bluuebunny/crossref_metadata_2025_split huggingface dataset:
An analysis of the vertical structure equation for arbitrary thermal profiles
Authors: Cohn Stephen E. , Dee Dick P. 
Published: February 2007

AbstractThe vertical structure equation is a singular Sturm‐Liouville problem whose eigenfunctions describe the vertical dependence of the normal modes of the primitive equations linearized about a given thermal profile. The eigenvalues give the equivalent depths of the modes. We study, for arbitrary thermal profiles, the spectrum of the vertical structure equation and the appropriateness of various upper boundary conditions.Our results depend critically upon whether or not the thermal profile is such that the basic state atmosphere is bounded. This is not surprising since, as we point out, the vertical structure equation is not meaningful at large heights because of the traditional shallowness approximations which are used to derive the primitive equations. The nature of the spectrum of a singular Sturm‐Liouville problem depends only on the behaviour of the coefficients of the differential equation near the singular boundary. Spectral results therefore have no physical significance for unbounded atmospheres.For all bounded atmospheres we show that the spectrum is totally discrete, regardless of details of the thermal profile. For the barotropic equivalent depth, which corresponds to the lowest eigenvalue, we obtain upper and lower bounds which depend only on the surface temperature and the atmosphere height. All eigenfunctions are bounded, but always have first derivatives which become unbounded near the top. We prove that the commonly invoked upper boundary condition that vertical velocity must vanish as pressure tends to zero, as well as a number of alternative conditions, are well posed.For unbounded atmospheres, on the other hand, we show that typically there is a continuous spectrum, that the boundary condition of vanishing vertical velocity is not well posed, and that the eigenfunctions. if any. are unbounded.
Kinetic equations for 2‐D turbulent flow
Authors: Tapp M. C. 
Published: February 2007

AbstractA formal BBGKY kinetic theory for inhomogeneous 2‐D fluid turbulence is developed from the Liouville equation for Fourier mode amplitudes. The analysis indicates that all members of the hierarchy are present except in the special case of unforced barotropic flow where only the two‐coefficient distributions are explicitly absent. A graphical representation of the BBGKY hierarchy is presented, providing a vivid picture of the interaction processes involved. Equations representing the turbulence dynamics of correlations are derived for binary‐ and triad‐wave groups and exhibit a highly nonlinear structure. It is shown that, in the inhomogeneous case, binary‐wave interaction via the mean flow plays an important role, in contrast with accepted homogeneous theories. A phenomenological approximation belonging to the eddy‐damped class of turbulence models is developed and yields a set of kinetic equations for which conservation laws and an H‐theorem can be established. The kinetic equations can be recast into a familiar Fokker‐Planck form written in intrinsic coordinates and demonstrate the approach to absolute equilibrium for the case of forced barotropic inhomogeneous flow. A certain information measure is shown to be a maximum at t = 0 for the exact dynamics and suggests that forcing gives rise to an increased rate at which information is lost.
Tidal fluctuations as seen in ECMWF data
Authors: Hsu Huang‐Hsiung , Hoskins Brian J. 
Published: February 2007

AbstractDiurnal and semi‐diurnal fluctuations in the troposphere and lower stratosphere during the 1986 summer and 1986/87 winter, deduced from data of the European Centre for Medium Range Weather Forecasts, are documented. In general, the tidal fluctuations are consistent with the results of previous investigators based on station data. It thus suggests the adequacy of the data generated by the analysis‐forecast system for presenting the global structure of tidal fluctuations in the troposphere.The semi‐diurnal fluctuations in ECMWF data are consistent with the predictions of tidal theory. The diurnal fluctuations, which are more sensitive to local factors such as the sea‐land contrast and topography, exhibit a complicated geographical dependence. It appears that the current tidal theory, by neglecting the lower boundary effects and zonally varying factors, is incomplete for explaining the diurnal tidal motions in the troposphere.
A comparison of satellite‐retrieved parameters with mesoscale model analyses
Authors: Saunders R. W. 
Published: February 2007

AbstractA scheme to retrieve quantitative products from radiances recorded by the AVHRR (Advanced Very High Resolution Radiometer) on NOAA satellites over the same area as the Meteorological Office mesoscale model is outlined. Satellite products and model fields are both averaged over fifth of a degree latitude/longitude grid squares. Results for one satellite overpass over the British Isles on 21 June 1986 are described in detail. Fields of surface temperature over land and sea, fractional cloud cover and cloud top temperature retrieved from the AVHRR data are compared with the corresponding model‐analysed fields nearest in time to the satellite overpass. The AVHRR and model sea surface temperatures are in reasonable agreement (∼0.7K), although the satellite data provide the more detailed field. The satellite land surface brightness temperatures could not be easily compared with the model ground temperatures as the former are radiative skin temperatures whereas the latter are related to screen temperatures and near‐surface lapse rates. Differences of up to 10 K are noted in clear sky situations. The satellite and model fractional cloud cover fields are in reasonable agreement though the satellite fields did highlight one or two areas where the model analysis is in error. This is mainly in areas devoid of conventional observations. Fields of satellite cloud top temperatures show more detail and are more realistic than the corresponding model fields. This is not surprising as cloud top temperatures cannot be easily inferred from the surface observations analysed by the model. Satellite products, such as the ones described here, could in the future be used operationally to improve mesoscale or regional model analyses.
A study of air particle motions during a stratospheric warming and their influence on photochemistry
Authors: Austin John , Butchart Neal 
Published: February 2007

AbstractMiddle stratospheric ten‐day trajectories are calculated using satellite data for air particles initially situated on the edge of the circumpolar vortex during the minor warming of January 1979. The trajectory behaviour is consistent with the observed evolution of the isentropic distribution of potential vorticity. Further, from an analysis of a large number of trajectories calculated from closely spaced initial positions it is argued that the trajectories are realistic indicators of transport. In particular, the trajectories divide naturally into a limited number of distinct categories representing either vortex erosion or the continued advection of air around the distorted vortex edge. None of the categories indicate horizontal transport of air into the vortex.A comprehensive photochemical model is integrated along the trajectories and the results are compared with similar integrations along trajectories calculated for a suitably chosen axially symmetric reference atmosphere. In this way it is established that, for the late January warming, the distortion of the vortex from zonal symmetry led to substantial decreases in the destruction of ozone and in the production of nitric acid on the edge of the vortex. Once the particles are stripped from the vortex by the wave breaking, they experience only small changes in photochemistry if the particles remain in middle latitudes, or much larger changes if they are transported as far as the subtropics. Results from the photochemical model are also compared with co‐located satellite observations of the constituents ozone, water vapour and nitric acid for the different categories of trajectories. In many cases there are statistically significant discrepancies between observed and modelled concentrations. It is suggested that these discrepancies are due to the presence of other particles transported along trajectories from different locations, and therefore with different chemical properties, contaminating the coarse resolution co‐located observations. The findings are entirely consistent with the view that outside the vortex the wave breaking will mix or generate small unresolvable scales in the tracer fields. Some implications of the results for the photochemical modelling of the stratosphere are discussed.
The equatorial Pacific Ocean prior to and during El Niño of 1982/83—a normal mode model view
Authors: Bigg Grant R. , Blundell Jeffrey R. 
Published: February 2007

AbstractAn equatorial ocean model with multiple vertical and horizontal modes is developed for the Pacific Ocean, and run for the five years 1979 to 1983. In the year prior to the warm event, or El Niño, of 1982/83 the model shows an unusually strong and delayed seasonal Kelvin wave generated in the western Pacific. By analogy with the ocean‐atmosphere coupling mechanism postulated by several recent authors for explaining the El Niño/Southern Oscillation cycle, it is suggested that this wave could ultimately have been instrumental in causing the subsequent El Niño to be so strong. The course of events during El Niño is described in terms of equatorial normal modes and related to the evolving wind field. The strength and importance of the second vertical mode during the event is emphasized. Evidence for a coupled system decay mechanism is also found. The model is compared with several types of observations, but principally the expendable bathythermograph (XBT) data of the Pacific Ship of Opportunity Programme. These data are analysed from a normal mode perspective which is related to that of the model, supporting the main conclusions derived from it. Such data have not been analysed previously in this manner, converting observed variables directly to theoretical equatorial wave amplitudes. The adequacy of the data for such analysis is discussed.
Response to small‐scale forcing on two staggered grids used in finite‐difference models of the atmosphere
Authors: Janjić Zaviša I. , Mesinger Fedor 
Published: February 2007

Abstract‘Source‐sink’ shallow water integrations are performed, using two limited area models with the Coriolis terms included. One of the models was defined on the staggered C grid, and the other on the semi‐staggered E/B grid. For each grid, experiments were performed with grid sizes of 250, 125 and 62.5 km. A source and a sink, of constant intensity, were placed symmetrically in the central part of the domain. They covered, for the three resolutions considered, areas of one, four and sixteen grid points, respectively.At the sink ‘point’, after 24 hours, substantial differences in the depth of the water existed between the lowest resolution experiments on the two grids. The differences were very much reduced in the two 125 km experiments, and were virtually absent in the highest resolution, 62.5 km, experiments. With increasing resolution, the depths at the sink on the two grids converged with about equal rapidity towards a value in between the two lowest resolution values. As to be expected on the basis of the geostrophic adjustment theory when applied to the spatially discretized systems, the depth on the low resolution C grid solution was found to be an underestimate, and that on the E grid an overestimate, of the true value. It is demonstrated that the rate of convergence on the E grid can be improved by increasing the weight of the modification introduced to cope with the grid separation problem.An implication of the results obtained in the simple experiments reported here is that forcing schemes should be considered which would avoid forcing at single grid points. Where appropriate, simultaneous forcing at several neighbouring points may be an attractive alternative approach.
An extended version of a nonhydrostatic, pressure coordinate model
Authors: White A. A. 
Published: February 2007

AbstractThe acoustically‐filtered, nonhydrostatic flow model formulated by M. J. Miller and R. P. Pearce in 1974 may be modified so as to remove dependence on a certain thermodynamic reference state. The modified model seems appropriate for mesoscale integrations. It implies an elliptic diagnostic equation for the height field, a satisfactory energy equation and an analogue of Ertel's potential vorticity theorem. These properties are also possessed by Miller and Pearce's original model, but in different forms (which have been obtained previously by various authors).
A scheme for predicting layer clouds and their water content in a general circulation model
Authors: Smith R. N. B. 
Published: February 2007

AbstractA cloud water variable has been included in the Meteorological Office atmospheric general circulation model. the cloud amount and water content are generated by a scheme which assumes a distribution of thermodynamic and water content variables about their grid‐box‐mean values. A parametrization of the rate of depletion of cloud water by precipitation is included. the model's boundary layer mixing is reformulated in terms of variables conserved during changes of state of water, allowing a more realistic calculation of atmospheric stability in cloudy regions.The scheme produces realistic cloud distributions and the predicted cloud water content verifies well against microwave radiometer data in all regions except the mid‐latitudes of the summer hemisphere. the parametrization of cloud optical properties in terms of the cloud water content will be an important future development of the model.
The theoretical relationship between foliage temperature and canopy resistance in sparse crops
Authors: Shuttleworth W. James , Gurney Robert J. 
Published: February 2007

AbstractOne‐dimensional, sparse‐crop interaction theory is reformulated to allow calculation of the canopy resistance from measurements of foliage temperature. A submodel is introduced to describe eddy diffusion within the canopy which provides a simple, empirical simulation of the reported behaviour obtained from a second‐order closure model. the sensitivity of the calculated canopy resistance to the parameters and formulae assumed in the model is investigated. the calculation is shown to exhibit a significant but acceptable sensitivity to extreme changes in canopy aerodynamics, and to changes in the surface resistance of the substrate beneath the canopy at high and intermediate values of leaf area index. In very sparse crops changes in the surface resistance of the substrate are shown to contaminate the calculated canopy resistance, tending to amplify the apparent response to changes in water availability. the theory is developed to allow the use of a measurement of substrate temperature as an option to mitigate this contamination.
Eastward travelling waves in the southern hemisphere stratosphere during the spring of 1983
Authors: Shiotani M. , Kuroi K. , Hirota I. 
Published: February 2007

AbstractThis paper presents a case study of planetary waves in the southern hemisphere stratosphere during the spring of 1983, using the global temperature and geopotential height data provided by the United States National Meteorological Center (NMC). the eastward travelling wave of zonal wavenumber 2 with a period of 9.2 days persisted from mid September to early November. It had a phase structure which showed only slight tilt with height and latitude, while the quasi‐stationary wave 1 tilted westwards with increasing height and towards the equator. During the vigorous period of the eastward travelling wave 2, the amplitude of the quasi‐stationary wave 1 varied with a period of about 10 days. This variation was due to a coincidence between the phases of the stationary wave 1 and the eastward travelling wave 2; when the ridges of waves 1 and 2 overlap, wave‐1 amplitude reaches its maximum. the active period of the travelling wave 2 is closely related to the seasonal change in the mean zonal winds. In 1983 the eastward travelling wave 2 became vigorous after the zonal‐mean westerly jet moved rapidly poleward and downward. Compared with the other 5 years for which we have data, the wind structure for the month of September 1983 was peculiar because of the stronger meridional curvature of the westerly jet.
Computational aspects of moisture transport in global models of the atmosphere
Authors: Rasch Philip J. , Williamson David L. 
Published: February 2007

AbstractComputational aspects of methods used to simulate the transport of water vapour in a global atmospheric general circulation model are examined. A set of properties useful in characterizing numerical methods for modelling atmospheric transport are identified. Spectral and semi‐Lagrangian methods, which are very different in terms of these desired properties are compared.The extent to which the schemes do not satisfy certain properties of the continuous equations provides a measure of one component of the error of the solution. For the spectral scheme, negative specific humidities q indicate such an error component. Conventional semi‐Lagrangian schemes are also susceptible to generating negative values. In addition, they are not inherently conservative. Shape‐preserving semi‐Lagrangian methods do not generate negative values, but still are non‐conservative. the degree to which the advection process does not conserve mass provides a measure of another error associated with the numerical solution. the negative error is shown to be large for the spectral transport scheme, measured either locally or globally. Measured globally, the semi‐Lagrangian transport schemes' conservation errors are equally large. Locally, the correction of this error can be made very much smaller, relative to physical processes in the model.The study highlights the computational problems which still exist within the better numerical methods used to simulate the transport of water vapour, and demonstrates the care with which one must apply computational constraints to the solution. the spectral and semi‐Lagrangian transport schemes produce very different climatologies in model simulations. A comparison of these climatologies will appear elsewhere.
A study of aerosol properties and solar radiation during a strawburning episode using aircraft and satellite measurements
Authors: Kilsby C. G. 
Published: February 2007

AbstractAircraft measurements of radiative fluxes over the North Sea during a heavy straw‐burning pollution episode are presented and compared with results from a radiative transfer model.The model uses aerosol properties derived from aircraft and satellite measurements, and acceptable agreement with the measured fluxes is obtained.High levels of aerosol concentration are described, with correspondingly large radiative effects. Boundary layer heating rates of 3K day−1 are measured, and rates of 5‐10K day−1 in regions of higher aerosol loading are inferred.
Parametrization of tropical ocean heat flux
Authors: Wells N. C. , King‐Hele S. 
Published: February 2007

AbstractA heat‐flux scheme, based on the assumption of a constant relative humidity and air‐sea temperature difference, is tested for the tropical oceans. It is compared with heat fluxes over the Pacific Ocean, determined by empirical formulae and maritime observations, both for long‐term monthly‐mean fluxes and for interannual variations in heat fluxes over the equatorial Pacific Ocean.It is shown that the differences in latent heat flux between values obtained by the scheme and those calculated from maritime observations are generally no greater than the uncertainty in the bulk aerodynamic method. the method is also shown to be successful in reproducing the interannual variation in surface heat loss in the eastern Pacific Ocean, though the variability is less than that obtained from maritime observations in the coastal upwelling zone adjacent to South America.Finally, the sensitivity of the heat‐flux scheme to sea surface temperature, for wind speeds typical over the tropical Pacific Ocean, is compared to other schemes. It is shown that the sensitivity coefficient is a function of both wind speed and sea surface temperature, and has values in the range 2‐16 W m−2 K−1 for the tropical ocean.
North atlantic blocking during January 1979: Linear theory
Authors: Frederiksen J. S. , Bell R. C. 
Published: February 2007

AbstractThe second half of January 1979 was a time of persistent and large‐scale blocking in the North Atlantic region. In this study we examine the instability properties of the flow during this period and relate the growing modes to the development that occurs. We have analysed the growth rates, phase frequencies and structures of the ten fastest‐growing modes, within a five‐level quasi‐geostrophic model incorporating spherical geometry, on each day for the period between 10 and 30 January 1979.Large‐scale equivalent barotropic dipole or multipole instability modes are prevalent in the North Atlantic during the second half of January. These modes are stationary or slowly propagating and grow rapidly with efolding times as short as 1.7 days in some cases. We find that there is a close connection between these modes and the dynamical developments observed subsequently a few days later. Associated with the early stages of block formation, there are large‐scale eastward propagating westward tilting onset‐of‐blocking wave‐train modes. the onset‐of‐blocking modes also appear to be associated with rapid cyclogenesis off the east coast of North America.The formation of the North Atlantic blocking system and diffluent flow also produces corresponding changes in the storm tracks. During the early parts of this period the fastest‐growing monopole cyciogenesis instability modes have regions of preferential development in the climatological storm tracks over the Pacific, Atlantic and Siberian regions. Subsequently, however, the Atlantic cyclogenesis modes split into wave trains poleward and equatorward of the block.In the developments during this period, we also examine the roles of the adjoint eigenmodes and of optimal perturbations which produce the largest initial tendency of the L2 norm squared of the streamfunction.
Airborne studies of the altitudinal variability of the microphysical structure of small, ice‐free, montanan cumulus clouds
Authors: Blyth Alan M. , Latham John 
Published: February 2007

AbstractThe University of Wyoming's instrumented King Air aeroplane made special multiple‐altitude passes through seven non‐precipitating, essentially ice‐free summertime cumulus clouds, as part of CCOPE, conducted in Montana in 1981.The clouds were found to possess substantial microphysical variability on all detectable scales (down to 2 m) at all levels and distances from cloud boundaries. This was attributed to entrainment of environmental air, and evidence was found for a two‐stage process of turbulent mixing of cloudy and entrained air. Evaporation of the cloud drops was found to occur in a manner consistent with the ‘inhomogeneous mixing’ theory.Generally the clouds were negatively buoyant and more turbulent in the upper regions and in near equilibrium with the environment near cloud base. the normalized liquid water content was found to be roughly independent of altitude.Four of the clouds studied were found to contain embryonic raindrops (diameter 40 to 190 μm) in concentrations around 10 to 10001−1. Calculations and analysis showed that these particles could not have been formed in adiabatic clouds of the same base‐temperatures and depths, nor could they have been formed on ‘giant nuclei’. Their presence was shown to be explicable in terms of the ‘extreme inhomogeneous mixing’ model of accelerated growth. This mechanism also appears to offer an explanation for the small concentrations (≈0.1 m−3) of millimetric‐sized drops, shown in dual‐polarization studies often to be initiatory to substantial cloud development.