2D thermospheric wind fields, at both E- and F-region altitudes within a common vertical volume, were made using a Scanning Doppler Imager (SDI) at Poker Flat, Alaska, during a substorm event. Coinciding with these observations were F-region plasma velocity measurements from the Super Dual Auroral Radar Network (SuperDARN), and estimations of the total downward and upward field-aligned current density from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE). This combination of instruments gives an excellent opportunity to examine the spatial characteristics of high latitude ionosphere-thermosphere coupling, and how a process which is triggered in the magnetosphere (the substorm) affects that coupling at different altitudes. We find that during the substorm growth phase, the F-region thermospheric winds respond readily to an expanding ionospheric plasma convection pattern, whilst the E-region winds appear to take a much longer period of time. The differing response timescales of the E- and F-region winds is likely due to differences in neutral density at those altitudes, resulting in E-region neutrals being much more ‘sluggish’ with regards to ion-drag. We also observe increases in the F-region neutral temperature, associated with neutral winds accelerating during both substorm growth and recovery phases.

Our limited understanding of millennial-scale variability in the context of the last glacial period can be explained by the lack of a reliable modelling framework to study abrupt climate changes under realistic glacial backgrounds. In this article, we describe a new set of long-run Last Glacial Maximum experiments where such climate shifts were triggered by different snapshots of ice-sheet meltwater derived from the early stages of the last deglaciation. Depending on the location and the magnitude of the forcing, we observe three distinct dynamical regimes and highlight a subtle window of opportunity where the climate can sustain oscillations between cold and warm modes. We identify the European-Arctic and Nordic Seas regions as being most sensitive to meltwater discharge in the context of switching to a cold mode, compared to freshwater fluxes from the Laurentide ice sheets. These cold climates follow a consistent pattern in temperature, sea ice and convection, and are largely independent from freshwater release as a result of effective AMOC collapse. Warm modes, on the other hand, show more complexity in their response to the regional pattern of the meltwater input, and within them, we observe significant differences linked to the reorganisation of deep water formation sites and the subpolar gyre. Broadly, the main characteristics of the oscillations, obtained under full-glacial conditions with realistically low meltwater discharge, are comparable to δ18O records of the last glacial period, although our simplified experiment design prevents detailed conclusions from being drawn on whether these represent actual Dansgaard-Oeschger events.

Urbanization manifests rainfall anomalies both in and around the city. While prior studies have assessed the effect of urban expansion as well as city size, little is known about the impact of city shape on rainfall. Here, idealized large eddy simulations (LES) coupled with the Weather Research and Forecasting (WRF) model are conducted to bridge the knowledge gap. Results indicate difference in the timing for urban-induced rainfall in the inland versus coastal environment. This is associated with contrasting diurnal cycles of vertical velocity and cloud water mixing ratio driven by the land-sea breeze. The impact of city shape on rainfall is more evident in the coastal environment, as buoyancy flows arise from cities alter the interactions between urban-rural circulation and sea breeze. A circular city show nearly 22.0% larger rainfall accumulation and 78.6% greater rainfall intensity compared to a triangular city over urban surfaces. Results highlight the need for considering city shape for coastal urban planning, as a potential adaptation strategy to manage rainfall under future climate. Results indicate that coarser models which represent impervious surface area but not the right shape would have a higher uncertainty in simulating rainfall changes.

Hydrological modeling of wetlands is important for reliable estimation of biogeochemical processes in soils subject to periodically inundating conditions. The present study has developed a wetland module in the Richards-equation-based SWAT model to fully couple the surface water storage and soil water dynamics. The wetland module was tested using observed daily water level data from four wetlands (including restored and natural wetlands with and without impermeable soil layers) in the Choptank River Watershed, Maryland, USA. After the wetland module was calibrated, simulated daily water level and observed data were compared and evaluated using three statistics, i.e., percent bias (Pbias), coefficient of determination (R2), and Nash-Sutcliffe coefficient (NS) from 2016 to 2017. The results showed that, in general, the wetland module regenerated hydroperiods for both restored and natural wetlands with and without impermeable soil layers; specifically, the module was able to accurately model saturation conditions for different soil layers corresponding to wet and dry periods in plant growing seasons; the wetland module had the tendency to generate better results for natural wetlands because restored wetlands tended to have mixed plant types which caused difficulty for accurate estimation of evapotranspiration; the ability to accurately describe inundation conditions for wetlands is important for biogeochemical modeling so that the newly developed wetland module has a great potential in enhancing simulation of biogeochemical cycles not only at the site scale but also at the watershed scale.

A technique for predicting the secular variation (SV) of the geomagnetic field based on the echo state network (ESN) model is proposed. SV is controlled by the geodynamo process in the Earth’s outer core. However, it is difficult to model the realistic nonlinear behaviors of the geodynamo due mainly to the very small Ekman number of the actual outer core. This study employs the ESN to represent the temporal evolution of the geomagnetic field on the Earth’s surface. The hindcast results of SV demonstrate that the ESN enables us to predict SV for several years with satisfactory accuracy. In particular, the nonlinear behaviors of SV is accurately predicted for the case where accurate geomagnetic data with a 1-year time resolution are available. It is found that an increase in the number of training data does not necessarily improve prediction accuracy. The results suggest that the information on the latest temporal variations is important for the short-term prediciton by the ESN valid for, say 5 years.

Historically multiyear sea ice (MYI) covered a majority of the Arctic and circulated through the Beaufort Gyre for years. However, increased ice melt in the Beaufort Sea during the early-2000s was proposed to have severed this circulation. Constructing a regional MYI budget from 1997-2021 reveals that MYI import into the Beaufort Sea has increased year-round, yet less MYI now survives through summer and is transported onwards in the Gyre. Annual average MYI loss quadrupled over the study period and increased from ~7% to ~33% of annual Fram Strait MYI export, while the peak in 2018 (385,000 km^2) was similar to Fram Strait MYI export. An accelerating ice-albedo feedback coupled with dynamic conditioning towards younger thinner MYI is responsible for the increased MYI loss. MYI transport through the Beaufort Gyre has not been severed, but it has been reduced so severely to prevent it from being redistributed throughout the Arctic Ocean

This study considers 28 geomagnetic storms with Dst $\leq-50$ nT driven by high-speed streams (HSSs) and associated stream interaction regions (SIRs) during 2010-2017. Their impact on ionospheric horizontal and field-aligned currents (FACs) have been investigated using superposed epoch analysis of SuperMAG and AMPERE data, respectively. The zero epoch ($t_0$) was set to the onset of the storm main phase. Storms begin in the SIR with enhanced solar wind density and compressed southward oriented magnetic field. The integrated FAC and equivalent currents maximise 40 and 58 min after $t_0$, respectively, followed by a small peak in the middle of the main phase ($t_0$+4h), and a slightly larger peak just before the Dst minimum ($t_0$+5.3h). The currents are strongly driven by the solar wind, and the correlation between the Akasofu $\varepsilon$ and integrated FAC is $0.90$. The number of substorm onsets maximises near $t_0$. The storms were also separated into two groups based on the solar wind dynamic pressure p_dyn in the vicinity of the SIR. High p_dyn storms reach solar wind velocity maxima earlier and have shorter lead times from the HSS arrival to storm onset compared with low p_dyn events. The high p_dyn events also have sudden storm commencements, stronger solar wind driving and ionospheric response at $t_0$, and are primarily responsible for the first peak in the currents after $t_0$. After $t_0+2$ days, the currents and number of substorm onsets become higher for low compared with high p_dyn events, which may be related to higher solar wind speed.

A document by Aurélien Podglajen. Click on the document to view its contents.

The lifetime of excess atmospheric CO2 (above preindustrial) xCO2 governs the future consequences of present xCO2 and those of future CO2 emissions. Recent assessments of the decrease of xCO2 following abrupt cessation of anthropogenic emissions (zero emissions commitment, ZEC) inferred from studies with carbon-cycle (CC) models (e.g., Joos et al., ACP, 2013; MacDougall et al., BG, 2020) vary substantially, with the fraction of xCO2 remaining in the atmosphere 100 years after cessation, fCO2(100) = xCO2(100)/xCO2(0), ranging from 0.55 to 0.85 (Figure 1a; Schwartz, JGR, 2018; Schwartz, in review). In this study prior atmospheric and oceanic CO2 and future xCO2 for ZEC were calculated with a 5-compartment global model. Model compartments are the atmosphere, upper and deep ocean, and labile and obdurate terrestrial biosphere (TB). Model parameters are obtained mainly from observation (e.g., rate of uptake of heat by the deep ocean) and theory (e.g., CO2-dependent solubility of CO2 in seawater); uptake of CO2 by the two TB compartments is apportioned by parameterization, with parameters rather narrowly constrained by observations of CO2 and radiocarbon. CO2 is found to decay much more rapidly than in CC models; fCO2(100) = 0.41 ± 0.8 (1 σ), Figure 1a. These results indicate that cessation of anthropogenic CO2 emissions would result in discernible decrease in atmospheric CO2 on a time scale as short as a human lifetime, much faster than in current CC models. Shown in Figure 1b is a quantity denoted τE(t), the equivalent 1/e lifetime of xCO2, as a function of time subsequent to cessation of emissions t, evaluated as τE(t) = -1/ln fCO2(t). τE(t) is a generalization of the relation between half-life of a decaying quantity and its 1/e lifetime and is an integral measure of decay over time t. The present model yields τE(t) of excess atmospheric CO2 about 100 years, much shorter than obtained with current CC models. Figure 1. a, Fractional excess CO2 fCO2(t) as function of time t following abrupt cessation of anthropogenic CO2 emissions as calculated in a recent model intercomparison (MacDougall et al., 2020) and with present model (best estimate, thick red, and uncertainty range); dotted black lines denote exponential decay with lifetime indicated at right. b, Equivalent 1/e lifetimes as function of t.

We present the results of the analysis of the measurements data of amplitudes and phases of signals from GQD (19.58 kHz) and GBZ (22.1 kHz) VLF transmitters in the geophysical observatory of IDG RAS “Mikhnevo” (GO MIK) to assess the impact of solar flares of different classes that occurred in June 2014 on the height profile of the electronic concentration $Ne$ in the lower ionosphere. The use of two-channel data from the GOES satellite (0.05-0.4 nm and 0.1-0.8 nm) allowed us to estimate the fluxes of X-rays in harder spectral ranges. The analysis of the dynamics of the $Ne$ height profile and X-ray fluxes in different wavelength ranges allowed us to estimate the ionization and recombination rates and determine the spectral ranges of radiation that have the greatest influence on the dynamics of the electron concentration at considered heights.

The region between a shallow mixed layer and a deep isothermal layer resulting from salinity stratification is called a barrier layer (BL). Since BLs hinder the surface heat and momentum exchange with the ocean subsurface, they play an important role in air-sea interaction. Synoptic features and formation of BLs and associated temperature inversions (TIs) in the Eastern Pacific Fresh Pool (EPFP) were investigated using shipboard observations. BLs and TIs were found in the high precipitation EPFP, a frontal transition zone (FTZ) and in the sea surface salinity (SSS) front during a 2016 boreal summer cruise. During a 2017 boreal autumn cruise, BLs and TIs were found from the southern part of the EPFP to the SSS front. The BLs in the SSS front and the FTZ were meridionally displaced from the precipitation maximum during both the 2016 and 2017 cruises. Freshening and cooling by tilting of both the isohalines and isotherms of the SSS front occurred via Ekman and geostrophic flow during both cruises, and also within the EPFP during 2016. The SSS front was a dominant contributor to the density gradient in the horizontal plane. A high dissolved oxygen content observed within BLs confirmed that the surface flow plays an important role in the BL and TI formation through the tilting process. Patchy distribution of freshening and cooling by tilting implies an intermittent BL and TI formation with short time scales.

Seismic event classification can be challenging in the regions where different types of seismicity overlap in space, time, and magnitude. In this paper, I evaluate the performance of a supervised machine learning technique called Random Forest for the discrimination of icequakes and earthquakes in southeast Alaska at 15 stations surrounding the region. I train the Random Forest on about 3000 icequakes and earthquakes that occurred in the region over the last 17 years. For each event, absolute frequency spectrum values are considered as input features. The accuracies at different stations range from 75 to 95% with an average of about 90%. I conducted tests for selecting the optimum number of decision trees in the RF model and compared the results obtained by applying bandpass filters of different frequency bands on input waveforms. I further experiment by reducing the dimensions of input features by applying Principal Component Analysis (PCA), and conducted test for selecting the minimum number of components and the frequency band that gives the best results. The application of PCA resulted in slightly better results and a final model that gave the best results among all the tests was chosen. The accuracy results of the final model were further analyzed with respect to the amount of available dataset, the average distance of a station from all the glaciers, and the local geology.

Radiation-topography interaction plays an important role in the surface energy balance over the Tibetan Plateau (TP). However, the impacts of such interaction over the TP on climate locally and in the Asian regions remain unclear. This study uses the Energy Exascale Earth System Model (E3SM) to evaluate the regional and teleconnected impacts of radiation-topography interaction over the TP. Land-atmosphere coupled experiments show that topography regulates the surface energy balance, snow processes, and surface climate over the TP across seasons. Accounting for radiation-topography interaction overall improves E3SM’s performance in simulating surface climate. The winter cold bias in air temperature decreases from -4.48 K to -3.70 K, and the wet bias in summer precipitation is mitigated in southern TP. The TP’s radiation-topography interaction further reduces the South and East Asian summer precipitation biases. Our results demonstrate the topographic roles in regional climate over the TP and highlight its teleconnected climate impacts.

Temporal changes of S-wave velocities at shallow depth on Mars are derived using seismic data from the InSight mission. Autocorrelation functions are computed for three-component seismic recordings to retrieve zero-offset reflection seismograms. Observed S-wave reflection phase with two-way travel time of ~1.2 s and its multiples indicate an interface at ~200 m depth. Daily relative travel time changes (dt/t) with ~5% variations are correlated well with the surface temperature. A top ~1m-thick regolith layer produces a delay of about one Martian day between the dt/t and surface temperature. Assuming the travel time changes are produced primarily in the top ~18 m sand layer, the daily velocity variations in that layer are ~40%. The dominant mechanisms driving the changes are thermoelastic strain in the shallow structure generating the time delays and possible material failures in the regolith layer.

Bedload flux is notoriously challenging to measure and model. The dynamics of bedload therefore remains largely unknown in most fluvial systems worldwide. We present a global scale bedload flux model as part of the WBMsed modeling framework. Our results show that the model can very well predict the distribution of water discharge and suspended sediment and well predict bedload. We analyze the model’s bedload predictions sensitivity to river slope, particle size, discharge, river width and suspended sediment. We found that the model is most responsive to spatial dynamics in river discharge and slope. We analyze the relationship between bedload and total sediment flux globally and in representative longitudinal river profiles (Amazon, Mississippi, and Lena Rivers). We show that while, as expected, the proportion of bedload is decreasing from headwater to the coasts, there is considerable variability between basins and along river corridors. The latter is largely responsive to changes in suspended sediment and river slope due to dams and reservoirs. We provide a new estimate of water and sediment fluxes to global oceans from 2,067 largest river outlets (draining 67% of the global continental mass). Estimated water discharge (30,579 km3/y) corresponds well to past estimates however sediment flux is considerably higher. Of the total 22 Gt/y estimated average sediment flux to global oceans, 19 Gt/y is transported as washload, 1 Gt/y as bedload, and 2 Gt/y as suspended bed material. The largest 25 rivers are predicted to transport over 55% of total sediment flux to global oceans.

The ROTI index based on the variation of the TEC is used to detect and characterize the ionospheric irregularities. In the present work, we present a comparative study of five different methodologies to ROTI calculation in order to evaluate the most suitable for the Brazilian region. This was performed over three GNSS stations at different latitudes: São Luís (SALU, 2°31′ S, 44°16′ W; dip: -6.60°) that is located near the dip equator; Cachoeira Paulista (CHPI, 22°40’ S, 44°59’ W; dip: -35.99°) which set close to the southern crest of the EIA at low latitude); and Santa Maria (SMAR, 29° 41′ S, 53° 48′ W, dip: -43.51°) a low-to-mid latitude station close to center of the SAMA region. The period of analysis covered January and December 2015. Our results show that only one out of the five techniques proposed seems to be appropriated for ROTI construction in the Brazilian sector. Our results are supported by comparison of the ROTI with TEC maps obtained over Brazil, ionograms acquired at Fortaleza (FZA0M), SALU, and CHPI ionosonde stations, and All-Sky imagers collected at the São João do Cariri, and CHPI. In addition, we were able to observe the typical irregularities of the Brazilian ionosphere by using the ROTI which we have classified as EPB.

We present a novel Overlapped Cloud Motion Vectors (OCMVs) deriving algorithm using the Himawari-8 satellite. A multi-layer Cloud Top Heights (CTHs) retrieving model based on multi-spectral observed radiances is constructed using neural networks to reduce the substantial uncertainty of CMVs over multi-layer clouds. The retrieved CTHs are assigned to upper ice and lower water cloud layers, then they are used as respective tracers for deriving OCMVs based on an optical flow algorithm. OCMVs reveal the asymmetric kinematic structure of Typhoon Mulan, particularly in the typhoon’s inner-core. The vortex center of lower water CMVs is significantly closer to the typhoon center than that of the upper ice CMVs. Additionally, the direction of CMVs in the lower inner-core show excellent agreement with dropsonde observations. This marks the first time that a geostationary satellite successfully captured the lower layer CMVs of typhoons, providing a valuable reference for the typhoon center identification.

In the framework of EGS, using hydraulic stimulation increases pore pressure into the existing fracture networks and can be responsible for slip reactivations. In order to predict the seismic or aseismic character of these slips, it is important to determine the influence of the gouge characteristics on the slip behaviour. Lab or in-situ testing can be very helpful, but experience has shown that it is complicated to install local sensors inside a mechanical contact and that some data remains impossible to obtain. In this study, we are focusing on the physics of the contact inside a granular gouge to understand the mechanisms of slip triggering by using numerical modelling. We implement a 2D granular fault gouge with Discrete Element Modelling (DEM). The model involves two rough surfaces representing the rock walls separated by a granular gouge with realistic angular particles created by the wear of previous slips. A displacement-driven model with dry contact is primarily studied to observe the peak of static friction (shape, slope or duration). In order to spotlight the aseismic and seismic patterns, a second model is carried out to add the stiffness of the rocks (or loading apparatus). Dedicated post processing tools are also used in order to analyse the spatial distribution of the solid fraction and the force chains patterns bringing a new understanding of physics from a granular point of view. Representing realistic morphology of particles (angular shapes) brings an additional level of control with respect to most of the simulations reported in the literature using circular grains. Angular grains lead to higher friction coefficients with different global behaviours. The initial solid fraction of the gouge appears to be a good indicator of the friction peak we have at a slip triggering (Fig.). The more compacted the gouge is, the more effort is needed to disturb the initial grains assembly and to reach a steady state regime. With fractal size distribution, we have also studied the mechanical contribution of small particles in the media. Friction coefficient results let us conclude that the presence or not of very small particles is not as influent as the initial solid fraction of the gouge. Finally, adding a stiffness on the upper rock wall provides information on the transition from stable sliding to slow-slip/stick-slip behaviour.