bluuebunny/crossref_metadata_2025_split huggingface dataset:
Effects of entrainment and droplet sedimentation on the microphysical structure of status and stratocumulus clouds
Authors: Considine Geoffrey , Curry Judith A. 
Published: February 2007

AbstractA new cloud model has been developed based upon a reconsideration of cloud‐top entrainment mechanisms. A finite‐thickness entrainment layer at cloud top allows the cloudy air to lose liquid water to dry entrained air by sedimentation. The model use a steady‐state ensemble of one‐dimensional updraughts and downdraughts, combined with a simplified traetment of droplet sedimentation, autoconversion, and collection of cloud drops by drizzle, to examine vertical profiles of cloud liquid‐water content and droplet concentration. The model is demonstrated both for idealized cases and in comparison with a nocturnal stratocumulus case from the ASTEX. The sedimentation flux divergence in the entrainment layer at cloud top is shown to have a large impact on the cloud microphysics.
Microphysical and short‐wave radiative structure of stratocumulus clouds over the Southern Ocean: Summer results and seasonal differences
Authors: Boers R. , Jensen J. B. , Krummel P. B. 
Published: February 2007

AbstractSix case‐studies are presented of stratocumulus clouds observed during the summer phase of the Southern Ocean Cloud EXperiment (SOCEX). The experiment was conceived to investigate the microphysical properties of clouds in the unpolluted background region over the Southern Ocean. The case‐studies show evidence of decoupled stratocumulus layers, i.e. situations where cumulus clouds were covered by a uniform sheet of stratocumulus cloud. Optical depths of the clouds ranged from 2 to 23. Cloud droplet concentrations varied from 50 to 180cm−3, and mostly decreased with altitude, indicating the importance of droplet coalescence in reducing the number of large cloud droplets. Drizzle was observed on four of the six flights.A comparison of the summer results with those published from the winter phase of SOCEX indicates that the droplet concentration in summer was three times that of the concentration in winter. The differences between summer and winter are so large that errors in sampling and measurement uncertainties hardly influence the results. Because these results were obtained under conditions where the air parcels had traversed long distances over the unpolluted Southern Ocean, it suggests a natural cause, the most likely being oxidation products of oceanic dimethylsulphide acting as cloud condensation nuclei.Model results show that the natural variation in droplet number concentration is responsible for a natural variation in cloud albedo of around 30% at a constant solar zenith angle. This may indicate that there is a natural negative albedo feedback counteracting the enhanced greenhouse effect. The feedback links increased emission of dimethylsulphide at higher oceanic temperatures to an increase in cloud condensation nuclei, and thus to an increase in cloud albedo.
Potential vorticity inversion of a two‐dimensional steady flow: Application to symmetric instability
Authors: Arbogast Philippe , Joly Alain 
Published: February 2007

AbstractThe potential vorticity inversion concept is extended to obtain two‐dimensional symmetrically unstable basic states. The goal is to invert a lens‐like patch of negative potential vorticity embedded within a large area of uniform positive potential vorticity to create the basic state. In order to circumvent the non‐ellipticity in areas of negative potential vorticity, a variational formulation of the invertibility principle is proposed. The functional incorporates the thermal wind balance as a constraint between the mass and the wind field. This condition is necessary and sufficient to define a steady solution to the inversion problem that is suitable to serve as a basic state. The uniqueness of this solution, however, is not addressed. Inversions with different potential vorticity distributions are performed, enabling us to highlight the dependence of the behaviour of the symmetric rolls (initial linear growth rate, wavelength) on the shape of the negative potential vorticity area (aspect ratio, amplitude and surface). Particular attention is paid to defining the domain of parameters (frontal environment and anomalies) that enables symmetric instability in a convectively‐ and inertially‐stable flow. Results from the stability analysis for embedded zones are compared to those obtained for uniform potential vorticity in the linear regime. Results are generally confirmed and new ones are found. It is shown, for example, that the linear growth rate increases when the unstable surface decreases. The inversion technique is finally extended to the moist (saturated) case. The variational approach is, therefore, shown to be able to handle areas of negative potential vorticity, opening new areas for a more systematic use of the concept of inversion in synoptic and dynamic meteorology.
Skill assessment for ENSO using ensemble prediction
Authors: Moore A. M. , Kleeman R. 
Published: February 2007

AbstractA crucial component of any prediction system is the ability to estimate the predictive skill of a forecast so that the degree of confidence that can be placed in an individual forecast can be assessed. In this paper we have used ensemble prediction techniques to develop a means of estimating a priori the predictive skill of forecasts of El Niñ Southern Oscillation (ENSO) using an intermediate coupled ocean–atmosphere model. Each member of an ensemble forecast is perturbed using either noise forcing fields or perturbations that are known to increase the low‐frequency variability of the coupled model. These are respectively the so‐called stochastic optimals and optimal perturbations of the coupled system; they are added to the model to mimic the presence of initial‐condition errors and high‐frequency stochastic noise and their effect on the predictability of the coupled system in the tropics.By performing ensemble predictions in hindcast mode, we have identified a usable relation between the skill of a model hindcast and the spread of the ensemble measured relative to some control hindcast. The practical nature of ensemble prediction is demonstrated by computing the relationship between model skill and the spread of an ensemble from hindcasts of ENSO for the period 1972–86, and then comparing the actual hindcast skills for the period 1987–93 with those suggested by the skill‐spread relation from the preceding period. The relationship that we find between the model skill and spread of an ensemble appears to be robust in the sense that it is relatively insensitive to variations in the ensemble prediction procedure and to changes in some model parameters. However, crucial to the success of the ensemble predictions is the use of perturbations in the ensembles that are known to increase the low‐frequency variability of the coupled model. These perturbations can efficiently probe the probability density function of possible coupled‐model states. If randomly chosen perturbations are used in the ensembles, however, no practical relation between model skill and the spread of an ensemble emerges.
The oceanic mesoscale convective system observed with airborne Doppler radars on 9 February 1993 during TOGA–COARE: Structure, evolution and budgets
Authors: Roux Frank 
Published: February 2007

AbstractData, collected on 9 February 1993 during the Tropical Ocean and Global Atmosphere Programme—Coupled Ocean‐Atmosphere Response Experiment (TOGA‐COARE) with the Doppler radars on board the two National Oceanic and Atmospheric Administration WP‐3D and the National Center for Atmospheric Research Electra aircraft, are analysed to study the structure and evolution of an equatorial oceanic mesoscale convective system (MCS). Observations lasted more than five hours, a period during which the cloud‐cluster characteristics changed from convective to stratiform. This mesoscale system was approximately aligned along the tropospheric wind shear, but it was nearly perpendicular to the low‐level shear.Series of three‐dimensional fields of reflectivity, air‐velocity components, retrieved pressure and temperature perturbations show similarities with previously observed convective systems over the tropical oceans. In particular, although the cloud top reached 18 km altitude, relatively weak vertical motions and temperature perturbations resulted from a moderate convective instability. In the low levels, a cold front‐to‐rear flow intensified and deepened as the overall structure evolved from convective to stratiform. The vertical profiles of mean horizontal divergence evolved from low‐level convergence in the cross‐line direction for the earliest analyses to mid‐level convergence from all directions during the later stratiform phase. Vertical momentum fluxes were important and led to an increase in the cross‐line wind shear and a decrease in the along‐line one.Analysis of radiosounding data from the nearest stations of the TOGA‐COARE array shows that the period during which airborne Doppler observations were conducted was characterized by relatively strong large‐scale ascent and ‘apparent’ sources of heat, moisture and momentum. These ‘diagnosed’ values and the mean radarderived ones agree fairly well. This shows that the early convective and late stratiform phases of the MCS are equally important for the ‘apparent’ sources of heat and momentum.
The effective roughness length for scalar transfer in neutral conditions over hilly terrain
Authors: Hewer F. E. , Wood N. 
Published: February 2007

AbstractNumerical weather prediction models are required to represent the area‐averaged effects of hills on temperature and water vapour in the atmospheric boundary layer. The transfer of these scalars within the boundary layer is affected by bouyancy. However, we have simplified the problem by simulating the behaviour of a passive scalar in a neutrally stratified turbulent boundary layer over hills. The numerical model used for these simulations was fully nonlinear and flows were simulated over a range of slopes, including ones where separation occurred.The area‐averaged effects of hills are small over low slopes but significant at steeper slopes due to the impact of the separated region. The area‐averaged scalar concentration varies logarithmically with height, but in a region that is shallower than the logarithmic wind region. We have found that, using an effective roughness length for scalar transfer, ZOSeff, the effects of hills on area‐averaged scalars may be parametrized. ZOSeff is smaller than the local value of roughness length for scalar transfer, and for hills of certain wavelengths and roughness it is three orders of magnitude smaller. A simplified interpretation of the physical processes simulated by the numerical model has been used to derive an expression for ZOSeff in terms of hill characteristics only. This parametrization should be useful for representing the influence of hills on scalar transfer in numerical weather prediction and climate models.
The retrieval of cirrus cloud properties from aircraft multi‐spectral reflectance measurements during EUCREX'93
Authors: Francis P. N. , Hignett P. , Macke A. 
Published: February 2007

AbstractWe present details of a scheme for retrieving cirrus cloud optical thickness and effective particle size from nadir‐viewing reflectance measurements. Two near‐infrared wavelengths are used, one with negligible ice absorption (1.04 μm) and one with significant ice absorption (1.55 μm). Four ice crystal shapes are used in the scheme: ice spheres, hexagonal columns, hexagonal plates and randomized polycrystals. We highlight the sensitivity of the retrievals to the shape of the phase function, which follows from the particular ice crystal shape assumed.Five aircraft‐based retrievals are presented, all cases occurring during the 1993 intensive field campaign of the European Cloud Radiation Experiment (EUCREX'93). The retrieved effective sizes are compared with the in situ measurements taken from the same cases. In these comparisons, account has to be taken of the fact that (a) in situ replicator data indicate that the presence of small (<100 μm) ice crystals can have a significant effect on the effective crystal size, and (b) the measured crystal sizes consistently show a systematic decrease with height in all cases, the effective sizes at cloud base being typically two or three times larger than at cloud top. Taking these considerations into account, we conclude that the assumption of polycrystals gives the most consistent agreement with the in situ measurements.
Assessment of a gravity‐wave drag parametrization with PYREX data
Authors: Beau Isabelle , Bougeault Philippe 
Published: February 2007

AbstractAn original methodology is developed to assess the behaviour of gravity‐wave drag parametrizations. It relies on the computation of differences between numerical simulations at various horizontal resolutions. The terms to parametrize are extracted from comparisons between the full momentum budgets of the high‐resolution and low‐resolution simulations. The parametrization under assessment is firstly calibrated off‐line to reproduce at best the residual vertical momentum flux, then it is introduced in the numerical model for an on‐line verification simulation. The methodology is applied to the ECMWF parametrization (in its 1990 version), using four meteorological situations well documented during the PYREX field experiment. The results show that the classical formulation, limiting the effect of orographic waves to the vertical divergence of the momentum flux, is well founded. However, it is shown that the optimal coefficients of the tested scheme are in general much larger than the value recommended at the time, and quite variable from one case to another. The variability of the low‐level blocking effect with the Froude number of the incident flow is suggested as a possible explanation.
Four‐dimensional variational assimilation of SSM/I precipitable water content data
Authors: Filiberti M.‐A. , Eymard L. , Rabier F. , Courtier P. , Thépaut J. N. 
Published: February 2007

AbstractSatellite microwave radiometers provide measurements of precipitable water content (PWC) over the oceans, with a horizontal resolution of a few tens of kilometres. These data represent the water vapour content integrated over the atmospheric column. We assess the value of this source of information for numerical weather prediction systems. We used the European Centre for Medium‐Range Weather Forecasts Integrated Forecasting System with a four‐dimensional variational method to assimilate the Special Sensor Microwave/Imager (SSM/I) PWC data over a 24‐hour period. Our experiments use an incremental variational method. Comparison with observations is made using a multi‐level global primitive‐equation T106 spectral model with physical parametrizations. Minimization is performed using a T63 adiabatic dynamics model which includes only simplified physics (horizontal diffusion, a simple surface drag and a vertical diffusion scheme).Comparing the control and assimilation experiments with aircraft and other data shows that the use of PWC data from SSM/I improves the analysis. We also obtain a slight improvement in short‐range forecasts of almost all parameters when SSM/I‐PWC data are used in the assimilation.
The ECMWF implementation of three‐dimensional variational assimilation (3D‐Var). III: Experimental results
Authors: Andersson Erik , Haseler Jan , Undén Per , Courtier Philippe , Kelly Graeme , Vasiljevic Drasko , Brankovic Cedo , Gaffard Catherine , Hollingsworth Anthony , Jakob Christian , Janssen Peter , Klinker Ernst , Lanzinger Andreas , Miller Martin , Rabier Florence , Simmons Adrian , Strauss Bernard , Viterbo Pedro , Cardinali Carla , Thépaut Jean‐Noël 
Published: February 2007

AbstractIn this third and final paper of a series, we assess the performance of the three‐dimensional variational data assimilation scheme, in the light of the results from the extensive pre‐operational programme of numerical experimentation. Its performance is compared with that of the previous operational scheme at the European Centre for Medium‐Range Weather Forecasts, which was based on Optimal Interpolation. The main features of the new scheme are illustrated, in particular the effects of non‐separable structure functions and the improved data usage. TIROS‐N Operational Vertical Sounder cloud‐cleared radiances, for example, are used directly without a separate retrieval step. Scatterometer data are assimilated in the form of ambiguous winds with the ambiguity removal taking place within the analysis itself. Problems encountered during the tests are discussed and the solutions implemented are explained.The overall impact on forecast accuracy in the troposphere of the northern hemisphere extratropics is neutral for geopotential and positive for wind and temperature. The impact is neutral in the tropics, and significantly positive in the southern hemisphere. Analyses and forecasts for the stratosphere have improved in all regions. Other positive results include a clear improvement in analyses of near‐surface winds over oceans, particularly in the vicinity of tropical storms. This is predominantly because of the assimilation of scatterometer wind‐data.
Multiple‐truncation incremental approach for four‐dimensional variational data assimilation
Authors: Veerse F. , Thepaut J.‐N. 
Published: February 2007

AbstractThe incremental approach provides an approximate solution to four‐dimensional variational data assimilation (4D‐Var) at a reasonable CPU cost. An extension of this approach is studied, namely the multiple‐truncation incremental technique, and is compared to the standard and single‐truncation incremental implementations of 4D‐Var, using a two‐dimensional barotropic vorticity‐equation model.The quasi‐continuous approach deals with the cut‐off problem and spreads the computer requirements over time. Quasi‐continuous incremental and quasi‐continuous multiple‐truncation incremental techniques are assessed.The same number of iterations is specified in all the experiments. The multiple‐truncation incremental technique is found to produce a good approximate analysis at a lower CPU cost. Applying the quasi‐continuous approach to the incremental methods adds some benefit in terms of computer time.
Development of perturbations within growing baroclinic waves
Authors: Snyder C. , Joly A. 
Published: February 2007

AbstractWe explore the linear stability of a growing, three‐dimensional baroclinic wave by calculating the perturbation that grows most rapidly over various time intervals and at various stages in the development of the parent wave and its fronts. Three norms are used to measure growth: volume‐integrated energy, enstrophy and stream function variance. The flow is assumed adiabatic and quasi‐geostrophic for simplicity, and perturbations are required to have uniform potential vorticity.These rapidly growing perturbations can produce realistic sub‐structures within the parent wave, such as upper‐level vorticity maxima that propagate relative to a synoptic‐scale parent wave or packets of synoptic‐scale waves within a planetary‐wave basic state. For a synoptic‐scale parent wave and the energy or enstrophy norms, however, the dominant characteristic of the fastest growing perturbations is that they rapidly evolve toward a final structure corresponding to a phase shift and slight change of shape of the original wave—in essence, the initial perturbation modifies the parent wave and the jet on which it propagates, which results in a modification, which grows in time, of the phase and amplitude of the parent wave. Amplifications in energy or enstrophy are also small compared to what would be estimated based on the locally large shears and baroclinicity within the parent wave. The fronts appear to be stabilized by the combined influences of synoptic‐scale horizontal deformation and the natural movement of perturbations relative to the parent wave.
The annual cycle of persistence in the El Nño/Southern Oscillation
Authors: Torrence Christopher , Webster Peter J. 
Published: February 2007

AbstractA spring ‘predictability barrier’ exists in both data and models of the El Niño/Southern Oscillation (ENSO) phenomenon. In statistical analyses this barrier manifests itself as a drop‐off in monthly persistence (lagged correlation) while in coupled ocean‐atmosphere models it appears as a decrease in forecast skill.The ‘persistence barrier’ for ENSO indices is investigated using historical sea surface temperature and sealevel pressure data. Simple statistical models are used to show that the persistence barrier occurs because the boreal spring is the transition time from one climate state to another, when the ‘signal‐to‐noise’ of the system is lowest and the system is most susceptible to perturbations. The strength of the persistence barrier is shown to depend on the degree of phase locking of the ENSO to the annual cycle.The phase locking of the ENSO to the annual cycle, as well as the ENSO variance, is shown to vary on interdecadal time‐scales. During 1871–1920 and 1960–90 the ENSO variance was high, while during 1920–50 it was low. Using wavelet analysis, this interdecadal variability in ENSO is shown to be correlated with changes in Indian summer monsoon strength. Finally, the change in persistence‐barrier strength between 1960–79 and 1980–95 is related to changes in the phase locking of ENSO to the annual cycle. These changes in persistence and phase locking appear to be related to the increased forecast skill seen from recent coupled ocean‐atmosphere models.
Radar estimates of rainfall rates at the ground in bright band and non‐bright band events
Authors: Smyth T. J. , Illingworth A. J. 
Published: February 2007

AbstractThe major error in estimating precipitation rates at the ground from radar arises from the change in radar reflectivity (Z) with height. In stratiform precipitation Z rises in the ‘bright band’, where the snow is melting, and then falls steeply in the dry ice above. In regions of vigorous convection containing graupel, the bright band is absent and the fall in Z with height is less marked. If accurate estimates of surface precipitation are to be made, based upon radar observations taken within or above the bright band, it is important to use a correction scheme which uses a different vertical profile of reflectivity (VPR) for the two precipitation types. Using a large radar data set, this work addresses the issue of VPR variability by constructing average vertical profiles and sorting them into snow and graupel categories using the value of the Linear Depolarization Ratio at the melting layer. These observations snow that convection is often embedded within stratiform precipitation, and some convective showers containing snow have bright bands. As a result, the use of an average profile over the whole region scanned by radar leads to only marginal improvement in rain estimates. Instead, this work proposes that a bright band correction should always be applied unless the value of Z at 1.5 km above the bright band exceeds 30 dBZ, in which case a ‘convective’ vertical profile of ‐2 dBZ km−1 should be used. This Z threshold indicates that graupel only forms when the ice water content exceeds 0.35 gm−3 which has implications for ice parametrization schemes in mesoscale models.