Vertical Velocity Profiles Research Articles

Evidence that Horizontal Moisture Advection Regulates the Ubiquitous Amplification of Rainfall Variability over Tropical Oceans

Abstract The column moist static energy (MSE) budget equation approximates the processes associated with column moistening and drying in the tropics, and is therefore predictive of precipitation amplification and decay. We use ERA-Interim (ERA-I) and TRMM 3B42 data to investigate day-to-day convective variability and distinguish the roles of horizontal MSE (or moisture) advection versus vertical advection, sources, and sinks. Over tropical convergence zones, results suggest that horizontal moisture advection is a primary driver of day-to-day precipitation fluctuations; when drying via horizontal moisture advection is smaller (greater) than Chikira’s “column process,” precipitation tends to amplify (decay). In the absence of horizontal moisture advection, precipitation tends to increase spontaneously almost universally through a positive column process feedback. This bulk positive feedback is characterized by negative effective gross moist stability (GMS), which is maintained throughout the tropical convergence zones. How this positive feedback is achieved varies geographically, depending on the shape of vertical velocity (omega) profiles. In regions where omega profiles are top-heavy, the effective GMS is negative primarily owing to strong feedbacks between convection and diabatic MSE sources (radiative and surface fluxes). In these regions, vertical MSE advection stabilizes the atmosphere (positive vertical GMS). Where omega profiles are bottom-heavy, by contrast, a positive feedback is primarily driven by import of MSE through a shallow circulation (negative vertical GMS). The diabatic feedback and vertical GMS are in a seesaw balance, offsetting one another. Our results suggest that ubiquitous convective variability is amplified by the same mechanism as moisture-mode instability.

On the Influence of the Beta Effect on the Spectral Characteristics of Unstable Perturbations of Ocean Currents

Based on the equation for potential vorticity in the quasi-geostrophic approximation, an analysis of stable and unstable perturbations of ocean currents of a finite transverse scale with a vertical linear velocity profile (Couette-type flows) is presented. The model takes into account the influence of vertical diffusion of buoyancy, friction, and the beta effect (the change in the Coriolis parameter with latitude). The analysis is based on the small perturbation method. The problem depends on several physical parameters and reduces to solving a spectral non-self-adjoint problem for a fourth-order equation with a small parameter at the highest derivative. Asymptotic expansions of eigenfunctions and eigenvalues are constructed for small values of the wavenumber $$k$$ . Using the continuation in the parameter $$k$$ , trajectories of the eigenvalues are calculated, which made it possible to compare the influence of the beta effect on unstable perturbations of the first and higher order modes. It is shown that the flow instability depends in a complex way on the physical parameters of the flow.

Effects of Friction and Buoyancy Diffusion on the Dynamics of Geostrophic Oceanic Currents with a Linear Vertical Velocity Profile

A spectral problem of the Orr–Sommerfeld type is considered to describe unstable disturbances of oceanic geostrophic currents with a linear vertical velocity profile by taking into account the vertical diffusion of buoyancy and friction. Numerical solutions are obtained for different values of the dimensionless parameters of the problem. Calculations of the spectra of eigenvalues and growth rates are compared with those of a similar problem for an ideal fluid. It is shown that (a) dissipation expands the range of wave numbers of unstable disturbances, (b) dissipation can increase the growth rates of baroclinic disturbances, (c) disturbances driven by the instability of the critical layer can grow faster than baroclinic disturbances, and (d) currents with a width equal to or less than the Rossby radius can be unstable; nearly circular (axisymmetric) unstable disturbances (submesoscale eddies) can develop in narrow currents or frontal zones.

A laboratory experiment of rip currents between the ends of breaking wave crests

abstract In order to investigate the rip current formed between the ends of breaking wave crests, a laboratory experiment was conducted over a bathymetry with an equilibrium profile. For the experiment, pseudo intersecting wave trains were produced in a wave basin, where regular waves were generated by running two parts of wave maker out of phase. The pseudo intersecting wave trains were compared with the intersecting wave trains, which are formed by the interference of two wave trains with slightly different directions and are the cause of the rip current, by conducting numerical simulations using FUNWAVE. The rip current developments between the ends of breaking wave crests through their node regions were confirmed similarly in both numerical simulations. In the hydraulic experiment using the pseudo intersecting wave trains, the rip current flow was observed by utilizing Large Scale Particle Image Velocimetry and Acoustic Doppler Velocimetry, and the wave height and the mean surface were measured. From the observations, it was confirmed that the rip current was developed through the node region with relatively low wave height, and it was found that its strength was varied by the refraction and dissipation of the ends of wave crests due to the wave-current interaction. The horizontal and vertical velocity profiles of the rip current were presented in the present experiment. The comparison between the measured results and the numerical results showed a reasonable agreement, however, also confirmed the limitation of the numerical model on breaking and mixing process. Using particle trajectories, it was observed that vortices were developed at the ends of breaking wave crests, which played a role of generating and accelerating the rip current.

Sediment Transport Model Quantifies Plume Length and Light Conditions From Mussel Dredging

Mussel dredging causes resuspension of sediment particles that reduce water clarity and potentially leads to reduced eelgrass growth. In order to study the impact of resuspension from mussel dredging on light conditions in the water column, field experiments were conducted at two sites in the Limfjorden. Light loggers were placed in two circular arrays around the dredge area. Vertical profiles of current velocity were measured by an ADCP and the sediment particle size composition was obtained from sediment core samples. The field data was used to force, calibrate and validate a sediment transport model developed in the FlexSem model system. Changes in sediment concentrations during and after mussel dredging were modelled for the two sites and for seven scenarios. We found that the distance and direction of the plume in the model was in good agreement with light logger data. The plume duration was less than 1 hour, and the impact range was between 260-540 m. The scenarios showed that fishing intensity and current speeds were most important for shaping the sediment plumes. Changes in suspended sediment concentrations were 0.62 to 1.79 mg/l on median average and 1.22 to 11.61 mg/l for the upper quantile of the plume, which were on the same order of magnitude as background values in the Limfjorden. The amount of fishing days during the eelgrass growth season was 6-8% in Lovns Bredning and 16-35% in Logstor Bredning and less than 1-2% of the total area was dredged per season. Even though there are substantial changes in the light conditions from the sediment plumes, the overall spatio-temporal impact in the study area is considered low. We recommend that management plans in other areas could sustain a shellfish fishery by limiting fishing intensity and frequency near eelgrass beds. The presented approach combines observational data, sediment transport modelling and reported fishing activity. It is a step forward within sediment transport modelling and could be incorporated into environmental impact assessments. The results have recently been used as scientific background for recommendations to improve the management plans according to the Danish Mussel Policy and relevant EU Directives.

Assessing the Horizontal Homogeneity of the Atmospheric Boundary Layer (HHABL) Profile Using Different CFD Software

One of the main factors affecting the reliability of computational fluid dynamics (CFD) simulations for the urban environment is the Horizontal Homogeneity of the Atmospheric Boundary Layer (HHABL) profile—meaning the vertical profiles of the mean streamwise velocity, the turbulent kinetic energy, and dissipation rate are maintained throughout the streamwise direction of the computational domain. This paper investigates the preservation of the HHABL profile using three different commercial CFD codes—the ANSYS Fluent, the ANSYS CFD, and the Siemens STAR-CCM+ software. Three different cases were considered, identified by their different inlet conditions for the inlet velocity, turbulent kinetic energy, and dissipation rate profiles. Simulations were carried out using the RANS k-ε turbulence model. Slight variations in the eddy viscosity models, as well as in the wall boundary conditions, were identified in the different software, with the standard wall function with roughness being implemented in the Fluent applications, the scalable wall function with roughness in the CFX applications, and the blended wall function option in the STAR-CCM+ simulations. There was a slight difference in the meshing approach in the three different software, with a prism-layer option in the STAR-CCM+ software, which allowed a finer mesh near the wall/ground boundary. The results show all three software are able to preserve the horizontal homogeneity of the ABL—less than 0.5% difference between the software—indicating very similar degrees of accuracy.

The mechanical origin of snow avalanche dynamics and flow regime transitions

Abstract. Snow avalanches cause fatalities and economic damage. Key to their mitigation is the understanding of snow avalanche dynamics. This study investigates the dynamic behavior of snow avalanches, using the material point method (MPM) and an elastoplastic constitutive law for porous cohesive materials. By virtue of the hybrid Eulerian–Lagrangian nature of the MPM, we can handle processes involving large deformations, collisions and fractures. Meanwhile, the elastoplastic model enables us to capture the mixed-mode failure of snow, including tensile, shear and compressive failure. Using the proposed numerical approach, distinct behaviors of snow avalanches, from fluid-like to solid-like, are examined with varied snow mechanical properties. In particular, four flow regimes reported from real observations are identified, namely, cold dense, warm shear, warm plug and sliding slab regimes. Moreover, notable surges and roll waves are observed peculiarly for flows in transition from cold dense to warm shear regimes. Each of the flow regimes shows unique flow characteristics in terms of the evolution of the avalanche front, the free-surface shape, and the vertical velocity profile. We further explore the influence of slope geometry on the behavior of snow avalanches, including the effect of slope angle and path length on the maximum flow velocity, the runout angle and the deposit height. Unified trends are obtained between the normalized maximum flow velocity and the scaled runout angle as well as the scaled deposit height, reflecting analogous rules with different geometry conditions of the slope. It is found that the maximum flow velocity is mainly controlled by the friction between the bed and the flow, the geometry of the slope, and the snow properties. We reveal the crucial effect of both flow and deposition behaviors on the runout angle. Furthermore, our MPM modeling is calibrated and tested with simulations of real snow avalanches. The evolution of the avalanche front position and velocity from the MPM modeling shows reasonable agreement with the measurement data from the literature. The MPM approach serves as a novel and promising tool to offer systematic and quantitative analysis for mitigation of gravitational hazards like snow avalanches.

Performance and Wake Characterization of a Model Hydrokinetic Turbine: The Reference Model 1 (RM1) Dual Rotor Tidal Energy Converter

The mechanical power and wake flow field of a 1:40 scale model of the US Department of Energy’s Reference Model 1 (RM1) dual rotor tidal energy converter are characterized in an open-channel flume to evaluate power performance and wake flow recovery. The NACA-63(4)-24 hydrofoil profile in the original RM1 design is replaced with a NACA-4415 profile to minimize the Reynolds dependency of lift and drag characteristics at the test chord Reynolds number. Precise blade angular position and torque measurements were synchronized with three acoustic Doppler velocimeters (ADV) aligned with each rotor centerline and the midpoint between the rotor axes. Flow conditions for each case were controlled to maintain a hub height velocity, uhub= 1.04 ms−1, a flow Reynolds number, ReD= 4.4 × 105, and a blade chord length Reynolds number, Rec= 3.1 × 105. Performance was measured for a range of tip-speed ratios by varying rotor angular velocity. Peak power coefficients, CP= 0.48 (right rotor) and CP= 0.43 (left rotor), were observed at a tip speed ratio, λ= 5.1. Vertical velocity profiles collected in the wake of each rotor between 1 and 10 rotor diameters are used to estimate the turbulent flow recovery in the wake, as well as the interaction of the counter-rotating rotor wakes. The observed performance characteristics of the dual rotor configuration in the present study are found to be similar to those for single rotor investigations in other studies. Similarities between dual and single rotor far-wake characteristics are also observed.

Vertical current structure in a macro-tidal, well mixed Sundarban ecosystem, India

The present study reports on the vertical structure of the current profile of the Jharkhali of Sundarban ecosystem. The clay embankments constructed along the Sundarbans for coastal protection are facing severe erosion problem due to tidal current, extreme water levels, storm surges, overtopping and breaches in embankments which lead to widespread flooding. The understanding of the current profile of Sundarban aquatic system with respect to depth of water column is essential for suitable engineering design for embankment. It will be used for conservation of coastal resources and inhabitants of the Sundarbans from the natural disasters. The tidal range during the present study was 6.1 m and 5 tidal constituents viz. M2, S2, K1, O1, and N2 are predominant at Jharkhali. The minimum (114.55 cm/s) and maximum (134.45 cm/s) current speed were observed at bottom and mid depth respectively. The maximum speed at bottom is also due to longitudinal salinity gradients, which may induce larger velocities in the lower part of the water column. The vertical velocity profiles during flood are characterized by a near bed maximum. The maximum speed usually occurs during mid-flood and mid-ebb periods and also varying with moon phase, usually maximum and minimum speed were observed during spring tide and neap tide respectively. This trend is persisting in the entire water column from the surface to bottom. The direction of the current is controlled by orientation of the river channel and adjacent tributaries. In general, water travels towards North and WNW direction during flood tide and E & ESE direction during ebb tide. Interestingly, about 80-degree difference in current direction was noticed between surface and bottom current with clockwise spiral movement.

Experimental investigation of the interaction between near-surface atmospheric boundary layer winds and downburst outflows

This article focuses on experimental investigation of the interaction between downburst (DB) and near-surface atmospheric boundary layer (ABL) winds. The flow field of DB immersed into ABL winds (DBABL) is compared against the outflow produced by an isolated DB without ABL winds. The diameter of investigated downdrafts was 3.2 ​m. The study demonstrates that there is a profound difference between the radial velocity components in the DBABL and DB outflows in terms of peak and mean velocities, vertical profile of radial velocity, as well as the overall temporal signature of the velocity records. The asymmetry of the DBABL outflow in the along-ABL wind direction is similar to that observed in real traveling downbursts. The turbulence intensity in the near-surface DBABL winds can exceed 40% in some parts of the outflow where the downburst propagates against ABL winds. The DBABL wind profile is characterized by smaller nose-shape curvature than the DB winds in the part of the outflow where downburst and ABL winds have the same direction. The steady-state ABL wind segments and transient downburst segments in the DBABL velocity records are similar to anemometer records of real downbursts. The paper also shows that the velocity addition DBABL→≠DB→+ABL→ throughout the outflow.

A Compact, Flexible, and Robust Micropulsed Doppler Lidar

AbstractThis work details a master oscillator power amplifier (MOPA) microjoule-class pulsed coherent Doppler lidar system configuration designed to measure line-of-sight wind velocities and backscatter intensity of atmospheric aerosols. The instrument is unique in its form factor. It consists of two physically separated modules connected by a 10 m umbilical cable. One module hosts the transceiver, which is composed of the telescope, transmit/receive (T/R) switch, and high-gain optical amplifier, and is housed in a small box (34.3 cm × 34.3 cm × 17.8 cm). The second module contains the data acquisition system and several electro-optical components. This form factor enables deployments on platforms that are otherwise inaccessible by commercial and research instruments of similar design. In this work, optical, electrical, and data acquisition components and configurations of the lidar are detailed and two example deployments are presented. The first deployment describes measurements of a controlled wildfire burn from a small aircraft to measure vertical plume dynamics and fire inflow conditions during summer in Florida. The second presents measurements of the marine boundary layer height and vertical velocity and variance profiles from the Research Vessel (R/V) Thomas Thompson. The new instrument has enabled greater flexibility in field campaigns where previous instruments would have been too costly or space prohibitive to deploy.

Investigating the turbulent flow behaviour through partially distributed discontinuous rigid vegetation in an open channel

AbstractRiparian vegetation does not only affect the channel flow carrying capacity, but also plays significant roles in water management, stream restoration, and river rehabilitation. This study numerically investigates the flow characteristics through longitudinally discontinuous rigid vegetation occupying half width of the channel, with the help of three‐dimensional software FLUENT in which a Reynolds stress turbulence model was adopted. Three varying conditions of vegetation were considered comprising of vertically double‐layered vegetation (DLV), submerged vegetation (SV), and emergent vegetation (EV) while keeping the same vegetation density, as well as a varying discharge condition against DLV was also tested. The results indicated that the flow distribution becomes more complex through DLV and SV followed by multiple layers with an inflectional instability in the vertical velocity profile around submerged canopy top, as compared to the flow through EV where the uniform distribution of flow over the canopy column was observed. The velocity in the canopy zone decreased considerably because of the resistance due to vegetation, which influenced the channel carrying capacity, in comparison to that in the non‐vegetated zone. The flow velocities through the obstructed part of the channel, that is, canopy zone, in DLV arrangement reduced by a percentage difference of approximately 42 and 37% compared to that of SV and EV arrangements, respectively; whereas it was reduced by approximately 55% when the discharge was twice while keeping the same configuration of DLV. The inflectional instabilities and estimated mixing layer over the interfacial zone suggested a stronger lateral exchange of momentum for DLV configuration in comparison to that of SV and EV. Within the gaps between the patch zones, the flow velocity, turbulent kinetic energy, and turbulent intensity reduced significantly due to blockage effect and sheltering offered by the vegetation patches, signifying a positive flow response towards aquatic life and sediment deposition.

The Cyclonic Eddy Train in the Indian Ocean Sector of the Southern Ocean as Revealed by Satellite Radar Altimeters and In Situ Measurements

AbstractWe investigated dynamic ocean topography (DOT) to reveal the oceanic subpolar circulation and possible pathway of modified Circumpolar Deep Water (CDW) in the Indian Ocean sector of the Southern Ocean. Satellite radar altimeter data sets were employed to develop monthly DOT in both open water and ice‐covered areas throughout the course of a year. A 0.2° × 0.2° gridded DOT was reconstructed for investigation of the oceanic structure at fine scale. The reconstructed DOT exhibited a “cyclonic eddy train” consisting of the Vincennes eddy, Poinsett eddy, and Sabrina eddies. The relationship between the depth of each eddy and the wind curl showed negative correlations for the Vincennes eddy and Sabrina eddies during wintertime only. Therefore, winds are not the main drivers of the eddies. A hydrographic section in 2017 clearly revealed the vertical structure of the Vincennes eddy. The absolute velocity estimated by the DOT and in situ geostrophic velocity showed equatorward Antarctic Bottom Water (AABW) transport at 6–8 cm s−1 on the western side and poleward transport of CDW at 3–4 cm s−1 on the eastern side of the Vincennes eddy. The net transports of the AABW and CDW layers in the Vincennes eddy were 0.6 ± 0.4 Sv (equatorward) and 0.5 ± 0.36 Sv (poleward), respectively. The order of magnitude of the AABW current speed was confirmed by in situ mooring data. The vertical profiles of absolute velocity suggest the possibility of estimating CDW transport using a satellite altimeter.

Modelling the Dynamics of Granular Particle Interactions in a Vortex Reactor using a Coupled DPM-KTGF Model

In this work, a shear-driven two-phase particulate flow of monodispersed and polydispersed granular materials has been studied experimentally and numerically as a function of solids concentration and restitution coefficients for different operating speeds N (70 -130 rpm) in a lab-scale rotor-stator agglomeration reactor. A coupled computational fluid-particle dynamics (CFPD) model was developed consisting of a steady-state flow field of the continuous phase coupled to a transient particle tracking of the discrete phase. This was achieved via a one-way coupling between the continuous and the discrete phase by including the effect of drag, lift, pressure gradient, virtual mass forces, as well as granular collisional forces in describing the particle-particle, particle-wall and the fluid-particle interactions. The spatiotemporal evolution of the flow pattern, discrete phase properties, and influence of the operating conditions on the granular properties were characterized. The validation of the numerical model developed in this study was carried out based on the theoretical analysis of the rotor-stator flow and the PIV flow measurements. The results showed that the particle sizes were uniformly distributed within the reactor after steady-state conditions, while a small region of high particle concentration was observed near the rotor due to low vorticity and turbulent intensity around the region. In terms of the operating conditions, the restitution coefficients and the operating speeds do not have a significant influence on the granular properties apart from the small region around the shaft where there is a correlation between these parameters. The particle sizes, however, show a positive correlation with the granular properties. Also, a wider particle size distribution was observed axially towards the stator, which might be attributed to the pumping effect of the Batchelor flow in this direction. It was also concluded that the discrete phase velocity does not seem to vary significantly with the restitution coefficients. Furthermore, the vertical velocity and vorticity profiles give a reasonably good agreement between the CFPD model predictions and PIV measurements. The minor observed deviations were mainly due to some of the experimental limitations rather than the robustness of the CFPD model or the numerical code.

Vertical Velocity Profiles in Convectively Coupled Equatorial Waves and MJO: New Diagnoses of Vertical Velocity Profiles in the Wavenumber–Frequency Domain

Abstract A new diagnostic framework is developed and applied to ERA-Interim to quantitatively assess vertical velocity (omega) profiles in the wavenumber–frequency domain. Two quantities are defined with the first two EOF–PC pairs of omega profiles in the tropical ocean: a top-heaviness ratio and a tilt ratio. The top-heaviness and tilt ratios are defined, respectively, as the cospectrum and quadrature spectrum of PC1 and PC2 divided by the power spectrum of PC1. They represent how top-heavy an omega profile is at the convective maximum, and how much tilt omega profiles contain in the spatiotemporal evolution of a wave. The top-heaviness ratio reveals that omega profiles become more top-heavy as the time scale (spatial scale) becomes longer (larger). The MJO has the most top-heavy profile while the eastward inertio-gravity (EIG) and westward inertio-gravity (WIG) waves have the most bottom-heavy profiles. The tilt ratio reveals that the Kelvin, WIG, EIG, and mixed Rossby–gravity (MRG) waves, categorized as convectively coupled gravity waves, have significant tilt in the omega profiles, while the equatorial Rossby (ER) wave and MJO, categorized as slow-moving moisture modes, have less tilt. The gross moist stability (GMS), cloud–radiation feedback, and effective GMS were also computed for each wave. The MJO with the most top-heavy omega profile exhibits high GMS, but has negative effective GMS due to strong cloud–radiation feedbacks. Similarly, the ER wave also exhibits negative effective GMS with a top-heavy omega profile. These results may indicate that top-heavy omega profiles build up more moist static energy via strong cloud–radiation feedbacks, and as a result, are more preferable for the moisture mode instability.

Experimental and numerical investigations of spatially-varying dispersion tensors based on vertical velocity profile and depth-averaged flow field

The dispersion coefficient is the most crucial parameter that governs the model prediction of pollutant mixing in rivers. However, the spatial variability of dispersion coefficient associated with velocity structures has not yet been thoroughly investigated. In this study, two types of spatially varying dispersion tensors were considered using both the vertical velocity profiles (DF) and the depth-averaged flow fields (DA), and the pollutant transports using DF and DA were compared with tracer test results in a meandering channel. The results showed that the peak concentration and arrival time were more accurately predicted by using DF than DA, because the flow properties associated with the shear induced dispersion were explicitly incorporated in the DF. The spatial distributions of the DF showed variations in both stream-wise and span-wise directions due to the creation and destruction of secondary flow in the meandering channel. In contrast, the DA only changed in the stream-wise direction. Therefore, the DF is advantageous to reproduce shear dispersion processes.

Field measurements and wind tunnel investigation of wind characteristics at a bridge site in a Y-shaped valley

It is essential in a bridge design to study the wind parameters at the bridge site, particularly in a complex mountain area where the wind characteristics vary from location to location. The aim of this paper is to study the wind characteristics in a Y-shaped valley in mountainous terrain, the site of a proposed long-span bridge. Data recorded continuously for almost two years by a Doppler Sodar observation system installed at the bridge site was analysed. The results show that the terrain-induced effect plays a significant role in the wind direction variation and there are regular changes in characteristics depending on the direction. To supplement the full-scale measurements, wind tunnel tests were also carried out using a high-precision terrain model at a scale of 1:1500. Vertical profiles of velocity and turbulence at key points along the bridge, and the lateral profile along the bridge axis were studied. The results from the different test directions showed the influence of the incoming flow field particularly shielding and channelling effects which contribute to wind deceleration and acceleration at the bridge location respectively. Furthermore, the speed-up of the valley and mountaintop is evident and there is greater acceleration at the mountaintop.

Observation of Turbulent Mixing Characteristics in the Typical Daytime Cloud-Topped Boundary Layer over Hong Kong in 2019

Turbulent mixing is critical in affecting urban climate and air pollution. Nevertheless, our understanding of it, especially in a cloud-topped boundary layer (CTBL), remains limited. High-temporal resolution observations provide sufficient information of vertical velocity profiles, which is essential for turbulence studies in the atmospheric boundary layer (ABL). We conducted Doppler Light Detection and Ranging (LiDAR) measurements in 2019 using the 3-Dimensional Real-time Atmospheric Monitoring System (3DREAMS) to reveal the characteristics of typical daytime turbulent mixing processes in CTBL over Hong Kong. We assessed the contribution of cloud radiative cooling on turbulent mixing and determined the altitudinal dependence of the contribution of surface heating and vertical wind shear to turbulent mixing. Our results show that more downdrafts and updrafts in spring and autumn were observed and positively associated with seasonal cloud fraction. These results reveal that cloud radiative cooling was the main source of downdraft, which was also confirmed by our detailed case study of vertical velocity. Compared to winter and autumn, cloud base heights were lower in spring and summer. Cloud radiative cooling contributed ~32% to turbulent mixing even near the surface, although the contribution was relatively weaker compared to surface heating and vertical wind shear. Surface heating and vertical wind shear together contributed to ~45% of turbulent mixing near the surface, but wind shear can affect up to ~1100 m while surface heating can only reach ~450 m. Despite the fact that more research is still needed to further understand the processes, our findings provide useful references for local weather forecast and air quality studies.

Reducing of Bend Scour by Setting Spurs in a Curved Channel

In Taiwan, the rivers not only are fast-flowing with high discharge, but they badly erode their beds during the typhoon seasons. In addition, erosion on the concave bank in a meandering channel is the primary cause of levee break. Therefore, the study conducted a down-scale experiment on erosion induced by oblique flow in a laboratory. It was similar to number 27–34 cross sections of the Fengshan river of Hsinchu County, Taiwan. The region was chosen because there are some special attacking angles of water flow and historical precedents of levee break. The study adopted the discharges of return periods of 10 and 20 years and measured the flow field by laser doppler velocimetry (LDV). Then the protective effects with different spur types were examined. The results indicate that increasing velocity induces side erosion when the flow impacts with the adjacent angle on the concave bank. However, the decreasing of velocity causes deposition of sediment on the concave bank. Furthermore, based on the vertical velocity profile of water flow, a higher flow rate is measured in the downstream on the concave bank. After the spurs are installed, the velocity at the spurs in the downstream is reduced, and the cross section with the larger velocity is moved to upstream. In addition, after setting the spurs, the reduction rates in volume of scour are 7.97% of a 10 year return period and 4.65% of a 20 year return period, respectively. That demonstrates the scour is effectively reduced as long as the spurs are set. Although the erosion mitigation rate and protection effect are decreased when the velocity is high, there is still a good protection effect at the bank. The setting of spurs has the following effects: First, the maximum scour depth generates in the front spur, while the maximum scour position keeps away from the bank. Then, the overall flow rate can be reduced to approximately 35%–40% comparing with the original flow field. Lastly, the spur on the slope of 1/30 degrees demonstrated the best function of stretching the distance from the embankment.

Evaluation of a method to retrieve temperature and wind velocity profiles of the Venusian nightside mesosphere from mid-infrared CO2 absorption line observed by heterodyne spectroscopy

We evaluated a method for retrieving vertical temperature and Doppler wind velocity profiles of the Venusian nightside mesosphere from the CO2 absorption line resolved by mid-infrared heterodyne spectroscopy. The achievable sensitive altitude and retrieval accuracy were derived with multiple model spectra generated from various temperature and wind velocity profiles with several noise levels. The temperature profiles were retrieved at altitudes of 70–100 km with a vertical resolution of 5 km and a retrieval accuracy of ± 15 K. The wind velocity was also retrieved at an altitude of approximately 85 km with a vertical resolution of 10 km and a retrieval accuracy of ± 25–50 m/s. In addition, we studied an event and applied our method to spectra obtained by the HIPWAC instrument attached to the NASA/IRTF 3-m telescope on May 19–22, 2012. Retrieved wind velocities in a latitude of 33° S at 3:00 LT were interpreted as subsolar-to-antisolar (SS-AS) flows at altitudes of 84 ± 6 km and 94 ± 7 km, and they were stronger than expected. This result suggested that the transition between the retrograde superrotational zonal (RSZ) wind and SS-AS flow may occur at altitudes below 90 km which previously was predicted to be the transition region. This work provides a basis for our analysis of further observations obtained by a mid-infrared heterodyne spectrometer MILAHI attached to the Tohoku University 60-cm telescope at Haleakalā, Hawaii.