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Aerosol Cloud Interactions in Southeast Pacific Stratocumulus

Aerosol Cloud Interactions in Southeast Pacific Stratocumulus
Author: Rhea George
Publisher:
Total Pages: 145
Release: 2013
Genre: Aerosols
ISBN:
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The influence of anthropogenic aerosols on cloud radiative properties in the persistent southeast Pacific stratocumulus deck is investigated using MODIS satellite observations, in situ data from the VAMOS Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS-REx), and WRF-Chem, a regional model with interactive chemistry and aerosols. An albedo proxy is derived based on the fractional coverage of low cloud (a macrophysical field) and the cloud albedo, with the latter broken down into contributions from microphysics (cloud droplet concentration, N[subscript d] and macrophysics (liquid water path). Albedo variability is dominated by low cloud fraction variability, except within 10-15° of the South American coast, where cloud albedo variability contributes significantly. Covariance between cloud fraction and cloud albedo also contributes significantly to the variance in albedo, which highlights how complex and inseparable the factors controlling albedo are. N[subscript d] variability contributes only weakly, which emphasizes that attributing albedo variability to the indirect effects of aerosols against the backdrop of natural meteorological variability is extremely challenging. Specific cases of aerosol changes can have strong impacts on albedo. We identify a pathway for periodic anthropogenic aerosol transport to the unpolluted marine stratocumulus>1000 km offshore, which strongly enhances N[subscript d] and albedo in in zonally-elongated `hook'-shaped arc. Hook development occurs with N[subscript d] increasing to polluted levels over the remote ocean primarily due to entrainment of a large number of small aerosols from the free troposphere that contribute a relatively small amount of aerosol mass to the marine boundary layer. Strong, deep offshore flow needed to transport continental aerosols to the remote ocean is favored by a trough approaching the South American coast and a southeastward shift of the climatological subtropical high pressure system. DMS significantly influences the aerosol number and size distributions, but does not cause hooks. The Twomey effect contributes 50-80% of the total aerosol indirect effect (AIE) both near sources and offshore during hook events. Meteorological variability between simulations can swamp the signal of AIEs, particularly due to the binary model cloud fraction field and distinguishing AIE requires determination of appropriate spatial and temporal averaging scales over which AIE is significant above this noise.

Investigation of the Cloud Microphysics and Albedo Susceptibility of the Southeast Pacific Stratocumulus Cloud Deck

Investigation of the Cloud Microphysics and Albedo Susceptibility of the Southeast Pacific Stratocumulus Cloud Deck
Author: David Painemal
Publisher:
Total Pages:
Release: 2011
Genre:
ISBN:
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Marine stratocumulus cloud regimes exert a strong climatic influence through their high solar reflectivity. Human-induced changes in stratocumulus clouds, attributed to an increase of the aerosol burden (indirect effects), can be significant given the cloud decks proximity to the continents; nevertheless, the magnitude and the final climatic consequences of these changes are uncertain. This thesis investigates further the interactions between aerosols, cloud microphysics, regional circulation, and radiative response in the Southeast Pacific stratocumulus cloud deck, one of the largest and most persistent cloud regimes in the planet. Specifically, three different aspects are addressed by this thesis: The importance of the synoptic atmospheric variability in controlling cloud microphysical and radiative changes, a validation analysis of satellite retrievals of cloud microphysics from MOderate Resolution Imaging Spectroradiometer (MODIS), and the quantitative assessments of cloud aerosol interactions along with their associated radiative forcing using primarily aircraft remote sensing data. Synoptic and satellite-derived cloud property variations for the Southeast Pacific region associated with changes in coastal satellite-derived cloud droplet number concentration (Nd) are analyzed through a composite technique. MAX and MIN Nd composites are defined by the top and bottom terciles of daily area-mean Nd values over the Arica Bight, the region with the largest mean oceanic Nd, for the five October months of 2001, 2005, 2006, 2007, and 2008. The MAX-Nd composite is characterized by a weaker subtropical anticyclone and weaker winds than the MIN-Nd composite. Additionally, the MAX-Nd composite clouds over the Arica Bight are thinner than the MIN-Nd composite clouds, have lower cloud tops, lower near-coastal cloud albedos, and occur below warmer and drier free tropospheres. At 85 ̊W, the top-of-atmosphere shortwave fluxes are significantly higher (50%) for the MAX-Nd, with thicker, lower clouds and higher cloud fractions than for the MIN-Nd. The change in Nd at this location is small, suggesting that the MAX-MIN Nd composite differences in radiative properties primarily reflects synoptic changes. The ability of MODIS level 2 retrievals to represent the cloud microphysics is assessed with in-situ measurements of droplet size distributions, collected during the VAMOS Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS-REx). The MODIS cloud optical thickness (t) correlates well with the in-situ values with a positive bias (1.42). In contrast, the standard 2.1 micron-derived MODIS cloud effective radius (r_e) is found to systematically exceed the in-situ cloud-top r_e, with a mean bias of 2.08 um. Three sources of errors that could contribute to the MODIS r_e positive bias are investigated further: the spread of the cloud droplet size distribution, the presence of a separate drizzle mode, and the sensor viewing angles. The sensor zenith viewing angles were found to have little impact, while the algorithm assumption about the cloud droplet spectra and presence of a precipitation mode could affect the retrievals but not by enough to fully explain the positive MODIS r_e bias. The droplet spectra effects account for r_e offsets smaller than 0.6 um, 0.9 um, and 1.6 um for non-drizzling, light-drizzling, and heavy-drizzling clouds respectively. An explanation for the observed MODIS bias is lacking although three-dimensional radiative effects were not considered. This investigation supports earlier studies documenting a similar bias, this time using data from newer probes. MODIS r_e and t were also combined to estimate a liquid water path (LWP) and Nd. A positive bias was also apparent in LWP, and attributed to r_e. However, when selected appropriate parameters a priori, the MODIS Nd estimate was found to agree the best with the insitu aircraft observations of the four MODIS variables. Lastly, the first aerosol indirect effect (Twomey effect) is explicitly investigated with VOCALS-REx observations, collected during three daytime research flights (Nov 9, 11, and 13), utilizing an aerosol-cloud interactions metric, and defined as ACI=dln(t)/dln(Na), with Na corresponding to the accumulation mode aerosol concentration, t derived from a broadband pyranometer, and ACI binned by cloud LWP derived from a millimeter-wavelength radiometer. Aircraft remote sensing estimates of the ACI, during sub-cloud transects, show that the cloud aerosol-interactions are strong and close to the maximum theoretical value for thin clouds, with a decrease of ACI with LWP. Although an explanation for the dependence of ACI on LWP is lacking, we found that a decrease in ACI with LWP is associated with decreases in both surface meridional winds and Nd. Similar to ACI, albedo fractional changes due to Nd fractional changes also tended to be smaller for higher LWPs, but with an overall radiative forcing larger than conservative global estimates obtained in global circulation models. The findings of this thesis emphasize the strong stratocumulus albedo response to an aerosol perturbation and its dependence on the regional scale atmospheric configuration. The results presented here can be used as a benchmark for testing regional and climate models, as well as helping to improve the current parameterizations of the first aerosol indirect effect.

A Lagrangian Study of Southeast Pacific Boundary Layer Clouds

A Lagrangian Study of Southeast Pacific Boundary Layer Clouds
Author: Gallia Painter
Publisher:
Total Pages: 47
Release: 2012
Genre: Boundary layer (Meteorology)
ISBN:
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Low clouds lie at the heart of climate feedback uncertainties. The representation of clouds in global climate models relies on parameterization of many sub-grid scale processes that are crucial to understanding cloud responses to climate; low clouds in particular exist as a result of tightly coupled microphysical, mesoscale, and synoptic mechanisms. The influence of anthropogenic aerosols on cloud properties could have important ramifications for our understanding of how clouds respond to a changing climate. The VAMOS Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS REx) sampled the persistent stratocumulus cloud deck located off the coast of Peru and Chile in the southeastern Pacific ocean. Several cloud features found in the stratocumulus deck during VOCALS exhibit signs of interesting aerosol-cloud interactions, including pockets of open cells (POCs). POCs are regions of open-cellular convection surrounded by closed cell stratocumulus, exhibiting not only a marked transition in mesoscale organization and cloud morphology, but also sharp microphysical gradients (especially in droplet concentration) across the boundary between open-cellular and closed cellular convection. In addition, precipitation is often higher at the POC boundaries, hinting at the importance of precipitation in driving their formation. In order to evaluate the microphysical characteristics of POCs prior cloud breakup, we use Lagrangian trajectories coupled with geostationary satellite imagery and cloud retrievals, as well as observational data from VOCALS REx and model data. In three of our case studies, we found regions of anomalously low droplet concentration 18-24 hours prior to POC formation (coupled with liquid water path similar to or higher than surrounding cloud), supporting a precipitation driven mechanism for POC formation. Another group of features with interesting aerosol-cloud interactions observed during VOCALS were mesoscale hook-like features of high droplet concentration which extend far offshore into regions of normally very clean cloud. We use Lagrangian trajectories to investigate the source of the high droplet concentrations of the mesoscale "hooks," and evaluate whether boundary layer transport of coastal pollutants alone can account for their extent. We find that boundary layer trajectories past 85 W do not pass sufficiently close to the coastline to explain high aerosol concentrations offshore.

Aerosol-Cloud Interactions from Urban, Regional, to Global Scales

Aerosol-Cloud Interactions from Urban, Regional, to Global Scales
Author: Yuan Wang
Publisher: Springer
Total Pages: 100
Release: 2015-05-05
Genre: Science
ISBN: 3662471752
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The studies in this dissertation aim at advancing our scientific understandings about physical processes involved in the aerosol-cloud-precipitation interaction and quantitatively assessing the impacts of aerosols on the cloud systems with diverse scales over the globe on the basis of the observational data analysis and various modeling studies. As recognized in the Fifth Assessment Report by the Inter-government Panel on Climate Change, the magnitude of radiative forcing by atmospheric aerosols is highly uncertain, representing the largest uncertainty in projections of future climate by anthropogenic activities. By using a newly implemented cloud microphysical scheme in the cloud-resolving model, the thesis assesses aerosol-cloud interaction for distinct weather systems, ranging from individual cumulus to mesoscale convective systems. This thesis also introduces a novel hierarchical modeling approach that solves a long outstanding mismatch between simulations by regional weather models and global climate models in the climate modeling community. More importantly, the thesis provides key scientific solutions to several challenging questions in climate science, including the global impacts of the Asian pollution. As scientists wrestle with the complexities of climate change in response to varied anthropogenic forcing, perhaps no problem is more challenging than the understanding of the impacts of atmospheric aerosols from air pollution on clouds and the global circulation.

Observational and Numerical Studies of the Boundary Layer, Cloud, and Aerosol Variability in the Southeast Pacific Coastal Marine Stratocumulus

Observational and Numerical Studies of the Boundary Layer, Cloud, and Aerosol Variability in the Southeast Pacific Coastal Marine Stratocumulus
Author: Xue Zheng
Publisher:
Total Pages:
Release: 2012
Genre:
ISBN:
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This dissertation investigates the impacts of meteorological factors and aerosol indirect effects on the costal marine stratocumulus (Sc) variations in the southeast Pacific, a region that has been largely unexplored and is a major challenge of the modeling community, through both observational and numerical studies. This study provides a unique dataset for documenting the characteristics of the marine Sc-topped BL off the coast of Northern Chile. The observational study shows that the boundary layer (BL) over this region was well mixed and topped by a thin and non-drizzling Sc layer on days synoptically-quiescent with little variability between this region and the coast. The surface wind, the surface fluxes and the BL turbulence appeared to be weaker than those over other ocean regions where stratocumulus clouds exist. The weaker turbulence in the BL may contribute to a relatively low entrainment rate calculated from the near cloud top fluxes. This in-situ data set can help us better understand cloud processes within this coastal regime, and also be valuable for the calibration of the satellite retrievals and the evaluation of numerical models operating at a variety of scales. A strong positive correlation between the liquid water path (LWP) and the cloud condensation nuclei (CCN) was observed under similar boundary layer conditions. This correlation cannot be explained by some of the hypotheses based on previous modeling studies. The satellite retrievals obtained upstream one day prior to the flight observations reveal some sign that the clouds under the high CCN concentrations have minimal LWP loss due to precipitation suppression effects. The results from large eddy simulations with a two-momentum bulk microphysics scheme under different idealized environment scenarios based on aircraft observations indicate that 1) the simulated Sc responds more quickly to changes in large-scale subsidence than to those changes in surface fluxes, free-tropospheric humidity, and the BL-top stability; 2) large-scale vertical wind shear clearly induces cloud-top mixing and enhances entrainment rate; 3) the solar radiation could weaken the BL turbulence, reduce the entrainment rate and decouple the BL; and 4) the impact of the reduced cloud sedimentation due to increasing aerosol on the cloud is small.

Exploring Meteorological and Biomass Burning Aerosol Influences on Marine Stratocumulus in the Southeast Atlantic Using WRF-chem

Exploring Meteorological and Biomass Burning Aerosol Influences on Marine Stratocumulus in the Southeast Atlantic Using WRF-chem
Author: Eowyn Baughman
Publisher:
Total Pages: 153
Release: 2014
Genre: Atmospheric aerosols
ISBN:
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Biomass burning in southern Africa exists in a complex series of relationships with its adjacent and remote environments. Gas and aerosol emissions can be lofted into the free troposphere and undergo long-range transport in the prevailing easterlies. The same large-scale subsidence that drives continental fires during the dry season also promotes extensive stratocumulus clouds in the subtropical southeast Atlantic Ocean. Biomass burning aerosols often exist as layers lofted above stratocumulus; where clouds and aerosol layers intersect, the aerosols can influence cloud microphysics and radiative properties. Meteorological patterns also modulate aerosol transport and clouds, but there is ongoing debate in the literature regarding whether this ultimately makes a difference or if aerosol effects are dominant. Few efforts have been made to model this system. In this research, the regional weather-chemistry model WRF-Chem is used to simulate the interaction of biomass burning aerosols with stratocumulus clouds in the Southeast Atlantic to explore how clouds respond to aerosols and to synoptic-scale meteorological variability. At present, absorbing aerosols are excluded; their role will be explored separately in future research. Comparing simulations with biomass burning present or absent, we found increased liquid water path (LWP), reduced lower tropospheric stability (LTS), reduced low cloud, and reduced effective cloud droplet radius. Comparing strong with weak offshore flow indicative of synoptic-scale meteorological variability, we found similarly strong but opposite sign response in LWP, LTS, and low cloud amount. Intriguingly, the offshore flow pattern was associated with greater distance between aerosol and cloud layers, reducing the incidence of cloud-aerosol interactions during peak aerosol loading. We conclude that the meteorological variability driving both clouds and aerosols is an important element in this system that adds interesting complications to the interpretation of observational datasets.

Investigating Passive Cloud Retrievals of Marine Stratocumulus Over the South East Pacific

Investigating Passive Cloud Retrievals of Marine Stratocumulus Over the South East Pacific
Author: Nicholas James King
Publisher:
Total Pages:
Release: 2013
Genre:
ISBN:
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Clouds are an important modulator of the global radiation budget and yet representing their formation, evolution and interaction with aerosols still remains as one of the largest uncertainties in modelling future climate. An important requirement to understanding the processes which govern clouds is accurate measurement of their global distribution and microphysical properties over a wide range of spatial and temporal scales which can only be satisfied by passive remote sensing measurements from satellite platforms. As such the development and validation of cloud remote sensing techniques is an important ongoing task. Of particular radiative importance are marine stratocumulus clouds, due to their large global extent and high solar reflectance. This thesis uses a range of in situ and remote sensing observations of marine stratocumulus over the South East Pacific taken during the Variability of the American Monsoon Systems (VAMOS) Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS-REx) to investigate some outstanding issues relating to passive remote sensing. In particular answers to two questions are sought: 1) Do measurements of solar reflectance at multiple wavelengths with different absorption properties allow information about the vertical structure of the cloud to be derived? 2) Is there a high bias in passive retrievals of droplet effective radius? A unique airborne hyperspectral data set is evaluated for its potential to provide insight into these problems but through extensive comparison to collocated in situ and satellite observations along with an analysis of historical calibrations, it is concluded that the calibration quality of this dataset is not sufficient to meet its scientific objectives. A theoretical study into the information content of multi-wavelength measurements to retrieve the vertical variation of droplet size is presented. Measurements from the Moderate Resolution Imaging Spectroradiometer (MODIS) satellite instrument are shown to contain little information related to the vertical structure of typical marine stratocumulus. The information content of hyperspectral measurements is shown to be significantly larger, indicating the potential to perform profile retrievals from future measurements. A comparison of in situ profile measurements to collocated MODIS cloud retrievals adds to the existing body of evidence that passive retrievals of the droplet effective radius of marine stratocumulus are high biased when compared to other measurement sources. Potential sources of this bias are investigated and many of the previously postulated reasons behind the bias are ruled out. It is also shown that the differences between MODIS retrievals of effective radius performed at different wavelengths bear no relation to the in situ observed vertical structure of the cloud.