Annual Variation and Global Structures of The DE3 Tide

The SABER/TIMED temperatures taken in 2002 2006 are used to delineate the tidal signals in the middle and upper atmosphere. Then the Hough mode decomposition is applied with the DE3 tide, and the overall features of the seasonal variations and the complete global structures of the tide are observed. Investigation results show that the tide is most prominent at 110km with the maximal amplitude of 〉 9K, and exhibits significant seasonal variation with its maximum amplitude always occurring in July every year. Results from the Hough mode decomposition reveal that the tide is composed primarily of two leading propagating Hough modes, i.e., the （-3, 3） and the （-3, 4） modes, thus is equatorially trapped. Estimation of the mean amplitude of the Hough modes show that the （-3, 3） mode and （-3, 4） mode exhibit maxima at 110km and 90km, respectively. The （-3, 3） mode plays a predominant role in shaping the global latitude-height structure of the tide, e.g., the vertical scale of 〉 50 km at the equator, and the annual course. Significant influence of the （-3, 4） mode is found below 90 km, where the tide exhibits anti-symmetric structure about the equator; meanwhile the tide at northern tropical latitudes exhibits smaller vertical wavelength of about 30 km.     

Actual Tropical Waves Identified by the Wavenumber-Frequency Spectrum Analysis during Boreal Summer

Based on daily satellite-observed outgoing longwave radiation （OLR） data, the tropical intraseasonal oscillation （ISO） associated with the Asian summer monsoon is found to be dominant during the 2005 summer. Such an ISO along with other tropical waves are identified using a wavenumber-frequency spectrum analysis. Some of Significant spectral peaks coincide well with the dispersion curves of the equatorially trapped wave modes from shallow＇water theory, with various equivalent depths of 12-200 m, suggesting that actual tropical waves can be detected from observational data. Although the formation mechanism of the ISO is more complicated than those of other tropical waves with higher frequencies, significant spectral peaks that correspond to the ISO with a period of 30-60 days distinctly appear in the OLR spectra. The selective wavenumber-frequency filtering of OLR data is thus performed to isolate the ISO, Kelvin and equatoriai Rossby waves, and the properties and signatures of such tropical waves that control tropical atmospheric variability for the 2005 summer are clearly identified.     

Detection of small forest fires by lidar

The possibility of detecting small forest fires with the help of a simple and cheap lidar operating at 0.532-μm wavelength up to distances of about 6.5 km is demonstrated. The values of the signal-to-noise ratio (SNR) achieved in the experiments are consistent with theoretical estimations obtained by computational modeling of the lidar detection process, including simulation of the smoke-plume shape and of the laser beam–plume interaction. This model was used to assess the potential of the lidar technique for fire surveillance in large forest areas. In particular, the upper limiting range for effective detection (SNR>5) of small localized fires in dry- and clear-weather conditions is estimated at 7–15 km depending on operation mode, burning rate, and observation geometry. Received: 29 August 2001 / Published online: 29 November 2001     

Generation of zonal flows by interchange modes in a plasma

The generation of zonal flows by flute-like interchange modes in a nonuniform magnetoplasma is considered. The guiding center particle drifts are then used to derive a system of coupled mode equations. The latter are Fourier analyzed to obtain a nonlinear dispersion relation, which exhibits the excitation of zonal flows by the ponderomotive force of the interchange modes. The growth rate of the parametrically driven zonal flows is obtained. Received 26 July 2002 Published online 24 September 2002 RID="a" ID="a"e-mail: ps@tp4.ruhr-uni-bochum.de     

Horizontal refraction of propagating sound due to seafloor scours over a range-dependent layered bottom on the New Jersey shelf

Three-dimensional propagation effects of low frequency sound from 100 to 400 Hz caused by seafloor topography and range-dependent bottom structure over a 20 km range along the New Jersey shelf are investigated using a hybrid modeling approach. Normal modes are used in the vertical dimension, and a parabolic-equation approximate model is applied to solve the horizontal refraction equation. Examination of modal amplitudes demonstrates the effect of environmental range dependence on modes trapped in the water column, modes interacting with the bottom, and modes trapped in the bottom. Using normal mode ray tracing, topographic features responsible for three-dimensional effects of horizontal refraction and focusing are identified. These effects are observed in the measurements from the Shallow Water 2006 experiment. Specifically, signals from a pair of fixed sources recorded on a horizontal line array sitting on the seafloor show an intensification caused by horizontal focusing due to the seabed topography of 4 dB along the array.     

Application of lidar in ultraviolet,visible and infrared ranges for early forest fire detection

The efficiencies of direct lidars operating at 355, 532, 1064 and 1540-nm radiation wavelengths for early forest fire detection were compared. For each wavelength, the range for reliable smoke-plume detection was estimated on the basis of a computer simulation plume using a one-dimensional “top-hat” gas dynamic model for the calculation of the backscattering and extinction-coefficient profiles within the plume. The agreement between the predicted signal-to-noise ratio (SNR) and experimental results for 532 and 1064-nm wavelength radiation is good. The decrease of the signal-to-noise ratio with distance is maximum for 355 nm and minimum for 1064 nm. At 1540 nm, the decay of SNR with distance is slightly faster, but the SNR is higher than for other wavelengths, leading to the highest detection efficiency for the same energy of the probing laser pulse. For a burning rate of 2 kg/s and a laser beam divergence of 2.5 mr, the maximum distance for reliable detection varies between 6 and 12 km, depending on the wavelength. Received: 15 May 2002 / Revised version: 13 September 2002 / Published online: 22 January 2003 RID="*" ID="*"Corresponding author. Fax: +351-21/841-8120, E-mail: rui.vilar@ist.utl.pt     

Acoustic sounding of diurnal variations and gravity waves in the planetary boundary layer

In the introductory part the paper presents some SODAR results on diurnal variations of different boundary layer structures from measurements during the KOPEX-experiment at a city station and a suburban station. In Sect. 1 the investigations about wave processes in the planetary boundary layer are discussed. The SODAR measurements were carried out in different international cooperations at the Kopisty Atmospheric Observatory of the Institute of Physics of the Atmosphere Prague (northern Czechoslovakia) in the year 1986 (KOPEX-86), at the Observatory Juliusruh of the Heinrich-Hertz-Institut Berlin at the island of Rügen (JULEX-87) and at the Meteorological Observatory of the nuclear power plant Jaslovske Bohunice in Slovakia, about 60 km north-east of Bratislava (JABEX-88).Formerly: Heinrich-Hertz-Institute, Berlin-Adlershof, Germany     

Zonal flows, GAM, and radial electric field in the H-1 heliac

Experimental results on the role of zonal flows and geodesic acoustic modes in formation of transport barriers in the H-1 heliac are reviewed. Presented at the Workshop “Electric Fields, Structures and Relaxation in Edge Plasmas”, Roma, Italy, June 26–27, 2006.     

Sub-part-per-billion detection of nitric oxide in air using a thermoelectrically cooled mid-infrared quantum cascade laser spectrometer

Non-cryogenic, laser-absorption spectroscopy in the mid-infrared has wide applications for practical detection of trace gases in the atmosphere. We report measurements of nitric oxide in air with a detection limit less than 1 nmole/mole (<1 ppbv) using a thermoelectrically cooled quantum cascade laser operated in pulsed mode at 5.26 μm and coupled to a 210-m path length multiple-pass absorption cell at reduced pressure (50 Torr). The sensitivity of the system is enhanced by operating under pulsing conditions which reduce the laser line width to 0.010 cm^-1 (300 MHz) HWHM, and by normalizing pulse-to-pulse intensity variations with temporal gating on a single HgCdTe detector. The system is demonstrated by detecting nitric oxide in outside air and comparing results to a conventional tunable diode laser spectrometer sampling from a common inlet. A detection precision of 0.12 ppb Hz^-1/2 is achieved with a liquid-nitrogen-cooled detector. This detection precision corresponds to an absorbance precision of 1×10^-5 Hz^-1/2 or an absorbance precision per unit path length of 5×10^-10 cm^-1 Hz^-1/2. A precision of 0.3 ppb Hz^-1/2 is obtained using a thermoelectrically cooled detector, which allows continuous unattended operation over extended time periods with a totally cryogen-free instrument. Received: 1 May 2002 / Revised version: 6 June 2002 / Published online: 21 August 2002 RID="*" ID="*"Corresponding author. Fax: +1-978/663-4918, E-mail: ddn@aerodyne.com     

Change of zonal flow spectra in the JIPP T-IIU tokamak plasmas

When Ohmically heated low-density plasmas are additionally heated by higher-harmonics ion-cyclotron-range-of frequency heating, heated by neutral beam injection, or strongly gas puffed, the intensity of zonal flows in the geodesic acoustic mode frequency range in the tokamak core plasma decreases sharply and that of low-frequency zonal flow grows drastically. This is accompanied by a damping of the drift wave propagating in the electron diamagnetic drift direction, turbulence by trapped electron mode (TEM), and the increase of the mode propagating to ion diamagnetic drift direction (ITG). In the half-radius region, TEM and high-frequency zonal flows remain intense in both OH and heated phases. ITG and low-frequency zonal flows grow in heated plasmas, suggesting a strong coupling between ITG and low-frequency zonal flow.     

Measurement of the parameters of the natural waveguide mode

The possibilities of using the decomposition of natural waveguide modes in a shallow-water sea in case there is a sound velocity gradient into sinusoidal modes of an ideal waveguide is grounded. The applicability range of such decomposition is shown. Dispersion in signals of the modes presented in such a way is determined by mathematical reversal without a test source. The structure of discrete modes in a natural waveguide is determined without utilizing the bottom parameters and sound velocity’s distribution over the waveguide depth. The coefficient of the mode signal’s correlation with the measured parameters of the mode signal and a real signal, introduced into it, is shown to be 0.973. The signals from a point emitter positioned at the depth of 50 m in the frequency range of 90–280 Hz in a shallow-water sea (the Barents Sea, a 120 m depth, a 7 km distance), received by a vertical antenna array comprising 32 receivers spaced equidistantly with a 3-m step are used in the experiment. A real signal has been successfully reversed using a mathematical model.     

Virtual modes and the surface boundary condition in underwater acoustics

An explicit expression for the contribution of continuous normal modes to the acoustic pressure field in shallow water is derived under the assumption of constant sound speed in the bottom. An approximate evaluation of the resulting integral leads to a summation over “virtual” modes. These virtual modes are similar to the trapped modes except that their amplitudes decay with range. Their contribution is important in certain situations, for example at short ranges and where there may be few or no trapped modes. The validity of the “pressure release” boundary condition at the surface is investigated. This approximate boundary condition is justified by allowing for energy leakage into the air and examining the virtual mode contribution to the normal mode expression. These virtual modes converge identically to the usual trapped mode expression in the limit of zero air density.     

On the use of SODAR data to estimate mixing height

Estimation of the mixing height is one of the most common applications of SODAR data for more than twenty years and SODAR has been shown to provide reliable results under a great variety of meteorological and site conditions. Nevertheless there are still some unsolved questions. The paper gives a brief review of results obtained during the last two decades. Additionally some of the open problems are discussed in more detail. These are the determination of the stable boundary layer height from digitized SODAR data, its relation to other common height scales, its parameterization, and finally the estimation of the height of a deep convective boundary layer, exceeding the SODAR probing range.     

Modal decomposition method for acoustic impedance testing in square ducts

Accurate duct acoustic propagation models are required to predict and reduce aircraft engine noise. These models ultimately rely on measurements of the acoustic impedance to characterize candidate engine nacelle liners. This research effort increases the frequency range of normal-incidence acoustic impedance testing in square ducts by extending the standard two-microphone method (TMM), which is limited to plane wave propagation, to include higher-order modes. The modal decomposition method (MDM) presented includes four normal modes in the model of the sound field, thus increasing the bandwidth from 6.7 to 13.5 kHz for a 25.4 mm square waveguide. The MDM characterizes the test specimen for normal- and oblique-incident acoustic impedance and mode scattering coefficients. The MDM is first formulated and then applied to the measurement of the reflection coefficient matrix for a ceramic tubular specimen. The experimental results are consistent with results from the TMM for the same specimen to within the 95% confidence intervals for the TMM. The MDM results show a series of resonances for the ceramic tubular material exhibiting a monotonic decrease in the resonant peaks of the acoustic resistance with increasing frequency, resembling a rigidly-terminated viscous tube, and also evidence of mode scattering is visible at the higher frequencies.     

Nonlinear interactions between drift waves and zonal flows

Phase coherent interactions between drift waves and zonal flows are considered. For this purpose, mode coupling equations are derived by using a two-fluid model and the guiding center drifts. The equations are then Fourier analyzed to deduce the nonlinear dispersion relations. The latter depict the excitation of zonal flows due to the ponderomotive forces of drift waves. The flute-like zonal flows with insignificant density fluctuations have faster growth rates than those which have a finite wavelength along the magnetic field direction. The relevance of our investigation to drift wave driven zonal flows in computer simulations and laboratory plasmas is discussed. Received 5 April 2002 Published online 28 June 2002     

轴对称环状静电模的漂移波湍流参量激发理论研究

本文所论述的轴对称环状静电模是指环形磁约束等离子体(如托卡马克)中环向模数为零的近理想静电流体模,它包含有测地声模和基频率与之较低的声模;也含有所谓的‘近零频带状流’.本文根据冷离子流体模型在圆形磁面构成的准环坐标系中的表示,对涉及以上三种模式的漂移波湍流参量激发理论,在一级环形效应近似下,进行了系统讨论,并证明了带状流的四个新命题.利用对漂移波能谱的参数化描写,注意到由漂移波能谱径向有限宽度所引发的特性,如波能传播量的双Landau奇点,揭示了有限宽度对径向δ谱所得结果的重要修正:如,对近零频带状流和测地声模的参量激发条件带来的严格限制.此外,还讨论了密度带状流在高q条件下被激发的可能性.本文选用合理的物理参数.采用图示方法详细地讨论了有关的数值结果.分析表明,测地声模和近零频带状流的参量激发不可能发生在同一小半径处;如果测地声模被参量激发,也应能观察到密度带状流.     

On numerical realizability of thermal convection

Astounded at the regularity of convective structures observed in simulations of mesoscale flow past realistic topography, we investigate the computational aspects of a classical problem of flow over a heated plane. We find that the numerical solutions are sensitive to viscosity, either incorporated a priori or effectively realized in computational models. In particular, anisotropic viscosity can lead to regular convective structures that mimic naturally realizable Rayleigh–Bénard cells, which are unphysical for the specified external parameter range. Details of the viscosity appear to play a secondary role; that is, similar structures can occur for prescribed constant viscosities, explicit subgrid-scale turbulence models, ad-hoc numerical filters, or implicit dissipation of numerical schemes. This implies the need for a careful selection of numerical tools suitable for convection-resolving simulations of atmospheric circulations. The implicit large-eddy-simulation (ILES) approach using non-oscillatory schemes is especially attractive, as for under-resolved calculations it reproduces well the coarsened results of finely-resolved boundary layer convection.     

Improving the description of sunglint for accurate prediction of remotely sensed radiances

The bidirectional reflection distribution function (BRDF) of the ocean is a critical boundary condition for radiative transfer calculations in the coupled atmosphere-ocean system. Existing models express the extent of the glint-contaminated region and its contribution to the radiance essentially as a function of the wind speed. An accurate treatment of the glint contribution and its propagation in the atmosphere would improve current correction schemes and hence rescue a significant portion of data presently discarded as “glint contaminated”. In current satellite imagery, a correction to the sensor-measured radiances is limited to the region at the edge of the glint, where the contribution is below a certain threshold. This correction assumes the sunglint radiance to be directly transmitted through the atmosphere. To quantify the error introduced by this approximation we employ a radiative transfer code that allows for a user-specified BRDF at the atmosphere-ocean interface and rigorously accounts for multiple scattering. We show that the errors incurred by ignoring multiple scattering are very significant and typically lie in the range 10-90%. Multiple reflections and shadowing at the surface can also be accounted for, and we illustrate the importance of such processes at grazing geometries.     

Nonlinear saturation of trapped electron modes via perpendicular particle diffusion

In magnetized fusion plasmas, trapped electron mode (TEM) turbulence constitutes, together with ion temperature gradient (ITG) turbulence, the dominant source of anomalous transport on ion scales. While ITG modes are known to saturate via nonlinear zonal flow generation, this mechanism is shown to be of little importance for TEM turbulence in the parameter regime explored here. Instead, a careful analysis of the statistical properties of the ExB nonlinearity in the context of gyrokinetic turbulence simulations reveals that perpendicular particle diffusion is the dominant saturation mechanism. These findings allow for the construction of a rather realistic quasilinear model of TEM induced transport.