bluuebunny/crossref_metadata_2025_split huggingface dataset:
A mass‐flux convection scheme for regional and global models
Authors: Bechtold P. , Bazile E. , Guichard F. , Mascart P. , Richard E. 
Published: February 2007

AbstractA bulk mass‐flux convection parametrization for deep and shallow convection is presented that includes an efficient and straightforward treatment of numerics, moist thermodynamics and convective downdraughts. The scheme is evaluated in a single‐column model context for a tropical deep‐convective period and a trade‐wind cumulus case. Preliminary applications in a global numerical weather‐prediction model and a mesoscale model are also discussed.The results suggest that the present scheme provides reasonable solutions in terms of predicted rainfall, and tropical temperature and moisture structures. The application of the scheme to various scales is supported by the use of a convective available potential energy convective closure that assures a smooth interaction with the large‐scale environment and efficiently suppresses conditional instability of the second kind‐like spin‐up processes on the grid‐scale.Finally, the theoretical and practical limits of the present approach are discussed together with possible future developments.
The correlation between tropical total ozone and outgoing long‐wave radiation
Authors: Williams Victoria , Toumi Ralf 
Published: February 2007

AbstractInterannual variations in tropical column ozone, as observed by the Total Ozone Mapping Spectrometer, are analysed for the period 1979–94 using linear regression to extract signals related to the solar cycle, quasi‐biennial oscillation, and El Niño Southern Oscillation (ENSO). Using time series of outgoing long‐wave radiation (OLR) as a proxy for ENSO variations, rather than the widely used Southern Oscillation Index is found to improve the fit to observed ozone. The variability in the ozone and OLR datasets is investigated using empirical orthogonal function analysis. The most strongly correlated OLR and ozone modes exhibit ENSO‐like variability.
Filtered non‐hydrostatic models in pressure‐related coordinates
Authors: Rõõm Rein , Miranda Pedro M. A. , Thorpe Alan J. 
Published: February 2007

AbstractThe anelastic sigma‐coordinate version of the pseudo‐anelastic Miller‐Pearce model is developed. The pseudo‐anelastic model is characterized by the presence of external‐mode acoustic perturbations which propagate in the horizontal, compromising the model performance by the need to use smaller time steps than would otherwise be required. The approach developed in this study allows for the filtering of acoustic waves and adjustment of surface pressure perturbations. The approach is tested in a new numerical implementation of an adiabatic sigma‐coordinate non‐hydrostatic model and results are found to be comparable to the unadjusted version, and to theory when applicable. The new code is found to allow for significantly larger time steps whilst still retaining numerical stability, showing the possibility of further model improvements.
Influence of the β‐effect on non‐modal baroclinic instability
Authors: Rivière G. , Hua B. L. , Klein P. 
Published: February 2007

AbstractWe focus on the influence of the β‐effect on the singular modes of baroclinic instability in the Phillips model. An analytical intercomparison between normal and singular modes for the Eady problem on an f‐plane has already been performed, showing that the amplification rate of the singular mode for the kinetic‐energy norm after a dimensional time of three days is about 1.2 times larger than for the normal mode. We show here that, in the presence of the β‐effect, the maximum amplification rate of the singular mode can be 1.5 times larger than for the normal‐mode case. This difference is due to the lesser stabilizing effect of β on the singular modes compared with the normal modes. This result is obtained for the case of equal layer depths which is relevant for the atmospheric situation. For the oceanic situation with unequal layer depths the amplification due to β is further increased leading to a factor of 1.8 in amplification rates between the two types of modes after a dimensional time of about 20 days. This behaviour is explained by the fact that β only intervenes in the anti‐Hermitian part of the linear dynamical operator, the main effect of which is to alter the vertical phase tilt of the perturbations. As a consequence, the structure of the bi‐orthogonal mode becomes more inclined to the vertical in the presence of β while, on the contrary, the structure of the unstable normal mode becomes less inclined.
Model studies of the interannual variability of the northern‐hemisphere stratospheric winter circulation: The role of the quasi‐biennial oscillation
Authors: Gray L. J. , Drysdale E. F. , Lawrence B. N. , Dunkerton T. J. 
Published: February 2007

AbstractA series of experiments are described that examine the sensitivity of the northern‐hemisphere winter evolution to the equatorial quasi‐biennial oscillation (QBO). The prime tool for the experiments is a stratosphere‐mesosphere model. The model is integrated over many years with the modelled equatorial winds relaxed towards observed values in order to simulate a realistic QBO. In experiment A the equatorial winds are relaxed towards Singapore radiosonde observations in the height region 16‐32 km. In contrast to previous modelling studies, the Holton‐Tan relationship (warm/cold winters associated with easterly/westerly QBO winds in the lower stratosphere) is absent. However, in a second experiment (run B) in which the equatorial winds are relaxed towards rocketsonde data over the extended height range 16‐58 km, a realistic Holton‐Tan relationship is reproduced. A series of further studies are described that explore in more detail the sensitivity to various equatorial height regions and to the bottom‐boundary forcing. The experiments suggest that the evolution of the northern‐hemisphere winter circulation is sensitive to equatorial winds throughout the whole depth of the stratosphere and not just to the lower‐stratospheric wind direction as previously assumed.
The microphysical properties of tropical convective anvil cirrus: A comparison of models and observations
Authors: Brown P. R. A. , Heymsfield A. J. 
Published: February 2007

AbstractThis paper describes the use of two‐dimensional (2‐D) cloud‐resolving numerical model simulations of a period of active convection from the Tropical Ocean‐Global Atmosphere Coupled Ocean‐Atmosphere Response Experiment to generate statistics of the distribution of total ice water content (IWC) and of the relative contribution to IWC of small (sub‐200 μm) particles with temperature. The model data are sampled in a way which excludes contributions from active convective updraught regions, so that the model results may be compared with similar such distributions derived from in situ sampling of a number of tropical anvil ice clouds in the vicinity of Kwajalein in the west Pacific.The model gives a good description of the mean and mode IWC in each of seven temperature ranges and, for temperatures warmer than –40°C, correctly predicts the dominant contribution of large particles to the IWC. At lower temperatures, the model retains an excessive fraction of its IWC in large particles. Estimates of the maximum crystal length (MCL) that would be sampled by a 2‐D Optical Array Probe exceed the observed values in this temperature range but are otherwise in good agreement. Tests of the sensitivity of model results to the cloud ice bulk density, and the absence of a homogeneous freezing source of ice crystals, suggest that the excessive MCL values are due in part to excessive rates of autoconversion and aggregation of cloud ice.
Parametrization of solar radiation in inhomogeneous stratocumulus: Albedo bias
Authors: Los Alexander , Duynkerke Peter G. 
Published: February 2007

AbstractLow‐level cloud fields with typical albedos between 50% and 70% contribute significantly to short‐wave cloud radiative forcing. Consequently, the earth's radiative budget is sensitive to extended stratocumulus which typically occurs in sub‐tropical subsidence regions over the oceans. It is therefore vital to represent the cloud albedo of such cloud fields accurately in climate models. Nevertheless, cloud fields are modelled as homogeneous plane‐parallel clouds, which leads to an overestimation of the albedo by up to 15% compared with real inhomogeneous clouds, both types of cloud fields having the same mean cloud optical depth. This so‐called plane‐parallel cloud albedo bias has given rise to a number of studies that present methods for reducing the albedo bias for stratiform clouds in climate models.Using Monte Carlo simulations and the Independent Pixel Approximation (IPA) the inhomogeneous cloud albedo is investigated with the aim of deriving a parametrization of the albedo bias. The new approach used here is that the variance of the cloud optical depth is parametrized as a function of grid size and mean cloud liquid‐water content. The variance of the cloud optical depth is derived from turbulence processes. For the determination of the albedo bias an analytical solution is presented which is based on a simplified distribution function of the cloud optical depth and the IPA method. When used in conjunction with the parametrized variance of the cloud optical depth, a box function, which replaces the distribution function, produces comparable results to Monte Carlo simulations within an acceptable accuracy of a few per cent.
The impact of new water vapour spectral line parameters on the calculation of atmospheric absorption
Authors: Zhong Wenyi , Haigh Joanna D. , Belmiloud Djedjiga , Schermaul Roland , Tennyson Jonathan 
Published: February 2007

AbstractNew laboratory measurements and theoretical calculations of integrated line intensities for water vapour bands in the near‐infrared and visible regions (8600–15 000 cm−1) show a systematic 6–26% increase in band intensities compared to the HITRAN96 database. We have used the GENLN2 line‐by‐line code to assess the effects of such changes in the water vapour spectral database on calculations of clear‐sky short‐wave fluxes and heating rates. Three standard atmospheres were used and it was found that, compared with HITRAN96 results, the absorbed downward solar fluxes increase (including the effects of theoretically predicted weak water lines in the region) by 5.5, 4.8 and 2.2 W m−2 (solar zenith angle = 30°) and by 2.4, 2.1 and 1.1 W m−2 (solar zenith angle = 75°) respectively. The maximum change in heating rate is about 4%. The effects are about five to eight times larger than those produced by using the Giver et al. corrections. The combined effects of the revised description of the spectroscopy of water vapour account for approximately 30–90% of the absorption currently ascribed to the water vapour continuum in this spectral region.
Teleconnections between the tropical Pacific and the Sahel
Authors: Rowell David P. 
Published: February 2007

AbstractA greater understanding of equatorial teleconnections is a key aspect of research into seasonal prediction and future climate change for tropical regions. Here the impact of Pacific sea surface temperature (SST) anomalies on rainfall fluctuations over the semi‐arid Sahel is explored, using a combination of observed and model data.The first key topic of this study is the identification of those aspects of anomalous Pacific SST variability that are most strongly linked to the Sahel. One of these, also illuminated by earlier studies, is similar to the classic El Niño Southern Oscillation pattern in the central and east Pacific, which in its El Niño phase increases the likelihood of Sahel drought. It is shown here that, although a part of this link is indirect (operating via Atlantic SSTs), its main effect appears to be through a direct atmospheric teleconnection. The other critical pattern, of equal importance and revealed here by a novel analysis technique, is the large‐scale zonal gradient of SSTs from the west Pacific to the east Indian Ocean. If weakened, this too enhances the likelihood of Sahel drought. Atmospheric general circulation model experiments, forced either by observed or idealized SSTs, are used to confirm these two influences on the Sahel. Crucially, their Sahelian impact is substantially reinforced when both are present and, additionally, further empirical analysis shows them to be largely independent.The second key topic is an investigation of the mechanisms for this Pacific‐Sahel teleconnection. These appear to involve anomalous stationary equatorial waves, with communication occurring in both the eastward and westward directions. In El Niño years (for example), a Kelvin wave emanates across the Atlantic from east Pacific convective heating anomalies, and an equatorial Rossby wave appears over the Indian Ocean in response to the anomalous west Pacific‐Indian Ocean SST gradients via convective heating anomalies over the Indian Ocean. These interact over Africa to enhance large‐scale subsidence over the Sahel, thus reducing seasonal rainfall totals. Interannual changes in propagating equatorial waves or in the residence of subseasonal regimes appear not to play a substantial role.
Bispectral analysis of geopotential‐height anomalies for synoptic‐intraseasonal time‐scales
Authors: Efimov V. V. , Shokurov M. V. 
Published: February 2007

AbstractSpectral decomposition of the third moment of geopotential‐height fluctuations, bispectral analysis, is considered as a convenient and straightforward method of studying the nonlinear three‐wave interactions between spectral constituencies of anomalies. The real part of the bispectrum directly indicates an asymmetry of the probability density function (PDF), and the imaginary part indicates lack of time reversability of the time series. NCEP reanalysis data for 1982–96 and NMC analysis data for 1946–89 are used. Different frequency domains of bispectra are considered, namely, the synoptic frequencies, 2–6 days, lower intraseasonal frequencies, 32–128 days, and joint synoptic‐intraseasonal frequency ranges.The spatial distribution of the integral over all frequencies of the real part of the bispectra, characterizing asymmetry of the PDF, on the whole, is the same as that obtained earlier by Holtzer. A similar integral of the imaginary part essentially differs from zero and has more non‐zonal behaviour. The first of them, asymmetry, accounts for different contributions of nonlinear and linear terms in the balance equation for geopotential anomalies.The spatial distribution of the bispectra in the low‐frequency intraseasonal domain is nearly the same as for their integrals, because of the predominant contribution of low frequencies in the spectra of geopotential oscillations. The distribution of the real and imaginary parts of the bispectra in the synoptic frequency range indicates an asymmetry between positive and negative deviations of high‐frequency transients, and a spatial asymmetry between the front and back slopes of the eastward‐moving synoptic cyclones. This behaviour is clearly evident in storm‐track regions.Most interesting is the synoptic‐intraseasonal frequency domain. Non‐zero values of real and imaginary parts of the bispectra imply an existence of low‐frequency oscillations of geopotential heights bounded by envelopes of synoptic fluctuations. This phenomenon is seen for all regions in both hemispheres and during both seasons, but takes place especially clearly in the storm‐track regions over the Pacific and Atlantic oceans. Phase shifts between the low‐frequency component and the variance of synoptic transients in most regions are nearly out of phase, but in the regions of storm‐track activity the phase relationship changes regularly along the tracks. Meaningful treatment of many of the results of the bispectral analysis needs special consideration.
The effect of the island of Crete on the Etesian winds over the Aegean Sea
Authors: Kotroni Vassiljki , Lagouvardos Kostas , Lalas Dimitris 
Published: February 2007

AbstractThe Etesians (northern sector winds), which blow over the Aegean Sea during summer, affect human activities in the area. The numerous islands of the Aegean and especially Crete (a mountainous island in the southern Aegean oriented perpendicular to the surface flow) seem to play an important role in the modification of the wind field during the Etesians. The Crete mountain ranges, surrounded as they are by water, are an excellent example of a major isolated topographic feature which significantly modifies the regional airflow and pressure; however, this modification can hardly be defined due to the lack of observing stations over the sea. For this reason, the available land surface and ship synoptic observations are used, together with ERS scatterometer wind data in order to identify the regions over the Aegean where the wind reaches its maximum intensity, and to assess the influence of Crete on the wind field. Moreover, numerical modelling is used to provide some further insight on the orographically disturbed wind flow. Sensitivity tests performed with the hydrostatic model BOLAM show that the interaction of the Etesian wind flow with the mountains of Crete produces deceleration of the Etesians up to almost 120 km upstream, the leftward deflection of the air as it approaches the mountains, and the associated intensification of the flow east of the island.
Modelling the local surface exchange over a grass‐field site under stable conditions
Authors: Best M. J. , Hopwood W. P. 
Published: February 2007

AbstractObservations of near‐surface variables are used with the Met Office Surface Exchange Scheme to calculate the exchange of heat and moisture between the surface and the atmosphere for a grass‐field site at the Met Office, Cardington, UK. Different methods for modelling the stability near the surface are investigated with the surface temperatures and turbulent heat fluxes compared with data collected from the site. The results show that, whilst using Monin‐Obukhov similarity theory can give good results in stable conditions, the form of the stability functions can significantly affect the accuracy in certain stability regimes. The use of log‐linear stability functions can cause the turbulent fluxes of heat and moisture to cut off at stabilities which are too close to neutral compared with the observations. The results also show that unrealistically high surface temperatures are obtained in clear‐sky nocturnal conditions with the standard surface energy balance. An alternative energy balance which includes a canopy of vegetation above the underlying soil gives better agreement with the observations in these conditions.
A strategy for high‐resolution ensemble prediction. II: Limited‐area experiments in four Alpine flood events
Authors: Marsigli C. , Montani A. , Nerozzi F. , Paccagnella T. , Tibaldi S. , Molteni F. , Buizza R. 
Published: February 2007

AbstractA high‐resolution ensemble system, based on five runs of a limited‐area model (LAM), is described. The initial and boundary conditions for the LAM integrations are provided by the representative members (RMs) selected from the European Centre for Medium‐Range Weather Forecasts Ensemble Prediction System (EPS). EPS members are grouped in five clusters; then, from each cluster, an RM is selected, according to the methodology described in the companion paper. The ability of the high‐resolution ensemble system to predict the occurrence of heavy rainfall events (either five or six days ahead) is tested for four cases of floods over the Alpine region. Results show that, in two case‐studies, the LAM integration corresponding to the RM of the highly populated cluster predicts the observed rainfall with a very good degree of time and spatial accuracy. In the other two cases, the extreme events are captured by at least one of the runs nested on the members of the less populated clusters. Probability maps constructed from LAM integrations provide great detail on the location of the regions affected by heavy precipitation and the information gained with respect to EPS probability maps and LAM deterministic forecasts is highlighted. The probabilistic estimates based on the LAM ensembles are also shown to be of valuable assistance to forecasters in issuing early flood alerts, contributing to the definition of a flood‐risk alarm system.
A method for testing a prespecified subgroup in clinical trials
Authors: Song Yang , Chi George Y. H. 
Published: February 2007

AbstractIn clinical trials, investigators are often interested in the effect of a given study treatment on a subgroup of patients with certain clinical or biological attributes in addition to its effect on the overall study population. Such a subgroup analysis would become even more important to the study sponsor if an efficacy claim can be made for the subgroup when the test for the overall study population fails at a prespecified alpha level. In practice, such a claim is often dependent on prespecification of the subgroup and certain implicit or explicit requirements placed on the study results due to ethical or regulatory concerns. By carefully considering these requirements, we propose a general statistical methodology for testing both the overall and subgroup hypotheses, which has optimal power and strongly controls the familywise Type I error rate. Copyright © 2007 John Wiley & Sons, Ltd.
A comparison between acoustic output indices in 2D and 3D/4D ultrasound in obstetrics
Authors: Sheiner E. , Hackmon R. , Shoham‐Vardi I. , Pombar X. , Hussey M. J. , Strassner H. T. , Abramowicz J. S. 
Published: February 2007

AbstractObjectiveThree‐dimensional (3D) ultrasound is gaining popularity in prenatal diagnosis. While there are no studies regarding the safety of 3D ultrasound, it is now widely performed in non‐medical facilities, for non‐diagnostic purposes. The present study was aimed at comparing the acoustic output, as expressed by thermal index (TI) and mechanical index (MI), of conventional two‐dimensional (2D) and 3D/4D ultrasound during pregnancy.MethodsA prospective, observational study was conducted, using three different commercially available machines (iU22, Philips Medical Systems; Prosound Alfa‐10, Aloka; and Voluson 730 Expert, General Electric). Patients undergoing additional 3D/4D ultrasound examinations were recruited from those scheduled for fetal anatomy and follow‐up exams. Fetuses with anomalies were excluded from the analysis. Data were collected regarding duration of the exam, and each MI and TI during 2D and 3D/4D ultrasound exams.ResultsA total of 40 ultrasound examinations were evaluated. Mean gestational age was 31.1 ± 5.8 weeks, and mean duration of the exam was 20.1 ± 9.9 min. Mean TIs during the 3D (0.27 ± 0.1) and 4D examinations (0.24 ± 0.1) were comparable with the TI during B‐mode scanning (0.28 ± 0.1, P = 0.343). The MIs during the 3D volume acquisitions were significantly lower than those in the 2D B‐mode ultrasound studies (0.89 ± 0.2 vs. 1.12 ± 0.1, P = 0.018). The 3D volume acquisitions added 2.0 ± 1.8 min of actual ultrasound scanning time (i.e. not including data processing and manipulation, or 3D displays, which are all post‐processing steps). The 4D added 2.2 ± 1.2 min.ConclusionsAcoustic exposure levels during 3D/4D ultrasound examination, as expressed by TI, are comparable with those of 2D B‐mode ultrasound. However, it is very difficult to evaluate the additional scanning time needed to choose an adequate scanning plane and to acquire a diagnostic 3D volume. Copyright © 2007 ISUOG. Published by John Wiley & Sons, Ltd.