A consistent derivation of the wave-energy equation from basic hydrodynamic principles

Based on a decomposition of the velocity into mean flow, turbulent and wave components, momentum and hereafter a wave-energy equation is derived. It contains a turbulent energy dissipation term which is closed by applying a wave-related mixing length model and linear wave theory solutions. This closure produces a non-linear turbulent wave-energy dissipation including the wave energy in a 5/2 power law. The theory is able to predict correctly the shape of deep-water wave spectra according to Phillips' similarity law.Responsible Editor: Hans Burchard     

Influence of wave groups on SSC patterns over vortex ripples

Estimates of spatial and temporal variations in suspended sand concentrations (SSC) made with a multi-transducer Acoustic Backscatter Sensor (ABS) under a repeated wave group over a mobile rippled bed in the wave research flume at the National Hydraulics Laboratory in Ottawa, Canada, reveal an number of complex and intriguing patterns. Ensemble averages of 8 nearly identical wave groups provided much more robust estimates of SSC and allowed a detailed examination of the wave group effects. The largest SSC near the bed (< 0.10 m) occurs in phase with the largest waves in the group. Above approximately 0.10 m elevation, SSC lags behind the near bed SSC by as much as 2–3 waves; introducing significant curvature (on a semi-log plot) to the SSC profile. The log linear segments of the SSC profile grow and decay systematically on the scale of the wave group. The range in lengths of log-linear profile segments ( 0.03–0.355 m) suggest that the boundary layer thickness also fluctuates throughout the passage of the wave group. Furthermore, there are significant variations in the patterns of SSC, which occur under the largest and smallest waves in the group. Under the largest waves vertical bands of alternating high and low SSC produce an intra-wave modulation in the upper water column ( 0.075–0.30 m). The equivalent horizontal excursion of these bands scales to the ripple length. Under the smaller waves the intra-wave modulation of the SSC disappears and is replaced by temporally homogenous suspension that expands vertically through several individual wave cycles. The former pattern of homogenous suspension appears to be associated with growth of a boundary layer due to the persistent uni-directional horizontal flow during this part of the group together with the persistence of antecedent bed generated turbulence and vorticity which maintains the suspension. The latter pattern of bands of high and low SSC indicates a strong temporal and spatial constraint on the SSC (phase coupling) induced by the presence of the bedforms which may be enhanced by strong reversals in both flow and vorticity under the large waves in the group.     

Some observations of bipolar flashes during summer thunderstorms near Warsaw

Lightning discharges monitored by the SAFIR network system in Poland have been additionally identified over the 100×100 km area near Warsaw by single-point independent recordings of electric field and Maxwell current rapid changes. The data collected in summer thunderstorm days of 2002 showed some untypical properties of the lightning discharges which are rarely observed. Especially remarkable was a number of ground multi-stroke flashes with the return strokes (RS) which transported to the earth charges of opposite signs. Bipolar flashes (BF) of this kind were mostly involved in the events in which the nearby intracloud (ic) and cloud-to-ground (c-g) discharges were very closely associated in time. Events of such a close collocation of two different types of lightning discharges, previously called the complex lightning discharge events (CLDE), were quite often observed during summer thunderstorms in Poland. The events of this kind, i.e. 8 flashes, identified by the SAFIR detection system as BF’s present the multiple stroke flashes of the mean horizontal separation distance between striking points of particular RS equal to (2.8 ± 2.1) km and of the mean time interval between strokes of (46.8 ± 74.4) ms. The time separation between the observed BF and the adjacent ic flashes was from 0.1 to 335 ms, and horizontal separation distance between them ranged from 1.8 to 14.5 km. The multiplicity of the recorded BF’s ranged from 2 to 4 strokes. Four of these BF’s followed the ic discharge, but the other three preceded the ic and one was alone with no close ic.     

Diagnostic analyses of convective events — The effect of propagating gust fronts

This paper is devoted to the testing experiment of a postprocessing tool aimed at the objective analysis of propagating gust fronts in a given convective environment. The tool is being developed to be applicable in the operational mode by utilizing NWP model outputs. The experiment was carried out on two summer convective cases which occurred in the Czech Republic. The cases were numerically simulated by the limited area NWP model LM COSMO with the horizontal resolution of 2.8 km. They represent different types of convective systems, both accompanied by objectively identifiable gust fronts and causing heavy precipitation. The event from July 2000 was characterised by the development of isolated thunderstorms. The other event from July 1998 was a long-lasting and organised convective system — a squall line. The hypothesis was that the developed postprocessing tool is capable to evaluate the role which downdraft outflows played in the decay and initiation of convective cells by interaction with convective environment and thus in prolongation of convection lifetime.The procedures of the Objective Analysis of Gust Fronts (OAGF) were applied to the thermodynamic outputs of the LM COSMO. The aim was to determine the position of gust fronts within the domain and to assess their speed of movement and the potential to initiate convection according to the properties of ambient vertical shear and stability as well as humidity conditions ahead of the respective downdraft outflows. In addition, the Radar Simulation Model (RSM) was employed to monitor the simulated convective systems in arbitrary PPI and RHI scans and to verify qualitatively the LM COSMO precipitation forecasts.The case studies has confirmed the applicability of the LM COSMO–OAGF chain and RSM, which may represent the potential for improving the operational nowcasting of hazardous convection phenomena. In both simulations, the objective gust fronts moved on into the vertical shear-favourable environment for triggering new convection. In addition to the dynamical organisation, there were also favourable stability and humidity conditions in the area of forced upward motions in the simulation of the event from July 2000.     

The thermodynamic structure atop a penetrating convective thunderstorm

The thermodynamic structure on top of a numerically simulated severe storm is examined to explain the satellite observed plume formation above thunderstorm anvils. The same mechanism also explains the formation of jumping cirrus observed by Fujita on board of a research aircraft. A three-dimensional, non-hydrostatic cloud model is used to perform numerical simulation of a supercell that occurred in Montana in 1981. Analysis of the model results shows that both the plume and the jumping cirrus phenomena are produced by the high instability and breaking of the gravity waves excited by the strong convection inside the storm. These mechanisms dramatically enhance the turbulent diffusion process and cause some moisture to detach from the storm cloud and jump into the stratosphere. The thermodynamic structure in terms of the potential temperature isotherms above the simulated thunderstorm is examined to reveal the instability and wave breaking structure. The plumes and jumping cirrus phenomena represent an irreversible transport mechanism of materials from the troposphere to the stratosphere that may have global climatic implications.     

Boulders of MIS 5 age deposited by a tsunami on the coast of Otago, New Zealand

A series of elevated imbricated boulders were investigated on the Otago coastline, southeast New Zealand, through field surveying and optical luminescence dating. By using established hydrodynamic relationships of sediment transport the energy required to move the clasts was calculated and compared to the historic record of marine inundations of that coast. The boulders are platy in shape and are over 2 m long in some cases, and are sourced from a locally outcropping conglomerate unit which appears to be the only lithology on this section of coast that erodes to produce clasts of this size. It is estimated that the boulders were deposited by a tsunami between 2 and 3 m high during the latter part of Marine Isotope Stage 5. They therefore represent the first pre-Holocene tsunami deposit and one composed of large boulders described on the New Zealand coastline.     

腾冲盆地阻水构造的发现

腾冲来凤山北麓前缘,潜伏一段阻水构造——地下水位坡降带。从研究13个水文钻井、119口民间井、2个泉水露头地下水位特征入手,采用古水文和现代水文地质研究方法,论证此段阻水构造的存在。     

太阳常数的变化与太阳软x射线辐射流量及低频天波时延的关系

本文对1982年在陕西临潼接收的美国罗兰-C西北大平洋链Y台发射的100kHz低频一跳天波时延的实测资料及SMS－GOES测量的太阳1～8A软x射线的每日辐射流量与SMM／ACRIMI测量的太阳常数之间的关系进行了分析。结果表明：太阳1～8A软x射线辐射流量与太阳常数之间存在较强的负相关;低频一跳天波时延与太阳常数之间存在着较强的正相关。并对此进行了讨论。     

Discrimination of small earthquakes and artificial explosions in the Korean Peninsula using Pg/Lg ratios

The purpose of this study is to develop a technique to discriminate artificial explosions from local small earthquakes ( M ≤ 4.0) in the time–frequency domain. In order to obtain spectral features of artificial explosions and earthquakes, 3-D spectrograms (frequency, time and amplitude) have been used. They represent a useful tool for studying the frequency content of entire seismic waveforms observed at local and regional distances (Kim, Simpson & Richards 1994). P and S(L g ) waves from quarry blasts show that the frequency content associated with the dominant amplitude appears above 10  Hz and Rg phases are observed at close distances. P and S(L g ) waves from the Tongosan earthquake have strong amplitudes below 10  Hz. For the Munkyong earthquake, however, a broader frequency content up to 20  Hz is found.
  For discrimination between small earthquakes and explosions, Pg/L g spectral ratios are used below 10  Hz, and through spectrogram analysis we can see different frequency contents of explosions and earthquakes. Unfortunately, because explosion data recorded at KSRS array are digitized at 20  sps, we cannot avoid analysing below 10  Hz because of the Nyquist frequency. In order to select time windows, the group velocity was computed using multiple-filter analysis (MFA), and free-surface effects have been removed from all three-component data in order to improve data quality. Using FFT, a log-average spectral amplitude is calculated over seven frequency bands: 0.5 to 3, 2 to 4, 3 to 5, 4 to 6, 5 to 7, 6 to 8 and 8 to 10  Hz. The best separation between explosions and earthquakes is observed from 6 to 8  Hz. In this frequency band we can separate explosions with log ( Pg/L g ) above −0.5, except EXP1 recorded at SIHY1-1, and earthquakes below −0.5, except the Munkyong earthquake record at station KMH.     

An analytical source function for a coupled hybrid wave model

An analytical form for the source function is formulated by comparing the fetch-limited approximation of the Ocean Wave Transport equation and the empirical equation for the fetch-dependent wave forecast nomograms. The source function thus generated has been utilised in the numerical model based on Toba’s formulation of wave transport equation and tested for the seas around the Indian subcontinent (5°S to 25°N latitude; 45°E to 100°E longitude). The grid averaged hindcast wave heights are found to be moderately matching with the GEOSAT altimeter measured significant wave heights of the 1987–1989 period, particularly for waves higher than 1 meter.