ALBERT

Description: The core-mantle boundary (CMB) is one of the most stable interfaces inside the Earth due to the large density and viscosity contrasts. Despite significant differences in physical, chemical, and transport properties, the possible mechanisms where the core and mantle can interact has become an active area of research. Detection of chemical signatures from the CMB can provide an unprecedented glimpse into the Earth’s deep interior and ancient past, with some studies suggesting certain isotopic and elemental anomalies in ocean island basalts to be core tracers. However, there is still uncertainty regarding processes that can convey chemical signatures from the core to the mantle. A recent study proposed a new hybrid mechanism that results from collaborative feedback between dynamic topography, percolation of liquid metal into submerged rock, the gravitational collapse of a metal-silicate mush, and induced small-scale mantle circulation above this mushy layer. The grain-scale intrusion of liquid iron into mantle rocks offers an opportunity for chemical and isotopic exchange to take place, while the gravitational collapse of the mushy layer can “soften” the CMB and enhance downwellings, thereby encouraging further chemical exchange. Using a mantle convection model coupled to the gravitational spreading of a thin layer at the CMB, we will show how the enhancement of downwellings change with the rheology of the mantle, and if the reacted mantle materials emerge from the mushy layer with a certain buoyancy ratio, how much of it will be entrained in upwelling plumes.

Description: On the timescales of mantle convection (~100 Myrs), downwelling mantle flows due to buoyancy-driven flows depress the core-mantle boundary (CMB) into the outer core (OC) creating a dynamic topography. Seismological studies have suggested that the peak-to-peak amplitudes are less than 10 km. If the solid grain boundaries of mantle silicates and oxides are sufficiently wetted, percolation of liquid iron alloy into the matrix can create an interconnected network to form a metal-silicate mush at CMB topography lows. Since timescales of large-scale flows at the top of the OC are much smaller than viscous compaction timescales of the silicate matrix in the mushy layer, hydrodynamic and magnetic pressure gradients in the OC can drive Darcy flow inside the porous region. This will not only create an electrically conducting region at the solid side of the CMB that distorts the magnetic field, but also produce stronger mechanical coupling within the mushy layer via viscous stresses between the matrix and fluid. These combined effects can result in torques at the CMB that transfers angular momentum between the mantle and OC, thus affecting variations in the length of day. Lastly, flows in the mushy layer facilitate chemical and isotopic exchange between the liquid iron and silicate matrix which has implications on the chemical evolution of the Earth. We will present a study that analyses the pressure torques at the CMB due to the presence of a porous layer, and how surface core flows can be affected by permeability and porosity of the layer.

Description: The core-mantle boundary (CMB) is isothermal as lateral variations in temperature are removed by core flows. However, mantle convection and thermochemical heterogeneities situated above the CMB can produce variations in heat flux. Direct seismic imaging of the lower mantle has identified different anomalous wave-speed features. Some seismically slow anomalies (e.g., large low-seismic-velocity provinces, LLVPs, and ultralow-velocity zones, ULVZs) have been interpreted to be structures of either thermal or thermochemical origin which are warmer than the ambient mantle. Meanwhile, fast anomalies are interpreted to be remnants of cold subducted slabs. Heat flow variations across the CMB are thought to influence core surface flows and the resulting magnetic secular variation. It has also been proposed that the frequency likelihood of geomagnetic reversals could be influenced by these heterogeneities. Other studies suggest that LLVPs and ULVZs might be partially molten, containing iron-rich melts due to chemical interactions with the core or remnants of a basal magma ocean. Meanwhile, metal-silicate mushy zones might exist beneath mantle downwellings due to percolation of liquid metal. Depending on the melt fraction present, the electrical conductivity at the CMB can be more significant than previously thought, possibly correlating with low heat flux regions. In this study, we will present results from geodynamo simulations with lateral variations in electrical conductivity and heat flux at the CMB, focussing on regional impacts on the geomagnetic field and secular variations. We aim to show that conditions at the CMB influence both the field generated and the resulting field at the Earth’s surface.

Description: Deep convective clouds (DCCs) are characterized by their extensive temporal and spatial coverage. These clouds have a significant impact on the balance of the Earth’s radiation. To quantify the impact of DCCs on the Earth’s radiation, we need to obtain the microphysical and single-scattering properties of the ice crystals in DCCs. There are multiple observations showed that quasi-spherical single frozen droplets (SFD) which were formed by homogeneous freezing and frozen droplet aggregates (FDA), were the common features in DCCs. Based on this, idealized models representing the SFD and FDA were developed using Gaussian random spheres (GSs) and droxtals. Habit mixture models (i.e., mixtures of GSs and droxtals) were also developed. Scattering-phase function P11 and asymmetry parameter g as single-scattering properties were calculated for idealized models using a geometric optics method at the operating wavelength (i.e., 0.80 μm) of a polar nephelometer (PN). The calculated single-scattering properties were compared with those obtained by the PN during the 2007 Cirrus Cloud Experiment field campaign. The accuracy of theoretically calculated single-scattering properties was determined by calculating a root mean square error (RMSE) between the theory and the in-situ measurements. It was found that the single-scattering properties of SFD and FDA models with identical components could not reproduce the in-situ measurements. Overall, the habit mixture models minimized the differences in P11 and consequently, provided better agreement with the in-situ estimated g. The average RMSE of the habit mixture models decreased by down to 19% compared with those of the SFD and the FDA models.