bluuebunny/crossref_metadata_2025_split huggingface dataset:
A comparison of two radiation schemes for calculating ultraviolet radiation
Authors: De Forster Piers M. F. , Shine Keith P. 
Published: February 2007

AbstractIncreases in ultraviolet radiation (UV) caused by ozone depletion in the stratosphere are expected to have physiological effects on plants and animals. Biologists require that the UV data have high wavelength resolution to assess these effects adequately. Numerical simulations of scattering and absorption in the atmosphere provide a useful way of predicting the UV irradiance reaching the ground and the results from such radiative transfer calculations are presented. Two different methods were used to calculate the diffuse irradiances: the Discrete‐Ordinate (D‐O) method and the Delta‐Eddington (D‐E) approximation. Surface UV irradiances for a wide variety of atmospheric conditions, obtained with the D‐O method were compared with those obtained using the D‐E approximation to assess how changes in tropospheric aerosol and cloud affect the accuracy of the D‐E approximation. Stratospheric aerosol has been shown to be capable of increasing the UV irradiances at the ground, the magnitude of these increases depending heavily on the absorption by ozone of multiply‐scattered photons. the prediction of these increases by means of numerical models provides a good test of the accuracy of the radiative transfer approximations used. It is shown that the D‐E approximation appears to be deficient for the prediction of surface irradiance at high spectral resolution, under certain circumstances.
Observations of the turbulent fluxes in fields of cumulus clouds
Authors: Smith Samantha A. , Jonas Peter R. 
Published: February 2007

AbstractAircraft observations in fields of small maritime cumulus clouds around the British Isles have been analysed to yield kinetic energy spectra and vertical turbulent fluxes, both within clouds and in the clear regions between clouds, for five cases where a cool airmass flowed over a warmer sea surface. the flights were made at various times of the year. Profiles of the kinetic energy and turbulent fluxes have been obtained from observations made at several levels. It is shown that, while the dominant source of turbulent kinetic energy may be due to evaporative cooling following entrainment at cloud top, in some cloud fields production by latent heating near cloud base dominates. the location of the dominant energy source depends on whether the clouds penetrate the inversion sufficiently for instability due to cloud‐top entrainment to occur. While the resulting flux profiles observed in some of the cases examined here resemble those observed in stratocumulus, profiles of similar shape are not always observed, and there are significant differences between cumulus layers and stratocumulus in the magnitudes of the fluxes.
Meso‐β‐scale circulations in realistic fronts. II: Frontogenetically forced basic states
Authors: Fischer Claude , Lalaurette François 
Published: February 2007

AbstractThis paper deals with the mesoscale circulations that may appear in a frontogenetically forced flow. Two mechanisms are studied: conditional symmetric instability (CSI) and transient adjustment of a finite momentum anomaly. the study is focused on two situations selected from frontal events that were documented during the MFDP/FRONTS 87 field experiment.In Part I of this paper, a successful numerical simulation of CSI during the second intensive observing period (IOP2) was presented. However, the growth rate of the perturbation was very small (≈ f/4) and 42 hours were needed before vertical velocities reached their maximum value ≈5 cm s−1. Therefore, interactions between CSI and the large‐scale frontogenetic forcing are expected; these are investigated in this Part. CSI is released (without significant increase of the growth rates) for small forcing only since confluence is found to shrink the area fulfilling the pure CSI criterion rapidly. It is thus not likely to occur in active fronts.The IOP7 is then investigated. This very anisotropic front was embedded in a strong large‐scale confluence. the creation and maintenance of the observed ‘Wide Cold‐Frontal Rainband’ (WCFR) (some 100 km wide) above the sloping frontal surface and behind the surface cold front are investigated. Using a hierarchy of threedimensional (3D) and two‐dimensional (2D) versions of the model, this band is shown to be produced by the inertial, transient adjustment of low pseudo‐angular‐momentum air that was advected into the mid‐troposphere by the frontal convective jet. the 3D, as well as the 2D, forced simulations produce a negative equivalent potential vorticity chimney‐like area which matches well with the observational data. This area evolves on the same time‐and space‐scales as the roll‐like circulation associated with the WCFR. This mechanism bears similarities with mature CSI and their distinction in diagnostic studies may be difficult. However, the presence of a momentum source (the frontal convection) is found here to be the important criterion.
Comparison between the orographic response of the ECMWF model and the pyrex 1990 data
Authors: Lott Francois 
Published: February 2007

AbstractRecent observations and analyses of the atmospheric flow in the vicinity of the Pyrénées made during PYREX are compared with numerical experiments carried out with the T213L31 European Centre for Medium‐range Weather Forecasts (ECMWF) global model. This study shows that the model simulates a certain proportion of the measured pressure drag, but always underestimates it. In the current model, this deficit is not adequately made up by any of the parametrized subgrid‐scale processes. When the flow is along the range, the pressure‐drag value depends on the synoptic background flow structure. It is close to zero when the flow impinging on the range is baroclinic. It is not zero when the atmosphere is more barotropic (e.g. in the absence of any fronts) and the model misses important lift processes. When the flow is normal to the ridge, the model response to the mountain forcing is essentially ageostrophic. In this case comparison between the vertical profiles of the model stresses and the measured profile suggests that further drag should be applied in the model at low level. This is supported by comparisons of isentropic diagnostics, from the ECMWF model and the French mesoscale model, Peridot. These diagnostics show that the ECMWF model underestimates irreversible low‐level processes, such as the flow separation on the mountain flanks and the associated downstream wake, which can be related to the drag. They also show that low‐level parametrized drags have a realistic impact on the model simulation of the mountain wake. This analysis supports the current developments at the ECMWF of a new subgrid‐scale orographic drag scheme which parametrizes the low‐level impact of mesoscale mountains.
Simulation of climate change over europe using a nested regional‐climate model. I: Assessment of control climate, including sensitivity to location of lateral boundaries
Authors: Jones R. G. , Murphy J. M. , Noguer M. 
Published: February 2007

AbstractPresent‐day climate simulations for Europe are presented, based on a 50 km regional‐climate model (RCM) driven by output from a global general‐circulation model (GCM) using a one‐way nesting approach. Both models are components of the Meteorological Office Unified Forecast/Climate Model and use the same subgrid‐scale physics. the relationship between the RCM circulation and that of the driving GCM was assessed in seasonal RCM integrations using domains of different sizes. In the larger domains, both the mean flow and the day‐to‐day variability in the RCM diverge from that of the GCM on the synoptic scale, rendering the RCM solution physically inconsistent with the GCM solution external to the RCM domain. At the grid‐point scale the RCM freely generates its own features, even in the smaller domains—only at points adjacent to the boundary buffer zone is there evidence of significant distortion by the lateral boundary forcing from the GCM.Using one of the smaller domains, a 10‐year RCM simulation was carried out, driven by a coupled atmosphere/mixed‐layer‐ocean version of the GCM. Over the region of interest the general circulation and daily synoptic variability is realistically simulated by the GCM and, therefore, also by the RCM (see above). Stronger vertical motions in the RCM lead to a general increase in dynamical precipitation relative to the GCM, and thus a drier and warmer troposphere and reduced convective cloud and precipitation. Layer‐cloud cover is also reduced in the RCM, due to a time‐step dependence in the treatment of the dissipation of ice cloud. Significant changes occur in the surface heat balance. the spatial patterns of surface air temperature and precipitation over Europe are well simulated by both the GCM and the RCM on scales resolved by the former. At finer scales the RCM contains a strong signal which is related to orographic height. Validation against a detailed observed climatology for Great Britain demonstrates that this signal contains considerable skill.
The effect of orography on wet deposition in an industrial area
Authors: Inglis D. W. F. , Choularton T. W. , Wicks A. J. 
Published: February 2007

AbstractTwo field experiments were carried out to observe the variation in wet deposition of ions caused by orography in a polluted region of the UK. the site chosen was Winter Hill which lies on the edge of the industrialised Mersey valley. Rain and cloud samples were collected daily and chemically analysed. Synoptic and meteorological data were used to classify individual rain events as frontal or convective.For the frontal events the altitude related increase in wet deposition was found to be entirely caused by the seeder‐feeder mechanism (the scavenging of cap cloud by rain from above). the concentration of marine ions in the water scavenged from the cap cloud was found to be between five and six times that in the seeder rain whereas for anthropogenically produced ions it was about twice as great. the efficiency of the seeder‐feeder mechanism is correspondingly affected.Convective events accounted for less than a quarter of the rainfall volume. However, this contained over half of the deposition of marine ions in the valley. Deposition of anthropogenically produced ions in convective events was small probably due to a lack of convection over land. A small decrease in deposition with altitude was observed for these events although this is expected to be a highly local effect.It is concluded that reliable predictions of the wet deposition of non‐marine ions can be made on the basis of the seeder‐feeder effect for northern and western regions of the UK. Elsewhere in the UK and for much of continental Europe wet deposition from convective rain should be taken into account.
Error growth and estimates of predictability from the ECMWF forecasting system
Authors: Simmons A. J. , Mureau R. , Petroliagis T. 
Published: February 2007

AbstractExamination has been made of the skill of ECMWF forecasts of the 500 hPa height field produced daily out to ten days ahead, verifying in the period from 1 December 1980 to 31 May 1994. Over this time accuracy has been improved substantially over the first half of the forecast range. the systematic (seasonal‐mean) component of the error has been greatly reduced at all forecast times, but there has been little reduction in the non‐systematic (transient) component later in the range.The simple model proposed by Lorenz for the intrinsic growth of forecast error has been applied to the evolution of differences between consecutive forecasts. the implied growth‐rates of small forecast errors have increased significantly since 1981. They do not show much variation with season, and are a little lower in the southern than in the northern hemisphere. the most recent error‐doubling times are around 1.5 days for the northern hemisphere and 1.7 days for the southern hemisphere. Error saturation levels are at present similar to or greater than those of the 1981 version of the model, having been significantly lower in intermediate years. the accuracy of recent short‐ and early medium‐range forecasts and realism of the climatology of the forecast model support the view that estimates of intrinsic error‐growth parameters from the current forecasting system are more reliable than those obtained earlier. Forecast accuracy later in the medium range may thus not have benefited fully from improvements earlier in the range because of the faster error‐growth associated with a more active, though more realistic, forecast model. Overprediction of variance may nevertheless detrimentally affect present levels of skill and estimates of predictability in all seasons other than summer.The error‐growth model currently indicates that it is possible, in principle, to make deterministic mediumrange forecasts for the extratropical 500 hPa height field of the northern hemisphere that are as accurate five days ahead as present forecasts are three days ahead, provided the one‐day forecast error can be reduced by the same factor in the future as has actually been achieved in the years since 1981. the level of error currently reached at day seven would then be reached at around day ten. the scope for improvement of forecasts for the southern hemisphere appears to be rather larger. Improvements seem to be possible throughout the spectral range studied, up to total wave‐number 40. This is found also for the rotational and divergent wind components at 850 and 200 hPa. For these components, particularly the divergent component, there is a quite pronounced error in the representation of the largest scales.
Computed conditions of corona emission from two interacting raindrops
Authors: Georgis Jean‐François , Chauzy Serge , Coquillat Sylvain 
Published: February 2007

AbstractThe conditions of corona emission from two interacting drops located in a vertical ambient field parallel to their line of centres are established by combining the corona experimental results obtained by Dawson (1969) with the distortion model from Georgis et al. (1995). Given the drop shapes and electrical fields at any points on the surface then, thanks to the distortion model, it is possible to calculate the altitude of corona occurrence from a fitting of Dawson's data. Unexpectedly it was found that an increase in the ambient field and/or a decrease in the distance between the two drops is not always equivalent to a lowering of the altitude of the corona occurrence, thus confirming that distortion plays a key part in this highly nonlinear mechanism. Nevertheless, corona emission is now generally possible at quite realistic altitudes in permanent ambient fields provided that the drops are close enough to each other. the mutual approach of the two drops certainly favours corona emission at low altitudes, but it also entails such a reduction in the ambient field for corona onset that conditions may be no longer propitious for corona propagation.
An intercomparison of three vegetation/soil models for a sparse vineyard canopy
Authors: Van Hurk B. J. J. M. Den , Verhoef A. , Van Berg A. R. Den , De Bruin H. A. R. 
Published: February 2007

AbstractThree Soil‐Vegetation‐Atmosphere Transfer models simulating fluxes collected at a dry sparse vineyard site in La Mancha, Spain, are intercompared. the algorithms used to describe soil heat‐flux density, aerodynamic transfer of water and heat between the surface and the atmosphere, and crop evaporation all have a different physical basis in these models. A common feature of the models is that interaction of many processes takes place via the surface temperature.Large differences are found between the predicted soil heat‐flux densities, caused by a poor physical description of heat storage in the soil and by the choices of soil coefficient values. Significant differences in the parametrization of the aerodynamic transfer between the surface and the atmosphere also exist, with a clear impact on predictions of surface temperature. Crop evaporation is underestimated by models taking the effect of soil moisture depletion or atmospheric humidity deficit on crop resistance into account.A suggestion is made for an optimal choice of various model components to describe the observed fluxes best.
Three‐dimensional transport simulations of the dispersal of volcanic aerosol from Mount Pinatubo
Authors: Fairlie T. Duncan A. 
Published: February 2007

AbstractThree‐dimensional Eulerian and Lagrangian transport simulations are used to investigate aspects of the initial dispersal of the volcanic aerosol plume from the 1991 eruption of Mount Pinatubo. A global finite‐difference model that simulates the large‐scale dynamics of the earth's stratosphere is combined with an Eulerian spectral transport model and a Lagrangian trajectory model. the simulated transport of the volcanic plume is compared with observations of the volcanic plume made from the SAGE II satellite instrument. Dynamical quantities, diagnosed from the simulated wind‐ and temperature‐fields, provide insight into the evolution of the volcanic plume. Lagrangian trajectories help assess the performance of the Eulerian transport model and highlight smallscale structure that is masked by the finite resolution of the Eulerian model. Exponential material‐stretching rates, symptomatic of efficient mixing, are found when air is peeled off the volcanic plume into southern mid‐latitudes. On‐line simulations show that radiative heating of the cloud has a major impact on its transport. Details of the local and far‐field dynamical responses to the heating are discussed.
Representation errors in various grid and spectral truncations for a symmetric feature on the sphere
Authors: Naughton Michael , Courtier Philippe , Bourke William 
Published: February 2007

AbstractA smooth Gaussian bell‐shaped feature is used for a test case to examine the accuracy properties of finite spectral representations on the sphere. Firstly, the effect of different degrees of thinning of the number of longitudes in the Gaussian grid in the vicinity of the poles is illustrated. Secondly, it is shown that the inhomogeneity of the Gaussian grid leads to variations in accuracy as the feature location is changed, despite the fact that the triangular truncation spectral representation is homogeneous. Thirdly, the accuracy of triangular, rhomboidal and other spectral truncations are compared, with triangular truncation proving the most accurate for this type of feature.
Vorticity asymmetries in Hurricane Josephine (1984)
Authors: Glatz Arno , Smith Roger K. 
Published: February 2007

AbstractA further analysis of omega dropwindsonde observations from three synoptic‐flow experiments in the environment of Hurricane Josephine is described, complementing an earlier study by Franklin. The intensity, structure and evolution of azimuthal wave‐number components of the 850–500 hPa layer‐mean and deep‐layer mean (surface to 100 hPa) vorticity fields, including the symmetric component, are investigated, and the determination of the extent to which the asymmetries may be interpreted in terms of the barotropic theory of vortex motion in simple large‐scale environments is sought. The latter endeavour calls for a (necessarily arbitrary) partitioning of the flow between vortex and environment. As a starting point for this it is hypothesized that the environmental flow has a linear variation across a domain that appears to be encompassed by the storm circulation. Based on this assumption it is shown that, on two of the three days on which data were available, the structure and strength of the wave‐number 1 vorticity asymmetry were similar to the beta‐gyres that are a feature of model calculations. The day‐to‐day changes in the orientation and strength of these asymmetries in the 850–500 hPa layer‐mean calculations are difficult to interpret in the light of barotropic theory, and are possibly associated with vertical shear in the storm environment, as described in a recent paper by Jones. Such rapid changes are not seen in the corresponding deep‐layer mean calculations, but the wave‐number 1 asymmetries in these fields are significantly influenced by the contributions from upper tropospheric levels where there are few data and where the vortex circulation centre is dispatched horizontally far from that at the surface.The orientation of the wave‐number 2 asymmetry varied little over the three‐day observation period and was broadly consistent with the orientation of the large‐scale deformation field. Moreover, its strength weakened during the period and varied monotonically with that of the deformation field, a result that is consistent also with theoretical expectations.The attempted analysis highlights certain intrinsic problems in interpreting hurricane motion in terms of current theories. This is due partly to the non‐uniqueness of the partition required to separate the storm environment from the vortex asymmetries and partly because of the difficulty of determining a representative model‐equivalent value for the environmental absolute‐vorticity gradient from the data.
Reversible and irreversible sources of radiation entropy
Authors: Goody Richard , Abdou Wedad 
Published: February 2007

AbstractWe discuss two practical issues concerning the relationship between the entropies of the atmosphere and of the radiation field. The first is whether the radiative flux of entropy can be determined from a satellite with sufficient accuracy for climate studies. We conclude that an accuracy much better than 1% is required, but that this can be achieved in the thermal spectrum with spectrometers having spectral resolutions ∼ 1 cm−1; it is also possible that the required accuracy can be achieved with a non‐spectral approach. The second is whether the rate of change of the atmospheric entropy inventory can be inferred from the outgoing radiative‐entropy flux. We conclude that, for clear skies in the thermal region of the spectrum, the two are directly proportional, and that the atmospheric term can be inferred with useful accuracy from an empirical relation.
Quasi‐continuous variational data assimilation
Authors: Järvinen Heikki , Thépaut Jean‐Noël , Courtier Philippe 
Published: February 2007

AbstractAll the computations associated with the operational meteorological data assimilation are usually carried out after the so called cut‐off‐time, i.e. a few hours after the end of the assimilation period. In this article we propose an alternative implementation of the four‐dimensional variational assimilation where some of the computations occur during the 24‐hour assimilation period. The feasibility of the approach is first validated with an assimilation system built around a low resolution barotropic grid point model. The results are then confirmed with a multi‐level primitive equation model using real observations. The main result of these experiments is that the peak computer power requirement of the four‐dimensional variational data assimilation may be significantly reduced by the suggested approach.