bluuebunny/crossref_metadata_2025 huggingface dataset:
Forced modulations of Sahel rainfall at decadal timescale over the 20th Century.
Authors: Ndiaye Cassien Diabe , Mignot Juliette , Mohino Elsa
Published: March 2021

The semiarid region of the Sahel was marked during the 20th Century by significant modulations of its rainfall regime. Part of these modulations has been associated with the internal variability of the climate system, mediated by changes in oceanic sea surface temperature (SST). We show here that the external forcings, and in particular anthropogenic aerosols, might have played a role more important than previously thought in setting these variations. The study is based on the recent simulations performed for CMIP6 with the IPSL-CM6A-LR coupled model. As in most coupled models, the maximum precipitation is limited to the southern Sahel during boreal summer in the IPSL-CM6A-LR model. A novel definition of the Sahel precipitation region is proposed in order to take this bias into account. Our results show that external forcings induce decadal modulations of Sahel precipitation that correlate significantly at 0.6 with the observed precipitations and that the anthropogenic aerosols explain more than 70% of these modulations. These results confirm recent results of CMIP6 highlighting an important role of aerosol forcing for the decadal climate in and around the North Atlantic ocean.
Impact of warm air mass intrusions on atmospheric chemistry and microphysics – Observations during MOSAiC
Authors: Schmale Julia , Dada Lubna , Beck Ivo , Jokinen Tuija , Quélélever Lauriane , Laurila Tii
Published: March 2021

The Arctic aerosol and trace gas regime features strong seasonal differences. The haze season in winter is dominated by long-range transported mid-latitude anthropogenic emissions, while the cleaner summer season is characterized by more local and natural trace gas and aerosol sources. Aerosols and trace gases have been shown to be important for the Arctic radiative balance, inducing an overall net positive radiative forcing through direct radiation interactions.

Aerosols and trace gases are fundamentally different between seasons in terms of chemical composition and microphysical properties. In winter, accumulation mode particles - with a concentration between 100 and 300 cm-3- composed mainly of sulfate, sodium and organics occur, and trace gases relevant for aerosol formation have very low concentrations. In summer, the aerosol number concentration is highly variable reaching from a few to thousands per cubic centimeter, in case of new particle formation. Their size distribution contains nucleation, Aitken and accumulation modes. Trace gases become more abundant, particularly in the marginal ice zone where marine microbial activity emits dimethylsulfide, which leads to the formation of the trace gases sulfuric acid and methanesulfonic acid, which in turn form secondary aerosol mass.

The transition seasons, i.e. spring and autumn, have been studied much less in terms of aerosol and trace gas chemical and microphysical properties in the past , except for ozone depletion events in spring where halogenated trace gases are predominantly involved. The transition between the two aerosol regimes is relatively short. Focusing on spring, the season is characterized by the arrival of warmer and moister air masses from the south, which transport different aerosols and trace gases up north. Cloud formation and precipitation en route have a strong impact on the aerosol number concentrations and size distribution, as well as on the chemical composition due to heterogeneous chemistry in cloud droplets.

Here, we present first results from observations of warm air mass intrusions reaching the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition in mid-April 2020. The period before arrival was characterized by persistent northerly winds, hence aged and dry Arctic air masses, where a very stable accumulation mode composed of sulfate and organics with traces of halogens was measured. With the arrival of southerly air masses, the size distribution started featuring several modes, which increased and decreased in diameter and concentration. Moreover, the chemical composition was significantly changed, featuring methanesulfonic acid from algal blooms in the north Atlantic.
Effects of spatial heterogeneity of leaf density and crown spacing of canopy patches on dry deposition rates
Authors: Yazbeck Theresia , Bohrer Gil , Vines Chante' , De Roo Frederik , Mauder Matthias , Bakshi Bhavik
Published: March 2021

Trees have a large role in improving urban air quality, among other mechanisms, through dry deposition of scalars and aerosols on leaf surfaces. We tested the role of leaf density and canopy structure in modulating the rate of dry deposition. We simulated the interactions between a virtual forest patch and deposition rate of an arbitrary scalar using the Parallelized Large Eddy Simulation Model (PALM). Two canopy structures were considered: a homogenous canopy; and canopy stripes perpendicular to the wind direction. For each canopy stripe scenario, we considered thin, intermediate, and wide stripes, while the space between stripes equals the stripes’ width. Four leaf area densities were considered for each case. The results showed that stripes perpendicular to the wind direction had a larger deposition per leaf area than homogeneous canopies, and denser canopies had more total deposition, but lower per-leaf area rate. Our results can be explained by canopy-induced turbulence structures that couple the air within and above the canopy and lead to more effective leaf area where this coupling is stronger. We aggregate our results to the whole-patch scale and suggest a canopy-structure and leaf-area dependent correction to the canopy resistance parameter so to be used in coarse models that resolve dry deposition. 
A Coupled CFD-DEM Simulation of Immersed Granular Collapse
Authors: Sun Yunhui , Wang Xiaoliang , Liu Qingquan
Published: March 2021

Natural disasters normally involve the flow of polydispersed granular materials with interstitial fluid which may change the flow dramatically. Here we focus on a typical small-scale case of fluid–particle mixture flows, i.e., the immersed granular collapse using computational fluid dynamics coupled with discrete element method (CFD-DEM). The simulation parameters are calibrated with laboratory experiments and the immersed granular collapse process is reproduced in terms of different aspect ratios. We present a deeper investigation of the collapse based on simulation results. The granular front evolves in three stages, i.e., acceleration, steady propagation, and deceleration. We found that the constant propagation stage is maintained by the transition of particles’ motion from vertical to horizontal and the drag of the fluid. The constant propagation velocity is proportional to the free-fall velocity with a Stokes-number-dependent coefficient and the normalized final runout is linearly correlated with the densimetric Froude number. These conclusions may find its significance in geophysical applications.
GFZ contribution to the third IGS reprocessing campaign
Authors: Männel Benjamin , Brandt Andre , Bradke Markus , Brack Andreas , Sakic Pierre , Nischan Thomas
Published: March 2021

Based on a large network of continuously operated GNSS tracking stations, the International GNSS Service (IGS) has a valuable contribution to the realization of the International Terrestrial Reference System. To provide the most accurate and consistent solution, the IGS refined the strategy and the set of associated models for the ongoing third reprocessing campaign. Beyond updated background models, a significant improvement is the combined reprocessing of three GNSS, namely GPS, GLONASS, and Galileo. Furthermore, signal-specific receiving antenna calibrations for Galileo and scale-free transmission phase center positions are applied. These modifications will allow exciting new investigations based on the delivered products.

In this contribution, we will present the final GFZ contribution to the IGS reprocessing efforts. We will present selected aspects of the station selection, parametrization, and processing scheme in the first part. Secondly, we will focus on the results by discussing the derived orbits, Earth rotation parameters, and station coordinates. Thirdly, the first results of our TIGA (IGS Tide Gauge Benchmark Monitoring Project) reprocessing will be presented, which are based on the previously discussed orbits and satellite clocks.
Transtensional Flanking Structures
Authors: Mayrhofer Franziska , Grasemann Bernhard , Schöpfer Martin , Adamuszek Marta
Published: March 2021

Flanking structures are deflections of an existing planar fabric (e.g., foliation) alongside a cross-cutting element (e.g., a vein) that can develop in a wide range of rock types, ranging from eclogites to unconsolidated sediments, and also glacier ice, which deforms in temperate glaciers dominantly by dislocation creep and can be considered as a monomineralic metamorphic rock analogue. The finite geometry of flanking structures depends on several factors, such as initial orientation of the cross-cutting element (CE) relative to the shear zone boundary and the kinematic vorticity of the shear zone flow. However, nearly all published examples of flanking structures are interpreted to have formed either under simple shear or transpressional general shear, although in theory flanking structures should also form under transtensional general shear. Here we describe the geometry and development of transtensional flanking structures in glacial ice of the Pasterze, Austria’s largest alpine valley glacier. Mapping was carried out with the aid of high-resolution drone photography and the structures’ attitudes were determined using traditional field techniques. The studied flanking structures develop in an area situated on the orographic right side of the glacier tongue and downstream of a transverse crevasse field. The CEs are closed crevasses containing granular ice and rotate clockwise (when viewed from above), consistent with the large-scale flow field of the glacier. The penetrative foliation, which is regionally parallel to the glacier’s flow direction, is locally deflected alongside the CEs, forming a- (antithetic) and s-type (synthetic) flanking structures. The variability of the cross-cutting elements’ orientation systematically decreases downstream as they rotate into a stable position. We compare the mapped flanking structures with model results of a semi-analytical modified Eshelby solutions for a frictionless CE embedded in an isotropic linear viscous matrix. The model results demonstrate that a variety of a- and s-type flanking structures form under transtensional shear flow for a broad range of kinematic vorticity numbers and initial orientations of the CE but also show that shear bands do not form a stable structure. On the other hand, s-type flanking folds may be diagnostic for transtension because they form stable structures (but still accumulate displacement) when the CE has been rotated parallel to the fabric attractor, which is oblique to the shear zone boundary under transtension. Because of the abundance of shear bands and the lack of s-type flanking structures in natural rocks we speculate that transtensional ductile shear zones rarely occur in nature.

 
Evolution of Tidal Flats in the Northern Part of the Po Delta (Italy):  A Strategy for Future Buiding-with-Nature Management
Authors: Brunetta Riccardo , Ciavola Paolo
Published: March 2021

In the period 1950s-60s, the Po river Delta (Northern Italy) was hit by several floods. Agricultural fields were covered by water and many of them remained submerged since. As a consequence of the massive sediment injection into the system, this lead to the birth of new tidal flats around the tip of the Delta. The evolution of these environments over 50 years was studied, as they may be taken as an example for future reconstruction of intertidal areas. The sediment distribution and the morphological evolution of a young tidal flat of about 10 ha located in the Northern part of the Po della Pila branch were studied by undertaking fieldwork since October 2018, including detailed topographic surveys using a UAV, sedimentological analyses, and a study of sediment deposition rates. An extended crevasse splay covers the central part of the flat. The granulometry is predominately fine (Silty clay and Clayey silt), except for the central area, where the sand percentage increases (Loam and Silty sand). This surface distribution is uniform down to ~10 cm; the sand percentage increases instead within the sediment column from ~10 to 25 cm next to the mouths of the channels. The tidal flat experienced a positive sediment budget and it was characterized by higher rates of accretion after the Po river floods. These observations suggest that the tidal channels are fed by sediment from the Po River branch. Orthophotos from the 1950s show that the tidal flat is about 17 - 20 years old and its formation was influenced by human intervention and river floods. The work aims at finally comparing this case study with other tidal flats and salt marshes worldwide characterized by similar and different tidal regimes, to identify the optimal elevation for vegetation to establish and flourish, to support the future restoration of these environments.
Response of soil nutrient structure and aggregate strength to land use in a typical area in the Loess Plateau, China 
Authors: Zhang Yixin , Li Peng , Xu Guoce
Published: March 2021

Changes in land use patterns have important implications for soil structure and soil nutrient transport processes. This paper is based on the project of returning farmland to forests in the Loess Plateau-Ziwuling area of China. Explore the phase changes of the aggregate structure and nutrients content and its effect on soil infiltration and erodibility during the changing land use process. Identify the effective time domain for soil management in the area, which provide a scientific basis for coordinating regional land use and efficient soil erosion control. The results showed that with the increase of soil recovery/opening time, the content of soil water stable aggregates (SWAG), soil organic matter (SOM), total nitrogen (TN) and total phosphorus (TP) showed an increasing /decreasing trend. In the process of soil reclamation, the content of organic matter in large-size aggregates (>5mm) decreases first at a higher rate, while in land restoration process, the content of organic matter in small-sized (2-5mm) aggregates increases rapidly. With the increase of soil reclamation time, the initial time of runoff production is advanced. In the 30-year of the land restoration process, the erodibility K decreased by 87%, and the SOC content has reached 96% to the common forest level, indicate that the soil quality is greatly improved when the land returns to this moment, the ability to resist erosion reaches a certain level and tends to be stable, and it is considered that the input and output of the governance before the time node is relatively high.
Application of DPSIR method for integrated management of the transboundary aquifer in Prespa-Ohrid basin
Authors: Voudouris Konstantinos , Kiri Emanuela , Reci Hamza , Kolokytha Elpida
Published: March 2021

The transboundary aquifers are of utmost importance to sustain water supply in present days. The exploitation of these aquifers has to be only in a sustainable and rational way, in order to sustain the quantity and quality of the aquifers' groundwater. A transboundary aquifer is developed in karstified limestones between the lakes Great Prespa (shared between North Macedonia, Albania, and Greece ) and Ohrid (shared between North Macedonia and Albania). The karst mountainous area is characterized by the presence of numerous crevices, sinkholes, as surface forms, and caves, canals, etc., as underground forms. The elevation of Prespa Lake is about 153 m higher than that of Ohrid Lake, and the two lakes represent a common hydraulic system, protected by international conventions. Prespa Lake is characterized by a continuous decline in water level during the last decades. The transboundary karstic aquifer is a complex system, discharging through numerous karstic springs (e.g., Saint Naum, Tushemisht, etc.), and operating as a hydrocollector and hydroconductor between the lakes. The aquifer is vulnerable to external pollution, as well as to climate changes affecting the lakes’ ecosystems. Agriculture, livestock, and tourism are the main activities in the wider area.

The DPSIR (Driver-Pressure-State-Impact-Response) model, adopted by European Environmental Agency, was applied in GIS context, in order to study and analyze the main causes of pressures and to optimize the measures for integrated aquifer management. In addition, the aim of DPSIR technique is the correlation of the driving forces and pressures with the present status of the complex hydraulic system and finally to optimize the aquifer management. The major driving forces that affect the Prespa-Ohrid basin are the irrigated agriculture, the livestock, the tourism development, and population growth. The main pressures are the overexploitation of the aquifer, water pollution, and the decline of the water level of Lake Prespa.

Based on the results of DPSIR, a set of measures and appropriate policy responses are proposed. Finally, DPSIR is a valuable tool for local authorities and administrators in order to plan and implement strategies for integrated and sustainable management of the transboundary karst aquifer and its dependent ecosystems.
Mobility of potentially toxic elements in family garden soils of the Riotinto mining area
Authors: Romero-Baena Antonio , Barba-Brioso Cinta , Ross Alicia , González Isabel
Published: March 2021

Agricultural soils in mining areas usually accumulate potentially toxic elements (PTEs) that can become a health risk to humans by entering the trophic chain. In this study, five small agricultural plots close to Riotinto mines (SW Spain) were studied, with the aims of comparing the concentration of PTEs with respect to the regional (South Portuguese Zone) baseline and conducting availability studies in order to determine the contamination of soils. Chemical composition, total and clay mineralogy, and edaphic parameters were determined in topsoil and subsoil samples to characterize the soils, and single extractions were conducted to assess the mobility. The mineralogy of the soils was composed of quartz and phyllosilicates, with small amounts of feldspars and occasionally containing hematite and calcite. The texture ranged from sandy to silty loam, the pH was slightly acidic, and high contents of organic matter were found. Total concentrations of trace elements correlated with the texture, the content in iron oxy-hydroxides and the pH. The values of As, Pb, Cu, and Zn exceeded the regional baseline even in sites unaffected by mining. The results suggest that a widespread sampling is necessary to determine the local background. The most water-soluble element was As, due to the competition of organic matter for sorption sites. The content of Cu, Cr and Zn extracted with different methods were higher in sandy soils with low iron oxy-hydroxides content. Monoammonium phosphate and EDTA extractions seemed to remove elements from organic matter and iron oxy-hydroxides. The extracted fractions of As and metals reached up to 10-30 wt%.  Despite the high total concentrations of the element in soils, they generally showed low available proportions, especially with water and ammonium acetate extractants. The results suggest that the soils are not necessarily a risk to humans and higher investigation efforts are necessary to assess the availability of PTEs and their transfer to plants.
Using short-term ensemble weather forecast to evaluate outcomes of irrigation
Authors: Guo Danlu , Western Andrew , Wang Quan , Ryu Dongryeol , Moller Peter , Aughton David
Published: March 2021

Irrigation water is an expensive and limited resource. Previous studies show that irrigation scheduling can boost efficiency by 20-60%, while improving water productivity by at least 10%. In practice, scheduling decisions are often needed several days prior to an irrigation event, so a key aspect of irrigation scheduling is the accurate prediction of crop water use and soil water status ahead of time. This prediction relies on several key inputs such as soil water, weather and crop conditions. Since each input can be subject to its own uncertainty, it is important to understand how these uncertainties impact soil water prediction and subsequent irrigation scheduling decisions.

This study aims to evaluate the outcomes of alternative irrigation scheduling decisions under uncertainty, with a focus on the uncertainties arising from short-term weather forecast. To achieve this, we performed a model-based study to simulate crop root-zone soil water content, in which we comprehensively explored different combinations of ensemble short-term rainfall forecast and alternative decisions of irrigation scheduling. This modelling produced an ensemble of soil water contents to enable quantification of risks of over- and under-irrigation; these ensemble estimates were summarized to inform optimal timing of next irrigation event to minimize both the risks of stressing crop and wasting water. With inclusion of other sources of uncertainty (e.g. soil water observation, crop factor), this approach shows good potential to be extended to a comprehensive framework to support practical irrigation decision-making for farmers.
UAV-based aeromagnetic gradient measurements and inversion
Authors: Kulüke Christian , Virgil Christopher , Stoll Johannes , Hördt Andreas
Published: March 2021

Rotary wing UAV’s are used in aeromagnetic measurements for UXO detection. That way, contaminated areas can be mapped fast and with high resolution. Until today, only the total magnetic intensity (TMI) is evaluated, even when a three axis fluxgate magnetometer is flown. In this project, we use two three component fluxgate sensors, an inertial measurement unit (IMU) and a GPS antenna. The IMU allows for a projection of the magnetic data into the geographic coordinate system as well as the calculation of the sensor positions relative to the GPS antenna. With this system, it is possible for the first time to evaluate the component gradients between the magnetometers.

The sensors are attached to the UAV via a versatile, T-shaped boom hanging below the UAV with the sensors positioned in a horizontal distance of 50 cm. The total mass of the flight system is about 5 kg with an air time of 15 minutes.

For the inversion, we use a dipole model which calculates the magnetic data for all sensor positions. Because the sources of the magnetic anomalies are unknown as a general rule, there is no distinction between induced and remanent magnetisation. Instead, the three components of the magnetic moment are fitted alongside the positions of the anomaly sources. The number of dipoles to be fitted and their initial parameters are arbitrary. For the inversion, the TMI and component gradients between the sensors are considered.

In order to analyse the accuracy of the complete system, we conducted surveys over a test field of 100 x 20 m, separated into four sections with varying anomaly configurations. As anomaly sources, we used neodymium magnets which we characterised in laboratory measurements. For optimal coverage and to compare flight directions, the test field was surveyed both lengthways and crossways with a sensor height of 1.5 m above ground. Inversion results show that when component gradients are used, overlapping anomalies can be separated and parameterised. The mean errors of the derived anomaly positions are 5 cm, the total magnetic moment can be determined with an accuracy of 0.35 Am2, whereby the errors in direction (declination and inclination) are 4 ° and 2 °, respectively.
Influence of the accumulation chamber insertion depth to measure surface radon exhalation rates
Authors: Gutiérrez Álvarez Isidoro , Guerrero José Luis , Martín José Enrique , Adame José Antonio , Bolívar Juan Pedro
Published: March 2021

A common method to measure radon exhalation rates relies on the accumulation chamber technique. Usually, this approach only considers one-dimensional gas transport within the soil that neglects lateral diffusion. However, this lateral transport could reduce the reliability of the method. In this work, several cylindrical- shaped accumulation chambers were built with different heights to test if the insertion depth of the chamber into the soil improves the reliability of the method and, in that case, if it could limit the radon lateral diffusion effects. To check this hypothesis in laboratory, two reference exhalation boxes were manufactured using phospho- gypsum from a repository located nearby the city of Huelva, in the southwest of Spain. Laboratory experiments showed that insertion depth had a deep impact in reducing the effective decay constant of the system, extending the interval where the linear fitting can be applied, and consistently obtaining reliable exhalation measurements once a minimum insertion depth is employed. Field experiments carried out in the phosphogypsum repository showed that increasing the insertion depth could reduce the influence of external effects, increasing the re- peatability of the method. These experiments provided a method to obtain consistent radon exhalation mea- surements over the phosphogypsum repository.
Effects of vegetation as an ecosystem service on the changes in runoff and sediment yield in a Mediterranean semi-arid basin
Authors: Vidal-Abarca Gutiérrez María Rosario , Martínez-Salvador Alberto , Conesa-García Carmelo , Suárez-Alonso María Luisa , Alonso-Sarria Francisco , Pérez-Cutillas Pedro , Gomáriz-Castillo Francisco J.
Published: March 2021

Semiarid basins contribute significantly to sediment loads, as they are often characterized by torrential flows, source areas with high sediment-producing rates, great availability of erodible material subjected to intense weathering processes, and poor vegetation cover. Vegetation, despite its scarce presence, is a dynamic component of this environment, which provides a range of important ecosystem services such as biodiversity, flood retention, nutrient sink, erosion control and groundwater recharge. This study examines the vegetation responses to the magnitude of peak flows and its contribution to the changes in runoff and sediment yield during the period 1997-2020 in a catchment Mediterranean semiarid basin: The Rambla de la Azohía (southeastern Spain).Vegetation type, density, preferred location and degree of permanence in each sub-basin were analyzed in order to determine their degree of influence on surface runoff and erosion control. Changes in riparian vegetation cover was quantified at large scale for the analysis period (1997-2020), using remotely sensed spatial information, such as satellite images and aerial photographs separated by two years on average (at scales from 1:15000 to 1:30000, and resolution between 0.22 and 0.50 m/pixel). A geo-spatial erosion prediction model was applied to estimate the runoff and sediment load generated at the event scale, taking into account the variability of the vegetation cover in each sub-basin. The simulated outputs of this model were previously calibrated with water levels measured by pressure sensors and suspended sediment records.The results showed both a poor response of vegetation (low incidence in the runoff coefficient) in steep metamorphic watersheds, capable of supplying large sediment loads, and functioned as an efficient ecosystem service (stabilization of slopes and decrease in peak flow) in less steep sub-basins with slopes in the shadow, composed of limestone formations and alluvial fans. This suggests important spatial differences in the vegetation impact, according to other environmental conditions intrinsic to each sub-basin, but also a low overall influence on the temporal variability of sediment fluxes at the event scale. This research was funded by FEDER/Spanish Ministry of Science, Innovation and Universities—State Research Agency (AEI)/Projects CGL2017-84625-C2-1-R and CGL2017-84625-C2-2-R; State Program for Research, Development and Innovation Focused on the Challenges of Society.
A new ensemble-based statistical methodology to verify changes in weather and climate models
Authors: Zeman Christian , Schär Christoph
Published: March 2021

Since their first operational application in the 1950s, atmospheric numerical models have become essential tools in weather and climate prediction. As such, they are a constant subject to changes, thanks to advances in computer systems, numerical methods, and the ever increasing knowledge about the atmosphere of Earth. Many of the changes in today's models relate to seemingly unsuspicious modifications, associated with minor code rearrangements, changes in hardware infrastructure, or software upgrades. Such changes are meant to preserve the model formulation, yet the verification of such changes is challenged by the chaotic nature of our atmosphere - any small change, even rounding errors, can have a big impact on individual simulations. Overall this represents a serious challenge to a consistent model development and maintenance framework.

Here we propose a new methodology for quantifying and verifying the impacts of minor atmospheric model changes, or its underlying hardware/software system, by using ensemble simulations in combination with a statistical hypothesis test. The methodology can assess effects of model changes on almost any output variable over time, and can also be used with different hypothesis tests.

We present first applications of the methodology with the regional weather and climate model COSMO. The changes considered include a major system upgrade of the supercomputer used, the change from double to single precision floating-point representation, changes in the update frequency of the lateral boundary conditions, and tiny changes to selected model parameters. While providing very robust results, the methodology also shows a large sensitivity to more significant model changes, making it a good candidate for an automated tool to guarantee model consistency in the development cycle.
A computationally efficient two-phase hydro-sediment-morphdynamic model and its preliminary field applications  
Authors: Lyu Binghan , Hu Peng , Li Ji , Cao Zhixian , Li Wei , He Zhiguo
Published: March 2021

While fluvial flows carrying relatively coarse sediments involve strong two-phase interactions, existing numerical modeling in the field-scale is mostly based on quasi-single phase flow model. Here a computationally efficient two-phase hydro-sediment-morphodynamic model is developed with a special focus on field applications. The hybrid LTS/GMaTS method originally developed for quasi-single flow model is extended to the present two-phase flow model, of which the achieved reduction in the computational cost facilitates the present field applications in the Taipingkou Waterway, Middle Yangtze River. To overcome numerical instabilities arising from the relatively large spatial and time steps in field case that lead to an issue of stiff source term, the following numerical treatments are proposed: implementation of theoretically-derived lower and upper limits for the inter-phase interactive forces. Moreover, to improve the numerical accuracy, the HLLC approximate Riemann solver is used for the water phase, whereas the FORCE solver is used for the sediment phase. Both the present two-phase flow model and the existing quasi-single-phase flow model are applied to a series of typical cases, including refilling of a dredged trench, a full dam-break flow in an abruptly widening channel and reproduction of the Taipingkou waterway, Middle Yangtze River. Compared with the quasi-single-phase flow model, the two-phase flow model has better performance as compared to the measure data and has more profound physical significance.
Contribution of forcings to Holocene climate evolution
Authors: Hopcroft Peter , Valdes Paul
Published: March 2021

The climate evolution of the past few thousand years is essential for understanding the context in which civilisation arose and for understanding the natural background of anthropogenic influence. Proxy-inferred records show a complex picture of earlier warming and later cooling during the Holocene depending on region and reconstruction method. In contrast climate model simulations almost uniformly show warming throughout the past 10,000 years and for example also fail to reproduce a major advance of rainbelt over the Sahara.  These discrepancies raise questions about the reliability of climate models on longer-time scales.

We present a suite of four new transient Holocene simulations covering the last 8500 years using the HadCM3B-M21aD coupled general circulation. We use an optimised version of this model which is able to replicate the greening of the Sahara through changes to the atmospheric convection and vegetation schemes. We apply transient changes in Earth’s orbit, ice-sheets and sea-level and greenhouse gases, and optionally solar output, volcanic eruptions and anthropogenic land-use change.  The simulations without land-use show a warming throughout the Holocene, albeit with significantly higher variability once volcanic eruptions are included. With the inclusion of land-use change temperature trends in Northern Hemisphere are reversed from around 4000 years before present.

We explore the contribution of different forcings to the regional trends in the model ensemble and we compare the simulations against the Holocene reconstructions to evaluate the relative importance of each forcing. We also use the model ensemble to quantify the terrestrial coverage of proxy locations that is required to reliably infer global mean temperature variations.
Coupling Hydrus-1D and METRIC models to estimate soil water balance of a highly heterogeneous agricultural field
Authors: Zancanaro Ester , Dal Ferro Nicola , Furlanetto Jacopo , Longo Matteo , Teatini Pietro , Morari Francesco
Published: March 2021

Understanding spatiotemporal variability of agricultural fields is fundamental for the definition of dynamic management zones and enhancing precision irrigation techniques. Accurate crop evapotranspiration (ET) estimation helps understanding the hydrological dynamics that characterize the root zone, particularly with highly heterogeneous conditions. The FAO method for the ET estimation is the most widely used in hydrological modeling. However, this approach is only partly effective in describing the spatial variability of water and salt stress at the subfield scale. Recently, remote sensing has been utilized as a viable tool for capturing the actual crop ET (ETa) at different scales but its coupling with hydrological modeling is still challenging. In this study, an original method was developed to integrate the hydrological model Hydrus-1D with the satellite-based energy balance METRIC (Mapping Evapotranspiration at High Resolution with Internalized Calibration) model. To this end, a two-year trial was conducted in a maize field located in the southern margin of the Venice Lagoon, characterized by heterogeneous soil properties and seawater intrusion. Five automatic monitoring stations were installed to investigate soil physical characteristics and hydrological dynamics. Undisturbed soil cores were collected and analyzed to determine soil water retention curves and hydraulic conductivity. Disturbed soil sample were analyzed for texture and chemical properties. Volumetric water content and matric potential were hourly monitored at 0.1, 0.3, 0.5 and 0.7 m, while data of depth to the water table were collected every week. Meteorological data were retrieved from an on-site weather station. Two approaches were used for modeling and optimizing the water dynamics. Firstly, water content and pressure head data collected in the field were used as input to Hydrus-1D and the inverse method was applied for the optimization of the water retention curve. Then, METRIC was coupled with Hydrus-1D, by using the ETa calculated from Landsat 8 images as forcing variable to enhance the inverse solution. Preliminary results highlighted that the integration of Hydrus-1D and METRIC model allowed to capture the different hydrological dynamics found at the five stations.
Regulation and Critical Threshold of Grass Cover on Slope Runoff in LoessHilly Gully Region under Artificial
Authors: Zhang Qiufen , Lv Xizhi , Chen Rongxin , Ni Yongxin , Ma Li
Published: March 2021

The slope runoff caused by rainstorm is the main cause of serious soil and water loss in the loess hilly area, the grassland vegetation has a good inhibitory effect on the slope runoff, it is of great significance to reveal the role of grassland vegetation in the process of runoff generation and control mechanism for controlling soil erosion in this area. In this study, typical grassland slopes in hilly and gully regions of the loess plateau were taken as research objects. Through artificial rainfall in the field, the response rules of slope rainfall-runoff process to different grass coverage were explored. The results show that: (1) The time for the slope flow to stabilize is prolonged with the increase of vegetation coverage, and shortened with the increase of rainfall intensity; (2) At 60 mm·h −1 rainfall intensity, the threshold of grassland vegetation coverage is 75.38%; at 90 mm·h −1 rainfall intensity, the threshold of grassland vegetation coverage is 90.54%; at 120 mm·h −1 rainfall intensity, the impact of grassland vegetation coverage on runoff is not significant; (3) the Reynolds number and Froude number of slope flow are 40.07‒695.22 and 0.33‒1.56 respectively, the drag coefficient is 1.42‒43.53. Under conditions of heavy rainfall, the ability of grassland to regulate slope runoff is limited. If only turf protection is considered, about 90% of grassland coverage can effectively cope with soil erosion caused by climatic conditions in loess hilly and gully regions. Therefore, in loess hilly areas where heavy rains frequently occur, grassland's protective effect on soil erosion is obviously insufficient, and investment in vegetation measures for trees and shrubs should be strengthened.
Spatially coherent postprocessing of cloud cover forecasts using generative adversarial networks
Authors: Dai Yinghao , Hemri Stephan
Published: March 2021

Despite considerable improvements over the last few decades, numerical weather prediction (NWP) models still tend to exhibit bias and dispersion errors. Statistical postprocessing reduces these errors and allows quantifying predictive uncertainty. However, classical postprocessing approaches such as ensemble model output statistics (EMOS) destroy any physical dependence structure of the NWP raw ensemble forecasts. Ensemble copula coupling (ECC) is a commonly used state-of-the-art method to map the spatio-temporal dependence structure of the raw ensemble to the postprocessed predictive distributions. However, if the variable of interest exhibits many ties, ECC may not be optimal. Here, the variable investigated is hourly cloud cover over Switzerland. The climatological distribution of cloud cover shows considerable point masses at both zero and one, hence ties are a major issue when it comes to applying ECC. 

We compare a variant of ECC, which is tailored to variables with many ties, applied to postprocessed forecast ensembles obtained by either EMOS or a dense neural network (dense NN) with postprocessed scenarios generated by a conditional generative adversarial network (cGAN). In particular, cGANs are appealing as they directly generate maps of postprocessed cloud cover forecast scenarios without the need of any dependence template. We trained the postprocessing models for COSMO-E and ECMWF IFS raw ensemble forecasts against hourly EUMETSAT CM SAF satellite data with a spatial resolution of around 2 km over Switzerland. For all the approaches, EMOS, dense NN, and cGANs, basic setups with a minimal set of raw ensemble predictors already allowed us to obtain a significantly better univariate performance (in terms of continuous ranked probability score) than the raw NWP ensembles. We present and discuss the advantages and drawbacks of EMOS+ECC, dense NN+ECC, and cGANs with respect to both univariate forecast skill and the ability to produce realistic cloud cover forecast scenario maps. 
Strong following earthquake forecasting by a pattern recognition approach in California 
Authors: Gentili Stefania , Di Giovambattista Rita
Published: March 2021

During seismic clusters, strong earthquakes (e.g. the mainshocks) are sometimes followed by another strong following earthquake, very dangerous because it strikes already damaged structures. To forecast the occurrence of such subsequent large earthquakes (SLE), we proposed a pattern recognition approach based on seismological features. The method, called NESTORE, has been successfully applied to northeastern Italy and western Slovenia (Gentili and Di Giovambattista, 2020) and to all of Italy (Gentili and Di Giovambattista, 2017). In this study, we will present the results of the application of NESTORE to California seismicity. NESTORE method is adaptive and depends on the region analyzed. During the supervised training phase, some features are selected as the best-performing ones in the analyzed area, which will be used for classification. Tests of this method demonstrate good performance for California seismicity.

 
Sensitivity Analysis of the Hyperparameters of CNN for Precipitation Downscaling
Authors: Nagasato Takeyoshi , Ishida Kei , Yokoo Kazuki , Kiyama Masato , Amagasaki Motoki
Published: March 2021

Nowadays, a convolutional neural network (CNN), which is a kind of deep neural network has been shown to have high applicability to precipitation downscaling in previous studies. CNN has various hyperparameters, which greatly affect the estimation accuracy. In the field of computer science, researches on hyperparameter settings have been conducted, especially for image recognition. However, there are few studies that investigated the sensitivity analysis of hyperparameters on precipitation downscaling by means of CNN. Therefore, this study conducted a sensitivity analysis of the hyperparameters of CNN for precipitation downscaling. For this study, atmospheric reanalysis data were used as the inputs for precipitation downscaling by means of CNN, and daily average precipitation at the basin level were used as the target data. Then, this study focused on the hyperparameters of CNN that have a great influence on the feature extraction of input data (such as kernel size, number of output channels in the convolutional layer, etc.). Considering that the learning process of CNN has randomness, CNN was trained 200 times for the setting conditions of each hyperparameter and evaluated the estimation accuracy. As the results of detailed sensitivity analysis, it was shown that the estimation accuracy may not be improved even if the structure of CNN deeper. Contrarily, It was also shown that initial conditions such as batch selection and bias in the CNN learning process may have relatively large effects on the learning results.
Soil texture can predominantly control organic matter mineralization in temperate climates by regulating soil moisture rather than through direct stabilization
Authors: Li Haichao , Van den Bulcke Jan , Mendoza Orly , Deroo Heleen , Haesaert Geert , Dewitte Kevin , De Neve Stefaan , Sleutel Steven
Published: March 2021

Soil organic carbon (OC) levels generally increase with increasing clay and silt content under a similar climatic zone because of increased association of OC to clay minerals and stronger occlusion inside aggregates. Surprisingly though, in Western Europe many silt loam soils actually bear low topsoil OC levels compared to lighter textured soils. Soil texture obviously also strongly controls moisture availability with consequent indirect impact on heterotrophic activity. We hypothesized that with increasingly frequent summer drought: 1) soil microbial activity in sandy soils is more likely impeded due to their limited water holding capacity retention during droughts, while soil OC mineralization in silty soils remain be less drought-limited; 2) capillary rise from sufficiently shallow groundwater would, on the other hand, alleviate the water stress in lighter textures. To test these hypotheses, we established a one-year field trial with manipulation of soil texture, monitoring of soil moisture and maize-C decomposition via 13/12C-CO2 emissions. The upper 0.5 m soil layer was replaced by sand, sandy loam and silt loam soil with low soil OC. Another sandy soil treatment with a gravel layer was also included beneath the sand layer to exclude capillary rise. Soil texture did not affect maize-C mineralization (Cmaize-min) until April 2019 and thereafter Cmaize-min rates were higher in the silt loam than in the sandy soils (P=0.01). θv correlated positively with the Cmaize-min rate for the sand-textured soils only but not for the finer textures. These results clearly highlight that soil texture controlled Cmaize-min indirectly through regulating moisture under the field conditions starting from about May, when soils faced a period of drought. By the end of the experiment, more added Cmaize was mineralized in the silt loam soil (81%) (P<0.05) than in the sandy soil (56%). Capillary rise did not result in a significant increase in cumulative Cmaize-min in the sandy soil, seemingly because the capillary fringe did not reach the sandy topsoil layer. These results imply that, under future climate scenarios the frequency of drought is expected to increase, the largely unimpeded microbial activity in silty soils might lead to a further stronger difference in soil OC with coarser textured soils under similar management.
Investigating tropical squall lines with a cloud resolving model
Authors: Abramian Sophie , Muller Caroline , Risi Camille
Published: March 2021

Investigating tropical squall lines with a cloud resolving model

Using a cloud resolving model, we attempt to clarify the physical processes responsible for the organization of deep clouds into squall lines in the tropics. To do so, we impose a vertical wind shear, and investigate the response of deep convection to different shear strengths in radiative convective equilibrium. As the magnitude of the shear increases, the convection becomes more and more organized into a line, perpendicular to the shear. It is due to the interaction of the low-level shear with the cold pools associated with convective downdrafts. Beyond a certain shear, called optimal shear, the line tends to orient at an angle to the shear. The existing literature suggests that this angle conserves the projection of the shear on the direction perpendicular to the squall line near the optimal value, a hypothesis that we further investigate here.

In this work, we propose a systematic method, based on image auto-correlation, to determine the angle of the squall line with respect to the shear. We highlight the existence of the sub-critical and super-critical regime, as predicted by earlier studies. In the sub-critical regime, squall lines are indeed perpendicular to the shear. Yet, angles of squall lines in the super-critical regime do not clearly correspond to the conservation of the projected component of the shear near the optimal value. In particular, squall lines often remain more perpendicular to the shear than expected.

We thus investigate the balance between shear and cold pool winds to explain this difference. Using statistical methods on extreme events, we find that this difference is due to an intensification of cold pool potential energy with shear. Cold pool intensification allows the squall line to better resist to the shear, and thus reduces its angle of orientation. This new feature leads us to conclude that two mechanisms maintain a squall line in wind shear : the orientation of clouds and the intensification of cold pools.