bluuebunny/crossref_metadata_2025_split huggingface dataset:
Coupled processes for equatorial Pacific interannual variability
Authors: Pontaud Marc , Thual Olivier 
Published: February 2007

AbstractDiagnostics for coupled processes relevant to equatorial Pacific interannual variability are defined, using a simplified analytical model, and applied to general circulation model simulations. The analytical model is a simplification of the Neelin model (Neelin 1991). The atmospheric component is represented by a direct zonal thermal circulation. The oceanic dynamics take into account a shear layer and a Kelvin wave. Analytical calculations lead to an unstable slow‐propagating coupled mode, referred to as the TK‐mode. All the processes of this mode are explicitly identified, as are their contributions to the resulting total growth rate. These contributions are computed in forced oceanic (covering the 1979–93 period) and in coupled ocean/atmosphere general circulation model simulations. The results are sensitive to the parameters but allow relative comparisons between general circulation model simulations.The forced oceanic simulation suggests that the total instability is nearly constant all over the equatorial Pacific basin, but the central part is dominated by the zonal advective coupled process, while the eastern part is controlled by both zonal and vertical advective coupled processes. The thermocline coupled process appears largest in the eastern part of the basin, but its contribution is still weaker than the other two.The coupled simulation appears globally more unstable than the forced one. The total instability is larger in the eastern part of the Pacific basin. This result is consistent with the location and the frequent occurrence of the simulated interannual signals in the eastern part of the basin.The seasonal cycle of the forced simulation shows maximum instability between July and November. This could suggest an approach towards the understanding of the seasonal phase lock of the onset of warm El Niño/Southern Oscillation events. Possible interactions between the delayed oscillator mechanism or the westerly wind bursts with the TK‐mode processes are discussed. In order to understand the real system, these diagnostics should be applied to the observations and not just models. Nonetheless, analysis of coupled processes allows objective comparisons between different general circulation model simulations, and the results presented in this paper should be considered as a first step in this direction.
Smooth quasi‐homogeneous gridding of the sphere
Authors: Purser R. J. , Rančcić M. 
Published: February 2007

AbstractWe describe a variational technique for generating smooth quasi‐homogeneous numerical grids covering the sphere. This work extends our earlier studies of non‐standard global grids by relaxing the requirement of conformality to admit a greater degree of homogeneity. The motivation for this extension is to increase the minimum grid distance and thus to enable a larger time step to be used within an Eulerian grid‐point model. The grids described here are specified by balancing terms in a variational principle that penalizes departures of grid‐point density from smoothness and homogeneity. Grids generated by the new procedure are tested in both the cubic and octagonal configurations we have described in our earlier studies. The time‐step restriction is successfully alleviated without significant loss of solution accuracy.
A first‐order closure for diturbed plant‐canopy flows, and its application to winds in a canopy on a ridge
Authors: Wilson John D. , Finnigan John J. , Raupach Michael R. 
Published: February 2007

AbstractTo calculate disturbed wind flows in plant canopies, we studied a variant of the K α λk1/2 first‐order closure (where K is the eddy viscosity, k is the turbulent kinetic energy calculated from its simplified governing differential equation, and λ is an algebraic length‐scale). We compare numerical solutions using this closure (which admits only three empirical constants) with earlier obervations of mean wind, shear stress, and turbulent kinetic energy in three, quite different, uniform canopy flows, and in disturbed canopy flow over a ridge. The closure performs as well as or better than the solutions of others based on modifications of the ‘k‐ϵ’ closure, where ϵ is the rate of dissipation of k.
A thermal oscillation under a restorative forcing
Authors: Cai Wenju , Chu Peter C. 
Published: February 2007

AbstractThe authors report an interdecadal oscillation in a wind‐ and thermally‐driven ocean general circulation model (OGCM). The oscillation is tantalizing in that it occurs under a relatively strong thermal damping (26.3 W m−2 K−1). Examinations involving a two‐dimensional OGCM, a simple thermal ‘flip–flop’ model, and a three‐dimensional OGCM with and without the nonlinear effect of temperature in the state equation of sea water demonstrate that the oscillation is not driven by mechanisms such as the so‐called convective oscillator, or the advective overshooting oscillator. Instead, the oscillation is associated with the propagation of modelled long Rossby‐waves with westward phase velocity. It is found that the north–south basin extent of the OGCM is an important factor; a larger north–south basin extent is conducive to the generation of thermal wind flows crucial for initiating the propagation of disturbances. The geophysical relevance of these results is discussed.
Boundary‐layer mean dynamic characteristics of cold fronts over Brittany
Authors: Lefloch Claire , Amory‐Mazaudier Christine 
Published: February 2007

AbstractThe European Mesoscale Frontal Dynamic Project/Fronts 87 lasted from 18 October 1987 to 13 January 1988. Its aim was to study the dynamics of activ cold fronts observed over the area centred on the English Channel between England and France.In this paper the mean dynamic characteristics derived from the five intensive observation phases during which the acoustic sodar at Brest was operating are presented. Observations of wind at ground level, sodar wind in the surface and the boundary layers, sodar friction velocity, and radiosonde wind measurements are combined to determine the dynamics of a ‘mean cold front’ in the boundary layer. In spite of the important variability of the frontal phenomena, ‘mean’ frontal characteristics (over Brittany, in autumn) can be identified. The analysis shows that for all the cases, except for the dry cold front, the variations of the dynamical parameters at the frontal passage are closely related. Variations of the friction velocity and the horizontal wind in the surface and the boundary layers are linearly related.
Humidity in the ECMWF model: Monitoring of operational analyses and forecasts using SSM/I observations
Authors: Vesperini Michèle 
Published: February 2007

AbstractOver oceans, total column water vapour (TCWV) estimates derived from brightness temperatures of the Special Sensor Microwave Imager (SSM/I) have been shown to give accuracies similar to radiosonde measurements. Data provided by the SSM/I on the DMSP satellites are not available in real‐time, and so they are not assimilated into the ECMWF model. They can thus be used for independent validation of this important aspect of the model's hydrological cycle. The quality of ECMWF operational analyses of humidity is assessed for the period between August 1992 and November 1994. At that time, assimilation was performed through an Optimal Interpolation scheme. For this work, 2‐week averages of TCWV have been computed from analysis humidity fields of the ECMWF model and compared with simultaneous SSM/I estimates. Discrepancies between model and SSM/I estimates associated with different meteorological regimes are discussed. According to the season, the underestimation of analysed humidity in the tropics can reach 30 to 50%, whereas oceanic subsidence areas located off western coasts of subtropical continents appear too wet by as much as 60%. Mid‐latitudes are generally too moist, but to a lesser extent. An example is also shown of the utilization of SSM/I TCWV to evaluate the impact of an experimental prognostic cloud scheme on analyses and forecasts.
Linearizing iterative processes for four‐dimensional data‐assimilation schemes
Authors: Polavarapu Saroja , Tanguay Monique 
Published: February 2007

AbstractThe tangent linear model is used in applications including Kalman filters and the growth of perturbations. It is also used to define adjoint models in applications such as four‐dimensional variational assimilation or sensitivity studies. The validity of the tangent linear model for all of these applications is determined by the period of time and initial amplitudes for which perturbation growth remains linear. In this work we examine the validity of various linearizations of a class of iterations known as fixed‐point iterations. The tangent linearization is found to converge more slowly than the nonlinear iteration, and also requires all iterates of the nonlinear process. The tangent linearization can also be invalid if too few iterations of the nonlinear process are taken. An approximate linearization which requires only the last iterate of the nonlinear process was found to be accurate when the correct linearization has converged. Application of these results to iterative processes occurring in a numerical weather‐prediction model reveals that the approximate linearization can be effective. Specifically, the linearization of the iteration for the mid‐trajectory position for semi‐Lagrangian advection was found to converge more slowly than the nonlinear iteration, for our choice of initial guess. For the iterative solution of an elliptic equation, the maximum number of iterations allowed by the model was sufficient for both the nonlinear and linearized iterations. Thus the approximate linearization could be used to save CPU time or memory space.
Baroclinic instability of semi‐geostrophic fronts with uniform potential vorticity. I: An analytic solution
Authors: Juckes M. N. 
Published: February 2007

AbstractBaroclinic instability of uniform potential‐vorticity flow between solid upper and lower boundaries is analysed. The instability is driven by meridional temperature gradients on the boundaries. The classic Eady model of baroclinic instability uses just this system with the further idealization that the temperature gradients in the basic state are uniform. Here, this last idealization will be replaced with a complementary one in which the basic‐state temperature gradients are taken to be concentrated in a front. The analysis then takes advantage of the fact that the boundary temperature anomalies created by the growing baroclinic wave are localized at the front. The dependence of the growth rates and phase structure on wave number are remarkably similar to those of the Eady model. The wave number of maximum instability and the short‐wave cut‐off differ from those of the Eady model by less than 2% and 10% respectively. The solution is asymptotic in the limit of zero frontal width in geostrophic coordinates. For a physical flow this limit can never be achieved, but comparison with direct numerical solutions shows that the analytic solution is still accurate at physically relevant frontal widths. Part I develops the solution based on an equation for the evolution of the displacement of the surface and upper fronts. Part II will look at the three‐dimensional structure of the disturbance in more detail.
The detection of hurricane asymmetries from aircraft reconnaissance flight data: Some simulation experiments
Authors: Smith Roger K. , Glatz Arno 
Published: February 2007

AbstractA series of observational system simulation experiments designed to investigate the data requirements for determining the flow asymmetry of a hurricane are described. Of particular interest is the azimuthal wave‐number‐one component of the asymmetry, which, together with the mean flow across the storm, is responsible for the storm motion. A principal finding is that the accuracy of retrieval of this component from wind data obtained from airborne omega dropwindsonde soundings depends on the orientation of aircraft flight patterns relative to that of the asymmetry. Since the orientation of the asymmetry is not known in advance, there will be an inherent uncertainty in the accuracy with which the asymmetry is determined. The simulation experiments enable one to estimate probable errors in the determination of the asymmetry and their implications for hurricane track errors.The track errors are assessed by comparing the vortex track in a high‐resolution control experiment with those that result from initial analyses based on simulated data sets of varying data density and quality. In some of the experiments, an increase in resolution along the flight tracks, or additional data in the storm environment, does not lead to an improvement in the forecast track, in contrast to increased data coverage on a regular grid. This at first sight surprising result is likely to be a consequence of the constant influence radius used in the objective analysis scheme, a factor which is currently being investigated.The addition of random wind‐measurement errors to the simulated data, comparable in magnitude to those in dropwindsonde measurements, does not lead to significantly inferior analyses or track forecasts based on them, suggesting that such errors are not the major cause of inaccuracy in diagnosing the vortex asymmetry. Moreover, the accuracy of the analysis is nearly independent of whether the data are taken at an instant of time, or over a typical nine‐hour flight duration and then corrected to a storm‐centred coordinate system.
The impact of the height of the model top on the simulation of tropospheric stationary waves
Authors: Ruosteenoja Kimmo 
Published: February 2007

AbstractWintertime stationary waves in the northern hemisphere were simulated with a linear model, employing an observational zonal‐mean state and recent estimates for thermal damping. the sensitivity of the simulations to the height of the model top, which was varied between 0.1 and 100 hPa, proved to be much smaller than in previous research. As long as the model top was located at 10 hPa or higher, the tropospheric stationary waves were well simulated, the correlation with the observed pattern at various levels being ∼ 0.8 or higher. Lowering the top to 30 or 100 hPa reduced the correlation by less than 0.1 units in the upper troposphere, and by even less near the surface. In an experiment with a local idealized high‐latitude forcing, the response was somewhat more sensitive to the placement of the top. Eliassen‐Palm fluxes proved an excellent tool in tracing the influence of the top on wave propagation. the distribution of the fluxes shows that the interaction with the model top takes place mainly in the extratropics, irrespective of the latitude of the forcing.Applying a sponge layer with artificially enhanced damping near the top improved the solution significantly in most of the experiments in which the top was located at 30 hPa or lower.
A second‐order Lagrangian stochastic model for particle trajectories in inhomogeneous turbulence
Authors: Reynolds A. M. 
Published: February 2007

AbstractThe second‐order Lagrangian stochastic model of Du et al. for particle trajectories in homogeneous turbulence is extended to inhomogeneous turbulence. the model is shown to be in good agreement with experimental dispersion data for laboratory‐scale convective boundary layers and for a neutrally stable wind‐tunnel boundary layer. the results of numerical simulations suggest that finite Reynolds number effects are not important in determining the mean concentrations of tracer particles dispersing in these flows. Estimates for the value of the Lagrangian velocity structure function constant, obtained by optimizing model agreement with experimental dispersion data, are found to be flow‐dependent.
Entropy production and dynamical complexity in a low‐order atmospheric model
Authors: Nicolis C. 
Published: February 2007

AbstractLorenz's classical model of thermal convection is analysed from the standpoint of irreversible thermodynamics. the entropy production, describing how dissipative processes are operating within the system, is expressed in terms of the model variables. It is subsequently evaluated in different parts of the attractor and the values so obtained are compared with various indicators of the complexity of the dynamics, such as the local Lyapunov exponents and a local generalization of the Kolmogorov entropy. As it turns out, dissipative processes are most efficient when the number of locally stable directions increases. In particular, when the local Kolmogorov entropy is close to zero (three stable directions) the system can dissipate over the entire spectrum of values available. Finally, it is shown that the evolution on the attractor does not correspond to any extremal property of the entropy production.
Momentum budgets over idealized orography with a non‐hydrostatic anelastic model. II: Three‐dimensional flows
Authors: Héreil Philippe , Stein Joël 
Published: February 2007

AbstractThe study of the interaction between two‐dimensional nonlinear orographic effects and the synoptic flow presented in Part I of this paper is extended to three‐dimensional (3D) flows. Numerical simulations of idealized 3D orographic flows are carried out with a non‐hydrostatic anelastic model to test the hypothesis of current drag parametrizations through calculations of momentum budgets in different domains of variable size representative of a grid mesh of a general‐circulation model (GCM). Parametrizations are tested in three nonlinear regimes of 3D orographic flows. In the regime of quasi‐symmetrical lee vortices, the large‐scale flow is decelerated in a restricted region located below the maximum height of the obstacle. When the symmetry of the lee vortices is altered, by a long model integration or by introduction of a perturbation, the flow evolves towards the vortex‐shedding regime. A very long integration of the model numerically shows the saturation of the disturbance after 140 units of dimensionless time. Budgets of momentum indicate that vortex shedding significantly enhances the large‐scale flow deceleration, still located below the maximum height of the obstacle. For a case of non‐breaking 3D trapped lee waves, the mean flow is decelerated in the trapping region. Comparison of the predictions of two gravity‐wave drag parametrizations with the results of fine‐mesh simulations shows that the parametrization of 3D subgrid‐scale orographic effects in a GCM takes into account most of the physical phenomena existing in the set of idealized flows.
The sensitivity of domain‐averaged solar fluxes to assumptions about cloud geometry
Authors: Barker Howard W. , Stephens Graeme L. , Fu Qiang 
Published: February 2007

AbstractThe effects on domain‐averaged broad‐band solar fluxes due to assumptions about geometry of connective clouds are explored using a Monte Carlo algorithm and 3D distributions of water generated by a cloud‐resolving model (CRM). Domains are (400 km) with 2 km horizontal grid‐spacing, δx, and 35 layers of varying thickness. Optical properties are computed based on single‐scattering parametrizations for hydrometeors and the correlatedk‐distribution method for gaseous absorption. Benchmark fluxes are established using the CRM fields at δx= 2 km. Four plane‐parallel versions of these fields (affected by letting δx‐oo) are considered which mimic 1D algorithms: the independent column approximation (ICA) uses the full CRM fields; for the others, mixing ratios of cloudy cells are reset to associated layer‐mean values thus conserving water mass and cloud fraction in each layer.For the ICA, errors in reflected flux to space and surface irradiance rarely exceed 20 W m‐. Total atmospheric absorption and heating rates are almost always within 5 W m−2and ∼3%, respectively. This demonstrates that cloud sides and horizontal fluxes are unimportant for averages over large domains. However, when clouds are homogenized horizontally yet exact overlap is retained, errors increase by almost an order of magnitude. This demonstrates the importance of horizontal variability. When the same clouds are randomly overlapped, errors in boundary fluxes can exceed 250 W m‐ at high sun, and heating rates can be off by 50% to 100%. When these clouds follow maximal/random overlap, albedo is oftenunderestimatedbecause overlap of CRM liquid clouds falls between maximal and random. This demonstrates the importance of cloud overlap and ultimately the need for ID models to account equally well for both subgrid‐scale variability in cloud extinction and overlap.
On the Removal of the Effect of Horizontal Fluxes In Two‐Aircraft Measurements of Cloud Absorption
Authors: Marshak Alexander , Wiscombe Warren , Davis Anthony , Oreopoulos Lazaros , Cahalan Robert 
Published: February 2007

AbstractCloud absorption inferred from the difference between the net fluxes measured by stacked aircraft below and above clouds is strongly affected by the uncertainties due to cloud horizontal inhomogeneity. the simplest way to get rid of these uncertainties is to perform grand averages over flight legs; if flight legs are long enough, grand averaging may lead to a reliable estimate of cloud absorption. However, the amount of information on ‘true’ cloud absorption returned from such an expensive measurement program will be very limited‐often one number per flight leg.This paper contains a discussion on how to enhance the harvest of true absorption data using two related methods: (a) subtraction and (b) conditional sampling. Both methods assume that, simultaneously with broadband measurements, some narrow non‐absorbing‐band net flux measurements are also available. Both methods are related to Ackerman‐Cox type corrections, where subtracting fluxes in a transparent spectral band from those in an absorbing band partially removes the radiative effects of horizontal inhomogeneity and allows the recovery of spatially resolved cloud absorption. the output of the two methods is different: while the subtraction method provides a contiguous record of recovered cloud absorption, the conditional sampling method yields a discrete set of data points where the vertical net flux divergence reliably estimates true cloud absorption.