bluuebunny/crossref_metadata_2025_split huggingface dataset:
Optimal perturbation time evolution and sensitivity of ensemble prediction to perturbation amplitude
Authors: Buizza Roberto 
Published: February 2007

AbstractCertain characteristics of the perturbations which grow most rapidly over a finite time interval in a primitiveequation atmospheric model are discussed. They are the singular vectors of a linear approximation of the European Centre for Medium‐Range Weather Forecasts primitive‐equation model. They are computed using the adjoint technique at horizontal spectral truncation T21 with 19 vertical levels. Linear combinations of singular vectors, named optimal perturbations, can be used in ensemble prediction to generate the initial conditions of perturbed integrations.Firstly, having specified the initial amplitude to be comparable with the amplitude of analysis‐error estimates, the nonlinear time evolution of optimal perturbations when added to the control initial conditions are studied. In particular, estimates are made of the time limit, TNL, after which nonlinear processes cannot be neglected. Considering optimal perturbations generated using singular vectors with maximum growth over a 36‐hour time interval, and characterized by amplitudes comparable with analysis‐error estimates, two different methods estimate TNL ≈︁ 2‐2.5 days.Secondly, the sensitivity of ensemble predictions to the optimal perturbation amplitude is analysed. This sensitivity study suggests that an increase of the root‐mean‐square amplitude of the initial perturbation can give a more realistic ensemble spread.Lastly, an estimate of the possible impact of the reduction of the amplitude of analysis errors on the skill of numerical weather prediction is deduced from the comparison of ensemble experiments run with T21 initial perturbations characterized by different amplitudes. Results indicate that a reduction of the root‐mean‐square amplitude of the analysis error by a factor √2 may lead to an improvement of medium‐range predictability up to 1 day, and that a reduction by a factor 2√2 may reduce the errors of the 7‐day forecast to values shown, at present, at forecast‐day 5.
Winds in the strait of gibraltar
Authors: Dorman Clive E. , Beardsley Robert C. , Limeburner Richard 
Published: February 2007

AbstractAutomated meteorological observations were made from October 1985 to September 1986 in the narrows of the Strait of Gibraltar. Additional surface and upper‐air measurements were made in the Strait from May to July 1986. the main wind events in summer were mostly mesoscale levanters (winds from the east through the Strait), and a lesser number of synoptic‐scale levanters and westerlies. During mesoscale levanters, the wind speed increased and the pressure decreased from Gibraltar downstream through the narrows as far as Tangier. A mesoscale low and the highest winds were found over the axis of the Strait, north of Tangier. High‐velocity (20 m s−1) winds occurred only under 1 ‐km height. the air‐temperature profile became progressively warmer moving from Gibraltar towards Tangier. A weak air‐temperature inversion decreased in height to the west of the narrows.Venturi flow as an explanation for the downstream increase of the low‐level winds during a levanter is inconsistent with observations. A new model is proposed to explain the wind field associated with levanter winds in the Strait that includes some orographic blocking and low‐level convergence into the narrows, some west‐side divergence due to drag over water, and leeside heating that generates a leeside, mesoscale low pressure. the surface low accelerates the surface winds to their maximum strength well downstream of the narrows. A similar flow regime has been observed recently in the sea‐level channel between Hawaii and Maui.
Inertial instability on an asymmetric low‐latitude flow
Authors: Clark P. D. , Haynes P. H. 
Published: February 2007

AbstractBoth satellite data and numerical model simulations have suggested a link between propagation of planetary waves into the low‐latitude middle atmosphere and the occurrence of dynamical structures with small vertical scale similar to those predicted by the theory of symmetric inertial instability. It is here argued that existing theories that consider the growth of disturbances to a basic flow that is independent of longitude are unlikely to be directly relevant to such observations. A theory is presented that takes account of longitudinal variations in the structure of the basic flow, using WKB methods and the notion of absolute instability to find localized unstable modes. In the limit of large vertical wavenumber it is demonstrated analytically that the flow is absolutely unstable. For finite vertical wavenumber the calculation is based on numerical derivation of the dispersion relation. The growth rates and the spatial structure of the unstable modes relative to the variation of the basic flow are predicted. The results are compared to those from numerical solution of the linear stability problem and with full numerical simulation in a three‐dimensional primitive‐equation model.It is found that under some circumstances the longitudinal variation causes the maximum growth rate to occur at finite vertical wavenumber, rather than at infinite vertical wavenumber as is the case when the basic flow is independent of longitude.
The rapid glaciation of slightly supercooled cumulus clouds
Authors: Mason B. J. 
Published: February 2007

AbstractIt is demonstrated that, contrary to current opinion, the rapid glaciation of shallow supercooled cumulus clouds with tops no colder than −8°C can be accounted for by the growth of a very low concentration (<10 m−3) of graupel pellets in a cloud of modest liquid‐water content of ∼1 g m−3 and a modest updraught of ∼3 m s−1. In an illustrative calculation, a graupel pellet, in ascending from the −3°C to the −7°C level and returning to the −5°C level, attains a radius of 3.6 mm and sheds > 104 ice splinters by the Hallett—Mossop mechanism in 750 s, one‐third of them in only 150 s. Ice splinters shed in the early stages of graupel growth develop into small graupel pellets of diameter ∼1 mm and account for these being observed in concentrations of ∼1 per litre.The whole process, from activation of the ice nuclei at the −8°C level to the final stage of explosive glaciation, takes about one hour and therefore occurs only in aged cumulus with weak updraughts and not in rapidly ascending towers.
A two‐dimensional Lagrangian stochastic dispersion model for daytime conditions
Authors: Rotach Mathias W. , Gryning Sven‐Erik , Tassone Caterina 
Published: February 2007

AbstractA two‐dimensional (x, z) Lagrangian stochastic dispersion model is presented that is correct (i.e. fulfils the well‐mixed condition) for neutral to convective conditions. The probability density function (pdf) of the particle velocities is constructed as a weighted sum of a neutral pdf (u and w jointly Gaussian) and a convective pdf (w skewed, u and w uncorrelated). The transition function ℱ varies continuously with stability and therefore ensures that the model results are not confined to a finite number of stability classes.The model is described in full detail and some sensitivity tests are presented. In particular, the role of Co, the universal constant in the Lagrangian structure function for the inertial subrange, in determining average plume characteristics is discussed. Furthermore, the evolution of average plume‐height and ‐width is investigated for different boundary‐layer stabilities ranging from ideally neutral to fully convective. Finally, the model is applied to the situation of a tracer experiment in Copenhagen, and it is shown that the measured surface‐concentrations can be well simulated.
Some aspects of the variation with time of katabatic flow over a simple slope
Authors: Helmis C. G. , Papadopoulos K. H. 
Published: February 2007

AbstractFeatures of the variations in katabatic flow along a simple slope in Athens, Greece are examined both observationally and from a simplified theoretical basis. When undisturbed periods of established flows are selected it is observed that the velocity and temperature fluctuations reveal characteristics which are in agreement with the theory proposed by Fleagle. Interesting implications of the theory are discussed, such as the effects of prescribing a decreasing cooling rate throughout the night or of taking into account the vertical structure of the katabatic layer.
Studies with a flexible new radiation code. I: Choosing a configuration for a large‐scale model
Authors: Edwards J. M. , Slingo A. 
Published: February 2007

AbstractA comprehensive new radiation code based on the two‐stream equations in both the long‐wave and short‐wave spectral regions is described. The spectral resolution of the code is variable, enabling it to be used in a wide range of applications. Because of its flexibility, the code is well‐suited to the investigation of the sensitivity of radiative calculations to changes in the way in which physical processes are parametrized. The gaseous transmission data are derived from a line‐by‐line model. Particular attention is directed towards the treatment of the water vapour continuum, the overlap between gases, and the sensitivity to changing the carbon dioxide concentrations.The performance of the code is examined both at high spectral resolution and in a lower‐resolution configuration designed for the UK Meteorological Office Unified Forecast/Climate Model (UM). Particularly for use in the UM, the code must be shown to perform satisfactorily across the whole range of atmospheric conditions. Comparisons are therefore made with reference calculations in both the long‐wave and the short‐wave, in clear and cloudy skies, and the accuracy with which various processes may be represented is studied.For the cloudy calculations in the short‐wave, a new method is presented for deriving the single‐scattering properties in broad bands, based on the analytic expression for the reflectivity of an optically thick cloud. This minimizes the errors in calculating the short‐wave radiative properties of water clouds when the spectral resolution is reduced to that designed for the UM. In contrast, for ice clouds the errors are minimized by deriving the single‐scattering properties using linear averaging, as appropriate for optically thin clouds. In the long‐wave, the vertical distribution of the radiative heating in cirrus clouds is examined at high spectral resolution. The effect of scattering of long‐wave radiation, usually ignored in large‐scale models, is examined in some detail and is explained using a simple model. Taking all these studies into account, it is concluded that the configuration designed for the UM retains the generality of the code, without significantly compromising the overall accuracy.
The response of a variable resolution semi‐Lagrangian NWP model to changes in horizontal interpolation
Authors: Tolstykh Mikhail A. 
Published: February 2007

AbstractThe effect of changing the horizontal interpolation for evaluating the quantities at departure points in the global spectral semi‐Lagrangian variable resolution NWP model ARPEGE/IFS is described. The two interpolations involved are conventional cubic Lagrangian and Hermite with estimates of derivatives by fifth‐order compact upwind differencing (CUD‐5). Their properties are compared with examples of different one‐dimensional and two‐dimensional linear‐advection tests. With each kind of horizontal interpolation, twelve 5‐day forecasts are performed, beginning on the fifth day of each month from February 1994 to January 1995, using the same vertical non‐interpolating scheme.On average, the difference between forecasts with these interpolations in geopotential and mean sea‐level pressure fields is negligible within the first three days, but noticeable on the fifth day, especially in the mean sea‐level pressure field. However, these improvements are present beyond the range for which the variable resolution strategy is designed.The root‐mean‐square errors for the relative‐humidity fields at 1000 hPa and 850 hPa demonstrate some improvement as a result of the change of interpolation, from the second day of the forecasts onwards. Hermite CUD‐5 interpolation also results in a reduction of the amount of negative moisture.
A new long‐wave formula for estimating downward clear‐sky radiation at the surface
Authors: Prata A. J. 
Published: February 2007

AbstractA new formula is proposed for estimating the long‐wave radiation from clear skies. The formula depends on screen‐level air temperature and water vapour pressure. The formula has been extensively tested using long‐wave measurements covering a large range of environmental temperatures (–40 to +40°C), and by using radiosonde profiles and an accurate radiative‐transfer model (Lowtran–7). It is shown that the new formula out‐performs five other widely used formulas.
Stratospheric inertia–gravity waves generated in a numerical model of frontogenesis. I: Model solutions
Authors: Griffiths Morwenna , Reeder Michael J. 
Published: February 2007

AbstractThe generation of stratospheric inertia–gravity waves is investigated with the aid of a dry two‐dimensional anelastic numerical model. The model calculations follow the development of upper‐level and low‐level fronts together with their attendant jets. Long‐wavelength low‐frequency inertia–gravity waves are generated in the model runs. The key constituents of the model basic‐state are: (1) a time‐dependent shear flow normal to the front in thermal‐wind balance with the along‐front potential temperature; and (2) a time‐ and height‐independent confluent‐deformation wind‐field with its axis of dilation parallel to the front.When the cross‐front geostrophic wind shear A is negative, and consequently the upper‐level jet advects cold air along the upper front, prominent vertically propagating inertia–gravity waves are evident in the lower stratosphere. Like those observed in the atmosphere, the modelled waves have frequencies close to the inertial frequency, horizontal wavelengths of order 400 – 1200km, and vertical wavelengths in the range 2 – 10km. Numerical experiments in which Λ ⩾ 0 showed little gravity‐wave activity.
A simple semi‐Lagrangian scheme for advection equations
Authors: Sun Wen‐Yih , Yeh Kao‐San , Sun Rueen‐Yuh 
Published: February 2007

AbstractA semi‐Lagrangian scheme is proposed by using forward trajectories and a splitting approach to the interpolation procedure. Several polynomial interpolation schemes are tested, including cubic spline and Lagrange polynomials of degree 3, 5 and 7. A simple filter is also proposed to eliminate spurious short waves and to achieve positive‐definiteness. Uniform, rotational and Smolarkiewicz's deformational flows are tested, a solution to the inviscid Burger's equation is also provided. This new algorithm employing cubic‐spline interpolation and the new filter yields efficient and accurate short‐term simulations. The advantage in efficiency of adopting split trajectories for interpolation is accompanied by a restriction which is slightly more stringent than the stability criterion for conventional semi‐Lagrangian advection schemes. It is also found that the simple filter can eliminate spurious short waves very effectively without degrading the solutions.
Radar measurements of critical‐layer absorption in mountain waves
Authors: Worthington R. M. , Thomas L. 
Published: February 2007

AbstractThe conditions controlling the vertical propagation of mountain waves in the troposphere and lower stratosphere have been examined using radar observations at 46.5 MHz of vertical‐velocity perturbations at Aberystwyth (52.4°N, 4.1°W) and radiosonde observations from Aberporth, some 50 km to the south‐west. Attention has been paid to the influence of the variation with height of the mean wind vector (U), the Brunt–Väisälä frequency and the Scorer parameter. Four case‐studies show the absorption of mountain waves at heights where the mean wind component normal to the wave front vanishes; this corresponds to U = 0 if the wind direction does not change with increasing height, and to the wind vector being orthogonal to the wave vector if the wind rotates with increasing height. For two cases, enhanced turbulence is found in the upper few hundred metres of the mountain‐wave field, directly below the height at which the waves disappear; for a third case, the enhancement extends over a broader range of heights, and for the fourth no clear identification of the enhancement is possible. The frequency power‐spectra of the vertical‐velocity oscillations show a ωn dependence, with n approaching −5/3 for regions of mountain‐wave activity below the critical layers, and n = 0 for heights above the critical layers. Statistically, mountain waves generated by low‐level easterly winds are often confined to tropospheric heights. An analysis of a large sample of radiosonde data indicates that critical‐layer absorption associated with rotation of the background wind in addition to wave reflection and trapping, plays an important role in this confinement. The results are discussed in terms of the momentum flux associated with gravity waves, their turbulent breakdown near the critical layer, and the implications of the frequency spectra below and above such layers.
Southern hemisphere climate system recovery from ‘instantaneous’ sea‐ice removal
Authors: Wu Xingren , Simmonds Ian , Budd W. F. 
Published: February 2007

AbstractA coupled atmosphere–sea‐ice model has been used to investigate the temporal response to instantaneous Antarctic sea‐ice removal under the present external climate conditions. In all cases the system was seen to be transitive, in that the sea ice eventually grew back to the present climate conditions. Here we examine the timescale and nature of the ice and atmospheric relaxation back to ‘control’ conditions. Several experiments were carried out, either supposing that all the sea ice was removed at certain times of year, or that it was removed only over certain large areas. The model in each case was run for six realizations for the equivalent of several years to a new equilibrium.For the ‘whole sea‐ice removal’ experiment the maximum impact was found to occur when the removal was made at the time of sea‐ice maximum (September) and least reaction near the minimum. In both cases the return to normal occured by the middle of the following winter. The atmospheric responses comprised reduced surface pressure over the area from which ice had been removed, greatly increased turbulent fluxes, reduced difference between precipitation and evaporation (P — E) over the ocean and increased (P — E) over the Antarctic continent. The air temperature changes spread through the troposphere and tended to reduce the circumpolar westerlies north of and over the sea‐ice zone. The reaction to the regional sea‐ice removals was more local and depended on where the region was located relative to the mean circulation pattern around and over Antarctica.
A study of the behaviour of semi‐Lagrangian models in the presence of orography
Authors: Li Yong , Bates J. R. 
Published: February 2007

AbstractOrography is incorporated into two global shallow‐water numerical models, the first (which is termed the (u, u) model) based on a semi‐Lagrangian vector discretization of the momentum equation, the second (which is termed the PV–D model) based on the semi‐Lagrangian advection of potential vorticity. Both models are two‐time‐level and use the finite‐difference approach.Linear analyses of both models are performed for the idealized situation of stationary flow over orography on an f‐plane. In extension of the analysis of Rivest et al. the effects of spatial discretization are included. Spurious orographic resonance in the sense of infinite amplitude is eliminated by the damping effects of spatial interpolation, but spurious enhanced responses in the neighbourhood of the points at which resonance previously occurred still remain. The linear analyses indicate similar spurious enhanced responses for both models, which is confirmed by the numerical experiments.The spurious orographic noise in both models can be eliminated by a sufficient amount of uncentring, though it is shown that uncentring of either first order or second order can have negative impacts on the large‐scale Rossby waves. The PV–D model formulation, however, allows the use of a scheme in which only the divergence and continuity equations are uncentred, which minimizes such negative impacts.
The mass flux across the tropopause: Quasi‐geostrophic theory
Authors: Juckes M. N. 
Published: February 2007

AbstractThe mean meridional circulation in the troposphere is driven by eddy momentum fluxes associated with baroclinic waves. the dynamics of these waves can be described qualitatively by quasi‐geostrophic theory. Here, the flow in the vicinity of the storm tracks is modelled using f‐plane quasi‐geostrophic theory with the tropopause represented as a nearly horizontal surface separating a troposphere of uniform potential vorticity from a similarly uniform stratosphere. Combining an analysis of the mean meridional mass‐flux associated with decaying eddies and the cross‐tropopause mass‐flux generated in the same process, it is possible to estimate the net flux both in the zonal mean and in a quasi‐Lagrangian mean framework. the quasi‐Lagrangian mean is here defined as an average along potential temperature contours on the tropopause. Consistent with earlier numerical and observational studies, it is found that the zonal mean mass‐flux is upwards poleward of the jet, whereas the quasi‐Lagrangian mean is downwards.
Entropy of a convecting water‐air system and the interpretation of cloud morphogenesis
Authors: Duane Gregory S. , Curry Judith A. 
Published: February 2007

AbstractCloud‐top entrainment in the stratocumulus‐to‐cumulus transition can be viewed as an entropy‐decreasing, self‐organizing process. an expression is derived for the entropy of a two‐phase convecting water‐air system which locates the entropy deficit in the region of the convection itself, generalizing a similar formulation of Dutton's for the one‐component case. This expression is applied to a buoyancy‐reversal model that simulates entrainment through convection driven by evaporative cooling. the overall entropy budget suggests that the evaporation/precipitation cycle is an essential ingredient of the stratocumulus‐to‐cumulus transition, underlying the difference between the behaviour of this system and that of the buoyancy‐reversal system. the cumulus cloud system is thus rendered as a dissipative structure. the class of dynamical mechanisms that can explain the stratocumulus‐to‐cumulus transition, together with the associated high entrainment rates, is restricted to those which depend on the water cycle. the relocalized entropy expression, in conjunction with the second law of thermodynamics, implies an upper bound on the possible extent of change in cloud form for given flux of water substance through the evaporation/precipitation cycle over the history of cloud morphogenesis.
Identification of cyclone‐track regimes in the North Atlantic
Authors: Blender R. , Fraedrich K. , Lunkeit F. 
Published: February 2007

AbstractA Langrangian‐type climatology of North Atlantic cyclones is established based on the high‐resolution European Centre for Medium‐Range Weather Forecasts data‐set of the 1000 hPa height‐field. First, an algorithm is introduced to identify mid‐latitude cyclones and cyclone paths with as few constraints as possible. Cluster analysis of relative cyclone displacements yields three types of cyclone tracks characterizing stationary‐, north‐eastward‐and zonally‐travelling storms. the internal Lagrangian statistics of these cyclone‐track types reveal representative life‐cycles for central pressure and geopotential‐height gradients and a power‐law scaling behaviour of cyclone displacements. Finally, a basic climatology of North Atlantic cyclone‐track regimes is deduced in terms of a time‐series and circulation statistics.