bluuebunny/crossref_metadata_2025 huggingface dataset:
Dual seismic migration velocities reveal imbricated fluid diffusion and aseismic slip In a Corinth Gulf  swarm (Greece)
Authors: De Barros Louis , Dublanchet Pierre , Cappa Frédéric , Deschamps Anne
Published: March 2021

Fluid induced earthquake sequences generally appear as expanding swarms activating a particular fault. Such swarms are generally interpreted as fluid diffusion, which ignores the possibility of static, dynamic or aseismic triggering, and the existence of rapid migration. Here, we study the temporal evolution of a seismic swarm that occurred over a 10-day period in October 2015 in the extensional rift of the Corinth Gulf (Greece) using high-resolution earthquakes relocations. The seismicity radially migrates on a normal fault at a fluid diffusion velocity (~125 m/day). However, this migration occurs intermittently, with periods of fast expansion (2-to-10 km/day) during short seismic bursts alternating with quiescent periods. Moreover, the growing phases of the swarm illuminate a high number of repeaters. Therefore, we propose a new model to explain the combination of multiple driving processes for such swarms.  Fluid up flow in the fault may induce aseismic slip episodes, separated by phases of fluid pressure build-up. The stress perturbation due to aseismic slip may activate small asperities in the fault that produce bursts of seismicity during the most intense phase of the swarm. We then validated this model through hydro-mechanical modeling, where earthquakes consist in the failure of asperities on a creeping fault infiltrated by fluid. For that, we couple rate‐and‐state friction, non‐linear diffusivity and elasticity along a 1D interface. This model reproduces the dual migration speeds observed in real swarms. We show that migration speeds increase linearly with the mean pressurization, and are not dependent on the hydraulic diffusivity, as traditionally suggested.
Isolating internal secular variation in Geomagnetic Virtual Observatory time series using Principle Component Analysis
Authors: Brown William , Beggan Ciarán , Hammer Magnus , Finlay Chris , Cox Grace
Published: March 2021

Geomagnetic Virtual Observatories (GVOs) are a method for processing magnetic satellite data in order to simulate the observed behaviour of the geomagnetic field at a fixed location. As low-Earth orbit satellites move at around 8 km/s and have an infrequent re-visit time to the same location, a trade-off must be made between spatial and temporal coverage, typically averaging over half the local time orbit precession period, within a radius of influence of 700 km. The annual differences (secular variation, SV) of residuals between GVO time series data and an internal field model at a single GVO location will be strongly correlated with its neighbours due to the influence of large-scale external field sources and the effect of local time precession of the satellite orbit. Using Principal Component Analysis we identify and remove signals related to these noise sources to better resolve internal field variations on sub-annual timescales.

We apply our methodology to global grids of monthly GVOs for the Ørsted, CHAMP, CryoSat-2 and Swarm missions, covering the past two decades. We identify common principle components representing orbit precession rate dependent local time biases, and major external field sources, for all satellites. We find that the analysis is enhanced by focussing on regions of geomagnetic latitude where different external field sources dominate, identifying distinct influences in polar, auroral and low-to-mid latitude regions. Annual differences are traditionally used to calculate SV so as to remove annual and semi-annual external field signals, but these signals can be re-introduced if our corrected SV is re-integrated. We find that by representing secular variation with monthly first differences, rather than annual differences, we can identify and remove annual and semi-annual external field variations from the SV, which then improves the use of re-integrated main field GVO time series. By better accounting for contaminating signals from correlated external fields and aliasing, we are able to produce a global grid of GVO time series which better represents internal secular variation at monthly time resolution.
A micromechanically calibrated numerical model reproducing earthquake cycle in the lab
Authors: Mollon Guilhem , Aubry Jérôme , Schubnel Alexandre
Published: March 2021

In this communication, we present a novel numerical framework which consists in a direct coupling between a discrete micromechanical modelling of rock damaging processes and a continuous modelling of elastic deformation and acoustic waves. It includes a polygon-based conforming Discrete Element Method (DEM) with a cohesive zone model (CZM, [1]) for the discrete part and a meshfree formulation for the continuum part. This framework is applied to the numerical reproduction of sawcut triaxial tests performed in the lab on marble samples under seismogenic conditions [2]. Realistic boundary conditions (in terms of the elasticity of the loading system, of the absorption of the elastic waves and of the fluid pressure applied on the lateral boundaries) are introduced. Constitutive laws (in the continuum part) and micromechanical parameters (in the discrete part) are calibrated by performing independant simulations based on experimental results found in the literature [3].

Upon loading, this model provides information on the system behavior that nicely complement the experimental data, such as (i) the progressive damaging of the contacting surfaces, leading to the emission of granular matter in the interface, to the formation of a gouge layer, and to a modification of the interface rheology, (ii) the space and time distribution and statistics and the detailed kinematics of the slip events related to the interface evolution, and (iii) the acoustic wave emission and propagation in the medium associated with such events.

The model shows that, depending on the experimental conditions (confining pressure, loading rate, surface roughness, etc.), and without relying to any prior choice of slip- or rate-dependent friction laws, a large number of sliding regimes can emerge from this system. This includes large stress drops, regular stick-slip, or stable sliding. This model thus provides an unprecedented view of both local and global phenomena at stake during lab earthquakes, at sampling rates in both space and time which remain out of reach for experimental instrumentation.

[1]. Mollon, G. (2015). “A numerical framework for discrete modelling of friction and wear using Voronoi polyhedrons”, Tribology International, 90, 343-355
[2]. Aubry, J. (2019). “Séismes au laboratoire: friction, plasticité et bilan énergétique”, PhD Thesis, Ecole Normale Supérieure.
[3]. Fredrich, J. T.; Evans, B. & Wong, T.-F., (1989). “Micromechanics of the brittle to plastic transition in Carrara marble”, Journal of Geophysical Research: Solid Earth,
Seismic doublets and a complex seismic sequence controlled by the rotation of the Juan Fernández microplate
Authors: Cesca Simone , Valenzuela Malebrán Carla , López-Comino José Ángel , Davis Timothy , Tassara Carlos , Oncken Onno , Dahm Torsten
Published: March 2021

 A complex seismic sequence took place in 2014 at the Juan Fernández microplate, a small microplate located between Pacific, Nazca and Antarctica plates. Despite the remoteness of the study region and the lack of local data, we were able to resolve earthquake source parameters and to reconstruct the complex seismic sequence, by using modern waveform-based seismological techniques. The sequence started with an exceptional Mw 7.1-6.7 thrust – strike slip earthquake doublet, the first subevent being the largest earthquake ever recorded in the region and one of the few rare thrust earthquakes in a region otherwise characterized by normal faulting and strike slip earthquakes. The joint analysis of seismicity and focal mechanisms suggest the activation of E-W and NE-SW faults or of an internal curved pseudofault, which is formed in response to the microplate rotation, with alternation of thrust and strike-slip earthquakes. Seismicity migrated Northward in its final phase, towards the microplate edge, where a second doublet with uneven focal mechanisms occurred. The sequence rupture kinematics is well explained by Coulomb stress changes imparted by the first subevent. Our analysis show that compressional stresses, which have been mapped at the northern boundary of the microplate, but never accompanied by large thrust earthquakes, can be accommodated by the rare occurrence of large, impulsive, shallow thrust earthquakes, with a considerable tsunamigenic potential.
Selecting indicators of drought impacts: the importance of context
Authors: Kchouk Sarra , van Oel Pieter , Melsen Lieke
Published: March 2021

Drought Early Warning Systems (DEWS) and Drought Monitoring Systems (DMS) are the principal tools used to tackle drought at an early stage and reduce the possibility of harm or loss. They are based on the use of drought indicators attributed to either : meteorological, agricultural and hydrological drought. This means that it is mostly hydro-climatic variables that are used to determine the onset, end and severity of a drought.  Drought impacts are rarely continuously monitored or even not included in DEWS and DMS. In this configuration, the likelihood of experiencing impacts is linearly linked to the severity of climatic features only. The aim of our study is to question the direct linkage between the delivery of hydro-climatic information and the detection of drought impacts and their severity. We reviewed scientific literature on drought drivers and impacts and analyzed how these two compare. We conducted a bibliometric analysis based on 4000+ scientific studies sorted by geographic area in which selected (i) drought indicators and (ii) impacts of drought were mentioned. Our review points toward an attachment to a conceptual view of drought by the main and broader use of meteorological (computed and remotely sensed) drought indicators. Studies reporting impacts related to food and water securities are more localized, respectively in Sub-Saharan Africa and Australasia. This mismatch suggests a tendency to translate hydroclimatic indicators of drought directly into impacts while neglecting relevant local contextual information. With the aim of sharpening the information provided by DEWS and DMS, we argue in favor of an additional consideration of drought indicators oriented towards the SDGs.
Connected or disconnected: functioning and management of (dis)connectivity in fluvial systems.
Authors: Hooke Janet
Published: March 2021

Connectivity provides a powerful framework for analysis of fluvial system functioning at a range of scales but identification of disconnectivity and application to management presents challenges. Three stages of evaluation are suggested, depending on the purpose of study and aims of management, and on the underlying strategy and approach. An overall premise of management is assumed of working with nature and allowing or enabling the system to function as sustainably as possible.

The first stage entails identification of the connectivity status and the disconnectors in the system. This includes analysis of what is being disconnected (water, fine sediment, coarse sediment, pollutants), under what conditions and when the disconnector operates, and the types and locations of disconnector. Various methods can be used but present modelling has deficiencies and requires field validation. That presents challenges in recognition of disconnectors across a range of spatial and temporal scales, but recommended strategies are discussed. 

A second stage for management is to analyse to what extent the present functioning is influenced and controlled by human actions, such as land use and structures, and how it departs from a more natural or less disturbed functioning.  This can then provide a platform for developing a strategy that amends past disturbances and moves the functioning towards identified management goals. However, these goals can differ and may be in conflict for different components of the same system.  For example, for many ecological restoration goals the strategy may be to remove disconnectors such as dams and weirs, to allow greater connectivity of species, e.g. fish. On the other hand, if the problem to be addressed is the increased soil erosion or flooding caused by human disturbance then the goal may be to increase the disconnectivity and reduce fluxes through the system.  Our research has proposed that use of vegetation is a sustainable and adaptable method of fulfilling this goal, and discusses how analysis of connectivity and identification of key points for disconnection allows the development of spatial strategies for management.
HIDRA 1.0: Deep-Learning-Based Ensemble Sea Level Forecasting in the Northern Adriatic 
Authors: Žust Lojze , Ličer Matjaž , Fettich Anja , Kristan Matej
Published: March 2021

Interactions between atmospheric forcing, topographic constraints to air and water flow, and resonant character of the basin make sea level modeling in Adriatic a challenging problem. In this study we present an ensemble deep-neural-network-based sea level forecasting method HIDRA, which outperforms our setup of the general ocean circulation model ensemble (NEMO v3.6) for all forecast lead times and at a minuscule fraction of the numerical cost (order of 2 × 10-6). HIDRA exhibits larger bias but lower RMSE than our setup of NEMO over most of the residual sea level bins. It introduces a trainable atmospheric spatial encoder and employs fusion of atmospheric and sea level features into a self-contained network which enables discriminative feature learning. HIDRA architecture building blocks are experimentally analyzed in detail and compared to alternative approaches. Results show the importance of sea level input for forecast lead times below 24 h and the importance of atmospheric input for longer lead times. The best performance is achieved by considering the input as the total sea level, split into disjoint sets of tidal and residual signals. This enables HIDRA to optimize the prediction fidelity with respect to atmospheric forcing while compensating for the errors in the tidal model. HIDRA is trained and analysed on a ten-year (2006-2016) timeseries of atmospheric surface fields from a single member of ECMWF atmospheric ensemble. In the testing phase, both HIDRA and NEMO ensemble systems are forced by the ECMWF atmospheric ensemble. Their performance is evaluated on a one-year (2019) hourly time series from tide gauge in Koper (Slovenia). Spectral and continuous wavelet analysis of the forecasts at the semi-diurnal frequency (12 h)-1 and at the ground-state basin seiche frequency (21.5 h)-1 is performed. The energy at the basin seiche in the HIDRA forecast is close to the observed, while our setup of NEMO underestimates it. Analyses of the January 2015 and November 2019 storm surges indicate that HIDRA has learned to mimic the timing and amplitude of basin seiches.
Decomposing the time-mean Atlantic Meridional Overturning Circulation and its variability with latitude.
Authors: Jonathan Tomas , Bell Mike , Johnson Helen , Marshall David
Published: March 2021

The Atlantic Meridional Overturning Circulations (AMOC) is crucial to our global climate, transporting heat and nutrients around the globe. Detecting  potential climate change signals first requires a careful characterisation of inherent natural AMOC variability. Using a hierarchy of global coupled model  control runs (HadGEM-GC3.1, HighResMIP) we decompose the overturning circulation as the sum of (near surface) Ekman, (depth-dependent) bottom velocity, eastern and western boundary density components, as a function of latitude. This decomposition proves a useful low-dimensional characterisation of the full 3-D overturning circulation. In particular, the decomposition provides a means to investigate and quantify the constraints which boundary information imposes on the overturning, and the relative role of eastern versus western contributions on different timescales. 

The basin-wide time-mean contribution of each boundary component to the expected streamfunction is investigated as a function of depth, latitude and spatial resolution. Regression modelling supplemented by Correlation Adjusted coRrelation (CAR) score diagnostics provide a natural ranking of the contributions of the various components in explaining the variability of the total streamfunction. Results reveal the dominant role of the bottom component, western boundary and Ekman components at short time-scales, and of boundary density components at decadal and longer timescales.
Future climate risk to UK agriculture from heat and humidity changes
Authors: Garry Freya , Bernie Dan , Davie Jemma , Pope Edward
Published: March 2021

Assessments of current and future climate risk are required for adaptation planning to increase resilience and enable society to cope with future climate hazards. Here we identify case studies of compound hazard (involving heat and humidity) events of interest to the UK agricultural sector and present a framework for comparing the frequency and duration of compound events now to those projected in 50 years’ time. We use high resolution (12 km) simulations from the UK Climate Projections to explore how the frequency and duration of instances of potato blight and thermal heat stress to dairy cattle may change locally under RCP 8.5 emissions forcing. We combine hazard (temperature and humidity data) with vulnerability (specific threshold exceedance) and exposure (regional dairy cattle numbers/potato growing area) to estimate risk. Regions where most potatoes are grown, and where the potato blight risk is greatest in both the current and future climate, include the East of England, Yorkshire and the Humber and Eastern Scotland. By 2070, potato blight occurrences may increase by 70 % in East Scotland and between 20 - 30 % across the East of England, the Midlands and Yorkshire and the Humber. Assuming dairy cattle spatial distributions remain the same, the area of greatest risk now and in the future is South West England, with notable increases in risk across Northern Ireland, Wales, the Midlands, North West England and North West Scotland. Dairy cattle heat stress (using a temperature-humidity index) is projected to increase by over 1000 % in South West England, the region with the most dairy cattle. Finally, we consider projected changes to UK seasons, using 2018 as a template, where a cold spring followed by a warm/dry summer resulted in hay/silage shortages. In addition to reduced crop yields in 2018, cattle were kept inside for longer in the cold spring and in the warm/dry summer, due to heat stress and poor grass quality. UK Climate Projections indicate that the annual probability of cold spring/warm summer conditions will decrease in future, but the annual probability of longer dry/warm summers will increase. We conclude that the agricultural sector should consider suitable climate adaptation measures to minimise the risk of dairy cattle thermal heat stress, increased potato blight, and longer dry/warm summer conditions.
Analysis of global trends of total column water vapour from multiple years of OMI observations
Authors: Borger Christian , Beirle Steffen , Wagner Thomas
Published: March 2021

Atmospheric water plays a key role for the Earth’s energy budget and temperature distribution via radiative effects (clouds and vapour) and latent heat transport. Thus, the distribution and transport of water vapour are closely linked to atmospheric dynamics on different spatiotemporal scales. In this context, global monitoring of the water vapour distribution is essential for numerical weather prediction, climate modelling, and a better understanding of climate feedbacks.

Total column water vapour (TCWV), or integrated water vapour, can be retrieved from satellite spectra in the visible “blue” spectral range (430-450nm) using Differential Optical Absorption Spectroscopy (DOAS). The UV-vis spectral range offers several advantages for monitoring the global water vapour distribution: for instance it allows for accurate, straightforward retrievals over ocean and land even under partly-cloudy conditions.

To investigate changes in the TCWV distribution from space, the Ozone Monitoring Instrument (OMI) on board NASA’s Aura satellite is particularly promising as it provides long-term measurements (late 2004-ongoing) with daily global coverage.

Here, we present a global analysis of trends of total column water vapour retrieved from multiple years of OMI observations (2005-2020). Furthermore, we put our results in context to trends of other climate data records and validate the OMI TCWV data by comparisons to additional reference data sets.
Does the trail matters? - In search of the environmental impact of hiking trails on the hillslope runoff.
Authors: Kopyść Piotr
Published: March 2021

Background: In the late 20th century great advancement was made in the field of hiking trail studies. The main concerns for these trails are susceptibility to soil erosion and dynamic landform changes in their nearest area. The impacts of the trails for vegetation were also studied with trampling and soil exposure being the most destructive. Today, With the higher and higher resolution for digital terrain models being available and advanced simulation algorithms and computing, it is possible to perform advanced modeling of a hillslope runoff.

The aim: The aim of these studies is to determine the impact of hiking trails on hillslope water runoff by comparison of the water discharge values during simulated precipitation events.

Method: The comparison is made possible, with the use of digital terrain modeling to create hillslope with and without a hiking trail, in its pre-erosion state. Both models then are subject to Simulated Water Erosion (SiMWE) based on the Monte Carlo simulation method. The algorithm is implemented in an open-source GRASS GIS program in the form of r.sim.water and r.sim.sediment modules. Exemplary hillslopes also present different scenarios based on the trail to slope alignment angle (0° - 22°; 22,1° - 45°; 45,1° - 67° and 67,1° - 90°). 

During the session a case study area of a suburban mountain range within Kielce city (Poland) will be introduced, together with the results of simulation values and their spatial distribution for the exemplary hillslopes. With it, the role of the hiking trail and its environmental impact on water runoff will be better understood and easier to predict.
Reconstructing Antarctic winter sea-ice extent during Marine Isotope Stage 5e
Authors: Chadwick Matthew , Allen Claire , Sime Louise , Crosta Xavier
Published: March 2021

Marine Isotope Stage (MIS) 5e marks the peak of the last interglacial (130-116 ka) and is an important ‘process analogue’ for understanding the high latitude climatic feedbacks and forcings active under future anthropogenic warming. Antarctic sea-ice extent is a critical component of the Earth’s climate system through its impact on global albedo and its roles in Southern Hemisphere atmospheric and ocean circulation. Published marine sediment core records are located too far north to accurately constrain the timing and extent of the winter sea-ice (WSI) minimum during MIS 5e (Chadwick et al., 2020) and researchers/models have therefore assumed that this minimum occurs synchronously with the Antarctic peak warming in ice core records (Holloway et al., 2017).

This study presents new reconstructions of Southern Ocean WSI extent for MIS 5e based on the diatom species assemblage records in marine sediment cores. These records have robust age models, which allow for the different timings and patterns of WSI retreat throughout the Southern Ocean to be examined. In particular, the difference between the relatively stable WSI extent in the Pacific sector of the Southern Ocean and the more dynamic WSI extent in the Atlantic sector of the Southern Ocean. Using sediment cores located south of 55 oS creates a novel synthesis for assessing the evidence for the considerable MIS 5e WSI reduction (67% in the Atlantic sector) predicted by model simulations (Holloway et al., 2017).

References

Chadwick M., Allen C.S., Sime L.C., Hillenbrand C-D. (2020). Analysing the timing of peak warming and minimum winter sea-ice extent in the Southern Ocean during MIS 5e. Quaternary Science Reviews, 229: 106134.

Holloway M.D., Sime L.C., Allen C.S., Hillenbrand C-D., Bunch P., Wolff E., Valdes P.J. (2017). The Spatial Structure of the 128 ka Antarctic Sea Ice Minimum. Geophysical Research Letters, 44 (21): 11129-11139.
3D Computer-Assisted Geological Mapping with FaultTrace: Results of the Richât Structure (Mauritania) and the Vineh Structure (Iran)
Authors: Faber Robert , Domej Gisela
Published: March 2021

To evaluate the functionality of FaultTrace – a tool for semi-automatic structural geological mapping of faults and bedding planes within the software WinGeol by TerraMath –, we demonstrate its detailed use in two different case studies: The Richât Structure in Mauritania is characterized by a volcanogenic anticline with associated fault systems and shows relatively planar fault structures within low-relief topography; the Vineh Structure in Iran consists of complexly faulted sequences of sedimentary and igneous layers in high mountainous terrain affected by the fault systems of the Purkan-Vardij Fault and the North Tehran Fault. The studies discuss which structural geological settings let expect a satisfying performance of FaultTrace, and what factors limit the achievement of meaningful results. 

Used data is freely available and consists of digital elevation models (e.g., SRTM or ALOS Data) and satellite imagery (e.g., Sentinel-2 or Landsat ETM+ Imagery). Where available, additional data such as, for instance, borehole logs and geological cross-sections were displayed to support the mapping process. Results from the structural geological assessments of both case studies were finally compared to previously published studies in order to validate the performance of FaultTrace on the one hand, and to discuss differences on the other hand.

We show that FaultTrace aims to provide a virtual environment allowing for fast-track and optimized data generation for 3D geological models. It can be used for a first remote structural geological assessment without the requirement of being at the site. Therefore, it is well suited for inaccessible terrain – for instance, due to transportation, political restrictions, warfare, natural hazards, or lack of funding. Nevertheless, and to take full advantage of the software, users have to be aware of the limitations and strengths, which are discussed in this work based on two very different case studies.
Thermal Influence on the LRI Scale Factor
Authors: Misfeldt Malte , Müller Vitali , Müller Laura , Wegener Henry , Heinzel Gerhard
Published: March 2021

In this talk we present the current status of our investigations regarding variations of the LRI scale factor with spotlight on the thermal environment of the LRI. Furthermore, we describe an alternative derivation of the scale factor through LRI telemetry data.

In the current SDS processing scheme for deriving the LRI scale factor, a cross-calibration to the KBR range is employed, which numerically estimates the LRI scale and a time-tag offset by minimizing the difference between KBR and LRI range. Typical numerical values are in the order of 2.2*10-6 for the scale and a few tens of microseconds for the time-tag offset. The scale shows some recurring features on large time scales, which we were investigating in depth.

At first, we use a LRI telemetry based model for the nominal laser frequency (see https://doi.org/10.5194/egusphere-egu2020-15569), which already reduces the LRI scale to below ±5*10-8, but does not suppress the features occurring on a 3-month time scale. To address these, we investigate thermal variations of the LRI instrument and its subsystems. We do not expect the temperatures to directly influence the laser frequency, but rather assume a proportionality of temperature and phase, which would manifest in tone errors at 1 CPR and 2 CPR. With our analysis, we were able to derive linear coupling factors for mapping temperature variations to errors in the LRI ranging data. With this tone error correction applied, the difference between LRI and KBR range can be reduced by about 60% at low frerquencies. Currently, we’re investigating the influence of our correction on the 1 CPR and 2 CPR amplitudes and their differences w.r.t. the KBR range.

Our goal is to derive a model for the LRI scale factor, which uses only LRI telemetry and temperature sensors. That would be especially beneficial in case the KBR observation become unavailable in GRACE-FO, and is furthermore helpful for the design of future laser ranging instruments.
An Update on the Impact of Aeolus Doppler Wind Lidar Observations for Use in Numerical Weather Prediction at ECMWF
Authors: Rennie Michael P. , Isaksen Lars
Published: March 2021

The latest results on the assessment of the impact of Aeolus Level-2B horizontal line-of-sight wind retrievals in global Numerical Weather Prediction at ECMWF will be presented.  Aeolus has been operationally assimilated at ECMWF since 9 January 2020.
Random and systematic error estimates were derived from observation minus background departure statistics.  The HLOS wind random error standard deviation is estimated to vary over the range 4.0-7.0 m/s for the Rayleigh-clear and 2.8-3.6 m/s for the Mie-cloudy; depending on atmospheric signal levels which in turn depends on instrument performance, atmospheric backscatter properties and the processing algorithms.
In Observing System Experiments (OSEs) Aeolus provides statistically significant improvement in short-range forecasts as verified by observations sensitive to temperature, wind and humidity.  Longer forecast range verification shows positive impact that is strongest at the 2-3 day forecast range; ~2% improvement in root mean square error for vector wind and temperature in the tropical upper troposphere and lower stratosphere and polar troposphere.  Positive impact up to 9 days is found in the tropical lower stratosphere.  Both Rayleigh-clear and Mie-cloudy winds provide positive impact, but the Rayleigh accounts for most tropical impact. The Forecast Sensitivity Observation Impact (FSOI) metric is available since Aeolus was operationally assimilated, which confirms Aeolus is a useful contribution to the global observing system; with the Rayleigh-clear and Mie-cloudy winds providing similar overall short-range impact in 2020.  If the OSEs are ready in time, we will present the impact of the first reprocessed Aeolus data for the July-December 2019 period.
Hydrodynamic modelling of outburst flood hazard in the Pokhara Valley, Nepal
Authors: Fischer Melanie , Korup Oliver , Veh Georg , Walz Ariane
Published: March 2021

In May 2012, a sudden outburst flood along the Seti Khola river caused 72 fatalities and damaged infrastructure in the northern Pokhara Valley, Nepal. This destructive event raised concerns about possible future landslide- or glacier-surge-related outburst floods from the Higher Himalayas. The Seti Khola runs along one of the steepest topographic gradients in this mountain belt. The river is fed by the debris-covered Sabche glacier, Nepal’s only observed surging glacier, below the flanks of Annapurna III (c. 7500 m asl) and reaches Pokhara, the country’s second largest city, at about 850 m asl. Over a course of some 40 km, the Seti Khola shaped the Pokhara Valley’s distinctive landscape of unpaired, several tens of meters to >100-m high alluvial terraces that alternate with deep slot gorges of <1 km length, all mostly cut into deposits of medieval and earlier outburst and outwash deposits. These abrupt changes in channel cross section provide many potential locations of hydraulic ponding during floods. We present a reanalysis of the 2012 Seti Khola outburst flood, and combine field-based surveys of valley geometry, flood markers, and surface roughness (i.e. Manning’s n value estimates) with landform mapping from high-resolution satellite images and digital elevation models. These components form the input for a one-dimensional steady flow simulation in HEC-RAS that allows us to reconstruct the dynamics, stage height, and runout from the 2012 Seti Khola flood. Validated by both this recent and the catastrophic historic events, we use our model to simulate future scenarios of inundation by these infrequent but potentially highly destructive outburst floods and compare them to the Pokhara Valley’s recurring monsoonal floods.
PASTRI - Pilot for Aerial Surface Temperature Retrieval
Authors: Spengler Daniel , Gulde Max , Marius Bierdel , Cassi Welling , Alex Stottmeister , Jain Atin , Thomas Ruhtz
Published: March 2021

PASTRI will be a joint pilot study of ConstellR, GFZ Potsdam and FU Berlin for airborne based aerial surface temperature retrieval. The mission is planned as a preparation and demonstrator mission for the upcoming spaceborne thermal satellite microsatellites of ConstellR. ConstellR will provide a land surface temperature (LST) monitoring service with an initial focus on companies in the precision farming industry. The initial minimal viable constellation (MVC) of four microsatellites will offer global, daily LST monitoring at 50 m spatial resolution with 1.5 K radiometric accuracy for a monitoring area capacity comparable to the size of Germany’s agricultural area.

The authors intend to use a six-week airborne campaign in May/June 2021 as a data delivery pilot to develop and validate the provision of an LST product. In total 18 flights are planned (3 flights/week every second week, 2 flights/day) with the FU Berlin Cessna T207A. On the technical side, the project includes the payload development and adaptation to the mechanical interface of the airplane, the actual (airborne) recording or imagery, as well as setting up the data processing pipeline. The aircraft will be instrumented with an adapted ConstellR Sensor and a thermal hyperspectral Telops HyperCam. This enables a performance evaluation of the microsatellite sensor performance against a hyperspectral reference instrument. Based on that results, final adaptions could be made for the spaceborne sensors. The flights will be performed in Central Germany at agricultural sites and will be supplemented by in-situ reference measurements.

The concept and the status of preparation of the campaign will be presented.
Exploring horizontal pressure gradient (HPG) schemes for general vertical coordinates.
Authors: Young Amy , Bell Mike
Published: March 2021

Terrain following coordinates allow for better representation of physics at the sea-bed than traditional z-coordinates but result in numerical discretisation errors in the calculation of the horizontal pressure gradient (HPG) which manifest as spurious currents.  As of NEMO r4.0.4, there were two HPG schemes available for use with terrain following coordinates, the traditional 2nd order sco scheme and the 3rd order prj scheme.  The prj scheme, while highly accurate in the ocean interior, shows unphysical behaviour at the sea-bed for steeply sloping bathymetry.  A task in the IMMERSE project was set up to identify, implement and test promising HPG schemes suitable for general vertical coordinates that are accurate, robust and physically consistent.  As part of this task, the 3rd-order accurate density Jacobian scheme (djc) as proposed by Shchepetkin and McWilliams (2003) has now been implemented in the NEMO trunk (as a rewrite of the previously existing but non-operational djc scheme).  Idealised testing has shown this scheme to be significantly more accurate than the sco scheme, and more robust than the prj scheme in coping with steeply sloping bathymetry.  Initial results from applying the djc scheme in a challenging realistic configuration (the AMM7 with hybrid s-z-coordinates and non-uniform vertical discretisation) show a reduction in spurious currents with respect to the sco scheme.  The prj scheme is highly sensitive to the rmax (maximum permitted slope) criterion.  In cases where the bathymetry is so steep that a velocity-point may lie multiple levels below one of its neighbouring tracer-points, the nature of the prj near-bed HPG calculation leads to sudden spin-ups of spurious velocities which can exceed those of the djc scheme in the longer-term.  Performance-wise, the djc scheme is 3 times slower than the sco scheme, but less expensive than the prj.  Further work is planned to reduce the memory footprint.  In addition to continued testing of the djc scheme, further work will look at alternative formulation (finite volume) HPG schemes, and high order variants.

This work is distributed under the Creative Commons Attribution 4.0 License. This licence does not affect the Crown copyright work, which is re-usable under the Open Government Licence (OGL). The Creative Commons Attribution 4.0 License and the OGL are interoperable and do not conflict with, reduce or limit each other.
Modeling of Mesoscale-Convective Systems Downstream of Mountain Regions
Authors: Prein Andreas F.
Published: March 2021

Mesoscale-Convective Systems (MCSs) are prolific rain-producers and are responsible for most flash flood events in mid-latitudes. Global hotspots of MCS occurrence are downstream of major mountain regions such as the Rocky Mountains, the Andes, and the Himalayas. This is because of the effects of mountain barriers on circulation patterns, moisture transport, and convective initiation. Realistically simulating MCSs in climate models is essential for representing the water and energy cycle and flood and severe convective weather assessments. However, state-of-the-art climate models have substantial biases in simulating MCSs and orographic impacts on downstream environments resulting in large uncertainties and errors in assessing climate change impacts on water availability and extreme events. Here we present that kilometer-scale models, which have an improved representation of orography and can represent deep convective processes explicitly, show a step improvement in simulating organized convective storms compared to coarser-resolution models. We will show examples of these improvements from kilometer-scale simulations over the Tibetan Plateau, North- and South America. We will also show sensitivities to the model setup and feedback processes and end with discussing remaining challenges and future prospects.
Using high-resolution groundwater data for the validation of a global hydrological model: evaluating WaterGAP and calibration/data assimilation (C/DA) performance over France
Authors: Hsu Kuei-Hua , Longuevergne Laurent , Eicker Annette , Hasan Mehedi , Güntner Andreas , Engels Olga , Schulze Kerstin , Kusche Jürgen
Published: March 2021

The dynamic global water cycle is of ecological and societal importance as it affects the availability of freshwater resources and influences extreme events such as floods and droughts. This work is set in the frame of the GlobalCDA Research Unit, which has the goal of developing a calibration/data assimilation approach (C/DA) to improve the quantification of freshwater resources by combining the global hydrological model WaterGAP with geodetic (GRACE, altimetry) and remote sensing data. This presentation focuses on the validation of the C/DA results using an independent in-situ groundwater data set based on ~1500 monitoring boreholes in France.

The resulting validation data set is applied to independently assess the output of several C/DA experiments: data assimilation using different combinations of the available geodetic and remote sensing data sets and different methods of model calibration, based on either an ensemble Kalman filter approach or a Pareto-optimal calibration algorithm.

To further understand in-situ groundwater and WaterGAP data set, we subtract the coherent signals using Empirical orthogonal function (EOF).  Over 85% variances can be explained by the first 3 EOFs for both data sets.
Bayesian Full-waveform Inversion with Weak Priors
Authors: Zhang Xin , Curtis Andrew
Published: March 2021

Seismic full waveform inversion (FWI) produces high resolution images of the subsurface directly from seismic waveforms, and has been applied at global, regional and industrial spatial scales. FWI is traditionally solved using optimization methods, by iteratively updating a model of the subsurface so as to minimize the misfit between observed waveforms and those predicted by linearised (approximate) physics. Due to the nonlinearity of the physical relation between model parameters and seismic waveforms, a good starting model (derived from other methods) and hence strong prior information is required in order that the linearised physical relationships are reasonable in the vicinity of the true solution. Such linearised methods cannot provide accurate estimates of uncertainties, which are required if we are to understand and interpret the results appropriately.

To estimate uncertainties more accurately, nonlinear Bayesian methods have been deployed to solve the FWI problem. Monte Carlo sampling is one such algorithm but it is computationally expensive, and all Markov chain Monte Carlo-based methods are difficult to parallelise fully. Variational inference provides an efficient, fully parallelisable alternative methodology. This is a class of methods that optimize an approximation to a probability distribution describing post-inversion parameter uncertainties. Both Monte Carlo and variational full waveform inversion (VFWI) have been applied previously to solve FWI problems, but only with strong prior information about the velocity structure to limit the space of possible models. Unfortunately such strong information is almost never available in practice. In addition, VFWI has only been applied to wavefield transmission problems in which seismic data are recorded on a receiver array that lies above the structure to be imaged, given known, double-couple (earthquake-like) sources located underneath the same structure. In practice, knowledge of such sources is never definitive, and usually depends circularly on the unknown structure itself. In this study, we present the first application of VFWI to seismic reflection data acquired from known (deliberately fired) near-surface sources. We also apply variational inference (specifically, Stein variational gradient descent) to solve FWI problems with realistic prior information. We perform multiple inversions using data from different frequency ranges, and show that VFWI produces high resolution mean and uncertainty models using both low and high frequency data, and given only weak prior information. This is usually impossible using traditional linearised methods. We conclude that VFWI may be a useful method to produce high resolution images and reliable uncertainties, at least in smaller FWI problems.
Clock modelling techniques for an enhanced GNSS orbit determination
Authors: Roldan Pedro , Guerin Pierre , Anton Julie , Laurenti Marco , Trilles Sebastien
Published: March 2021

The determination of GNSS orbits is generally based on the processing of pseudorange and carrier phase measurements from a station network, with an Orbit Determination and Time Synchronization (ODTS) process. This process involves the satellite and ground station clocks as part of the GNSS measurement reconstruction. The clocks are generally estimated as a snapshot parameter, without assuming any correlation between epochs. However, the stability of satellite and some station clocks, based on technologies of hydrogen, cesium or rubidium, allows for a significant predictability. Taking advantage of this predictability the ODTS process can be improved, especially in those cases where the station network is limited or does not provide a good coverage for certain areas.

The clock modelling can be directly done by estimating additional parameters in the filter. A quadratic model is generally estimated for each clock, keeping a small snapshot contribution to account for the stochastic part and for potential deviations with respect to the theoretical behavior of the clock. The detection of this kind of deviations in the satellite and station clocks becomes a major factor for achieving a good performance with these techniques. In case the clock experiences feared events like phase or frequency jumps, the estimated clock model stops being valid and the estimation of model parameters needs to be reset.

In case a composite clock algorithm is used to provide the reference timescale for the ODTS, the estimation of clock models can rely on this algorithm. Algorithms of composite clock are generally based on a Kalman filter that estimates as part of the state vector the differences between each contributing clock and the composite timescale. These differences can be used not only to define the reference timescale of the ODTS, but also to remove the deterministic part of the clocks in the measurement reconstruction. As for the case of clock modelling, for algorithms of composite clock the detection and correction of anomalies in the contributing clocks becomes a critical point.

In this work, the integration of orbit determination, clock modelling and composite clock algorithms will be described. The impact of clock modeling techniques on the GNSS orbit determination accuracy will be presented, both considering a direct estimation of clock models in the ODTS and the estimation provided by the composite clock algorithm. These analyses will be based on NEODIS, the orbit determination software developed by Thales Alenia Space, which integrates with a Kalman filter approach GNSS orbit determination and composite clock algorithms.

 
The Unusual Widespread Solar Energetic Particle Event on 2013 August 19: Solar origin, CME-driven shock evolution and particle longitudinal distribution
Authors: Rodríguez-García Laura , Gómez-Herrero Raúl , Zouganelis Yannis , Balmaceda Laura , Nieves-Chinchilla Teresa , Dresing Nina , Dumbovic Mateja , Nitta Nariaki , Carcaboso Fernando , dos Santos Luiz Fernando Guedes , Jian Lan , Mays Leila , Williams David , Rodríguez-Pacheco Javier
Published: March 2021

Context: Late on 2013 August 19, STEREO-A, STEREO-B, MESSENGER, Mars Odyssey, and L1 spacecraft, spanning a longitudinal range of 222° in the ecliptic plane, observed an energetic particle flux increase. The widespread solar energetic particle (SEP) event was associated with a coronal mass ejection (CME) that came from a region located near the far-side central meridian from Earth's perspective. The CME appeared to consist of two eruptions, and was accompanied by a ~M3 flare as a post-eruption arcade, and low-frequency (interplanetary) type II and shock-accelerated type III radio bursts.

Aims: The main objectives of this study are two, disentangling the reasons of the different intensity-time profiles observed by MESSENGER and STEREO-A, longitudinally separated by only 15°, and unravelling the single solar source related with the SEP event.

Results: The solar source associated with the widespread SEP event is the shock driven by the two-stages CME, as the flare observed as a posteruptive arcade is too late to explain the estimated particle onset. The different intensity-time profiles observed by STEREO-A, located at 0.97 au, and MESSENGER, at 0.33 au, can be interpreted as enhanced particle scattering beyond Mercury's orbit. The longitudinal extent of the shock does not explain by itself the wide spread of particles in the heliosphere. The particle increase observed at L1 may be attributed to cross-field diffusion transport, and this is also the case for STEREO-B, at least until the spacecraft is eventually magnetically connected to the shock at ~0.6 au. The CME-driven shock may have suffered distortion in its evolution in the heliosphere, such that the shock flank overtakes the shock nose at 1 au.