Stationary Front Research Articles

Falling jet of dry granular material in water

Abstract

Backstreaming Pickup Ions

Abstract Ions that are reflected at the shock front and escape back into the upstream region can play the role of ions that start to be accelerated by a diffusive shock acceleration mechanism. Backstreaming ions have been shown to be generated from a superthermal tail of the solar wind at sufficiently high upstream temperatures. The number of such ions was found to be low and they were not found at shock angles exceeding 50°. The mechanism of production is multiple reflection when an ion changes the direction of motion inside the ramp for the first time, due to the cross-shock potential. Since pickup ions (PUIs) constitute a strongly superthermal population of protons a substantially stronger production of backstreaming PUIs can be expected. We study the reflection of PUIs in a planar stationary shock front using test particle analysis. The used model is inspired by the observed profile of the termination shock. The influence of magnetic compression, the shock angle, and the overshoot are analyzed. It is found that generation of backstreaming PUIs in this shock is substantially more efficient than the generation of backstreaming protons from thermal solar wind. The fraction of backstreaming PUIs rapidly increases with the increase of magnetic compression and the decrease of the shock angle. Overshoot enhances production of backstreaming PUIs and allows it for larger shock angles. No backstreaming ions have been found for shock angles larger than 60°. The results of the test particle analysis are supported by full-particle simulations.

Ensemble‐based singular value decomposition analysis to clarify the causes of heavy rainfall

AbstractThe complex relationships between rainfall amounts and their causes require further clarification through analytical research. This study utilizes ensemble‐based singular value decomposition (ESVD) analysis that decomposes the ensemble‐based cross‐covariance matrix between datasets related to atmospheric states and hydrometeors. ESVD analysis is applied to the “Heavy Rain Event of July 2018 in Japan.” The initial states of 301‐member ensemble forecasts are created using a local ensemble transform Kalman filter, and the ensemble forecasts are obtained by the regional nonhydrostatic model (2 km horizontal grid interval). The ESVD analysis results indicate that the heterogeneous correlation maps of the first mode (maximum squared singular value) and second mode exhibit high correlations between the synoptic‐scale atmospheric states (the location of the stationary front and the baroclinically enhanced updraft) and rainfall characteristics. The results obtained using the sixth mode show that local water vapor fluxes in the lower layer are correlated with mesoscale rainfall characteristics. Therefore, ESVD analysis can be used to clarify multiple, independent relationships between multiscale atmospheric states and heavy rainfall.

Characterization of intra-continental smoke transport and impact on New York State air quality using aerosol reanalysis and multi-platform observations

Smoke aerosols emitted by wildfires can ascend to the free troposphere, travel over long distances and descend to affect local air quality (AQ) in downwind areas. This study investigates the AQ impact of long-range transported (LRT) smoke aerosols from western North America during summer 2017 in New York State (NYS) using observations and numerical products. Analysis of total fine particulate matter (PM2.5) and black carbon measurements at Queens and Buffalo shows that about 38% and 43% of the polluted events are related to the LRT smoke aerosols, respectively. Two LRT smoke events transported from the Pacific Northwest to NYS on Sep. 5 and 16, 2017, are analyzed. The transport path is determined by the large-scale flow (positive tilted trough for the 1st case and stationary front for the 2nd case). During both events, smoke aerosols were entrained into the boundary layer during the growth of the planetary boundary layer (PBL) and accumulated near surface when the PBL collapsed. The enhanced aerosol mass is primarily due to carbonaceous and secondary sulfate aerosols. Although the two events differed in aerosol loadings (150 versus 80 mg m−2 for column mass density), weather conditions (cold front passage versus high-pressure system), and entrainment rates (6 versus 12 cm s−1), the AQ impacts were comparable for two events (about 10 μg m−3 increase of total PM2.5). Our study of two smoke cases indicates that the LRT smoke events even with moderate intensity would degrade local AQ if the underlying meteorological conditions are favorable for downward mixing.

Hexagon Invasion Fronts Outside the Homoclinic Snaking Region in the Planar Swift--Hohenberg Equation

Stationary fronts connecting the trivial state and a cellular (distorted) hexagonal pattern in the Swift-Hohenberg equation with a quadratic-cubic nonlinearity are known to undergo a process of infinitely many folds as a parameter is varied, known as homoclinic snaking, where new hexagon cells are added to the core, leading to a region of infinitely-many, co-existing localized states. Outside the homoclinic snaking region, the hexagon fronts can invade the trivial state in a bursting fashion. In this paper, we use a far-field core decomposition to set up a numerical path-following routine to trace out the bifurcation diagrams of hexagon fronts for the two main orientations of cellular hexagon pattern with respect to the interface in the bistable region. We find for one orientation that the hexagon fronts can destabilize as the distorted hexagons are stretched in the transverse direction leading to defects occurring in the deposited cellular pattern. We then plot diagrams showing when the selected fronts for the two main orientations, aligned perpendicular to each other, are compatible leading to a hexagon wavenumber selection prediction for hexagon patches on the plane. Finally, we verify the compatibility criterion for hexagon patches in the Swift-Hohenberg equation with a quadratic-cubic nonlinearity and a large non-variational perturbation. The numerical algorithms presented in this paper can be adapted to general reaction-diffusion systems.

Stationary shock-like transition fronts in dispersive systems

We show that, contrary to popular belief, lower order dispersive regularization of hyperbolic systems does not exclude the development of the localized shock-like transition fronts. To guide the numerical search of such solutions, we generalize Rankine–Hugoniot relations to cover the case of higher order dispersive discontinuities and study their properties in an idealized case of a transition between two periodic wave trains with different wave lengths. We present evidence that smoothed stationary fronts of this type are numerically stable in the case when regularization is temporal and one of the adjacent states is homogeneous. In the zero dispersion limit such shock-like transition fronts, that are not travelling waves and apparently require for their description more complex anzats, evolve into travelling wave type jump discontinuities.

One-dimensional moving window atomistic framework to model long-time shock wave propagation

We develop a long-time moving window framework using Molecular Dynamics (MD) to model shock wave propagation through a one-dimensional chain of atoms. The moving window formulation “follows” a propagating shock wave allowing us to model shock wave propagation much longer than conventional non-equilibrium MD (NEMD) simulations. This formulation also significantly decreases the required domain size and thus reduces the overall computational cost. The domain is divided into a purely atomistic “window” region containing the shock wave flanked by boundary or “continuum” regions on either end which incorporate continuum shock conditions. Spurious wave reflections are removed by employing a damping band method using the Langevin thermostat applied locally to the atoms in each continuum region. The moving window effect is achieved by adding/removing atoms to/from the window and boundary regions, and thus the shock wave front is focused at the center of the window region indefinitely. We simulate the shock through a one-dimensional chain of copper atoms using either the Lennard-Jones, modified Morse, or Embedded Atom Model (EAM) interatomic potential. We first perform verification studies to ensure proper implementation of the thermostat, potential functions, and damping band method, respectively. Next, we track the propagating shock and compare the actual shock velocity and average particle velocity to their corresponding analytical input values. From these comparisons, we optimize the linear shock Hugoniot relation for the given “lattice” orientation and compare these results to those in literature. When incorporated into the linear shock equation, these new Hugoniot parameters are shown to produce a stationary shock wave front. Finally, we perform one-dimensional moving window simulations of an unsteady, structured shock up to a few nanoseconds and characterize the increase in the shock front’s width.

GNSS Meteorology for Disastrous Rainfalls in 2017–2019 Summer in SW Japan: A New Approach Utilizing Atmospheric Delay Gradients

We studied disastrous heavy rainfall episodes in 2017-2019 summer in SW Japan, especially in the Kyushu region using tropospheric delay data from the dense Global Satellite Navigation System network GEONET (GNSS Earth Observation Network). This region often suffers from extremely heavy rains associated with stationary fronts during summer. In this study, we first analyze behaviors of water vapor on July 6, 2018, using tropospheric parameters obtained from the database at University of Nevada Reno. The data set includes tropospheric delay gradient vectors (G), as well as zenith tropospheric delays, estimated every 5 minutes. At first, we interpolated G to obtain those at grid points and calculated their convergence, similar to the quantity proposed by Shoji (2013) as water vapor concentration index. We obtained zenith wet delay from ZTD by removing zenith hydrostatic delay. The raw ZWD values do not really reflect the wetness of the atmosphere above the GNSS station because they largely depend on the station altitudes. To study the dynamics of water vapor before heavy rains, we estimated ZWD converted to the values at sea-level. In the inversion scheme, we used G at all GEONET stations and ZWD data at low-altitude ( 50 mm/hour) events occurred, i.e., July 5, 2017, July 6, 2018, and August 27, 2019. Next, we performed high time resolution analysis (every 5 minutes) on the days of heavy rain. The results suggest that both WVC and sea-level ZWD go up prior to the onset of the rain, and ZWD decreases rapidly once the heavy rain started.

Local and global energy densities associated with welding residual stresses of cracked structures

The residual stresses associated with fusion welding processes have recently become a popular issue considering the reliable design of structural components. In a welding problem, it is quite common that fracture is observed to initiate either directly from the welds or from fatigue cracks that develop in the weld’s heat affected zone (HAZ). A recent study has investigated the effects of subsequent dynamic loading with stationary crack fronts, which assumes load being applied to the structure following the welding process. To analyze these types of problems, a computational procedure based on finite element methodology is developed to integrate the welding residual stresses with subsequent dynamic fracture behavior. The welding analysis is performed with a welding simulation program (SYSWELD). The inclusion of residual stresses and the determination of classical fracture parameters are handled with a specialized finite element program, called FRAC3D. The calculation of energy densities that are recently used in the analysis of cracked structures was shown to be an effective method for the solution of fracture problems. In this paper, a new methodology that is developed to determine local and global energy densities for the simulation of dynamic fracture behavior with initial welding residual stresses is presented.

Hindcasting the First Tornado Forecast in Europe: 25 June 1967

Abstract The tornado outbreak of 24–25 June 1967 was the most damaging in the history of western Europe, producing 7 F2–F5 tornadoes, 232 injuries, and 15 fatalities across France, Belgium, and the Netherlands. Following tornadoes in France on 24 June, the Royal Netherlands Meteorological Institute (KNMI) issued a tornado forecast for 25 June, which became the first ever—and first verified—tornado forecast in Europe. Fifty-two years later, tornadoes are still not usually forecast by most European national meteorological services, and a pan-European counterpart to the NOAA/NWS/Storm Prediction Center (SPC) does not exist to provide convective outlook guidance; yet, tornadoes remain an extant threat. This article asks, “What would a modern-day forecast of the 24–25 June 1967 outbreak look like?” To answer this question, a model simulation of the event is used in three ways: 20-km grid-spacing output to produce a SPC-style convective outlook provided by the European Storm Forecast Experiment (ESTOFEX), 800-m grid-spacing output to analyze simulated reflectivity and surface winds in a nowcasting analog, and 800-m grid-spacing output to produce storm-total footprints of updraft helicity maxima to compare to observed tornado tracks. The model simulates a large supercell on 24 June and weaker embedded mesocyclones on 25 June forming along a stationary front, allowing the ESTOFEX outlooks to correctly identify the threat. Updraft helicity footprints indicate multiple mesocyclones on both days within 40–50 km and 3–4 h of observed tornado tracks, demonstrating the ability to hindcast a large European tornado outbreak.

An Observational Analysis Quantifying the Distance of Supercell-Boundary Interactions in the Great Plains

Several case studies and numerical simulations have hypothesized that baroclinic boundaries provide enhanced horizontal and vertical vorticity, wind shear, helicity, and moisture that induce stronger updrafts, higher reflectivity, and stronger low-level rotation in supercells. However, the distance at which a surface boundary will provide such enhancement is less well-defined. Previous studies have identified enhancement at distances ranging from 10 km to 200 km, and only focused on tornado production and intensity, rather than all forms of severe weather. To better aid short-term forecasts, the observed distances at which supercells produce severe weather in proximity to a boundary needs to be assessed. In this study, the distance between a large number of observed supercells and nearby surface boundaries (including warm fronts, stationary fronts, and outflow boundaries) is measured throughout the lifetime of each storm; the distance at which associated reports of large hail and tornadoes occur is also collected. Statistical analyses assess the sensitivity of report distributions to report type, boundary type, boundary strength, angle of interaction, and direction of storm motion relative to the boundary. Additionally, the range at which each type of severe weather is produced for each boundary is identified to provide a useful operational tool for forecasters. Notably, tornadoes are more likely to be produced closer to a boundary than severe hail. Overall, the observations point to a unique range at which severe weather occurs for each boundary and report type.

On the Origin of Optical Radiation during the Impulsive Phase of Flares on dMe Stars. I. Discussion of Gas Dynamic Models

In connection with a published critique, the author justifies the use of a motionless homogeneous plane layer of pure hydrogen plasma that is near local thermodynamic equilibrium (LTE) for analyzing the characteristics of the radiation from a chromospheric condensation of thickness $\Delta{}z_m=10\text{ km}$ in a gas dynamic model of stellar flares. It is shown that the shock-wave model of flares proposed by Belova and Bychkov, as opposed to the model of Kostyuk and Pikelner, has irremovable internal defects owing to exclusion of the interaction between a thermal wave (temperature jump) and a non-stationary radiative shock. In particular, this model (a) does not make it possible to increase the geometric thickness of a chromospheric condensation owing to divergence of the fronts of the thermal and shock waves during impulsive heating, (b) cannot provide heating of the chromospheres of red dwarfs over significant distances, and (c) predicts $\mathrm{H}_\alpha$ line profiles in conflict with observational data. It is argued that: (a) the shock-wave model by Belova and Bychkov represents a development of the kinematic model of solar flares (Nakagawa et al.) and its application to dMe stars, specifically: a study of the radiative response of the chromosphere of a red dwarf to impulsive heating in the simplest gas dynamic statement of the problem (a thermal wave is excluded, a stationary approach is used); (b) in terms of the Kostyuk and Pikelner model, the regions behind the stationary shock fronts do not correspond to a chromospheric condensation with time-varying thickness but to zones in which the plasma relaxes to a state of thermal equilibrium. It is emphasized that the separation of the Kostyuk and Pikelner model into "thermal" and "shock-wave" components is fundamentally impossible.

Channel-levee evolution in combined contour current–turbidity current flows from flume-tank experiments

Abstract Turbidity currents and contour currents are common sedimentary and oceanographic processes in deep-marine settings that affect continental margins worldwide. Their simultaneous interaction can form asymmetric and unidirectionally migrating channels, which can lead to opposite interpretations of paleocontour current direction: channels migrating against the contour current or in the direction of the contour current. In this study, we performed three-dimensional flume-tank experiments of the synchronous interaction between contour currents and turbidity currents to understand the effect of these combined currents on channel architecture and evolution. Our results show that contour currents with a velocity of 10–19 cm s−1 can substantially deflect the direction of turbidity currents with a maximum velocity of 76–96 cm s−1, and modify the channel-levee system architecture. A lateral and nearly stationary front formed on the levee located upstream of the contour current, reduced overspill and thus restrained the development of a levee on this side of the channel. Sediment was preferentially carried out of the channel at the flank located downstream of the contour current. An increase in contour-current velocity resulted in an increase in channel-levee asymmetry, with the development of a wider levee and more abundant bedforms downstream of the contour current. This asymmetric deposition along the channel suggests that the direction of long-term migration of the channel form should go against the direction of the contour current due to levee growth downstream of the contour current, in agreement with one of the previously proposed conceptual models.

Ensemble simulations of the influence of regionally warm sea surface on moisture and rainfall in Tsushima Strait during August 2013

This study investigates the short-term (four days) atmospheric response to regionally high sea-surface temperature (SST) in the Tsushima Strait. During 18–22 August 2013, SST in the strait increased and cloud cover was scarce (sunny period). During 23–26 August, SST decreased and frequent rainfall was associated with a stationary front (rainy period). The moisture response to SST differed between the sunny and rainy periods. Ensemble-mean moisture variation induced by a regionally warm sea surface is well correlated with SST increases during the sunny period. However, they are not clearly correlated with SST increases during the rainy period when water vapor fluctuated because of frequent rainfall. The high SST resulted in locally enhanced precipitation in the central area of the warm core. Unlike the climatological response of precipitation to SST, the ensemble experiment shows that warm SSTs do not always enhance hourly rainfall because of SST-related changes in moisture from prior rainfall events. In a simulation that performs well in reproducing precipitation at Izuhara observatory (located in Tsushima Strait), high SSTs resulted in enhanced precipitation in the morning. Subsequently, water vapor decreased, leading to lower precipitation in the afternoon. In contrast, a low-SST experiment with the warm-SST core removed produced moisture concentrations that were higher than those in the high-SST experiment after weak rainfall during the morning. As a result, low SST led to greater precipitation in the afternoon. Thus, responses of hourly precipitation to SST should be carefully investigated by considering transient moisture variations during each rainfall event and related uncertainties in ensemble simulations.

Synoptic Mechanisms behind Extreme Precipitation Event Frequency in Omaha, Nebraska, 1948–2018

Daily extreme precipitation events (EPEs) within the top 1% of the 1948–2018 precipitation event distribution are identified for Omaha, NE. The frequency of EPEs significantly increased over the 71- year period by approximately 1.1 events decade–1, based on a linear regression of 11-year running sums. Embedded within the long-term increase is a multidecadal oscillation where over 40% of its variance can be explained by the phase of the Atlantic Multidecadal Oscillation. Using a temporal synoptic index classification methodology, the synoptic weather types associated with the 71 EPEs are identified and analyzed. Eight weather types, which encompass over 75% of the events, are examined in detail. Weather types are assigned to general categories of extratropical cyclones, frontal passages, stationary fronts, and southeasterly flow. Each of these broad categories contributed near equally to extreme event totals, ranging between 22% and 28%. The frequency of one type, Stationary Front #1 (STF_1), can explain approximately 12% of the variation in EPEs. STF_1 significantly increased in frequency from 1948 to 2018, contributing to the overall increase in event frequency. The increasing occurrence of EPEs may have far- reaching implications to various societal sectors, particularly with respect to water resource management and agricultural productivity.

Identification and Classification of Heavy Rainfall Areas and their Characteristic Features in Japan

We propose a new procedure for the objective identification and classification of heavy rainfall areas (HRAs) to advance the understanding of mesoscale convective systems (MCSs) in Japan. The distributions of accumulated precipitation amounts are evaluated from the radar/raingauge-analyzed precipitation amounts and characteristic features of HRAs are examined. The HRAs extracted during the warm seasons (April–November) in 2009–2018 are classified into four types (e.g., linear-stationary, linear, stationary, and others) based on their morphological features and temporal variations. HRAs are frequently observed on the Pacific sides of eastern and western Japan; 80 % of HRAs appeared from June to September and 60 % of the HRAs were observed in association with stationary fronts and tropical cyclones. Approximately 80 % of those HRAs of the linear-stationary type corresponded to typical elongated and stagnated MCSs, as suggested in previous studies.

Seven-Doppler Radar and In Situ Analysis of the 25–26 June 2015 Kansas MCS during PECAN

AbstractThis case study analyzes a nocturnal mesoscale convective system (MCS) that was observed on 25–26 June 2015 in northeastern Kansas during the Plains Elevated Convection At Night (PECAN) project. Over the course of the observational period, a broken line of elevated nocturnal convective cells initiated around 0230 UTC on the cool side of a stationary front and subsequently merged to form a quasi-linear MCS that later developed strong, surface-based outflow and a trailing stratiform region. This study combines radar observations with mobile and fixed mesonet and sounding data taken during PECAN to analyze the kinematics and thermodynamics of the MCS from 0300 to 0630 UTC. This study is unique in that 38 consecutive multi-Doppler wind analyses are examined over the 3.5 h observation period, facilitating a long-duration analysis of the kinematic evolution of the nocturnal MCS. Radar analyses reveal that the initial convective cells and linear MCS are elevated and sustained by an elevated residual layer formed via weak ascent over the stationary front. During upscale growth, individual convective cells develop storm-scale cold pools due to pockets of descending rear-to-front flow that are measured by mobile mesonets. By 0500 UTC, kinematic analysis and mesonet observations show that the MCS has a surface-based cold pool and that convective line updrafts are ingesting parcels from below the stable layer. In this environment, the elevated system has become surface based since the cold pool lifting is sufficient for surface-based parcels to overcome the CIN associated with the frontal stable layer.

The characteristics of air pollution induced by the quasi-stationary front: Formation processes and influencing factors

The quasi-stationary front is a significant weather system which influences East Asia in spring. The air quality deteriorated along with the moist circumstance when the quasi stationary front dominated the area. Surface meteorological parameters, air pollutants and PM2.5 chemical species were observed during the air pollution episode. Liquid water content and aerosol acidity were calculated by thermodynamic model in order to investigate heterogeneous/aqueous reactions for secondary aerosol formation. The episode was divided into four stages based on quasi-stationary front influences. Hourly PM2.5 concentrations were up to 150.2 μg·m−3 while O3 concentrations reached the minimum value of 1.27 μg·m−3, indicating that the precursor gas NOx participated in the different reactions during the episode. Nitrate proportion of water-soluble inorganic ions was 42.2%. High concentrations of secondary inorganic aerosol ions and the high sulfur oxidation ratio (SOR) and nitrogen oxidation ratio (NOR) indicated the increasing conversions from SO2 and NOx to their corresponding particulate phases. Ratios of [NO3−]/[SO42−] and [NH4+]/[SO42−] in the four stages declared that nitrate formation preferred heterogeneous conversions. A series of liquid water content (LWC) fitting equations between relative humidity and inorganic ions were conducted to verify heterogeneous aqueous reactions of NO2 and secondary nitrate formation. The results of this study highlighted the significance of LWC and chemical reactions associated with acidity during the specific synoptic situation in South China.

Processes Shaping the Frontal-Scale Time-Mean Surface Wind Convergence Patterns around the Kuroshio Extension in Winter

AbstractHigh-resolution satellite observations and numerical simulations have revealed that climatological-mean surface wind convergence and precipitation are enhanced locally around the midlatitude warm western boundary currents (WBCs) with divergence slightly to their poleward side. While steep sea surface temperature (SST) fronts along the WBCs have been believed to play an important role in shaping those frontal-scale atmospheric structures, the mechanisms and processes involved are still under debate. The present study explores specific daily scale atmospheric processes that are essential for shaping the frontal-scale atmospheric structure around the Kuroshio Extension (KE) in winter, taking advantage of a new product of global atmospheric reanalysis. Cluster analysis and case studies reveal that a zonally extending narrow band of surface wind convergence frequently emerges along the KE, which is typically observed under the surface northerlies after the passage of a developed synoptic-scale cyclone. Unlike its counterpart around the cyclone center and associated cold front, the surface convergence tends to be in moderate strength and more persistent, contributing dominantly to the distinct time-mean convergence/divergence contrast across the SST front. Accompanying ascent and convective precipitation, the band of convergence is a manifestation of a weak stationary atmospheric front anchored along the SST front or generation of a weak meso-α-scale cyclone. By reinforcing the ascent and convergence, latent heating through convective processes induced by surface convergence plays an important role in shaping the frontal-scale atmospheric structure around the KE.

Changes in southeastern USA summer precipitation event types using instrumental (1940–2018) and tree-ring (1790–2018) data

We examined short- and long-term changes in precipitation event types using instrumental (1940–2018) and tree-ring (1790–2018) data from North Carolina, USA. We documented the amount and frequency of summer (July–September) precipitation events using daily weather station data. Stationary front precipitation (SFP) represented 71% of total summer rainfall and SFP and convective uplift combined (i.e., quasi-stationary precipitation, QSP) represented 87%. SFP (r = 0.52, p < 0.01) and QSP (r = 0.61, p < 0.01) precipitation reconstructions from a montane longleaf pine latewood chronology both recorded significant declines during 1940–2018, matching the instrumental record. Conversely, no significant change in either SFP or QSP occured during the full reconstruction indicating the instrumental decline was unmatched throughout 1790–1939. Our method demonstrates that variations in latewood growth can be attributed to specific precipitation event types and that the relative contribution of each event type can be quantified over a multi-century period.