bluuebunny/crossref_metadata_2025 huggingface dataset:
Does forest growth acceleration lead to denser stands? Insights from Swiss forests and mechanistic modelling
Authors: Marques Laura , Weng Ensheng , Bugmann Harald , Forrester David I. , Hobi Martina , Kettunen Henna-Riikka , Rohner Brigitte , Thuerig Esther , Trotsiuk Volodymyr , Stocker Benjamin D.
Published: March 2021

Forest demographic processes - growth, recruitment and mortality - are being altered by global change. The changing balance between growth and mortality strongly influences forest dynamics and the carbon balance. Elevated atmospheric carbon dioxide (eCO2) has been reported to enhance photosynthesis and tree growth rates by increasing both light-use efficiency (LUE) and water-use efficiency (WUE). Tree growth enhancement could be translated into an increase in biomass stocks or could be associated with a reduction in the longevity of trees, thus reducing the ability of forest ecosystems to act as carbon sinks over long timescales. These links between growth and mortality, and the implications for forest stand density and self-thinning relationships are still debated. Scarce empirical evidence exists for how changing drivers affect tree mortality due to existing data and modelling limitations. Understanding the causes of observed mortality trends and the mechanisms underlying these processes is critical for accurate projections of global terrestrial carbon storage and its feedbacks to anthropogenic climate change.

Here, we combine a mechanistic model with empirical forest data to better understand the causes of changes in tree mortality and the implications for past and future trends in forest tree density. Specifically, we test the Grow-Fast-Die-Young hypothesis to investigate if a leaf-level CO2 fertilization effect may lead to an increase in the biomass stock in forest stands. We use a novel vegetation demography model (LM3-PPA) which includes vegetation dynamics with biogeochemical processes allowing for explicit representation of individuals and a mechanistic treatment of tree mortality. The key links between leaf-level assimilation and stand dynamics depend on the carbon turnover time. In this sense, we investigate alternative mortality assumptions about the functional dependence of mortality on tree size, tree carbon balance or growth rate. These formulations represent typical approaches to simulate mortality in mechanistic forest models. Model simulations show that increasing photosynthetic LUE leads to higher biomass stocks, with contrasting behavior among mortality assumptions. Empirical data from Swiss forest inventories support the results from the model simulations showing a shift upwards in the self-thinning relationships, with denser stands and bigger trees. This data-supported mortality-modelling helps to identify links between forest responses and environmental changes at the leaf, tree and stand levels and yields new insight into the causes of currently observed terrestrial carbon sinks and future responses.
Characteristics of earthquake sequences: comparison from 0D to 3D
Authors: Li Meng , Pranger Casper , van Dinther Ylona
Published: March 2021

Numerical models are well-suited to overcome limited spatial-temporal observations to understand earthquake sequences, which is fundamental to ultimately better assess seismic hazard. However, high-resolution numerical models in 3D are computationally time and memory consuming. This is not optimal if the aspects of lateral or depth variations within the results are not needed to answer a particular objective. In this study we quantify and summarize the limitations and advantages for simulating earthquake sequences in all spatial dimensions.

 

We simulate earthquake sequences on a strike-slip fault with rate-and-state friction from 0D to 3D using both quasi-dynamic and fully dynamic approaches. This cross-dimensional comparison is facilitated by our newly developed, flexible code library Garnet, which adopts a finite difference method with a fully staggered grid. We have validated our models using problems BP1-QD & FD and BP4-QD & FD of the SEAS (Sequences of Earthquakes and Aseismic Slip) benchmarks from the Southern California Earthquake Center.

 

Our results demonstrate that lower-dimensional/quasi-dynamic models are qualitatively similar in terms of earthquake cycle characteristics to their higher-dimensional/fully-dynamic counterparts, while they could be hundreds to millions times faster at the same time. Quantitatively, we observe that certain earthquake parameters, such as stress drop and fracture energy release, can be accurately reproduced in each of these simpler models as well. However, higher dimensional models generally produce lower maximum slip velocities and hence longer coseismic durations. This is mainly due to lower rupture speeds, which result from increased energy consumption along added rupture front directions. In the long term, higher dimensional models produce shorter recurrence interval and hence smaller total slip, which is mainly caused by the higher interseismic stress loading rate inside the nucleation zone. The same trend is also observed when comparing quasi-dynamic models to fully dynamic ones. We extend a theoretical calculation that to first order approximates the aforementioned physical observables in 3D to all other dimensions. These theoretical considerations confirm the same trend as what is observed for stress drop, recurrence interval and total slip across dimensions. These findings on similarities and differences of different dimensional models and a corresponding quantification of computational efficiency can guide model design and facilitate result interpretation in future studies.
Modelling landfast sea ice and its influence on ocean-ice interactions in the area of the Totten Glacier, East Antarctica
Authors: Van Achter Guillian , Fichefet Thierry , Goosse Hugues , Pelletier Charles , Sterlin Jean , Huot Pierre-Vincent , Lemieux Jean-François , Fraser Alexander , Porter-Smith Richard , Haubner Konstanze
Published: March 2021

The Totten Glacier in East Antarctica is of major climate interest because of the large fluctuation of its grounding line and of its potential vulnerability to climate change. The ocean above the continental shelf in front of the Totten ice shelf exhibits large extents of landfast sea ice with low interannual variability. Landfast sea ice is mostly not or sole crudely represented in current climate models. These models are potentially omitting or misrepresenting important effects related to this type of sea ice, such as its influence on coastal polynya locations. Yet, the impact of the landfast sea
ice on the ocean – ice shelf interactions is poorly understood. Using a series of high-resolution, regional NEMO-LIM-based experiments including an
explicit treatment of ocean – ice shelf interactions over the years 2001-2010, we simulate a realistic landfast sea ice extent in the area of Totten Glacier
through a combination of a sea ice tensile strength parameterisation and a grounded iceberg representation. We show that the presence of landfast sea
ice impacts seriously both the location of coastal polynyas and the ocean mixed layer depth along the coast, in addition to favouring the intrusion of
mixed Circumpolar Deep Water into the ice shelf cavities. Depending on the local bathymetry and the landfast sea ice distribution, landfast sea ice affects ice shelf cavities in different ways, either by increasing the ice melt (+28% for the Moscow University ice shelf) or by reducing its seasonal cycle
(+10% in March-May for the Totten ice shelf). This highlights the importance of including an accurate landfast sea ice representation in regional and
eventually global climate models
Connecting the Land Surface to Droughts: How Transpiration, Canopy Evaporation, and Ground Evaporation Impact Droughts Across the North American Continent
Authors: Harrington Tyler , Skinner Christopher , Nusbaumer Jesse
Published: March 2021

Land surface moisture plays a crucial role in precipitation patterns across the globe. Evapotranspiration (the combination of ground evaporation (E), canopy evaporation (I), and transpiration (T)) from the land surface can influence precipitation through local recycling and the propagation of moisture to downwind regions. However, the role of the land surface and of T, E, and I individually in these two processes are not well understood and limit our understanding of the role of the land surface for both drought onset and intensification. Here we use a version of the Community Earth System Model (CESM1.2 with the Community Atmosphere Model CAM5 and the Community Land Model CLM5) with online water tracers to directly track and quantify the movement of T, E and I moisture across North America for the 1985–2015 period. Initial findings suggest that over 50% of summer precipitation for much of central and northern US and Canada comes from the land surface. The tracers also suggest that, with the exception of the US west coast and desert southwest, 40-60% of land precipitation across the continent comes from the T component. The connection between land surface moisture and drought episodes are examined for different regions of North America. The individual roles of T, E, and I in shaping droughts are also examined.
Investigation of atmospheric hydrogen cyanide: a modelling perspective
Authors: Bruno Antonio G. , Harrison Jeremy J. , Moore David P. , Chipperfield Martyn P. , Pope Richard J.
Published: March 2021

Hydrogen cyanide (HCN) is one of the most abundant cyanides present in the global atmosphere, and is a tracer of biomass burning, especially for peatland fires. The HCN lifetime is 2–5 months in the troposphere but several years in the stratosphere. Understanding the physical and chemical mechanisms of HCN variability is important due to its non-negligible role in the nitrogen cycle. The main source of tropospheric HCN is biomass burning with minor contributions from industry and transport. The main loss mechanism of atmospheric HCN is the reaction with the hydroxyl radical (OH). Ocean uptake is also important, while in the stratosphere oxidation by reaction with O(1D) needs to be considered.

HCN variability can be investigated using chemical model simulations, such as three-dimensional (3-D) chemical transport models (CTMs). Here we use an adapted version of the TOMCAT 3-D CTM at a 1.2°x1.2° spatial resolution from the surface to ~60 km for 12 idealised HCN tracers which quantify the main loss mechanisms of HCN, including ocean uptake, atmospheric oxidation reactions and their combinations. The TOMCAT output of the HCN distribution in the period 2004-2020 has been compared with HCN profiles measured by the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) over an altitude grid from 6 to 42 km. HCN model data have also been compared with ground-based measurements of HCN columns from NDACC FTIR stations and with in-situ volume mixing ratios (VMRs) from NOAA ground-based measurement sites.

The model outputs for the HCN tracer with full treatment of the loss processes generally agree well with ACE-FTS measurements, as long as we use recent laboratory values for the atmospheric loss reactions. Diagnosis of the individual loss terms shows that decay of the HCN profile in the upper stratosphere is due mainly to the O(1D) sink. In order to test the magnitude of the tropospheric OH sink and the magnitude of the ocean sink, we also show the comparisons of the model tracers with surface-based observations. The implications of our results for understanding HCN and its variability are then discussed.
A novel data-driven method to estimate GIA signal from Earth observation data
Authors: Vishwakarma Bramha Dutt , Ziegler Yann , Royston Sam , Bamber Jonathan L.
Published: March 2021

Geophysical inversions are usually solved with the help of a-priori constraints and several assumptions that simplify the physics of the problem. This is true for all the inversion approaches that estimate GIA signal from contemporary datasets such as GNSS vertical land motion (VLM) time-series and GRACE geopotential time-series. One of the assumptions in these GIA inversions is that the change in VLM due to GIA can be written in terms of surface mass change and average mantle density. Furthermore, the surface density change is obtained from GRACE data using the relations derived in Wahr et al., 1998, which actually is only applicable for surface processes (such as hydrology) and not for sub-surface processes such as GIA. This leaves us with a tricky signal-separation problem. Although many studies try to overcome this by constraining the inversion with the help of constrains from a priori GIA models, the output is not free from influence of GIA models that are known to have huge uncertainties. In this presentation, we discuss this problem in detail, then provide a novel mathematical framework that solves for GIA without any a priori GIA model. We validate our method in a synthetic environment first and then estimate a completely data-driven GIA field from contemporary Earth-observation data.
Evaluating linear trends with respect to past decadal variability can help us assess the severity of GRACE observed water storage change
Authors: Bamber Jonathan L. , Vishwakarma Bramha Dutt , Bates Paul , Sneeuw Nico , Westaway Richard M.
Published: March 2021

Often in geosciences, short time series are investigated to infer a linear trend (secular change), and the magnitude of trend is used to infer severity of change without considering the spatiotemporal variability of the observable. Therefore, it is not always known to what extent resultant trends are truly representative of severity of change, or whether the trends in short time series are driven by long wavelength signals that can only become apparent when additional years of data are available. Furthermore, same value of trend can have different interpretation over different regions.

GRACE, a novel satellite mission to monitor water mass redistribution, was launched in 2002 and a decade later several studies analyzed linear trends in GRACE time series to claim that some regions were experiencing unprecedented changes in regional water storage. Studies published more recently further suggest that some of those regions have recovered and some new regions have emerged as endangered. This update in our knowledge is driven by additional GRACE data that became available in the last five years, demonstrating that as the time series became longer, inferences from studying trends changed. In this presentation, we demonstrate that multi-decadal natural variability in water cycle influenced previous interpretations of linear trends from relatively short (<20 year) GRACE time series. We propose a new metric (trend to variability ratio or TVR) that incorporates standard deviation of historical natural variability to better interpret the severity of trends inferred from GRACE. Since, natural hydrological variability is different for different regions, same value of trend has different interpretation for different river catchments. Using this metric, we find that several regions that were thought to be losing water at a moderate rate are actually more endangered and vice-versa. We also provide a map that demarcates river catchments that have experienced severe water storage change between 2003 to 2015.
Investigating the forecast predictability for fluvial flooding from tropical cyclones
Authors: Titley Helen , Cloke Hannah , Harrigan Shaun , Pappenberger Florian , Prudhomme Christel , Robbins Joanne , Stephens Elizabeth , Zsoter Ervin
Published: March 2021

Global ensemble forecast models have been shown to have good skill in forecasting the track probabilities of tropical cyclones worldwide, but less well-studied is their ability to predict the hazards resulting from tropical cyclones, which in the case of fluvial flooding can extend far from the landfall location traditionally focussed on in operational tropical cyclone warnings. This work aims to investigate the key factors that influence the predictability of fluvial flood severity from tropical cyclones, using forecasts from the Global Flood Awareness System (GloFAS). GloFAS is jointly developed by the European Commission and the European Centre for Medium-Range Weather Forecasts (ECMWF) and is designed to provide a global overview of upcoming flood events to decision makers as part of the Copernicus Emergency Management Service, producing probabilistic river discharge forecasts driven by global ECMWF ensemble forecasts coupled to a hydrological model. This presentation will explore the chain of uncertainty through the forecasting process for several recent tropical cyclone flood events including Hurricane Iota and Cyclone Nivar. It investigates the influence on the overall predictability and uncertainty of the fluvial flood forecasts of various components of the forecasting chain, including the track, intensity, and precipitation forecasts for the tropical cyclone, and the hydrological catchment conditions and modelling.
Residual bootstraps for the uncertainty analysis of tidal models with temporally correlated noise 
Authors: Innocenti Silvia , Matte Pascal , Fortin Vincent , Bernier Natacha
Published: March 2021

The accurate characterization of the uncertainty associated with the estimation of tidal constituents is critical to provide accurate water level reconstructions and predictions. However, this represents a challenge in applications since the sparse sampling and finite series length prevent sharply distinguishing between the deterministic tidal signal and the stochastic fluctuations present in the ob- served records. Specifically, the presence of various unresolved sources of vari- ability (e.g., the tide-surge, tide-tide, and tide-river flow interactions, as well as errors and in-homogeneities associated with data measurements) results in sig- nificant broad-spectrum variability of the recorded signals, as well as harmonic analysis parameter modulations from sub-daily to decadal temporal scales. As a result, the residuals obtained after performing regression harmonic analysis are temporally correlated. Conventional methods for assessing the harmonic model uncertainty typically ignore this autocorrelation. A Monte Carlo exper- iment is used to evaluate the effect of neglecting the residual autocorrelation in the estimation of tidal constituent uncertainty. The estimation of regression parameter variability from three commonly used analytical techniques (from the UTide and NS Tide packages, and the IRLS method) and two residual resam- pling (moving-block and semi-parametric bootstrap) are compared. We show that conventional methods (e.g., UTide and the IRLS) may largely underesti- mate the parameter uncertainty when relying on simplified assumptions, such as normality and independence of the regression residuals. This may lead to in- correct assessments about the significance of one or more predictors. We showed improved performance by using the two bootstrap strategies and NS Tide, as a result of a better representation of the autocorrelation structure of residuals. The moving-block bootstrap approach provides a simple alternative that can be easily applied to a large range of (unknown) autocorrelation structures of the observed residuals.
Aerosol emission estimation using SPEXone observational capabilities and Observing System Simulation Experiments (OSSEs) 
Authors: Tsikerdekis Athanasios , Schutgens Nick , Fu Guangliang , Hasekamp Otto
Published: March 2021

A top-down approach for aerosol emission estimation from polarimetric retrievals of aerosol amount, size, and absorption is employed . The method uses a fixed-lag ensemble Kalman smoother (LETKF-Smoother) under the framework of Observing System Simulation Experiments (OSSEs), in order to evaluate the observational capabilities of a satellite with near perfect global coverage as well as of the future multi-angle polarimeter instrument, SPEXone. ECHAM-HAM is used for the nature runs (NATs), the control (CTL) and the data assimilation (DAS) experiments. The ensemble is composed by 32 simulations where the default aerosol emissions for all species are perturbed with factors taken from a Gaussian distribution. Synthetic observations, specifically Aerosol Optical Depth at 550nm (AOD550), Angstrom Exponent 550nm to 865nm (AE550-865) and Single Scattering Albedo at 550nm (SSA550) are assimilated in order to estimate the aerosol emission fluxes of desert dust (DU), sea salt (SS), organic carbon (OC), black carbon (BC) and sulphates (SO4), along with the emission fluxes of two SO4 precursor gases (SO2, DMS). The synthetic observations are sampled from the NATs according to two satellite observing systems, with different spatial coverage capabilities. The first, is an idealized sensor that retrieves observations over the whole globe in 2days even under cloudy conditions, hence is named PERFECT. The second, is the sensor SPEXone, a hyperspectral multi-angle polarimeter with a narrow swath (100km), that will be a part of the NASA PACE mission. The assimilated observations sampled using the PERFECT sensor, estimate the emission of all aerosol species with a global relative Mean Absolute Error (MAE) equal or lower than 5% (except SO4). Despite its limited coverage, the SPEXone sampling bares similar results, although MAE is a bit larger for Dust and Sea Salt. Further, experiments show that doubling the measurement error on the assimilated observations, increases additionally the global relative MAE by less than 10%. In addition, the role of biased meteorology on emission estimation was quantified by using two different datasets (ERA5 and ERAi) to nudge the U and V wind components of the model. The results reveal that when the wind of NAT and DAS are nudged to different datasets the global relative MAE of SS grows by 24%, while the estimated emissions of DU, OC, BC and SO2 are negatively affected to a smaller extent (~10%). The upcoming SPEXone sensor will provide observations related to aerosol amount size and absorption, with sufficient coverage and accuracy, in order to estimate aerosol emission accurately.
Generic concepts for organising data management in research projects
Authors: Anders Ivonne , Gehlot Swati , Lammert Andrea , Peters-von Gehlen Karsten
Published: March 2021

Since few years Research Data Management is becoming an increasingly important part of scientific projects regardless of the number of topics or subjects, researchers or institutions involved. The bigger the project, the more are the data organization and data management requirements in order to assure the best outcome of the project. Despite this, projects rarely have clear structures or responsibilities for data management. The importance of clearly defining data management and also budgeting for it is often underestimated and/or neglected. A rather scarce number of reports and documentations explaining the research data management in certain projects and detailing best practice examples can be found in the current literature.  Additionally, these are often mixed up with topics of the general project management. Furthermore, these examples are very focused on the certain issues of the described projects and thus, a transferability (or general application) of provided methods is very difficult.

This contribution presents generic concepts of research data management with an effort to separate them from general project management tasks. Project size, details among the diversity of topics and the involved researcher, play an important role in shaping data management and determining which methods of data management can add value to the outcome of a project. We especially focus on different organisation types, including roles and responsibilities for data management in projects of different sizes. Additionally, we show how and when also education should be included, but also how important agreements in a project are.
NASA Space Weather Science Application Strategy and Activities
Authors: Spann James
Published: March 2021

The NASA Heliophysics Division Space Weather Science Application (SWxSA) program has as its strategic mission to establish a preeminent space weather capability that supports human and robotic space exploration and meets national, international, and societal needs. This is done by advancing measurement and analysis techniques and expanding knowledge and understanding that improves space weather forecasts and nowcasts. Ultimately, the SWxSA program enables space weather forecasting capability that the Agency and Nation and international community require, in partnership with NASA’s Artemis Program and other Federal agencies, and international partners. This includes the development and launch of missions/instruments that advance our knowledge of space weather and improve its prediction, and the transitioning of technology, tools, models, data, and knowledge from research to operational environments. This presentation will provide an update on NASA’s SWxSA space weather strategy and activities.
Unravelling the ozone-weather relationship: the role of vegetation and radical reactions
Authors: Emmerichs Tamara , Franco Bruno , Wespes Catherine , Rosanka Simon , Taraborrelli Domenico
Published: March 2021

Near-surface ozone is a harmful air pollutant, which is not only controlled by chemical production and loss processes.  The major removal process of near-surface ozone is dry deposition accounting for 20 % of the total tropospheric ozone loss. Due to its significance, parameterizations used in atmospheric chemistry models represent a major source of uncertainty for tropospheric ozone simulations. This uncertainty might be one of the reasons why global models tend to overestimate ozone, when compared to observations. The model used in this study, the global atmospheric model ECHAM5/MESSy (EMAC), is no exception. Like most global models, EMAC employs a “resistances in series” scheme, which is hardly sensitive to local meteorological conditions (e.g. humidity) and lacks non-stomatal deposition. In this study, these missing features have been implemented in EMAC affecting not only the deposition of ozone but also the removal of ozone precursors, resulting in lower chemical production of ozone.

Furthermore, near-surface ozone may be significantly impacted by water vapour forming complexes with peroxy radicals. The role of water in the reaction of HO2 radical with itself and nitrogen oxides is known from the literature. However, in current models only the former is considered by assuming a linear dependence on water concentrations. Recent experimental evidence for the significant role of water on the kinetics of one of the most important reaction for ozone chemistry, namely NO2 + OH, has been published. Here, the available kinetic data for the HOx + NOx reactions have been critically re-assessed and included in EMAC to test its global significance. Additionally, we considered the representation of isoprene and nitrous acid (HONO) as important oxidants for lower tropospheric chemistry. Namely, for isoprene emissions we added a drought stress factor which enables a higher sensitivity to meteorology leading to reduced emissions. Also, we firstly implemented soil emissions of HONO which is known as a missing source in models. The implications of these modifications on the global tropospheric composition are analysed, focusing on near-surface ozone and related precursors. The improved representation of ozone in EMAC is demonstrated using measurements from the Infrared Atmospheric Sounding Interferometers (IASI), the Tropospheric Ozone Assessment Report (TOAR) database and from the Trajectory-mapped Ozonesonde dataset for the Stratosphere and Troposphere (TOST). The overall changes might help to reduce the uncertainty and overestimation of models predicting near-surface ozone.
Dirty sea ice drives higher Mn concentrations in the Canada Basin
Authors: Rogalla Birgit , Allen Susan E. , Colombo Manuel , Myers Paul G. , Orians Kristin J.
Published: March 2021

The rapidly changing conditions of the Arctic sea ice system have cascading impacts on the biogeochemical cycles of the ocean. Sea ice transports sediments, nutrients, trace metals, pollutants, and gases from the extensive continental shelves into the more isolated central basins. However, it is difficult to assess the net contribution of this supply mechanism on nutrients in the surface ocean. In this study, we used Manganese (Mn), a micronutrient and tracer which can integrate source fluctuations in space and time, to understand the net impact of the long range transport of sea ice for Mn.

We developed a three-dimensional dissolved Mn model within a subdomain of the 1/12 degree Arctic and Northern Hemispheric Atlantic (ANHA12) configuration of NEMO centred on the Canadian Arctic Archipelago, and evaluated this model with in situ observations from the 2015 Canadian GEOTRACES cruises. The Mn model incorporates parameterizations for the contributions from river discharge, sediment resuspension, atmospheric deposition of aerosols directly to the ocean and via melt from sea ice, release of sediment from sea ice, and reversible scavenging, while the NEMO-TOP engine takes care of the advection and diffusion of the tracers. 

Simulations with this model from 2002 to 2019 indicate that the majority of external Mn contributed annually to the Canada Basin surface is released by sediment from sea ice, much of which originates from the Siberian shelves. Reduced sea ice longevity in the Siberian shelf regions has been postulated to result in the disruption of the long range transport of sea ice by the transpolar drift. This reduced sea ice supply has the potential to decrease the Canada Basin Mn surface maximum and downstream Mn supply, with implications for other nutrients (such as Fe) contained in ice-rafted sediments as well. These results demonstrate some of the many changes to the biogeochemical supply mechanisms expected in the near-future in the Arctic Ocean and the subpolar seas.
Analysis of Vertical Velocities Development through High-resolution Simulation and Glider Observations in the Alboran Sea
Authors: Garcia-Jove Maximo , Mourre Baptiste , Zarokanellos Nikolaos , Lermusiaux Pierre F. J. , Rudnick Daniel L. , Allen John , Tintoré Joaquín
Published: March 2021

Vertical velocities associated with meso- and submeso-scale structures generate important vertical fluxes of carbon and other biogeochemical tracers from the surface layer to depths below the mixed layer. Vertical velocities are very weak and characterized by small scales which make them difficult to measure. The project entitled Coherent Lagrangian Pathways from the Surface Ocean to Interior (CALYPSO, Office of Naval Research initiative) addresses the challenge of observing, understanding, and predicting the vertical velocities and three-dimensional pathways on subduction processes in the frontal regions of the Alboran Sea. Within the framework of the CALYPSO project, we analysed the processes that give rise to vertical velocities in the Western Alboran Gyre Front (WAGF) and Eastern Alboran Gyre Front (EAGF). The periods of frontal intensification were analyzed in the perspective of the frontogenesis, instabilities, non-linear Ekman effects, and filamentogenesis using multi-platform in-situ observations and a high-resolution simulation in spring 2018. The spatio-temporal characteristics of the WAGF indicate a wider, deeper, and longer-lasting front than the EAGF. The WAGF intensification and vertical velocities development are explained through i) frontogenesis, ii) conditions for symmetric and ageostrophic baroclinic instabilities generation, and iii) nonlinear Ekman effects. These mechanisms participate to generate and strengthen an ageostrophic secondary circulation responsible for vertical velocities intensification in the front. In the case of the EAGF, the intensification and vertical velocities development are explained by filamentogenesis in both the model and glider observations. The EAGF intensification is characterized by a sharp and outcropping density gradient at the center of the filament, where two asymmetrical ageostrophic cells develop across the front with narrow upwelling region in the middle.
The role of tides on the carbonate chemistry of a coastal polynya in the south-eastern Weddell Sea
Authors: Droste Elise , González Dávila Melchor , Santana Casiano Juana Magdalena , Hoppema Mario , Rohardt Gerd , Queste Bastien , Venables Hugh , Dall'Olmo Giorgio , Bakker Dorothee C. E.
Published: March 2021

Tides have a large impact on coastal polynyas around Antarctica. We investigate the effect of semi-diurnal tidal cycles on the seawater carbonate chemistry in a coastal polynya hugging the Ekström Ice Shelf in the south-eastern Weddell Sea. This region experiences some of the strongest tides in the Southern Ocean. We assess the implications for the contribution of coastal polynyas to the carbon dioxide (CO2) air-sea flux of the Weddell Sea.

Two site visits, in January 2015 and January 2019, are intercompared in terms of the dissolved inorganic carbon (DIC) concentration, total alkalinity, pH, and CO2 partial pressure (pCO2). The tides induce large variability in the carbonate chemistry of the coastal polynya in the austral summer: DIC concentrations vary between 2174 and 2223 umol kg-1.

The tidal fluctuation in the DIC concentration can swing the polynya from a sink to a source of atmospheric CO2 on a semi-diurnal timescale. We attribute these changes to the mixing of different water masses. The amount of variability induced by tides depends on – and is associated with – large scale oceanographic and biogeochemical processes that affect the characteristics and presence of the water masses being mixed, such as the rate of sea ice melt.

Sampling strategies in Antarctic coastal polynyas should always take tidal influences into account. This would help to reduce biases in our understanding of how coastal polynyas contribute to the CO2 uptake by the Southern Ocean.
Multi-frequency and multi-GNSS PPP-RTK for vehicle navigation in urban environments
Authors: Wang Bo , Li Xin , Huang Jiaxin , Feng Guolong , Lv Hongbo , Han Xinjuan , Zhong Yaxin , Li Xingxing
Published: March 2021

PPP-RTK which combines the advantages of real-time kinematic (RTK) and precise point positioning (PPP), is able to provide centimeter-level positioning accuracy with rapid integer ambiguity resolution. In recent years, with the development of BDS and Galileo as well as the modernization of GPS and GLONASS, more than 130 GNSS satellites are available and new-generation GNSS satellites are capable of transmitting signals at three or more frequencies. Multi-GNSS and multi-frequency observations bring more possibilities for enhancing the performance of PPP-RTK. In this contribution, we develop a multi-frequency and multi-GNSS PPP-RTK model aiming to achieve rapid centimeter-level positioning for vehicle navigation in urban environments. The precise undifferenced atmospheric corrections are derived from multi-frequency and multi-GNSS observations of regional networks. Then the corrections are distributed to users to achieve PPP rapid ambiguity resolution. Vehicle experiments in different scenarios such as suburbs, overpasses, tunnels are conducted to validate the proposed method. Our results indicate that the multi-frequency and multi-GNSS PPP-RTK can achieve 2~3 cm positioning accuracy in the horizontal direction, 5~6 cm positioning accuracy in the vertical direction with the time to first fix of 5~7 s. In the urban environments where signals are interrupted frequently, a fast ambiguity recovery can be achieved within 5 s. Moreover, the PPP-RTK performance is significantly improved with multi-GNSS and multi-frequency observations. Compared to GPS-only solution, the positioning accuracy can be improved by 75%, and the fixing percentage can be up to 90% with this new method.
How many grains do we need for tracer thermochronology?
Authors: Madella Andrea , Glotzbach Christoph , Ehlers Todd A.
Published: March 2021

Detrital tracer thermochronology exploits the relationship between bedrock thermochronometric ages and elevation to study spatial variations of upstream erosion from the distribution of detrital grain ages. If ages increase linearly with elevation and analytical uncertainties are negligible, spatially uniform erosion is expected to yield a detrital age distribution that mirrors the catchment’s hypsometric curve. Alternatively, a mismatch between detrital and hypsometric distributions may indicate non-uniform erosion within a catchment. For studies of this sort, measured age populations rarely exceed 100 grains, because applying thermochronology is time consuming and expensive. With such limited sample sizes, discerning between two detrital age distributions produced by different catchment erosion scenarios may be statistically impossible with high confidence. However, there is no established method to quantify the sample-size-dependent uncertainty inherent to detrital tracer thermochronology. Here, we investigate how sample size affects the uncertainty of detrital age distributions and how such uncertainty affects the ability to uniquely infer the erosional pattern of the upstream area. We developed a new tool to consistently report confidence levels as a function of sample size and case-specific variables. The proposed tool will be made available as open-source script along with test data. Testing the hypothesized erosion scenarios will help tracer thermochronologists define the minimum sample size (i.e. number of grain ages) to answer their specific scientific question with high level of statistical confidence. Alternatively, in cases of unavoidable small sample size, the related confidence level can be quantified.
Development for wind friction velocity retrieval algorithm based on the SFMR and NOAA dropwindsondes measurements in hurricane conditions
Authors: Poplavsky Evgeny , Rusakov Nikita , Ermakova Olga , Sergeev Daniil , Troitskaya Yuliya , Balandina Galina
Published: March 2021

The work is concerned with the development of a method for the retrieval of tropical cyclones boundary atmospheric layer parameters, namely the wind friction velocity and wind speed at meteorological height. For the analysis, we used the results of field measurements of wind speed profiles from dropwindsondes launched from National Oceanic and Atmospheric Administration (NOAA) aircraft and collocated data from the Stepped-Frequency Microwave Radiometer (SFMR) located onboard of the same aircraft.

The results of radiometric measurements were used to obtain the emissivity values, which were compared with the field data obtained from the falling dropwindsondes. Using the algorithm taking into account the self-similarity of the velocity defect profile (Ermakova et al., 2019), the parameters of the atmospheric boundary layer were determined from the data measured by dropwindsondes. This algorithm gives an opportunity to obtain the wind speed value at meteorological height and wind friction velocity from the averaged data in the wake part of the profiles of the marine atmospheric boundary layer.

A comparison of the wind speed U10 dependencies, retrieved from the SFMR data and measurements from dropwindsondes, with the similar dependencies obtained in (Uhlhorn et al., 2007), was made, and their satisfactory agreement was demonstrated. This work was supported by the RFBR projects No. 19-05-00249, 19-05-00366.
 Ground-based GNSS for climate research: review and perspectives
Authors: Pacione Rosa , Santos Marcelo , Dick Galina , Jones Jonathan , Pottiaux Eric , Rinke Annette , Van Malderen Roeland , Elgered Gunnar
Published: March 2021

In climate research, the role of water vapour can hardly be overestimated. Water vapour is the most important natural greenhouse gas and is responsible for the largest known feedback mechanism for amplifying climate change. It also strongly influences atmospheric dynamics and the hydrologic cycle through surface evaporation, latent heat transport and diabatic heating, and is, in particular, a source of clouds and precipitation.

Atmospheric water vapour is highly variable, both in space and in time. Therefore, measuring it remains a demanding and challenging task. The Zenith Total Delay (ZTD) estimated from GNSS observations, provided at a temporal resolution of minutes and under all weather conditions, can be converted to Integrated Water Vapour (IWV), if additional meteorological variables are available. Inconsistencies introduced into long-term time series from improved GNSS processing algorithms, instrumental, and environmental changes at GNSS stations make climate trend analyses challenging. Ongoing re-processing efforts using state-of-the-art models aim at providing consistent time series of tropospheric data, using 24+ years of GNSS observations from global and regional networks. GNSS is reaching the “maturity age” of 30 years when climate normal of ZTD/IWV (and horizontal gradients) can be derived. Being not assimilated in numerical weather prediction model reanalyses, GNSS products can also be used as independent datasets to validate climate model outputs (ZTD/IWV). However, what is the actual use of GNSS ZTDs in climate monitoring? What are the advantages of using GNSS ZTDs for climate monitoring? In addition, what would be the best ZTD time series to serve the climate community?

The presentation will provide a review of the progress made in and the status of using GNSS tropospheric datasets for climate research, highlighting the challenges and pitfalls, and outlining the major remaining steps ahead. We will show examples demonstrating the benefits for climate monitoring brought by using GNSS ZTD and/or IWV datasets in complement to other observations.

This contribution is related to the activities of JWG C.2: Quality control methods for climate applications of geodetic tropospheric parameters, https://iccc.iag-aig.org/joint-work-groups/216, of the IAG Inter-Commission Committee on "Geodesy for Climate Research" (ICCC).
Building a new high-density air temperature measurement network in two Swiss cities
Authors: Anet Julien G. , Schlögl Sebastian , Spirig Curdin , Frey Martin P. , Renold Manuel , Gutbrod Karl G.
Published: March 2021

With progressive climate change, weather extremes are very likely to become more frequent. While rural regions may suffer from more intense and longer drought periods, urban spaces are going to be particularly affected by severe heat waves. This urban temperature anomaly, also known as “urban heat island” (UHI), can be traced back to different factors, the most prominent being soil sealing, lower albedo and lack of effective ventilation.

City planners have started developing mitigation strategies to reduce future forecasted heat stress in urban regions. While some heat reduction strategies are currently intensely scrutinized and applied within pilot projects, the efficiency of latter mitigation actions can be overseen due to the low density of reference in situ air temperature measurements in urban environments. The same problem applies when trying to benchmark modeling studies of UHI as the amount of benchmarking data may be insufficient.

To overcome this lack of data, over the last two years, a dense air temperature measurement network has been installed in the Swiss cities of Basel and Zurich, counting more than 450 sensors. The low-cost air temperature sensors are installed on street lamps and traffic signs in different local climate zones of the city with an emphasis on street canyons, where air temperatures are expected to be the largest and most of the city’s population lives and works. These low-cost sensors add valuable meteorological information in cities and complement the WMO reference stations.

Air temperature measurements from the low-cost sensor network were controlled for accuracy, reliability and robustness and homogenized in order to minimize radiation errors, although 40% of the stations were equipped with self-built radiation shields, allowing an efficient passive ventilation of the installed sensors.

We demonstrate the strength of our network by presenting first results of two exemplary heat waves that occurred in July 2019 and August 2020 and show that a) the radiation-error corrected datasets correlate well with different high-quality reference WMO stations, and b) the existence of urban heat islands in Zurich and Basel can be well confirmed, showing significant air temperature differences of several degrees between rural and urban areas.

The results demonstrate the advantages of a high-density low-cost air temperature network as a benchmark for future urban heat islands modelling studies.
Statistical Distribution of Energetic Electron Precipitation due to Hiss Waves In the Earth’s Inner Magnetosphere
Authors: Ma Qianli
Published: March 2021

We investigate the statistical distribution of energetic electron precipitation from the equatorial magnetosphere due to hiss waves in the plasmasphere and plumes. Using Van Allen Probes measurements, we calculate the pitch angle diffusion coefficients at the pitch angle of bounce loss cone, and evaluate the energy spectrum of precipitating electron flux using quasi-linear theory. Our ~6.5 years survey shows that, during disturbed times, the plasmaspheric hiss mostly causes the electron precipitation at L > 3 near the dayside in the plasmasphere, and hiss waves in plume cause the precipitation at L > 5 near dayside and L > 3.5 near the dusk side. The precipitating energy flux increases with increasing geomagnetic index, and is typically higher in the plasmaspheric plume than the plasmasphere. The characteristic energy of precipitation increases from ~20 keV at L = 6 to ~100 keV at L = 3, potentially causing the loss of electrons at several hundred keV. Although the total precipitating energy flux due to hiss waves is generally lower than the precipitation due to whistler mode chorus waves, the characteristic energy of precipitation due to hiss is higher, and the precipitation extends closer to the Earth.
A relation between extreme precipitation and surface air temperature over Russia in the last four decades
Authors: Aleshina Maria , Semenov Vladimir , Chernokulsky Alexander
Published: March 2021

Precipitation extremes are widely thought to intensify with the global warming due to exponential growth, following the Clausius-Clapeyron (C-C) equation of atmosphere water holding capacity with rising temperatures. However, a number of recent studies based on station and reanalysis data for the contemporary period showed that scaling rates between extreme precipitation and temperature are strongly dependent on temperature range, region and moisture availability. Here, we examine the scaling between daily precipitation extremes and surface air temperature over Russian territory for the last four decades using meteorological stations data and ERA-Interim reanalysis. The precipitation-temperature relation is examined for total precipitation amount and, separately, for convective and large-scale precipitation types. In winter, a general increase of extreme precipitation of all types according to C-C relation is revealed. For the Russian Far East region, the stratiform precipitation extremes scale with surface air temperature following even super C-C rates, about two times as fast as C-C. However, in summer we find a peak-like structure of the precipitation-temperature scaling, especially for the convective precipitation in the southern regions of the country. Being consistent with the C-C relationship, extreme precipitation peaks at the temperature range between 15 °C and 20 °C. For the higher temperatures, the negative scaling prevails. Furthermore, it was shown that relative humidity in general decreases with growing temperature in summer. Notably, there appears to be a temperature threshold in the 15-20 °C range, beyond that relative humidity begins to decline more rapidly. This indicates that moisture availability can be the major factor for the peak-shaped relationship between extreme precipitation and temperature revealed by our analysis.