bluuebunny/crossref_metadata_2025 huggingface dataset:
Low‐frequency variability in a simplified atmospheric global circulation model: Storm‐track induced ‘spatial resonance’
Authors: Franzke Christian , Fraedrich Klaus , Lunkeit Frank
Published: February 2007

The interaction between two storm tracks is analysed by using idealized experiments with a simplified global‐circulation model. A zonal distance of 180° between the storm tracks drives a retrograde travelling zonal wave‐number‐two wave with an amplitude modulation of about 50 days. The stream‐function tendency equation demonstrates that the amplitude modulation is excited in the storm‐track regions by the high‐frequency (periods between two and eight days) eddy forcing; ‘spatial resonance’ of a retrograde propagating zonal wave‐number‐two Rossby wave and high‐frequency eddy forcing are proposed as the amplifying mechanisms, while friction and self interaction between the low‐frequency eddies ensure damping. Sensitivity experiments indicate that the pronounced modulation of 50 days vanishes with decreasing zonal distance between the storm tracks, thereby enhancing the intensity and duration of blocking events.
On the effects of the Hadley circulation and westerly equatorial flow on planetary‐wave reflection
Authors: Magnusdottir Gudrun , Walker Christopher C.
Published: February 2007

The nonlinear behaviour of quasi‐stationary planetary waves that are excited by mid‐latitude orographic forcing is considered in a global shallow‐water model taken to represent the upper troposphere. The waves propagate toward low latitudes where the background flow is normally weak and the waves are therefore likely to break. Nonlinear pseudomomentum conservation relations are used to quantify the absorption‐reflection behaviour of the wave‐breaking region. Two different flow scenarios are represented: (i) initial states without a representation of the Hadley circulation, but where the axisymmetric equatorial background flow changes from being weak and easterly to moderate westerly; (ii) initial states that include a representation of the Hadley circulation and that have weak equatorial easterlies.

Based on linear arguments, both (i) and (ii) are expected to influence the progression of the wave train. The nonlinear behaviour in the presence of low‐latitude westerly background flows is different from linear predictions. For large‐amplitude forcing, wave breaking takes place even though there is no zero‐wind line in the initial slate, and the cross‐equatorial wave propagation that look place for small‐amplitude forcing is stopped before it can reach the equator. Nonlinear reflection is found to take place back into the hemisphere of origin but not across to the other hemisphere.

In the presence of a Hadley circulation representative of winter conditions, the nonlinear reflection takes longer to get established, i.e. it requires more forcing, but a reflected wave train is still present in the numerical simulations, both for a longitudinally symmetric forcing and for the more realistic case of an isolated forcing. A summer Hadley circulation allows wave activity to get to the winter hemisphere. As the forcing is increased, wave breaking occurs and eventually nonlinear reflection.
Observations and numerical modelling of mountain waves over the Southern Alps of New Zealand
Authors: Lane Todd P. , Reeder Michael J. , Morton Bruce R. , Clark Terry L.
Published: February 2007

A detailed case study of mountain waves observed over the South Island of New Zealand during the Southern Alps Experiment is presented. The purpose of the study is to document the waves generated over this region, to explore the extent to which high‐resolution radiosonde data can be used to determine gravity‐wave parameters, and to estimate the vertical flux of horizontal momentum produced by flow over the Southern Alps.

Organized wave clouds were observed in a visible satellite image on 20 October 1996 and are thought to have been produced by partially trapped lee waves. Using background flow fields derived from a radiosonde sounding, the theoretical horizontal wavelength of the partially trapped mode is calculated and agrees well with that measured from the satellite image.

A technique, called the horizontal projection method, is developed to analyse radiosonde soundings for mountain waves. Specifically, the technique determines the dominant horizontal wavelength over the entire depth of a radiosonde sounding. This method uses a coordinate transformation to take account of vertical changes in the background flow and, hence, vertical variations in the vertical wave number, as well as changes in the path of the radiosonde. This method is applied to a radiosonde sounding to identify the dominant wave mode, and emphasizes that radiosonde soundings should not, in general, be treated as instantaneous vertical profiles when analysing mountain waves.

The mountain waves observed are simulated using a high‐resolution numerical model and the results are used to check the consistency of the analysis. The numerical model produces a horizontally averaged value of momentum flux in the lower stratosphere of −0.02 N m−2 although this value is probably underestimated by at least a factor of four.
Observations of atmospheric density currents using a tethered balloon‐borne turbulence probe system
Authors: Alapworth
Published: February 2007

A tethered kite balloon with multiple instrumented probes clamped to the tethering cable is used to investigate atmospheric density‐current fronts passing through three sites at varying distances from the coast. The system allows high‐frequency measurements of the three components of wind, temperature and humidity to be made simultaneously at up to ten levels as the front passes through. This enables a two‐dimensional visualization of the current and its circulation to be constructed. As a result the structure of some of the wide variety of atmospheric density currents is revealed by direct observation. The internal circulations and the effects of varying environments on the currents and their movement are described, together with turbulence statistics. One current was associated with what may have been a coastal trapped disturbance, and in one case two opposing sea breezes appear to have collided inland. Two currents penetrating into surface inversions were dissociated from the surface. One of these was travelling downwind and formed a strong near‐surface jet. Associated wave and bore structures were found to be travelling in layers above the currents in two cases. In one case environmental air was found to intrude into the head region and convect internally, while, in another, air from the head region extruded into the environmental flow. The measurement array enabled the different terms of the dynamic equations to be evaluated and their relative contribution assessed. A remarkable correlation is found between thermal and dynamic variables. A study of the internal flux balance of daytime density currents shows that the internal flow plays a significant part in maintaining the capping inversion of the current. Eddies were observed at interfacial regions in which Richardson numbers were significantly greater than 0.25, confirming some modelling studies. Frontal slopes in most cases are in accord with theoretical values but in one downwind case the slope is considerably less than predicted. The inland currents are found to be deeper than coastal currents which may explain the increase in speed observed later in the day. Water vapour frontal structures are generally less marked than thermal fronts.
A comparison of the impact of TOVS and ATOVS satellite sounding data on the accuracy of numerical weather forecasts
Authors: English S. J. , Renshaw R. J. , Dibben P. C. , Smith A. J. , Rayer P. J. , Poulsen C. , Saunders F. W. , Eyre J. R.
Published: February 2007

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.
Multi‐resolution analysis, entropic information and the performance of atmosoheric sounding radiometers
Authors: Peckham G. E.
Published: February 2007

The performance of remote‐sounding radiometers measuring properties of the earth's atmosphere is analysed through a multi‐resolution wavelet transform. This technique allows the uncertainty in retrieved atmospheric profiles to be determined as a function of both altitude and scale of patterns in the profile. Multi‐resolution analysis may be applied to a number of indicators of the quality of a measurement including entropic information. The apportionment of performance indicators to specific altitude ranges and pattern scales facilitates a comparison with performance requirements. The analysis is illustrated through a simple model of a remote‐sounding radiometer and by application to the Infrared Atmospheric Sounding Interferometer.
The Met. Office global three‐dimensional variational data assimilation scheme
Authors: Lorenc A. C. , Ballard S. P. , Bell R. S. , Ingleby N. B. , Andrews P. L. F. , Barker D. M. , Bray J. R. , Clayton A. M. , Dalby T. , Li D. , Payne T. J. , Saunders F. W.
Published: February 2007

The Met. Office has developed a variational assimilation for its Unified Model forecast system, which contains a grid‐point mode) that is run operationally in global, mesoscale, and stratospheric configuration. Key characteristics of the design are:

a development path from three‐dimensional to four‐dimensional variational assimilation;

global and limited‐area configurations;

variational analysis of perturbations;

and a carefully designed, well conditioned background term.

The background term is implemented using a sequence of variable transforms to independent balanced and unbalanced variables, to vertical modes, and to spectral coefficients. The coefficients used are based on statistics from differences of one‐ and two‐day forecasts valid at the same time. The covariance model represents many of the features seen in the covariances of forecast differences.

The three‐dimensional variational data assimilation (3D‐Var) system was implemented in the operational global forecast system on 29 March 1999. In parallel trials, the 3D‐Var system gave a 2.7% improvement in a composite skill score (verified against observations and weighted according to the importance of each field).
Instability and large‐scale circulations in a two‐column model of the tropical troposphere
Authors: Raymond D. J. , Zeng X.
Published: February 2007

A two‐column model of the tropical atmosphere is developed. The two columns are kept in buoyancy equilibrium by mass exchange driven by the inter‐column pressure gradient, and diabatic processes are parametrized by highly simplified schemes. This model is used to investigate whether spontaneous large‐scale circulations will develop in the tropical atmosphere when (he sea surface temperature and solar radiative forcing are uniform. Such circulations do indeed develop in the model, with ascent in one column and descent in the other, when one of the columns is slightly perturbed from the initial state of radiative‐convective equilibrium. A key element of the circulation dynamics is that increased equivalent potential temperature in a column leads to enhanced convection and rainfall in that column, which further increases the equivalent potential temperature there. The latter effect arises because convection enhances surface heat fluxes and decreases outgoing long‐wave radiation by virtue of the increase in stratiform cloudiness. These results form an attractive explanation for the observed patchiness of deep convection over warm tropical oceans and suggest that further modelling and observational work be directed towards understanding the budget of equivalent potential temperature in the tropical atmosphere and its relationship to rainfall.
The evolution of vortices in vertical shear. II: Large‐scale asymmetries
Authors: Jones Sarah C.
Published: February 2007

The role of large‐scale asymmetries in the evolution of a tropical‐eye lone‐like vortex in vertical shear on an f‐plane is investigated. Idealized numerical calculations using a primitive‐equation model are used to illustrate the development of the asymmetries and their role in the evolution of the vortex lilt. The asymmetries develop in the outer region of the vortex, well outside the radius of maximum wind. For the vortex profile used in most of the calculations, the region where the asymmetries develop has anomalously negative potential vorticity (PV) compared with the undisturbed environment, and the flow fields associated with the asymmetries consist of large‐scale anticyclonic gyres. The orientation of the gyres changes with height and they are located on opposite sides of the vortex at lower and upper levels. The influence of the asymmetries on the vortex evolution depends on both the structure and location of the asymmetries and on the orientation of the tilted vortex within the asymmetries.

It is hypothesized that the asymmetries arise due to the distortion of the initially symmetric vortex by the horizontally sheared flow associated with the vertical projection of the tilted PV anomaly. Calculations using a barotropic model are presented in support of this hypothesis. The sensitivity of the large‐scale asymmetries to the initial vortex structure is investigated. For a vortex profile in which the tangential wind decreases more slowly with radius, the asymmetries form in a region of positive PV anomaly and the associated flow field contains large‐scale cyclonic gyres. The implications of the large‐scale asymmetries for tropical‐cyclone motion and intensity change are discussed.
A two‐time‐level semi‐Lagrangian global spectral model
Authors: Temperton Clive , Hortal Mariano , Simmons Adrian
Published: February 2007

A three‐time‐level semi‐Lagrangian global spectral model was introduced operationally at the European Centre for Medium‐Range Weather Forecasts in 1991. This paper first documents some later refinements to the three‐time‐level scheme, and then describes its conversion to a two‐time‐level scheme. Experimental results are presented to show that the two‐time‐level scheme maintains the accuracy of its three‐time‐level predecessor, while being considerably more computationally efficient.
A new representation of data‐assimilation methods: The PALM flow‐charting approach
Authors: Lagarde Thierry , Piacentini Andrea , Thual Olivier
Published: February 2007

We introduce a new graphical representation for data‐assimilation methods which allows the visualization of the following items: control spaces, dual spaces, elementary operators and flow charts, for the computation of the gradient of the cost function. It is illustrated on a large variety of data‐assimilation methods used in meteorology and oceanography: three‐dimensional variational, four‐dimensional (4‐D) variational, 4D‐dual, Kalman filters, incremental formulations, formulations with imperfect models, addition of weak constraints, and reductions of the control space, etc.

The flow‐charting approach is a powerful tool for decomposing a data‐assimilation method into elementary steps, for pointing out the elementary steps common to several methods or for building new and more efficient algorithms. In addition to the use of a common system of notation for assimilation methods, flow charting allows different algorithms to be easily compared, and is a necessary step prior to the development of any operational suite coupling data and models through one or several assimilation methods. This approach has been successfully followed to design a universal coupler called PALM (Projet d'Assimilation par Logiciel Multi‐méthodes) for the purpose of operational oceanography in the framework of the MERCATOR project.
A note on the roughness length for temperature over melting snow and ice
Authors: Calanca Pierluigi
Published: February 2007

The characteristics of roughness length for temperature, zT, over melting snow and ice are examined on the base of data gathered during one summer season on the Greenland ice sheet. Despite fixed surface temperature and homogeneous surface conditions, zT is highly variable, taking on values between 10−5 and 1 m. Mean value and median are computed as 2 × 10−2 m and 1 × 10−3 m. It thus turns out that zT is about 10 to 100 times larger than the aerodynamic roughness length, z0. This disparity cannot be explained in terms of the roughness Reynolds number. Rather, the data show that over melting snow and ice, and for slightly stable conditions, zT is uniquely determined by the temperature defect.
A nonlinear dynamical perspective on model error: A proposal for non‐local stochastic‐dynamic parametrization in weather and climate prediction models
Authors: Palmer T. N.
Published: February 2007

Conventional parametrization schemes in weather and climate prediction models describe the effects of subgrid‐scale processes by deterministic bulk formulae which depend on local resolved‐scale variables and a number of adjustable parameters. Despite the unquestionable success of such models for weather and climate prediction, it is impossible to justify the use of such formulae from first principles. Using low‐order dynamical‐systems models, and elementary results from dynamical‐systems and turbulence theory, it is shown that even if unresolved scales only describe a small fraction of the total variance of the system, neglecting their variability can, in some circumstances, lead to gross errors in the climatology of the dominant scales. It is suggested that some of the remaining errors in weather and climate prediction models may have their origin in the neglect of subgrid‐scale variability, and that such variability should be parametrized by non‐local dynamically based stochastic parametrization schemes. Results from existing schemes are described, and mechanisms which might account for the impact of random parametrization error on planetary‐scale motions are discussed. Proposals for the development of non‐local stochastic‐dynamic parametrization schemes are outlined, based on potential‐vorticity diagnosis, singular‐vector analysis and a simple stochastic cellular automaton model.
Mesoscale structure of a polar low with strong upper‐level forcing
Authors: Browning K. A. , Dicks E. M.
Published: February 2007

Dry air descending within the circulation of a polar low was observed to give a striking signature in the echo return from an upward‐looking VHF radar. Data from this radar, along with weather‐radar data, satellite imagery and output from limited‐area and mesoscale versions of the Met Office Unified Model, are synthesized to depict the mesoscale structure of the weather system. The polar low had a compact vortex centred near 3 km, a cold core below this level, and a warm core above. Attribution of quasi‐geostrophic forcing showed that on the larger scale the polar low was dominated by upper‐level forcing. The detailed mesoscale analysis showed that the polar low, during its mature phase, was affected by a stratospheric intrusion that brought air from near tropopause level down to 3 km to give the striking signature in the VHF radar echo pattern. The echo pattern also suggested that thin tongues of this stratospheric, or near‐stratospheric, air were penetrating slantwise down a further several hundred metres, above the shallow cloud constituting the southern end of an archetypal cloud head. It is argued that this may be a locally important region of mixing between air originating near the tropopause and the boundary layer. In more general terms, the study demonstrates the ability of the operational mesoscale model to represent various small‐scale features. The study also helps in developing the interpretation of the patterns of clear‐air radar echo observed with VHF radar.
Life cycle of Sahelian mesoscale convective cloud systems
Authors: Mathon Vincent , Laurent Henri
Published: February 2007

This paper provides an eight‐year high‐resolution climatology of Sahelian mesoscale convective systems (MCSs) during the summer. MCSs are defined as convective cloud clusters larger than 5000 km2. They are extensively tracked from METEOSAT full‐resolution infrared images (time resolution 0.5 h and spatial resolution about 5 km). The method enables every MCS to be tracked throughout its entire lifetime. For each time step, the MCS location and its morphological and radiative characteristics are computed for three different brightness temperature thresholds. The methodology is presented, evaluated and compared with previous studies using low‐resolution data.

Statistical MCS distributions, diurnal cycle and spatial variability of MCS characteristics are analysed on the basis of this high‐resolution tracking. It is shown that a few large and long‐lived cloud clusters contribute most of the total cloud cover. Sahelian cloud clusters propagate westward at a greater speed when very deep convection is well developed. The diurnal organization of the convection has been analysed, and has proved that the merging of MCSs is partly explained by the actual merging of independent convective entities, whereas the splitting of MCSs is mostly associated with weakening of convection. The importance of mesoscale convective complexes for the total MCS coverage has also been studied.
Cloud‐base fluxes in the cumulus‐capped boundary layer
Authors: Grant A. L. M.
Published: February 2007

A closure for the fluxes of mass, heat, and moisture at cloud base in the cumulus‐capped boundary layer is developed. The cloud‐base mass flux is obtained from a simplifed turbulence kinetic energy (TKE) budget for the sub‐cloud layer, in which cumulus convection is assumed to be associated with a transport of TKE from the sub‐cloud layer to the cloud layer.

The heat and moisture fluxes are obtained from a jump model based on the virtual‐potential‐temperature equation. A key part of this parametrization is the parametrization of the virtual‐temperature flux at the top of the transition zone between the sub‐cloud and cloud layers.

It is argued that pressure fluctuations must be responsible for the transport of TKE from the cloud layer to the sub‐cloud layer.
Atmospheric multiple equilibria and non‐Gaussian behaviour in model simulations
Authors: Hannachi A. , O'Neill A.
Published: February 2007

Multiple equilibria from general circulation models (GCMs) are investigated via Gaussian mixture models. The cross‐validation technique based on bootstrap, used to find the possible number of clusters, is shown to have a tendency to display a bias in the number of clusters when the sample is not statistically independent. The method is shown, however, to give satisfactory results when applied with a strategy based on statistical independence. The mixture model is successfully applied to a few dynamical systems and data from the UK Universities Global Atmospheric Modelling Programme (UGAMP) GCM. The same analysis is then carried out and applied to the wintertime 500 mb geopotential height field taken from a 50‐year run of the Hadley Centre HadAM3 GCM. Three regions, the North Pacific‐American, the North Atlantic, and the northern hemisphere are investigated for nonlinear circulation regimes. It is shown that the model displays robust regime behaviour over the North Pacific‐American sector, while only marginal regime behaviour is observed over the North Atlantic, and no trace of hemispheric regimes.
The GEOS ozone data assimilation system: Specification of error statistics
Authors: Štajner Ivanka , Riishøjgaard Lars Peter , Rood Richard B.
Published: February 2007

A global three‐dimensional ozone data assimilation system has been developed at the Data Assimilation Office of the NASA Goddard Space Flight Center. The Total Ozone Mapping Spectrometer (TOMS) total ozone data and the Solar Backscatter Ultraviolet/2 (SBUV/2) partial ozone profile observations are assimilated. The assimilation, into an off‐line ozone transport model, is done using the global Physical‐space Statistical Analysis Scheme. This system became operational in December 1999.

A detailed description of the statistical analysis scheme and, in particular, of the forecast‐ and observation‐error covariance models is given. A new global anisotropic horizontal forecast‐error correlation model accounts for a varying distribution of observations with latitude. Correlations are largest in the zonal direction in the tropics where data are sparse. Forecast‐error variance is assumed to be proportional to the ozone field. The forecast‐error covariance parameters were determined by maximum‐likelihood estimation. The error covariance models are validated using χ2 statistics.

The analysed ozone fields in the winter 1992 are validated against independent observations from ozone sondes and the Halogen Occultation Experiment (HALOE). The difference between the mean HALOE observations and the analysis fields is less than 10% at pressure levels between 70 and 0.2 hPa. The global root‐mean‐square difference between TOMS observed and forecast values is less than 4%. The global root‐mean‐square difference between SBUV observed and analysed ozone between 50 and 3 hPa is less than 15%.
Diagnosis and adaptive tuning of observation‐error parameters in a variational assimilation
Authors: Desroziers Gerald , Ivanov Serguei
Published: February 2007

Following the a posteriori diagnosis approach proposed by some authors, a practical computation of the expectation of sub‐parts of the value of a cost function at the minimum is shown to be feasible by using a randomization technique based on a perturbation of observations or background fields. These computations allow the tuning of observation‐error weighting parameters by applying a simple iterative fixed‐point procedure. The procedure is first tested in a simplified variational scheme on a circular domain and then in a similar scheme but with the addition of the vertical coordinate. The relationship between the proposed approach and the Generalized Cross Validation is also shown. A test in the French Action de Recherche Petite Echelle Grande Echelle (ARPEGE) three‐dimensional variational framework with both simulated observations and background fields is finally performed. It shows that a complete description of observation‐error parameters can be retrieved with only a few iterations and, thus, at a reasonable cost.
A satellite radiance‐bias correction scheme for data assimilation
Authors: Harris B. A. , Kelly G.
Published: February 2007

Recently, much progress has been made in the direct assimilation of satellite radiance measurements in numerical weather‐prediction systems. In order to use radiances from the TIROS Operational Vertical Sounder (TOVS), biases between the observed radiances and those simulated from the model first guess must be corrected. The original scheme for TOVS radiance‐bias correction at the European Centre for Medium‐Range Weather Forecasts utilized a global scan correction, and a linear air‐mass correction, with the observed radiances from the Microwave Sounding Unit channels 2, 3 and 4 as predictors. The new scheme differs in two fundamental ways. Analysis of radiance data shows a significant residual scan bias which depends strongly on latitude for some channels. The new scheme applies a latitudinally dependent scan correction to take this into account. The air‐mass predictors are now computed from the background field, since the background field contains a more consistent representation of the air mass and surface characteristics than the observed microwave radiances. Four new predictors are used, 1000–300 hPa thickness, 200–50 hPa thickness, model surface skin temperature and total precipitable water. In particular, the skin‐temperature predictor is able to differentiate between ocean and sea‐ice, performing much better than the old scheme in the winter hemisphere. The use of model predictors is a change in philosophy away from correction of the observations to correction of the computed forward radiances. This leads to a natural extension where the gradient of the bias correction can be taken into account in variational retrieval schemes.
A gravity‐wave induced quasi‐biennial oscillation in a three‐dimensional mechanistic model
Authors: Lawrence Bryan N.
Published: February 2007

Previous two‐dimensional model simulations using the Hines parametrization of unresolved gravity waves have produced long‐period oscillations in the tropics. Such oscillations include the quasi‐biennial oscillation (QBO) and the semi‐annual oscillation (SAO). In this paper simulations with a three‐dimensional mechanistic model are reported which show an even more realistic representation of the QBO. These simulations show the expected dependence of the period of the QBO on the momentum flux entering the stratosphere, but also show a hitherto unreported dependency of the period on the upper boundary condition. In these simulations the dependency is linked to changes in mesospheric vertical velocities associated with the SAO.
The role of sea‐salt particles as cloud condensation nuclei over the remote oceans
Authors: Mason B. J.
Published: February 2007

If recent measurements of the fluxes of sea‐salt particles over the oceans as a function of wind speed are extrapolated to include the smallest particles detected over the oceans and in laboratory experiments on bursting bubbles, the flux F of particles having radii between r0 = 0.1 μm and r μm at 80% relative humidity may be represented by F = 2U3.5(e−0.2r0) – e−0.2r) m−2) s−1, where U is the near‐surface wind speed in m s−1. The flux of particles of r ≤ 5 μm in winds of 16 m s−1 is 20 000 m−2s−1. The corresponding local rate of production by air bubbles bursting in the foam patches of breaking waves that occupy, on average, only 5% of the ocean surface at this wind speed is 40 cm−2s−1. The average concentration of particles over the sea area producing whitecaps is:

where vr is the deposition velocity for particles of radius r. In winds of 16 m s−1 the total concentration of sea‐salt particles is calculated to be 40 cm−3. These, and the fluxes and concentrations calculated for other wind speeds, are consistent with measurements made at sea and in the laboratory by many scientists during the last 50 years.

Recently O'Dowd et al. have reported on sea‐salt particles of dry radius as small as 0.03 μm, with radius 0.06 μm at 80% relative humidity and mass 5 × 10−16 g. If the above equation is extended to include such small particles, it predicts a total concentration of 60 cm−3 in winds of 17 m s−1, fairly close to the reported 70 cm−3.

In general these computed and observed concentrations of sea‐salt particles are adequate to account for observed droplet concentrations in clouds over remote oceans uncontaminated by continental aerosols. However, on rather rare occasions droplet concentrations in maritime cumulus exceed 100 cm−3. Observations are cited to suggest that, in such cases, sea‐salt nuclei may be augmented by biogenic particles originating at the sea surface and by sulphate/sulphuric acid nuclei produced by the absorption of derivatives of dimethyl sulphide in cloud droplets.

Locations such as Hawaii and Tasmania are occasionally contaminated by continental or local pollution which may account for the rare reports of droplet populations being as high as 400 cm−3.
Measures of skill and value of ensemble prediction systems, their interrelationship and the effect of ensemble size
Authors: Richardson David S.
Published: February 2007

Ensemble forecasts provide probabilistic predictions for the future state of the atmosphere. Usually the probability of a given event E is determined from the fraction of ensemble members which predict the event. Hence there is a degree of sampling error inherent in the predictions. In this paper a theoretical study is made of the effect of ensemble size on forecast performance as measured by a reliability diagram and Brier (skill) score, and on users by using a simple cost‐loss decision model. The relationship between skill and value, and a generalized skill score, dependent on the distribution of users, are discussed. The Brier skill score is reduced from its potential level for all finite‐sized ensembles. The impact is most significant for small ensembles, especially when the variance of forecast probabilities is also small. The Brier score for a set of deterministic forecasts is a measure of potential predictability, assuming the forecasts are representative selections from a reliable ensemble prediction system (EPS). There is a consistent effect of finite ensemble size on the reliability diagram. Even if the underlying distribution is perfectly reliable, sampling this using only a small number of ensemble members introduces considerable unreliability. There is a consistent over‐forecasting which appears as a clockwise tilt of the reliability diagram. It is important to be aware of the expected effect of ensemble size to avoid misinterpreting results. An ensemble of ten or so members should not be expected to provide reliable probability forecasts. Equally, when comparing the performance of different ensemble systems, any difference in ensemble size should be considered before attributing performance differences to other differences between the systems.

The usefulness of an EPS to individual users cannot be deduced from the Brier skill score (nor even directly from the reliability diagram). An EPS with minimal Brier skill may nevertheless be of substantial value to some users, while small differences in skill may hide substantial variation in value. Using a simple cost‐loss decision model, the sensitivity of users to differences in ensemble size is shown to depend on the predictability and frequency of the event and on the cost‐loss ratio of the user. For an extreme event with low predictability, users with low cost‐loss ratio will gain significant benefits from increasing ensemble size from 50 to 100 members, with potential for substantial additional value from further increases in number of members. This sensitivity to large ensemble size is not evident in the Brier skill score. A generalized skill score, dependent on the distribution of users, allows a summary performance measure to be tuned to a particular aspect of EPS performance.
Liquid water path variability in unbroken marine stratocumulus cloud
Authors: Wood Robert , Taylor Jonathan P.
Published: February 2007

Liquid water path variability in unbroken marine stratocumulus clouds is examined using a simple adiabatic model and both ground‐based and airborne observations. Observations from FIRE suggest that fluctuations in liquid water path at scales smaller than around 60 km can be modelled reasonably well as a function of fluctuations in cloud‐base height. A simple adiabatic model of cloud liquid water content with a normally distributed cloud base and constant inversion height results in liquid water path distributions that compare favourably with observations. Further, it is shown how cloud‐base height fluctuations can be predicted from the sub‐cloud turbulent fluctuations of temperature, specific humidity and their covariance. The relative importance of these three contributing terms is assessed. Aircraft observations of these fluctuating variables are used to predict the widths of liquid water path distributions observed radiometrically. The observations also suggest that the standard deviation of cloud‐base height and hence liquid water path variability increases with increasing boundary‐layer depth. The model liquid water path distributions could be used to derive plane‐parallel albedo biases and offer a possible framework for future parametrizations of subgrid‐scale variability in general circulation models.
Alternative implementations of the semi‐Lagrangian semi‐implicit schemes in the ECMWF model
Authors: Cullen M. J. P.
Published: February 2007

An important issue for the future development of global models is the degree of implicit solution required for optimum performance, and whether this will change the relative cost‐effectiveness of spectral and finite‐difference models. This is explored by using a predictor‐corrector integration scheme in the European Centre for Medium‐Range Weather Forecasts model, where both steps are integrated using the current semi‐implicit procedure. This is equivalent to taking a first iteration towards a fully implicit scheme, using the existing semi‐implicit method as a preconditioner. The results show that benefit can be obtained provided that both the nonlinear part of the gravity‐wave terms and the semi‐Lagrangian trajectory are recalculated. This suggests that more fully implicit schemes will be beneficial in numerical models. However, the cost of elliptic solvers will not dominate as significant recalculation of tendencies will be required at each iteration.

Another strategic issue is the robustness of semi‐Lagrangian schemes at high resolution. These are the only advection schemes that preserve monotonicity that are available for spectral models. As resolution is increased, solutions of the equations always contain motions with the wind direction varying rapidly along the trajectory. These are typically not well treated at current operational resolutions, but limit the time step that can be used with semi‐Lagrangian schemes. We illustrate the effect of selective damping of these motions, which may be desirable until much higher resolutions are reached.
Influenza viruses and MAP kinase cascades – Novel targets for an antiviral intervention?
Authors: Ludwig Stephan
Published: February 2007

Influenza virus infections still pose a major threat to human and animal health worldwide. On one hand, like any other viral pathogen influenza viruses are strictly dependent on their host cell for efficient replication. On the other hand cells respond to infection by mounting an innate immune program to fight the invader. This constant battle between the virus and the host‐cell is also reflected on the level of the intracellular signalling events that are induced upon infection. While these signalling pathways are primarily initiated as a cellular defence program, the virus has acquired the capability to exploit some of these activities to support its replication. This review aims to illustrate this principle with regard to the diverse functions of different MAP kinase cascades during influenza virus infection. Some of these cascades act within cell protective pathways while others are required for efficient viral replication and thus, may serve as novel targets for antiviral intervention.
Human T cell leukemia virus type 1 Tax‐induced signals in cell survival, proliferation, and transformation
Authors: Silbermann Katrin , Grassmann Ralph
Published: February 2007

Human T cell leukemia virus type 1 (HTLV‐1), a delta‐retrovirus, causes an aggressive malignancy of T lymphocytes called adult T cell leukemia/lymphoma and stimulates permanent cell growth in culture. The virus encodes a nonstructural regulatory protein, Tax, which is both transforming in cell culture and oncogenic in vivo. This multifunctional protein controls viral transcription and in multiple ways interferes with cellular control of survival, proliferation, and genomic stability. Tax, by activation of NF‐κB, AP‐1, and other transcriptional pathways, enhances expression of cellular genes encoding cytokines (e.g. IL‐13, IL‐15), cytokine receptors (e.g. IL‐2Rα), and antiapoptotic factors (Hiap‐1, Bcl‐xL, OX40), leading to altered signal transduction (e. g. Jak/Stat, PI3K, Caspase 3/7). Cellular proliferation is stimulated by direct targeting of the cell cycle kinase (Cdk4, Cdk6) holoenzymes, repression of Cdk inhibitors, and the functional inactivation of the tumor suppressor p53. Finally, Tax, by promoting genomic instability through interference with DNA‐damage signaling, checkpoint control (G2/M, mitotic spindle assembly), chromosome segregation, and cellular DNA repair pathways could contribute to malignant conversion of infected cells.
HIPIMS: Die neue PVD‐Technologie
Authors: Münz W.‐D.
Published: February 2007

HIPIMS (High Power Impulse Magnetron Sputtering) stellt die neueste Entwicklung auf dem Gebiet der PVD‐Puls‐Sputter‐Technik dar. Unter dem Einfluss extrem hoher Puls‐Leistungen (4–6 MW) gelingt es, Gasentladungen zu erzeugen, die der weithin bekannten Arc‐Entladung, unter völliger Vermeidung von Droplets, sehr ähnlich sind. Hohe Metallionen‐Konzentrationen und Mehrfach‐Ionisierung tragen dazu bei, dass besonders dichte, homogene Schichten mit hoher Haftfestigkeit abgeschieden werden. Die industrielle Realisierung steht noch am Anfang. Mittlerweile sind bereits erste, auf HIPIMS spezialisierte Beschichtungsanlagen im Markt erhältlich. Erste Ergebnisse mit CrN–Schichten mit einer Härte von HV 3000 weisen auf das hohe Potential dieser neuen Technologievariante hin.
Preparation of dibasic acid‐containing soy phospholipids by chemical and enzyme‐catalyzed transesterification and evaluation of their surface‐active properties
Authors: Das Soma , Das Kali Pada , Bhattacharyya D. K.
Published: February 2007

The preparation of dibasic acid‐containing soy phospholipid was made by transesterification reaction with alkyl ester of diabasic acid with both lipase and alkoxide as a catalyst. The extent of incorporation of a dibasic acid varied with the molecular size of the dibasic acid. The extent of incorporation in soy phospholipids was 4–13% in the case of adipic acid and 9–20% in the case of sebacic acid. The surface‐active properties of these modified soy phospholipids were examined and were found to be different from those of the original (unmodified) soy phospholipid. The interfacial properties such as critical micelle concentration (CMC), γCMC, surface excess concentration Γmax, and minimum area per molecule (Å), and thermodynamic parameters such as standard free energy of micellization, were found to depend on the hydrophobic part of the dibasic acids.
Synthesis and performance of surfactants based on epoxidized methyl oleate and glycerol
Authors: Doll Kenneth M. , Erhan Sevim Z.
Published: February 2007

A small series of surfactants based on methyl oleate and glyceroe was synthesized. The synthesis utilizes an epoxidation reaction of methyl oleate followed by a simple esterification. The resultant products have between two and seven glyceride units, and their performance properties, including aqueous surface tensions and dynamic aqueous surface tensions, were studied. The droplet size of soybean oil/water emulsions made with each surfactant was also studied. The surfactants show properties similar to alcohol ethoxylates, such as the reduction of aquous surface tension to ∼34 mN m−1. Additionally, because the synthesis leaves the epoxide functionality in the surfactant, further modification for performance optimization is possible.
Low‐density lipoproteins are more electronegatively charged in type 1 than in type 2 diabetes mellitus
Authors: Gambino R. , Pisu E. , Pagano G. , Cassader M.
Published: February 2007

Multifactorial etiology is involved in premature atherosclerosis related to diabetes. Most of the mechanisms that are responsible for the etiology in diabetes have remained unsolved so far. Type 1 diabetes is associated with a favorable lipid pattern and with microangiopathy, which is not true for type 2 diabetes, which is related to dyslipidemia and macroangiopathy. The aim of this work was to evaluate the degree of LDL modification related to the types of diabetes. The question is whether the LDL could be differently modified since the pathogenesis of type 1 and type 2 diabetes is different. Thirtyone type 1 (19 male and 12 female) and thirty type 2 (18 male and 12 female) diabetic patients were included in this study. Isolated LDL was analyzed by capillary electrophoresis for diene conjugate content and for electronegativity. LDL from type 1 diabetes subjects showed the highest electrophoretic mobility (P=0.000). Instead, the diene conjugates contents were higher in the type 2 patients with HbA1c levels >8% (P=0.007). In conclusion, the increased diene content in type 2 diabetic subjects in poor glycemic control and the highest LDL mobility found in type 1 subjects show that the LDL undergoes different modifications. In type 2 patients, electronegative LDL are in a state of higher susceptibility to oxidation, whereas in type 1 subjects the finding of electronegative lipoproteins could provide an index of the relative atherogenicity of circulating LDL, especially as LDL has higher electrophoretic mobility than normal subjects.
One‐step analysis of major bile components in human bile using 1H NMR spectroscopy
Authors: Nagana Gowda G. A. , Somashekar B. S. , Ijare Omkar B. , Sharma Ajay , Kapoor V. K. , Khetrapal C. L.
Published: February 2007

Human gallbladder bile dissolved in dimethyl‐sulfoxide provides sharp and resolved signals for major bile components in 1H NMR spectra. Characteristic well‐resolved marker signals that invariably appear in 1H NMR spectra of bile were identified for cholesterol (H18 methyl signal at 0.643 ppm), lipids (glycerol CH signal at 5.064 ppm), total bile acids (H18 signals in the range 0.520–0.626 ppm), total glycine conjugated bile acids (NH signal at 6.958 ppm), total taurine conjugated bile acids (NH signal at 7.646 ppm), and urea (NH2 signal near 5.48 ppm), which enabled their rapid and accurate analysis. Excellent linearity and precision of quantitative analysis was observed for all the identified bile components (R2>0.99 for all). The method was demonstrated on gallbladder bile from 19 patients with gallbladder diseases. Urea in bile was identified by NMR for the first time and its quantitative analysis, along with several other bile components, is presented. The majority of the bile components could be analyzed in a single step. Accurate and rapid quantification of several bile components noninvasively by using the method presented herein may have far‐reaching implications in the study of bile acid metabolism and pathophysiology of various hepatobiliary and gastrointestinal diseases.
Singular limits for a 4-dimensional semilinear elliptic problem with exponential nonlinearity
Authors: Baraket Sami , Dammak Makkia , Ouni Taieb , Pacard Frank
Published: February 2007

Using some nonlinear domain decomposition method, we prove the existence of branches of solutions having singular limits for some 4-dimensional semilinear elliptic problem with exponential nonlinearity.

En utilisant une variante non linéaire de la méthode de décomposition de domaines, nous démontrons l'existence de branches de solutions ayant une limite singulière, pour une équation semilinéaire elliptique avec nonlinéarité exponentielle, en dimension 4.