bluuebunny/crossref_metadata_2025_split huggingface dataset:
A dynamic chemical model of bi‐directional ammonia exchange between semi‐natural vegetation and the atmosphere
Authors: Flechard C. R. , Fowler D. , Sutton M. A. , Cape J. N. 
Published: February 2007

AbstractA mechanistic, dynamic compensation point model to simulate the vegetation/atmosphere exchange of ammonia (NH3) is described. the model is applied to long‐term micrometeorological measurements of NH3 exchange obtained over moorland in southern Scotland (1995‐96). the model describes the gaseous bi‐directional exchange between the atmosphere, leaf surface water films, plant stomata and apoplast. A simple chemistry module is included to simulate the exchange of water‐soluble atmospheric pollutants at the air‐water interface on wet plant surfaces. Initialization of the chemistry module is achieved during rain events using measured rain chemical composition. the exchange of NH3 with stomata is based on the concept of a stomatal compensation point parametrized by means of the apoplast ammonium/hydronium I ratio. the trans‐cuticular transfer of ammonium may also constitute a sink for dissolved ammonia on plant cuticular water films, and is parametrized using a trans‐cuticular resistance and the concentration difference between leaf surface water and the apoplast. the leaching of base cations from the inside of the plant to foliar surfaces is simulated in a similar fashion to ammonium transfer, and the heterogeneous oxidation of sulphur dioxide (SO2) in thin water films is also treated. Numerical iterative procedures at each time‐step allow the calculation of pH and dissolved ion concentrations.Modelled NH3 fluxes were compared with 3259 half‐hourly micrometeorological measurements over moorland, and with two existing modelling approaches, the static canopy compensation point model and the canopy resistance model. the dynamic and static canopy compensation point models both gave long‐term estimates of the NH3 dry deposition flux to moorland within 10% of actual measurements, while the canopy resistance approach overestimated deposition by about 30%. the dynamic model performed best during wet conditions for which it was designed, and performed reasonably well during dry conditions using a more empirical resistance approach.The model was also capable of simulating SO2 dry deposition fluxes to within 20% of measured fluxes. the model provides a tool that may also be used to simulate scenarios, whereby NH3/SO2 concentration ratios in the atmosphere vary, and examine ‘co‐deposition’ interactions of these two species.
A case‐study of pronounced perturbations to cloud properties and boundary‐layer dynamics due to aerosol emissions
Authors: Taylor J. P. , Ackerman A. S. 
Published: February 2007

AbstractDuring June 1994 the UK Meteorological Office C‐130 aircraft took part in the Monterey Area Ship Tracks experiment. On 13 June 1994 a merchant vessel, the Sanko Peace, was observed steaming below a layer of broken stratus clouds in a shallow (300 m deep) boundary layer. These clean maritime clouds were significantly modified by the aerosol emissions from the ship. Droplet concentrations increased from around 10 cm−3 in the background cloud to a peak of 120 cm−3 in the ship track. This resulted in a roughly 50% reduction in average droplet size and drizzle flux. Less than an hour downwind of the Sanko Peace, the cloud top in the ship track was observed to be elevated by 100 m above the surrounding cloud tops.A one‐dimensional (1‐D) model with size‐resolved cloud microphysics, a detailed radiation code, and a turbulence closure scheme is used to simulate the observed ship track. the model predicts the observed microphysical changes reasonably well and also predicts a deepening of the boundary layer, though the simulated deepening is considerably slower than was observed. the driving force in the modelled response is an increased buoyancy flux due to enhanced radiative cooling in the cloud layer. the modelled boundary layer deepens in response to an increase in cloud water, which results from suppression of the drizzle flux. Differences between the modelled and observed profiles of cloud water in the ship track are difficult to reconcile with the modelled mechanism for the deepening, and are attributed to shortcoming in the 1‐D model representation.
Cloud bands induced by isolated mountains
Authors: Sang J. G. 
Published: February 2007

AbstractThe wave equation is solved analytically to obtain the solutions of diverging types of trapped waves, caused by an isolated hill, at the top of a convective boundary layer with an overlying stable layer above. Wind veering in the upper stable layer favours the wave trains to be to the left of the mean wind direction. the elevated lifting latent heat may make the crest line of the diverging wave a diabatic heating band, if the water vapour supply is sufficient.Analytical solution is also used to analyse the effects of a heating band, which makes a small angle with the mean wind direction. the trapped components of the induced waves may spread horizontally and excite wave trains running parallel to the original heating band. the untrapped waves hold an in‐phase relation with the heating. the resultants of trapped and untrapped waves possess multiscale structure and properties similar to those of cloud streets. These analyses may provide an explanation for the formation and propagation of cloud streets.
Development and quality control of the FASTEX data archive
Authors: Jaubert Geneviève , Piriou Catherine , Loehrer Scot M. , Petitpa Alain , Moore James A. 
Published: February 2007

AbstractThe Fronts and Atlantic Storm Track EXperiment (FASTEX) was conducted over the North Atlantic during January and February 1997. the FASTEX Data Archive is a combination of the datasets collected as a part of the two‐month intensive field season and many supplementary datasets and products retrieved from the participating nations and cooperating agencies. the archive is composed of the FASTEX Central Archive (FCA) and a group of specialized databases.The technical design of the FCA allowed rigorous management of the data and made the checking of the data easy to accomplish. This rigorous management was accomplished using customized quality control evaluation techniques as determined by the individual datasets received at the FCA.An important aspect of FASTEX data archival activities was the collection, processing and production of a high‐resolution rawinsonde and dropsonde dataset. These data are of importance to model‐verification studies and mesoscale analysis in FASTEX. Data from rawinsondes released from land sites and operational and research ships as well as dropsondes from five aircraft are included. the activities of two groups involved in the preparation of this dataset are described.The humidity measurements by the rawinsondes and dropsondes were examined in some detail using two methods. First, the three primary types of sondes used over the North Atlantic Ocean (Vaisala rawinsondes and the new and old generation Vaisala dropsondes) were intercompared. Some initial evidence was found of biases in the dropsonde data depending on the type of expendable used and the temperature regime. Second, to further clarify these biases a comparison of the precipitable water content calculated from the rawinsondes and dropsondes with that derived from SSM/I measurements was conducted. the results from this verified many of the conclusions of the initial intercomparison and brought additional insight.The FASTEX dataset is one of the most rich and varied datasets focusing on the North Atlantic. the data are readily accessible to the scientific community at the FCA via the World Wide Web at: http://www.cnrm.meteo.fr/fastex/.
Impact of the additional FASTEX radiosonde observations on the high‐resolution limited‐area model (HIRLAM) data‐assimilation and forecasting system
Authors: Amstrup Bjarne , Huang Xiang‐Yu 
Published: February 2007

AbstractDuring the Fronts and Atlantic Storm‐Track EXperiment (FASTEX) the radiosonde network was considerably enhanced over the North Atlantic area, making this an ideal period for assessing the impact of additional radiosonde observations, in data‐sparse regions, on the atmospheric data‐assimilation and numerical weather‐forecast systems.We have carried out an observing‐system experiment (OSE) with a limited‐area data‐assimilation system. Two parallel experiments have been run, one with and one without the extra radiosonde data.We have used both observation verification and field (or analysis) verification to compare the analyses and forecasts from the parallel experiments. On a daily basis, the extra data can have both positive and negative impacts on the different meteorological variables considered here. However, averaged over the whole period the impact is definitely positive, especially on upper model levels. Our results strongly support the inclusion of extra radiosonde data, like those provided by FASTEX. They could be of significant benefit in reducing the errors in operational numerical weather‐prediction products if they were available routinely.
Bimodal ice spectra in frontal clouds
Authors: Field Paul R. 
Published: February 2007

AbstractAircraft observations from five Lagrangian descents in frontal clouds from around the British Isles are presented. A common bimodal ice size spectral evolution is observed using two‐dimensional optical array probes, and it is demonstrated that depositional growth alone is not responsible for the observed evolution. It is suggested that the process of aggregation is chiefly responsible for the large‐particle mode of bimodal ice size spectra. Synthetic bimodal spectra that are a function of depth below cloud top and ice‐water content are presented.
Observations of microphysics pertaining to the development of drizzle in warm, shallow cumulus clouds
Authors: French Jeffrey R. , Vali Gabor , Kelly Robert D. 
Published: February 2007

AbstractThe evolution of drizzle in small cumulus clouds in a subtropical environment was studied. Observations were made using three instrumented aircraft, a ground‐based radar, and a 95 GHz airborne Doppler radar during the Small Cumulus Microphysics Study in east central Florida in summer 1995. Data from six clouds, from two days, are examined in this paper. Both sets of clouds were less than 2 km in depth at observation times; in‐cloud temperatures were > 10 °C.Droplet spectra observed in these clouds were generally bi‐modal. The sizes of the droplets within the large mode are consistent with growth through condensation. It remains unclear how droplets within the small mode formed, but their existence appears to be tied to entrapment and mixing.Drizzle drops (drops with diameters > 50 μm) were found at all levels in the clouds. The penetration‐averaged concentrations of drizzle drops were less than 15 L−1, except in the uppermost regions of clouds on one of the days where drizzle drop concentrations exceeded 100L−1. In general, the presence of drizzle drops in the upper regions of the clouds is consistent with models of droplet growth through condensation and stochastic collection. The existence of drizzle at low and mid‐levels may be due to collection initiated by ultra‐giant aerosols, or to the redistribution of drizzle drops from neighbouring or earlier cloud elements.
Observations and simulations of a non‐stationary coastal atmospheric boundary layer
Authors: Sunuararajan Ragothaman , Tjernström Michael 
Published: February 2007

AbstractThis paper studies the turbulent and mesoscale structures of a coastal marine atmospheric boundary layer in the southern Baltic Sea. Data from a field experiment are examined for a case during autumn, and this case is subsequently modelled with a mesoscale numerical model.During this day, the marine atmospheric boundary layer develops from a well‐mixed cloud‐capped state into a stably stratified, sheared boundary layer. This development is due to the dissipation of the cloud layer by radiation processes and by synoptic‐scale subsidence. The boundary layer becomes non‐stationary, and spectral analysis shows that the turbulence structure deviates from the ideal steady‐state homogeneous boundary‐layer turbulence, which forms the basis for most turbulence closures in numerical models. It is shown that the subsiding boundary‐layer top and the increasing stability alter the spatial scale of the turbulent eddies; they become vertically distorted and smaller than those expected during steady‐state conditions. In spite of these deviations, the turbulence statistics are sufficiently well behaved for simplified higher‐order closure modelling to be applicable.When the mixed layer collapses due to the dissipating clouds, the transition triggers an inertial oscillation and a non‐classical low‐level jet forms. At the same time, the changes in depth and stability of the marine boundary layer are perturbed by the mesoscale interaction between the flow and the coastal terrain. Thus, the upstream terrain increasingly blocks the flow, so that the inertia) jet forms only for the part of the flow that overcomes the terrain obstacle. A numerical simulation of the case reveals the primary mechanisms, and several sensitivity simulations, wherein the terrain and the time‐evolution of the forcing are manipulated, confirm the hypotheses proposed as causing the observed (low structure.
COLA AGCM simulation of the effect of anomalous spring snow over Eurasia on the Indian summer monsoon
Authors: Bamzai A. S. , Marx L. 
Published: February 2007

AbstractThe ability of the Center for Ocean‐Land‐Atmosphere Studies Atmospheric General Circulation Model to capture the inverse snow‐monsoon relation observed in satellite‐derived data is tested. The effect of initial enhanced and reduced spring Eurasian snow mass on the evolution of the subsequent Indian summer monsoon is examined through 11‐member ensemble integrations from 1 March to 1 October. The results of the sensitivity experiment indicate that remote responses from the snow‐mass anomalies include significant changes in the sensible‐heat flux, latent‐heat flux and sea level pressure, leading to changes in summer precipitation over the land mass of the Indian subcontinent. For the pre‐monsoon season, the ensemble average of the temperature of the atmospheric column for enhanced snow mass is anomalously colder over large‐scale regions of the Eurasian continent compared with the ensemble average for the reduced snow mass. Low‐level south‐westerlies associated with monsoonal circulation are increased. The results indicate the importance of land‐surface processes in the evolution of the monsoon circulation and lend hope to the use of Eurasian spring snow mass towards improved forecasting of the summer monsoon.
Northern hemisphere winter stratospheric variability in The Met. Office Unified Model
Authors: Lahoz W. A. 
Published: February 2007

AbstractThis paper uses the ensemble approach to study the simulated northern winter stratospheric variability in a tropospheric‐stratosphere version of The Met. Office Unified Model (UM). The runs are for the December‐March periods from 1979/80 to 1997/98; the ensemble for each winter has nine members. We use observed sea surface temperatures (SSTs), a fixed ozone climatology and fixed greenhouse gases. This paper concerns itself with the following questions: (1) Does the UM reproduce observed temperature trends with only SST variations? and (2) Can any significant trend be attributed to SST variations? The results of this paper suggest that the answer to question (1) is that the UM reproduces the observed temperature trend with only SST variations. In particular, northern winter in the simulated 1990s (1989/90 to 1997/98) is colder than northern winter in the simulated 1980s (1979/80 to 1988/89) at both 10 hPa and 50 hPa, with the model cooling trend (0.5‐1.5 K between the decades for 30°N‐70°N) comparable to that from observations (0.5‐1 K decade−1 for 30°N‐70°N). The temperature differences between the simulated 1990s and 1980s generally are not significant at 10 hPa, but are generally significant at 50 hPa at the 95% significance level. The results in this paper provide a first step toward answering question (2) by suggesting that SST variations alone may explain the cooling trend at 50 hPa. Because the model results show that there is no significant cooling trend at 10 hPa outside a natural variability, one cannot rule this out as an explanation. A complete answer to question (2) requires the inclusion of all likely influences (in particular ozone and water vapour trends) and is beyond the scope of this paper.
A hybrid method for wave propagation from a localized source, with application to mountain waves
Authors: Broutman Dave , Rottman James W. , Eckermann Stephen D. 
Published: February 2007

AbstractWave propagation from a localized source into a non‐uniform medium is analysed using a hybrid method. This consists of an integral representation in the near‐field and a numerical ray tracing in the far‐field. The ray tracing includes the calculation of wave‐action density and is initialized from the stationary‐phase approximation to the near‐field integral. The method is tested on the problem of mountain waves in a turning wind, as studied by Shutts. A ray calculation with rotation added to the Shutts model is also presented and is interpreted with a simple analytical model in which the horizontal divergence of rays balances the vertical convergence of rays. In the example presented here, the inclusion of rotation speeds up the approach to wave‐breaking amplitudes.
Southern hemisphere winter ozone fluctuations
Authors: Vigliarolo P. K. , Vera C. S. , Diaz S. B. 
Published: February 2007

AbstractIn this paper the relationship between ozone and atmospheric variability is explored over the southern hemisphere during the austral winter season, with special emphasis on synoptic transient fluctuations.The analysis of ozone tracks (or high‐frequency ozone variability) shows that they have a significant correspondence with storm tracks at middle and high latitudes. Moreover, ozone tracks maximize over the Indian Ocean slightly downstream of the storm‐track maximum, while over the Pacific region both ozone and storm tracks show decreased amplitudes.In particular, over southern South America (a region of climatological winter ozone minima and moderate to high ozone variability) the leading winter synoptic‐scale variability mode was identified through a rotated extended empirical orthogonal function analysis applied to the meridional‐wind perturbation at 300 hPa. The resulting mode is characterized by a baroclinic wave travelling eastward along subpolar latitudes, which maximizes near the tropopause level. Composite ozone fields based on this mode confirm, from a statistical point of view, the classical relationship between ridges (troughs) and minimum (maximum) ozone content. Furthermore, it is shown that dynamical processes in the upper troposphere and lower stratosphere associated with subpolar waves are responsible for the observed ozone distribution. This happens due to the barotropic equivalent vertical structure of the wave, together with the fact that ozone partial pressure maximizes near the level where the waves attain maximum amplitudes.
Coriolis effects on orographic and mesoscale flows
Authors: Hunt J. C. R. , Olafsson H. , Bougeault P. 
Published: February 2007

AbstractA perturbation theory is developed for the effects of rotation on stably stratified flow over mountains at low Froude number, F = U0/(NH) where N is the buoyancy frequency, U0 is the wind speed and H is the mountain height. The Rossby number, R0 = U0/(f D) where f is the Coriolis parameter, and D the along‐wind length of the mountain, is assumed to be a large number. The mountain width, βD, is assumed to be larger than D. Typically R0 is found to lie in the range 3–10. The results are compared with the recent numerical simulations. It is found that as the flow impacts on the mountain, in the northern hemisphere it turns to the left (with your back to the wind); also wave activity over the top of the mountain is greatest on the left side but the pressure drop is greatest on the right in the northern hemisphere. Over the Rossby deformation distance, LR, of the order of HN/f, e.g. 150 km for the Pyrenees, a new wake structure develops that can extend downwind over 1000 km (or a spin‐down distance). There is a momentum defect within the wake but the wind speed increases either side of the wake. Coriolis forces induce a deflection upwards of the isopycnals (and hence more precipitation) on the left, and downwards on the right: this is consistent with some of the differences in mesoscale weather and climate phenomena that are observed on the different flanks of elongated mountains and between the different side of wide valleys, and also in the wakes downwind of mesoscale convection cells. The large perturbation pressure change predicted by the theory is of the order of pU20/F (where p is the density), which is consistent with the magnitude of the terms introduced into the recent European Centre for Medium‐Range Weather Forecasts orographic parametrizations, but it should be noted that these large Rossby‐Froude orographic effects on drag and wave flux are asymmetric with respect to the mountain's centre line. The theory shows how ‘lift’ forces on the mountain are caused, and how these are related to circulation in horizontal planes around the mountains.
Tropical singular vectors computed with linearized diabatic physics
Authors: Barkmeijer J. , Buizza R. , Palmer T. N. , Puri K. , Mahfouf J.‐F. 
Published: February 2007

AbstractWith the introduction of diabatic processes in the forward and adjoint tangent models of the European Centre for Medium‐Range Weather Forecasts's model, it is possible to determine singular vectors (SVs) for situations where diabatic physics may be important in producing perturbation growth. In this paper, the linear physical parametrizations are used to compute SVs for the tropical region, or subsets thereof, with an optimization time of 48 h. Perturbation growth is measured in terms of the so‐called total energy norm, augmented with a term for specific humidity.Difficulties that may arise in computing tropical SVs, such as associated with spurious upper‐tropospheric perturbation growth, are described. Also, the impact on the SV structure by including a specific‐humidity term in the defining norm is discussed. Using a term for specific humidity based on background‐error statistics in the norm at initial time yields SVs with a more realistic specific‐humidity vertical profile.SVs are determined in various configurations for two tropical cyclones. Results show that targeting in the vicinity of the cyclone is required to obtain SVs associated with the cyclone dynamics. The dominant targeted SVs for tropical cyclones show resemblance to fast‐growing structures found for idealized vortices.