bluuebunny/crossref_metadata_2025_split huggingface dataset:
The spectral signature of global warming
Authors: Slingo A. , Webb M. J. 
Published: February 2007

AbstractSimulations are presented of the change in the spectrum of the clear‐sky outgoing long‐wave radiation (OLR) associated with the global warming produced by increases in greenhouse‐gas concentrations. the input data for the present day and for the middle of the next century were taken from a recent climate‐prediction run of the Hadley Centre Climate Model. the simulations focus on the spectral signature of the warming as opposed to that of the forcing. Stratospheric cooling causes decreases of the OLR in the carbon‐dioxide and ozone bands, whilst surface warming increases the OLR in the region of the atmospheric window. the signal in the water‐vapour bands is more subtle, due to a near cancellation between the effects of changes in atmospheric temperatures and specific humidities that have little impact on the relative humidity. the residual signal is shown to be related to small changes in upper‐tropospheric relative humidity, although at some latitudes this relationship breaks down. It is suggested that satellite observations in the water‐vapour bands could be used to provide a quantitative measure of the water‐vapour feedback during global warming.
The global population of mesoscale convective complexes
Authors: Laing Arlene G. , Michael Fritsch J. 
Published: February 2007

AbstractA global set of 714 mesoscale convective complexes is compiled and some of the common properties of the convective systems are identified and examined from a global perspective. the data set includes date of occurrence, time of first storms, initiation, maximum extent, termination, duration, cold‐cloud shield areas, and tracks from initiation to termination.It is found that the typical convective complex is nocturnal, generates a cold‐cloud shield area of approximately 350 000 km2, and persists for about 10 h. the largest systems and most persistent systems tend to occur near the summer solstices. For the globe, about 400 systems occur each year, primarily over land areas. Most systems develop in favoured zones, although some activity occurs over every continent (except Antarctica) and all major oceans. the concentration of activity into favoured zones indicates that there must be special dynamic and/or thermodynamic conditions necessary for convection to organize into convective complexes.Activity is strongly tied to the solar day, and shifts from 35°S in early January to about 50°N during the boreal summer and back to 35°S by December. Within the northern hemisphere there is a pronounced poleward migration as the jet stream shifts northward. Relatively little migration occurs in the ocean‐dominated southern hemisphere where the subtropical jet remains quasi‐stationary over the convective‐complex regions.The nocturnal life cycles, copious rainfall, large cloud shields, and great frequency of mesoscale convective complexes suggest that they may be significant contributors to the global hydrologic cycle and earth‐system energy budget.
Organized convective systems in the tropical western pacific as a process in general circulation models: A toga coare case‐study
Authors: Moncrieff Mitchell W. , Klinker Ernst 
Published: February 2007

AbstractWe examine the large‐scale effects of organized convective systems in the tropical western Pacific observed during the Tropical‐Ocean Global‐Atmosphere Coupled Ocean‐Atmosphere Response Experiment (TOGA COARE). In a case‐study approach, we examine realizations of a supercluster, associated with the onset of the December 1992 westerly wind burst, in the T213 operational medium‐range weather forecasting model of the European Centre for Medium‐Range Weather Forecasts (ECMWF). We idealize a supercluster as a hierarchy of three interacting scales, namely organized cumulonimbu 𝒞1 mesoscale convective systems 𝒞2, and the supercluster component 𝒞3. It is shown that the ECMWF model represents this hierarchy as a 𝒞3‐like surrogate whose influence dominates the effect of parametrized convection. This causes over‐prediction of the model tendencies which, in the case of zonal momentum, is explained in elementary terms.The structure of the resolved‐scale momentum flux is explained by Moncrieff's (1992) archetypal theory of organized convection which has been verified against observations and cloud‐resolving model data‐sets. the parametrization of subgrid‐scale convective momentum‐flux in the ECMWF model, based on a momentum mixing concept, produces subgrid‐scale tendencies that are physically different from transports associated with cumulonimbus convection in a shear flow.We outline a strategy for parametrizing the momentum flux by the 𝒞1 component based on the archetypal model. the 𝒞2 component, which is part‐resolved and part‐parametrized, is at odds with the assumptions of scale separation underpinning parametrization. It is argued that this component should be represented as part of the prognostic treatment of convectively generated cirrus.Finally, we suggest cloud‐resolving modelling studies to further quantify the structure and large‐scale impact of superclusters in a westerly‐wind‐burst environment, ranging from idealized models to models having data assimilation capability.
A multi‐thermal model of cumulus glaciation via the hallett‐Mossop process
Authors: Blyth Alan M. , Latham John 
Published: February 2007

AbstractAnalysis of field observations has yielded the conclusion that the Hallett‐Mossop process (H‐M) of secondary ice production plays a major role in the glaciation of summertime cumulus clouds over New Mexico. Other studies have revealed that these clouds possess a characteristic multi‐thermal structure.In an effort to quantify more fully the role of H‐M in such clouds, and to establish which of the salient dynamical and microphysical parameters play important roles in the glaciation process, a model of ice‐particle growth and splinter production in a simple multi‐thermal framework is developed. the characteristics of the model are prescribed with values that are based on the above‐mentioned field studies. the model cloud possesses four distinct regions: the main updraught, a quiescent (debris) region, the cloud top, and a downdraught region.The trajectories of all primary ice particles introduced into the cloud at t = O, together with those created as a consequence of the operation of H‐M, are followed as they grow and are transported around the cloud. the sensitivities of these trajectories and a multiplication factor f to variations in parameters such as updraught speed, liquid‐water content, L, thermal depth, inter‐thermal interval, and downdraught characteristics are examined.These tests reveal that f is particularly sensitive to the values of L in the distinct regions of the cloud. Basically, combinations of parameter values which produce rapid growth of graupel pellets, large number of thermals, and efficient transport between cloud top and the Hallett‐Mossop temperature band yield the most rapid ice‐particle multiplication.
Balanced mesoscale motion and stratified turbulence forced by convection
Authors: Vallis G. K. , Shutts G. J. , Gray M. E. B. 
Published: February 2007

AbstractNumerical experiments are carried out with a large‐eddy simulation model to investigate the production of stratified turbulence and gravity wave energy in the atmospheric mesoscale by deep convection. Relatively long integrations (typically about 2.5 days) are carried out, so that statistical equilibrium might be achieved for an atmosphere subject to fixed cooling and a ground surface maintained at constant temperature. The main simulations may be considered as representing convective activity in a cold airstream passing over warm seas. The transfer of updraught kinetic energy into quasi‐horizontal rotational energy, and its subsequent cascade to larger scales, is examined in the context of the production of balanced motion and stratified turbulence theory. Experiments are carried out to examine the effects of the Coriolis force, a wave‐damping stratosphere, boundary‐layer vertical wind shear, and precipitation.An inverse mesoscale energy cascade is observed in experiments both with and without background rotation (the Coriolis effect), although weaker in the latter case. Mean boundary‐layer vorticity is found to be the principal source of the horizontal rotational energy created by deep convection, although the dipole character of the vorticity thereby produced frustrates the inverse energy cascade. At the level of convective detrainment, potential‐vorticity anomalies have a small monopolar element in the rotating case and this implies a direct forcing of energy at large scales. It also promotes a more efficient inverse energy cascade through long‐range influence. The inverse energy cascade arises primarily because of the quasi‐two‐dimensional, divergence‐free nature of the flow at scales larger than the convective forcing, which is maintained by the stable stratification. Even in the absence of background rotation balanced flow arises, especially in the upper troposphere and in the decay phase of the simulations when the surface fluxes and atmospheric cooling driving the convection are removed. In these cases potential‐vorticity inversion can be quite successful in determining the other field variables, with contributions from both geostrophic and cyclostrophic balance. Suppression of precipitation (i.e. removing the liquid‐water phase) increases the net convective mass transfer through the elimination of convective downdraughts, which leads to a higher efficiency of balanced‐flow energy production and upscale energy transfer. Intense ‘meso‐vortices’, with warm core structure, form spontaneously after about 2.5 days integration in these runs.
Secondary cyclogenesis: The initiation phase of a frontal wave observed over the eastern Atlantic
Authors: Rivals Hélène , Cammas Jean‐Pierre , Renfrew Ian A. 
Published: February 2007

AbstractThe initiation phase of a secondary frontal wave in the wake of a primary cyclogenesis event over the Atlantic is documented. North of the trailing surface cold front and south‐west of the primary cyclone, an intense upper‐level jet generates a direct and transverse ageostrophic circulation in its entrance region. Evidence is shown for two conflicting roles of this circulation: first, stabilization of the front through frontogenetic cross‐frontal low‐level convergence; and second, an increase of the potential for instability of the front through diabatic (e.g. latent‐heat release) potential‐vorticity (PV) redistribution in the upward branch of the circulation. Following saturation of the primary cyclogenesis and the associated along‐front north‐eastwards shift of the upper‐level jet, the low‐level branch of the ageostrophic circulation behaves like a zipper closing on the front, while in the upward branch an anomalous strip of PV builds along the cold front. Isentropic PV maps show that parcles of stratospheric origin embedded in the downward branch of the ageostrophic circulation (a tropopause fold) may have ended in the vicinity of the strip of anomalous low‐level PV, perhaps contributing to an increase in the potential for instability of the surface front.Using the domain‐independent attribution method of Bishop, it is shown that the frontal wave forms on a part of the cold front where: first, the along‐front stretching rate due to the environmental flow decays below the critical threshold value (0.6 × 10−5 s−1) prescribed by theoretical studies; and second, the across‐front frontogenetic component due to the ageostrophic circulation decays. This is consistent with semi‐geostrophic theory, where the cross‐frontal convergence and the stretching deformation are linked through the Sawyer‐Eliassen equation. Results add to other recent observational findings: that the potential for instability, the evolving environmental flow and the induced ageostrophic convergence each have a crucial role in differentiating between frontal waves that will grow and those that will be suppressed.
The frequency spectrum of mountain waves
Authors: Worthington R. M. , Thomas L. 
Published: February 2007

AbstractMountain waves are often assumed to remain steady with respect to time, but measurements of their vertical wind velocity by radar are known to show a clear time dependence. Specifically, the vertical wind W obeys a power law, where the spectral density varies as ω−5/3 over at least three orders of magnitude, and ω is the frequency. Power laws in the frequency and wave‐number spectra of parameters such as horizontal wind have been widely studied, but the frequency spectrum of mountain waves remains, by comparison, poorly understood. It is found that errors arising from the VHF radar measurement technique are not sufficient to explain the power law, and a previous model invoking vortical modes or two‐dimensional turbulence does not, on its own, explain entirely the W data. However, even a surprisingly simple model, in which the mountain‐wave pattern drifts upwind or downwind in a random walk, gives a similar time dependence to the observed W data. The radar data are contrasted with other observations and models, where W time variations have been interpreted in terms of lee‐wave trains with non‐zero phase speed; the relationship to other systems showing time‐scale invariant ωβ behaviour, such as 1/f noise, is also discussed.
The dynamics of Euro‐Atlantic blocking onsets
Authors: Michelangeli Paul‐Antoine , Vautard Robert 
Published: February 2007

AbstractThe mechanisms leading to Euro‐Atlantic blocking are investigated using 31 winters of 500 hPa geopotential height from NMC and ECMWF daily analyses, and a 30 000‐day perpetual winter run from a simple 3‐level quasi‐geostropic model. A composite analysis of the atmospheric flow during days preceding blocking events is carried out. It indicates that Euro‐Atlantic blocking events have two simultaneous main precursors: a high‐latitude retrograding wave‐number 1 pattern at planetary scale, and an enhanced baroclinic wavetrain travelling across the North Atlantic at synoptic scale. These phenomena are identified both in observed and in simulated data.The nonlinear interactions between small scale, large scale and the mean flow, during the establishment of the blocking anomaly, are examined by a budget analysis of the quasi‐geostrophic potential‐vorticity equation. These diagnostics demonstrate that wave‐wave interactions contribute to about 50% of the local amplification of the blocking anomaly while wave/mean‐flow interactions contribute to propagation of the large‐ and small‐scale anomalies and their phase locking.Finally, the question is addressed as to whether the two precursors are necessary and sufficient conditions for blocking onsets by performing various sets of initial‐value experiments performed with the quasi‐geostrophic model. These experiments show that the two precursors occur simultaneously but only the synoptic‐scale baroclinic wavetrain is necessary to trigger Euro‐Atlantic blocking.
Estimating downward clear sky long‐wave irradiance at the surface from screen temperature and precipitable water
Authors: Dilley A. C. , O'brien D. M. 
Published: February 2007

AbstractThis paper presents two parametrizations of the downward clear sky long‐wave irradiance at the surface of the earth in terms of screen temperature and precipitable water. The first parametrization is based on a simple physical model, while the second is an extension of Swinbank's model by the addition of a term involving precipitable water. The parametrizations achieve standard deviations of approximately 5 W m−2 when compared with detailed radiative‐transfer calculations for temperature and moisture profiles in the TOVS Initial Guess Retrieval data set. Both models are more accurate than Swinbank's model, and other published models that are expressed in terms of the screen temperature and surface vapour pressure.
Efficient methods to assimilate remotely sensed data based on information content
Authors: Joiner J. , Da Silva A. M. 
Published: February 2007

AbstractTwo basic approaches have evolved to utilize measurements of radiance (i.e. thermal or scattered solar radiation) by satellite‐borne instruments in data assimilation systems: radiances (raw or cloud‐corrected) may be assimilated directly, or they may be pre‐processed to retrieve geophysical parameters for subsequent assimilation. The retrieval process is often ill‐posed, and therefore requires the use of prior information to constrain the solution. For example, temperature and humidity profiles retrieved using radiances from nadir‐viewing infrared and microwave sounders often incorporate prior information in the form of climatology or forecasts. The use of prior information presents difficulties when assimilating retrievals. Here we present methods to remove prior information from retrievals in order to achieve a more consistent assimilation of the data. In addition, these methods can be used as a data compression device, which can reduce the amount of computation required by some analysis systems compared with radiance assimilation. The methods are implemented and compared in a one‐dimensional assimilation system using simulated data from current and future infrared‐temperature profiling instruments.
Singular vectors and estimates of the analysis‐error covariance metric
Authors: Barkmeijer Jan , Bouttier François , Van Gijzen Martin 
Published: February 2007

AbstractAn important ingredient of ensemble forecasting is the computation of initial perturbations. Various techniques exist to generate initial perturbations. All these aim to produce an ensemble that, at initial time, reflects the uncertainty in the initial condition. In this paper a method for computing singular vectors consistent with current estimates of the analysis‐error statistics is proposed and studied. The singular‐vector computation is constrained at initial time by the Hessian of the three‐dimensional variational assimilation (3D‐Var) cost function in a way which is consistent with the operational analysis procedure. The Hessian is affected by the approximations made in the implementation of 3D‐Var; however, it provides a more objective representation of the analysis‐error covariances than other metrics previously used to constrain singular vectors.Experiments are performed with a T21L5 Primitive‐Equation model. To compute the singular vectors we solve a generalized eigenvalue problem using a recently developed algorithm. It is shown that use of the Hessian of the cost function can significantly influence such properties of singular vectors as horizontal location, vertical structure and growth rate. The impact of using statistics of observational errors is clearly visible in that the amplitude of the singular vectors reduces in data‐rich areas. Finally, the use of an approximation to the Hessian is discussed.
Radiative–convective equilibrium in a three‐dimensional cloud‐ensemble model
Authors: Tompkins Adrian M. , Craig George C. 
Published: February 2007

AbstractA knowledge of radiative convective interactions is key to an understanding of the tropical climate. In an attempt to address this a cloud‐resolving model has been run to a radiative‐convective equilibrium state in three dimensions. The model includes a three‐phase bulk microphysical scheme and a fully interactive two‐stream broadband radiative‐transfer scheme for both the infrared and solar radiation. The simulation is performed using a fixed sea surface temperature, and cyclic lateral boundary conditions. No ‘large‐scale’ convergence, mean wind shear or background vorticity was imposed.The total integration lasted 70 days, and a statistical equilibrium state was reached at all heights after 30 days of simulation in all model variables. It is seen that some variables, such as vertical mass flux, adjust quickly to their equilibrium values while others, such as column‐integrated water amount, domain‐mean temperature and convective available potential energy (CAPE) display variation on a longer 30‐day time‐scale. The equilibrium state had a column‐integrated vapour amount of 42.3 kg m−2, a mean temperature of 258.7 K and a pseudo‐adiabatic CAPE value of 1900 J kg−1. The equilibrium‐state statistics are consistent with tropical observations.The convection does not remain randomly distributed but instead becomes organized, aligning in a band structure associated with high moisture values in the boundary layer. This organization seems to result from interactions between radiation, convection and surface fluxes. The surface‐flux feedback is due to higher boundary‐layer winds, associated with convection, increasing surface fluxes of moisture locally. Horizontally inhomogeneous radiation can act to make clouds longer lasting and also increase convergence into cloudy region. Replacing the wind‐sensitive surface‐flux calculation with a linear relaxation to surface values appeared to largely destroy this organization, as did the use of an imposed horizontally uniform radiative‐heating rate.
Predictive skill of an NWP system in the southern lower stratosphere
Authors: Waugh D. W. , Sisson J. M. , Karoly D. J. 
Published: February 2007

AbstractThe predictive skill of the Australian Bureau of Meteorology's Global Assimilation and Prediction (GASP) system in the southern lower stratosphere is examined using two different sets of diagnostics: (i) conventional verification statistics used in numerical weather‐prediction studies (namely, root‐mean‐square (RMS) error, anomaly correlation, and bias), and (ii) elliptical diagnostics of the polar vortex (defined using potential vorticity on isen‐tropic surfaces). Both sets of diagnostics indicate the same variation in predictive skill for forecasts during October 1994. The stratospheric forecasts are a large improvement over persistence even at seven days, with the performance at seven days being comparable to that in the troposphere of three‐day forecasts. There is large daily variability in the forecast scores for seven‐day forecasts, and the days with below‐average scores occur when the flow (vortex) is rapidly changing. Examination of the differences in the elliptical diagnostics show that the forecast vortex is weaker, less disturbed (i.e. closer to the pole and less elongated), and rotates faster than the analysed vortex. Consistent with a weaker forecast vortex, the minimum polar temperature and maximum zonal wind are underpredicted in the forecasts. The verification statistics in the stratosphere have a large seasonal variation, although the variation is different for different statistics. The GASP RMS errors are largest (smallest) in late‐spring (summer) whereas both the ratio of GASP to persistence RMS error and the anomaly correlation indicate that the performance relative to persistence is best (worst) in late‐spring (summer).