Downwind Boundary Research Articles

Initiation and evolution of urban-induced precipitation under different background wind speeds: Roles of urban breeze circulation and cold pool

Urban-enhanced precipitation is often observed far downwind of cities, suggesting important roles of background wind. This study examines the impacts of background wind on the initiation and evolution of urban-induced precipitation through idealized ensemble large-eddy simulations with different initial background wind speeds of 0, 1, 2, 3, 4, and 5 m s−1. Without background wind, a strong urban breeze circulation (UBC) initiates a relatively large number of cumulus clouds in the urban area, which exhibit greater cloud top heights than those in the rural area. The early-initiated clouds do not produce precipitation but moisten the middle troposphere, assisting later-initiated clouds to develop into deep convective clouds and produce concentrated precipitation in the urban area. As the background wind speed increases, the UBC weakens. Also, the high-humidity area generated by early-initiated clouds is continuously advected away from the urban area. Consequently, the precipitation initiation is delayed as the background wind speed increases. Precipitating clouds are advected downwind and generate a cold pool, and updrafts are repeatedly produced at the downwind boundary of the cold pool. The downwind-advected clouds are fed by the newly produced updrafts, being a long-lived organized precipitation system that produces a considerable amount of precipitation in the downwind area. This feeding effect is most prominent for a moderate background wind (3 m s−1), causing the precipitation amount and duration to be maximized. For stronger background winds, precipitation is initiated by the mechanical lifting of the warm air of the urban boundary layer over the cool rural boundary layer around sunset.

Impacts of background wind on the interactions between urban breeze circulation and convective cells: Ensemble large‐eddy simulations

AbstractUrban breeze is produced by the temperature difference between urban and surrounding rural areas, and its interactions with other atmospheric phenomena are of great interest. In this study, the impacts of background wind on the interactions between urban breeze circulation (UBC) and convective cells are numerically investigated. For this, a large number of idealized ensemble simulations are performed using the Weather Research and Forecasting‐large‐eddy simulation model, where the initial background wind speed varies from 0 to 5 m·s−1 in 0.5 m·s−1 intervals. As the background wind speed increases, the location of the strongest updraft moves from the urban center to the downwind boundary of the urban area. This is associated with different interactions between the UBC and convective cells under different background wind speeds. For no background wind, the UBC is relatively strong and its center is located at the urban center, producing a strong updraft there. Convective cells are advected toward the UBC center and merge with the strong updraft, further intensifying it. For weak background wind, the UBC is relatively weak and its center is located near the downwind urban boundary. Convective cells passing the urban area merge with each other and intensify, and after they pass the downwind urban boundary, the strong vertical wind shear in the right part of the UBC tilts and weakens them. For strong background wind, the UBC is very weak and convective cells are advected mainly by the strong background wind. These changes in the characteristics of convective cells in and around the urban area with background wind speed may lead to the changes in the preferred location of cloud initiation and cloud development.

Trace volatile compounds in the air of domestic waste landfill site: Identification, olfactory effect and cancer risk

Landfill sites are regarded as sources of volatile compounds (VOCs) and odors emitted to the atmosphere. Surface emissions of VOCs and odors were investigated in a rural domestic waste landfill site located in southwest China. A total of 76 chemical compounds belonging to 3 chemical families were identified and quantified. The total number of VOCs (TVOC) detected ranged from 18.1 to 806.3 mg/m3, while odorous gases and greenhouse gases ranged from 0.4 to 21.2 and 0–100.5 mg/m3, respectively. High emissions were found in the air surrounding the leachate storage pool (LSP) and dumping area (DPA). The dominant species of VOCs were hexaldehyde, m-xylene, propylene oxide, acetophenone, and 2-butanone. The traceability analysis showed that the odors and VOCs diffused to the downwind boundary mainly came from the DPA and LSP. According to the olfactory effect analysis and cancer risk assessment, the main odor-causing gaseous pollutants were hydrogen sulfide, propionic acid, styrene, and 2-pentanone, while benzene, trichlorethylene, and 1,3-butadiene were the dominant carcinogens. This study provides new insights into the emission characteristics, olfactory effects, and cancer risks of VOCs and odors emitted from rural domestic solid waste landfill sites.

Nonlinear amplification of ocean waves in straits

Using the exact Hasselmann equation, we study wind-driven deep-water ocean waves in a strait with the wind directed orthogonally to the shore. The strait has “dissipative” shores with no reflection from the shorelines. We show that the evolution of wave turbulence can be divided into two different regimes in time. During the first regime, waves propagate with the wind, and the wind-driven sea can be described by self-similar solutions of the Hasselmann equation. The second regime starts after a sufficiently significant accumulation of wave energy at the downwind boundary. From this instant, an ensemble of waves propagating against the wind starts to form. Moreover, waves orthogonal to the wind arise and propagate along the strait. The wave system eventually reaches an asymptotic stationary state in which two types of wave motion coexist: an ensemble of self-similar waves propagating with the wind and quasimonochromatic waves propagating almost orthogonally to the wind direction and tending to slant against the wind at the angle of 15° with respect to the shore of turbulence origination. These “secondary waves” arise only as a result of an intensive nonlinear wave interaction. The total wave energy exceeds its expected value approximately by a factor of two compared with the energy calculated in the absence of shores. We expect that this amplification increases substantially in the presence of reflective shores. We propose calling this “secondary” laser-like mechanism “nonlinear ocean wave amplification” (abbreviated NOWA).

Geomorphic and sediment volume responses of a coastal dune complex following invasive vegetation removal

ABSTRACTThis paper documents application of an established geostatistical methodology to detect significant changes in a foredune–transgressive dune complex where Parks Canada Agency (PCA) implemented a dynamic restoration program to remove invasive marram grasses (Ammophila spp.) and enhance dynamic dune habitat for an endangered species. Detailed topographic surveys of a 10 320 m2 site in the Wickaninnish Dunes in Pacific Rim National Park, British Columbia, Canada for the first year post‐treatment are compared to a pre‐restoration LiDAR baseline survey. The method incorporates inherent spatial structure in measured elevation datasets at the sub‐landscape scale and models statistically significant change surfaces within distinct, linked geomorphic units (beach, foredune, transgressive dune complex). Seasonal and annual responses within the complex are discussed and interpreted.All geomorphic units experienced positive sediment budgets following restoration treatment. The beach experienced the highest differential volumetric change (+1656 m3) and net sediment influx (+834 m3, 0 · 19 m3 m–2) mostly from supply to the supratidal beach and incipient dune. This sediment influx occurred independent of the restoration effort and was available as a buffer against wave erosion and as supply to the landward dunes. The foredune received +200 m3 (0 · 13 m3 m‐2) and its seaward profile returned to a similar pre‐restoration form following erosion at the crest from vegetation removal and scarping by high water events. Sediment bypassing and minimal change was evident at the mid‐stoss slope with appreciable extension of depositional lobes in the lee. The transgressive dune complex experienced high accretion following restoration activity (+201 m3) and over the year (+284 m3, 0 · 07 m3 m–2) mostly from depositional lobes from the foredune, precipitation ridge growth along the downwind boundary, and growth of existing lobes within the complex. Further integration of this methodology to detect significant geomorphic changes is recommended, particularly for applications where sampling densities are limited or logistically defined. Copyright © 2013 John Wiley & Sons, Ltd.

Single SnO2 gas sensor as a practical tool for evaluating the efficiency of odor control engineering at food waste composting plants

The objective of this study was to evaluate the feasibility of using a single tin oxide (SnO2) gas sensor as a simple and reliable tool for evaluating the efficiency of odor control engineering at food waste composting plants, by correlating sensor responses with chemical concentrations of critical odorants and olfactometric data obtained under laboratory and field conditions. Three critical odorants, including dimethylsulfide, trimethylamine and acetic acid, were prepared in various concentrations, ranging from parts per billion, to parts per million levels. Field samples were collected from two large food waste composting plants in Taiwan. The results indicated that the sensor responses showed significant linear correlation with the chemical concentration of the three target odorants (P<0.01) and with the olfactometric data for these odorants at various concentrations (P<0.01). The correlation coefficients were all above 0.940. For field odor measurement, the SnO2 gas sensor responses showed a good linear correlation with the olfactometric data for samples inside the composting plants, at exhaust outlets and at downwind boundaries (P<0.01), and the correlation coefficient was 0.963; the coefficient of variation (CV%) of the sensor for triplicate measurements was 0.9–8.4%.

Endotoxin Levels at Swine Farms Using Different Waste Treatment and Management Technologies

Concentrated animal feeding operations (CAFOs) are a major source of airborne endotoxins, which are air pollutants that can cause adverse health effects to both on-site farmers and neighbors. Release of airborne endotoxins to the environment can be reduced using proper waste treatment and management technologies. In this study, the levels of endotoxins released from two swine CAFOs using conventional lagoon-sprayfield technology were compared to those of 15 farms using various alternative waste management technologies in North Carolina. Over a 2-year period, 236 endotoxin samples were collected from the 17 farm units and analyzed using the Limulus amebocyte lysate (LAL) test. Concentrations of airborne endotoxins near barn exhaust fans were significantly higher than at the upwind boundary of the farm and at other farm sites. For most of the study sites, mean concentrations of endotoxins at the downwind boundary of the farm were higher than those at the upwind boundary of the farm, indicating the release of endotoxins from swine CAFOs to the neighboring environment. Endotoxin levels were significantly associated with concentrations of airborne bacteria but not fungi. Environmental factors, such as temperature, relative humidity, and wind velocity, affected the levels of airborne endotoxins at the farms. Based on the ratios of airborne endotoxins in downwind and upwind samples from the farm units, at least five different alternative waste management technologies significantly reduced the release of endotoxins from swine CAFOs. These results suggest that swine CAFOs are important sources of airborne endotoxins, the levels of which can be reduced by applying more robust and effective waste management technologies.

Ambient and indoor particulate aerosols generated by dairies in the southern High Plains

The objectives were to quantify and size ambient aerosolized dust in and around the facilities of 4 southern High Plains dairies of New Mexico and to determine where health of workers might be vulnerable to particulate aerosols, based on aerosol concentrations that exceed national air quality standards. Ambient dust air samples were collected upwind (background) and downwind of 3 dairy location sites (loafing pen boundary, commodity, and compost field). The indoor milking parlor, a fourth site, was monitored immediately upwind and downwind. Aerosolized particulate samples were collected using high-volume sequential reference air samplers, laser aerosol monitors, and cyclone air samplers. The overall (main effects and estimable interactions) statistical general linear model statement for particulate matter (PM10; particulate matter with an aerodynamic diameter of up to 10 μm) and PM2.5 resulted in a greater mean concentration of dust in the winter (PM10=97.4±4.4μg/m3; PM2.5=32.6±2.6μg/m3) compared with the summer (PM10=71.9±5.0μg/m3; PM2.5=18.1±1.2μg/m3). The upwind and downwind boundary PM10 concentrations were significantly higher in the winter (upwind=64.3±9.5μg/m3; downwind=119.8±13.0μg/m3) compared with the summer (upwind=35.2±7.5μg/m3; downwind=66.8±11.8μg/m3). The milking parlor PM10 and PM2.5 concentration data were significantly higher in the winter (PM10=119.5±5.8μg/m3; PM2.5=55.3±5.8μg/m3) compared with the summer (PM10=88.6.0±5.8μg/m3; PM2.5=21.0±2.1μg/m3). Personnel should be protected from high aerosol concentrations found at the commodity barn, compost field, and milking parlor during the winter.

The pollution characteristics of odor, volatile organochlorinated compounds and polycyclic aromatic hydrocarbons emitted from plastic waste recycling plants

Plastic waste treatment trends toward recycling in many countries; however, the melting process in the facilities which adopt material recycling method for treating plastic waste may emit toxicants and cause sensory annoyance. The objectives of this study were to analyze the pollution characteristics of the emissions from the plastic waste recycling plants, particularly in harmful volatile organochlorinated compounds, polycyclic aromatic hydrocarbons (PAHs), odor levels and critical odorants. Ten large recycling plants were selected for analysis of odor concentration (OC), volatile organic compounds (VOCs) and PAHs inside and outside the plants using olfactometry, gas chromatography–mass spectrometry and high performance liquid chromatography–fluorescence detector, respectively. The olfactometric results showed that the melting processes used for treating polyethylene/polypropylene (PE/PP) and polyvinyl chloride (PVC) plastic waste significantly produced malodor, and the odor levels at downwind boundaries were 100–229 OC, which all exceeded Taiwan’s EPA standard of 50 OC. Toluene, ethylbenzene, 4-methyl-2-pentanone, methyl methacrylate and acrolein accounted for most odors compared to numerous VOCs. Sixteen organochlorinated compounds were measured in the ambient air emitted from the PVC plastic waste recycling plant and total concentrations were 245–553 μg m −3; most were vinyl chloride, chloroform and trichloroethylene. Concentrations of PAHs inside the PE/PP plant were 8.97–252.16 ng m −3, in which the maximum level were 20-fold higher than the levels detected from boundaries. Most of these recycling plants simply used filter to treat the melting fumes, and this could not efficiently eliminate the gaseous compounds and malodor. Improved exhaust air pollution control were strongly recommended in these industries.

Investigation of Bioaerosols Released from Swine Farms using Conventional and Alternative Waste Treatment and Management Technologies

Microbial air pollution from concentrated animal feeding operations (CAFOs) has raised concerns about potential public health and environmental impacts. We investigated the levels of bioaerosols released from two swine farms using conventional lagoon-sprayfield technology and ten farms using alternative waste treatment and management technologies in the United States. In total, 424 microbial air samples taken at the 12 CAFOs were analyzed for several indicator and pathogenic microorganisms, including culturable bacteria and fungi, fecal coliform, Escherichia coli, Clostridium perfringens, bacteriophage, and Salmonella. At all of the investigated farms, bacterial concentrations at the downwind boundary were higher than those at the upwind boundary, suggesting that the farms are sources of microbial air contamination. In addition, fecal indicator microorganisms were found more frequently near barns and treatmenttechnology sites than upwind or downwind of the farms. Approximately 4.5% (19/424), 1.2% (5/424), 22.2% (94/424), and 12.3% (53/424) of samples were positive for fecal coliform, E. coli, Clostridium, and total coliphage, respectively. Based on statistical comparison of airborne fecal indicator concentrations at alternative treatment technology farms compared to control farms with conventional technology, three alternative waste treatment technologies appear to perform better at reducing the airborne release of fecal indicator microorganisms during on-farm treatment and management processes. These results demonstrate that airborne microbial contaminants are released from swine farms and pose possible exposure risks to farm workers and nearby neighbors. However, the release of airborne microorganisms appears to decrease significantly through the use of certain alternative waste management and treatment technologies.

Approaches to measuring fluxes of methane and nitrous oxide between landscapes and the atmosphere

The theory, applications, strengths and weaknesses of approaches commonly used for measuring trace gas fluxes are reviewed. Chambers, representing the smallest scale (∼1 m2), are the most common tools. Their operating principle is simple, they can be highly sensitive, the cost can be low and field requirements small. Problems include leaks, stickiness of some gases, inhibition of fluxes through concentration build-up, pressure effects and spatial and temporal variability in gas fluxes. Mass balance techniques are suitable for small, defined source areas, typically tens to thousands of square metres in extent. Emissions are calculated from the difference in the rates at which the gas is carried into a control volume above the source area by the wind and carried out. The required primary data are profiles of gas concentration on the downwind boundaries as well as the wind speed profile, the wind direction and the upwind background gas concentration. They have been used to measure gas emissions from landfills, treated fields and small animal herds. Circular test areas make the method independent of wind direction. A newly developed technique based on a backward Lagrangian stochastic dispersion model is also applicable to small, well-defined source areas of any shape. The surface flux is calculated form measurements of atmospheric turbulence and stability and the gas concentration at any height downwind. Implementation of the method is aided greatly by a software package WindTrax. The combination provides a powerful new tool for measuring gas emissions from treated areas and intensive animal production systems. Finally, techniques suitable for measuring gas emissions on large landscape scales (ha) are discussed. Eddy covariance is the micrometeorologist’s preferred technique for this scale. The method uses fast response anemometers and gas sensors to make direct measurements of the vertical gas flux at a point, several times a second. However, it is not feasible for many trace gases for a variety of reasons. These are discussed. Relaxed eddy accumulation is an alternative technique that retains the attraction of eddy covariance by providing a direct point measurement. It removes the need for a fast response gas sensor by substituting for it a fast solenoid valve sampling system. Flux–gradient methods are in more common use. Fluxes are calculated as the product of an eddy diffusivity and the vertical concentration gradient of the gas or the product of a transfer coefficient and the difference in gas concentration between two heights. Assumptions of the method and precautions in its application are discussed.

Open-Path Transmissometry to Determine Atmospheric Extinction Efficiency Associated with Feedyard Dust

Open-lot, concentrated animal feeding operations (CAFOs) in the southern High Plains, such as cattle feedyards and open-lot dairies, emit fugitive particulate matter (PM) that occasionally reduces downwind visibility. The long-path visibility transmissometer (LPV) can be used to measure changes in total atmospheric extinction, a direct measure of path-averaged visibility impairment. To our knowledge, no researchers have used transmissometry as a surrogate to estimate aerosol concentrations downwind of open-lot livestock facilities. We compare time-resolved PM mass concentrations (g m-3) and atmospheric extinction coefficients (km-1) measured simultaneously along the downwind boundary of a commercial cattle feedyard to compute extinction efficiency, the change in atmospheric extinction that results from a unit change in PM mass concentration. Expected values for the in-situ extinction efficiency of total suspended particulate (TSP) and its fraction less than 10 microns (PM10) aerodynamic equivalent diameter (AED) are 0.2 to 0.5 m2 g-1 and 0.4 to 0.8 m2 g-1, respectively. Determination of the atmospheric extinction efficiency of feedyard dust will enable transmissometry to be used as an intuitive, real-time surrogate for measuring time-averaged PM10 and/or TSP concentrations.

Measurement and modelling of air pollution and atmospheric chemistry in the U.K. West Midlands conurbation: Overview of the PUMA Consortium project

The PUMA (Pollution of the Urban Midlands Atmosphere) Consortium project involved intensive measurement campaigns in the Summer of 1999 and Winter of 1999/2000, respectively, in which a wide variety of air pollutants were measured in the UK West Midlands conurbation including detailed speciation of VOCs and major component analysis of aerosol. Measurements of the OH and HO 2 free radicals by the FAGE technique demonstrated that winter concentrations of OH were approximately half of those measured during the summer despite a factor of 15 reduction in production through the photolysis of ozone. Detailed box modelling of the fast reaction chemistry revealed the decomposition of Criegee intermediates formed from ozone–alkene reactions to be responsible for the majority of the formation of hydroxyl in both the summer and winter campaigns, in contrast to earlier rural measurements in which ozone photolysis was predominant. The main sinks for hydroxyl are reactions with NO 2, alkenes and oxygenates. Concentrations of the more stable hydrocarbons were found to be relatively invariant across the conurbation, but the impacts of photochemistry were evident through analyses of formaldehyde which showed the majority to be photochemical in origin as opposed to emitted from road traffic. Measurements on the upwind and downwind boundaries of the conurbation revealed substantial enhancements in NO x as a result of emissions within the conurbation, especially during westerly winds which carried relatively clean air. Using calcium as a tracer for crustal particles, it proved possible to reconstruct aerosol mass from the major chemical components with a fairly high degree of success. The organic to elemental carbon ratios showed a far greater influence of photochemistry in summer than winter, presumably resulting mainly from the greater availability of biogenic precursors during the summer campaign. Two urban airshed models were developed and applied to the conurbation, one Eulerian, the other Lagrangian. Both were able to give a good simulation of concentrations of both primary and secondary pollutants at urban background locations.

Three-dimensional numerical simulation of wind-driven flows in closed channels and basins

In the paper, a fully 3D finite-volume numerical model is developed and employed for the prediction of wind-induced flows in a regular channel and in a square basin with a complex bathymetry. Numerical results are compared with laboratory experiments.Numerical tests are then performed to investigate whether simplifying assumptions about the pressure distribution and the turbulente stresses representation can be employed in the simulation of wind-driven flows. The hydrostatic pressure assumption, resulting in the use of “quasi-3D” models, proved to be reasonably acceptable in order to obtain the vertical profile of the streamwise velocity component away from the boundaries. The quasi-3D model employed, nevertheless, provided incorrect velocity patterns near the upwind and downwind boundaries. The zero-equation turbulence model proposed by Tsanis [J. Hydraul. Div. ASCE 115 (1990) 1113] is also investigated by comparing the parabolic vertical profiles of the eddy viscosity coefficient assumed in this model with those obtained with a more refined k-ε model. The eddy viscosity coefficient obtained with the latter is in good agreement with the Tsanis' parabolic assumption in the most part of the studied domains. A strong overestimation of the eddy viscosity was however observed again at the upwind and downstream boundaries of the domains.

Wind‐generated transport of fictitious passive larvae into shallow tidal estuaries

Four 14‐day field experiments were conducted to determine the densities of postlarval white shrimp and blue crab megelopae moving into a tidal inlet along the south‐eastern United States. Certain winds directions were statistically associated with peaks in abundance, a result which motivated us to use a high resolution finite‐element numerical model to simulate passive larval transport under a variety of wind directions into the inlet. The passive particles were initially distributed uniformly in a zone of the continental shelf which extended 20 km offshore and 20 km alongshore in either direction. Each simulation was conducted for five tidal cycles (2.5 days) under constant wind stress. These simulations indicated that larvae are withdrawn from the continental shelf into the inlet from a narrow zone parallel to the shoreline but extending less that 5 km offshore. The withdrawal zone changed to one directly offshore of the inlet mouth only for a wind direction that pointed directly toward the inlet mouth. Under downwelling‐favourable winds, particles originating in the surface accumulate along the downwind boundary and drift shoreward with time thus causing a ‘pooling’ of larvae along the coast. This scenario is repeated with less efficiency for upwelling‐favourable winds with particles originating near bottom. The ‘pooling’ process occurs over the scale of the particle domain. A second and smaller scale is indicated by the relatively few particles which are withdrawn into the inlet as they pass inside the 7‐m isobath (within 5 km of the coast). Those that do pass become available for inlets farther downstream.

A mass balance method for non-intrusive measurements of surface-air trace gas exchange

A mass balance method is described for calculating gas production from a surface or volume source in a small test plot from measurements of differences in the horizontal fluxes of the gas across upwind and downwind boundaries. It employs a square plot, 24 m×24 m, with measurements of gas concentration at four heights (up to 3.5 m) along each of the four boundaries. Gas concentrations are multiplied by the appropriate vector winds to yield the horizontal fluxes at each height on each boundary. The difference between these fluxes integrated over downwind and upwind boundaries represents production. Illustrations of the method, which involve exchanges of methane and carbon dioxide, are drawn from experiments with landfills, pastures and grazing animals. Tests included calculation of recovery rates from known gas releases and comparisons with a conventional micrometeorological approach and a backward dispersion model. The method performed satisfactorily in all cases. Its sensitivity for measuring exchanges of CO 2, CH 4 and N 2O in various scenarios was examined. As employed by us, the mass balance method can suffer from errors arising from the large number of gas analyses required for a flux determination, and becomes unreliable when there are light winds and variable wind directions. On the other hand, it is non-disturbing, has a simple theoretical basis, is independent of atmospheric stability or the shape of the wind profile, and is appropriate for flux measurement in situations where conventional micrometeorological methods can not be used, e.g. for small plots, elevated point sources, and heterogeneous surface sources.

Dynamic Aspects of a Distinctly Three-Dimensional Mesoscale Convective System

The dynamics of a mesoscale convective system that matured in central Kansas on 3–4 June 1985 is investigated based upon data from ground-based dual-Doppler radar and other sources. The system was distinctly three-dimensional, evolving to a wavelike shape owing to the intersection of two convective bands. The two bands, one oriented north–south and the other east-north–west-southwest, are compared and contrasted with respect to their velocity, pressure, and buoyancy structure and the consequent attributes of their momentum transports and budgets. Dynamic retrieval of pressure and buoyancy from the wind fields provides insight into system structure and allows for the calculation of pressure gradients needed for the horizontal momentum budget. Several independent checks are carried out to ensure the quality of the pressure solution. The solution is found to be more accurate if the velocity time derivatives are included in the retrieval process. Dissimilar structure of the two bands is highlighted by a reversal of the low-level pressure gradient in the line-normal direction that can be related to the presence of a baroclinic zone along the more northern band. In both convective lines momentum fluxes at all levels are negative in the line-normal direction and positive along the line in agreement with the results of past studies. Line-parallel fluxes are comparable in magnitude to line-normal fluxes owing to the strong line-parallel shear. The calculated wind component tendencics resulting from the line-normal momentum budget for the north–south line include increases in rear-to-front momentum, at all altitudes, that is most pronounced at low and high levels, similar to results of past studies of lines oriented normal to the environmental shear. For the northeast–southwest line, increases in front-to-rear momentum are found at all levels except for a thin layer near the system top. This difference stems from a pronounced low-level pressure decrease toward the rear of the line. In the north–south line, budget calculations show an increase in along-line momentum with time at low and high levels and a decrease at midlevels. The results for the northeast–southwest band are quite different in that a decrease with time in along-line momentum is calculated at all levels, with a pronounced decrease at high levels owing to outward flux through the downwind boundaries. Along-line pressure gradients are quite weak in both convective bands. The significant influence of the horizontal flux divergence is attributed in part to the three-dimensional character of the system. The momentum budget results for each band are discussed in relation to maintenance of trailing stratiform precipitation, provision of favorable environments for severe weather, and their potential effects upon system evolution.

Novel meteorological methods for measuring trace gas fluxes

The paper deals with flux measurements in two contexts: small plots and plant canopies. Mass balance methods have been developed for small experimental plots with lateral dimensions of tens of metres rather than the 1 m typical of chambers or the hundreds of metres required for conventional micrometeorological estimates. The general method relies on the conservation of mass to equate the differences in horizontal fluxes across upwind and downwind boundaries of a test plot with the surface flux within the plot along the line of the wind. Applications to soil and animal experiments are discussed. Lagrangian descriptions of transport now supplant older, but inappropriate gradient-diffusion theory for inferring fluxes and source-sink distributions of scalars in plant canopies. An inverse Lagrangian theory due to M. R. Raupach provides a relatively simple observational and computational scheme for making such inferences from measurements of mean concentration profiles and canopy turbulence. The scheme and a range of applications are described.

Water Exchange Characteristics of Wind-Driven Density Current System in Semi-Enclosed Stratified Basins

In order to examine mixing and turbulent mass transport processes in a wind-driven density current, a turbulence model analysis is performed. The system considered here is a 2-D semi-enclosed stratified basin whose downwind boundary is opened to an ocean; this flow system is analogous to the wind-enerated currents in a small inlet, a bay, etc., Focus is placed on water mass exchange rate across the open boundary, which is found to have a strong functional dependency on the so-called “overall Richardson number”. The present analysis give us useful information on what sort of hydrodynamic conditions could generate upwelling of polluted bottom layer fluid and water quality exchange between the basin and the external water area.

Advection in the Coastal Hudson Bay Lowlands, Canada. II. Impact of Atmospheric Divergence on the Surface Energy Balance

A box model, contiguous with the coastline, 250 m in height, and with variable dimensions is employed to study the storage and the divergence/convergence of heat, water vapor, and mass in the internal boundary layer and to identify their impacts on the surface energy balance. Simultaneous measurements with tethersondes were made at the upwind and downwind boundaries of the box to facilitate these calculations. The analysis includes 2 d of onshore winds and 2 d of offshore winds. Storage terms are negligible and are ignored. The absolute magnitude of the horizontal divergence of latent heat was less than 5% that of sensible heat and the vertical divergence was less than 2% that of sensible heat. Thus the sensible heat flux predominates in divergence within the internal boundary layer. It always coincides in direction with the vertical flux of mass, whereas the latent heat flux is strongly influenced by the vapor pressure gradient and behaves more independently. During onshore winds, a well-developed thermal inversion of marine origin promotes a strong downward flux of mass and sensible heat near the coast. This entrains cold, moist air of marine origin into the surface boundary layer which leads to larger Bowen ratios by enhancing the sensible heat flux and diminishing the latent heat flux. This becomes particularly pronounced during the high sun period around solar noon and helps explain the exponential increase in Bowen ratios toward the coast during onshore winds. During offshore winds, there may be either vertical divergence or convergence near the coast. Divergence is due to backing up of the horizontal wind by denser marine air at the coast. Convergence is due to synoptic scale subsidence and it is hypothesized that, near the coast, it entrains cold marine air into the surface boundary layer. This can explain the observed increase in Bowen ratios toward the coast during offshore winds.