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Vertical Resolution (vertical + resolution)

Distribution by Scientific Domains

Earth and Environmental Science	75%
Engineering	10%
4 Other Domains	15%

Distribution within Earth and Environmental Science

Atmospheric Sciences	70%
Geology & Geophysics	13%

Selected Abstracts

Increasing the vertical resolution of conventional sub-bottom profilers by parametric equalization

GEOPHYSICAL PROSPECTING, Issue 2 2002
P. Cobo
ABSTRACT Vertical resolution, i.e. the ability to resolve two close reflectors, is a crucial aspect of pulses used in geo-acoustic exploration of sea sub-bottoms. This paper deals with the problem of exploring the shallowest unconsolidated layers of the seafloor with conventional piezo-electric sonar pulses. Such transducers do not have a sufficiently broad transmission response to enable them to radiate short high-resolution pulses. Therefore, some kind of equalization process must be applied to broaden the transmission response. Here, inverse filtering is used to calculate the transducer driving waveform so that the subsequent acoustic pulse has a zero-phase cosine-magnitude nature. Within a specified bandwidth, this pulse has minimum length, i.e. maximum resolution. The method has been applied to compress the acoustic pulses radiated by two piezo-electric transducers. In conventional performance, these transducers radiate narrowband pulses which contain several cycles at the natural resonance frequency. Under equalized driving, both transducers emit broadband pulses, with resolving power greatly increased, at the cost of some amplitude loss. That is, the pulses radiated by both transducers have been shortened from 1 ms (low-frequency transducer) and 0.274 ms (high-frequency transducer) in conventional performance to 0.13 ms and 0.038 ms in equalized mode, with amplitude losses of 33% and 56%, respectively. The great improvement in the resolution of this technique is demonstrated by comparing the synthetic echograms that should be obtained when exploring a wedge model using zero-phase cosine-magnitude pulses with conventional ping pulses. [source]

High resolution quantification of gully erosion in upland peatlands at the landscape scale

EARTH SURFACE PROCESSES AND LANDFORMS, Issue 8 2010
Martin Evans
Abstract The upland peatlands of the UK are severely eroded, with large areas affected by gully erosion. The peatlands are important areas of carbon storage and provide a range of other ecosystem services including water supply and biodiversity all of which are negatively impacted by erosion of the upland surface. The magnitude of the gully erosion, and consequent adjustment of the peatland morphology, is such that in degraded peatlands the extent and magnitude of erosion is a major control on peatland function. Accurate mapping of gully form is therefore a necessary precondition to the understanding and management of these systems. This paper develops an approach to extracting gully maps from high resolution digital elevation models (DEMs). Gully maps of the Bleaklow Plateau in northern England were derived from a 2,m LiDAR DEM by combining areas of low difference from mean elevation and high positive plan curvature. Gully depth was modelled by interpolating between gully edges. Testing of the gully mapping and depth modelling against aerial photography, manual interpretation of the DEM and ground survey revealed that gully plan form is well represented and gully width and depth are modelled with tolerances close to the horizontal and vertical resolution of the LiDAR imagery. Estimates of gully width and depth were less reliable for gullies with total width of less than four pixels. The approach allows for the first time the derivation of accurate estimates of gully extent and magnitude over large areas and provides the basis for modelling a range of processes controlled by gullying. The approach has wider applicability to mapping gully erosion in a wide range of environments. Copyright © 2010 John Wiley & Sons, Ltd. [source]

Computational fluid dynamics modelling of boundary roughness in gravel-bed rivers: an investigation of the effects of random variability in bed elevation

EARTH SURFACE PROCESSES AND LANDFORMS, Issue 4 2001
A.P. Nicholas
Abstract Results from a series of numerical simulations of two-dimensional open-channel flow, conducted using the computational fluid dynamics (CFD) code FLUENT, are compared with data quantifying the mean and turbulent characteristics of open-channel flow over two contrasting gravel beds. Boundary roughness effects are represented using both the conventional wall function approach and a random elevation model that simulates the effects of supra-grid-scale roughness elements (e.g. particle clusters and small bedforms). Results obtained using the random elevation model are characterized by a peak in turbulent kinetic energy located well above the bed (typically at y/h,=,0·1,0·3). This is consistent with the field data and in contrast to the results obtained using the wall function approach for which maximum turbulent kinetic energy levels occur at the bed. Use of the random elevation model to represent supra-grid-scale roughness also allows a reduction in the height of the near-bed mesh cell and therefore offers some potential to overcome problems experienced by the wall function approach in flows characterized by high relative roughness. Despite these benefits, the results of simulations conducted using the random elevation model are sensitive to the horizontal and vertical mesh resolution. Increasing the horizontal mesh resolution results in an increase in the near-bed velocity gradient and turbulent kinetic energy, effectively roughening the bed. Varying the vertical resolution of the mesh has little effect on simulated mean velocity profiles, but results in substantial changes to the shape of the turbulent kinetic energy profile. These findings have significant implications for the application of CFD within natural gravel-bed channels, particularly with regard to issues of topographic data collection, roughness parameterization and the derivation of mesh-independent solutions. Copyright © 2001 John Wiley & Sons, Ltd. [source]

High-resolution seismic imaging in deep sea from a joint deep-towed/OBH reflection experiment: application to a Mass Transport Complex offshore Nigeria

GEOPHYSICAL JOURNAL INTERNATIONAL, Issue 3 2010
S. Ker
SUMMARY We assess the feasibility of high-resolution seismic depth imaging in deep water based on a new geophysical approach involving the joint use of a deep-towed seismic device (SYSIF) and ocean bottom hydrophones (OBHs). Source signature measurement enables signature deconvolution to be used to improve the vertical resolution and signal-to-noise ratio. The source signature was also used to precisely determine direct traveltimes that were inverted to relocate source and receiver positions. The very high accuracy of the positioning that was obtained enabled depth imaging and a stack of the OBH data to be performed. The determination of the P -wave velocity distribution was realized by the adaptation of an iterative focusing approach to the specific acquisition geometry. This innovative experiment combined with advanced processing succeeded in reaching lateral and vertical resolution (2.5 and 1 m) in accordance with the objectives of imaging fine scale structures and correlation with in situ measurements. To illustrate the technological and processing advances of the approach, we present a first application performed during the ERIG3D cruise offshore Nigeria with the seismic data acquired over NG1, a buried Mass Transport Complex (MTC) interpreted as a debris flow by conventional data. Evidence for a slide nature of a part of the MTC was provided by the high resolution of the OBH depth images. Rigid behaviour may be inferred from movement of coherent material inside the MTC and thrust structures at the base of the MTC. Furthermore, a silt layer that was disrupted during emplacement but has maintained its stratigraphic position supports a short transport distance. [source]

Increasing the vertical resolution of conventional sub-bottom profilers by parametric equalization

Process Considerations for Trolling Borehole Flow Logs

GROUND WATER MONITORING & REMEDIATION, Issue 3 2006
Phil L. Oberlander
Horizontal hydraulic conductivity with depth is often understood only as a depth-integrated property based on pumping tests or estimated from geophysical logs and the lithology. A more explicit method exists for determining hydraulic conductivity over small vertical intervals by collecting borehole flow measurements while the well is being pumped. Borehole flow rates were collected from 15 deep monitoring wells on the Nevada Test Site and the Nevada Test and Training Range while continuously raising and lowering a high-precision impeller borehole flowmeter. Repeated logging passes at different logging speeds and pumping rates typically provided nine unique flow logs for each well. Over 60 km of borehole flow logs were collected at a 6.1-cm vertical resolution. Processing these data necessitated developing a methodology to delete anomalous values, smooth small-scale flow variations, combine multiple borehole flow logs, characterize measurement uncertainty, and determine the interval-specific lower limit to flow rate quantification. There are decision points in the data processing where judgment and ancillary analyses are needed to extract subtle hydrogeologic information. The analysis methodology indicates that processed measurements from a high-precision trolling impeller flowmeter in a screened well can confidently detect changes in borehole flow rate of ,0.7% of the combined trolling and borehole flow rate. An advantage of trolling the flowmeter is that the impeller is nearly always spinning as it is raised and lowered in the well and borehole flow rates can be measured at lower values than if measurements were taken while the flowmeter was held at a fixed depth. [source]

Further experiences with computing non-hydrostatic free-surface flows involving water waves

INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 2 2005
Marcel Zijlema
Abstract A semi-implicit, staggered finite volume technique for non-hydrostatic, free-surface flow governed by the incompressible Euler equations is presented that has a proper balance between accuracy, robustness and computing time. The procedure is intended to be used for predicting wave propagation in coastal areas. The splitting of the pressure into hydrostatic and non-hydrostatic components is utilized. To ease the task of discretization and to enhance the accuracy of the scheme, a vertical boundary-fitted co-ordinate system is employed, permitting more resolution near the bottom as well as near the free surface. The issue of the implementation of boundary conditions is addressed. As recently proposed by the present authors, the Keller-box scheme for accurate approximation of frequency wave dispersion requiring a limited vertical resolution is incorporated. The both locally and globally mass conserved solution is achieved with the aid of a projection method in the discrete sense. An efficient preconditioned Krylov subspace technique to solve the discretized Poisson equation for pressure correction with an unsymmetric matrix is treated. Some numerical experiments to show the accuracy, robustness and efficiency of the proposed method are presented. Copyright © 2004 John Wiley & Sons, Ltd. [source]

Three-Dimensional Microstructural Characterization of Porous Hydroxyapatite Using Confocal Laser Scanning Microscopy

INTERNATIONAL JOURNAL OF APPLIED CERAMIC TECHNOLOGY, Issue 3 2005
Fei Ren
The characterization of porosity is crucial in the development and commercialization of ceramic bone replacement technology, since the pore size and interconnectivity play a central role in both biological function (bone ingrowth and nutrient flow) as well as mechanical properties of bone scaffolds. The ability of confocal laser scanning microscopy (CLSM) to image three-dimensional (3-D) structures with large vertical depths (,2 mm) and fine vertical resolution (,1 ,m) is utilized in this article to characterize the 3-D microstructures of hydroxyapatite (HA) bone scaffold specimens with porosity ranging from roughly 60,70 vol%. Various CLSM techniques are applied to image and interpret the HA pore structure, including Z -series stacking, topographic profiling, and Phi- Z scanning and contour mapping. [source]

The impact of vertical resolution on regional model simulation of the west African summer monsoon

INTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 10 2008
Leonard M. Druyan
Abstract The RM3 regional climate model is used to simulate the west African summer monsoon for six June,September seasons using NCEP reanalysis data for lateral boundary forcing. The study compares the performance of the previously published 16-level version with a newly tested 28-level version, both running on a horizontal grid with 0.5° spacing, in order to determine what improvements in simulations are achieved by increased vertical resolution. Comparisons between the performances include diagnostics of seasonal mean precipitation rates and circulation, vertical profiles of cumulus heating rates, frequencies of shallow and deep convection and diagnostics related to transient African easterly waves (AEWs). The characteristics of a composite AEW simulated at both vertical resolutions are presented. Results show that the most significant impact of increasing the vertical resolution is stronger circulation, stronger vertical wind shear and higher amplitude AEWs. The simulations with higher vertical resolution also achieve higher peaks of cumulus latent heating rates. Spatial,temporal correlations between simulated daily 700 mb meridional winds versus corresponding NCEP reanalysis data and simulated daily precipitation versus estimates from the Tropical Rainfall Measurement Mission (TRMM) archive were equally high at both vertical resolutions. Copyright © 2007 Royal Meteorological Society [source]

CHARACTERIZATION OF FOOD SURFACES USING SCALE-SENSITIVE FRACTAL ANALYSIS

JOURNAL OF FOOD PROCESS ENGINEERING, Issue 2 2000
FRANCO PEDRESCHI
ABSTRACT Length-scale and area-scale analyses, two of the scale-sensitive fractal analyses performed by the software Surfraxhttp://www.surfract.com, were used to study food surfaces measured with a scanning laser microscope (SLM). The SLM measures surfaces, or textures (i.e., acquires topographical data as a collection of heights as a function of position), at a spatial and vertical resolution of 25 ,m. The measured textures are analyzed by using linear and areal tiling (length-scale and area-scale analysis) and by conventional statistical analyses. Area-scale and length-scale fractal complexities (Lsfc and Asfc) and the smooth-rough crossover (SRC) are derived from the scale-sensitive fractal analyses. Both measures proved adequate to quantify and differentiate surfaces of foods (e.g., chocolate and a slice of bread), which were smooth or porous to the naked eye. Surfaces generated after frying of potato products (e.g., potato chips and French fries) had similar values of Asfc and SRC, and larger (implying more complex and rougher surfaces) than those of the raw potato. Variability of surface texture characterization parameters as a function of the size of the measured region was used in selecting the size of the measured regions for further analysis. The length-scale method of profile analysis (also called the Richardson or compass method) was useful in determining the directionality or lay of the anisotropic texture on food surfaces. [source]

Wind speed-up in the Dover Straits with the Met Office New Dynamics Model

METEOROLOGICAL APPLICATIONS, Issue 3 2003
Rachel Anne Capon
It is part of British sailing and forecasting folklore that the wind speed increases in the Dover Straits when there is an established wind-flow ,westerly/south-westerly or easterly/north-easterly ,along the English Channel. However the underlying mechanism of the phenomenon is unclear. We have used the Met Office ,New Dynamics' mesoscale model to perform a case study on an occasion when this phenomenon was observed in the Channel but not forecast well by the operational model, UM 4.5. Results are presented showing the sensitivity of forecasts to horizontal resolution (down to 2 km) and to vertical resolution. We probe the physical mechanism of the Channel jet by altering the surrounding orography and the land or sea surface roughness. Both the orography and the surface roughness are shown to influence the jet formation. Copyright © 2003 Royal Meteorological Society [source]

Optical turbulence vertical distribution with standard and high resolution at Mt Graham

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2010
E. Masciadri
ABSTRACT A characterization of the optical turbulence vertical distribution (C2N profiles) and all the main integrated astroclimatic parameters derived from the C2N and the wind speed profiles above the site of the Large Binocular Telescope (LBT) (Mt Graham, Arizona, USA) is presented. The statistics include measurements related to 43 nights done with a Generalized SCIDAR (GS) used in standard configuration with a vertical resolution ,H, 1 km on the whole 20 km and with the new technique (High Vertical Resolution GS) in the first kilometre. The latter achieves a resolution ,H, 20,30 m in this region of the atmosphere. Measurements done in different periods of the year permit us to provide a seasonal variation analysis of the C2N. A discretized distribution of C2N, useful for the Ground Layer Adaptive Optics (GLAO) simulations, is provided and a specific analysis for the LBT Laser Guide Star system ARGOS (running in GLAO configuration) case is done including the calculation of the ,grey zones' for J, H and K bands. Mt Graham is confirmed to be an excellent site with median values of the seeing without dome contribution ,= 0.72 arcsec, the isoplanatic angle ,0= 2.5 arcsec and the wavefront coherence time ,0= 4.8 ms. We find that the OT vertical distribution decreases in a much sharper way than what has been believed so far in the proximity of the ground above astronomical sites. We find that 50 per cent of the whole turbulence develops in the first 80 ± 15 m from the ground. We finally prove that the error in the normalization of the scintillation that has been recently demonstrated in the principle of the GS technique affects these measurements by an absolutely negligible quantity (0.04 arcsec). [source]

High-resolution seismic and ground penetrating radar,geophysical profiling of a thermokarst lake in the western Lena Delta, Northern Siberia

PERMAFROST AND PERIGLACIAL PROCESSES, Issue 4 2002
G. J. Schwamborn
Abstract High-resolution seismic and ground-penetrating-radar (GPR) data have been acquired over Lake Nikolay in the western Lena Delta in order to study the uppermost basin fill and the bordering frozen margins. GPR (100 MHz antenna pair) measurements were completed on the frozen lake and its permafrost margins, while high-resolution seismic data were acquired from the lake during open-water conditions in summer using a 1.5,11.5 kHz Chirp profiler. The combined use of the two profiling systems allows stratigraphic profiling in both frozen and unfrozen parts of the lake. Shallow seismic reflection images of the uppermost 4 to 5 m of sediments are compared to GPR sections, which have approximately the same horizontal and vertical resolution. Short sediment cores aid calibrate the geophysical data. Copyright © 2002 John Wiley & Sons, Ltd. [source]

An improved PDF cloud scheme for climate simulations

THE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 651 2010
Akira Kuwano-Yoshida
Abstract An efficient grid-scale cloud scheme for climate simulation is implemented in the atmospheric general circulation model for the Earth Simulator (AFES). The new cloud scheme uses statistical partial condensation using joint-Gaussian probability distribution functions (PDFs) of the liquid water potential temperature and total water content, with standard deviations estimated by the moist Mellor,Yamada level-2 turbulence scheme. It also adopts improved closure parameters based on large-eddy simulations and a revised mixing length that varies with the stability and turbulent kinetic energy. These changes not only enable better representation of low-level boundary layer clouds, but also improve the atmospheric boundary layer structure. Sensitivity experiments for vertical resolution suggest that O(100,200 m) intervals are adequate to represent well-mixed boundary layers with the new scheme. The new scheme performs well at relatively low horizontal resolution (about 150 km), although inversion layers near the coast become more intense at a higher horizontal resolution (about 50 km). Copyright © 2010 Royal Meteorological Society [source]

Ensemble simulations of the cold European winter of 2005-2006

THE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 636 2008
A. A. Scaife
Abstract There is only limited understanding of the processes driving year-to-year variability in European winter climate and the skill of seasonal forecasts for Europe in winter is generally low. The winter of 2005-2006 is a useful case-study because it was the coldest winter in large parts of western Europe for over a decade, and the coldest in central England since 1995-1996. Here, we present results of experiments with a range of general circulation models to investigate the importance of both the Atlantic Ocean and stratospheric circulation in producing the unusually cold winter of 2005-2006. We use models with different combinations of horizontal and stratospheric vertical resolution, allowing the sensitivity of the response to model formulation to be tested. The response to Atlantic sea-surface temperature (SST) anomalies is improved in a more recent model with higher horizontal resolution. The results show that both Atlantic SSTs and the January 2006 sudden stratospheric warming are likely to have contributed to the cold 2005-2006 European winter. © Crown Copyright 2008. Reproduced with the permission of HMSO. Published by John Wiley & Sons Ltd. [source]

Evaluation of a large-eddy model simulation of a mixed-phase altocumulus cloud using microwave radiometer, lidar and Doppler radar data

THE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 618 2006
J. H. Marsham
Abstract Using the Met Office large-eddy model (LEM) we simulate a mixed-phase altocumulus cloud that was observed from Chilbolton in southern England by a 94 GHz Doppler radar, a 905 nm lidar, a dual-wavelength microwave radiometer and also by four radiosondes. It is important to test and evaluate such simulations with observations, since there are significant differences between results from different cloud-resolving models for ice clouds. Simulating the Doppler radar and lidar data within the LEM allows us to compare observed and modelled quantities directly, and allows us to explore the relationships between observed and unobserved variables. For general-circulation models, which currently tend to give poor representations of mixed-phase clouds, the case shows the importance of using: (i) separate prognostic ice and liquid water, (ii) a vertical resolution that captures the thin layers of liquid water, and (iii) an accurate representation the subgrid vertical velocities that allow liquid water to form. It is shown that large-scale ascents and descents are significant for this case, and so the horizontally averaged LEM profiles are relaxed towards observed profiles to account for these. The LEM simulation then gives a reasonable cloud, with an ice-water path approximately two thirds of that observed, with liquid water at the cloud top, as observed. However, the liquid-water cells that form in the updraughts at cloud top in the LEM have liquid-water paths (LWPs) up to half those observed, and there are too few cells, giving a mean LWP five to ten times smaller than observed. In reality, ice nucleation and fallout may deplete ice-nuclei concentrations at the cloud top, allowing more liquid water to form there, but this process is not represented in the model. Decreasing the heterogeneous nucleation rate in the LEM increased the LWP, which supports this hypothesis. The LEM captures the increase in the standard deviation in Doppler velocities (and so vertical winds) with height, but values are 1.5 to 4 times smaller than observed (although values are larger in an unforced model run, this only increases the modelled LWP by a factor of approximately two). The LEM data show that, for values larger than approximately 12 cm s,1, the standard deviation in Doppler velocities provides an almost unbiased estimate of the standard deviation in vertical winds, but provides an overestimate for smaller values. Time-smoothing the observed Doppler velocities and modelled mass-squared-weighted fallspeeds shows that observed fallspeeds are approximately two-thirds of the modelled values. Decreasing the modelled fallspeeds to those observed increases the modelled IWC, giving an IWP 1.6 times that observed. Copyright © 2006 Royal Meteorological Society [source]

The diurnal cycle of shallow cumulus clouds over land: A single-column model intercomparison study

THE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 604 2004
Geert Lenderink
Abstract An intercomparison study for single-column models (SCMs) of the diurnal cycle of shallow cumulus convection is reported. The case, based on measurements at the Atmospheric Radiation Measurement program Southern Great Plains site on 21 June 1997, has been used in a large-eddy simulation intercomparison study before. Results of the SCMs reveal the following general deficiencies: too large values of cloud cover and cloud liquid water, unrealistic thermodynamic profiles, and high amounts of numerical noise. Results are also strongly dependent on vertical resolution. These results are analysed in terms of the behaviour of the different parametrization schemes involved: the convection scheme, the turbulence scheme, and the cloud scheme. In general the behaviour of the SCMs can be grouped in two different classes: one class with too strong mixing by the turbulence scheme, the other class with too strong activity by the convection scheme. The coupling between (subcloud) turbulence and the convection scheme plays a crucial role. Finally, (in part) motivated by these results several models have been successfully updated with new parametrization schemes and/or their present schemes have been successfully modified. © Royal Meteorological Society, 2004. S. Irons's contribution is Crown copyright [source]

Three-dimensional simulation of the ASTEX Lagrangian 1 field experiment with a regional numerical weather prediction model

THE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 597 2004
Robert Sigg
Abstract The Atlantic Stratocumulus Transition Experiment (ASTEX) first Lagrangian experiment (Lagrangian 1) is here simulated with a modified version of the regional forecast model HIRLAM (High Resolution Limited Area Model). The main modification is that moist turbulent fluxes are accounted for in the model. Trajectory calculations show good agreement with earlier estimations. The initially rather shallow stratocumulus topped marine boundary layer is deepening along the trajectory, and in the end cumulus clouds are formed that penetrate the boundary-layer top. The model predicts this change in cloudiness, but the boundary layer is too shallow in the model. A simulation with modified initial conditions shows improved results, but is still too slow in increasing the boundary-layer depth. Additional factors that influence the boundary-layer growth are: the increase in sea surface temperatures, lower modelled wind speeds, low entrainment rates due to coarse vertical resolution, and synoptic-scale subsidence. An anticyclone at the surface moved slightly northward during the simulation. The anticyclone was accompanied at 500 hPa by a deepening cyclone and, therefore, one would expect synoptic subsidence in the area of the Lagrangian 1. The modelled negative vertical wind component at the boundary-layer top oscillates, and this is examined using spectral analysis. The results show that the vertical velocity is influenced by cumulus clouds on time-scales up to 15 h with a peak at 9 h. The horizontal and vertical wavelengths of the vertical velocity disturbances are estimated from model output to be 400,500 km and 6,10 km, respectively. Using the estimated vertical wavelength and linear theory for hydrostatic inertia,gravity waves, a horizontal wavelength of 350,550 km was calculated for a frequency of 9 h. The model results thus indicate that these types of waves are responsible for the undulating vertical velocity. Finally, an estimation of the synoptic-scale vertical velocity is calculated by filtering out all scales smaller than 15 h from the vertical velocity signal. This results in subsidence both at the beginning and the end of the Lagrangian with vertical velocities between ,0.1 and ,0.4 cm s,1. Copyright © 2004 Royal Meteorological Society [source]

Deriving cloud overlap statistics from radar

THE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 569 2000
Robin J. Hogan
Abstract The predictions of general-circulation models (GCMs) are sensitive to the assumed cloud overlap within a vertical column of model grid boxes, but until now no reliable observations of the degree of cloud overlap have been available. In this note we derive the overlap characteristics of clouds from 71 days of high vertical resolution 94 GHz cloud radar data in the UK. It is found that, contrary to the assumption made in most models, vertically continuous clouds tend not to be maximally overlapped. Rather, the overlap of clouds at two levels tends to fall rapidly as their vertical separation is increased, and for levels more than 4 km apart, overlap is essentially random. A simple inverse-exponential expression for the degree of overlap as a function of level separation is proposed that could, once results become available from a variety of other locations and seasons, be implemented in current GCMs with relatively little difficulty. [source]

A comparison of the impact of TOVS arid ATOVS satellite sounding data on the accuracy of numerical weather forecasts

THE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 569 2000
S. J. English
Abstract The Advanced TIROS Operational Vertical Sounder (ATOVS) was launched on the NOAA-15 satellite in May 1998. This provided a very significant improvement in the information available from meteorological polar-orbiting satellites compared with the previous TIROS Operational Vertical Sounder system, particularly for humidity and vertical resolution of temperature in cloudy areas. In preparation for assimilation of the observations into a three-dimensional analysis of atmospheric temperature and humidity, the observations have been compared with calculated top-of-atmosphere brightness temperatures computed from numerical weather prediction model profiles of temperature and humidity. Differences between observed and modelled brightness temperature are small. In some parts of the tropics and northern hemisphere the standard deviation of these differences for the tropospheric Advanced Microwave Sounding Unit sounding channels is only marginally higher than the radiometric noise of the observations. Early in 1999 a series of observation-system experiments were completed in which ATOVS observations were assimilated using a one-dimensional variational analysis. No use of the new humidity information could be made because of interference problems experienced by the microwave humidity sounder on ATOVS. Nonetheless, these experiments showed that the assimilation of the new temperature information provided by the radiance observations reduces forecast errors by as much as 20% in the southern hemisphere and 5% in the northern hemisphere. Further improvements have been found by assimilating more data over land. The major impact arises from the microwave channels. Whilst forward-model errors may be slightly lower for the microwave channels than the infrared channels the primary reason is the provision of sounding information in active weather systems, which are usually cloudy. [source]

The vertical resolution sensitivity of simulated equilibrium temperature and water-vapour profiles

THE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 565 2000
Adrian.
Abstract Variability of atmospheric water vapour is the most important climate feedback in present climate models. Thus, it is of crucial importance to understand the sensitivity of water vapour to model attributes, such as physical parametrizations and resolution. Here we attempt to determine the minimum vertical resolution necessary for accurate prediction of water vapour. To address this issue, we have run two single-column models to tropical radiative,convective equilibrium states and have examined the sensitivity of the equilibrium profiles to vertical resolution. Both column models produce reasonable equilibrium states of temperature and moisture. Convergence of the profiles was achieved in both models using a uniform vertical resolution of around 25 hPa. Coarser resolution leads to significant errors in both the water vapour and temperature profiles, with a resolution of 100 hPa proving completely inadequate. However, fixing the boundary-layer resolution and altering only the free-tropospheric resolution significantly reduces sensitivity to vertical resolution in one of the column models, in both water and temperature, highlighting the importance of resolving boundary-layer processes. Additional experiments show that the height of the simulated tropopause is sensitive to upper-tropospheric vertical resolution. At resolutions higher than 33 hPa, one of the models developed a high degree of vertical structure in the vapour profile, resulting directly from the complex array of microphysical processes included in the stratiform cloud parametrization, some of which were only resolved at high resolutions. This structure was completely absent at lower resolutions, casting some doubt on the approach of using relatively complicated cloud schemes at low vertical resolutions. [source]

Simulation of the Asian summer monsoon in five European general circulation models

ATMOSPHERIC SCIENCE LETTERS, Issue 1 2000
G. M. Martin
Abstract A comparison is made of the mean monsoon climatology in five different general circulation models (GCMs) which have been used by the participants of a project, funded by the European Union, entitled Studies of the Influence, Hydrology and Variability of the Asian summer monsoon (SHIVA). The models differ considerably, in horizontal and vertical resolution, numerical schemes and physical parametrizations, so that it is impossible to isolate the cause of differences in their monsoon simulations. Instead, the purpose of this comparison is to document and compare the representation of the mean monsoon in models which are being used to investigate the characteristics of the monsoon, its variability and its response to different boundary forcings. All of the models produce a reasonable representation of the monsoon circulation, although there are regional variations in the magnitude and pattern of the flow at both 850 hPa and 200 hPa. Considerable differences between the models are seen in the amount and distribution of precipitation. The models all reproduce the basic monsoon seasonal variation, although the timing of the onset and retreat, and the maxima in the winds and precipitation during the established phase, differ between them. There are corresponding differences in the evolution of the atmospheric structure between the pre-monsoon season and its established phase. It is hoped that this study will set in context the investigations of the monsoon system and its impacts carried out using these models, both during SHIVA and in the future. Copyright © 2000 Royal Meteorological Society. [source]

Seismogenic Structure around the Epicenter of the May 12, 2008 Wenchuan Earthquake from Micro-seismic Tomography

ACTA GEOLOGICA SINICA (ENGLISH EDITION), Issue 4 2009
Meijian AN
Abstract: A three-dimensional local-scale P -velocity model down to 25 km depth around the main shock epicenter region was constructed using 83821 event-to-receiver seismic rays from 5856 aftershocks recorded by a newly deployed temporary seismic network. Checkerboard tests show that our tomographic model has lateral and vertical resolution of ,2 km. The high-resolution P -velocity model revealed interesting structures in the seismogenic layer: (1) The Guanxian-Anxian fault, Yingxiu-Beichuan fault and Wenchuan-Maoxian fault of the Longmen Shan fault zone are well delineated by sharp upper crustal velocity changes; (2) The Pengguan massif has generally higher velocity than its surrounding areas, and may extend down to at least ,10 km from the surface; (3) A sharp lateral velocity variation beneath the Wenchuan-Maoxian fault may indicate that the Pengguan massif's western boundary and/or the Wenchuan-Maoxian fault is vertical, and the hypocenter of the Wenchuan earthquake possibly located at the conjunction point of the NW dipping Yingxiu-Beichuan and Guanxian-Anxian faults, and vertical Wenchuan-Maoxian fault; (4) Vicinity along the Yingxiu-Beichuan fault is characterized by very low velocity and low seismicity at shallow depths, possibly due to high content of porosity and fractures; (5) Two blocks of low-velocity anomaly are respe tively imaged in the hanging wall and foot wall of the Guanxian-Anxian fault with a ,7 km offset with ,5 km vertical component. [source]

Appropriate vertical discretization of Richards' equation for two-dimensional watershed-scale modelling

HYDROLOGICAL PROCESSES, Issue 1 2004
Charles W. Downer
Abstract A number of watershed-scale hydrological models include Richards' equation (RE) solutions, but the literature is sparse on information as to the appropriate application of RE at the watershed scale. In most published applications of RE in distributed watershed-scale hydrological modelling, coarse vertical resolutions are used to decrease the computational burden. Compared to point- or field-scale studies, application at the watershed scale is complicated by diverse runoff production mechanisms, groundwater effects on runoff production, runon phenomena and heterogeneous watershed characteristics. An essential element of the numerical solution of RE is that the solution converges as the spatial resolution increases. Spatial convergence studies can be used to identify the proper resolution that accurately describes the solution with maximum computational efficiency, when using physically realistic parameter values. In this study, spatial convergence studies are conducted using the two-dimensional, distributed-parameter, gridded surface subsurface hydrological analysis (GSSHA) model, which solves RE to simulate vadose zone fluxes. Tests to determine if the required discretization is strongly a function of dominant runoff production mechanism are conducted using data from two very different watersheds, the Hortonian Goodwin Creek Experimental Watershed and the non-Hortonian Muddy Brook watershed. Total infiltration, stream flow and evapotranspiration for the entire simulation period are used to compute comparison statistics. The influences of upper and lower boundary conditions on the solution accuracy are also explored. Results indicate that to simulate hydrological fluxes accurately at both watersheds small vertical cell sizes, of the order of 1 cm, are required near the soil surface, but not throughout the soil column. The appropriate choice of approximations for calculating the near soil-surface unsaturated hydraulic conductivity can yield modest increases in the required cell size. Results for both watersheds are quite similar, even though the soils and runoff production mechanisms differ greatly between the two catchments. Copyright © 2003 John Wiley & Sons, Ltd. [source]