bluuebunny/crossref_metadata_2025_split huggingface dataset:
The UCD Advanced Canopy‐Atmosphere‐Soil Algorithm: Comparisons with observations from different climate and vegetation regimes
Authors: Pyles R. David , Weare Bryan C. , Pawu Kyaw Tha 
Published: February 2007

AbstractThe University of California, Davis (UCD), Advanced Canopy‐Atmosphere‐Soil Algorithm (ACASA) is presented and its output is compared with a comprehensive set of observations at six diverse sites. ACASA is a multi‐layer canopy‐surface‐layer model that solves the steady‐slate Reynolds‐averaged fluid flow equations to the third‐order. These equations include an explicit representation of the steady‐state, horizontally homogeneous, diabatic set of vector and scalar fluxes and flux transports. ACASA includes a fourth‐order, near‐exact technique to calculate leaf, stem, and soil surface temperatures and surface energy fluxes at various levels within the canopy. Plan! physiological response to micro‐environmental conditions is also included using Ball‐Berry/von Caemmerer‐Farquhar formulations. Observed energy fluxes and microenvironmental conditions from a grass field in the Netherlands, deciduous and coniferous forests in Canada, tropical pasture and forest in Brazil, and an ancient temperate rainforest in the USA are compared with simulated values.Results indicate that simulated and observed estimates of monthly to annual means of all surface fluxes agree within 95% confidence thresholds for all six sites. Observed and simulated hourly estimates of net radiation are also in excellent agreement for all sites considered. Observed and simulated hourly sensible‐ and latent‐heat flux estimates are in very good statistical agreement in most cases. Differences that exist between ACASA and observed sensible‐and latent‐heat flux estimates are of the same magnitudes as observational uncertainties. Estimates of observed and simulated hourly values of canopy and ground heal storage are within 95% statistical confidence limits of agreement with observations in most cases. Simulated and measured values of daytime intra‐canopy mean wind speed, temperature, and specific humidity agree with 95% confidence within both a tropical and temperate rainforest at all levels. Results also indicate that, in general, ACASA produces flux estimates closer to observations with significantly less scatter than does the Biosphere‐Atmosphere Transfer Scheme. Sensitivity tests show that reducing the vertical resolution, linearizing surface temperature calculations, and/or simplifying the treatment of surface‐layer turbulence each altered mean sensible‐ and latent‐heat flux estimates by amounts that are statistically significant in many cases. Results show that simplifying the model alters flux predictions in manners not simply related to vegetation character, and that using ACASA at its full complexity for all vegetation regimes is warranted. Increasing the vertical resolution beyond 20 layers improved flux predictions at tropical locations but had little impact elsewhere.
The numerical stability of a parametrization of convective momentum transport
Authors: Kershaw R. , Grant A. L. M. , Derbyshire S. H. , Cusack S. 
Published: February 2007

AbstractThe Gregory el at. parametrization of momentum transport by convection is shown to be more prone to numerical instability than other parts of the convection scheme, because of an additional element of numerical diffusion. The limiting Courant number for linear stability is (I + c)−1, where c is the parameter which controls the dependence of the pressure gradient on environment shear. Some revised numerical treatments cure the instability problem but reduce forecast skill, apparently because the diffusive element of the original formulation is beneficial, even though it is artificial. A possible solution to the problem is to use an implicit treatment of the original formulation which is absolutely stable.
Internal wave reflection in uniform shear
Authors: Sutherland B. R. 
Published: February 2007

AbstractIf nonhydrostatic internal waves are of sufficiently large amplitude, they undergo significant dispersion due to interactions between the waves and wave‐induced mean flow. The effect of these interactions is investigated for internal waves propagating upward in a uniformly stratified Boussinesq flow with uniform shear. The sign of the shear is established so that the wave intrinsic frequency increases as the wave packet propagates upward; hence linear theory predicts that the waves should reflect at some level. Fully nonlinear numerical simulations of two‐dimensional wave packets are performed to study the wave‐packet evolution as a function of the initial amplitude and spatial extent of the wave packet. It is shown that if the waves are horizontally periodic, and of sufficiently large amplitude, momentum is permanently deposited to the mean flow at altitudes near, but below, the reflecting level predicted by linear theory. If the waves are horizontally compact, the waves propagate upward well above the reflection level.
The statistical structure of forecast errors and its representation in The Met. Office Global 3‐D Variational Data Assimilation Scheme
Authors: Ingleby N. Bruce 
Published: February 2007

AbstractPrevious studies and different methods of estimating short‐range forecast errors are summarized. Zonally and temporally averaged statistics based on differences of one‐ and two‐day forecasts valid at the same time are presented and an attempt is made to explain many of the features by reference to dynamical concepts.Vertical correlation length‐scale tends to increase with horizontal correlation scale but to be very short in the Tropics; horizontal scale is longest in the Tropics and in the stratosphere. The variations in vertical correlation are much more pronounced for largely balanced variables such as rotational wind and temperature than they are for divergent wind or humidity. The extratropics are dominated by an equivalent barotropic mode with the level of the maximum wind amplitude (and the zero crossing of the temperature correlation) being determined by the tropopause. Surface pressure is negatively correlated with low‐level temperature as expected (except over the Antarctic plateau where it is positively correlated); it is also negatively correlated with temperatures near/above the tropopause in the extratropics.The covariance model used in The Met. Office Global Three‐Dimensional Variational (3D‐Var) Data Assimilation system represents the variation of vertical covariances with latitude reasonably well, but the longer horizontal scales in the stratosphere are not currently reproduced. The implied covariances used operationally have been modified so that the correlation length‐scales, both horizontal and vertical, are somewhat shorter than those direct from the forecast differences. Recent changes to the representation are briefly described, with an indication of their impact on the forecasts. The impacts are significant relative to other changes tested, and the covariance model has played a major role in the successful implementation and subsequent improvement of our 3D‐Var system.
A physical basis for a maximum of thermodynamic dissipation of the climate system
Authors: Paltridge Garth W. 
Published: February 2007

AbstractA mechanism is proposed by which the energy flow through a turbulent medium might be constrained to maximize its dissipation or (equivalently) its thermodynamic efficiency. The mechanism may provide a physical basis for the various findings over the years that the earth‐atmosphere system has adopted a format which maximizes its overall rate of entropy production. The qualitative picture is of a system which, because of the asymmetry of its turbulent fluctuations about the locus of possible steady states determined by energy balance, moves to a preferred steady state and therefore to a preferred turbulent transfer coefficient.
Ammonia dry deposition over grassland‐micrometeorological flux‐gradient measurements and bidirectional flux calculations using an inferential model
Authors: Spindler G. , Teichmann U. , Sutton M. A. 
Published: February 2007

AbstractFor a 17‐day period in late summer 1995 ammonia (NH3) fluxes over an extensive area of semi‐natural ungrazed short grassland were measured and modelled. The measurements were carried out at the Melpitz field research station near Torgau in the state of Saxony in eastern Germany (86 m above sea level, 51°31′N, 12°56′E). For the calculation of the fluxes the micrometeorological flux‐gradient technique was applied using a sensitive continuous‐flow wet annular denuder system for NH3at three heights. Both downward fluxes to the canopy and upward fluxes from the grassland to the atmosphere were observed. On the basis of the direction of the flux in relation to air concentrations, the ammonia canopy compensation point was estimated to vary within the range 0.2 to 3.0 μg m−3. Because the simple ‘canopy/surface resistance’ (Rc) model presupposes a surface concentration of zero for the observed trace gas, which cannot explain bidirectional fluxes, the results of the measurements were compared with a static ‘canopy compensation point‐cuticular resistance’ (Xc–Rw) model, which is able to quantify the stomatal compensation point (Xs) and allows bidirectional fluxes. The agreement between the model and the measured fluxes for the whole period was relatively good. Given realistic ratios of ammonium to proton concentrations (T = [NH4+]/[H+]) for the intercellular fluid of between 150 and 1000 the minimum in the difference was found for Γ ≈︁ 1000 (correlation coefficient r2= 0.23) and even bidirectional fluxes were obtained. Possible reasons for the differences between the measurements and model estimates are discussed.
The organization of tropical convection by intraseasonal sea surface temperature anomalies
Authors: Woolnough S. J. , Slingo J. M. , Hoskins B. J. 
Published: February 2007

AbstractThe organization of convection by intraseasonal sea surface temperature (SST) anomalies is investigated to examine the role that atmosphere‐ocean coupling may play in the maintenance of the Madden‐Julian Oscillation. An atmospheric general‐circulation model with an ocean‐covered surface is used to investigate the response of tropical convection to idealized imposed intraseasonal SST anomalies, and the sensitivity of this response to their propagation speed. The convection is found to be organized on the spatial and temporal scales of the imposed SST anomalies and the location of the maximum in precipitation relative to the SST anomaly is in good agreement with observations, and different from that for a stationary SST anomaly. The magnitude of the precipitation anomalies increases with decreasing propagation speed of the SST anomalies. The role of the free‐tropospheric humidity is crucial for determining the location and magnitude of the precipitation response.
On a fundamental problem in implementing flux‐form advection schemes for tracer transport in 3‐dimensional general circulation and chemistry transport models
Authors: Jöckel P. , von Kuhlmann R. , Lawrence M. G. , Steil B. , Brenninkmeijer C. A. M. , Crutzen P. J. , Rasch P. J. , Eaton B. 
Published: February 2007

AbstractA fundamental difficulty in 3‐D models is addressed which can arise due to inconsistencies between advection schemes and winds. It is shown that a model will fail to meet certain basic criteria, desired of 3‐D transport models, when the density field computed by the advection scheme from the winds differs from the implied density field based on the surface pressure and the sigma (or hybrid) coordinates. To allow a rigorous mathematical formulation, the focus is on the example of a mass flux advection scheme in a model where the winds and surface pressure are derived from different advection schemes (e.g. a spectral scheme in a climate model or a weather centre model); however, in principle the discussion applies to nearly any situation in which the pressure levels change in a model. To illustrate the potential severity of such problems, a mass conserving gridto‐grid transformation scheme is constructed which only uses the current tracer mass mixing‐ratio distribution. It is shown that only one solution exists that is comprehensively valid for any arbitrary tracer distribution, and that this type of correction introduces an additional undesired artificial vertical diffusion component into the model transport that increases with increasing tracer mass mixing‐ratio gradients and may exceed the physical vertical transport itself. It is demonstrated that the results of any supplementary fix, either mass fixer or grid‐to‐grid transformation, are generally unacceptable for global modelling applications. From this, it is concluded that the only alternative which can produce reliable results for any arbitrary tracer is to maintain a consistent grid throughout the entire model time step, where all changes in pressure levels due to modelled advection exactly match the changes implied by the surface pressure at the next time step. Although this is already done in some models, this would require significant changes in the structure of the advection scheme or its input wind fields in several other contemporary general circulation and chemistry transport models.
Assessment of which scales of orography can be credibly resolved in a numerical model
Authors: Davies L. A. , Brown A. R. 
Published: February 2007

AbstractIdealized simulations are performed in order to examine which scales of orography (relative to the grid scale) can credibly be represented in numerical models. A wide range of flow regimes is considered. These include neutral flow over small hills embedded in the boundary layer and highly stable flow over and around large mountains. In none of these cases do simulations with only two grid points per hill show any appreciable skill. Four and six points per hill give qualitatively reasonable results, but still tend to underestimate the drag relative to that obtained in the best resolved simulations. Implications for the smoothing of the resolved orography fields (and for the inputs to the sub‐grid parametrizations) in large‐scale models are discussed.
Observing‐system experiments in the ECMWF 4D‐Var data assimilation system
Authors: Bouttier F. , Kelly G. 
Published: February 2007

AbstractA set of global observing‐system experiments is run by excluding classes of observations from the European Centre for Medium‐Range Weather Forecasts (ECMWF) four‐dimensional variational (4D‐Var) data assimilation and forecast system. This indicates how efficiently the observations are used in this particular system. The observing systems tested were mainly TIROS (Television Infra‐Red Observation Satellite) Operational Vertical Sounder (TOVS) radiances, SATellite OBservation (SATOB) atmospheric‐motion winds, radiosondes, aircraft, drifting buoys and Australian pseudo observations (PAOBs). The impact is assessed in both summer and winter 1999, over a total period of six weeks, in terms of the average objective quality of the deterministic forecast of tropospheric wind and geopotential height at medium (5–7 day) and short (1–3 day) ranges. It is shown that the choice of verification method matters in data‐poor areas such as the tropics and the southern hemisphere.Some interesting conclusions can be drawn about the ECMWF 4D‐Var system. Each observing system tested has a notable positive impact on the medium‐range forecast performance. This is not always true at shorter ranges. The TOVS radiances have a large impact on all areas and ranges. Drifting buoys and PAOBs both have some impact in the southern hemisphere. In the northern hemisphere, the relative impact of TOVS, aircraft and radiosondes depends on the area considered. For Europe, the statistical significance of the results is limited, but the most important observing system among those tested appears to be the radiosondes.
The glaciation of a cumulus cloud over New Mexico
Authors: Phillips Vaughan T. J. , Blyth Alan M. , Brown Philip R. A. , Choularton Thomas W. , Latham John 
Published: February 2007

AbstractThe Met Office Cloud Resolving Model (CRM) and the UMIST Explicit Microphysics Model (EMM) have been employed in the analysis of data from airborne studies of a multi‐thermal cumulus cloud which developed over New Mexico in the summer of 1987. The principal goal was to establish a quantitative understanding of the observed development of glaciation of this cloud.The EMM was utilized in a series of tests designed to assess the sensitivity of cloud glaciation via the Hallett‐Mossop (H‐M) process to cloud parameters such as the concentration of cloud condensation nuclei, the cloud‐base temperature, entrainment, and the freezing and splintering of supercooled raindrops. These tests with the EMM demonstrate that reductions in the mean droplet diameter can inhibit the rates of H‐M splinter production and auto‐conversion, reducing the rate of accumulation of precipitation at the ground and reducing the concentration of ice particles. The warm‐rain process in the EMM is fundamental to the production of graupel, H‐M splinters and precipitation.Good agreement was found between the predictions of the CRM and the available dynamical and microphysical field observations. Analysis of results from both models indicated that the cloud glaciation is explicable in terms of the H‐M process, with ice production being dominated by the freezing of supercooled raindrops in the H‐M band, and the immediate and continuous production of ice splinters as supercooled droplets freeze onto them.
Wavelet analysis and the governing dynamics of a large‐amplitude mesoscale gravity‐wave event along the East Coast of the United States
Authors: Zhang Fuqing , Davis Christopher A. , Kaplan Michael L. , Koch Steven E. 
Published: February 2007

AbstractDetailed diagnostic analyses are performed upon a mesoscale numerical simulation of a well‐observed gravity‐wave event that occurred on 4 January 1994 along the East Coast of the United States. The value of using wavelet analysis to investigate the evolving gravity‐wave structure and of using potential vorticity (PV) inversion to study the nature of the flow imbalance in the wave generation region is demonstrated. The cross‐stream Lagrangian Rossby number, the residual in the nonlinear balance equation, and the unbalanced geopotential‐height field obtained from PV inversion are each evaluated for their usefulness in diagnosing the flow imbalance. All of these fields showed clear evidence of strong imbalance associated with a middle‐to‐upper tropospheric jet streak, and tropopause fold upstream of the large‐amplitude gravity wave several hours before the wave became apparent at the surface.Analysis indicates that a train of gravity waves was continuously generated by geostrophic adjustment in the exit region of the unbalanced upper‐level jet streak as it approached the inflection axis in the height field immediately downstream of the maximum imbalance associated with the tropopause fold. A split front in the middle troposphere, characterized by the advance of the dry conveyor belt above the warm front, was overtaken by one of these propagating waves. During this merger process, a resonant interaction resulted, which promoted the rapid amplification and scale contraction of both the incipient wave (nonlinear wave development) and the split front (frontogenesis). The gravity wave and front aloft became inseparable following this merger. The situation became even more complex within a few hours as the vertical motion enhanced by this front‐wave interaction acted upon a saturated, potentially unstable layer to produce elevated moist convection. An analysis of the temporal changes in the vertical profile of wave energy flux suggests that moist convective downdraughts efficiently transported the wave energy from the midlevels downward beneath the warm‐front surface, where the wave became ducted. However, pure ducting was not sufficient for maintaining and amplifying the waves; rather, wave‐CISK (Conditional Instability of the Second Kind) was crucial.This complex sequence of nonlinear interactions produced a long‐lived, large‐amplitude gravity wave that created hazardous winter weather and disrupted society over a broad and highly populated area. Although gravity waves with similar appearance to this large‐amplitude wave of depression occasionally have been seen in other strong cyclogenesis cases involving a jet streak ahead of the upper‐level trough axis, it is unknown whether other such events share this same sequence of interactions.
A scattering phase function for ice cloud: Tests of applicability using aircraft and satellite multi‐angle multi‐wavelength radiance measurements of cirrus
Authors: Baran A. J. , Francis P. N. , Labonnote L.‐C. , Doutriaux‐Boucher M. 
Published: February 2007

AbstractA parametrization of the Henyey‐Greenstein scattering phase function is presented for application to aircraft and satellite remote sensing of cirrus‐cloud bulk and microphysical properties at non‐absorbing and absorbing wavelengths. The phase function is based initially around a non‐absorbing laboratory‐measured phase function between scattering angles of 30° and 180°, although at scattering angles greater than 95° the phase function is made constant with scattering angle. This ‘analytic’ phase function has an asymmetry parameter value of 0.80 at non‐absorbing wavelengths. This analytic phase function is tested using aircraft‐based transmission radiance measurements at the wavelengths of 0.55 μm, 0.87 μm, 1.6 μm and 3.7 μm between scattering angles of 10° and 120°, and satellite retrievals of cirrus‐cloud spherical albedo at the wavelength of 0.67 μm between scattering angles of 60° and 180°. The analytic phase function reproduces both aircraft and satellite measurements well, and is shown to be a distinct improvement over single‐crystal model phase functions representing both small and large crystals in terms of angular scattering pattern and retrieved optical thickness.
A theory for the estimation of surface fluxes in simple katabatic flows
Authors: Grisogono Branko , Oerlemans Johannes 
Published: February 2007

AbstractThe classical Prandtl model describes the basic dynamics of katabatic flows, but it fails in determining the gradients near the surface because the eddy diffusivity, K, is independent of height, z. An improvement is proposed to the Prandtl model by using the WKB method to solve the dynamic equation for any prescribed, reasonably smooth, K(z) profile.The surface fluxes are obtained by extrapolation from the katabatic jet down to the surface, thus the fluxes depend primarily on the jet which is well captured by the WKB method. The profiles of wind, temperature, momentum and heat fluxes, as well as the surface fluxes calculated using the model, are tested against observations from a meteorological field campaign on the Pasterze glacier, Austria, in 1994, and their numerical simulations. Generally there is a good agreement between observations and theory.