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Geophysical Research Letters | Page 2

Kathrin Wehrli, Benoit P Guillod, Mathias Hauser, Matthieu Leclair, Sonia I Seneviratne
Global climate models present systematic biases, among others, a tendency to overestimate hot and dry summers in midlatitude regions. Here we investigate the origin of such biases in the Community Earth System Model. To disentangle the contribution of dynamics and thermodynamics, we perform simulations that include nudging of horizontal wind and compare them to simulations with a free atmosphere. Prescribing the observed large-scale circulation improves the modeled weather patterns as well as many related fields...
August 28, 2018: Geophysical Research Letters
H K Nowell, C D Holmes, K Robertson, C Teske, J K Hiers
Florida, United States, government records provide a new resource for studying fire in landscapes managed with prescribed fire. In Florida, most fire area (92%) is prescribed. Current satellite fire products, which underpin most air pollution emission inventories, detect only 25% of burned area, which alters airborne emissions and environmental impacts. Moreover, these satellite products can misdiagnose spatiotemporal variability of fires. Overall fire area in Florida decreases during drought conditions as prescribed fires are avoided, but satellite data do not reflect this pattern...
August 16, 2018: Geophysical Research Letters
Lukas Brunner, Nathalie Schaller, James Anstey, Jana Sillmann, Andrea K Steiner
The impact of atmospheric blocking on European heat waves (HWs) and cold spells (CSs) is investigated for present and future conditions . A 50-member ensemble of the second generation Canadian Earth System Model is used to quantify the role of internal variability in the response to blocking. We find that the present blocking-extreme temperature link is well represented compared to ERA-Interim, despite a significant underestimation of blocking frequency in most ensemble members. Our results show a strong correlation of blocking with northern European HWs in summer, spring, and fall...
June 28, 2018: Geophysical Research Letters
Piercarlo Giacomel, Elena Spagnuolo, Manuela Nazzari, Andrea Marzoli, François Passelegue, Nasrrddine Youbi, Giulio Di Toro
The safe application of geological carbon storage depends also on the seismic hazard associated with fluid injection. In this regard, we performed friction experiments using a rotary shear apparatus on precut basalts with variable degree of hydrothermal alteration by injecting distilled H2 O, pure CO2 , and H2 O + CO2 fluid mixtures under temperature, fluid pressure, and stress conditions relevant for large-scale subsurface CO2 storage reservoirs. In all experiments, seismic slip was preceded by short-lived slip bursts...
June 28, 2018: Geophysical Research Letters
X Ma, H Jia, F Yu, J Quaas
The Moderate Resolution Imaging Spectroradiometer (MODIS) C6 L3 and the European Centre for Medium-Range Weather Forecasts (ECMWF) ERA-Interim reanalysis data from 2003 to 2016 are employed to study aerosol-cloud correlations over three industrial regions and their adjacent oceans, as well as explore the impact of meteorological conditions on the correlations. The analysis focusing on liquid and single-layer clouds indicates an opposite aerosol-cloud correlation between land and ocean; namely, cloud effective radius is positively correlated with aerosol index over industrial regions (positive slopes), but negatively correlated over their adjacent oceans (negative slopes), for a quasi-constant liquid water path...
June 16, 2018: Geophysical Research Letters
Catherine A Rychert, Nick Harmon
The ocean lithosphere is classically described by the thermal half-space cooling (HSC) or the plate models, both characterized by a gradual transition to the asthenosphere beneath. Scattered waves find sharp seismic discontinuities beneath the oceans, possibly from the base of the plate. Active source studies suggest sharp discontinuities from a melt channel. We calculate synthetic S -to- P receiver functions and SS precursors for the HSC and plate models and also for channels. We find that the HSC and plate model velocity gradients are too gradual to create interpretable scattered waves from the base of the plate...
June 16, 2018: Geophysical Research Letters
J P Eastwood, R Mistry, T D Phan, S J Schwartz, R E Ergun, J F Drake, M Øieroset, J E Stawarz, M V Goldman, C Haggerty, M A Shay, J L Burch, D J Gershman, B L Giles, P A Lindqvist, R B Torbert, R J Strangeway, C T Russell
Magnetospheric Multiscale observations are used to probe the structure and temperature profile of a guide field reconnection exhaust ~100 ion inertial lengths downstream from the X-line in the Earth's magnetosheath. Asymmetric Hall electric and magnetic field signatures were detected, together with a density cavity confined near 1 edge of the exhaust and containing electron flow toward the X-line. Electron holes were also detected both on the cavity edge and at the Hall magnetic field reversal. Predominantly parallel ion and electron heating was observed in the main exhaust, but within the cavity, electron cooling and enhanced parallel ion heating were found...
May 28, 2018: Geophysical Research Letters
Krzysztof Wargan, Clara Orbe, Steven Pawson, Jerald R Ziemke, Luke D Oman, Mark A Olsen, Lawrence Coy, K Emma Knowland
1998-2016 ozone trends in the lower stratosphere (LS) are examined using the Modern-Era Retrospective Analysis for Research and Applications Version 2 (MERRA-2) and related NASA products. After removing biases resulting from step-changes in the MERRA-2 ozone observations, a discernible negative trend of -1.67±0.54 Dobson units per decade (DU/decade) is found in the 10-km layer above the tropopause between 20°N and 60°N. A weaker but statistically significant trend of -1.17±0.33 DU/decade exists between 50°S and 20°S...
May 28, 2018: Geophysical Research Letters
W T Crow, F Chen, R H Reichle, Y Xia, Q Liu
Accurate partitioning of precipitation into infiltration and runoff is a fundamental objective of land surface models tasked with characterizing the surface water and energy balance. Temporal variability in this partitioning is due, in part, to changes in pre-storm soil moisture, which determine soil infiltration capacity and unsaturated storage. Utilizing the NASA Soil Moisture Active Passive Level-4 soil moisture product in combination with streamflow and precipitation observations, we demonstrate that land surface models (LSMs) generally underestimate the strength of the positive rank correlation between pre-storm soil moisture and event runoff coefficients (i...
May 28, 2018: Geophysical Research Letters
C Köhn, O Chanrion, T Neubert
Bursts of X-rays and γ -rays are observed from lightning and laboratory sparks. They are bremsstrahlung from energetic electrons interacting with neutral air molecules, but it is still unclear how the electrons achieve the required energies. It has been proposed that the enhanced electric field of streamers, found in the corona of leader tips, may account for the acceleration; however, their efficiency is questioned because of the relatively low production rate found in simulations. Here we emphasize that streamers usually are simulated with the assumption of homogeneous gas, which may not be the case on the small temporal and spatial scales of discharges...
May 28, 2018: Geophysical Research Letters
Tom Goren, Daniel Rosenfeld, Odran Sourdeval, Johannes Quaas
This study examines the relationships between marine stratocumulus clouds (MSC) coupling state with the ocean surface, their precipitation rate and fractional cloud cover (CF). This was possible by developing a novel methodology for satellite retrieval of the clouds coupling state. Decks of overcast MSC were reported in previous studies to break up often as their precipitation rate increases significantly, thus reducing CF and cloud radiative effect substantially. Here we show that decks of precipitating decoupled MSC have larger CF compared to similarly precipitating coupled MSC...
May 28, 2018: Geophysical Research Letters
Clemens Schwingshackl, Martin Hirschi, Sonia I Seneviratne
The evolution of near-surface air temperature is influenced by various dynamical, radiative, and surface-atmosphere exchange processes whose contributions are still not completely quantified. Applying stepwise multiple linear regression to Coupled Model Intercomparison Project phase 5 (CMIP5) model simulations and focusing on radiation (diagnosed by incoming shortwave and incoming longwave radiation) and land surface conditions (diagnosed by soil moisture and albedo) about 79% of the interannual variability and 99% of the multidecadal trend of monthly mean daily maximum temperature over land can be explained...
May 28, 2018: Geophysical Research Letters
Tianle Yuan, Lazaros Oreopoulos, Steven E Platnick, Kerry Meyer
Modeling studies have shown that cloud feedbacks are sensitive to the spatial pattern of sea surface temperature (SST) anomalies, while cloud feedbacks themselves strongly influence the magnitude of SST anomalies. Observational counterparts to such patterned interactions are still needed. Here we show that distinct large-scale patterns of SST and low-cloud cover (LCC) emerge naturally from objective analyses of observations and demonstrate their close coupling in a positive local SST-LCC feedback loop that may be important for both internal variability and climate change...
May 16, 2018: Geophysical Research Letters
Andreas Fichtner, Dirk-Philip van Herwaarden, Michael Afanasiev, Saulė Simutė, Lion Krischer, Yeşim Çubuk-Sabuncu, Tuncay Taymaz, Lorenzo Colli, Erdinc Saygin, Antonio Villaseñor, Jeannot Trampert, Paul Cupillard, Hans-Peter Bunge, Heiner Igel
We present a general concept for evolutionary, collaborative, multiscale inversion of geophysical data, specifically applied to the construction of a first-generation Collaborative Seismic Earth Model. This is intended to address the limited resources of individual researchers and the often limited use of previously accumulated knowledge. Model evolution rests on a Bayesian updating scheme, simplified into a deterministic method that honors today's computational restrictions. The scheme is able to harness distributed human and computing power...
May 16, 2018: Geophysical Research Letters
O Wurl, K Bird, M Cunliffe, W M Landing, U Miller, N I H Mustaffa, M Ribas-Ribas, C Witte, C J Zappa
This paper describes high-resolution in situ observations of temperature and, for the first time, of salinity in the uppermost skin layer of the ocean, including the influence of large surface blooms of cyanobacteria on those skin properties. In the presence of the blooms, large anomalies of skin temperature and salinity of 0.95°C and -0.49 practical salinity unit were found, but a substantially cooler (-0.22°C) and saltier skin layer (0.19 practical salinity unit) was found in the absence of surface blooms...
May 16, 2018: Geophysical Research Letters
Yi-Hsin Liu, M Hesse, P A Cassak, M A Shay, S Wang, L-J Chen
A prediction of the steady state reconnection electric field in asymmetric reconnection is obtained by maximizing the reconnection rate as a function of the opening angle made by the upstream magnetic field on the weak magnetic field (magnetosheath) side. The prediction is within a factor of 2 of the widely examined asymmetric reconnection model (Cassak & Shay, 2007, in the collisionless limit, and they scale the same over a wide parameter regime. The previous model had the effective aspect ratio of the diffusion region as a free parameter, which simulations and observations suggest is on the order of 0...
April 28, 2018: Geophysical Research Letters
E Darelius, J B Sallée
The ice shelf water (ISW) found in the Filchner Trough, located in the southern Weddell Sea, Antarctica, is climatically important; it descends into the deep Weddell Sea contributing to bottom water formation, and it blocks warm off-shelf waters from accessing the Filchner ice shelf cavity. Yet the circulation of ISW within the Filchner Trough and the processes determining its exchange across the ice shelf front are to a large degree unknown. Here mooring records from the ice shelf front are presented, the longest of which is 4 years long...
April 28, 2018: Geophysical Research Letters
J B Garvin, D A Slayback, V Ferrini, J Frawley, C Giguere, G R Asrar, K Andersen
We have monitored a newly erupted volcanic island in the Kingdom of Tonga, unofficially known as Hunga Tonga Hunga Ha'apai, by means of relatively frequent high spatial resolution (~50 cm) satellite observations. The new ~1.8 km2 island formed as a tuff cone over the course of a month-long hydromagmatic eruption in early 2015 in the Tonga-Kermadec volcanic arc. Such ash-dominated eruptions usually produce fragile subaerial landscapes that wash away rapidly due to marine erosion, as occurred nearby in 2009...
April 28, 2018: Geophysical Research Letters
Peter O Hopcroft, Paul J Valdes, Jed O Kaplan
The observed rise in atmospheric methane (CH4 ) from 375 ppbv during the Last Glacial Maximum (LGM: 21,000 years ago) to 680 ppbv during the late preindustrial era is not well understood. Atmospheric chemistry considerations implicate an increase in CH4 sources, but process-based estimates fail to reproduce the required amplitude. CH4 stable isotopes provide complementary information that can help constrain the underlying causes of the increase. We combine Earth System model simulations of the late preindustrial and LGM CH4 cycles, including process-based estimates of the isotopic discrimination of vegetation, in a box model of atmospheric CH4 and its isotopes...
April 28, 2018: Geophysical Research Letters
P Gentine, S H Alemohammad
Solar-induced fluorescence (SIF) observations from space have resulted in major advancements in estimating gross primary productivity (GPP). However, current SIF observations remain spatially coarse, infrequent, and noisy. Here we develop a machine learning approach using surface reflectances from Moderate Resolution Imaging Spectroradiometer (MODIS) channels to reproduce SIF normalized by clear sky surface irradiance from the Global Ozone Monitoring Experiment-2 (GOME-2). The resulting product is a proxy for ecosystem photosynthetically active radiation absorbed by chlorophyll (fAPARCh )...
April 16, 2018: Geophysical Research Letters
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