bluuebunny/crossref_metadata_2025_split huggingface dataset:
Impact of Aircraft Wind Data On Ecmwf Analyses and Forecasts During the Fgge Period, 8‐19 November 1979
Authors: Baede A. P. M. , Kållberg P. , Uppala S. 
Published: July 2010

AbstractThis paper describes the results of an observing system experiment, set up to assess the impact of aircraft wind data on ECMWF analyses and forecasts. Two parallel data assimilation runs for an 11‐day period during the Global Weather Experiment (FGGE) in November 1979 show that the ECMWF system is able to extract valuable and sometimes essential information from such single‐level data. the impact was particularly significant in the tropics, though limited elsewhere. Especially for the aviation community it is interesting to note that a positive impact of aircraft data was found on both wind analyses and 6‐hour forecasts along flight tracks. On average an improvement of about 1 m s−1 is found for both wind components but locally and in individual cases this may be considerably larger.In order to assess the possible redundancy of aircraft and satellite data outside the tropics, the with‐ and without‐aircraft data assimilations were repeated with satellite winds and satellite temperatures excluded from the data base. This experiment confirmed our suspicion but suggested that the redundancy may be less serious in operational non‐FGGE conditions and may also be a consequence of the particular synoptic situation during the period.From an intercomparison with a similar experiment carried out at the Meteorological Office it is inferred that a multi‐variate scheme with a proper vertical coupling is advantageous in extracting and directing information to the rotational modes of the model.A quality control on the data showed that the reliability of the automatically transmitted aircraft data (ASDAR and AIDS) is high. Therefore the results of this experiment may be unfavourably biased by the more abundant but less reliable conventional aircraft (AIREP) data. This leads us to conclude that a future system of aircraft data, based entirely on ASDAR, will be a valuable contribution to the global observing system.
The Development and Verification of A Cloud Prediction Scheme For the Ecmwf Model
Authors: Slingo J. M. 
Published: July 2010

AbstractThis paper describes the development of a fractional cloud cover scheme which was implemented operationally in the ECMWF medium range forecast model in May 1985. the scheme is based on a diagnostic approach in which cloudiness is related empirically to the large‐scale model variables, including convective activity. an example of the performance of the scheme is given, showing that a fair degree of skill is achieved in forecasting tropical and extratropical cloudiness. an attempt to verify the results using retrieved cloudiness from Nimbus 7 is described. Considerable difficulties were experienced due to the different height classifications of clouds in the model and in the satellite data. the importance of cloud radiative properties as well as cloud amount is considered briefly, based on a comparison of model earth radiation budget diagnostics with similar data from the NOAA polar‐orbiting satellites. the sensitivity of the simulated outgoing long‐wave radiance to changes in the prescribed cloud liquid water content is discussed. Two examples of the effects of cloudradiation interaction on boundary layer processes are described which demonstrate the importance of an integrated approach to the treatment of clouds, radiation and turbulent fluxes.
A Study of the Diurnal Variation of Stratocumulus Using A Multiple Mixed Layer Model
Authors: Turton J. D. , Nicholls S. 
Published: July 2010

AbstractA multiple mixed layer model of the cloud‐topped boundary layer is developed to investigate the diurnal variation of stratocumulus. A simple representation of the microphysical properties of the cloud layer is included which enables high resolution, interactive radiation calculations to be made. A constraint on the buoyancy flux profile is introduced which permits the previously well‐mixed layer to separate into two independently driven layers, thereby avoiding many of the unrealistic aspects of single‐layer models and enabling the model to reproduce features seen in observational studies. the diagnosis of this decoupling, which generally occurs during the morning, is discussed and is shown to have a seasonal and latitudinal sensitivity due to its dependence on cloud layer short‐wave absorption.Comparisons of results with those obtained when separation is not permitted show significant differences. In particular, the cloud layer displays a much enhanced diurnal variation in thickness when separation is allowed, with the minimum occurring in the afternoon. These results are shown to produce much better agreement with observations. the likely consequences of separation for boundary layer evolution are discussed. the surface energy balance can be quite strongly affected, suggesting that it is important to resolve this diurnal variation, especially in areas where stratocumulus is a dominant climatological feature.The sensitivity of the results to model assumptions are investigated and the limitations of the model are also assessed.
A Study of the Short‐Wave Radiative Properties of Marine Stratus: Aircraft Measurements and Model Comparisons
Authors: Hignett P. 
Published: July 2010

AbstractAircraft measurements of the short‐wave albedo of a marine stratus cloud are described and compared with results from radiative transfer models of plane‐parallel clouds. the albedos are measured in the visible and near‐infrared parts of the solar spectrum using clear dome and red dome pyranometers. Using all the available liquid water content data and the measured microphysical properties, two model clouds are constructed to represent the maximum and minimum liquid water paths encountered during the experiment. Model albedo calculations based on these clouds are compared with the range of albedos measured just above cloud top. the natural variation of liquid water path at the cloud edge is exploited as a further qualitative assessment of the models' behaviour.
An Efficient Two‐Time‐Level Semi‐Lagrangian Semi‐Implicit Integration Scheme
Authors: Temperton Clive , Staniforth Andrew 
Published: July 2010

AbstractThe semi‐implicit semi‐Lagrangian integration technique enables numerical weather prediction models to be run with much longer timesteps than permitted by a semi‐implicit Eulerian scheme. the choice of timestep can then be made on the basis of accuracy rather than stability requirements. to realize the full potential of the technique, it is important to maintain second‐order accuracy in time; this has previously been achieved by applying it in the context of a three‐time‐level integration scheme. In this paper we present a two‐time‐level version of the technique which yields the same level of accuracy for half the computational effort. Unlike other efficient two‐time‐level schemes, ours does not rely on operator splitting.We apply this scheme to a variable‐resolution barotropic finite‐element regional model with a minimum gridlength of 100 km, using timesteps of up to three hours. the results are verified against a control run with uniformly high resolution, and are shown to be of similar accuracy to those of a semi‐implicit Eulerian integration with a timestep of 10 minutes.
Angular Momentum Variation In A Translating Cyclone
Authors: Skubis Steven , Molinari John 
Published: July 2010

AbstractAn expression for the time rate of change of relative angular momentum in a moving cyclone is derived which allows a kinematic interpretation of the effects of motion. One term arises from the convergence of flux of momentum by the storm motion vector as the storm moves through an environmental gradient of momentum, and has a counterpart in the equation for the time change of any scalar in a moving volume. A second term, which appears because angular momentum depends upon distance from the axis of rotation, arises purely due to the motion of the axis. This latter term is shown to be proportional to the deviation of the storm motion vector from the basic current, and to make a positive contribution when the storm moves to the left of the mean flow.The Lagrangian terms were calculated for an observed rapidly moving hurricane about to make landfall in a baroclinic environment. Their overall contribution was negative and equal in magnitude to the largest terms in the Eulerian momentum budget, indicating that the rapid observed decay of the storm was influenced not only by its landfall but also by unfavourable environmental interactions as the storm moved into a region of reduced cyclonic momentum, the results suggest that the effects of translation cannot be neglected for a vortex which is rapidly moving or embedded in a non‐uniform environment.
Diurnal Variations In Flow Over A Succession of Ridges and Valleys
Authors: Mason P. J. 
Published: July 2010

AbstractObservations of flow in a nearly two‐dimensional valley located within a succession of similar ridges and valleys are presented. Measurements of the mean flow have been made at all times of the day and are found to be well characterized by a suitably defined Froude number. the unstable and neutral daytime flow shows a flow separation whilst when the Froude number has a value of about two (usually around evening and dawn transitions) the flow is attached and the valley well ventilated. For smaller values of Froude number the flow speed in the valley reduces and a characteristic ‘drainage’ flow occurs. In addition to mean flow measurements and soundings of temperature structure, measurements were also made of turbulence energy and stress. This allows an estimate of the effective value of the surface roughness, zo, and an indication of the stability dependence of the momentum transfer to be made.
A Model of Drizzle Growth In Warm, Turbulent, Stratiform Clouds
Authors: Nicholls S. 
Published: July 2010

AbstractA model to describe the growth of precipitation sized drops in layers of warm, stratiform cloud is developed which combines the effects of stochastic turbulent diffusion with explicit microphysical calculations. the representation of diffusion is consistent with measured turbulence statistics, reproduces standard results and is used to define an ensemble of Lagrangian trajectories within which the effects of phase change, stochastic coalescence and vertical exchange due to gravitational settling can be calculated. Mean drop size distributions are predicted as functions of height within the cloud.The inclusion of realistic levels of turbulence is found to produce greatly increased numbers of larger (r >100 μm) drops throughout the depth of the cloud. the modelled steady‐state drop size distributions show good agreement with corresponding aircraft measurements in a case study. the effects of changing the turbulence intensity, cloud depth and liquid water content on the distribution of larger drops are investigated and implications for the cloud layer water balance and the cloud radiative properties are also considered.
Wave Structure and Evolution In Baroclinic Flow Regimes
Authors: Weng H.‐Y. , Barcilon A. 
Published: July 2010

AbstractA uniform potential vorticity model, modified by two Ekman layers of different strengths, is used to study analytically and numerically the structure and evolution of strongly nonlinear, baroclinic waves. the model consists of a single zonal wave with its lowest two meridional modes, and of the mean flow correction with its lowest four meridional modes. Each meridional mode consists of a baroclinic and a barotropic pattern.Travelling steady waves, amplitude vacillation and structural vacillation are found in the model. Steady waves are energetically balanced between baroclinicity and Ekman dissipation through nonlinearity. In a typical amplitude vacillation, the wave potential energy vacillates with time, via the interference of nonlinear barotropic and baroclinic patterns of the lowest meridional mode of the wave. In a typical structural vacillation, the distribution of wave kinetic energy in the meridional direction vacillates with time, via interference of the lowest two nonlinear meridional modes of the wave. the results are qualitatively consistent with those observed in annulus experiments. the mechanisms of amplitude and structural vacillations may be responsible for blocking in middle and high latitudes.
Lateral Boundaries and the Heat‐Induced Flow In the Tropical Atmosphere
Authors: Phlips P. J. , Gill A. E. 
Published: July 2010

AbstractThe effect on the heat‐induced circulation of lateral vertical boundaries is studied by means of a simple model. the basic state in the absence of boundaries is a Hadley circulation driven by a narrow band of heating at a specified latitude. Lateral friction is included in the model. This has little effect on the meridional circulation but smooths out the zonal flow. the eastern boundary modifies the Hadley flow through planetary waves needed to make the flow at the boundary meridional, and the flow pattern is similar to that of the east Pacific trades. the western boundary generates a Kelvin wave in order to make the total flux normal to the boundary vanish. This can give rise to patterns similar to those encountered in the Indian Ocean.
Variational Assimilation of Meteorological Observations With the Adjoint Vorticity Equation. I: Theory
Authors: Talagrand Olivier , Courtier Philippe 
Published: July 2010

AbstractThe following variational approach is taken to the problem of assimilation of meteorological observations: find the solution of the assimilating model which minimizes a given scalar function measuring the ‘distance’ between a model solution and the available observations. It is shown how the ‘adjoint equations’ of the model can be used to compute explicitly the ‘gradient’ of the distance function with respect to the model's initial conditions. the computation of one gradient requires one forward integration of the full model equations over the time interval on which the observations are available, followed by one backward integration of the adjoint equations. Successive gradients thus computed are introduced into a descent algorithm in order to determine the initial conditions which define the minimizing model solution.The theory is applied to the vorticity equation. Successful numerical experiments performed on a Haurwitz wave are described.
Comparison of Southern Hemisphere Radiosonde and Lims Temperatures At 100 Mb
Authors: Miles T. , Grose W. L. , Russell J. M. , Remsberg E. E. 
Published: July 2010

AbstractRadiosonde and satellite‐derived 100 mb temperatures over New Zealand at 12 GMT are compared for the 1978‐79 summer. the satellite measurements are made by the Nimbus‐7 Limb Infrared Monitor of the Stratosphere (LIMS) instrument. the co‐located LIMS temperature information consists of synoptically‐mapped values (for 12 GMT), as well as the primary nighttime orbital retrievals valid at ∼1030 GMT.The radiosonde time series of temperature is dominated by temporal fluctuations associated mainly with the eastward passage of waves which have characteristic periods of 4‐5 and 11‐12 days and peak‐to‐peak amplitudes of 10‐15 K. In agreement with previous (northern hemisphere) 100 mb LIMS validation studies, the New Zealand orbital LIMS temperatures for 100 mb compare quite favourably with the ‘ground‐truth’ radiosonde measurements (r.m.s. difference of ∼1.6K). the corresponding LIMS‐mapped temperature time series is also found to exhibit quite close agreement, in terms of temporal phase, with the radiosonde information. However, it is shown that the LIMS‐mapped temperature fluctuations suffer from a noticeable attenuation in amplitude (approaching 50% for higher frequency fluctuations), which will therefore impact on the accuracy of LIMS‐derived estimates of dynamical quantities such as wind velocity and relative vorticity in the lower stratosphere.
Uncertainty dynamics and predictability in chaotic systems
Authors: Smith L. A. , Ziehmann C. , Fraedrich K. 
Published: July 2010

AbstractAn initial uncertainty in the state of a chaotic system is expected to grow even under a perfect model; the dynamics of this uncertainty during the early stages of its evolution are investigated. A variety of ‘error growth’ statistics are contrasted, illustrating their relative strengths when applied to chaotic systems, all within a perfect‐model scenario. A procedure is introduced which can establish the existence of regions of a strange attractor within which all infinitesimal uncertainties decrease with time. It is proven that such regions exist in the Lorenz attractor, and a number of previous numerical observations are interpreted in the light of this result; similar regions of decreasing uncertainty exist in the Ikeda attractor. It is proven that no such regions exist in either the Rössler system or the Moore‐Spiegel system. Numerically, strange attractors in each of these systems are observed to sample regions of state space where the Jacobians have eigenvalues with negative real parts, yet when the Jacobians are not normal matrices this does not guarantee that uncertainties will decrease. Discussions of predictability often focus on the evolution of infinitesimal uncertainties; clearly, as long as an uncertainty remains infinitesimal it cannot pose a limit to predictability. to reflect realistic boundaries, any proposed ‘limit of predictability’ must be defined with respect to the nonlinear behaviour of perfect ensembles. Such limits may vary significantly with the initial state of the system, the accuracy of the observations, and the aim of the forecaster. Perfect‐model analogues of operational weather forecasting ensemble schemes with finite initial uncertainties are contrasted both with perfect ensembles and uncertainty statistics based upon the dynamics infinitesimal uncertainties.
Observations of subtropical air in the european mid‐latitude lower stratosphere
Authors: O'Connor F. M. , Vaughan G. , De Backer H. 
Published: July 2010

AbstractEvidence of two wave‐breaking events, during which subtropical air was transported into the northern mid latitudes, is presented. the first event took place in January 1997 when ozone mixing ratios of the order of 120 parts per billion (109) by volume (p.p.b.v.) were measured in the lower stratosphere above Aberystwyth (52.4°N, 4.1°W). the amount of subtropical air involved was 13 × 1014 kg. A second, more dramatic, wavebreaking event took place in March 1997 involving 36 × 1014 kg of air. This case contained a region with potential vorticity ⩽ 2 potential vorticity units (1 PVU=1 × 10‐6 K kg‐1m2s‐1), consistent with ozone mixing ratios of 80 p.p.b.v. observed at Aberystwyth at 365 K. Trajectory analysis showed that this air was advected northwards from near the subtropical tropopause. the extent to which irreversible transport into the mid‐latitude lower stratosphere followed both events is evaluated using trajectory analysis and potential‐vorticity evolution.
Parametrization of effective sizes of cirrus‐cloud particles and its verification against observations
Authors: Sun Zhian , Rikus Lawrie 
Published: July 2010

AbstractA simplified parametrization of effective sizes of cirrus‐cloud particles is developed in terms of the ice‐water content and the cloud temperature based on some earlier studies. the effective particle sizes of the cirrus clouds determined using the simplified parametrization are compared with the results determined from observations. the results are generally encouraging.The simplified parametrization is further tested by incorporating it into a new ice‐cloud optical‐property parametrization scheme and performing radiative‐transfer calculations. the results show that this scheme in conjunction with the ice‐cloud optical‐properties scheme can generate a reasonably good estimate of cirrus‐cloud radiative properties compared with the observations.
Cloud‐to‐ground lightning activity and radar observations of storms in the pyrénées range area
Authors: Molinie G. , Soula S. , Chauzy S. 
Published: July 2010

AbstractThis paper analyses radar observations and cloud‐to‐ground (CG) lightning flash activity of several thunderstorms occurring over the Pyrénées range during the night of 23‐24 August 1993 and the afternoon of 24 August 1993. the available data consisted of the European meteorological bulletin, the plan position indicator radar scans at three elevations performed by the 5.5 cm wavelength Rodin radar of Toulouse, and the characteristics of the CG lightning strikes.The first thunderclouds began to develop at around 2200 UTC 23 August. Up to 0100 UTC 24 August, we observed three isolated convective cells. We also studied a mesoscale convective system (MCS) occurring during the afternoon of 24 August, and chose to investigate three convective cells embedded within it. These different thunderclouds developed over the Spanish side of the Pyrénées and moved north‐eastwards over the mountains. the locations of CG flashes in relation to the reflectivity fields revealed two kinds of CG lightning activity: flashes associated with intense rainfall, and those located in stratiform cloud areas far from the intense precipitating zone. the flash density was larger within the first type and the positive flash proportion was larger in the second type. the amount of large precipitating particles was characterized by the area A(τ) covered by reflectivities larger than τ dBZ, with τ taken as 33 dBZ. the study of the time evolution of A (33) at three altitudes showed that, for the lightning activity associated with intense rainfall, strong reflectivities at high altitude (4.5 km, and sometimes 7.5 km) coincided with large lightning flash rates, though it was the time evolution of the vertical profile of the area covered by precipitation that drove the CG lightning flash rate (or vice‐versa). the maximum flash rate coincided with the fall of large precipitating particles. Furthermore, a time lag was observed at the ground between the lightning flash rate and the rainfall rate. the time evolutions of the flash rate and A (τ) at the lowest elevation of the radar beam (0.6°) confirmed this delay. the correlation coefficient between these two parameters for the isolated convective cells reached 0.81 in the best case. the correlation between the lightning activity and precipitation of the whole MCS was less good but, nevertheless, the volume of precipitation per CG lightning flash was of the same order of magnitude for all the cells or MCSs observed. It ranged from 3.2 × 103 to 46.8 × 103 m3 flash‐1, in accordance with the results of other authors.