bluuebunny/crossref_metadata_2025_split huggingface dataset:
Freezing modes of riming droplets with application to ice splinter production
Authors: Griggs D. J. , Choularton T. W. 
Published: February 2007

AbstractLaboratory studies of riming have shown that, provided the heat loss is fairly symmetrical, the internal freezing pattern of an accreted drop shows two distinct modes in the temperature range −3°C to −9°C and a transition to a third below this temperature. (1) At high temperatures the freezing is dominated by an ice front growing outwards from the rimer with a somewhat enhanced growth rate around the drop edge. (2) At temperatures below about −5°C two ice fronts are evident, one from the substrate, one from the remote edge of the drop. (3) At temperatures of −9°C and below the freezing is essentially symmetrically inwards. Freezing modes (1) and (2) sometimes give rise to protuberances which may be associated with the Hallett‐Mossop ice splinter production. These are suppressed by the addition of ammonia to the drop. The three modes of freezing are accounted for in terms of the pattern of dendritic growth during the first stage of freezing, and it is suggested that this explains the low temperature cut‐off in secondary ice particle production.
A laboratory experiment on momentum and heat transfer in the stratified surface layer
Authors: Fukui Keisuke , Nakajima Masamoto , Ueda Hiromasa 
Published: February 2007

AbstractStratification effects on transport processes in the atmospheric surface layer or in the wall region are investigated by a laboratory experiment. A fully developed stratified condition was attained in turbulent flow between horizontal parallel plates kept at different temperatures. The experiment was performed both for stable and unstable conditions and provided conclusive data on stratification effects. A local gradient model is proposed, based on the concept that stratification effects may be evaluated at the same non‐dimensional distance from the wall z+ (= zu*w/vw u*, vw being friction velocity and kinematic viscosity, respectively, at the wall), the effects depending only on ε (ε z/L; L being the Monin‐Obukhov length). A comparison of the shear functions with previous field and laboratory experiments shows excellent agreement. By use of this concept, the measured intensities of velocity and temperature fluctuations near the wall are also well correlated with ε.
The propagation of eddies in diffluent jetstreams: Eddy vorticity forcing of ‘blocking’ flow fields
Authors: Shutts G. J. 
Published: February 2007

AbstractNumerical experiments are performed with barotropic models to test a hypothesized mechanism by which barotropic eddies can, through Reynolds stresses, reinforce blocking flow patterns. Eddies propagating into a split Jetstream suffer an east‐west compression and north‐south extension of their vorticity fields and this enhanced, local enstrophy cascade is associated with energy transmission to the straining flow (i.e. the blocking flow field) and a characteristic pattern of vorticity forcing by transient motion.As a first step in demonstrating this mechanism, the barotropic vorticity equation is integrated in a form linearized about a state chosen to represent a split Jetstream with a prescribed eddy forcing function upstream. After thirty days of model integration, the time‐mean eddy kinetic energy, enstrophy and vorticity flux divergence are calculated, together with mean eddy vorticity flux vectors. The resulting eddy vorticity forcing field can be used to determine the second‐order induced flow.Experiments with the nonlinear barotropic vorticity equation show that dipole blocking patterns can be created simply by the introduction of an eddy generator into sufficiently weak, uniform westerly flow. Eddies alone are responsible for the block since the time‐mean vorticity equation has no external forcing functions such as an orographic term. Monthly mean statistics are again calculated and terms in the time‐mean vorticity equation are compared. Anticyclonic eddy vorticity forcing appears just upstream of the ‘blocking high’ in agreement with data analysed by Illari (1982) for the anomalous circulation over Europe in July 1976. Model flow fields fluctuate in a manner highly reminiscent of blocking episodes as observed in 500mb contour maps with strong bursts of southerly winds on the western flank of blocking highs during the approach and subsequent stagnation of each depression. The proposed straining mechanism and model simulations are in agreement with early observational studies of blocking.
The anisotropy of diffuse solar radiation determined from shade‐ring measurements
Authors: Steven M. D. 
Published: February 2007

AbstractWhen a shade‐ring is used to shield a pyranometer from direct solar radiation, a correction to the measured diffuse radiation is necessary to account for diffuse radiation occluded by the ring. The shade‐ring correction depends on the dimensions and positioning of the ring and on the anisotropy of sky radiance. In this analysis these factors are separated using a simple model of sky radiance as the sum of a uniform background and a circumsolar component. The predictions of the model are tested against standard distributions of radiance and measured shade‐ring corrections for clear skies. The model is also used to describe average distributions of radiance for cloudy skies and is in good agreement with the limited measurements available. This analysis enables measurements of solar radiation to be partitioned more exactly into direct and diffuse components and may also be applicable to the estimation of diffuse irradiance on other shaded surfaces.
Baroclinic instability in a long wave environment. Part II: Ageostrophic energy conversions
Authors: Grotjahn Richard 
Published: February 2007

AbstractA linear, quasi‐geostrophic, numerical, spectral, initial value, model is used to examine the stability properties of two long wave flows on a mid‐latitude β plane. In the ‘barotropic case’ the long wave amplitude is independent of height; in the ‘baroclinic case’ the long wave amplitude varies from zero at the bottom and top to a maximum near the model's ‘tropopause’. The initial structure of the unstable short waves incorporates the eigenfunctions for a similar, straight‐flow version of this problem. The solutions rapidly converge to a repeatable structure.This report emphasizes the eddy energetics and especially its horizontal variation due to the long wave. In deriving the eddy energy equation three ‘ageostrophic’ conversions are found along with the two quasigeostrophic conversions. While the ageostrophic conversions vanish on a volume average, they are locally important; they redistribute the eddy properties in each of the three directions; none are described by any previous author. The vertical ageostrophic conversion is the difference between the conversion terms shared by the eddy available potential and eddy kinetic energy equations. The horizontal ageostrophic conversions steer the eddies along storm tracks which would not be anticipated on the basis of the basic flow's direction or vertical shear. These conversions create significant cross‐isobaric motion of eddies even though the long wave amplitude is modest compared with the zonal mean flow. This motion is not reproduced in a standard barotropic model wherein the ageostrophic conversions are all identically zero. On a global average the eddies are baroclinically growing; the barotropic conversion is small and damping.
Intercomparison of planetary‐scale diagnostics derived from separate satellite and radiosonde time‐mean temperature fields
Authors: Miles T. , Chapman W. A. 
Published: February 2007

AbstractThe planetary‐scale components of the extratropical northern hemisphere troposphere‐stratosphere 1973–74 winter circulation are diagnosed using separate time‐mean temperature fields based on radiosonde and satellite observations. Meridional cross‐sections of zonal wind together with, for zonal wavenumbers 1, 2 and 3, the streamfunction amplitude, phase and Eliassen‐Palm flux are displayed, with the relative accuracy of the satellite‐derived diagnostics assessed through comparison with the ‘ground‐truth’ radiosonde information. The satellite and radiosonde diagnostics compare most favourably in terms of zonal wind speed and shear, direction of wave propagation and meridional wave structure—all of which are closely related to the differential properties of the atmospheric temperature field. The intensity of the satellite‐derived patterns of tropospheric wave propagation is underestimated due to the effects of spatial smoothing and residual cloud contamination present in the satellite radiance measurements.
Mean state of the troposphere over south‐east Asia and the East Indies, December 1978
Authors: Mower R. N. , Chu J.‐H. , Martin D. W. , Auvine B. 
Published: February 2007

AbstractThe mean state of the troposphere over south‐east Asia from 8–30 December 1978 is described.In the lower troposphere, a jet was observed over the South China Sea extending south‐westward from the Philippines. Evidence suggests that this north‐easterly jet was supported by downstream blocking induced by the Borneo land mass. At the equator, the jet had the appearance of being channelled between the highlands of Borneo and Sumatra. On its left flank, near the west coast of Borneo, was a semi‐permanent and quasi‐stationary vortex. Being warm core, this vortex was probably maintained, in part, by latent heat release in cumulus convection.By partitioning the data into means at 00 and 12 GMT, the intensity of the rising branch of the Hadley circulation over Borneo was shown to increase by a factor of two, from −100 × 10−5 to −200 × 10−5mb s−1, from 00 to 12 GMT.
Super‐rotation and diffusion of axial angular momentum: II. A review of quasi‐axisymmetric models of planetary atmospheres
Authors: Read P. L. 
Published: February 2007

AbstractThe role of angular momentum quasi‐conservation in generating and maintaining axisymmetric flows characterized by a net global and/or local super‐rotation is reviewed. The conditions under which angular momentum per unit mass, m, can be diffused by Newtonian viscosity against its mean gradient in the meridional plane in a compressible, spherical fluid shell are explored in detail, and integral constraints on the form of the diffusive flux of m are derived for a steady flow. The implications of the results for simple models of planetary and stellar atmospheres are explored, with particular reference to the analogy between molecular and ‘eddy’ viscosity (as represented in mixing‐length theory), and various mixing hypotheses for eddy transfer properties. The results are relevant to zonally‐averaged models of the circulation of planetary and stellar atmospheres since, in the absence of molecular viscosity, the Eliassen—Palm flux of m must obey similar integral constraints to those applicable to the viscous flux in a purely axisymmetric system. Despite the non‐Newtonian character of realistic eddies in a baroclinic atmosphere, therefore, studies of simple axisymmetric systems with viscous diffusion can shed some useful insight into ways in which atmospheres, such as those of the earth, Venus, Titan, the Jovian planets and the sun, may acquire significant components of super‐rotation and prograde differential rotation, especially (though not exclusively) near the equator.
Comments on the computational stability of Gadd's adjustment and advection schemes for a split explicit model
Authors: Riphagen H. A. , Burger A. P. 
Published: February 2007

AbstractIn this note we investigate two‐level difference schemes for the solution of the adjustment and advection stages of a split atmospheric model by means of von Neumann stability analyses. The stability properties of the schemes proposed by Gadd are discussed using generalized formulations of which his schemes are special cases. In particular it is shown (in one and two dimensions) that the criterion for linear computational stability for the modified Lax—Wendroff advection scheme can be relaxed somewhat if Gadd's choice of parameter is extended to allow negative as well as positive values.
Convectively forced internal gravity waves: Results from two‐dimensional numerical experiments
Authors: Clark Terry L. , Hauf Thomas , Kuettner Joachim P. 
Published: February 2007

AbstractTwo‐dimensional numerical simulations were performed to investigate the nature of tropospheric internal gravity waves of the type which are observed to occur above active thermal convection over an unstable boundary layer. These gravity waves are believed to be excited by a combination of pure thermal forcing and by the boundary layer eddies and cumulus clouds acting as obstacles to the flow in the presence of mean environmental wind shear.Large amplitude internal gravity waves were obtained in the simulations with amplitudes and horizontal scales similar to the 12 June 1984 aircraft observations over western Nebraska. This was a day with strong wind shear in the lowest 3 km above the ground and with scattered cumulus clouds topping the boundary layer. The simulations show that there is significant difference between the early time solutions (as might be predicted by linear theory) and late time solutions for the boundary layer eddy structure. A layer interaction occurs in which gravity waves of the stable layer are excited by the boundary layer convection. There is evidence to suggest that this layer interaction occurs both with and without shear but that it is stronger in the presence of low‐level shear. Results indicate that shear (or the obstacle) effect is a more efficient generator of gravity waves than is the pure thermal forcing. The simulations show that the gravity waves initially forced by the boundary layer eddies lead to a feedback mechanism that acts to organize the boundary layer eddies and the cumulus convection. The solutions suggest that the character of fair weather convection (moist or dry) is a non‐local problem involving at times the full depth of the troposphere.The clouds produced in the simulations have very little influence on the wave field or boundary layer eddy structure as they are relatively small cumuli. On the other hand the clouds are strongly influenced by the interactions between the wave and eddy fields. Upshear growth of cumulus clouds similar to that which is frequently observed in nature is reproduced in the simulations. The development of feeder (‘feeder’ is used here in a dynamical sense only) clouds on (typically) the upshear side of the cloud is found to be a result of the interaction between the gravity wave field and the dry and moist convection. The relative phase velocity between the gravity waves and the cloud plays a crucial role in determining the character of the cumulus cloud growth in the present simulations. These simulations suggest that the dynamics both internal and external to the boundary of a cumulus cloud is a complicated mix between wave dynamics and the usually considered convection dynamics. A brief discussion of the implications of the present results to cloud boundary baroclinic instability dynamics is also presented.
Objective quality control of observations using Bayesian methods. Theory, and a practical implementation
Authors: Lorenc A. C. , Hammon O. 
Published: February 2007

AbstractThis work attempts to provide a theoretical framework for the quality control of data from a large variety of types of observations, with different accuracies and reliabilities. Bayes' theorem is introduced, and is used in a simple example with Gaussian error distributions to derive the well‐known formula for the combination of data with errors. A simple model is proposed whereby the error in each datum is either from a known Gaussian distribution, or a gross error, in which case the observation gives no useful information. Bayes' theorem is applied to this, and it is shown that usual operational practice, which is to reject outlying data and to treat the rest as if their errors are Gaussian, is a reasonable approximation to the correct Bayesian analysis. Appropriate rejection criteria are derived in terms of the observational error and the prior probability of a gross error.These ideas have been implemented in a computer program to check pressure, wind, temperature and position data from ships, weather ships, buoys and coastal synoptic reports. Historical information on the accuracies and reliabilities of various classifications of observation is used to provide prior estimates of observational errors and the prior probabilities of gross error. The latter are then updated in the light of information from a current forecast, and from nearby observations (allowing for the inaccuracies and possible gross errors in these) to give new estimates. The final probabilities can be used to reject or accept the data in an objective analysis.Results from trials of this system are given. It is shown to be possible using an archive generated by the system to update the prior error statistics necessary to make the method truly objective. Some practical case studies are shown, and compared with careful human quality control.
Observations within two regions of charge during initial thunderstorm electrification
Authors: Dye J. E. , Jones J. J. , Weinheimer A. J. , Winn W. P. 
Published: February 2007

AbstractAirborne electric field measurements in two small thunderstorms in New Mexico show the existence of a narrow region of charge in each storm during the early stage of electrification. In one case a net negative region of charge was observed at about 7 km (‐ 12°C) about 1.5 km below the radar cloud top, when the electric field was only 600 V m−1, and could be accounted for by total charge of −0–01 C with a maximum net space charge density of −0.15 nCm−3, assuming spherical symmetry. This region of charge was about 500 m across and appears to have been associated with an updraught‐downdraught transition zone. In the other cloud, a region of net positive charge, also about 500m across, was detected at 7.7 km (‐20°C) about 500 m below the radar cloud top, when the electric field was about 2000 Vm−1. In both regions of charge, supercooled liquid water and ice particles including graupel were present, and ice particle concentrations, sizes, and collision rates were at a relative maximum, suggesting that the charge generation occurred via a precipitation‐based mechanism.
Estimates of cyclone track predictability. I: Tropical cyclones in the Australian region
Authors: Fraedrich K. , Leslie L. M. 
Published: February 2007

AbstractA nonlinear systems analysis is applied to the tracks of 249 tropical cyclones (with a six‐hour sampling time) in the Australian tropics for the period 1959–1980. First estimates are obtained of the degree of their chaotic, or irregular, behaviour. The degree of chaos is estimated by normalizing all trajectories to a common initial position and measuring the average rate at which initially close pairs of pieces of trajectories diverge.It was found from the correlation integrals calculated for the tropical cyclone tracks that the dimensionality of the underlying dynamical processes appears to be between six and eight, and that the time scale for e‐folding error growth is about one day. The results of this study therefore suggest that the movement of Australian region tropical cyclones is predictable deterministically up to about 24 hours in advance. Beyond that limit, consideration must be given to statistically based techniques.These findings were supported further by comparing the rate of growth of the observed Australian region tropical cyclone position variance with that derived from a random walk model superimposed on a mean drift. The correspondence was very close, with both the empirical and theoretical position variances growing linearly in time after approximately the first 18 to 24 hours, confirming that stochastic models have a role to play in forecasts beyond 24 hours.
Three‐dimensional numerical experiments on convectively forced internal gravity waves
Authors: Hauf Thomas , Clark Terry L. 
Published: February 2007

AbstractResults are presented of thermally forced dry and moist convection and the associated gravity wave fields from three‐dimensional numerical simulations using a non‐hydrostatic anelastic model. This paper extends earlier two‐dimensional simulations to include effects of the third spatial dimension employing a very similar environmental speed‐shear case for the study. The present simulations produce scattered fair weather cumuli in agreement with observations. In many important respects, the physical response is quite similar to that obtained in the earlier two‐dimensional calculations. The near‐uniform surface sensible heat flux results in Rayleigh modes filling the convective boundary layer (CBL) to begin with, whereas later, after convective motions start interacting with the overlying stable layer, larger horizontal scale deep modes become evident and in some cases dominant. The eigenfunction structure of these dominant forced normal modes consists of boundary layer eddies in the lower levels and gravity waves above. They are important organizers of the cumulus convection. As in the earlier two‐dimensional simulations, the efficiency of gravity wave excitation was found to be very sensitive to the mean wind shear in the region spanning the CBL and the overlying stable layer. The dominant horizontal wavelength in the shear direction ranges between 10 and 15 km in the free atmosphere whereas it peaks at about 6 km in the CBL.The strong difference between the preferred directions of alignment for the eddies in the CBL (rolls aligned with the mean shear) and the overlying waves (aligned with lines of constant phase normal to the shear) results in overall broken conditions. The boundary layer motions are organized in broken ‘varicose‐like’ rolls aligned approximately with the mean shear. The overlying waves show a somewhat more scattered pattern. This scattered‐type dominant forced modal response combined with the nonlinear effect of the clouds themselves results in a cloud pattern revealing a high degree of randomness. This cloud field randomness occurs in spite of a near‐zero horizontal wavenumber structure to the surface sensible heat flux.Exchanges of momentum between convective and mean motions in the CBL result in strongly curved stress profiles and a mixing out of the initial boundary layer shear. Sensitivity tests were performed where the mixing of momentum was partially compensated by the addition of low‐level pressure gradient terms.
The chemistry of a mesoscale rainband
Authors: Hegg Dean A. , Rutledge Steven A. , Hobbs Peter V. , Barth Mary C. , Hertzman Owen 
Published: February 2007

AbstractA diagnostic modelling study is presented of the chemistry of a cold‐frontal rainband observed on the mid‐Atlantic coast of the United States. Both the chemical and physical data sets were sufficient to allow realistic evaluation of the 2‐D model employed. Utilizing the model, assessment is made of the importance of in‐cloud chemical processes to the deposition on the ground of selected trace chemicals; in‐cloud sulphate production was found to be of great importance. The effects of the rainband on the vertical transport of trace chemicals, and the effects of precipitation‐formation mechanisms on chemical scavenging, are also discussed.