| 1. |
- Eriksson, Patrick, 1964, et al.
(författare)
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Diurnal variations of humidity and ice water content in the tropical upper troposphere
- 2010
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Ingår i: Atmospheric Chemistry and Physics. - : Copernicus GmbH. - 1680-7316 .- 1680-7324. ; 10:23, s. 11519-11533
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Tidskriftsartikel (refereegranskat)abstract
- Observational results of diurnal variations of humidity from Odin-SMR and AURA-MLS, and cloud ice mass from Odin-SMR and CloudSat are presented for the first time. Comparisons show that the retrievals of humidity and cloud ice from these two satellite combinations are in good agreement. The retrieved data are combined from four almost evenly distributed times of the day allowing mean values, amplitudes and phases of the diurnal variations around 200 hpa to be estimated. This analysis is applied to six climatologically distinct regions, five located in the tropics and one over the subtropical northern Pacific Ocean. The strongest diurnal cycles are found over tropical land regions, where the amplitude is ∼7 RHi for humidity and ∼50% for ice mass. The greatest ice mass for these regions is found during the afternoon, and the humidity maximum is observed to lag this peak by ∼6 h. Over tropical ocean regions the variations are smaller and the maxima in both ice mass and humidity are found during the early morning. Observed results are compared with output from three climate models (ECHAM, EC-EARTH and CAM3). Direct measurement-model comparisons were not possible because the measured and modelled cloud ice masses represent different quantities. To make a meaningful comparison, the amount of snow had to be estimated from diagnostic parameters of the models. There is a high probability that the models underestimate the average ice mass (outside the 1-σ uncertainty). The models also show clear deficiencies when it comes to amplitude and phase of the regional variations, but to varying degrees. © 2010 Author(s).
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| 2. |
- Ghan, Steven, et al.
(författare)
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Challenges in constraining anthropogenic aerosol effects on cloud radiative forcing using present-day spatiotemporal variability
- 2016
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Ingår i: Proceedings of the National Academy of Sciences of the United States of America. - : Proceedings of the National Academy of Sciences. - 0027-8424 .- 1091-6490. ; 113:21, s. 5804-5811
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Tidskriftsartikel (refereegranskat)abstract
- A large number of processes are involved in the chain from emissions of aerosol precursor gases and primary particles to impacts on cloud radiative forcing. Those processes are manifest in a number of relationships that can be expressed as factors dlnX/dlnY driving aerosol effects on cloud radiative forcing. These factors include the relationships between cloud condensation nuclei (CCN) concentration and emissions, droplet number and CCN concentration, cloud fraction and droplet number, cloud optical depth and droplet number, and cloud radiative forcing and cloud optical depth. The relationship between cloud optical depth and droplet number can be further decomposed into the sum of two terms involving the relationship of droplet effective radius and cloud liquid water path with droplet number. These relationships can be constrained using observations of recent spatial and temporal variability of these quantities. However, we are most interested in the radiative forcing since the preindustrial era. Because few relevant measurements are available from that era, relationships from recent variability have been assumed to be applicable to the preindustrial to present-day change. Our analysis of Aerosol Comparisons between Observations and Models (AeroCom) model simulations suggests that estimates of relationships from recent variability are poor constraints on relationships from anthropogenic change for some terms, with even the sign of some relationships differing in many regions. Proxies connecting recent spatial/temporal variability to anthropogenic change, or sustained measurements in regions where emissions have changed, are needed to constrain estimates of anthropogenic aerosol impacts on cloud radiative forcing.
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| 3. |
- Gryspeerdt, Edward, et al.
(författare)
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Constraining the instantaneous aerosol influence on cloud albedo
- 2017
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Ingår i: Proceedings of the National Academy of Sciences of the United States of America. - : Proceedings of the National Academy of Sciences. - 0027-8424 .- 1091-6490. ; 114:19, s. 4899-4904
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Tidskriftsartikel (refereegranskat)abstract
- Much of the uncertainty in estimates of the anthropogenic forcing of climate change comes from uncertainties in the instantaneous effect of aerosols on cloud albedo, known as the Twomey effect or the radiative forcing from aerosol-cloud interactions (RFaci), a component of the total or effective radiative forcing. Because aerosols serving as cloud condensation nuclei can have a strong influence on the cloud droplet number concentration (Nd), previous studies have used the sensitivity of the Nd to aerosol properties as a constraint on the strength of the RFaci. However, recent studies have suggested that relationships between aerosol and cloud properties in the present-day climate may not be suitable for determining the sensitivity of the Nd to anthropogenic aerosol perturbations. Using an ensemble of global aerosol-climate models, this study demonstrates how joint histograms between Nd and aerosol properties can account for many of the issues raised by previous studies. It shows that if the anthropogenic contribution to the aerosol is known, the RFaci can be diagnosed to within 20% of its actual value. The accuracy of different aerosol proxies for diagnosing the RFaci is investigated, confirming that using the aerosol optical depth significantly underestimates the strength of the aerosol-cloud interactions in satellite data.
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| 4. |
- Zhang, Shipeng, et al.
(författare)
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On the characteristics of aerosol indirect effect based on dynamic regimes in global climate models
- 2016
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Ingår i: Atmospheric Chemistry And Physics. - : Copernicus GmbH. - 1680-7316 .- 1680-7324. ; 16:5, s. 2765-2783
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Tidskriftsartikel (refereegranskat)abstract
- Aerosol-cloud interactions continue to constitute a major source of uncertainty for the estimate of climate radiative forcing. The variation of aerosol indirect effects (AIE) in climate models is investigated across different dynamical regimes, determined by monthly mean 500 hPa vertical pressure velocity (omega(500)), lower-tropospheric stability (LTS) and large-scale surface precipitation rate derived from several global climate models (GCMs), with a focus on liquid water path (LWP) response to cloud condensation nuclei (CCN) concentrations. The LWP sensitivity to aerosol perturbation within dynamic regimes is found to exhibit a large spread among these GCMs. It is in regimes of strong large-scale ascent (omega(500)aEuro-aEuro parts per thousand < aEuro-a'25 hPa day(-1)) and low clouds (stratocumulus and trade wind cumulus) where the models differ most. Shortwave aerosol indirect forcing is also found to differ significantly among different regimes. Shortwave aerosol indirect forcing in ascending regimes is close to that in subsidence regimes, which indicates that regimes with strong large-scale ascent are as important as stratocumulus regimes in studying AIE. It is further shown that shortwave aerosol indirect forcing over regions with high monthly large-scale surface precipitation rate (> 0.1 mm day(-1)) contributes the most to the total aerosol indirect forcing (from 64 to nearly 100 %). Results show that the uncertainty in AIE is even larger within specific dynamical regimes compared to the uncertainty in its global mean values, pointing to the need to reduce the uncertainty in AIE in different dynamical regimes.
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| 5. |
- Zheng, Minjie, et al.
(författare)
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Modeling Atmospheric Transport of Cosmogenic Radionuclide 10Be Using GEOS-Chem 14.1.1 and ECHAM6.3-HAM2.3 : Implications for Solar and Geomagnetic Reconstructions
- 2024
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Ingår i: Geophysical Research Letters. - 0094-8276. ; 51:2
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Tidskriftsartikel (refereegranskat)abstract
- A prerequisite to applying 10Be in natural archives for solar and geomagnetic reconstructions is to know how 10Be deposition reflects atmospheric production changes. However, this relationship remains debated. To address this, we use two state-of-the-art global models GEOS-Chem and ECHAM6.3-HAM2.3 with the latest beryllium production model. During solar modulation, both models suggest that 10Be deposition reacts proportionally to global production changes, with minor latitudinal deposition biases (<5%). During geomagnetic modulation, however, 10Be deposition changes are enhanced by ∼15% in the tropics and attenuated by 20%–35% in subtropical and polar regions compared to global production changes. Such changes are also hemispherically asymmetric, attributed to asymmetric production between hemispheres. For the solar proton event in 774/5 CE, 10Be shows a 15% higher deposition increase in polar regions than in tropics. This study highlights the importance of atmospheric mixing when comparing 10Be from different locations or to independent geomagnetic field records.
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