bluuebunny/crossref_metadata_2025_split huggingface dataset:
Upper Tropospheric Equatorial Waves In Ecmwf Analyses
Authors: Randel William J. 
Published: February 2007

AbstractTropical wind fields from the European Centre for Medium‐range Weather Forecasts operational analyses are analysed to search for equatorially trapped waves in the upper troposphere. Data are studied for 1980‐87. Zonal wind spectra show a predominance of power at low zonal wave numbers and low frequencies; these data do not show distinct spectral peaks corresponding to eastward‐propagating Kelvin waves or westward‐propagating equatorially‐trapped Rossby waves. the zonal wind spectra do show episodic evidence for westward‐propagating 5‐day zonal wave‐1 features with maxima over the equator; these are likely related to Rossby normal‐mode oscillations previously reported.Meridional wind spectra show a predominance of westward‐propagating power in the 6‐ to 10‐day‐period range, primarily concentrated in zonal waves 4‐7. Enhanced meridional wind activity at the equator occurs episodically in time, with events persisting for one to several months duration. Wave packets are longitudinally localized, with preferred occurrence over the eastern Pacific Ocean. Most cases show evidence of meridional wind maxima in the tropics, suggesting a degree of equatorial trapping. the horizontal structure of these oscillations is that of mixed Rossby‐gravity waves, although they are not necessarily centred directly over the equator. These waves have largest amplitudes in the upper troposphere, showing weak coherence with the surface; they appear to be fundamentally distinct from fast 4‐ to 5‐day Rossby‐gravity waves observed in the western Pacific.A detailed study of a group of intense Rossby‐gravity waves observed over the eastern Pacific during August‐September 1985 is presented; the reality of these features is confirmed in direct rawinsonde observations. Correlations are found between bursts of equatorial wave activity and enhanced meridional momentum fluxes from the subtropics, suggesting extratropical forcing as at least one mechanism for their generation. the waves are also strongly coupled to the upper tropospheric zonal wind field, which likely accounts for their longitudinal and temporal confinement.
A Study of Mountain Lee Waves Using Clear‐Air Radar
Authors: Ralph F. M. , Crochet M. , Venkateswaran S. V. 
Published: February 2007

AbstractThe tropospheric and lower stratospheric response to flow over complex terrain is investigated using observations of vertical and horizontal motions made by a clear‐air Doppler radar stationed at Toulon in southern France. Analysis of the data has disclosed that episodic enhancements in the variability of vertical and horizontal velocities are due to lee waves excited by obstacles located within 5 km of the radar. Three separate cases of lee‐wave activity are studied. Two‐dimensional linear perturbation theory and existing three‐dimensional simulation results, for appropriate parameter values, are used as guides to check against the vertical structure observed by radar. the magnitudes of vertical velocities observed during lee‐wave incidence are well within the range reported in previous observational and numerical simulation studies.From a detailed examination of one of the cases of trapped waves, the time variation, observed by the radar, of timescales of less than an hour, has emerged as the likely result of small directional changes in the upstream winds causing a bodily rotation of the downstream three‐dimensional wave pattern centred on the wave‐generating obstacle. the observed tendency of the vertical‐velocity perturbation to change sign with time above the radar location follows as a consequence of the presence of large horizontal gradients in the motion field, which are inherent in the structure of three‐dimensional lee waves.The temporal variability of the lee‐wave pattern has allowed the radar to sweep out a partial horizontal cross‐section, revealing vertical circulation features that are broadly consistent with gravity‐wave behaviour. However, analysis of the effects of finite antenna beam spacing on the sampled data indicates that a spurious phase‐shift might have been introduced between the vertical and horizontal velocity perturbations observed in the cross‐section. In contrast, the measured vertical velocities are not seriously affected by finite beam‐width effects.The lee waves examined in this study are quite moderate, nevertheless, this circumstance has provided a favourable opportunity to conduct a critical evaluation of the capabilities and limitations of this radar technique for observing lee‐wave phenomena in general. We have been able to show clearly how the incidence and vertical structure of the waves respond to the synoptic‐scale situation. In particular we have been able to distinguish between trapped and vertically propagating waves and to relate a transition between these wave types to synoptic‐scale changes in the static stability structure.
On the Shape and Fall Velocities of Raindrops
Authors: Brazier‐Smith P. R. , Brazier‐Smith P. R. 
Published: February 2007

AbstractA numerical model for the calculation of the shapes and fall velocities of raindrops is presented. Central to the operation of the model is the calculation of the flow over the front portion of the drop. This is done on the basis that the flow in that region is potential and that the surface streamline separates at the waist. In its final form the model predicts fall velocities within 4% of the experimental values for drops of 1 mm radius and greater, and shapes that correspond very closely to observed shapes. the incorporation of electrical forces and their effects are briefly explored.
On the spectral behaviour of atmospheric boundary‐layer parameters at Thumba, India
Authors: Winston Prakash J. Jeeva , Ramchandran Radhika , Nair K. Narayanan , Gupta K. Sen , Kunhikrishnan P. K. 
Published: February 2007

AbstractThe features of the spectra of meteorological fields of wind velocity and temperature observed at Thumba, a site on the western coast of India, are discussed. Data utilized for the study are mostly from an instrumented tower, sodar and long‐period temperature data available for the site. The influence of local circulations like the sea breeze is manifested also in the general spectral characteristics of horizontal wind, as with the spectral gap in the mesoscale region between the synoptic‐scale region and the turbulent‐scale region of the spectra. The width of the spectral gap observed between the wave‐active region and the microscale region in the wind velocity spectra is seen to be dependent on the stability of the medium, and the gap is found to shift to the high‐frequency side with increasing stability. The composite spectrum of temperature also reveals a prominent gap at about an hour. It is seen that with the advection of a sea breeze over the land, a shallow thermal internal boundary layer is formed and regions above one tenth of this depth obey mixed‐layer similarity.
Surface‐layer description under free‐convection conditions
Authors: Sykes R. I. , Henn D. S. , Lewellen W. S. 
Published: February 2007

AbstractLarge‐eddy simulation is used to study the structure of the boundary layer close to the surface under free‐convection conditions. The magnitude of the surface friction velocity induced by the large convective eddies is determined as a function of the convective velocity scale, w*, and the surface, roughness length, z0. A simple analytic scaling form is obtained from a balance of forces in the wall layer induced by the convective eddies, giving the effective thickness of the layer and the effective surface stress and temperature dependence. The depth of the wall layer varies significantly with surface roughness and is extremely thin for small roughness, e.g. 0.5% of the mixed layer depth for a roughness length of 10−4zi. The analytic forms are consistent with the numerical predictions over the range of roughness length considered. The probability distribution of the instantaneous surface friction velocity (magnitude only) is examined and found to be reasonably close to Gaussian.
A GCM simulation of the impact of Amazonian deforestation on climate using an improved canopy representation
Authors: Lean J. , Rowntree P. R. 
Published: February 2007

AbstractTo obtain a estimate of the impcat of Amazonian deforestation on local climate it is critical that the representation of the forest canopy within general circulation models (GCMs) is as realistic as possible. Recent measurements from the Amazonian forest have highlighted major weaknesses in the Meteorological Office GCM simulation of the interception of rainfall from the forest canopy. Here, we present results for Amazonia from a new 3‐year control experiment which incorporates an improved representation of micrometeorological processes within the forest. A detailed assessment of the control simulation reveals that the adjusted GCM provides a realistic description of the climate of Amazonia. In determining the impact of Amazonian deforestation on climate we present a comprehensive analysis of the simulated climate following the replacement of Amazonian forest by pasture%.A comparison of the new results with those from the earlier deforestation experiment carried out by Lean and Warrilow (1989) suggests that the reductions in local rainfall (14%) and evaporation (24%) are smaller than those obtained with the previous formulation of interception. It was concluded by Lean and Warrilow that with a wet canopy, decreases in roughness in the deforested case reduce evaporation. With the introduction of the new interception formulation the canopy is less often wet, and so the effect of deforestation is reduced.
The density and structure of ice accretion predicted by a random‐walk model
Authors: Szilder Krzysztof 
Published: February 2007

AbstractA random‐walk, ballistic‐type model is used to simulate the ice accretion process due to impinging supercooled droplets. the model is applicable over a wide range of growth conditions including, in the limit, growth where the droplets freeze virtually as spheres (dry growth), and growth where the droplets spread into and across the ice surface losing their identity (wet growth). The structure and density of the ice are determined by the probabilities governing droplet motion and freezing during their random walk along the ice deposit. A relationship between the Macklin parameter, commonly used to relate atmospheric conditions to ice density, and the probabilities of droplet motion has been established. The ice density and accretion structure perdicted by the random‐walk model agree qualitatively with experimental observations at the stagnation line on a fixed circular cylinder. The model also predicts finger‐like ice structures for small values of droplet momentum, also in keeping with observations.
Turbulent transfer relationships over an urban surface. II: Integral statistics
Authors: Roth M. 
Published: February 2007

AbstractA comprehensive study of atmospheric turbulence over an urban surface has been carried out. Part II presents the normalized standard deviations of all important atmospheric variables from unstable conditions Measured at z'/z0=37 and analysed within the Monin‐Obukhov similarity framework. They differ from the rural reference data in the following ways.
The normalized standard deviation of vertical velocity is systematically smaller.
The efficiency of urban transfer of heat and especially momentum are larger.
The humidity statistics cannot be described within the similarity framework.
It is suggested that the roughness sublayer effects and large‐scale space and time inhomogeneities of the motions in the suburban atmosphere are responsible for the observed dissimilarities.
Microphysical and radiative properties of small cumulus clouds over the sea
Authors: Raga G. B. , Jonas P. R. 
Published: February 2007

AbstractMicrophysical and radiative data obtained in fields of cumulus clouds over the sea around the United Kingdom are presented. The Meteorological Research Flight C‐130 aircraft was used to make radiation and cloud microphysical measurementsin situon four different occasions. The sub‐cloud aerosol concentrations ranged from 100 to 5000 cm−3, from fairly maritime to heavily polluted. Vertical profiles of microphysical variables indicate that small cumuli over the sea have some properties that are more similar to marine stratocumuli than to continental cumuli. Nonetheless, variables, such as liquid‐water content and droplet‐number concentration, exhibit a larger variability than in stratocumuli, owing to more vigorous entrainment in cumuli.Mie theory was used to compute extinction cross‐sections, single scattering albedo, back‐scattering cross‐sections, and asymmetry parameters for all in‐cloud samples. The average vertical profiles are in agreement with values computed by previous authors for trade‐wind cumuli. Our results show that these properties depend upon the background aerosol concentrations%.A simple two‐stream radiative‐transfer model, using the back‐scattering cross‐section, is presented and shows good agreement with run‐averaged radiometric observations.
A multiple sounding technique for the study of gravity waves
Authors: Shutts G. J. , Healey P. , Mobbs S. D. 
Published: February 2007

AbstractRadiosondes released in rapid succession, with a time separation of a few minutes, were launched during a field experiment in central Wales, U.K. to study orographic gravity waves. Rate of ascent fluctuations were used to infer the existence of vertical‐velocity fluctuations associated with orographic gravity waves, and differences between consecutive ascents provided information on the steadiness and phase‐line slope of the waves.The potential of the multiple sounding technique is examined here and an analysis strategy proposed. Three triple‐sounding cases from the field experiment are used to illustrate the strengths and weaknesses of the technique. In one case it was possible to estimate a phase‐line tilt with height and this, combined with the inferred vertical‐velocity amplitude, allowed the estimation of the vertical momentum flux.It is suggested that the near‐simultaneous release of between three and five sondes upstream of mountainous terrain could provide a useful method for calculating the vertical flux of momentum due to gravity waves.
Studies of the radiative properties of ice and mixed‐phase clouds
Authors: Sun Zhian , Shine Keith P. 
Published: February 2007

AbstractRadiative parametrizations for both ice and water clouds are developed in terms of liquid/ice water content, based on Mie scattering theory. For ice crystals the application of Mie theory is guided by the hexagonal‐crystal/equivalent‐spheres comparison of Takano and Liou. These parametrizations are extensively tested against measurements from aircraft and are shown to perform satisfactorily, although corrections for unobserved small crystals and the effect of crystal shape are large and not currently well defined. The parametrizations are then used to investigate the effect of mixed‐phase clouds on radiative transfer. It is found that, because the radiative properties of ice crystals and liquid droplets are significantly different, the radiative properties of mixed‐phase clouds cannot be simulated successfully if the ice in clouds is converted into liquid water. Both the albedo and the rate of change of albedo with ice fraction are significantly dependent on the method by which the phases are mixed; these factors may be of especial importance in climate‐sensitivity experiments that incorporate mixed‐phase clouds. The presence of ice in clouds below the cirrus level is often ignored in climate‐model and radiation‐budget studies. The calculations presented here indicate that this neglect may lead to a serious bias in cloud albedo for a given path of condensed water.
On the reflectance of cellular cloud layers
Authors: Jonas Peter R. 
Published: February 2007

AbstractA Monte Carlo model is used to investigate the effects on the reflectance of low cloud layers of organized cellular structure, which is often observed in such layers. It is assumed that the cloud is composed of a series of cells each of which comprises a core region surrounded by a wall region of reduced water content. It is shown that the sensitivity of reflectance to the structure is largest for wavelengths that are not absorbed by liquid water. The reflectance of the cloud layer is reduced when the water content in the regions of low water content is less than about 30% of the water content in the remainder of the layer. The effect on the cloud‐layer reflectance of organized structure in the water content is large compared with the effect of changes in the effective radius at cloud top. It is further shown that, for narrow (∼100 m) regions of low water content, estimations of the layer average reflectance based on the area weighted average of the reflectance of layer clouds with high and low water contents are significantly in error. For small solar zenith angles the regions of low water content tend to permit incident radiation to pass through the cloud, reducing the reflectance, while for large solar zenith angles the reflectance is increased by the effects of cloud shadowing.
Dynamics of a low‐level jet observed during the Fronts 87 experiment
Authors: Kotroni V. , Lemaitre Y. , Petitdidier M. 
Published: February 2007

AbstractA study of the internal dynamics of a low‐level jet, its role on the frontal discontinuity evolution, and its relevance to the organization of convection at mesoscale and small scale is presented. The low‐level jet studied is associated with an active cold front that was observed during the French—British co‐operative field experiment Fronts 87. The originality of the present low‐level‐jet description lies in the fact that it is observed not only by means of soundings but also by means of a new tool for mesoscale meteorology observations, a wind profiler (stratospheric—tropospheric UHF radar). The high resolution of the wind‐profiler data in both time and height allows one to assess, in precipitating as well as in clear‐sky areas, a detailed image of the low‐level jet and the associated mesoscale and convective‐scale circulations. The comparison between observations and predictions of various theories provides information on the physical mechanisms responsible for the initiation and maintenance of the observed mesoscale features. The flow resulting from the presence of the low‐level jet produces inertial acceleration and baroclinic symmetric instability which lead to a narrow mesoscale line of sloping motions. Moreover, the interaction of the cold surface outflow with the low‐level vertical shear below the jet maximum is found to be crucial in the intensification and maintenance of the observed narrow cold‐frontal rainband.
Sensitivity of optimal unstable structures
Authors: Buizza Roberto 
Published: February 2007

AbstractOptimal unstable perturbations, i.e. structures that grow fastest over a finite time interval, can be used in numerical weather prediction to construct the initial conditions of an ensemble of integrations. Optimal perturbations superimposed on a basic state provide a representation of the uncertainty of the initial state of the atmospheric flow. The computation of the fastest‐growing perturbations over a finite time interval can be achieved in the linear approximation by using the forward and adjoint tangent version of a full nonlinear model.Previous results, obtained using a version of a primitive‐equation model with a reasonable horizontal and vertical resolution, showed that the lack of parametrization of turbulent processes can lead to fast‐growing perturbations characterized by ‘non‐meteorological’ structures. The first part of this paper focuses on this problem. Numerical experiments have been performed to study the impact of a simple surface‐drag and vertical‐diffusion scheme on the most unstable perturbations. It is shown how the very simple parametrization of the turbulent processes implemented inhibits the growth of non‐meteorological structures close to the surface.A second very important problem to face when constructing the initial conditions of an ensemble of forecasts, using optimal perturbations, is the definition of the optimization time interval over which the growth of these unstable structures is maximized. This problem will be investigated in the second part of this work, where the impact of the optimization time interval on the definition of the unstable sub‐space is studied for time periods up to three days. Results from different cases seem to indicate that unstable sub‐spaces computed with an optimization time interval longer than 36 hours are very similar.
A three‐dimensional model study of nitrogen oxides in the stratosphere
Authors: Lary D. J. , Pyle J. A. , Carver G. 
Published: February 2007

AbstractA new three‐dimensional model for stratospheric chemistry studies is described. The model, a spectral general circulation model, is developed from the model used at the European Centre for Medium‐range Weather Forecasts. Schemes to describe atmospheric chemistry and photochemistry as well as advection of trace gases have been implemented. In this study a perturbed period in January 1987 is studied. The model performs well. An interesting feature, similar to the Noxon cliff, is reproduced in which the NO2 column is reduced significantly in high latitudes. In this calculation the cliff is produced by gas phase conversion of NO2 to N2O5.
Comments on ‘Spatial structure of ultra‐low frequency variability of the flow in a simple atmospheric circulation model’ by I. N. James and P. M. James (October 1992, 118, 1211–1233.)
Authors: Feldstein Steven B. , Robinson Walter A. 
Published: February 2007

AbstractJames and James (1992) examine internally generated interannual variability in a multi‐level primitive‐equation model. The authors claim that the variability in the global mean relative angular velocity is associated with fluctuations in the mid‐latitude zonally‐averaged zonal winds. We question this relationship, and present results from an aqua‐planet GCM which indicate that, instead, the global mean relative angular velocity is primarily related to changes in the tropical zonally‐averaged zonal winds. Furthermore, we also question whether the ‘ultra‐low‐frequency variability’ examined by James and James is not simply just ‘climate noise’.