Vertical Exchange Processes Research Articles

Methodology for evaluating the effectiveness of system solutions in the oil and gas industry: approaches and practices

This article is dedicated to the study of the degree of development of the system approach in literary sources. The work highlights the issue that the system approach and its individual elements are not sufficiently and fully utilised in activities related to the development of oil reservoirs. The essence of the system approach is identified, stating that it is a method based on considering individual processes and objects in the development of an oil field as interacting and closely interrelated elements. The object of the study is the system approach and the methodology of assessment in its use. Methodology: research of scientific-methodological literature, methods of inductive-deductive reasoning, analysis, synthesis, abstraction, and theoretical generalization. In accordance with the reviewed sources, the following conclusions are drawn: the development and operation of a specific field are carried out according to the type of two-circuit system controlled by enterprise workers and a research institute; it is determined that the most promising approach would be the use of 3D hydrodynamic models, which allow for the consideration of data on vertical exchange processes or filtration flows; the proposed EPS concept minimises inaccuracies in the field of oil and gas science and has a positive impact on the stability and hierarchy of knowledge of subsurface formations, the formation of 3D models based on this, and the accuracy of calculating reserves.

Identifying hydraulic connectivity among the vadose zone, unconfined and confined aquifers in the thick loess deposits using multiple tracers

Low water flux and a thick vadose zone increase the likelihood that water bearing units are hydraulically disconnected in shallow groundwater systems. Particularly for the thick loess deposit of China, hydraulic connectivity among the vadose zone, unconfined and confined aquifers is still unclear since it is controversial whether the unconfined groundwater is recharged by precipitation or confined groundwater. Taking several loess tablelands as example, we collected precipitation, soil water, unconfined and confined groundwater to measure the contents of multiple tracers (e.g., hydrochemistry, δD, δ18Owater, δ17Owater, δ34S, δ18Osulfate, δ13C and 14C) to evaluate hydraulic connectivity among the vadose zone, unconfined and confined aquifers. Deep soil water can be recharged by precipitation with intensities ≥ 30–40 mm/day, and then vertically discharge to unconfined water primarily through matrix flow with an infiltration velocity of 0.15 m/year. Matrix flow dominates the water flux from the vadose zone to the unconfined aquifer with the rates of 85.4 ± 39.6 mm/year, reflecting the regional hydraulic equilibrium. The unconfined and confined aquifers exhibit negligible vertical hydraulic connectivity because of different recharge periods, flow patterns, water − rock interaction, water residence time and sulfate evolution behaviors. The identified connectivity supports local precipitation as the recharge source, which conflicts with the external source hypothesis. The utilization of multiple tracers in this study can complement each other to provide more comprehensive results, and the results greatly improve the conceptualization of vertical water exchange processes in the loess deposits.

Important role of turbulent wind gust and its coherent structure in the rapid removal of urban air pollution

The removal process of urban air pollution is very rapid even under the strong obstruction of various buildings in the city. In this study, based on high-accuracy ultrasonic anemometer observations at seven different altitudes on a 325 m tower, we proposed a physical mechanism in which turbulent wind gust and its coherent structure may play important roles in this phenomenon. Pollutants suspended within the urban canopy layer UCL cannot be quickly removed by means of basic flow but diffuse and crosse the UCL by means of the coherent structure of gust wind and then propagate further under strong basic flows above the UCL. With a continuous increase in wind speed in the whole layer, combined with gust-wind and turbulence mixing, the rapid transformation from heavily polluted days to clean days in urban areas can take place in just a few hours. Turbulent wind gust and its coherent structure were extracted by third-order Daubechies wavelet transformation. The downwind wind was decomposed into three separate components, basic flow gust-wind and turbulence The results show that in the early rapid drop of urban surface pollutant concentration, horizontal wind speed at 8 m was still low, and sinking movements dominated in the lower layer. Friction velocity calculated from gust-wind significantly affected the surface PM2.5 concentration, and was the most obvious factor with an inverse correlation coefficient −0.56. The correlation coefficient of coherent structure of gustiness Ru g w g at 8 m basically changed to positive values before the rapid decline of the surface PM2.5 concentration. The concentration of PM2.5 decreased significantly when Ru g w g > 0, and exhibited an increasing trend when Ru g w g < 0. The close relationship between the concentration of PM2.5 and Ru g w g indicated the important role of the coherent structure of gustiness in the rapid removal process of urban pollutants. The research deepens the vertical exchange process of matter and energy in the UCL, and provides a scientific reference for the rapid dissipation mechanism of urban air pollution.

Horizontal and vertical spreading of dye in the coastal ocean of the northern Mid-Atlantic bight

The paper investigates the horizontal and vertical spreading of a neutrally-buoyant chemical tracer (rhodamine) that was released into the surface mixed layer in the coastal ocean. Rhodamine concentrations from the dye release experiment were combined with observations of water column properties and velocity measurements from the high-frequency radar (HFR) and ship-based ADCP in order to investigate and interpret the downward penetration of dye from the mixed layer into the stratified part of the water column and to quantify the horizontal diffusivity associated with the lateral dye spreading. Of the total amount of dye encountered during the 33-h long survey, 99% was observed within the mixed layer, suggesting that the downward penetration of dye from the mixed layer into the pycnocline is a slow process that occurs on the time scale of at least multiple days. The observed elevated dye concentrations below the mixed layer were always co-located with the elevated concentrations within the mixed layer. However, the distribution of dye below the mixed layer was patchy, and the below-mixed-layer dye concentrations were poorly correlated with those within the mixed layer. Combined with the absence of strong vertical shear of horizontal velocity across the base of the mixed layer, this suggests that the observed dye could have been ejected from the mixed layer earlier and at a different location, and then advected to the observed location by the horizontal currents below the mixed layer. The vertical diffusivity in the mixed layer is estimated to be approximately 7x10−4m2s, about two orders of magnitude larger than the diffusivity across the base of the mixed layer, which is 5x10−6m2s. Lateral spread of dye was found to be well represented by the HFR velocities (with roughly 2-km resolution) with the added small-scale turbulent diffusivity of 5 m2s.Plain Language Summary.The top part of the water column, called the surface mixed layer, is well mixed to a nearly uniform density. Below lays the pycnocline where density changes rapidly with depth. The exchange of water and properties between the mixed layer and pycnocline is an important but poorly understood oceanographic process and is the main subject of the current observational study. Specifically, this paper describes and interprets the initial 36 h of the vertical and horizontal spreading of a dye plume, released into the surface mixed layer of the coastal ocean south of Martha's Vineyard, MA. Our results indicated that the ejection and penetration of dye down from the mixed layer is a slow process that occurs over a time scales longer than multiple days. The episodic nature of the downward dye ejection from the mixed layer and the resulting patchy distribution of the subsurface dye is another important finding from this work, which has far-reaching implications for the parameterization of these small-scale vertical exchange processes in numerical models. The vertical diffusivity within the mixed layer (7x10−4m2s) is estimated to be about two orders of magnitude larger than the diffusivity across the base of the mixed layer (5x10−6m2s). Finally, the lateral spreading of the dye plume was found to be well represented by advection by the high-frequency-radar-based velocities with the added small-scale turbulent diffusivity of 5 m2s – the value that is in line with our prior estimates for this coastal region, but is larger than many prior estimates from different regions of the ocean.

Современные исследования динамики вод в северо-западной части Черного моря по данным LADCP

In this work a result of preliminary analysis of current measurements collected in fourteen large-scale expeditions of the Marine Hydrophysical Institute in 2016–2019 in the north-eastern part of the Black Sea is presented. It is shown that the use of the Lowered Acoustic Doppler Current Profiler (LADCP) significantly increases the information content of the expedition materials, opening up new opportunities in the experimental study of water dynamics. Based on the materials of the expeditions, it was found that the upper layer of shear baroclinic currents in the measurement area is limited by the isopycnic depth with a potential density of ~16.75 kg/m3 (~350 m). At this depth, the variability of the buoyancy frequency with the depth changes from a power-law to an exponential dependence. The same depth limits the penetration of seasonal variations of the average kinetic energy into the water column. The highest values of the average kinetic energy are observed in April-May and November–December, the lowest—in June–August. The ratio of the maximum energy values (summer) to its minimum values (spring, late autumn, winter) was ~3. The Rim Black Sea Current is most pronounced near the Crimean Peninsula in the spring and winter seasons. In summer and autumn, the horizontal distribution of the current velocity in depths of 30–80 m is less structured, which is caused by the seasonal intensification of mesoscale processes. At a number of stations, the current velocity profiles contain fragments with a well-defined harmonic variability of the current velocity components with depth, which can be interpreted as the manifestation of near-inertial internal waves. In most cases, the rotation of the current velocity vector with depth occurs clockwise, which determines the main direction of propagation of the inertial internal waves into the water column. The example of one of the stations shows the significant contribution of such waves to the value of shear. This fact suggests a significant influence of near-inertial internal waves on the processes of vertical turbulent exchange in the water column.

Numerical simulation of turbulent mixing and transport of biochemical substances in inland waters

Numerical modeling of inland water objects (lakes and reservoirs) and, in particular, mixing processes and transport of biochemical substances in these basins is considered. A three-dimensional hydrostatic model and a one-dimensional model LAKE based on averaging of three-dimensional equations over a horizontal section of a reservoir are used as tools for the calculations. In the LAKE model, seiche oscillations are taken into account by parameterizing the pressure gradient and horizontal eddy viscosity. A k-ε closure is used to describe the vertical exchange processes in both models. A parameterization of the Prandtl number is implemented in the 3D model which allows turbulence to persist at any values of the gradient Richardson number. The 3D model is also supplemented by equations for calculating biochemical substances by analogy with the one-dimensional biochemistry equations used in the LAKE model, which, in particular, allow us to study the transport of substances such as oxygen (O2) and methane (CH4). Numerical experiments aimed at studying turbulent mixing and transport of substances are carried out.

Numerical Models of the Vertical Turbulent Exchange in Stably Stratified Water Body: I. Mathematical Models

Improved second-order mathematical models are developed for description of the processes of vertical turbulent exchange in stably stratified water body. The models are based on algebraic representations of Reynolds stresses and fluxes and the use of a differential transport equation of dispersion of fluctuations of the vertical velocity component. A numerical model of the vertical turbulent exchange under simultaneous stratification in temperature and salinity is developed. In the case of stable stratification defined by only a variation in salinity, role ofWeinstock modification of the relaxation time scale of a scalar field is evaluated.

Evaluation of fast atmospheric dispersion models in a regular street network

The need to balance computational speed and simulation accuracy is a key challenge in designing atmospheric dispersion models that can be used in scenarios where near real-time hazard predictions are needed. This challenge is aggravated in cities, where models need to have some degree of building-awareness, alongside the ability to capture effects of dominant urban flow processes. We use a combination of high-resolution large-eddy simulation (LES) and wind-tunnel data of flow and dispersion in an idealised, equal-height urban canopy to highlight important dispersion processes and evaluate how these are reproduced by representatives of the most prevalent modelling approaches: (i) a Gaussian plume model, (ii) a Lagrangian stochastic model and (iii) street-network dispersion models. Concentration data from the LES, validated against the wind-tunnel data, were averaged over the volumes of streets in order to provide a high-fidelity reference suitable for evaluating the different models on the same footing. For the particular combination of forcing wind direction and source location studied here, the strongest deviations from the LES reference were associated with mean over-predictions of concentrations by approximately a factor of 2 and with a relative scatter larger than a factor of 4 of the mean, corresponding to cases where the mean plume centreline also deviated significantly from the LES. This was linked to low accuracy of the underlying flow models/parameters that resulted in a misrepresentation of pollutant channelling along streets and of the uneven plume branching observed in intersections. The agreement of model predictions with the LES (which explicitly resolves the turbulent flow and dispersion processes) greatly improved by increasing the accuracy of building-induced modifications of the driving flow field. When provided with a limited set of representative velocity parameters, the comparatively simple street-network models performed equally well or better compared to the Lagrangian model run on full 3D wind fields. The study showed that street-network models capture the dominant building-induced dispersion processes in the canopy layer through parametrisations of horizontal advection and vertical exchange processes at scales of practical interest. At the same time, computational costs and computing times associated with the network approach are ideally suited for emergency-response applications.

Characteristics of vertical exchange process in the Pearl River estuary

Being an estuary with the occasional appearance of hypoxia, the Pearl River estuary was selected to investigate its vertical exchange process by a validated 3-D numerical model. The concept of vertical exchange time was used to quantify the vertical exchange process and was calculated by adding a conservative tracer to the hydrodynamic model. The results revealed vertical exchange time has significant seasonal variation modulated by seasonally changed external forcing, and has similar structure with spatial variation in salinity. During the wet season, remarkably long vertical exchange times (&amp;gt;50 days) occurred at the lower estuary under 10 m depth, which was coincident with the frequently reported hypoxic zone. The entire Pearl River estuary was occupied by water mass with a vertical exchange time of &amp;lt;5 days during the dry season, which indicated the estuary's health condition during this period. Further analyses revealed the seasonal trend of vertical exchange time was closely related with salinity stratification and river runoff in which the lowest vertical exchange times can be estimated by linear regressions in certain stages. Tide modulated these exchange times in different time scales. Spring tide better mixed the water column and thus facilitated the vertical exchange process compared with neap tide. As a result, minimum vertical exchange time during spring tide was about 10 days shorter than that during neap tide. Flood-ebb tide also modulated vertical exchange time in the Pearl River estuary. Vertical exchange time varies with the tide and reached minimum and maximum values during valley and high tides, respectively.

The roles of MCDW and deep water iron supply in sustaining a recurrent phytoplankton bloom on central Pennell Bank (Ross Sea)

During January–February 2011 standing stocks of phytoplankton (chl a) in the Pennell Bank region of the Ross Sea were variable over 10–100km spatial scales. One area of elevated chl a on central Pennell Bank (CPB) appeared to be a recurrent mid-summer feature. The western flank (WF) of Pennell Bank had pronounced signatures of Modified Circumpolar Deep Water (MCDW). We evaluated the spatial extent of Fe limitation and net primary production and tested whether MCDW may provide elevated amounts of Fe to the CPB region, through a combination of in situ measurements, shipboard incubations and a horizontally resolved physical model. Regional fluxes of dissolved Fe from deep to surface waters were compared to calculated Fe demands. Low in situ variable to maximum fluorescence (Fv/Fm; 0.24–0.37) and surface water dissolved Fe concentrations (~0.12–0.21nM) were suggestive of widespread limitation, corroborated by the consistent responses (Fv/Fm, growth, and nutrient removal ratios) of incubation treatments to Fe addition. MCDW from the WF region had lower dissolved Fe concentrations than that measured in CDW (Circumpolar Deep Water), which suggests on-shelf modification with Fe deplete surface waters and is consistent with the lack of stimulation due to incubation amendments with filtered MCDW. Model results and empirical data suggest MCDW from the WF region is further modified and mixed en route to the CPB region, leading to both the erosion of the canonical MCDW signature and an elevated dissolved Fe inventory of CPB region deep water. This suggests the addition of Fe possibly via diagenesis, as suggested by Marsay et al. (2014). Calculated deep water supply rates to the surface waters of CPB were ~0.18–0.43md−1, while calculated rates at the WF or northern Pennell Bank (NPB) regions were negative. The CPB populations exhibited ~4.5-fold higher net production rates compared to those in the WF and NPB regions and required 520–3200nmolFem−2d−1. The modeled vertical supply rates seem to provide ~2–15% of the estimated Fe requirement. Since this flux is based on subsurface dissolved Fe inventories, it does not account for any bioavailable Fe from deep water particulate sources or for Fe recycling in the upper water column. These data suggest the recurrent productivity hotspots at CPB are not fueled by Fe-rich MCDW but are partially supported by the delivery of Fe through vertical exchange processes.

The impact of valley geometry on daytime thermally driven flows and vertical transport processes

The influence of valley geometry on thermally driven flows is studied by means of high‐resolution simulations. An idealized valley–plain topography and a spatially constant but time‐dependent surface sensible heat flux are used to generate upslope, upvalley and plain‐to‐mountain winds. A systematic variation of valley depth, width and length induces differences in the cross‐ and along‐valley flow field and thermal structure of the boundary layer. The deeper the valley, the stronger the upvalley winds and the more favoured the formation of vertically stacked circulation cells and an elevated valley inversion layer. Upvalley winds become weaker for wide valleys. The development of plain‐to‐mountain circulations increases vertical exchange processes between the boundary layer and the free atmosphere considerably, compared with vertical transport processes over a plain. The analysis of mass‐flux budgets and forward trajectories indicates that mass is transported three to four times more effectively from the surface to the free atmosphere over valleys than over flat terrain. Vertical transport processes are strongest for deep and narrow valleys.

Modelling-based assessment of suspended sediment dynamics in a hypertidal estuarine channel

We investigate the dynamics of suspended sediment transport in a hypertidal estuarine channel which displays a vertically sheared exchange flow. We apply a three-dimensional process-based model coupling hydrodynamics, turbulence and sediment transport to the Dee Estuary, in the north-west region of the UK. The numerical model is used to reproduce observations of suspended sediment and to assess physical processes responsible for the observed suspended sediment concentration patterns. The study period focuses on a calm period during which wave-current interactions can reasonably be neglected. Good agreement between model and observations has been obtained. A series of numerical experiments aim to isolate specific processes and confirm that the suspended sediment dynamics result primarily from advection of a longitudinal gradient in concentration during our study period, combined with resuspension and vertical exchange processes. Horizontal advection of sediment presents a strong semi-diurnal variability, while vertical exchange processes (including time-varying settling as a proxy for flocculation) exhibit a quarter-diurnal variability. Sediment input from the river is found to have very little importance, and spatial gradients in suspended concentration are generated by spatial heterogeneity in bed sediment characteristics and spatial variations in turbulence and bed shear stress.

Intensity of thunderstorm-generated turbulence revealed by large-eddy simulation

Thunderstorms are characterized by turbulent processes that constitute an important aviation hazard and cause vertical transport of atmospheric constituents. Turbulence occurs within cloud and in the surrounding clear air, but, despite its importance, the characteristics of thunderstorm-generated turbulence and its spatial distribution are poorly understood, especially outside of cloud. Here we use large-eddy simulation to characterize turbulence generated by a canonical thunderstorm. The simulation identifies regions of notable three-dimensional anisotropic turbulence more than 5 km above the storm, in a shallow layer above the storm's anvil, and a horizontally asymmetric pattern of weaker turbulence that extends more than 50 km horizontally away from the cloud. Our results provide the first continuous estimate of turbulence intensity in and around thunderstorms and represent a major step toward improved turbulence avoidance methods. The results have broader implications for understanding the fundamental aspects of how thunderstorms affect their environment through vertical exchange processes.

Using timescales to interpret dissolved oxygen distributions in the bottom waters of Chesapeake Bay

A simplified conceptual model based on timescales of gravitational circulation, vertical exchange, and total oxygen consumption rate of the biochemical processes is presented to provide insight into the relationships between estuarine dynamics and bottom water dissolved oxygen (DO). Two dimensionless parameters are introduced to diagnose the relationship between the vertical exchange process and the biochemical DO consumption and the influence of gravitational circulation on replenishment of bottom DO. The relative magnitudes of these timescales provide a linkage between the physical and biochemical processes. The hypoxic and anoxic conditions in deep waters of Chesapeake Bay are successfully interpreted with these three proposed timescales. Because the Bay is a long estuary, the replenishment of the bottom DO due to gravitational circulation diminishes as the bottom water travels farther upstream. The bottom DO is mainly modulated by the vertical exchange process in the middle and upper portions of the Bay. In addition to other physical processes that affect vertical exchange, wind and freshwater are the major predictors of the vertical exchange time. The model is applicable to Chesapeake Bay and other estuaries with persistent gravitational circulation if the dimensionless parameters can be appropriately estimated.

Linking dynamics of transport timescale and variations of hypoxia in the Chesapeake Bay

[1] Dissolved oxygen (DO) replenishment in the bottom waters of an estuary depends on physical processes that are significantly influenced by external forcings. The vertical exchange time (VET) is introduced in this study to quantify the physical processes that regulate the DO replenishment in the Chesapeake Bay. A 3-D numerical model was applied to simulate the circulation, VET, and DO. Results indicate that VET is a suitable parameter for evaluating the bottom DO condition over both seasonal and interannual timescales. The VET is negatively correlated with the bottom DO. Hypoxia (DO 23 days) can account for 77% of variations of hypoxic volume in the main Bay. The VET cannot explain all the DO variations as it can only account for the contribution of physical processes that regulate DO replenishment. It is found that the short-term vertical exchange process is highly controlled by the wind forcing. The VET volume decreases when the high-speed wind events are frequent. The summertime VET volume is less sensitive to short-term variations (pulses) of river discharge. It is sensitive to the total amount of river discharge and the high VET volume can be expected in the wet year.

A New Sensor Platform for Investigating Turbulence in Stratified Coastal Environments

Abstract Characterizing and quantifying vertical exchange processes is essential for understanding physical and biological dynamics in stratified lakes and oceans. Unfortunately, the role of mixing is still poorly understood because of the challenges of conducting field research on small-scale turbulence, especially in the vicinity of a thermocline. This study presents a new moored sensor platform that was designed to investigate small-scale turbulence structures in time and space. The objective is to determine all terms of the turbulent energy equation separately and simultaneously. The platform is equipped with a microstructure package for measuring shear, temperature, and temperature gradients, as well as with a vertical array of high-precision thermistor probes and acoustic Doppler velocimeters. The platform can be moved vertically in the water column using a bottom-resting winch that is connected to a shore station by a 1800-m-long communication cable. This cable allows real-time data access and control of the winch, and thus optimization of the measurement strategy. A field study in Lake Geneva, located between Switzerland and France, shows that this system is ideally suited for the analysis of the dynamics of baroclinic motions, such as internal Kelvin waves. First results indicate a clear relationship between low Richardson number and elevated dissipation, and suggest a mean flux Richardson number Rf = 0.14 ± 0.4. However, this measurement campaign was not as conclusive for the reason of the variability of Rf.

Significant downward trend of SO2 observed from 2005 to 2010 at a background station in the Yangtze Delta region, China

SO2 is an important gas in atmosphere with great environmental and climate implications. SO2 emission in China has been receiving great attention as the economy grows and the amount of coal consumption has increased in the past few decades. SO2 has been observed from July 2005 to June 2010 at Linan WMO GAW regional station (30.3 °N, 119.73 °E, 138 m a.s.l.) located in the Yangtze Delta region in eastern China. These observation data are analyzed to understand the trend of regional SO2 background concentration. Strict quality controls are conducted to ensure the temporal comparability of the data. Significant downward trend with −2.4 ppb/yr (P < 0.0001) of surface SO2 is observed from 2005 to 2010, especially after 2008. The average concentration of SO2 from July 2005 to June 2008 is 14.2±3.1 ppb, which is slightly higher than the mean values of 13.5±5.1 ppb during 1999–2000 and is two folds of the average value (7.1±3.1 ppb) from July 2008 to June 2010. More than 50% of the SO2 has been cut down after 2008 in the Yangtze Delta region due to the implementation of stricter emission control measures. The peak SO2 concentration appears around 10 o’clock in the morning after 2009 while appears at night before 2009. These diurnal variations of SO2 might indicate that after 2009, more SO2 is from the vertical exchange process than from the local accumulation.

Frequency dispersion of losses during acoustic sounding of water in Lake Baikal

The processes of horizontal and vertical water exchange play the most important role in the ecosys� tem of Lake Baikal providing the transport of organic and inorganic matter including oxygen in the entire volume of water up to the limiting depths. The investi� gation of the main processes of water transport in Lake Baikal based on the traditional methods of episodic profiling from research ships does not supply sufficient information about many inhomogeneous and nonsta� tionary processes. The difficulties are also related to the insufficient sensitivity of the existing instruments for measuring the vertical motion of water whose velocity usually does not exceed a few fractions of a millimeter per second. An infrastructure was created within the project of the Baikal neutrino telescope (1), and new technolo� gies were developed that make it possible to set up sys� tems of longterm multiparameter monitoring of dif� ferent processes in the lake including precise acoustic measurements. The initial goal of the investigations presented in this paper was to develop the methods of acoustic tomography (2) as a tool for investigation of the hydrophysical processes in Lake Baikal. It is possi� ble to study multiscale processes using acoustic path� ways with a length from tens of meters to tens of kilo� meters. The preliminary results of acoustic sounding in the water under ice over basic lengths on the order of hun� dreds of meters based on pulse signals were published in (3, 4). The key idea of the method is to measure the intrapulse phase distortions, which allows us not only to increase the sensitivity (as during typical phase measurements) but also to take into account the dis� persion distortions of the signals related to the depen� dence of attenuation on frequency. The amplitude, delay, and phase of the pulse signal and also the addi� tional phase modulation that results from the fre� quency dependence of attenuation in the signal band were recorded in the experiments. This dependence is manifested in the slope of the amplitudefrequency characteristic of the channel. In this case, the spec� trum of the amplitudemodulated pulse no longer remains symmetric with respect to the carrier fre� quency, which is manifested as additional intrapulse phase modulation. We note that this effect for the pulse narrow band signals is the effect of the first order of magnitude with respect to the bandwidth, while the usual dispersion (the dependence of the signal propa� gation velocity on frequency) leads to the effects of the second order with respect to the bandwidth. The appearance of additional phase modulation is not manifested on the envelope curve of the received sig� nal, but an additional quadrature component propor� tional to the derivative of the inphase component whose amplitude is proportional to the slope of the amplitudefrequency characteristic appears during detection by a synchronous phase detector.

Vertical air mass exchange driven by the local circulation on the northern slope of Mount Everest

To better understand vertical air mass exchange driven by local circulation in the Himalayas, the volume flux of air mass is estimated in the Rongbuk Valley on the northern slope of Mount Everest, based on a volume closure method and wind-profiler measurements during the HEST2006 campaign in June 2006. Vertical air mass exchange was found to be dominated by a strong downward mass transfer from the late morning to late night. The average vertical air volume flux was 0.09 m s−1, which could be equivalent to a daily ventilation of 30 times the enclosed valley volume. This vertical air mass exchange process was greatly affected by the evolution of the South Asian summer monsoon (SASM), with a strong downward transfer during the SASM break stage, and a weak transfer during the SASM active stage.

Sensitivity study of the effects of wave-induced vertical mixing on vertical exchange processes

The hydrodynamic model COHERENS-SED, developed by the present authors through introducing wave-enhanced bottom stress, wave dependent surface drag coefficient, wave-induced surface mixing, SWAN to COHERENS, is modified to account for wave-induced vertical mixing. The COHERENS-SED model can also be used for one-dimensional, two-dimensional, three-dimensional current and salinity calculations. One-dimensional model and three-dimensional model are used to study the effects of the wave-induced vertical mixing. The horizontal current velocity profiles obtained by the model are in good agreement with the analytical velocity profiles under the same input conditions. Numerical results show that higher wave height would generally generate larger vertical eddy viscosity and lower horizontal velocity. The results for fresh water in Yellow River Delta show that the wave-induced vertical mixing increases the momentum of fresh water transferring ability downwards to seabed and salt water's mixing with upper fresh water. Fresh water flume length is compressed considerably.