bluuebunny/crossref_metadata_2025_split huggingface dataset:
Planetary wave modelling of the middle atmosphere: The importance of transients
Authors: Austin John 
Published: February 2007

AbstractA linear planetary wave model of the middle atmosphere is used to diagnose the wavenumber‐1 structure for a day during December 1979 given the wave forcing and the time‐derivative terms. The time derivatives are calculated using data from the stratospheric sounding unit (SSU) on NOAA 6 for days on either side of the principal day of interest. The effect of the time‐derivative terms is significant and the model is found to be in fair agreement with the data in high latitudes although low latitude features are poorly reproduced. The influence of the transient wave components is discussed and in particular some dissipation‐like effects are demonstrated which might clarify the role of Rayleigh friction in planetary wave models.
Statistical mechanics of a finite difference approximation to the barotropic vorticity equation
Authors: Bennett A. F. , Middleton J. F. 
Published: February 2007

AbstractA conventional finite‐difference representation of the barotropic vorticity equation is shown to possess a statistical mechanical equilibrium state, which is attained as a quasi‐equilibrium in the presence of weak dissipation. The state is isotropic only in suitably chosen wavenumber coordinates. The autocorrelation functions of the discrete complex Fourier amplitudes have integral time scales which are O(wave number)−1 at middle wavenumber, but which are infinite at the highest wavenumbers. This is demonstrated experimentally and confirmed by inspection of the interaction coefficients for the finite‐difference Jacobian. The results emphasize the dependence of predictability upon the choice of numerical method.
Aircraft measurements of mountain waves and their associated momentum flux over the British Isles
Authors: Brown Philip R. A. 
Published: February 2007

AbstractThe Hercules and Canberra aircraft of the Meteorological Research Flight have made a number of flights to investigate mountain waves over different mountainous areas of the British Isles. Results from five flight days are presented in this study. For each flight day the observed values of the dominant wavelength and the wave amplitude as a function of height were compared with the values predicted by two‐dimensional linear lee wave theory. Good agreement was obtained in four of the five cases. Measurements of the vertical flux of horizontal momentum show that the largest values were obtained when the wavelength was 20·9 km and the waves were only weakly trapped. On this occasion, the flux was observed to be constant with height in the range over which measurements were made, supporting a theoretical result of Eliassen and Palm. The value –0·35Nm−2 is comparable with some aircraft measurements made over the Rocky Mountains.
Ice nucleus measurement with a continuous flow chamber
Authors: Hussain K. , Saunders C. P. R. 
Published: February 2007

AbstractA continuous flow vapour diffusion chamber has been designed and constructed so that ice nuclei can be activated over ranges of temperature and supersaturation similar to those found in natural clouds. A conventional filter‐processing chamber was also built for carrying out comparative measurements. Simultaneous use of the two chambers under similar conditions of temperature and supersaturation has shown that the filter method leads to an underestimate of the concentration of ice nuclei in an air sample. The ratio of the two simultaneous measurements is strongly dependent on the concentration of Aitken nuclei in the air sample and is also dependent on the chamber supersaturation. Some tentative evidence of the presence of condensation‐freezing nuclei active above water saturation is presented.
A study of mesoscale convective bands behind cold fronts. Part I: Mesoscale organization
Authors: Bennetts D. A. , Ryder P. 
Published: February 2007

AbstractThis paper reports observations made over the North Atlantic of a system of convective cloud bands found within the cold air behind an occluded front. The observations were made from an instrumented aircraft which additionally released a pattern of dropsondes, measuring temperature, relative humidity and wind, as functions of pressure. These data are analysed to establish some of the mesoscale, cloud‐scale and microphysical properties of the system.Here, in part I, the mesoscale structure is described and compared with the predictions of various theoretical models. The convective cloud structure emerges as a consequence of the mesoscale organization but appears to play only a minor role in its creation and maintenance. One of the theoretical models considered, conditional symmetric instability, provides a piausible explanation of the mesoscale observations.
Radiative heating in the lower stratosphere and the distribution of ozone in a two‐dimensional model
Authors: Haigh J. D. 
Published: February 2007

AbstractThe sensitivity of a two‐dimensional model to the diabatic heating rates used in the lower stratosphere is discussed. Different radiation schemes are described; in particular a longwave scheme which allows for interaction between surface temperature, ozone amounts and the heating rates in the ozone 9·6 μm band. Near‐infrared heating is also included.A model run including the new radiation schemes is compared with a control run in which radiative equilibrium is assumed in the 150–30 mb region (as in previous versions of this model) and with a run in which fixed heating rates are adopted in the same region. The temperature and ozone distributions of each run are discussed. The ozone structure is very sensitive to the scheme used and the inclusion of the new schemes improves the model's ozone distribution. The importance of the diabatic heating in determining the mean meridional circulation and the transport of trace constituents is stressed.Perturbation experiments of Haigh and Pyle are repeated in the model including the new schemes. The effect on ozone of increasing CO2 is found to have a stronger latitudinal and seasonal variation. The effect of including chlorofluorocarbons is a larger ozone depletion at all latitudes. The problems associated with assessing the uncertainties in model predictions are discussed.A simple scheme for representing cloud cover in the longwave scheme is described. The effect of inclusion of clouds on heating in the ozone 9·6 μm band, and thus on the ozone distribution, is discussed. The results of changing the effective albedo in the solar heating scheme are considered briefly.
Baroclinic instability in a long wave environment. Part I: Review
Authors: Grotjahn Richard 
Published: February 2007

AbstractA brief survey of past theoretical studies of the instability of curving flows is presented. Most of the studies consider linear instability. Finite amplitude, nonlinear studies are not considered here. Most of the early papers were concerned with finding instabilities; later papers include more discussion of the structural and other properties of the unstable modes. It is found that nearly all these studies lack a proper discussion of the energetics of the unstable modes. Proper energy conversions are derived and examined in part II.
Parametrization of the shortwave flux over high albedo surfaces as a function of cloud thickness and surface albedo
Authors: Shine K. P. 
Published: February 2007

AbstractA simple method for the calculation of the net shortwave flux at the surface for overcast situations is presented. Explicit account is taken of the effect of cloud optical thickness and multiple reflections between the surface and cloud base. Using simple two‐stream theory, a theoretical form for the flux is proposed, and then results from a detailed radiative transfer scheme are used to determine the values of a number of empirical coefficients. Over a range of cosines of the solar zenith angle from 0.1 to 0.7, of cloud optical thickness from 1.8 to 20 and of surface albedo from 0.0 to 0.9, the empirical equation reproduces results from the radiative transfer scheme to within 2.7%, and is shown to perform well outside the range of fit. The method was derived with high latitudes in mind but is applicable generally. It is shown that the spectral properties of snow and ice surfaces play an important role in surface net flux prediction. For totally overcast skies the influence of cloud height and atmospheric water vapour content are shown to be minor. Finally, for clear skies, an empirical equation due to J. W. Zillman is investigated and by adjustments to his coefficients, is brought into good agreement with the detailed calculations described here.
A study of the structure of mid‐latitude depressions in a numerical model using trajectory techniques. I: Development of ideal baroclinic waves in dry and moist atmospheres
Authors: Golding B. 
Published: February 2007

AbstractTrajectory‐tracing techniques are used to investigate the structure and air flow of developing baroclinic waves simulated by a numerical atmospheric forecast model. Three stages are described. The structure of the linear stage is explained with reference to the normal mode theory of baroclinic instability. In the mature stage, flow separation zones form which are identified as warm and cold fronts. At the occluded stage, circulation of cold air round the centre of the depression leads to changes in the structure of the cold front. When the initial state is nearly saturated, the overall development is speeded up, and at the mature stage the rate of deepening of the depression is doubled at the surface. In the occluded stage formation of a warm air conveyor belt is observed. Comparisons are made with previously published integrations by specially constructed models in order to establish a basis for comparison of forecasts from real data which will appear in a future paper.
The formation of multiple fronts
Authors: Hoskins B. J. , Neto E. Caetano , Cho H.‐R. 
Published: February 2007

AbstractUsing two‐dimensional semi‐geostrophic frontogenesis models and three‐dimensional semi‐geostrophic baroclinic wave models, it is shown that atmospheric development in the presence of non‐uniformities in pre‐existing temperature distributions may be expected to lead to the frequent occurrence of multiple frontal structures. In general, such non‐uniformities in the temperature distribution will be important if they significantly change the ambient temperature gradient. The frontal structures associated with the irregularities tend to be very shallow. They are possible candidates for the triggering mechanism for frontal troughs well behind a cold front and for shallow rainbands ahead of such a front. However, it seems that a separate mechanism is still necessary for the production of the middle‐level banded structures that are frequently observed.
The seasonal mean flow and distribution of large‐scale weather systems in the southern hemisphere: The effects of moisture transports
Authors: James I. N. , Anderson D. L. T. 
Published: February 2007

AbstractCirculation statistics, based on seasonal mean fields calculated from archived analyses at ECMWF, are presented for the southern hemisphere. Particular emphasis is placed on the large‐scale distribution of the high frequency (periods 2–6 days) transient eddy activity during the period July–August 1982. This was found to be a maximum over the Atlantic and Indian Oceans, and a minimum over the Pacific. The relative distribution of the temperature and momentum fluxes suggests that the high frequency transients are baroclinic waves, which reach large amplitudes and then decay, mainly as a result of Rossby wave radiation.The location at which the transients began to grow cannot be related to the regional variations of the mean flow. In particular, it is poorly related to the distribution of low‐ and middle‐level baroclinicity defined on the basis of dry, linear theory. It is suggested that the entrainment of moisture into the low‐level westerlies over the Atlantic could account for the large increase of transient activity there. It is shown that the major low‐level flux of moisture from the tropics into the southern hemisphere mid‐latitudes occurred over South America.Since moisture data cannot be extracted reliably from the data archives, moisture fluxes are deduced indirectly. Use is made of a general circulation model of the southern hemisphere March flow to illustrate the entrainment of moisture into the baroclinic zone over the mid‐Atlantic.
Eddy correlation measurements of energy partition for Amazonian forest
Authors: Shuttleworth W. James , Gash John H. C. , Lloyd Colin R. , Moore Christopher J. , Roberts John , Filho Ari De O. Marques , Fisch Gilberto , De Paula Silva Filho Vicente , Ribeiro Maria De Nazaré Góes , Molion Luiz C. B. , De Abreu Sá Leonardo D. , Nobre J. Carlos A. , Cabral Osvaldo M. R. , Patel Sukaran R. , Carvalho De Moraes J. 
Published: February 2007

AbstractMeasurements of energy partition for Amazonian forest made with novel eddy correlation equipment are presented for eight dry days in September 1983. These are interpreted to provide estimates of the aerodynamic and surface resistance for this vegetation type. Daily total evaporation for a transpiring canopy accounts for 70% of the available radiant energy, and is two thirds of conventional estimates of potential evaporation. The results are used to provide an initial calibration of a simple, physically based model of daily evaporation for Amazonian rain forest.
Observations of radiation exchange above and below Amazonian forest
Authors: Shuttleworth W. James 
Published: February 2007

AbstractMeasurements of shortwave and longwave components of the radiation budget were made above and below tropical forest in the Amazon Basin for a total duration of 203 hours in September 1983. Albedo was (12.25±0.2)%, and showed a small variation with solar altitude. The net outward flux of longwave radiation was around 30 W m−2, and fairly constant over the day. The relationship between net radiation, R, and solar radiation, S, was adequately described by the expression R = (0.858±0.006)S ‐ (35.0±1.9) (W m−2). The fraction of solar radiation reaching the bottom of the canopy was low, about one per cent: net radiation in this position was consistent with low soil heat flux and low soil evaporation.
The influence of the spectral response of satellite sensors on estimates of broadband albedo
Authors: Shine K. P. , Henderson‐Sellers A. , Slingo A. 
Published: February 2007

AbstractThe radiative impact of cloudiness variations on climate depends on the relative magnitudes of changes in the planetary albedo and the outgoing longwave radiation. R. D. Cess and co‐workers showed in 1982 that the calculated direction of the cloud‐climate feedback (i.e. whether the earth‐atmosphere system warms or cools with changing cloud amount) depended on the satellite data sets used in their analysis. It was suggested that the different spectral resolutions of the satellite sensors might be responsible. This note examines that suggestion. It is shown that the change in planetary albedo measured by the narrow band scanning radiometers on board NOAAs 2 to 5 as a result of cloudiness variation can be an overestimate of the actual change by a factor of up to 1.38 at low latitudes. The overestimate is not sufficient to explain completely the differences amongst the satellite data. Nevertheless, the results indicate that satellite spectral response must be carefully considered if the strength (and possibly even the sign) of the cloud‐climate feedback is to be correctly assessed from narrow band satellite data. Over snow‐covered surfaces it is shown that the NOAA data may underestimate the effect of cloud cover changes by over 70%, and overestimate the planetary albedo by over 0.2.
An experimental study of the remote location of lightning flashes using a VLF arrival time difference technique
Authors: Lee A. C. L. 
Published: February 2007

AbstractThe operational ‘cathode ray direction finding’ (CRDF) network for the location of lightning flashes at ranges up to thousands of kilometres can be replaced by a system using the ‘arrival time difference’ (ATD) technique. The ATD technique has theoretical advantages over CRDF, which could lead to substantially improved flash location, but the extent to which it suffers from certain propagation effects in practice must be quantified.In an experiment using the ATD technique, equipments were deployed in the U.K. and Gibraltar. These captured the vertical electric field of VLF electromagnetic radiation from individual lightning flashes, synchronized to CRDF‐selected flashes and recorded against an accurate timebase. By analysing the data using techniques analogous to those of hyperbolic navigation, the flash location was inferred, together with a measure of the data consistency.ATD location of independent flashes observed in time proximity frequently gave the tight grouping expected from an isolated active thunderstorm cell. Groups of diameter 2–10 km were observed at megametre ranges.ATD flash locations were compared with CRDF and other meteorological data. Positioning agreed within the limited accuracy of current techniques, but the CRDF system's potential for poor accuracy was highlighted.An analysis method was developed relating the internal consistency of the ATD flash data to geographical location accuracy, and ATD accuracy of less than 5 μs was measured experimentally. On this basis, an ATD system can locate flashes over a synoptic‐scale area with errors comparable with a storm cell diameter.
The semi‐annual oscillation and equatorial tracer distributions
Authors: Gray L. J. , Pyle J. A. 
Published: February 2007

AbstractSome interesting low latitude features of tracer distributions revealed by recent satellite experiments have been studied using a two‐dimensional atmospheric model. It is found that the equatorial semi‐annual oscillation, forced in the model by simply prescribing a suitable momentum convergence, is important in determining the tracer behaviour. In particular, the meridional circulation associated with the semi‐annual oscillation leads to a successful model simulation of the ‘double peak’ in N2O and CH4 mixing ratios, observations of which show, at certain times of the year, maxima on a fixed pressure surface in the subtropics with a local minimum at the equator.
Metastable stratus tops
Authors: Rogers David P. , Telford James W. 
Published: February 2007

AbstractPreviously, it has been shown that a saturated or near‐saturated mixture of cloud top air with cloud‐free air which has a somewhat lower wet‐bulb potential temperature, θw, will be denser than the surrounding cloud air, and will descend through the cloud layer. This has been referred to as entrainment instability. It is often presumed that this mechanism results in the rapid entrainment of dry air, which may lead to the breakup of a continuous stratocumulus deck.Airborne observations of the marine planetary boundary layer are presented. These, along with previously published observations, indicate that θw at cloud top may exceed that of the air above by as much as 5°C without strong entrainment being active. The fact that potential instability is not always accompanied by rapid entrainment suggests that the process is in a metastable state until sufficiently disturbed. These observations show the vertical transfer mechanisms in stratus cloud and reveal the presence of a case with a drop‐free region between cloud top and the base of the inversion. Theoretical arguments are proffered to explain these phenomena and to account for the fact that entrainment episodes are delayed and intermittent.
Simulations of the Saharan atmosphere—dependence on moisture and albedo
Authors: Cunnington W. M. , Rowntree P. R. 
Published: February 2007

AbstractThe sensitivity of an atmospheric general circulation model to changes in atmospheric water content, soil moisture content, radiation scheme and albedo have been investigated using the Meteorological Office global 11‐layer model. Previous integrations had produced excessive rainfall in the Saharan region. This paper reports experiments which show the importance for a good Saharan simulation of the initial atmosphere and soil moisture specifications, of the radiation scheme and of the surface albedo specification. It is shown that a dry Sahara with realistic circulation patterns is simulated only if the surface and atmosphere are initially realistically dry and the model's longwave radiation is dependent on atmospheric water vapour content. With a moist atmosphere, introduced either initially or by rapid evaporation from a wet soil, or by using a radiation scheme without water vapour dependence, there is increased net downward radiation which enhances ascent and convective activity and leads eventually to excessive rainfall in the desert region with a reversal of the usual circulation pattern to give 700 mb westerly winds at 5°–15°N associated with a cyclonic circulation over the Saharan region which draws in moist air from the south. As expected from Charney's hypothesis on the maintenance of deserts, the use of more realistic albedos tends to transfer rainfall from regions of higher albedo (e.g. deserts) to regions of lower albedo (e.g. forests).An extension of Charney's hypothesis to include moisture is proposed in which any increase in diabatic heating, whether due to reduced albedo or increased moisture availability, must be compensated by increased cold advection; in the tropics this typically occurs through ascent of potentially colder air leading to rainfall if the air is moist enough or the ascending layer sufficiently deep. Some implications of the possibility of two distinct summer circulation patterns for the theory of past North African climate are discussed.
Ice accretions on fixed cylinders
Authors: Prodi F. , Levi L. , Levizzani V. 
Published: February 2007

AbstractA comparison of ice accretions obtained on fixed and on rotating deposits was expected to provide a better understanding of the basic accretion mechanisms as well as being useful in many practical applications. Therefore, the morphological and physical properties of ice deposits accreted in a wind tunnel on fixed cylinders have been studied at temperatures, liquid water contents and air speed ranges previously tested for rotating cylinders.The deposits obtained are studied as to their external shape and internal morphology, crystallographic properties and local density distributions.The external shape is mainly dependent on the growth regime (dry or wet‐spongy) and the wind speed. In dry growth and high wind speed the experimental profile is lenticular in agreement with prediction; however, at lower wind speeds, the growth rate at the stagnation line is reduced with respect to the flanks, and the deposit develops a central depression. In wet or spongy growth, run back is effective at high wind speeds, while shedding becomes important at lower speeds.Observation of the crystal structure of fixed deposits shows that grain size and orientation depend on the growth regime and temperature in a similar way as with the rotating cylinders. However, the grain elongation is more pronounced in the fixed deposits, especially at lower temperatures.The local density distribution of the deposits is measured from X‐ray micrographs. The stagnation line density is maximum or close to the maximum at impact speeds of 26 and 10 ms−1, while in deposits obtained at 5 and 2.5 ms−1 it drops to remarkably lower values.The deposit profiles are compared with previous model investigations, and information is provided on the behaviour of some growth parameters which should be introduced into the model in order to simulate the experimental process better.The angular profile of the local density is also compared with predictions by previous investigators. It is shown that the best representation of the experimental points is achieved by calculating the mean volume radius of droplets at the collection angle θ and the corresponding impact speed, and using a new relationship between ice density and the Macklin parameter.