Showing papers in "Quarterly Journal of the Royal Meteorological Society in 1970"

The influence of nitrogen oxides on the atmospheric ozone content

Abstract: The probable importance of NO and NO2 in controlling the ozone concentrations and production rates in the stratosphere is pointed out. Observations on and determinations of nitric acid concentrations in the stratosphere by Murcray et al. (1968) and Rhine et al. (1969) support the high NO and NO2 concentrations indicated by Bates/Hays (1967). Some processes which may lead to production of nitric acid are discussed. The importance of O (1S), possibly produced in the ozone photolysis below 2340 A, on the ozone photochemistry is mentioned.

Flux‐gradient relationships in the constant flux layer

Abstract: An analysis is made of the Monin-Obukhov function ΦM in the familiar wind profile equation, using data from two recent expeditions to Gurley (New South Wales) and Hay (New South Wales). In one, the friction velocity u* is determined directly by the eddy correlation method, and in the other, conducted during mid-winter when small heat-fluxes were experienced, by the use of a friction coefficient applied to a low-level wind. By collating with a similar earlier analysis for heat and water vapour transfer, the variations of ΦM, ΦH and ΦW with stability are presented in tabular form in the z/L range − 0.01 to − 1.0. Within this range the empirical relationships ΦM = (1 − 16 z/L)−1/4 and ΦH, W = (1 − 16 z/L)−1/2, and the implied equality between Ri and z/L, are found to approximate the data to within a few per cent.

Profile relationships: The log-linear range, and extension to strong stability

Abstract: The diabatic mean profile forms in the surface layer are studied, by applying analysis methods having high resolving power to data from O'Neill, U.S.A. (heights up to 6.4 m) and from Kerang and Hay, Australia (heights mostly up to 16 m). It is found, concordantly from the O'Neill and Australian data, that the log-linear law is valid for z/L values between — 0.03 and + 1, which includes a small range of unstable and a surprisingly wide range of stable conditions. For all quantities studied (wind, potential temperature, and specific humidity), it is concluded that the Monin-Obukhov coefficient α is near 4.5 in unstable and 5.2 in stable conditions, within a standard error of about 10 per cent. The ratios KH/KM and KW/KM evidently remain constant, equal to unity, over the whole of the log-linear range (and somewhat beyond). In stable conditions, the log-linear law implies that Ri approaches a critical value α−1, approximately 0.2, as z/L → ∞. However, at z/L a second regime sets in, in which the profiles are only quasi-determinate, approximating, on the average, a simple logarithmic form (gradients proportional to z−1); this regime covers the range approximately 1 1, is practically unrepresented in the data examined. The major part of the unstable range, for z/L < − 0.03, will be discussed in a later paper.

A wind tunnel investigation of the internal circulation and shape of water drops falling at terminal velocity in air

Abstract: The internal circulation and the shape of water drops falling at terminal velocity in air of 20°C at sea level pressure, and nearly water saturated, were studied by means of a wind tunnel. Drops with an equivalent radius a0 smaller than 140 um had within the experimental error no detectable deformation from spherical shape. Drops of sizes 140 μm ≤ a0 ≤ 500 μm were slightly deformed into an oblate spheroid. The deviation of these drops from spherical shape was found to be in fair agreement with that theoretically predicted by Imai (1950) and others. The deformation of drops of sizes 0.5 mm < a0 < 4.5 mm was found to be linearly related to the drop size. Such a linear relationship is predicted by the semi-empirical calculations of Savic (1953). By means of a tracer technique it was established that water drops falling at terminal velocity in air have a well developed internal circulation. The flow pattern inside a drop was found to be consistent with the flow pattern of the air around the drop and that predicted theoretically by Hadamard (1911) and by Hamielec and Johnson (1962). The surface velocity at the equator of a drop was found to be about 1/100 of the drop's terminal velocity. The experimentally determined internal velocities were compared with those predicted theoretically by McDonald (1954) from boundary layer theory and by Hadamard (1911) based on Stokes flow.

Transfer properties of the large-scale eddies and the general circulation of the atmosphere

Abstract: The transfer of heat and momentum by motion on the scale of cyclone waves and long waves is deduced from a knowledge of their mechanics. The law of horizontal transfer of entropy is independent of notions of mixing but is similar to that for non-isotropic diffusion. The law accurately represents the annual variation of heat-flux in the Northern Hemisphere. Perturbation theory suggests realistic spatial variations for the transfer and provides a rational basis for calculating transfer coefficients which can then be used to find the flux of other conservative quantities. Most importantly the Equatorward flux of potential vorticity is well defined and is shown to determine the horizontal flux of momentum uv. The mean surface winds, and the mean meridional circulation (which is seen to be frictionally driven) are then determined. A model of the general circulation is defined by these properties, and this is integrated analytically for a simplified case and compared with motion observed in the Gulf Stream, the mesosphere, and dishpan experiments as well as the troposphere.

Observational studies in the atmospheric boundary layer

Abstract: Winds and temperatures in the boundary layer measured during two Australian expeditions are analysed according to the similarity scheme, with the use of four stability classifications. Under conditions of deep convection there is a minimum potential temperature and a maximum velocity component in the direction of the surface wind, at a height of about 0.12 u*/f. In the very stable cases, the temperature gradient follows rather closely a z−2 law for a considerable height range from 0.08 u*/f upwards. An expression closely fitting the mean data in stable conditions is suggested for the vertical temperature structure at all levels in the boundary layer. Wind data processed in this way show, for all four stability classes, a rudimentary Ekman spiral. With deep convection the spiral is found to be reversed in sense, but the flow in the convecting layer is down the gradient of pressure. If the convective limit lies broadly within the Ekman layer, a spiral of the expected sense is found. The upper limit of the Ekman layer (as defined by the ‘spiral’) is found to lie at a height of 0.17 to 0.24 (increasing with stability) in units of u*/f. Stress and heat-flux are apparently considerable above this level, with sub-geostrophic wind, when deep convection is occurring. Approximations to the universal distributions of stress, eddy coefficients, mixing length and rate of degradation of mean flow kinetic energy are computed for the various stabilities. The mixing length in unstable conditions increases almost as height up to a level of about 0.08 u*/f, and then decreases, but in general appears not to vanish in stable layers above the boundary layer. In the unstable boundary layer with deep convection, the eddy transfer coefficient for heat exceeds that for momentum up to 0.12 u*/f, where it becomes infinite, and is negative at higher levels. In stable conditions the transfer coefficients for a small sample of soundings were estimated to be closely similar. The universal functions of stability, A, B and C, which enable one to compute free atmosphere wind vector and temperature, given surface conditions, have been evaluated with moderate success, although B, which essentially describes the change of wind direction with height, exhibits excessive variability. A method is suggested for computing horizontal advection in the boundary layer when this is to be ‘parameterized’ in mathematical models. The drag coefficient, in terms of free atmosphere wind, has almost a 50-fold range, due to stability variation only. Most of the variation occurs relatively close to neutral, so warning against too ready an assumption of neutrality in practical applications. It is suggested that, for modelling purposes, it is preferable to adopt boundary layer formulations which are not too sensitive to departures from ideal conditions, and eddy coefficients, perhaps based on mixing lengths, may well provide the best approach currently available.

The water‐vapour infra‐red continuum

Abstract: All published work on the water-vapour continuum in the region 8 μ to 50 μ is reviewed in the light of some new experimental results in the 11 to 21 -μ region. This new work employed a 15.5 m multiple-reflection cell, giving total path lengths up to 500 m, and a high-resolution (1–2 cm−1) grating spectrograph. Control of temperature (20–40°C), and water-vapour partial pressure (3–35 mb) allowed simulation of tropospheric conditions and their extension to rather higher humidities, but the total pressure could not be varied from atmospheric. The results suggest that two mechanisms of absorption are involved. One of them is almost certainly the foreign-broadening of water-vapour lines and it is responsible for most of the continuum absorption noted by previous workers. The origin of the other is not clear; it gives rise to absorption which increases with partial water-vapour pressure, e, equalling the foreign-broadened component at e ∼ 15 mb. This behaviour was first reported by Bignell, Saiedy and Sheppard (1963) who then attributed it to self-broadening of H2O lines. However, the present more sensitive work casts doubt on this interpretation because the absorption is found to have a negative temperature dependence of 2 per cent per °C over the range 21° to 45°C, which is greater than that of any water-vapour line. Absorption of the same type, though weaker, is present near 4 μ. Reinterpretation of previous open-air measurements in atmospheric conditions ranging from warm and damp to cold and dry shows that the systematic differences they exhibit might be a manifestation of the new absorption. No firm explanation can be given, but the possibility of continuum absorption by the recently reported water dimer molecule (H2O)2 should be investigated.

New lag associations between North Atlantic sea temperature and European pressure applied to long‐range weather forecasting

Abstract: A feed-back relationship, at least in the statistical mean, between Atlantic sea surface temperatures on the one hand and monthly atmospheric circulation anomalies on the other hand, is demonstrated for the north-east Atlantic and western Europe. The key area in the Atlantic is a wide area south of Newfoundland; colder than usual ocean surfaces in this area are shown to be associated with blocked atmospheric patterns the following month over northern and western Europe while a warmer than usual ocean in the same general area favours more progressive synoptic types to follow. Month to month changes in this general relationship are described in some detail and the application to long-range forecasting in the British Isles is discussed. One other pattern of Atlantic sea surface temperature anomaly is also described and associated with atmospheric circulation patterns to follow. Possible inter-relationship between the Atlantic and Pacific Oceans in respect of the effects of ocean surface temperatures on circulation patterns is also briefly considered.

Air motion and precipitation growth at a cold front

Abstract: A case study of a cold front has shown that, although appreciable ascent occurred over a deep layer, practically all precipitation growth was associated with the ascent of air which originated within the friction layer ahead of the front. Doppler radar observations showed that the ascent was accomplished in two phases; first through near-vertical line convection to between 1 and 3 km at the surface cold front, and thence through shallow-slope convection of the same air to between 3 and 6 km. The line convection was 2-dimensional rather than cellular and occurred in the absence of appreciable hydrostatic instability. Horizontal convergence at low levels was very intense (10−2 s−1 averaged over 500 m vertically and horizontally), so that despite the shallowness of the line convection, the updraught attained a rising speed of 8 m s−1 which was sufficient to generate hail and thunder. The subsequent slope convection produced a period of moderate precipitation behind the surface cold front, and was associated with a pronounced transverse circulation, with strong gradients of velocity separating the weak downdraughts in (and beneath) the sloping frontal zone from the overlying updraught. The overall efficiency of precipitation production was high, 60 per cent of the water vapour flux in the rising air reaching the ground as precipitation.

Spectra and cospectra of turbulence over water

Abstract: Composite diagrams of spectra and cospectra were constructed from eight cases of velocity component and temperature measurements made over the water by an acoustic anemometer. Their universal form is discussed.

Reflection coefficient of natural vegetation, crops and urban surfaces in Nigeria

Abstract: Reflection coefficients measured over a wide variety of natural and agricultural vegetation surfaces in Nigeria (lat. 4° 40′ to 13° 30′N) were between 0.057 to 0.251; and were inversely related to the vegetation height. Considerable seasonal differences were found with unexpected low values of reflectivity during the harmattan haze in the dry season. Possible reasons for these seasonal differences are discussed. The weighted mean annual reflection coefficient for Nigeria was calculated to be 0.178. This value was consistent with the relationship between surface reflection coefficient and latitude found from similar results from six other locations in the Northern Hemisphere. Urban coefficients were found to be lower than those for rural settlements or temperate zone urban centres.

Ice particles in maritime clouds near Tasmania

Abstract: An instrumented aircraft was used to sample particles in the upper regions of small supercooled cumulus clouds in maritime air off the Tasmanian coast in May 1968. 114 clouds were studied on nine days, and it was found on average that clouds had a 50 per cent chance of containing ice particles when a summit temperature of about − 8°C was reached. Not only cloud temperature but also cloud width had a profound effect on whether ice particles were found. The cloud summit temperature at which ice particles first reached detectable concentrations in narrow clouds on a particular day was related to the ice nucleus concentration as measured in a cloud chamber, being, in general, highest on days of high ice nucleus concentration. However, in some glaciated clouds ice crystal concentrations of the order of 100 1−1 were found. These were greater by a factor of ∼ 104 than the estimated number of active ice nuclei. The process of multiplication of ice crystals may be associated with the riming of ice particles. Riming was assisted by the presence of large droplets, up to drizzle size, in these clouds. Multiplication of the number of ice crystals by a factor of 10 took place in about 8 minutes. The various stages in the glaciation process are described.

Surface exchanges of sensible and latent heat in a 10-level model atmosphere

Abstract: This paper describes a representation of the distribution of sensible and latent heat from the surface through the atmospheric boundary layer which has been formulated for use in a 10-level primitive equation model atmosphere. The transfer process is represented in two parts : (i) the transfer of energy across the Earth's surface into the lowermost 100 mb layer of the model atmosphere; and (ii) the subsequent redistribution of this energy through two or more such layers by small-scale convection. The fluxes of energy across the surface are calculated using empirical ‘bulk aerodynamic’ relationships. In land regions consideration of the energy balance at the surface is also necessary, and diurnal variations of radiation are taken into account. The redistribution of energy by small-scale convection is represented by convective adjustments which ensure that a certain neutral lapse rate of temperature is never exceeded. Some results of the incorporation of these effects into the 10-level model are described.

Solar radiation and solar activity

Abstract: Using high-altitude balloon soundings (1961 to 1968) and other data, it is shown that the most reliable maximum value of the solar constant is at present 1.94 cal cm−2 min−1, to an accuracy of 1 per cent. This value is observed with Wolf numbers between 80 and 100, below or above which the solar constant decreases. The maximum decrease does not exceed 2 to 2.5 per cent. The observations reveal the influence of nuclear explosions and volcanic eruptions which caused an anomalous turbidity of the upper atmospheric layers. These anomalies have been studied. The above conclusions require further confirmation using direct long-term measurements of the solar constant outside the atmosphere.

On the influence of synoptic development on the production of high level turbulence

Abstract: This paper considers free shear layer turbulence in the atmosphere (clear air turbulence) as a mechanism for relieving the formation of discontinuities of wind and temperature in the vertical plane by shearing and stretching deformation. An expression for the rate of change of the logarithm of the gradient Richardson number following the air motion is derived and the assumption is made that turbulence counteracts the dynamical processes which are reducing the Richardson number, maintaining it at a limiting value. The rate of working required to do this is equated to the turbulent energy dissipation rate and in the model used is given by Φ (ΔV)2/24 (where the logarithmic rate of reduction of Ri which would take place in the absence of turbulence, and ΔV is the velocity difference across the turbulent layer). Two preliminary tests of the theory as a forecasting tool using the Bushby-Timpson 10-level numerical model show that the dynamical processes changing Ri are largest in areas where clear air turbulence might be expected from synoptic experience.

Remote sounding of the atmospheric temperature profile in the presence of cloud

Abstract: The present state of the art of retrieving the atmospheric temperature profile from remote measurements is briefly described, and methods based on maximum probability and minimum variance criteria are discussed. A maximum probability method is derived for the case of a cloudy atmosphere and synthetic examples of its use are given. The method is numerically fast and is capable of being generalized to incorporate other kinds of information. Such extra information might be for example surface temperature, cloud estimates from television pictures or forecast profiles.

Comparison of turbulent fluxes over water determined by profile and eddy correlation techniques

Abstract: The turbulent fluxes of momentum and heat were determined by the eddy correlation and profile techniques for over water, fetch limited, near neutral conditions. The estimates agree well within the experimental error. The o11 technique estimates of stress are systematically higher.

The temperature profile near the ground on calm clear nights

Abstract: An observational study of the temperature profile over grass, snow and bare soil surfaces in strong stability is presented It was found that in nearly calm, clear conditions the minimum temperature occurred 1 to 50 cm above the surface The height and intensity of the minimum is strongly correlated with wind speed, surface roughness and stability A discussion of existing hypotheses suggests that the most likely explanation for the phenomenon should involve both turbulence and infra-red flux divergence

Observational characteristics of atmospheric fluctuations with a time scale of a month

Abstract: 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 derivation of Swinbank's long-wave radiation formula

Abstract: It is shown that Swinbank's empirical formula for the long-wave radiation from clear skies can be derived from existing knowledge of atmospheric emission by taking account of the moderately strong correlation at most places between screen temperature and the amount of water vapour overhead.

An observational study of the energy balance of the stationary disturbances in the atmosphere

Abstract: Different terms in the equations describing the balance of kinetic energy and available potential energy in the so-called stationary disturbances or standing waves (deviation of the atmosphere's time-mean flow from axial symmetry) are evaluated approximately by using the observational statistics of the atmospheric flow. Because many simplifications and assumptions have been necessary in order to get estimates of certain terms, the results only give a rough first-estimate picture about the maintenance of the stationary disturbances. The results indicate that in winter the stationary disturbances are typically baroclinic waves, which get available potential energy from the temperature distribution of the zonally-averaged mean flow and partly convert it into kinetic energy to offset the destructive effect on the latter by small-scale turbulent friction, large-scale transient motions and conversion into zonally-averaged mean motion. The most important ‘ external ’ forcing seems, from the energy point of view, to be the stationary heat sources and sinks, which in winter destroy the available potential energy of the stationary disturbances; the effect of mountains in the energy balance of the standing waves seems to be relatively minute and actually smaller than the kinematic and thermal effects of large-scale transient flow systems. In summer the stationary disturbances appear to form a thermally-driven system, where the frictional loss of kinetic energy is compensated for by a conversion from the available potential energy, which in turn is maintained by generation due to diabatic heating.

Non‐linear baroclinic instability of a continuous zonal flow

Abstract: In this theoretical paper, Eady's linear model of baroclinic instability of a zonal basic flow of an inviscid non-conducting Boussinesq fluid in quasi-geostrophic motion is extended by consideration of non-linearity. The method of normal mode cascade is used to find the non-linear development of a slightly unstable linear disturbance. Amplitude vacillations are found, similar to some observed in experiments on a differentially-heated rotating annulus. Interactions of the zonal flow and non-linear waves are described analytically and related to instability of the westerlies in mid-latitudes.

Dynamics of disturbances on the Intertropical Convergence Zone

Abstract: The dynamics of disturbances on a theoretically derived Intertropical Convergence Zone (ITCZ) are studied. Radiative cooling over a hemisphere is parameterized by a Newtonian cooling law, relative to a radiative equilibrium temperature which decreases from Equator to Pole. Release of latent heat of condensation in the Tropics is treated as a function of convergence in the planetary boundary layer. The zonally symmetric field of motion which evolves in response to these sources of energy shows a concentrated region of rising motion near, but not at, the Equator. The associated low-level wind field possesses a strong cyclonic shear. Asymmetric perturbations periodic in longitude are introduced by means of truncated Fourier series. The wavelengths are chosen to correspond to maximum instability in the ITCZ, and as such are too small to permit baroclinic instability in middle latitudes. In this way, the stability of the low latitude flow is examined while excluding middle latitude perturbations. The low-level wind field in the vicinity of the ITCZ is found to be barotropically unstable, the wavelength of maximum growth rate being about 2,000 km. The corresponding e-folding time is found to be of the order of two days, depending on the frictional and heating coefficients. The perturbations are allowed to grow to finite amplitude. In the initial stages of growth, the Reynolds stresses supply most of the perturbation energy. At the mature stage, the energy is provided mainly by direct conversion of condensationally produced eddy available potential energy. Further growth is then limited by frictional dissipation of kinetic energy. The mean flow is in turn influenced by the disturbance through the mechanisms of Reynolds stresses, eddy conduction and the modification of the mean flow condensational heating through boundary-layer pumping. The influence is seen on the mean temperature and zonal wind fields, and may extent to latitudes poleward of where the perturbation amplitude in situ has decreased to zero. A framework is thus provided for viewing tropical disturbances as an integral component of the general circulation of the atmosphere.

Mid‐tropospheric frontogenesis

Abstract: Mid-tropospheric frontogenesis is investigated for a case study in which special three-hourly rawinsonde data are available. Isentropic analyses are made by objective means. Isentropic trajectories lead to the three-dimensional description of the velocity field in space and time. Potential vorticity is used to check trajectories and to distinguish stratospheric from tropospheric air. Miller's frontogenesis equation is applied in two and three dimensions to an intensifying mid-tropospheric baroclinic zone. Frontogenetical and frontolytical regions arising from twisting, horizontal confluence, vertical shear of vertical velocity and ageostrophic contributions are delineated. Typical frontogenesis magnitudes are 2°C (100 km)−1 (3 hr)−1 and 2°C (km)−1 (3 hr)−1 in two and three dimensions respectively. Previous observations of the extrusion of stratospheric air into the troposphere are confirmed. A synoptic sequence common to many occurrences of mid-tropospheric frontogenesis is described. A simple explanation is given of the initial development of a thermally indirect circulation accompanying the early frontogenesis. Mid-tropospheric frontogenesis seems to accompany the extrusion of stratospheric air into the troposphere during which momentum, potential vorticity, ozone and radioactivity are transported downward and southward.

The temperature fluctuations in stable stratification

Abstract: The intensity and spectrum of temperature fluctuations recorded at a height of 2 m over flat grassland in stable conditions are examined. Thr results show a demarcation between two distinct regimes, ‘ turbulent ’ for Ri less than a critical value of about 0.2 and ‘ quiet ’ for larger Ri. The magnitude of the temperature fluctuations, scaled relative to z δz, is comparatively large in the turbulent regime, decreases as Ri increases towards the critical value, and remains small in the quiet regime. The temperature, while always of a typically turbulent character in the turbulent regime, is of variable behaviour in the quiet regime - intermittently quiescent with occasional isolated smooth pulses (usually negative), or wave-like with assorted frequencies, or turbulent. In the turbulent regime, the spectrum approximates to the − 5/3 power form for normalized frequencies nz/U greater than 0.6. In the quiet regime, the form of the spectrum varies unpredictably from one occasion to another.

Experimental measurements of the collection efficiencies of ice crystals in electric fields

Abstract: Experimental studies have been made of the aggregation of ice crystals of mean diameter about 5 μm upon an ice sphere of diameter 0.2 cm at the same temperature. The growth rate was found to be roughly independent of temperature over the range − 7°C to − 27°C and increased with electric field strength E to exceed the field-free value by about 15 per cent, 40 per cent and 80 per cent in fields of 500, 1,000 and 1,500 V cm−1 respectively. At higher values of E the growth-rate was reduced because of the ejection of clusters of ice crystals from the aggregate, probably under the influence of the intense electric forces. An appreciable amount of aggregation was observed at temperatures as low as − 37°C, in the presence or in the absence of electric fields. Absolute values of collection efficiencies at a velocity of 3 m s−1 were determined at the temperatures − 7°C, − 11°C, − 17°C and − 27°C. In the absence of an electric field collection efficiencies of about 0.3 were determined at all temperatures studied. Experiments with charged ice crystals indicated that electric fields increase the growth rate by increasing the adhesion efficiency and not the collision efficiency.

Richardson number limited shear zones in the free atmosphere

Abstract: Two case studies are presented of the detailed structure of frontal zones with strong vertical shear. Temperature structure has been obtained from sequential radiosonde ascents. Wind structure, including the fields of horizontal divergence and deformation, has been obtained on scales of 1 to 10 km by means of Doppler radar techniques. The results are consistent with the frontal zones being at least partly controlled by shearing instability, in so far as there was a balance between a wind field tending to diminish the Richardson number (Ri) and shearing instability tending to impose a lower limit to Ri. One of the main factors tending to diminish Ri was stretching deformation in a direction normal to the local shear vector. A typical rate of decrease of Ri which this would have produced in the absence of shearing instability was 10−4 s−1. The Richardson number itself within the frontal zones was mainly between 0.2 and 0.4 when averaged vertically over 200 m and horizontally over circles of diameter 10 km; however, Ri diminished to about half of these values locally on horizontal scales of about 1 km. As a result shearing instability is thought to have led to turbulent breakdown in only isolated patches rather than in a sustained fashion over the entire depth of the frontal zones.