bluuebunny/crossref_metadata_2025_split huggingface dataset:
The Role of Diffusive Effects On Potential Vorticity In Fronts
Authors: Cooper Iain M. , Thorpe Alan J. , Bishop Craig H. 
Published: February 2007

AbstractTheoretical studies of two‐dimensional fronts have shown that frictional processes at the time of occlusion and/or diabatic effects imply low‐level, positive potential vorticity (PV) anomalies. It is shown here that significant amounts of PV can be produced in an evolving baroclinic wave (taken to be the two‐dimensional semigeostrophic Eady wave) purely owing to the presence of a no‐slip, insulating lower boundary. A simple analytic expression is given for the time evolution of the mass‐weighted volume‐average PV, (PV), up to the point of frontal collapse. For the most unstable linear normal mode it is found, at the time of the formation of the frontal discontinuity, that the PV has increased by approximately a third of its basic‐state value.The bulk formulation for surface fluxes of heat and momentum is used to illustrate the general role of boundary‐layer processes in modifying PV locally. Turbulent fluxes which decrease linearly from maximum values at the surface to zero at the top of the planetary boundary layer (PBL) are used to recover a simple analytical form for the PV generation tendency. the magnitudes of the PV anomalies produced by flows typical of mid‐latitude cyclones and associated fronts are highlighted by vertically integrating the tendency equation over the PBL. It is shown that the generation of PV in the PBL depends on the vertical differences of velocity and potential temperature over the depth of the PBL; and on the vertical vorticity and horizontal potential‐temperature gradient at the top of the PBL. For example, there is a local source of PV if there is cooler air at the top of the boundary layer to the right of the surface wind vector, i.e. if the thermal wind has a component opposing the wind at anemometer level. the production of PV anomalies by surface processes, using this formulation, is independent of the occlusion process.Purely horizontal mixing of heat in the vicinity of a potential‐temperature anomaly is examined to predict the magnitude of local PV anomalies, consistent with the internal ‘eddy’ viscosity often used in numerical models.Numerical integrations of a two‐dimensional Eady wave with a detailed surface‐layer description and stability‐dependent eddy viscosity coefficients also illustrate that significant growth in the PV occurs before the warm sector is lifted off the surface. There is a direct correspondence between this growth and the decrease of the horizontally‐averaged surface potential temperature, as predicted from the no‐slip, insulating boundary theory. Also, the local PV anomalies predicted by the bulk PBL representation are shown to be in close agreement with those produced in the numerical simulation, which uses a more comprehensive scheme.
Four‐Dimensional Assimilation In the Presence of Baroclinic Instability
Authors: Rabier Florence , Courtier Philippe 
Published: February 2007

AbstractCurrent operational assimilation methods have revealed deficiencies in cases of strong baroclinic development. Baroclinic conditions are therefore appropriate for evaluating the potential for improvement which could be achieved through the implementation of a fully four‐dimensional data assimilation. In this paper the behaviour of a variational scheme is investigated for a typical baroclinic instability problem, where a wave develops and retroacts on to the basic zonal flow. the tangent‐linear model is shown to lead to a good approximation of the time evolution of the wave over a range of 48 hours, even at the most intense cyclogenesis period. For the assimilation experiments the twin experiment approach is applied. Only part of the flow evolution is observed, either the zonal component or the eddy component. In either case the method proves successful in reconstructing the unobserved part of the flow, taking advantage of the nonlinear coupling that exists between those components. However, nonlinearities can lead to difficulties when the range of validity of the tangent‐linear model is exceeded or when the cost function exhibits multiple minima.
Four‐Dimensional Data Assimilation: Comparison of Variational and Sequential Algorithms
Authors: Rabier F. , Courtier P. , Ehrendorfer Martin 
Published: February 2007

AbstractThe problem of data assimilation that is concerned with the complete and accurate specification of the atmospheric state based upon observations and other types of information can be approached either by variational or sequential algorithms. While variational techniques proceed by the global fitting of an assimilating model to the available information, sequential assimilation involves a statistical minimum mean‐square‐error estimation approach.In this paper both algorithms are compared in a systematic manner with regard to assimilation/forecast accuracy, computational efficiency, and storage requirements based on a limited series of observing‐systems simulation experiments. the barotropic vorticity equation on a rotating sphere is used as the assimilating model.The results indicate that under a variety of conditions the variational algorithm performs at least as well as the sequential algorithm. the variational algorithm is also found to be more successful than the sequential algorithm in the reconstruction of the physical fields in data‐void regions. Limitations and possible extensions, as well as operational implications of this work, are briefly discussed.
Measurements of Dry Deposition of No2 to A Dutch Heathland Using the Eddy‐Correlation Technique
Authors: Coe H. , Gallagher M. W. 
Published: February 2007

AbstractNitrogen dioxide (NO2) fluxes were determined over a dry heather moor by the eddy‐correlation technique using a sonic anemometer and two Scintrex LMA‐3 chemiluminescent analysers. Using power spectral analysis virtually all the fluctuations in NO2 concentration were of frequencies less than 1 Hz at the measuring height of 5.32m. Even with the non‐ideal response of the LMA‐3 little information about NO2 concentration was lost. Co‐spectral analysis showed there was no loss of flux at high frequencies arising from the slow response of the LMA‐3.NO2 was deposited at a constant rate of 0.02 μg m‐2 ‐1 for nearly all of the measuring period. the deposition velocity maintained a mean value of 1 mm s‐1 throughout with minimum values nearing zero and maximum values of 3.5 mms‐1. There was no diurnal variation in canopy resistances to NO2. Mean values were 548 s m‐1 and 400 sm‐1 with and without the inclusion of several brief periods when the resistance increased markedly. Except for a brief period on 1 May the deposition was to the leaf cuticle as temperature and water stress caused the stomata to be closed. During the afternoon of 1 May there was some stomatal activity when the canopy resistance to water vapour fell and NO2 flux increased.Finally a simulation of the cumulative sampling method was performed on the data set as described in Businger and Oncley (1990). They calculated a constant of proportionality, b, and showed it to have a value of 0.6 when considering heat and water‐vapour fluxes. A value of b was calculated for NO2 flux using the data set presented here and was found to be 0.58.
A Quasi‐Analytic Model of Droplet and Ccn Lifetimes, Optical and Chemical Characteristics of Ice‐Free Cumulus Clouds
Authors: Baker Marcia B. , Latham John 
Published: February 2007

AbstractWe have formulated a semi‐analytic model of the microphysical and passive chemical development within entraining ice‐free cumulus clouds growing in surroundings characteristic of Hawaii and Montana in summertime. Calculations were made of liquid‐water contents, age distributions of droplets and cloud condensation nuclei (CCN), optical parameters (effective radius and optical depth), the concentration of a passive chemical tracer in both cloud and rainwater; and the variability of the concentration of cloud‐water tracer over the droplet spectrum, all as functions of altitude within the clouds, entrainment scale, droplet number concentration and altitudinal variability in environmental tracer concentration.All of these microphysical and chemical parameters were found to be sensitive to the entrainment scale II. Droplets at any level above cloud base may have been formed on primary nuclei or entrained nuclei, either of which may already have activated droplets which subsequently evaporated as a consequence of entrainment. the average nuclear ages are greater (typically by about 50%) than that of the average droplets, the latter being in conformity with those inferred from observations. the effective radius at a given level was only mildly dependent on II, whereas the optical depth increased rapidly as II increased. the concentration of tracer in both cloud water and rainwater decreased markedly as the entrainment scale increased.
Comparisons of Multi‐Year Statistics of Selected Variables From the Coads and the Nimbus‐7 and Ecmwf Data Sets
Authors: Weare Bryan C. 
Published: February 2007

AbstractCoarse spatial resolution monthly statistics of selected variables of the Consolidated Ocean‐Atmosphere Data Set (COADS) have been compared with Nimbus‐7 satellite analyses and the European Centre for Medium‐range Weather Forecasts (ECMWF) operational analyses. Comparisons are made for the important cloud/climate variables of total cloudiness, surface air temperature and surface relative humidity. to ease these comparisons the COADS variances have been adjusted to account for small numbers of observations, and the COADS temperature and humidity data have been transformed to the 1000 mb level of the ECMWF analyses.The results show that the Nimbus‐7 cloud fractions tend to underestimate total cloudiness for much of the earth, especially in regions of substantial low cloud cover. After account has been made for the different observation heights, the COADS and the ECMWF annual mean temperature statistics are found to be in generally good agreement. However, the annual mean ECMWF relative humidities are significantly lower over large portions of the oceans.The interrelations between the Nimbus‐7 and the COADS total cloudiness and the ECMWF and the COADS lower‐level temperature and relative humidity are compared. Significant regional correlation patterns are evident in all cloud‐surface data pairs. On the other hand, some patterns of correlation using the Nimbus‐7 and the COADS cloud data are quite different, with those of the COADS generally appearing to be the more realistic.
Assessing the Sensitivity of A Land‐Surface Scheme to Parameters Used In Tropical‐Deforestation Experiments
Authors: Henderson‐Sellers A. 
Published: February 2007

AbstractA series of factorial experiments is used to elucidate the factors and multifactor interactions important for the performance of one complex land‐surface scheme in a tropical environment. the BATS (Biosphere‐Atmosphere Transfer Scheme) is found to be sensitive to atmospheric factors which are currently poorly predicted by global climate models (GCMs) and, more importantly, to combinations of factors rendering tuning for good performance when coupled into today's global models (a worthless practice if the tropical area is subjected to different conditions such as those induced by deforestation). Similarly, two‐factor interactions, between coded ecological parameters (here roughness length and short wave, <0.7 μm, vegetation albedo), mean even if the BATS characterization and its driving GCM perform well in today's tropical‐forest environment, altering either or both parameters, as will occur in a deforestation simulation, must greatly diminish confidence in representations of future climate.
The Investigation of Regional Climate Anomalies With A Linear Barotropic Model and an Adjoint Technique
Authors: Robertson Andrew W. 
Published: February 2007

AbstractThe adjoint of a steady‐state linear barotropic model is presented as a tool for diagnosing regional upper‐level vorticity anomalies occurring in general circulation model (GCM) experiments. As a starting point, a simulation of a chosen GCM vorticity anomaly is made using a standard linear barotropic model forced by the corresponding anomalies in the GCM's upper‐level divergence and transient‐forcing fields. the associated adjoint model is then used to interpret this linear simulation over a chosen geographical region, in terms of the linear model's global forcing field. to do this, the adjoint model is forced by a local information source situated over the chosen region. the resulting adjoint solution consists of a global spatial weighting function which can then be used to identify regions of the forcing—local or remote—associated with the regional anomaly. the validity of barotropic linear theory can also be quantified as a by‐product.As an illustration, the model is applied to a large interdecadal anomaly over Europe, arising in a 100‐year doubled CO2 integration of the Hamburg coupled ocean‐atmosphere GCM. Linear barotropic theory is found to account for over 60% of the European interdecadal anomaly amplitude in terms of anomalous divergence and the effects of transients. Although a large part of the effects of anomalous divergence are spatially concurrent with the European anomaly and just upstream of it, remote forcing regions over the east Pacific and tropical Atlantic are also implicated. Anomalous transients are found to play a secondary role. the linear results are compared with some one‐point correlation maps of the GCM's interdecadal variability.
The sea breeze and gravity‐current frontogenesis
Authors: Reible D. D. , Simpson J. E. , Linden P. F. 
Published: February 2007

AbstractThe development of a sea‐breeze circulation into a sea‐breeze front in often observed late in the afternoon. Measurements of horizontal temperature and humidity profiles in a number of sea‐breeze circulation, made using an instrumented light aircraft, are presented. On each of the four days for which data are presented an initial weak horizontal temperature gradient, extending over a distance of 5 to 10 kilometres, developed later in the into a sharp front only a few hundred metres across. The data are discussed in terms of simple gravity‐current frontogenesis models applied to the development of sharp sea‐breeze front from an initial diffuse transition between the air over the land sea. Examination of the equations for frontogenesis, and the result of some previous laboratory experiments on gravity‐current frontogenesis, lead to the proposal that, given sufficient contrast between land and sea temperatures, the formation of a sea‐breeze front depends on the balance between convergent horizontal winds which act to generate a front, and turbulent convective mixing over the land which tends to prevent its formation. the differences in driving force, wind convergence and turbulent intensity are discussed for each day on which data are available and are shown to be consistent with the theory proposed.
Entropy and climate. I: ERBE observations of the entropy production of the earth
Authors: Stephens G. L. , O'Brien D. M. 
Published: February 2007

AbstractThis paper introduces an approximate method for estimating the global distributions of the entropy fluxed flowing through the upper boundary of the climate systems, and provides an estimate of the entropy exchange between the earth and space and the entropy production of the planet. The entropy fluxes are those associated with the exchange of radiation between the earth and space and a specific method is described to derive these entropy fluxes from available measurements of the earth's radiation budget. The distribution of these entropy fluxes and the entropy production derived from these fluxes differ in significant ways from those reported previously. The major source of these differences arises from the incorrect estimate of entropy exchange associated with reflected solar radiation in these earlier studies. This problem is overcome by the application of a simple physical model for the entropy exchange. Radiative‐transfer simulations of the entropy exchange demonstrate that this model reasonably approximates the entropy exchange associated with reflected solar radiation, and that the long‐wave fluxes of entropy are closely approximated by the equivalent black‐body entropy flux. Entropy fluxes calculated from the Earth Radiation Budget Experiment measurements show how the long‐wave entropy flux densities dominate the total entropy fluxes at all latitudes compared with the entropy flux densities associated with reflected sunlight, although the short‐wave flux densities are important in the context of clear sky‐cloudy sky net entropy flux differences. It appears from the analysis presented in this paper that the entropy production of the planet is both constant for the 36 months of data considered and very near its maximum possible value. The mean value of this production is 0.68 × 1015 W K−1 and the amplitude of the annual cycle is approximately 1 to 2% of this value, which is within the uncertainty of our estimate. Climatic fluctuations of entropy production are shown to be less than about 2% of the total production and therefore cannot be detected using the method described in this paper. The production of global entropy is dominated entirely by radiative processes. It remains a considerable challenge to detect these climatic fluctuations above such a large background of entropy production.
A model of the development of droplet effective radius in convective cloud
Authors: Choularton T. W. , Bower K. N. 
Published: February 2007

AbstractA model is presented of a growing cumulus turret which is used to investigate the sensitivity of the droplet effective radius in a convective cloud to variations in the cloud‐base temperature, the liquid‐water content, the vertical wind speed at cloud base, the cloud condensation nucleus (CCN) spectrum entering the cloud base, and dry‐air entrainment. It is found that the existence of large quantities of dry‐air entrainment has a major impact on the evolution of the cloud microphysics. Near to the cloud base the growth of newly nucleated droplets causes a rapid increase in the droplet effective radius with height. Higher in the cloud, dry‐air entrainment results in new nucleation of small droplets at all heights in the cloud and the complete evaporation of some of the pre‐existing droplets. The effect of this is that is that the horizontally averaged effective radius becomes a very weak function of height. The value of the effective radius is most strongly influenced by the CCN spectrum entering through the cloud base. This mean that differences between oceanic and continental clouds are reflected in different values for the effective radius. The effects of entrainment do, however, tend to reduce the sensitivity of the effective radius to the initial CCN spectrum. In addition, the very broad droplet‐size distribution produced by the effects of entrainment may be very important in the development of the ice phase in these clouds. The droplent effective radius produced by the entraining model is less sensitive to temperature than the effective radius predicted by adiabatic growth. The result. The result is also insensitive to differences between the CCN distribution entering the cloud base and the CNC population entrained from the environment within reasonable limits.
An analogue‐dynamical long‐range numerical weather prediction system incorporating historical evolution
Authors: Jianping Huang , Yuhong Yi , Shaowu Wang , Jifen Chou 
Published: February 2007

AbstractIt is convenient to regard a predicted dynamical field as a small disturbance superimposed on a historical analogue field. A Statistical technique can then be used in combination with a dynamical forecast. Using this approach, a coupled atmosphere/earth‐surface analogue‐dynamical model has been formulated and applied to seasonal prediction. This approach facilitates the use of information contained both in the historical data‐set and in the initial field in improving the dynamical model (based solely on the latter) and shows some predictive skill.
Comments on ‘The density of natural ice accretions related to non‐dimensional icing parameters’ by K. F. Jones (January B 1990, 116, 477‐496)
Authors: Levi L. , Prodi F. 
Published: February 2007

AbstractThe results obtained by applying the various equations relating the accreted ice density, p, to the growth parameters are compared with each other and with direct experimental results. Special attention is paid to the equation recently proposed by Jones, in which, taking into account an extensive series of results derived from Mt Washington time experiments, an attempt was made to relate p to several thermal and dynamcial non‐dimensional parameters, πi. It is shown that this equation can lead to large errors, especially at low densities, it the parameter ranges are not those that prevailed in the Mr Washington experiments. This disparity arises from basic defects in the physical arguments used by Jones to establish the dependence of p on related phenomena.Nevertheless, the attempt at relating p explicitly to the main system parameters through the dimensionless products πi is considered to be of interest as a possible basis for new studies directed at obtaining a more general equation for p(πi).
Reply to comments by L. Levi and F. Prodi: Another look at the dependence of ice accretion density on non‐dimensional parameters
Authors: Jones Kathleen F. 
Published: February 2007

AbstractIn response to the comments of Levi and Prodi I have made a comparison between their rime density data and a similar subset of my data. The choice of air temperature rather than accretion temperature to characterize droplet freezing is further explained, and the convective‐heat‐flux π‐product is modified by using the droplet diameter rather than the cylinder diameter as the length scale. A limitation to the regression analysis associated with the form of the function relating ice density to non‐dimensional parameters associated with ice accretion is discussed.
Entrainment in cumulus clouds. I: Thermodynamics and buoyancy
Authors: Telford James W. , Chai Steven K. 
Published: February 2007

AbstractConclusions drawn from thermodynamic composition diagrams are examined in terms of their applicability in real conditions. Displays show the temperature and wet‐bulb temperature of samples calculated at 1000 mb. Additional lines show total mixing ratio and saturation pressure. A point giving the composition where there is a minimum of liquid water in cloud can be taken as a saturation pressure point. Such a point can be selected with a PC ‘mouse’ on the monitor display of the thermodynamic diagram, whereupon the program draws the associated constant virtual‐temperature (density and buoyancy) or the constant temperature line.Buoyancy can then be used to eliminate some sources of outside air as possible components for in‐cloud mixtures. Similarly, constant temperature lines associated with constant‐altitude‐aircraft cloud measurements provide an estimate of the composition which would remain after precipitation removed the condensed water and ice.Examination of Cooperative Convective Precipitation Experiment data show that many cloud compositions consist of cloud‐base air mixed with air remaining from previous clouds. Sometimes this air is from clouds which have precipitated, even though the sampled cloud is not precipitating. Similarly, balloon soundings in clear air may show parcels containing most patches from previous clouds. Such soundings, not representing air near the cloud under study, cannot be used to give unambiguous information about the level of mixing. Studies based on balloon soundings often suggest that the entrained air originates from above the level where the cloud is sampled, but several claim otherwise. The moist air may be important in radiation studies.