bluuebunny/crossref_metadata_2025_split huggingface dataset:
Structure and evolution of a mesoscale convective system near the British Isles
Authors: Browning K. A. , Hill F. F. 
Published: February 2007

AbstractFrequent digital cloud imagery from Meteosat and precipitation patterns from radar have been analysed together with surface observations and conventional and satellite soundings, to develop a conceptual model of a small mesoscale convective system. The system, observed to the south‐west of England, had many of the features previously found in the United States, where the term mesoscale convective system or complex is applied to a cluster of thunderstorms that generates a common cirrus shield of mesoscale dimensions. A mesoscale region of fairly uniform, essentially stratiform, rain developed beneath the cirrus shield downwind of the active convection. Thunder was widespread throughout the precipitation area which was also characterized by a surface mesohigh. The mesoscale convective system, and another similar one before it, lasted for about half a day each, during which time they produced large falls of rain over an extensive area. The mesoscale convective systems developed in a region of locally strong baroclinicity on the eastern flank of a cut‐off low. They happened to form over the sea and were decoupled from the surface by a cool easterly boundary layer flow. Their development could be traced to the onset of advection from France of warm moist air with a wet‐bulb potential temperature in excess of 18 °C at 850 mb. The arrival of this air coincided with the intensification of the SE‐ly flow around the approaching cut‐off low.
Characteristics of southern hemisphere blocking as determined from a time series of observational data
Authors: Lejenäs Harald 
Published: February 2007

AbstractA time series of 500 mb geopotential heights has been used to assess characteristics of southern hemisphere blocking. A zonal index similar to the index used in an analogous study of northern hemisphere blocking has been used. Characteristics of blocking situations have been computed with a simple algorithm, which has been programmed, and are presented as statistics.We found one preferred region for blocking action: the Australian‐New Zealand region. Two other regions where blocks occur are the Atlantic east of South America, and the Indian Ocean south‐east of South Africa. The number of days with blocked flow is considerably less over the Atlantic and the Indian Ocean than in the Australian‐New Zealand region. The annual variation shows two maxima: one peak value of 19% in April and one extended maximum of c. 20% during June–August in the Australian‐New Zealand region. There is no clear indication of a seasonal variation in the Atlantic and the Indian Ocean region.Investigation of individual blocking episodes reveals that the majority of the blocks have a mean eastward velocity, although the longer they last the more of them are westward‐moving. Longlasting episodes are almost exclusively found in the Australian region.
Errors in atmospheric tidal determination from surface pressure observations
Authors: Cooper Neill S. 
Published: February 2007

AbstractIt has previously been assumed that by taking a sufficient length of data the mean atmospheric tide at any tropical station could be determined to an accuracy of a few percentage points by using surface pressure observations. However, evidence is herein presented which shows that the inferred amplitude of the tide can be changed by over 20% just by changing the instrument used to measure atmospheric pressure. This has occurred both when one barometer has replaced another and when one barograph has replaced another at the same station. It is suggested that the main reason for the inaccuracy is that diurnal temperature variations cause systematic instrumental errors. There are important implications for the accuracy of all tidal determinations that have been made and for the reliability of tidal theory.
The response of the upper ocean to solar heating II: The wind‐driven current
Authors: Woods J. D. , Strass V. 
Published: February 2007

AbstractThe current profile generated by a steady wind stress is disturbed by the diurnal variation of mixed layer depth forced by solar heating. Momentum diffused deep at night is abandoned to rotate inertially during the day when the mixed layer is shallow and then re‐entrained next night when it deepens. The resulting variation of current profile has been calculated with a one‐dimensional model in which power supply to turbulence determines the profile of eddy viscosity. The resulting variations of current velocity at fixed depths are so complicated that it is not surprising that current meter measurements have seldom yielded the classical Ekman solution. However, the progressive vector diagrams do exhibit an Ekman‐like response (albeit with superimposed inertial disturbances) suggesting that the model might be tested by tracking drifters designed to follow the flow at fixed depths. The inertial rotation of the current in the diurnal thermocline leads to a diurnal jet, the dynamical equivalent of the nocturnal jet in the atmospheric boundary layer over land. The role of inertial currents in deepening the mixed layer is clarified, leading to proposals for improving the turbulence parametrizations used in models of the upper ocean. The model predicts that the diurnal thermocline contains two layers of persistent vigorous turbulence separated by a thicker band of patchy turbulence in otherwise laminar flow.
A laboratory study of secondary ice particle production by the fragmentation of rime and vapour‐grown ice crystals
Authors: Griggs D. J. , Choularton T. W. 
Published: February 2007

AbstractLaboratory experiments have been performed to study the strength of rime and vapour‐grown ice crystals to investigate the possibility that fragmentation may occur following collisions in cloud. The results show that fragmentation of rime is very unlikely to occur in natural clouds. Vapour‐grown crystals are found to be considerably more fragile, with dendrites the weakest crystal habit. In the experiments one or two microscopic fragments were produced. Collisions between dendritic crystals and graupel particles may be able substantially to enhance ice crystal concentrations in some clouds.
A model of the development of the droplet spectrum in a growing cumulus turret
Authors: Hill T. A. , Choularton T. W. 
Published: February 2007

AbstractA model is presented which describes the growth of the droplet spectrum in a growing cumulus turret in which the vertical velocities are determined by buoyancy forces. It is found that if the entrainment of dry air occurs continuously and homogeneously across the cloud then no enhanced growth of the largest droplets in the spectrum occurs unless the environment is assumed to be saturated with a lower concentration of CCN than in the air entering cloud base. This model is used as the basis for a comparison between recent cloud microphysical models, with a view to resolving some of the discrepancies in the results associated with them. Emphasis is placed on the propensity of a particular formulation to produce large droplets. If entrainment is allowed to occur discontinuously and non‐uniformly across the cloud then substantial enhanced growth can occur, in agreement with observations, provided that each part of the cloud experiences several distinct entrainment events. It is found that a highly buoyant turret is required to provide the momentum needed for the continual ascent of these regions.
A study of wintertime circulation anomalies during past El Niño events using a high resolution general circulation model. II: Variability of the seasonal mean response
Authors: Palmer T. N. , Mansfield D. A. 
Published: February 2007

AbstractIn this paper the response of a (relatively) high‐resolution 11‐layer general circulation model (GCM) to a number of different sea surface temperature (s.s.t.) anomalies in the tropical East and West Pacific is studied. The integrations were all run for 540 days in a perpetual January mode and had either mean or envelope orography.The s.s.t. anomalies included an enhanced composite El Niño anomaly, the observed anomalies during December 1982, and December‐February 1982–3, and a weak (∼1 K) positive s.s.t. anomaly in the tropical West Pacific.The objective of this study is to assess whether the observed variability in wintertime circulation anomalies between past El Niño events can be modelled by considering the 90‐day mean (‘interannual’) variability of the integration with the composite El Niño s.s.t. anomaly. It is concluded that such observed variability cannot be modelled in this way and that variations in tropical Pacific s.s.t. anomaly from one event to another are crucial in accounting for observed extratropical variability over the Pacific and North America. In particular it is shown that the model's response to the 1982/3 anomaly and the weak West Pacific anomaly are significantly different from the response to the composite El Niño anomaly over the Pacific and North America, and agree well with the appropriate observations for 1982/3 and 1976/7 respectively. It is also concluded that the relatively weak extratropical response during the winter of 1972/3 was related to the fact the s.s.t. anomalies were confined to the East Pacific. By contrast the response to the weak West Pacific anomaly was very strong and statistically significant over the Pacific and North America.
On the relative dispersion of two particles in homogeneous stationary turbulence and the implications for the size of concentration fluctuations at large times
Authors: Thomson D. J. 
Published: February 2007

AbstractA model for the separation of two particles in homogeneous stationary turbulence is derived by considering the analogous process of one‐particle dispersion in inhomogeneous turbulence. Numerical simulations suggest that the intensity of concentration fluctuations tends to zero at large times.
On the development of orographic cyclones
Authors: Radinovic Djuro 
Published: February 2007

AbstractCertain characteristics of cyclone development are compared and, besides the two types (type A and type B) described by S. Petterssen and S. J. Smebye, orographic cyclone development of a third type (type C) is identified. The analysis of dynamic features, thermal structure and kinetic energy change of cyclones which developed in the lee of the Alps during the special observing period of the ALPEX support the view that orographic cyclogenesis should be considered as a specific mechanism of extratropical cyclone development.
A further study of the surface zonal flow predicted by an eddy flux parametrization scheme
Authors: Wu G.‐X. , White A. A. 
Published: February 2007

AbstractThe problem of calculating the surface zonal flow using Green's large‐scale eddy flux parametrizations in spherical geometry is re‐examined. In a 2‐level, quasi‐geostrophic (QG1) model, low‐latitude easterlies and mid‐latitude westerlies of reasonable intensity can be obtained by assuming a more realistic baroclinicity than that applied in a study reported by White in 1977. This result also depends on the use of a more realistic value for the latitude average of a certain transfer coefficient; but no detailed treatment of its spatial variation is found to be necessary. Some further solutions are obtained using a strictly inconsistent formulation in which the Coriolis parameter is allowed its true latitude variation in all terms (in contrast to the QG1 model). In line with the conclusions of White's earlier study it seems in this case that detailed specification of the relevant transfer coefficient's spatial variation would be needed in order to produce a realistic surface zonal flow. Regarding the surface flow problem as a test of Green's parametrization scheme, we conclude that the latter performs quite well if QG1 approximations are applied to the Coriolis parameter. However, further refinement of the scheme is evidently required for good performance when these approximations are not applied.
Analysis methods for numerical weather prediction
Authors: Lorenc A. C. 
Published: February 2007

AbstractBayesian probabilistic arguments are used to derive idealized equations for finding the best analysis for numerical weather prediction. These equations are compared with those from other published methods in the light of the physical characteristics of the NWP analysis problem; namely the predetermined nature of the basis for the analysis, the need for approximation because of large‐order systems, the underdeterminacy of the problem when using observations alone, and the availability of prior relationships to resolve the underdeterminacy.Prior relationships result from (1) knowledge of the time evolution of the model (which together with the use of a time distribution of observations constitutes four‐dimensional data assimilation); (2) knowledge that the atmosphere varies slowly (leading to balance relationships); (3) other nonlinear relationships coupling parameters and scales in the atmosphere.Methods discussed include variational techniques, smoothing splines, Kriging, optimal interpolation, successive corrections, constrained initialization, the Kalman‐Bucy filter, and adjoint model data assimilation. They are all shown to relate to the idealized analysis, and hence to each other. Opinions are given on when particular methods might be more appropriate. By comparison with the idealized method some insight is gained into appropriate choices of parameters in the practical methods.
On the consequences of non‐negative absolute vorticity
Authors: Lunnon R. W. 
Published: February 2007

AbstractIt is shown that if the flow on an isobaric surface is such that the wind and mass fields satisfy the gradient wind equation, then the gradient of the Bernoulli function (the sum of the kinetic energy per unit mass and the geopotential) is everywhere perpendicular to the local wind vector and that, furthermore, if the absolute vorticity is non‐negative, the direction of increase of the Bernoulli function bears the same relation to the wind vector as does the direction of increase of the geopotential to the geostrophic wind. Applications of this relationship are discussed.
Linear wavetrains in models of the stratosphere
Authors: Marks C. J. 
Published: February 2007

AbstractExperiments with a primitive equation model of the stratosphere and mesosphere are described which investigate the effect on the stratosphere of the growth of localized, stationary forcing in the troposphere. Two basic states are used which are representative of stratospheric flow in early and late winter in the southern hemisphere. Simple mechanistic models are then introduced to examine the wave phenomena which occur during the first 10 days of the experiments when the dynamics are known to be linear.The localized forcing grows to a peak amplitude of 600 gpm and half‐width of 15° of latitude, values typical of the mid‐latitude upper troposphere. Stratospheric travelling waves are excited with phase velocities which are sensitive to the zonal mean winds and to the speed of the switch‐on of the forcing, and wave activity spreads quickly from the localized forcing with a typical vertical group velocity of 10 km d−1. Strong horizontal curvature in the basic states can profoundly affect the vertical and horizontal structures of the wavetrains: mid‐latitude regions of negative northward potential vorticity gradient in the stratosphere (as seen in the southern hemisphere in late winter) lead to the confinement and amplification of waves in high latitudes. Three‐dimensional diagnostics of wave activity propagation reveal wave reflections missed by zonally‐averaged EP flux diagnostics.Significant wave reflection and trapping occur on the flow from late winter, eventually producing a steady linear wavetrain with an intense local centre of circulation in the upper stratosphere, the maximum anomaly being ∼1000gpm at 1mb. It is argued that such wavetrains are unrealistic because nonlinear ageostrophic winds drive the dynamics away from any linear steady state.
Stability of cloud‐topped boundary layers
Authors: Kuo Hung‐Chi , Schubert Wayne H. 
Published: February 2007

AbstractAccording to the thermodynamic theory of cloud top evaporative instability, persistent stratocumulus should be observed only when the cloud top jumps in equivalent potential temperature θ and total water mixing ratio r satisfy the stability condition Δθ > k(L/cp)Δr, where k ± 0.23. Using observations of persistent mid‐latitude and subtropical stratocumulus we find that the above stability condition is violated in many cases. In an attempt to understand how stratocumulus can persist under apparently unstable conditions we first review the thermodynamic instability theory and then develop a dynamical framework using a two‐dimensional Boussinesq moist convection model with spectral discretization and with resolution sufficient to simulate cloud top processes. Idealized initial value experiments confirm that, when the above condition is violated, evaporative instability leads to cloud breakup through sequential dissections of existing cloud. However, initial conditions close to the critical stability line (e.g. Δθ, (L/cp)Δr = –6K, –15 K) lead to cloud breakup with a characteristic cloud half‐life of several hours, while physically realistic initial conditions far from the critical stability line (e.g. Δθ, (L/cp)Δr = –16 K, –25 K) lead to more rapid breakup, with a cloud half‐life on the order of 1/2 hour. When evaporative instability is so weak that the associated cloud half‐life is as long as several hours, other physical processes (such as surface evaporation) can apparently moisten the boundary layer rapidly enough to mask the cloud breakup process.
Extension of explicit radiance observations by the Stratospheric Sounding Unit into the lower stratosphere and lower mesosphere
Authors: Nash J. 
Published: February 2007

AbstractIt is shown that radiance observations centred at four additional levels in the stratosphere and lower mesosphere can be synthesized utilizing the measured limb‐brightening of the radiance observations of the Stratospheric Sounding Unit (SSU). These additional radiance measurements double the vertical range of explicit radiance observations from the SSU. As a series of Stratospheric Sounding Units have been flown on the TIROS‐N series of NOAA satellites, observations of zonal mean radiance, with higher vertical resolution than has generally been obtainable with nadir‐viewing satellite radiometers, are available since 1979 from the lower stratosphere to the lower mesosphere.The additional radiances have allowed operational monitoring of the long term climatology of the lower mesosphere by means of satellite observations to commence.
A Lagrange multiplier approach for the metric terms of semi‐Lagrangian models on the sphere
Authors: Coté J. 
Published: February 2007

AbstractThe horizontal momentum equation of the shallow‐water equations on the sphere is written in 3‐D vector form using the undetermined Lagrange multiplier method. The direct time discretization of this equation is proposed as an alternative to the usual approach, i.e. time discretization after explicit removal of the constraint by the use of generalized coordinates. The proposed procedure is applicable to any coordinate system and eliminates an instability associated with the metric term in a spherical grid spectral semi‐Lagrangian model. For a polar‐stereographic grid finite‐element model, equivalent results and a gain in efficiency are obtained. The formalism can be extended to multilevel models.
Turbulent shear flows over low hills
Authors: Hunt J. C. R. , Leibovich S. , Richards K. J. 
Published: February 2007

AbstractA general analysis is developed for turbulent shear flows over two‐ and three‐dimensional hills with low‐slopes which allows for a wide range of upwind velocity profiles, such as those caused by wakes of upwind hills, roughness changes, or changes in stratification. In this paper the atmosphere is assumed to be neutrally stable and the half‐lengths of the hills, L, are large compared with their heights, H, which are very large compared with the roughness length zo. The general structure of the solution is defined by dividing the flow into two regions, each of which is divided into two sublayers: an inviscid outer region composed of an upper layer in which there is potential flow when the atmosphere is neutrally stable, and a middle layer in which the wind shear dominates; and an inner region of thickness l ≤ L in which the effects of perturbation shear stresses are confined. The latter region is divided into two: a shear stress layer where the shear stresses, although weak, determine that the maximum of the perturbation velocity is located in this layer; and an inner surface layer of thickness ls where the shear stress gradient varies rapidly and the perturbation velocity tends to zero. The details of the middle layer are given here for different kinds of upwind profiles, including logarithmic, ‘power law’ and linear profiles. It is shown that the analysis can be extended to allow for nonlinear inertial effects in the middle layer. Analytical solutions are derived for the inner region as asymptotic expansions in δ = [ln(l/zo)]−1, which is assumed to be small, and this shows that ls ∼ zo(l/zo)1/2.The results of the analytical model are compared with our own and with previously published numerical computations of the full equations (applying the same assumptions used for calculating the turbulent shear stresses as used in the analytical work), which have largely been validated against full‐scale measurements. These results confirm that the relative increase of surface stress is significantly greater than the increase of wind speed near the surface except when there is no upwind shear (as for example in a logarithmic boundary layer when the roughness length tends to zero).Finally, the paper shows that the outer regions of laminar (or constant eddy viscosity) and of turbulent flows over hills are broadly similar, but that the effects of the flow in the inner region on the outer regions are much smaller in the latter case.
A practical Lagrangian method for relating scalar concentrations to source distributions in vegetation canopies
Authors: Raupach M. R. 
Published: February 2007

AbstractThe transfer of scalar atmospheric constituents within vegetation canopies cannot be predicted from simple gradient‐diffusion theory. To replace gradient‐diffusion theory in this context, an analytic Lagrangian theory is developed which predicts the concentration field of a scalar atmospheric constituent emanating from a spatially extensive source in an inhomogeneous turbulent flow. In this ‘localized near‐field’ theory, the mean scalar concentration C is expressed as the sum of a diffusive far‐field contribution Cf which obeys gradient‐diffusion theory, and a non‐diffusive near‐field contribution Cn which is calculated for each source element by assuming the turbulence to be locally homogeneous. It follows that Cf provides the large‐scale background variation and Cn the detailed local structure of the C profile. The theory applies to both non‐advective and advective situations. Comparison with random‐flight predictions of C shows that the assumptions of the theory (which are exact in homogeneous turbulence) are adequate for inhomogeneous turbulence typical of real canopies. The random‐flight predictions also show that vertical velocity skewness (not accounted for in the localized near‐field theory but typically of order −0.5 to −1 in canopies) has only a small effect on C.
Numerical experiments on urban heat island intensity
Authors: Richiardone R. , Brusasca G. 
Published: February 2007

AbstractNumerical experiments with a two‐dimensional model have been performed to study diurnal urban circulation and heat islands in calm conditions. Vertical structures of rural and urban planetary boundary layers, heat island intensities and circulations are examined in different conditions of stability, surface heat fluxes and city width. A simple expression to predict diurnal evolution of the urban‐rural temperature difference is proposed.