bluuebunny/crossref_metadata_2025_split huggingface dataset:
Ensemble‐based targeting experiments during FASTEX: The effect of dropsonde data from the lear jet
Authors: Szunyogh I. , Toth Z. , Emanuel K. A. , Bishop C. H. , Snyder C. , Morss R. E. , Woolen J. , Marchok T. 
Published: February 2007

AbstractIn this study we evaluate the performance of the Ensemble Transform (ET) technique, which is one of several targeting methods used in real time during the Fronts and Atlantic Storm‐Track EXperiment (FASTEX). ‘Targeted’ observations were taken adaptively in those upstream areas identified in real time as most relevant for improving the initial conditions for forecasts of synoptic‐scale storms developing downstream. the upstream areas were identified as regions where the effect of extra observations at a future analysis time could produce the largest decrease in the largest likely forecast error at a preselected later verification time at a given downstream location. the ET technique selects these observational areas out of a large number of possible deployment locations of observational resources via a linear transformation of an ensemble of forecasts.The analysis and forecast effects of special targeted observations associated with seven Intensive Observing Periods (IOPs) during FASTEX were investigated. the most important result of the present study is that the ET technique, based on the National Centers for Environmental Prediction (NCEP) operational global ensemble, was able to identify upstream areas that had significant contribution to the quality of selected future downstream forecast features. Moreover, the technique could reliably distinguish between the areas of greatest contribution.Though the overall impact of the targeted data on forecast quality is positive, there were cases when the extra data degraded the forecasts. Our analysis indicates that large amplification of ensemble perturbations from the targeted area into the verification area is a good indicator of potential forecast improvement.
Adaptive observations during FASTEX: A systematic survey of upstream flights
Authors: Bergot Thierry 
Published: February 2007

AbstractThe effectiveness of the recent concept of adaptive observations is tested on 20 cases for which Fronts and Atlantic Storm‐Track EXperiment (FASTEX) upstream dropsondes were available.In the first stage, the influence of the targeted data is tested without taking into account the real evolution of the atmosphere. On average, the inclusion of these targeted data significantly influences the forecasts (mean impact of about 9 hPa at 36 h). However, a large spread of impacts is observed and at 36 h (close to the mean verifying time) the mean impact is about 7.8 hPa for National Center for Atmospheric Research Learjet 36 (L36) flights and 10.8 hPa for National Oceanic and Atmospheric Administration Gulfstream IV (GIV) flights, with different behaviours. These tests also indicate that the targeted observations may indirectly affect the mediumrange forecast of the subsequent lows. the redundancy of targeted data with respect to conventional no‐FASTEX data (defined as the difference in impact between forecasts with and without conventional data), is about 40% for L36 flights and 14% for GIV flights, at 36 h. These differences between L36 and GIV flights (for the temporal characteristics of the impact signals and the redundancy) might be due either to the location of the data (L36 data were closer to the observationally dense North American continent), or to the difference between targeting techniques used to guide the planes. It seems also that adaptive observations are clearly constrained by the conventional observing network, and the sampling strategy used is also a crucial problem to solve.In the second stage, the skill of forecasts with targeted dropsondes is evaluated. the inclusion of these targeted data leads to a mixture of improvement and degradation cases, the mean improvement is very weak and has the same order of magnitude as the verifying‐analysis errors. However, a study of cases suggests that the adaptive‐observation effectiveness is high when the predictability is weak. Otherwise, the assimilation system seems unable to reduce the errors in the most unstable directions, and these deficiencies blur the impact of the targeted data. This suggests that the success of adaptive observations also depends on the assimilation system used.
As assessment of the singular‐vector approach to targeted observing using the FASTEX dataset
Authors: Gelaro R. , Langland R. H. , Rohaly G. D. , Rosmond T. E. 
Published: February 2007

AbstractIn this study, we investigate whether the results of assimilating special targeted observations from the Fronts and Atlantic Storm‐Track EXperiment (FASTEX) in an operational forecast model support the underlying principles of the singular‐vector (SV) approach to targeted observing. A simple framework is presented that allows explicit examination of the changes made to the analysis in the subspace of the leading SVs from assimilation of the observations. the impact of this component on the forecast provides a key measure of the effectiveness of SV‐based targeting.Results confirm that the impact of the additional observations occurs primarily as a result of changes to the analysis in the subspace of the leading SVs. These changes account for a small fraction of the total targeting increment at initial time, but explain a large fraction of the response of the forecast at the verification time. the results also confirm that analysis errors in the middle and lower troposphere are an important source of error in forecasts of extratropical cyclones.While moist processes can play an important role in the forecast‐error evolution, SVs that exclude these processes can remain an effective targeting tool. This is because the location of maximum sensitivity will not necessarily differ from that identified by the dry SVs. It is also shown that the locations of the leading (target) SVs can be computed accurately with lead times of up to 48 hours, allowing ample time for the deployment of observational resources.
Four‐dimensional variational analyses of FASTEX situations using special observations
Authors: Desroziers Gerald , Pouponneau Béatrice , Thépaut Jean‐Noel , Janisková Marta , Veersé Fabrice 
Published: February 2007

AbstractThis paper focuses on Intensive Observation Period 17 of the Fronts and Atlantic Storm‐Track EXperiment (FASTEX): the FASTEX special soundings obtained during that period are added to the conventional dataset and assimilated with a 4‐dimensional variational (4D‐Var) scheme under development at Météo‐France. Results show a consistent use of the high density FASTEX ship soundings by the 4D‐Var formulation and an improvement brought by this new scheme over 3D‐Var when using only the vertical profiles synchronous with synoptic time. the application of the 4D‐Var analysis to dropsondes launched by a system‐relative flight designed by the Joint Centre for Mesoscale Meteorology also produces a fine‐scale description of sub‐structures of the mature system.
Pacific and atlantic ‘bomb‐like’ deepenings in mature phase: A comparative study
Authors: Lemaǐtre Y. , Protat A. , Bouniol D. 
Published: February 2007

AbstractThe mesoscale dynamics of a secondary cyclone sampled during the field phase of the Fronts and Atlantic Storm‐Track Experiment (FASTEX) are documented using airborne Doppler radar data. the high quality of the mesoscale airborne Doppler dataset collected within this secondary cyclone is highlighted. This study demonstrates, however, the need to combine in the near future airborne Doppler radar and dropsonde measurements, in order to access multiscale processes involved in the mature stage of this FASTEX cyclone both in clear‐air and in precipitating regions. the studied secondary cyclone developed on the trailing cold front of a parent low situated over Greenland, and underwent the most explosive deepening (roughly −54 mb in 24 hours) sampled in the multiscale sampling area during FASTEX. the main flows responsible for the mesoscale organization of the cyclone are: a warm airflow emerging from the base of the synoptic warm conveyor belt that generates the most active part of the cloud head; a dry intrusion approaching in the dry slot the cyclone centre; a cold conveyor belt that contributes to the upward mass transport towards the upper part of the polar‐front cloud band; and a cold northerly flow in the western part of the cloud head, which tends to wrap around the low centre and is likely to release convective instability through air‐sea interactions. This study also documents a slight shift in position between the high vertical‐vorticity core associated with the active part of the cloud head and the location of the pressure minimum.The detailed three‐dimensional structure of this Atlantic secondary cyclone is also compared with that gathered within another ‘bomb‐like’ deepening (defined as more than a 24 mb surface pressure drop in 24 hours) observed over the Pacific ocean during the Coastal Observation and Simulation with Topography (COAST) experiment. Some similarities are found. the main differences appear to be linked to the warm seclusion that is exclusive to the studied Pacific cyclone. More pronounced ascending motions are found within the cloud head of the Pacific ‘reference’ case, and the deepening results from a more constructive effect of geostrophic and non‐geostrophic processes than for the Atlantic ‘reference’ case.
Mesoscale dynamics of a deepening secondary cyclone in FASTEX IOP16: Three‐dimensional structure retrieved from dropsonde data
Authors: Bouniol D. , Protat A. , Lemaǐtre Y. 
Published: February 2007

AbstractThe mesoscale dynamics of a secondary cyclone sampled during the field phase of the Fronts and Atlantic Storm‐Track EXperiment (FASTEX) Intensive Observation Period (IOP) 16 are documented using airborne dropsonde data. the method proposed to recover the mesoscale analytical three‐dimensional (3‐D) dynamic fields from the dropsonde measurements is evaluated using simulated fields, airborne in situ measurements, and airborne Doppler radar measurements. These simulations and comparisons indicate that the method is able to resolve the mesoscale structures within the studied secondary cyclone both in the regions of clear air and precipitation. It appears, however, that the most accurate description of the 3‐D dynamic fields in both these regions would be obtained by combining Doppler and dropsonde measurements.The studied secondary cyclone is a fast moving frontal wave that developed on the trailing cold front of a parent low situated over Greenland. Using the dropsonde‐derived 3‐D fields, the mesoscale flows involved in the organization of the secondary cyclone are described. the potential‐vorticity and equivalent‐potential‐vorticity fields (deduced from the basic analytical fields) indicate that the atmosphere is roughly neutral to moist slantwise instability. This configuration is known to significantly promote frontogenesis. the vertical configuration of the jets and circulations within this secondary cyclone are compared with the coupling‐uncoupling scheme of Shapiro. A slight coupling is found between the thermally indirect circulation and the ageostrophic circulation. the obtained coupling is, however, not as developed as in the ideal case described by Shapiro, which may explain the small upward velocities in our case.Airborne Doppler radar and dropsonde measurements are combined so as to infer the multi‐scale processes involved in the mesoscale and convective‐scale organization of the Atlantic secondary cyclones and to evaluate the new emerging theoretical interpretations of secondary cyclogenesis.
Intensification of tropical‐cyclone‐like vortices in uniform zonal background‐flows
Authors: Dengler K. , Keyser D. 
Published: February 2007

AbstractThe effect of uniform zonal background‐flows on the intensification of tropical‐cyclone‐like vortices is investigated using a numerical three‐layer shallow‐water model that includes parametrizations of convection, sea‐surface energy‐exchange, and boundary‐layer friction.Calculations on an f ‐plane showed that during the developing stage, an initially weak vortex intensified more rapidly in the presence of a uniform zonal background‐flow. Compared to an environment at rest, the uniform zonal background‐flow resulted in increased boundary‐layer convergence collocated with increased boundary‐layer mixing ratio (as a result of surface moisture fluxes), producing stronger convection and more rapid intensification. After the vortex achieved hurricane strength in the presence of an easterly (westerly) background‐flow, a region of convectively stable air formed to the south (north) of the vortex centre as a result of small values of surface moisture flux and the downward transport of relatively dry middle‐layer air. This convectively stable air penetrated slowly into regions of boundary‐layer convergence, thereby weakening convection and reducing the maximum intensity of the vortex compared to the case without background flow. This effect was more pronounced for stronger background‐flows.Calculations on a β‐plane showed that a westerly background‐flow is more favourable for intensification than an easterly background‐flow of the same strength. In the former, the vortex always moved into a region of convectively unstable air, whereas, in the latter, the vortex progressively moved into a region of convectively stable air produced by subsidence and drying of the boundary layer. Because of the relatively high vortex drift speed in the easterly background‐flow case, there was not sufficient time for surface moisture fluxes to moisten the boundary‐layer air rapidly enough to eliminate the convectively stable region ahead of the vortex, resulting in the overall weakening of convective activity and a less intense storm than in the westerly flow case.The foregoing calculations suggest that the imposed background‐flow affects the development of the vortices in the shallow‐water model by establishing an environment in which boundary‐layer convergence and surface moisture fluxes may interact to produce stratifications that either enhance or suppress convection, the occurrence of which is necessary for the intensification of the vortices in the model.
Nonlinear equatorial Kelvin waves and CISK. I: Small‐amplitude approximation and the trailing edge of a cloud region
Authors: Cho Han‐Ru 
Published: February 2007

AbstractKelvin wave‐CISK (Conditional Instability of the Second Kind) is studied as a possible explanation for the intraseasonal oscillations of the equatorial troposphere. The small‐amplitude approximation will be made in part I of this paper. The governing equations of the small‐amplitude approximation are nonlinear, however, because of the cumulus parametrization scheme used. It was found that: (1) Kelvin waves when coupled with cumulus healing are never unstable if the cumulus heating rate is smaller than the adiabatic cooling rate; (2) the solution is not equatorially confined if the cumulus heating rate is larger than the adiabatic cooling rate; (3) since a Kelvin wave is not a dispersive wave, the method of characteristics is used to solve the governing equations when the cumulus heating rate is smaller than the adiabatic cooling rate. It was found that the cumulus parametrization scheme often used in theoretical studies of CISK is incompatible with the so called 'jump condition' of the generalized solutions often required by hyperbolic systems of equations.
The use of TOVS level‐1b radiances in the NCEP SSI analysis system
Authors: McNally A. P. , Derber J. C. , Wu W. , Katz B. B. 
Published: February 2007

AbstractA methodology is described for the direct assimilation of TOVS (TIROS‐N (Television Infra‐Red Observation Satellite) Operational Vertical Sounder) radiance data that have not undergone the pre‐processing (such as limb adjustment and cloud clearing) that is usually applied to make radiances suitable for use in linear retrieval algorithms. Inherent nonlinearities in the observations can be treated successfully within a variations analysis scheme such as Spectral Statistical Interpolation (SSI) thus making radiance pre‐processing unnecessary. The benefit of using unprocessed (or level‐lb) data is demonstrated in terms of an improved ability to quality control and bias correct the observations within the analysis. The assimilation of level‐1b radiance data in the SSI has shown a significant positive impact on the quality of the analysis compared with the use of pre‐processed radiances, and a significant improvement in forecast skill for the tropics and southern hemisphere.
Use of a neural‐network‐based long‐wave radiative‐transfer scheme in the ECMWF atmospheric model
Authors: Chevallier F. , Morcrette J.‐J. , Chéruy F. , Scott N. A. 
Published: February 2007

AbstractThe definition of an approach for radiative‐transfer modelling that would enable computation limes suitable for climate studies and a satisfactory accuracy, has proved to be a challenge for modellers. A fast radiative‐transfer model is tested at ECMWF: NeuroFlux. It is based on an artificial neural‐network technique used in conjunction with a classical cloud approximation (the multilayer grey‐body model). The accuracy of (he method is assessed through code‐by‐code comparisons, climate simulations and ten‐day forecasts with the ECMWF model. The accuracy of NeuroFlux appears to be comparable to the accuracy of the ECMWF operational scheme, with a negligible impact on the simulations, while its computing time is seven times faster.
TIGR‐like atmospheric‐profile databases for accurate radiative‐flux computation
Authors: Chevallier F. , Chédin A. , Cheruy F. , Morcrette J.‐J. 
Published: February 2007

AbstractThis note summarizes the characteristics of different topological methods for sampling vertical profiles of heterogeneous variables, like atmospheric temperature and water‐vapour concentration. The methods presented follow the approach developed for the successive Thermodynamic Initial Guess Retrieval (TIGR) databases at Laboratoire de Météorologie Dynamique. The most recent one is applied to select limited numbers of profiles (some thousands) by the sampling of a much larger dataset (more than one million profiles) from the European Centre for Medium‐Range Weather Forecasts short‐range forecasts. The sampled datasets are then used for (raining a neural‐net work‐based radiative‐flux profile computation model (NeuroFlux).
Three‐dimensional modelling of ice accretion density
Authors: Szilder Krzysztof , Lozowski Edward P. 
Published: February 2007

AbstractA three‐dimensional, random‐walk, ballistic model is used to simulate the density and structure of an ice accretion formed by impinging supercooled droplets. The model is calibrated using experimental observations of the ice density at the stagnation line on a fixed circular cylinder. A relationship has been established between the Macklin parameter, used commonly to relate atmospheric conditions with ice accretion density, and model parameters. The model is applicable over a wide range of growth conditions, including, in the limits, growth where the droplets freeze on impact and growth where the droplets spread over and percolate into the existing ice structure.
Evolution of disturbances and singular vectors in the shallow‐water semi‐geostrophic model
Authors: Ren Shuzhan 
Published: February 2007

AbstractThe first part of this work is devoted to exploring the instability mechanism of the shallow‐water semi‐geostrophic model by investigating the evolution of disturbances under constant‐shear and cosine‐type basic flows. For the constant‐shear basic‐flow case, the wave‐packet analysis method is applied to study the evolution of potential and total energy and the meridional structure of disturbances under me assumption of a spatially slowly varying basic state. The results are found to be in good agreement with numerical calculations even when the assumption is slightly relaxed. For the cosine‐type basic‐flow case, both unstable normal modes and continuous spectra exist if the stability criterion is violated. Numerical results show that the relative importance of regular normal‐mode and continuous‐spectrum disturbances in the evolution process depends on the configuration of initial disturbances and their projections onto the normal modes and continuous spectra.In the second part of this work, in order to search for initial disturbances based on which the growth of certain norms of disturbances can be optimized, singular vectors of different norms (metrics) are computed. It is found that of all the norms, the structures of singular vectors are mainly determined by the configuration of the zonal basic flow and less affected by model parameters such as the Rossby number, rotating Froude number, and amplitude of topography. However, when the shear of the basic flow is strong, the optimal singular value can be sensitive to the model parameters. When the stability criterion is violated, the optimal singular values of all the norms have clear wave‐number preferences. The peaks of spectra of singular values of the energy‐type norms, including potential, kinetic and total‐energy norms, correspond to very weak normal‐mode instability. However, the optimal singular value of the enstrophy norm has almost the same wave‐number preference as unstable normal modes.
A parametrization of the effects of cloud and precipitation overlap for use in general‐circulation models
Authors: Jakob Christian , Klein Stephen A. 
Published: February 2007

AbstractThe necessity for treating the effects of vertically varying cloud fraction when parametrizing microphysical processes in general‐circulation models (GCMs) was recently highlighted by Jakob and Klein. In this study a parametrization to include such effects in a GCM is developed, and the new scheme is applied in the ECMWF global model. The basic idea of the new scheme is to separate the model's rain and snow fluxes into a cloudy and a clear‐sky part. The scheme is tested using the subgrid‐scale precipitation model of Jakob and Klein as a benchmark. The impact of the new scheme on the model climate is also investigated.It is shown that the new parametrization leads to a better representation of the effects of cloud and precipitation overlap, and that it alleviates most of the problems connected with their treatment in the current scheme. Due to the better treatment of cloud and precipitation overlap the new parametrization leads to a reduction in precipitation evaporation and an increase in accretion rates. When tested in seasonal model integrations the new scheme produces a drier tropical mid‐troposphere with consequences for the hydrological cycle.