Showing papers in "Quarterly Journal of the Royal Meteorological Society in 1973"

Non-precipitating cumulus convection and its parameterization

Abstract: This paper discusses the thermodynamic transports of heat, liquid water and (briefly) water vapour by non-precipitating cumulus convection. It is shown that because of the irreversible mixing between cloud and environment, there is a downward transport of enthalphy in the cumulus layer. A lapse-rate adjustment model relates stratification to the life-cycle of a model cloud parcel. A sub-cloud layer model specifies the lower boundary of the lapse-rate model, and the convective transports through cloud-base. Budget equations together with the lapse-rate model, and its time dependent boundary conditions, predict the time development of the cumulus layer, and show the dependence on large-scale mean vertical motion, cloud-base variations, and the surface sensible heat flux.

Momentum, heat and water vapour transfer to and from natural and artificial surfaces

Abstract: Recent atmospheric and wind tunnel data on the heat, mass and momentum transfer above natural and artificial surfaces indicate a general increase in the quantity kB−1 (= In(zo/zp), where zp is the roughness length appropriate to the quantity p) between aerodynamically smooth and fully rough flow. Below a roughness Reynolds number, u*zo/v, of about 100, kB−1 appears to be a single valued function of u*zo/v, closely approximated by the assumption of a molecular sub-layer for heat and water vapour fluxes. Beyond u*zo/v = 100, there is no unique value of kB−1, but rather the observations suggest that the behaviour falls into either of two categories. For surfaces comprised of arrays of widely spaced solid roughness elements, kB−1 continues to increase (with some evidence for a plateau being reached by about u*zo/v = 1,000, when kB−1 ≃ 10), whilst for surfaces comprised of randomly distributed fibrous roughness elements, kB−1 gradually decreases with increasing roughness Reynolds number beyond about 100.

Energy budgets in pine forest

Abstract: Hourly energy budgets measured in Thetford Forest, Norfolk, are analysed for seven fine days in months from May to September Values found for the Bowen ratio β ranging from near 1 to 4 or more, are used to show that the bulk physiological resistance rST of the forest exhibits a consistent diurnal trend, from near 1·2 s cm−1 in the forenoon (once the trees are dry) to as much as 4 s cm−1 by late afternoon - consistent with independent biological measurements (Robins, personal communication) In contrast, the forest's bulk aerodynamic resistance r generally lies between 0·05 and 0·10 s cm−1 The ratio rST/r alone, of order 20:1, is shown to imply that transpiration from the forest must occur at rates much less dependent, primarily, on net radiation, Rn, than on ambient vapour pressure deficit (vpd), (provided that the latter is not too small, ie ≮ 1 mb per 100 W m−2 of Rn, say); and also that the evaporation of intercepted rainfall from the trees must occur at about 5 times the corresponding transpiration rate under the same meteorological conditions In addition, because lighter winds during fine weather tend to favour larger vpd's the observed decrease in transpiration rate with increasing wind speed is much larger than that expected from the accompanying decrease in r alone Other conclusions are (i) that the storage associated with a 1° C change in air temperature per hour, within the canopy, is 18 W m−2, which is not always negligible, as it is for short vegetation; and (ii) that values derived for the dimensionless excess resistance parameter, B−1, although remarkably small, 2 to 3 for u* ∼ 0·75 m s−1, are commensurate with other bulk aerodynamic resistances in the system: accordingly, the easily-derived surface resistance parameter, rs, (Monteith 1965; Thom 1972) provides an estimate of rST for the forest within 10 per cent for all β > 0

Mechanisms influencing the distribution of precipitation within baroclinic disturbances

Abstract: An analysis of the flow within baroclinic disturbances over and near the British Isles is made assuming that the wet bulb potential temperature (θw is a conserved property of the flow. Conservation of mass and, when the flow is dry, of mixing ratio, are also used as constraints in the analysis. A model of the flows significant for the production and distribution of precipitation is derived. It is shown that most of the precipitation reaching the surface initially forms within a single well-defined flow, labelled the ‘conveyor-belt’ which is typically a few hundred km wide and a few km deep and which flows parallel to and immediately ahead of the surface cold front. The speed of ascent above the warm frontal zone is of the order of 10 cm s−1. Ascent often begins within the warm sector so that there is no well-marked discontinuity between the precipitation within the warm sector and that in advance of the surface warm front. This flow has been identified as the most significant flow producing widespread precipitation in a variety of synoptic situations, including some which would be analysed conventionally as non-frontal. Potential instability is continually generated by differential advection in advance of the cold front as a result of over-running of the low level flow by mid-tropospheric air of lower θw. The instability is gradually released as the low-level flow ascends. The leading edge of the warm-frontal precipitation at the surface is eroded by the evaporation within a descending flow beneath the warm-frontal zone. When the melting level is low, or the air entering the system on its forward side is dry, the evaporation causes the width of the area of precipitation at the surface to be several hundred km less than that aloft. Some of the moisture which is evaporated is eventually re-precipitated farther north as the air ascends close to the surface warm front. The mesoscale variability of the airflow within the large-scale flow modifies the distribution of precipitation. Various structures which have been identified in many studies are summarized and a model of the mesoscale airflow within the conveyor belt is deduced.

Structure of low-level jet streams ahead of mid-latitude cold fronts

Abstract: Several case studies are presented showing the structure of ana-cold fronts over the British Isles. One of the cases is analysed in detail using data acquired on the Isles of Scilly to avoid any confusing effects due to topography. All of the fronts described are characterized by a narrow band of shallow but vigorous convection at the surface cold front; this convection is essentially two-dimensional and is termed line convection. In each case the line convection is bounded on its forward side by a low-level jet reaching 25 to 30 m s−1; behind the line convection the winds decrease abruptly. The low-level jet is embedded within a ‘convective boundary layer’, reaching its maximum velocity at 900 to 850 mb, and it consists of a tongue of anomalously warm, moist air, which has had a trajectory over an even warmer sea. It is shown that the line convection can be regarded as part of a mesoscale ‘right-hand corkscrew circulation’ within the low-level jet. The line convection constitutes one flank of the jet; it is characterized by very strong cyclonic shear and is fed by frictional convergence of air from beneath the jet core within the lowest 100 mb. Some of the air which ascends within the line convection subsequently flows forward and gently subsides within the upper part of the low-level jet as part of the corkscrew circulation; however, most of the air which ascends within the line convection is extruded from the boundary layer and ascends in a rearward direction as slantwise convection above the inclined cold frontal zone. Precipitation grown in the slantwise convection falls into the cold air behind the surface cold front and the heat sink resulting from the partial evaporation of this precipitation probably accentuates the sharpness of the boundary of the low-level jet at the surface cold front.

Radiative properties of terrestrial clouds at visible and infra-red thermal window wavelengths

Abstract: A detailed theoretical study of the radiative properties of water droplet and ice clouds at visible and infra-red wavelengths of 2·3, 3·5, 3·8 and 8·5 and 11 μm is presented. The radiative transfer computations have used a model atmosphere in which the microphysical properties of the clouds have been accurately incorporated. A range of physically realistic size distributions for the cloud particles have been used. The results have been presented in the form of tables of the emissivity, reflectivity and transmissivity, and their equivalent fluxes, at a particular wavelength, as a function of the optical thickness of the cloud layer. A simple scaling relationship enables these extensive tables to be used for any water content of the cloud layers. The results provide a detailed understanding of the radiative properties of terrestrial clouds at thermal infra-red window wavelengths. The theoretical results are used to interpret infra-red observations of stratocumulus and cirrus clouds.

A wind tunnel investigation of freezing of small water drops falling at terminal velocity in air

Abstract: 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.

Momentum and mass transfers in the surface boundary layer

Abstract: Results are presented from six micrometeorological studies conducted over a grass turf at Davis, California, in 1966 and 1967. Highly reliable surface drag and evaporation data from very sensitive lysimeters of 6·1 m diameter afforded excellent opportunity to evaluate several parameters important to aerodynamic-prediction equations. For the six studies the mean von Karman constant, k, ranged from 0·40 to 0·44, strongly supporting continued acceptance of k at around 0·42. The Monin-Obukhov (1954) universal ϕM function was found to vary as ∣Ri∣−1/3 under near-free convection, indicating significantly greater diabatic profile effects than suggested in the form of the KEYPS profile as given by Sellers (1965). Empirical relationships providing excellent fit to experimental data for the range −3·5 < Ri < 0·3, were ϕM = (1–16 Ri)−1/3 and ϕM = (1 + 16Ri)1/3 respectively for unstable and stable conditions. For ϕw corresponding expressions were ϕw = ·885 (1–22 Ri)−.40 and ϕw = ·885 (1+34 Ri).40. The ratio KW/KM showed a systematic drop from 1·13 at neutral to a value around 0·75 under strongly stable conditions, which conflicts with recent reports of no change of KH/KM within a wide range of stable conditions (Laykhtman and Panomareva 1969; Webb 1970; Oke 1970). The UCD results are in general agreement with the literature in that KW/KM systematically increased with increasing instability. In the range 0 > Ri > −2·0, KW/KM increased from 1·13 to 1·6. No expressions relating ϕM and ϕW or KW/KM to z/L are presented due to uncertainties in Davis z/L data. However, use of Davis Ri data in several published z/L (Ri) relationships, allowed a limited comparison with results of previous studies.

The laminated structure of the ozone in the atmosphere

Abstract: Ozonesonde ascents have shown that frequently the vertical distribution of the ozone in the stratosphere is very complicated with strong maxima and minima. Such a complicated distribution was not expected. In an attempt to find the cause of this highly laminated structure of the ozone, the variations in the intensity of the lamination with both season and latitude have been studied. It is found that a highly laminated structure is most frequent in spring and in the higher latitudes. It is also shown that well marked minima in the vertical distribution of the ozone are found at a height of about 15 km. The frequency of occurrence of these minima is also greatest in spring and in the higher latitudes. It is suggested that the ozone-weak air in these minima has entered the stratosphere at the sub-tropical tropopause break and has been carried polewards. A region of minimum ozone is also sometimes found at heights of around 23 km but less frequently than at 15 km. These high level minima are only found in late summer or autumn and in very high latitudes.

The structure of rainbands within a mid-latitude depression

Abstract: A case study is presented of a winter depression over the British Isles in which extensive banded structure was observed within precipitation ahead of the surface warm front. Measurements of the mesoscale airflow and precipitation structure of the rainbands were made using a variety of radar techniques together with multiple radiosonde and aircraft observations. The measurements were made over the sea to avoid the confusing effects of topography. The dominant rainbands were oriented parallel to the surface cold front and were typically 100 km wide. They moved with a velocity faster than the underlying warm front. For the most part the bands were characterized by clusters of weak small-scale convective cells due to the release of potential instability produced where tongues of relatively dry air of low θw in the middle troposphere overran low-level moist air undergoing slantwise ascent above the warm frontal zone. Although there was the usual large-scale, and thermally-direct, circulation associated with the active warm front, the air which ascended as small-scale convection within the rainbands entered a region of weak cold frontal baroclinicity, whereupon it participated in a thermally direct circulation of its own. This led to each rainband having a rearward-sloping anvil cloud canopy characterized by ascending air with colder drier air descending beneath. Precipitation falling from the canopy evaporated within the underlying drier air thereby probably intensifying the descending branch of the circulation. Very large ageostrophic winds were measured in association with these circulations. The important ingredient responsible for the convective nature of the rainbands appears to have been the incursion of tongues of relatively dry air of low θw in the middle troposphere above the moist warm-sector air in a region where the resulting instability could be realized by large-scale ascent. Although the potential instability was very weak in the present case, the origin of the rainbands appears to have been similar to that of pre-frontal squall lines. The intensity of the convection within rainbands depends on the stability but the very existence of any precipitation in the first place depends on other dynamical factors leading to widespread ascent.

Stability of the Rossby-Haurwitz wave

Abstract: A spectral method is used to integrate the primitive equations for the motion on a sphere of a shallow layer of fluid with a free surface. A simple Rossby-Haurwitz wave of zonal wavenumber 4 is found to change its form little over 24 days, whilst one of wavenumber 8 breaks down completely in 5 days. Inspired by the integrations, the barotropic stability of a wave to a perturbation composed of another wave and a zonal flow is considered. The theory suggests that waves of zonal wavenumber greater than 5 may be unstable because the feedback of the perturbation on itself is positive. Those of wavenumber 5 and less are stable, the feedback being negative. The theory and the general barotropic instability of Rossby waves is tested by integrations of the non-divergent barotropic model, in which the wave is perturbed by a zonal flow. Some agreement with theory is found. Waves of zonal wavenumber 8 rapidly break down to produce jets in the same direction as the perturbation zonal flow. The stability of these jets is also examined.

A synoptic climatology of satellite observed cloud vortices over the Southern Hemisphere

Abstract: A classification scheme for satellite observed cloud vortices is used to assess some general features of the behaviour of Southern Hemisphere depressions. From coincident conventional observations the mean variation of surface pressure and upper level geopotential anomaly near the centre of, and in west-east cross-section through an extratropical vortex is determined in relation to its age. Data are obtained on the geographical variation of occurrence of vortex type, duration, and frequency of change between development stages. The percentage of vortices developing without a pre-existing cloud band is found to be greater at higher latitudes but, in general, over 50 per cent of developments occur in this way. Persistent maxima of early vortex development are found in the western South Atlantic and in the central South Pacific, both regions extending into lower latitudes than elsewhere in the hemisphere. Two less prominent maxima are located in the central Indian Ocean, and south of Australia. Generalized vortex track and speed data are obtained, and several areas of high frequency of termination of depression track are observed at high latitudes near the Antarctic coast. In total, the observations tend to point to a circulation pattern having a high frequency of major long wave troughs east of the Andes, and in the central Pacific, with a further frequent trough more variable in space and time being located between the central Indian Ocean and the longitude of the Great Australian Bight.

Global meridional interactions in stratosphere and troposphere

Abstract: Significant correlations have been found between year to year mean latitudes of the surface high pressure belt, L, over eastern Australia and atmospheric parameters such as total ozone, winds and temperatures at various levels in the troposphere and stratosphere, and rainfall. In those years when, in late winter and spring, the high pressure belt lies nearer to the Equator it appears that the Hadley circulation and poleward transport processes in the lower stratosphere and upper troposphere in middle latitudes are stronger. This is probably also associated with the earlier breakdown of the winter stratospheric polar vortex. Evidence is presented to support the view that L is determined to a good approximation by the criterion for baroclinic instability which was derived by Phillips and Smagorinsky and applied to climatology by Flohn and Henning. Thus variations in L are related to variations in the driving force of the Hadley circulation, which in turn may well be related to variations in sea surface temperature. This encourages the view that an empirical and theoretical basis for long-range seasonal forecasts of temperate zone climate may be in prospect. The results should lead to a greater understanding of the nature of global climatic change.

Water vapour budget and precipitation over the Arabian Sea during the northern summer

Abstract: 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.

Flooding of Venice: Phenomenology and prediction of the Adriatic storm surge

Abstract: Since both observations and theory indicate weak dissipation, a frictionless one-dimensional model elucidates the dynamics of the surge and is used to hindcast and forecast. Wind over the shallow northern Adriatic dominates the forcing. The relative phases of the astronomical tide, the storm tide, and the preexisting seiche determines flooding. Good real time predictions are now in progress.

Air motion within Kelvin-Helmholtz billows determined from simultaneous Doppler radar and aircraft measurements

Abstract: A new linked mode of operation has been used to obtain virtually simultaneous measurements by a fully instrumented aircraft and a ground-based high power radar within a train of large amplitude Kelvin-Helmholtz billows in the optically clear atmosphere. Both sets of data were analysed to show the detailed distribution of air motion and turbulence within the billows. The resulting pattern was consistent with a train of vortices each of which was characterized by a vertical wind shear of 6 m s−1 over 100 m, with downward motion of about 1 m s−1 on the downshear side and upward motion of about 1 m s−1 on the upshear side. Between the vortices the vertical shear decreased to almost half the maximum value. The most intense radar echoes occurred within inclined layers connecting the top of one region of maximum vorticity to the bottom of the next. The aircraft, which flew through the upper parts of the billows, encountered turbulence of up to moderate intensity as it penetrated the crests of the billows. Between successive penetrations of the billow crests the aircraft flew close to but just above the region of intense radar echo and encountered smooth air with a slowly varying vertical component of motion.

Equatorial Kelvin waves at 100 millibars

Abstract: A marked oscillation with a periodicity slightly greater than a month occurred in the zonal wind at 100 mb over Gan (00°41′S, 73°09′E) through the months January to May 1966. Corresponding oscillations took place during this period over other equatorial stations round the globe, but on the whole the oscillations became weaker with increasing latitude. The meridional wind was not directly involved in the oscillations, which were manifestations of a wave which generally progressed eastward relative both to the earth and to the mean flow with, however, a somewhat irregular rate of progression. The wave had zonal global wave number 1. Although it also advanced southward over the eastern hemisphere, it was not far from being symmetrical (as opposed to antisymmetrical) about the Equator. It was largely confined to the region between 150 mb and 70 mb, but it generally propagated downwards, Over the eastern hemisphere the 100 mb temperature tended to lead the 100 mb zonal wind by about 1/4 cycle. At least over Gan the 100 mb height and the 100 mb zonal wind tended to be in phase. The effect as a whole was sufficiently like an equatorial Kelvin wave to be considered as such. especially over the eastern hemisphere. The wave was linked very indirectly to concurrent events at 100 mb at 40°N. Less marked equatorial Kelvin waves occurred at 100 mb in 1960, 1967 and 1969. The incidence of equatorial Kelvin waves at 100 mb did not appear to be closely related to the phase at 50 mb of the tropical quasi-biennial zonal wind oscillation.

Observation of Kelvin‐Helmholtz billows and their mesoscale environment by radar, instrumented aircraft, and a dense radiosonde network

Abstract: On 19 February 1970, a T-33 jet aircraft, equipped with fast-response meteorological instruments, obtained measurements of a wave structure and clear air turbulence. The clear air structure was simultaneously detected with a sensitive 10·7 cm wavelength radar at Wallops Island, Virginia. From a special network of radiosonde stations with separations of about 100 km, the detailed mesoscale synoptic conditions associated with the clear air structures were determined. The turbulence and wave structure occurred between heights of 2 and 3 km within an upper level frontal zone which had advanced well ahead of the surface cold front. The wave pattern consisted of two-dimensional Kelvin-Helmholtz billows. The billow velocity was close to the mean wind velocity of the sheared layer, and the Richardson number for the layer was about 0·15 which is consistent with the criterion for instability based on theoretical and wind tunnel results. The most intense turbulence occurred where the billows were rolling up or breaking.

Raindrop interactions and rainfall rates within clouds

Abstract: Experiments showed that when a pair of water drops of raindrop dimensions collide and separate in air at their relative terminal velocities satellite drops are generally produced. Although the numbers and sizes of satellites resulting from individual interactions are highly variable a typical event produces about 3 satellites each having a volume of 0.04 ViVj/(Vi + Vj), where Vi and Vj are the volumes of the parent drops. A full stochastic treatment is presented of the development of rainfall in a volume within a cloud where water is being released by condensation at a constant rate J. The production of satellites and an experimentally verified expression for the coalescence efficiency e are incorporated into these calculations. Computations are made of the variation with time of the cloud water content, C, the rainwater content, L, the radar reflectivity, Z, the rainfall rate, P, and the drop-size distribution. The calculations show that the crucial parameter in governing the intensity and duration of rainfall is J, and that the microphysical processes are of secondary importance; C, L and P are all insensitive to e and the production of satellite drops. The influence upon rainfall development of the break-up of large raindrops on attaining their maximum size within a cloud is found to be even less than that of the satellites. The drop size distribution after several minutes of growth is insensitive to the initial spectrum but is markedly dependent upon e and the production of satellites, which tend eventually to produce a bimodal spectrum. The radar reflectivity increases very rapidly with time (typically by about an order of magnitude every 2 1/2 minutes) and depends quite strongly on e, but is unaffected by the production of satellites.

Some comparisons between observed wind profiles at Risø and theoretical predictions for flow over inhomogeneous terrain

Abstract: Velocity profiles observed in near neutral conditions at the Riso 125 m tower are presented. The upwind terrain contains, depending on wind direction, either one or two distinct changes in surface roughness. Comparisons are made between the observed profiles and those predicted using models based on mixing-length theory, and, in the case of a single roughness change, the turbulent energy equation. Comparisons with the mixing-length model are moderately good but some features of the observed profiles are missing from the theoretical predictions. The particular turbulent energy equation model used appears to give slightly less accurate predictions than the mixing-length model.

The shedding of accreted water during hailstone growth

Abstract: Icing tunnel experiments have shown that loss of accreted water occurs in both the dry and wet growth regimes at ambient temperatures above about −12°C. The loss in the dry growth regime is probably due to splashing while that in the wet growth regime is undoubtedly due to shedding of water from the liquid layer which covers the surface of the accreting object. The latter can affect appreciably the growth rates of hail stones. These data, together with calculations of the collection efficiency, have shown that the structure of the hailstone analysed by Macklin, Merlivat and Stevenson (1970) is consistent with growth in an adiabatic updraught.

Infra‐red heating calculations with a water vapour pressure broadened continuum

Abstract: Previous heat budget studies relying on infra-red radiation calculations have not used a water vapour pressure dependent continuum absorption in the 830-1,250 cm−1 region of the infra-red spectrum. The purpose of this study was to investigate the differences in the infra-red divergence profiles caused by inclusion of the vapour pressure dependent continuum absorption. The results indicate only a very small change in infra-red divergence from previous work for a typical midlatitude temperature, water vapour profile. However, for tropical temperature and moisture profiles, the new continuum data result in approximately 30 per cent more total tropospheric infra-red cooling than obtained using earlier continuum absorption data. In addition, it is shown that within a realistic range of tropical atmosphere moisture content, the continuum infra-red cooling may act to either stabilize or destabilize the lower layers of the troposphere.

A study of the continuum absorption within the 8–13 μm atmospheric window

Abstract: The spectral continuum within the 8–13 μm atmospheric window was studied using a tethered balloon borne radiometer, while simultaneous measurements of temperature and humidity profiles were made. Aerosol absorption was estimated as negligible as the measurements were made in conditions of high visual range. The results from a range of situations were tested separately against a line wing continuum model and a water dimer continuum model, both with appropriate spectral line band models. These results were found to agree well with laboratory data for the dimer model.

Some experiments in dynamic initialization for a simple primitive equation model

Abstract: 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.

Measurements and interpretation of the polarization of radiation emerging from the atmosphere at an altitude of 28 km over South—Western New Mexico (USA)

Abstract: The degree of linear polarization of the upward radiation emerging from the atmosphere at an altitude of 28 km was measured over south-western New Mexico (USA) with a balloon-borne photoelectric polarimeter. Measurements were made in four spectral intervals (bandwidth ∼ 15 nm) centred on λλ362, 401, 501 and 599 nm. The objective was to estimate the turbidity factors of simple, homogeneous models of turbid atmospheres from a comparison of the experimental results with computed values of the polarization of the radiation emerging from the top of the model atmospheres. The computations were based on the solution of the radiative transfer problem in a homogeneous turbid atmosphere. The ‘doubling’ method was used to obtain the solution. It is found that the optimal values of the turbidity factors required to establish good agreement between theory and experiment vary directly as the phase function asymmetry factors associated with the two types of aerosols used in the models. The uncertainty in the interpretation of the polarization data due to the similar effects that the atmospheric aerosols and the unpolarized part of the radiation reflected by the ground have on the polarization of the scattered radiation is briefly discussed.

A numerical experiment using a general circulation model of the atmosphere

Abstract: The general circulation model whose formulation is described in Corby et al. (1972) was integrated for 60 days from real data with January radiation and sea surface temperatures. The deficiencies of the model as a representation of the atmosphere were revealed by the extent and rate of the systematic changes as an equilibrium state was approached. For the first ten days, the model lost eddy kinetic energy, then gradually recovered and from about day 30 its tropospheric simulations both synoptically and in its time-mean cross-sections at various longitudes, had many realistic features; it failed, however, to maintain a substantial Siberian winter anticyclone. A major weakness, resulting from the use of a single level to represent the 0–200 mb layer, was the progressive deterioration of the model stratosphere.

Coalescence in a weakly turbulent cloud

Abstract: In a recent paper on the effect of turbulence on droplet collisions in clouds, Woods, Drake and Goldsmith (1972) proposed that the principal factor was the strong spectral peak of shear near the Kolmogoroff micro-scale. This hypothesis led to an important simplification, namely that spatial and temporal fluctuations in the turbulent shear field could be ignored during an encounter between a pair of cloud droplets. However, the authors pointed out that their simple model of (isotropic, homogeneous) turbulence contained an assumption, namely that the shears are normally distributed, which would have to be re-examined if it turned out that droplet collision efficiency increased rapidly after a threshold value is exceeded. Subsequent measurements by Jonas and Goldsmith (1972) showed that this is indeed the case. In this paper we reconsider the efficiency of turbulent coalescence in the light of the new experimental data, and taking account of the intermittency of shear distribution. It is concluded that significant increases of collection efficiency will occur in clouds which are only weakly turbulent. For example, the collection efficiency for droplets of radii R = 20 μm and γ = 9 μm in a cloud with energy dissipation rate e = 55·5 cm2 s−3 will be approximately 14 per cent compared with 2 per cent for the same droplets falling though still air. The shear zone in the wind tunnel experiment designed by Woods et al. (1972) is a realistic approximation to the shear zones in cloud turbulence.

On the summer hemisphere Hadley cell

Abstract: In the extreme seasons the winter hemisphere Hadley cell dominates the entire zonally averaged mean meridional circulation by extending well into the summer hemisphere, and the summer hemisphere Hadley cell practically disappears. The hypothesis that this apparent disappearance is due to the process of zonal averaging, and that a summer hemisphere Hadley cell does exist when the averaging is restricted to the longitudes not occupied by the Asiatic monsoon, is tested. Using data for 1957–1964 covering the months June-August, it is found that the summer hemisphere Hadley cell was indeed present over the longitudes 160°E-0–30°E, or two thirds of the Tropics. It is argued that some of the most interesting features of the meridional circulation and their associated transport processes are obscured by the zonal averaging process.

Time spectra and cross‐spectra of kinetic energy in the planetary boundary layer

Abstract: Temporal spectra and cross-spectra of the three turbulent wind components for meso-synoptic scales at 250 m vertical intervals in the planetary boundary layer are presented. The data were derived from hourly simultaneous balloon ascents from a 5-point grid at Hay, New South Wales, Australia (lat. 34 ½S), during the Wangara expedition. The spectra indicate a meso-scale gap for u (eastward component of wind) and v (northward component). The computed w-spectra suggest that no marked gap is present. For periods less than 1 day the u- and v-spectra, with a correction applied for high frequency noise, fall off approximately as n−2·6, where n is the frequency, and this result appears to be invariant with height within and just above the planetary boundary layer. The corrected w-spectra exhibit n−1 dependency in this frequency range. The uv, uw, and vw cospectra also show only small contributions on the meso-scale (periods less than a day) compared to the synoptic scale, indicating that meso-scale motions may not have to be explicitly modelled in atmospheric general circulation models. An inertial peak is found for u and v but the w-spectra show a diurnal peak. The phase angles showed that u and v were in approximate quadrature at the inertial frequency, demonstrating the existence of the inertial cycle. Tests for horizontally two-dimensional isotropy indicated that the turbulence did not obey this simplification.