bluuebunny/crossref_metadata_2025_split huggingface dataset:
The Three‐Dimensional Nature of ‘Symmetric’ Instability
Authors: Jones Sarah C. , Thorpe Alan J. 
Published: February 2007

AbstractThe importance of three‐dimensional effects in a flow with negative potential vorticity is considered here; this is the three‐dimensional counterpart of symmetric instability. the term symmetric instability refers to a two‐dimensional flow with negative potential vorticity which develops roll circulations aligned along the thermal wind. the requirement for exact two‐dimensionality is here relaxed by using three‐dimensional numerical simulations. Two classes of simulation are described; both take a two‐dimensional basic flow.In the first part we explore the response of a flow with uniform negative potential vorticity to localized initial perturbations. It is found that circulations become established which elongate at an angle to the thermal wind. This angle, it is shown, is determined by viscous effects and is in accord with the structure of linear viscous tilted modes for this flow. If, as happens on occasions, such an angle formed by rainbands to the thermal wind is observed in nature, it gives an indication of the importance of viscous effects. In the cases presented here the orientation is such that the rolls tilt towards the warm air as viewed in the direction of the thermal wind; i.e. they are rotated anticyclonically relative to the front. It is shown that these structures, when at finite amplitude, are stable to perturbations imposed along their length.In the second part localized regions of instability are produced which are confined along the thermal wind direction by prescribing in these zones a reduced static stability for vertical motion. This represents the effects of latent‐heat release and it is only in these zones that the effective potential vorticity is negative. It is found that if the length of the zone along the thermal wind direction is smaller than about twice the roll wavelength across that direction then the growth rate is substantially reduced; otherwise it is relatively unaffected by this confinement. It therefore appears that a flow with negative potential vorticity is not only unstable in the pathological two‐dimensional case.
Iterative Analysis Using Covariance Functions and Filters
Authors: Lorenc Andrew 
Published: February 2007

AbstractAn earlier review paper of analysis methods for numerical weather prediction showed how many methods could be derived from the same basic analysis equation, expressing the Bayesian probability that any state is the true state. Prior probabilities depend on the error covariance of the background prior estimate, and these covariances figure in most solutions. This paper extends the analysis of iterative solution methods (one of which is known as the ‘Successive Correction’ method). In a simple example the methods are compared with a direct solution: the ‘Optimal Interpolation’ method. It is shown that the iterative methods can be equally ‘optimal’. If the number of iterations is limited, or the background weighting is omitted, then the methods normalizing the increments in observation space are shown to be more reliable than those normalizing in grid‐point space. the traditional successive‐correction method has a grid‐point space normalization.The covariances in these methods perform a filtering function. It is possible to replace them by a filter acting on the analysis increments. Iterative methods using such a filter are derived, and are shown to correspond exactly to the iterative methods using covariance functions. A simple and efficient recursive filter is described and applied to the same example. the analysis using a two‐pass filter is almost identical to that using a ‘second‐order auto‐regressive’ covariance function. A filter with many passes corresponds to a Gaussian‐shaped covariance function.Approximate filters can be devised to model the effect of observational‐error correlations, with an accuracy adequate in view of the lack of knowledge of the real correlations. With this filter, the iterative methods can be extended to deal effectively with data from remote‐sensing instruments.Published iterative methods (many originally empirically derived) are reviewed, and fitted into the optimal theory. Practical applications are discussed.
Water‐Vapour Continuum Absorption In the Tropics: Aircraft Measurements and Model Comparisons
Authors: Kilsby C. G. , Edwards D. P. , Saunders R. W. , Foot J. S. 
Published: February 2007

AbstractMeasurements of water‐vapour continuum absorption in the 10‐12 μm window have been made in the tropical atmosphere using a multi‐channel narrow field‐of‐view radiometer mounted on an aircraft. Comparisons with two radiative transfer models (GENLN 2 and LOWTRAN7) are made which show that the model underestimates the absorption by some thirty percent. Two methods of modifying the existing empirical continuum parametrization to obtain better agreement with the aircraft measurements are assessed: one involves increasing the self‐broadened absorption coefficient, the other increasing the temperature‐dependence parameter.
A Numerical Investigation of Cloud‐Top Entrainment Instability and Related Experiments
Authors: Siems Steven T. , Bretherton Christopher S. 
Published: February 2007

AbstractBuoyancy reversal occurs between two homogeneous fluids when as a result of non‐linear mixing the mixture becomes denser than either of the fluids. In 1980 both Randall and Deardorff suggested that such nonlinear mixing, which may occur across a stratocumulus cloud‐top as a consequence of evaporative cooling. could create an entrainment instability; the ensuing rapid entrainment could lead to stratocumulus break‐up. This break‐up mechanism is known as cloud‐top entrainment instability or CTEI.In 1990 Shy and Breidenthal investigated buoyancy reversal within a laboratory environment and proposed a model for turbulent entrainment, and conditions under which buoyancy reversal may lead to an entrainment instability. They found that an instability could occur only if D (the ratio of the density difference between the most cooled mixed parcel and the lower fluid to the density difference across the unmixed layers) was approximately one or larger. This restriction of a strong buoyancy reversal for an instability (hence SBRI) is more stringent than Randall's condition, D ≥ 0, and is almost never satisfied at the inversion topping a marine stratocumulus layer.We have used numerical simulations of a single eddy at an interface between two fluids to explore the entrainment model of Shy and Breidenthal. Consistent with SBRI. we find that in all cases a critical value of D, Dc, between 0.3 and 2.0 exists. For D Dc, kinetic energy is found to increase on the scale of the entraining eddy. We expand the domain to model the interaction of the multiple eddies with an entrainment interface. We find a similar Dc where unstable entrainment is exhibited, and a tendency for larger eddies to develop if Dc. > D. We explore the influence of the fluid and flow characteristics on Dc to bridge the gap between the laboratory and the atmosphere. We find that Dc is lower at Schmidt numbers of order unity, characteristic of the atmosphere, than for those equal to 103, typical of the laboratory fluid. In all cases Dc remains too large to be a factor in stratocumulus break‐up.
Nonlinear Balance and Potential‐Vorticity Thinking At Large Rossby Number
Authors: Raymond D. J. 
Published: February 2007

AbstractTwo rational approximations are made to the divergence, potential‐vorticity, and potential‐temperature equations resulting in two different nonlinear balance models. Semi‐balance is similar (but not identical to) the nonlinear balance model of Lorenz. Quasi‐balance is a simpler model which is equivalent to quasi‐geostrophy at low Rossby number and to the barotropic model at high Rossby number. Practical solutions involving methods that work for all Rossby numbers are outlined for both models. A variety of simple initial‐value problems are then solved with the aim of fortifying our insight into the behaviour of flows at large Rossby numbers. the flows associated with a potential‐vorticity anomaly at very large Rossby numbers differ in significant ways from the corresponding low Rossby number results. In particular, an isolated anomaly has zero vertical radius of influence at infinite Rossby number, while the induced tangential velocity in a horizontal plane containing the anomaly decreases inversely as, rather than inversely as the square of, the radius. the effects of heating and frictional forces are approached from a point of view somewhat different from that recently expressed by Haynes and McIntyre, though the physical content is the same.
A Three‐Dimensional Model of Airflow and Orographic Rainfall Enhancement
Authors: Dore A. J. , Choularton T. W. 
Published: February 2007

AbstractA three‐dimensional model of airflow over hills has been used in conjunction with a two‐dimensional model of orographic rainfall enhancement. This development has allowed the accurate production of rainfall contour maps in areas of complex terrain. A comparison has been made, using a variety of atmospheric stability profiles, between the wind fields and rainfall distributions over a bell‐shaped hill and those over a two dimensional ridge of similar height. the results show that, in conditions of either stably stratified air or with an inversion layer aloft, three‐dimensional airflow effects can lead to much reduced rainfall over a bell‐shaped hill. It is shown that in such situations the use of a two‐dimensional approximation can introduce significant errors. an example is presented of the rainfall distribution in complex hill terrain over south Kintyre on the western coast of Scotland.
Simulation of Clear‐Sky Outgoing Longwave Radiation Over the Oceans Using Operational Analyses
Authors: Slingo A. , Webb M. J. 
Published: February 2007

AbstractWe have developed a system for the Simulation and Analysis of Measurements from Satellites using Operational a Nalyses (SAMSON). In this initial study, simulations of clear‐sky outgoing longwave radiation (OLR) over the oceans are compared with data from the Earth Radiation Budget Experiment (ERBE). the calculations were performed with a Malkmus narrow‐band radiation scheme, including the effects of all the radiatively important gases. Sea surface temperatures were taken from the National Meteorological Center's blended analyses and ozone data from the NIMBUS‐7 Solar Backscatter Ultra Violet experiment. Atmospheric temperatures and humidities came from operational analyses. Parallel simulations were performed with the analyses produced both by the European Centre for Medium‐range Weather Forecasts (ECMWF) and by the Meteorological Office.Simulations for the period February 1985 to October 1986 are compared with ERBE data. Differences between the humidity data in the analyses have a significant impact on the calculated clear‐sky OLR. the changes to the ECMWF model in May 1985 reduce the differences compared with ERBE. After this time, the differences are consistent with two known systematic errors; a small overestimate in the ERBE data and a tendency for the ECMWF analyses to be too dry in the ITCZ and too moist in the sub‐tropics. the simulations with the Meteorological Office's analyses generally show slightly larger differences compared with ERBE. Tests show the sensitivity of the ECMWF results to diurnal sampling of the analyses and to the time resolution employed to calculate the monthly means. These tests reveal instances of extremely low humidities in the ECMWF analyses in the upper troposphere over the tropical east Pacific.Over most of the domain, the simulations are within 5‐10 W m−2 of the ERBE data. the clear‐sky OLR can thus be computed from operational analyses with an accuracy comparable with that of ERBE. This indicates the potential of such simulations and has implications for the planning of future radiation‐budget observations.
An Optimal Nudging Data Assimilation Scheme Using Parameter Estimation
Authors: Zou X. , Navon I. M. , Ledimet F. X. 
Published: February 2007

AbstractA new optimal nudging dynamical relaxation technique is tested in the framework of 4‐dimensional variational data assimilation, applied to an adiabatic T40 version of the National Meteorological Center (NMC) spectral model with 18 vertical layers. Several experiments are performed using the NMC operationally analysed data. the variational data assimilation algorithm is also employed in a parameter‐estimation mode to determine the vector of optimal nudging coefficients. Results of data‐assimilation experiments involving estimated nudging, optimal nudging and variational data assimilation are compared. Issues are addressed related to the dependence of the assimilation on the length of the assimilation period as well as to the ability of retrieving high‐quality model initial conditions.The study outlines the ability to obtain optimal nudging coefficients, which can vary in space, in the framework of a parameter‐estimation approach using variational data assimilation. Based on our preliminary results the optimal nudging seems to be a most promising data‐assimilation scheme.
Variational assimilation of conventional meteorological observations with a multilevel primitive‐equation model
Authors: Thépart Jean‐Noël , Vasiljevic Drasko , Courtier Philippe , Pailleux Jean 
Published: February 2007

AbstractThe paper describes a four‐dimensional variational assimilation using a multilevel global primitive‐equation spectral model. The experiments consist in minimizing the distance between the model solution and conventional observations spread over 24 hours.A first set of experiments was performed at low resolution (spectral truncation T21). The model solution converges to fit the observations satisfactorily over the whole assimilation period. The meteorological quality of the analyses, in terms of the large‐scale part of the flow, is comparable with the ECMWF operational analysis. Confirming early results, the control of gravity waves in the solution can be easily handled using a combination of a penalty term and a normal‐mode initialization scheme within the forward‐backward process of the variational assimilation. Experiments excluding data over an area with a strong baroclinic development show that the information contained in the dynamics of the model is used successfully in the analysis over this area. In addition, the dynamics are able to inter implicitly flow‐dependent structure functions.Higher‐resolution experiments (spectral truncation T42) were performed and compared with optimum interpolation (OI) analyses performed under the same conditions. Although the lack of a first‐guess term in the cost function leads to small‐scale noise generation if the minimization is pursued, the quality of the variational assimilation is comparable with OI, and the error growth of the following 24‐hour forecast is smaller when performed from the variational analysis. Moreover, experiments excluding wind data from aircraft show a clear advantage for the variational approach against OI in using, in a consistent way, the information coming from the model and the observations.
Ensemble prediction using dynamically conditioned perturbations
Authors: Mureau R. , Molteni Franco , Palmer T. N. 
Published: February 2007

AbstractWe apply the technique developed in the companion paper by Molteni and Palmer (1993) as a means of providing dynamically conditioned perturbations for ensemble forecasting with a primitive‐equation model. Four wintertime initial states are chosen—three at random and one because of substantial development in the large‐scale flow within four days, which the control forecast completely missed. A set of singular vectors are created using a quasi‐geostrophic model linearized about basic states taken from data close to the chosen initial dates. These are interpolated onto the primitive‐equation‐model grid, and used as perturbations to the initial state. An ensemble forecast is made form the perturbed initial states. The dispersion of this ensemble is compared, for each date, with that from a second ensemble with initial perturbations constructed from 6‐hour‐forecast errors. Throughout the forecast period, it is found that the amplitude of the perturbations is noticeably larger using the singular vectors. The dispersion of the ensembles using the forecast‐error perturbations did not indicate that the control forecast form the case with substantial development was likely to be poor. By contrast, the (envelope) dispersion of the ensemble using the singular vectors was notably larger for this case than the other three. A number of members of this ensemble were particularly skilful in predicting weather‐related elements of the flow, such as low‐level temperature change. It is found that the evolution of perturbations which are initially localized over the western Pacific, or western Atlantic, can develop blocking‐like structures several days later over the eastern oceans. With the initial amplitudes used in this paper, the development of these structures is in part nonlinear.The growth of the singular‐vector perturbations was not as large in the primitive‐model as in the quasi‐geostrophic model, probably due to interpolation problems exacerbated by inconsistent orographic representation. Further work to overcome these, and other, problems is indicated.
Linke and Unsworth‐Monteith turbidity parameters in Athens
Authors: Kambezidis H. D. , Founda D. H. , Papanikolaou N. S. 
Published: February 2007

AbstractThe Linke, TL, and Unsworth‐Monteith, TU, turbidity parameters were chosen to be studied in Athens. They were derived from measurements of direct solar radiation and exhibit similar behaviour in the decade 1975‐84. The increase in air pollution in Athens does not reveal itself in the trends of the turbidity coefficients during the decade. An interdependence between TL and TU was found and is expressed as a mathematical function. The seasonal, monthly and daily variation of the parameters are investigated.
Use of a flux‐limited scheme for vertical advection in a GCM
Authors: Thuburn John 
Published: February 2007

AbstractA general method of constructing one‐dimensional total‐variation diminishing advection scehmes using flux limiters is presented. A simple scheme that uses the Van Leer limiter is demonstrated in some idealized tests. The scheme is compared with the more usual centred‐difference scheme in a global circulation model where the schemes are used to calculate the vertical advection of tracers, moisture, temperature and momentum. In climate simulations noise is generated by the centred‐difference scheme in tracer and temperature profiles near the tropopause, and in the moisture profile near the tropical boundary‐layer inversion; this is eliminated when the flux‐limited scheme is used. The use of this improved scheme for vertical advection is a cheap and simple way to remove these errors from global circulation models.
Estimation of area‐average rainfall for frontal rain using the threshold method
Authors: Cheng M. , Brown R. 
Published: February 2007

AbstractThe relationship between area‐average rainfall rate and the fractional area covered by radar echoes of intensity greater than a predefined rainfall‐rate threshold is investigated using quality‐controlled data from the United Kingdom and Irish radars for the period 26 January to 25 February 1990. During this period the precipitation was predominantly frontal, unlike previous studies that have concentrated upon convective precipitation. A high degree of correlation is found for the composite radar field and for the areas covered by individual radars within the composite. The correlation coefficient varies with rainfall‐rate threshold, peaking at a threshold of 2.0 mm h−1 when it is higher than 0.97 for all individual and composite radar data. The order of merit of the six thresholds investigated is 2.0, 1.0, 4.0, 0.5, 0.06 and 80. mm h−1. The spatial and temporal stability of the correlation over the area and period studied is high, especially for the threshold of 2.0 mm h−1%.A linear‐regression relationship is used to estimate the area‐average rainfall from the observed fractional area. The average error in the estimation of instantaneous areal rainfall rate for the single thresholds of 1.0 and 2.0 mm h−1 is 18.0% to 20.0% for observed area‐average rainfall rates ⩾0.05 mm h−1. Use of combined thresholds of 1.0 and 2.0 mm h−1 reduces the average error to about 14%. Rainfall accumulations are estimated with greater accuracy than rainfall rates, the accuracy increasing with accumulation period. Five‐day areal‐average accumulations obtained using thresholds of 0.5, 1 and 2 mm h−1 have an average error in the range 8% to 11%. Over this period even a threshold of 0.06 mm h−1 produces a useful estimate, with an average error of 18%.
A comparison between four‐dimensional variational assimilation and simplified sequential assimilation relying on three‐dimensional variational analysis
Authors: Rabier Florence , Courtier Philippe , Pailleux Jean , Talagrand Olivier , Vasiljevic Drasko 
Published: February 2007

AbstractThe aim of this study is to make a strict comparison between two assimilation algorithms, sequential and four‐dimensional variational, on a 24‐hour period extracted from a baroclinic instability situation representative of mid‐latitude dynamics. In the case of linear dynamics, and under the hypothesis of a perfect model, these two four‐dimensional algorithms are known to lead to the same optimal estimate of the atmosphere at the end of the assimilation period, and both methods can be generalized in the nonlinear case. Because the full sequential algorithm is too resource‐demanding to be implemented as such, we shall test the four‐dimensional variational method (4D‐VAR), and a simplified sequential method based on three‐dimensional variational analysis (3D‐VAR), deliberately not exceeding the range of validity of the tangent‐linear model in the experiments. 4D‐VAR is then expected to be almost equivalent to the generalization of the sequential Kalman filter in the nonlinear case, i.e. the extended Kalman filter. As for the simplified sequential algorithm, it can be seen as an approximation of this full extended Kalman filter, for which the forecast error matrices are evaluated only approximately before each analysis, instead of being explicitly computed from the complete dynamical equations. In the four‐dimensional variational scheme, the consistency of the propagation of information with the dynamics is illustrated in an experiment assimilating some localized AIREP data. The large impact which these additional observations have over a large geographical area appears to be very beneficial for the quality of the analysis. Comparing the results of both methods in various configurations, we found that 4D‐VAR systematically behaved substantially better than the simplified sequential algorithm, and had a more accurate analysis at the end of the assimilation period and a much smaller error growth rate in subsequent forecasts. On the one hand, extremely bad specifications of initial forecast errors were found to be detrimental to both algorithms. On the other hand, the four‐dimensional variational algorithm proves to be more robust to the way by which gravity‐wave control is implemented.