Turbulence structure of the boundary layer below marine clouds in the SOFIA experiment

The SOFIA (Surface of the Ocean: Flux and Interaction with the Atmosphere) experiment, included in the ASTEX (Atlantic Stratocumulus Transition EXperiment) field program, was conducted in June 1992 in the Azores region in order to investigate air-sea exchanges, as well as the structure of the atmospheric boundary layer and its capping low-level cloud cover. We present an analysis of the vertical structure of the marine atmospheric boundary layer (MABL), and especially of its turbulence characteristics, deduced from the aircraft missions performed during SOFIA. The meteorological situations were characteristic of a temperate latitude under anticyclonic conditions, i.e., with weak to moderate winds, weak surface sensible heat flux, and broken capping low-altitude cloud cover topped by a strong trade inversion. We show that the mixed layer, driven by the surface fluxes, is decoupled from the above cloud layer. Although weak, the surface buoyancy flux, and the convective velocity scale deduced from it, are relevant for scaling the turbulence moments. The mixed layer then follows the behavior of a continental convective boundary layer, with the exception of the entrainment process, which is weak in the SOFIA data. These results are confirmed by conditional sampling analysis, which shows that the major turbulence source lies in the buoyant moist updrafts at the surface.     

The November 2004 superstorm: Comparison of low-latitude TEC observations with LLIONS model results

We investigate the effects of penetration electric fields, meridional thermospheric neutral winds, and composition perturbation zones (CPZs) on the distribution of low-latitude plasma during the 7–11 November 2004 geomagnetic superstorm. The impact on low-latitude plasma was assessed using total electron content (TEC) measurements from a latitudinally distributed array of ground-based GPS receivers in South America. Jicamarca Radio Observatory incoherent scatter radar measurements of vertical E×B drift are used in combination with the Low-Latitude IONospheric Sector (LLIONS) model to examine how penetration electric fields and meridional neutral winds shape low-latitude TEC. It is found that superfountain conditions pertain between ～1900 and 2100 UT on 9 November, creating enhanced equatorial ionization anomaly (EIA) crests at ±20° geomagnetic latitude. Large-amplitude and/or long-duration changes in the electric field were found to produce significant changes in EIA plasma density and latitudinal location, with a delay time of ～2–2.5 h. Superfountain drifts were primarily responsible for EIA TEC levels; meridional winds were needed only to create hemispherical crest TEC asymmetries. The [O/N₂] density ratio (derived from the GUVI instrument, flown on the TIMED satellite) and measurements of total atmospheric density (from the GRACE satellites), combined with TEC measurements, yield information regarding a likely CPZ that appeared on 10 November, suppressing TEC for over 16 h.     

The boundary layer of the residence time field

The residence time of a tracer in a control domain is usually computed by releasing tracer parcels and registering the time when each of these tracer parcels cross the boundary of the control domain. In this Lagrangian procedure, the particles are discarded or omitted as soon as they leave the control domain. In a Eulerian approach, the same approach can be implemented by integrating forward in time the advection–diffusion equation for a tracer. So far, the conditions to be applied at the boundary of the control domain were uncertain. We show here that it is necessary to prescribe that the tracer concentration vanishes at the boundary of the control domain to ensure the compatibility between the Lagrangian and Eulerian approaches. When we use the Constituent oriented Age and Residence time Theory (CART), this amounts to solving the differential equation for the residence time with boundary conditions forcing the residence time to vanish at the open boundaries of the control domain. Such boundary conditions are likely to induce the development of boundary layers (at outflow boundaries for the tracer concentration and at inflow boundaries for the residence time). The thickness of these boundary layers is of the order of the ratio of the diffusivity to the velocity. They can however be partly smoothed by tidal and other oscillating flows.     

Solar wind—magnetosphere coupling: A review of recent results

This paper reviews some aspects of solar wind–magnetosphere–ionosphere interaction. It is shown that in addition to the interplanetary electric field, the solar wind dynamic pressure also has a significant role in determining the state, dynamics, and energetics of the system. It is demonstrated how the state of the magnetosphere and the prior driving affect the amount of energy input to the system, which highlights the capability of the magnetosphere to control the energy flow. The active role of the magnetosphere in determining the dynamics is illustrated by statistical results of the flux balance in the magnetotail and the various dynamic cycles the system can enter. The inner magnetosphere processes during storms are shown to be a result of a complex interplay of processes at the magnetopause and in the magnetotail in response to the solar wind driving. The conclusions are drawn from statistical observational results, empirical models, and global MHD simulations.     

南京城市化进程对大气边界层的影响研究

为了研究城市化进程对城市边界层结构的影响,通过高分辨率的卫星遥感资料获得土地利用类型,以及地表反照率、植被指数等地表参数,以南京为例,运用数值模拟手段进行了研究,结果显示这是一种可行的手段.数值模拟结果表明,随着城市的发展,城市反照率减小、植被减少、地表湿度减小,蒸发耗热减少,感热通量增加140 W/m2,城市波恩比增加.地表和大气之间热量交换加强,湍流热量通量增大了一倍,湍流交换发展加剧,14：00混合层高度抬高了500 m.地表湍流水汽通量和空气中水汽含量都减小,这也使更多的热量用于加热地表和大气,使地温、气温的日变化幅度增加.     

The effect of vibrationally excited nitrogen on the low-latitude ionosphere

The first five vibrationally excited states of molecular nitrogen have been included in the Sheffield University plasmasphere ionosphere model. Vibrationally excited molecular nitrogen reacts much more strongly with atomic oxygen ions than ground-state nitrogen; this means that more O+ ions are converted to NO+ ions, which in turn combine with the electrons to give reduced electron densities. Model calculations have been carried out to investigate the effect of including vibrationally excited molecular nitrogen on the low-latitude ionosphere. In contrast to mid-latitudes, a reduction in electron density is seen in all seasons during solar maximum, the greatest effect being at the location of the equatorial trough.     

边界层参数化方案对边界层热力和动力结构特征影响的比较

本文利用高分辨率中尺度WRF模式,通过改变边界层参数化方案进行多组试验,评估该模式对美国北部森林地区边界层结构的模拟能力,同时比较了五种不同边界层参数化方案模拟得出的边界层热力和动力结构.结果表明：除个别方案外,配合不同边界层方案的WRF模式都能成功模拟出白天对流边界层强湍流混合特征和夜间稳定边界层内强逆温、逆湿和低空急流等热力和动力结构.非局地YSU、ACM2方案在白天表现出强的湍流混合和卷夹,相比于局地MYJ、UW方案,模拟的对流边界层温度更高、湿度更低、混合层高度更高、感热通量更大,更接近实际观测,这表明在不稳定层结下考虑非局地大涡输送更为合理,但局地方案在风速和风向的预报上存在一定优势.TEMF方案得到的白天局地湍流混合强度为所有方案中最弱,混合层难以发展,无法体现对流边界层内气象要素垂直分布均匀的特点.对于夜间稳定边界层的模拟,不同参数化方案之间的差异较小,但是YSU方案在一定程度上高估了机械湍流,导致局地湍流混合偏强,从而影响了其对稳定边界层的模拟能力.     

Physical problems of the benthic boundary layer

Since the boundary layer at the sea bed has a number of features in common with boundary layers found in laboratory scale flows and in meteology, a brief review is given first of the properties which may be inferred from experience in these fields or from theoretical studies. Measurements of velocity profiles, turbulence, and shearing stress which have been made near the bottom, in deep water, and on the continental shelf, are described in relation to this background. In particular, the logarithmic form of the velocity profile near the bed and deductions from it appear to be valid in certain conditions, but the occurrence of ripples and other bed forms is a complicating feature. The relation of the dynamical aspects of the flow to the transport of sediment as bed load and in suspension is discussed. The diflusive properties of the layer are then considered, in relation to fluxes near the sea-sediment interface and to the formation of nepheloid layers or layers well mixed in temperature and salinity.At Dept. of Oceanography, Univ. of Washington, Seattle, WA, 98195, U.S.A., Jan. to Aug. 1978.     

Dynamics of the auroral Es layer during weak and strong disturbances in the magnetosphere

The paper is dedicated to studying the dynamics of the auroral ionosphere at the level of the sporadic Es layer during magnetospheric disturbances. A new approach to this problem, proposed in the paper, uses the geomagnetic PC index, which is calculated using the magnetic data in the polar caps of the northern and southern hemispheres and manifests the geoefficiency of the interplanetary electric field. It is shown that variations in the sporadic electron concentration in the auroral Es layer could be related to changes in the PC index with a high degree of statistical reliability. However, the character of precipitations of sporadic particles into the ionosphere under high (PC > 2 mV/m) and low (PC < 2 mV/m) magnetic activity differs substantially. During strong magnetic disturbances and under intensified electric fields in the interplanetary environment, the intensity of particle precipitation from the magnetosphere into the E region of the high-latitude ionosphere is governed by the values of the PC magnetic index. During weak magnetic disturbances, short-time pulses of an increase in the PC values, caused by the variability in electric field in the magnetosphere, are the main factor in the occurrence of sporadic ionization in the Es layer.     

The nighttime distribution of ozone in the low-latitude mesosphere

The intensity of stars at wavelengths in the Hartley continuum region of ozone has been monitored by the University of Wisconsin stellar photometers aboard the OAO-2 satellite during occultation of the star by the earth's atmosphere. These occultation data have been used to determine the ozone number density profile at the occultation tangent point. The nighttime ozone number density profile has a bulge in its vertical profile with a peak of 1 to 3×10⁸ cm^–3 at approximately 83 km and a minimum near 75 km. The ozone number density at high altitudes varies by as much as a factor of 4, but does not show any clear seasonal variation or nighttime variation. The retrieved ozone number density profiles define a data envelope that is compared with other nighttime observations of the ozone number density profile and also the results of theoretical models.Calculations are also presented that illustrate the difference in retrieving the bulge in the ozone number density profile from stellar and solar occultation data.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

The magnetosphere of Jupiter: Coupling the equator to the poles

Jupiter is a planet of superlatives: the most massive planet in the solar system, rotates the fastest, has the strongest magnetic field, and has the most massive satellite system of any planet. These unique properties lead to volcanoes on Io and a population of energetic plasma trapped in the magnetic field that provides a physical link between the satellites, particularly Io, and the planet Jupiter. There are strong differences between the magnetospheres of Earth and Jupiter but there are also underlying basic physical principles that all magnetospheres share in common. This paper provides a rough sketch of the magnetosphere of Jupiter, briefly describes the current understanding and lists outstanding issues. As at Earth, a major issue of the jovian system is how the magnetospheric plasma is coupled to the planet's ionosphere.     

The diffusive boundary layer of sediments: oxygen microgradients over a microbial mat

Oxygen microelectrodes were used to analyze the distribution of the diffusive boundary layer (DBL) at the sediment-water interface in relation to surface topography and flow velocity. The sediment, collected from saline ponds, was covered by a microbial mat that had high oxygen consumption rate and well-defined surface structure. Diffusion through the DBL constituted an important rate limitation to the oxygen uptake of the sediment. The mean effective DBL thickness decreased from 0.59 to 0.16 mm as the flow velocity of the overlying water was increased from 0.3 to 7.7 cm s-1 (measured 1 cm above the mat). The oxygen uptake rate concurrently increased from 3.9 to 9.4 nmol cm-2 min-1. The effects of surface roughness and topography on the thickness and distribution of the DBL were studied by three-dimensional mapping of the sediment-water interface and the upper DBL boundary at 0.1-mm spatial resolution. The DBL boundary followed mat structures that had characteristic dimensions > 1/2 DBL thickness but the DBL had a dampened relief relative to the mat. The effective surface area of the sediment-water interface and of the upper DBL boundary were 31 and 14% larger, respectively, than a flat plane. Surface topography thereby increased the oxygen flux across the sediment-water interface by 49% relative to a one-dimensional diffusion flux calculated from the vertical oxygen microgradients.     

A numerical model of the wave boundary layer

A numerical model of deep, uniform, oscillatory, rough-turbulent boundary-layer flow is described. The model is based upon the governing horizontal momentum equation and a closure scheme involving the turbulent-energy equation and various turbulence-scaling laws. Finite difference solutions of these equations are obtained for a range of values of the ‘relative roughness’ (A₀/k_s), whereA₀ is the excursion amplitude of the water particles in the free-stream flow andk_s is the ‘equivalent bed roughness’. Typical vertical profiles of horizontal velocity, turbulence energy and eddy viscosity, and time-series of the bed shear stress are presented. The model results are then used to determine the wave drag coefficient, boundary-layer thickness and phase lead of the bed shear stress over the free-stream velocity, each as a function ofA₀/k_s. These results are shown to be in generally good agreement with previous experimental and theoretical results. Finally, the model is used to test for the existence of a universal velocity distribution for uniform oscillatory (sinusoidal) rough-turbulent flow. The ‘law of the wall’ and the ‘defect law’ proposed by Jonsson (1980, Ocean Engineering, 7, 109–152) are well supported by the model, and the existence of a logarithmic ‘velocity overlap layer’ in which both of these laws are valid is demonstrated forA_o/k_s30.     

Unified solution of the unsteady boundary layer equations

Summary The momentum conservation equation for a compressible boundary layer flow, may be transformed to the corresponding equation for the incompressible flow, in some cases, and the resulting equation can be solved by series expansion.     

Wake effects on turbulent transport in the convective boundary layer

The effect of the downstream propagation of a wake on the transport of momentum, energy and scalars (such as humidity) in the convective boundary layer (CBL) is studied using a direct numerical simulation. The incompressible Navier–Stokes and energy equations are integrated under neutral and unstable thermal stratification conditions in a rotating coordinate frame with the Ekman layer approximation. Wake effects are introduced by modifying the mean velocity field as an initial condition on a converged turbulent Ekman layer flow. With this initial velocity distribution, the governing equations are integrated in time to determine how turbulent transport in the CBL is affected by the wake. Through the use of Taylor’s hypothesis, temporal evolution of the flow field in a doubly periodic computational domain is transformed into a spatial evolution. The results clearly indicate an increase in the scalar flux at the surface for the neutrally stratified case. An increase in wall scalar and heat flux is also noted for the CBL under unstable stratification, though the effects are diminished given the enhanced buoyant mixing associated with the hot wall.     

The effect of an interface boundary layer on the resonance properties of a buried structure

This paper presents a model for the analysis of the diffraction of plane waves at a cavity in an infinite homogeneous linear elastic medium supported by a segmented lining. An elastic boundary layer is introduced between the cavity lining and the infinite medium. The boundary layer is simulated by ‘elastic boundary conditions’ in which the stress is proportional to the relative displacement of the lining and of the surrounding medium boundary. A closed‐form analytical solution of the problem was obtained using the Fourier–Bessel series, the convergence of which was proven. It was shown that the number of series terms required to obtain a desired level of accuracy can be determined in advance. Using amplitude–frequency response analysis it was shown that the boundary layer produces additional ‘pseudo‐resonance’ frequencies that depend on the layer properties. These frequencies are almost identical to the eigenvalues obtained from the simple analysis of a segmented elastically supported lining. Copyright © 2003 John Wiley & Sons, Ltd.     

The effect of turbulent flow structures on saltation sand transport in the atmospheric boundary layer

The effect of turbulent flow structures on saltation sand transport was studied during two convective storms in Niger, West Africa. Continuous, synchronous measurements of saltation fluxes and turbulent velocity fluctuations were made with a sampling frequency of 1 Hz. The shear stress production was determined from the vertical and streamwise velocity fluctuations. The greatest stress-bearing events were classified as turbulent structures, with sweep, ejection, inward interaction, and outward interaction described according to the quadrant technique. The classified turbulent structures accounted for 63·5 per cent of the average shear stress during the first storm, and 56·0 per cent during the second storm. The percentage of active time was only 20·6 per cent and 15·8 per cent, respectively. High saltation fluxes were associated with sweeps and outward interactions. These two structures contribute positively (sweeps) and negatively (outward interactions) to the shear stress, but have in common that the streamwise velocity component is higher than average. Therefore, the horizontal drag force seems primarily responsible for saltation sand transport, and not the shear stress. This was also reflected by the low correlation coefficients (r) between shear stress and saltation flux (0·12 and 0·14, respectively), while the correlation coefficients between the streamwise velocity component and saltation flux were much higher (0·65 and 0·57, respectively). © 1998 John Wiley & Sons, Ltd.     

Structure of the low-latitude magnetopause: MAGION-4 observations

The aims of this paper are (1) briefly to describe the plasma devices onboard the MAGION-4 satellite, launched on 3 August 1995, of the INTER-BALL project, and (2) to discuss first observations made near the magnetopause region. During the presented boundary crossings the MAGION–4 observed quasi-periodic pulses of magnetosheath-like plasma in a region of low plasma density. This region is located just earthwards of the magnetopause and is populated by a plasma which, except for the density, has the same parameters as in the magnetosheath. Deeper in the magnetosphere, the encounter of a layer of hot electrons and high-energy ions was interpreted as low-latitude boundary layer.