bluuebunny/crossref_metadata_2025_split huggingface dataset:
Seasonal and interannual variability of the stratosphere diagnosed from UKMO TOVS analyses
Authors: Scaife A. A. , Austin J. , Butchart N. , Pawson S. , Keil M. , Nash J. , James I. N. 
Published: February 2007

AbstractUK Meteorological Office (UKMO) TOVS analyses are used to show that the record of satellite radiance measurements is now long enough to capture the vast majority of year‐to‐year variability in the stratosphere. Given this, a climatology of the stratosphere is constructed and used to identify robust but detailed features of the seasonal evolution in the northern and southern hemispheres. Interannual variability is also investigated. In the northern winter stratosphere, the interannual variability in geopotential height increases monotonically towards the pole. In contrast, in the southern hemisphere the largest interannual variability occurs in a collar of high values surrounding a low‐variability region over the pole. For the Tropics, we show that quasi‐biennial and semiannual oscillations in the tropical zonal wind can be derived from the geopotential‐height analyses. However, comparisons of tropical winds from three different methods of analysis show that the current meridional resolution of the analysis limits the accuracy of the stratospheric winds over the equator. Finally, the satellite radiance data are used to derive updated temperature trends for the stratosphere. These show a cooling trend throughout most of the stratosphere that increases with height above the tropopause and exceeds 2 K per decade at the stratopause.
Small‐amplitude coastally trapped disturbances and the reduced‐gravity shallow‐water approximation
Authors: Durran Dale R. 
Published: February 2007

AbstractSolutions are obtained for linear hydrostatic disturbances propagating parallel to the face of an uninterrupted topographic step in an infinitely deep, stably stratified fluid on an β‐plane. These waves are vertically trapped because their frequencies are smaller than the Coriolis parameter and the height of the topographic step is finite. These waves are referred to as step‐trapped Kelvin waves, because they are dynamically similar to internal Kelvin waves throughout the layer of fluid below the top of the topographic step. These waves appear to provide an idealized, semi‐analytic model for the coastally trapped disturbances observed to propagate parallel to mountainous coastlines in several parts of the world.Computations are performed for a basic state with uniform static stability and for a three‐layer basic state in which the two lowest layers represent the marine boundary layer and a strong capping inversion. One might suppose that the linear dynamics of hydrostatic disturbances in the three‐layer basic state could be well approximated by a reduced‐gravity shallow‐water model, but this is not the case. In particular, the reduced‐gravity shallow‐water model does not provide reliable estimates for the phase speed of linear step‐trapped Kelvin waves. This defect suggests that detailed quantitative comparisons between marine boundary‐layer flows and the reduced‐gravity shallow‐water system may not have any intrinsic physical significance. Nevertheless, these results do not preclude the possibility of constructing useful qualitative analogies between marine boundary‐layer flows and the reduced‐gravity shallow‐water model.
Low‐frequency variability in a simplified atmospheric global circulation model: Storm‐track induced ‘spatial resonance’
Authors: Franzke Christian , Fraedrich Klaus , Lunkeit Frank 
Published: February 2007

AbstractThe interaction between two storm tracks is analysed by using idealized experiments with a simplified global‐circulation model. A zonal distance of 180° between the storm tracks drives a retrograde travelling zonal wave‐number‐two wave with an amplitude modulation of about 50 days. The stream‐function tendency equation demonstrates that the amplitude modulation is excited in the storm‐track regions by the high‐frequency (periods between two and eight days) eddy forcing; ‘spatial resonance’ of a retrograde propagating zonal wave‐number‐two Rossby wave and high‐frequency eddy forcing are proposed as the amplifying mechanisms, while friction and self interaction between the low‐frequency eddies ensure damping. Sensitivity experiments indicate that the pronounced modulation of 50 days vanishes with decreasing zonal distance between the storm tracks, thereby enhancing the intensity and duration of blocking events.
On the effects of the Hadley circulation and westerly equatorial flow on planetary‐wave reflection
Authors: Magnusdottir Gudrun , Walker Christopher C. 
Published: February 2007

AbstractThe nonlinear behaviour of quasi‐stationary planetary waves that are excited by mid‐latitude orographic forcing is considered in a global shallow‐water model taken to represent the upper troposphere. The waves propagate toward low latitudes where the background flow is normally weak and the waves are therefore likely to break. Nonlinear pseudomomentum conservation relations are used to quantify the absorption‐reflection behaviour of the wave‐breaking region. Two different flow scenarios are represented: (i) initial states without a representation of the Hadley circulation, but where the axisymmetric equatorial background flow changes from being weak and easterly to moderate westerly; (ii) initial states that include a representation of the Hadley circulation and that have weak equatorial easterlies.Based on linear arguments, both (i) and (ii) are expected to influence the progression of the wave train. The nonlinear behaviour in the presence of low‐latitude westerly background flows is different from linear predictions. For large‐amplitude forcing, wave breaking takes place even though there is no zero‐wind line in the initial slate, and the cross‐equatorial wave propagation that look place for small‐amplitude forcing is stopped before it can reach the equator. Nonlinear reflection is found to take place back into the hemisphere of origin but not across to the other hemisphere.In the presence of a Hadley circulation representative of winter conditions, the nonlinear reflection takes longer to get established, i.e. it requires more forcing, but a reflected wave train is still present in the numerical simulations, both for a longitudinally symmetric forcing and for the more realistic case of an isolated forcing. A summer Hadley circulation allows wave activity to get to the winter hemisphere. As the forcing is increased, wave breaking occurs and eventually nonlinear reflection.
Observations and numerical modelling of mountain waves over the Southern Alps of New Zealand
Authors: Lane Todd P. , Reeder Michael J. , Morton Bruce R. , Clark Terry L. 
Published: February 2007

AbstractA detailed case study of mountain waves observed over the South Island of New Zealand during the Southern Alps Experiment is presented. The purpose of the study is to document the waves generated over this region, to explore the extent to which high‐resolution radiosonde data can be used to determine gravity‐wave parameters, and to estimate the vertical flux of horizontal momentum produced by flow over the Southern Alps.Organized wave clouds were observed in a visible satellite image on 20 October 1996 and are thought to have been produced by partially trapped lee waves. Using background flow fields derived from a radiosonde sounding, the theoretical horizontal wavelength of the partially trapped mode is calculated and agrees well with that measured from the satellite image.A technique, called the horizontal projection method, is developed to analyse radiosonde soundings for mountain waves. Specifically, the technique determines the dominant horizontal wavelength over the entire depth of a radiosonde sounding. This method uses a coordinate transformation to take account of vertical changes in the background flow and, hence, vertical variations in the vertical wave number, as well as changes in the path of the radiosonde. This method is applied to a radiosonde sounding to identify the dominant wave mode, and emphasizes that radiosonde soundings should not, in general, be treated as instantaneous vertical profiles when analysing mountain waves.The mountain waves observed are simulated using a high‐resolution numerical model and the results are used to check the consistency of the analysis. The numerical model produces a horizontally averaged value of momentum flux in the lower stratosphere of −0.02 N m−2 although this value is probably underestimated by at least a factor of four.
Observations of atmospheric density currents using a tethered balloon‐borne turbulence probe system
Authors: Alapworth  
Published: February 2007

AbstractA tethered kite balloon with multiple instrumented probes clamped to the tethering cable is used to investigate atmospheric density‐current fronts passing through three sites at varying distances from the coast. The system allows high‐frequency measurements of the three components of wind, temperature and humidity to be made simultaneously at up to ten levels as the front passes through. This enables a two‐dimensional visualization of the current and its circulation to be constructed. As a result the structure of some of the wide variety of atmospheric density currents is revealed by direct observation. The internal circulations and the effects of varying environments on the currents and their movement are described, together with turbulence statistics. One current was associated with what may have been a coastal trapped disturbance, and in one case two opposing sea breezes appear to have collided inland. Two currents penetrating into surface inversions were dissociated from the surface. One of these was travelling downwind and formed a strong near‐surface jet. Associated wave and bore structures were found to be travelling in layers above the currents in two cases. In one case environmental air was found to intrude into the head region and convect internally, while, in another, air from the head region extruded into the environmental flow. The measurement array enabled the different terms of the dynamic equations to be evaluated and their relative contribution assessed. A remarkable correlation is found between thermal and dynamic variables. A study of the internal flux balance of daytime density currents shows that the internal flow plays a significant part in maintaining the capping inversion of the current. Eddies were observed at interfacial regions in which Richardson numbers were significantly greater than 0.25, confirming some modelling studies. Frontal slopes in most cases are in accord with theoretical values but in one downwind case the slope is considerably less than predicted. The inland currents are found to be deeper than coastal currents which may explain the increase in speed observed later in the day. Water vapour frontal structures are generally less marked than thermal fronts.
A comparison of the impact of TOVS and ATOVS satellite sounding data on the accuracy of numerical weather forecasts
Authors: English S. J. , Renshaw R. J. , Dibben P. C. , Smith A. J. , Rayer P. J. , Poulsen C. , Saunders F. W. , Eyre J. R. 
Published: February 2007

AbstractThe Advanced TIROS Operational Vertical Sounder (ATOVS) was launched on the NOAA‐15 satellite in May 1998. This provided a very significant improvement in the information available from meteorological polar‐orbiting satellites compared with the previous TIROS Operational Vertical Sounder system, particularly for humidity and vertical resolution of temperature in cloudy areas. In preparation for assimilation of the observations into a three‐dimensional analysis of atmospheric temperature and humidity, the observations have been compared with calculated top‐of‐atmosphere brightness temperatures computed from numerical weather prediction model profiles of temperature and humidity. Differences between observed and modelled brightness temperature are small. In some parts of the tropics and northern hemisphere the standard deviation of these differences for the tropospheric Advanced Microwave Sounding Unit sounding channels is only marginally higher than the radiometric noise of the observations. Early in 1999 a series of observation‐system experiments were completed in which ATOVS observations were assimilated using a one‐dimensional variational analysis. No use of the new humidity information could be made because of interference problems experienced by the microwave humidity sounder on ATOVS. Nonetheless, these experiments showed that the assimilation of the new temperature information provided by the radiance observations reduces forecast errors by as much as 20% in the southern hemisphere and 5% in the northern hemisphere. Further improvements have been found by assimilating more data over land. The major impact arises from the microwave channels. Whilst forward‐model errors may be slightly lower for the microwave channels than the infrared channels the primary reason is the provision of sounding information in active weather systems, which are usually cloudy.
Multi‐resolution analysis, entropic information and the performance of atmosoheric sounding radiometers
Authors: Peckham G. E. 
Published: February 2007

AbstractThe performance of remote‐sounding radiometers measuring properties of the earth's atmosphere is analysed through a multi‐resolution wavelet transform. This technique allows the uncertainty in retrieved atmospheric profiles to be determined as a function of both altitude and scale of patterns in the profile. Multi‐resolution analysis may be applied to a number of indicators of the quality of a measurement including entropic information. The apportionment of performance indicators to specific altitude ranges and pattern scales facilitates a comparison with performance requirements. The analysis is illustrated through a simple model of a remote‐sounding radiometer and by application to the Infrared Atmospheric Sounding Interferometer.
The Met. Office global three‐dimensional variational data assimilation scheme
Authors: Lorenc A. C. , Ballard S. P. , Bell R. S. , Ingleby N. B. , Andrews P. L. F. , Barker D. M. , Bray J. R. , Clayton A. M. , Dalby T. , Li D. , Payne T. J. , Saunders F. W. 
Published: February 2007

AbstractThe Met. Office has developed a variational assimilation for its Unified Model forecast system, which contains a grid‐point mode) that is run operationally in global, mesoscale, and stratospheric configuration. Key characteristics of the design are:

a development path from three‐dimensional to four‐dimensional variational assimilation;
global and limited‐area configurations;
variational analysis of perturbations;
and a carefully designed, well conditioned background term.
The background term is implemented using a sequence of variable transforms to independent balanced and unbalanced variables, to vertical modes, and to spectral coefficients. The coefficients used are based on statistics from differences of one‐ and two‐day forecasts valid at the same time. The covariance model represents many of the features seen in the covariances of forecast differences.The three‐dimensional variational data assimilation (3D‐Var) system was implemented in the operational global forecast system on 29 March 1999. In parallel trials, the 3D‐Var system gave a 2.7% improvement in a composite skill score (verified against observations and weighted according to the importance of each field).
Instability and large‐scale circulations in a two‐column model of the tropical troposphere
Authors: Raymond D. J. , Zeng X. 
Published: February 2007

AbstractA two‐column model of the tropical atmosphere is developed. The two columns are kept in buoyancy equilibrium by mass exchange driven by the inter‐column pressure gradient, and diabatic processes are parametrized by highly simplified schemes. This model is used to investigate whether spontaneous large‐scale circulations will develop in the tropical atmosphere when (he sea surface temperature and solar radiative forcing are uniform. Such circulations do indeed develop in the model, with ascent in one column and descent in the other, when one of the columns is slightly perturbed from the initial state of radiative‐convective equilibrium. A key element of the circulation dynamics is that increased equivalent potential temperature in a column leads to enhanced convection and rainfall in that column, which further increases the equivalent potential temperature there. The latter effect arises because convection enhances surface heat fluxes and decreases outgoing long‐wave radiation by virtue of the increase in stratiform cloudiness. These results form an attractive explanation for the observed patchiness of deep convection over warm tropical oceans and suggest that further modelling and observational work be directed towards understanding the budget of equivalent potential temperature in the tropical atmosphere and its relationship to rainfall.
The evolution of vortices in vertical shear. II: Large‐scale asymmetries
Authors: Jones Sarah C. 
Published: February 2007

AbstractThe role of large‐scale asymmetries in the evolution of a tropical‐eye lone‐like vortex in vertical shear on an f‐plane is investigated. Idealized numerical calculations using a primitive‐equation model are used to illustrate the development of the asymmetries and their role in the evolution of the vortex lilt. The asymmetries develop in the outer region of the vortex, well outside the radius of maximum wind. For the vortex profile used in most of the calculations, the region where the asymmetries develop has anomalously negative potential vorticity (PV) compared with the undisturbed environment, and the flow fields associated with the asymmetries consist of large‐scale anticyclonic gyres. The orientation of the gyres changes with height and they are located on opposite sides of the vortex at lower and upper levels. The influence of the asymmetries on the vortex evolution depends on both the structure and location of the asymmetries and on the orientation of the tilted vortex within the asymmetries.It is hypothesized that the asymmetries arise due to the distortion of the initially symmetric vortex by the horizontally sheared flow associated with the vertical projection of the tilted PV anomaly. Calculations using a barotropic model are presented in support of this hypothesis. The sensitivity of the large‐scale asymmetries to the initial vortex structure is investigated. For a vortex profile in which the tangential wind decreases more slowly with radius, the asymmetries form in a region of positive PV anomaly and the associated flow field contains large‐scale cyclonic gyres. The implications of the large‐scale asymmetries for tropical‐cyclone motion and intensity change are discussed.
A two‐time‐level semi‐Lagrangian global spectral model
Authors: Temperton Clive , Hortal Mariano , Simmons Adrian 
Published: February 2007

AbstractA three‐time‐level semi‐Lagrangian global spectral model was introduced operationally at the European Centre for Medium‐Range Weather Forecasts in 1991. This paper first documents some later refinements to the three‐time‐level scheme, and then describes its conversion to a two‐time‐level scheme. Experimental results are presented to show that the two‐time‐level scheme maintains the accuracy of its three‐time‐level predecessor, while being considerably more computationally efficient.
A new representation of data‐assimilation methods: The PALM flow‐charting approach
Authors: Lagarde Thierry , Piacentini Andrea , Thual Olivier 
Published: February 2007

AbstractWe introduce a new graphical representation for data‐assimilation methods which allows the visualization of the following items: control spaces, dual spaces, elementary operators and flow charts, for the computation of the gradient of the cost function. It is illustrated on a large variety of data‐assimilation methods used in meteorology and oceanography: three‐dimensional variational, four‐dimensional (4‐D) variational, 4D‐dual, Kalman filters, incremental formulations, formulations with imperfect models, addition of weak constraints, and reductions of the control space, etc.The flow‐charting approach is a powerful tool for decomposing a data‐assimilation method into elementary steps, for pointing out the elementary steps common to several methods or for building new and more efficient algorithms. In addition to the use of a common system of notation for assimilation methods, flow charting allows different algorithms to be easily compared, and is a necessary step prior to the development of any operational suite coupling data and models through one or several assimilation methods. This approach has been successfully followed to design a universal coupler called PALM (Projet d'Assimilation par Logiciel Multi‐méthodes) for the purpose of operational oceanography in the framework of the MERCATOR project.
A note on the roughness length for temperature over melting snow and ice
Authors: Calanca Pierluigi 
Published: February 2007

AbstractThe characteristics of roughness length for temperature, zT, over melting snow and ice are examined on the base of data gathered during one summer season on the Greenland ice sheet. Despite fixed surface temperature and homogeneous surface conditions, zT is highly variable, taking on values between 10−5 and 1 m. Mean value and median are computed as 2 × 10−2 m and 1 × 10−3 m. It thus turns out that zT is about 10 to 100 times larger than the aerodynamic roughness length, z0. This disparity cannot be explained in terms of the roughness Reynolds number. Rather, the data show that over melting snow and ice, and for slightly stable conditions, zT is uniquely determined by the temperature defect.
A nonlinear dynamical perspective on model error: A proposal for non‐local stochastic‐dynamic parametrization in weather and climate prediction models
Authors: Palmer T. N. 
Published: February 2007

AbstractConventional parametrization schemes in weather and climate prediction models describe the effects of subgrid‐scale processes by deterministic bulk formulae which depend on local resolved‐scale variables and a number of adjustable parameters. Despite the unquestionable success of such models for weather and climate prediction, it is impossible to justify the use of such formulae from first principles. Using low‐order dynamical‐systems models, and elementary results from dynamical‐systems and turbulence theory, it is shown that even if unresolved scales only describe a small fraction of the total variance of the system, neglecting their variability can, in some circumstances, lead to gross errors in the climatology of the dominant scales. It is suggested that some of the remaining errors in weather and climate prediction models may have their origin in the neglect of subgrid‐scale variability, and that such variability should be parametrized by non‐local dynamically based stochastic parametrization schemes. Results from existing schemes are described, and mechanisms which might account for the impact of random parametrization error on planetary‐scale motions are discussed. Proposals for the development of non‐local stochastic‐dynamic parametrization schemes are outlined, based on potential‐vorticity diagnosis, singular‐vector analysis and a simple stochastic cellular automaton model.