An empirical algorithm to map perennial firn aquifers and ice slabs within the Greenland Ice Sheet using satellite L-band microwave radiometry

Perennial firn aquifers are subsurface meltwater reservoirs consisting of a meters-thick water-saturated firn layer that can form on spatial scales as large as tens of kilometers. They have been observed within the percolation facies of glaciated regions experiencing intense seasonal surface melting and high snow accumulation. Widespread perennial firn aquifers have been identified within the Greenland Ice Sheet (GrIS) via field expeditions, airborne ice-penetrating radar surveys, and satellite microwave sensors. In contrast, ice slabs are nearly continuous ice layers that can also form on spatial scales as large as tens of kilometers as a result of surface and subsurface water-saturated snow and firn layers sequentially refreezing following multiple melting seasons. They have been observed within the percolation facies of glaciated regions experiencing intense seasonal surface melting but in areas where snow accumulation is at least 25 % lower as compared to perennial firn aquifer areas. Widespread ice slabs have recently been identified within the GrIS via field expeditions and airborne ice-penetrating radar surveys, specifically in areas where perennial firn aquifers typically do not form. However, ice slabs have yet to be identified from space. Together, these two ice sheet features represent distinct, but related, sub-facies within the broader percolation facies of the GrIS that can be defined primarily by differences in snow accumulation, which influences the englacial hydrology and thermal characteristics of firn layers at depth. Here, for the first time, we use enhanced-resolution vertically polarized L-band brightness temperature (TVB) imagery (2015–2019) generated using observations collected over the GrIS by NASA's Soil Moisture Active Passive (SMAP) satellite to map perennial firn aquifer and ice slab areas together as a continuous englacial hydrological system. We use an empirical algorithm previously developed to map the extent of Greenland's perennial firn aquifers via fitting exponentially decreasing temporal L-band signatures to a set of sigmoidal curves. This algorithm is recalibrated to also map the extent of ice slab areas using airborne ice-penetrating radar surveys collected by NASA's Operation IceBridge (OIB) campaigns (2010–2017). Our SMAP-derived maps show that between 2015 and 2019, perennial firn aquifer areas extended over 64 000 km2, and ice slab areas extended over 76 000 km2. Combined together, these sub-facies are the equivalent of 24 % of the percolation facies of the GrIS. As Greenland's climate continues to warm, seasonal surface melting will increase in extent, intensity, and duration. Quantifying the possible rapid expansion of these sub-facies using satellite L-band microwave radiometry has significant implications for understanding ice-sheet-wide variability in englacial hydrology that may drive meltwater-induced hydrofracturing and accelerated ice flow as well as high-elevation meltwater runoff that can impact the mass balance and stability of the GrIS.

Investigation of the chemical ordering and thermal properties of the Rh–Ag–Au nanoalloys

In this paper, the melting behaviors of Rh–Ag–Au nanoalloys are investigated with MD simulation. For Rh–Ag–Au nanoalloys, icosahedron structure was considered. The local optimizations of Rh–Ag–Au nanoalloys were carried out with the BH algorithm. The interatomic interactions were modeled with the Gupta potential. The local optimization results of Rh–Ag–Au nanoalloys show that Au and Ag atoms prefer to locate on the surface, and Rh atoms prefer to locate in the inner shells. The bond order parameter result is compatible with the excess energy analysis. It is noted that structures with more Ag–Au bonds are more energetically stable. Caloric curve, heat capacity, Lindemann index, and RMSD methods were used for estimating the melting temperatures of Rh–Ag–Au nanoalloys. According to the simulation results, melting temperatures depend on the composition. Also, it is discovered that nanoalloys are generally melting in two stages. Surface melting of the third shell is occupied by Ag and Au atoms, and then homogeneous melting of the inner shells is occupied by Rh atoms. It is found that the difference between surface melting temperatures and homogeneous melting temperatures in Ag-poor compositions is more significant than that of Ag-rich nanoalloys. In addition, the melting temperatures of the nanoalloys are found to be increased as the size of nanoalloys increases.

Impact of the melt–albedo feedback on the future evolution of the Greenland Ice Sheet with PISM-dEBM-simple

Surface melting of the Greenland Ice Sheet contributes a large amount to current and future sea level rise. Increased surface melt may lower the reflectivity of the ice sheet surface and thereby increase melt rates: the so-called melt–albedo feedback describes this self-sustaining increase in surface melting. In order to test the effect of the melt–albedo feedback in a prognostic ice sheet model, we implement dEBM-simple, a simplified version of the diurnal Energy Balance Model dEBM, in the Parallel Ice Sheet Model (PISM). The implementation includes a simple representation of the melt–albedo feedback and can thereby replace the positive-degree-day melt scheme. Using PISM-dEBM-simple, we find that this feedback increases ice loss through surface warming by 60 % until 2300 for the high-emission scenario RCP8.5 when compared to a scenario in which the albedo remains constant at its present-day values. With an increase of 90 % compared to a fixed-albedo scenario, the effect is more pronounced for lower surface warming under RCP2.6. Furthermore, assuming an immediate darkening of the ice surface over all summer months, we estimate an upper bound for this effect to be 70 % in the RCP8.5 scenario and a more than 4-fold increase under RCP2.6. With dEBM-simple implemented in PISM, we find that the melt–albedo feedback is an essential contributor to mass loss in dynamic simulations of the Greenland Ice Sheet under future warming.

Premelting and the Machanisms of Melting in the Alkali Halides

<p>An experimental and theoretical study of premelting behaviour and mechanisms of melting in the alkali-halides is presented. Theories of melting and previous premelting experiments are first reviewed, then an elastic strain theory of melting is developed, which includes dilatation and shear contributions to the elastic energy and to the vibrational entropy, as well as a communal entropy and an entropy due to the isothermal expansion on melting. By fitting experimental melting parameters, dislocation-like local strains are implicated. The bulk and shear moduli are shown to be continuous with respect to dilatation through the melting expansion and one of the shear moduli vanishes at the dilatation of the melt at the melting temperature. A modified Born instability theory of melting is thus valid. Premelting rises in the apparent specific heat and electrical conductivity within 6 K of the melting point are studied and are shown to occur at the surfaces only. The use of guard rings to eliminate surface conduction is essential at all temperatures above the extrinsic/intrinsic conductivity 'knee', and electrical fringing must be taken into account for typical specimen sizes. For various surface orientations, the rises in surface conductivity occur at lower temperatures the lower the surface packing density, and for deformed specimens, the greater the deformation. The results are interpreted in terms of an atomic-scale surface melting below the melting point, and a consequent rapid rise in vaporisation rate. A dislocation theory of surface melting, melting and the solid-liquid interface is developed which gives good agreement with experimental values for the melting temperatures and the interfacial energies.</p>

Premelting and the Machanisms of Melting in the Alkali Halides

<p>An experimental and theoretical study of premelting behaviour and mechanisms of melting in the alkali-halides is presented. Theories of melting and previous premelting experiments are first reviewed, then an elastic strain theory of melting is developed, which includes dilatation and shear contributions to the elastic energy and to the vibrational entropy, as well as a communal entropy and an entropy due to the isothermal expansion on melting. By fitting experimental melting parameters, dislocation-like local strains are implicated. The bulk and shear moduli are shown to be continuous with respect to dilatation through the melting expansion and one of the shear moduli vanishes at the dilatation of the melt at the melting temperature. A modified Born instability theory of melting is thus valid. Premelting rises in the apparent specific heat and electrical conductivity within 6 K of the melting point are studied and are shown to occur at the surfaces only. The use of guard rings to eliminate surface conduction is essential at all temperatures above the extrinsic/intrinsic conductivity 'knee', and electrical fringing must be taken into account for typical specimen sizes. For various surface orientations, the rises in surface conductivity occur at lower temperatures the lower the surface packing density, and for deformed specimens, the greater the deformation. The results are interpreted in terms of an atomic-scale surface melting below the melting point, and a consequent rapid rise in vaporisation rate. A dislocation theory of surface melting, melting and the solid-liquid interface is developed which gives good agreement with experimental values for the melting temperatures and the interfacial energies.</p>