bluuebunny/crossref_metadata_2025 huggingface dataset:
A wind tunnel investigation of collection kernels for small water drops in air
Authors: Beard K. V. , Pruppacher H. R.
Published: February 2007

The rate at which small water drops grow by collision and coalescence has been investigated experimentally by means of a wind tunnel constructed for cloud physics research. Individual water drops of radii 70 < A <300μ were freely suspended in the tunnel airstream carrying a continuous water cloud with droplets of radii 2 < a < 8μ and with a liquid water content between 0.8 and 1.2 g m−3. The rate at which the suspended water drops were growing by collision and coalescence was determined as a function of the drop size of the suspended drop, the liquid water content of the cloud and the size distribution of the drops in the cloud. From the observed parameters, specific collision kernels were computed and compared with those derived from theoretical and experimental collision efficiencies reported in literature. The results show that for p = a/A <0.1, i.e. for large cloud drops growing by collision with small cloud drops, it is justified to compute the growth rate of cloud drops of A < 136μ on the basis of the theoretical collision efficiencies of Shafrir and Neiburger as given analytically by Berry, and the growth rate of cloud drops of A > 136μ on the basis of the theoretical collision efficiencies given by Mason.
The vertical distribution of atmospheric ozone over Christchurch, New Zealand
Authors: Hutchings J. W. , Farkas Edith
Published: February 2007

The results of a limited series of ozone‐sonde ascents made at Christchurch, New Zealand (latitude 43.5°S) during 1965‐66 with Brewer‐Mast electro‐chemical sondes are discussed. It is found that the vertical distribution of ozone and the pattern of its seasonal variation is similar to that found at Aspendale (southeastern Australia, latitude 38.0°S), but that the Christchurch ozone concentrations are greater at all atmospheric levels. It is further shown that at Christchurch the horizontal transport of ozone by transient eddies is southward at all levels and that the annual average vertically integrated flux is not very different from that observed at a comparable latitude in the Northern Hemisphere. Consideration of the middle latitude ozone budget tends to indicate a coefficient of vertical eddy diffusion in the lower stratosphere of the order of 105 cm2 s−1 for the Southern Hemisphere middle latitudes. Major assumptions involved in this estimate are pointed out.
A note on the log‐linear velocity profile in stable conditions
Authors: Taylor P. A.
Published: February 2007

If we consider a stably stratified (H<0) constant flux layer, 0 <z <zr, within which we know the heat flux, Hi, and have measured only the mean velocity Ur at one height zr we may wish to determine the friction velocity, u*, by assuming a log‐linear velocity profile with a fixed value for the Monin‐Obukhov parameter, a. This approach leads to a cubic equation for u* which may have either two or no real positive roots. A condition for the existence of positive roots is derived and it is conjectured that the larger of the two roots is the one required. This condition leads to an upper bound, In \documentclass{article}\pagestyle{empty}\begin{document}$ \left({\frac{{z_r + z_o}}{{z_o}}} \right) $\end{document} / (2α) on z/L for the applicability of a ‘log‐ linear’ velocity profile.
Precipitation growth at a cold front
Authors: Jonas P. R. , Tabony R. C.
Published: February 2007

A simple model of the growth of precipitation particles in cloud systems in which the air flow is known has been used to calculate the growth of hail pellets at a cold front. The calculated pellet sizes and densities, and surface rainfall rates, are consistent with observations. It is shown that in the 3 km deep frontal cloud the larger hail pellets were grown during at least two ascents in the strong updraught. The growth of large pellets depends on the airflow being such as to cause two ascents, and small changes in the airflow can prevent the growth of large hail pellets.
Water vapour budget and precipitation over the Arabian Sea during the northern summer
Authors: Saha K. R. , Bavadekar S. N.
Published: February 2007

Water vapour budget of the lower troposphere over the Arabian Sea is studied by computing fluxes across the boundary walls of a rectangular volume around the Arabian Sea during September 1963 and June until September 1964. The net flux divergence is used to estimate total precipitation within the volume after taking into account the total estimated evaporation from the surface of the Arabian Sea. Relative magnitudes of net cross‐equatorial flux of water vapour into the volume and evaporation from the surface of the Arabian Sea appear to suggest that on an average the former may be about 30 per cent larger than the latter.
Some experiments in dynamic initialization for a simple primitive equation model
Authors: Temperton C.
Published: February 2007

A number of experiments in dynamic initialization are carried out within the framework of a relatively simple primitive equation model. Some previously suggested variants of initialization schemes based on the Euler‐backward timestep are tested and found wanting. A new scheme is proposed, based on averaging the solutions of the primitive equations at time‐levels separated by a small interval. This scheme is tested, with encouraging results.

Theoretical considerations and experimental evidence indicate that both mass‐to‐wind and wind‐to‐mass adjustment should be catered for in a dynamic initialization scheme. This facility can easily be incorporated into the scheme proposed here, and tests confirm the advantages of doing so.
Bounds upon the growth rate of errors in quasi‐nondivergent prediction models
Authors: Davies H. C.
Published: February 2007

An upper bound is placed upon the amplification rate of an arbitrary perturbation imposed upon the observed, initial state of a barotropic flow and a two‐level, quasi‐geostrophic baroclinic flow. The implied lower bound to the predictability of the barotropic flow is a function of the mean energy and the mean enstrophy of the initial perturbation, together with certain mean properties of the observed state. Similar results are obtained for the baroclinic flow.

Estimates of the doubling time of an initial perturbation of an observed state that is characteristic of largescale atmospheric motion yields values not far removed from those attained in corresponding numerical predictions. Thus the derived bounds may be useful in assessing the growth of errors in quasi‐nondivergent prediction models.
The evolution of droplet spectra and large droplets by condensation in cumulus clouds
Authors: Mason B. J. , Jonas P. R.
Published: February 2007

The paper describes a relatively simple model of a non‐precipitating cumulus cloud that grows by the ascent of successive spherical thermals through the residues of their predecessors, mixing with the surroundings being determined by the relative velocity and the radius of the thermal. The model appears to account for several important features of the cloud structure and the droplet‐size spectrum that are not produced in models of a single parcel or plume. In particular, it produces modest clouds in 30‐40 min in which the vertical velocity, cloud depth and liquid‐water content agree quite well with observations. The computed droplet spectra closely resemble measured spectra and reproduce the bimodal structure observed by Warner. In model maritime clouds containing small concentrations of droplets, the spectra broaden quite rapidly and produce droplets of r = 25 μm by condensation on nuclei of m = 10−11g in concentrations of order 100 m−3 within half an hour, beyond which size they may continue to grow rapidly by coalescence to precipitation size. However, in continental clouds containing droplet concentrations of a higher order, it is difficult to produce a significant number of droplets of even 20μm radius.
A three‐dimensional primitive equation model of cumulonimbus convection
Authors: Miller M. J. , Pearce R. P.
Published: February 2007

The model is described and the results of three integrations presented. Its main novel features are the inclusion of non‐hydrostatic terms in a pressure co‐ordinate system, and the treatment of flow through the lateral boundaries. The results of the integrations, one with an ambient vertical wind shear and two without, indicate that the cloud dynamics is quite sensitive to assumptions made regarding the cloud microphysics, particularly at the stage when the downdraught is produced.
Periodic variations in stratospheric zonal wind from 20 to 65 km, at 80° N to 70° S
Authors: Belmont A. D. , Dartt D. G. , Nastrom G. D.
Published: February 2007

To help understand the observed variability of zonal wind, the amplitude and phase of five component waves are displayed on height‐latitude sections, 20 to 65 km, 80° N to 70° S using rawinsonde and meteorological rocket data for twelve years. Among the new features are the polar semiannual wave in the Southern Hemisphere, matching that in the Northern Hemisphere. The tropic semiannual wave seems displaced south of the Equator. A terannual wave with 20 m s−1 amplitude is possibly related to the square wave properties of the annual insolational heating at high latitudes. Both the annual and terannual waves appear to have minima north of the Equator.
Mean meridional winds estimated from constant level balloon flights in Southern Hemisphere temperate latitudes
Authors: Angell J. K.
Published: February 2007

Temporal and longitudinal variations in mean meridional wind are estimated from GHOST and EOLE constant level balloon flights at 100 and 200 mb in south temperate latitudes during the period 1966/72. At these latitudes there is evidence of about a 0.3 m s−1 poleward flow in winter‐spring and equatorward flow in summer‐autumn, with the oscillation at 100 mb preceding that at 200 mb by 1 to 2 months. This annual oscillation may reflect a direct circulation between summer and winter hemispheres. At 200 mb in south temperate latitudes there is also evidence for about a 0.1 m s−1 poleward flow during the west wind phase, and equatorward flow during the east wind phase, of the quasi‐biennial zonal wind oscillation in the low tropical stratosphere. The significance of this relation has not yet been ascertained. The overall mean equatorward drift of 0.04 m s−1 points up the existence of the Ferrel Cell in the long‐term average. A breakdown of the data by longitude shows that the annual variation in zonal‐average meridional wind is due to large seasonal variations in meridional wind over the South Atlantic.
Secular variations of regional rainfall over India
Authors: Parthasarathy B. , Dhar O. N.
Published: February 2007

Trends and periodicities in the annual rainfall of 31 meteorological sub‐divisions of India based on data for over 60 years have been examined by various latest techniques like Mann‐Kendall rank method, low‐pass filter and power spectrum analysis. It is seen that the frequency distribution of annual rainfall for the period of 1901 to 1960 is not normally distributed in some sub‐divisions of the country. Positive trend is noticed over central India and the adjoining parts of the peninsula and also over two smaller areas in north‐west and north‐east India. Only over some parts of eastern India there is negative trend. Increase in mean rainfall for 30 years shows a gradient from the west coast to the interior parts of the country with a maximum over the west coast.

The spectral analysis of the annual rainfall of these sub‐divisions shows a significant cycle of range 8.5 to 12.0 years which is mainly seen in and around the arid and semi‐arid regions of Rajasthan, some central parts of India and extreme south peninsula. A cycle of 2.0 to 3.5 years is also observed over large parts of the country mainly over north‐west India, central parts of peninsula and some parts of northeast India.
Planetary waves in the winter mesosphere ‐ preliminary analysis of nimbus 6 PMR results
Authors: Hirota I. , Barnett J. J.
Published: February 2007

The temperature field of the northern hemisphere mesosphere during the mid‐winter of 1975/1976 is described by the use of radiance observations made by the Pressure Modulator Radiometer (PMR) on the Nimbus 6 satellite.

A month's average of the radiance field indicates that planetary waves of wavenumbers one and two are of significant amplitude in the winter hemisphere up to the highest level observed by the PMR (∼85 km). These waves show a westward tilt of wave axis with height, and the maximum amplitude is observed in the middle mesosphere to the north of the polar‐night westerly jet.

Time series analysis of the zonal mean radiance during the period of a minor stratospheric warming in early 1976 indicates that the stratospheric and lower mesospheric temperature increases with time in the higher latitudes to the north of 30°N, associated with the amplification of wavenumber one, while the upper mesospheric temperature decreases with time at all latitudes.

A rough estimate is also given of the time change of the mid‐latitude mean zonal wind during the period of the warming.
Water vapour absorption in the 10–12 μm atmospheric window
Authors: Coffey M. T.
Published: February 2007

Results are presented from an experimental programme in the atmosphere to determine water vapour absorption in the 10–12μm atmospheric window. Radiation measurements have been made with upward and downward looking multi‐channel radiometers from an aircraft. Midlatitude and tropical conditions have been studied. the wavelength and temperature dependence of the absorption are presented. Possible causes of the absorption are investigated and dimers of the water molecule are concluded to be the responsible absorbing species.
Hailstorms in the Transvaal on 29 November 1972
Authors: Carte A. E. , Mader G. N. , Triegaardt D. O.
Published: February 2007

A broad band of non‐frontal storms moved northwards across a dense hail‐reporting network. the precipitation patterns of two isolated storms that produced hailstones > 5cm in diameter were studied in detail. Both storms gave a burst of large hailstones and then declined in severity, and ultimately merged. Radar reflectivity structures showed both to be essentially unicellular but one showed evidence of pulsating growth. Internal structures of several hundred hailstones from these two storms and a third less severe storm were examined. the growth centres of most were found to be rime but a few were concluded to have been frozen drops. the layering of the hailstones, the fallout pattern on the ground and features of the radar echoes all indicated greater complexity of the two severe storms than a single updraught that steadily built up to a maximum speed then faded out. Deuterium contents, reported in a companion paper, were determined for many of the hailstones. Storms on this day showed a spectrum of behaviour; some were organized multicellular complexes.
The relation between the flux ratio and energy ratio in convectively mixed layers
Authors: Manins P. C. , Turner J. S.
Published: February 2007

Two alternative formulations of the problem of entrainment at the top of a convecting surface layer are in current use. One expresses the mixing in terms of the ratio of the buoyancy flux at the top of the convecting layer to that at the ground, and the other (which is the more fundamental) compares the energy used for entrainment with that produced by the destabilizing flux at the ground. Several authors have proposed similar methods of calculating the latter and relating the two ratios, but we believe these authors are in error because of a fallacious assumption about the nature of the mixing in the practically uniform layer. The present note attempts to resolve this difficulty by using a simple model which considers explicitly the separate contributions of the environment and the surface flux to the density of the mixed layer. We suggest that the physically relevant energy ratio is the increase in potential energy due to the redistribution of the initial density profile compared with the potential energy made available by heating from below.
A limited area nested numerical weather prediction model: Formulation and preliminary results
Authors: Gauntlett D. J. , Leslie L. M. , McGregor J. L. , Hincksman D. R.
Published: February 2007

A limited area, six‐level, primitive equations numerical weather prediction model has been developed to provide improved 24‐ to 36‐hour forecasts for the Australian region. Important features of the model include semi‐implicit time differencing, and a nesting procedure which enables boundary values to be updated from a hemispheric spectral model. Preliminary results from the model indicate considerable operational potential.
Estimation of outgoing longwave radiation from meteorological variables accessible from numerical models
Authors: Jensenius John S. , Cahir John J. , Panofsky Hans A.
Published: February 2007

A regression equation was developed and tested for directly estimating infrared flux density at the top of the atmosphere from variables available in numerical forecast models. The relation was developed in a stepwise procedure which used observed radiant flux density from the NOAA‐3 satellite in the 10.5 to 12.5 μ window as the predictand and predictors derived from the US National Meteorological Center's Limited Fine Meshm model which are believed to have associations with cloudiness and with temperature.

The equation was developed from continental data for the winter of 1974/5 and tested on data for February 1976, for which the weather regime was significantly different. By far the best predictor was the Planck function calculated from a temperature which approaches the boundary layer temperature when the 500mb dewpoint depression is large, and an upper tropospheric temperature when it is small. This variable alone accounted for almost 61% of the variance of the predictand in the initial sample. Four additional predictors increased the explained variance to over 67%. The corresponding variance explained in the test sample increased to over 71%. The distribution of mean fluxes at each gridpoint was especially well described. The mean field estimated by the equation explained over 97% of the variance of the mean observed field in the initial sample and almost 95% in the test sample.

On a daily basis, predicted fields of radiant flux density appeared to be somewhat smoother than the observed fields, but the regression equation rather accurately predicted the location of relative radiant flux density minima caused by clouds having cold tops.

When combined with a similar relation for reflected shortwave radiation, this equation offers a method of estimating the radiation balance at the top of a numerical model atmosphere which allows for cloudiness change without explicit prediction of cloudiness. The present paper illustrates the method for the restricted case of longwave radiation over a continent in winter.
The application of complex demodulation to meteorological satellite data
Authors: Chapman W. A. , McGregor J.
Published: February 2007

A method of performing zonal harmonic analyses on near‐polar orbiting satellite data is presented. This method, known as complex demodulation, yields the time variation of the spectral components of global scale meteorological parameters. Experimental data from the selective chopper radiometers on board the Nimbus 4 and 5 weather satellites are examined in order to gain an insight into the temporal behaviour of the stationary and travelling components of stratospheric zonal temperature fields.
Longwave radiation at the ground: III. A radiometer for the ‘representative angle’
Authors: Dalrymple G. J. , Unsworth M. H.
Published: February 2007

The construction of a directional radiometer for measuring longwave radiation from the atmosphere is described. The instrument had a filter of black polyethylene to exclude solar radiation, and the detector was a thermopile. By analysis of the radiation and heat balances of the thermopile, correction terms are derived for (i) the flux density of radiation received by the thermopile from the filter, and (ii) the dependence of instrument sensitivity on temperature. Measurements with the new instrument of the equivalent flux density below cloudless skies at zenith angle 52.5° agree well with measurements of hemispheric flux density derived from simultaneous measurements with a Linke‐Feussner actinometer.
The Hampstead storm: A numerical simulation of a quasi‐stationary cumulonimbus system
Authors: Miller M. J.
Published: February 2007

A numerical simulation of the storm which occurred over north London on 14 August 1975 is presented. This storm was notable in giving record rainfall totals for the London area while confining the rainfall to an area of about 100km2 despite lasting three hours. The results of the simulation define a distinctive model of the storm dynamics, featuring an impulsive yet quasi‐stationary system of cumulonimbus which contributed successively to the rainfall over a limited area. Furthermore the simulation emphasizes the dominant role of the downdraught outflow which establishes and sustains the storm through its interaction with the low‐level flow. A mechanism by which the prevailing vector shear maintains the storm is presented. Trajectories computed for the simulated storm show an interlocking updraught/downdraught structure similar to that proposed by Browning (1964).

In order to simulate the observed high surface rainfall it was found necessary to model the high fallspeeds associated with hail by a simple modification to the ‘warm’ rain parameterization. The simulated and observed rainfall patterns compare remarkably well.

The localization of the storm is considered but the local orography is thought to have no significant influence. Dynamical points arising from the model identify aspects of storm modelling that require further investigation.
Structural and stochastic analysis of a zero‐dimensional climate system
Authors: Fraedrich Klaus
Published: February 2007

An ‘almost trivial’ climate system of geometrical dimension zero is analysed, the complexity of which has been reduced to a minimum. It can be simply described as the globally averaged energy flux balance between infrared emission and solar heat input, expanded by a linear albedo‐temperature feedback. This nonlinear and time‐dependent climate model is formulated as a gradient system of a potential and can be analysed without explicit time integration. It includes many of the results which are also exhibited by one‐dimensional energy balance models. Two equilibrium solutions appear. The stable one is characterized by the interglacial, whereas the unstable equilibrium defines a lower bound for temperature (state variable) changes which the system can absorb. Beyond a threshold of an external parameter combination (fold catastrophe) no equilibria exist so that the system attains a ‘deep freeze’ climate situation. A −2 power law describes the linear response of the (internally stable) system to weather fluctuations.
On the interaction of topography and Ekman boundary layer pumping in a stratified atmosphere
Authors: Mason P. J. , Sykes R. I.
Published: February 2007

A simple theory of the interaction of topography and Ekman boundary layer pumping in a stratified atmosphere is presented, together with numerical solutions of the corresponding two‐dimensional Navier‐Stokes equation problem. Numerical results for flow over a two‐dimensional ridge confirm theoretical predictions that stratification enhances momentum coupling and produces a low‐level jet parallel to the ridge.
A numerical advection scheme with small phase speed errors
Authors: Gadd A. J.
Published: February 2007

It is demonstrated that, for linear advection in one space dimension, a simple modification to the well‐known Lax‐Wendroff integration scheme leads to a substantial reduction in phase speed errors. The modification is designed in such a way that no additional restriction is placed on the timestep used in an integration. The improved performance is comparable or superior to that of previously proposed advection schemes, yet it is achieved without recourse to expensive or cumbersome computations.

The new scheme is readily extended to two space dimensions and has been used in numerical weather prediction and sea wave modelling in the Meteorological Office. An example is given of an improved numerical forecast of the 500 mb height field which was obtained by applying the new scheme in the operational 10‐level numerical weather prediction model.
Generalized treatment of particulate scattering in Dobson ozone spectrophotometer calculations
Authors: Gardiner B. G.
Published: February 2007

In the calculation of atmospheric total ozone from ultraviolet spectrometry, the particulate scattering term is usually regarded as a linear function of wavelength. A method due to R. E. Basher, in which it is treated as a quadratic polynomial, is here generalized to include a polynomial of any degree, and the quadratic approximation is seen to be somewhat arbitrary.

It is shown that the determination of the quadratic and higher coefficients from Dobson spectrophotometer observations is limited by systematic errors of measurement, and that these errors are doubled or trebled by attempting to correct even for the quadratic coefficient. It is concluded that Basher's method of calculation is at present inadvisable.
An assessment of the Keily probe for the ground‐based measurement of drop size distributions in clouds
Authors: Corbin R. C. , Latham J. , Mill C. S. , Stromberg I. M.
Published: February 2007

A series of experiments has been performed to calibrate and assess the reliability of the Keily probe for ground‐based measurements of droplet size distributions in cloud. Tests were conducted in natural and artificially produced water clouds at our field research station on Great Dun Fell, and in a specially constructed cloud tunnel. Comparisons were made with drop size distributions measured with a Knollenberg ASSP device and with MgO coated slides. Collection efficiencies were obtained for the Keily probe for various values of ventilation speed and orifice size. It was concluded that the instrument operated consistently and reliably within the range of conditions applicable to our ground‐based field research.
Differences between morning and evening temperatures of cloud tops over tropical continents and oceans
Authors: Riehl Herbert , Miller Alan H.
Published: February 2007

Longwave radiation measurements made in the water vapour window by the satellite NOAA‐2 are examined for the period 15 June to 30 September 1974, in two oceanic and two continental tropical areas. Satellite overpasses are near 09 and 21 local time. It is found that convective cloud tops over land are much colder (and higher) in the evening than in the morning. The reverse is true over sea; further, the absolute frequency of very cold clouds is less there than over land. A specific cloud mass which could be followed for one week shows a morning‐evening variation in cloud top temperature in agreement with statistical deductions.
A temperature correction for Tucker's ocean rainfall estimates
Authors: Dorman Clive E. , Bourke Robert H.
Published: February 2007

A method suggested by G. B. Tucker of estimating rainfall from present weather observations is tested on 28 coastal stations. Tucker's method was developed from a group of stations in Great Britain; at other latitudes, due to air temperature differences, the Tucker estimates may be several times too high or too low. Regression formulae are presented to adjust yearly, quarterly, and monthly mean precipitation values for air temperature. Corrected rainfall maps based upon ocean weather station data reveal a formerly undetected rainfall maximum in western mid‐latitudes of the North Pacific and North Atlantic Oceans.
Effects of aerosol optical properties and size distributions on heating rates induced by stratospheric aerosols
Authors: Mugnai A. , Fiocco G. , Grams G.
Published: February 2007

A previous analysis of the energy balance of small particles in the earth's atmosphere is extended to substances that have special significance in the stratospheric aerosol layer, such as sulphuric acid and volcanic ash. The analysis is based on establishing a balance between the power absorbed by the particles from solar and planetary radiation fields, the power emitted by the particles as thermal radiation and the power exchanged through collisions with the ambient gas. The planetary radiation field is calculated as a function of altitude and includes radiation from the surface as well as emission and absorption by the infrared bands of carbon dioxide, ozone and water vapour. The various terms in the energy balance equation change as a function of the composition, radius and altitude of the aerosol particles, and of the latitude, season, time of day and albedo of the underlying earth‐atmosphere system; aerosols may heat or cool the atmosphere and their temperature may differ from the ambient gas temperature. Average values for rates of heating induced in the ambient gas by monodisperse and polydisperse aerosol particles are computed for various substances. The rates of heating induced by volcanic ash particles and sulphuric acid droplets are calculated for equatorial, mid‐latitude summer and mid‐latitude winter conditions for altitudes to 40 km.

Our results illustrate the effects on atmospheric heating rates of changing the size distribution of the particles and the particle composition. We also show that heating rates of at least 0.25 K day−1 were induced by the stratospheric aerosol layer after the 1963 eruption of the Gunung Agung volcano in Bali.
A parameterization of heat convection for a numerical sea breeze model
Authors: Dalu G. A.
Published: February 2007

A hydrostatic model of a sea breeze circulation is developed through numerical integration of the primitive equations. Air flow is assumed inviscid and external gravity waves are filtered by using a rigid‐top boundary condition. Heat is fed directly into the model, and a parameterization of its distribution is developed which does not use diffusion coefficients explicitly. The model is stable for an integration timestep of up to 14 minutes. Computations are carried out for the latitude of Rome. The Apennines are parameterized as a vertical rigid wall, parallel to the shore.

The results show good agreement with wind profiles given by pilot balloon soundings at Rome Airport in conditions of a sea breeze circulation. A comparison is made with the two‐dimensional model of Neumann and Marher.
Resonance characteristics of linear lee waves in an atmosphere of three layers
Authors: Berkshire F. H. , Pickersgill A. O.
Published: February 2007

The upward leakage of wave energy from a two‐layer troposphere into the stratosphere is examined; the changes produced in the natural complex modes of the system by altering the parameters are found to give rise to some quasi‐resonance phenomena. Under special circumstances there may exist modes which decay extremely rapidly downstream and these provide a qualitative explanation for some features of observed flows.
Low‐level air flow circulation over the Arabian Sea during the summer monsoon as deduced from satellite‐tracked superpressure balloons. Part II‐Analysis of the flow field
Authors: Cadet Daniel , Olory‐Togbé Paul
Published: February 2007

Trajectories of superpressure balloon flights in the tropical boundary layer over the Indian Ocean during the 1975 summer monsoon are analysed in relation to conventional meteorological observations, mainly ship reports. An assimilation of these randomly distributed observations is performed to compute mean gridded fields of wind and pressure during four periods defined according to monsoon activity over the Indian sub‐continent. Mean cloud cover fields over a 2.5° × 2.5° square are also determined from satellite images. The main features of the low‐level monsoon circulation are deduced from the two sets of data; these are in conflict with earlier conclusions deduced from the interpretation of local conditions. The differences in meteorological fields between the various phases of the activity of the monsoon over India are studied and discussed.
A study of the major stratospheric warming of 1976/77
Authors: O'neill A. , Taylor B. F.
Published: February 2007

The unusual atmospheric behaviour observed at middle and high latitudes during the winter of 1976/77 is discussed in both synoptic and dynamical terms, using routine, objectively analysed data. During this period the growth of large amplitude, planetary‐scale waves in the troposphere resulted in a marked wave growth in the stratosphere. A qualitative discussion of the interaction between these waves and the zonal flow is followed by referring to the temporal evolution of the relevant terms in the zonally averaged momentum and heat equations. It is shown that the export of momentum from high latitudes by the waves was crucial in effecting a zonal wind reversal in both troposphere and stratosphere. The importance of this mechanism during the period of study indicates the need for a revision of the currently favoured dynamical model of the stratospheric sudden warming.
The observed zonal wind cycle in the southern hemisphere stratosphere
Authors: Tucker G. B.
Published: February 2007

Zonal winds in the stratosphere for a period of 20 years have been analysed at nine stations between 7°S and 66°S centred on the longitude of eastern Australia. The amplitude, period and phase of the third (‘quasi‐biennial’) cycle are determined. Although a coherent pattern emerges in the variation of period with latitude and height it is concluded that, at least in the tropics, the representative period is 832 days (27.35 months). The well‐known maximum in amplitude of this third cycle in the tropics decreases to a minimum at about 30°S but increases again polewards, reaching a maximum at about 50°S at the highest levels of observation (6mb). The phase of the cycle ‘leads’ at high levels and low latitudes and also at the height and latitude of the polar night jet. Below these maxima there is a downward propagation of the cycle but little phase variation with height exists in middle latitudes. For the tropics, the variation with latitude of amplitude and phase is consistent with lateral diffusion of a forced equatorial cycle, with eddy diffusion coefficients of 1 × 109cm2s−1 at 80mb, decreasing to 3 × 108cm2s−1 at 25mb.
Mesoscale structure of line convection at surface cold fronts
Authors: James P. K. , Browning K. A.
Published: February 2007

In this paper we consider 15 cases of line convection at ana‐cold fronts which have been observed by radar. Although line convection can be a simple two‐dimensional feature, it is usually broken up into line elements having lengths from a few to many tens of kilometres separated by somewhat smaller gaps. The passage of a line element is accompanied by a short burst of heavy precipitation, a temperature drop of about 2degC, a sudden veer and decrease in wind speed, and a pressure jump of 1 to 2 mb. If a gap between line elements passes over a station, however, the changes can occur gradually over a period of up to an hour. Despite the variable size of the line elements, they are found to share common characteristics. They are always oriented slightly clockwise with respect to the synoptic cold front. They all move with the same velocity as their neighbours on a given front, with a component parallel to the front in the direction of the strong low‐level flow which occurs ahead of the front. Moreover, the line elements tend to travel in a fairly predictable manner, largely unaffected by topography and with the major features having lifetimes of several hours.
Interpretation of Nimbus 5 SCR data for obtaining vertical temperature profiles
Authors: Molnár Gy.
Published: February 2007

In this paper a statistical estimator is used to retrieve vertical temperature profiles above an area for which autocorrelation matrices of the temperature profile have been computed for each season using radiosonde data. The temperature dependence of atmospheric transmittance has been taken into account. We find that in cloudless cases the retrieved vertical temperature profiles have an average r.m.s. error of 2–3.5 K, when compared with appropriate radiosonde measurements. We note that to achieve this rather satisfactory result it is not necessary to divide the atmosphere into more than 18 levels; and we will show that it is advantageous to use the total covariance matrices instead of only the diagonal elements. To obtain further comparisons between retrieved and radiosonde profiles, the analysed gridded radiance values have been used, and in this case the r.m.s. error is 2.5–3.8 K.
A high resolution infrared radiative transfer scheme to study the interaction of radiation with cloud
Authors: Roach W. T. , Slingo A.
Published: February 2007

This paper describes a high resolution radiative transfer scheme which calculates infrared fluxes and heating rates in a cloudy atmosphere. It is intended as a research tool for use in studies of the role of radiation in the physics of fog and layer cloud. The spectrum is split into five bands and allowance is made for molecular absorption by the water vapour 6.3 μm band and the rotation band beyond 20 μm, the 15 μm carbon dioxide band, the 9.6 μm ozone band, and the continuum absorption by water vapour in the 10 μm atmospheric window. The strong absorption by cloud water droplets is calculated directly from the droplet size distribution function. The scheme has been run with a model stratocumulus cloud included, using data based on tethered balloon observations at Cardington. A large cooling rate of 8.7 K per hour is shown to exist at the cloud top.
Recent variations in annual‐mean maximum temperatures over Australia
Authors: Coughlan M. J.
Published: February 2007

A data set of annual‐mean maximum temperature variations over the Australian continent, including Tasmania, for the period 1946 to 1975 has been prepared, using all available data. From this data set, trends over the 30‐year period indicate a space scale of variability typically less than 1500km. Most of Australia has experienced a rise in annual‐mean maximum temperature over the period although the proportion which has experienced a fall is not insignificant. This has implications for the determination of hemispheric trends in temperature south of the equator.

Comparisons between annual‐mean maximum temperature anomalies and annual rainfall are made, which show that there is no strong overall relationship. Typically negative correlations do emerge, however, when comparisons are made between temperature variations and the two major patterns of rainfall derived from a principal component analysis. It is further demonstrated that the pattern of trends in temperature can be related to a large extent to the trends in circulation features to which these two major patterns have been ascribed: the Southern Oscillation, and the mean latitude of the high pressure belt over eastern Australia.
The cellular structure of narrow cold‐frontal rainbands
Authors: Hobbs Peter V. , Biswas Kumud R.
Published: February 2007

The heaviest precipitation in narrow rainbands situated along surface cold fronts has been observed on radar PPI displays to be organized into regularly spaced small mesoscale elements having shapes which are roughly elliptical. The long axes of the ellipses are essentially parallel to each other and oriented at an angle of about 35° to the front.
Aircraft observations of the structure of the lower boundary layer over the sea
Authors: Nicholls S. , Readings C. J.
Published: February 2007

Results from an investigation of the turbulence structure of the lower half of the convective boundary layer over the sea around the UK in a wide variety of meteorological conditions are presented. The data were obtained on eight flights made by the Hercules aircraft of the Meteorological Research Flight.

Differences in structure between boundary layers over sea and over land are emphasized, the most notable being the relatively increased importance of mechanically driven mixing over the sea. This means that a more general scaling scheme is required which retains both u* and zi as scaling parameters. Such a scheme orders the results quite effectively. Dimensionless profiles of momentum, sensible heat and latent heat fluxes are presented, together with dimensionless variance profiles. Excellent agreement is found between these and theoretical predictions from numerical models. Surface fluxes are estimated and compared with bulk aerodynamic formulae.

The turbulence statistics are shown to be in good agreement with those published by other workers.
A diagnostic study of the long‐term budget of momentum of atmospheric large scale motion over the British Isles
Authors: Holopainen E. O.
Published: February 2007

Different terms in the equations of zonal and meridional motion for large scale flow in the atmosphere have been evaluated using British aerological data for the period 1974‐76; the effect of motion on scales smaller than a few hundred kilometres on large scale flow is evaluated as the residual term in these equations.

In the long‐term mean there is horizontal advection of zonal momentum into the area of the British Isles. This advection, which is characteristic of both the time‐mean flow and transient disturbances and takes place mainly in the zonal plane, is to a large extent compensated for by Ferrel‐type ageostrophic meridional circulation. The momentum budget in the meridional direction reflects the high degree of geostrophic balance in the time‐mean zonal flow in the free atmosphere.

The mean residual forces in the atmosphere below 700mb are used to infer the vertical sub‐grid scale flux of momentum by assuming that the residual forces at these levels are due to vertical eddy stresses (i.e. stresses involving the vertical velocity component) only, and by evaluating the stress at the earth's surface with the aid of the geostrophic drag law. The results indicate that the annual mean vertical flux of zonal momentum above about 900mb is downwards but relatively small; the corresponding flux of meridional momentum is relatively large and upwards. Mesoscale circulations in connection with fronts are suggested as a possible explanation of the latter phenomenon which, however, may also partly arise in calculations as the result of the poor vertical resolution of the routine upper wind observations in the atmospheric boundary layer.

The residual force in the upper troposphere is very variable. However, consistent with an earlier analysis of the kinetic energy budget using the same data, it exhibits the ‘negative viscosity’ phenomenon, implying acceleration of the large scale flow by motion on scales smaller than the average distance between aerological stations.
Zonal and meridional variations of eddy fluxes induced by long planetary waves
Authors: Frederiksen J. S.
Published: February 2007

A study is made of the horizontal variations of eddy momentum and heat fluxes induced by long planetary waves in a linear two‐layer quasi‐geostrophic spherical model. A wide range of basic flows, consisting of upper‐layer, pure baroclinic or pure barotropic long waves together with solid‐body rotation or either of two zonal jet profiles, characteristic of the troposphere, are considered. The superposition of long planetary waves on otherwise zonal flow profiles has an effect on the eddy fluxes which is profound but is highly variable, depending on the basic flow. It is found, however, that for all profiles involving basic upper‐layer or baroclinic waves, the horizontal structure of both momentum and heat fluxes may be understood in terms of a generalization of Phillips's criterion for incipient instability. For these profiles, the regions of preferential development of baroclinic eddies are also found to be related to the general stability criterion. When barotropic planetary waves are present in the basic flows, the structure of the eddy fluxes is more complicated, reflecting the strong zonal wavenumber coupling in the disturbance streamfunctions.
The estimation of rain from satellite‐borne microwave radiometers
Authors: Lovejoy S. , Austin G. L.
Published: February 2007

Recent advances in satellite‐borne microwave radiometry have led to interest in their use for rainfall amount estimation. The potential accuracy of such techniques is investigated in detail, and is estimated to be about ±70% for rainfall rate estimates, even when many wavelengths are used. The effects of the low spatial and temporal resolution of the most recently launched microwave radiometer (on Nimbus 7) are also investigated, using radar‐based computer simulations. The effects are shown to produce r.m.s. errors of ±200, ±310% respectively, yielding a total error of ±370% for independent processes.
Eddy‐fluxes and spectra in the GATE sub‐cloud layer
Authors: Thompson N. , Webber K. L. , Norris B. P.
Published: February 2007

An investigation of the structure of the GATE sub‐cloud layer carried out using turbulence sensors attached to a tethered balloon cable is described. The observations were made at one or two levels up to 400m near the centre of the C‐scale ship array during the final phase of the experiment. The data were used to derive the vertical eddy fluxes of momentum, sensible and latent heat and the spectra and cospectra of the turbulence parameters, all over one‐hour long periods. Surface fluxes were estimated using the bulk aerodynamic formulae.

The results were classified according to the convective activity prevailing during each one‐hour period. Marked changes in the vertical flux profiles with increasing convective activity are demonstrated. Spectra and cospectra in conditions of suppressed convection (small cumulus clouds only, no showers) show the increasing importance of larger scale mixing processes as heights increase to about 300m, but above this (near the top of the mixed layer) processes on a smaller scale are found to predominate. On these latter scales there is an upward flux of momentum which is a consequence of rather shallow vertical mixing in a region where wind speed decreases with height: in more disturbed conditions deeper convection, coupled with the reversed vertical gradient of wind, causes an upward momentum transfer on larger scales.
Mass divergence in tropical weather systems Paper I: Diurnal variation
Authors: McBride John L. , Gray William M.
Published: February 2007

The diurnal variation of mass divergence and vertical velocity is documented for tropical summertime oceanic weather systems in the Western Pacific, Western Atlantic and the GATE region. It is shown that it is large and has the same basic character in all regions.

Gray and Jacobson (1977) proposed that the diurnal variation of mass convergence is a result of diurnal differences in the net radiative and convective heating rates in the thick cirrus‐shield covered weather systems and their surrounding clear areas. Fingerhut (1978) has developed a numerical model which appears to substantiate this hypothesis. A comparison of observations with results from his model reveals that a simple radiation‐condensation model simulates most of the observed diurnal variations of convergence. The hypothesis is that radiational forcing is one of the major contributors to the maintenance and modulation of tropical weather systems.
Energetics of diabatic mesoscale circulation: A numerical study
Authors: Dalu G. A. , Green J. S. A.
Published: February 2007

Mesoscale circulations, induced by mesoscale available potential energy diabatically generated in the boundary layer are studied through the numerical integration of primitive equations. The energetics of a sea‐breeze simulation is studied in some detail. After 12 hours of simulation the kinetic energy is about a quarter of the mesoscale available potential energy diabatically produced. We comment on the energy lost by the upstream difference scheme, and on some aspects of the unavailability of available potential energy in such a system that together describe this level of inefficiency.