bluuebunny/crossref_metadata_2025 huggingface dataset:
Observations of non‐dimensional wind shear in the coastal zone
Authors: Vickers Dean , Mahrt L.
Published: February 2007

Vertical profiles of the time‐averaged wind stress, wind speed and buoyancy flux from the off‐shore tower site in the Risø Air Sea Experiment are used to evaluate similarity theory in the coastal zone. the observed dependence of the non‐dimensional wind shear on stability is compared to the traditional parametrization. Relationships between the non‐dimensional shear, development of internal boundary layers and wave state are explored.

We find that the largest‐scale turbulent eddies are suppressed in shallow convective internal boundary layers, leading to larger non‐dimensional shear than that of the traditional prediction based only on stability. In shallow stable boundary layers, elevated generation of turbulence leads to smaller non‐dimensional shear compared to the traditional prediction. Over young growing waves in stable stratification, the observed non‐dimensional shear is less than that over older more mature waves in otherwise similar conditions. the non‐dimensional shear is a function of wave state for stable conditions even though the observations are well above the wave boundary layer. We conclude that development of shallow internal boundary layers and young growing‐wave fields, both of which are common in the coastal zone, can lead to substantial departures of the non‐dimensional shear from the prediction based only on stability.
Direct retrieval of wind from total ozone mapping spectrometer (TOMS) data: Examples from FASTEX
Authors: Davis Christopher , Low‐Nam Simon , Shapiro Melvyn A. , Zou Xiaolei , Krueger Arlin J.
Published: February 2007

The quantitative relationship between ozone and potential vorticity, and hence balanced wind and temperature, is examined using observations collected for a one‐month period by the total ozone mapping spectrometer (TOMS). Ozone data are compared with Ertel potential vorticity (PV) integrated vertically from 500 to 50 mb, obtained from the NCEP/NCAR (National Centers for Environmental Prediction/National Center for Atmospheric Research) reanalysis archive. By performing a simple time filtering we isolate synoptic‐scale transients in the ozone and PV. We show that the previously reported high correlation between PV and ozone is actually variable from day to day. the correlation also shows strong latitudinal dependence, with essentially no correlation in the tropics and high correlation in middle latitudes.

A method is introduced to derive a three‐dimensional PV representation from ozone data and a background field taken from the reanalysis data. PV‐inversion methods are then used to recover balanced winds and temperatures and geopotential heights which may be compared to the analysed fields. Results suggest that many of the upper‐tropospheric synoptic‐scale wind perturbations may be recovered from the ozone data. A systematic error in the retrieval method used here is the understimation of the strength of upper‐level ridges, corresponding to an excess of vertically integrated ozone compared with integrated PV. We discuss reasons for this difference.
FASTEX IOP17 cyclone: Introductory synoptic study with field data
Authors: Cammas Jean‐Pierre , Pouponneau Béatrice , Desroziers Gerald , Santurette Patrick , Joly Alain , Arbogast Philippe , Mallet Isabelle , Caniaux Guy , Mascart Patrick
Published: February 2007

This paper introduces the study of the early development phase of the IOP17 frontal cyclone of the FASTEX field experiment (Fronts and Atlantic Storm‐Track EXperiment). With the support of observations from FASTEX observing platforms a synoptic description highlights the development of mesoscale features of interest in the operational Météo‐France ARPEGE (Action de Recherche Petite Echelle Grand Echelle) analysis cycle, i.e. cloud head, dry wedge and upper‐level jet outflow. A discussion attempts to isolate two significant differences of the present cyclone life cycle compared with previous well‐known conceptual models of maritime cyclone life cycles: (i) the indirect role of an upper‐level precursor in triggering the surface low; (ii) the existence of a diabatic regime taking over from the baroclinic development to sustain the deepening rate of the cyclone. These two mechanisms are the subject of two companion papers.
Effects of a low‐level precursor and frontal stability on cyclogenesis during FASTEX IOP17
Authors: Mallet Isabelle , Arbogast Philippe , Baehr Christophe , Cammas Jean‐Pierre , Mascart Patrick
Published: February 2007

The Fronts and Atlantic Storm‐Track EXperiment (FASTEX) has provided comprehensive data to document the life‐cycle of secondary frontal cyclones over the North Atlantic, and to improve the understanding of cyclogenesis mechanisms. This study analyses the processes leading to the triggering of a particularly well‐sampled frontal cyclone that developed on the trailing cold front of a mature primary cyclone during FASTEX Intensive Observation Period 17 (IOP17) between 16 and 20 February 1997.

The case features both a classical low‐level vorticity strip along the primary front where the frontal cyclone develops and, more unexpectedly, a pre‐existing continental surface low further west, the importance of which is revealed in the pilot study of Arbogast and Joly. In order to study possible cyclogenetic contributions along the primary front, the domain‐independent vorticity‐divergence attribution technique of Bishop is used to partition the flow into contributions from the continental low and from the large‐scale environment. Results indicate that the frontal cyclone is triggered where and when along‐front stretching decreases below the theoretical critical threshold of Bishop and Thorpe. It is shown that the cyclone is developing under normalized strain less than 1, in agreement with other case‐studies. Finally, the role of the continental low is investigated using a series of numerical forecasts made from different initial conditions manipulated with a potential‐vorticity inversion tool. the wind attributable to the continental low accounts for roughly 30% of the along‐front stretching decrease. the continental low also has a frontolytic effect on the western part of the primary front where its thermal advection pattern favours cyclogenesis.

In general terms, the frontal‐cyclone triggering seems to support the Bishop and Thorpe theoretical framework of frontal instability in low‐strain areas. However, a more complex picture arises when considering the role of the continental surface low, which also acts as a low‐level precursor.
Mesoscale analysis of arc rainbands in a dry slot
Authors: Browning K. A. , Roberts N. M.
Published: February 2007

A series of almost concentric, are‐shaped narrow rainbands was observed travelling at 140 km h−1 and producing severe wind gusts over parts of England. Their structure and environment have been analysed using radar and satellite imagery, mesoscale‐model diagnostics, surface observations and additional rawinsonde ascents obtained during the Fronts and Atlantic Storm‐Track EXperiment (FASTEX). the multiple‐rainband event is shown to have been associated with part of a dry intrusion characterized by a mesoscale vortex/cold pool. the event occurred within the ‘dry slot’ region of a small frontal cyclone where the mesoscale pool of cold dry air was overrunning warm‐sector air. Continuous wind profiles from a UHF radar, together with a well‐located rawinsonde at the ‘epicentre’ of the arc rainbands, showed that the major anomaly in the cold pool was an area of locally stronger winds (>40 m s−1) at 800 hPa colocated with the temperature minimum at that level. the observed orientation of the rainbands is explained in terms of the cold‐frontal structure at this level triggering ascent of the underlying warm moist air. the multiple nature of the arc rainbands remains unexplained; the possibilities of mesoscale gravity waves and conditional symmetric instability are both considered.
Improved retrieval of tropospheric temperatures from remote measurements of thermal radiation using the adiabatic lapse rate as a constraint
Authors: Peckham G. E. , Grippa M.
Published: February 2007

Space‐borne radiometers now form an important part of the global network of atmospheric observing systems which provide data for weather forecasting and studies of climate change. However, achieving adequate vertical resolution remains a problem, particularly for the retrieval of profiles of temperature and of water‐vapour concentrations in the troposphere. The problem of retrieving these profiles from radiance measurements is ill‐posed in that there is no unique answer‐solutions may be unstable and excessively sensitive to measurement noise. A technique known as regularization may be used to stabilize the solution by biasing the retrieved profiles toward an acceptable form. Similar inverse problems occur in many areas of science and engineering. Work on image enhancement has shown that regularization which includes a constraint, such as non‐negativity of the solution, can provide retrievals with improved accuracy over other regularization methods. We note that the adiabatic lapse rate provides an effective constraint on tropospheric temperature profiles and we exploit this to develop a regularization scheme for the retrieval of atmospheric temperatures. We demonstrate that the retrieved profiles are indeed more accurate than those obtained without this constraint.
Remote sensing of bubble clouds in sea water
Authors: Flatau Piotr J. , Flatau Maria , Zaneveld J. R. V. , Mobley Curtis D.
Published: February 2007

We report on the influence of submerged bubble clouds on the re mote‐sensing properties of water. We show that the optical effect of bubbles on radiative transfer and on the estimate of the ocean colour is significant. We present a global map of the volume fraction of air in water derived from daily wind speed data. This map, together with the parametrization of the microphysical properties, shows the possible significance of bubble clouds on the albedo to incoming solar energy.
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

The 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

This 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

Detailed 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

The 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

The 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

Salmon'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

The 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

A 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

The 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

A 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

A 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

The 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

This 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.
Combination theory and equilibrium evaporation
Authors: Raupach M. R.
Published: February 2007

This paper is an analysis of equilibrium evaporation and its role in the energy balance of a terrestrial surface, as described by combination theory. Three themes are covered: first, a brief historical review identifies multiple definitions of the concept of equilibrium evaporation. Second, these are formalized by developing the basic principles of combination theory with minimum approximation. Several measures are utilized to do this: linearization is avoided, radiative and storage coupling are incorporated systematically, and actual and linearized saturation deficits are distinguished. The formalism is used to analyse several algebraically defined states and limits for the surface energy balance. Third, the thermodynamic foundation of equilibrium evaporation is analysed by studying surface‐atmosphere feedbacks in arbitrary closed and open evaporating systems. It is shown that under steady energy supply any closed evaporating system evolves towards a quasi‐steady state in which the Bowen ratio takes the equilibrium value 1/εv, where εv is the ratio of the latent‐ and sensible‐heat contents of saturated air with temperature, evaluated at the volume‐averaged temperature in the closed system. This applies whether the system is well‐mixed or imperfectly mixed, and whatever the internal distribution of surface fluxes and surface and aerodynamic resistances. In contrast, open systems cannot reach such an equilibrium. This evolutionary definition of equilibrium evaporation differs from an alternative algebraic definition, the fully decoupled limit. The differences between the two definitions are identified, and the evolutionary definition is shown to be more fundamental. Thus, the correct temperature for evaluating ε in determining equilibrium evaporation is the volume‐averaged temperature in a closed region, which in the case of a convective boundary layer is well approximated by the mixed‐layer temperature.
Impact of field‐calibrated vegetation parameters on GCM climate simulations
Authors: Sen Omer L. , Bastidas Luis A. , Shuttleworth W. James , Yang Zong‐Liang , Gupta Hoshin V. , Sorooshian Soroosh
Published: February 2007

This paper describes a study in which, for the first time, advanced systems‐engineering parameter‐estimation techniques were applied to data from several field studies to estimate the preferred set of parameters for some of the most common biomes represented in an advanced Soil‐Vegetation‐Atmosphere Transfer (SVAT) scheme (BATS2, a recent version of the Biosphere‐Atmosphere Transfer Scheme); the effect on modelled climate was also investigated. Observational data from field sites in Brazil, Canada, Arizona and Kansas/Oklahoma in the USA, and the Netherlands were chosen as representative of tropical rain forest, coniferous forest, semi‐arid vegetation, agricultural crops, and grassland biomes, respectively. Together, these five biomes make up 50% of the land area represented in BATS2. Multi‐criteria calibration algorithms do not produce a unique set of model parameters and, when different combinations of the available objective functions at each site are considered, the number of solutions increases substantially. The need for a single parameter‐set for each site (biome) is an important practical issue that was necessarily addressed in this study. A procedure was defined in which optimized parameter‐sets were successively discarded by successively applying a cut‐off threshold to single observable objective functions following a preference hierarchy. In this study, only the vegetation‐related parameters are calibrated for each of the five biomes and implemented into BATS2; however, in a separate experiment, the effect of including soil parameters in the optimization was investigated. When the calibrated parameters are adopted and used in BATS2, there are significant changes between the climates calculated in an eight‐year run with Version 3 of the Community Climate Model and in an equivalent eight‐year run in which the original default parameters were used. The overall conclusion of this exploratory study is that advanced parameter‐estimation techniques and appropriate field data can be used successfully to improve representation of surface exchanges and the modelled climate given by a GCM, by defining appropriate values for vegetation‐related parameters in an advanced SVAT scheme.
Baroclinic developments in jet entrances and exits. II: Nonlinear development
Authors: Hare Sylvia H. E. , James I. N.
Published: February 2007

This study examines the effect of the ageostrophic circulations associated with jet entrance and exit regions on the nonlinear development of normal modes. The nonlinear growth and decay phases of the baroclinic life cycle and, in particular, the vertical propagation and subsequent meridional advection of Rossby waves are very sensitive to the exact form of the zonally symmetric basic state. This leads to changes in the associated energetics and eddy fluxes. In order to represent the passage of a disturbance though a jet, a time‐varying zonally symmetric meridional circulation was implemented. Many of the factors identified in the time invariant work, such as the normal‐mode latitudinal and meridional advection, were again found to be important. Ageostrophic meridional circulations may play an important role in determining the magnitude and length of mid‐latitude storm tracks.
Extratropical low‐frequency variability as a low‐dimensional problem I: A simplified model
Authors: D'Andrea Fabio , Vautard Robert
Published: February 2007

It is shown that the low‐frequency and large‐scale variability of an intermediate complexity reference model can be reproduced faithfully by a simplified model of 10 independent variables and 10 equations. The reference model is quasi‐geostrophic and baroclinic. The low‐order model is based on the truncated projection of the reference‐model equations on the empirical orthogonal functions of its output. A closure term is shown to be essential for good performance of the low‐order model. This closure term is meant to reproduce all the neglected scales and all the scale interactions, mainly baroclinic eddy forcing, that drive the large‐scale flow. The closure is built as an empirically defined function of the large‐scale flow of the model, relying on an extensive previously computed library of tendency differences between the full and truncated model. Two other parametrization schemes, a time‐mean and a stochastic one, are tested; comparisons of these two with the former underline the importance of the flow dependence of the formulation. The low‐order and reference models exhibit the same climate, as well as the same low‐frequency variability and weather regimes.
Ice nucleation in orographic wave clouds: Measurements made during INTACC
Authors: Field Paul R. , Cotton R. J. , Johnson D. , Noone K. , Glantz P. , Kaye P. H. , Hirst E. , Greenaway R. S. , Jost C. , Gabriel R. , Reiner T. , Andreae M. , Saunders C. P. R. , Archer A. , Choularton T. , Smith M. , Brooks B. , Hoell C. , Bandy B. , Heymsfield A.
Published: February 2007

This paper reports on measurements made during the INTACC (INTeraction of Aerosol and Cold Clouds) experiment. Observations are shown from six wave cloud flights in the temperature range −12°C to −40 °C. In nearly all cases ice nucleation does not occur until after droplets have formed. Parcel trajectories that experience temperatures below −35°C are dominated by homogeneous nucleation, while parcel trajectories that stay above −35°C are affected by heterogeneous nucleation. Ice particles appear to be nucleated after the parent droplets reach their largest size and coldest temperature at the wave peak. The most likely heterogeneous nucleation mechanisms are immersion and/or contact nucleation.
Polar stratospheric cloud impacts on Antarctic stratospheric heating rates
Authors: Hicke Jeffrey , Tuck Adrian
Published: February 2007

The impact of polar stratospheric clouds (PSCs) on the stratospheric radiative heating rate is analysed by including a nominal PSC in heating‐rate calculations that incorporate realistic atmospheric variables including tropospheric clouds. The use of realistic atmospheric conditions constrains the possible radiative effects of PSCs, which previous studies have shown to be very sensitive to such variables as temperature, tropospheric clouds, and solar zenith angle. Over the pole, winter heating rates within the stratospheric polar vortex are decreased substantially by the presence of a PSC, while a PSC increases the heating rates equatorward of 75–85°S. Although the PSC always increases the short‐wave heating, the effect in the long‐wave region depends on the ground temperature, the stratospheric temperature, and presence of a tropospheric cloud. For the thickest PSCs (Type II), the effect in August 1994 varies from cooling by 0.25 K d−1 (potential‐temperature difference Δθ = 0.5 K d−1) at the pole to heating by 0.3 K d−1 (Δθ = 0.6 K d−1) at 65°S to slight cooling equatorward of 57°S. September 1994 results are similar. Calculated heating rates over the pole including PSCs are near –0.5 K d−1 (θ = –1 K d−1) for both months, and positive heating rates of up to 0.25 K d−1 (θ = 0.5 K d−1) occur near the vortex edge. Thinner PSCs (Type I) have less of an effect; for example, heating rates of 0.375 K d−1 (θ = 0.75 K d−1) occur over the pole in August when a Type I PSC is included. These results should be viewed as an upper bound to the effect of PSCs since the calculations specify 100% PSC cover; satellite results show that this assumption is not unreasonable within the vortex during winter and early spring, however. The increased latitudinal gradient in descent rates in the presence of a PSC is consistent with the behaviour of long‐lived trace‐gas observations, and strengthens the vortex relative to a PSC‐free case.
Canonical frontal circulation patterns in terms of Green's functions for the Sawyer‐Eliassen equation
Authors: Hakim G. J. , Keyser D.
Published: February 2007

Green's function solutions for the Sawyer‐Eliassen equation are derived and compared for quasi‐geostrophic and semi‐geostrophic two‐dimensional frontal dynamics in vertically bounded domains. The semi‐geostrophic Green's functions provide a framework for understanding canonical frontal circulation patterns arising from interactions, or coupling, between upper‐ and lower‐tropospheric jet‐front systems. Previously documented conceptual models for these circulation patterns are reproduced by the solutions. A noteworthy finding is that vertical coupling produces a deep and narrow region of ascent when the upper‐ and lower‐tropospheric point sources for the Green's functions share the same sign and absolute momentum surface.
Correlation modelling on the sphere using a generalized diffusion equation
Authors: Weaver Anthony , Courtier Philippe
Published: February 2007

An important element of a data assimilation system is the statistical model used for representing the correlations of background error. This paper describes a practical algorithm that can be used to model a large class of two‐ and three‐dimensional, univariate correlation functions on the sphere. Application of the algorithm involves a numerical integration of a generalized diffusion‐type equation (GDE). The GDE is formed by replacing the Laplacian operator in the classical diffusion equation by a polynomial in the Laplacian. The integral solution of the GDE defines, after appropriate normalization, a correlation operator on the sphere. The kernel of the correlation operator is an isotropic correlation function. The free parameters controlling the shape and length‐scale of the correlation function are the products kpT, p = 1, 2, …, where (‐1)pkp is a weighting (‘diffusion’) coefficient (kp > 0) attached to the Laplacian with exponent p, and T is the total integration ‘time’. For the classical diffusion equation (a special case of the GDE with kp = 0 for all p > 1) the correlation function is shown to be well approximated by a Gaussian with length‐scale equal to (2k1T)1/2.

The Laplacian‐based correlation model is particularly well suited for ocean models as it can be easily generalized to account for complex boundaries imposed by coastlines. Furthermore, a one‐dimensional analogue of the GDE can be used to model a family of vertical correlation functions, which in combination with the two‐dimensional GDE forms the basis of a three‐dimensional, (generally) non‐separable correlation model. Generalizations to account for anisotropic correlations are also possible by stretching and/or rotating the computational coordinates via a ‘diffusion’ tensor. Examples are presented from a variational assimilation system currently under development for the OPA ocean general‐circulation model of the Laboratoire d'Oceanographie Dynamique et de Climatologie.
Observations of atmosphere‐ocean coupling in the North Atlantic
Authors: Czaja Arnaud , Marshall John
Published: February 2007

Anindex of sea surface temperature (SST) variability, δT, is introduced that measures the difference in SST across the separated Gulf Stream in late winter. By analysing a long observational record of SST and sea‐level pressure (SLP), it is shown that δT exhibits damped oscillations of decadal period, and covaries with the strength of a dipolar SLP anomaly reminiscent of the North Atlantic Oscillation (NAO). Analysis in the frequency domain shows a broad‐band ‘peak’ at 10–20 years in δT, with a continuous decrease of power on longer time‐scales. Similar spectral signatures are found in the northern part of the SLP dipole (the Greenland‐Icelandic Low region) but not in its southern part (the subtropical High region), whose power increases on long time‐scales.

The observations are interpreted in the framework of a delayed‐oscillator model in which the ocean circulation introduces the delay, and modulates δT on decadal time‐scales. The decrease of power seen on long time‐scales (>25 years) in the δT index is captured by a model including wind‐driven ocean circulation, and arises primarily as a passive response of the latter to the NAO forcing. Variability of the ocean's meridional overturning circulation could also play a role in modulating δT on decadal time‐scales. If a small feedback of δT on the NAO pattern is introduced, the simple model can also reproduce the spectral structures seen in the SLP anomaly in the Greenland‐Iceland region.
Measurements of dissipation rate in frontal zones
Authors: Chapman D. , Browning K. A.
Published: February 2007

This paper demonstrates that it is possible to use measurements of the Doppler spectrum width from a high‐resolution radar to obtain quantitative estimates of dissipation rate during the passage of precipitating frontal zones. Using the high‐resolution Chilbolton radar, this method has been applied, for the first time, to infer the rate at which kinetic energy is dissipated by turbulence within mid‐ and lower‐tropospheric frontal zones. Analysis of detailed measurements for one cold‐frontal region showed multiple shallow sheets of high dissipation rate, with spatially averaged values around 1 x 10−3 m2s−3. Comparable values were also observed in a warm‐frontal zone. The magnitude of the observed dissipation rate supports the hypothesis that mixing played a major role in determining the mean properties of the fronts, in that the turbulence acted to destroy the frontal shear layer on a time‐scale of several hours. These issues have particular relevance to the representation of fronts within future very high‐resolution numerical weather‐prediction models.