Transfer Of Horizontal Momentum Research Articles

Bioinspired light-fueled water-walking soft robots based on liquid crystal network actuators with polymerizable miniaturized gold nanorods

Walking on water is a unique locomotion mode found in aquatic living creatures. However, it remains challenging to create light-driven aquatic soft robots that can walk freely on water surface. Herein, water strider-inspired water-walking soft robots that can be remotely controlled by light are demonstrated through combining superhydrophobic floating surface with hierarchical nanostructures and light-driven soft-actuation legs that are fabricated through in situ embedding of judiciously designed polymerizable miniaturized gold nanorods (MiniGNR nanomonomer) in liquid crystal network (LCN)-based soft actuators. The MiniGNRs-LCN soft actuators act as driving paddles to enable propulsive motion on the water surface, and the superhydrophobic surface facilitates the aquatic soft robots with weight-bearing and drag-reducing ability. Upon cyclic exposure to near-infrared light, the LCN-based actuation leg quickly exhibits a photoinduced shape-bending deformation, enabling the one-legged soft robot to walk on water surface unidirectionally through horizontal momentum transfer of photoinduced actuation force to the water. Taking advantage of tailorable photoresponsiveness of MiniGNR nanomonomer, a tri-legged water-walking soft robot that can execute visible/infrared wavelength-dependent multi-directional motions is further demonstrated for cargo transportation. This research may provide a new toolbox for the development of untethered soft robots with sophisticated spatiotemporal actuation toward aquatic complex environments.

Low-Level Atmospheric Flow at the Central North Coast of Brazil

The planetary-boundary-layer (PBL) flow and above is investigated for the central north coast of Brazil, an equatorial region spanning from 8° to 2°S. The daily PBL flow is controlled by vertical entrainment of horizontal momentum from a southerly large-scale flow associated with the Hadley cell, and by a mesoscale pressure gradient force (PGF) created by the differential heating between land and ocean. Near the coast, the flow is from the north-east quadrant comprising a small rotation, probably caused by a weak mesoscale PGF and a weak Coriolis force. Inland it is north-easterly in the morning, but deep mixing during the afternoon brings down momentum from above causing it to become south-easterly. The mesoscale PGF executes a daily 360° rotation at most of the stations. In the afternoon it points to land due to continuous heating of the land, and a sea breeze develops in the presence of the background flow. Once convection dies out, the transfer of horizontal momentum is reduced, and the marine-air layer can flow faster into the continent as a nocturnal jet. As the stable boundary layer grows thicker, this flow tends to be eliminated at the surface. By morning, the mesoscale PGF points north, forcing the inland flow to become south-easterly, while on the coast flow becomes almost easterly. This scenario repeats during dry and wet seasons and can be understood as a consequence of the south–north propagation of an atmospheric circulation resembling a helix with its rotation axis oriented parallel to the shoreline.

An analytical model for rapid estimation of hurricane supergradient winds

The supergradient winds that may have severe implications on the wind design of high-rise buildings have been commonly observed in the hurricane boundary layer. However, the widely-used log-law or power-law wind profile excludes the supergradient-wind region in which the tangential winds are larger than the gradient winds. Although high-fidelity, nonlinear hurricane wind models may well capture the supergradient winds, high computational demand is needed for each simulation. Recently developed linear, height-resolving hurricane wind models, while can efficiently consider the existence of supergradient winds, significantly underestimate them due essentially to the ignorance of vertical advection term in the governing equations. A number of studies have actually demonstrated that the vertical advection is a major contributor to the transfer of horizontal momentum to the supergradient region. To this end, a refined analytical model that simultaneously integrates the horizontal advection, vertical advection and vertical diffusion terms into the governing equations is developed for accurately and efficiently estimating the hurricane supergradient winds. The important role of the vertical wind speed in determining the horizontal wind speeds (including supergradient winds) in the hurricane boundary layer is highlighted. Since the horizontal and vertical wind components are mutually dependent, the iteration technique is utilized to solve the proposed analytical model. The consideration of the vertical advection results in intensified supergradient winds that are consistent with the observations. Furthermore, a strong outflow region in the vicinity of the radius of maximum winds due to the supergradient winds can be obtained. Due to its simplicity and computational efficiency, the developed analytical model can be easily implemented in the Monte Carlo simulations for the rapid assessment of hurricane wind risk to coastal structures, especially to high-rise buildings.

Modeling rain-induced effects on boundary-layer wind field of tropical cyclones

Despite the significant impacts of heavy rainfall on the tropical cyclone intensity due to the transfer of horizontal momentum between air and raindrops, the comprehensive modeling of rain-induced effects on the boundary-layer wind field remains a challenge. The wind shear zone developed surrounding the falling precipitation results in complicated dynamic interactions between the wind and rain fields. The solution of dynamically coupled, intensively interactive wind and rain fields may be achieved using high-fidelity air-water interaction simulations but needs extremely high computational costs. To consider the wind-rain interactions with a first-order approximation, the fully-coupled dynamic system governing the raindrop motion and the wind field has been simplified herein to a weakly-coupled one represented by aerodynamic drag force. The drag-induced horizontal momentum transfer is integrated into the governing equations of the linear, height-resolving wind field, and an analytical model is accordingly developed to effectively consider the rain-induced effects on the boundary-layer winds of tropical cyclones. The results generated by the present model are consistent with the field measurements. It has been demonstrated that, while the wind speed can be either accelerated or decelerated depending on the location in the tropical cyclones and the rain parameters (e.g., rain rate, relative motion between the air and raindrops, drag coefficient and raindrop size distribution), the rain-induced effects on the boundary-layer wind directions (and hence the inflow angle) also have important significance on the tropical cyclone wind hazard on tall buildings and other structures. Due to its simplicity and high computational efficiency, the proposed model could be easily implemented in the risk assessments for tropical-cyclone wind hazards in engineering applications.

Discharge Estimation at the Apex of Compound Meandering Channels

A new method is proposed to calculate flow in a compound meandering channel by considering the momentum transfer between different zones. Vertical momentum transfer, at the horizontal interface of the lower main channel and the meander belt; and the horizontal momentum transfer, at the vertical interfaces between the meander belt and the adjoining outer floodplains. Modelling of this analytical method for the bend apex section is carried out, for a control volume of unit length. Calibration has been carried out for different laboratory channels and a natural river, demonstrating that the present model is capable of providing an adequate prediction of discharge in experimental as well as in field study. A constant value of 0.01 as the momentum transfer coefficient is verified to act at the interacting interfaces for the meandering channels. The zonal velocity for each subsection is determined, which facilitates prediction of discharge distribution.

Fully Coupled Simulations of Megathrust Earthquakes and Tsunamis in the Japan Trench, Nankai Trough, and Cascadia Subduction Zone

Subduction zone earthquakes can produce significant seafloor deformation and devastating tsunamis. Real subduction zones display remarkable diversity in fault geometry and structure, and accordingly exhibit a variety of styles of earthquake rupture and tsunamigenic behavior. We perform fully coupled earthquake and tsunami simulations for three subduction zones: the Japan Trench, the Nankai Trough, and the Cascadia Subduction Zone. We use data from seismic surveys, drilling expeditions, and laboratory experiments to construct detailed 2D models of the subduction zones with realistic geometry, structure, friction, and prestress. Greater prestress and rate-and-state friction parameters that are more velocity-weakening generally lead to enhanced slip, seafloor deformation, and tsunami amplitude. The Japan Trench’s small sedimentary prism enhances shallow slip, but has only a small effect on tsunami height. In Nankai where there is a prominent splay fault, frictional parameters and off-fault material properties both influence the choice of rupture pathway in complex ways. The splay generates tsunami waves more efficiently than the decollement. Rupture in Cascadia is buried beneath the seafloor, but causes a tsunami that is highly complex due to the rough seafloor bathymetry. Neglecting compliant sediment layers leads to substantially different rupture behavior and tsunami height. We demonstrate that horizontal seafloor displacement is a major contributor to tsunami generation in all subduction zones studied. We document how the nonhydrostatic response of the ocean at short wavelengths smooths the initial tsunami source relative to commonly used approach for setting tsunami initial conditions. Finally, we determine self-consistent tsunami initial conditions by isolating tsunami waves from seismic and acoustic waves at a final simulation time and backpropagating them to their initial state using an adjoint method. We find no evidence to support claims that horizontal momentum transfer from the solid Earth to the ocean is important in tsunami generation.

Flow structure and turbulence characteristics downstream of a spanwise suspended linear array

Laboratory experiments are conducted to quantify the mean flow structure and turbulence properties downstream of a spanwise suspended linear array in a uniform ambient water flow using Particle Tracking Velocimetry. Eighteen experimental scenarios, with four depth ratios (array depth to water column depth) of 0.35, 0.52, 0.78, and 0.95 and bulk Reynolds number (length scale is the array depth) from 11,600 to 68,170, are investigated. Three sub-layers form downstream of the array: (1) an internal wake zone, where the time-averaged velocity decreases with increasing distance downstream, (2) a shear layer which increases in vertical extent with increasing distance downstream of the array, and the rate of the increase is independent of the bulk Reynolds number or the depth ratio, and (3) an external wake layer with enhanced velocity under the array. The location of the shear layer is dependent on the depth ratio. The spatially averaged and normalized TKE of the wake has a short production region, followed by a decay region which is comparable to grid turbulence decay and is dependent on the depth ratio. The results suggest that the shear layer increases the transfer of horizontal momentum into the internal wake zone from the fluid outside of the array and that the turbulence in the internal wake zone can be modeled similarly to that of grid turbulence.

Prognostic Residual Mean Flow in an Ocean General Circulation Model and its Relation to Prognostic Eulerian Mean Flow

AbstractRecent work has shown that taking the thickness-weighted average (TWA) of the Boussinesq equations in buoyancy coordinates results in exact equations governing the prognostic residual mean flow where eddy–mean flow interactions appear in the horizontal momentum equations as the divergence of the Eliassen–Palm flux tensor (EPFT). It has been proposed that, given the mathematical tractability of the TWA equations, the physical interpretation of the EPFT, and its relation to potential vorticity fluxes, the TWA is an appropriate framework for modeling ocean circulation with parameterized eddies. The authors test the feasibility of this proposition and investigate the connections between the TWA framework and the conventional framework used in models, where Eulerian mean flow prognostic variables are solved for. Using the TWA framework as a starting point, this study explores the well-known connections between vertical transfer of horizontal momentum by eddy form drag and eddy overturning by the bolus velocity, used by Greatbatch and Lamb and Gent and McWilliams to parameterize eddies. After implementing the TWA framework in an ocean general circulation model, the analysis is verified by comparing the flows in an idealized Southern Ocean configuration simulated using the TWA and conventional frameworks with the same mesoscale eddy parameterization.

Spatial variability of the wave bottom boundary layer over movable rippled beds

[1] Observations of the spatially dependent velocity field over movable bed forms subjected to slightly skewed and asymmetric regular wave forcing were collected. The dynamics between the ripple elements is dominated by coherent vortices, characterized by the swirling strength, and evidenced in the temporal and spectral characterization. Within the boundary layer, spectral energy in the second harmonic (3f0) is amplified at the ripple slopes and is consistent with the location of the expected strongest pressure gradients. First-moment and second-moment velocity statistics were used to address the spatial variability of the intra-ripple hydrodynamics. Estimates of displacement and momentum thicknesses (δ* and δmom) are smaller than suggested by the higher harmonics, but consistently highlight areas of adverse and favorable pressure gradients. Shear stress and roughness estimates were inferred by fitting a logarithmic model to first-moment and second-moment statistics of the velocity field. The maximum Shields parameter was observed to peak at the stoss slope of asymmetric ripples during the strongest and shorter half-wave period (onshore). First-moment roughness estimates are similar in magnitude to bed load parameterizations provided by Li et al. (1997), and about a factor of 3 larger than second-moment estimates. Assessment of the vertical transfer of horizontal momentum derived using a Reynolds decomposition suggests that stresses inferred from the logarithmic law using first-moment velocity statistics appropriately reproduce the mean momentum transfer for the longer and weaker offshore half-wave period.

Irreversible compressible work and available potential energy dissipation in turbulent stratified fluids

Although it plays a key role in the theory of stratified turbulence, the concept of available potential energy (APE) dissipation has remained until now a rather mysterious quantity, owing to the lack of rigorous result about its irreversible character or energy conversion type. Here, we show by using rigorous energetics considerations rooted in the analysis of the Navier–Stokes for a fully compressible fluid with a nonlinear equation of state that the APE dissipation is an irreversible energy conversion that dissipates kinetic energy into internal energy, exactly as viscous dissipation. These results are established by showing that APE dissipation contributes to the irreversible production of entropy, and by showing that it is a part of the work of expansion/contraction. Our results provide a new interpretation of the entropy budget, that leads to a new exact definition of turbulent effective diffusivity, which generalizes the Osborn–Cox model, as well as a rigorous decomposition of the work of expansion/contraction into reversible and irreversible components. In the context of turbulent mixing associated with parallel shear flow instability, our results suggests that there is no irreversible transfer of horizontal momentum into vertical momentum, as seems to be required when compressible effects are neglected, with potential consequences for the parameterizations of momentum dissipation in the coarse-grained Navier–Stokes equations.

MERIX—Next generation medium energy resolution inelastic X-ray scattering instrument at the APS

MERIX (Medium Energy Resolution Inelastic X-ray Scattering) is an instrument for Resonant Inelastic X-ray Scattering (RIXS) studies, in the hard X-ray regime, designed to work with photons in the 5–12keV range, spanning atomic resonances near the K-edges of 3d elements, and the L-edges of 4f and 5d elements. The energy analysis of inelastically scattered photons is performed with segmented spherical crystal analyzers in close to Bragg backscattering geometry. For each resonance (edge) a specially designed analyzer is used, fabricated from Ge, Si, or LiNbO3 crystals. MERIX uses a position sensitive (micro-strip) detector to take snapshots of IXS spectra which are dispersed in space and over an energy range of a few eV with ≃20–40meV energy resolution. The spectral resolution of the MERIX spectrometer depends on the analyzer and varies from ≃45meV to ≃170meV, while the momentum transfer resolution is ≃1–4nm−1. Samples are illuminated by micro-focused beams of size ≃10μm×45μm, allowing for studies at high-pressure and other extreme conditions. Polarization selectivity is ensured by vertical or horizontal momentum transfer scans. MERIX features ≃100 times higher count-rates compared to previously built RIXS instruments.

Improving storm surge estimates: Increased downwards transfer of horizontal momentum by wind-driven waves

The numerical predictions of historical storm surges in which the low pressure meteorological system remains well offshore requires a factor three, or more, wind stress than that provided using the standard wind stress coefficients found in the literature. The literature values are from the ‘air side’, where wind stress coefficients are determined either from the measured vertical velocity gradient of the wind over the water or more commonly from wind Reynolds stresses. On the other hand, the wind stress required to match the observed storm surges is consistent with the momentum transfer observed during wind-wave growth experiments (i.e., the inferred wind stress coefficients from experiments measuring wave height growth due to wind). Both the wind-wave growth data and our storm surge modelling are consistent with field and laboratory measurements of Reynolds stresses of wind-driven waves. The possibility that there is more momentum being transferred downwards urges the development of a storm surge model capable of using the wind stress inferred from the wind-wave growth data. This requires a Reynolds stress model covering deep through to shallow water depths and at arbitrary levels (3D model implementation), which has been formulated herein. Application of this wind stress to Tropical Cyclone Roger using the steady shallow water equations qualitatively explains the observed storm surge. This new approach also raises the question of why storm surge estimates are generally acceptable for weather systems that cross the coastline despite the use of momentum transfer based on air side wind stress.

Evolutionary characteristics of a dust storm over Oman on 2 February 2008

The evolutionary characteristics of a dust storm over Oman on 2 February 2008 were studied by analyzing the weather associated with it. The National Center for Environmental Prediction/National Centre for Atmospheric Research (NCEP/NCAR) reanalysis wind flow pattern at 1,000 hPa showed a clockwise and outward wind flow pattern over the study region, a manifestation of a high-pressure cold air mass. In addition, ground truth observations for surface temperature and surface winds showed cold northerly winds until the early morning of 2 February 2008. A strong wind shear resulted from differences in wind speed between warm air and trapped cold air. This vertical wind shear enhanced instability. Furthermore, the weakening of the inversion in the lower troposphere and the formation of a mixed layer due to transfer of horizontal momentum from upper air towards the surface led to strong surface winds. These strong winds lifted a large amount of dust particles off the ground, resulting in the dust event of 2 February 2008.

Design of a Nonhydrostatic Atmospheric Model Based on a Generalized Vertical Coordinate

The isentropic system of equations has particular advantages in the numerical modeling of weather and climate. These include the elimination of the vertical velocity in adiabatic flow, which simplifies the motion to a two-dimensional problem and greatly reduces the numerical errors associated with vertical advection. The mechanism for the vertical transfer of horizontal momentum is simply the pressure drag acting on isentropic coordinate surfaces under frictionless, adiabatic conditions. In addition, vertical resolution is enhanced in regions of high static stability, which leads to better resolution of features such as the tropopause. Negative static stability and isentropic overturning frequently occur in finescale atmospheric motion. This presents a challenge to nonhydrostatic modeling with the isentropic vertical coordinate. This paper presents a new nonhydrostatic atmospheric model based on a generalized vertical coordinate. The coordinate is specified in a manner similar to that of Konor and Arakawa, but “arbitrary Eulerian–Lagrangian” (ALE) methods are used to maintain coordinate monotonicity in regions of negative static stability and to return the coordinate surfaces to their isentropic “targets” in statically stable regions. The model is mass conserving and implements a vertical differencing scheme that satisfies two additional integral constraints for the limiting case of z coordinates. The hybrid vertical coordinate model is tested with mountain-wave experiments including a downslope windstorm with breaking gravity waves. The results show that the advantages of the isentropic coordinate are realized in the model with regard to vertical tracer and momentum transport. Also, the isentropic overturning associated with the wave breaking is successfully handled by the coordinate formulation.

Vibration-induced arching in a deep granular bed

Forced vertical vibration of a granular layer can drive flow phenomena such as heaping, convection, fluidization, densification, surface waves and arching. Food, mineral processing, and pharmaceuticals industries all utilize vibratory processes for the handling and transport of granular materials. Understanding how a granular material responds when subjected to vibration is essential for equipment design. Three-dimensional discrete element simulations have been used in this study to investigate the convective motion leading to arching in a vertically vibrated, deep granular bed. The undulating granular layer contains alternating regions that first compact and then relax. The dynamics of these regions may depend on material properties such as restitution and friction coefficients; as well as particle shape. The effects of these factors on the kinematics and dynamics of the arching pattern are investigated here. The arching pattern is found to arise from synchronised momentum transfer between the rise and fall of the deforming granular layer and horizontally travelling waves. The arching pattern was found to be stable across a broad range of restitution and friction levels and particle shapes. Particles with high restitution tend to disrupt the timing between the vertical and horizontal periodic flows and affect the stability of the pattern selection. Large friction results in shear resistance, higher bed pressures, lower bulk densities, and delays in the timing of the vertical and horizontal momentum transfer. Non-sphericity leads to increased dilation of the bed, slower sideways velocities, and increased loading on the floor and dissipation rate in the bed.

Numerical study of the effects of the breaking intensity on wave breaking flows

The flow generated by the breaking of free-surface waves of different initial steepnesses is simulated numerically. The aim is to investigate the role played by the breaking intensity on the resulting flow. The study, which assumes a two-dimensional flow, makes use of a two-fluids Navier–Stokes solver combined with a Level-Set technique for the interface capturing. The evolution of periodic wavetrains is considered. Depending on the initial steepness ϵ, the wavetrain remains regular or develops a breaking, which can be either of spilling or plunging type. From the analysis of the local strain fields it is shown that, in the most energetic phase of plunging breaking, dissipation is mainly localized about the small air bubbles generated by the fragmentation of the air cavity entrapped by the plunging of the jet. The downward transfer of the horizontal momentum is evaluated by integrating the flux of momentum through horizontal planes lying at different depths beneath the still water level. From weak to moderate breaking, increase in the breaking intensity results in growing transfer of horizontal momentum, as well as thickening of the surface layer. Beyond a certain breaking intensity, the larger amount of air entrapped causes a reduction in the momentum transferred and the shrinkage of the layer. Quantitative estimates of the amount of air entrapped by the breaking and of the degassing process are provided. A scaling dependence of the amount of air entrapped by the first plunging event on the initial steepness is found. A careful analysis of the circulation induced in water by the breaking process is carried out. It is seen that in the plunging regime the primary circulation induced by the breaking process scales with the velocity jump between the crest and the trough of the wave.The limits of the main assumptions of the numerical calculations are analysed. It is shown that up to half-wave period after the breaking onset, the Reynolds number of the simulation does not significantly affect the solution. In order to further support the findings, an estimate of the uncertainty of the numerical results is derived through several repetitions of the numerical simulation with small perturbations of the initial conditions.

Mesoscale wind climate modelling in steep mountains

Although the Mesoscale Community Compressible (MC2) model successfully reproduces the wind climate (for wind energy development purposes) of the Gaspé region, equivalent simulations for the steep mountainous southern Yukon have been unsatisfactory. An important part of the problem lies in the provision of suitable boundary conditions in the lower troposphere. This paper will describe an alternative provision of boundary conditions to the MC2 model based partly on standard National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) Reanalysis statistics, however, with modified lower tropospheric conditions based on local radiosonde measurements. The MC2 model is part of the AnemoScope wind energy simulation toolkit which applies statistical‐dynamical downscaling of basic large‐scale weather situations (i.e., the NCEP/NCAR Reanalysis) to simulate the steadystate wind climate of a complex region. A case study summarized here imposes a typical mean winter temperature inversion on the boundary conditions to reduce downward momentum transfer in the MC2 model over the Whitehorse region. In conjunction with this step, the geostrophic wind at the boundaries is held constant (with height) in speed and direction, based on the (observed) dominant southwesterly winds above the mountaintops. The resulting simulation produces wind directions within the modelled domain that are in much better agreement with the available measurements. However, despite the imposed atmospheric stability, downward transfer of horizontal momentum from aloft still appears to exceed that occurring in nature. It is recommended that (in future studies of this type regarding mountain wind climate) the input statistics processed from the NCEP/NCAR Reanalysis be modified by referencing the geostrophic winds to a level above the mountaintops. It is also suggested that converting to a height (z) coordinate system may reduce the erroneous downward momentum transfer found in the present terrain‐following grid.

Investigation of Vertical and Horizontal Momentum Transfer in the Gulf of Mexico Using Empirical Mode Decomposition Method

Abstract Data from a series of deep mooring stations in the Gulf of Mexico (GOM) have been analyzed with the newly developed empirical mode decomposition and Hilbert spectral analysis method, abbreviated as Hilbert–Huang transformation (HHT). The flows in the GOM near the shelf/slope region are treated as a two-layer system, with the 800-m permanent thermocline as the dividing depth. When the data are treated with HHT, motions of different temporal scales are identified. The top layer (depth less than 800 m) is controlled by inertia flow with episodic Loop Current eddies, while the lower layer (depth greater than 800 m) is controlled primarily by the topographic Rossby waves and small-scale cyclonic and anticyclonic eddies. Using a cross-correlation analysis between the appropriate intrinsic mode components from the data, the wavelength, the phase velocity, and the vertical trapping depth for the topographic Rossby waves were determined. Observations are in general agreement with the modeled results by Oey and Lee.

Drag-Induced Transfer of Horizontal Momentum between Air and Raindrops

Abstract A theoretical model for unsteady drag-induced transfer of horizontal momentum between air and raindrops in moderate to heavy rainfall is presented. The model accounts for a two-way coupling in which the relative horizontal motion between air and raindrops appears as a drag forcing in both the air and raindrop equations of motion. Analytical solutions of these coupled equations are obtained for the case of rain falling through (i) an initial step change in environmental wind, (ii) a uniform shear profile, and (iii) periodically varying vertical shears of various wavenumbers (a crude proxy for turbulent eddies). Formulas for the propagation (descent) speeds of the shear zones are obtained for (ii), (iii), and for the later stage of (i). However, these speeds are generally quite small—on the order of a few centimeters per second even for heavy rainfall. More importantly, the solutions of (i) and (iii) indicate that the drag interaction leads to a decay of the velocity gradients. A formula for the e-folding decay time of the periodically varying shear profiles indicates that at small wavelengths, the smallest decay times are found for the smaller drops, but at large wavelengths, the smallest decay times are found for the larger drops. The decay times decrease with decreasing wavelength, and approach a value equal to the reciprocal of the product of the rainwater mixing ratio and a drag parameter in the limit of vanishing wavelength. For parameters typical of moderate to heavy rainfall, the small-scale decay times are on the order of a few minutes.

A Case Study of Severe Storm Development along a Dryline within a Synoptically Active Environment. Part I: Dryline Motion and an Eta Model Forecast

Through a case study approach the motion of a dryline (on 16 May 1991) within a synoptically active environment in the southern plains, along which severe storms ultimately developed, is examined in detail. Observations from research aircraft, surface mesonetwork stations, mobile ballooning vehicles, radar, wind profilers, and operational surface and upper air networks are examined and combined. Additionally, output from the operational mesoscale Eta Model is examined to compare predictions of dryline motion with observations and to aid in interpretation of observations. The dryline on this day advanced rapidly eastward and included formation of a bulge; additionally, in at least two instances it exhibited redevelopment (loss of definition at one location and gain at another). Aircraft observations revealed that an eastward redevelopment occurred in the early afternoon and was characterized by a series of four “steps” along the western edge of the boundary layer moisture. The westernmost and easternmost steps coincide with the locations of the dryline before and after redevelopment, respectively. The retreat of the dryline in the central and southern portion of the analysis domain in the late afternoon included both continuous motion and redevelopment toward the west-northwest. This dual-mode retreat of the dryline was accompanied by gradual backing of the winds and moistening in low levels. The Eta Model forecast initialized at 1200 UTC produced dryline features that were qualitatively similar to observed fields. The eastward motion of a broad area of enhanced moisture gradient agreed well with observations following an initial spinup period. A north–south moisture convergence axis preceded the rapid eastward motion of the dryline by several hours. Lack of subsidence in the air behind the modeled dryline leads to the conclusion that processes other than downward transfer of horizontal momentum by larger-scale motions (that would support eastward advection) produced the rapid dryline motion and observed eastward dryline bulge. Results of diagnosing physical processes affecting model dryline motion point toward boundary layer vertical mixing coupled with advection of dry air aloft as vital components in rapid advance of the dryline eastward in this synoptically active case.