bluuebunny/crossref_metadata_2025 huggingface dataset:
Natural convection in water over an ice surface
Authors: Townsend A. A.
Published: February 2007

A tank of water with an ice‐covered bottom and an upper surface maintained at about 25°C provides a simple way of studying the disturbance of a layer of stably‐stratified fluid by convective motion in an underlying layer of unstable fluid. Observations of temperature fluctuations and of the motion of dye streaks and suspended particles show that vigorous overturning is confined to a thin layer on the ice‐covered bottom, from which emerge rising columns of buoyant fluid. the columns penetrate a region of constant mean temperature (3·2°C) which occupies about two‐thirds of the tank volume but are deflected horizontally as they reach the region of stable stratification. the layer just beyond the farthest penetration of the columns is remarkable for large and continuous fluctuations of temperature, apparently caused by internal gravity waves excited by the impacts of columns on the stable fluid. Calculations show that the disturbance from an impulse at the interface spreads comparatively slowly into the stable fluid and the observed concentration of wave energy can be explained if the life‐time of each ripple is limited by energy loss either by ‘wave breaking’ or by viscous forces. Shallow layers with strong internal wave motion may occur in the atmosphere near the interface between an upper region of stable fluid and a lower region in convective motion.
Evaporation and evapotranspiration in southern nigeria
Authors: Chapas L. C. , Rees A. R.
Published: February 2007

Data are presented on a 4‐weekly basis over the two years 1961 and 1962 for evaporation from three types of evaporation pan, and an evaporation estimate by Penman's method. Evapotranspiration was estimated by the methods of Penman, Blaney‐Criddle, Thornthwaite, and Garnier's modification of Thornthwaite's method. Grass covered lysimeters receiving only rainfall gave actual evapotranspiration, or were watered daily to give potential evapotranspiration. Actual evapotranspiration measurements are compared with theoretical estimates.

Potential evapotranspiration is overestimated by Thornthwaite's method and slightly underestimated by that of Penman. the true value is about 120 cm annually with a marked seasonal variation which follows the bi‐modal radiation cycle. Actual evapotranspiration is 50 cm less than this, marked departure from the potential rate occurring during the main and subsidiary dry seasons.
A note on the relationship between net radiation and solar radiation
Authors: Davies J. A.
Published: February 2007

Linear relationships between daytime net radiation (Rn) and solar radiation (Q) have been determined for West Africa and for selected stations throughout the world. For three West African stations.

Rn = 0·612 Q −28

And for fourteen stations throughout the world

Rn = 0·617 Q −24

Where Rn and Q are in g cal cm−2 day−1.

The surfaces represented have albedos ranging from 0·20 to 0·30. On average, net radiation is 55 per cent of the solar radiation. Values for a heating coefficient β, inferred from the relationships, lie between 0·14 and 0·30. The results are not consistent with some previous work. The application of the relationship to estimating evaporation and potential evapotranspiration is discussed.
The turbulent transport of heat and water vapour in an unstable atmosphere
Authors: Dyer A. J.
Published: February 2007

Micro‐meteorological data acquired during five expeditions to carefully chosen sites are analysed to determine the flux‐gradient relation for the transfer of heat and water vapour in the lower atmosphere. The analysis takes the form of a direct assessment of the Monin‐Obukhov universal functions φH and φW. Data for φH were available from all five expeditions, and for φW from two of these.

It was found that φH = φW over the whole of the z/L range available, indicating an identity in mechanism for the turbulent transport of heat and water vapour over a freely evaporating surface. Over the range 0.02 < |z/L| < 0.6, both φH and φW were found to vary approximately as |z/L|−1/3. In this range of |z/L|, Priestley's H* was assessed as 1.15·0.07 and Crawford's E* as 1.10·0.12. This value of H* leads to the formulae H = 21.2 δθ3/2 and E = 51.7 δq δθ1/2 where E and H are in mWcm−2, δθ is the potential temperature difference (°C) and δq the specific humidity difference (gm kg−1) between 1 and 4 metres height. For |z/L| > 0.2, φH was found to vary as |z/L|−1/2. Insufficient data limited the value of the corresponding analysis for φW Excellent agreement was found between the φH and φW data, and φ‐curves assessed from a previous shape‐function analysis (Swinbank and Dyer 1967). The empirical relation φ = (1–15 z/L)−0·55 agrees with the experimental values of φH and φW over the whole of the |z/L| range to within a few per cent, thus permitting numerical evaluation of φ.
Windfinding data from radar tracking of high altitude sensors
Authors: Hyson P.
Published: February 2007

The poor wind response of parachute‐borne sensors at altitudes above 50 km leads to significant errors in the determination of wind speed, unless a correction is applied. Errors vary considerably, depending on the type of radar.
An example of synoptic development in a general circulation model
Authors: Gilchrist A.
Published: February 2007

The structure of an artificial depression which developed during a long numerical integration is examined and it is shown that many of the features of a real depression are reproduced.
Momentum and mass transfers in the surface boundary layer
Authors: Pruitt W. O. , Morgan D. L. , Lourence F. J.
Published: February 2007

Results are presented from six micrometeorological studies conducted over a grass turf at Davis, California, in 1966 and 1967. Highly reliable surface drag and evaporation data from very sensitive lysimeters of 6·1 m diameter afforded excellent opportunity to evaluate several parameters important to aerodynamic‐prediction equations.

For the six studies the mean von Kármán constant, k, ranged from 0·40 to 0·44, strongly supporting continued acceptance of k at around 0·42.

The Monin‐Obukhov (1954) universal ϕM function was found to vary as ∣Ri∣−1/3 under near‐free convection, indicating significantly greater diabatic profile effects than suggested in the form of the KEYPS profile as given by Sellers (1965).

Empirical relationships providing excellent fit to experimental data for the range −3·5 < Ri < 0·3, were ϕM = (1–16 Ri)−1/3 and ϕM = (1 + 16Ri)1/3 respectively for unstable and stable conditions. For ϕw corresponding expressions were ϕw = ·885 (1–22 Ri)−.40 and ϕw = ·885 (1+34 Ri).40.

The ratio KW/KM showed a systematic drop from 1·13 at neutral to a value around 0·75 under strongly stable conditions, which conflicts with recent reports of no change of KH/KM within a wide range of stable conditions (Laykhtman and Panomareva 1969; Webb 1970; Oke 1970). The UCD results are in general agreement with the literature in that KW/KM systematically increased with increasing instability. In the range 0 > Ri > −2·0, KW/KM increased from 1·13 to 1·6.

No expressions relating ϕM and ϕW or KW/KM to z/L are presented due to uncertainties in Davis z/L data. However, use of Davis Ri data in several published z/L (Ri) relationships, allowed a limited comparison with results of previous studies.
On the use of pressure as vertical co‐ordinate in modelling convection
Authors: Miller M. J.
Published: February 2007

A primitive equation system using pressure as the vertical coordinate has been derived, and this system has been applied to modelling both deep and shallow convetion. It has been demostrated that this system, including suitable boundary conditions, filters out the acoustic mode entirely. There exist certain advantage due to having gridpoints at constant pressure.
Modified quasi‐geostrophic equations using geometric height as vertical coordinate
Authors: White A. A.
Published: February 2007

A quasi‐geostrophic β‐plane set using geometric height as vertical coordinate is derived without the conventional assumption that fractional changes of basic state potential temperature over a scale height of the atmosphere are small. Although the hydrostatic, continuity and vorticity equations contain terms not present in the usual set, and the rigid horizontal boundary condition is modified, the form of the implied potential vorticity equation in terms of the stream function is the same as before. Some of these conclusions are extensions to the nonlinear case of results obtained in 1968 by Lindzen in a linearized analysis. the modified equations have reasonable integral conservation properties. As regards accuracy they are closely comparable with the quasi‐geostrophic pressure coordinate equations, but since rigid horizontal boundary conditions are easily applied exactly in the modified set the most nearly equivalent pressure‐based system is evidently a (quasi‐geostrophic) σ‐coordinate set rather than the p‐coordinate set. the modified rigid horizontal boundary condition has the property of allowing for the change in the apparent vertical when a coordinate transformation to a frame moving with a uniform zonal velocity relative to the β‐plane is carried out. Application of the modified equations to an f‐plane baroclinic instability problem reveals a dependence of stability properties on the height‐averaged flow Uo as well as the shear (as found also in 1947 by Charney and by Geisler and Dickinson in 1975 in β‐plane analyses), the dependence on Uo being most marked for the longer waves.
Dynamics of stationary ultra‐long waves in middle latitudes
Authors: Bates J. R.
Published: February 2007

The dynamics of stationary ultra‐long waves in middle latitudes are studied using a set of approximate linear equations for small perturbations about a zonal flow. the equations are derived by an expansion procedure based on the assumptions of planetary scales of variation and small mean superrotation. the zero‐order equations reduce to a vertical structure equation containing no meridional derivatives. Analytical solutions are obtained for forced stationary waves in a model atmosphere consisting of three continuous layers, representing the troposphere, the lower stratosphere and the upper stratosphere.

The waves are generated by thermal and topographic forcing in the model troposphere and propagate wave energy at zonal wavenumbers 1 and 2 into the upper stratosphere, where they are damped by Newtonian cooling. General relationships are found involving the zero‐order fluxes of sensible heat and wave energy. These greatly clarify the dynamics of the waves, particularly in the lower stratospheric layer which links the regions of generation and dissipation.

Taking realistic values of the parameters for northern middle latitudes in winter, the solutions for wavenumbers 1 and 2 show good agreement with observation. an examination of the sensitivity of the solution to changes in various model parameters shows that the meridional flux of heat by ultra‐long waves in the troposphere is extremely sensitive to changes in the stratospheric wind profile and static stability. Since these waves are known observationally to account for a large fraction of the total poleward heat transport in northern middle latitudes in winter, a dynamical mechanism is suggested whereby variations in solar ultraviolet radiation or changes in the ozone content of the upper atmosphere could lead to climate change.
Swell waves at Saint Helena related to distant storms
Authors: Cartwright D. E. , Driver J. S. , Tranter Joyce E. , Charnock H.
Published: February 2007

This paper discusses the relationship between a frequency‐time plot of energy density of swell, from records taken at St Helena in December 1970 and January 1971, and meteorological records of distant storms. A similar exercise in 1969 had suggested that some of the swells originated from the southern hemisphere, even possibly from the Pacific Ocean, but wave amplitudes were all very low. Higher waves were recorded in the present exercise, but all the major events in the energy spectra were attributable to northern hemisphere storms, a few minor events only to the southern hemisphere, and none to the Pacific Ocean. It is concluded that large northwesterly storms in the area southeast of Newfoundland are probably responsible for the spectacular ‘rollers’ which are frequently reported from St Helena.
Deuterium in hailstones collected on 29 November 1972
Authors: Roos D. v. D. S. , Schooling H. , Vogel J. C. , Triegaardt D. O.
Published: February 2007

The deuterium content of more than 400 samples cut from 34 hailstones was determined. These hailstones came from three storms the structures of which were studied by radar. the range in values of deuterium excess (δD) of about 60‰ (extending as low as −86‰) was compatible with the moist adiabatic model only if a significant increase in δD of atmospheric vapour occurred at some stage. Individual growth layers were concluded isotopically uniform. Opaque ice was associated with relatively low δD, especially for the bulk of the hailstones beyond the embryo stage. the correlation found between SD and crystal size was used to derive the relationship between δD and ambient temperature. It was hence concluded that embryos mostly grew between 6 and 7km above ground level. Hailstones of all sizes acquired most of their mass between 7 and 8 km (−20 to −25°C) during the early stages of the severe storms when radar echo tops reached up to 16km. However, during later stages most of the growth occurred during descent. Hailstones from places no more than a few kilometres apart in one storm were concluded 10 have grown in very different conditions: while some must have experienced updraughts in which the speed in the environment of the hailstone increased steadily with time, others encountered an updraught that pulsated with a period of about 8min. Updraught maxima often occurred at about 7.5km, approaching values of 40ms−1.
Stratospheric water vapour measurements over Australia 1973–1976
Authors: Hyson P.
Published: February 2007

Results are presented of measurements of stratospheric water vapour over Mildura (34°S 140°E) Australia. The mean mixing ratio between 15 and 21 km is approximately 2–7ppm and is closely approximated below 15km by an exponential profile with a height constant of 2–4km.
Infrared fluctuation hygrometry in the atmospheric surface layer
Authors: Raupach M. R.
Published: February 2007

This paper describes a single‐beam infrared fluctuation hygrometer, operating in the 2.7 μm water vapour band. By restricting attention to frequencies making up the atmospheric eddy spectrum, a calibration equation for fluctuations is obtained which is independent of moderately stable instrumental gain parameters. A transmission function appropriate to this type of hygrometer is obtained from the random statistical model of band absorption, and shown to imply: (i) a weak but significant dependence of the calibration upon mean humidity; (ii) a very small contribution to transmission fluctuations from temperature fluctuations, which can often be neglected but which may require correction when the Bowen ratio is high. A suitable calibration method is outlined. The results of a field trial over a shallow lake demonstrate the hygrometer's reliability and operational effectiveness for experimental periods of many weeks, and lead to an estimate of 0.88 for the Kolmogorov coefficient for humidity fluctuations.
Instability of planetary waves and zonal flows in two‐layer models on a sphere
Authors: Frederiksen J. S.
Published: February 2007

A study is made of the stability of finite amplitude planetary waves by themselves and together with zonal flows in two‐layer quasi‐geostrophic models on a sphere. Critical amplitudes for incipient instability of baroclinic waves, growth rates, perturbation streamfunctions and momentum fluxes are obtained and are compared with the corresponding properties for barotropic waves in the nondivergent barotropic model. The change in the growth rates, perturbation streamfunctions, momentum and heat fluxes due to the superposition of basic planetary waves on zonal flow profiles is examined in two‐layer quasi‐geostrophic models. The presence of the long planetary waves is found to produce regions of preferential development of cyclones and anticyclones. The position of the most intense development, downstream from the long wave troughs or ridges depending on the basic flow profile, is shown to be related to Phillips's criterion for incipient instability. It is also found that the largest of all the growth rates is increased in the presence of baroclinic waves but decreased in the presence of barotropic waves and it is concluded that baroclinic instability, rather than barotropic instability, is the most important factor in the unpredictability of largescale atmospheric motions. Barotropic waves, however, produce larger changes in the disturbance stream‐functions, momentum and heat fluxes; in particular, when superimposed on solid body rotation zonal flow profiles, they may produce zonally averaged perturbation momentum fluxes with both poleward and equatorward components. The presence of planetary waves in the basic profile removes the short wave instability cutoff characteristic of two‐layer quasi‐geostrophic models with purely zonal flow basic profiles.
The diffuse solar irradiance of slopes under cloudless skies
Authors: Steven M. D. , Unsworth M. H.
Published: February 2007

The diffuse irradiance of slopes relative to the diffuse irradiance of a horizontal surface is calculated by integrating the mean radiance distributions of cloudless skies in Britain for a wide range of solar elevations, azimuths and slope angles. There is reasonable agreement with measurements reported from a number of stations; discrepancies are probably due to errors in measurements and to the influence of aerosol on the radiance distributions. A simple model of the diffuse irradiance of slopes, which takes account of circumsolar radiation, is proposed, and agrees well with integrated values of irradiance.
Transient synoptic systems as mechanisms for meridional transport: An observational study in the southern hemisphere
Authors: Tucker G. B.
Published: February 2007

A classification of satellite cloud vortices compiled for the southern hemisphere by Streten and Troup is used to provide an inventory of the incidence of various stages in the life‐cycle of depressions as a function of latitude. These figures are then allied with estimated meridional transient eddy fluxes of both heat and momentum to assess the varying effectiveness of the systems during their life‐cycles as mechanisms of meridional transport, in comparison with the net contribution from other transient systems (travelling long waves and temporal variations in stationary waves). The developing stage of the cloud vortex is shown to be associated with a strong poleward heat flux, decreasing with the age of the system, with only a small (poleward) contribution from other transient mechanisms. For momentum, the developing stage is associated with a moderate poleward flux and the mature stage with relatively little net flux, while strong equatorward transport occurs during the decaying stage. A major poleward momentum flux is attributed to other transient mechanisms, particularly in the upper troposphere. The results are consistent with the hypothesis that the occurrence at preferred latitudes of different stages in the life‐cycle of systems is a major factor controlling the latitude profile of meridional transient eddy flux. A postulated meridional eddy flux distribution within a frame of influence for a mid‐latitude disturbance during its life‐cycle, when related to the broad‐scale zonal current, can account for salient features in the observed global distribution of meridional eddy fluxes of both heat and momentum. An indirect toroidal circulation is inferred when such a system interacts with the mean zonal current, and other synoptic implications are discussed.
A cloud parametrization scheme derived from GATE data for use with a numerical model
Authors: Slingo Julia M.
Published: February 2007

A cloud parametrization scheme which allows for low, medium, high and convective clouds has been developed from GATE data for use in the Meteorological Office 11‐layer tropical model. The problems involved in using synoptic observations to derive methods of predicting clouds are discussed. Only limited success was obtained in relating observed cloud amounts to relative humidity and atmospheric temperature structure. The restrictions imposed on the cloud scheme by the model's resolution and by its inability to produce a perfect simulation are considered. In the light of these difficulties a simple approach was adopted based on the assumption that condensation on the smallest scales is part of a larger‐scale condensation regime related to the synoptic scale situation. The scheme has been designed to reproduce the main features of a cloud field by relating the large‐scale meteorological features associated with a cloud distribution to model variables. Low, medium and high cloud amounts are determined from a quadratic relationship with relative humidity. Low cloud has also been related to the temperature lapse rate in an attempt to model the persistent areas of sub‐tropical stratocumulus occurring under inversions. A relative humidity relationship is inappropriate for convective cloud which has, therefore, been related to the convective mass flux calculated in the convection scheme of the model. The scheme has been reasonably successful in predicting the cloudiness associated with the ITCZ and the NE. and SE. trades. The cloud fields showed a good degree of coherence from day to day and there were no signs of unrealistic feedbacks between radiation, cloud and dynamics.
On the vertical structure of damped steady circulation in the tropics
Authors: Geisler J. E. , Stevens Duane E.
Published: February 2007

This paper presents an analysis of the vertical structure of steady motion in a dissipative tropical atmosphere forced by steady isolated diabatic heating. Vertical modes appropriate to the problem are obtained, and the forcing is projected onto these modes. With the use of an analytic expression obtained by Gill (1980) for the horizontal structure, these modes are summed to obtain the amplitude and the vertical structure of the response in the region to the east of the heating.
Solar heating by ozone in the tropical stratosphere
Authors: Edwards D. P.
Published: February 2007

The solar heating caused by ozone absorption in the tropical stratosphere has been calculated by direct integration over wavelength. Ozone concentration profile, cloud cover and albedo are shown to be the main parameters affecting the heating rate. Variations in the heating rate due to the temperature dependence of ozone absorption, are found to be small.
Equatorial waves in steady zonal shear flow
Authors: Plumb R. Alan , Bell Robert C.
Published: February 2007

The properties of equatorial Kelvin and mixed Rossby‐gravity waves are investigated in an equatorial beta‐plane model of the lower stratosphere. The basic state and wave amplitudes are assumed to be independent of time; the waves are dissipated by thermal and/or mechanical damping.

Particular emphasis is placed on the forcing of the mean flow by the waves. In agreement with earlier studies the latitudinal structure of the mean flow forcing is found to be sensitive to the dissipation mechanism; when the mean state is one of no zonal motion the easterly mixed Rossby‐gravity wave‐induced acceleration of the zonal flow is zero on the equator when only thermal dissipation acts, but maximizes on the equator when dissipation is included. This latter property is also found in the presence of equatorial jets.

Results from this model, with its explicit resolution of the wave structure, are compared with those from the multiple‐scales approach of Boyd; good agreement is found.
Simulations of an observed stratospheric warming with quasigeostrophic refractive index as a model diagnostic
Authors: Butchart N. , Clough S. A. , Palmer T. N. , Trevelyan P. J.
Published: February 2007

A three‐dimensional primitive equation model of the stratosphere and mesosphere is described. The model was forced at its lower boundary by observed 100 mb height fields for the wavenumber 2 stratospheric warming period of February 1979, and correctly simulated the reversal of the high latitude circulation. This behaviour contrasts with earlier model simulations of wavenumber 2 warmings in which forcing of a climatological zonally symmetric initial circulation by a stationary wave perturbation led to an initial reversal of the circulation in low latitudes. In a series of idealized experiments we show that wave‐wave interaction played no essential role in this simulation and that the ingredients leading to its success were firstly the non‐climatological initial wind structure and secondly an imposed longitudinal phase speed for the upward propagating planetary wave at the lower boundary. These studies also demonstrate that in the prewarming period the direction of propagation of planetary wave activity, as represented by integral curves of the so‐called Eliassen‐Palm flux, may be qualitatively described by the WKBJ limit of the quasi‐geostrophic potential vorticity equation. In this limit, the Eliassen‐Palm flux is simply related to the zonal mean refractive index, leading to two complementary diagnostics for studying wave, mean‐flow interaction.
Ozone perturbation experiments in a two‐dimensional circulation model
Authors: Haigh J. D. , Pyle J. A.
Published: February 2007

A two‐dimensional circulation model is used to investigate the possible perturbation of the ozone layer by CO2 and chlorofluorocarbons (CFC's). Four experiments are described: a control run; a run including increasing levels of CO2 up to 625 ppm; a run including a source of chlorine compounds from the CFCs; and a run to investigate the coupled perturbation due to both CO2 and the CFC's. The dynamical and chemical behaviour of the runs is described.

The control run reaffirms the conclusions of earlier experiments with this model (Harwood and Pyle 1980) that the summer stratopause is very close to radiative equilibrium. The strength of the polar night jet is also discussed.

The perturbations due to CO2 and the CFC's have opposite effects on ozone with increasing CO2 producing an increase in column ozone which is greatest in high latitudes.

The coupled perturbation is not a linear combination of the two individual experiments. This is investigated in terms of the photochemical scheme used. The reduced temperature dependence of the ozone concentration when chlorine compounds play a significant role is shown to produce the non‐linearity.
The Morning Glory: An extraordinary atmospheric undular bore
Authors: Smith Roger K. , Roff Greg , Crook Norman
Published: February 2007

Observations made during an expedition in October 1980 to study ‘morning glory’ wind squalls in the Gulf of Carpentaria region of northern Australia are described. During a ten day period in Burketown, North Queensland, six squalls were documented on consecutive mornings (8–13 October), including two moving from the south, an apparently unusual direction. The unique data gathered from the latter confirm that these have the same character as their north‐easterly counterparts; that is, they are undular bores. The origin of southerly glories remains uncertain, but, at least, in some cases, appears to be linked with the northeast advance of a frontal trough across central Australia. While the data are equivocal, it seems possible that the motion of the front into the developing nocturnal inversion could generate waves, or hydraulic‐type disturbances, which run along the inversion layer, ahead of the front, as proposed by Tepper (1950). The efficacy of such a mechanism is demonstrated by a simple laboratory experiment.

Evidence is presented which suggests that morning glory type events occur elsewhere in the world, sometimes as precursors to cold fronts. Observations in the Gulf of Carpentaria region show that they are effective triggers of deep convection when they advance into air with a sufficient degree of conditional instability.
Solar variability, weather and climate: An update
Authors: Barrie Csiro A. Pittock
Published: February 2007

This review substantially updates the earlier review by Pittock (1978) in the light of more than 100 papers published in the last few years. Note is made of an encouraging trend towards the study of physical mechanisms, as well as important new evidence on the short, intermediate, and longer time‐scales which must be taken into account. Notable amongst these are increasing doubts and controversy surrounding the claimed effects of solar magnetic field sector boundary crossings, new studies and ideas concerning correlations on the decadal time‐scales, and the availability of long series of proxy data on solar activity and climatic fluctuations on a thousand‐year time‐scale. The review concludes with a discussion of the controversy over solar variability, weather and climate as an example of scientific decision‐making in the face of inadequate data and other uncertainties, with reference to the statistical ideas of Type I and Type II errors and Weinberg's concept of a ‘republic of trans‐science’.
Some features of the structure of the tropical tropopause
Authors: Atticks M. G. , Robinson G. D.
Published: February 2007

Statistics of the minimum saturation humidity mixing ratio are extracted from 2 500 radiosonde ascents in the tropics (10°N to 10°S) and in the region of the Asian summer monsoon. Two periods of approximately 20 days are examined, one in the northern winter and one in the northern summer. The relation of the observations to an hypothesis, proposed by A. W. Brewer in 1949, of mass exchange between troposphere and stratosphere is briefly discussed.
The steady, linear response of the stratosphere to tropospheric forcing
Authors: Karoly David J. , Hoskins Brian J.
Published: February 2007

A linearized, steady‐state, 15–level numerical model is used to study the stationary planetary wave response of the stratosphere to thermal and orographic forcing in the troposphere. Zonally symmetric basic states representative of northern hemisphere winter and summer conditions are used. Solutions for each zonal wave‐number are considered separately. Eliassen‐Palm cross‐sections are used as a diagnostic of wave propagation.

First, several simple thermal and orographic forcing distributions are used in the winter basic state. The response in the stratosphere is sensitive to changes in the zonal wind but its structure is insensitive to the forcing mechanism and to the dissipation used in the model. For representative northern hemisphere thermal and orographic forcing, solutions in winter agree well with observations except for wavenumber one, which has too small an amplitude and an incorrect structure.

The solutions for wavenumbers one and two in the troposphere have small differences from those obtained using a similar 5–level model described by us earlier. For higher wavenumbers, the 15–level and 5–level solutions in the troposphere converge, as these waves are trapped in the troposphere.

For the summer basic state, the planetary wave response has large amplitude in the troposphere only. It is necessary to use vorticity forcing in the upper troposphere to give a solution with a phase variation similar to that observed in summer.
A spectral model of the zonally averaged circulation in the middle atmosphere
Authors: Volland Hans
Published: February 2007

Most papers dealing with numerical simulation of the zonally averaged atmospheric circulation start with the solar heat input as an external source that drives the circulation. In this paper, the reverse way will be followed. First, the meridional structure of the wave modes with zonal wavenumber m = 0 within the atmospheric waveguide is determined from tidal theory. Then the height structure of these modes is calculated numerically applying a heat source configuration per mode by trial and error such that the observed global zonal wind and temperature fields between 0 and 100km can be reproduced reasonably well.

Observations during solstice and equinox conditions outside the equatorial regions can be grossly simulated by the four wave modes of lowest degree. These wave modes are driven by two heat sources, one centred near 3 km, the other near 50 km altitude. The symmetric component of the lower atmospheric heat source is strongly enhanced when compared with the expected direct solar heat input into the atmosphere, indicating the dominant influence of latent heat in redistributing solar energy. The antisymmetric component of the middle atmospheric heat source is enhanced by a factor of more than two when compared with direct solar heating. Here, dissipated energy of planetary waves may be the origin of the additional source. The absence of that source leads to wind and temperature changes within the middle atmosphere which strongly resemble the situation during a minor stratospheric warming.
Nonlinear mountain waves in two and three spatial dimensions
Authors: Peltier W. R. , Clark T. L.
Published: February 2007

A three‐dimensional, time‐dependent, anelastic model is employed to simulate the evolution of nonlinear internal waves which are forced by stratified flow over isolated topography. Both two‐dimensional and three‐dimensional flows are considered. In the former case the discussion focusses upon the development of the wave field induced by a uniform mean flow with constant stability. The full three‐dimensional model is employed to study the nonlinear development of atmospheric ‘ship‐waves’ which have recently been observed in satellite photographs to the lee of oceanic islands in the Norwegian and Barents seas. The model is able to reproduce the characteristics of the observed wave patterns rather nicely when it is initialized with vertical profiles of wind and stability obtained through upper air ascents from stations located on the islands themselves. Detailed comparisons of the output of the nonlinear model with the results of three‐dimensional linear steady state theory are also provided. As is found to be the case in two‐dimensional analyses, linear theory may considerably underestimate the forced wave amplitude for symmetric topographic excitation.
Solutions to a model of a front forced by deformation
Authors: Cullen M. J. P.
Published: February 2007

A semi‐geostrophic deformation model of a front studied by B. J. Hoskins and F. P. Bretherton is studied further. It is shown that a unique solution of the Lagrangian conservation laws governing the motion can still be constructed after the front has formed. This solution shows that separate upper and lower fronts propagate into the fluid, and are separated by a region where only smooth variations occur. Finite difference solutions of the primitive equations are also given which converge to a front with the correct mean slope and maximum long‐front velocity but without the correct variation in slope through the depth of the fluid.
The scavenging of particles by electrified drops: Radar echo intensification following lightning
Authors: Barker E. , Bicknell J. A. , Griffiths R. F. , Latham J. , Verma T. S.
Published: February 2007

A series of laboratory experiments were performed in an effort to improve our understanding of the physics of the echo‐intensification phenomenon, in which the radar reflectivity of a region of a thundercloud increases rapidly immediately following the occurrence of lightning within it. Measurements were made of the charges, q, deposited on falling water drops when they engage in direct interactions with positive corona streamers in a uniform electric field, E, just above the limit for propagation, E0. As the drop radius increased from 12 to 950/μm, q increased from about 10−13 to about 10−10C; and q increased as E was raised above E0. These measurements yielded an estimate of 30μm and 108, respectively, for the radius of, and the number of elementary charges contained in, the tip of a propagating streamer at a pressure of one atmosphere.

The collection efficiencies of drops in the radius range 40 to 120 μm carrying charges appropriate to the direct interaction process, for uncharged droplets of radius around 12 μm, were found to be about four times the non‐electrical values. It was concluded that this enhancement in collection resulted largely from dipole rather than coulomb forces. A simple analysis indicates that reported echo‐intensification observations cannot be explained in terms of these enhanced collection efficiencies, but could possibly be a consequence of the greatly enhanced velocities (and hence growth rates) of drops of radius ∼ 100 μm highly charged by direct interaction with corona streamers in the intense electric fields of a thundercloud.
Turbulent dispersion from an elevated line source: Markov chain simulations of concentration and flux profiles
Authors: Legg B. J.
Published: February 2007

A Markov chain model that simulates the trajectories of fluid particles in turbulent flow is described. It incorporates fluctuations in vertical and streamwise velocity, and can be used with velocities that have a skewed probability distribution. The model is used to estimate the distribution of heat downwind from an elevated line source in the boundary layer above a rough surface, and the results are compared with those from a wind tunnel experiment. The simplest model, with fluctuations in vertical velocity only, underestimates the plume depth by 4% to 22% at various distances downwind, and underestimates the rise in the mean height of the plume by 1·5 mm in 25·5 mm. The inclusion of streamwise velocity fluctuations increases the vertical plume width by 6% and also improves the prediction of the mean height of the plume. The model also gives profiles of streamwise eddy flux of heat u'θ that agree closely with those measured. Skewed velocity distributions can be included in the model, but in the boundary layer above a rough surface Gaussian velocity distributions are quite adequate.
An observational study of the anvil of a winter maritime cumulonimbus cloud
Authors: Bennetts D. A. , Ouldridge M.
Published: February 2007

This paper reports on microphysical and dynamical measurements made from an aircraft in the anvil of a winter cumulonimbus cloud which grew in maritime air near the United Kingdom. The anvil had a well‐organized structure, with ice content in the central region exceeding 1 gm−3. The ice appeared to be distributed uniformly over the size range of crystals, with half the total mass contained in particles larger than 1 mm. The total particle concentration may have exceeded 1501−1 in places. A widespread supersaturation of typically 10% with respect to ice ensured that the ice crystals continued to grow by deposition. The microphysical and dynamical structures of the anvil are shown to be closely linked, and semi‐quantitative explanations are given for the evolution of particle size distributions in terms of deposition and gravitational settling.
Tropical cyclogenesis: A comparative study of two depressions in the northwest of Australia
Authors: Foster I. J. , Lyons T. J.
Published: February 2007

A comparative study of two cloud clusters in the north‐west of Australia highlights synoptic influences on tropical cyclogenesis. Although both clusters developed over land in the same region, the large‐scale flow was conducive to the subsequent development of one cluster into tropical cyclone Enid and suppressed the development of the other cluster. The non‐developing low was under strong upper easterlies from an upper tropospheric ridge. An upper trough in the subtropical westerlies later split the ridge, bringing light and divergent upper winds over the low‐level monsoonal trough. This facilitated the development and intensification of a low‐level cyclonic circulation, tropical cyclone Enid.
Simulation of hydraulic jumps in the presence of rotation and mountains
Authors: Parrett C. A. , Cullen M. J. P.
Published: February 2007

Numerical solutions of the shallow water equations in one dimension are presented. The formation and evolution of hydraulic jumps are predicted with a simple centred finite difference model and with a model utilizing Glimm's method. Glimm's method is a special one‐dimensional method, which has been proved to converge to the physically relevant solution of a number of equations in the presence of discontinuities. The solution from it is used to check the finite difference method.

Cases presented include examples of jumps produced in the presence of rotation, and a variety of jumps forced by flow over orography. It is shown that a conservative finite difference method, with the addition of artificial diffusion, converges to the correct solution for almost all cases. However, the use of multipoint filters gives poor results. At low resolution, the errors in modelling the flow induced by the orography are larger than errors caused by not resolving the shape of the orography.
Mesospheric ozone — theory and observation
Authors: Vaughan G.
Published: February 2007

The predictions of a radiative‐photochemical model are compared with observations of the diurnal variation of mesospheric ozone obtained from rocket‐borne experiments. Temperature observations from the SAMS instrument on the Nimbus‐7 satellite are used to define the basic atmospheric state. Good agreement is found, provided that low water vapour abundances are accepted, together with slow rate coefficients for the recombination of odd hydrogen species. Sensitivity tests performed with the model show that uncertainties in the ozone calculations arise chiefly from uncertainties in water vapour concentrations, solar irradiance, O2 absorption cross‐sections and certain rate coefficients, and that the sensitivity of ozone to these parameters may be markedly different at different times of day, especially above 60 km. The model's representation of the ozone variation around dawn is also used to estimate the error caused by the assumption of spherical uniformity when deriving ozone concentrations from solar occultation experiments.
Transport of water vapour in a stratosphere‐troposphere general circulation model. II: Trajectories
Authors: Allam R. J. , Tuck A. F.
Published: February 2007

Trajectories have been computed during a 10‐day period in January for approximately zonal rings of parcels in the upper troposphere and lower stratosphere of a general circulation model. Time histories of ozone and water vapour highlight the central role of the subtropical jet streams as mixing agents for tropospheric and stratospheric air. There is some evidence of inertial oscillations in the upper tropical troposphere, particularly at longitudes where there is no large‐scale convergence or divergence. The western tropical Pacific from about 150°E to 160°W appeared to be a centre of large‐scale motion, with pronounced upper tropospheric descent below about 10 km and ascent at and above 16 km, apparently linked to large‐scale ascent in the middle and high latitude stratosphere up to 30 km.
Interannual moisture variations near the surface of the tropical Pacific Ocean
Authors: Weare Bryan C.
Published: February 2007

Interannual variations of surface specific humidity, surface latent heat flux out of the ocean and surface layer moisture divergence are analysed for the period 1957–76. Special emphasis is given to understanding the relationship between variations in these quantities and the El Niño/Southern Oscillation phenomenon. The analyses include eigenvectors, composites and time series. The results indicate spatially coherent temporally persistent variations, some of which are clearly linked to El Nino. The composite and time series analyses both suggest that during El Niño in the eastern Pacific moisture convergence changes precede those of sea surface temperature by one or more months, which in turn precede those of surface latent heat flux by a further one or more months. A hypothesis is suggested to relate these changes to variations of the South Pacific high.
Gradient distributions and flux profile relations above a rough forest
Authors: Lindroth A.
Published: February 2007

Temperature and humidity gradients above a sparse pine forest (400 trees per hectare) in Sweden were measured using movable sensors. Under daytime conditions the gradients were remarkably small with a median value of −0.007 K m−1 for the potential temperature gradient and −3.8 × 10−6m−1 for the specific humidity gradient. The range of inferred Bowen ratios was −0.3 to 1.8 with a median value of 0.72.

The starting point for the paper was a discrepancy between energy balance/Bowen ratio (EBBR) and water balance estimates of the forest evapotranspiration.

There was no sign that insufficient fetch influenced the results but some evidence was obtained to suggest that the lowest measurement level, about one mean distance between trees above the mean tree level, was situated in a region of inhomogeneity in the heat and water flux field. Excluding this level from the calculations increased the inferred Bowen ratio by 30%. This change is not sufficient to account for the difference in estimates of evapotranspiration by the two methods. It is, however, concluded that the discrepancy could be explained by rather small, quite realistic errors in the gradient measurements.
On the dynamics of tropical cyclone structural changes
Authors: Holland Greg. J. , Merrill Robert T.
Published: February 2007

Tropical cyclone structural change is separated into three modes: intensity, strength and size. The possible physical mechanisms behind these three modes are examined using observations in the Australian/south‐west Pacific region and an axisymmetric diagnostic model.

We propose that, though dependent on moist convection and other internal processes, the ultimate intensity, strength or size of a cyclone is regulated by interactions with its environment. Possible mechanisms whereby these interactions occur are described.
A diagnostic test of the closure assumption in Arakawa and Schubert's cumulus parametrization scheme
Authors: Dümenil Lydia , Ruprecht Eberhard
Published: February 2007

The parametrization of cumulus convection by Arakawa and Schubert is based on the assumption of quasi‐equilibrium between cumulus‐scale and large‐scale processes. Under these conditions, the cloud work function A(λ) should be constant. This assumption is tested in a diagnostic approach by using GATE data. Rawinsonde observations at the A/B‐scale ships were analysed for the period 30 August to 8 September 1974 and the cloud work functions are calculated for each observation time.

The sensitivity of the cloud work function to temperature and humidity is studied. It is concluded that for the limited range of variations observed during GATE, the cloud work function can be regarded as constant for periods of up to 24 hours. Observed time changes of the large‐scale forcing indicate that a coupling between the time changes of the large‐scale temperatures and the moisture fields ensures that A(λ) remains constant.