bluuebunny/crossref_metadata_2025 huggingface dataset:
Updating prediction models by dynamical relaxation: an examination of the technique
Authors: Davies H. C. , Turner R. E.
Published: February 2007

A study is undertaken of a particular method of updating numerical prediction models. the method constrains the time development of a subset of the model fields toward a prescribed space‐time estimate of those fields, while the other model variables are allowed to evolve in an explicitly unconstrained fashion. It represents an attempt to update the model by a form of dynamical relaxation.

An analysis of the method is carried out in the context of the primitive equations linearized about an isothermal basic state of no motion. It is shown that, for a particular form of the scheme and the availability of the time history of the mass field (or the wind field) on an f‐plane, all the time‐ and space‐scale error fields in the initial specification suffer an amplitude reduction of at least the order e−1 on the timescale of one day. On an equatorial β‐plane it is shown that the same rate of amplitude reduction can be achieved if an accurate time history is known for both the mass field and the zonal‐wind field. Numerical experiments performed with the nonlinear shallow‐water equations for a mid‐latitude β‐plane geometry support these results and demonstrate that the technique compares favourably with the conventional direct insertion update method. Consideration is also given to the possible effects of errors in the prescribed fields and it is shown that the relaxation scheme can to some extent be tuned to offset the effect of this particular source of error.

This study of the relaxation update scheme, although not comprehensive, is nevertheless sufficient to indicate its potential. However, it is stressed that a trenchant assessment of the scheme's usefulness should be based at least in part upon its performance under more testing and realistic conditions.
Orographic rainfall in warm sectors of depressions
Authors: Bader M. J. , Roach W. T.
Published: February 2007

Results of some calculations are presented which show that the washout of droplets in a low‐level, orographically produced cloud by raindrops falling from a higher‐level cloud (formed by large‐scale ascent) can augment the rainfall rate by several mm h−1 over a hill a few hundred metres high. The dependence of this enhancement upon different meteorological parameters is discussed and calculated values are compared with some observed rainfall rates over south Wales. It is considered that this mechanism is most effective in enhancing warm‐sector precipitation over hills.
Angular momentum advection by axisymmetric motions
Authors: Plumb R. A.
Published: February 2007

It has been shown by Starr that net absolute angular momentum cannot be advected by axisymmetric motions alone across a closed surface of constant absolute angular momentum. Hence it has been claimed that certain systems possessing differential rotation about a central axis must be non‐axisymmetric. The theorem is analysed in detail and such a conclusion is shown to be invalid.
On the choice of radial boundary conditions for numerical models of sub‐synoptic vortex flows in the atmosphere, with application to dust devils
Authors: Leslie L. M. , Smith R. K.
Published: February 2007

This paper is a sequel to the recent numerical study of thermally driven vortices by the authors. the model developed earlier is used as a basis for comparing a variety of radial boundary conditions and to assess their suitability for models of ‘tall‐thin’ atmospheric vortices, in particular dust devils. We have also determined those aspects of vortex dynamics which are least sensitive to the choice of radial boundary condition, and those which have a stronger dependence. the results confirm the suitability of the boundary conditions used in the earlier study, for the purposes for which that study was intended, but more realistic boundary conditions are obtained for a dust devil model.

The discussion should provide useful guidance in the design of future observational studies of dust devils and their immediate environment, and in the formulation of models for other types of sub‐synoptic‐scale vortices such as tornadoes and waterspouts.
Relative role of atmosphere and ocean in the global heat budget: Tropical atlantic and eastern Pacific
Authors: Hastenrath Stefan
Published: February 2007

Calculations of the oceanic heat budget for the tropical Atlantic and eastern Pacific, in conjunction with satellite measurements of net radiation at the top of the atmosphere, permit a partitioning of heat export performed by the oceanic water body v. the atmospheric column. For individual Marsden squares, the divergence of heat transport within the oceanic water body ranges from +82% (export) to −120% (import) of the net radiative input to the system at the top of the atmosphere. Oceanic heat export is particularly conspicuous in the realm of the cold currents off the west coasts and in the cold water tongue immediately to the south of the equator.
On air‐borne transport of sulphur over the North Atlantic
Authors: Nyberg Alf , Mason B. J.
Published: February 2007

The sulphur content in the precipitation collected on Ocean weather ships in the North Atlantic has been analysed. From these data and from the results of trajectory computations it is concluded that considerable amounts of anthropogenic sulphur are transported from North America towards Europe. the dry deposition of sulphur over the Atlantic must be small except close to the emitting sources. the amount of sulphur in precipitation from air coming from the high pressure area near the Azores is very small. It is concluded that the normal global value of natural sulphur in precipitation can hardly be larger than 0.1 mg 1−1.
A field study on the representativeness of turbulent fluxes of heat and water vapour at various sites in southern England
Authors: Wood N. L. H.
Published: February 2007

Extensive measurements were made of surface flux densities of sensible and latent heat from short grass in unstable conditions using the combined Bowen ratio and surface energy balance method. Simultaneous measurements were taken during anticyclonic periods at sites in southern England, and these were examined for differences due to different environments. Differences were not evident within the accuracy of measurement, although one of the sites (Harlington), being noticeably drier, gave anomalous fluxes during the summer of 1975. Sites with separation on the 10km scale, and later, on approximately the 100km scale, revealed no evidence that mesoscale organization of the convection affects the fluxes of heat and water vapour from the surface. Finally, this paper presents a comparison between energy fluxes determined by the Bowen ratio method at one site and the average flux estimated from a boundary‐layer heat budget study for a large area which included the site.
The nocturnal jet
Authors: Thorpe Alan J. , Guymer Trevor H.
Published: February 2007

Three case studies of the nocturnal jet at dawn are presented. Observations were made over southern England where the terrain is considerably less homogeneous than that over which previous published data concerning the jet have been obtained, for example the Great Plains and Wangara.

It is shown that previously suggested layer models of the boundary layer can be usefully quantified to give the diurnal evolution of a layer‐average wind. This model, which involves a decoupling of the flow in middle levels from surface constraints by thermal stratification at night, exhibits a nocturnal jet. Two surface‐stress parameterizations, which make the stress proportional to the velocity and to the velocity squared, give a similar wind evolution.

The observed abrupt transition in boundary layer structure soon after sunrise is studied by considering the growth and transfer characteristics of perturbations to a homogeneous Boussinesq fluid, which is stratified and possesses an idealized jet wind structure. Depending on the value of an average Richardson number, Ri, two regimes of dynamically unstable eddies are indicated. When +0.35 ≳ Ri −0.03 the preferred eddies are oriented transverse to the shear and when Ri ≲−0.03 they are oriented parallel to the shear. the latter regime, associated with longitudinal convective circulations, is efficient at smoothing the jet momentum distribution to give the characteristic well‐mixed daytime wind profile.
A numerical study of the initiation and early development of convection in temperate atmospheres
Authors: Bennetts D. A. , Roach W. T.
Published: February 2007

A recent three‐dimensional numerical model of a tropical cumulonimbus cloud by Miller and Pearce has been adapted to temperate latitudes and it has been found that the mode of development is substantially altered. the tropical cloud has a fairly smooth development whereas the temperate‐latitude cloud is built up by successive thermals. Several integrations have been run and three particularly interesting cases are presented.
On the spectral scale of wind fluctuations within and above the surface layer
Authors: Wamser C. , Moller H. , Panofsky H. A.
Published: February 2007

The spectral scales of the longitudinal and vertical velocity components are investigated by tower measurements up to a height of 250 m at sites of different roughness features. the results are compared with those from the 1968 Kansas experiments. It is shown that the local Monin‐Obukhov parameter z/L at 50 m sufficiently describes the stability dependence of the spectral scales also at greater heights. the influence of roughness on the w scale is investigated qualitatively by classifying the comprehensive data according to stability, height and roughness.
Relations between surface fluxes and mean profiles of velocity, temperature and concentration, downwind of a change in surface roughness
Authors: Mulhearn P. J.
Published: February 2007

Changes in surface fluxes (of momentum, heat, and mass) downwind of a surface roughness change, are related to changes in mean profiles (of velocity, temperature, and concentration). the theory assumes that changes in mean quantities are self‐preserving. At fetches sufficiently large for the underlying surface to be specified by an aerodynamic surface roughness, the changes in surface fluxes are related to changes in this quantity. the theoretical results are supported by available experimental data.
The aerodynamics of oblate hailstones
Authors: Thwaites S. , Carras J. N. , McKlin W. C.
Published: February 2007

Artifical hailstones having shapes and structures closely resembling natural oblate hailstones have been grown in a vertical icing tunnel. These hailstones were grown by spinning them about the minor axis and at the same time causing the minor axis to move in a conical manner about the horizontal, a motion termed symmetric gyration. It is inferred that natural oblate hailstones undergo essentially this type of motion during growth. the oblateness of the hailstones is determined by the angle of the cone and the observed oblateness of natural hailstones indicates that the most common half‐angles of the cone lie in the range 20° to 45°. the lobes in the hailstones grow in the local direction of impingement of the accreted droplets and this produces characteristic non‐radial divergent lobes along the minor axis.
Baroclinic instability on a sphere in two‐layer models
Authors: Baines P. G. , Frederiksen J. S.
Published: February 2007

The baroclinic instability problem is studied for various two‐layer zonal velocity profiles on a sphere for both quasi‐geostrophic (P) and modified‐geostrophic (L) models, the latter containing nongeostrophic advection of temperature. Critical amplitudes for instability and other properties for the rigid rotation profile are obtained for a wide range of values of the static stability, and results of previous studies on the sphere are related and shown to be mutually consistent. Critical amplitudes are also obtained for some jet profiles, and unstable midlatitude jet profiles with values of the static stability and shear representative of the atmosphere are studied in detail. In particular it is found for the latter that heat fluxes (in the L model) are larger in the lower layer than in the upper, and that their equatorward components (at high and low latitudes) are small; also, there are no growing modes with large equatorial flows as found for the rigid rotation profile. A simple hypothesis which relates the pattern of momentum fluxes to the β‐plane stability criterion, and is consistent with all known results, is proposed.

The results, particularly growth rates and stream functions, are compared with those of simplified and multilevel models. For the most unstable mode the differences between the two‐layer and multilevel results are not as large as might be expected on the basis of previous studies, and reasons for this are stated.
Some relationships between sea surface temperature anomalies and surface pressure anomalies
Authors: Haworth C.
Published: February 2007

Possible relationships between sea surface temperature anomalies in the tropical east Atlantic and surface pressure anomalies in the Atlantic‐European sector are investigated. From principal component analysis of the sea surface temperature time series in conjunction with the mean surface pressure anomaly distribution, it is deduced that there is a long‐term change in tropical sea surface temperatures associated with variations in westerly component in higher latitudes. Variations of sea surface temperature are shown to be related, through spatial surface pressure eigenvectors, to the surface pressure distribution in most months of the year. Indications are that the atmosphere initially drives the ocean and that the two media subsequently vary in phase. A method of making quantitative forecasts of the sea surface temperature effect on surface pressure is suggested with special reference to the winter season.
The response of North Atlantic sea surface temperature to atmospheric forcing processes
Authors: Daly A. W.
Published: February 2007

A depth‐integrated heat energy conservation equation is used with input data from three case studies of dissimilar meteorological forcing to provide values for the month‐to‐month change of mean monthly sea surface temperature anomalies. Evaluation of the model is made by comparison of predicted anomaly changes with those observed. The separate contributions of anomalous surface cooling and anomalous advection to the sea temperature anomaly change, give agreement of varying closeness: in all three studies best results are achieved by a combination of these processes; in each case inclusion of a horizontal diffusivity term produces deteriorations over sea temperature anomaly changes based on a combination of anomalous heat loss and advection terms. In regions of significant anomalous windstress curl, neither sea temperature anomaly changes produced in part by advection due to an alternative meridionally directed flow, nor changes otherwise consequent on the associated effects of induced vertical motion, are observed to be consistent with actual changes.
Maintenance of kinetic energy in large‐scale tropical disturbances over the eastern Atlantic
Authors: Kung E. C. , Burgdorf H. A.
Published: February 2007

With the GATE upper air data an A‐B scale area and a B scale area are defined over the eastern Atlantic for the energetics analysis of tropical disturbances in the space domain. Computed gross energy budgets at A‐B scale for large‐scale disturbances and at B scale for cloud cluster scale disturbances clearly indicate that these possess distinctly different energetics regimes in and above the mid troposphere.

It is shown that the eddy available potential energy is added to its tropical reservoir at B scale. However, at A‐B scale the eddy available potential energy is released as the source of kinetic energy. The released energy in large‐scale disturbances appears as eddy kinetic energy in the upper layer above 200mb, and cascades to smaller scales. Examination of the residual term of the eddy kinetic energy equation with other terms also indicates that A‐B and B scale disturbances significantly interact with other scales.
Airborne studies of electric fields and the charge and size of precipitation elements in thunderstorms
Authors: Gaskell W. , Illingworth A. J. , Latham J. , Moore C. B.
Published: February 2007

During the summer of 1976 the ONR/NMIMT research aeroplane was employed in studies of the electrical properties of thunderstorms. Flights through clouds were made in Florida, as part of the Thunderstorm Research International Project, and in New Mexico. The most important measurements were of electric field and the charge, Q, and size, d, of individual precipitation elements. A novel device was constructed for the Q and d measurements. The charge carried on a particle passing through a metal cylinder was sensed by induction, and its size by a shadowgraph technique involving a linear array of photo‐diodes. The penetrations were generally through the lower regions of the clouds.

The major findings of the studies in New Mexico were as follows:

Volume charge densities on precipitation, pp, were often around ‐ 5nCm −3 over horizontal distances of several kilometres. pP was almost always negative, but positive charge densities, of lower magnitude, were occasionally observed over shorter distances. The major contribution to the measured values of pP was made by particles of size around 1 mm, or smaller.

Simultaneous measurements of Q and d showed that no simple relationship existed between them. Charges of about 100pC were commonly observed on particles around 1 mm in size. These are much too high to be explicable in terms of the inductive theory.

Positive and negative charges were found to coexist, except when the precipitation rate, p, was very low. However, charge of one sign (almost invariably negative) was always strongly dominant.

Values of p could be estimated crudely from the d pulses. In regions of high pP they were rarely in excess of 10mmh−1; on some occasios when pP was substantial p was below 1 mmh−1.

Our primary conclusions are that in the clouds studied substantial currents were often carried on precipitation, and that the charges on individual precipitation elements are not explicable in terms of the inductive mechanism of thunderstorm electrification.
A systematic error in the measurement of frost point using a Meteorological Office Mk 3 hygrometer
Authors: Oliver M. J. , Cluley A. P.
Published: February 2007

A series of laboratory and airborne experiments were undertaken to investigate a possible systematic error in the measurement of frost point using a Meteorological Office Mk 3 hygrometer. It was found that temperature gradients in the hygrometer thimble could cause the temperature sensor to read frost points too low by over 2 degC. The error was greater at low thimble temperatures. A table of corrections is presented for frost points in the range −40 to −85°C.
Measurements of turbulence by an instrumented aircraft in a convective atmospheric boundary layer over the sea
Authors: Nicholls S.
Published: February 2007

The turbulence measurement instrumentation system installed on the Meteorological Research Flight Hercules aircraft is described and discussed with particular reference to the measurement of atmospheric boundary layer parameters. As an illustration, the structure of a simple, shallow, unstable boundary layer over the sea is discussed. Semi‐organized motions, perhaps in the form of convective rolls or bands aligned roughly parallel to the wind direction, are shown strongly to influence the transport of water vapour and momentum above the surface layer. Where possible, measurements are compared with previous results.
On the concept of available potential energy
Authors: Pearce R. P.
Published: February 2007

The available potential energy of the atmosphere is defined as the property of the atmosphere for which the source is energy generated by the distribution of the heat sources and sinks, i.e. their deviations from the mean heating rate, and the sink its conversion to kinetic energy; the unavailable potential energy is defined as having as its source the mean (uniform) heating rate and its sink the energy made available for conversion by the heating distribution, i.e. the APE source. These definitions lead to an expression for the APE which, to a good approximation, is proportional to the global mean temperature variance. This partitions into zonal and eddy components and, in addition, a component involving the mean temperature distribution in the vertical. Associated with the latter is a source term determined by the mean heating distribution in the vertical and sink terms corresponding to its conversion to the other two components, these terms being proportional to the conversions to KE. This additional component is referred to as the static stability component of available potential energy (SAPE). The zonal and eddy components, which depend on the temperature variance on isobaric surfaces, are together referred to as the baroclinicity component of the APE.

The relationship between this form of the APE and that proposed by Lorenz is discussed in detail. The physical interpretation of the conversion between zonal and eddy APE, associated mainly with poleward heat (enthalpy) transport down the mean meridional temperature gradient is found to have its analogue in the conversion from the static stability component to the other two components which is associated with upward heat transport down the vertical temperature gradient. The concepts are illustrated for a numerical simulation of the life cycle of an idealized baroclinic wave. It is further shown that this definition of APE extends quite naturally to the description of the energetics of dry and moist convection and its conversions can be calculated without difficulty even if there exist regions in which the lapse rate is dry adiabatic. The roles of frictional and non‐frictional heating in the general atmospheric circulation are discussed and, finally, comments are made on the concept of adiabatic mass redistribution and the definition of a reference state for APE in the light of the approach adopted here.
Direct evaluation of aerosol mass loadings from multispectral extinction data
Authors: Box Michael A. , McKellar Bruce H. J.
Published: February 2007

We derive a formula for the evaluation of the total volume of aerosol in a column, and hence for the aerosol columnar mass loading, from multispectral extinction data. This formula is exact in the ‘anomalous diffraction’ approximation, and reasonably accurate for Mie scattering, over a fairly wide range of refractive indices typical of real aerosols.
Numerical simulations showing the role of the downdraught in cumulonimbus motion and splitting
Authors: Thorpe Alan J. , Miller Martin J.
Published: February 2007

The results of two numerical simulations are presented. The first shows the classic deviatory motion of a multicell system, to the right of the mean tropospheric wind. The second, having a more highly sheared environment, produces a storm which splits into two after the formation of a wedge‐shaped downdraught outflow.

Both simulations highlight the important role played by the downdraught dynamics in establishing a regenerative forcing mechanism. In the higher shear case the updraught core exhibits a large downshear slope with rain falling into the inflow region, thus producing a split updraught core and eventually two distinct cells.

The strength and direction of the relative inflow lead to the different structures and life histories of the two storm systems. The importance of the flow relative to the cell is demonstrated by considering the effects of Galilean transformations of the coordinate system.
The steady‐state format of global climate
Authors: Paltridge G. W.
Published: February 2007

It can be proved for certain small‐scale convective heat transfer processes that the preferred steady‐state mode is one of maximum entropy production. The constraint is more or less equivalent to one of maximum kinetic energy dissipation or of maximum convective heat transport. Evidence is accumulating that the same constraint may apply on the much larger scale of the earth‐atmosphere system. The concept is accepted as the basis of a purely thermodynamic model of the mean annual global climate. The model allows a prieri calculation of the broad‐scale geographic distributions of cloud, surface temperature, horizontal energy fluxes in the ocean and in the atmosphere, net radiant energy inputs, etc. The agreement with observation strongly supports the basic concept. It suggests also that, to the extent allowed by the degrees of freedom in the dynamics, the partition of atmospheric and oceanic energy flow is determined by a requirement to equalize the local dissipations in the two media.
A numerical investigation of variations in the drag coefficient for air flow above water waves
Authors: Taylor P. A. , Gent P. R.
Published: February 2007

Numerical predictions of the drag coefficient for air flow above a train of monochromatic water waves are presented. The air is assumed to be neutrally stratified, and the water surface aerodynamically rough, characterized by a local small‐scale roughness length Zo associated primarily with capillary waves. Details of the numerical model are described in an earlier paper. The variation of the drag coefficient with wave slope and with the ratio of wave phase speed to friction velocity is analysed. The applicability of the results to the sea is discussed in relation to the many observations of the drag coefficient.
Separation effects in Ekman layer flow over ridges
Authors: Mason P. J. , Sykes R. I.
Published: February 2007

Two‐dimensional numerical integrations of the Navier‐Stokes equations for flow of an Ekman layer ove a ridge on a range of length scales corresponding to large Rossby numbers are presented. The results are compared, where appropriate, with numerical solutions of the triple deck equations of boundary layer flow For steady flow agreement is good and, as predicted by triple deck theory, the net change in total force or the domain is small for homogeneous flow. An increase in force arises either through gravity wave radiation when triple deck predictions agree well, or when the ridge is sufficiently steep to give eddy shedding.
Destruction at the ground and the diurnal cycle of concentration of ozone and other gases
Authors: Garland J. A. , Derwent R. G.
Published: February 2007

Several gaseous pollutants show marked diurnal variations, with daily maxima and nightly minima of concentration. The nocturnal decrease is held to be due to deposition at the ground, with isolation of the lowest layers of the atmosphere from the air above by the nocturnal inversion. In the morning, deep mixing is re‐established and brings material down to the surface from higher layers. Ozone exhibits this behaviour clearly and is convenient to study as an example, but it has been suggested by some authors that changes in the rates of gas‐phase reactions that produce and destroy ozone are responsible for the daily cycle of concentration. We have studied the destruction of ozone at the surface and conclude that this process can probably explain the nocturnal decline of concentration near the ground.

Deposition rates for ozone were deduced from profiles of concentration, windspeed and temperature measured over grassland in southern England. The mean deposition velocity was 0.58cms−1 by day and 0.29cms−1 by night. The aerodynamic and surface components of resistance both increased at night, but the behaviour of the surface resistance was not consistent with stomatal control.

Observations of the nocturnal decrease of ozone concentration at the same site are reported. A computer simulation of the behaviour of ozone, with a description of the nocturnal boundary layer due to Wyngaard, duplicated the major features observed, and a procedure for predicting the height of the mixed layer and the rate of decline of ozone concentration at the ground gave good agreement with observations on many occasions.

The nocturnal behaviour has important consequences for the measurement of mean concentrations in the atmosphere, and for long‐range transport. Observation of the daily cycle of concentration may indicate the likely importance of the surface sink for many gases.
Measurements of the kinematics of natural hailstones near the ground
Authors: Lozowski E. P. , Beattie A. G.
Published: February 2007

High speed photography of 48 natural hailstones falling in a hailshaft near the ground has led to new results concerning the vertical, horizontal and tumbling motions of quasi‐conical hail in the size range 0.5 to 1 cm. The relation vz = 12.43 D1/2 max provides an estimate (±1.5ms−1) of the sea level fall speed of these hailstones, while deviations from this relation and variations in the horizontal velocity components indicate the possible influence of air turbulence, particle shape and density, and tumbling motion on the free‐fall hailstone kinematics. At least one third of the hailstones were tumbling or oscillating while falling.
Annual and semiannual waves in the temperature of the mesosphere as deduced from Nimbus 6 PMR measurements
Authors: Crane A. J.
Published: February 2007

The annual and semiannual waves in zonal mean temperature in the mesosphere are analysed using data from the Nimbus 6 pressure modulator radiometer (PMR). The observation that the annual temperature cycle in the extratropical upper mesosphere has almost exactly opposite phase to the solar cycle and explains over 90% of the total variance suggests that the mechanism which maintains the temperature structure is very closely linked with the annual solar cycle. The occurrence of a peak in the semiannual oscillation around 64° latitude in the southern hemisphere contrasts with an amplitude maximum nearer to the pole in the northern hemisphere, reflecting the different characteristics of sudden warmings between the hemispheres which are the cause of this wave. In the upper mesosphere the semiannual wave has similar phase at all latitudes. This gives rise to a pronounced semiannual wave in the global mean temperature of the upper mesosphere.
The polar low as an extratropical CISK disturbance
Authors: Rasmussen E.
Published: February 2007

It is suggested that the polar low is basically a thermal instability phenomenon much akin to a small tropical cyclone. A quasi‐geostrophic model is employed in order to find small‐scale unstable CISK‐forced disturbances in a barotropic basic flow with a vertical structure similar to that actually observed during a polar air outbreak. Case studies of some polar low developments in some ways support a convective hypothesis according to which the polar low is driven by latent heat released in convection clouds.
Forecasting and general circulation results from finite element models
Authors: Cullen M. J. P. , Hall C. D.
Published: February 2007

A 5‐level global model is described in which finite element methods are used to describe the variations of fields in the horizontal. Three versions of the model are used: two use velocity components as dependent variables but differ in horizontal resolution; the third uses stream function and velocity potential. The results show that the finite element models are competitive with existing finite difference models but proper comparison is difficult because of the large effect of certain special features of the models, for instance the treatment of the poles. The change in dependent variable has a much greater impact on the results than a change in resolution with no change in formulation.
A one‐dimensional time‐dependent model for small cumulus
Authors: Ryan B. F. , Lalousis P.
Published: February 2007

A one‐dimensional cloud model with a variable radius has been developed. The model is capable of being tuned to give internal cloud properties (such as liquid water content) that are in reasonable agreement with field observations. Unlike many other one‐dimensional cloud models, the cloud base rises only slowly over the lifetime of the cloud. The model is compared with two other types of one‐dimensional model: a steady‐state model and an Eulerian time‐dependent model.
A diagnostic study of the long‐term balance of kinetic energy of atmospheric large scale motion over the British Isles
Authors: Holopainen E. O. , Eerola K.
Published: February 2007

Calculations of the various terms in the kinetic energy budget of the atmosphere were carried out for the area of the British Isles using aerological data for the period 1974–76. The residual term (‘dissipation’) necessary to balance the equation is interpreted as representing the forcing effect of sub‐grid scale processes. The results are compared with earlier calculations for the same area, and with corresponding results for the area of North America.

The results show that the annual mean dissipation of kinetic energy in the boundary layer (below 850 mb) is 3.4 Wm−2, which is considerably larger than that found for the area of North America. The corresponding values for winter, spring, summer and autumn are 6.2, 2.3, 2.0 and 3.2 Wm−2, respectively.

Results for the British Isles in the free atmosphere are very different from those for North America. Whereas there is, on average, generation of kinetic energy by pressure forces (−V.▽Φ > 0) and net export of kinetic energy over North America, the contrary is true over the British Isles.

The mean residual term in the kinetic energy equation over the British Isles is relatively small between about 850 and 400 mb. However, higher up, the term is definitely positive, indicating (if interpreted physically) an energy input from sub‐grid scales to the synoptic scale: a situation opposite to that prevailing, in the annual mean, over North America. It is suggested that this ‘negative viscosity’ phenomenon may in general be typical for diffluent flows (such as are found in the exit areas of jet streams) and could be due to horizontal and vertical convergence of momentum flux associated with jet streaks and other sub‐grid scale phenomena.
On the broadening of a droplet distribution by turbulence near cloud base
Authors: Manton M. J.
Published: February 2007

The effect of turbulence on the development of the distribution of droplets growing by condensation in a cumulus cloud is investigated. Both small scale and large scale turbulence are shown to produce some broadening of the distribution. Turbulence with high vertical coherence is found to generate a bimodal distribution for which the dispersion increases with height above cloud base while the mean radius increases slowly.
Numerical weather prediction for GATE
Authors: Krishnamurti T. N. , Pan Hua Lu , Chang Chia Bo , Ploshay Jeff , Walker David , Oodally A. Wahed
Published: February 2007

The results of a few numerical weather prediction experiments that utilize GATE data are presented. Aside from some simple forecasts based on single‐level models, experiments with a multilevel model examine the roles of some of the physical processes. They include the influences of diurnally varying radiative effects, effects of cloud feedback on shortwave and longwave radiation, the heat balance of land surfaces, and the influence of West African orography in the 3‐ to 4‐day range of numerical weather prediction. Data sources for these experiments include surface and upper air observations from the World Weather Watch, GATE ships, upper winds from commercial aircraft, low‐ and high‐level cloud winds from geostationary satellites, winds from GATE research aircraft, and special surface observations from a ship data collection. The data analysis is based on a successive correction method where a subjective interface is also incorporated. The initial data are further subjected to a static as well as a dynamic initialization process.

The major results are that single‐level forecasts are very promising with this data set if they are carried out at 700 mb. The multilevel adiabatic experiment produces very poor predictions due to lack of adequate vertical coupling between the lower and the upper troposphere and due to its inability to describe the broad‐scale monsoons. The diurnally varying heat balance of the earth's surface and cloudiness in radiative calculations appear to be important in the 3‐ to 4‐day range of prediction of African disturbances. In this paper we show some interesting vertical structure diagrams of African waves based on the results of numerical weather prediction, which exhibit a close similarity to the structures of composite GATE disturbances constructed by Professor R. J. Reed and his associates.
Katabatic winds: A field case study
Authors: Manins P. C. , Sawford B. L.
Published: February 2007

Results of a case study of katabatic winds utilizing tethered balloon profiling equipment are presented and discussed. The data are unique in the detail they provide of both vector wind and potential virtual temperature throughout the flow.

It is concluded from the data that the one‐dimensional models so far proposed in the literature are inappropriate. Direct computation shows that the divergence of radiation responsible for cooling the air is much smaller than the surface radiation flux used in these models. The inadequacy of these models is not due to the underlying assumption of temporal invariance, but rather the implication is that the flow is significantly three dimensional.

Use of surface stress as the retarding stress in the dynamical balance is also shown to be erroneous, at least so long as ambient wind does not assist the katabatic flow. The stress between the flow and the ambient air is much more important and this stress is inextricably tied in with the downslope development of the flow which is thus inferred to play a major part in the dynamics of katabatic winds.

Katabatic flow is an example of an internal mixing process. The data are shown to suggest a critical gradient Richardson number of about 1/4 for the maintenance of internal mixing above slope flows.
Non‐selective absorption by atmospheric water vapour at visible and near infrared wavelengths
Authors: Tomasi Claudio
Published: February 2007

Measurements of the atmospheric optical thickness at ‘window’ wavelengths in the visible and near infrared region have been obtained from data of direct solar radiation through clear and hazy atmospheres. At these wavelengths, the weak attenuation of the atmosphere is due to Rayleigh scattering, non‐selective absorption by water vapour and extinction by particulate matter.

Since non‐linear dependence may exist between the particulate matter optical thickness and the preci‐pitable water vapour, seasonal sets of atmospheric optical thicknesses have been selected, which present linear and direct correlation with the precipitable water vapour, at each wavelength. Thus the slope of the regression line of optical thickness on precipitable water vapour is the sum of (i) the non‐selective absorption coefficient of atmospheric water vapour and (ii) the linear variation coefficient of the particulate matter optical thickness as a function of precipitable water vapour. The latter coefficient has been evaluated at the various wavelengths for each set of data, by examining the spectral series of data in terms of Angström's expression and determining linear relationships between atmospheric turbidity parameters and precipitable water vapour. Each monochromatic estimate of the non‐selective absorption coefficient of water vapour has been obtained as the difference between the slope of the regression line and the linear variation coefficient of the particulate matter optical thickness. The resulting estimate has been related to the vertical distribution of absolute humidity, in order to determine the mass absorption coefficient of water vapour at unit air pressure.
Evaluation of empirical expressions for the estimation of hourly and daily totals of atmospheric longwave emission under all sky conditions
Authors: Arnfield A. J.
Published: February 2007

Measurements of hourly totals of atmospheric longwave emission, made during summer and autumn seasons in southern Ontario, are used to evaluate the performance of several empirical estimation equations. Under clear sky conditions, the formulae of Swinbank and of Idso and Jackson gave similar results, with daily totals estimated to within measurement error, but with systematic deviations in hourly values which resembled those reported in earlier studies. The timing of these errors, however, suggests that previous hypotheses for such deviations may be invalid. The accuracy of estimates made with the formula of Brunt depended very strongly upon the regression parameters selected. Under all sky conditions, use of the Idso and Jackson formula, with adjustment for cloud type and amount following Bolz, yielded estimates of variable accuracy on an hourly basis, but daily (and similar period) totals are estimated with an error similar to that of the original measurements.
Velocity spectra and cospectra and integral statistics over Arctic leads
Authors: Andreas Edgar L. , Paulson Clayton A.
Published: February 2007

From the high frequency velocity measurements of the AIDJEX Lead Experiment we derive estimates of u and w spectra and uw cospectra both above Arctic leads and over relatively smooth pack ice. The measurements over the leads are unique for two reasons: they were made within a geophysical internal boundary layer (IBL) resulting from a large step change in surface temperature; and all were made within 50cm of the surface.

The intense mixing within the IBL and the nearness of the measurements to the surface affect the shapes of all the over‐lead spectra and cospectra. The u and w spectra have rounded peaks and no inertial subrange. The cospectra contain substantial energy at frequencies typically in the inertial subrange. Coherence spectra are greater than zero out to nondimensional frequencies of order ten. Despite the anisotropy of their origin, however, normalized spectra and cospectra plotted against the nondimensional frequency fz/U and grouped according to Reynolds number collapse into single curves.

The integral statistics (w2/ − uw)1/2 and (u2/ −uw)1/2 have values smaller than those usually reported, as a consequence of both the mixing and the low measurement height.
Comparison and combination of two recent proposals for a generalized Penman equation
Authors: Stigter C. J.
Published: February 2007

Two recent proposals for the generalization of the Penman equation based on completely different calibration attempts appear to yield a strong basis for such a generalization, if combined. Consequences of the combination and its use are discussed in the light of each of the original proposals.
Stratospheric O3‐CO2 coupling in a photochemical‐radiative column model. I: Without chlorine chemistry
Authors: Groves K. S. , Tuck A. F.
Published: February 2007

A column model coupling photochemistry and radiative transfer has been used to examine the relationship between carbon dioxide and ozone amounts in the stratosphere in a chlorine‐free atmosphere. The annually averaged ozone column density increases by about 10% as the CO2 volume mixing ratio is increased from 250 to 700ppmv. These increases result from cooling of the upper stratosphere by up to 20 K over this range. The phase of the seasonal variation of ozone in the upper stratosphere is correctly reproduced, with the annual maximum occurring at the time of minimum temperature rather than at the time of maximum insolation.
Airborne and ground‐based studies of thunderstorms in the vicinity of Langmuir Laboratory
Authors: Christian H. , Holmes C. R. , Bullock J. W. , Gaskell W. , Illingworth A. J. , Latham J.
Published: February 2007

Flights through the central regions of thunderstorms were made over New Mexico on 6 and 15 August 1977 with the ONR/NMIMT Schweitzer aeroplane which carried equipment designed to measure all three components of the electric field, and the charge, Q, and diameter, d, of individual precipitation elements. On the earlier day, information was also obtained with: a rain‐gauge network surrounding Langmuir Laboratory; a 3 cm radar; an acoustic system for locating lightning channels; a ground‐based field‐change meter.

The first cell on 6 August produced precipitation at the ground but no lightning. Vertical fields, Ex, of up to about 50kVm−1 and precipitation charge densities ρ of up to −0.5 C km−3 were recorded within the cloud. The second cell, which grew as the first one decayed, produced 7 lightning strokes in 9 minutes during which time the radar revealed vigorous vertical growth in a narrow zone containing precipitation.

Thunder reconstructions showed the acoustic sources for the first flash of this cell to be very near the top of the cloud at an altitude of 10 km a.s.1. The subsequent flashes produced acoustic signals from progressively lower in the cloud. When the radar echo reached its maximum height lightning activity ceased. Ex values of up to about 50kVm−1 and pp values of down to −1 Ckm−3 were measured. ρp was consistently negative, individual charges being less than ±40 pC. Q values were within the inductive limit for a thundercloud at breakdown but no systematic relation between Q and d was found.

Six penetrations were made through the thundercloud of 15 August, which produced only two lightning strokes. The Ex records were indicative of a (±) dipole located near the cloud top, at around –13°C. Fields of up to about 100kVm−1 and ρp values (positive and negative) of around 5Ckm−3 were measured. Q values of up to ±250 pC were recorded, with charges around ±50 pC being commonly found. No systematic Q‐d relation was revealed, and smaller precipitation particles frequently carried charges (positive or negative) in excess of the inductive limit.

On both days estimated precipitation rates were of order 10mmh−1 and on most occasions the pilot reported precipitation particles to be either ‘ice’ or ‘mixed liquid water and ice’.
A model to derive precipitation patterns and vertical velocities in large‐scale rain systems
Authors: Ryan B. F.
Published: February 2007

A simple model is proposed that enables a prediction to be made of precipitation patterns and vertical velocities from single‐station serial ascents. Micro physical processes are parametrized by dividing the liquid water generated by the model into cloud and rain water. The ice phase is not explicitly modelled. Rainfall is predicted reasonably well when the cloud cover is extensive and convection is not dominant. Calculations show that the precipitation developed by the model is not strongly dependent on changes in the microphysical parametrization.
Two refinements of the split explicit integration scheme
Authors: Gadd A. J.
Published: February 2007

Two minor numerical instabilities in the split explicit integration scheme are identified and remedies which have been developed are described. Some results of these refinements are presented.