Surface Convergence Research Articles

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.

Convection On the Edge

AbstractDeep convection over tropical oceans often appears intensified at the edge of convectively active regions, both in idealized studies and in observations. This edge intensification of convection is studied in detail here, using the steady state of a radiative‐convective equilibrium study, marked by a single convective cluster with deep convection intensified at the edge of this cluster. The cause for edge intensification and its dependence on the cluster area is investigated by comparing the spatial distribution of deep convection to different variables known to be important for convection. Analysis of the simulation suggests that the edge is marked by an increased probability for the triggering of convection rather than by stronger updrafts. In particular, while the edge of the moist region is not thermodynamically more favorable, we find strong surface convergence and therefore dynamical lifting at this edge. The surface convergence is shown to result from two opposing flows. On the one hand, there is, as expected from previous radiative‐convective equilibrium simulations, a low‐level inflow directed toward the moist region. On the other hand, there is a positive density anomaly at the surface which is the result of continuously forming cold pools within the convectively active region, creating a super‐cold‐pool. As the velocity of the low‐level inflow approximately matches the potential propagation speed of the super‐cold‐pool boundary, these opposing flows explain the presence of strong convergence at the edge of this region. Whether the resulting lifting induces the formation of deep convection is shown to depend on the large‐scale instability.

Impact of SCATSat‐1 Retrieved Wind Vectors on Short‐Range WRF Model Predictions Over the South Asia Region

AbstractThe Indian Space Research Organization launched SCATterometer Satellite‐1 (SCATSat­1) satellite by PSLV‐C35 on 26 September 2016 from Sriharikota. In this study, SCATSat‐1 retrieved winds are assimilated in the regional atmospheric model subsequent to preliminary evaluation of SCATSat‐1 retrieved winds against National Centers for Environmental Prediction final analyses. The Weather Research and Forecasting (WRF) model and its three‐ and four‐dimensional variational (3D‐Var and 4D‐Var) data assimilation system are used to demonstrate the importance of SCATSat‐1 retrieved winds on short‐range weather prediction. In the first set of experiments, two parallel runs are performed with and without assimilation of SCATSat‐1 retrieved winds using 6‐hourly cycling during entire month of December 2016. Results suggest that assimilation of SCATSat‐1 retrieved winds improved the WRF model analyses and subsequent forecasts of winds and moisture. In the second set of experiments, parallel assimilation experiments are performed with SCATSat‐1 retrieved winds using 3D‐Var and 4D‐Var method for the entire lifetime of tropical cyclone Ockhi as a case study. Results suggest that assimilation of SCATSat‐1 winds has positive impact on Ockhi tropical cyclone prediction with slightly better forecast skill for 4D‐Var technique. Moreover, assimilation of SCATSat‐1 retrieved winds improved predictions of surface convergence and accumulated rainfall. In the last set of experiments, forecast sensitivity to observations are performed for SCATSat‐1 retrieved different resolution winds using the WRF tangent linear and adjoint models for the entire lifetime of tropical cyclone Ockhi. Results suggest that SCATSat‐1 retrieved winds have positive impact on tropical cyclone simulation.

Exploring the Potential Impact of Louisville's Urban Heat Island on the Structure and Intensity of an Organized Convective Systems

Simulations of an organized convective system interacting with Louisville, KY were conducted to determine the impact of urban heat island (UHI) magnitude on the structure and intensity of convective storms. Four different simulations are presented: A control, two simulations in which the UHI is enhanced through observational nudging, and a simulation where the urban area is removed (no-city simulation). Results show that the downwind enhancement of convective cells increases with increasing UHI magnitude. However, large differences are also apparent between the control and the no-city simulations. This suggests that surface convergence caused by flow obstruction may be as influential to the modification of organized storms as the UHI, although the locations of these two effects are offset. Convergence due to flow obstruction is most noticeable on the upwind edge and over the center of the city while the impact of UHI magnitude is most noticeable on the downwind edge of the city.

The large-scale meteorological condition during the critical stage of Mesoscale Convective Complexes (MCCs) over Indian Ocean

This study analyzed the large-scale meteorological condition during Mesoscale Convective Complexes (MCCs) over the Indian Ocean. The MCCs were identified by infrared satellite imagery using an algorithm that combined information about cloud coverage, eccentricity, and the cloud lifetimes of MCCs. The data used were a combination of satellite data and reanalysis data, while the Brightness temperature (TBB) was obtained from the MTSAT IR1 satellite data. The results of this study showed that, in general, the initial stage of the MCC was characterized by strong low-level convergence and vertical convection, and largely driven by the convergence of the moisture flux in the lower troposphere. The mature stage of the MCC was characterized by weak surface convergence, strong upper-level divergence, and a shortwave ridge in the mid- and upper levels. Where there was strong surface divergence, the decay and dissipation stages were very similar, and the surface convergence left the system. The movement of most MCCs resulted from the combined contributions of advection and the propagation of surface convergence. Results from this research showed that these large convective systems tended to be formed in the vicinity of the terminus of a low-level jet that transported moist and warm air to the originating regions of the MCCs. Shortwave troughs and baroclinic zones were associated with MCC development.

Lower-Tropospheric Influences on the Timing and Intensity of Afternoon Severe Convection over Modest Terrain in a Convection-Allowing Ensemble

AbstractA 50-member convection-allowing ensemble is used to examine effects of daytime PBL evolution and ambient flow interacting with modest terrain features on convection initiation (CI) in the lee of the Rocky Mountains. The examined case (4 June 2015) has isolated supercell storms that initiate during mid- to late afternoon along the northern portion of the Palmer Lake Divide, which is a ~0.5-km-deep zonally oriented terrain feature in east-central Colorado that extends eastward from the Rocky Mountains. To diagnose factors most crucial to storm development, two 10-member subensembles are constructed from the full 50-member ensemble. One subensemble (STRONG) has storm locations with mature storm intensities, and average timing of CI similar to that observed. The other subensemble (WEAK) has fewer storms, with generally weaker intensity, and delayed CI. Environmental composites constructed from these subensembles reveal a stronger surface horizontal convergence zone and moisture gradient in STRONG, resulting from 2–3.5 m s−1 stronger southerly winds on the south flank of the convergence zone. The stronger southerlies result from accelerated PBL growth and momentum mixing in the presence of strong low-to-midtropospheric vertical shear, which is facilitated by reduced above-PBL static stability in the composite STRONG initial condition. Stronger time-averaged low-to-midtropospheric upward motion coincides with the surface convergence zone in STRONG, and individual CI locations occur at the northeastern edge of the composite vertical motion maximum. Trajectory analysis with STRONG members confirms that the CI locations are consistent with large vertical displacements, and corresponding relative humidity increases leading to decreases in convective inhibition, as the southerly airstream ascends across the convergence zone.

Coincident Observations of Dye and Drifter Relative Dispersion over the Inner Shelf

AbstractCoincident Lagrangian observations of coastal circulation with surface drifters and dye tracer were collected to better understand small-scale physical processes controlling transport and dispersion over the inner shelf in the Gulf of Mexico. Patches of rhodamine dye and clusters of surface drifters at scales of O(100) m were deployed in a cross-shelf array within 12 km from the coast and tracked for up to 5 h with airborne and in situ observations. The airborne remote sensing system includes a hyperspectral sensor to track the evolution of dye patches and a lidar to measure directional wavenumber spectra of surface waves. Supporting in situ measurements include a CTD with a fluorometer to inform on the stratification and vertical extent of the dye and a real-time towed fluorometer for calibration of the dye concentration from hyperspectral imagery. Experiments were conducted over a wide range of conditions with surface wind speed between 3 and 10 m s−1 and varying sea states. Cross-shelf density gradients due to freshwater runoff resulted in active submesoscale flows. The airborne data allow characterization of the dominant physical processes controlling the dispersion of passive tracers such as freshwater fronts and Langmuir circulation. Langmuir circulation was identified in dye concentration maps on most sampling days except when the near surface stratification was strong. The observed relative dispersion is anisotropic with eddy diffusivities O(1) m2 s−1. Near-surface horizontal dispersion is largest along fronts and in conditions dominated by Langmuir circulation is larger in the crosswind direction. Surface convergence at fronts resulted in strong vertical velocities of up to −66 m day−1.

Adhesive sliding and interfacial property of YAlO3/TiC interface: A first principles investigation

With consideration about the different termination conditions of atomic layer, six kinds of YAlO3(001)/TiC(100) coherent interface models were selectively constructed based on the results of lattice mismatch calculation and surface convergence test. The work of adhesion and interfacial energy were calculated by first principles method, which is used to analyze their interface adhesive strength and stability to find out interface structures with excellent property. Charge density difference and electron localization functions were calculated to analyze interfacial atomic bonding characteristics. The YAlO3(001)/TiC(100) interfacial structure with the best grain boundary crack and displacement resistance were predicted by the adhesive sliding simulation and potential barrier calculation of preferable formed interfacial structures. The calculation results show that the lattice mismatch of YAlO3(001)/TiC(100) interfacial structures is very small, about 2.5%, which indicates they have the advantage of forming coherent interfaces. The work of adhesion of O1-Ti interface is the largest (Wad = 5.74 J/m2), which is followed by that of the O1-C interface (Wad = 5.73 J/m2). Their interface structures are similar and interfacial atomic bond type are both mixture of covalent bond (between O atom and C atom) and ionic bond (between O atom and Ti atom). However, the interfacial energy of O1-C is the smallest (γ = 2.16 J/m2), which reveals that the O1-C type YAlO3(001)/TiC(100) interface is the most stable. In addition, the sliding direction static friction force of O1-C model is 1.24 times larger than that of O1-Ti, and most of the O1-C interfacial barrier is larger than that of O1-Ti structure in a sliding period. Therefore, it can be predicted that a YAlO3(001)/TiC(100) interface with excellent combination property will be preferable formed in type of O1-C structure. The densification and mechanical property are expected to be greatly improved in TiC reinforced wear-resistance coating and other functional ceramics after adding YAlO3 additive.

Influences of Lake Malawi on the spatial and diurnal variability of local precipitation

Abstract. We investigate how the intensity and spatial distribution of precipitation vary around Lake Malawi on a diurnal timescale, which can be valuable information for water resource management in tropical south-eastern African nations. Using a state-of-the-art satellite product and regional atmospheric model, the well-defined diurnal cycle is detected around Lake Malawi with harmonic and principle component analyses: the precipitation is intense during midnight to morning over Lake Malawi and the precipitation peaks in the daytime over the surrounding area. This diurnal cycle in the precipitation around the lake is associated with the lake–land breeze circulation. Comparisons between the benchmark simulation and an idealized simulation in which Lake Malawi is removed reveal that the diurnal variations in precipitation are substantially amplified by the presence of Lake Malawi. This is most evident over the lake and surrounding coastal regions. Lake Malawi also enhances the lake–land breeze circulation; the nocturnal lakeward land breeze generates surface convergence effectively and precipitation intensifies over the lake. Conversely, the daytime landward lake breeze generates the intense divergence over the lake and precipitation is strongly depressed over the lake. The lake–land breeze and the background vapour enriched by Lake Malawi drive primarily a diurnal variation in the surface moisture flux divergence/convergence over the lake and surrounding area which contributes to the diurnal cycle of precipitation in this region.

Structural characteristics of the soil fauna community in beach wetlands of the Poyang Lake region

We examined the soil fauna community composition and structure of Poyang Lake beach wetlands using the modified Tullgren and Baermann methods during September and December 2015. A total of 3755 individuals were obtained from 2 samplings, which were found to belong to 7 classes, 8 orders, 3 suborders, and 17 families. Among these, the Oribatida, Prostigmata, Nematoda, and Onychiuridae were dominant. The functional groups of soil animals included omnivores, saprozoics, predators, and phytophages. Analysis results of the soil fauna composition and diversity showed that the soil animals were mainly concentrated at the 0-10 cm depth, presenting as a significant surface convergence phenomenon. The numbers of individual soil animals differed significantly between sampling sites (P < 0.05), with the beach wetland in Lianhu Township having the highest numbers of individuals and groups. The effect of water level variability on numbers of soil animals was not significant (P > 0.05). Habitat type and sampling time both had highly significant effects on diversity and evenness (P < 0.01), and significant effects on species richness (P < 0.05). During water level changes in Poyang Lake, the individual number, group number, and diversity of the soil animals were closely linked to the sampling site. These results provide basic data for further studies on soil fauna ecology in Poyang Lake beach wetlands.

Idealized Modeling of the Atmospheric Boundary Layer Response to SST Forcing in the Western Indian Ocean

Abstract The atmospheric response to sea surface temperature (SST) variations forced by oceanic downwelling equatorial Rossby waves is investigated using an idealized convection-resolving model. Downwelling equatorial Rossby waves sharpen SST gradients in the western Indian Ocean. Changes in SST cause the atmosphere to hydrostatically adjust, subsequently modulating the low-level wind field. In an idealized cloud model, surface wind speeds, surface moisture fluxes, and low-level precipitable water maximize near regions of strongest SST gradients, not necessarily in regions of warmest SST. Simulations utilizing the steepened SST gradient representative of periods with oceanic downwelling equatorial Rossby waves show enhanced patterns of surface convergence and precipitation that are linked to a strengthened zonally overturning circulation. During these conditions, convection is highly organized, clustering near the maximum SST gradient and ascending branch of the SST-induced overturning circulation. When the SST gradient is reduced, as occurs during periods of weak or absent oceanic equatorial Rossby waves, convection is much less organized and total rainfall is decreased. This demonstrates the previously observed upscale organization of convection and rainfall associated with oceanic downwelling equatorial Rossby waves in the western Indian Ocean. These results suggest that the enhancement of surface fluxes that results from a steepening of the SST gradient is the leading mechanism by which oceanic equatorial Rossby waves prime the atmospheric boundary layer for rapid convective development.

Effects of a Cold Ocean Eddy on Local Atmospheric Boundary Layer Near the Kuroshio Extension: In Situ Observations and Model Experiments

AbstractShipboard observations captured clear modulations of the marine atmospheric boundary layer (MABL) over a 300‐km‐diameter cold eddy in the Kuroshio Extension in April 2014. Regional atmospheric model experiments, with 30‐km and 10‐km horizontal resolutions for outer and inner domains, respectively, are conducted to aid the interpretations of the in situ observations. Enhanced turbulent heat fluxes promoted a well‐mixed MABL up to 1900‐m deep and a stratocumulus layer over the warm water west of the eddy. Over the cold sea surface of the eddy, the MABL was free of cloud and shrank to 1,200‐m deep. In the transition zone from the warm water in the west to the cold eddy, the MABL deepened and convective cumulus developed under a temperature inversion at 2,800 m. Model results indicate that, via the vertical mixing and pressure adjustment mechanisms, low sea surface temperatures of the cold eddy drives anomalous easterly winds in the transition zone, causing surface convergence to deepen the MABL and clouds. The model further shows that the atmospheric asymmetric responses in surface divergence and secondary circulation are due to the combined effects of the vertical mixing and pressure adjustment.

Investigation of the atmospheric boundary layer during an unexpected summertime persistent severe haze pollution period in Beijing

Persistent severe haze pollution seldom happens in summer in Beijing. In this study, a persistent severe haze pollution event, defined as daily PM2.5 concentration higher than 150 μg m−3 on 5 consecutive days, was observed in Beijing from 26 to 30 in July 2010. It was not caused by crop residue burning during the harvest period. Weak pressure systems dominated at surface and implicated weak advection. Regional weak southerly winds with speeds of 2–3 m s−1 consistently brought pollutants from southern large-emission regions to Beijing. Surface convergence resulted from northerly winds prevailing in northern regions induced pollutants remaining in Beijing, which contributed to the maximum daily PM2.5 concentration on 26 in July. A continental high-pressure system persisted in the northwest of Beijing at 500 hPa, which led to significant sinking motion. Elevated inversion was found in the form of subsidence inversion, and this was confirmed by a skew T-logp diagram and vertical velocity analysis. It is much different from the vertical boundary layer structure in wintertime pollution period that surface-based inversion dominates. The subsidence inversion was an extremely stable layer with an average depth of hundred meters and strength of 1.4 °C. The capping effect of the inversion layer at low altitude of average 720 m limited vertical diffusion of pollutants and trapped them in a shallow layer, and thus, extremely high concentration of PM2.5 remained. WRF-Chem model simulation demonstrated that about 70% of PM2.5 was transported to Beijing from its southern regions.

Use of WorldView-2 Along-Track Stereo Imagery to Probe a Baltic Sea Algal Spiral

The general topic here is the application of very high-resolution satellite imagery to the study of ocean phenomena having horizontal spatial scales of the order of 1 kilometer, which is the realm of the ocean submesoscale. The focus of the present study is the use of WorldView-2 along-track stereo imagery to probe a submesoscale feature in the Baltic Sea that consists of an apparent inward spiraling of surface aggregations of algae. In this case, a single pair of images is analyzed using an optical-flow velocity algorithm. Because such image data generally have a much lower dynamic range than in land applications, the impact of residual instrument noise (e.g., data striping) is more severe and requires attention; we use a simple scheme to reduce the impact of such noise. The results show that the spiral feature has at its core a cyclonic vortex, about 1 km in radius and having a vertical vorticity of about three times the Coriolis frequency. Analysis also reveals that an individual algal aggregation corresponds to a velocity front having both horizontal shear and convergence, while wind-accelerated clumps of surface algae can introduce fine-scale signatures into the velocity field. Overall, the analysis supports the interpretation of algal spirals as evidence of a submesoscale eddy and of algal aggregations as indicating areas of surface convergence.

Response of the Diurnal Cycle of Summer Rainfall to Large‐Scale Circulation and Coastal Upwelling at Hainan, South China

AbstractThe modulation of coastal rainfall at Hainan by large‐scale circulation and coastal upwelling is studied using observations and numerical modeling. Tropical Rainfall Measuring Mission data show that the monthly mean rainfall off southern Hainan was considerably higher in August 2010 than in August 2011. The main cause of this difference is an intensification of offshore rainfall from midnight to early morning. Compared with the multiyear (2000–2017) average field, reanalysis data show that there is an apparent increase in atmospheric humidity in August 2010. During this time, a moderate El Niño was decaying and a new La Niña event was developing, so a significant adjustment of the large‐scale circulation was observed in the western Pacific. The resulting anomalous onshore wind depresses the development of upwelling, leading to a relatively warm sea surface temperature off the south coast of Hainan, which in turn enhances the land‐sea thermal contrast and land breeze at night. Decreases in upwelling intensity and asymmetric intensification of nocturnal and daytime offshore rainfall are confirmed not only in the summer of 2010 but also in the summer of 2003. Numerical simulations using the Weather Research and Forecasting model verify that the enhanced land breeze ultimately leads to pronounced coastal rainfall off southern Hainan at night. The results indicate that localized surface convergence associated with the sea and land breezes may be more important than the atmospheric humidity and convective instability in modulating the diurnal cycle of rainfall for tropical islands.

Efficient Control of a Non-Linear System Using a Modified Sliding Mode Control

Trajectory tracking is an essential requirement in robot manipulator movement and localization applications. It is a current research topic of interest, and several researchers have proposed different schemes to achieve the task accurately. This research proposes efficient control of a hydraulic non-linear robot manipulator using a modified sliding mode control, named proportional derivative sliding mode control with sliding perturbation observer (PDSMCSPO), to overcome parameter uncertainties and non-linearity. The proposed new control strategy achieves higher accuracy and better time convergence than the previous one. A positive derivative gain, which has a value less than one, is multiplied with the velocity error term of the sliding surface. The proposed control (PDSMCSPO) also achieves robustness. Results show that by introducing the derivative gain, the chattering from the system has been reduced more than classical sliding mode control (SMC). The reason is that during reaching phase this small gain multiplies with the perturbation and minimizes the effect of perturbation on the system. A smaller value of switching gain K is required as compared to SMC, and the transfer function between sliding surface and perturbation in proportional derivative sliding mode control (PDSMC)has low pass filter characteristics. The proposed PDSMCSPO has a faster response than previous sliding mode control with sliding perturbation observer (SMCSPO), and the output and sliding surface convergence to the desired value is much quicker than conventional logic. Some other characteristics such as error in the output are small because of more attenuation of the perturbation signal. Simulation and experimental results are presented for a link between the hydraulic robot manipulator and the mass damper system.

Impacts of Convergence on Structure Functions from Surface Drifters in the Gulf of Mexico

AbstractThere are limitations in approximating Eulerian statistics from surface drifters, due to biases from surface convergences. By contrasting second- and third-order Eulerian and surface drifter structure functions obtained from a model of the Gulf of Mexico, the consequences of the semi-Lagrangian nature of observations during the summer Grand Lagrangian Deployment (GLAD) and winter Lagrangian Submesoscale Experiment (LASER) are estimated. By varying launch pattern and location, the robustness and sensitivity of these statistics are evaluated. Over scales less than 10 km, second-order structure functions of surface drifters consistently have shallower slopes (~r2/3) than Eulerian statistics (~r), suggesting that surface drifter structure functions differ systematically and do not reproduce the scalings of the Eulerian fields. Medians of Eulerian and cluster release second-order statistics are also significantly different across all scales. Synthetic cluster release statistics depend on launch location and weakly on launch pattern. The observations suggest little seasonal difference in the second-order statistics, but the LASER third-order structure function shows a sign change around 1 km, while GLAD and the synthetic cluster releases show a third-order structure function sign change around 10 km. Further, synthetic surface drifter cluster releases (and therefore likely the GLAD observations) show robust biases in the negative third-order structure functions, which may lead to significant overestimation of the spectral energy flux and underestimation of the transition scale to a forward energy cascade. The Helmholtz decomposition, and curl and divergence statistics, of Eulerian and cluster releases differ, particularly on scales less than 10 km, in agreement with observations of drifters preferentially sampling convergences in coherent structures.

Evaluation of Supercell Storm Triggering Factors Based on a Cloud Resolving Model Simulation

An attempt has been made in this study to examine the conditional instability parameters in the selected area and to determine the main ingredients responsible for initiation and evolution of supercell storm over Skopje, Macedonia on 6 August 2016. WRF model forecasts provide the basic meteorological parameters for cloud model initialization and the detail information about atmospheric instability potential as triggering factor for severe convection. The cloud model simulation has been performed with very fine spatial and temporal resolution capable to resolve the detail aspects of convection. The results utilizing this novel method suggest that, upper level lifting, moisture advection, large CAPE, near surface convergence and increased potential vortices in the selected area play substantial role in early assessment of the atmospheric status, convective instability and storm potential. In addition the directional wind shear (veering) at the near surface layer, high storm helicity index, differential heating induced by the strong local forcing environment serve as triggering factors for initiation of supercell storm with rotational updrafts-mesocyclone. The cloud model simulation with fine resolution allows more detail insight into the storm dynamics and the mechanism of generation of rotational updrafts and mesocyclone, a hook echo signature and the presence of bounded weak echo region as ingredients for supercellular structure and evolution. The overshooting top of 15 km, peak updraft speed of 40 m/s, wind gust of 35 m/s and reflectivity which exceeds 70 dBZ indicates to the occurrence of a very severe storm. A longer live cycle of storm and the intense water production, with extreme rainfall rate of 38 mm/5 min, contribute to formation of excessive torrential rainfall and local catastrophic flooding.

Urban Influences on the Spatiotemporal Characteristics of Runoff and Precipitation during the 2009 Atlanta Flood

AbstractThe 2009 Atlanta flood was a historic event that resulted in catastrophic damage throughout the metropolitan area. The flood was the product of several hydrometeorological processes, including moist antecedent conditions, ample atmospheric moisture, and mesoscale training. Additionally, previous studies hypothesized that the urban environment of Atlanta altered the location and/or overall quantities of precipitation and runoff that ultimately produced the flood. This hypothesis was quantitatively evaluated by conducting a modeling case study that utilized the Weather Research and Forecasting Model. Two model runs were performed: 1) an urban run designed to accurately depict the flood event and 2) a nonurban simulation where the urban footprint of Atlanta was replaced with natural vegetation. Comparing the output from the two simulations revealed that interactions with the urban environment enhanced the precipitation and runoff associated with the flood. Specifically, the nonurban model underestimated the cumulative precipitation by approximately 100 mm in the area downwind of Atlanta where urban rainfall enhancement was hypothesized. This notable difference was due to the increased surface convergence observed in the urban simulation, which was likely attributable to the enhanced surface roughness and thermal properties of the urban environment. The findings expand upon previous research focused on urban rainfall effects since they demonstrate that urban interactions can influence mesoscale hydrometeorological characteristics during events with prominent synoptic-scale forcing. Finally, from an urban planning perspective, the results highlight a potential two-pronged vulnerability of urban environments to extreme rainfall, as they may enhance both the initial precipitation and subsequent runoff.

Large deformation of tunnels in longwall coal mines

Large deformation of tunnels in longwall coal mines creates massive cracks in the surrounding rocks, which may cause groundwater loss and threaten the coal mine safety. Rock mechanical characterization is crucial for understanding the large deformation of underground openings. Based on the theoretical analysis and laboratory triaxial tests, this study aims to propose a strength degradation model of the rocks for a deep understanding on the large deformation of underground openings. The proposed model captures the non-linear post-peak rock strength degradation which is dependent on the plastic strains accumulated under the decreasing confining pressures. The stress-dependent elastic properties were not considered in this study. The proposed model was incorporated in a finite difference package (FLAC3D) for engineering application. The simulated surface convergence of the underground opening matches the in-mine measurements, which supports the proposed model. More importantly, the simulated crack zone after tunnel excavation agrees with the observations from borehole images. Therefore, the proposed model provides a helpful tool to evaluate the safe margin of an underground tunnel if it is close to a subsurface water-bearing formation. It also sheds lights on the underground tunnel design to prevent the adjacent groundwater from inrush.