bluuebunny/crossref_metadata_2025 huggingface dataset:
The singular vectors of a coupled ocean‐atmosphere model of Enso. II: Sensitivity studies and dynamical interpretation
Authors: Moore A. M. , Kleeman R.
Published: February 2007

We have studied the singular vectors of an intermediate coupled ocean‐atmosphere model of El Niño Southern Oscillation in an attempt to understand the factors which influence the growth of perturbations in the coupled system in the tropics. the singular vectors are important because they are the fastest growing perturbations that can exist in the system before nonlinearity becomes important. In this study, we have examined how the model singular‐vectors are affected by variations in certain model parameters, some of which mathematically define the singular vectors. the important findings of this study can be summarized as follows:

the behaviour of the model singular‐vectors can be understood in terms of a superposition of the linear modes of the coupled system which are non‐orthogonal. In some cases, the gravest modes are oscillatory in time and this leads to resonances in the singular‐vector growth‐factors.

the singular‐vector spectrum is generally dominated by its fastest‐growing member whose wind structure is relatively insensitive to the parameters that define the singular vectors mathematically. This result is in accord with the findings of Part I, and suggests that, in the tropics, the coupled system has a preferred response to perturbations, and that this response will readily manifest itself if conditions are favourable.

Favourably configured perturbations can undergo rapid transient growth over the entire regime in which the coupled model is asymptotically stable (i.e. when there are no unstable modes in the system). In this regime, rapid transient growth can occur over a wide range of timescales from 1 week to 24 months.

The results and ideas presented here have important ramifications for the way in which the real coupled oceanatmosphere system will respond in the tropics to perturbations arising from stochastic forcing inherent in the system. the dynamical operators that govern the early stages of linear perturbation‐development are non‐normal which means that the ubiquitous stochastic noise present in the tropics can be amplified. the dynamical mechanisms by which this can occur are the same as those responsible for the growth of the system singular‐vectors. the dominant singular‐vectors of the coupled model bear a remarkable resemblance to the tropical‐cyclone pairs that are often observed spanning the equator in the western Pacific Ocean accompanied by strong westerly wind bursts, and thought to be precursors of El Niño events. We propose that both westerly and easterly wind bursts occuring in nature are a preferred response of the coupled system in the tropics to its inherent stochastic noise component.
Parametrization of momentum transport by convection. II: Tests in single‐column and general circulation models
Authors: Gregory D. , Kershaw R. , Inness P. M.
Published: February 2007

Diagnostics derived from cloud‐resolving‐model simulations in part I of this study, relating to the vertical transport of horizontal momentum by convection, are used to develop a parametrization of convective momentum‐transports for deep convection based upon the mass‐flux convection‐scheme discussed by Gregory and Rowntree. the importance of cloud pressure‐gradients to the flow within the cloud is emphasised, and a simple method of representing their effect is suggested. the scheme is able to reproduce the fluxes derived from the cloud‐resolving model studies where cloud organization by the flow is unimportant. Inclusion of the parametrization in a version of the Meteorological Office Unified Model demonstrates that convective momentum‐transports play a large role in the momentum balance of the atmosphere. Generally, simulation of the mean atmospheric circulation by the Unified Model is improved by the inclusion of such transports.
A comparison of two bulk microphysical schemes and their effects on radiative transfer using a single‐column model
Authors: Petch J. C. , Craig G. C. , Shine K. P.
Published: February 2007

Increasingly, numerical models in climate studies are using prognostic bulk microphysical schemes to predict grid‐scale cloud cover and properties. These schemes provide information which can lead to improved calculations of radiative transfer, and a better understanding of the interaction of radiation with cloud microphysics on the large scale. In this study a one‐dimensional, hydrostatic column model with fixed vertical velocities includes two different bulk microphysical schemes to investigate how the type of scheme influences the hydrometeor content in a cloud, and the effect of this on the radiative heating rates through the cloud.

Two test cases are performed, one representing the stratiform region of a tropical cloud cluster, the other a dissipating tropical cirrus cloud. Each test is first performed using a microphysical scheme that carries only one variable for solid water (MSI), and then using a scheme that separates solid water into ice crystals, snow, and graupel (MS2). Further sensitivity tests are made using MS2 to examine the effects of excluding graupel, and of allowing the ice crystals to fall. The influence of these modifications on the hydrometeor contents of the clouds and the corresponding radiative heating rates is considered.

In a simulation of the stratiform region of a tropical cloud cluster, MS1 is shown to produce significantly larger hydrometeor contents than MS2. However, in the simulation of a dissipating cirrus cloud, the ice content predicted by MS2 remains much larger than the ice content predicted by MS1, throughout a 24‐hour integration. This is because there is a non‐precipitating ice category in MS2 which is very slow to convert to snow at low ice‐water contents. The use of a non‐precipitating ice variable is shown to have a major impact on both the solar and the infrared radiative heating rates at the cloud top, and in some cases to give unrealistic predictions of cloud ice contents.
The role of the environmental flow in the development of secondary frontal cyclones
Authors: Renfrew Ian A. , Thorpe Alan J. , Bishop Craig H.
Published: February 2007

The impact of the environmental flow on the development of secondary frontal cyclones is investigated. Several case‐studies are examined as examples of secondary frontal‐cyclone events observed in the North Atlantic‐western Europe sector. A simple measure of growth is defined to chart their development. The vorticity attribution technique of Bishop is utilized to calculate the action of the large‐scale (environmental) flow on the fronts. In particular the environmental along‐front stretching—shown to be important in theoretical models of frontal instabilities—is calculated. The role of the environmental deformation appears to be crucial: as part of a baroclinic life cycle, stretching deformation acts to build up a front but suppress along‐front waves; if the stretching rate diminishes, barotropic instabilities may then break out. Diagnostics are examined to try to ascertain the growth mechanisms at work in each frontal‐cyclone case. A range of values for the commonly prescribed deformation‐frontogenesis and shearing‐frontogenesis parameters are calculated.
The tropical stratopause in the UKMO stratospheric analyses: Evidence for a 2‐day wave and inertial circulations
Authors: Crsolini Yvan J. , Limpasuvan Varavut , Leovy Conway B.
Published: February 2007

Large‐scale atmospheric motions near the low‐latitude stratopause in the UK Meteorological Office stratospheric analyses during the northern hemisphere winter are investigated, focusing in particular on the 2‐day wave and inertial eddies.

The 2‐day wave is a westward‐propagating planetary wave which appears recurrently in the summer subtropical upper stratosphere and mesosphere. Space‐time spectral analysis of the assimilated fields of temperature and winds, and high‐resolution transport of water vapour, reveal large‐amplitude breaking planetary waves in the summer subtropics, principally a 2‐day wave 3, and a wave 1 of a period near 7 days. A region of anomalous zonal‐mean meridional gradient of potential vorticity also exists, pointing to a barotropic instability mechanism for the origin of the 2‐day wave.

By contrast, the winter subtropics are characterized by mixing on less‐than‐planetary scales during the period considered, occurring during the easterly phase of the stratospheric semi‐annual oscillation. Water vapour is mixed turbulently into gyres, and drawn in narrow filaments out of the southern hemisphere. The potential‐vorticity field displays vertically layered, zonally‐asymmetric variations.

The behaviour of the model assimilated fields is supported by along‐track water vapour distribution observed by the Microwave Limb Sounder aboard the Upper Atmosphere Research Satellite which shows evidences for intermittent filamentary mixing in the northern subtropics, and a 2‐day wave signature in the summer subtropics.

Based on these results, a possible mechanism connecting the initiation of the austral 2‐day wave events to strong planetary‐wave activity in the winter is proposed. The winter planetary waves provide the background conditions for development of inertial instabilities at low latitudes of the winter hemisphere, which in turn create conditions favourable for barotropic instability of the summer subtropical easterly jet.
Modelling fluxes of momentum, sensible heat and latent heat over heterogeneous snow cover
Authors: Essery Richard
Published: February 2007

A boundary‐layer model is used to simulate turbulent fluxes of momentum, sensible heat and latent heat over heterogeneous surfaces. Results for heterogeneous snow cover are compared with results for heterogeneous snowfree vegetation, and effective parameters characterizing the surfaces are discussed. A tile model, which calculates area‐average fluxes from given area‐average windspeeds, temperatures and humidities at a reference height in the atmosphere, is assessed in comparison with the boundary‐layer model. the performance of the tile model is found to be degraded for surfaces with regions of both upward and downward sensible‐heat flux, as can occur over heterogeneous snow cover. Better results can be obtained by using different reference heights for heat flux and momentum flux calculations.
Atmospheric ammonia at a moorland site. I: The meteorological control of ambient ammonia concentrations and the influence of local sources
Authors: Flechard C. R. , Fowler D.
Published: February 2007

Ammonia (NH3) concentrations were continuously monitored at a moorland site in southern Scotland for 13 months from February 1995 to February 1996, providing about 13 500 half‐hourly average measurements (71% of the time). Concentrations were found to be log‐normally distributed (with a geometric mean of 0.43 μg m−3 and a geometric standard devation of 3.35), and were strongly dependent on wind direction because of local agricultural point soources within 3 km of the site. These sources contributed approximately 60% of the NH3 advected over the measurement site, while more distant sources accounted for the remaining 40%. Meteorological conditions also influenced the concentration of NH3 because of their effects on emissions from vegetation, on dispersion by the atmosphere, and on rats of removal from the atmosphere. The largest concentrations were measured in warm (>15 °C), dry, summer conditions and in cold (<°C) weather, typically 1–5 μg m−3. Smaller concentratins, typically in the range 0.1–1 μg m−3 were found in intermediate (1–15 °C), humid and wind conditions. Ammonia concentrations generally decreased with increasing wind speed and were very variable at windspeeds <3 m s−1. Concentrations also decreased with increasing relative humidity, reflecting the partitioning of NHx toward the liquid phase at large (>90%) relative humidities, while evaporation from aerosols seemed more likely at lower (40–90%) relative humidities. Seasonal variations in meteorological patterns resulted in marked seasonal variations in concentrations, with the largest mean‐values observed during the warmer and drier summer months, and the smallest values measured in humid and windy, atumn and winter conditions. Measurements indicated a short atmospheric residence time for NH3, of the order of 1–2 hours. The atmospheric residenc time for dry deposition of gaseous NH3 was estimated from these data and ranges from 2 to 20 hours depending on atmospheric and surface conditions, with a mean value of 3.5 hours for the 13 months of monitoring.
Atmospheric ammonia at a moorland site. II: Long‐term surface‐atmosphere micrometeorological flux measurements
Authors: Flechard C. R. , Fowler D.
Published: February 2007

Long term micrometeorological measurements of the net vertical exchange of atmospheric ammonia (NH3) over moorland vegetation at a site in southern Scotland from February 1995 through to February 1996 are reported. The measurments, made using a combination of eddy covariance and aerodynamic flux‐gradient methods, provided satisfactory 30‐minute average fluxes approximately 50% of the time using continuous annular wet denuders for NH3 sampling at three heights between 0.4 and 3.4 m above the ground.

The moorland vegetation was a net sink for atmospheric NH3 as 93.4% of all half‐hourly measured fluxes indicated dry deposition to the surface with mean flux of −5.2 ng m−2 S−1 and a mean deposition velocity Vd of 11.7 mm S−1 at (z ‐ d) = m. Despite the dominance of deposition to the moorland, emission fluxes were observed approximately 6.6% of the time with a median of +1.1 ng m−2 S−2, and most commonly resulting from drying water films on foliar surfaces.

The mean aerodynamic resistance at a reference height of (z ‐ d) = 1 m and the mean viscous sub‐layer resistance were 36.6 s m−1 and 11.6 s m−1 respectively, and of a similar magnitude to the mean canopy or surface resistance Rc of 37 s m−1. When the moorland surfaces were wettend by rain or dew, canopy resistances to NH3 deposition were generally smaller (averaging 23 s m−1) than in dry conditions (when Rc was 61 s m−1) or when surfaces were covered in snow (Rc = 56 s m−1. Therefore, moorland surfaces cannot be treated as perfect sinks for NH3 deposition.

Diurnal and seasonal patterns in rates of deposition and net fluxes to the moorland surface area were determined mainly by a combination of wind speed, surface wetness and ambient NH3 concentration. Thus, during winter, depositions rates expressed as a flux or deposition velocity were generally much larger than in summer by factors of 2 or 3 respectively, as a result of stronger winds and the presence of surface water for extensive periods which kept canopy resistances small except in frozen conditions. At temperatures below 0°C, the surface became progressively dry and canopy resistances were large (50–100 s m−1).

The measurements provided direct estimates of the net annual gaseous NH3 input to the moorland, which was 2.5 kg NH3‐N ha−1 a−1 and was almost identical to the measured wet deposition input at the site of 2.4 kg NH4+‐N ha−1 a−1.
Zonal extent of oceans, high‐latitude fresh water supplies and the thermohaline circulation
Authors: Cai Wenju
Published: February 2007

One of the most distinctive features of the present‐day global thermohaline circulation is the absence in the North Pacific Ocean, and the presence in the North Atlantic Ocean, of deep‐water formation. In this paper, the different zonal extents and the associated high‐latitude fresh water supplies of the Pacific and the Atlantic when under a zonally uniform fresh water flux forcing are examined, as a possible cause for the asymmetry. A series of numerical experiments using the Bryan–Cox–Semtner ocean general circulation model is performed. In order to achieve a true independence, the model configuration and the model forcing are so constructed that zonal extent is the only built‐in difference between the two oceans. Three configurations are used. They are (a) an idealized Atlantic Ocean (narrow zonal extent) alone, (b) an idealized Pacific Ocean (wider zonal extent) alone, and (c) a connected Pacific–Atlantic system.

Upon spin‐up, the model ocean is forced by a zonally uniform Schopf boundary‐condition for temperature and a zonally uniform fresh water flux for salinity. Both forcings are diagnosed from a spin‐up. All diagnosed fresh water fluxes have net precipitation in polar regions and net evaporation in low‐latitude regions as observed. This feature means that the high‐latitude model Pacific is subject to a fresh water flux twice that in the high‐latitude model Atlantic. This causes the northern sinking cell in the model Pacific of the spin‐up state to collapse; a southern sinking cell settles in. The process is similar to that in many model studies examining the response of the thermohaline circulation to a high‐latitude freshening. By contrast, the northern sinking cell in the North Atlantic of the spin‐up state persists. In the connected Pacific–Atlantic system, these produce the ‘conveyor belt’ type circulation, and the asymmetry between the North Atlantic and the North Pacific. It is found that the high‐latitude fresh water supply is the crucial parameter, and that the difference in basin width is not directly important. It is also found that the connection by the Antarctic Circumpolar Current is not important.

A large number of experiments is performed in the model Pacific–Atlantic system to test the sensitivity to initial conditions. The system shows a clear tendency to favour the conveyor‐belt circulation, although the strength of the North Atlantic northern sinking cell shows considerable variations. Thus, although in nature there are many factors that can contribute to the asymmetry, the difference in total available fresh water at the northern high latitudes between the two oceans as a result of the width difference may have played a significant role.
Storm‐track organization and variability in a simplified atmospheric global circulation model
Authors: Frisius Thomas , Lunkeit Frank , Fraedrich Klaus , James Ian N.
Published: February 2007

A storm track is simulated in a coarse‐resolution multilevel primitive‐equation model with linear surface friction and heating terms. A restoration temperature distribution consisting of a dipole embedded in a zonally symmetric profile forces the model to simulate the surface heating. Three simulations, each with a different dipole orientation, are performed to test the sensitivity of storm‐track organization to the external forcing field. The climatological time‐mean circulation and the transient disturbances of the reference simulation agree well with observations of northern hemisphere wintertime storm tracks. Local energetics show that baroclinic instability is responsible for the enhanced eddy kinetic energy downstream of the jet, and the downstream end of the storm track results from the barotropic conversions of eddy kinetic energy to the time‐mean flow.

Low‐frequency fluctuations with a period of about 50 days associated with a retrograding large‐scale wave pattern are identified by a complex empirical‐orthogonal‐function analysis of the vertically averaged stream function. A composite life cycle of the low‐frequency variability reveals the growth and decay of a blocking anticyclone downstream of the storm track. A cyclogenesis initiated by an eastward‐propagating wave train is observed a few‐days before the amplitude of the low‐frequency anomaly attains its peak. The net forcing of the high‐frequency eddies contributes to the growth and decay of the blocking anticyclone. The results suggest that the occurrence of the blocking‐like event is part of the storm‐track dynamics.
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

When 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

In 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

Dynamical 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

This 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

Recent 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

Results 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

As 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

A 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

The 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

The 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

In 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

Detailed 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).
Climate modelling: Achievements and prospects
Authors: Carson D. J.
Published: February 2007

The significant progress made with coupled (atmosphere‐ocean) general‐circulation models (CGCMs) over the past decade or so is discussed in the context of the reports of the Intergovernmental Panel on Climate Change (IPCC). In their first report in 1990, most climate‐change GCMs were atmospheric GCMs coupled to ‘slab’ ocean models and being used only to determine the ‘equilibrium’ climate response to specified increases in carbon dioxide. the wide range of estimates of their ‘climate sensitivity’ is discussed, especially in the context of the uncertainty introduced by model sensitivity to representations of cloud. the modelling focus then moved to transient‐response, climate‐change experiments. These and the related modelling problems are noted; as are the simultaneous growing awareness and improved quantification of a range of natural and human‐induced radiative forcings. the consequent inclusion of this information in CGCMs to study the detection and attribution of climate change, leading to the IPCC statement in their 1995 report that ‘the balance of evidence suggests that there is a discernible human influence on global climate’, is also discussed.

A significant modelling breakthrough since the 1995 report is the capability of a few CGCMs to produce stable and realistic climate (control) simulations without using artificial ‘flux adjustments’. This is illustrated with results from such a model, the Hadley Centre HadCM3. In conclusion, a brief perspective is given of high‐priority modelling developments and applications to be addressed over the next decade.
An examination of the accuracy of the linearization of a mesoscale model with moist physics
Authors: Errico Ronald M. , Raeder Kevin D.
Published: February 2007

The accuracy of tangent linear and adjoint versions of a primitive‐equation model with moist physics is examined with respect to growing perturbations having significant initial magnitudes. the Jacobians for the convective parametrizations are approximated using a perturbation method. These Jacobians are then quality controlled to ensure that the approximations are suitable. Results show that: (1) linearization of the diabatic moist physics can have a significant impact; (2) even where such impacts are large, the linearized versions of the model can yield good approximations to the nonlinear behaviour for significant perturbations, especially if there is sufficient dynamical influence; (3) poor approximations can be obtained when convection dominates the results; and (4) a straightforward linearization of some parametrization schemes may be inadequate. the results are encouraging for quantitative applications of some moist adjoint models to extratropical cyclones in the winter, but suggest some tangent linear approximations may be unsuitable in the tropics or over continents in the summer, except if qualitative agreements with nonlinear results are suficient. Detailed comparisons of linear and nonlinear results should be made, particularly using optimal perturbations, prior to any applications of tangent linear or adjoint models.
An intercomparison of radiatively driven entrainment and turbulence in a smoke cloud, as simulated by different numerical models
Authors: Bretherton C. S. , Macvean M. K. , Bechtold P. , Chlond A. , Cotton W. R. , Cuxart J. , Cuijpers H. , Mhairoutdinov M. , Kosovic B. , Lewellen D. , Moeng C.‐H. , Siebesma P. , Stevens B. , Stevens D. E. , Sykes I. , Wyant M. C.
Published: February 2007

As part of a programme of intercomparison of eddy‐resolving and one‐dimensional (1‐D) boundary‐layer models, a convective boundary‐layer filled with radiatively active ‘smoke’ was simulated. the programme is sponsored by the Global Energy and Water Experiment Cloud Systems Study. Cloud‐top‐cooling rates weire chosen to be comparable with those observed in marine stratocumulus, while avoiding evaporative feedbacks on entrainment and turbulence that are also important in liquid‐water clouds. the radiative‐cooling rate hacl a specified dependence on the smoke profile, so that differences between simulations could only be a result of different numerical representations of fluid motion and subgrid‐scale turbulence. At a workshop in De Bilt, the Netherlands in August 1995, results from numerous groups around the world were compared with each other and with a previously investigated laboratory analogue to the smoke cloud.

The intercomparison results show that models must be run with higher vertical resolution in the inversion than is customary at present, in order to accurately simulate the entrainment rate into cloud‐topped bounday‐layers under strong inversions. In three‐dimensional (3‐D) models using a vertical grid spacing of 5‐12.5 m, sufficient to resolve the horizontal variability of inversion height, entrainment rates were 10‐50% larger than: he range consistent with the laboratory experiments. With a larger vertical grid spacing of 25 m, 1‐D, 2‐D and 3‐D models all overestimated the entrainment rate by more than 50%. 3‐D models with monotone advection‐schemes overestimated entrainment slightly more than those with non‐monotone schemes, at least when 25 m vertical grid‐spacing was used. However, results from non‐monotone schemes had several undesirable features associated hith the generation of undershoots and overshoots, most notably spurious turbulent mixing above the smoke layer. the 1‐D models tended to underestimate turbulent kinetic energy (TKE) but performed reasonably well given tkeir simplicity. 2‐D models produced too much entrainment and considerably overestimated TKE, compared with 3‐D models with the same numerical formulation.

Based on a simple scaling‐argument, we propose that the minimum vertical grid‐spacing required to obtain an accurate entrainment‐rate is of the order of the horizontal fluctuations in inversion height, which is proportional to the layer‐averaged TKE and inversely proportional to the inversion strength.