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Modeled response in radiative properties of isolated shallow convective clouds due to perturbations in meteorological state variables and atmospheric aerosol loading
Stockholm University, Faculty of Science, Department of Meteorology .
Stockholm University, Faculty of Science, Department of Meteorology .
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The aim of the present study is to estimate the range of variability in observational data of meteorological variables permitted to determine a clear and unambiguous signal in cloud fraction and cloud albedo due to changes in aerosol concentrations. Three different cases of tropical shallow convection have been simulated using a two-dimensional cloud resolving model with aerosol-cloud interactions. For each case, 30 different small perturbations were added to the initial meteorological profiles of zonal wind, potential temperature and specific humidity. For each meteorological per- turbation an additional 14 simulations were performed with different boundary layer accumulation mode aerosol concentrations ranging from clean (50 cm-3) to polluted (1350 cm-3) conditions. A total of 5040 simulations were performed to elucidate the impact of aerosols on the simulated cloud fraction and cloud albedo and to compare this to the corresponding impact to the influence of small meteorological perturbations. For the simulated cases, we find that for cloud fraction, the aerosol signal is in general much weaker than the change due to small changes in meteorology. This was especially true for parameters that influence the relative humidity of the environment, i.e. tempera- ture and specific humidity. For cloud albedo, the aerosol signal surpasses that of the relatively small meteorological perturbations. We find up to 40% difference in cloud albedo going from clean to polluted conditions. The corresponding maximum change due to the meteorological perturbations was only 14%. We conclude that for the simulated shallow convective clouds, isolating an aerosol effect on cloud fraction is not possible if using meteorological analysis data containing errors of the same order of magnitude as the imposed meteorological perturbations. However, the meteorological constraints may be sufficient to isolate an aerosol effect on cloud albedo.

National Category
Meteorology and Atmospheric Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
URN: urn:nbn:se:su:diva-62821OAI: oai:DiVA.org:su-62821DiVA: diva2:445051
Available from: 2011-10-01 Created: 2011-10-01 Last updated: 2011-10-21
In thesis
1. Aerosol-cloud interaction from an observational and modeling perspective
Open this publication in new window or tab >>Aerosol-cloud interaction from an observational and modeling perspective
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Clouds may respond strongly to changes in the atmospheric aerosol population, and the response of clouds to an increased global aerosol burden could to some extent mask the warming caused by enhanced greenhouse gas concentrations. However, estimates of the impact of aerosols on cloud properties are associated with large uncertainties, both because of difficulties representing the aerosol-cloud interaction within models, and because of problems of unequivocally isolating the effect of aerosols on cloud properties in observational data. This thesis focuses in part on underlying meteorological factors that significantly correlate with both aerosol and cloud properties, and on how sensitive clouds are to small variations in meteorological conditions. It was found that meteorological covariations must be taken into account when estimating the strength of the relationship between aerosols and cloud properties. By studying the response of shallow convective clouds to perturbations in meteorological conditions and aerosol concentration, it was further concluded that variations in meteorological conditions can enhance or mask the relationship between aerosols and cloud properties, making it difficult to isolate the aerosol signature from small meteorological differences. Additionally, the impact of deep convective clouds on the redistribution of aerosols within a cloud life cycle is examined. It was found that mid-tropospheric aerosols can have a substantial source in evaporating cloud droplets within deep convection. Lastly, this thesis focuses on the implications of meteorological analysis uncertainties, in part related to the difficulties of constraining meteorological variability in observational data of clouds and aerosols, but mainly the impact of analysis errors on atmospheric trajectory calculations. A method is presented to consistently estimate the uncertainty in trajectory calculations. It was concluded that the spatial and temporal trajectory error can be substantially underestimated if the analysis error is not taken into account.

Place, publisher, year, edition, pages
Stockholm: Department of Meteorology, Stockholm University, 2011. 49 p.
National Category
Meteorology and Atmospheric Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-62322 (URN)978-91-7447-355-1 (ISBN)
Public defence
2011-11-25, Nordenskiöldsalen, Geovetenskapens hus, Svante Arrhenius väg 12, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 4: Manuscript. Available from: 2011-11-02 Created: 2011-09-14 Last updated: 2011-10-21Bibliographically approved

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