bluuebunny/crossref_metadata_2025 huggingface dataset:
A split explicit integration scheme for numerical weather prediction
Authors: Gadd A. J.
Published: February 2007

An economical explicit integration scheme for numerical weather prediction models is described. A splitting technique is used, in which the horizontal advection terms in the governing equations are integrated with a timestep limited by the wind speed, whilst the terms which describe gravity‐inertia oscillations are integrated in a succession of shorter steps. A two‐level numerical scheme with small phase speed errors is used for the advection stage and a forward‐backward method for the gravity‐inertia terms.

The split explicit scheme has been applied to the Meteorological Office operational 10‐level model, and to a similar sigma coordinate model, to compute forecasts for periods up to six days ahead. The quality of the numerical forecasts obtained is not reduced when the timesteps used are close to the limits set by linear computational stability criteria. The use of such timesteps leads to a substantial computational economy relative to previously available integration schemes. For the northern hemisphere version of the 10‐level model the computing time required is one‐third of that for a split semi‐implicit scheme and one‐sixth of that for the original explicit Lax‐Wendroff scheme. Verification statistics for split explicit forecasts indicate improved accuracy when compared with those for earlier operational models.
Some effects of meridional shear and spherical geometry on long stratospheric waves
Authors: Simmons A. J.
Published: February 2007

Forced, low‐wavenumber disturbances of normal‐mode form are determined for zonal flows with meridional shear, but no vertical shear, using the linearized primitive equations with spherical geometry. Results are presented both for stationary waves in the winter westerlies and for travelling, equatorially trapped modes.

For flow profiles representative of the winter lower stratosphere results are in agreement with some earlier studies, curvature effects giving a maximum geopotential perturbation close to the axis of the polar night jet. Wave structure is more variable for upper stratospheric profiles of larger meridional scale, and more sensitive to zonal wavenumber, spherical geometry and ageostrophy. Geopotential maxima are typically located polewards of mean flow maxima, and meridional structures are in general agreement with observation. The vertical penetration of disturbances to these broader flows is significantly reduced by singular‐line absorption.

Equatorial wave solutions show the vertical wavelength of the Rossby–gravity wave to be sensitive to the meridional scale of the mean flow, and for and asymmetric jet the wave structure is approximately symmetric about the latitude of zero zonal mean absolute vorticity. The Kelvin wave is much less sensitive to meridional shear. A simple expression is given for the zonal mean acceleration due to waves dissipated by Newtonian cooling, and results confirm a suggestion that asymmetries in the zonal mean flow profile will be enhanced by the presence of such waves.
Aircraft measurements of ice in Florida cumuli
Authors: Hallett John , Sax Robert I. , Lamb Dennis , Murty A. S. Ramachandra
Published: February 2007

Aircraft observations of cloud and precipitation particles just below the tops of Florida cumuli at temperatures near −4 and −9°C show that fresh, vigorously growing towers (updraughts of order 10 ms−1) tend to contain low concentrations (<0.11−1) of graupel, whereas older towers with lower updraught speeds contain much higher concentrations (>101−1). The ice phase, mostly as graupel, was often found to appear within a period of five minutes. Cloud drops of diameter >25μm were always found to be present in concentrations of 10 to 100cm−3 between −4 and −6°C, a condition, according to criteria established by laboratory studies, necessary for secondary ice crystal production by the riming of graupel particles. Vapour‐grown columns in concentrations as high as 501−1 were sometimes observed at the colder level, an observation consistent with the concept of a copious production zone below. Supercooled raindrops in concentrations > 11−1 occur in many regions of the clouds, often concurrently with graupel and crystalline ice. It is shown by means of a simple analytical model that aerodynamic capture of secondary ice particles by the supercooled raindrops and the subsequent transformation of the raindrops to new splinter‐producing graupel may be the main mechanism responsible for the rapid increase in observed graupel concentrations. The coalescence processes are therefore seen to be instrumental in the evolution of the ice phase. This process is favoured in lower updraught regions where the particles can be retained in the −4 to −6°C production zone long enough (≃100s) for the entire feedback process of splinter production, capture, drop freezing and renewed splinter production to take place.
Longwave radiation at the ground: IV. Comparison of measurement and calculation of radiation from cloudless skies
Authors: Dalrymple G. J. , Unsworth M. H.
Published: February 2007

On cloudless days in summer, longwave irradiance was measured with a radiometer and was calculated from a radiation chart using records from radiosondes released at regular intervals. The results show that the chart underestimated the longwave flux density at the ground by up to 40 Wm−2, corresponding to an extra atmospheric emissivity of up to 0.12. The extra emissivity was correlated with turbidity during the day, and on turbid days it showed a diurnal variation with a maximum at noon. It is suggested that emission in the atmospheric window from dust, pollen and spores may account for some of the observed excess.
An analytical model of rainfall interception by forests
Authors: Gash J. H. C.
Published: February 2007

The description of the evaporation of rainfall intercepted by forests in terms of a regression of evaporation loss on incident rainfall is discussed and some of the assumptions implicit in that method are re‐examined. The two major factors which control the evaporation of intercepted rainfall are identified. These are: (i) the amount of time that the canopy spends saturated during rainfall and the evaporation rate applicable under these conditions; and (ii) the canopy saturation capacity and the number of times this store is emptied, by drying out after the cessation of rainfall. A model is then constructed which is conceptually similar to the Rutter model, but which replaces that model's numerical approach with an analysis by storm events. The evaporation from a saturated canopy during rainfall is estimated from the Penman‐Monteith equation; the evaporation after rain has ceased, the effect of small storms insufficient to saturate the canopy, wetting‐up the canopy and evaporation from the trunks are added as separate terms. The model has been tested against data from Thetford Forest in East Anglia, with satisfactory agreement between observation and estimation. It is suggested that the model may be capable of making useful estimates of the evaporation of intercepted rainfall, solely from rainfall measurements.
A simple air‐sea interaction model
Authors: Nicholls N.
Published: February 2007

The way in which equatorial sea surface temperature (s.s.t.) anomalies may interact with the atmosphere is discussed. A simple model is developed to simulate this air‐sea interaction in the Australian region. It is shown that the model can correctly simulate many interesting aspects of the observed statistical behaviour of the ocean‐atmosphere system, including the seasonal variation in persistence of pressure anomalies, the seasonal variation in pressure v. s.s.t correlation, a biennial oscillation in the atmosphere and ocean, and certain lag correlations between pressure, temperature and rainfall. It is concluded that the air‐sea interaction simulated by the model is an important control on the behaviour of the atmosphere and ocean.
Aspects of the energetics of the upper stratosphere during the January‐February 1973 major sudden warming
Authors: Crane A. J.
Published: February 2007

Data from the Nimbus 5 selective chopper radiometer have been used to study certain features of the dynamics and energetics of the January‐February 1973 sudden warming. Daily values of the energetics of the upper stratosphere during a sudden warming have been calculated. Analyses of the rates of change of temperature and geopotential wave amplitude give no support to the suggestion that the disturbance suffers a time lag with increasing height. Maximum rate of warming is shown to descend from the upper stratosphere. Interaction between travelling and quasi‐stationary waves did not appear to be important in the development of the warming. Two main results arise from an analysis of the latitudinal distributions of the energetics. Firstly there is good evidence to suggest that the amplification of the temperature wave between 10 and 2mb results from a transfer of eddy potential energy from the 50–10mb layer, and not from the lower stratosphere or troposphere below 50mb. However, energy propagation above 2mb is initiated by a large increase in energy flux through the 50mb level. Secondly, despite considerable uncertainty in the kinetic energy budgets, it appears that eddy kinetic energy increases in high latitudes as a result of convergence of the vertical energy flux but increases in middle latitudes as a result of barotropic conversion from the zonal flow. The westerly zonal flow is decelerated in mid‐latitudes and initially accelerated in high latitudes by divergence and convergence of large poleward eddy momentum transport. Eventually the effect of the induced mean meridional circulation leads to easterly acceleration in high as well as middle latitudes. Some strong resemblances are found in the numerically simulated zonal flow changes of some model warmings which show a meridional lag in the onset of easterly acceleration.
Occurrence and structure of the equatorial trough zone in Venezuela
Authors: Riehl Herbert
Published: February 2007

Aircraft traverses were made from north to south across Venezuela during the rainy season of 1972 to determine whether a convectively active equatorial low pressure trough occurred and, if so, its structure, average properties and mechanics. The trough could indeed be located regularly near latitudes 5−7°N flying along the Orinoco River. Near cloud base (920mb) there was a wind shift from east to west and from north to south components. Farther south, winds from southeast were encountered. The low troposphere was warm with a maximum equivalent potential temperature at the axis. At 700mb neither the warm layer nor any wind shift could be found. The structure closely resembled that of the equatorial trough observed in the Garp Atlantic Tropical Experiment of 1974.

The traverses largely indicated negative correlation between thermodynamic energy, mainly controlled by water vapour, and buoyancy, given by virtual temperature, on mesoscale distances of 30–50km. Occasional ‘hot tower’ penetrations were encountered. Surface air reached cloud base undiluted. Absolute vorticity was highest at cloud base just north of and at the trough, decreasing sharply to near zero to its south, just where heaviest convection was concentrated. Here also the largest vertical shear of the zonal wind component was located, again in agreement with GATE. The pressure‐height field was calculated from the wind data and indeed yielded a minimum value at the trough axis. To the north, the zonal wind increased with height though temperature decreased northwards. Calculations on the balance of forces with frictional retardation and on the origin of the low‐level warm layer give indications of the mechanics of the structure; the latter appears to be suggestive of a widely applicable equatorial trough model.
Stratospheric temperatures and geostrophic winds during 1973–1974
Authors: McGregor James , Chapman W. A.
Published: February 2007

Radiance measurements derived from the Nimbus 5 selective chopper radiometer during the two years 1973–1974 are utilized to produce estimates of the global stratospheric temperature structure. Results are presented as polar stereographic maps of equivalent temperature for various of the instrument channels, and as meridional cross‐sections. Seasonal, monthly and daily averages, as well as monthly and daily averages of zonal wavenumbers one and two in temperature, are analysed. Monthly mean zonal winds are also computed, using the radiance data together with conventional 200mb height fields.

The seasonal analysis confirms that the southern hemisphere experiences greater extremes of temperature than does the northern. In the monthly analysis considerable differences in temperature and zonal wind are found between the two hemispheres. Maximum wavenumber‐one amplitudes are generally found in regions of weak westerly winds. Wavenumber‐two amplitudes are usually less than half those of wave‐number one. A peak in the wavenumber‐two amplitude at the equator during January 1974 suggests that cross‐equatorial wave propagation occurs. Daily variations of zonal mean temperatures show that equatorial coolings occur at times of sudden warmings in the winter hemisphere and the daily variations of wavenumbers one and two reveal the more regular nature of wave activity in the southern hemisphere compared to the northern hemisphere.
Spectrophone measurements of the absorption of solar radiation by aerosol
Authors: Foot J. S.
Published: February 2007

Absorption of the direct solar beam was measured using an instrument based on the spectrophone. The method is described and results obtained from filtered and unfiltered air samples are presented. This enabled the aerosol's contribution to the absorption to be determined. Comparison of this absorption was made with the local visibility: with a visibility of 10km the heating rates observed were typically 0.2K day−1, increasing to 0.5K day−1 when smoke made a major contribution to the aerosol.
Phase‐lagged wave‐CISK
Authors: Davies H. C.
Published: February 2007

The conventional formulation of the wave‐CISK mechanism involves a quasi‐equilibrium assumption whereby the instantaneous large‐scale diabatic heating that is induced by cloud‐scale motions is assumed to be related to the instantaneous distribution of the large‐scale flow, and in particular to the low‐level moisture convergence of the large‐scale flow. Thus in this context the deep convective clouds of the tropical atmosphere are viewed as instantaneous processors of the large‐scale moisture supply.

Herein a conventional, inviscid wave‐CISK model is developed and an examination undertaken of the properties of the analytically determined solutions. The quasi‐equilibrium assumption is then discarded by incorporating into the CISK model's formulation a simple time‐dependent model of the dynamics of a cumulus cloud ensemble. The latter model has a finite adjustment time (TR) to changes in the large‐scale forcing field. Hence the apparent diabatic heat source experienced by the large‐scale fields becomes a function of the time history of that same forcing field.

Neutral solutions of the refined wave‐CISK model are derived as functions of the diabatic heating amplitude and the adjustment time. These solutions illustrate the frequency‐selective stabilizing effect of the hypothesized cloud‐scale dynamics. Again, the marginally unstable waves of the refined model are shown to exhibit properties that overcome some of the debilitating features of conventional wave‐CISK. These solutions also reproduce some of the observed features of tropical wave motions. In particular, for Tr ∼ 12 h and with a physically reasonable value of the cumulus diabatic heating amplitude, it is shown that all the inertia‐gravity modes are stable, and that the most unstable Kelvin wave, mixed Rossby‐gravity wave and Rossby (n = 1 and n = 2) waves have finite preferred spatial length scales corresponding respectively to zonal wavenumbers (m) of 2, 4, 6 and 8. The periods of these four waves correspond approximately to 10, 4/1/2 12/1/2 and 15 days but the growth rates are comparatively small. The agreement with tropical wave data is not unreasonable when allowance is made for the Doppler shift of the Rossby waves.

Also the vertical structures of these preferred modes are different: the Kelvin and mixed Rossby‐gravity waves are associated respectively with the shortest and longest vertical wavelengths, and the meridional wind field of the Rossby wave (n = 1) has a maximum in both the upper and lower troposphere. The degree of mutual compensation in the model solutions between diabatic heating due to cumulus clouds and cooling due to large‐scale ascent is not as strong as that observed in the atmosphere.
Measurements of boundary layer structure and development over SE England using aircraft and tethered balloon instrumentation
Authors: Moores W. H. , Caughey S. J. , Readings C. J. , Milford J. R. , Mansfield D. A. , Abdulla S. , Guymer T. H. , Johnston W. B.
Published: February 2007

During August 1974 the Co‐operative Atmospheric Boundary Layer Experiment (CABLE) took place in which several experimental groups combined resources to study spatial and temporal characteristics of the convective atmospheric boundary layer over a fairly uniform area of SE England. The results presented concentrate on the four‐day period 19–22 August. Mesoscale variations in boundary layer development and the influence of synoptic scale features on boundary layer structure are described and discussed. Profiles of sensible heat flux, obtained by several independent methods, are examined in some detail and are shown to imply significant heating due to absorption of solar radiation.
A note on momentum transfer above very rough surfaces
Authors: Mulhearn P. J.
Published: February 2007

Data from wind tunnel experiments on the deviations from unity of the non‐dimensional velocity gradient, ΦM = (kz/u*)(δU/δz), close to very rough surfaces are reviewed and compared with field data discussed by J. R. Garratt. It is found for z/z0 < 102 that ΦM < 1 in field data and > 1 in the wind tunnel experiments. This difference is discussed tentatively in terms of roughness element flexibility and porosity.
Baroclinic instability governed by the modified quasi‐geostrophic equations
Authors: White A. A. , Gadian A. M.
Published: February 2007

Results are presented for the baroclinic instability of zonal flows on a β‐plane according to the modified quasi‐geostrophic equations (which retain a non‐Doppler term in the rigid horizontal boundary condition).Numerical techniques, with up to 180 interior levels, are used. In the absence of a β‐effect the instability of the longer waves is fairly sensitive to the non‐Doppler term, the more so if static compressibility is neglected. A realistic β‐effect markedly reduces this sensitivity: the weakly unstable long waves are essentially internal mode phenomena and are consequently little affected by the non‐Doppler term. It is confirmed that the stable external mode Rossby waves, where they exist, are radically altered when the term is included..
Incorporation of climatology into a Backus‐Gilbert variational scheme for atmospheric temperature retrieval
Authors: Palmer T. N.
Published: February 2007

A method is proposed for explicitly incorporating climatological information into a Backus‐Gilbert variational scheme for atmospheric temperature retrieval by a remote sounding radiometer. The method is applied to a study of the retrieval ability of a ground‐based microwave radiometer.
Some aspects of turbulence structure through the depth of the convective boundary layer
Authors: Caughey S. J. , Palmer S. G.
Published: February 2007

Results from a series of boundary layer measurements carried out at Ashchurch, Worcestershire during July 1976 are combined with those from the 1973 Minnesota experiment. This data set provides a more complete description of the behaviour of some turbulence statistics through the depth of the convective boundary layer and into the stable air of the free atmosphere. Although the two experimental regions differ quite markedly topographically, the two sets of data are found to merge together quite well in the middle of the boundary layer and do not reveal any systematic differences that might be attributable to surface effects. The vertical profiles of turbulence statistics are compared, where possible, with other results from numerical models and laboratory experiments.
On the net forces produced by surface‐mounted obstacles
Authors: Mason P. J. , Sykes R. I.
Published: February 2007

Numerical calculations of the forces involved in Ekman layer flow past three‐dimensional topography are presented. The forces are obtained from finite‐difference solutions of the Navier‐Stokes equations. The results confirm expectations from earlier two‐dimensional work that many flows produce no significant change in total momentum transfer between the fluid and the surface. Only flows generating some form of trailing vortex system appear capable of changing the total force on the lower boundary. The effects of Ekman boundary layer instabilities, and their interaction with topography, are also discussed.
Empirical orthogonal functions of 500 mb height in the northern hemisphere determined from a large data sample
Authors: Rinne Juhani , Karhila Vesa
Published: February 2007

Horizontal empirical orthogonal functions (EOFs) were determined from a sample of 11 876 500 mb analyses given at 1404 gridpoints covering the northern hemisphere north of 20°N. Due to computer restrictions a joining method had to be applied. The set of 175 EOFs determined explains 99.5% of the total variance in the dependent sample and 99.6 to 99.7% in the independent samples.
Truncation of the EOF series representing 500 mb heights
Authors: Rinne Juhani , Järvenoja Simo
Published: February 2007

The representativeness of different sets of EOFs describing 500 mb heights over the northern hemisphere north of 20° is discussed. Some criteria for truncating series given in EOFs are introduced. A case study is presented to find out how well charts constructed with the aid of EOFs represent the original synoptic analysis.

In the EOF series, about 160, 130 and 120 terms are needed when applying functions based on analyses from one year, 4 years and 20 years, respectively. Thus the degrees of freedom of the EOF series are cut down to one tenth of those in the corresponding gridpoint presentation (1404 points). From the error variance of the analyses, assumed to be 1.5% of the total sample variance, 0.5 percentage units are included in the EOF series, while the remaining 1.0% is rejected into the residual. The true information lost when truncating the EOF series is 0.5% of the total variance.

The EOFs are stable; functions based on data from 1946 work reasonably well in independent data from 1965.
Conditional symmetric instability ‐ a possible explanation for frontal rainbands
Authors: Bennetts D. A. , Hoskins B. J.
Published: February 2007

In order to study the possible importance of symmetric baroclinic instability in the formation of frontal rainbands, the existing theory is reviewed and the inclusion of the effects of latent heat release attempted. When an atmosphere is rendered symmetrically unstable by latent heat release, it may be said to be conditionally symmetrically unstable. Simple numerical experiments support the extended theory and describe the structure of the finite amplitude cells. These exhibit conditional gravitational instability in preferred linear regions. It is shown that such a gravitational destabilization is possible only when a wet bulb potential vorticity is initially negative. This latter is a necessary and possibly sufficient condition for ‘conditional symmetric instability’ (CSI). Limited comparison with observed frontal rainbands lends some support to the hypothesis that CSI can be a dominant formative mechanism, though more sophisticated numerical modelling and observational studies are required.
Growth and disappearance of evaporation fog during the transformation of a cold air mass
Authors: Wessels H. R. A.
Published: February 2007

Evaporation fog (sea smoke, steam fog) occurs during the advection of cold air over warm water. Current knowledge of the structure of unstable surface layers is used to predict the height of the fog patches as a function of the meteorological conditions. A possible application is the estimation of an increase of fog frequency due to discharge of cooling water.

An inversion‐capped mixed layer model is developed, which describes the transformation of the temperature and humidity characteristics of the cold air mass. The results agree with the observation that evaporation fog is usually restricted to coastal waters.
The numerical modelling of storm surges in the Bay of Bengal
Authors: Johns B. , Ali M. Anwar
Published: February 2007

A numerical model is developed for the simulation of surges generated by a tropical cyclone in the Bay of Bengal. The analysis area extends from approximately 11°N to 22°N and, in the northeastern sector of the Bay, includes a representation of the Ganges‐Brahmaputra‐Meghna river system in Bangladesh. The extent of the analysis area allows three days of the surge‐generating capacity of a cyclone originating in the southern Indian Ocean to be recorded before landfall at the Bangladesh coast. The incorporation of the river system permits a potentially deep inland penetration of surges originating in the Bay. The model is non‐linear and this allows a determination of the interactive effect between surge and the astronomical tide. Numerical experiments are described that relate to the change in surge response resulting from a change in the cyclone track during the 24 hours preceding landfall. An account is given of the interaction between surge and tide in the Bay and the river system.
Stratospheric O3‐CO2 coupling in a photochemical‐radiative column model. II: With chlorine chemistry
Authors: Groves K. S. , Tuck A. F.
Published: February 2007

It is shown that the inclusion of a chlorine chemistry mechanism, necessitated by present abundances of methyl chloride and carbon tetrachloride, reduces the modelled natural stratospheric ozone above about 25 km by 20 to 40% compared with part I, resulting in substantial agreement with observations. The effects of simultaneous increases in the atmospheric mixing ratios of carbon dioxide and chlorofluoromethanes (CFMs) are investigated. It is found that there is alleviation of the CFM‐induced ozone reductions attributable to increased CO2, and that the effects of the two perturbations upon ozone column density are not quite linearly additive. The net effect on ozone column density consists of larger percentage decreases in the upper stratosphere almost compensated by smaller percentage increases in the lower stratosphere, where most of the ozone is resident.
Volcanic eruptions and long‐term temperature records: An empirical search for cause and effect
Authors: Taylor Billie L. , Gal‐Chen Tzvi , Schneider Stephen H.
Published: February 2007

The ‘superposed epoch’ analysis method of compositing temperatures is employed to aid in the search for evidence of a drop in surface temperature due to large volcanic dust veils. The temperature records examined have been chosen from the archives at the National Center for Atmospheric Research according to the length and completeness of the record. Various groupings of the data stations are used as data bases and the strength of the ‘volcanic signal’ determined in the different cases. A seasonal (winter‐summer) investigation of the signal is also conducted. A significant dip in temperature can be found within a few years after the major eruption dates in most of the dust veil temperature superpositions, whereas the noise appears to vary randomly from superposition to superposition. The evidence points to a stronger, but less statistically significant, response to the effects of large volcanic dust veils in higher latitudes, although a definite conconclusion requires, among other things, a larger and more representative data base. The seasonal investigation reveals a stronger summer signal in the majority of cases.
Redistribution of scalars during upper ocean frontogenesis: A numerical model
Authors: Macvean M. K. , Woods J. D.
Published: February 2007

A two‐dimensional frontogenesis model forced by a barotropic horizontal deformation field has been developed. Turbulent mixing is neglected and the long‐front flow is assumed geostrophic. The horizontal gradients at the surface become infinite after 3–4 days, assuming a deformation rate of 10−5s−1. In reality we expect turbulent mixing to prevent the final collapse to a singularity. The inclination of cross‐front streamlines to isopycnals decreases rapidly below the surface; the horizontal density gradients do not therefore increase rapidly there. However, such flow can effectively increase the horizontal gradients of passive scalars (we consider temperature here) whose isopleths are inclined to the isopycnals. In a typical example the isotherm pattern makes a front appear to be an order of magnitude thinner and to penetrate much deeper than does the isopycnal pattern. The isotherm patterns produced vary markedly with the initial inclination of isopycnals and isotherms.
Laboratory studies of riming, and its relation to ice splinter production
Authors: Choularton T. W. , Griggs D. J. , Humood B. Y. , Latham J.
Published: February 2007

Experiments have shown that if a supercooled drop is accreted on to a rimer in such a way that it lands on an already frozen smaller droplet then it may develop a protuberance as it freezes, presumably because the heat loss is fairly symmetrical. These protuberances were found in the temperature range –3 to –8°C, and their probability of production reached a maximum at –5 to –6°C of 1 for every 20 drops accreted. The protuberances tended to have a larger aspect ratio and to be longer relative to the drop diameter at lower temperatures. All these effects were found to be independent of speed of collection over the range 1.5 to 3ms−1. Due to the strong parallelism between the conditions required for protuberance production and ice splinter ejection during riming we suggest that the splinters are a result of explosive fragmentation of about 1% of the protuberances formed. This idea is supported by some experimental evidence.
The dynamical behaviour of a two‐dimensional model of the stratosphere
Authors: Harwood R. S. , Pyle J. A.
Published: February 2007

For the atmosphere below approximately 60 km the dynamical behaviour of the zonal mean circulation model described by Harwood and Pyle (1975, 1977) is discussed in detail and compared with observations. The major features of the observed extra‐tropical circulation are reproduced by the model, although the principal jets are overestimated. Equatorial temperatures are representative but horizontal temperature gradients are too large, in keeping with the excess wind speed.

The roles played in forcing the mean meridional circulation by the diabatic heating and the eddy fluxes of angular momentum and heat are considered.

The temperature structure of the summer stratopause is discussed in detail; the model results indicate that this region is in radiative equilibrium to a good approximation.

The model has been used to investigate stratospheric ozone and in this paper the sensitivity of the ozone distribution to a number of the model variables is discussed. Reasonable changes in the specified eddy diffusion coefficients and momentum fluxes can alter the total ozone column by around 10%. For small changes (∼ a few tenths K d−1) in the radiative heating of the lower stratosphere even larger changes in the ozone amount are found in the model. The net radiative heating in the lower stratosphere is difficult to calculate. The implication of these sensitivity experiments for pollution perturbation calculations is stressed. Unless all feedback processes are included calculations of pollution‐induced changes in ozone of around 10% must be regarded with caution.
Charge transfer accompanying the splashing of supercooled raindrops on hailstones
Authors: Latham J. , Warwicker R.
Published: February 2007

Measurements have been made of the charge transfer accompanying the splashing of a supercooled water drop, of raindrop dimensions, onto a simulated hailstone. The important variables were: temperature of drop and hailstone (–4°C to –20°C); impact velocity (1 to 4ms−1); impact parameter (0 to ±1); drop composition (pure water; NaCl and (NH4)2CO3 solution; CO2‐saturated water; water containing detergent); and surface structure of the hailstone.

It was found that the hailstone always acquired a positive charge, of magnitude invariably less than 200 fC, and usually around 20 fC. These values are much less than those measured by other workers (in experiments where the drops were not significantly supercooled), and are far too small to be of significance in thunderstorm electrification.

When these experiments were performed in the presence of an electric field substantial charges (up to 300 fC m V−1) were measured, but their sign was such as to dissipate the field.
Three‐dimensional lee waves
Authors: Marthinsen T.
Published: February 2007

Linear wave theory is used to find the stationary, trapped lee waves behind an isolated mountain. The lower atmosphere is approximated by a three‐layer model with Brunt‐Väisälä frequency and wind velocity constant in each layer. The Fourier‐integrals are solved by a uniformly valid asymptotic expansion and also by numerical methods.

The wave pattern is found to be strongly dependent on the atmospheric stratification. The way the waves change when the parameters describing the atmosphere and the shape of the mountain vary, is studied. Further, the results predicted by the theory are compared with waves observed on satellite photographs. It is found that the observed wave patterns are described well by the linear theory, and there is good agreement between observed and computed wavelengths.
Numerical experiments on the inland penetration of the sea breeze
Authors: Physick W. L.
Published: February 2007

A two‐dimensional primitive‐equation model is used to investigate various aspects of the sea‐breeze phenomenon. The influence of soil moisture content is examined by varying the Bowen ratio of the underlying land surface. This is equivalent to controlling the magnitude of the sensible heat flux at the ground since the diurnal variation of net radiation minus ground heat flux is approximately the same in all cases. It is found that the inland penetration rate of the sea breeze varies significantly with the degree of soil moisture. The model predicts a significant slowing of the frontal advance around midday for the case of ‘relatively weak’ sensible heating over the land, as well as reproducing the frontal acceleration observed in late afternoon for all cases. The sea‐breeze circulation detaches itself from the coastline around sunset and moves rapidly inland with diminishing magnitude and this aspect is discussed in relation to observations in the south of England.

An attempt is made to model the southern England sea breeze of 14 June 1973. Various experiments are discussed, including one in which different values of Bowen ratio are specified ahead of and behind the front. Fairly good agreement is found, although it is not possible to reproduce all features of the inland penetration curve for that day.

Some implications of these experiments for the production of daily forecasts by mesoscale models are briefly discussed.
Thermal radiation in Arctic stratus clouds
Authors: Herman G. F.
Published: February 2007

Infrared radiative properties of Arctic stratus clouds in the 10.1 to 12.7μm wavelength region are determined from a series of aircraft measurements. The average emissivity of the clouds when scattering is neglected is unity for cloud depths greater than 350m. Under the assumption that the cloud layers are homogeneous the cloud droplet volume absorption coefficient is estimated as 17±5km−1. Several mass absorption coefficients are estimated for several assumed liquid water distributions. It is also concluded that the cloud reflectance is not larger than 2(£5%).
Charge transfer accompanying individual collisions between ice particles and its role in thunderstorm electrification
Authors: Gaskell W. , Illingworth A. J.
Published: February 2007

Experiments were conducted with a wind tunnel in a cold‐room in order to investigate the individual charges transferred when ice spheres collided with an artificial hailstone. The charge transferred to a sublimating hailstone was negative and had a magnitude proportional to the velocity of impact and to the diameter of the ice sphere to the power 1.7. The charge transferred when 100μm diameter ice spheres collided with the hailstone at a velocity of 8ms−1 was typically –15 fC. No variation of charging could be detected over the temperature range –5 to –25 °C, or when the ice was doped with impurities, or when hailstones of different surface roughness were prepared.

Hailstones which were cooled below the ambient cold‐room temperature and subsequently grew by deposition, charged positively by typically 100 fC per collision. However, a hailstone which was cooled by the same amount but was maintained in a sublimating condition charged negatively. These experiments show that thermal effects did not play a direct role in the charge transfer process. Experiments were performed with a riming hailstone in order to simulate the conditions found in a natural cloud. The hailstone charged positively at temperatures of –5°C and –10°C and negatively at –15°C and –20°C with liquid water contents in the range 0.05 gm−3 to 0.85gm−3, the magnitudes of the charges being typically 30 fC.

It is suggested that the charge transfer occurs from one ice surface to the other at the contact interface and that the driving force for the charge transfer at the contact interfaces is the different surface potentials and/or different charge carrier densities of ice surfaces formed in different ways. It is possible that the charge carriers may be surface ions of a liquid like layer on the ice surface.

The results with the riming hailstone are consistent with thunderstorm observations that the negative charge centre is above the –10°C isotherm and that, although precipitation particles are predominantly negatively charged, a mixture of signs is usually present.
Tropospheric gravity waves: Their detection by and influence on rawinsonde balloon data
Authors: Lalas D. P. , Einaudi F.
Published: February 2007

In this paper we discuss the interaction of a tropospheric gravity wave, detected by a radar and a microbarograph array, with a normal rawinsonde balloon. By calculating the wave properties and comparing them to balloon data, we show that such data can provide information about tropospheric gravity waves. We also draw attention to the consequences of the wave‐induced differences between the measured and the actual background state of the atmosphere. The differences may be critical, for example, in predicting clear air turbulence and in evaluating remote sensing instrumentation.
The simulation of a fair weather marine boundary layer in GATE using a three‐dimensional model
Authors: Nicholls S. , Lemone M. A. , Sommeria G.
Published: February 2007

A detailed three‐dimensional model has been used to simulate a cumulus‐topped tropical oceanic boundary layer. A particularly dense set of observational data from surface‐based and airborne instrumentation collected during one day of the GARP Atlantic Tropical Experiment (GATE) has been used enabling comprehensive initial and boundary conditions to be specified. This also permits detailed comparisons to be drawn between the model and observations.

The model is found to reproduce the structure of the observed undisturbed boundary layer quite realistically. The clouds developed in the model are similar to those observed in both amount and physical size and the modelled turbulent fluxes in the subcloud layer agree well with observations. Also, the evolution of the modelled boundary layer is similar to that in which the observations were made. The model confirms that the major effects of the turbulent mixing are to warm the subcloud layer but to moisten and cool the cloud layer. The limited scale of the model (2 km x 2 km x 2 km) does, however, lead to systematic differences between measured and modelled variances.

The model's turbulent kinetic energy balance for the coupled cloud‐subcloud layers is presented. Although reliable measurements are only available in the subcloud layer, they confirm that the model's values are qualitatively correct except when very close to the bottom boundary.

The model's temperature and humidity profiles are strongly affected by mixing occurring across cloud‐base throughout much of the subcloud layer. This is reflected in the shape of the θv profile which displays a characteristic minimum midway through the subcloud layer. When scaled using cloudbase parameters, the resulting profiles are similar to those obtained in more unstable conditions over land.
Comparison of some different approximations in the statistical theory of relative dispersion
Authors: Sawford B. L.
Published: February 2007

The effect on the theory of relative dispersion of some different approximations to the two‐particle Lagrangian correlation function is examined. Two of these, one due to Taylor, the other to Smith and Hay, are treated in detail.

Through numerical solution of the dispersion equations, the influence of the initial cluster size, a number of simple variations of the spatial separation argument of the two‐particle correlation function, the number of particles in the cluster and the ratio of Lagrangian to Eulerian integral scales are examined. With the exception of the initial cluster size, which is important in the early stage of growth, these parameters are relatively unimportant, particularly compared to the overall difference between the two approximations.

In qualitative agreement with Batchelor's inertial range theory, both the Taylor and Smith‐Hay approximations show linear growth at small time with an accelerated growth region at intermediate time. However, between these regions, the Smith‐Hay solution shows a region of less‐than‐linear growth for which there appears to be no observational or theoretical support. This regime is more pronounced, and the difference between the two approximations greater, for initially small clusters.

Comparison with suitably documented observations, while not entirely definitive, shows a degree of consistency and suggests the Taylor approximation to be the more appropriate.
A study of the earth's radiation budget using a general circulation model
Authors: Slingo Julia M.
Published: February 2007

Observations by satellites have shown that the various components of the global mean radiation budget have a significant seasonal variation, the causes of which are not properly understood. Results from an annual cycle integration with the Meteorological Office 5‐level general circulation model have been used to clarify the roles played by clouds and the surface in determining the seasonal variations in the radiation budget. The model uses an interactive radiation scheme with seasonally varying climatological sea surface temperatures, incoming solar radiation and zonal mean climatological cloud amounts. Its simulation of the various components of the earth's radiation compares favourably with results from satellites and it has therefore been possible to use its results to explain some aspects of the observed budgets.

Many of the features of the annual cycle of the global mean radiation can be associated with the asymmetry in the distribution of land and sea between the two hemispheres. Variations in the global mean planetary albedo can be attributed partly to the illumination of Antarctica near the solstice and partly to the southwards spread of winter snow cover over the northern hemisphere continents. For the globe as a whole variations in cloudiness have little effect. The main determinants of the global mean outgoing flux appear to be the surface and associated tropospheric temperatures. Since the seasonal change in the surface temperatures of the land masses of the northern hemisphere is greater than that of the extensive oceans of the southern hemisphere, the variation in the global mean outgoing flux is dominated by that in the northern hemisphere. Again the effect of seasonal variations in cloudiness is small.

The model results have also been used to estimate the earth's annual mean total energy budget and, in particular, the poleward flux of heat by the oceans. The oceanic heat flux implied by the model compares quite favourably with estimates based on observations, the main differences being at the equator and near the North Pole. The model gives a cross‐equatorial oceanic flux from the southern to the northern hemisphere and a significant flux of heat by the oceans into the Arctic Basin.
Moisture analysis of the A‐scale phase means during GATE
Authors: Huang Huo‐Jin , Vincent Dayton G.
Published: February 2007

A moisture analysis, based on GATE A‐scale phase means, is presented for western Africa (land) and the eastern Atlantic (water) area. Vertically‐integrated moisture budget terms are calculated for the whole area, as well as for land, water and A/B ship array areas. Precipitation minus evaporation (P̄−Ē) is solved as the residual. Upward vertical motion, associated with organised convective disturbances and westward‐propagating waves, causes the vertical advection term to dominate. Over land P̄−Ē decreases slightly from Phase I to Phase III, whereas over water it increases greatly during the same time span. When the evaporation rate of 3.8mmd−1 from the Phase III B‐scale results of Thompson et al. (1979) is applied to our A/B scale results, it yields a precipitation rate of 10.8mmd−1. This compares favourably with the remotely‐sensed estimates of 12.7mmd−1 for the B‐scale array obtained by radar and 14.0mmd−1 for the A/B ship array derived from satellite observations. Furthermore, P̄−Ē in the GATE area is found to be considerably greater than the global estimate for approximately the same latitude band.
A Boolean approach to climate dynamics
Authors: Nicolis C.
Published: February 2007

One of the main characteristics of the climatic system is the existence of numerous feedback and jump phenomena. It is suggested that a discontinuous formulation based on Boolean logic can be fruitfully used to analyse this behaviour. The approach is first applied to simple examples, and the logical predictions are compared with the results of the more familiar continuous formalism. The qualitative insights that such an analysis can add are then illustrated on more complex situations.
The diurnal thermocline due to buoyant convection
Authors: Dalu G. A. , Purini R.
Published: February 2007

A simple model, including solar radiation, surface colling and buoyant convective adjustment is used to study the temperature distribution in the uppermost layers of the ocean, assuming that the salinity is uniformly distributed and that wind mixing is negligible.

It is found that the depth of convection cannot be identified with the thermal compensation depth: time dependent solutions show that the lags are such that equilibrium solutions may be misleading.
The diurnal variation of ozone in the upper stratosphere as a test of photochemical theory
Authors: Pallister R. C. , Tuck A. F.
Published: February 2007

It is argued that detailed consideration of the 24‐hour waveform of ozone in the upper stratosphere provides a means of testing current photochemical kinetic mechanisms, with measurements requiring good relative rather than good absolute accuracy. The implications of the pronounced changes with height of the waveform for remote sounding and monitoring are pointed out. Diurnal, semidiurnal and higher harmonics of ozone and temperature waveforms are obtained by Fourier analysis.