bluuebunny/crossref_metadata_2025 huggingface dataset:
Localization of optimal perturbations using a projection operator
Authors: Buizza Roberto
Published: February 2007

Ensemble prediction is an attempt to estimate the probability distribution of forecast states through a finite sample of nonlinear deterministic integrations of a numerical weather‐prediction model. At the European Centre for Medium‐range Weather Forecasts (ECMWF) it is based on 32+1 (control) model integrations at horizontal spectral triangular truncation T63, with 19 vertical levels. The initial conditions of the perturbed forecasts are generated from optimal perturbations, which identify the directions in the phase space of the system that guarantee the maximum growth of the total energy of the perturbation over a fixed time interval. Since the ECMWF is mainly interested in predicting the atmospheric flow over the northern hemisphere, in particular over the European region, optimal perturbations are chosen to give different forecasts in this region.

One of the problems faced during the first months of ensemble prediction was that, on some occasions, the spread between the perturbed and the control forecasts appeared to be small. One case, for 14 February 1993, which represents an extreme among these cases, is analysed in detail. For that period, for three consecutive days, the two unperturbed forecasts (the control T63L19 and the high‐resolution operational forecast) and all the perturbed forecasts of the ensemble system were very similar over the European region.

A second problem, closely related to the first, that had to be prevented, was the inability of the system to identify optimal perturbations which amplify over the northern hemisphere during the warm seasons, when the relative instability of the northern hemisphere is smaller than the instability of the southern hemisphere.

It is shown how the introduction of a local projection operator, which confines the region over which the optimal perturbation growth is maximized, can improve the spread among the ensemble members. Moreover, it is proven that its application avoids the occurrence of the second problem.
Solar heating rates after a volcanic eruption: The importance of SO2 absorption
Authors: Lary D. J. , Balluch M. , Bekki S.
Published: February 2007

Studies of the impact of volcanic eruptions on the climate have mainly focused on the radiative effects of enhanced aerosol loadings. However, initially, volcanoes release not aerosols, but large amounts of sulphur dioxide (SO2), which subsequently leads to the production of sulphate aerosols. SO2 absorbs strongly in the ultraviolet and the visible region of the spectrum. This paper shows that accounting for this absorption is important if an accurate determination of change in heating rates is to be made in the period shortly after a volcanic eruption.
Diabatic heating in an axisymmetric cut‐off cyclone and related stratosphere‐troposphere exchange
Authors: Wirth Volkmar
Published: February 2007

A mechanistic axisymmetric Eliassen balanced vortex model is presented for the investigation of the role of diabatic heating in the dynamic evolution of a cut‐off cyclone and the related stratosphere‐troposphere exchange. As an initial state, a balanced upper‐level vortex is chosen representing an idealized cut‐off cyclone. After specifying a diabatic heating field, which is supposed to simulate latent heat release in deep convection, one can diagnose the induced cross‐vortex circulation and calculate the ensuing vortex evolution. Tropospheric heating with a maximum of some 10 K d−1 underneath the lowered tropopause leads to the decay of the upper‐level vortex within a few days. During the decay there is a sizeable, diabatically induced mass flux across the PV‐defined tropopause: air in the vortex centre which initially was part of the lower stratosphere gradually turns into tropospheric air. This stratosphere‐troposphere exchange depends sensitively on the heating profile around the tropopause level and, hence, on the maximum height reached by the convection. The inclusion of diabatic cooling to account for the effect of radiation at cloud tops leaves the vortex evolution almost unchanged but increases the rate of stratosphere‐troposphere exchange dramatically. Cross‐isentropic mixing alone may result in strengthening rather than in weakening the vortex thus re‐emphasizing the role of diabatic heating during decay.
Entrainment and mixing in buoyancy‐reversing convection with applications to cloud‐top entrainment instability
Authors: Grabowski Wojciech W.
Published: February 2007

A consensus seems to exist throughout the cloud‐physics community that buoyancy reversal associated with evaporative cooling affects not only the global (cloud‐scale) dynamics of a convective cloud, but also the rate of mixing between the cloud and its environment. The latter effect is associated with the concept of the so‐called ‘cloud‐top entrainment instability’ (CTEI), which assumes a positive feedback between buoyancy reversal and the rate of entrainment. In this paper, effects of buoyancy reversal on cloud dynamics are discussed in the context of an unstratified anelastic two‐fluid system. Convection in this system mimics some essential features of cumulus convection. Two‐dimensional numerical experiments, with and without the effects of buoyancy reversal, have been performed. It was found that buoyancy reversal has a dramatic impact on the overall flow evolution, but that its effect on the rate of mixing between the two fluids is small, i.e. flows which differed dramatically depending on whether there was buoyancy reversal or not still resulted in similar amounts of mass being mixed. This result casts doubt on the concept of CTEI in cumulus dynamics. The distinction between strong global effects of buoyancy reversal and minor effects on the rate of mixing in buoyancy‐reversing convection is supported by laboratory experiments with classical and buoyancy‐reversing thermals.
Unsymmetrical symmetric instability
Authors: Sun Wen‐Yih
Published: February 2007

A northward component of the earth's rotation (b‐effect) is included in a set of linearized, non‐hydrostatic equations to study symmetric instability. The results show that the b‐effect can be important if the perturbations are functions of the latitude. In comparison with the conventional symmetric instability, results also show that, in the northern hemisphere, when the temperature increases with latitude, the growth rate can be enhanced by the b‐effect, and the growth rate decreases when the temperature decreases with latitude. We also find that when the perturbations are independent of latitude, the b‐effect disappears. The physical explanation is provided.
Second‐order spectral local isotropy of the humidity and temperature fields in an urban area
Authors: Kalogiros J. A. , Helmis C. G.
Published: February 2007

A theoretical and experimental analysis of the humidity and temperature spatial differentials (δ/δxj,) in an urban area is presented. The local isotropy form of the spectra of the streamwise and vertical differentials of a scalar field is derived. The experimental results for the humidity and temperature fields in the lowest part of the urban boundary layer are in satisfactory agreement (at least in form under stable conditions) with the local isotropy model indicating that these fields can be locally isotropic. A strong anisotropy was observed at small wavenumbers of the vertical differentials spectra and it is probably connected with surface‐layer plumes. The analysis of the humidity and temperature differentials is extended to their co‐spectra and the spectra of the vertical differentials are compared to the streamwise ones. The comparison of the structure parameters of the temperature or the humidity field in the streamwise and the vertical direction, estimated using a spectral and a variance (structure function) method, showed no systematic departure from the isotropic prediction of equality.
Flow over mountains: Coriolis force, transient troughs and three dimensionality
Authors: Peng Melinda S. , Li Shang‐Wu , Chang Simon W. , Williams R. T.
Published: February 2007

Some issues, concerning uniform, stratified flow over a three‐dimensional mountain, that have not been fully explored are studied using primitive‐equation models. When the Froude number (Fr) is small (e.g. Fr < 0.5), we find that the Coriolis force cannot be neglected for flow over small‐scale mountains (characteristic length L < 50 km) even though the Rossby number (Ro) is large. When the Coriolis force is neglected, a pair of symmetric vortices is induced on the lee side, irrespective of the scale of the mountain, as long as Fr lies roughly between 0.1 and 0.5. The major effect of the Coriolis force is to modify or diminish the lee vortices due to the leftward deflection on the upstream side of the mountain. The Coriolis deflection also reduces the upstream propagation. When Fr is ≥ 1, most of the flow goes over the mountain and the Coriolis effect is small for the same Ro. Therefore, the importance of the Coriolis effect is determined by both Ro and Fr.

As the scale of the mountain increases from small‐scale to mesoscale (100 km < L < 300 km), the upstream flow is deflected more to the left by the Coriolis effect, and a trough develops on the lee side. The length‐scale of the trough depends on the scale of the mountain in the along‐flow direction. For smaller‐scale mountains, small vortices can be generated within the lee‐side trough when Fr is small. When the mountain scale is increased, the lee‐side vortices disappear, and only a lee‐side trough remains. Depending on the mean wind speed and the mountain scale, the lee‐side trough can be advected by the anticyclonic flow over the mountain to the south, and remain attached to the mountain for a long time. A train of troughs is generated on the lee side of an elliptic mountain associated with the lee‐side gravity waves.

Simulations of flows over mountains in two‐dimensional and three‐dimensional models with the same cross‐sectional profile are compared. For small‐scale mountains (L < 100km), the two‐dimensional model overestimates the amount of airflow over the mountain and also the lee‐side downslope wind.
Observations of absorbing layers in the antarctic stratosphere in October 1991
Authors: Harries J. E. , Russell J. M. , Park J. , Tuck A. F. , Drayson S. R.
Published: February 2007

Measurements of the transmission of solar infrared radiation through the earth's atmosphere, by the HALOE experiment on the UARS spacecraft, were made at high southern latitudes during October 1991. These observations are direct measurements of atmospheric transmission, and do not need to be passed through a composition or temperature retrieval process; they are, therefore, amenable to direct interpretation. During October 1991 the profiles of transmittance versus height in the atmosphere show clear evidence for the arrival (from more northerly latitudes) of layers of some material which absorbs infrared radiation, at heights of up to 28 km. The spatial structure of the absorbing material shows considerable variability with longitude at a given latitude. The spectral properties of the detected absorption, measured at the various HALOE wavelengths, are consistent with absorption by sulphate aerosol, and clearly implicate the volcanic eruptions from Mts Pinatubo and Hudson during 1991. These results provide direct and detailed evidence for the arrival of layers of what appears to be sulphate aerosol during the 1991 southern spring, at latitudes as high as 80 degrees south.
Short‐range predictability of the atmosphere: Mechanisms for superexponential error growth
Authors: Nicolis C. , Vannitsem S. , Royer J.‐F.
Published: February 2007

Recently it has been established, using simple mathematical models of chaos, low‐order models, and large numerical models of the atmosphere, that small errors grow in the mean in a superexponential manner. In this paper the mechanisms behind this behaviour are examined with special emphasis on the non‐orthogonality of the eigenvectors of the linearized evolution operator and the variability of the local Lyapunov exponents on the attractor. The study reveals a picture that is far more complex and system‐dependent than what has been advanced so far in the literature. The general ideas are illustrated by Lorenz's low‐order atmospheric model for which a simple phenomenological model of error growth is developed and tested successfully against the simulations.
The structure of stably stratified atmospheric boundary layers: A large‐eddy simulation study
Authors: Andren A.
Published: February 2007

Dry stably‐stratified planetary boundary layers have been studied by large‐eddy simulation. Results from simulations using two different subgrid‐scale models are compared. A recently suggested improved subgrid‐scale model version is shown to give improvement in the near‐surface region comparable to what have previously been obtained with inclusion of random subgrid‐scale stresses.

A comparison of two flows with or without wave motions aloft shows that effects of these waves are negligible within the boundary layer. This is attributed to the small amplitudes of these waves, being forced by the turbulent eddies in the planetary boundary layer. an analysis of second moments and their budgets shows that a quasi‐steady regime has been reached for turbulence statistics, whereas mean fields are, as expected, still evolving. Accordingly profiles of vertical fluxes deviate from the forms assumed within local scaling theories.

A picture of eddy structure in the turbulent layer is obtained through a combination of quadrant analysis of vertical fluxes and distributions of vorticity vector orientations. It is found that bursts dominate the fluxes. A few intense events make up for most of the flux. This effect of intermittency increases towards the top of the boundary layer.

Distributions of vorticity vectors indicate a transition from a typical shear‐flow structure near the surface to what might be expected for an internal‐wave field above the boundary layer.
A comparison of two radiation schemes for calculating ultraviolet radiation
Authors: De Forster Piers M. F. , Shine Keith P.
Published: February 2007

Increases in ultraviolet radiation (UV) caused by ozone depletion in the stratosphere are expected to have physiological effects on plants and animals. Biologists require that the UV data have high wavelength resolution to assess these effects adequately. Numerical simulations of scattering and absorption in the atmosphere provide a useful way of predicting the UV irradiance reaching the ground and the results from such radiative transfer calculations are presented. Two different methods were used to calculate the diffuse irradiances: the Discrete‐Ordinate (D‐O) method and the Delta‐Eddington (D‐E) approximation. Surface UV irradiances for a wide variety of atmospheric conditions, obtained with the D‐O method were compared with those obtained using the D‐E approximation to assess how changes in tropospheric aerosol and cloud affect the accuracy of the D‐E approximation. Stratospheric aerosol has been shown to be capable of increasing the UV irradiances at the ground, the magnitude of these increases depending heavily on the absorption by ozone of multiply‐scattered photons. the prediction of these increases by means of numerical models provides a good test of the accuracy of the radiative transfer approximations used. It is shown that the D‐E approximation appears to be deficient for the prediction of surface irradiance at high spectral resolution, under certain circumstances.
Observations of the turbulent fluxes in fields of cumulus clouds
Authors: Smith Samantha A. , Jonas Peter R.
Published: February 2007

Aircraft observations in fields of small maritime cumulus clouds around the British Isles have been analysed to yield kinetic energy spectra and vertical turbulent fluxes, both within clouds and in the clear regions between clouds, for five cases where a cool airmass flowed over a warmer sea surface. the flights were made at various times of the year. Profiles of the kinetic energy and turbulent fluxes have been obtained from observations made at several levels. It is shown that, while the dominant source of turbulent kinetic energy may be due to evaporative cooling following entrainment at cloud top, in some cloud fields production by latent heating near cloud base dominates. the location of the dominant energy source depends on whether the clouds penetrate the inversion sufficiently for instability due to cloud‐top entrainment to occur. While the resulting flux profiles observed in some of the cases examined here resemble those observed in stratocumulus, profiles of similar shape are not always observed, and there are significant differences between cumulus layers and stratocumulus in the magnitudes of the fluxes.
Meso‐β‐scale circulations in realistic fronts. II: Frontogenetically forced basic states
Authors: Fischer Claude , Lalaurette François
Published: February 2007

This paper deals with the mesoscale circulations that may appear in a frontogenetically forced flow. Two mechanisms are studied: conditional symmetric instability (CSI) and transient adjustment of a finite momentum anomaly. the study is focused on two situations selected from frontal events that were documented during the MFDP/FRONTS 87 field experiment.

In Part I of this paper, a successful numerical simulation of CSI during the second intensive observing period (IOP2) was presented. However, the growth rate of the perturbation was very small (≈ f/4) and 42 hours were needed before vertical velocities reached their maximum value ≈5 cm s−1. Therefore, interactions between CSI and the large‐scale frontogenetic forcing are expected; these are investigated in this Part. CSI is released (without significant increase of the growth rates) for small forcing only since confluence is found to shrink the area fulfilling the pure CSI criterion rapidly. It is thus not likely to occur in active fronts.

The IOP7 is then investigated. This very anisotropic front was embedded in a strong large‐scale confluence. the creation and maintenance of the observed ‘Wide Cold‐Frontal Rainband’ (WCFR) (some 100 km wide) above the sloping frontal surface and behind the surface cold front are investigated. Using a hierarchy of threedimensional (3D) and two‐dimensional (2D) versions of the model, this band is shown to be produced by the inertial, transient adjustment of low pseudo‐angular‐momentum air that was advected into the mid‐troposphere by the frontal convective jet. the 3D, as well as the 2D, forced simulations produce a negative equivalent potential vorticity chimney‐like area which matches well with the observational data. This area evolves on the same time‐and space‐scales as the roll‐like circulation associated with the WCFR. This mechanism bears similarities with mature CSI and their distinction in diagnostic studies may be difficult. However, the presence of a momentum source (the frontal convection) is found here to be the important criterion.
Comparison between the orographic response of the ECMWF model and the pyrex 1990 data
Authors: Lott Francois
Published: February 2007

Recent observations and analyses of the atmospheric flow in the vicinity of the Pyrénées made during PYREX are compared with numerical experiments carried out with the T213L31 European Centre for Medium‐range Weather Forecasts (ECMWF) global model. This study shows that the model simulates a certain proportion of the measured pressure drag, but always underestimates it. In the current model, this deficit is not adequately made up by any of the parametrized subgrid‐scale processes. When the flow is along the range, the pressure‐drag value depends on the synoptic background flow structure. It is close to zero when the flow impinging on the range is baroclinic. It is not zero when the atmosphere is more barotropic (e.g. in the absence of any fronts) and the model misses important lift processes. When the flow is normal to the ridge, the model response to the mountain forcing is essentially ageostrophic. In this case comparison between the vertical profiles of the model stresses and the measured profile suggests that further drag should be applied in the model at low level. This is supported by comparisons of isentropic diagnostics, from the ECMWF model and the French mesoscale model, Peridot. These diagnostics show that the ECMWF model underestimates irreversible low‐level processes, such as the flow separation on the mountain flanks and the associated downstream wake, which can be related to the drag. They also show that low‐level parametrized drags have a realistic impact on the model simulation of the mountain wake. This analysis supports the current developments at the ECMWF of a new subgrid‐scale orographic drag scheme which parametrizes the low‐level impact of mesoscale mountains.
Simulation of climate change over europe using a nested regional‐climate model. I: Assessment of control climate, including sensitivity to location of lateral boundaries
Authors: Jones R. G. , Murphy J. M. , Noguer M.
Published: February 2007

Present‐day climate simulations for Europe are presented, based on a 50 km regional‐climate model (RCM) driven by output from a global general‐circulation model (GCM) using a one‐way nesting approach. Both models are components of the Meteorological Office Unified Forecast/Climate Model and use the same subgrid‐scale physics. the relationship between the RCM circulation and that of the driving GCM was assessed in seasonal RCM integrations using domains of different sizes. In the larger domains, both the mean flow and the day‐to‐day variability in the RCM diverge from that of the GCM on the synoptic scale, rendering the RCM solution physically inconsistent with the GCM solution external to the RCM domain. At the grid‐point scale the RCM freely generates its own features, even in the smaller domains—only at points adjacent to the boundary buffer zone is there evidence of significant distortion by the lateral boundary forcing from the GCM.

Using one of the smaller domains, a 10‐year RCM simulation was carried out, driven by a coupled atmosphere/mixed‐layer‐ocean version of the GCM. Over the region of interest the general circulation and daily synoptic variability is realistically simulated by the GCM and, therefore, also by the RCM (see above). Stronger vertical motions in the RCM lead to a general increase in dynamical precipitation relative to the GCM, and thus a drier and warmer troposphere and reduced convective cloud and precipitation. Layer‐cloud cover is also reduced in the RCM, due to a time‐step dependence in the treatment of the dissipation of ice cloud. Significant changes occur in the surface heat balance. the spatial patterns of surface air temperature and precipitation over Europe are well simulated by both the GCM and the RCM on scales resolved by the former. At finer scales the RCM contains a strong signal which is related to orographic height. Validation against a detailed observed climatology for Great Britain demonstrates that this signal contains considerable skill.
The effect of orography on wet deposition in an industrial area
Authors: Inglis D. W. F. , Choularton T. W. , Wicks A. J.
Published: February 2007

Two field experiments were carried out to observe the variation in wet deposition of ions caused by orography in a polluted region of the UK. the site chosen was Winter Hill which lies on the edge of the industrialised Mersey valley. Rain and cloud samples were collected daily and chemically analysed. Synoptic and meteorological data were used to classify individual rain events as frontal or convective.

For the frontal events the altitude related increase in wet deposition was found to be entirely caused by the seeder‐feeder mechanism (the scavenging of cap cloud by rain from above). the concentration of marine ions in the water scavenged from the cap cloud was found to be between five and six times that in the seeder rain whereas for anthropogenically produced ions it was about twice as great. the efficiency of the seeder‐feeder mechanism is correspondingly affected.

Convective events accounted for less than a quarter of the rainfall volume. However, this contained over half of the deposition of marine ions in the valley. Deposition of anthropogenically produced ions in convective events was small probably due to a lack of convection over land. A small decrease in deposition with altitude was observed for these events although this is expected to be a highly local effect.

It is concluded that reliable predictions of the wet deposition of non‐marine ions can be made on the basis of the seeder‐feeder effect for northern and western regions of the UK. Elsewhere in the UK and for much of continental Europe wet deposition from convective rain should be taken into account.
Error growth and estimates of predictability from the ECMWF forecasting system
Authors: Simmons A. J. , Mureau R. , Petroliagis T.
Published: February 2007

Examination has been made of the skill of ECMWF forecasts of the 500 hPa height field produced daily out to ten days ahead, verifying in the period from 1 December 1980 to 31 May 1994. Over this time accuracy has been improved substantially over the first half of the forecast range. the systematic (seasonal‐mean) component of the error has been greatly reduced at all forecast times, but there has been little reduction in the non‐systematic (transient) component later in the range.

The simple model proposed by Lorenz for the intrinsic growth of forecast error has been applied to the evolution of differences between consecutive forecasts. the implied growth‐rates of small forecast errors have increased significantly since 1981. They do not show much variation with season, and are a little lower in the southern than in the northern hemisphere. the most recent error‐doubling times are around 1.5 days for the northern hemisphere and 1.7 days for the southern hemisphere. Error saturation levels are at present similar to or greater than those of the 1981 version of the model, having been significantly lower in intermediate years. the accuracy of recent short‐ and early medium‐range forecasts and realism of the climatology of the forecast model support the view that estimates of intrinsic error‐growth parameters from the current forecasting system are more reliable than those obtained earlier. Forecast accuracy later in the medium range may thus not have benefited fully from improvements earlier in the range because of the faster error‐growth associated with a more active, though more realistic, forecast model. Overprediction of variance may nevertheless detrimentally affect present levels of skill and estimates of predictability in all seasons other than summer.

The error‐growth model currently indicates that it is possible, in principle, to make deterministic mediumrange forecasts for the extratropical 500 hPa height field of the northern hemisphere that are as accurate five days ahead as present forecasts are three days ahead, provided the one‐day forecast error can be reduced by the same factor in the future as has actually been achieved in the years since 1981. the level of error currently reached at day seven would then be reached at around day ten. the scope for improvement of forecasts for the southern hemisphere appears to be rather larger. Improvements seem to be possible throughout the spectral range studied, up to total wave‐number 40. This is found also for the rotational and divergent wind components at 850 and 200 hPa. For these components, particularly the divergent component, there is a quite pronounced error in the representation of the largest scales.
Computed conditions of corona emission from two interacting raindrops
Authors: Georgis Jean‐François , Chauzy Serge , Coquillat Sylvain
Published: February 2007

The conditions of corona emission from two interacting drops located in a vertical ambient field parallel to their line of centres are established by combining the corona experimental results obtained by Dawson (1969) with the distortion model from Georgis et al. (1995). Given the drop shapes and electrical fields at any points on the surface then, thanks to the distortion model, it is possible to calculate the altitude of corona occurrence from a fitting of Dawson's data. Unexpectedly it was found that an increase in the ambient field and/or a decrease in the distance between the two drops is not always equivalent to a lowering of the altitude of the corona occurrence, thus confirming that distortion plays a key part in this highly nonlinear mechanism. Nevertheless, corona emission is now generally possible at quite realistic altitudes in permanent ambient fields provided that the drops are close enough to each other. the mutual approach of the two drops certainly favours corona emission at low altitudes, but it also entails such a reduction in the ambient field for corona onset that conditions may be no longer propitious for corona propagation.
An intercomparison of three vegetation/soil models for a sparse vineyard canopy
Authors: Van Hurk B. J. J. M. Den , Verhoef A. , Van Berg A. R. Den , De Bruin H. A. R.
Published: February 2007

Three Soil‐Vegetation‐Atmosphere Transfer models simulating fluxes collected at a dry sparse vineyard site in La Mancha, Spain, are intercompared. the algorithms used to describe soil heat‐flux density, aerodynamic transfer of water and heat between the surface and the atmosphere, and crop evaporation all have a different physical basis in these models. A common feature of the models is that interaction of many processes takes place via the surface temperature.

Large differences are found between the predicted soil heat‐flux densities, caused by a poor physical description of heat storage in the soil and by the choices of soil coefficient values. Significant differences in the parametrization of the aerodynamic transfer between the surface and the atmosphere also exist, with a clear impact on predictions of surface temperature. Crop evaporation is underestimated by models taking the effect of soil moisture depletion or atmospheric humidity deficit on crop resistance into account.

A suggestion is made for an optimal choice of various model components to describe the observed fluxes best.
Three‐dimensional transport simulations of the dispersal of volcanic aerosol from Mount Pinatubo
Authors: Fairlie T. Duncan A.
Published: February 2007

Three‐dimensional Eulerian and Lagrangian transport simulations are used to investigate aspects of the initial dispersal of the volcanic aerosol plume from the 1991 eruption of Mount Pinatubo. A global finite‐difference model that simulates the large‐scale dynamics of the earth's stratosphere is combined with an Eulerian spectral transport model and a Lagrangian trajectory model. the simulated transport of the volcanic plume is compared with observations of the volcanic plume made from the SAGE II satellite instrument. Dynamical quantities, diagnosed from the simulated wind‐ and temperature‐fields, provide insight into the evolution of the volcanic plume. Lagrangian trajectories help assess the performance of the Eulerian transport model and highlight smallscale structure that is masked by the finite resolution of the Eulerian model. Exponential material‐stretching rates, symptomatic of efficient mixing, are found when air is peeled off the volcanic plume into southern mid‐latitudes. On‐line simulations show that radiative heating of the cloud has a major impact on its transport. Details of the local and far‐field dynamical responses to the heating are discussed.
Representation errors in various grid and spectral truncations for a symmetric feature on the sphere
Authors: Naughton Michael , Courtier Philippe , Bourke William
Published: February 2007

A smooth Gaussian bell‐shaped feature is used for a test case to examine the accuracy properties of finite spectral representations on the sphere. Firstly, the effect of different degrees of thinning of the number of longitudes in the Gaussian grid in the vicinity of the poles is illustrated. Secondly, it is shown that the inhomogeneity of the Gaussian grid leads to variations in accuracy as the feature location is changed, despite the fact that the triangular truncation spectral representation is homogeneous. Thirdly, the accuracy of triangular, rhomboidal and other spectral truncations are compared, with triangular truncation proving the most accurate for this type of feature.
Vorticity asymmetries in Hurricane Josephine (1984)
Authors: Glatz Arno , Smith Roger K.
Published: February 2007

A further analysis of omega dropwindsonde observations from three synoptic‐flow experiments in the environment of Hurricane Josephine is described, complementing an earlier study by Franklin. The intensity, structure and evolution of azimuthal wave‐number components of the 850–500 hPa layer‐mean and deep‐layer mean (surface to 100 hPa) vorticity fields, including the symmetric component, are investigated, and the determination of the extent to which the asymmetries may be interpreted in terms of the barotropic theory of vortex motion in simple large‐scale environments is sought. The latter endeavour calls for a (necessarily arbitrary) partitioning of the flow between vortex and environment. As a starting point for this it is hypothesized that the environmental flow has a linear variation across a domain that appears to be encompassed by the storm circulation. Based on this assumption it is shown that, on two of the three days on which data were available, the structure and strength of the wave‐number 1 vorticity asymmetry were similar to the beta‐gyres that are a feature of model calculations. The day‐to‐day changes in the orientation and strength of these asymmetries in the 850–500 hPa layer‐mean calculations are difficult to interpret in the light of barotropic theory, and are possibly associated with vertical shear in the storm environment, as described in a recent paper by Jones. Such rapid changes are not seen in the corresponding deep‐layer mean calculations, but the wave‐number 1 asymmetries in these fields are significantly influenced by the contributions from upper tropospheric levels where there are few data and where the vortex circulation centre is dispatched horizontally far from that at the surface.

The orientation of the wave‐number 2 asymmetry varied little over the three‐day observation period and was broadly consistent with the orientation of the large‐scale deformation field. Moreover, its strength weakened during the period and varied monotonically with that of the deformation field, a result that is consistent also with theoretical expectations.

The attempted analysis highlights certain intrinsic problems in interpreting hurricane motion in terms of current theories. This is due partly to the non‐uniqueness of the partition required to separate the storm environment from the vortex asymmetries and partly because of the difficulty of determining a representative model‐equivalent value for the environmental absolute‐vorticity gradient from the data.
Reversible and irreversible sources of radiation entropy
Authors: Goody Richard , Abdou Wedad
Published: February 2007

We discuss two practical issues concerning the relationship between the entropies of the atmosphere and of the radiation field. The first is whether the radiative flux of entropy can be determined from a satellite with sufficient accuracy for climate studies. We conclude that an accuracy much better than 1% is required, but that this can be achieved in the thermal spectrum with spectrometers having spectral resolutions ∼ 1 cm−1; it is also possible that the required accuracy can be achieved with a non‐spectral approach. The second is whether the rate of change of the atmospheric entropy inventory can be inferred from the outgoing radiative‐entropy flux. We conclude that, for clear skies in the thermal region of the spectrum, the two are directly proportional, and that the atmospheric term can be inferred with useful accuracy from an empirical relation.
Quasi‐continuous variational data assimilation
Authors: Järvinen Heikki , Thépaut Jean‐Noël , Courtier Philippe
Published: February 2007

All the computations associated with the operational meteorological data assimilation are usually carried out after the so called cut‐off‐time, i.e. a few hours after the end of the assimilation period. In this article we propose an alternative implementation of the four‐dimensional variational assimilation where some of the computations occur during the 24‐hour assimilation period. The feasibility of the approach is first validated with an assimilation system built around a low resolution barotropic grid point model. The results are then confirmed with a multi‐level primitive equation model using real observations. The main result of these experiments is that the peak computer power requirement of the four‐dimensional variational data assimilation may be significantly reduced by the suggested approach.
A vegetation‐atmosphere interaction study for Amazonia deforestation using field data and a ‘single column’ model
Authors: da Rocha Humberto R. , Nobre Carlos A. , Bonatti José P. , Wright Ivan R. , Sellers Piers J.
Published: February 2007

The recent high deforestation rates in Amazonia have raised the question of how climate, mainly the precipitation patterns, might be affected as pasture land replaces the primary forest. This work initially shows how the dry season soil moisture and water stress in typical Amazonian pasture modifies the energy partitioning at the surface. Low‐level moisture convergence is a primary mechanism to trigger convective cloud formation and precipitation. The relationship between moisture convergence and local surface evaporation in generating precipitation in Amazonia is studied utilizing a one‐dimensional ‘single column’ model (SiB‐1D). SiB‐1D couples a surface‐vegetation model (SiB) to a physical parametrization of deep convection (Kuo scheme), radiation, turbulent diffusion and large‐scale precipitation. Model simulations for short periods (2 days) show its ability to calculate the Amazonian surface energy‐balance components and boundary‐layer dynamics when compared with field observations from the Rondǒnia Boundary‐Layer Experiment and the Anglo‐Brazilian Amazonian Climate Observation Study. The model was further used to test the sensitivity of the deep convection scheme to a range of typical low‐level moisture convergence situations in a second experiment, and a third test investigated the local convective precipitation generated over forest and grass vegetation as a function of available soil moisture. Results showed that the rainfall over forest vegetation appeared to be almost insensitive to soil water stress whereas reduced precipitation was generated over pasture. When available soil water fell below a threshold of 60% the calculated precipitation over the pasture sites rapidly declined. Although these results are confined by the short integration period and the initial atmospheric profiles, they help to strengthen the notion that deforestation reduces evaporation and convective precipitation, especially during the dry season: a result already indicated by some previous general‐circulation model experiments.
Potential vorticities in semi‐geostrophic theory
Authors: Roulstone Ian , Sewell Michael J.
Published: February 2007

A family of kinematically possible motions in shallow‐water versions of semi‐geostrophic theory is devised, with variable Coriolis parameter, for which two distinct measures of potential vorticity take the same value. Conditions are described for which this value is conserved following the actual motion of a particle. The results are related to the existing literature, in particular for a constant Coriolis parameter.
Stratospheric inertia–gravity waves generated in a numerical model of frontogenesis. II: Wave sources, generation mechanisms and momentum fluxes
Authors: Reeder Michael J. , Griffiths Morwenna
Published: February 2007

Using a dry two‐dimensional numerical model, we investigate the source regions and generation mechanisms for stratospheric inertia–gravity waves accompanying frontogenesis. Attention is focused on run 3 from Part I of this paper (Griffiths and Reeder 1996), here referred to as our ‘standard calculation’.

Ray‐tracing techniques are used to identify the source regions of the waves. Although these results are strictly valid only in the far field away from the frontal region, they suggest that the waves originate in the frontal region. The ray equations also show that the horizontal wavelength decreases exponentially with time because of the action of the basic‐state deformation, whereas the vertical wavelength increases linearly with time because of the shear of the basic‐state cross‐front wind.

It is argued that the cross‐front circulation may be viewed, for the most part, as the unsteady source of stratospheric gravity‐waves in the standard calculation. This position is supported by a linear forced‐wave calculation, in which the forcing is provided by the cross‐front circulation from the standard calculation. Comparison of the vertical‐motion fields in the stratosphere for the standard and forced‐wave calculations show good agreement. An estimate of the nonlinear interaction between the frontal circulation and the wave field is made. It is shown that the back‐reaction of the waves on the forcing may be neglected, at least, qualitatively.

The time–space mean of the vertical flux of horizontal momentum in the stratosphere accompanying the modelled waves is approximately 1.4 × 10−4 N m−2 where the average is taken over −2000 km ⩽ x ⩽ 3000 km, 15 km ⩽ z ⩽ 29 km and 24 h ⩽ t ⩽ 48 h.
Lee waves over the English Lake District
Authors: Vosper S. B. , Mobbs S. D.
Published: February 2007

This paper presents the results of a field experiment which was conducted for three weeks during November 1991. The aim was to investigate the properties of internal gravity waves in the troposphere and lower stratosphere which were generated by the mountains of the Lake District in north‐west England, UK. The main technique used was that of multiple sounding using radiosondes.

Throughout the experiment measurements obtained from groups of five radiosondes launched nearly simultaneously indicate that the waves are quasi‐steady and trapped within the troposphere. This is illustrated by a single detailed case study for 26 November in which the radiosonde ascent‐rate fluctuations are as large as 6.5 m s−1 peak to peak. These measurements are supported by aircraft observations.

Simple techniques for analysing the structure of the wave‐field are presented and these include the calculation of a phase‐line tilt and phase speed and comparisons with simple analytical and numerical solutions for trapped gravity waves.

A three‐dimensional linear numerical model is used to simulate the waves observed on 26 November and three other cases also. The model predictions are shown to compare well with the observations. This is particularly true in the mid‐troposphere, where the model reproduces both the positions and amplitudes of the peaks and troughs in the vertical‐velocity field.

Spectral analysis of the local orography and examination of typical variations of the wind and buoyancy frequency with height show that gravity waves generated by the orography in the Lake District will frequently be trapped in the troposphere.
Variational analysis of humidity information from TOVS radiances
Authors: McNally A. P. , Vesperini M.
Published: February 2007

The impact of assimilating TOVS radiance data in the European Centre for Medium‐Range Weather Forecasts humidity analysis is evaluated. It has been found that the introduction of a one‐dimensional variational analysis scheme (1DVAR) applied to the TOVS radiances significantly improves the representation of many aspects of the hydrological cycle. The theoretical information content of the TOVS radiance data is discussed and found to be consistent with significant changes observed in the mid upper‐tropospheric moisture fields when the radiance data are assimilated. In particular, a tendency of the model (without radiance assimilation) to produce a tropical humidity structure that is far too dry is removed, and excessively moist conditions in the southern sub‐tropics are improved. It is argued that the latter problem originates from the use of operational NESDIS retrieved products in the analysis. The humidity adjustments caused by the assimilation of TOVS radiances are accompanied by significant changes in the model dynamics, especially the description of the tropical Hadley circulation. One such case is described in detail, where the moistening of the tropics and drying of the sub‐tropics has resulted in a stronger mean analysed meridional circulation in the Atlantic. The analysis changes are also shown to improve the medium‐range forecasting of humidity, together with some associated benefit in the prediction of cloud and precipitation.
On four‐dimensional variational assimilation of ozone data in weather‐prediction models
Authors: Riishøjgaard Lars Peter
Published: February 2007

Routinely made satellite observations of total ozone contain information about the dynamical state of the atmosphere in the regions where the ozone gradients are strong. It is argued that four‐dimensional variational assimilation (4D‐VAR) of these data with a meteorological model may be a way of extracting this information in order to use it for determining the initial condition of the model. In the first part of this paper the problem of assimilating ozone data in a variational context is discussed. In the second part a series of simple experiments is presented in which ozone pseudo‐observations are assimilated with a barotropic vorticity‐equation model. The aim of these experiments is to examine whether the atmospheric flow at a given level near the tropopause can be reconstructed by a 4D‐VAR scheme using a series of simulated observations of passive tracer mixing ratios. Both data produced by the assimilating model itself and data generated by a more realistic three‐dimensional model have been tested. It is found that when started from a purely zonal flow field, 4D‐VAR can to a large extent reconstruct the flow field, using only observations of the mixing ratio of the tracer. The quality of the result depends on the resolution of the model and on the choice of the length of the assimilation window.
Dynamics of cumulus rising into stratocumulus as observed during the first ‘Lagrangian’ experiment of ASTEX
Authors: de Roode Stephan R. , Duynkerke Peter G.
Published: February 2007

The Atlantic Stratocumulus Transition Experiment took place during June 1992 in the eastern Atlantic Ocean in an area extending south‐east of the Azores to Madeira. One of the goals of this experiment was to study the physical processes responsible for the transition from stratocumulus to shallow cumulus clouds. The cloud type most frequently observed was cumulus rising into stratocumulus. Two ‘Lagrangian’ experiments were carried out to study the evolution of the boundary layer and to observe any transition in the cloud characteristics downstream.

Here we concentrate on the last flight of the first ‘Lagrangian’ experiment during which cumulus cloud was penetrating thin and broken stratocumulus from below. The immediate cause of the cumulus convection was condensation at the top of the well‐mixed surface‐based boundary layer. The lifting condensation level was observed to be at about 500 m and above this height there was a conditionally unstable layer up to 1600 m. In the upper part of the conditionally unstable layer the horizontally averaged thermodynamical variables showed that the air was very close to saturation, indicating that the upper part of the cloud deck consisted of broken clouds. The vertical structure of mean variables, variances, fluxes and turbulent kinetic energy budget is discussed. One of the findings is that the cumulus convection is an important factor in counteracting the gradual dissipation of the stratocumulus deck in that it supplies moisture to the latter. Furthermore, the net longwave radiative loss at cloud top was observed to be about 40 ± 15 W m−2, which tended to destabilize the cloud layer from above. The skewness factor S showed that the upward convection was quite intense owing to the rising cumuli, the factor S reaching a maximum of about 2 ± 0.7 in the middle of the cloud layer. The skewness was found to be slightly negative near the cloud top.