A model intercomparison of CCN-limited tenuous clouds in the high Arctic

We perform a model intercomparison of summertime high Arctic ( > 80°N) clouds observed during the 2008 Arctic Summer Cloud Ocean Study (ASCOS) campaign, when observed cloud condensation nuclei (CCN) concentrations fell below 1cm‾³. Previous analyses have suggested that at these low CCN concentrations the liquid water content (LWC) and radiative properties of the clouds are determined primarily by the CCN concentrations, conditions that have previously been referred to as the tenuous cloud regime. The intercomparison includes results from three large eddy simulation models (UCLALES-SALSA, COSMO-LES, and MIMICA) and three numerical weather prediction models (COSMO-NWP, WRF, and UM-CASIM). We test the sensitivities of the model results to different treatments of cloud droplet activation, including prescribed cloud droplet number concentrations (CDNCs) and diagnostic CCN activation based on either fixed aerosol concentrations or prognostic aerosol with in-cloud processing. There remains considerable diversity even in experiments with prescribed CDNCs and prescribed ice crystal number concentrations (ICNC). The sensitivity of mixed-phase Arctic cloud properties to changes in CDNC depends on the representation of the cloud droplet size distribution within each model, which impacts autoconversion rates. Our results therefore suggest that properly estimating aerosol–cloud interactions requires an appropriate treatment of the cloud droplet size distribution within models, as well as in situ observations of hydrometeor size distributions to constrain them. The results strongly support the hypothesis that the liquid water content of these clouds is CCN limited. For the observed meteorological conditions, the cloud generally did not collapse when the CCN concentration was held constant at the relatively high CCN concentrations measured during the cloudy period, but the cloud thins or collapses as the CCN concentration is reduced. The CCN concentration at which collapse occurs varies substantially between models. Only one model predicts complete dissipation of the cloud due to glaciation, and this occurs only for the largest prescribed ICNC tested in this study. Global and regional models with either prescribed CDNCs or prescribed aerosol concentrations would not reproduce these dissipation events. Additionally, future increases in Arctic aerosol concentrations would be expected to decrease the frequency of occurrence of such cloud dissipation events, with implications for the radiative balance at the surface. Our results also show that cooling of the sea-ice surface following cloud dissipation increases atmospheric stability near the surface, further suppressing cloud formation. Therefore, this suggests that linkages between aerosol and clouds, as well as linkages between clouds, surface temperatures, and atmospheric stability need to be considered for weather and climate predictions in this region

Bounding global aerosol radiative forcing of climate change

Aerosols interact with radiation and clouds. Substantial progress made over the past 40 years in observing, understanding, and modeling these processes helped quantify the imbalance in the Earth's radiation budget caused by anthropogenic aerosols, called aerosol radiative forcing, but uncertainties remain large. This review provides a new range of aerosol radiative forcing over the industrial era based on multiple, traceable, and arguable lines of evidence, including modeling approaches, theoretical considerations, and observations. Improved understanding of aerosol absorption and the causes of trends in surface radiative fluxes constrain the forcing from aerosol-radiation interactions. A robust theoretical foundation and convincing evidence constrain the forcing caused by aerosol-driven increases in liquid cloud droplet number concentration. However, the influence of anthropogenic aerosols on cloud liquid water content and cloud fraction is less clear, and the influence on mixed-phase and ice clouds remains poorly constrained. Observed changes in surface temperature and radiative fluxes provide additional constraints. These multiple lines of evidence lead to a 68% confidence interval for the total aerosol effective radiative forcing of -1.6 to -0.6 W m−2, or -2.0 to -0.4 W m−2 with a 90% likelihood. Those intervals are of similar width to the last Intergovernmental Panel on Climate Change assessment but shifted toward more negative values. The uncertainty will narrow in the future by continuing to critically combine multiple lines of evidence, especially those addressing industrial-era changes in aerosol sources and aerosol effects on liquid cloud amount and on ice clouds

Diffraction-limited gradient-index (GRIN) microlenses with high numerical apertures produced by silver ion exchange in glass: Diffusion modeling and process optimization

Cylindrical and rod gradient-index lenses with numerical apertures of 0.5 are produced by silver ion exchange in a sodium-aluminosilicate glass. Choosing the appropriate glass composition enables the generation of refractive index changes of 0.145 in the glass without coloration in the visible range. Diffraction-limited optical performance of lenses of up to 1.3 mm in thickness or diameter is achieved by ion exchange modeling which comprises the following steps: (1) The concentration-dependent diffusion coefficient of the glass is experimentally determined by the Boltzmann-Matano method, (2) Process control is provided by the measured dopant concentration/refractive index relation and the glass/salt equilibrium dependence, (3) The ion exchange process is optimized by solving the non-linear diffusion equation for two steps under different boundary conditions, (4) Diffraction-limited lenses up to numerical apertures of 0.5 are generated by applying the optimized process parameters to the ion exchange

Diffraction-limited gradient-index lenses with high numerical aperture for applications in laser beam-shaping and medicine

Diffraction-limited focusing lenses with a numerical aperture of 0.52 and diverging lenses with a numerical aperture of 0.62 were fabricated by silver-sodium ion exchange in cylindrical and rod geometry. The silver ion exchange in a sodium-aluminosilicate glass was optimized by the simulation of the diffusion equation for concentration-dependent diffusion coefficients and by direct measurement of the index profile by the refracted -near-field-method. The performance of the lenses was proved by interferometrical measurements of wavefront aberrations. Applications of gradient-index lenses to the fast-axis-collimation (FAC) of high-power lasef diodes and in endoscopic systems are discussed

A comparison of two chemistry and aerosol schemes on the regional scale and the resulting impact on radiative properties and liquid- and ice-phase aerosol–cloud interactions

The complexity of atmospheric aerosol causes large uncertainties in its parameterization in atmospheric models. In a process-based comparison of two aerosol and chemistry schemes within the regional atmospheric modeling framework COSMO-ART (Consortium for Small-Scale Modelling, Aersosol and Reactive Trace gases extension), we identify key sensitivities of aerosol parameterizations. We consider the aerosol module MADE (Modal Aerosol Dynamics model for Europe) in combination with full gas-phase chemistry and the aerosol module M7 in combination with a constant-oxidant-field-based sulfur cycle. For a Saharan dust outbreak reaching Europe, modeled aerosol populations are more sensitive to structural differences between the schemes, in particular the consideration of aqueous-phase sulfate production, the selection of aerosol species and modes, and modal composition, than to parametric choices like modal standard deviation and the parameterization of aerosol dynamics. The same observation applies to aerosol optical depth (AOD) and the concentrations of cloud condensation nuclei (CCN). Differences in the concentrations of ice-nucleating particles (INPs) are masked by uncertainties between two ice-nucleation parameterizations and their coupling to the aerosol scheme. Differences in cloud droplet and ice crystal number concentrations are buffered by cloud microphysics as we show in a susceptibility analysis

A comparison of two chemistry and aerosol schemes on the regional scale and the resulting impact on radiative properties and liquid- and ice-phase aerosol–cloud interactions

The complexity of atmospheric aerosol causes large uncertainties in its parameterization in atmospheric models. In a process-based comparison of two aerosol and chemistry schemes within the regional atmospheric modeling framework COSMO-ART (Consortium for Small-Scale Modelling, Aersosol and Reactive Trace gases extension), we identify key sensitivities of aerosol parameterizations. We consider the aerosol module MADE (Modal Aerosol Dynamics model for Europe) in combination with full gas-phase chemistry and the aerosol module M7 in combination with a constant-oxidant-field-based sulfur cycle. For a Saharan dust outbreak reaching Europe, modeled aerosol populations are more sensitive to structural differences between the schemes, in particular the consideration of aqueous-phase sulfate production, the selection of aerosol species and modes, and modal composition, than to parametric choices like modal standard deviation and the parameterization of aerosol dynamics. The same observation applies to aerosol optical depth (AOD) and the concentrations of cloud condensation nuclei (CCN). Differences in the concentrations of ice-nucleating particles (INPs) are masked by uncertainties between two ice-nucleation parameterizations and their coupling to the aerosol scheme. Differences in cloud droplet and ice crystal number concentrations are buffered by cloud microphysics as we show in a susceptibility analysis

Real-case simulations of aerosol–cloud interactions in ship tracks over the Bay of Biscay

Ship tracks provide an ideal test bed for studying aerosol–cloud interactions (ACIs) and for evaluating their representation in model parameterisations. Regional modelling can be of particular use for this task, as this approach provides sufficient resolution to resolve the structure of the produced track including their meteorological environment whilst relying on the same formulations of parameterisations as many general circulation models. In this work we simulate a particular case of ship tracks embedded in an optically thin stratus cloud sheet which was observed by a polar orbiting satellite at 12:00 UTC on 26 January 2003 around the Bay of Biscay. <br><br> The simulations, which include moving ship emissions, show that the model is indeed able to capture the structure of the track at a horizontal grid spacing of 2 km and to qualitatively capture the observed cloud response in all simulations performed. At least a doubling of the cloud optical thickness was simulated in all simulations together with an increase in cloud droplet number concentration by about 40 cm^&minus;3 (300%) and decrease in effective radius by about 5 <abbr>μm</abbr> (40%). Furthermore, the ship emissions lead to an increase in liquid water path in at least 25% of the track regions. <br><br> We are confident in the model's ability to capture key processes of ship track formation. However, it was found that realistic ship emissions lead to unrealistic aerosol perturbations near the source regions within the simulated tracks due to grid-scale dilution and homogeneity. <br><br> Combining the regional-modelling approach with comprehensive field studies could likely improve our understanding of the sensitivities and biases in ACI parameterisations, and could therefore help to constrain global ACI estimates, which strongly rely on these parameterisations

Reliable profile reconstruction of GRIN lenses produced by ion-exchange processes

We propose a method that allows for a fast and accurate reconstruction of the refractive index profile of radially symmetric gradient index lenses fabricated by ion-exchange processes. The presented method enables the reconstruction of the profile up to the 10th polynomial order without direct spatially resolved refractive index measurements. It requires as input a working distance measurement at the paraxial limit and an accurate wavefront aberration measurement at full aperture. In addition, the approach combines the information about the optical behavior with the knowledge about the overall mass density changes of the glass rods during the ion exchange production processes to refine the reconstruction. Finally, the reconstruction of multiple profiles produced with different boundary conditions is demonstrated and confirms the functionality of the method