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The Mantle Transition Zone Structure Beneath the Pamir Plateau and Western Tian Shan and Adjacent Regions

JGR–Solid Earth - Fri, 07/19/2024 - 14:30

Abstract

Systematical investigation of deep mantle structure beneath the Pamir Plateau, western Tian Shan and their surroundings is of great significance to understand dynamics of continental collision, intracontinental orogenesis and deformation in response to the Indo-Eurasian collision. In this research, we imaged the mantle transition zone (MTZ) structure beneath these regions using 42,560 P-wave receiver functions obtained from 352 seismic stations and 6,173 teleseismic events. Our results reveal significant 15–20 km depression of the 410-km discontinuity (d410) mainly beneath the southern Kazakh Shield, which is consistent with the low-velocity anomaly in tomographic models and thus attributed to the mantle upwelling from the MTZ, providing evidence for the fossil Tian Shan plume responsible for the Late Cretaceous-Paleocene basaltic magmatism (74–52 Ma) at the western Tian Shan. Considering that the d410 is slightly depressed by ∼8 km beneath the western Tian Shan, deep subduction of the Tarim lithosphere is likely excluded and its subhorizontal indentation into the Tian Shan is preferred. As a result, segments of thickened Tian Shan lithosphere delaminated and accumulated near the 660-km discontinuity (d660), which induce small-scale upwelling across the d410 there. The d410 is depressed by ∼10–15 km beneath Tarim, which is interpreted to be caused by the mantle upwelling originating from beneath the d410. The d660 below the central Hindu Kush is extremely depressed by 25–30 km, providing direct evidence for the deep subduction of Indian lithosphere into the bottom of the MTZ and suggesting different mechanisms for continental collision between the Hindu Kush and Pamir Plateau.

Estimating Post‐Depositional Detrital Remanent Magnetization (pDRM) Effects for Several Lacustrine and Marine Sediment Records Using a Flexible Lock‐In Function Approach

JGR–Solid Earth - Fri, 07/19/2024 - 14:20

Abstract

Geomagnetic field models over past millennia rely on two main data sources: archeomagnetic data provide snapshots of the geomagnetic field at specific locations, and sediment records deliver time series of the geomagnetic field at specific locations. The limited temporal and spatial coverage of archeomagnetic data necessitates the incorporation of sediment data especially when models go further back in time. When working with sediment data one should consider the post-depositional detrital remanent magnetization (pDRM) process, which can cause delayed and smoothed signals. To address the distortion associated with the pDRM process a Bayesian modeling technique incorporating archeomagnetic data and a class of flexible parameterized lock-in functions has been proposed. In this study, we investigate this method in more detail and apply it to declination and inclination of several lacustrine and marine sediment records. Data-driven results support the hypothesis that the pDRM process can introduce distortions, including offsets and smoothing, in some lacustrine and marine sediment records. We demonstrate a correction approach to minimize the distortion caused by the pDRM process and its impact on geomagnetic field reconstructions. The variability in the results observed across the nine records points to a potential dependence on sedimentological characteristics. To explore this further, we plan to systematically apply our novel method to a larger number of records in future studies.

Comparative Analysis of Gravity Wave Characteristics in China and the United States Using High Vertical Resolution Radiosonde Observations

JGR–Atmospheres - Fri, 07/19/2024 - 13:44

Abstract

The characteristics of gravity waves in China are investigated through an extensive analysis of high vertical resolution radiosonde observations collected over eight years across 120 stations, and are subsequently compared to those in the United States. These characteristics encompass energy density, intrinsic frequencies, horizontal and vertical wavelengths, as well as vertical and horizonal propagation directions. China and the United States, situated in mid-latitude regions with prominent western topographical features, the Qinghai-Tibet Plateau and Rocky Mountains respectively, demonstrate striking similarities in the generation and distribution of gravity waves. Both landmasses exhibit the strongest gravity waves during winter and the weakest during summer. And within the troposphere, the maximum energy of gravity waves is generated over and immediately downstream of the topographies. In addition, the energy level is amplified in the lower stratosphere. However, unique regional contrasts in summer are result from the differences of summer monsoon influence and the distinct western topographies. The maximum gravity wave energy in summer troposphere is observed over the north side of the Qinghai-Tibet Plateau in China, contrasting with its location downstream of the Rockies in the United States.

Impact of Marine Shipping Emissions on Ozone Pollution During the Warm Seasons in China

JGR–Atmospheres - Fri, 07/19/2024 - 09:31

Abstract

As China's land-based anthropogenic emissions are decreasing, the impact of marine shipping emissions (MSEs) on the atmosphere, especially in coastal areas, deserves further attention. This study investigates the impact of MSEs on MDA8 ozone (O3) levels during the warm seasons of 2017 in China, considering different seasons and synoptic patterns. The results indicate that the average impact of MSEs on O3 decreases from offshore to inland, peaking at over 29.0 ppb at sea and 13.8 ppb along the coast of mainland China. Influenced by precursor emissions, meteorology and other factors, MSEs contribute differently to O3 in Beijing-Tianjin-Hebei (BTH), Yangtze River Delta (YRD) and Pearl River Delta (PRD), with contributions of 3.0, 5.2, and 4.9 ppb, respectively, and ranging from 2.7 to 7.3 ppb in 13 coastal port cities. The O3 impacts of MSEs are higher on polluted days than on clean days, especially during onshore winds. In the BTH, MSEs increase O3 by 5.5 ppb on polluted days and 3.0 ppb on clean days with northeast winds from the Bohai Sea. In the YRD, MSEs increase O3 by 9.4 ppb on polluted days and 7.3 ppb on clean days with southeast winds. MSEs significantly increase O3 levels in the PRD by 11.0 ppb on polluted days and 5.0 ppb on clean days with southeast winds. Although the emission inventories, initial and boundary conditions, etc. may introduce uncertainties, our results still provide useful information for O3 pollution management in coastal cities as a reasonable way to track mass contributions.

Influence of Terrestrial Nitrogen Dynamics on Mesoscale Near‐Surface Meteorological Fields

JGR–Atmospheres - Fri, 07/19/2024 - 09:05

Abstract

The influence of biogeochemical cycles, particularly the nitrogen cycle, on near-surface meteorological fields is a critical yet understudied aspect of regional climate modeling. Neglecting such interactions may compromise the accurate representation of vegetation growth and hydrological processes in climate models, consequently affecting the simulated regional near-surface climate conditions. In order to quantify such effects, we coupled the nitrogen-augmented Noah-MP land surface model with the Weather Research and Forecasting (WRF) model v4.1.2 (hereafter WRF-CN) for regional climate modeling. Compared to the default WRF simulation without nitrogen dynamics, the WRF-CN simulated net primary productivity, gross primary productivity (GPP), and leaf area index (LAI) were all higher in the study region. Because WRF underestimated the observed GPP and LAI due to the fixed nitrogen limitation of plant growth, these higher estimations improved WRF-CN's performance in modeling GPP and LAI, which translated into improved simulations of near-surface climate. Specifically, for the 2-m air temperature, compared to WRF, WRF-CN reduced the mean absolute error and root mean square error by 14.45% and 14.19%, respectively, while increased the Nash-Sutcliffe efficiency coefficient by 7.23%, with the most pronounced improvements in the regions dominated by croplands. Our findings shed light on the crucial interactions between biogeochemical processes and near-surface meteorological conditions, emphasizing the significance of incorporating terrestrial nitrogen dynamics in regional climate models. These insights contribute to advancing our understanding of climate system dynamics and improving the accuracy of climate predictions at the mesoscale.

Cloud Susceptibility to Aerosols: Comparing Cloud‐Appearance Versus Cloud‐Controlling Factors Regimes

JGR–Atmospheres - Fri, 07/19/2024 - 08:30

Abstract

Clouds can be classified into regimes based on their appearance or meteorological controlling factors. The cloud appearance regimes inherently include adjustments to aerosol effects, such as transitions between closed and open cells. Therefore, calculating cloud susceptibilities to aerosols for each cloud-appearance regime individually and then aggregating them excludes much of the cloud adjustment component of the susceptibilities. In contrast, aggregating susceptibilities over regimes defined by cloud-controlling factors includes the full effects of cloud adjustments. Here we compared the susceptibilities of the two kinds of cloud regimes and demonstrated this effect. Overall, increasing cloud droplet number concentration (N d ) consistently correlates to weaker precipitation, higher cloud fraction (CF), and reduced liquid water path, regardless of how the regime is defined. However, their susceptibilities to N d aggregated over cloud-appearance regimes are significantly lower than those aggregated over cloud-controlling factors regimes, with lower-tropospheric stability (LTS) serving as an example to define cloud-controlling factors regimes. This underestimation is more pronounced for CF susceptibility, where the susceptibility for cloud appearance regimes is only 1/4 of the susceptibility for cloud controlling regimes. These findings imply that relying solely on cloud-appearance regimes may underestimate the effective radiative forcing produced by cloud adjustment (ERFaci). Nevertheless, the substantial variability in the magnitude of cloud adjustment across appearance regimes at similar LTS also suggests that a single cloud-controlling factor is not sufficient to fully separate cloud regimes to quantify cloud adjustment. Therefore, identifying a comprehensive set of cloud-controlling factors is essential for accurately quantifying cloud adjustments in future studies.

Multi‐Scale Seismic Imaging of the Ridgecrest, CA, Region With Waveform Inversion of Regional and Dense Array Data

JGR–Solid Earth - Fri, 07/19/2024 - 07:00

Abstract

We develop an inversion procedure for deriving multi-scale velocity models with waveform inversions of earthquake and ambient noise data at multi-frequency bands recorded by regional and dense sensor configurations. The method is applied for the area around the 2019 Ridgecrest earthquake rupture zones, utilizing data recorded by regional stations and dense 2D and 1D arrays with station spacings of ∼5 km and ∼100 m, respectively. Starting with regional Vp, Vs models and locations of Ridgecrest aftershocks, the velocity models and event locations are improved iteratively by inversions of waveforms recorded by regional stations and the 2D array, using a minimum spectral element size of ∼600 m. Waveforms from local events recorded by dense 1D arrays across the M7.1 rupture zone with frequencies of up to 10 Hz are used to resolve small-scale features of the rupture zone and shallow crust with a local spectral element size of 80 m. The refined models provide self-consistent descriptions of the rupture zone and the shallow crust embedded in the regional structures. The results reveal pronounced low Vs and high Vp/Vs in the M6.4 and M7.1 rupture zones coinciding with concentrations of seismicity, and also around the Garlock fault and in several local basins. We also observe clear velocity contrasts across the Garlock fault with polarity reversals along strike and with depth. The obtained multi-scale velocity models can be used to improve derivations of earthquake source properties, simulations of dynamic ruptures and ground motions, and the understanding of fault and tectonic processes in the region.

What Drove the GICs >10 A During the 17 March 2013 Event at Mäntsälä? A Novel Framework for Distinguishing the Magnetospheric Sources

Space Weather - Fri, 07/19/2024 - 07:00

Abstract

We combine wavelet analysis and data fusion to investigate geomagnetically induced currents (GICs) on the Mäntsälä pipeline and the associated horizontal geomagnetic field, BH, variations during the late main phase of the 17 March 2013 geomagnetic storm. The wavelet analysis decomposes the GIC and BH signals at increasing “scales” to show distinct multi-minute spectral features around the GIC spikes. Four GIC spikes >10 A occurred while the pipeline was in the dusk sector—the first sine-wave-like spike at ∼16 UT was “compound.” It was followed by three “self-similar” spikes 2 hr later. The contemporaneous multi-resolution observations from ground-(magnetometer, SuperMAG, SuperDARN), and space-based (AMPERE, Two Wide-Angle Imaging Neutral-atom Spectrometers) platforms capture multi-scale activity to reveal two magnetospheric modes causing the spikes. The GIC at ∼16 UT occurred in two parts with the negative spike associated with a transient sub-auroral eastward electrojet that closed a developing partial ring current loop, whereas the positive spike developed with the arrival of the associated mesoscale flow-channel in the auroral zone. The three spikes between 18 and 19 UT were due to bursty bulk flows (BBFs). We attribute all spikes to flow-channel injections (substorms) of varying scales. We use previously published MHD simulations of the event to substantiate our conclusions, given the dearth of timely in-situ satellite observations. Our results show that multi-scale magnetosphere-ionosphere activity that drives GICs can be understood using multi-resolution analysis. This new framework of combining wavelet analysis with multi-platform observations opens a research avenue for GIC investigations and other space weather impacts.