On 1 September 1859, occurred one of the most intense geomagnetic storm that has been documented in recent history. This storm is known as the Carrington Event. On the morning September 1st at around 11:15 UT, Richard Carrington and Richard Hodgson observed in England, independently and for the first time, an intense white light solar flare. About 17 hours after this solar event, occurred the strongest geomagnetic perturbation ever recorded as well as a greatly extended red aurora, which covered unusually low latitudes. The red auroral display on September 2nd was reported in regions where this kind of phenomena are very rare, like in Cuba and Hawaii. Until now however, it was not known to scientists that the low latitude red aurora also registered in Mexico. At that time, Mexico was in a civil war, and there were very difficult conditions in which to establish astronomical and magnetic observatories. Nevertheless, the geomagnetic storm was observed with a maximum of intensity between 7:00-8:00 UTC and re- ported to a Mexican newspaper from five different locations (Mexico City, Queretaro, Guadalajara, Hidalgo, and Guanajuato) and registered also from at least in two additional sites (Michoacan and San Luis Potosı) in other historical documents. These records confirm that the Carrington geomagnetic storm was a global event with planetary repercussions, and that the Mexican low latitude region is susceptible to significant effects associated with intense space weather events.

The Yarlung Tsangpo Grand Canyon in southern Tibetan Plateau typically features incised bedrock and wide braided rivers. Yet, a recent discovery of two unique meandering streams within a young floodplain Cuoka offers insights into the formation of meandering rivers. Within the Cuoka floodplain, stratified layers of silty clay and root soil composite of 1.56m are deposited on the original lateral moraine basin. The sedimentation can be related to the damming of the floodplain’s mainstream, the Parlung Tsangpo River, in 1950s’ that backwaters the floodplain while promoting deposition of fine particles. This study highlights the primary factors behind the formation of these meandering streams: floodplain formation, valley slope leveling, cohesive deposition with dense riparian vegetation, and meandering initiation due to alternate bars or local discontinuities. Notably, the main and tributary streams within the floodplain exhibit distinct geometric and morphological features (topography, wavelength, curvature) that suggest different formative causes.

Physics-based numerical modeling of earthquake source processes strives to predict quantities of interest for seismic hazard, such as the probability of an earthquake rupture jumping between fault segments. How to assess the predictive power of numerical models remains a topic of ongoing debate. Here, we investigate how sensitive are the outcomes of numerical simulations of sequences of earthquakes and aseismic slip to choices in numerical discretization and treatment of inertial effects, using a simplified 2-D crustal fault model with two co-planar segments separated by a creeping barrier. Our simulations demonstrate that simplifying inertial effects and using oversized cells significantly affects the resulting earthquake sequences, including the rate of two-segment ruptures. We find that a number of fault models with different properties and modeling assumptions can produce comparable frequency-magnitude statistics and static stress drops but have rates of two-segment ruptures ranging from 0 (single-segment ruptures only) to 1 (two-segment ruptures only). For sufficiently long faults, we find that long-term sequences of events can substantially differ even among simulations that are well-resolved by standard considerations. In such simulations, some outcomes, such as static stress drops, are stable among adequately-resolved simulations, whereas others, such as the rate of two-segment ruptures, can be highly sensitive to numerical procedures and physical assumptions, and hence cannot be reliably inferred. Our results emphasize the need to examine the potential dependence of simulation outcomes on the modeling procedures and resolution, particularly when assessing their predictive value for seismic hazard assessment.

Clouds exert a significant impact on global temperatures and climate change. Cloud-radiative feedback (CRF) is one of the major sources of climate change uncertainty. Understanding CRF is therefore crucial for accurate climate projections. Biases like the double-ITCZ problem in Global Climate Models (GCMs) hampers precise climate projections. Here, we employ a bias-corrected downscaling method to constrain the cloud feedback uncertainties in the tropical and sub-tropical Atlantic region. We use high-resolution regional climate model (RCM) simulations with resolved deep convection, driven by debiased driving fields from three different global climate models (GCMs). Our simulations project a narrower range of the CRF over the tropical and sub-tropical Atlantic (-0.7 to 0.6 $Wm^{-2}K^{-1}$) compared with the driving GCMs (-0.9 to 1.0 $Wm^{-2}K^{-1}$). Furthermore, using linear regression between equilibrium climate sensitivity (ECS) and the CRF over the studied domain, a slightly narrower range of ECS (3.05 - 4.87 K) is inferred after the downscaling, compared to the GCMs’ result (2.98 - 5.15 K). The narrowed range is mainly due to a reduced range of stratocumulus feedback.   Our study highlights the potential of bias-corrected downscaling in constraining the uncertainty of simulations and estimates of cloud feedback and equilibrium climate sensitivity. The results advocate for further simulations with additional RCMs and domains for a more comprehensive analysis.

A document by Shihan Li. Click on the document to view its contents.

Earth system modeling of climate geoengineering proposals suggests that the physical outcomes of such interventions will depend on the particulars of the implementation. Here, we present a first attempt to “geoengineer” a well-known teleconnection between sea surface temperatures (SSTs) and Sahelian precipitation. Using idealized earth system model simulations, we show that selectively cooling the Indian Ocean efficiently increases precipitation in the Sahel region, widening the seasonally migrating rainband over Africa. Applying the SST perturbations derived from the idealized experiments to observationally constrained historical ones, we find that our intervention can reverse conditions as extreme as the mid-20th century Sahelian drought, albeit less efficiently than in the idealized simulations. Side effects include changes in the seasonal distribution of Sahelian precipitation and substantial precipitation reductions in sub-Saharan East Africa. This work represents a proof-of-concept illustration of effects that might be expected with a tailored, regional approach to climate intervention.

Ground-level ozone(O3) and atmospheric fine particulate matter (PM2.5) pollution are the major challenges for continually improving air quality in the Yangtze River Delta (YRD) region of China (Lu et al., 2020; Wei et al., 2020). Understanding regional transport pattern of PM2.5 and O3 pollution is essential for the development of regional cooperative prevention and control strategies. This study shows annual concentration of PM2.5 in the YRD decreased by 18.5% from 2015 to 2018, while average of daily maximum 8-h average(MDA8) O3 concentration during March to October increased by 16.3%. A complex network method is developed to investigate regional transport of PM2.5 and O3 in different grid cells (nodes). Source apportionment technology in regional transport model is applied for comparing with the results from the complex network method. Interregional and intraregional transportation play an important role in both PM2.5 and O3 over the YRD. The northern part of the YRD contributes much more than other areas for PM2.5 in winter, which accounts for about 60%,while the central part of the YRD, especially the northern part of Zhejiang, is the largest contributor of O3 in the YRD in summer,which accounts for about 70%. Intraregional transport played major role in PM2.5 than O3. The study focusing on heavily pollution episodes not only show results between source apportionment technology and complex network method are comparable, and also reveals both two methods pose great potential in understanding transport pattern and air pollution relationship, which is the foundation for emission mitigation in the YRD region.

The climatological atmospheric circulation is key to establishing the tropical ‘pattern effect’, whereby cloud feedbacks induced by sea surface temperature (SST) warming depend on the spatial structure of that warming. But how patterned warming-induced circulation changes affect cloud responses is less clear. Here we use idealized simulations with prescribed SST perturbations to understand the contributions to changes in tropical-mean cloud radiative effects (CRE) from different circulation regimes. We develop a novel framework based on moist static energy to understand the circulation response, targeting in particular the bulk circulation metric of ascent fraction. Warming concentrated in regions of ascent leads to a strong ‘upped-ante’ effect and spatial contraction of the ascending region. Our framework reveals substantial contributions to tropical-mean CRE changes not only from traditional ‘pattern effect’ regimes, but also from the intensification of convection in ascent regions as well as a smaller contribution from cloud changes in convective margins.

A document by Gang Lin. Click on the document to view its contents.

The uncertainty of climate projection is significantly contributed by warm cloud feedback, which involves a complex interplay of various mechanisms. However, it is hard to unentangle temperature’s impact on a single cloud with experiments, since the cloud dynamics always covaries with environmental thermodynamical conditions. In this study, we investigate a simulated single shallow cumulus cloud’s response to temperature using two perturbation methods, namely “uniform” and “buoyancy-fixed”, the latter of which keeps the buoyancy profile unchanged in temperature perturbation. High-resolution large eddy simulation shows that uniform warming significantly increases cloud buoyancy, reducing cloud adiabaticity. If buoyancy is fixed, warming only reduces cloud area, leaving adiabatic fraction almost unchanged. Such response can be explained by Clausius-Clapeyron effect with an idealized 1D diffusion model, showing that warming increases the cloud-environment absolute humidity difference more than the increase in cloud liquid water content, resulting in a faster loss in both cloud coverage and total liquid water solely by lateral mixing. The responses of cloud coverage and total liquid water counteract, making adiabatic fraction insensitive to temperature change. Our works shows that cloud adiabatic fraction’s response to temperature is sensitive to the perturbed structure of the boundary layer, and the cloud coverage reduction by diffusion acts as positive cloud feedback mechanism in addition to the adjustment processes of the boundary layer.

Seismic wave propagation in porous rocks that are saturated with a liquid exhibits significant dispersion and attenuation due to fluid flow at the pore scale, so-called squirt flow. This phenomenon takes place in compliant flat pores such as microcracks and grain contacts that are connected to stiffer isometric pores. Accurate quantitative description is crucial for inverting rock and fluid properties from seismic attributes such as attenuation. Up to now, many analytical models for squirt flow were proposed based on simplified geometries of the pore space. These models were either not compared with a numerical solution or showed poor accuracy. We present a new analytical model for squirt flow which is validated against a three-dimensional numerical solution for a simple pore geometry that has been classically used to explain squirt flow; that is why we refer to it as classical geometry. The pore space is represented by a flat cylindrical (penny-shaped) pore whose curved edge is fully connected to a toroidal (stiff) pore. Compared with correct numerical solutions, our analytical model provides very accurate predictions for the attenuation and dispersion across the whole frequency range. This includes correct low-and high-frequency limits of the stiffness modulus, the characteristic frequency, and the shape of the dispersion and attenuation curves. In a companion paper (Part 2), we extend our analytical model to more complex pore geometries. We provide as supplementary material Matlab and symbolic Maple routines to reproduce our main results.

We present indirect measurements of size-resolved ultrafine particle composition conducted during the Ocean–Atmosphere–Sea Ice–Snowpack (OASIS) Campaign in Utqiagvik, Alaska, during March 2009. This study focuses on measurements of size-resolved particle hygroscopicity and volatility measured over two periods of the campaign. During a period that represents background conditions in this location, particle hygroscopic growth factors (HGF) at 90% relative humidity ranged from 1.45-1.51, which combined with volatility measurements suggest a mixture of ~30% ammoniated sulfates and ~70% oxidized organics. Two separate regional ultrafine particle growth events were also observed during this campaign. Event 1 coincided with elevated levels of H2SO4 and solar radiation. These particles were highly hygroscopic (HGF=2.1 for 35 nm particles), but were almost fully volatilized at 160 °C. The air masses associated with both events originated over the Arctic Ocean. Event 1 was influenced by the upper marine boundary layer, while Event 2 spent more time closer to the surface and over open ocean leads, suggesting marine influence in growth processes. Event 2 particles were slightly less hygroscopic (HGF=1.94 for 35nm and 1.67 for 15nm particles), and similarly volatile. We hypothesize that particles formed during both events contained 60-70% hygroscopic salts by volume, with the balance for Event 1 being sulfates and oxidized organics for Event 2. These observations suggest that primary sea spray may be an important initiator of ultrafine particle formation events in the Arctic late-winter, but a variety of processes may be responsible for condensational growth.

Weak Temperature Gradient (WTG) modeling using a small cloud-resolving model (CRM) admits multiple equilibria depending upon the initial model conditions. A new kind of equilibrium is presented here which undergoes a periodic and steady-state oscillation that is emergent from static boundary conditions. The periodic oscillation is shown to be a moisture mode with an oscillating vertical motion profile shape generated by the interaction between vertical motion and radiation. A simple two-mode model based on decomposing the vertical motion profile and associated radiative cooling variations can describe the oscillation.

Sea fog strongly affects human activities in the Southern Ocean (SO), but little is known about the factors controlling its occurrence and properties. Using multi-year ship-based observations and state-of-the-art atmospheric reanalysis data, we summarize two methods to identify sea fog episodes on a climatologic scale and estimate the sea fog occurrence in SO from 1979 to 2019. The first method considers the sea-air temperature difference and atmospheric humidity, while the second explores the similarity between fog and low-level clouds. Both methods revealed three centers of sea fog occurrence in the SO. Further analysis shows that the downward surface sensible heat flux due to the southward heat advection in the lower troposphere determines the monthly climatology and linear trends of sea fog occurrence. Precipitation suppresses summer sea fog over the entire SO and partly determines the latter’s response to climate signals on an interannual scale associated with the Southern Annular Mode.

Short-term slow slip events (S-SSEs) intensively occur at the transition zone along the Nankai subduction zone, southwest Japan. Because crustal deformation due to a single S-SSE is small, the source fault is often represented using a planar uniform single-fault slip model, resulting to little constraint on the spatial heterogeneity in amounts of fault slip. To comprehensively investigate the detailed cumulative spatial distribution of S-SSEs in the entire Nankai subduction zone, we adopted a stacking approach of Global Navigation Satellite System (GNSS) data using low-frequency earthquakes as reference. We extracted cumulative displacements due to a series of S-SSEs from 2004 to 2009; coherent signals in almost opposite direction of plate subduction were obtained. The inverted slip indicated significant slip patches laterally elongated along the transition zone at ~30–35 km depth, and small patches in the shallow portions at ~15–20 km and ~10–15 km depth in eastern Shikoku and in Tokai as well as western Shikoku, respectively. The shallow patches in Shikoku were located on the downdip edge of the coseismic slip area of the 1946 Nankai earthquake, while the Tokai small slip was located on the shallower side of the anticipated source area of a large earthquake. Large slip patches of S-SSEs were complementary to the spatially dense low-frequency earthquake areas; in major S-SSE areas, the number of low-frequency earthquakes is small. This spatial dependence of fault slip style even within the transition zone provides new insights regarding the generation mechanism of slow earthquakes.

Glacier flow is typically modeled using a power law rheology known as the Glen–Nye flow law, with the power n assumed to be 3. However, recent research and past observations suggest that n=4 may better represent ice flow in some locations. We lack a quantitative understanding of how much this exponent affects ice loss projections, and its significance relative to other sources of uncertainty. Here, we test the effect of n=3 versus n=4 in a series of 300-year forward simulations of the Amundsen Sea Embayment, West Antarctica. We find that in periods of rapid grounding line retreat, uncertainty in n leads to a larger spread in ice loss projections than the spread due to uncertainty in climate forcing. The spatial sensitivity of n is generally proportional to the change in strain rates, so we expect regions undergoing more moderate dynamic change to be less sensitive to n.

The current paper investigates the shallow layers of the lunar regolith at the Chang'E-4 landing site. Four layers between 0-10 meters were identified using lunar penetrating radar. Based on these outputs, a revised stratigraphic model is suggested for the post-Imbrian ejecta at the Von Karman crater. The layers were previously unseen due to the smooth boundaries between them. The revised model was inferred using an advanced hyperbola-fitting scheme. Applying conventional hyperbola-fitting to non-homogeneous media results in errors and inaccuracies that are often wrongly assumed to be negligible. We propose a novel hyperbola-fitting scheme that is not constrained to homogeneous media and can be applied subject to any arbitrary one-dimensional permittivity distribution. Via this approach, we can estimate the permittivity profile of an investigated area and detect layered structures that were previously transparent to electromagnetic waves due to the gradational dielectric properties at their interfaces.

An ensemble of experiments based on a ¼° global NEMO configuration is presented, including tidally forced and non-tidal simulations, and using both the default z* geopotential vertical coordinate and the z~ filtered Arbitrary Lagrangian-Eulerian coordinate, the latter being known to reduce numerical mixing. This is used to investigate the sensitivity of numerical mixing, and the resulting model drifts and biases, to both tidal forcing and the choice of vertical coordinate. The model is found to simulate an acceptably realistic external tide, and the first-mode internal tide has a spatial distribution consistent with estimates from observations and high-resolution tidal models, with vertical velocities in the internal tide of over 50 meters per day. Tidal forcing with the z* coordinate increases numerical mixing in the upper ocean between 30°S and 30°N where strong internal tides occur, while the z~ coordinate substantially reduces numerical mixing and biases in tidal simulations to levels below those in the z* non-tidal control. The implications for the next generation of climate models are discussed.