ALBERT

Description: Ultra-low frequency (ULF) plasma waves propagate through the magnetosphere and ionosphere where they can alter the Doppler velocity of HF radar echoes. Data from the two Tasman International Geospace Environment Radars (TIGER) and the fluxgate and induction coil magnetometers located on Macquarie Island (54. 5 ∘ S, 158.95 ∘ E geographic) over 2006-2009 show that ULF wave signatures are common. Using coincident radar and magnetometer data selection criteria, 194 events representing a total of 233.4 hours were identified. The majority of ULF signatures seen in the radar data were detected between 06-12 UT (15-21 LT). Using the Maximum Entropy Method (MEM), the spectral content showed favored frequencies of 1.6, 2.1, 2.9 and 3.3 mHz but no obvious variation of frequency with latitude. Most of the observed frequencies were in the range 1-4 mHz. A class of Doppler velocity signatures that appeared as a zigzag shape in the radar range:time plots were identified and maybe related to the expected plasmapause latitudes. Regularly spaced, favored frequencies appeared in the ground magnetometer data during the afternoon, night and morning sectors for those days where ULF wave events were observed in the radar data.

Description: The spatially and temporally variable parameters and inputs to complex groundwater models typically result in long runtimes which hinder comprehensive calibration, sensitivity and uncertainty analysis. Surrogate modeling aims to provide a simpler, and hence faster, model which emulates the specified output of a more complex model in function of its inputs and parameters. In this review paper, we summarize surrogate modeling techniques in three categories: data-driven, projection, and hierarchical-based approaches. Data-driven surrogates approximate a groundwater model through an empirical model that captures the input-output mapping of the original model. Projection based models reduce the dimensionality of the parameter space by projecting the governing equations onto a basis of orthonormal vectors. In hierarchical or multi-fidelity methods the surrogate is created by simplifying the representation of the physical system, such as by ignoring certain processes, or reducing the numerical resolution. In discussing the application to groundwater modeling of these methods, we note several imbalances in the existing literature: a large body of work on data-driven approaches seemingly ignores major drawbacks to the methods; only a fraction of the literature focuses on creating surrogates to reproduce outputs of fully distributed groundwater models, despite these being ubiquitous in practice; and a number of the more advanced surrogate modeling methods are yet to be fully applied in a groundwater modeling context. This article is protected by copyright. All rights reserved.

Description: The diffusion and energization of electrons in the equatorial plane of Earth's magnetosphere by ULF waves under different ionosphere boundary conditions is examined. Using test-particle simulations and considering intervals of weak geomagnetic activity, we find that the highest energization and minimum diffusion rates correspond to nightside ionosphere conditions. Conversely, the highest diffusion rates and minimum energization are seen for a perfectly reflecting ionosphere boundary. The maximum energies gained under dayside conditions, when Hall conductivity is included, are slightly greater than the maximum energy for similar conditions without Hall conductivity. The diffusion rates for dayside ionosphere conditions with only Pedersen conductivity are greater than the diffusion rates when Hall conductivity is included. These findings show that ULF-wave-particle interactions in Earth's magnetosphere depend on the ionosphere conductance.

Description: Abstract The number of paleoprecipitation records from the South American Monsoon domain that cover the last millennium has increased substantially in past years. However, hitherto most studies focused only on regional aspects, thereby neglecting the role of large‐scale monsoon variability and the mechanisms that link proxy locations in space and time. Here we decompose the South American Monsoon into its main modes of variability by applying a Monte Carlo principal component analysis to a compilation of 11 well‐dated summer paleoprecipitation records from tropical South America. The first mode represents changes in precipitation over the core monsoon domain, while the second mode is characterized by high loadings along the fringes of the South American Monsoon over Southeastern South America and the northern monsoon limit. Composite analysis reveals an enhanced monsoon with a wider, rather than a southward displaced, South Atlantic Convergence Zone during the early Little Ice Age, in contrast to previous interpretations.

Description: Abstract Accurate estimates of ocean mass change are necessary to infer steric sea level change from sea level changes measured with satellite altimeters. Published studies using the Gravity Recovery and Climate Experiment (GRACE) satellite mission indicated a large range in trends (∼1–2 mm/year) with reported standard errors of 0.1–0.3 mm/year. Here we show that a large part of this discrepancy (up to 0.6 mm/year) can be explained by which model is used to account for the effect of glacial isostatic adjustment (GIA). The second largest contribution (0.3–0.4 mm/year) is related to the way how different studies have restored atmospheric and oceanic signals which have been removed during the GRACE gravity estimation process. Here two processing strategies, which previously resulted in differing ocean mass trends, are considered. The “direct” method uses the standard GRACE Stokes coefficients, while the “inverse” method applies a joint inversion of data from GRACE and altimetry. After accounting for differences in processing corrections, global mean ocean mass estimates from the direct, the mascon, and inverse approach agree with each other on global scales within less than 0.1 mm/year. Using the A et al. (2013; https://doi.org/10.1093/gji/ggs030) GIA model, we provide a reconciled monthly time series of global mean ocean mass, which suggests that ocean mass has increased by 1.43 mm/year over 2002.6–2014.5, with an amplified rate of 1.75 mm/year over 2002.6–2016.5 which covers almost the complete GRACE time span. However, we note that estimates as low as 1.05 mm/year cannot be ruled out when other published GIA corrections with lower mass‐equivalent signals over Antarctica are used.

Description: Abstract Snow‐dominated watersheds are bellwethers of climate change. Hydroclimate projections in such basins often find reductions in annual peak runoff due to decreased snowpack under global warming. British Columbia's Fraser River Basin (FRB) is a large, nival basin with exposure to moisture‐laden atmospheric rivers originating in the Pacific Ocean. Landfalling atmospheric rivers over the region in winter are projected to increase in both strength and frequency in Coupled Model Intercomparison Project Phase 5 climate models. We investigate future changes in hydrology and annual peak daily streamflow in the FRB using a hydrologic model driven by a bias‐corrected Coupled Model Intercomparison Project Phase 5 ensemble. Under Representative Concentration Pathway (8.5), the FRB evolves toward a nival‐pluvial regime featuring an increasing association of extreme rainfall with annual peak daily flow, a doubling in cold season peak discharge, and a decrease in the return period of the largest historical flow, from a 1‐in‐200‐year to 1‐in‐50‐year event by the late 21st century.

Description: The cover image is based on the Research Article Effects of estuarine mudflat formation on tidal prism and large-scale morphology in experiments by Lisanne Braat et al., https://doi.org/10.1002/esp.4504.

Description: Mudflats form on the sides of estuaries and on top shoals. The mud accretion elevates high intertidal areas up to supratidal elevations, which decreases tidal prism. This leads to a decrease in channel migration and to a near‐equilibrium planform that is smaller in volume and especially narrower upstream with increased bar heights and no channel deepening. Abstract Human interference in estuaries has led to increasing problems of mud, such as hyper‐turbidity with adverse ecological effects and siltation of navigation channels and harbours. To deal with this mud sustainably, it is important to understand its long‐term effects on the morphology and dynamics of estuaries. The aim of this study is to understand how mud affects the morphological evolution of estuaries. We focus on the effects of fluvial mud supply on the spatial distribution of mudflats and on how this influences estuary width, depth, surface area and dynamics over time. Three physical experiments with self‐forming channels and shoals were conducted in a new flume type suitable for tidal experiments: the Metronome. In two of the experiments, we added nutshell grains as mud simulant, which is transported in suspension. Time‐lapse images of every tidal cycle and digital elevation models for every 500 cycles were analysed for the three experiments. Mud settles in distinct locations, forming mudflats on bars and sides of the estuary, where the bed elevation is higher. Two important effects of mud were observed: the first is the slight cohesiveness of mud that causes stability on bars limiting vertical erosion, although the bank erosion rate by migrating channels is unaffected. Secondly, mud fills inactive areas and deposits at higher elevations up to the high‐water level and therefore decreases the tidal prism. These combined effects cause a decrease in dynamics in the estuary and lead to near‐equilibrium planforms that are smaller in volume and especially narrower upstream, with increased bar heights and no channel deepening. This trend is in contrast to channel deepening in rivers by muddier floodplain formation. These results imply large consequences for long‐term morphodynamics in estuaries that become muddier due to management practices, which deteriorate ecological quality of intertidal habitats but may create potential area for marshes. © 2018 The Authors. Earth Surface Processes and Landforms Published by John Wiley & Sons Ltd.

Description: Predicting debris-flow runout is of major importance for hazard mitigation. Apart from topography and volume, runout distance and area depends on debris-flow composition and rheology, but how is poorly understood. We experimentally investigated effects of composition on debris-flow runout, depositional mechanisms and deposit geometry. The small-scale experimental debris flows were largely similar to natural debris flows in terms of flow behavior, deposit morphology, grain-size sorting, channel width-depth ratio and runout. Deposit geometry (lobe thickness and width) in our experimental debris flows is largely determined by composition, while the effects of initial conditions of topography (i.e., outflow plain slope and channel slope and width) and volume are negligible. We find a clear optimum in the relations of runout with coarse-material fraction and clay fraction. Increasing coarse material concentration leads to larger runout. However, excess coarse material results in a large accumulation of coarse debris at the flow front and enhances diffusivity, increasing frontal friction and decreasing runout. Increasing clay content initially enhances runout, but too much clay leads to very viscous flows, reducing runout. Runout increases with channel slope and width, outflow plain slope, debris-flow volume and water fraction. These results imply that debris-flow runout depends at least as much on composition as on topography. This study improves understanding of the effects of debris-flow composition on runout, and may aid future debris-flow hazard assessments.

Description: Comparisons of collocated Atmospheric Infrared Sounder (AIRS) and Moderate Resolution Imaging Spectroradiometer (MODIS) ice cloud optical thickness (τ) and effective radius (r e ) retrievals and their uncertainty estimates are described at the pixel-scale. While an estimated 27% of all AIRS fields of view contain ice cloud, only 7% contain spatially uniform ice according to the MODIS 1-km optical properties phase mask. The ice cloud comparisons are partitioned by horizontal variability in cloud amount, cloud-top thermodynamic phase, vertical layering of clouds, and other parameters. The magnitudes of τ and r e and their relative uncertainties are compared for a wide variety of pixel-scale cloud complexity. The correlations of τ and r e between the two instruments are strong functions of horizontal cloud heterogeneity and vertical cloud structure, with the highest correlations found in single-layer, horizontally homogeneous clouds over the low-latitude tropical oceans. While the τ comparisons are essentially unbiased for homogeneous ice cloud with variability that depends on scene complexity, a bias of 5–10μm remains in r e within the most homogeneous scenes identified, consistent with known radiative transfer differences in the visible and infrared bands. The AIRS and MODIS uncertainty estimates reflect the wide variety of cloud complexity, with greater magnitudes in scenes with larger horizontal variability. The AIRS averaging kernels suggest scene-dependent information content that is consistent with infrared sensitivity to ice clouds. The AIRS normalized-χ 2 radiance fits suggest that accounting for horizontal cloud variability is likely to improve the AIRS ice cloud retrievals.