bluuebunny/crossref_metadata_2025_split huggingface dataset:
Some limits to the variable‐mesh solution and comparison with the nested‐lam solution
Authors: Caian Mihaela , Geleyn Jean‐François 
Published: February 2007

AbstractThe solution at high resolution of a global spectral model with a conformally‐variable mesh, obtained through a stretched coordinate, is compared as fairly as possible to the ‘classical’ solution of a limited‐area nested model. We compare the global, variable‐mesh, ARPEGE model at hyper‐stretched values (which is integrated operationally at Météo‐France with a ‘low’ value of stretching) with its limited‐area version, the ALADIN model. Having the same physics, the same grid‐point dynamics and a high code‐compatibility, they allow clean comparisons on real situations. to conclude on their efficiency and accuracy, comparisons are made choosing configurations of the two models having either the same resolution or the same computational price.The limits of the variable‐mesh strategy on the sphere are also investigated through a set of experiments with increasing stretching. We found that there is indeed a limit to the stretching factor in the interval [7, 9]: a stretching factor c = 7 keeps an accurate spectral solution and gives satisfying results, even when compared with the limited‐area model at the same computational price. We show that the value c = 10.5 cannot be reached without paying the price of a spectral deformation of the fields which becomes too obvious. Additional tests showed the occurrence of this deformation, even if less pronounced, at the stretching value c = 8.75. At the same resolution ALADIN does not manifest any similar weakness.Although the solution remains stable for at least 72 hours, its accuracy when increasing the stretching above the critical value becomes questionable. This problem seems to be due to a particular behaviour of the truncation error, which does not decrease with increasing resolution through stretching, thus leading to an energy accumulation in the tail of the spectra.As a conclusion of this study, it follows that one can extend the idea of Courtier and Geleyn, currently used in operations at Météo‐France for c = 3.5, to ‘medium’ stretched configurations up to c = 7 at least (at the current computer's power). Above a value found to be about c = 9, our results show that increasing the resolution does not produce any improvement and, hence, that the use of a limited‐area model appears a better choice for very high resolutions.
Dynamical modelling of wintertime lidar observations in the arctic : Ozone laminae and ozone depletion
Authors: Orsolini Y. J. , Hansen G. , Hoppe U.‐P. , Manney G. L. , Fricke K. H. 
Published: February 2007

AbstractA series of lower stratospheric ozone profiles taken by a lidar during February 1995 in northern Norway are examined. the instrument location allowed continuous monitoring of rapidly evolving layered ozone structures, or laminae, near the polar vortex edge.Observations under the vortex edge revealed laminae extending up to 20 km. Moreover, when the lidar was sampling vortex air, a thicker layer, characterized by unusually low mixing ratios, was also observed, thereby confirming satellite observations of Arctic ozone depletion during the winter 1994/95.To unravel the respective role of dynamics and chemistry in generating these ozone‐depleted layers requires better understanding of trace constituent lamination and filamentation through numerical modelling.High‐resolution modelled realistic transport of idealized tracers and satellite‐derived ozone on many isentropes is therefore used to reconstruct fine‐scale three‐dimensional tracer fields. Tilted tracer or ozone sheets, peeled off near the vortex edge, lead to the formation of laminae in profiles. the model shows remarkable success in reproducing laminae intensification, thickening and lofting, analogous to the ones observed by the lidar over the course of a few hours.
Broadening of convective cells
Authors: Dörnbrack Andreas 
Published: February 2007

AbstractThe turbulent convection in a dry Boussinesq fluid above a heated surface is studied in a domain of large aspect ratio and long duration. Large‐eddy simulations are used to investigate the influence of the dynamical character of the lower boundary and the effect of cooling at the top on the growth of convective cells. the aim is to identify physical mechanisms that lead to the broadening of convective cells even under conditions where moisture processes play no role. Cooling at the top produces a vertically uniform heat‐flux, and it acts to enhance the total kinetic energy of the whole flow. Essentially, there are two competitive effects: first, the prescribed heat‐flux at the upper (lower) boundary produces very warm (very cool) fluid that is forced to rise (sink). the resulting large temperaturefluctuations are transported by large‐scale motions and cause broad temperature‐variances independent of height. On the other hand, the strong turbulent mixing (mainly near the boundaries where the turbulent kinetic energy is maximum) tries to homogenize the flow structure in such a way that the resulting temperature‐distribution is uniform. Except directly close to the walls, horizontal and vertical temperature‐gradients are reduced. Additionally, a trend to form an organized large‐scale horizontal drift in one direction near the bottom and in the opposite direction near the top was found for runs without surface friction. This horizontal streaming motion has two effects: firstly, the turbulent mixing is enhanced due to larger shear and, secondly, separated thermals approach faster and are able to merge more easily, forming gradually growing cells. an adiabatic upper boundary condition leads to a heat‐flux profile that is linear and decreasing with height, and to a moderate temporal growth of thermal structures. A considerable scale‐reduction of the temperature structures occurs because of friction at the lower surface.
Short‐wave optical properties of precipitating water clouds
Authors: Savijärvi Hannu , Arola Antti , Räisänen Petri 
Published: February 2007

AbstractWater cloud drop‐size distributions for non‐precipitating clouds are taken from various sources. the optical depth, single‐scattering co‐albedo and asymmetry parameter are calculated using Mie theory. By averaging over four spectral intervals in the short‐wave regime, a linear parametrization for the co‐albedo and the asymmetry parameter is made as a function of the cloud‐layer effective drop radius, re, in each interval; re can be related to the cloud‐layer liquid‐water content (LWC). the optical depth is parametrized quadratically on re−1. This gives an excellent fit. These parametrizations are useful for general‐circulation model (GCM) and weather‐forecast models, which produce estimates of cloud‐layer LWC.Large‐drop effects are studied by adding typical raindrop size distributions to the stratusand cumulonimbus‐type cloud‐drop distributions. the re of these precipitating clouds can be related to the rain intensity, R. It turns out that (1) the asymmetry parameter is insensitive to R, (2) the optical depth follows the quadratic parametrization on re−1, and (3) the co‐albedo increases nearly linearly with re (or R), so absorption of a cloud layer increases with R. A cloud layer with large drops can thus absorb more solar radiation than one without large drops. A simple four‐band GCM‐type parametrization of the large‐drop effect is presented.
The Berlin troposphere‐stratosphere‐mesosphere GCM: Climatology and forcing mechanisms
Authors: Langematz Ulrike , Pawson Steven 
Published: February 2007

AbstractThe climatological structure of the Berlin troposphere‐stratosphere‐mesosphere general circulation model is presented and compared with observations. the zonal‐mean zonal wind and temperature display some successes for the simulation in that all of the climatological features are captured, but there are some systematic errors regarding their location and strength. In particular, the location and structure of the polar‐night jet in the northern winter is incorrect: it is located too far polewards at high levels and is too weak in the middle and lower stratosphere. On the positive side, there is a separation between the polar‐night jet and the subtropical jet. the temperature in the winter polar upper troposphere and lower stratosphere is too high, presumably arising from the treatment of radiative processes. Consistent with hydrostatic balance, the 30 hPa (and other) isobaric surface lies too high. the polar vortex is both too asymmetric and too weak. This leads to an underestimate of the modelled zonal velocity, which is consistent with the poleward (rather than equatorward) propagation of the stationary planetary waves, in the lower and middle stratosphere. In the upper stratosphere the poleward heat and momentum fluxes due to these waves are too strong. Despite these systematic errors, the seasonal evolution of both hemispheres is quite well simulated: the large variability and the strong, episodic forcing of the northern winter contrasts well with the relatively stable southern winter and the breakdown of the southern polar vortex in springtime. the magnitude of the variability in the northern hemisphere is too small in the model and it is proportionally too large in the early winter.
Stability of equatorially symmetric and asymmetric climates under annual solar forcing
Authors: Xie Shang‐Ping 
Published: February 2007

AbstractA simple coupled ocean‐atmosphere model is developed to investigate the maintenance of the latitudinal asymmetries of the sea surface temperature (SST) field and the intertropical convergence zone (ITCZ). the model is of the minimum degree of freedom necessary to describe latitudinal asymmetries, computing only the difference between the hemispherical SST maxima. A wind‐evaporation‐SST feedback is found to destabilize the equatorial symmetry established by the annual‐mean solar radiation. By considering the nonlinear SST threshold for deep convection, equatorially asymmetric steady states are reached, with a single ITCZ off the equator.The annual component of solar radiation is anti‐symmetric about the equator, acting to force convection in the summer hemisphere independent of symmetry‐breaking ocean‐atmosphere interactions. A multiple time‐scales analysis is applied to separate the annual cycle and the slowly evolving annual‐mean climatology. It is found that the stability of an annual‐mean state varies with annual forcing amplitude. As forcing amplitude varies, the model displays three flow regimes: (1) an asymmetric regime for small seasonal forcing, featuring a permanent ITCZ in one hemisphere and a seasonal one in the other; (2) a mixed regime for intermediate forcing, with both symmetric and asymmetric solutions being stable; and (3) a symmetric regime for large annual forcing where the ITCZ migrates between the hemispheres following the seasonal march of the sun.
Modelling of stratospheric intrusions within a mid‐latitude synoptic‐scale disturbance
Authors: Cox B. D. , Bithell M. , Gray L. J. 
Published: February 2007

AbstractThe results of a run of the UK Universities' Global Atmospheric Modelling Programme (UGAMP) General Circulation Model (GCM) is presented, initialized with European Centre for Medium‐Range Weather Forecasts data for 6 October 1990. the dates correspond with observations of a trough made during a Transport of Ozone and Stratosphere/Troposphere Exchange campaign. This anticyclonic wrapping system is compared with a cyclonic wrapping system studied in a previous paper. the fate of stratospheric air is followed using potential vorticity (PV) and specific humidity. the run gives good agreement with radiosonde data for the development of the trough, which involves a tropopause fold and a ‘streamer’, both with regard to the position and evolution of a layer of dry air. Analysis of the height of the tropopause shows that PV values (in PV units) in the range 1‐5 agree well with the thermal tropopause outside the trough, but in order to capture the layer of dry air seen in the radiosonde data, a value of PV of order 1 is required within the trough. Use of PV values in the range 3‐4 may therefore not follow the evolution of dry stratospheric air entering the troposphere, even though such values may on average follow the thermal tropopause. Most of the dry region is visible as a dark band in the Meteosat data, although the lowest levels of dry air (at approx. 800 mb) are too low to be seen by the satellite. Thus Meteosat observations may not capture all processes involved in stratosphere‐troposphere exchange, and the images must be interpreted with caution when using them to follow the movement of dry stratospheric air.
Sensitivity of ozone and temperature to vertical resolution in a gcm with coupled stratospheric chemistry
Authors: Austin John , Butchart Neal , Swinbank Richard 
Published: February 2007

AbstractResults are presented from a general‐circulation model with comprehensive stratospheric photochemistry and which includes the coupling between radiative heating and simulated ozone. Each model integration covers the 60‐day period beginning 15 January during which episodes of polar stratospheric clouds (PSCs) are normally observed in the Arctic. Results from two versions of the model with different numbers of atmospheric levels near and above the tropopause are compared with observations. In the 19‐level model the ozone transport is poorly simulated and, in particular, there is a significant increase in the tropospheric column. In contrast, in the 49‐level model the simulated ozone distribution is in good general agreement with observations and reproduces well the steep vertical gradients in ozone mixing ratios in the tropical lower stratosphere, and the weak vertical gradients in the high‐latitude middle stratosphere. This version also maintains a virtually constant tropospheric ozone column. Since in the 49‐level model the ozone distribution is well simulated, including this ozone in the radiation calculation has only a moderate influence on the results. However, with the 19‐level model, it substantially increases the global stratospheric temperature error. Increasing the number of levels improves the simulation of stratospheric temperatures but the zonal‐mean temperatures in both versions of the model are generally lower than observed. Despite this, the modelled frequency of occurrences of PSCs is less than observed because of the underprediction of the zonally asymmetric component of the temperature distribution. the results suggest that if coupled chemistry‐climate simulations are to proceed, it is important to have both a high upper boundary and good vertical resolution in the lower stratosphere to ensure a realistic meridional circulation and a more accurate representation of ozone transport across the tropopause.
Radar observations of wind‐shear splitting within evolving atmospheric kelvin‐helmholtz billows
Authors: Chapman D. , Browning K. A. 
Published: February 2007

AbstractThe high‐resolution radar at Chilbolton was used to measure the velocity structure of large‐amplitude Kelvin‐Helmholtz billows within a precipitating warm‐frontal zone. A deep layer of frontal shear was observed to split into thin layers of more concentrated shear displaying the characteristic braided structure previously reported within clear‐air radar reflectivity patterns and attributed to a splitting of the thermal stratification. the billows were situated within a region of strong wind veer with height and they were orientated at right angles to the mean flow such that air parcels within them were tending to follow helical trajectories up the inclined frontal zone. Radar scans normal to the axes of the billows at different distances from their origin, close to the position of the surface warm front, provided an indication of the evolution of the split‐shear structure.
The ‘1/3’ power law between effective radius and liquid‐water content
Authors: Liu Yangang , Hallett John 
Published: February 2007

AbstractRadiative properties of water clouds are parametrized in terms of the effective radius, liquid‐water content and total droplet concentration. A new general ‘1/3’ power law between effective radius and the ratio of liquid‐water content to total droplet concentration is formulated theoretically. The prefactor in the ‘1/3’ power law depends upon, and relates, cloud droplet size distributions to physical mechanisms such as entrainment and mixing. It shows that the ‘1/3’ power law holds in general for non‐precipitating water clouds with any entrainment and mixing, including clouds with monodispersed cloud droplet size distributions as special cases. The formulation builds upon a system theory that is based on Shannon's maximum entropy principle. The theoretical ‘1/3’ power law is further demonstrated using observational data. The existence of such a ‘1/3’ power law indirectly justifies the system theory generalized in this paper.
Evaporation and radiation interception by neighbouring plants
Authors: Wallace J. S. 
Published: February 2007

AbstractThis paper presents the ERIN model, for estimating evaporation and radiation interception by neighbouring species of plants. the model contains a comparatively simple theoretical framework which couples transpiration and the interception of light in multi‐species mixtures, such as those which occur in many natural and man‐made plant communities including grasslands, savannahs, crop/weed mixtures and the results of intercropping and agroforestry. the light‐interception sub‐model can estimate the individual amounts of radiation intercepted by the components of a multi‐species mixture based on a knowledge of the canopy height, leaf area and extinction coefficient of each constituent. In homogeneous plant mixtures, the light sub‐model in ERIN compares well with other more complex models of light interception. A simple method for dealing with spatially heterogeneous canopies is also included, based on a knowledge of the canopy cover and the clumping of the foliage of each species within the canopy. the transpiration sub‐model in ERIN allows the evaporation from each component of a multi‐species mixture to be calculated. It also has the advantage that it allows for microclimatic interactions between the different canopies and that it is coupled to the competition for light. the way in which the fluxes of heat and water vapour from one canopy can affect the microclimate and transpiration from another canopy is also illustrated.
The effects of different radiation parametrizations on cloud evolution
Authors: Loughlin P. E. , Trautmann T. , Bott A. , Panhans W. G. , Zdunkowski W. 
Published: February 2007

AbstractWith the use of the microphysical stratus model (MISTRA), investigations into the effects of variations in the radiative‐transfer parametrizations on cloud development have been undertaken. Two radiative‐transfer schemes were coupled with the microphysical‐thermodynamical section of MISTRA, one based on the exponential sumfitting method and the other based on the correlated k‐distribution method of determining gaseous absorption properties. Model runs were initiated with parameter values in accordance with measurements made over the North Sea and comparisons made between model runs where only the radiation schemes were altered. Results indicated that differences between the two schemes had a significant effect on cloud evolution. Alterations were then made to the correlated k‐distribution method in an attempt to match the results from the original radiation method and so determine why there were differences between the two sets of results.The results showed that infrared scattering played an important role in cloud development as it extended the infrared cooling rates into the cloud top when compared with a case with no infrared scattering, and therefore helped to offset solar heating during daytime. Exclusion of infrared scattering produced a large increase in total cloud‐top cooling rates, almost 100%, but produced only minimal changes in other bulk cloud properties of typically 3%, partially due to the net increase in heating immediately below cloud top. the inclusion of e‐type water‐vapour absorption was also investigated. Results showed that by reducing the wavelength range where this component was included from 8.0‐35.7 μm to 8.0‐12.5 μm produced only a small change, 3%, in the downward infrared fluxes above the cloud. However, this was matched by larger changes in bulk cloud properties of typically 7%. This indicates that cloud development is very sensitive to changes in the downward infrared radiation field above the cloud. Most importantly, the changes in the radiation fields reported are smaller than the values given as experimental errors and what could be considered reasonable theoretical uncertainty.
Improvement in stratosphere simulation with a hybrid σ‐ϑ coordinate GCM
Authors: Zhu Z. , Schneider E. K. 
Published: February 2007

AbstractThis study demonstrates the practical use of a smoothly varying hybrid sigma‐isentropic (σ‐ϑ) coordinate in a general‐circulation model (GCM). Changes have been made to the vertical scheme developed in previous work in order to overcome a numerical problem associated with the crossing of levels. the hybrid coordinate, which changes gradually from a sigma coordinate at the bottom to a nearly isentropic one in the stratosphere, was implemented into the GCM. Multi‐year integrations were successfully conducted with the σ‐ϑ coordinate model.Improvements in the simulated stratospheric and upper‐tropospheric circulation are found compared with a control run using the original sigma‐pressure coordinate GCM. the greatest improvements are significant reductions of the cold biases in the winter polar stratosphere and of the wind speed of the polar‐night jet. These changes are shown to be related to a significant increase of the poleward eddy heat flux in the middle‐latitude stratosphere. Diagnostics show the wave kinetic energy, stationary‐wave phase structure, and EP flux are changed by using the nearly isentropic coordinate in the stratosphere. Possible reasons for the improvements of the stratospheric simulation are discussed, including the more accurate representation of the advection processes, and the effect of the isentropic model top.
On the concept of exergy and available potential energy
Authors: Kucharski Fred 
Published: February 2007

AbstractA systematic derivation of the concepts of available potential energy of Lorenz and Van Mieghem from an extended concept of exergy is performed. This procedure leads to an extension of Van Mieghem's global theory to a local formulation which can be used in limited‐area models. In the limit of small amplitude motion it is shown that the extended exergy reduces to the available potential energy of Boussinesq‐approximated models. Moreover, Shepherd's concept of pseudoenergy is extended in order to deal with diabatic and irreversible processes.
Superposing semi‐geostrophic potential‐vorticity anomalies
Authors: Birkett H. R. , Thorpe A. J. 
Published: February 2007

AbstractA numerical method for inversion of the semi‐geostrophic potential‐vorticity equation is described. It is used to examine the validity of a strictly linear superposition principle when applied to this high‐order balance model. These results are important when aiming to extend rigorously the value of recent ‘attribution’ ideas. Experiments conclude that the nonlinear components of the inversion have only a very short‐range effect, with a magnitude which increases with the magnitude of the potential‐vorticity anomalies.For balance of higher‐order than quasi‐geostrophic these results show that, whilst flow and temperature of an anomaly are nonlinear functions of potential‐vorticity magnitude, spatially separated anomalies are quasi‐linearly superposable.