bluuebunny/crossref_metadata_2025_split huggingface dataset:
A variational modification algorithm for three‐dimensional mass flux non‐divergence
Authors: Ehrendorfer Martin , Hantel Michael , Wang Yong 
Published: February 2007

AbstractThe diagnosed three‐dimensional (3D) wind field should strictly satisfy the continuity equation; however, because of analysis, interpolation, and numerical errors this requirement is not met in practice. The resulting noise divergence is of the order of 10−6s−1. Its impact upon energy budgets is fatal. In order to remove the noise divergence it is suggested that a 3D minimum mean‐square modifying field is added to the diagnosed wind under the constraint that the modified wind strictly satisfies mass continuity. It is shown that the solution of the resulting variational problem, namely the modifying wind field, has the property of conserving the 3D vorticity of the diagnosed wind field.In this paper the theory proposed for modification of the diagnosed wind field is rigorously developed for the first time for both the continuous and the discrete situation. The discrete problem—its solution being referred to as variational modification algorithm (VMA)—is approached by converting the diagnosed wind field into mass fluxes across the faces of finite boxes; the modifying mass flux components are obtained as differences of a potential function being the solution of a 3D Poisson equation which—owing to consideration of mass fluxes—is free of coordinate increments. A fast direct algorithm to solve this equation through decoupling coordinate directions is presented and compared with a fast Fourier algorithm.The VMA is applied to the wind field over Europe, as routinely analysed by the European Centre for Medium‐range Weather Forecasts (ECMWF), for the specific case of the strong rain episode over central Europe on 1 August 1991. The wind is converted into mass flux components across the faces of boxes (100 km)2 × 100 hPa in size. The modification of the mass flux enforced by the VMA is about 7 per cent of the analysed mass flux, and 6 per cent of the initialized mass flux; this similarity suggests that the original 3D noise divergence is not in the ECMWF data (analysed or initialized), but is generated by the interpolation procedure from the ECMWF model coordinates to pressure levels. Investigation of analysed and initialized mass fluxes before and after application of the VMA reveals that the modification of the mass flux field through the initialization procedure at the ECMWF is considerably larger than the modification required to enforce non‐divergence through application of the VMA.The impact of the modification upon the imbalance of the heat budgets in the atmosphere over Europe is investigated by a diagnostic model. It is found that the VMA reduces the root‐mean‐square values of the sensible‐heat imbalance from 518 (337) down to 66 (35) W m−2 for analysed (initialized) input data. The reduction is less dramatic for the latent‐heat imbalance (about 30 per cent).The effectiveness of the VMA as well as its generalization to arbitrary coordinate systems and variable resolution are briefly discussed.
The northern hemisphere tropospheric response to changes in the gravity‐wave drag scheme in a perpetual January GCM
Authors: Stephenson David B. 
Published: February 2007

AbstractThe sensitivity of a general‐circulation‐model climatology to changes in the gravity‐wave drag scheme is investigated with particular emphasis being placed upon the northern hemisphere wintertime tropospheric circulation. It is found that by weakening the upper‐level drag, the Ferrel‐cell circulation intensifies to the north, resulting in a greater than 10 K cooling in the region of the arctic tropopause. The poleward side of the northern hemisphere sub‐tropical jet intensifies in the region of 60–70°N, and a poleward shift is observed in the eddy meridional momentum flux. A large part of the response is zonally asymmetric, with the Aleutian and Icelandic lows becoming shallower with weakened upper‐level gravity‐wave drag. By comparing the results of a gravity‐wave drag scheme having a wave‐breaking vertical profile with those of a simple uniform drag‐profile scheme it appears that the tropospheric response to weakened upper‐level drag depends very little upon the choice of vertical drag profile in the troposphere.
Transport into the troposphere in a tropopause fold
Authors: Vaughan G. , Price J. D. , Howells A. 
Published: February 2007

AbstractA tropopause fold developed to the west of the British Isles on the western flank of a trough in the 300 mb flow on 6 October 1990. Radiosonde ascents over western Europe showed very dry stable layers beneath the jet stream in the potential temperature range 310‐315 K. These were evident on profiles from 12h 6 October to 00h 8 October. The European Centre for Medium‐range Weather Forecasts (ECMWF) model assimilations were examined for this period to determine how well they represented the radiosonde observations. Good agreement was found, especially for the humidity distribution. The model overestimated the vertical extent of the fold by a factor of about 2, so its representation of potential vorticity was less successful. Nevertheless, agreement was sufficiently good for the model to be used to evaluate the amount of stratosphere‐troposphere exchange in the event.Isentropic trajectories were calculated for the air in the fold as represented by the ECMWF assimilation at 00h on the 7th. Those on the western edge of the fold split from the jet stream and transferred to the troposphere, while those on the eastern side ended up in the cut‐off low. The model fields were used to estimate a lower bound of 1.1 × 1014 kg and2 × 1032 molecules for the amount of stratospheric air and ozone, respectively, transferred into the troposphere by this fold. Owing to the model's incomplete representation of the event, the actual amount transferred may be at least twice this estimate.
The development of potential vorticity in a hurricane‐like vortex
Authors: Möller J. D. , Smith R. K. 
Published: February 2007

AbstractWe investigate the time evolution of the potential‐vorticity distribution in an axisymmetric hurricane‐like vortex using a numerical model based on a formulation by Schubert and Alworth. In particular, we compare the vortex response to an annular heating function with that in the analytic calculation by Schubert and Alworth in which the maximum heating rate occurs on the vortex axis. The annular heating function is intended to better represent the latent‐heat release in the eye‐wall clouds of a hurricane. We show that after about four days of integration time in the Schubert—Alworth calculation, the isentropes near the vortex centre, and hence the prescribed heating function, become concentrated near the surface in physical space. In comparison, for the annular heating distribution the descent of the isentropes is less marked and the vertical distribution of the heating in physical space remains more realistic vis‐à‐vis a hurricane. It is significant that in this case the potential‐vorticity maximum lies not on the vortex axis, as in the Schubert—Alworth calculation, but close to the radius of maximum heating. This appears to be consistent with observations in hurricanes. Finally we show that, counter to intuition, there is greater subsidence on the vortex axis in the case where the heating is a maximum on the axis. A dynamical explanation is offered for this result.
Simulation of the semi‐annual oscillation of the equatorial middle atmosphere using the Extended UGAMP General Circulation Model
Authors: Jackson D. R. , Gray L. J. 
Published: February 2007

AbstractThe semi‐annual oscillation (SAO) is examined in two runs of a middle atmosphere general circulation model. The first (run PGW) includes a parametrization of gravity waves with zero phase speed only, whilst the second (run AGW) also includes gravity waves with phase speeds of 20 m s−1. Run PGW successfully simulates equatorial SAOs near the stratopause and in the mid to upper mesosphere. However, the zonal winds at all heights are easterly and the amplitude of the oscillation is too weak by up to 15 m s−1 near the stratopause and too strong by about 45 m s−1 in the upper mesosphere. The corresponding SAOs in run AGW, however, have amplitudes that are within 5 m s−1 of those observed by rocketsondes, and have a pattern of easterly and westerly winds that are in reasonable agreement with the observations. Zonal mean momentum‐budget studies show that both at the stratopause and in the mid to upper mesosphere, the westerly phase of the oscillation in run AGW is stronger than in run PGW owing to an increase in total eddy forcing associated with the introduction of the 20 m s−1 phase speed gravity wave. The easterly phase of the stratopause oscillation on the summer side of the equator is driven by advection of easterlies across the equator. However, on the winter side of the equator, the net zonal forcing is westerly in May but easterly in November, since easterly eddy forcing associated with extratropical planetary waves is stronger in November than in May. The easterly phase of the mid to upper mesosphere oscillation is forced by contributions from both the residual circulation and eddy forcing. The eddy‐forcing contribution is more easterly than in other modelling studies owing to weaker Kelvin‐wave activity and to easterly forcing associated with model diurnal tides. Introduction of the 20 m s−1 gravity waves also improves the simulation of an N2O‐like tracer. In run AGW a ‘double peak’ of tracer is present in April, in reasonable agreement with observations, but this feature is absent in run PGW.
Localization of optimal perturbations using a projection operator
Authors: Buizza Roberto 
Published: February 2007

AbstractEnsemble 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

AbstractStudies 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

AbstractA 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

AbstractA 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

AbstractA 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

AbstractA 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

AbstractSome 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

AbstractMeasurements 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

AbstractRecently 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

AbstractDry 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.