bluuebunny/crossref_metadata_2025 huggingface dataset:
The deepening of wave depressions in relation to geostrophic vorticity advection at 300 mb
Authors: Dixon K.
Published: February 2007

It is found that deepening waves have a characteristic pattern of vorticity advection at the 300 mb level. They have a centre of maximum vorticity advection in the cold air and a marked gradient of vorticity advection extending from this centre across the surface centre of the low into the warm sector. Likewise, non‐deepening and filling waves are also distinctively characterized by their vorticity advection patterns. It is shown that the deepening of waves is related to two parameters, the geostrophic vorticity advection and the scalar product of the gradients of geostrophic vorticity advection and of 300 mb contour height over the centre of the wave. It is further shown that this latter parameter is closely related to the value of the ‘twisting term’ at the level of non‐divergence.
A simple formula for confluent or diffluent currents, with special application to winds
Authors: Groen P.
Published: February 2007

In close formal analogy to the ‘gradient wind’ formula, a formula is derived for confluent or diffluent current (wind) patterns, having only longitudinal accelerations. In first approximation, if the accelerations are not too large, the velocity differs from the geostrophic velocity mainly in direction, the angle of deviation β being determined by:

Where vg = geostrophic velocity, f = 2ω sin ϕ = Coriolis parameter and s = ‘radius of confluence (diffluence)’ or ‘radius of longitudinal acceleration.’
Turbulent energy measurements by vanes
Authors: Maccready P. B. , Jex H. R.
Published: February 2007

Utilizing the — 5/3 law spectrum for the inertial subrange of eddy sizes, it is shown that the indicated variance from a direction vane will be the same for an actual vane with a damping ratio of about 0·43 as for a vane with perfect response. For this damping ratio the addition to the variance by overshooting at wavelengths near the natural wavelength compensates for the loss by vane attenuation at shorter wavelengths. the relation between the variance for short sampling times, using such sensors, and ·, the dissipation rate, which represents a basic parameter for inertial subrange eddies, is derived. an airborne method of indicating · continuously is described.
The use of pressure tendencies as an aid to forecasting in east africa
Authors: Sansom H. W.
Published: February 2007

It is shown from the tendency equation that the 3‐hr pressure tendency, corrected for normal diurnal variation, may be an indication of vertical motion. an analysis of twelve months records of weather at both Entebbe and Nairobi confirms the existence of a relationship between pressure tendency and impending weather. the influence of isallobaric convergence on the weather is also demonstrated.
Bearing and incidence of sub‐tropical convective rainfall
Authors: Hudson N. W.
Published: February 2007

The Rose/Farbrother three‐raingauge instrument was used to measure bearing and inclination of rainfall at Mazoe, Southern Rhodesia. Heavy falls of rain (more than 0·5 in.) were found to be associated with small angles of inclination with the vertical. Three‐quarters of the rainfall was from the east and south in the sector 045° to 195°.
Moisture transfer in free and forced convection
Authors: Crawford T. V.
Published: February 2007

The vertical transfer of moisture is examined under conditions where the Richardson number varies from − 2.5 to + 2.9. The use of a non‐dimensional evaporative flux, similar to Priestley's (1959) non‐dimensional sensible heat flux, and extensive data from the Davis lysimeter field show a smooth transition from forced to free convection. An equation for this transitional regime of mixed convection is derived by dimensional analysis and fitted to the data for different Richardson number ranges. A free convection interpretation of the data for −2.5 < Ri < −0.025 gives a proportionality constant of 1.40.

In stable conditions, the turbulent transfers are shown to decrease rapidly with increasing + Ri and this behaviour is consistent with some estimates of the initiation of decay of turbulence.
Determination of turbulence dissipation by Eulerian variance analysis
Authors: Frenzen P.
Published: February 2007

Using a ‘variance function’ to specify the variance retained by a time series smoothed via running means, it is shown that the rate of turbulence dissipation can be computed from an Eulerian wind record supplied by a single, 3‐cup anemometer through the relation ϵ = (A) [( ‐)/(Tk2/3 ‐ Tj2/3)]3/2 Here A ≈︁ 3.3 derives from the Kolmogoroff law, Ū is the mean flow, and the numerator of the bracketed term represents the additional reduction in variance effected by increasing the averaging interval from Tj to Tk. Preliminary verifications of the relation are presented, and an experimental programme designed to use the method is outlined.
The electrification of freezing water drops
Authors: Stott Doreen , Hutchinson W. C. A.
Published: February 2007

When 633 supercooled drops of mean diameter 1.3 mm were nucleated with ice crystals at −1°C and then frozen in air at −15°C the 91 measurable charges on the residues of the 118 drops which broke ranged from −17 × 10−3 to + 25 × 10−3 e.s.u. Spicules were always formed before the drops broke, and in 48 cases the break was across the spicule stem. The breaking spicules containing liquid water resulted mainly in negatively charged residues. Solid spicules usually left positively charged residues. When these results are combined with those of Evans and Hutchinson (1963) the average charge for liquid spicule residues is − 1.4 ± 0.9 × 10−3 e.s.u. (49 drops) and for solid spicules + 3.4 ± 2.2 × 10−3 e.s.u. (25 drops). Other reported observations are also discussed. The magnitudes of the charges are rather high to be explained entirely in terms of the temperature‐gradient theory of Latham and Mason. There appears to be considerable evidence for a charge separation across the boundary between the freezing water and the ice, water being positive and ice negative.
Instability in a geostrophic wind with a transverse wind‐speed gradient
Authors: Wilson S. D. R.
Published: February 2007

The stability of an airstream in geostrophic balance, with a transverse velocity gradient, to horizontal displacements of a single particle has previously been investigated. Here, the effect on such an airstream of a general three‐dimensional disturbance, taking account of the stable density gradient of the air, is considered. A necessary and sufficient condition for instability is obtained for an inviscid airstream and a two‐dimensional disturbance (independent of the downstream coordinate). Finally we discuss how this criterion is modified if viscosity is introduced, and also if the disturbance is allowed to be three‐dimensional.
The description of Australian pressure fields by principal components
Authors: Veitch L. G.
Published: February 2007

The statistical method of principal components, when applied to mean sea level pressure data from the Australian region, shows that, in addition to the well‐known eastward succession of cyclones and anticyclones and the zonal pressure index type of oscillation, an important source of pressure variability is a quasi‐periodic oscillation of pressure over the eastern Australian region, which has ‘periods’ considerably longer than those due to the normal succession of cyclones and anticyclones. These several effects are revealed in the first three principal components which together account for 75 per cent of the total pressure variation.
Cross‐arc deposit of particles from continuous and instantaneous sources
Authors: Walker E. R.
Published: February 2007

Experimental details are presented for two series of short‐range diffusion experiments in which glass spheres were emitted from continuous and instantaneous sources. Observed standard deviations of cross‐arc deposits were compared with predictions. For instantaneous sources these standard deviations were fairly well forecast by the simple method of Smith and Hay (1961), although predictions were improved by use of the most appropriate value of β, a Lagrangian‐Eulerian scale factor. Continuous source cross‐arc standard deviations were well forecast by Hay and Pasquill's (1959) method.
A laboratory investigation of the electrification of snowstorms
Authors: Latham J. , Stow C. D.
Published: February 2007

Artificial snowstorms were created inside a large cold room by blowing snow crystals over a snow surface. The charges acquired by the visible blown particles and the residual snow surface and the electric fields and concentrations of positive and negative ions produced in the air above the snow surface were measured as a function of the wind velocity, U, the relative humidity, the amount of snow introduced into the air jet, and the temperature of the jet and the snow surface, Tj and Ts respectively; all of these variables were monitored and recorded continuously. In a typical experiment with Tj = − 20°C, Ts = −40°C, U = 430 cm sec−1, and snow crystals introduced into the air jet at a rate of 1 g sec−1 the snow surface acquired a charge of 5 × 103 e.s.u. in 10 sec, the charge residing on the visible blown particles was approximately equal and opposite to this amount, and the concentrations of positive and negative ions created in the air were 4,600 and 4,800 cm−3 respectively, corresponding to a net charge carried on the air within the cold room of less than 1 e.s.u. The charging was found to be a sensitive function of (Tj — Ts) over a very wide range but was independent of Ts except at high temperatures when large‐scale melting and erratic charging were observed. The charging was found to increase rapidly as U was increased from 1 to 5 m sec−1, and was slightly reduced as the relative humidity was raised.

The measured electrification at temperatures below 0°C is shown to be entirely explicable, quantitatively and qualitatively, in terms of the temperature‐gradient effect. The charge acquired by the snow surface and the blown particles is a result of fragmentation of snow crystals which have temperature differences across them, asymmetric rubbing between snow particles and transient contact between crystals of different temperatures, all of which processes are manifestations of the temperature‐gradient effect. The primary source of the charge carried by the air is shown to be point discharge between highly charged particles.

The contributions to the measured electrification of the Workman‐Reynolds effect, the Dinger‐Gunn effect and the evaporation of ice are shown to be minimal.
The distribution of charge within ice specimens subjected to linear and non‐linear temperature gradients
Authors: Latham J. , Stow C. D.
Published: February 2007

Numerical solutions are presented of the differential equations derived by Latham and Mason (1961) describing the distribution of charge within ice specimens across which temperature gradients exist. In the case of specimens subjected to non‐linear temperature gradients the computations predict an extremely rapid increase of volume charge density with increasing temperature gradient and therefore provide an explanation for the reported dependence of the thermo‐electric effect in ice on the geometry of the specimen under investigation.
Evidence of singularities in radiosonde data for Rapid City, South Dakota
Authors: Dennis A. S.
Published: February 2007

A search for singularities in the weather at Rapid City, South Dakota is described. It differs from previous investigations of singularities in two respects : (i) the use of pressure and temperature data from radiosondes, and (ii), the application of analysis‐of‐variance techniques to departures of weekly means from fitted annual trends. These analyses lead to the rejection of the hypothesis that there are no weekly singularities in the climatic elements studied. The singularities are tentatively ascribed to week‐to‐week variations in the probability of Arctic outbreaks into the Great Plains region.
Freezing and shattering of supercooled water drops
Authors: Johnson D. A. , Hallett J.
Published: February 2007

Supercooled water drops of about 0·05 cm radius were frozen at temperatures between −4°C and −25°C under various conditions. Explosive shattering and ejected ice splinters were observed when supercooled drops were frozen under conditions of free convection in hydrogen and helium at 1·0 atm pressure, in air below 0·13 atm pressure and in carbon dioxide below 0·05 atm pressure. Drops did not fracture when ventilated at terminal velocity unless they were rotated about an axis normal to the airstream during freezing. Fracture was also observed when either the drops were not allowed sufficient time to attain thermal and solution equilibrium with the environment before nucleation, or when drops were frozen in carbon dioxide at a pressure above 0·3 atm.

A condition for drop fracture is the establishment of a strong shell of ice around the drop, which in turn depends on the distribution of heat transfer to the environment during freezing and on the effective thermal conductivity of the environment. The volume of gas dissolved in the drop is a secondary factor, except in the case of a very soluble gas such as carbon dioxide where splinters are produced by vigorous effervescence during freezing. In the atmosphere the shattering of an individual drop appears possible only if the drop rotates during freezing. This appears unlikely since any asymmetry in shape would tend to orient the drop as it fell.
The spectra of charging events due to the collision of natural ice particles with an ice surface
Authors: Scott W. D. , Hobbs P. V.
Published: February 2007

A sensitive, solid state electrometer has been built which is capable of measuring the individual charges acquired by an ice surface exposed to precipitation or cloud particles. Measurements made in natural snowfall show that the individual charges fluctuate widely both in sign and magnitude. Individual charges ranging from ‐10−2 e.s.u. to 10−2 e.s.u. have been recorded within an interval of a few seconds. In most cases the ice surface received a net positive charge, and the median charge per collision was about 2 × 10−4 e.s.u. The charging spectra commonly followed a log‐normal distribution.

It is suggested that two important mechanisms for the charging were the direct transfer to the ice surface of some of the charge residing on the ice particles in the air and the fragmentation of the ice particles as they collided with the ice surface.
Mass, heat and momentum exchange between stands of plants and their atmospheric environment
Authors: Cowan I. R.
Published: February 2007

A model of mass and momentum transfer in the air layer occupied by a stand of plants is presented which takes account of the differing mechanisms of mass exchange and momentum dissipation near to the leaf surfaces. An analysis of momentum transfer leads to expressions for the vertical transport coefficient and windspeed in terms of the mean drag coefficient of the leaves, the drag coefficient of the stand as a whole and the area of leaf per unit volume of space. These expressions are used in the solution of the equations describing mass transfer with constant concentration of mass at the leaf surface. The effect of a chemical or physical resistance to transfer at the leaf surface is examined. The resistance to mass exchange between the stand and the atmosphere above is compared with that for momentum exchange. The transfer of water vapour in the stand is considered, the boundary conditions at the leaf surfaces being linked with those for sensible heat transfer. It is shown that, when leaf resistance to transpiration is negligible, the vapour and heat‐fluxes are readily obtained from simple mass transfer theory and a knowledge of the distribution of radiation in the stand. The formal relations are extended to allow for finite leaf resistance. Computed profiles of the fluxes, temperature and humidity in a plant stand of assumed characteristics with defined boundary conditions are presented and compared with published observations. The interrelationship between leaf resistance, transpiration and leaf temperature is discussed. An expression for the total evaporation from a stand is derived and contrasted with a relation based on the analysis of Penman and Schofield (1951) and Monteith (1965).
Recent measurements of cospectra for heat‐flux and stress
Authors: Panofsky H. A. , Mares E.
Published: February 2007

Recent measurements of cospectra for stress at different sites show no significant difference when plotted in similarity coordinates, but vary with stability only. Further, there is no significant difference between heat‐flux and stress cospectra when plotted in such coordinates.

Cumulative cospectra derived from the recent data agree well with much earlier results compiled by Deacon.
The circulation driven by ring convection in a rotating system
Authors: Turner J. S.
Published: February 2007

A recent survey of hurricane dynamics has suggested an investigation of the motions which could be driven entirely by convection in the eye wall. A laboratory experiment based on this idea has been performed in a rotating tank of water, with convection in a ring produced using air bubbles. A steady circulation having many features of the mean hurricane circulation can be maintained by this means.
Observational characteristics of atmospheric fluctuations with a time scale of a month
Authors: Sawyer J. S.
Published: February 2007

The time series of 500 mb heights and 1,000‐500 mb thickness at a network of points over the Northern Hemisphere are filtered in a way which extracts those disturbances with periods in the range 15 to 60 days. Such disturbances are shown to have a maximum amplitude in latitudes 60 to 70 degrees north and be strongly influenced geographically. They are closely linked to the phenomenon of blocking.
The collision efficiency of small drops
Authors: Hocking L. M. , Jonas P. R.
Published: February 2007

The forces and couples on a pair of spheres rotating and translating in a viscous fluid have been obtained by Davis (1969) and O'Neill and Majumdar (1970) from accurate solutions of the Stokes equations. These forces and couples have been used to calculate the collision efficiencies of pairs of small cloud droplets. The results show that the collision efficiencies are finite for all the pairs of droplets considered, in contrast with the results of Hocking (1959) who predicted that for droplets of radius less than 19 μm the collision efficiencies were zero. The collision efficiencies of droplets of radius less than 20 μm is shown to vary considerably with the critical minimum gap between the droplets below which collision is assumed always to occur. The approximations involved are discussed and the effects of electric fields or charges on the drops are shown to be negligible in the early stages of cloud droplet growth.
Results of a comparison between Umkehr and ozone‐sonde data
Authors: Kulkarni R. N. , Pittock A. B.
Published: February 2007

Results of an intercomparison of 26 pairs of vertical distributions of ozone obtained at Aspendale (38°S) using the Umkehr technique and Mast‐Brewer ozone‐sondes on the same days are presented. Correlation coefficients between the integrated ozone amounts found by the two methods for Umkehr layers 2, 3 and 4 are found to be between 0.82 and 0.88, while the correlations for layers 1 and 5 are 0.47 and 0.37 respectively. On the average the Umkehr method indicates substantially more ozone in layers 1 and 2 than do the ozone‐sondes, but slightly less in layer 4. The Umkehr method thus tends to smooth out the vertical distribution of ozone relative to that observed by the ozone‐sondes. Results of comparisons at different times and places suggest a variation in the response of the Umkehr method relative to sonde methods.
Spectra and cospectra of turbulence in the atmospheric surface layer
Authors: Sitaraman V.
Published: February 2007

Spectra and cospectra of turbulence in the constant‐flux surface layer of the atmosphere are studied with measurements made by rapid response propeller anemometers and a bead thermistor. The results are described in comparison with those obtained at other sites.
Scale length in atmospheric turbulence as measured from an aircraft
Authors: Taylor R. J. , Warner J. , Bacon N. E.
Published: February 2007

Scale lengths, defined in four different ways, have been derived from previously reported spectra and autocorrelations of vertical air velocity component measured from an aircraft at heights between 10 and 1,310 m. Below about 200 m the scale length increases with height more slowly than linearly and it does not seem possible to attribute this result to stability effects. At higher levels, no systematic height dependence can be shown. The results offer some support for the suggestion that the scale length is greater in lapse conditions than otherwise.
Simultaneous airborne measurements of cloud condensation nuclei and sodium‐containing particles over the ocean
Authors: Hobbs P. V.
Published: February 2007

Simultaneous airborne measurements have been made over the Pacific Ocean of the concentrations of cloud condensation nuclei (CCN) active at 0.5 per cent supersaturation and the concentrations and size‐spectra of sodium‐containing particles (SCP) over a wide range of sizes. The concentrations of SCP decreased sharply with increasing height above the ocean and it is likely that most of them originated from the ocean. However, even at altitudes as low as 50 ft above the surface of the ocean, and over surf, the measured concentrations of SCP were only a few per cent of the concentrations of CCN at G.5 per cent supersaturation. Also the average concentrations of CCN did not change significantly with altitude up to at least 10,000 ft. These observations indicate that sea‐salt particles were not the major source of the CCN.

It is suggested that the production of sulphates in cloud droplets, followed by the release of these particles when the droplets evaporate, is an important mechanism for increasing the efficiency of CCN in the air.
Charge separation during splashing of large drops on ice
Authors: Shewchuk S. R. , Iribarne J. V.
Published: February 2007

An investigation was performed on the electrification associated with splashing of water drops at 0°C on ice coated rotating spheres. The influence of different parameters was studied: roughness of the surface, drop size, collision speed (up to 30 m s−1), liquid composition, including pH, and ice temperature (down to ‐ 20°C). It is shown that the sign of the charge acquired by the ice, usually negative for low temperatures and dilute solutions, becomes positive for temperatures close to 0°C, for large concentrations of electrolyte and for solutions containing ammonium ion. The electrification increases with drop size and with the violence of the impact, while it is strongly reduced by the concentration of hydrogen ion in acid solutions. Flash photographs at high speeds show the splashing process to be similar to that at room temperature on other solid surfaces.

The results are discussed in the light of a theory previously developed for splashing at room temperature and of the selective incorporation of ions into the ice. Some speculation is done on the relative importance of the process in clouds.
On an observation of turbulent waves on the tropopause surface
Authors: Axford D. N.
Published: February 2007

An analysis of aircraft measurements at a well defined tropopause inversion is presented. It is found that there is turbulence on the inversion which is closely correlated to some form of wave activity.

The observations show that any aircraft flying at the level of a strong inversion may expect sudden changes in both temperature and horizontal and vertical wind due to wave motions on the interface, and that these changes can amount to the full temperature excursion through the surface (in this case 10°C); and the full shear (in this case 7 ms−1) in a horizontal distance of about 1 km.
A wind tunnel investigation of freezing of small water drops falling at terminal velocity in air
Authors: Pitter R. L. , Pruppacher H. R.
Published: February 2007

A method utilizing a vertical wind tunnel is described, by which supercooled water drops and ice particles can be stably suspended at temperatures between 0 and −30°C. Using this method, we studied: (i) the freezing temperature of water drops of various purity and volume, (ii) the freezing temperature of drops nucleated by contact with dry clay particles and the freezing temperature of drops nucleated by clay particles which were suspended in them, (iii) the crystallographic characteristics of frozen drops, and (iv) the free fall motion of supercooled drops and frozen drops freely suspended in air. The first study showed that the median freezing temperature of a population of equal size water drops is a linear function of the logarithm of the drop volume, in agreement with the law found by Bigg for water drops in oil. The second study showed that the temperature at which a drop froze when nucleated by contact with dry clay particles was over 10°C warmer than the temperature at which a drop of the same size froze when nucleated by clay particles suspended within the drop. The third study showed that frozen drops, recovered from the wind tunnel airstream and investigated under a polarization microscope, were single crystalline or polycrystalline depending on the drop volume, the temperature of freezing, and the heat dissipative capability of the medium surrounding the drop. The lowest temperature for single crystalline freezing is significantly colder for drops which freeze while completely surrounded by air than for drops which freeze on contact with an ice crystal. The fourth study showed that a supercooled drop falls with a constant velocity and no horizontal movement, while a frozen drop often spins and tumbles, and falls along a helical path with a terminal velocity which is 6 to 7 per cent smaller than that of the supercooled drop before it froze. This means the life time of a cloud particle lengthens as a result of freezing.
Translation speed of convective plumes in the atmospheric surface layer
Authors: Kaimal J. C.
Published: February 2007

Experiments conducted over a flat uniform ploughed field indicate that the small convective plumes commonly observed in the surface layer travel as an entity with horizontal speeds proportional to the friction velocity, u. This proportionality, in turn, implies a direct relationship between the inferred height of the plume base and roughness length. For these experiments, the height of the plume base averaged 1.2 m.
On the low‐level wind structure in the Atlantic trade
Authors: Brümmer B. , Augstein E. , Riehl H.
Published: February 2007

From ship‐borne radiosonde and radar wind observations during the Atlantic Trade Wind Experiment 1969 (ATEX) the relation between actual and geostrophic wind in the subcloud layer is determined. The frictional force and the divergence of vertical transport of momentum between the sea surface and 500‐m height are computed by the ageostrophic method.

Along the surface wind direction, balance between pressure gradient and frictional force is closely established. Perpendicular to the surface wind the expected balance of pressure gradient and Coriolis force is not obtained but a substantial frictional force results from this component.

The boundary condition that the stress vanishes at the lowest wind maximum leads to an underestimation of surface stress by about 35 per cent. It is suggested that convective transport of momentum must take place in the subcloud layer.
The ionic conductivity of electrified clouds
Authors: Griffiths R. F. , Latham J. , Myers V.
Published: February 2007

Calculations have been made, on the basis of the ionic balance equations derived by Phillips (1967), of the electrical conductivity of clouds in the presence, and in the absence, of secondary ion production resulting from corona.

In weakly electrified clouds ‐ no corona ‐ the immobilization of ions on cloud droplets resulting from diffusional capture results in a reduced ionic concentration and conductivity. The normalized conductivity λ the ratio of the in‐cloud value to the clear‐air value at the same altitude Z was calculated for three different cloud types (cumulus congestus, strato‐cumulus and fog) as a function of Z, liquid water content L, droplet charge Q and field strength E. λ was found to be very sensitive to variations in L and E, but only slightly affected by changes in Z, Q and the manner in which the charge is distributed over the size‐spectrum. For example, when E = O λ possessed the values 0.034 and 0.17 in a cumulus congestus cloud for L = 0.5 gm/m3 and 0.1 gm/m3 respectively, whereas when E = 90 kV/m λ was 1.5 times; 10−3 and 7.6 times; 10−3 respectively for the same two values of L.

The experimental measurements of corona onset fields and currents from ice hydrometeors, Ec and ic respectively, made by Griffiths and Latham (1974) were used to calculate the normalized conductivity within strongly electrified clouds. Corona from ice provides a copious supply of positive and negative ions which causes λ to increase from between 10−3 and 10−4 to greater than 1 when Ec is achieved. The value of λ once corona is initiated depends on L and the concentration of corona sites, J. In this situation altitude Z plays an important role because Ec decreases quite rapidly with decreasing pressure. For example, with a typical distribution of temperature and pressure within a thundercloud, corona will be initiated from hailstones at Ec = 720 kV/m if Z = 2.5 km (0° C) and 540 kV/m at 5.5 km (‐18° C); the corresponding values of Ec for snowflakes are 540 and 390 kV/m respectively. At 2.5 km is slightly higher than and lower than 10 respectively, as L is increased from 1 to 5 gm/m3 with J = 1 m−3.

It is concluded that the introduction of an ionic leakage current into calculations of field growth within thunderclouds is unnecessary, and that the distribution of electric field is crucial in determining whether corona, once initiated, will trigger lightning or merely increase the ionic conductivity.
A case study of the structure and energetics of an inversion
Authors: Rayment R. , Readings C. J.
Published: February 2007

Using data obtained from a probe mounted on the flying cable of a tethered kite balloon, this paper presents a fairly comprehensive description of the structure of a ‘hummocky’ subsidence inversion and the associated air motions, including the formation and breakdown of billows at the crests of the hummocks. Below the inversion, the transfer of warm air downwards was detected and the rate at which this occurred estimated using the model proposed by Readings et al. (1973). This estimate is shown to be in good agreement with that derived from some radio‐sonde ascents. The fluxes of potential heat downwards at the inversion and upwards at the ground are estimated and their ratio is shown to be about 0.25.
A numerical investigation of polar lows
Authors: Duncan C. N.
Published: February 2007

A quasi‐geostrophic numerical model is employed to find normal mode solutions for unstable disturbances with low static stability near the earth's surface. Three occasions on which polar lows developed are analysed to provide the necessary data for the model. The normal mode solutions which are obtained are, therefore, those which have the highest growth rates under conditions in which polar‐air depressions have developed. Since the model is two‐dimensional, being a cross‐section through the mean flow, the perturbation amplitude and energy conversion rates of the growing disturbances in this plane are described for each case. It is shown that horizontal variations in static stability are an important factor in the forecasting problem and some criteria for polar‐low development are suggested.
A new parameterization of deep convection for use in the 10‐level model
Authors: Hayes F. R. , Jonas P. R.
Published: February 2007

A convection parameterization scheme recently incorporated into the Meteorological Office 10‐level model is described. A one‐dimensional cloud model incorporating entrainment and detrainment, together with a simple representation of cloud microphysics, is used to provide heat and moisture fluxes for the predictive equations of the parameterization. Results of the scheme in operational use are presented.
Empirical orthogonal analysis of atlantic ocean surface temperatures
Authors: Weare Bryan C. , Newell R. E.
Published: February 2007

An empirical orthogonal function analysis has been performed on monthly mean sea surface temperature observations in the Atlantic Ocean between 70°N and 30°S for the years 1949–69. This method derives the most important ‘modes’ of variation, defined as those functions which explain the largest total variance in the data field. Such modes are calculated for two sets of temperature deviations in which the annual cycle is both included and excluded. For the seasonal case, only four functions explain more than 90% of the total variance, whereas in the nonseasonal case, ten functions explain less than 50% of the total. the most important seasonal mode indicates lower minimum temperatures in winter in the later years of study. the dominant nonseasonal function shows a cooling trend starting in early 1951. Nonseasonal functions of the tropical region alone suggest large variations, having a characteristic period of a few months.
Climatology of the vertical distribution of ozone over Aspendale (38°S 145°)
Authors: Pittock A. B.
Published: February 2007

Data from the first eight years (1965–73) of an ozone sounding programme at Aspendale, Victoria, have been analysed. the mean annual cycle is compared with that derived from similar data from the northern hemisphere. Close agreement is found at similar latitudes in the two hemispheres for the stratosphere at and below the primary ozone maximum, whereas in the free troposphere there is a much smaller seasonal variation in the southern than in the northern hemisphere. There appears to be a marginally higher ozone concentration around the 10 mb region in the northern as compared with the southern summer.

The variance about the overall mean vertical distribution has been partitioned between various time scales. the seasonal contribution dominates the variance around the level of the primary ozone maximum, with substantial contributions also in the stratosphere at lower altitudes. the synoptic and other shorter timescale variance (including instrumental ‘noise’) is low above the 50 mb level, where ozone and pressure sensors are least reliable, but it exceeds the seasonal contribution below 70 mb. This suggests that the maximum contribution of the shorter timescales to the variance around the 100 mb level is largely synoptic in character.

Significant linear trends in the data are discussed briefly. No evidence for quasi‐biennial or 10 to 11 year solar cycle periodicities is found but significant correlations between ozone variations and the position of the subtropical surface high pressure belt (Pittock's L index) are shown to occur in the stratosphere at and below the level of 50 mb.
The rime‐splintering hypothesis of cumulus glaciation examined using a field‐of‐flow cloud model
Authors: Koenig L. Randall , Murray Francis W.
Published: February 2007

A mathematical experiment was conducted to test the hypothesis that the occurrence of unexpectedly large concentrations of ice particles in certain clouds is caused by the production of small ice particles (‘splinters’) during the riming of mother ice particles. the experiment used an axially symmetric two‐dimensional field‐of‐flow model incorporating parameterized liquid and ice microphysics. the behaviour of clouds, growing with either wide or narrow initial droplet size distributions, with and without splintering, was simulated. Splintering criteria were based on recent laboratory data.

The rapidity of liquid to ice conversion was found to be a strong, direct function of the prior presence of large liquid drops. the dynamical properties of the clouds influenced both splinter production and also the over‐all glaciating characteristics regardless of the presence or absence of an ‘ice multiplication’ mechanism. Cloud condensate at levels having strong updraughts and vigorous growth was more slowly converted to ice than condensate in regions of dissipating cloud. the simulations permitting ice generation by splintering more closely duplicated pertinent observations in nature than those without splinter production. Unexpectedly, the characteristics of the liquid phase exerted substantial influence on the dynamics of the simulated clouds.
The electrical structure of an inversion
Authors: Chisnell R. F. , Latham J. , Ryder P. , Stromberg I. M.
Published: February 2007

Measurements were made, using an instrumented aircraft, of the vertical electric field strength, E, and the ionic conductivity, λ, of the atmosphere in the vicinity of a temperature inversion. It was found—moving downwards through the inversion — that the reduction in λ associated with ionic immobilization upon aerosol particles occurred over an appreciably shorter vertical distance than the accompanying increase in E. the ionic current density was not preserved over the region of changing E. These observations suggested that some of the space‐charge created in the region of conductivity gradient was being transported downwards by eddy diffusion. A theoretical model of this process was constructed, based on the principle of current continuity. Excellent agreement between the particularly simple field structure measured on one flight and the theoretical structure existed if the characteristic diffusion length was about 60 m. This corresponds to an eddy diffusion coefficient, K, of about 3.0 m2s−1.

It is suggested that in some circumstances, the simultaneous measurement of E and λ affords a quantitative technique for the determination of K.
On the general circulation of the atmosphere without clouds
Authors: Hunt B. G.
Published: February 2007

An experiment has been performed with an 18‐level, hemispheric, stereographic general circulation model to clarify the radiative role of clouds in the general circulation of the atmosphere. All cloud properties were removed from the radiation scheme in the model, and it was then integrated to generate a new set of quasi‐equilibrium atmospheric statistics which were compared with a control run.

Apart from an expected warming of the surface and troposphere the overall changes to the general circulation characteristics following the removal of clouds were surprisingly small. For example, the zonal mean latitude‐height distributions of the zonal wind and eddy kinetic energy were almost identical to those of the control run, while synoptic distributions of most model variables revealed little change in character attributable to the removal of cloud. The largest modification occurred in the mean meridional stream function, with corresponding effects in surface pressure. This result was explicable in terms of the enhanced role of latent heat in the model which occurred at the expense of sensible heat. Despite the individual variations in latent and sensible heat, the poleward flux of total heat by large‐scale eddies was essentially unchanged in the two models. This surprising result indicated that the forcing function for the general circulations of the two models was also unchanged, and moreover that this forcing function was independent of cloud cover.

Where possible, explanations are advanced to account for the limited response of the model to the removal of cloud. It appears that the major function of clouds in radiative calculations, at least in extended general circulation simulations, is to provide an obscuration or blocking effect, rather than a more subtle radiative‐dynamical coupling mechanism.
A wind tunnel investigation of the hydrodynamic behaviour of growing, freely falling graupel
Authors: Pflaum J. C. , Martin J. J. , Pruppacher H. R.
Published: February 2007

The hydrodynamic behaviour of riming frozen drops and riming simulated ice crystal plates was studied by means of a vertical wind tunnel. At all times the ice particles were freely suspended in the vertical airstream of the wind tunnel where temperature and liquid water content could be accurately controlled. The ice particles studied had initial radii between 100 and 300 μm and final radii between 400 and 600 μm. The supercooled cloud in which riming of the ice particles was observed had a temperature near −15°C, a liquid water content of 2g m−3, and consisted of drops with a modal radius of 7μm.

Spinning, helical, and various oscillating motions, or simple straight fall with an almost fixed orientation of the ice particle, but no tumbling motions, were observed. These fall modes appeared to be closely related to the instantaneous shape, surface roughness, and mass loading of the ice particle. Frozen drops as well as ice crystal plates were found to develop into conically shaped graupel.

During the initial stages of the observed dry growth riming, the terminal velocity of rimed frozen drops decreased with increasing ice particle size as a result of a less rapid increase of the mass of the ice particle compared with its cross‐sectional area perpendicular to the airstream, i.e. as a result of a less rapid increase in time of the force the ice particle experiences in the earth's gravitational field compared with the hydrodynamic drag acting on it. This behaviour may have some important bearing on the lifetime of graupel and hailstones in atmospheric clouds.
Tornadogenesis
Authors: Smith R. K. , Leslie L. M.
Published: February 2007

In this paper we use a simple numerical model to study vortex growth in a flow configuration which broadly simulates the principal characteristics of a severe tornadic storm system, i.e. strong vertical forcing by intense cumulus or cumulonimbus convection in the presence of an organized field of rotation on the cloud scale. The model is similar to one described by L. M. Leslie in which the updraught in the main cloud cell is modelled by an imposed body force, but differs in that the rotation field is determined by specifying the vertical profile of swirling velocity at the lateral boundary of the flow domain and air is allowed to enter or leave the computational region through its radial and upper boundaries. In particular, we compare situations in which the imposed swirl is concentrated aloft, primarily above cloud base, and when it extends to lower levels. In the former case, solutions exhibit genesis to a steady suspended vortex provided that the forcing strength lies within a certain range, depending on the level of rotation. However, if the imposed swirl extends sufficiently far below cloud base, the vortex continues its downward growth and establishes contact with the ground.

We believe our results provide a plausible and consistent picture of the growth of a tornado beneath the main updraught of a severe thunderstorm and indicate why only a relatively small proportion of such clouds spawn pendant funnel clouds and why only a fraction of these develop into tornadoes. Moreover, they appear to be consistent with the observed development of circulation patterns deduced from single‐pulse Doppler radar measurements of a tornado‐producing storm system by Burgess, Lemon and Brown.
Quasi‐geostrophic planetary wave forcing
Authors: Shutts G. J.
Published: February 2007

The problem of stationary planetary wave forcing by orography and longitudinal asymmetry of heating is re‐examined in the light of recent observations and theories of stratospheric motion. It is deduced that the region between 30 and 60km is a sink of planetary wave energy for zonal wavenumbers one and two in winter and that, in contrast to many theoretical descriptions, the transmission of wave energy to these levels is unimpeded.

Quasi‐geostrophic theory expressed in spherical polar geometry suggests that in winter, wave energy should be able to propagate freely into the upper stratosphere for the largest scales of motion. Uncertainty in the absorption mechanism is avoided by the use of an energy‐transmitting upper boundary condition at a suitably chosen level and attention is concentrated on the structure of the wave motion below. Features of the January mean contour charts such as the Siberian and Aleutian anticyclones are shown to be consistent with the theory of untrapped, thermally forced motion. Close agreement of the structure of wavenumber one in theoretical solutions with existing observations is obtained for all heights below 30km, vindicating the use of energy‐transmitting boundary conditions.

An important consequence of the ability of forced waves to propagate energy into the stratosphere is the poleward transport of heat, which is particularly strong for thermally forced motion near the level of non‐adiabatic heating. These calculations suggest that the contribution of stationary, thermally generated waves to the total poleward transport of heat is a major one in the lower troposphere.

Some attempt to describe the monsoonal circulation of the northern hemisphere in summer is made with the inclusion of a stratosphere dominated by easterly mean zonal winds. The presence of a critical layer (where the mean zonal wind vanishes) introduces some interesting problems as regards the ‘realism’ of the linearized solution. Observations in July show that the phases of low wavenumber disturbances tilt rapidly eastwards with height in marked contrast to the westward tilt in winter. Solutions allowing critical‐level absorption are in poor agreement with observation in the troposphere with westward‐sloping phase lines, though the rapid attenuation of all waves in the stratospheric easterlies is well represented.
Transfer characteristics for a heterogeneous surface of large aerodynamic roughness
Authors: Garratt J. R.
Published: February 2007

Values of zo and ZT, roughness scaling lengths for momentum and heat, are found to be 0.4±0.2m and 0.035±0.02m, respectively, for a heterogeneous surface comprising 8m high trees with transpiring foliage, 1 m high dry grass, and sandy soil. Determination of ZT utilizes aircraft and ground‐based radiometric observations of surface temperature.

The ratio Zo/ZT ≈ 12 was found essentially constant over the observed roughness Reynolds number range 5000–25000, consistent with values published for homogeneous natural vegetated surfaces.
The dynamical structure of two‐dimensional steady convection in constant vertical shear
Authors: Moncrieff M. W.
Published: February 2007

A nonlinear regime of two‐dimensional steady convection in unidirectional flow of constant vertical shear is examined theoretically, the main objective being to determine the organization of the updraught and downdraught system. The asymptotic solution for the travel speed of the system and the remote flow obtained by Moncrieff and Green is used as lateral inflow and outflow boundary conditions. The internal flow and the shape of the interface separating the updraught and downdraught branches of the circulation are determined as solutions of a free‐boundary problem. The orientation of the flow is basically determined by the ratio of convective available potential energy (CAPE) to the square of the cloud layer shear expressed as R=CAPE/½ (Δ U)2. In all realistic ranges of R the interface slopes downshear, the slope being little affected by compressibility, by the negative vorticity generated in an interfacial boundary layer and by different values of R in the updraught and downdraught branches. Quasi‐steady storms are thus not adequately described by two‐dimensional dynamics. Certain three‐dimensional effects appear essential to significantly advance the theory of severe storms, and these are discussed with reference to existing numerical and observational models. Steady convection can exist only if the values of R in the updraught and downdraught branches lie within a limited range, a result of considerable significance in cloud physical terms.