bluuebunny/crossref_metadata_2025_split huggingface dataset:
Formation of inhomogeneity in drop concentration induced by the inertia of drops falling in a turbulent flow, and the influence of the inhomogeneity on the drop‐spectrum broadening
Authors: Pinsky M. , Khain A. 
Published: February 2007

AbstractThe mechanisms of drop‐concentration inhomogeneity formation are studied using both a numerical simulation with a model of isotropic and homogeneous turbulence, and analytical methods. It is shown that atmospheric turbulence can create a significant drop‐concentration inhomogeneity due to the effects of drop inertia. Two types of area in the turbulent flow are revealed. Drops tend to leave the areas of ‘drop vortices’ and collect within the zones out of the vortices. As a result, the zones of drop‐track collection turn out to be the zones of enhanced drop concentration. the rate of concentration enhancement is studied for drops of different sizes using the Monte Carlo method. It is shown that the drop flux velocity divergence and droplet‐concentration variations reach their maximum at 100 μm drop radius. Zones of enhanced drop concentrations are stretched along the drop tracks. Characteristic scales of drop‐concentration fluctuations along the drop tracks are of the order of several metres or even a few tens of metres. Across the drop tracks, the characteristic scale of concentration pulsations is of the order of a few centimetres.The existence of the areas of drop‐concentration enhancement and the areas of decreased concentration means that drop collisions are not distributed uniformly, but are concentrated within the areas with greater drop concentration. the possible effects of droplet inhomogeneity is simulated by solving a stochastic coalescence equation for the water drop‐size‐distribution function in areas of enhanced and decreased droplet concentration with the subsequent mixing of the corresponding spectra at each 10 s. It is shown that due to the nonlinear nature of collision processes the effects of the increase of collision frequency in the areas of enhanced concentration dominate over those of its decrease. the effects of the drop‐concentration inhomogeneity are accumulated with time and lead to substantial acceleration of droplet spectrum broadening. It is supposed that the inhomogeneity of rain rates with time and space can be attributed to these turbulence effects determining drop‐track collection and drop‐concentration inhomogeneity. These turbulence effects can contribute to the formation of inhomogeneity of ice and aerosol particles as well.
The singular vectors of a coupled ocean‐atmosphere model of Enso. II: Sensitivity studies and dynamical interpretation
Authors: Moore A. M. , Kleeman R. 
Published: February 2007

AbstractWe have studied the singular vectors of an intermediate coupled ocean‐atmosphere model of El Niño Southern Oscillation in an attempt to understand the factors which influence the growth of perturbations in the coupled system in the tropics. the singular vectors are important because they are the fastest growing perturbations that can exist in the system before nonlinearity becomes important. In this study, we have examined how the model singular‐vectors are affected by variations in certain model parameters, some of which mathematically define the singular vectors. the important findings of this study can be summarized as follows:
the behaviour of the model singular‐vectors can be understood in terms of a superposition of the linear modes of the coupled system which are non‐orthogonal. In some cases, the gravest modes are oscillatory in time and this leads to resonances in the singular‐vector growth‐factors.
the singular‐vector spectrum is generally dominated by its fastest‐growing member whose wind structure is relatively insensitive to the parameters that define the singular vectors mathematically. This result is in accord with the findings of Part I, and suggests that, in the tropics, the coupled system has a preferred response to perturbations, and that this response will readily manifest itself if conditions are favourable.
Favourably configured perturbations can undergo rapid transient growth over the entire regime in which the coupled model is asymptotically stable (i.e. when there are no unstable modes in the system). In this regime, rapid transient growth can occur over a wide range of timescales from 1 week to 24 months.
The results and ideas presented here have important ramifications for the way in which the real coupled oceanatmosphere system will respond in the tropics to perturbations arising from stochastic forcing inherent in the system. the dynamical operators that govern the early stages of linear perturbation‐development are non‐normal which means that the ubiquitous stochastic noise present in the tropics can be amplified. the dynamical mechanisms by which this can occur are the same as those responsible for the growth of the system singular‐vectors. the dominant singular‐vectors of the coupled model bear a remarkable resemblance to the tropical‐cyclone pairs that are often observed spanning the equator in the western Pacific Ocean accompanied by strong westerly wind bursts, and thought to be precursors of El Niño events. We propose that both westerly and easterly wind bursts occuring in nature are a preferred response of the coupled system in the tropics to its inherent stochastic noise component.
Parametrization of momentum transport by convection. II: Tests in single‐column and general circulation models
Authors: Gregory D. , Kershaw R. , Inness P. M. 
Published: February 2007

AbstractDiagnostics derived from cloud‐resolving‐model simulations in part I of this study, relating to the vertical transport of horizontal momentum by convection, are used to develop a parametrization of convective momentum‐transports for deep convection based upon the mass‐flux convection‐scheme discussed by Gregory and Rowntree. the importance of cloud pressure‐gradients to the flow within the cloud is emphasised, and a simple method of representing their effect is suggested. the scheme is able to reproduce the fluxes derived from the cloud‐resolving model studies where cloud organization by the flow is unimportant. Inclusion of the parametrization in a version of the Meteorological Office Unified Model demonstrates that convective momentum‐transports play a large role in the momentum balance of the atmosphere. Generally, simulation of the mean atmospheric circulation by the Unified Model is improved by the inclusion of such transports.
A comparison of two bulk microphysical schemes and their effects on radiative transfer using a single‐column model
Authors: Petch J. C. , Craig G. C. , Shine K. P. 
Published: February 2007

AbstractIncreasingly, numerical models in climate studies are using prognostic bulk microphysical schemes to predict grid‐scale cloud cover and properties. These schemes provide information which can lead to improved calculations of radiative transfer, and a better understanding of the interaction of radiation with cloud microphysics on the large scale. In this study a one‐dimensional, hydrostatic column model with fixed vertical velocities includes two different bulk microphysical schemes to investigate how the type of scheme influences the hydrometeor content in a cloud, and the effect of this on the radiative heating rates through the cloud.Two test cases are performed, one representing the stratiform region of a tropical cloud cluster, the other a dissipating tropical cirrus cloud. Each test is first performed using a microphysical scheme that carries only one variable for solid water (MSI), and then using a scheme that separates solid water into ice crystals, snow, and graupel (MS2). Further sensitivity tests are made using MS2 to examine the effects of excluding graupel, and of allowing the ice crystals to fall. The influence of these modifications on the hydrometeor contents of the clouds and the corresponding radiative heating rates is considered.In a simulation of the stratiform region of a tropical cloud cluster, MS1 is shown to produce significantly larger hydrometeor contents than MS2. However, in the simulation of a dissipating cirrus cloud, the ice content predicted by MS2 remains much larger than the ice content predicted by MS1, throughout a 24‐hour integration. This is because there is a non‐precipitating ice category in MS2 which is very slow to convert to snow at low ice‐water contents. The use of a non‐precipitating ice variable is shown to have a major impact on both the solar and the infrared radiative heating rates at the cloud top, and in some cases to give unrealistic predictions of cloud ice contents.
The role of the environmental flow in the development of secondary frontal cyclones
Authors: Renfrew Ian A. , Thorpe Alan J. , Bishop Craig H. 
Published: February 2007

AbstractThe impact of the environmental flow on the development of secondary frontal cyclones is investigated. Several case‐studies are examined as examples of secondary frontal‐cyclone events observed in the North Atlantic‐western Europe sector. A simple measure of growth is defined to chart their development. The vorticity attribution technique of Bishop is utilized to calculate the action of the large‐scale (environmental) flow on the fronts. In particular the environmental along‐front stretching—shown to be important in theoretical models of frontal instabilities—is calculated. The role of the environmental deformation appears to be crucial: as part of a baroclinic life cycle, stretching deformation acts to build up a front but suppress along‐front waves; if the stretching rate diminishes, barotropic instabilities may then break out. Diagnostics are examined to try to ascertain the growth mechanisms at work in each frontal‐cyclone case. A range of values for the commonly prescribed deformation‐frontogenesis and shearing‐frontogenesis parameters are calculated.
The tropical stratopause in the UKMO stratospheric analyses: Evidence for a 2‐day wave and inertial circulations
Authors: Crsolini Yvan J. , Limpasuvan Varavut , Leovy Conway B. 
Published: February 2007

AbstractLarge‐scale atmospheric motions near the low‐latitude stratopause in the UK Meteorological Office stratospheric analyses during the northern hemisphere winter are investigated, focusing in particular on the 2‐day wave and inertial eddies.The 2‐day wave is a westward‐propagating planetary wave which appears recurrently in the summer subtropical upper stratosphere and mesosphere. Space‐time spectral analysis of the assimilated fields of temperature and winds, and high‐resolution transport of water vapour, reveal large‐amplitude breaking planetary waves in the summer subtropics, principally a 2‐day wave 3, and a wave 1 of a period near 7 days. A region of anomalous zonal‐mean meridional gradient of potential vorticity also exists, pointing to a barotropic instability mechanism for the origin of the 2‐day wave.By contrast, the winter subtropics are characterized by mixing on less‐than‐planetary scales during the period considered, occurring during the easterly phase of the stratospheric semi‐annual oscillation. Water vapour is mixed turbulently into gyres, and drawn in narrow filaments out of the southern hemisphere. The potential‐vorticity field displays vertically layered, zonally‐asymmetric variations.The behaviour of the model assimilated fields is supported by along‐track water vapour distribution observed by the Microwave Limb Sounder aboard the Upper Atmosphere Research Satellite which shows evidences for intermittent filamentary mixing in the northern subtropics, and a 2‐day wave signature in the summer subtropics.Based on these results, a possible mechanism connecting the initiation of the austral 2‐day wave events to strong planetary‐wave activity in the winter is proposed. The winter planetary waves provide the background conditions for development of inertial instabilities at low latitudes of the winter hemisphere, which in turn create conditions favourable for barotropic instability of the summer subtropical easterly jet.
Modelling fluxes of momentum, sensible heat and latent heat over heterogeneous snow cover
Authors: Essery Richard 
Published: February 2007

AbstractA boundary‐layer model is used to simulate turbulent fluxes of momentum, sensible heat and latent heat over heterogeneous surfaces. Results for heterogeneous snow cover are compared with results for heterogeneous snowfree vegetation, and effective parameters characterizing the surfaces are discussed. A tile model, which calculates area‐average fluxes from given area‐average windspeeds, temperatures and humidities at a reference height in the atmosphere, is assessed in comparison with the boundary‐layer model. the performance of the tile model is found to be degraded for surfaces with regions of both upward and downward sensible‐heat flux, as can occur over heterogeneous snow cover. Better results can be obtained by using different reference heights for heat flux and momentum flux calculations.
Atmospheric ammonia at a moorland site. I: The meteorological control of ambient ammonia concentrations and the influence of local sources
Authors: Flechard C. R. , Fowler D. 
Published: February 2007

AbstractAmmonia (NH3) concentrations were continuously monitored at a moorland site in southern Scotland for 13 months from February 1995 to February 1996, providing about 13 500 half‐hourly average measurements (71% of the time). Concentrations were found to be log‐normally distributed (with a geometric mean of 0.43 μg m−3 and a geometric standard devation of 3.35), and were strongly dependent on wind direction because of local agricultural point soources within 3 km of the site. These sources contributed approximately 60% of the NH3 advected over the measurement site, while more distant sources accounted for the remaining 40%. Meteorological conditions also influenced the concentration of NH3 because of their effects on emissions from vegetation, on dispersion by the atmosphere, and on rats of removal from the atmosphere. The largest concentrations were measured in warm (>15 °C), dry, summer conditions and in cold (<°C) weather, typically 1–5 μg m−3. Smaller concentratins, typically in the range 0.1–1 μg m−3 were found in intermediate (1–15 °C), humid and wind conditions. Ammonia concentrations generally decreased with increasing wind speed and were very variable at windspeeds <3 m s−1. Concentrations also decreased with increasing relative humidity, reflecting the partitioning of NHx toward the liquid phase at large (>90%) relative humidities, while evaporation from aerosols seemed more likely at lower (40–90%) relative humidities. Seasonal variations in meteorological patterns resulted in marked seasonal variations in concentrations, with the largest mean‐values observed during the warmer and drier summer months, and the smallest values measured in humid and windy, atumn and winter conditions. Measurements indicated a short atmospheric residence time for NH3, of the order of 1–2 hours. The atmospheric residenc time for dry deposition of gaseous NH3 was estimated from these data and ranges from 2 to 20 hours depending on atmospheric and surface conditions, with a mean value of 3.5 hours for the 13 months of monitoring.
Atmospheric ammonia at a moorland site. II: Long‐term surface‐atmosphere micrometeorological flux measurements
Authors: Flechard C. R. , Fowler D. 
Published: February 2007

AbstractLong term micrometeorological measurements of the net vertical exchange of atmospheric ammonia (NH3) over moorland vegetation at a site in southern Scotland from February 1995 through to February 1996 are reported. The measurments, made using a combination of eddy covariance and aerodynamic flux‐gradient methods, provided satisfactory 30‐minute average fluxes approximately 50% of the time using continuous annular wet denuders for NH3 sampling at three heights between 0.4 and 3.4 m above the ground.The moorland vegetation was a net sink for atmospheric NH3 as 93.4% of all half‐hourly measured fluxes indicated dry deposition to the surface with mean flux of −5.2 ng m−2 S−1 and a mean deposition velocity Vd of 11.7 mm S−1 at (z ‐ d) = m. Despite the dominance of deposition to the moorland, emission fluxes were observed approximately 6.6% of the time with a median of +1.1 ng m−2 S−2, and most commonly resulting from drying water films on foliar surfaces.The mean aerodynamic resistance at a reference height of (z ‐ d) = 1 m and the mean viscous sub‐layer resistance were 36.6 s m−1 and 11.6 s m−1 respectively, and of a similar magnitude to the mean canopy or surface resistance Rc of 37 s m−1. When the moorland surfaces were wettend by rain or dew, canopy resistances to NH3 deposition were generally smaller (averaging 23 s m−1) than in dry conditions (when Rc was 61 s m−1) or when surfaces were covered in snow (Rc = 56 s m−1. Therefore, moorland surfaces cannot be treated as perfect sinks for NH3 deposition.Diurnal and seasonal patterns in rates of deposition and net fluxes to the moorland surface area were determined mainly by a combination of wind speed, surface wetness and ambient NH3 concentration. Thus, during winter, depositions rates expressed as a flux or deposition velocity were generally much larger than in summer by factors of 2 or 3 respectively, as a result of stronger winds and the presence of surface water for extensive periods which kept canopy resistances small except in frozen conditions. At temperatures below 0°C, the surface became progressively dry and canopy resistances were large (50–100 s m−1).The measurements provided direct estimates of the net annual gaseous NH3 input to the moorland, which was 2.5 kg NH3‐N ha−1 a−1 and was almost identical to the measured wet deposition input at the site of 2.4 kg NH4+‐N ha−1 a−1.
Zonal extent of oceans, high‐latitude fresh water supplies and the thermohaline circulation
Authors: Cai Wenju 
Published: February 2007

AbstractOne of the most distinctive features of the present‐day global thermohaline circulation is the absence in the North Pacific Ocean, and the presence in the North Atlantic Ocean, of deep‐water formation. In this paper, the different zonal extents and the associated high‐latitude fresh water supplies of the Pacific and the Atlantic when under a zonally uniform fresh water flux forcing are examined, as a possible cause for the asymmetry. A series of numerical experiments using the Bryan–Cox–Semtner ocean general circulation model is performed. In order to achieve a true independence, the model configuration and the model forcing are so constructed that zonal extent is the only built‐in difference between the two oceans. Three configurations are used. They are (a) an idealized Atlantic Ocean (narrow zonal extent) alone, (b) an idealized Pacific Ocean (wider zonal extent) alone, and (c) a connected Pacific–Atlantic system.Upon spin‐up, the model ocean is forced by a zonally uniform Schopf boundary‐condition for temperature and a zonally uniform fresh water flux for salinity. Both forcings are diagnosed from a spin‐up. All diagnosed fresh water fluxes have net precipitation in polar regions and net evaporation in low‐latitude regions as observed. This feature means that the high‐latitude model Pacific is subject to a fresh water flux twice that in the high‐latitude model Atlantic. This causes the northern sinking cell in the model Pacific of the spin‐up state to collapse; a southern sinking cell settles in. The process is similar to that in many model studies examining the response of the thermohaline circulation to a high‐latitude freshening. By contrast, the northern sinking cell in the North Atlantic of the spin‐up state persists. In the connected Pacific–Atlantic system, these produce the ‘conveyor belt’ type circulation, and the asymmetry between the North Atlantic and the North Pacific. It is found that the high‐latitude fresh water supply is the crucial parameter, and that the difference in basin width is not directly important. It is also found that the connection by the Antarctic Circumpolar Current is not important.A large number of experiments is performed in the model Pacific–Atlantic system to test the sensitivity to initial conditions. The system shows a clear tendency to favour the conveyor‐belt circulation, although the strength of the North Atlantic northern sinking cell shows considerable variations. Thus, although in nature there are many factors that can contribute to the asymmetry, the difference in total available fresh water at the northern high latitudes between the two oceans as a result of the width difference may have played a significant role.
Storm‐track organization and variability in a simplified atmospheric global circulation model
Authors: Frisius Thomas , Lunkeit Frank , Fraedrich Klaus , James Ian N. 
Published: February 2007

AbstractA storm track is simulated in a coarse‐resolution multilevel primitive‐equation model with linear surface friction and heating terms. A restoration temperature distribution consisting of a dipole embedded in a zonally symmetric profile forces the model to simulate the surface heating. Three simulations, each with a different dipole orientation, are performed to test the sensitivity of storm‐track organization to the external forcing field. The climatological time‐mean circulation and the transient disturbances of the reference simulation agree well with observations of northern hemisphere wintertime storm tracks. Local energetics show that baroclinic instability is responsible for the enhanced eddy kinetic energy downstream of the jet, and the downstream end of the storm track results from the barotropic conversions of eddy kinetic energy to the time‐mean flow.Low‐frequency fluctuations with a period of about 50 days associated with a retrograding large‐scale wave pattern are identified by a complex empirical‐orthogonal‐function analysis of the vertically averaged stream function. A composite life cycle of the low‐frequency variability reveals the growth and decay of a blocking anticyclone downstream of the storm track. A cyclogenesis initiated by an eastward‐propagating wave train is observed a few‐days before the amplitude of the low‐frequency anomaly attains its peak. The net forcing of the high‐frequency eddies contributes to the growth and decay of the blocking anticyclone. The results suggest that the occurrence of the blocking‐like event is part of the storm‐track dynamics.