bluuebunny/crossref_metadata_2025_split huggingface dataset:
Predictability and the relationship between subseasonal and interannual variability during the Asian summer monsoon
Authors: Sperber Kenneth R. , Slingo Julia M. , Annamalai H. 
Published: February 2007

AbstractThe relationship between subseasonal and interannual variability of the Asian summer monsoon has been investigated through analysis of the dominant modes of variability in the 40‐year NCEP/NCAR Re‐analysis, with complementary satellite and surface‐based precipitation data. The hypothesis that the characteristics of monsoon subseasonal variability (i.e. weather regimes) are modulated on interannual time‐scales in a systematic and therefore predictable manner has been tested. The null hypothesis is that predictability of the seasonal mean monsoon behaviour requires that the effects of the slowly varying components of the climate system be correctly simulated.An interannual mode of monsoon variability has been identified which is closely related to the observed seasonal mean all‐India Rainfall (AIR). A counterpart of this mode has also been identified at subseasonal time‐scales which projects strongly on to the daily AIR, confirming that a common mode of monsoon variability exists on sub‐seasonal and interannual time‐scales. It has been shown that the temporal behaviour of this subseasonal mode, as described by the probability distribution function (PDF) of the principal component time series, does not show any evidence of bimodality, the shape of the PDF being Gaussian. Further, it has been shown that the mean of the PDF is systematically and significantly perturbed towards negative (positive) values in weak (strong) monsoon years as categorized in terms of the seasonal mean AIR. This translation in the mean of the PDF, rather than a change in shape of the PDF, suggests that anomalous monsoons are not associated with changes in weather regimes. Further analysis has confirmed that low‐frequency modulation of the basic state is primarily responsible for these shifts in the subseasonal PDFs, supporting the null hypothesis that predictability of the seasonal mean monsoon requires that the effects of the slowly varying components of the climate system be correctly simulated. Thus, model improvements to reduce systematic errors in the mean simulation and the response to low‐frequency boundary forcing may improve the prospects for dynamical seasonal prediction.However, the results indicate that only a subset of the subseasonal modes are systematically perturbed either by the El Nino Southern Oscillation or in weak vs. strong monsoon years, suggesting that predictability is likely to be limited by the chaotic, internal variability of the monsoon system.
Initialization of a fine‐scale model for convective‐system prediction: A case study
Authors: Ducrocq Véronique , Lapore Jean‐Philippe , Redelsperger Jean‐Luc , Orain Françoise 
Published: February 2007

AbstractThis paper focuses on the problem of the initialization of a fine‐scale (2.5 km resolution) model with observational data. An approach combining optimal interpolation (OI) analysis and bogussing methods was designed. The information provided by surface mesonet observations (relative humidity, temperature and winds) is incorporated by means of a mesoscale (OI) analysis. Simple cloud and precipitation analyses based on conventional radar reflectivities and infrared satellite data help to adjust the humidity and hydrometeor fields of the initial state. With this initialization procedure the resulting initial state describes the signature of mesoscale convective systems.This technique is applied to one observed mesoscale convective system to produce an initial state describing a one‐hour‐old developing convective system. From this initial state, the mesoscale nonhydrostatic model, named MESO‐NH, succeeds in simulating a convective line organization which is close to observations. Experiments to determine the sensitivity to the initial state definition and to the model characteristics are carried out and the initialization procedure is found to be instrumental in a successful simulation; both the surface‐data analysis and the cloud analysis are required.
Microphysical and radiative properties of inhomogeneous stratocumulus: Observations and model simulations
Authors: Los A. , Duynkerke P. G. 
Published: February 2007

AbstractDetailed microphysical observations made from aircraft during the Atlantic Stratocumulus Transition Experiment are used to examine the optical properties of marine stratocumulus. Special attention is given to the relationship between the droplet size distribution (droplet‐number concentration, effective radius, and liquid‐water content) and the optical extinction parameter. The extinction parameter calculated with measurements from the forward‐scattering spectrometer probe (FSSP) differs only slightly from the extinction parameter obtained from the total droplet spectrum (measured with two optical probes, FSSP and 260X). In contrast, the results for the effective radius and the liquid‐water content show differences up to 40% between the two spectra. A Monte Carlo radiative‐transfer model is initialized with optical properties calculated from the observed microphysics. Monte Carlo simulations of radiative fluxes are in good agreement with observations for well‐mixed boundary‐layer cloud fields.
The impact of mesoscale convective‐system potential‐vorticity anomalies on numerical‐weather‐prediction forecasts
Authors: Gray M. E. B. 
Published: February 2007

AbstractThe effect of the potential vorticity (PV) anomalies formed by mesoscale convective systems (MCSs) on the downstream flow is investigated using a numerical weather prediction (NWP) model. Four case studies of North American MCSs from June and July 1999 are selected in which the MCS PV anomalies are not present in the model analysis. Idealized MCS PV anomalies are introduced into the model analyses and their impact on the NWP model forecast ascertained.In two of the four cases the presence of the MCS PV anomalies leads to significant perturbation in the forecast evolution. Sensitivity studies show that it is the presence of the mid‐level cyclonic vortex (MCV) that has the biggest impact on the NWP forecast. This positive PV anomaly is shown to cause areas of ascent and descent, as has been postulated following other studies. This can lead to the development of a growing mesoscale disturbance if the MCV becomes situated in a strong baroclinic gradient. These disturbances are shown to have direct and indirect impacts on the depths of synoptic‐scale depressions.The presence of the MCS PV anomalies leads to some reduction in the forecast errors. It is suggested that attempting to assimilate the MCS PV anomalies into the NWP model analysis may be of some benefit to forecast accuracy. However, uncertainties in the properties of the MCV may mean that a more practical application may be in perturbing the initial states of ensemble forecasts.
Pressure‐potential‐temperature covariance in convection with rotation
Authors: Mironov Dmitrii V. 
Published: February 2007

AbstractThe pressure‐potential‐temperature covariance in free and rotating turbulent convection with no mean velocity shear is analysed, using a dataset generated with a large‐eddy simulation (LES) model. The pressure field is resolved into turbulence‐turbulence, buoyancy and Coriolis components, and the contributions from these components to the pressure‐gradient‐potential‐temperature covariance in the budget equation for the potential‐temperature flux are examined.In non‐rotating convection, the buoyancy contribution compensates for about one half of the buoyant production term in the flux budget equation, and the turbulence‐turbulence contribution is well approximated by the Rotta‐type return‐to‐isotropy model with the relaxation time‐scale set proportional to the turbulence energy dissipation time‐scale.In convection with rotation, neither the simplest Rotta‐type model with the relaxation time‐scale proportional to the energy dissipation time‐scale nor the more sophisticated two‐component‐limit (TCL) nonlinear model are able to accurately describe the LES data. A somewhat better agreement is found when a limitation is imposed on the relaxation time‐scale due to the background rotation. The simplest model for the buoyancy contribution, where it is set proportional to the buoyant production term in the flux budget equation, fares poorly. The TCL model shows better agreement with LES data although some uncertainties remain. However, the relative importance of the buoyancy contribution to the pressure‐gradient‐potential‐temperature covariance decreases with increasing rotation rate.In contrast, the Coriolis contribution becomes more important as the rotation rate increases. Neither the simplest linear model for the Coriolis contribution nor the much more complex nonlinear TCL model are found to be adequate. Neither model appropriately accounts for the component of the angular velocity of rotation that is parallel to the component of the pressure gradient in question. In the seemingly simplest case considered in the present paper, when the rotation vector is aligned with the vector of gravity, no Coriolis contribution to the vertical‐pressure‐gradient‐potential‐temperature covariance is predicted by these models. This results in a strong underestimation of the pressure term in the flux budget equation and may lead to an erroneous prediction of the vertical potential‐temperature flux in convection with rotation. In an attempt to remedy the situation, an extension of the TCL model that contains only one extra empirical coefficient is developed and checked against LES data.
Salmon's Hamiltonian approach to balanced flow applied to a one‐layer isentropic model of the atmosphere
Authors: Verkley W. T. M. 
Published: February 2007

AbstractSalmon's Hamiltonian approach is applied to formulate a balanced approximation to a hydrostatic one‐layer isentropic model of the atmosphere. The model, referred to as the parent model, describes an idealized atmosphere of which the dynamics is closely analogous to a one‐layer shallow‐water model on the sphere. The balance used as input in Salmon's approach is a simplified form of linear balance, in which the balanced velocity vb is given by vb = k×δf−1(M–M). Here k is a vertical unit vector, f is the Coriolis parameter, M is the Montgomery potential and M is the value of the Montgomery potential at the state of rest. This form of balance is used in preference to standard geostrophic balance, vb = k × f−1δM, which forces the meridional wind velocity to be zero at the equator. Salmon's Hamiltonian technique is applied to obtain an equation for the time rate of change of the balanced velocity that guarantees both the material conservation of potential vorticity as well as conservation of energy. New in this application of Salmon's approach is a nonlinear relation between Montgomery potential and surface pressure (characteristic for an isentropic ideal gas in hydrostatic equilibrium) in combination with spherical geometry and a variable Coriolis parameter. We discuss how the unbalanced velocity va can be calculated in a practical way and how the model can be stepped forward in time by advecting the balanced potential vorticity with the total velocity v = vb + va. The balanced model is tested against a ten‐day integration of the parent model.
Ensemble prediction of tropical cyclones using targeted diabatic singular vectors
Authors: Puri K. , Barkmeijer J. , Palmer T. N. 
Published: February 2007

AbstractThe feasibility of using the European Centre for Medium‐Range Weather Forecasts's ensemble prediction system for tropical‐cyclone prediction is studied in terms of the spread in cyclone tracks and intensities. It is shown that significant spread in the tracks is obtained through the use of initial perturbations based on targeted singular vectors derived using linearized physics. Inclusion of stochastic physics in the forecast model leads to larger spread in the central pressures. The spread in the tracks shows a high level of sensitivity to the background state used in deriving the singular vectors.
Parametrizing the effects of orography on the boundary layer: An alternative to effective roughness lengths
Authors: Wood N. , Brown A. R. , Hewer F. E. 
Published: February 2007

AbstractA new method of parametrizing the effects of form drag due to unresolved orography is proposed in which an explicit orographic‐stress profile is specified. This approach is evaluated using the results of numerical model simulations and the performance is found to be at least as good as that obtained with the traditional effective‐roughness‐length parametrization for cases of neutral flow normal to ridges. The new method is more readily extended to represent the changes in form drag as a function of wind direction for flow over anisotropic orography, and also simplifies the problem of parametrizing the scalar transfer. It is also suggested that this more explicit approach will provide a more convenient framework for parametrizing form drag in stable conditions and for understanding and improving the interactions with gravity‐wave‐drag parametrization schemes.
Sensitivity of modelled Antarctic climate to surface and boundary‐layer flux parametrizations
Authors: King J. C. , Connolley W. M. , Derbyshire S. H. 
Published: February 2007

AbstractThe Hadley Centre climate model version HadAM2 is used to study the sensitivity of modelled Antarctic climate to the parametrization of surface and boundary‐layer heat fluxes under stable conditions. Specifically, the impact of changing the dependence of surface exchange coefficients and eddy diffusivities on the Richardson number is investigated. Three alternative parametrizations are implemented; in all of these the exchange coefficients decrease more rapidly with increasing stability than they do in the standard parametrization used in the model. When only the surface flux scheme is replaced by one of these alternatives, cooling is largely restricted to the surface, with some compensating warming occurring at the lowest atmospheric levels, and little change is seen in the low‐level wind field over Antarctica. If alternative schemes are implemented both at the surface and in the boundary layer, widespread cooling occurs at the surface and at the lowest one or two atmospheric levels. The increased negative buoyancy thus generated causes significant increases in the speed of katabatic winds blowing down the coastal slopes of Antarctica. Colder and stronger offshore winds lead to increased cooling of the Antarctic coastal waters. In a coupled model, this could impact on the production of sea ice and ocean‐bottom water. The modelled temperature changes appear to show both a direct response to changed boundary‐layer heat‐flux divergence and an indirect response as a result of the consequent changes to the low‐level circulation.
Three‐dimensional chemical model simulations of the ozone layer: 2015–55
Authors: Austin John , Butchart Neal , Knight Jeffrey 
Published: February 2007

AbstractA stratospheric‐chemistry model coupled to a general‐circulation model is used to investigate chemistry‐climate coupling processes and their influence on ozone. Simulations commence on 1 March in each of the years 2014, 2024, 2034, 2044 and 2054, and consist of a 4‐month spin‐up period, followed by a 1‐year integration. Projected values of halogen amounts and greenhouse gases are imposed on the model. During the period 2014–54, ozone generally increases but by 2054 has still not returned to 1980 conditions. In Antarctica, spring ozone recovers temporarily in the 2024 integration but the ozone hole deepens significantly again in the 2034 and 2044 integrations before finally disappearing in the 2054 integration. The results suggest that the deepening of the Antarctic ozone hole in the model in 2034 and 2044, despite a reduction in halogen loading, is due to enhanced cooling due to increased greenhouse gases. Model‐predicted temperature and ultraviolet (UV) changes are also investigated. It is found that recovery of ozone during the period of the simulations gives rise to reduced stratospheric temperature decreases and UV levels are still slightly higher in general than in previous calculations for 1980.
Reconstructing balloon trajectories in the tropical stratosphere with a hybrid model using analysed fields
Authors: Dvorkin Yona , Paldor Nathan , Basdevant Claude 
Published: February 2007

AbstractA hybrid Lagrangian‐Eulerian model of particle motion is employed to reproduce the observed trajectories of three constant level balloon flights along the 0.0987 kg m−3 and 0.110 kg m−3 isopycnic surfaces during the EQUATEUR experiment, conducted by Centre Nationale d'Etudes Spatiales in the summer of 1998. The hybrid model employs velocity, temperature and geopotential fields as provided by the European Centre for Medium‐Range Weather Forecasts analysed data for several three week periods of the flights. The balloon in the model is advected primarily by the ambient air velocity to which a correctional velocity, calculated on the basis of a Lagrangian dynamical model, is added. The latter is driven by the Montgomery stream function gradient, interpolated to the relevant isopycnic surface from the geopotential and temperature standard fields, and Coriolis and drag forces. This correctional velocity is free from the continuity constraint satisfied by the air flow. Altogether, the model employs two physical parameters: the Rayleigh friction coefficient and the fraction (weight) of correctional velocity in the advection. To quantify the difference between the calculated and observed trajectories we compute the temporal mean of the separation between the two trajectories. On time‐scales on the order of three weeks the best calculated trajectories are obtained when the correctional velocity fraction in the advection is between 5 and 25%, and these best fitting trajectories have a mean separation that is down to 20% of that of the advection by the ambient air velocity, only. The implication of our results is that, although a balloon is advected primarily (75 to 95%) by the ambient air velocity, the small deviations from this velocity result in a significantly more accurate calculated trajectory over periods of three weeks. When a trajectory remains in one hemisphere for the entire three‐week period, the best fitting fraction of the correctional velocity is smaller and the Rayleigh friction coefficient is larger than in the case when the trajectory straddles the equator, but the improvement factor does not differ significantly.
Four‐dimensional variational data assimilation with a diabatic version of the NCEP global spectral model: System development and preliminary results
Authors: Zou X. , Liu H. , Derber J. , Sela J. G. , Treadon R. , Navon I. M. , Wang B. 
Published: February 2007

AbstractThe formulation of the National Centers for Environmental Prediction four‐dimensional variational data‐assimilation (4D‐Var) system is described. Results of applying 4D‐Var over a one‐week assimilation period, with a full set of physical parametrizations, are presented and compared with those of 3D‐Var. The linearization has been performed without simplifications and, therefore, the tangent‐linear and adjoint codes are consistent with the nonlinear physical parametrizations. The 4D‐Var assimilation is similar in formulation to the 3D‐Var analysis, except that observations are used at the appropriate time in 4D‐Var. Compared with the 3D‐Var runs, the 4D‐Var results showed good convergences, smaller analysis increments, and a comparable fit of analyses and short‐range forecasts to observations. A consistent improvement with the 4D‐Var system is observed in short‐range (six‐hour) forecasts of all model variables except the specific humidity. The temperature analyses from 4D‐Var were found to be better in most of the areas where the analysis errors from 3D‐Var were largest, although the globally averaged root‐mean‐square difference in the 4D‐Var temperature analysis was larger due to a very small degradation in some parts of the globe that include data‐rich areas. The globally averaged root‐mean‐square difference in the 4D‐Var specific‐humidity analysis, compared with that of 3D‐Var, was larger and was found to result from slightly increased analysis‐error maxima in the 4D‐Var results over data‐sparse tropical regions. The 3–4 day forecasts from 4D‐Var analyses compared more favourably than forecasts from the 3D‐Var analyses with the targeted mid‐Pacific dropwindsonde observations available from the 1998 North Pacific Experiment. Compared with conventional observations, a consistent improvement in the 1‐5 day forecasts of wind and temperature was shown in the tropics and the southern hemisphere.
The comparative impact of the assimilation of SSM/I and TMI brightness temperatures in the ECMWF 4D‐Var system
Authors: Marécal Virginie , Gérard Élisabeth , Mahfouf Jean‐François , Bauer Peter 
Published: February 2007

AbstractThis paper studies the impact of assimilating TRMM (Tropical Rainfall Measuring Mission) Microwave Imager (TMI) brightness temperatures in the European Centre for Medium‐Range Weather Forecasts fourdimensional variational assimilation system (4D‐Var). The methodology is similar to the one developed operationally for Special Sensor Microwave/Imager (SSM/I) radiometers. First, a one‐dimensional variational (1D‐Var) approach is used to retrieve specific‐humidity profiles, sea‐surface wind speed and cloud liquid‐water path. Then, the first two quantities are assimilated in the 4D‐Var analysis system. Results show an improvement of 1D‐Var product accuracy when using TMI instead of SSM/I brightness temperatures. This is related to the characteristics of the TMI radiometer which allow a better estimate of surface parameters and of water vapour in tropical conditions. Consequently, 4D‐Var analyses of the total column water vapour are improved using TMI data. The experiment with TMI observations gives a smaller increase in global humidity than the experiment with SSM/I data, leading to a reduction of the model precipitation spin‐down at the beginning of the forecasts. The impact on the low‐level wind analysis is small with either SSM/I or TMI data. Forecast performance with TMI data is generally improved compared with a control experiment without any SSM/I or TMI data. The TMI experiment scores are better than those obtained in the SSM/I experiment for the 1000 hPa geopotential in the northern hemisphere and for tropical winds at 200 hPa.