Defining a Digital Earth System

In a 1998 speech before the California Science Center in Los Angeles, then US Vice‐President Al Gore called for a global undertaking to build a multi‐faceted computing system for education and research, which he termed “Digital Earth.” The vision was that of a system providing access to what is known about the planet and its inhabitants’ activities – currently and for any time in history – via responses to queries and exploratory tools. Furthermore, it would accommodate modeling extensions for predicting future conditions. Organized efforts towards realizing that vision have diminished significantly since 2001, but progress on key requisites has been made. As the 10 year anniversary of that influential speech approaches, we re‐examine it from the perspective of a systematic software design process and find the envisioned system to be in many respects inclusive of concepts of distributed geolibraries and digital atlases. A preliminary definition for a particular digital earth system as: “a comprehensive, distributed geographic information and knowledge organization system,” is offered and discussed. We suggest that resumption of earlier design and focused research efforts can and should be undertaken, and may prove a worthwhile “Grand Challenge” for the GIScience community.     

Performance of buildings in two Caribbean Territories: Dominica under Hurricane Maria in 2017 and Trinidad under the 2018 earthquake

Natural Hazards - There have been damaging hurricanes and earthquakes throughout the history of the Caribbean. However, the intensity and frequency of these natural events have increased. The...     

The initial mass function in clusters: theoretical and observational perspectives

We review the success of current simulations in reproducing the observed initial mass function (IMF) in clusters and show that whereas the two main types of simulation invoke the same physics to explain the point at which the IMF flattens from a Salpeter slope, they reproduce the Salpeter tail itself by quite different means. We highlight the fact that a proper modelling of the thermal properties of star-forming gas is probably crucial in explaining why the IMF is apparently quite insensitive to cloud parameters. We also explore how the mass of a cluster’s most massive star depends on cluster mass and how this issue impacts the relationship between the IMF in individual clusters and the integrated galactic IMF. We emphasise the importance of a careful statistical treatment of data in this area.     

Dye dispersion in the surf zone: Measurements and simple models

To examine the spatial and temporal effect of low-volume land-based runoff on beach contamination, discrete batches of dye were released at the shoreline at three beaches in Santa Monica Bay in 2000 (Malibu Creek, Santa Monica Canyon and Pico–Kenter drain). Dye concentration was measured at the shoreline 25, 50 and 100 m alongshore from the dye release point for up to 40 min after dye release. The shoreline concentration time series are characterized either by approximately exponential decay in concentration after passage of the dye patch maximum concentration or by persistent low concentration up to 30 min after passage of the initial dye patch front. In the absence of detailed measurements of physical conditions, several simple advection–diffusion models are used to simulate shoreline concentration time series for an idealized surf zone in order to probe the roles of alongshore current shear and rip currents in producing the observed characteristics in dye concentration time series. Favorable qualitative and quantitative comparison of measured and simulated time series suggest alongshore current shear and rip currents play key roles in generating the observed characteristics of nearshore dye patch dispersion. The models demonstrate the potential effects of these flow features on the extent and duration of beach contamination owing to a continuous contamination source.     

Integrating structure-from-motion photogrammetry into rock weathering field methodologies

Despite recent rapid advances in the field of structure-from-motion (SfM) photogrammetry, the use of high-resolution data to investigate small-scale processes is a relatively underdeveloped field. In particular, rock weathering is rarely investigated using this suite of techniques. This research uses a combination of traditional non-destructive rock weathering measurement techniques (rock surface hardness) and SfM to map deterioration and loss of cohesion of the surface using three-dimensional data. The results are used to interpret weathering behaviour across two different lithologies present on the site, namely shale and limestone. This new approach is tested on seven sites in Longyearbyen, Svalbard, where active weathering of a rock surface was measured after 13 years of exposure to extreme temperature regimes and snow cover. The surface loss was quantified with SfM and combined with rock surface hardness measurement distributions extrapolated in geographic information system (GIS). The combined results are used here to quantify the difference in response of both lithologies to these extreme temperatures. This research demonstrates the potential for further integration of SfM in rock weathering research and other small-scale geomorphological investigations, in particular in difficult field conditions where portability of field equipment is paramount. © 2019 John Wiley & Sons, Ltd. © 2019 John Wiley & Sons, Ltd.     

Late stage slope failure in the wake of the 1929 Grand Banks earthquake

Small volume (<15 km³) debris flows which were triggered by the 1929 earthquake postdate the period of high velocity turbidity current flow resulting from that earthquake. They thus could not have contributed sediment to the 1929 cable-breaking turbidity currents. Both the proposed “Grand Banks Slump” and another large scale debris flow also attributed to the 1929 event, are shown to be autochthonous. In light of the limited volume and late-stage timing of mass wasting on the upper Laurentian Fan in 1929, an additional mechanism must have existed which supplied further sediment to the turbidity current in 1929.     

Evaluating sampling efficiency when estimating sediment source contributions to suspended sediment in rivers by fingerprinting

A general method is proposed which measures the increase in uncertainty when sampling effort is reduced in sediment fingerprinting. The method gives quantitative measures of how reduced sampling of material in one of the source areas, and/or of suspended sediment in streams, increases the uncertainties in the proportions of sediment contributed from the sources. Because the proportions of sediment contributed by the source areas must add to one, standard errors of the estimated proportions cannot be used as the usual measures of uncertainty: the paper uses instead the volume of the joint 95% confidence region for the estimated proportions. The paper shows how the uncertainty in this volume changes as numbers of suspended sediment samples, and the numbers of samples collected from cropped fields, are reduced by successive steps from 24 (20, in the case of cropped fields) to 16, 12, 8, 4 and 2 samples. As expected, uncertainty increases rapidly as the number of samples – whether of suspended sediment or from cropped fields – is reduced drastically. The pattern of increasing uncertainty is similar both for reductions in suspended sediment sampling, and for reduced sampling from cropped areas. When the number of suspended sediment samples, and the number of samples from cropped fields, are reduced to the same values, the increase in uncertainty from fewer suspended sediment samples was always slightly greater than the increased uncertainty from the reduced sampling of cropped areas, although this finding took no account of differences in the costs of field sampling and laboratory analysis. Copyright © 2016 John Wiley & Sons, Ltd.     

Accretion in stellar clusters and the initial mass function

We present a simple physical mechanism that can account for the observed stellar mass spectrum for masses M ∗≳0.5 M_⊙ . The model depends solely on the competitive accretion that occurs in stellar clusters where each star's accretion rate depends on the local gas density and the square of the accretion radius. In a stellar cluster, there are two different regimes depending on whether the gas or the stars dominate the gravitational potential. When the cluster is dominated by cold gas, the accretion radius is given by a tidal-lobe radius. This occurs as the cluster collapses towards a ρ  ∝  R ⁻² distribution. Accretion in this regime results in a mass spectrum with an asymptotic limit of γ =−3/2 (where Salpeter is γ =−2.35) . Once the stars dominate the potential and are virialized, which occurs first in the cluster core, the accretion radius is the Bondi–Hoyle radius. The resultant mass spectrum has an asymptotic limit of γ =−2 with slightly steeper slopes ( γ ≈−2.5) if the stars are already mass-segregated. Simulations of accretion on to clusters containing 1000 stars show that, as expected, the low-mass stars accumulate the majority of their masses during the gas-dominated phase whereas the high-mass stars accumulate the majority of their masses during the stellar-dominated phase. This results in a mass spectrum with a relatively shallow γ ≈3/2 power law for low-mass stars and a steeper power law for high-mass stars −2.5≲ γ ≤−2 . This competitive accretion model also results in a mass-segregated cluster.