bluuebunny/crossref_metadata_2025_split huggingface dataset:
Growth of droplets in cloud edge downdraughts
Authors: Jonas Peter 
Published: February 2007

AbstractThermodynamic analysis of aircraft observations of the downdraughts at the edges of small maritime cumulus clouds is used to obtain estimates of the rate of entrainment of cloudy air into the downdraughts. The entrainment rates are around 10−4 to 10−2s−1. A simple entraining parcel model is used to investigate the fate of droplets entrained at this rate into such downdraughts. It is shown that the majority of small droplets evaporate totally, resulting in a subsaturation in the downdraught of less than 1%, except close to cloud top or cloud base. However, a few droplets which contain large soluble nuclei continue to grow, or undergo only a small amount of evaporation, during their descent. The limited evaporation ensures that some droplets undergoing a cycle of ascent and descent will grow even without net ascent. The largest droplets at some levels are therefore to be found in the downdraughts although the droplet concentration and liquid water content are much lower than in the cloud at the same level. The calculated growth and the thermodynamic properties of the downdraughts are in good agreement with observations of small maritime cumulus.
The initial value problem for tropical perturbations to a baroclinic atmosphere
Authors: Hoskins Brian J. , Jin Fei‐Fei 
Published: February 2007

AbstractLarge‐scale wave propagation ideas have proved extremely valuable in interpreting real and model data. These ideas have been developed in the context of barotropic models, and here the extension to baroclinic atmospheres is investigated. The dispersion of waves from an initial, large‐scale, tropical perturbation to a baroclinic basic flow is studied using a direct approach. A 15‐level, spectral, primitive equation model is integrated forward in time. Even in baroclinically unstable flows it is found that for a period, normally of order 12 days, the model yields what can be considered to be the direct response to the tropical perturbations. After this time baroclinic instability dominates. The basic flows are a resting atmosphere, a climatological westerly, an equatorial longitudinal modification to this, and a 3‐D climatological December‐February state. For the perturbations used here, equatorial Kelvin waves play only a very small role. The large‐scale equatorial Rossby wave shows only small dispersion and tends to move with a Doppler‐shifted phase speed which allows it to ‘accumulate’ to the east of an equatorial maximum of 10 ms−1 or more. Rossby wavetrains propagate into the middle and high latitude westerly regions. With the climatological flow, perturbations in the Indian Ocean‐west Pacific sector trigger a modal structure in the North Pacific and wavetrain across North America very much as barotropic theory would suggest and similar to observed Pacific‐North America (PNA) patterns. In such a flow the middle latitude waves tend to extend to the equator in the westerly regions of the east Pacific and Atlantic, in agreement with observed behaviour.
On the dependence of travelling wave stability on basic state wave phase speed
Authors: Manney Gloria L. 
Published: February 2007

AbstractThe dependence of stability on basic state wave phase speed is examined in a nondivergent barotropic model on a sphere for planetary scale waves with phase speeds typical of waves observed in the atmosphere. Triad interactions are examined analytically and compared to numerical results from a full spectral model. A number of triads may result in growth of the perturbation components for each basic state wave; for each triad there is a basic state wave phase speed where it is resonant, identified as the point where the critical amplitude for instability to that perturbation becomes zero. Critical amplitudes for instability obtained using the full spectral model generally agree well with triad results. Since the basic state wave phase speed determines which triad will grow, spatial structure and critical amplitudes for instability for the growing disturbances depend strongly on basic state wave phase speed. The results of this idealized study suggest that phase speed may be an important factor in determining the stability of planetary scale waves in the atmosphere.
Data sampling strategies for general circulation models
Authors: Thuburn John 
Published: February 2007

AbstractFor climate study purposes, complex general circulation models are often run for extended periods; the model state is sampled regularly and various diagnostics calculated. A convenient and often used sampling period is 1 day, but in this case diurnal and semi‐diurnal variations, for example those associated with the atmospheric tides, will alias into the time mean.Using the UK Universities Global Atmospheric Modelling Project model, adapted from the ECMWF forecast model, an attempt is made to quantify this aliasing in various diagnostics. A variety of data sampling/processing strategies are investigated, for example time‐averaging the model fields before writing them out, in an attempt to eliminate the aliasing problem. It is found that sampling the model state every 3/4 day or every 5/4 day is a simple and effective solution.
Role of detailed wind‐topography interaction in orographic rainfall
Authors: Alpert P. , Shafir H. 
Published: February 2007

AbstractA meso γ‐scale (Δx = 1 km) model for orographic rainfall was used to investigate the dependence of orographic enhancement on wind speed and direction and on detailed topography. Applying the model to cases reported in south Wales, high sensitivity to wind direction and the corresponding upwind topographical cross‐sections—factors neglected in some previous studies—was found. This suggests that studies of high‐resolution orographical rainfall characteristics (such as orographic enhancement as function of wind speed) or model verification should account for detailed interaction of the wind vector with the high‐resolution topography. Comparison with microphysical models illustrates that the effect of detailed wind‐topography interaction is at least of similar magnitude to that due to microphysical processes.
Variations common to the interplanetary magnetic field, the zonal atmospheric circulation and the earth's rotation
Authors: Djurovic D. , Páquet P. 
Published: February 2007

AbstractA study of the cyclic fluctuations in the earth's rotation, the geomagnetic field, the global atmospheric circulation and solar activity reveals the existence of 50‐day and 120‐day oscillations common to all. Our results, as well as the results obtained by radiometric measurements from the satellites Nimbus‐7 and the Solar Maximum Mission (Wilson 1982; Pap 1985), suggest that their origin lies in the physical processes in the sun. The mechanism remains unexplained but from this work it follows that the role of the interplanetary magnetic field could very well be important.
Lamination in ozone profiles in the lower stratosphere
Authors: Reid S. J. , Vaughan G. 
Published: February 2007

AbstractA survey of the distribution and scale of layers of enhanced and depleted ozone in the lower stratosphere was conducted over the altitude range 9.5–21.5 km using data recorded by balloon‐borne sondes released at various sites around the world.The results show clearly that lamination is primarily a winter‐spring phenomenon from subtropical to polar regions in both hemispheres, and seems to be linked to the presence of the polar vortex. A peak altitude of occurrence was found near 14 km. The interannual fluctuations in lamina abundance were found to be extremely large, varying at high latitudes by as much as a factor of three.Both the mean vertical thickness of the laminae and the associated rise or fall in ozone partial pressure within them vary progressively with latitude, season and altitude.
Dynamic initialization by repeated insertion of data
Authors: Macpherson B. 
Published: February 2007

AbstractIn this study we isolate and assess the impact of dynamic initialization by repeated insertion of data (RI), also known as ‘nudging’. A novel control experiment compares this iterative technique with an alternative intermittent analysis‐forecast cycle (AF) framework within the same assimilation system. Both schemes are versions of the operational global data assimilation system for numerical weather prediction at the Meteorological Office. A secondary issue is the impact of divergence damping as an extra initialization measure.The effectiveness of RI for control of high‐frequency gravity waves is demonstrated, together with its beneficial impact on the short‐period forecast of surface pressure. Changes to analysed fields due to RI are only a small fraction of the changes due to model evolution in six hours. RI is found to enhance upper‐level divergence in the equatorial west Pacific and the problem of spin‐up of the global mean precipitation is virtually absent, except when divergence damping is imposed during assimilation and removed during the forecast. The main benefit from divergence damping is a more accurate tropical wind field.RI is found to degrade the short‐period forecast of the wind field in the basic system studied, and the extra errors are largest in strong upper winds at mid latitudes. This fact is discussed in the context of an earlier idealized study of RI. Analysis constraints are a primary source of difference between various ‘nudging’ schemes used in numerical weather prediction. The sensitivity of the impact from RI to these is evaluated by relaxing constraints in the basic system. The result emerges that as the analysis step of a nudging scheme becomes better balanced, the scope for improvement by using RI for initialization becomes smaller. Also, if the better‐balanced analysis step entails stronger enforcement of non‐divergence on the wind increments, then the damaging effect of RI on the wind field through advection becomes larger and can outweigh the benefit through initialization. A scheme combining RI for mass data with an iterative AF method for wind data retains much of the benefit and avoids most of the problems in the scheme with RI of all data. This suggests a method of improving the contribution of an RI structure to the overall performance of an assimilation scheme based on the nudging approach.
The summertime variation of heat and freshwater contents between the Azores and Greenland
Authors: Bauer J. , Leach H. , Woods J. D. 
Published: February 2007

AbstractThe Kiel Sea Rover System was used to make a series of hydrographic surveys north of the Azores, which provided the first detailed description of the spatial structure of the summertime heat and freshwater contents. Our results confirm the conclusion based on earlier studies that advection plays a dominant role in the seasonal change in the freshwater content. Using the new data we have assessed the reliability of estimates of the various contributions to the summertime changes of fresh water content based on climatological estimates of the surface fluxes, on Ekman and geostrophic transport and on horizontal eddy diffusivity. We show that they contain substantial inconsistencies and consider how these might have arisen. Joint regression of the heat and freshwater variations recorded in our synoptic data suggest a balance of contributions that is significantly different from that derived from climatology, but, in view of the complexity of the region, we do not claim that they do more than reveal the difficulty of the problem.
A concise derivation of the semi‐geostrophic equations
Authors: Chynoweth S. , Sewell M. J. 
Published: February 2007

AbstractA self‐contained account of the semi‐geostrophic equations is given. This contains formulae not previously available in the literature. The equations are presented in terms of four alternative sets of independent variables, and the Legendre mappings between these variables are specified. The approach is facilitated by the introduction of a general theorem in vector analysis, which is proved, concerning the total or convected derivative of a Jacobian following the particle. An expression for the curvature of a front is included.
Four‐dimensional variational data assimilation using the adjoint of a multilevel primitive‐equation model
Authors: Thepaut Jean‐Noéul , Courtier Philippe 
Published: February 2007

AbstractThe aim of the paper is to demonstrate the numerical feasibility of 4‐D variational assimilation using a multilevel primitive‐equation model. The experiments consist in minimizing the distance between the model solution and the observations. The gradient of the cost function thus defined is computed by integrating the adjoint of the model.Here, assimilations are performed using model‐generated observations. In a preliminary set of experiments, assimilations were performed assuming that observations consisting of a full‐model‐state vector are available only at the end of the assimilation period. The numerical convergence of the method is proved and the results are meteorologically realistic. The use of the Machenhauer nonlinear normal mode initialization scheme and its adjoint turned out to have hastened the convergence and to have controlled to some extent the amount of gravity waves appearing in the solution. We identify a loss of conditioning of the minimization problem with an increase in the length of the assimilation period. The presence of horizontal diffusion in the model has the effect of degrading the convergence.The second set of experiments evaluates the impact of observations distributed over the whole assimilation period. Through different senarios of sets of observations, we demonstrate the efficiency of the 4‐D variational approach in extracting the information contained in the dynamics of the model, together with the information contained in the observations. In particular, observing only the small scales of the flow leads to a good reconstruction of both small scales and large scales. Observations of the mass‐field evolution lead to a good reconstruction of the vorticity field in mid latitudes but less so in the tropics.Increased resolution in the model in the experiments was found to have a negative impact on the speed of convergence of the minimization algorithm.
Three‐Dimensional Mesoscale Circulation Within A Convective Post‐Frontal System. Possible Role of Conditional Symmetric Instability For Triggering Convective Motion
Authors: Lemaitre Y. , Scialom G. 
Published: February 2007

AbstractThe role of the Conditional Symmetric Instability (CSI) in the formation of convective systems in the cold airmass located at the rear of a secondary cold‐frontal system is evaluated. This study is based on observations made on 4 June 1984 during the French experiment LANDES‐FRONTS 84. In particular, the mesoscale three‐dimensional circulation deduced from Doppler‐radar data within one of these convective systems is studied in detail and compared with that predicted by the CSI theory. In spite of the fact that, on the one hand, notable convective‐scale motion masks the mesoscale circulation, and that, on the other hand, the synoptic‐scale circulation is poorly described, this comparison shows that, among the various theories involved at the mesoscale, only CSI gives a proper account of the observed mesoscale circulation.
Rossby Waves and El NiñO During 1935‐46
Authors: Bigg Grant R. , Inoue Masamichi 
Published: February 2007

AbstractThe late 1930s and early 1940s saw strong climatic anomalies in the tropical Pacific basin. In this paper we combine an ocean‐modelling aproach to the investigation of the Pacific climate of this period with a multi‐parameter data analysis. It is shown that a large and deep pool of warm water in the western Pacific in the late 1930s acted as an energy source for the strong and prolonged El Niño event of the early 1940s and its rapidly following successor. Remote off‐equatorial Rossby‐wave forcing plays a prominent role in the generation of these events.
Observation of High Ice Particle Concentrations In Convective Cells and Cloud Glaciation Evolution
Authors: Gayet J.‐F. , Soulage R. G. 
Published: February 2007

AbstractDuring an investigation into convective cloud at a single temperature level near −6°C, observations by aircraft have found ice crystal concentrations significantly in excess (up to 600 L−1) of those to be expected from primary ice formation (i.e. ice multiplication). Despite the fact that the main conditions for the operation of the Hallet‐Mossop secondary ice multiplication process were fulfilled, the corresponding rate of ice production, as calculated, appeared to be lower than was observed in the cloud. This suggests that other ice‐enhancement processes might be at work during the ice‐forming stage.A small‐scale (100m) analysis of the data revealed that ice‐multiplication processes operated only in the fringes of the updraught regions of growing cells. It was also observed that a sudden occurrence of the ice usually coincided with the decay of the updraught. From these observations and from an evaluation of the amount of heat released by riming and vapour deposition processes, we suggest that the cell cores are auto‐seeded by the mixing or recycling of numerous ice crystals and that the rapid glaciation tends to be ineffective in increasing the buoyancy of the cell.
Gravity‐Wave Observations Using an Array of Microbarographs In the Alearic Islands
Authors: Monserrat S. , Thorpe A. J. 
Published: February 2007

AbstractTime series of surface pressure from an array of microbarographs in the Balearic Islands (western Mediterranean) are used to obtain wavelength, phase speed and direction of propagation of gravity waves on the Balearic Islands. These pressure waves are commonly found during the summer and they force the large seal‐level oscillations observed in the inlet of Ciutadella; a phenomenon locally known as ‘rissaga’. Four events are described here, each of which has a duration of between 30 and 48 hours. Radiosonde data from Palma de Mallorca are used to relate the waves to the vertical structure of the atmosphere. the various source mechanisms of these waves and also the possibility of the existence of a duct layer trapping the wave energy near the surface are discussed. the waves are found to have no significant variation of phase speed with frequency and so they are basically non‐dispersive. This is an indicator pointing either to a local generation of gravity waves by wind‐shear instability or to the existence of neutral trapped waves, in the lower troposphere.
Comments On ‘Observations of Two Regions of Charge During Initial Thunderstorm Electrification’ By J. E. Dye, J. J. Jones, A. J. Weinheimer and W. P. Winn (July 1988, 114, 1271‐1290)
Authors: Moore C. B. , Vonnegut B. 
Published: February 2007

AbstractDye et al., in a 1984 thunderstorm study, identified regions containing electric charge in two growing clouds from measurements of the atmospheric electric fields taken, during penetrations of these clouds, by an instrumented aeroplane. There are, however, problems concerning the locations they assigned to these charges relative to the radar echoes from the clouds. In one case the authors identified the pressure altitude reported by the aeroplane pilot with the altitude of the radar echo top; this introduced an error of about 450m. In another case, the location given for the charged region relative to the cloud echoes is not compatible with the pilot's report of an updraught. While the authors measurements add to our knowledge of cloud electrification, some of their interpretation is questionable, and other explanations for the charged regions are possible.
Vibrational Temperatures and Radiative Cooling of the Co2 15 Mum Bands In the Middle Atmosphere
Authors: LÓPez‐Puertas M. , LÓPez‐Valverde M. A. , Taylor F. W. 
Published: February 2007

AbstractAltitude and latitude distributions of the populations of the bending and symmetric stretching levels of CO2 and of the cooling rates induced by the bands originating from them in the middle atmosphere (40‐120km), have been calculated for the CIRA 1986 reference atmosphere using a non‐LTE radiative transfer model. A sensitivity study of the more uncertain parameters that strongly affect these quantities has been undertaken. These factors are the carbon dioxide and atomic oxygen volume mixing ratios, and the rate coefficient for the deactivation of the bending mode of CO2 by atomic oxygen. the model makes use of the most recent values of these parameters: the abundances of CO2 in the upper mesosphere and lower thermosphere as measured by, and derived from, rocket and satellite instruments; the global distribution of atomic oxygen density calculated by Rees and Fuller‐Rowell (1988) and the measurements of Thomas (1990); and the fast rate for the deactivation of CO2(0, v2, 0) by O(3P), recently derived by Sharma and Wintersteiner (1990). Among the most important results are the following: (i) the CO2 15 μm fundamental band is close to LTE (Tv‐ Tk ≤ 6K) up to about 100km altitude; (ii) the first and second hot bands exhibit high vibrational temperatures, especially in the daytime summer mesopause region; (iii) the cooling rates by the CO2 15μm bands in the lower thermosphere can be very large, depending on the CO2 abundance. Finally, our results have been compared with other recent non‐LTE models in order to quantify the effects of different radiative transfer schemes on the calculations of vibrational temperatures and cooling rates.
Point Vortices In A Low‐Order Model of Barotropic Flow On the Sphere
Authors: Egger Joseph 
Published: February 2007

AbstractA low‐order model of global barotropic flow with intrinsic long‐term variability is presented. the model is based on the barotropic vorticity equation with Newtonian forcing of the zonal flow and realistic orography. Subgrid eddy motion is represented by point vortices which move within the low‐order flow of the model and affect it through their own circulation. the vortices are inserted at locations of maximum flow speed and are removed after a prescribed lifetime has elapsed. In the absence of Newtonian forcing, mid‐latitude westerlies and strong polar easterlies are induced if there are only cyclonic vortices. Anticyclonic vortices create equatorial easterlies.If ‘realistic’ Newtonian forcing is added, two stationary orographic troughs dominate the model's mean flow in the northern hemisphere. Cyclones are generated downstream of the troughs, and pronounced storm tracks extend eastward from these genesis areas. the model atmosphere exhibits slow variability of a realistic order of magnitude with maxima over the ‘oceans’. This long‐term variability is due to the interaction of long‐lived circulation patterns, like blocks, with the point vortices. Moreover, cyclone generation is important in inducing slow motions. In the southern hemisphere there is little variation of flow statistics with longitude. All in all the low‐order model is surprisingly successful in simulating the observed patterns of atmospheric transience. This suggests that a low‐order model with point vortices might be a useful tool in climate research.