Geophysical Research Letters

Shen, Xiangjin; Liu, Binhui; Jiang, Ming; Lu, Xianguo

doi: 10.1029/2020GL087648pmid: N/A

China has the third largest area of marshland in the world. Due to the effects of human activities, marshland in China has been widely converted to built‐up land (BL) and cultivated land (CL) during the past decades. Using satellite measurements of land surface temperature (LST), this study investigated the biophysical effects of marshland loss on LST. The results showed that marshland conversion to BL could increase the LST during both the daytime and nighttime. Conversion from marshland to CL could increase the daytime LST in most months and nighttime LST in the nongrowing season but decrease the daytime LST from July to September and nighttime LST in the growing season. Conversion from marshland to paddy field and dry farmland had different effects on LST. These complex effects suggest that the biophysical effects of marshland conversions should be considered in evaluating the effects of marshland loss on climate change in China.

Wang, Lei; Kwiatek, Grzegorz; Rybacki, Erik; Bonnelye, Audrey; Bohnhoff, Marco; Dresen, Georg

doi: 10.1029/2019GL086627pmid: N/A

Understanding the physical mechanisms governing fluid‐induced fault slip is important for improved mitigation of seismic risks associated with large‐scale fluid injection. We conducted fluid‐induced fault slip experiments in the laboratory on critically stressed saw‐cut sandstone samples with high permeability using different fluid pressurization rates. Our experimental results demonstrate that fault slip behavior is governed by fluid pressurization rate rather than injection pressure. Slow stick‐slip episodes (peak slip velocity < 4 μm/s) are induced by fast fluid injection rate, whereas fault creep with slip velocity < 0.4 μm/s mainly occurs in response to slow fluid injection rate. Fluid‐induced fault slip may remain mechanically stable for loading stiffness larger than fault stiffness. Independent of fault slip mode, we observed dynamic frictional weakening of the artificial fault at elevated pore pressure. Our observations highlight that varying fluid injection rates may assist in reducing potential seismic hazards of field‐scale fluid injection projects.

Kaweeyanun, N.; Masters, A.; Jia, X.

doi: 10.1029/2019GL086228pmid: N/A

Ganymede is the only Solar System moon known to generate a permanent magnetic field. Jovian plasma motions around Ganymede create an upstream magnetopause, where energy flows are thought to be driven by magnetic reconnection. Simulations indicate Ganymedean reconnection events may be transient, but the nature of magnetopause reconnection at Ganymede remains poorly understood, requiring an assessment of reconnection onset theory. We present an analytical model of steady‐state conditions at Ganymede's magnetopause, from which the first Ganymedean reconnection onset assessment is conducted. We find that reconnection may occur wherever Ganymede's closed magnetic field encounters Jupiter's ambient magnetic field, regardless of variations in magnetopause conditions. Unrestricted reconnection onset highlights possibilities for multiple X lines or widespread transient reconnection at Ganymede. The reconnection rate is controlled by the ambient Jovian field orientation and hence driven by Jupiter's rotation. Future progress on this topic is highly relevant for the JUpiter ICy moon Explorer mission.

Weng, Libin; Lei, Jiuhou; Zhong, Jiahao; Dou, Xiankang; Fang, Hanxian

doi: 10.1029/2020GL087140pmid: N/A

In this study, we revisit the long‐term trend of thermospheric density by using a Machine‐Learning approach. Our Artificial Neural Network Model (ANNM) can better capture the variations in the satellite drag‐derived densities than earlier empirical models, especially during extremely solar minimum period of 2007–2009. The long‐term trends from ANNM are similar when density data during either 1967–2005 or 1967–2013 intervals are used in the calculation. In addition, our trend estimates relative to the ANNM are −1.5% to −2.0% per decade from 250 to 575 km without obvious solar activity dependence. The Machine‐Learning approach provides a good way to give stable long‐term trend estimates of thermospheric density.

Sherman, Peter; Tziperman, Eli; Deser, Clara; McElroy, Michael

doi: 10.1029/2019GL086913pmid: N/A

Understanding how internal atmospheric variability affects Greenland ice sheet (GrIS) summertime melting would improve understanding of future sea level rise. We analyze the Community Earth System Model Large Ensemble (CESM‐LE) over 1951–2000 and 2051–2100. We find that internal variability dominates the forced response on short timescales (~20 years) and that the area impacted by internal variability grows in the future, connecting internal variability and climate change. Unlike prior studies, we do not assume specific patterns of internal variability to affect GrIS melting but derive them from maximum covariance analysis. We find that the North Atlantic Oscillation (NAO) is the major source of internal atmospheric variability associated with GrIS melt conditions in CESM‐LE and reanalysis, with the positive phase (NAO+) linked to widespread cooling over the ice sheet. CESM‐LE and CMIP5 project an increase in the frequency of NAO+ events, suggesting a negative feedback to the GrIS under future climate change.

Ito, J.; Niino, H.; Yoshino, K.

doi: 10.1029/2020GL086999pmid: N/A

An accident occurred when an aircraft landed at Narita International Airport, Japan, on 20 June 2012. The aircraft encountered rapid changes of winds together with strong turbulence, although the weather was fair. In the present study, a two‐domain nested regional weather prediction models are used. The results in the outer domain show that southwesterly winds associated with a synoptic extratropical cyclone were locally accelerated to the southwest of the airport resulting in strong vertical shear. The simulation in the inner domain reproduces horizontal convective rolls, which are similar to those observed by a Doppler lidar at the airport. The wind velocity component parallel to the runway had a spatial variation of about 10 m s −1. The present approach using a large eddy simulation is useful for clarifying environments and features of horizontal convective rolls and forecasting low‐level wind shear associated with them, which can be a significant risk for aircraft.

Solgaard, A. M.; Simonsen, S. B.; Grinsted, A.; Mottram, R.; Karlsson, N. B.; Hansen, K.; Kusk, A.; Sørensen, L. S.

doi: 10.1029/2019GL085802pmid: 32713980

We use remotely sensed ice velocities in combination with observations of surface elevation and glacier area change to investigate the dynamics of Hagen Bræ, North Greenland in high detail over the last 35 years. From our data, we can establish for the first time that Hagen Bræ is a surge‐type glacier with characteristics of both Alaskan‐ and Svalbard‐type surging glaciers. We argue that the observed surge was preconditioned by the glacier geometry and triggered by englacially stored meltwater. At present, the glacier is in a transitional state between active and quiescence phases and is not building up to its pre‐surge geometry. We suggest that the glacier is adjusting to the loss of its floating section, general thinning, and changes in fjord conditions that occurred over the study period which are unrelated to the surge behavior. The high temporal resolution of the ice velocity data gives insight to the sub‐annual glacier flow.

Barahona, Donifan

doi: 10.1029/2019GL086033pmid: N/A

Ice nucleation efficiencies, expressed in terms of either the active site density or the ice nucleation rate coefficient, are widely used to estimate ice formation rates in atmospheric models. Most estimates are, however, subject to bias since composition and surface area variation between particles is commonly neglected. This may amount to several orders of magnitude error in active site densities and introduce substantial error in cloud freezing temperatures impacting the accuracy of atmospheric models. Here it is shown that by performing droplet freezing experiments, varying mean particle surface area along with the temperature removes such a bias. The proposed method offers, for the first time, a solution to the long‐standing problem of differentiating the “freezing rate” from the ice nucleation rate, or the apparent and the actual active site density of a material, and will likely improve the estimation of ice crystal formation rates in clouds.

Park, Yongsoo; Mousavi, S. Mostafa; Zhu, Weiqiang; Ellsworth, William L.; Beroza, Gregory C.

doi: 10.1029/2020GL087032pmid: N/A

We revisited the June 2010 to October 2011 Guy‐Greenbrier earthquake sequence in central Arkansas using PhaseNet, a deep neural network trained to pick P and S arrival times. We applied PhaseNet to continuous waveform data and used phase association and hypocenter relocation to locate nearly 90,000 events. Our catalog suggests that the sequence consists of two adjacent earthquake sequences on the same fault and that the second sequence may be associated with the wastewater disposal well to the west of the Guy‐Greenbrier Fault, rather than the wells to the north and the east that were previously implicated. We find that each sequence is composed of many small clusters that exhibit diffusion along the fault at shorter timescales. Our study demonstrates that machine‐learning‐based earthquake catalog development is now feasible and will yield new insights into earthquake behavior.

Rajewski, D. A.; Takle, E. S.; VanLoocke, A.; Purdy, S. L.

doi: 10.1029/2019GL086010pmid: N/A

Single wind turbines and large wind farms modify local scales of atmospheric boundary layer (ABL) turbulence through different mechanisms dependent on location within the wind farm. These changes in turbulence scales would most likely have notable influence on surface fluxes and microclimate during the afternoon and early evening stability transition. Profiles of Richardson number and shear and buoyancy from 1‐Hz tall tower measurements in and near a wind farm in an agricultural landscape were used to quantify departures in stability characteristics during the fallow seasons. A single turbine wake decoupled turbulent connection between the surface and above the wind turbine, changed the onset of near‐surface stabilization (earlier by a few hours), and lengthened the transition period (by up to an hour) within the rotor wake. Deep within a large wind farm, turbulence recovered to near‐ambient conditions and departures of the transition onset and duration were within 30 min of the natural ABL.