Multiresolution Flux Decomposition

Geophysical variables are orthogonally decomposed by averaging timeseries using different averaging lengths, referred to as a (Haar)multiresolution decomposition. This simple and economic decomposition isassociated with cospectra that formally satisfy Reynolds averaging rules foreach averaging length. The multiresolution decomposition provides a naturalestimate of the random error in estimating a mean turbulent flux. The Fourierand multiresolution decompositions are compared using aircraft data fromBOREAS.     

Countergradient Fluxes of Conserved Variables in the Clear convective and Stratocumulus-topped Boundary Layer: The role of the Entrainment Flux

Large-eddy simulations of a clear convective boundary layer (CBL)and a stratocumulus-topped boundary layer are studied. Bottom-upand a top-down scalars were included in the simulations, and theprinciple of linear superposition of variables was applied toreconstruct the fields of any arbitrary conserved variable.This approach allows a systematic analysis of countergradient fluxesas a function of the flux ratio, which is defined as the ratio betweenthe entrainment flux and the surface flux of the conserved quantity.In general, the turbulent flux of an arbitrary conserved quantityis counter to the mean vertical gradient if the heights where thevertical flux and the mean vertical gradient change sign do notcoincide. The regime where the flux is countergradient is thereforebounded by the so-called zero-flux and zero-gradient heights. Becausethe vertical flux changes sign only if the entrainment flux has anopposite sign to the surface flux, countergradient fluxes arepredominantly found for negative flux ratios. In the CBL the fluxratio for the virtual potential temperature is, to a good approximation,constant, and equal to -0.2. Only if the moisture contribution to thevirtual potential temperature is negligibly small will the flux ratio forthe potential temperature be equal to this value. Otherwise, theflux ratio for the potential temperature can have any arbitrary(negative) value, and, as a consequence, the fluxes for thepotential temperature and the virtual potential temperature willbe countergradient at different heights. As a practical application ofthe results, vertical profiles of the countergradient correction termfor different entrainment-to-surface-flux ratios are discussed.     

Variation of the plasma turbulence in the central plasma sheet during substorm phases observed by the interball/tail satellite

Fluctuations of the plasma bulk velocity across the plasma sheet are studied using single-point measurements from the Corall instrument on board the Interball/Tail satellite. Several hour-long intervals of continuous data corresponding to quiet geomagnetic conditions and different phases of isolated substorms are analyzed. The plasma sheet flow appears to be strongly turbulent, i.e. dominated by fluctuations that are unpredictable. Corresponding eddy diffusion coefficients were obtained as a function of the autocorrelation time and rms velocity of the fluctuations. It was found that the amplitude of the turbulence and the values of eddy-diffusion coefficients increase significantly during substorm growth and expansion phases and they decrease to their initial level during the recovery phase. We also studied a relationship between the eddy-diffusion coefficients and the absolute value of the geomagnetic field, also measured by the Interball/Tail satellite. It was found that this relationship varies depending on the phase of substorm, indicating possible change in the turbulence regimen with substorm phase.     

Spontaneous Emission of Gravity Waves by Interacting Vortex Dipoles in a Stratified Fluid: Laboratory Experiments

Results from a new series of experiments on the geophysically important issue of spontaneous emission of internal gravity waves during unsteady interactions of vortical structures are presented. Vortex dipoles are a common element of a quasi-two-dimensional turbulent flow. Vortex dipoles perform translational motion and can collide with other vortices. During collision events the flow is unsteady and unbalanced and a further adjustment process associated with these events can therefore result in the spontaneous emission of gravity waves. Our laboratory experiments demonstrate that gravity waves are emitted when two translating vortex dipoles interact (collide) in a layered fluid, in accord with the current theoretical results. The emission was evident both in a two-layer system and in a fluid with a linear distribution of density with depth. The waves were generated during the period of deceleration of the secondary dipoles which constitute a vortex quadrupole emerging immediately after the collision of the primary dipoles.     

黄海海雾WRF数值模拟中垂直分辨率的敏感性研究

基于WRF模式探究不同垂直分辨率下模式对黄海海雾的模拟表现。将3种垂直分层方案（35η、44η与63η）和2种边界层方案（YSU、MYNN）组合,对10次海雾做模拟研究。统计分析了水平雾区与雾顶高度对垂直分辨率的敏感性,并利用1次典型个例剖析了雾顶长波辐射与雾体湍流的作用。统计结果显示,提高垂直分辨率能显著改进水平雾区的模拟效果,改善明显的个例在不同垂直分辨率试验下的雾顶高度差异也较大;YSU方案相比于MYNN方案更敏感,从35η层增至44η层其试验个例的击中率（POD）和公正预兆得分（ETS）的平均改进率分别提高了13.29%和10.22%。典型个例研究结果揭示,模式对雾顶长波辐射和雾体湍流作用的模拟程度强烈依赖垂直分辨率：粗垂直分辨率给出的湍流强度很弱,导致模拟失败;雾顶存在“云水含量增多→长波辐射增强→降温加大→云水含量增多”的正反馈过程,细垂直分辨率比粗垂直分辨率更容易维持与增强此反馈;只有细垂直分辨率才能模拟出雾顶长波辐射冷却产生的贯穿雾体直抵海面、强度不弱于近海面机械湍流的浮力湍流,它导致雾体降温而出现符合观测事实的海温高于气温的现象。     

Effects of wave–current interaction on the current profile

Wave–current laboratory experiments have shown that the logarithmic current profile observed in pure current flows is modified due to the presence of waves. When waves propagate opposite the current, an increase in the current intensity is achieved near the mean water level, while a reduction is obtained for following waves and currents. With the aim of analyzing these nonlinear effects along the whole water column, an Eulerian wave–current model is presented. In contrast to previously presented wave–current models, the present is able to include the variation of the free surface elevation due to the wave motion and the effect of a non hydrostatic pressure field. Therefore it does not restrict its application to waves in shallow waters. Moreover, the model is able to simulate all the possible angles between waves and currents.     

Temperature and heat flux spectra in the turbulent buoyancy subrange

The physical nature of motions with scales intermediate between approximately isotropic turbulence and quasi-linear internal gravity waves is not understood at the present time. Such motions play an important role in the energetics of small scales processes, both in the ocean and in the atmosphere, and in vertical transport of heat and constituents. This scale range is currently interpreted either as a saturated gravity waves field or as a buoyancy range of turbulence.We first discuss some distinctive predictions of the classical (Lumley, Phillips) buoyancy range theory, recently improved (Weinstock, Dalaudier and Sidi) to describe potential energy associated with temperature fluctuations. This theory predicts the existence of a spectral gap in the temperature spectra and of an upward mass flux (downward buoyancy and heat fluxes), strongly increasing towards large scales. These predictions are contrasted with an alternate theory, assuming energetically insignificant buoyancy flux, proposed by Holloway.Then we present experimental evidences of such characteristic features obtained in the lower stratosphere with an instrumented balloon. Spectra of temperature, vertical velocity, and cospectra of both, obtained in homogeneous, weakly turbulent regions, are compared with theoretical predictions. These results are strongly consistent with the improved classical buoyancy range theory and support the existence of a significant downward heat flux in the buoyancy range.The theoretical implications of the understanding of this scale range are discussed. Many experimental evidences consistently show the need for an anisotropic theory of the buoyancy range of turbulence.     

HEIFE戈壁地区近地层大气的湍流结构和输送特征

根据黑河地区地气相互作用实验研究(HEIFE)1988年POP和1990年PIOP实验的观测资料,分析了戈壁地区近地层大气的湍流结构,主要是风速分量和温度方差的相似关系,以及各有关量的功率谱和湍流通量的协谱。1988年POP期间发现的戈壁近地层大气中白天经常出现的水汽由上而下输送等特殊现象,在1990年PIOP中得到进一步的验证。文中分析了有关物理机制,认为这一现象和众所周知的“绿洲效应”是戈壁-绿洲这一地区性中尺度环流影响的两个侧面。     

天津市郊大气边界层湍谱特征分析

用１９９０年７月在天津市郊的观测资料，计算了风速谱、温度谱及动量和热量通量的协谱。结果表明，市郊下垫面上的大气湍流谱特征与平坦地形上得到的典型结果基本一致，但满足各向同性的湍流尺度似乎比在平坦地形上的略大，谱的峰值频率区域略窄，谱的低频区（含能区）特征与平坦地形上的情况也不同。     

The granulometrical characteristics of a slowly-moving dust cloud

A torn-up road offered the opportunity to sample a dust cloud continuously at different heights during a time interval of 51 hours. In particular the granulometrical characteristics of the dust caught were investigated. It appears that if the air layer in the vicinity of a dust cloud is not too turbulent, the cloud will be clearly granulometrically stratified. The coarse material moves chiefly at the bottom of the cloud, whereas the fine material will occur both at the bottom and at the top. The experiment also shows that the higher the dust in the cloud, the worse it is sorted. Above a critical height, however, the degree of sorting increases again. The variation of the mean dust diameter d₅₀, as a function of the height z, can be expressed in a simple semi-logarithmic equation. The variation of the dust quantity G, as a function of the height z, can also be expressed in a semi-logarithmic form but the correlation is higher when a power equation is used. The ratio of fine silt to coarse silt seems to vary parabolically with height. One can only speak of an optimal transport height in the case of particles > 16 μm. For finer particles, the turbulence of a normal air stream is usually high enough to carry the particles anywhere in the dust cloud, so that one can no longer speak of an optimal transport height. The granulometrical border between aeolian transport in saltation and aeolian transport in suspension lies, according to the results of the experiment, at a diameter of about 63 μm.