment and evaporation. Finally, it is noted that at any given aerosol concentration the dynamical variability in bulk cloud parameters such as LWP tends to be larger than the aerosol-induced changes in these parameters, indicating that the second aerosol indirect effect may be hard to measure in this cloud type. The variability in cloud optical depth is, however, dominated by changes in aerosol, rather than dynamics.

scholarly journals Explicit representation of subgrid variability in cloud microphysics yields weaker aerosol indirect effect in the ECHAM5-HAM2 climate model

2014 ◽  
Vol 14 (10) ◽  
pp. 15523-15543
Author(s):  
J. Tonttila ◽  
H. Järvinen ◽  
P. Räisänen
Keyword(s):  
Indirect Effect ◽  
Climate Models ◽  
Climate Model ◽  
Cloud Microphysics ◽  
Anthropogenic Aerosols ◽  
Aerosol Indirect Effect ◽  
Cloud Properties ◽  
Microphysical Processes ◽  
Cloud Activation ◽  
The Impact

Abstract. Impacts of representing cloud microphysical processes in a stochastic subcolumn framework are investigated, with emphasis on estimating the aerosol indirect effect. It is shown that subgrid treatment of cloud activation and autoconversion of cloud water to rain reduce the impact of anthropogenic aerosols on cloud properties and thus reduce the global mean aerosol indirect effect by 18%, from 1.59 to 1.30 W m−2. Although the results show the importance of considering subgrid variability in the treatment of autoconversion, representing several processes in a self-consistent subgrid framework is emphasized. This paper provides direct evidence that omitting subgrid variability in cloud microphysics significantly contributes to the apparently chronic overestimation of the aerosol indirect effect by climate models, as compared to satellite-based estimates.

International Satellite Cloud Climatology Project: Extending the Record

2021 ◽  
pp. 1-62
Author(s):  
William B. Rossow ◽  
Kenneth R. Knapp ◽  
Alisa H Young
Keyword(s):  
Product Design ◽  
Cloud Amount ◽  
Cloud Microphysics ◽  
Temperature Threshold ◽  
Ancillary Data ◽  
Total Cloud Amount ◽  
High Cloud ◽  
Enso Events ◽  
Cloud Properties ◽  
Low Cloud

AbstractISCCP continues to quantify the global distribution and diurnal-to-interannual variations of cloud properties in a revised version. This paper summarizes assessments of the previous version, describes refinements of the analysis and enhanced features of the product design, discusses the few notable changes in the results, and illustrates the long-term variations of global mean cloud properties and differing high cloud changes associated with ENSO. The new product design includes a global, pixel-level product on a 0.1°?grid, all other gridded products at 1.0°-equivalent equal-area, separate-satellite products with ancillary data for regional studies, more detailed, embedded quality information, and all gridded products in netCDF format. All the data products including all input data), expanded documentation, the processing code and an Operations Guide are available online. Notable changes are: (1) a lowered ice-liquid temperature threshold, (2) a treatment of the radiative effects of aerosols and surface temperature inversions, (3) refined specification of the assumed cloud microphysics, and (4) interpolation of the main daytime cloud information overnight. The changes very slightly increase the global monthly mean cloud amount with a little more high and a little less middle and low cloud. Over the whole period, total cloud amount slowly decreases caused by decreases in cumulus/altocumulus; consequently, average cloud top temperature and optical thickness have increased. The diurnal and seasonal cloud variations are very similar to earlier versions. Analysis of the whole record shows that high cloud variations, but not low clouds, exhibit different patterns in different ENSO events.

scholarly journals Influence of Giant CCN on warm rain processes in the ECHAM5 GCM

2008 ◽  
Vol 8 (14) ◽  
pp. 3769-3788 ◽  
Author(s):  
R. Posselt ◽  
U. Lohmann
Keyword(s):  
Indirect Effect ◽  
General Circulation ◽  
Hydrological Cycle ◽  
Precipitation Amount ◽  
Circulation Model ◽  
Anthropogenic Activity ◽  
Total Water ◽  
Anthropogenic Aerosol ◽  
Aerosol Indirect Effect ◽  
Cloud Properties

Abstract. Increased Cloud Condensation Nuclei (CCN) load due to anthropogenic activity might lead to non-precipitating clouds because the cloud drops become smaller (for a constant liquid water content) and, therefore, less efficient in rain formation (aerosol indirect effect). Adding giant CCN (GCCN) into such a cloud can initiate precipitation (namely, drizzle) and, therefore, might counteract the aerosol indirect effect. The effect of GCCN on global climate on warm clouds and precipitation within the ECHAM5 General Circulation Model (GCM) is investigated. Therefore, the newly introduced prognostic rain scheme (Posselt and Lohmann, 2007) is applied so that GCCN are directly activated into rain drops. The ECHAM5 simulations with incorporated GCCN show that precipitation is affected only locally. On the global scale, the precipitation amount does not change. Cloud properties like total water (liquid + rain water) and cloud drop number show a larger sensitivity to GCCN. Depending on the amount of added GCCN, the reduction of total water and cloud drops account for up to 20% compared to the control run without GCCN. Thus, the incorporation of the GCCN accelerate the hydrological cycle so that clouds precipitate faster (but not more) and less condensed water is accumulated in the atmosphere. An estimate of the anthropogenic aerosol indirect effect on the climate is obtained by comparing simulations for present-day and pre-industrial climate. The introduction of the prognostic rain scheme lowered the anthropogenic aerosol indirect effect significantly compared to the standard ECHAM5 with the diagnostic rain scheme. The incorporation of the GCCN changes the model state, especially the cloud properties like TWP and Nl. The precipitation changes only locally but globally the precipitation is unaffected because it has to equal the global mean evaporation rate. Changing the cloud properties leads to a local reduction of the aerosol indirect effect and, hence, partly compensating for the increased anthropogenic CCN concentrations in that regions. Globally, the aerosol indirect effect is nearly the same for all simulations.

Indications of a Decrease in the Depth of Deep Convective Cores with Increasing Aerosol Concentration During the CACTI Campaign

2021 ◽  
Keyword(s):  
Indirect Effect ◽  
Deep Layer ◽  
Mixed Boundary ◽  
Convective Available Potential Energy ◽  
Vertical Wind Shear ◽  
Aerosol Concentration ◽  
Meteorological Variables ◽  
Condensation Nuclei ◽  
Study Region ◽  
Aerosol Indirect Effect

Abstract An aerosol indirect effect on deep convective cores (DCCs), by which increasing aerosol concentration increases cloud-top height via enhanced latent heating and updraft velocity, has been proposed in many studies. However, the magnitude of this effect remains uncertain due to aerosol measurement limitations, modulation of the effect by meteorological conditions, and difficulties untangling meteorological and aerosol effects on DCCs. The Cloud, Aerosol, and Complex Terrain Interactions (CACTI) campaign in 2018-19 produced concentrated aerosol and cloud observations in a location with frequent DCCs, providing an opportunity to examine the proposed aerosol indirect effect on DCC depth in a rigorous and robust manner. For periods throughout the campaign with well mixed boundary layers, we analyze relationships that exist between aerosol variables (condensation nuclei concentration >10 nm, 0.4% cloud condensation nuclei concentration, 55-1000 nm aerosol concentration, and aerosol optical depth) and meteorological variables [level of neutral buoyancy (LNB), convective available potential energy, mid-level relative humidity, and deep layer vertical wind shear] with the maximum radar echo top height and cloud-top temperature (CTT) of DCCs. Meteorological variables such as LNB and deep-layer shear are strongly correlated with DCC depth. LNB is also highly correlated with three of the aerosol variables. After accounting for meteorological correlations, increasing values of the aerosol variables (with the exception of one formulation of AOD) are generally correlated at a statistically significant level with a warmer CTT of DCCs. Therefore, for the study region and period considered, increasing aerosol concentration is mostly associated with a decrease in DCC depth.

scholarly journals The first aerosol indirect effect quantified through airborne remote sensing during VOCALS-REx

2012 ◽  
Vol 12 (9) ◽  
pp. 25441-25485
Author(s):  
D. Painemal ◽  
P. Zuidema
Keyword(s):  
Remote Sensing ◽  
Indirect Effect ◽  
Climate Models ◽  
Remotely Sensed ◽  
Cloud Microphysics ◽  
Liquid Water Path ◽  
Aerosol Indirect Effect ◽  
Southeast Pacific ◽  
Millimeter Wave Radiometer

Abstract. The first aerosol indirect effect (1AIE) is investigated using a combination of in situ and remotely-sensed aircraft (NCAR C-130) observations acquired during VOCALS-REx over the Southeast Pacific stratocumulus cloud regime. Satellite analyses have previously identified a high albedo susceptibitility to changes in cloud microphysics and aerosols over this region. The 1AIE was broken down into the product of two independently-estimated terms: the cloud aerosol interaction metric ACIτ =dln τ/dln Na|LWP, and the relative albedo (A) susceptibility SR-τ = dA/3dln τ|LWP, with τ and Na denoting retrieved cloud optical thickness and in-situ aerosol concentration, respectively and calculated for fixed intervals of liquid water path (LWP). ACIτ was estimated by combining in-situ Na sampled below the cloud, with τ and LWP derived from, respectively, simultaneous upward-looking broadband irradiance and narrow field-of-view millimeter-wave radiometer measurements, collected at 1 Hz during four eight-hour daytime flights by the C-130 aircraft. ACIτ values were typically large, close to the physical upper limit (0.33), increasing with LWP. The high ACIτ values were in agreement with other in-situ airborne studies in pristine marine stratocumulus and reflect the imposition of a LWP constraint and simultaneity of aerosol and cloud measurements. SR-τ increased with LWP and τ, reached a maximum SR-τ (0.086) for LWP (τ) of 58 g m−2 (13–14), decreasing slightly thereafter. The net first aerosol indirect effect thus increased over the LWP range of 30–80 g m−2. These values were consistent with satellite estimates derived from instantaneous, collocated CERES albedo and MODIS-retrieved droplet number concentrations at 50 km resolution. The consistency of the airborne and satellite estimates (for airborne remotely sensed Nd < 1100 cm−3), despite their independent approaches, differences in observational scales, and retrieval assumptions, is hypothesized to reflect the robust remote sensing conditions for these homogeneous clouds. We recommend the Southeast Pacific for a regional assessment of the first aerosol indirect effect in climate models on this basis.

scholarly journals Influence of Giant CCN on warm rain processes in the ECHAM5 GCM

2007 ◽  
Vol 7 (5) ◽  
pp. 14767-14811 ◽  
Author(s):  
R. Posselt ◽  
U. Lohmann
Keyword(s):  
Wind Speed ◽  
Liquid Water ◽  
Indirect Effect ◽  
Water Mass ◽  
Cloud Condensation Nuclei ◽  
Sea Salt ◽  
Rain Water ◽  
Anthropogenic Aerosol ◽  
Aerosol Indirect Effect ◽  
Cloud Properties

Abstract. Increased Cloud Condensation Nuclei (CCN) load due to anthropogenic activity might lead to non-precipitating clouds because the cloud drops become smaller (for a constant liquid water content) and, therefore, less efficient in rain formation (aerosol indirect effect). Adding giant CCN (GCCN) into such a cloud can initiate precipitation (namely, drizzle) and, therefore, might counteract the aerosol indirect effect. The effect of GCCN on global climate, especially on clouds and precipitation, within a General Circulation Model (GCM) is investigated. GCCN are aerosol particles larger than 5–10 μm in radius that can act as cloud condensation nuclei. One prominent GCCN species is sea salt. Sea salt concentrations depend mainly on wind speed but also on relative humidity, stability and precipitation history. Natural variability is much larger than the simulated one because sea salt emissions within ECHAM5 are a function of wind speed only. Giant sea salt concentrations in ECHAM5 are determined by using the tail of the coarse mode aerosol distribution with cutoff radii of 5 μm or 10 μm. It is assumed that activated GCCN particles directly form rain drops (of 25 μm size). Thereby, the added rain water mass and number stems from the redistribution of the condensed water into cloud and rain water according to the number of activated GCCN. As the formed precipitation is most likely drizzle with rather small drops a prognostic rain scheme is applied to account for the lower fall speeds and, therefore, slower sedimentation of the drizzle drops. The ECHAM5 simulations with incorporated GCCN show that precipitation is affected only locally. Cloud properties like liquid water and cloud drop number show a larger sensitivity to GCCN. On the one hand, the increased rain water mass causes an increase in the accretion rate and, therefore, in the rain production. On the other hand, very high GCCN concentrations can lead to an artificially exaggerated transfer of cloud water to the rain class which then results in a strong decrease of the conversion rate and the rain production. The introduction of the GCCN reduces the anthropogenic increase of liquid water in the atmosphere from pre-industrial to present day because clouds are precipitating faster in the presence of the GCCN. Hence, the accumulation of liquid water in the atmosphere is reduced. According to those changes in the cloud properties, the radiative budget is also changing. The GCCN cause a reduction of the anthropogenic aerosol indirect effect of about 0.1–0.25 W m−2 which corresponds to 5–10% of the total effect. Thus, the GCCN in ECHAM5 partly offset the anthropogenic aerosol indirect effect.

scholarly journals Long-term observation of aerosol–cloud relationships in the Mid-Atlantic of the United States

2014 ◽  
Vol 14 (13) ◽  
pp. 18943-18960 ◽  
Author(s):  
S. Li ◽  
E. Joseph ◽  
Q. Min ◽  
B. Yin
Keyword(s):  
Indirect Effect ◽  
The United States ◽  
Aerosol Size Distribution ◽  
Cloud Droplets ◽  
Field Station ◽  
Aerosol Indirect Effect ◽  
Aerosol Loading ◽  
Cloud Properties ◽  
The Mean

Abstract. Long-term ground-based observations (2006 to 2010) of aerosol and cloud properties derived from passive radiometric sensors deployed at an atmospheric measurement field station in the Baltimore–Washington corridor operated by Howard University were used to examine aerosol indirect effect on cloud optical depth (COD), liquid water path (LWP), cloud droplets effective radius (Re) and cloud droplets number concentration (Nd). A higher frequency of clouds with large COD (> 20) and small Re (< 7 m) was found during summer of 2006 and 2007 along with higher frequency of abundant aerosol loading. The five-year data are screened for summer months only and are separated into clean and polluted cases based on aerosol particulate matter with aerodynamic diameter ≤ 2.5 m (PM2.5) value. Evidence of aerosol indirect effect is found where for polluted cases the mean and median values of COD and Nd distributions were elevated while the mean and median values of Re were decreased. Further reinforcing this conclusion is the result that the mean and median values of LWP distributions did not show prominent difference between clean and polluted cases, this implies that differences between the two cases of influential factors on cloud properties were relatively controlled. Moreover aerosol indirect effects were found insignificant when LWP was small but significant when LWP was large through the analysis of sensitivity of Nd to LWP under different aerosol loading and the measurements of aerosol size distribution.

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