bluuebunny/crossref_metadata_2025_split huggingface dataset:
The role of mass transfer in describing the dynamics of mesoscale convective systems
Authors: Gray M. E. B. , Shutts G. J. , Craig G. C. 
Published: February 2007

AbstractWhen deep convection occurs in a rotating stratified fluid, potential vorticity (PV) anomalies form in response to the change in the mass field. This geostrophic adjustment process, and its importance in shaping the dynamics of long‐lived convective systems, is studied using a high‐resolution numerical model to simulate idealized precipitating and non‐precipitating convection.The first series of experiments considers the partitioning of energy between the balanced flow associated with the adjustment (EB), inertia‐gravity waves and turbulent dissipation and how this varies with convected mass (Mc). It is found, for two‐dimensional non‐precipitating convective plumes with a Coriolis parameter equal to 10−4 s−1, that, and, for the plumes which transport the largest quantities of mass, up to 60% of the energy is retained in the balanced flows – a much higher figure than previous estimates. For three‐dimensional (3–D) non‐precipitating plumes it is shown that EB α Mc2. It is suggested that these results indicate that the geostrophic adjustment to mass transfer is an important process in larger convective systems, such as Mesoscale Convective Systems, and that PV anomalies associated with such a system would be much stronger than the net anomaly associated with an ensemble of thunderstorms giving the same upward mass transfer. It is found, for the 3‐D runs, that inertia‐gravity waves comprise only 10% of the total energy released.A further series of experiments are carried out in which the effects of precipitation driven downdraughts are included. The downdraughts do not have the effect of simply reversing the mass transfer of the updraught, and the combination of the updraught and downdraught results in a complex PV anomaly structure with a mid‐level cyclonic vortex. Similar conclusions to those for the non‐precipitating convection case appear to follow for more realistic precipitating systems.
Graupel trajectories and charging: A new numerical approach for cloud electrification studies
Authors: Masuelli S. , Pulido M. A. , Scavuzzo C. M. , Caranti G. M. 
Published: February 2007

AbstractIn this work the growth, motion and charging of graupel particles are simulated by computer. The cloud fields are calculated by a three‐dimensional cloud model. Another program calculates the time evolution, inside these cloud fields, of graupel properties (size, position, charge and others). The electricity enters both in the charging of the particles through their interactions with other particles and in the motion itself, since in the equation of motion the electrical force is included. Embryos with 500 μm initial diameter are released at 656 locations uniformly distributed in the updraught centre at four altitudes. The trajectories of all the particles are followed all the way to the 0°C altitude. The graupel is charged by the ice‐crystal‐graupel non‐inductive mechanism.This kind of approach appears to be a powerful tool to study the electric charging of the graupel in greater detail, since a very high spatial resolution (about 1 m) is easily achieved. Some preliminary analysis of the charges on the precipitation particles is carried out and the results are related to in situ observations. For the sake of comparison, trajectories without the electrical interaction are also computed, and it is shown that in some cases the final radius with and without the electrical force can differ by a factor of two to three in either direction depending on the initial position.
Accuracy of trajectories as determined from the conservation of meteorological tracers
Authors: Stohl A. , Seibert P. 
Published: February 2007

AbstractDynamical structures as well as transport processes are often investigated using trajectories. Several alternative techniques are currently in use to calculate trajectories, which may produce significantly different results. In this study, three‐dimensional, isentropic, isobaric and isoeta (terrain‐following coordinate surfaces) trajectories are calculated. In the case of the isentropic trajectories, both the kinematic and the dynamic calculation method are applied.Using a tracer that is conserved along the real three‐dimensional trajectories, it is possible to determine which of these trajectories are the most accurate. Here, conservative meteorological quantities, namely potential vorticity, specific humidity and potential temperature, are used. As these tracers are conserved only in the absence of diabatic processes, regions of the atmosphere where these processes are important, i.e. the boundary layer and moist regions, are excluded. Tracer conservation along the different trajectories is studied to find the most accurate trajectory type.It is found that three‐dimensional trajectories are the most accurate ones, and kinematic isentropic trajectories, affected more by dynamical inconsistencies between meteorological fields, are the second best in the troposphere. In the stratosphere, three‐dimensional and isentropic trajectories may be of similar accuracy. Isobaric and isoeta trajectories are much less accurate, both in the troposphere and in the stratosphere. Dynamic trajectories tend to perform unrealistic inertial oscillations and thus give clearly worse results than any of the kinematic trajectories.Since there exists no direct relationship between the average errors in tracer conservation and average spatial position errors of the trajectories, a transformation of the tracer conservation errors into spatial position errors is difficult.
Middle atmosphere variability in the UK Meteorological Office Unified Model
Authors: Swinbank R. , Douglas C. S. , Lahoz W. A. , O'neill A. , Heaps A. 
Published: February 2007

AbstractThis paper describes the seasonal evolution of the stratospheric circulation simulated by a stratospheretroposphere configuration of the UK Meteorological Office (UKMO) Unified Model. Results are shown from a five‐year integration of the model. The model's simulation of the stratosphere and its seasonal evolution compares well with global analyses produced by the UKMO troposphere‐stratosphere data‐assimilation system. The contrast between the winter circulation in the two hemispheres is well simulated. The zonal‐mean winds show strong interannual variability in northern winter, while the southern hemisphere winter jet is much less variable. In northern wither the model spontaneously produces two major warmings and a number of minor warmings. In southern winter and spring the model reproduces well the break‐up of the polar vortex and elements of the flow regime that often precedes this break‐up. The model does, however, exhibit a number of shortcomings. Lack of conservation of potential vorticity prevents the model from capturing some of the ingredients of the flow regimes associated with stratospheric warmings and with the merger of anticyclones. There is a cold bias in the stratosphere throughout the year, with a maximum cold bias over the winter pole near the stratopause. This temperature bias appears to be due to a cooling bias in the long‐wave part of the radiation scheme. The model also has unrealistically strong planetary waves in the upper stratosphere, although amplitudes in the upper troposphere and lower stratosphere are in good agreement with those derived from the UKMO analyses. This paper discusses possible future model improvements that should alleviate these problems.
Barotropic instability in frontolytic strain
Authors: Parker D. J. 
Published: February 2007

AbstractRecent observational studies have indicated that a front which is undergoing frontolytic strain may be conducive to rapid growth of waves or secondary cyclones. Previous work has shown that a frontogenetic strain flow will tend to suppress the growth of waves on a two‐dimensional vortex strip: here, the opposite case of negative strain is studied. Justification for considering a basic state which is frontolytic is presented, given the natural transience of this state. The results show that the growth is sufficiently rapid for instability to be relevant on the time‐scale of evolution of the parent front. Waves grow kinematically with the front itself, at an exponential rate. More rapid growth is possible for transient barotropically unstable modes but only for limited time. These results are discussed with a view to their impact on the understanding of atmospheric flows.
Extended assimilation and forecast experiments with a four‐dimensional variational assimilation system
Authors: Rabier Florence , Thépaut Jean‐Noel , Courtier Philippe 
Published: February 2007

AbstractResults of four‐dimensional variational assimilations, 4D‐Var, in cycling mode, over a few two‐week assimilation periods are presented. 4D‐Var is implemented in its incremental formulation, with a high‐resolution model with the full physical parametrization package to compare the atmospheric states with the observations, and a low‐resolution model with simplified physics to minimize the cost‐function. The comparison of 4D‐Var using several assimilation windows (6, 12 and 24 hours) with 3D‐Var (the equivalent of 4D‐Var with no time‐dimension) over a two‐week period shows a clear benefit from using 4D‐Var over a 6 or 12—hour window compared to the static 3D‐Var scheme. It also exhibits some problems with the forecasts started using 4D‐Var over a 24‐hour window. The poorer performance of 4D‐Var over a relatively long assimilation window can be partly explained by the fact that, in these experiments, the tangent‐linear and adjoint models used in the minimization are only approximations of the assimilating model (having lower resolution and crude physics). The error these approximations introduce in the time evolution of a perturbation affects the convergence of the incremental 4D‐Var, with larger discontinuities in the values of the cost‐function when going from low to high resolution for longer assimilation windows. Additional experiments are performed comparing 4D‐Var using a 6‐hour window with the 3D‐Var system. Two additional 2‐week periods show a consistent improvement in extratropical forecast scores with the 4D‐Var system. The main 4D‐Var improvements occur in areas where the 3D‐Var errors were the largest. Local improvement can be as large as 35% for the root‐mean‐square of the 5‐day‐forecast error, averaged over a two‐week period. A comparison of key analysis errors shows that, indeed, 4D‐Var using a 6‐hour window is able to reduce substantially the amplitude of its fast‐growing error components. The overall fit to observations of analyses and short‐range forecasts from 3D‐Var and 4D‐Var is comparable. In active baroclinic areas, the fit of the background to the data is considerably better for the 4D‐Var system, resulting in smaller increments. It appears that in these areas (and in particular over the west Atlantic), 4D‐Var is able to better use the information contained in the observations. The ability of 4D‐Var to extrapolate some aircraft data in the vertical with a baroclinic tilt is illustrated. Problems exist in the tropics and mountainous areas due partly to a lack of physics in the tangent‐linear model. Possible improvements to the system (the introduction of more physics; better behaviour of the incremental approach owing to a line search at high resolution) are also discussed.
Permeability of the stratospheric vortex edge: On the mean mass flux due to thermally dissipating, steady, non‐breaking Rossby waves
Authors: Mo Ruping , Bühler Oliver , McIntyre Michael E. 
Published: February 2007

AbstractAs part of an assessment of the flowing‐processor hypothesis of Tuck et al. (1993) and references–see also Rosenlof et al. (1997)–this paper estimates possible contributions to flow through the edge of the stratospheric polar vortex due solely to distortion of the vortex by thermally dissipating Rossby waves forced from below. To isolate such contributions in a clear‐cut way, and to eliminate questions about numerical dissipation and truncation error, an idealized model is studied analytically. It assumes steady conditions and non‐breaking waves, the waves being stationary in some rotating frame such as that of the earth. The model is studied using two approaches: first via the generalized Lagrangian‐mean formalism of Andrews and McIntyre (1978), simplified by assuming small wave amplitude a; and second via a direct consideration of the three‐dimensional, finite‐amplitude undulations of the vortex edge, as defined by isentropic contours of potential vorticity, avoiding the use of any mean‐and‐deviation formalism. It is shown, in particular, that under quasi‐geostrophic scaling the Lagrangian‐mean meridional velocity V−L is given correct to O(a2) by
equation image
, where θB is the basic‐state potential temperature, z the altitude, η′ the meridional particle displacement and 𝒳 the wave‐induced fluctuation in the diabatic rate of change of potential temperature θ. The formula for V−L is shown to be consistent with the independently derived finite‐amplitude result; and the implication of both results is that, for disturbances dissipated by infrared radiative relaxation in the wintertime lower stratosphere, V−L may well be directed into rather than out of the vortex, though weak outward flow is possible in some cases. There is, in addition, a vertical mean flow w−L controlled by eddy dynamics above the altitude under consideration. This is usually directed downward V−L< 0), and can therefore push mass out of the vortex if the vortex edge has its usual upward equatorward slope. However, under typical parameter conditions for the winter stratosphere, the magnitudes are nowhere near large enough to be consistent, by themselves, with Tuck et al.'s statement that the vortex is ‘flushed several times’ during a single winter.
Internal interannual variability of the extratropical stratospheric circulation: The low‐latitude flywheel
Authors: Scott R. K. , Haynes P. H. 
Published: February 2007

AbstractA new mechanism for internal interannual variability of the extratropical stratospheric circulation is described. The variability is internal in the sense that it arises in a stratosphere‐only model without any interannual variability being imposed externally. In particular the wave forcing in the model, representing the effect of the tropospheric circulation, is kept constant from year‐to‐year and there is no imposed quasi‐biennial variability in the equatorial winds. It is argued that the internal variability arises because of the longer ‘memory’ of the stratospheric flow at low latitudes. The smaller Coriolis parameter at low latitudes means that a given wind signal has associated with it a smaller temperature signal and is therefore less affected by radiative damping than a corresponding signal in the extratropics. The circulation at low latitudes can therefore act as a large flywheel, retaining memory of zonal mean quantities on an annual time‐scale or longer.The mechanism for the variability is studied using a ‘mechanistic’ primitive‐equation model of the stratosphere. Waves are forced by a constant wavenumber one perturbation to the geopotential height field at the lower boundary and a seasonal cycle is prescribed through Newtonian cooling towards a time‐dependent temperature field. Interannual variability is found within a certain range of forcing amplitude. The variability typically takes the form of a biennial oscillation, but more complex behaviour is also found under some circumstances. Diagnostics suggest that zonal flow anomalies in the subtropics persisting from the end of one winter to the beginning of the next are responsible for the interannual variability. Further experiments, in which the zonal flow at low latitudes is constrained in a particular configuration, provide further evidence that the role of the subtropical flow is crucial since constraining the subtropical flow reduces or eliminates the forcing range for which interannual variability is obtained.The importance of the contrast between high latitudes and low latitudes for such internal modes of variability is consistent with their absence in stratospheric models such as the Holton‐Mass model with a single degree of freedom for representing latitudinal structure. To illustrate the mechanism for the interannual variability further, an extended version of the Holton‐Mass model is formulated that differentiates between high‐ and low‐latitude regions. As in the case with full latitudinal resolution, interannual variability is obtained when the amplitude of the wave forcing lies within a certain range.
Dynamics of fine‐scale variables versus averaged observables in a T21L3 quasi‐geostrophic model
Authors: Vannitsem S. , Nicolis C. 
Published: February 2007

AbstractThe dynamics of space‐ and time‐averaged observables of a 3‐level quasi‐geostrophic model is investigated and compared with that of the fine‐scale variables, with emphasis on the role of the initial error field on predictability. The invariance of the largest Lyapunov exponent against averaging of up to 10 days is established. Some improvements of predictability of the averaged variables turn out to be possible, at least for certain classes of initial perturbations. Finally, a map of error distribution in space and a classification of weather regimes are derived on the basis of the average properties of the Lyapunov vectors.
Baroclinic instability of semi‐geostrophic fronts with uniform potential vorticity. II: Comparison of analytic and numerical solutions
Authors: Juckes M. N. 
Published: February 2007

AbstractThe structure of an asymptotic analytic solution for linear baroclinic waves on a frontal basic state is analysed and compared with finite‐difference solutions. The accuracy of the geostrophic fields is good for fronts with widths less than about 400 km. There is direct cancellation between along‐front advection of the disturbance by the strong frontal jet, and disturbance cross‐front advection of the sharp basic‐state gradients. This cancellation of small‐scale effects leaves the larger scales to control the evolution of the disturbance. It follows that the structure is very similar to that of the classic, meridionally uniform, Eady model. Secondary features, such as the ageostrophic circulation, are more sensitive to the frontal width and hence less accurately reproduced in the asymptotic solution. The analysis illustrates significant differences between the ageostrophic circulation associated with deformation frontogenesis and that which occurs in the linear stage of the instability, forced by along‐front curvature. In the latter case, the horizontal ageostrophic wind associated with the rising motion is characterized by along‐front convergence and cross‐front divergence (i.e. ageostrophic frontolysis).
The semi‐annual oscillation in upper stratospheric and mesospheric water vapour as observed by HALOE
Authors: Jackson D. R. , Burrage M. D. , Harries J. E. , Gray L. J. , Russell J. M. 
Published: February 2007

AbstractIn this paper observations made by the Halogen Occultation Experiment (HALOE) instrument are used to investigate the semi‐annual oscillation (SAO) in water vapour mixing ratio in the upper stratosphere. We also present first observations of an SAO in water vapour in the mid to upper equatorial mesosphere. The HALOE data set, which spans a period of greater than four years, also facilitates the examination of interannual variability, and in particular the effects of quasi‐biennial oscillation (QBO)/SAO interaction on the water vapour distribution. The data show an SAO in equatorial water vapour near 1 mb, with troughs in February and August and peaks in May and November. This SAO travels downwards in time to around the 5 mb level, approximately in phase with contemporaneous zonal winds calculated from United Kingdom Meteorological Office (UKMO) analyses. In addition, the UKMO winds show a seasonal variation in the strength of the SAO, related to which is an annual cycle in HALOE water vapour near 1 mb. There is also an SAO in the upper mesosphere, which has a maximum amplitude approximately 2 1/2 times larger than that in the upper stratosphere. This SAO propagates downwards in time, approximately in phase with the zonal winds observed by the High‐Resolution Doppler Imager instrument. However, the pattern of the water vapour maxima between 0.04 and 0.1 mb does not appear to be consistent with the vertical transport that is induced by the SAO in zonal wind. There is also an annual cycle in mesospheric water vapour, which results from the modulation of the winds in the mesosphere by the seasonal cycle in the upper stratospheric SAO in zonal wind. Latitude/pressure sections show that the amplitude of both the upper stratospheric and mesospheric SAO in water vapour decreases away from the equator. However, between 30 and 35° S the SAO in the mesosphere is again large, possibly due to upward transport in winter associated with breaking gravity waves. There are also considerable bieninal cycles in upper stratospheric and mesospheric water vapour, which appear to result chiefly from the phase of the QBO modulating the SAO in zonal wind.
Decrease of total ozone at low latitudes in the southern hemisphere by a combination of linear and nonlinear processes
Authors: Teitelbaum H. , Moustaoui M. , Van Velthoven P. F. J. , Kelder H. 
Published: February 2007

AbstractDetailed analysis of the behaviour of the edge of the Antarctic polar vortex shows that it has to be considered as a region of finite latitudinal extent rather than as an infinitely narrow boundary. A new definition of the external boundary of the vortex edge allows the study of events where the edge becomes strongly dilated towards lower, even subtropical, latitudes. Such events have been observed by Argentinian subtropical ultra‐violet measurement stations and can also be found in the Total Ozone Mapping Spectrometer total‐ozone observations. The dilation of the vortex edge is due to a linear deformation of isentropic surfaces, but eventually leads to irreversible nonlinear mixing at low latitudes in the surf zone, where the horizontal gradients in Ertel potential vorticity are small.The dilation of the vortex edge in the studied events is demonstrated to be due to the action of planetary waves. Planetary waves not only affect the low‐latitude ozone by causing vortex‐edge dilation events, but may also by themselves lower the ozone column locally, as they adiabatically decrease the isentropic density in the ozone layer in some regions (and increase it in other regions).