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  • 1
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    Impacts of extreme 2013–2014 winter conditions on Lake Michigan's fall heat content, surface temperature, and evaporation (2015)
    Wiley
    Details
    Publication Date: 2015-04-08
    Description: Since the late 1990s, the Laurentian Great Lakes have experienced persistent low water levels and above-average over-lake evaporation rates. During the winter of 2013–2014, the lakes endured the most persistent, coldest temperatures and highest ice cover in recent history, fostering speculation that over-lake evaporation rates might decrease and that water levels might rise. To address this speculation, we examined inter-seasonal relationships in Lake Michigan's thermal regime. We find pronounced relationships between winter conditions and subsequent fall heat content, modest relationships with fall surface temperature, but essentially no correlation with fall evaporation rates. Our findings suggest that the extreme winter conditions of 2013–2014 may have induced a shift in Lake Michigan's thermal regime, and that this shift coincides with a recent (and ongoing) rise in Great Lakes water levels. If the shift persists, it could (assuming precipitation rates remain relatively constant) represent a return to thermal and hydrologic conditions not observed on Lake Michigan in over 15 years.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 2
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    Subduction Erosion, Crustal Structure, and an Evolutionary Model of the Northern Yap Subduction Zone: New Observations From the Latest Geophysical Survey (2019)
    Wiley
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    Publication Date: 2019
    Description: Abstract The Yap subduction zone is a distinctive erosive margin with an extremely slow convergence rate. The high relief of the subducting plate, generated by horst and graben structures and seamounts, leads to attenuation of the crust. In this study, we present the latest geophysical data, collected by the Chinese research vessel Kexue, to investigate subduction erosion at the Yap subduction zone and develop subduction models for Yap subduction zone structures. We show an anomalous distance between the Yap Trench and the adjacent volcanic arc, the steep slope of the trench arcward, a frontal prism, and rare sediment in the trench, all typical features of erosive margins. We propose that the high‐relief subducting plate has led to erosion of the overlying plate and that different subduction processes, controlled locally by the topography of the subducting plate, have modified the Yap subduction zone. Numerous normal faults increase the relief of the seafloor entering the trench, and an anomalously large slope angle along the trench reflects uplift of the fore‐arc high via seamount subduction. In addition, the thin crust of the subducted horst and graben structures, along with the normal faults, might have eroded the fore‐arc crust and subsequently eroded the Yap arc crust during subduction. These subduction erosional processes at the Yap Trench provide one of the best cases of an erosive margin in the world, particularly for a subduction zone with an extremely slow convergence rate.
    Electronic ISSN: 1525-2027
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 3
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    Propagation characteristics of plasmaspheric hiss: Van Allen Probe observations and global empirical models (2017)
    Wiley
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    Publication Date: 2017-03-14
    Description: Based on the Van Allen Probe A observations from 1 October 2012 to 31 December 2014, we develop two empirical models to respectively describe the hiss wave normal angle (WNA) and amplitude variations in the Earth's plasmasphere for different substorm activities. The long-term observations indicate that the plamsaspheric hiss amplitudes on the dayside increase when substorm activity is enhanced (AE index increases), and the dayside hiss amplitudes are greater than the nightside. However, the propagation angles (WNAs) of hiss waves in most regions do not depend strongly on substorm activity, except for the intense substorm-induced increase in WNAs in the nightside low-L region. The propagation angles of plasmaspheric hiss increase with increasing magnetic latitude (MLAT) or decreasing radial distance (L value). The global hiss WNAs (the power-weighted averages in each grid) and amplitudes (medians) can be well reproduced by our empirical models.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 4
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    Decreasing net primary production due to drought and slight decreases in solar radiation in China from 2000 to 2012 (2017)
    Wiley
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    Publication Date: 2017-01-10
    Description: Terrestrial ecosystems have continued to provide the critical service of slowing the atmospheric CO 2 growth rate. Terrestrial net primary productivity (NPP) is thought to be a major contributing factor to this trend. Yet, our ability to estimate NPP at the regional scale remains limited due to large uncertainties in the response of NPP to multiple interacting climate factors and uncertainties in the driver datasets needed to estimate NPP. In this study, we introduced an improved NPP algorithm that used local driver datasets and parameters in China. We found that bias decreased by 30% for gross primary production (GPP) and 17% for NPP compared with the widely-used global GPP and NPP products, respectively. From 2000-2012, a pixel-level analysis of our improved NPP for the region of China showed an overall decreasing NPP trend of 4.65 TgC a -1 . Reductions in NPP were largest for the southern forests of China (-5.38 TgC a -1 ); whereas minor increases in NPP were found for North China (0.65 TgC a -1 ). Surprisingly, reductions in NPP were largely due to decreases in solar radiation (82%), rather than the more commonly expected effects of drought (18%). This was because for southern China, the interannual variability of NPP was more sensitive to solar radiation (R 2 in 0.29 ~ 0.59) relative to precipitation (R 2  〈 0.13). These findings update our previous knowledge of carbon uptake responses to climate change in terrestrial ecosystems of China and highlight the importance of shortwave radiation in driving vegetation productivity for the region, especially for tropical forests.
    Print ISSN: 0148-0227
    Topics: Biology , Geosciences
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 5
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    Enhancement of oxygen in the magnetic island associated with dipolarization fronts (2016)
    Wiley
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    Publication Date: 2016-12-08
    Description: A significant enhancement of O + is observed by Cluster inside an earthward propagating magnetic island behind a dipolarization front (DF). Such enhancement, from 0.005 to 0.03 cm -3 , makes the O + flux inside the magnetic island ~20 times larger than that outside the magnetic island. In the meantime, the H + density is nearly a constant, 0.1 cm -3 , during the magnetic-island encounter. This results in a dramatic increase of the density ratio, , from 0.05 to 0.3 (about 10 times as large as the average value in the plasma sheet) and a dramatic decrease of the local Alfvén speed from V A  ≈ 770 km/s to V A  ≈ 430 km/s inside the magnetic island. The decrease of Alfvén speed indicates an asymmetric reconnection and a slow magnetic reconnection rate near the secondary X-line. Since the reconnection rates at the primary X-line and secondary X-line are imbalanced, the DFs and magnetic islands are pushed to propagate earthward by the outflow of the primary reconnection, as demonstrated in recent simulations.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 6
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    A case study on ionospheric scintillations at low latitude associated with a plasma blob observed in-situ (2015)
    Wiley
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    Publication Date: 2015-03-18
    Description: In general, ionospheric scintillations at low latitude are considered as signatures of equatorial plasma bubbles (depletions). However, some authors considered that scintillations may also be associated with plasma blobs (enhancements), but there was no in-situ measurement hitherto to confirm it. We performed a case study on the concurrent observation of an ionospheric plasma blob with in-situ measurements by ROCSAT-1 and of GPS amplitude scintillations in the low latitude ionosphere on June 1, 2003. The blob measured in situ had a scale size of about 800 km in the F-layer, and the ion density inside the blob was severely disturbed. Amplitude scintillation with S 4 〉0.3 were observed concurrently in the same longitude range as the blob measured. This case study provides evidence of simultaneously observed GPS amplitude scintillations and a blob in-situ, and it confirms that scintillations can be associated with plasma blobs in the low latitude ionosphere.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 7
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    Ionospheric plasma bubbles observed concurrently by multi-instruments over low latitude station Hainan (2015)
    Wiley
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    Publication Date: 2015-02-27
    Description: Previous studies have shown that the ionospheric “strong range spread F” (SSF) closely correlates with the occurrence of scintillations caused by equatorial plasma bubbles. However, there is no report on concurrent observations of SSF and bubbles with in situ measurement. This paper discusses two cases of concurrent observations with a DPS4 Digisonde and a collocated scintillation monitor at the low-latitude station Hainan (19.5° N, 109.1° E), and with in situ ion density measurements made by the ROCSAT-1 satellite. Two case studies were made for 10 and 23 April 2004, respectively. In both cases the SSF occurred before midnight and lasted more than 3.5 hours. The scintillations were accompanied with strong range SF. Concurrently, the ROCSAT-1 satellite observed plasma bubbles over Hainan station. In the first case, two bubbles were observed by the satellite with east–west sizes of more than ~200 km over Hainan station. Two bubbles were also observed in the second case with east–west extensions of about 220 km and 35 km, respectively. For the first time direct observational evidence is provided for the causal relationship between equatorial plasma bubbles with in situ measurement and the concurrent occurrence of SSF and strong scintillations.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 8
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    Slow shock and rotational discontinuity in MHD and Hall MHD models with anisotropic pressure (2016)
    Wiley
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    Publication Date: 2016-06-19
    Description: Pressure anisotropy may modify the characteristics of magnetohydrodynamic (MHD) waves, in particular, the slow mode wave and the corresponding shocks and discontinuities. In this study the formation of slow shocks (SSs) in anisotropic plasmas is examined by solving the gyrotropic MHD and Hall MHD equations numerically for one-dimensional Riemann problem. The MHD shocks and discontinuities are generated by imposing a finite normal magnetic field on the Harris type current sheet with a guide magnetic B y component. It is shown that anomalous SSs moving faster than the intermediate wave or with positive density-magnetic field correlation may be generated in gyrotropic MHD and Hall MHD models. Moreover, for some parameter values SSs may exhibit upstream wave trains with right-handed polarization in contrast with the earlier prediction that SSs shall possess downstream left-hand polarized wave trains based on the isotropic Hall MHD theory. For the cases of B y  ≠ 0, SSs with increased density and decreased magnetic field followed by noncoplanar intermediate mode or rotational discontinuity (RD)-like structures similar to the compound SS-RD structures observed in space plasma environments may possibly form in symmetric and asymmetric current layers. The Walén relation of these anomalous RDs without the correction of pressure anisotropy may significantly be violated.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 9
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    Responses of phytoplankton community to the input of different aerosols in the East China Sea (2016)
    Wiley
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    Publication Date: 2016-06-23
    Description: Atmospheric deposition can affect marine phytoplankton by supplying macronutrients and trace elements. We conducted mesocosm experiments by adding aerosols with different composition (dominated by mineral dust, biomass burning and high Cu, and secondary aerosol, respectively) to the surface seawater of the East China Sea. Chlorophyll a concentrations were found to be the highest and lowest after adding aerosols containing the highest Fe and dissolved inorganic nitrogen (DIN), respectively. The relative abundance of Haptophyceae increased significantly after adding mineral dust, whereas diatom, Dinophyceae and Cryptophyceae reached the maximum accompanied with the highest DIN. Our results suggest that Fe may be more important than DIN in promoting primary productivity in the sampled seawater. The input of mineral dust and anthropogenic aerosols may result in distinct changes of phytoplankton community structure.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 10
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    Assimilation of stream discharge for flood forecasting: Updating a semidistributed model with an integrated data assimilation scheme (2015)
    Wiley
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    Publication Date: 2015-04-09
    Description: Real-time discharge observations can be assimilated into flood models to improve forecast accuracy; however, the presence of time lags in the routing process and a lack of methods to quantitatively represent different sources of uncertainties challenge the implementation of data assimilation techniques for operational flood forecasting. To address these issues, an integrated error parameter estimation and lag-aware data assimilation (IEELA) scheme was recently developed for a lumped model. The scheme combines an ensemble-based maximum a posteriori (MAP) error estimation approach with a lag-aware ensemble Kalman smoother (EnKS). In this study, the IEELA scheme is extended to a semi-distributed model to provide for more general application in flood forecasting by including spatial and temporal correlations in model uncertainties between sub-catchments. The result reveals that using a semi-distributed model leads to more accurate forecasts than a lumped model in an open-loop scenario. The IEELA scheme improves the forecast accuracy significantly in both lumped and semi-distributed models, and the superiority of the semi-distributed model remains in the data assimilation scenario. However, the improvements resulting from IEELA are confined to the outlet of the catchment where the discharge observations are assimilated. Forecasts at “ungauged” internal locations are not improved, and in some instances, even become less accurate. This article is protected by copyright. All rights reserved.
    Print ISSN: 0043-1397
    Electronic ISSN: 1944-7973
    Topics: Architecture, Civil Engineering, Surveying , Geography
    Published by Wiley on behalf of American Geophysical Union (AGU).
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