bluuebunny/crossref_metadata_2025_split huggingface dataset:
Combination theory and equilibrium evaporation
Authors: Raupach M. R. 
Published: February 2007

AbstractThis paper is an analysis of equilibrium evaporation and its role in the energy balance of a terrestrial surface, as described by combination theory. Three themes are covered: first, a brief historical review identifies multiple definitions of the concept of equilibrium evaporation. Second, these are formalized by developing the basic principles of combination theory with minimum approximation. Several measures are utilized to do this: linearization is avoided, radiative and storage coupling are incorporated systematically, and actual and linearized saturation deficits are distinguished. The formalism is used to analyse several algebraically defined states and limits for the surface energy balance. Third, the thermodynamic foundation of equilibrium evaporation is analysed by studying surface‐atmosphere feedbacks in arbitrary closed and open evaporating systems. It is shown that under steady energy supply any closed evaporating system evolves towards a quasi‐steady state in which the Bowen ratio takes the equilibrium value 1/εv, where εv is the ratio of the latent‐ and sensible‐heat contents of saturated air with temperature, evaluated at the volume‐averaged temperature in the closed system. This applies whether the system is well‐mixed or imperfectly mixed, and whatever the internal distribution of surface fluxes and surface and aerodynamic resistances. In contrast, open systems cannot reach such an equilibrium. This evolutionary definition of equilibrium evaporation differs from an alternative algebraic definition, the fully decoupled limit. The differences between the two definitions are identified, and the evolutionary definition is shown to be more fundamental. Thus, the correct temperature for evaluating ε in determining equilibrium evaporation is the volume‐averaged temperature in a closed region, which in the case of a convective boundary layer is well approximated by the mixed‐layer temperature.
Impact of field‐calibrated vegetation parameters on GCM climate simulations
Authors: Sen Omer L. , Bastidas Luis A. , Shuttleworth W. James , Yang Zong‐Liang , Gupta Hoshin V. , Sorooshian Soroosh 
Published: February 2007

AbstractThis paper describes a study in which, for the first time, advanced systems‐engineering parameter‐estimation techniques were applied to data from several field studies to estimate the preferred set of parameters for some of the most common biomes represented in an advanced Soil‐Vegetation‐Atmosphere Transfer (SVAT) scheme (BATS2, a recent version of the Biosphere‐Atmosphere Transfer Scheme); the effect on modelled climate was also investigated. Observational data from field sites in Brazil, Canada, Arizona and Kansas/Oklahoma in the USA, and the Netherlands were chosen as representative of tropical rain forest, coniferous forest, semi‐arid vegetation, agricultural crops, and grassland biomes, respectively. Together, these five biomes make up 50% of the land area represented in BATS2. Multi‐criteria calibration algorithms do not produce a unique set of model parameters and, when different combinations of the available objective functions at each site are considered, the number of solutions increases substantially. The need for a single parameter‐set for each site (biome) is an important practical issue that was necessarily addressed in this study. A procedure was defined in which optimized parameter‐sets were successively discarded by successively applying a cut‐off threshold to single observable objective functions following a preference hierarchy. In this study, only the vegetation‐related parameters are calibrated for each of the five biomes and implemented into BATS2; however, in a separate experiment, the effect of including soil parameters in the optimization was investigated. When the calibrated parameters are adopted and used in BATS2, there are significant changes between the climates calculated in an eight‐year run with Version 3 of the Community Climate Model and in an equivalent eight‐year run in which the original default parameters were used. The overall conclusion of this exploratory study is that advanced parameter‐estimation techniques and appropriate field data can be used successfully to improve representation of surface exchanges and the modelled climate given by a GCM, by defining appropriate values for vegetation‐related parameters in an advanced SVAT scheme.
Baroclinic developments in jet entrances and exits. II: Nonlinear development
Authors: Hare Sylvia H. E. , James I. N. 
Published: February 2007

AbstractThis study examines the effect of the ageostrophic circulations associated with jet entrance and exit regions on the nonlinear development of normal modes. The nonlinear growth and decay phases of the baroclinic life cycle and, in particular, the vertical propagation and subsequent meridional advection of Rossby waves are very sensitive to the exact form of the zonally symmetric basic state. This leads to changes in the associated energetics and eddy fluxes. In order to represent the passage of a disturbance though a jet, a time‐varying zonally symmetric meridional circulation was implemented. Many of the factors identified in the time invariant work, such as the normal‐mode latitudinal and meridional advection, were again found to be important. Ageostrophic meridional circulations may play an important role in determining the magnitude and length of mid‐latitude storm tracks.
Extratropical low‐frequency variability as a low‐dimensional problem I: A simplified model
Authors: D'Andrea Fabio , Vautard Robert 
Published: February 2007

AbstractIt is shown that the low‐frequency and large‐scale variability of an intermediate complexity reference model can be reproduced faithfully by a simplified model of 10 independent variables and 10 equations. The reference model is quasi‐geostrophic and baroclinic. The low‐order model is based on the truncated projection of the reference‐model equations on the empirical orthogonal functions of its output. A closure term is shown to be essential for good performance of the low‐order model. This closure term is meant to reproduce all the neglected scales and all the scale interactions, mainly baroclinic eddy forcing, that drive the large‐scale flow. The closure is built as an empirically defined function of the large‐scale flow of the model, relying on an extensive previously computed library of tendency differences between the full and truncated model. Two other parametrization schemes, a time‐mean and a stochastic one, are tested; comparisons of these two with the former underline the importance of the flow dependence of the formulation. The low‐order and reference models exhibit the same climate, as well as the same low‐frequency variability and weather regimes.
Ice nucleation in orographic wave clouds: Measurements made during INTACC
Authors: Field Paul R. , Cotton R. J. , Johnson D. , Noone K. , Glantz P. , Kaye P. H. , Hirst E. , Greenaway R. S. , Jost C. , Gabriel R. , Reiner T. , Andreae M. , Saunders C. P. R. , Archer A. , Choularton T. , Smith M. , Brooks B. , Hoell C. , Bandy B. , Heymsfield A. 
Published: February 2007

AbstractThis paper reports on measurements made during the INTACC (INTeraction of Aerosol and Cold Clouds) experiment. Observations are shown from six wave cloud flights in the temperature range −12°C to −40 °C. In nearly all cases ice nucleation does not occur until after droplets have formed. Parcel trajectories that experience temperatures below −35°C are dominated by homogeneous nucleation, while parcel trajectories that stay above −35°C are affected by heterogeneous nucleation. Ice particles appear to be nucleated after the parent droplets reach their largest size and coldest temperature at the wave peak. The most likely heterogeneous nucleation mechanisms are immersion and/or contact nucleation.
Polar stratospheric cloud impacts on Antarctic stratospheric heating rates
Authors: Hicke Jeffrey , Tuck Adrian 
Published: February 2007

AbstractThe impact of polar stratospheric clouds (PSCs) on the stratospheric radiative heating rate is analysed by including a nominal PSC in heating‐rate calculations that incorporate realistic atmospheric variables including tropospheric clouds. The use of realistic atmospheric conditions constrains the possible radiative effects of PSCs, which previous studies have shown to be very sensitive to such variables as temperature, tropospheric clouds, and solar zenith angle. Over the pole, winter heating rates within the stratospheric polar vortex are decreased substantially by the presence of a PSC, while a PSC increases the heating rates equatorward of 75–85°S. Although the PSC always increases the short‐wave heating, the effect in the long‐wave region depends on the ground temperature, the stratospheric temperature, and presence of a tropospheric cloud. For the thickest PSCs (Type II), the effect in August 1994 varies from cooling by 0.25 K d−1 (potential‐temperature difference Δθ = 0.5 K d−1) at the pole to heating by 0.3 K d−1 (Δθ = 0.6 K d−1) at 65°S to slight cooling equatorward of 57°S. September 1994 results are similar. Calculated heating rates over the pole including PSCs are near –0.5 K d−1 (θ = –1 K d−1) for both months, and positive heating rates of up to 0.25 K d−1 (θ = 0.5 K d−1) occur near the vortex edge. Thinner PSCs (Type I) have less of an effect; for example, heating rates of 0.375 K d−1 (θ = 0.75 K d−1) occur over the pole in August when a Type I PSC is included. These results should be viewed as an upper bound to the effect of PSCs since the calculations specify 100% PSC cover; satellite results show that this assumption is not unreasonable within the vortex during winter and early spring, however. The increased latitudinal gradient in descent rates in the presence of a PSC is consistent with the behaviour of long‐lived trace‐gas observations, and strengthens the vortex relative to a PSC‐free case.
Canonical frontal circulation patterns in terms of Green's functions for the Sawyer‐Eliassen equation
Authors: Hakim G. J. , Keyser D. 
Published: February 2007

AbstractGreen's function solutions for the Sawyer‐Eliassen equation are derived and compared for quasi‐geostrophic and semi‐geostrophic two‐dimensional frontal dynamics in vertically bounded domains. The semi‐geostrophic Green's functions provide a framework for understanding canonical frontal circulation patterns arising from interactions, or coupling, between upper‐ and lower‐tropospheric jet‐front systems. Previously documented conceptual models for these circulation patterns are reproduced by the solutions. A noteworthy finding is that vertical coupling produces a deep and narrow region of ascent when the upper‐ and lower‐tropospheric point sources for the Green's functions share the same sign and absolute momentum surface.
Correlation modelling on the sphere using a generalized diffusion equation
Authors: Weaver Anthony , Courtier Philippe 
Published: February 2007

AbstractAn important element of a data assimilation system is the statistical model used for representing the correlations of background error. This paper describes a practical algorithm that can be used to model a large class of two‐ and three‐dimensional, univariate correlation functions on the sphere. Application of the algorithm involves a numerical integration of a generalized diffusion‐type equation (GDE). The GDE is formed by replacing the Laplacian operator in the classical diffusion equation by a polynomial in the Laplacian. The integral solution of the GDE defines, after appropriate normalization, a correlation operator on the sphere. The kernel of the correlation operator is an isotropic correlation function. The free parameters controlling the shape and length‐scale of the correlation function are the products kpT, p = 1, 2, …, where (‐1)pkp is a weighting (‘diffusion’) coefficient (kp > 0) attached to the Laplacian with exponent p, and T is the total integration ‘time’. For the classical diffusion equation (a special case of the GDE with kp = 0 for all p > 1) the correlation function is shown to be well approximated by a Gaussian with length‐scale equal to (2k1T)1/2.The Laplacian‐based correlation model is particularly well suited for ocean models as it can be easily generalized to account for complex boundaries imposed by coastlines. Furthermore, a one‐dimensional analogue of the GDE can be used to model a family of vertical correlation functions, which in combination with the two‐dimensional GDE forms the basis of a three‐dimensional, (generally) non‐separable correlation model. Generalizations to account for anisotropic correlations are also possible by stretching and/or rotating the computational coordinates via a ‘diffusion’ tensor. Examples are presented from a variational assimilation system currently under development for the OPA ocean general‐circulation model of the Laboratoire d'Oceanographie Dynamique et de Climatologie.
Observations of atmosphere‐ocean coupling in the North Atlantic
Authors: Czaja Arnaud , Marshall John 
Published: February 2007

AbstractAnindex of sea surface temperature (SST) variability, δT, is introduced that measures the difference in SST across the separated Gulf Stream in late winter. By analysing a long observational record of SST and sea‐level pressure (SLP), it is shown that δT exhibits damped oscillations of decadal period, and covaries with the strength of a dipolar SLP anomaly reminiscent of the North Atlantic Oscillation (NAO). Analysis in the frequency domain shows a broad‐band ‘peak’ at 10–20 years in δT, with a continuous decrease of power on longer time‐scales. Similar spectral signatures are found in the northern part of the SLP dipole (the Greenland‐Icelandic Low region) but not in its southern part (the subtropical High region), whose power increases on long time‐scales.The observations are interpreted in the framework of a delayed‐oscillator model in which the ocean circulation introduces the delay, and modulates δT on decadal time‐scales. The decrease of power seen on long time‐scales (>25 years) in the δT index is captured by a model including wind‐driven ocean circulation, and arises primarily as a passive response of the latter to the NAO forcing. Variability of the ocean's meridional overturning circulation could also play a role in modulating δT on decadal time‐scales. If a small feedback of δT on the NAO pattern is introduced, the simple model can also reproduce the spectral structures seen in the SLP anomaly in the Greenland‐Iceland region.
Measurements of dissipation rate in frontal zones
Authors: Chapman D. , Browning K. A. 
Published: February 2007

AbstractThis paper demonstrates that it is possible to use measurements of the Doppler spectrum width from a high‐resolution radar to obtain quantitative estimates of dissipation rate during the passage of precipitating frontal zones. Using the high‐resolution Chilbolton radar, this method has been applied, for the first time, to infer the rate at which kinetic energy is dissipated by turbulence within mid‐ and lower‐tropospheric frontal zones. Analysis of detailed measurements for one cold‐frontal region showed multiple shallow sheets of high dissipation rate, with spatially averaged values around 1 x 10−3 m2s−3. Comparable values were also observed in a warm‐frontal zone. The magnitude of the observed dissipation rate supports the hypothesis that mixing played a major role in determining the mean properties of the fronts, in that the turbulence acted to destroy the frontal shear layer on a time‐scale of several hours. These issues have particular relevance to the representation of fronts within future very high‐resolution numerical weather‐prediction models.
Two‐level primitive‐equation baroclinic instability on an f‐plane
Authors: Fraedrich Klaus , Frisius Thomas 
Published: February 2007

AbstractBaroclinic instability of a two‐level primitive equation model is represented by a dynamical system consisting of the four interacting fields of barotropic and baroclinic vorticity, divergence and temperature. The instability mechanism is illustrated in terms of a positive feedback loop with a succession of tendencies. It occurs at wavelengths longer than the short wave cut‐off, for which vortex stretching prevails over vertical temperature advection. Quasi‐geostrophy reduces the positive feedback loop to the interaction of the barotropic and baroclinic vorticity fields which are diagnostically linked to divergence and temperature by the omega equation and by geostrophy, respectively.
Retrieval of microphysical and morphological properties of volcanic ash plumes from satellite data: Application to Mt Ruapehu, New Zealand
Authors: Prata A. J. , Grant I. F. 
Published: February 2007

AbstractA quantitative analysis of the properties of several Mt Ruapehu, New Zealand, ash plumes has been performed using multispectral satellite data from the AVHRR‐2 and ATSR‐2 instruments. The analysis includes: identification of the plume from background clouds using the ‘reverse’ absorption effect in the thermal channels: modelling and retrieval of particle sizes; determination of the plume height from cloud shadows, stereoscopy and meteorological data; and estimates of the mass of fine particles (radii less than 10 μm). A new spectral technique for identifying opaque, silica‐rich ash clouds is demonstrated by utilizing the near‐infrared (1.6 μm) and visible (0.67 μm) channels of the ATSR‐2, and the optical properties of a simple volcanic cloud are presented for use in radiative transfer studies. It is found that the Ruapehu eruption cloud contained silica‐rich ash particles with radii generally less than a few micrometres. The distribution of fine particles is monomodal with a dominant mode peak of about 3 μm radius. Mass loadings of fine particles are found to be in the range ≈︁1 to ≈︁7 mg m−3, and are consistent with estimates of mass loadings of volcanic clouds from eruptions of other volcanoes. The height of the plume top, derived from radiosonde data and plume‐top temperatures in the opaque regions, was found to be between 7.5 and 8.5 km, while the plume thickness was estimated to be between 1.5 and 3 km. Cloud height derived from ATSR‐2 stereoscopy on a different plume gave heights in the range 5 to 8 km.The results of this study provide important information on the optical properties of nascent volcanic eruption plumes. This information may prove useful in determining the potential effects of volcanic clouds on local climate, and in assessing any hazard to aviation.
Sensitivity studies using a cloud‐resolving model simulation of the tropical west Pacific
Authors: Petch J. C. , Gray M. E. B. 
Published: February 2007

AbstractIt is important to understand the variability of results which can be produced in a cloud‐resolving model (CRM) before using its output to develop parametrizations. In the work presented here a broad range of sensitivity experiments are carried out so that the significance of each change to the model can be put into context. Changes to the parametrization of microphysics and radiation in the CRM are considered along with the influence of dimensionality, horizontal resolution and domain size. Further detailed sensitivity studies are then carried out to investigate the impact of the spatial and temporal distribution of the radiative heating.The results presented show that, out of all the sensitivity studies considered, no particular feature or physical parametrization in a CRM dominates. The domain size, horizontal grid length, use of a third dimension and parametrization of microphysics all significantly impact the simulation in some way. Radiative heating has a large impact on the simulation because it dominates over the large‐scale forcing above 8 km. However, the magnitude of this impact is strongly dependent on other aspects of the model, such as the choice of microphysical scheme. A diurnal cycle, which is inherent in the large‐scale forcing, is not significantly enhanced by radiation in the CRM. However, accounting for the temporal and spatial variability in the radiative heating is important because neglecting this is shown to overestimate the strength of convection.
Symmetrization of lopsided vorticity monopoles and offset hurricane eyes
Authors: Prieto R. , Kossin J. P. , Schubert W. H. 
Published: February 2007

AbstractThis paper is a contribution towards better understanding of the continually occurring symmetrization processes in tropical cyclones. Before a tropical cyclone develops an eye, it can often possess an asymmetric monopolar vorticity distribution. As an idealized initial condition in a non‐divergent barotropic model, we use a lopsided monopole with different degrees of asymmetry of the monotonic vorticity field. We then study the axi‐symmetrization process which involves the ejection of a winding spiral band. For extreme asymmetric initial conditions, the band can produce regions of barotropic instability, resulting in nonlinear mixing of vorticity and the formation of polygonal structures. In a second series of experiments, we study the case of a tropical cyclone with a developed eye, modelled as a hollow‐tower vorticity distribution, i.e. an annular region of elevated vorticity, with low vorticity in the eye. If the eye is offset and the annular region of elevated vorticity is not of uniform width, complex symmetrization processes can occur, sometimes leading to a tripole structure of the hurricane's vorticity field. Long‐lived hurricane eyes are found for initial conditions with a slight offset. For such initial conditions, passive tracers can remain in the eye for as long as 72 hours, showing that in this model it is possible for air inside the eye to remain there for long periods of time, while moving coherently with the storm. Predictions of the axi‐symmetric final equilibrium states of the flow are obtained using the statistical mechanics theory of maximum Boltzmann mixing entropy. These predictions are then compared with results from the direct numerical integrations for both the lopsided monopole and the offset hurricane eye. The distribution of air‐parcel tracers initially placed in selective regions of vorticity for the direct numerical integrations are compared with results from the statistical theory.
Possible links between cloud optical depth and effective radius in remote sensing observations
Authors: Boers R. , Rotstayn L. D. 
Published: February 2007

AbstractThe theoretical relationship between cloud optical depth and effective radius is examined. The relationship between cloud optical depth and effective radius is shown to depend on (a) the relative variability of droplet concentration, cloud depth, mixing and precipitation, and (b) the correlation between the two most important parameters, namely cloud depth and droplet concentration. It is possible to obtain positive, negative or zero correlation between optical depth and effective radius for different values of (a) and (b). Of all these parameters, mixing appears to be the least important. If clouds are not affected by precipitation, negative correlations between optical depth and effective radius (the expected signature of the first indirect aerosol effect) can be caused by a small spread in cloud depths relative to that in droplet concentration, or (more importantly) by a positive correlation between droplet concentration and cloud depth.Precipitation tends to reduce the correlation between optical depth and effective radius, because one of the main effects of precipitation is to increase the spread in droplet effective radius. If the effect of increases in aerosols is a reduction in precipitation, our results indicate that the correlation coefficient between optical depth and effective radius should increase, an effect that should be most clearly visible over the ocean. These findings illustrate the complexity of the physical processes that underpin the linkage of optical depth and effective radius.