Earth's Boundary Layer Research Articles

The spatial distribution of deposited seeding material in the Earth boundary layer during weather modification

Cloud seeding projects may have the time scale of half a century and cover the planetary-scale surface. Such activities among the positive also have negative consequences that include environmental pollution. Year after year cloud seeding over certain areas could produce large amounts of seeding agents washed out in precipitation. The sampling of these deposits is therefore important, but not applied in large space and time scales due to a high cost. As an alternative, the cloud seeding project measurements may be used for finding the deposit spatial pattern and locations of its maximum. In this study, we established the method for finding the spatial distribution of deposited silver iodide over a selected area after hail suppression using the observed characteristics of seeded hailstorms. The estimation of the silver iodide deposit maximum is 155 μg m−2 during a 6-year period. Our findings agree well with those obtained from sampling silver content in precipitation during the other convective cloud seeding experiments. On the other hand, our method gives an answer of where to place the samplers, and hence more detailed chemical analysis and monitoring can be done in the future. The proposed methodology may be applied for any other target area and cloud seeding scenario.

Velocity shear instability and plasma billows at the Earth's magnetic boundary

The Kelvin-Helmoltz instability (KH) with formation of vortices appears in a wide variety of terrestrial, interplanetary, and astrophysical contexts. We study a series of iterated rolled-up coherent plasma structures (15) that flow in the equatorial Earth's boundary layer (BL), observed on October 24, 2001. The data were recorded during a 1.5 hour-long Wind crossing of the BL at the dawn magnetospheric flank, tailward of the terminator (X≈−13 RE). The interplanetary magnetic field (IMF) was radially directed, almost antiparallel to the magnetosheath (MS) flow. This configuration is expected to be adverse to the KH instability because of the collinearity of field and flow, and the high compressibility of the MS. We analyze the BL stability with compressible MHD theory using continuous profiles for the physical quantities. Upstream, at near Earth sites, we input parameters derived from an exact MHD solution for collinear flows. Further downtail at Wind position we input measured parameters. The BL is found KH unstable in spite of unfavorable features of the external flow. On the experimental side, the passage of vortices is inferred from the presence of low density - hot plasma being accelerated to speeds higher than that of the contiguous MS. It is further supported by the peculiar correlation of relative motions (in the bulk velocity frame): cold-dense plasma drifts sunward, while hot-tenuous plasma moves tailward. This event differs from many other studies that reported BL vortices under strongly northward IMF orientations. This is a case of KH vortices observed under an almost radial IMF, with implicit significance for the more common Parker's spiral fields, and the problem of plasma entry in the magnetosphere.

Testing of Atmospheric Turbulence Effects on the Performance of Micro Air Vehicles

Micro Air Vehicles (MAV) are generally operated at low altitudes and within the earth boundary layer. This is a very dynamic environment with varying wind intensity and turbulence levels far greater than those experienced by traditional manned aircraft cruising at higher altitudes. Yet aerodynamic research on MAVs is often based on the assumption of steady aerodynamics. Little effort has been made to experimentally determine the validity of this assumption. In this paper, the effect of turbulence on the performance of a MAV is studied using flight testing in different wind conditions. Flight testing technique, data logging equipment and data reduction are explained. Additionally, a low cost technique for propeller performance measurement is presented. Results show that the flow around a MAV flying in windy conditions qualifies as highly unsteady, although the impact on its performance is surprisingly small for the kind of turbulence levels at which MAVs can be expected to operate. Accelerometer data from the flights reveals that if steady aerodynamic theory is assumed, increasing turbulence should have resulted in a measurable drag increase, thus indicating that the tested MAV to some extent passively manages to benefit from the turbulence.

Evolution of an MHD-scale Kelvin-Helmholtz vortex accompanied by magnetic reconnection: Two-dimensional particle simulations

[1] We have performed 2.5-dimensional full particle simulations of an MHD-scale Kelvin-Helmholtz (KH) vortex and accompanying magnetic reconnection. This is the first study of so-called vortex-induced reconnection (VIR) using kinetic simulations. First, as a key property of the VIR, we found that magnetic reconnection occurs at multiple points in the current sheet compressed by the flow of the KH vortex. The resulting multiple mesoscale islands are carried toward the vortex body along the vortex flow and then are incorporated into the vortex body via re-reconnection. The rates of the first reconnection and second re-reconnection are both generally higher than that of spontaneous reconnection; both reconnection processes are of driven nature. Noteworthy is that the high rate of the first reconnection leads to strong magnetic field pileup within the multiple islands. This characteristic magnetic structure of the islands could be used as new observational evidence for the occurrence of the VIR. Next, as a key kinetic aspect of the VIR, we found that a series of the multiple island formation and incorporation processes causes efficient plasma mixing in real space and bidirectional magnetic field-aligned acceleration of electrons simultaneously within the vortex. These kinetic effects of the VIR could account for observed features of the Earth's low-latitude boundary layer, where mixed ions and bidirectional field-aligned electrons generally coexist.

Wind turbine aerodynamics: analysis and design

In this paper, the classical work on wind turbine is reviewed, starting from the ground work of Rankine and Froude, then revisiting the minimum energy condition of Betz, and applying modern computing techniques to build codes, based on the vortex model of Goldstein that are both fast and reliable. Such numerical simulations can be used to help analyse and design modern wind turbines in regimes where the flow is attached. Much of the work has been developed under the impulsion of General Electric whose support is gratefully acknowledged. The vortex model has reached a mature state which includes capabilities to model unsteady flows due to yaw, tower interference and earth boundary layer as well as flows past rotors with advanced blade tips that have sweep and/or winglets. When separation occurs on the blades, a higher fidelity model is presented, called the hybrid method, which consists in coupling a Navier-Stokes solver with the vortex model, the Navier-Stokes code solving the near blade flow whereas the vortex model convects the circulation to the far field without dissipation and allows for accurate representation of the induced velocities. Further development of the vortex model includes its coupling with a blade structural model to perform aeroelasticity studies.

Mesoscale Associations Between Midwest Land Surface Properties and Convective Cloud Development in the Warm Season

The study of land surface-atmosphere interactions is vital to understanding climatic variations in the Earth's planetary boundary layer, particularly given continual land cover modifications by humans on local to regional scales. An agriculturally important region ideal for the study of land surface-atmosphere interactions is the Midwest United States "Corn Belt." To evaluate the mesoscale relationships between Corn Belt land surface properties and a key indicator of the surface-atmosphere feedback in humid climates—warm-season convective cloud development—conventional meteorological data, digital maps of land surface properties, and satellite data were examined in a GIS framework for the May-September periods of 1991 through 1999. The results indicate associations between the surface and atmospheric moisture content and the formation of convective clouds: cumulus clouds initiate first and persist longer over a dry (moist) surface with an initially dry (moist) atmosphere. These relationships are evident when forcing from the free atmosphere is either extremely weak (i.e., when fronts and other disturbances are absent and 500 mb winds are <7 ms-1), weak (same as extremely weak, except 500 mb winds are 7-13 ms-1), or strong (same as extremely weak, except 500 mb winds >18 ms-1). However, the association between convective cloud development and other land surface properties (e.g., land use-land cover [LULC], soil order, elevation, and slope) is not consistent spatially. We find that a surface moisture-convective cloud relationship dominates Corn Belt land surface-atmosphere interactions across a range of barotropic synoptic conditions under different atmospheric moisture contents. The study results can help lead to improved prediction of convective cloud formation, and more realistic modeling of land surface-atmospheric interactions for weather and climate forecasting.

Infrasonic ambient noise interferometry from correlations of microbaroms

We show that microbaroms, continuous infrasound fluctuations resulting from the interaction of the ocean with the atmosphere, have long‐range correlation properties that make it possible to estimate the impulse response between two microphones from passive recordings. The processing is analogous to methods employed in the emerging field of ambient noise seismology, where the random noise source is the ocean coupling with the solid Earth (microseisms) instead of the atmosphere (microbaroms). We find that time‐dependent temperature fields and temperature inversions determine the character of infrasonic impulse responses at Fourpeaked Volcano in Alaska. Applications include imaging and monitoring the gross structure of the Earth's atmospheric boundary layer.

Characteristics of some nonlinear coherent wave structures in a collisionless weakly relativistic plasma with nonthermal electrons

In some recent on board space observations, it has been studied that high streaming ions play a major role on the physical mechanism for nonlinear wave structures. We have investigated here ion-acoustic solitons in a plasma with nonthermal electrons and weakly relativistic ions. Such plasmas occur in plasma sheet boundary layer of earth’s magnetosphere, Van-Allen belts. We have set up and solved KdV equation in a stationary frame to obtain the expression for the peak soliton amplitude and width. These quantities are significantly influenced by the relativistic effect, the nonthermal parameters and ratio of ion to electron temperatures. It is observed that for specific range of nonthermal parameter values, we observe compressive solitons and above which the rarefactive solitons are observed. The nonthermal parameter also affects the double layers.

Particle-in-cell guiding center models of ion velocity space shell instabilities

Particle-in-cell simulations with full dynamics ions and drift kinetic electrons are essential tools to model kinetic instabilities in strongly magnetized plasmas. In these guiding center models, parallel acceleration of the electrons is retained while their perpendicular velocity is given to lowest order by the drift. These simulations are applied to a study of the electrostatic stability of energetic velocity-space ion shells observed in the Earth's plasma sheet boundary layer by the Cluster satellites.

Large‐eddy simulation of atmospheric convection on Mars

AbstractLarge‐eddy simulations are performed using the Mars Regional Atmospheric Modeling System (a nonhydrostatic, mesoscale model) in order to obtain a detailed, three‐dimensional understanding of the daytime Mars atmospheric boundary layer. These microscale runs utilize the full radiative‐transfer (including a static dust profile) routines of the mesoscale model and a multi‐level, prognostic subsurface thermal model. Surface albedo, thermal inertia, Coriolis parameter and solar forcing are homogeneously set to values at the Mars Pathfinder landing site (19.33°N,33.55°W (IAU1991); Ls=143°). The initial state is obtained from a previous mesoscale simulation, and is representative of the Mars Pathfinder landing site during summer.The convective boundary layer of Mars is found to exhibit structures and turbulent statistics outwardly similar to those of earth's convective boundary layer. However, direct infrared radiative heating of the near‐surface atmosphere is the primary mechanism for the transfer of energy from the solar‐heated surface, and affects the behaviour of the convection. Mars convection is more intense than that on earth, primarily due to lesser gravity and atmospheric mass. Also, certain empirical scaling constants within the subgrid‐scale turbulence parametrization (originally developed for terrestrial large‐eddy simulations) appear to require significant reduction in order for the scheme to perform adequately on Mars. The simulation results are used to further meaningfully interpret spacecraft images of convective clouds. The results also compare favourably with Mars Pathfinder in situ meteorological measurements, and help reconcile large daytime variances in that dataset. Copyright © 2004 Royal Meteorological Society

The nature of water on surfaces of laboratory systems and implications for heterogeneous chemistry in the troposphere

A number of heterogeneous reactions of atmospheric importance occur in thin water films on surfaces in the earth's boundary layer. It is therefore important to understand the interaction of water with various materials, both those used to study heterogeneous chemistry in laboratory systems, as well as those found in the atmosphere. We report here studies at 22 °C to characterize the interaction of water with such materials as a function of relative humidity from 0–100%. The surfaces studied include borosilicate glass, both untreated and after cleaning by three different methods (water, hydrogen peroxide and an argon plasma discharge), quartz, FEP Teflon film, a self assembled monolayer of n-octyltrichlorosilane (C8 SAM) on glass, halocarbon wax coatings prepared by two different methods, and several different types of Teflon coatings on solid substrates. Four types of measurements covering the range from the macroscopic level to the molecular scale were made: (1) contact angle measurements of water droplets on these surfaces to obtain macroscopic scale data on the water-surface interaction, (2) atomic force microscopy measurements to provide micron to sub-micron level data on the surface topography, (3) transmission FTIR of the surfaces in the presence of increasing water vapor concentrations to probe the interaction with the surface at a molecular level, and (4) X-ray photoelectron spectroscopy measurements of the elemental surface composition of the glass and quartz samples. Both borosilicate glass and the halocarbon wax coatings adsorbed significantly more water than the FEP Teflon film, which can be explained by a combination of the chemical nature of the surfaces and their physical topography. The C8 SAM, which is both hydrophobic and has a low surface roughness, takes up little water. The implications for the formation of thin water films on various surfaces in contact with the atmosphere, including building materials, soil, and vegetation, are discussed.

The implementation of land use and relief height data in predicting the evolution of the atmospheric boundary-layer within the urban environment

It is vitally important to understand the structure of the atmospheric boundary-layer within the urban environment and the local rural surroundings. Within the local environment the structure of the earth's boundary-layer is highly turbulent and diffusive, with distinctive inner and outer regions. The flow in the inner region is mainly influenced by the physical characteristics of the terrain, such as the relief and the land use, whereas in the outer region larger scale phenomena need to be taken into account. In this paper, novel methods are implemented to understand the structure of the atmospheric boundary-layer within the urban environment. These include a technique that allows the larger scale topographical effects on the atmospheric boundary-layer to be incorporated, yet still enabling fine resolution of the boundary-layer within the smaller scale of the urban environment. This is achieved by manipulating two large data sets for the land use and the relief height within the UK in conjunction with sophisticated mathematical modelling. In this paper a simple but effective algorithm is presented that allows these large data sets to be practically implemented within the framework of an atmospheric boundary-layer fluid flow model.

Transient Magnetic Reconnection and Unstable Shear Layers

We study three-dimensional magnetic reconnection caused by the Kelvin-Helmholtz (KH) instability and differential rotation in subsonic and sub-Alfvenic flows. The flows, which are modeled by the resistive magnetohydrodynamic equations with constant resistivity, are stable in the direction of the magnetic field but unstable perpendicular to the magnetic field. Localized transient reconnection is observed on the KH time scale, and kinetic energy increases with decreasing resistivity. As in flux-transfer events in the Earth's magnetopause boundary layer, bipolar structures in the normal flux and bidirectional jetting away from reconnection zones are observed.

UV 380 nm reflectivity of the Earth's surface, clouds and aerosols

The 380 nm radiance measurements of the Total Ozone Mapping Spectrometer (TOMS) have been converted into a global data set of daily (1979–1992) Lambert equivalent reflectivities R of the Earth's surface and boundary layer (clouds, aerosols, surface haze, and snow/ice) and then corrected to RPC for the presence of partly clouded scenes. Since UV surface reflectivity is between 2 and 8% for both land and water during all seasons of the year (except for ice and snow cover), reflectivities larger than the surface value indicate the presence of clouds, haze, or aerosols in the satellite field of view. A statistical analysis of 14 years of daily reflectivity data shows that most snow‐/ice‐free scenes observed by TOMS have a reflectivity less than 10% for the majority of days during a year. The 380 nm reflectivity data show that the true surface reflectivity is 2–3% lower than the most frequently occurring reflectivity value for each TOMS scene as seen from space. Most likely the cause is a combination of frequently occurring boundary layer water and/or aerosol haze. For most regions the observation of extremely clear conditions needed to estimate the surface reflectivity from space is a comparatively rare occurrence. Certain areas (e.g., Australia, southern Africa, portions of northern Africa) are cloud‐free more than 80% of the year, which exposes these regions to larger amounts of UV radiation than at comparable latitudes in the Northern Hemisphere. Regions over rain forests, jungle areas, Europe and Russia, the bands surrounding the Arctic and Antarctic regions, and many ocean areas have significant cloud cover (R&gt;15%) more than half of each year. In the low to middle latitudes the areas with the heaviest cloud cover (highest reflectivity for most of the year) are the forest areas of northern South America, southern Central America, the jungle areas of equatorial Africa, and high mountain regions such as the Himalayas or the Andes. The TOMS reflectivity data show both the presence of large nearly clear ocean areas and the effects of the major ocean currents on cloud production.

“Broadband” plasma waves in the boundary layers

Boundary layers are commonly encountered in space and astrophysical plasmas. For example, interaction of solar wind plasma with the planets and comets produces magnetopause and cometopause boundary layers, respectively. Generally, the boundary layers are formed when plasmas with different characteristics interact with each other. The plasma sheet boundary layer in the Earth's magnetotail is formed owing to the interaction of hot dense plasma in the plasma sheet region with the rarefied plasma of the lobe region. Boundary layers are the site where energy and momentum are exchanged between two distinct plasmas. Boundary layers occurring in space plasmas can support a wide spectrum of plasma waves spanning a frequency range of a few mHz to 100 kHz and beyond. The purpose of this review is to describe the main characteristics and the possible generation mechanisms of the broadband plasma waves (with frequencies of &gt; 1 Hz) observed in the Earth's magnetopause boundary layer, the Jovian magnetopause boundary layer, the plasma sheet boundary layer, and the Earth's polar cap boundary layer. The rapid pitch angle scattering of energetic particles via cyclotron resonant interactions with the waves can provide sufficient precipitated energy flux to the ionosphere to create the dayside aurora at Earth and a weak high‐latitude auroral ring at Jupiter. In general, the broadband plasma waves may play an important part in the processes of local heating/acceleration of the boundary layer plasma. Recent exciting high time resolution results on the broadband plasma waves coming from Geotail, Polar, and FAST will be discussed.

Concentrations and Mass Fluxes of Chloroacetic Acids and Trifluoroacetic Acid in Rain and Natural Waters in Switzerland

Chloroacetic acids (CAAs) and trifluoroacetic acid (TFA) are widely distributed environmental pollutants. The origin of CAAs is still under debate, while TFA is considered to be a major atmospheric degradation product of recently introduced hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs). Because of the phytotoxic potential of the CAAs, i.e., monochloroacetic acid (MCA), dichloroacetic acid (DCA), and trichloroacetic acid (TCA), and the persistence of TFA, these compounds are of considerable environmental concern. Hence, it is important to know their fluxes and their spatial and temporal distribution in the environment. In this study, the occurrence and mass fluxes of MCA, DCA, TCA, and TFA were assessed on a regional scale over Switzerland, based on more than 1000 concentration measurements in rain and snow, surface water, groundwater, and wastewater. Among different precipitation events, the measured concentrations varied significantly from <11 to 7100 ng/L. However, no statistically different average haloacetic acid (HAA) concentrations among six precipitation sampling sites located in various areas in Switzerland were observed (range of average concentrations: MCA 1430−2770 ng/L, DCA 390−1370 ng/L, TCA 95−380 ng/L, TFA 33−220 ng/L). The similar average HAA concentrations in precipitation at a remote site close to the free troposphere at an elevation of 3580 m above sea level (Jungfraujoch) and at a site that receives precipitation which scavenged the Earth's boundary layer (urban site Dübendorf/Zürich) suggests that HAAs are derived from well-mixed precursor(s) in the atmosphere. When moving from precipitation to surface waters (i.e., rivers), the TFA/CAA ratios increased by a factor of 10−11 for TFA/MCA and TFA/DCA and by a factor of 1.2 for TFA/TCA. Mass flux calculations show that precipitation is the dominant source of the HAAs, particularly of MCA, DCA, and TFA (>95%). In the case of TCA, wastewater effluents contributed 27% of the total input. The results further indicate that, as compared to the CAAs, TFA is quite persistent in the aquatic and terrestrial environment and may thus accumulate in soils and groundwater.

Exploration of the atmospheric lower layer thermal turbulences by means of microthermocouples

The experiments on the propagation of acoustic air waves in the low atmospheric layer show a large influence of aerological parameters. In particular, there were only few measurements carried out outdoors to approach thermal turbulent values [1]. There are many thermal sensors ranging from a simple platinum resistance to quartz crystal. Each technology has some advantages depending on the type of measurement one intends to perform. To explore the earth boundary layer, we chose a micro-thermocouple of type S. Its small size (1.27 µm) allows us to obtain a low calorific capacity and a high thermal conductance. On the other hand, its sensitivity is low and it was necessary to associate an amplifier with a gain of 100 000. The whole device was set outside on a bar 2 m above the ground. The different experiments carried out with one or several microthermocouples showed very small turbulences of different types depending on the role of the different layers in the low atmosphere. They also enabled to visualize convection due to the ground or due to the wind as a function of time.

Modification of the relaxed eddy accumulation technique to maximize measured scalar mixing ratio differences in updrafts and downdrafts

A modification to the relaxed eddy accumulation (REA) flux measurement technique is proposed which maximizes the scalar mixing ratio difference in updrafts and downdrafts. This technique was developed with the goal of measuring the stable isotope (13C/12C and 18O/16O) ratios of updraft and downdraft air and thus the net fluxes of 13C16O2 and 12C18O16O. Current mass spectrometer precision is small relative to measured isotopic gradients in CO2 in the Earth's boundary layer, and the conventional REA approach is likely to be ineffective. The new technique, which we refer to as hyperbolic relaxed eddy accumulation (HREA), uses the conditional sampling concept of hyperbolic hole analysis to control sampling of air during only those turbulent events which contribute most strongly to the flux. Instead of basing updraft/downdraft sampling decisions strictly on vertical wind velocity, CO2 mixing ratio ([CO2]) fluctuations or those of another scalar are also used. Simulations using 10‐Hz data show that a wind‐based/scalar‐based sampling threshold can achieve a factor of 2.7 increase in scalar updraft/downdraft [CO2] differences over simple REA. During midday periods with strong photosynthetic fluxes, up/down [CO2] differences with HREA of 8–10 ppm are possible, compared with 3–5 ppm for the best conventional REA case. Corresponding isotopic differences can likely be resolved with current mass spectrometers using this approach.

Broadband Plasma Waves In The MagnetopauseAnd Polar Cap Boundary Layers

Boundary layers occurring in the magnetosphere can support a wide spectrumof plasma waves spanning a frequency range of a few mHz to tens of kHz andbeyond. This review describes the main characteristics of the broadband plasma waves observed in the Earth's low-latitude magnetopause boundary layer (LLBL), in the polar cap boundary layer (PCBL), and the possible generation mechanisms. The broadband waves at the low-latitude boundary layer are sufficiently intense to cause the diffusion of the magnetosheath plasma across the closed magnetospheric field lines at a rate rapid enough to populate and maintain the boundary layer itself. The rapid pitch angle scattering of energetic particles via cyclotron resonant interactionswith the waves can provide sufficient precipitation energy flux to the ionosphere to create the dayside aurora. In general, the broadband plasma waves may play an important part in the processes of local heating/acceleration of the boundary layer plasma.

Implementation of newly developed algorithms in the simulation of atmospheric turbulent transports

The paper presents a three-dimensional transient numerical model for atmospheric wind flow and industry and/or traffic pollutant dispersion over terrains having a complex topography. The model is based on a finite-volume integration of the equations governing mass, momentum, heat and pollutant transport within the earth's atmospheric boundary layer, using a collocated grid arrangement. The instability provoked by such a formulation was avoided by using a special pressure-velocity coupling. Local refinement of the grid was achieved via a domain decomposition method. The technique of ‘porosity’ used to approximate curved three-dimensional boundaries is incorporated in the procedure, thus avoiding the less accurate and more common approximation by a broken surface with segments parallel to the coordinate lines. The method was validated by simulating the flow over the Attica peninsula for which measurements of wind speed and pollutant emissions are available.