bluuebunny/crossref_metadata_2025_split huggingface dataset:
Organized Convective Systems: Archetypal Dynamical Models, Mass and Momentum Flux Theory, and Parametrization
Authors: Moncrieff Mitchell W. 
Published: February 2007

AbstractA dynamical basis is established for understanding the structure and transport properties of organized convection and for expediting its parametrization in large‐scale models. A two‐dimensional model provides an exact theory of the momentum transport by mesoscale convective systems and similar phenomena. the system‐scale dynamics are supposed to dominate local processes and are modelled by a stationary triple‐branch airflow regime consisting of a double‐branch updraught and a downdraught. It is argued that a jump updraught is essential and has a fundamental effect on the momentum transport. the triple‐branch model degenerates into a form of conservative density current; another limiting case consists of a propagating positive pressure jump of maximum amplitude‐an overturning updraught but no downdraught. the functional relationship among dynamical parameters is determined by a characteristic regime equation derived from elementary nonlinear Lagrangian conservation properties and the volume integral of the horizontal‐momentum equation.The inflow to the archetypal jump updraught is constant. an example shows that the inclusion of shear in this region alters the detailed shape of the momentum flux profile but its fundamental character, namely the negative values of momentum flux for a system travelling in the positive x‐direction, is retained. This result, together with recent numerical simulations, implies that low‐level shear directly influences the initiation and evolution of convective elements, whereas the mature‐state fluxes for which the system‐scale flow organization and tilt is paramount is a product of the distribution of heat sources/sinks and deep tropospheric shear. the universal nature of the momentum flux profiles is explained in elementary terms by appealing to dynamical theory.The physical basis of the model and the momentum flux profiles are validated by using published results. the archetype emulates the basic character of the mass and momentum fluxes by mesoscale convective systems. For example an upper‐tropospheric flow deceleration is consistent with the observed effect of tropical cloud clusters on the mean flow but is distinct from the balanced response due to diabatic heating.The theory is used to develop a dynamical approach to the parametrization of organized convective processes that have hitherto been neglected in global models. Mass and momentum fluxes are obtained from the archetypal model in an approach that is fundamentally different from the statistical or averaging approximations that characterize present techniques. the activation of the parametrization scheme is also studied. Mass‐flux criteria are used to define an amplitude function for the mesoscale flux divergence to incorporate the flux laws into the large‐scale equations. the work can be extended to include thermodynamic fluxes by using generalized conservation properties.
Dry Season Micrometeorology of Central Amazonian Ranchland
Authors: Wright I. R. , Gash J. H. C. , Da Rocha H. R. , Shuttleworth W. J. , Nobre C. A. , Maitelli G. T. , Zamparoni C. A. G. P. , Carvalho P. R. A. 
Published: February 2007

AbstractThis paper presents the first comprehensive micrometeorological measurements to be recorded over post‐deforestation Amazonian ranchland. the ranch was managed for the production of beef cattle and had been created by felling and burning the original rainforest 12 years previously. the measurements allow derivation of the aerodynamic roughness, and a description of the response of the energy balance and surface conductance to the progressing dry season. Zero‐plane displacement and roughness length were derived from wind‐speed profiles as O.17 ± 0.03 m and 0.026 ± 0.003m respectively, while measurement of energy partition was achieved, with excellent agreement, between three independent measurement systems. During the 1990 dry season, average evaporation diminished from 3.8 to 2.1 mm d‐1 as the Bowen ratio increased from 0.43 to 0.67. Values of surface conductance were derived and these compare well with expected trends.
Spatial Structure of Ultra‐Low‐Frequency Variability of the Flow In A Simple Atmospheric Circulation Model
Authors: James I. N. , James P. M. 
Published: February 2007

AbstractThe internally generated variability of the global circulation on time‐scales in excess of one year is investigated using a multilevel baroclinic primitive‐equation model with moderate resolution of dynamical processes. the model includes idealized heating and friction. Maximum variability is found on time‐scales of around a decade. Empirical orthogonal‐function analysis reveals that the principal component of this variability is an alternate splitting and coalescence of the subtropical and mid‐latitude jets. Associated changes in the strength and distribution of eddy activity are found. the character of the ultra‐low‐frequency variability is completely changed for very low resolutions, but seems relatively insensitive to details of model formulation at higher resolution. the possibilities of deriving a low‐order model which includes ultra‐low‐frequency variability are considered.
Structure and evolution of an isolated semi‐geostrophic cyclone
Authors: Schär Christoph , Wernli Heini 
Published: February 2007

AbstractThe evolution and structure of an indealized low‐pressure system is studied within the framework of the semi‐geostrophic dynamics and in the limit of uniform potential vorticiy. An isolated cyclone is grown from suitably chosen initial conditions, rather than studying the evolution of a longitudinally periodic train of baroclinic systems.It is shown that the resulting development is able to produce a range of flow features that compare favourably with observationally based conceptual models of cyclogenesis. These features include in particular: (i) the simultaneous occurrence of both a cold and a warm front whose alignment shows some of the characteristics of occluded frontal systems and is skin to the notion of a frontal fracture, (ii) a dry descending air‐stream to the rear of the cyclone, (iii) a narrow region of maximum ascent within the warm front an its bent‐back portion, (iv) a poleward traveling air‐stream ahead of the cold front, and (v) a γ‐shaped pattern of vertical lifting comparable with the cloud patterns as commonly observed in satellite pictures.The evolving cyclone is analysed both form Lagrangian and Eulerian viewpoints. It is demonstrated that Lagrangian criteria exist that allow for the objective definition of air‐streams and flow patterns within developing cyclones. The structures of cold and warm fronts at low levels are significantly affected by the different nature of the Lagrangian trajectories within each of these regions. In particular, the air parcels in the warm‐frontal region are transported rapidly towards the centre of the low, resulting in a low‐level warm front with an intrinsically three‐dimensional structure and an associated vorticity gradient in the along‐front direction.
Development of ice and precipitation in New Mexican summertime cumulus clouds
Authors: Blyth Alan M. , Latham John 
Published: February 2007

AbstractAn experiment, involving the National Center for Atmospheric Research's Kin Air aeroplane, was conducted in order to measure the microphysical properties of New Mexican summertime cumulus clouds. Since the clouds formed and developed essentially in place, over the mountains, it was possible to make multiple penetrations through a single cloud, therby observing a significant fraction of the cloud's life cycle. In this paper, the questions of primary‐ and secondary‐ice production, and the development of precipitation particles, are addressed.Primary‐ice nucleation was found to occur when the temperature within the cloud reached a value of between −10 and −12°C irrespective of whether this was in the updraught or downdraught. Drops with diameters of about 0.5 mm were often observed in concentrations of about 10L−1 before the formation of ice. which suggests a nucleation mechanism involving large drops. The maximum concentrations of ice particles observed in these clouds (up to about 1300L−1) are much greater than typical concentrations of ice particles that can be attributed to primary‐ice nucleation. Evidence suggests that the most likely explanation is the Hallett‐Mossop process of secondary‐ice‐crystal production.Ice particles generally were first observed in the downdraughts. The development of precipitation is often thought to occur via downdraught transport. followed by sedimentation or mixing of ice particles into fresh. liquid‐laden turrets. The multi‐thermal nature of the cloud is considered to be central to this process.
The potential for stratosphere‐troposphere exchange in cut‐off‐low systems
Authors: Price J. D. , Vaughan G. 
Published: February 2007

AbstractThree mechanisms are discussed associated with cut‐off‐low (COL) systems which can transfer stratospheric air into the troposphere. These mechanisms are: (i) convective erosion of the tropopause, (ii) erosion of the tropopause by turbulence associated with a jet stream, and (iii) tropopause folding around the flank of a COL. Three case studies are presented which examine the first two of these processes and provide evidence that stratosphere‐troposphere exchange (STE) had occurred. Furthermore, results suggest that periodic reintensification of some COLs cause all these processes to recur or be prolonged. Their relative importance to STE is discussed and an estimate for that occurring by mechanism (i) is given for one of the case studies. Statistical data on annual COL numbers are used to calculate and annual mean transfer of ozone by this mechanism, which is found to be no more than 4% of the annual transfer for the northern hemisphere. This result suggests that convective STE in COLs is a minor contributor to the northern hemisphere STE.
Baroclinic‐wave life cycles, climate simulations and cross‐isentrope mass flow in a hybird isentropic coordinate GCM
Authors: Thuburn John 
Published: February 2007

AbstractResults are presented from a number of simulations using a global circulation model to compare the use of a δ or hybrid δ‐p vertical coordinate with a hybrid δ‐θ‐p vertical coordinate. In idealized baroclinic instability experiments, without diabatic processes, δ coordinates and δ‐θ‐p coordinates give very similar results, with some suggestion of slightly improved Lagrangian potential‐vorticity conservation in the δ‐θ‐p coordinate case. In climate simulations it has been necessary to include extra vertical diffusion in the presence of noisy temperature profiles in order to achieve stable integrations using δ‐θ‐p coordinates (though this may by unnecessary if improved schemes for vertical advection and radiation are used.) The extra diffusion leads to a noticeably less noisy temperature profile around the tropopause. Large differences between δ‐p coordinate integrations and δ‐θ‐p coordinate integrations appear in the moisture field because the diabatic terms and vertical‐advection terms are computed in a different order, though the net moisture budget is not affected. Other diagnostics show that the two types of vertical coordinate give very similar results. Some examples are presented of a novel diagnostic: the accumulated cross‐isentrope mass flow. Global tracer conservation may be poor when hybird isentropic coordinates are used, unless care is taken to put the horizontal scale‐selective dissipation into the form of a flux divergence.
Time‐varying zonal asymmetries in stratospheric nitrous oxide and methane
Authors: Gao H. , Stanford J. L. 
Published: February 2007

AbstractPreviously analyses of Stratospheric And Mesopheric Sounder (SAMS) data of atmospheric constituent gases have dealt almost exclusively with zonal means (and mostly monthly means), owing perhaps to concern over data quality. The purpose of this note is to show that, with care, time‐dependent zonally‐asymmetric features may be recovered from the SAMS nitrous oxide and methane data. As an example, we demonstrate the existence of zonal wave‐1 constituent perturbations with periods of a few weeks in the middle and upper stratosphere. When the perturbations are normalized by the constituent zonal‐mean mixing ratio to compensate for the slowly varying (in both space and time) background concentration of constituents, wavepacket‐like features are found over all latitudes and seasons in the three‐year SAMS record. One specific low‐latitude case discussed had features which appear to be consistent with constituent oscillations induced by episodic equatorial Kelvin waves. Further studies are needed to better identify the nature of the plethora of observed wave‐like phenomena.
An integrated view of the 1987 Australin monsoon and its mesoscale convective systems. II: Vertical structure
Authors: Mapes Brian , Houze Robert A. 
Published: February 2007

AbstractThe vertical structure of monsoon thermal forcing by precipitating convection is diagnosed in terms of horizontal divergence. Airborne Doppler‐radar divergence profiles from nine diverse mesoscale convective systems (MCSs) are presented. The MCSs consisted of multicellular convective elements which in time gave rise to areas of stratiform precipitation. Each of the three basic building blocks of the MCSs—convective, intermediary, and stratiform precipitation areas—has a consistent, characteristics divergence profile. Convective areas have low‐level convergence, with its peak at 2‐4 km altitude, and divergence above 6 km. Intermediatry areas have convergence aloft, peaked near 10 km, feeding into mean ascent high in the upper troposphere. Stratiform areas have mid‐level convergence, indicating a measoscale downdraught below the melting level, and a mososcale updraught aloft.Rawinsonde composite divergence profiles agree with the Doppler data in t least one important respect: the lower‐tropospheric convergence into the MCSs peaks 2‐4 km above the surface. Rawinsonde vorticity profiles show that monsoonal tropical cyclones spin‐up at these elevated levels first, then later descend to the surface. Rawinsonde observations on a larger, continental scale demonstrate that a large horizontal scales only the ‘gravest vertical mode’ of MCS heating is felt, while the effects of shallower components of the heating (or divergence) profiles are trapped near the heating, as predicted by geostrophic adjustment theory.
Moisture and heat budgets of a cirrus cloud from aircraft measurements during FIRE
Authors: Gültepe Ismail , Rao Gandikota V. 
Published: February 2007

AbstractThis study is based on the NCAR king Air aircraft and radiosonde on 31 October 1986 during the FIRE (First International Satellite Cloud Climatology Project Regional Experiment) in Wisconsin over Oshkosh. Aircraft step‐up and spiral descent flights are used to obtain kinematic and thermodynamic data. In the step‐up manoeuvre, six different penetrations were made between 1528 and 1616 UTC. Each penetration was about 30 km long separated in the vertical by about 300 m. The time difference between the two spiral soundings was about 43 minutes. The aircraft descended at a rate of 1.5 ms−1 during these spiral soundings. Kinematic, cloud physical, and radiometric observations from various instruments are used to estimate the different using the mean longitudinal wind and vertical radiative fluxes are important in forming budgets for the cirrus layers. Ice‐crystal growth is significant in the upper layers. The maintenance of cirrus can be attributed to relatively warm and moist air advection, radiative cooling at upper levels, and moisture advection in the vertical. Turbulent heat and moisture fluxes are found to be significant in the low levels of cirrus.
On the interaction of tropical‐cyclone‐scale vortices. I: Observations
Authors: Lander Mark , Holland Greg J. 
Published: February 2007

AbstractA detailed analysis is made of the observed behaviour in interaction, tropical‐cyclone‐scale vortices in the western North Pacific region. It is found that all multiple‐vortex interactions can be broken down into a common model of binary interaction. The classical Fujiwhara model of converging cyclonic rotation about a centroid followed by merger is rarely followed. Capture tends to occur rapidly, within several hours, and is followed by a period of relatively stable cyclonic orbit. Cyclone merger occurs by one vortex experiencing a loss of convective organization, followed by horizontal shearing and incorporation into the outer circulation of the other vortex. However, a substantial proportion of interacting cyclones escape from the interaction, and the changeover from a stable orbiting configuration occurs rapidly. A model of binary interaction is presented.Cases where swarms of mesoscale vortices are formed in convectively active regions without tropical cyclones are also documented. These vortices have a much narrower range of influence (a few hundred kilometres) then that observed for tropical cyclones. When groups of vortices come within this range they are observed to conform to the same interaction model as observed for tropical cyclones.
Airflow over a two‐dimensional escarpment. III: Nonhydrostatic flow
Authors: Blockley J. A. , Lyons T. J. 
Published: February 2007

AbstractForced flow over the asymmetric topography of the Darling Scarp, Western Australia is modelled, using a nonhydrostatic model, and compared with observations. Drag histories and sensitivity tests indicate that it is possible for the flow to be dominated by hydrostatic downslope windstorms even if the hydrostatic index indicates nonhydrostatic dominance. Experiments on a wide range of one‐ or two‐layered flows suggest that nonhydrostatic effects on windstorm events are very small. However, the development and location of trapped lee waves can be significantly affected if there is sufficient hydrostatic forcing in the flow. For hydrostatic forcing to dominate over trapped lee waves it is necessary for reflection, from a region of wave breaking or a critical layer, to occur at the right height.
Oscillations in the equatorial components of the atmosphere's angular momentum and torques on the earth's bulge
Authors: Bell M. J. 
Published: February 2007

AbstractThe angular momentum of the atmosphere is the sum of the wind term, W, due to the winds relative to the earth's surface, and the matter term, M. It is shown that if the earth were an oblate spheroid with an equipotential surface, the atmospheric torque on the earth would be ‐Ω ∧ M, Ω being the rotation rate of the solid earth. As a result, the equatorial components of M for a wave propagating at azimuthal angular velocity σ without changing shape are (Ω ‐ σ)/Ω times the equatorial components of W. Laplace's equation for tidal motions ‘on a sphere’ strictly applies to motions on such an oblate spheroid and its solutions apply a torque on the earth equal to ‐Ω ∧ M. It is shown that the resulting relationship between M and W also implies that inanyseparable wave solution of the tidal equations the surface wind is simply related to the vertical integral of the wind and the equivalent depth.Analyses of the equatorial components of the matter term by the weather forecast systems of the European Centre for Medium‐range Weather Forecasts and the Meteorological Office are dominated by chaotic oscillations with periods of between 8 and 10 days that are well forecast out to 5 days ahead. It is argued that these are essentially free solutions of the tidal equations which exert considerable torques on the earth. The main feature of the equatorial components of the wind term is a seasonally modulated diurnal oscillation. Analyses and 2‐day forecasts of this phenomenon are in less good agreement. It is argued that it is thermally forced and depends on the compressibility of the atmosphere.
Non‐hydrostatic simulations of a cold front observed during the Fronts 87 experiment
Authors: Redelsperger J.‐L. , Lafore J.‐P. 
Published: February 2007

AbstractA two‐dimensional non‐hydrostatic cloud model is used to simulate a frontal system observed on 11 November 1987 during the Fronts 87 experiment.The cloud‐resolving model is able to reproduce a large variety of frontal bands, starting from only the meso‐α‐scale fields produced by a hydrostatic forecast model. A narrow cold‐frontal rainband is identified at the surface cold front. Two classes of wide bands are also simulated: a wide cold‐frontal rainband and warm‐sector wide rainbands. The vertical‐velocity fields exhibit similar banded features in both the simulations and the Doppler‐radar observations, with bands having the same position, intensity and width.Several sensitivity experiments are presented. One of them shows that the initial knowledge of the ageostrophic circulation is not necessary to give a successful simulation. Other experiments show that the along‐front baroclinicity drives the wide‐band formation whereas the along‐front deformation is not important.The heat, moisture and horizontal momentum budgets are also investigated, in order to determine the rainband effects on larger scales.