ALBERT

Description: Tree-ring chronologies underpin the majority of annually-resolved reconstructions of Common Era climate. However, they are derived using different datasets and techniques, the ramifications of which have hitherto been little explored. Here, we report the results of a double-blind experiment that yielded 15 Northern Hemisphere summer temperature reconstructions from a common network of regional tree-ring width datasets. Taken together as an ensemble, the Common Era reconstruction mean correlates with instrumental temperatures from 1794–2016 CE at 0.79 (p 〈 0.001), reveals summer cooling in the years following large volcanic eruptions, and exhibits strong warming since the 1980s. Differing in their mean, variance, amplitude, sensitivity, and persistence, the ensemble members demonstrate the influence of subjectivity in the reconstruction process. We therefore recommend the routine use of ensemble reconstruction approaches to provide a more consensual picture of past climate variability.

Description: Cirrus clouds play a significant role in the Earth's global radiative budget. The radiative effect of cirrus clouds depends on the aspherical ice crystal composition present within, making a proper understanding of ice crystal single scattering properties crucial for accurate climatological modeling and forecasting. One of the most relevant cirrus ice crystal habits is a polycrystalline bullet rosette, where individual bullets are radiating from the same nucleation point. Comprehensive studies on the dependencies of ice crystal habit formation have shown that bullet rosettes grow at a range of ice supersaturations with temperatures below -40 °C; environmental conditions frequently found within high altitude cirrus clouds. Here, single scattering properties of atmospheric bullet rosettes were investigated using correlated high resolution in-situ stereo-images of individual bullet rosettes and their angular scattering functions measured by the airborne Particle Habit Imaging Polar Scattering (PHIPS-HALO) cloud probe during the CIRRUS-HL airborne campaign. The microphysical properties of individual bullet rosettes, such as their maximum dimensions, crystal complexity, and number of bullets per rosette, were analyzed and associated with environmental conditions and their measured angular light scattering functions and resulting asymmetry parameter and complexity parameter. Results show that the asymmetry parameters of atmospheric bullet rosettes are lower than what is expected by theoretical studies assuming smooth surfaces on solid and hollow bullet rosettes.

Description: Ice clouds play a key role in the atmospheric radiation budget, both by reflection of shortwave radiation and absorption-emission of longwave radiation. Through these radiative interactions, ice clouds can set atmospheric temperature gradients and thereby influence atmospheric circulation regimes. The radiative signature of ice clouds strongly depends on their macro- and microphysical characteristics, as well as their optical properties. While the new generation of storm-resolving models improves the representation of the vertical velocities that drive cloud formation, subgrid-scale differences, for example in ice optics and microphysics, generate large variability in the modeled atmospheric cloud-radiative heating (CRH) rates (Sullivan and Voigt, 2021). We propose both an idealized single-column and more realistic two-dimensional transect approach for investigating CRH, using the new ecRad radiative transfer module (Hogan and Bozzo, 2018). First, in a series of single-column calculations, we evaluate the impact of realistic perturbations in macro- and microphysical properties, such as cloud-top temperature and ice crystal effective radius, on CRH. For this approach, a heating sensitivity matrix visualization is presented as the response for the different levels of macro-micro properties perturbations. Secondly, we study the impact of using three different ice optical schemes (Fu, 1996; Fu et al., 1998; Yi et al., 2013; Baran et al., 2016) on CRH over three latitudinal transects located in the Eastern Pacific, Western Pacific, and passing over the Asian Monsoon Area. For each of these transects, ecRad is driven by realistic atmospheric conditions provided by ERA5.

Description: The interaction of solar radiation with ice particles in clouds plays a crucial role in redistributing solar light. Understanding this process is essential for accurate calculations of the shortwave radiative transfer in climate models and for retrieving cloud properties from satellite observations. However, current knowledge of the light scattering behavior of atmospheric ice particles is limited using simplified ice particle morphologies in optical models.To address this limitation, the Particle Habit Imaging and Polar Scattering (PHIPS) airborne probe was developed to obtain in-situ measurements of single atmospheric ice crystals. The PHIPS probe is a combination of a stereo microscopic bright field imager and a polar nephelometer that works on single particles. By using PHIPS, a unique and comprehensive dataset of microphysical properties and correlated angular light scattering functions of real atmospheric ice particles has been acquired from several aircraft campaigns.The acquired dataset is of high value for scientists developing and applying single particle light scattering models. By simulating the light scattering behavior of the observed crystals with the optical engineering software FRED, the study shows that even solid hexagonal crystals require surface roughness assumptions to accurately explain their light scattering function. The study tests two surface roughness models: the tilted-facet angle distribution method and the "smooth surface roughness" approach, which assumes scalar diffraction on wavelength-scale roughness features. The performance of these two models for the example crystals is discussed. Overall, this study demonstrates the importance of in-situ measurements for improving our understanding of the light scattering behavior of atmospheric ice particles.

Description: Cirrus clouds play an important role in the Earth's energy budget by reflecting solar radiation and trapping long wave radiation from the surface. While globally, they are believed to have a net warming effect on the atmosphere, the regional differences in the cirrus cloud radiative effect (CRE) can be significant. In the polar regions, where there is restricted solar radiation or high surface albedo, cirrus clouds have a strong warming effect on the atmosphere. However, the uncertainties in calculating the cirrus cloud CRE remains significant, partly due to the incomplete understanding of ice crystal optical properties. To address this issue, in-situ observations of Arctic cirrus ice crystal microphysical and optical properties were made using the Particle Habit Imaging and Polar Scattering (PHIPS) probe during the CIRRUS-HL airborne measurement campaign in the summer of 2021. The observations are used to generate a new parameterisation of ice crystal single scattering properties, which is tested in radiative transfer simulations to determine the CRE of Arctic cirrus during the polar day. The study highlights the key microphysical properties that define the magnitude of the ice cloud asymmetry parameter and provides a more accurate estimate of the CRE of cirrus clouds, particularly in the polar regions. The findings have significant implications for climate modelling and weather forecasting, and they demonstrate the importance of using direct in-situ observations to improve our understanding of the optical properties of cirrus clouds.

Description: Aims. For decades now, researchers have been looking for a way to tie the kinematic and dynamic reference frames. Certain worldwide organizations have looked to using co-location in space, combining various techniques. Given the long list of possible applications of the Global Navigation Satellite System (GNSS), it is worthwhile investigating the connection between the most accurate and stable International Celestial Reference Frame (ICRF) and the Earth-centered Celestial Inertial reference frame (ECI) used in GNSS data processing. Methods. We simulated phase-referencing observations of GNSS satellites and nearby radio source calibrators to realize the connection between the two celestial reference frames. We designed two schemes for observation plans. One scheme is to select the satellite target when it can be observed by the greatest number of stations in order to obtain high-precision positioning. During each scan, we employ four regional networks to simultaneously track four chosen satellites. The alternative scheme is to observe satellite orbits of as many satellites as possible on different daily observations. In addition, to test the two schemes, we used Monte Carlo methods to generate 1000 groups of random errors in the simulation. Results. Finally, we estimate the right ascension and declination offsets (∆α, ∆δ) of GNSS satellites in the ICRF, and then derive frame tie parameters based on those results: three global rotation angles (A1, A2, A3). The celestial angular offset results assessed from the former scheme show that this scheme leads to high precision of namely 1 mas, while the parameters of the frame tie determined from the second scheme can achieve an improved precision of better than 1.3 µas.

Description: Since 2015, when a cabled observatory was first set up on Axial Seamount, 〉150,000 earthquakes have been located within a 25 km³ block of crust. The volcanic system has also experienced the passage of seismic waves from 〉120 large remote earthquakes and periodic tidal loading. Therefore, it represents a unique laboratory for us to probe how earthquakes respond to static and dynamic stress changes. We find that the seismicity rate increases while the b-value decreases (larger proportion of large events) systematically with increasing tidal stress. The stress dependence of seismicity rate conforms to triggering theory over the whole tidal stress range, demonstrating that there is no triggering stress threshold and stress shadowing is simply a continuous function of stress decrease. We also observe statistically significant episodes of dynamic triggering for ~11% of large remote earthquakes. However, while some coincide with changes in permeability estimated from changes in the tidal phase lag of hydrothermal vent temperature, others are not. Therefore, permeability change is likely not the only mechanism underlying dynamic triggering of earthquakes.