bluuebunny/crossref_metadata_2025_split huggingface dataset:
Measurement of methane and carbon dioxide fluxes from peatland ecosystems by the conditional‐sampling technique
Authors: Beverland I. J. , Moncrieff J. B. , ÓNéill D. H. , Hargreaves K. J. , Milne R. 
Published: February 2007

AbstractThe conditional sampling method was used to measure methane (CH4) and carbon dioxide (CO2) fluxes from peatlands in northern Scotland. Preliminary data were obtained using a simple system. Subsequent automation made continuous flux‐measurements possible. Observed CH4 fluxes were in the range −70 to +110 μmol m−2 h−1 with a mean flux of 23 μmol m−2 h−1. Peak photosynthetic CO2 fluxes were in the range −10 to −30 mmol m−2 h−1. Nocturnal respiration ranged from 0 to +10 mmol m−2 h−1. The conditional sampling observations showed reasonable agreement with measurements of flux by eddy‐covariance, gradient and aircraft methods. Error analyses and laboratory tests were conducted to determine the precision of the flux‐measurement system. The dominant error was associated with the determination of the mixing‐ratio difference in conditionally‐sampled updraught and downdraught air. The standard error of the difference for CH4 was typically 0.15 ng g−1 (0.3 parts in 109 (p.p.b.)) using a careful high‐repetition sampling strategy with a modified gas chromatographic/flame ionization detector system. Under typical daytime atmospheric conditions this corresponded to a standard error in the flux measurement of 10 μmol m−2 h−1, which is consistent with field observations. The empirical beta factor in the conditional sampling equation was found to be insensitive to changes in turbulence intensity and atmospheric stability. Simple upscaling models were used to estimate annual carbon‐fluxes to Great Britain's peatlands of −0.5 Mt of carbon in the form of CO2 and 18 kt in the form of methane.
The dynamics of error growth and predictability in a coupled model of ENSO
Authors: Moore Andrew M. , Kleeman Richard 
Published: February 2007

AbstractUsing new and emerging ideas about the growth of singular vectors (‘optimal perturbations’) in dynamical systems, the dynamics of error growth and predictability in an intermediate coupled model of the El Niño Southern Oscillation (ENSO) is investigated. A mechanism is identified for error growth associated with penetrative‐convection anomalies in the atmosphere. Conditions for error growth via this mechanism are most favourable in the central Pacific where sea surface temperatures (SSTs) are relatively warm, and where changes in SST are moderately sensitive to vertical movements of the main oceanic thermocline.The singular vectors of the coupled system were computed using the tangent‐linear coupled model and its adjoint. The singular‐value spectrum was found to be dominated by one singular vector at all times of the year. The potential for error growth in the coupled model, measured in terms of the growth of energy of the dominant singular vector, is found to vary seasonally, being greatest during the boreal spring. These seasonal variations are associated with the seasonal cycle in SST. During boreal spring and early summer, the SST in the central Pacific is at its maximum, at which time conditions are most favourable for error growth. Springtime is also the time of the ‘predictability barrier’ for ENSO. The potential for error growth is also influenced by the ENSO cycle itself. The results suggest that error growth will be enhanced during the onset of El Niño and suppressed during the onset of La Niña, which indicates that El Niño may be less predictable than La Niña.
Introduction of a local mapping factor in the spectral part of the Météo‐France global variable mesh numerical forecast model
Authors: Yessad K. , Bénard P. 
Published: February 2007

AbstractFor variable horizontal resolution simulations, a new formulation of spectral computations has been implemented in the ARPEGE/IFS spectral global numerical weather forecast model. The formulation makes use of a property of the conformal transformation which defines the stretched geometry: the local mapping factor is a combination of zero and first‐order Legendre polynomials only. Consequently, the mapping factor dependency in the spectral part of the computations can be introduced by simple extra multi‐diagonal operator multiplication. This idea is applied to both the semi‐implicit and the horizontal diffusion schemes to eliminate problems occurring when variable resolution is used together with either long time‐steps or steep orographies. The positive impact of these modifications is presented for semi‐Lagrangian and high‐resolution integrations. The relatively small extra computation and memory cost of the modified schemes is well suited to operational applications.
A new subgrid‐scale orographic drag parametrization: Its formulation and testing
Authors: Lott François , Miller Martin J. 
Published: February 2007

AbstractA scheme for the representation of subgrid‐scale orography (SSO) in numerical weather prediction and climate models is presented. the new scheme arose in part from a desire to represent nonlinear low‐level mountain drag effects not currently parametrized. an important feature of the scheme is that it deals explicitly with a low‐level flow which is ‘blocked’, when the effective height of the subgrid‐scale orography is sufficiently high. In this new scheme, it is assumed that, for this ‘blocked’ flow, separation occurs at the mountain flanks, resulting in a form drag. This drag is parametrized on model levels which are intersected by the SSO, and provides a dynamically based replacement for envelope orography. the upper part of the low‐level flow goes over the orography and generates gravity waves. At the model resolutions considered (T106 and T213) it is assumed that the length scales characteristic of the SSO are sufficiently small for the Coriolis force to be neglected. the various parameters of the scheme are adjusted using an off‐line procedure in which the scheme is used to estimate the mountain drag and the momentum profiles above the Pyrenees; and these estimates are validated with the PYREX data. Forecasts using T106 and T213 resolutions with this new scheme, and with mean orography, show that the forecast mountain drag consistently reproduces the drag measured during PYREX whenever the flow component normal to the ridge is large. Isentropic flow diagnostics, further, show that the new scheme has a realistic impact on the flow dynamics, reinforcing the low‐level wake observed in mesoscale analyses of the flow. With this new scheme and a mean orography, the ECMWF model outperformed, in forecast skill, a version of the model which had an envelope orography and the old gravity‐wave‐drag scheme, while no longer suffering any disadvantages of envelope orography. the proposed low‐level drag parametrization should also be relevant at model horizontal resolutions much higher than T213.
A line of convection embedded in a stratocumulus‐topped boundary layer
Authors: Boers R. , Jensen J. B. , Krummel P. B. 
Published: February 2007

AbstractDuring the Southern Ocean Cloud Experiment a line of convection embedded in an otherwise homogeneous stratocumulus layer was sampled at four levels by means of an instrumented aircraft. At the lower levels there was an abrupt change in microphysical and thermodynamic properties from the updraught region to the environment. At the upper level the change was more gradual and extended over a larger region, indicating the spreading out of the convective region underneath the strong inversion capping the boundary layer. Precipitation was observed at all levels, even in the convective region except near its centre core. Vertical‐velocity fluctuations in the convective region near the inversion were indistinguishable from the surroundings. At these high levels the horizontal wind in the convective region was from the same direction as the air close to the surface, but was at an angle of up to 60° with the horizontal wind from the surroundings. the convective region was characterized by cloud droplet concentrations that were often more than double those recorded in the surrounding cloud. This indicates that the modifications to the cloud droplet spectra associated with the onset of precipitation starts in the convective region and is enhanced in the stratiform region.
Simulation of climate change over europe using a nested regional‐climate model. II: Comparison of driving and regional model responses to a doubling of carbon dioxide
Authors: Jones R. G. , Murphy J. M. , Noguer M. , Keen A. B. 
Published: February 2007

AbstractResults are assessed from a 10‐year simulation of the equilibrium response to doubled carbon dioxide (CO2) over Europe made with a nested high‐resolution regional‐climate model (RCM). the simulated changes are compared against those produced by the driving global general‐circulation model (GCM). the domain‐averaged increases in temperature and moisture content are similar in both models. Because of a stronger hydrological cycle the increases in precipitation and evaporation are larger in the RCM than in the GCM, whereas the reductions in lower and middle tropospheric relative humidity and cloud cover are smaller.The frequency of intense precipitation events increases substantially in both models; however, the fractional changes are significantly smaller in the RCM. the proportion of precipitation associated with convection also increases in both models; however, much of the increase in intense events is explained simply by increased atmospheric moisture concentrations, especially in the RCM.The time‐averaged precipitation changes in the RCM contain a substantial mesoscale component on scales not resolved by the GCM. Attempts are made to reconstruct this component from the changes in the large‐scale atmospheric circulation using empirical relationships calibrated from the 1 x CO2 integration. These are largely unsuccessful, indicating that simple downscaling schemes to generate high‐resolution scenarios of climate change from coarse‐grid GCM data may be of only limited applicability. Further statistical calculations suggest that longer integrations (∼ 30 years) are needed to reduce the sampling uncertainty associated with the simulated mesoscale component to an acceptable level.The large‐scale patterns of change of surface temperature and precipitation reveal significant regional contrasts which are influenced both by changes in atmospheric circulation and regional physical feedbacks. the RCM changes are similar to those of the driving GCM except in summer. the differences in the summer changes are traced to differences between the 1 x CO2 integrations, in which the influence of the lateral boundary forcing on the RCM simulation is found to be anomalously weak. It is argued that any development of significant divergence between the RCM and GCM solutions on scales resolved by the latter may imply the need to refine or replace the one‐way nesting technique currently used in many regional modelling experiments.
Sensitivity of the asian summer monsoon to aspects of sea‐surface‐temperature anomalies in the tropical pacific ocean
Authors: Soman M. K. , Slingo Julia 
Published: February 2007

AbstractThe response of both the time of onset, and the strength of the Asian summer monsoon, to regional aspects of the sea surface temperature (SST) anomalies in the tropical Pacific Ocean associated with El Niño/Southern Oscillation (ENSO) has been investigated through a series of sensitivity experiments with the Universities Global Atmospheric Modelling Programme (UGAMP) General Circulation Model (UGCM). This paper builds on the work of Ju and Slingo (1995) and on their hypothesis that the relationship between the Asian summer monsoon and ENSO involved the latitudinal position and strength of the tropical convective maximum (TCM) over Indonesia and the west Pacific in the preceding spring. the inference from their results was that the modulation of the TCM might be associated either with changes in the Walker circulation through the influence of the east Pacific SST anomalies, or with changes in the local Hadley circulation associated with the in situ SST anomalies in the west Pacific. the investigation has focused on the particular contrasting years of 1983 and 1984. the experiments described in this paper are designed to isolate the effects of the principal SST anomalies in the east and central Pacific, associated with El Niño/La Niño, from those of opposite sign which develop in the west Pacific as a complementary pattern during the mature phase of El Niño/La Niño.The results of the experimentation suggest that, at least for the test cases of 1983 and 1984, the modulation of the Walker circulation, with implied additional subsidence over the eastern hemisphere, is the dominant mechanism whereby the Asian summer monsoon is weakened during El Niñño years. However, the late onset during El Niño years may also be associated with the complementary cold SST anomalies in the west Pacific which delay the northwards transition of the TCM. During La Niño, the modulation of the Walker circulation appears not to be the controlling factor which determines the stronger monsoons. the UGCM results suggest that the complementary warm SST anomalies in the west Pacific enhance the TCM, and it is this in situ response by the TCM that leads to an early onset and stronger monsoon. the importance of warm anomalies in the west Pacific in the development of a strong monsoon has been investigated further through a case‐study of the 1994 season. the year 1994 was an El Niño year in which the monsoon was unexpectedly active, but which was also marked by warmer than normal SSTs in the west Pacific.The sensitivity experiments have also elucidated the role of El Niño in influencing the precursory signature of stronger subtropical westerlies over India and south‐east Asia during the winter and spring preceding weak monsoons. the results suggested that the equatorwards shift of the subtropical jet is a remote response to the warm SST anomalies in the central and east Pacific associated with El Niño.
Seasonal instability of pacific sea‐surface‐temperature anomalies
Authors: Efimov V. V. , Prusov A. V. , Shokurov M. V. 
Published: February 2007

AbstractThe main purpose of this study is to reveal the instability of sea‐surface‐temperature (SST) anomalies in connection with the ‘predictability barrier’ of El Niño established by Webster and Yang. the characteristics of linear instability of Pacific SST anomalies are computed using monthly COADS data and the method of Lyapunov exponents. Analysis is carried out separately for the northern and tropical part of the Pacific Ocean via approximating SST fields using a few leading empirical orthogonal functions. Changes of Lyapunov exponents throughout the year are computed and periods of the most unstable and stable seasons revealed. of most interest is the spring instability of SST anomalies both in the tropical and extratropical Pacific. This instability is crucial for the existence of a predictability barrier for an El Niño event.
A theory for vortex rossby‐waves and its application to spiral bands and intensity changes in hurricanes
Authors: Montgomery Michael T. , Kallenbach Randall J. 
Published: February 2007

AbstractIn this paper we examine further the physics of vortex axisymmetrization, with the goal of elucidating the dynamics of outward‐propagating spiral bands in hurricanes. the basic shysics is illustrated most simply for stable vorticity monopoles on an f‐plane. Unlike the dynamics of sheared disturbances in rectilinear shear flow, axisymmetrizing disturbances on a vortex are accompanied by outward‐propagating vortex Rossby‐waves whose restoring mechanism is associated with the radial gradient of storm vorticity. Expressions for both phase and group velocities are derived and verified; they confirm earlier speculations on the existence of vortex Rossbywaves in hurricanes. Effects of radially propagating vortex Rossby‐waves on the mean vortex are also analysed. In conjunction with sustained injection of vorticity near the radius of maximum winds, these results reveal a new mechanism of vortex intensification. the basic theory is then applied to a hurricane‐like vortex in a shallow‐water asymmetric‐balance model. the wave mechanics developed here shows promise in elucidating basic mechanisms of hurricane evolution and structure changes, such as the formation of secondary eye‐walls. Radar observations possessing adequate temporal resolution are consistent with the predictions of this work, though more refined observations are needed to quantify further the impact of mesoscale banded disturbances on the evolution of the hurricane vortex.
Stationary rossby‐wave propagation in a baroclinic atmosphere
Authors: Ambrizzi Tércio , Hoskins Brian J. 
Published: February 2007

AbstractObservational studies of teleconnections in both solsticial seasons have suggested various patterns of behaviour linked mainly to westerly jets and equatorial westerlies. These have been interpreted using barotropic Rossby‐wave theory, and simulated using stationary forcing in barotropic models linearized about 300 hPa flows. In this paper the relevant propagation behaviour is investigated using a baroclinic model with three‐dimensional, climatological basic states. Time integrations are performed using localized thermal forcing as a wavemaker. the propagation results are qualitatively very similar to those obtained with the barotropic model, though there are quantitative differences. the westerly jets still act as strong waveguides. the shorter wavelengths and smaller wave‐activity speeds found with the baroclinic model are generally in better agreement with observation. Propagation into and from the upper tropospheric equatorial westerlies in the east Pacific, propagation across North America and propagation from Europe across the Arabian Gulf are all found. the extent to which upper tropospheric westerlies reach towards the equator influences the propagation into those regions. Barotropic models can only represent such behaviour if the basic state reflects the near‐equatorial upper tropospheric zonal wind structure.
A contour‐advective semi‐lagrangian numerical algorithm for simulating fine‐scale conservative dynamical fields
Authors: Dritschel David G. , Ambaum Maarten H. P. 
Published: February 2007

AbstractThis paper describes a novel numerical algorithm for simulating the evolution of fine‐scale conservative fields in layer‐wise two‐dimensional flows, the most important examples of which are the earth's atmosphere and oceans. the algorithm combines two radically different algorithms, one Lagrangian and the other Eulerian, to achieve an unexpected gain in computational efficiency.The algorithm is demonstrated for multi‐layer quasi‐geostrophic flow, and results are presented for a simulation of a tilted stratospheric polar vortex and of nearly‐inviscid quasi‐geostrophic turbulence. the turbulence results contradict previous arguments and simulation results that have suggested an ultimate two‐dimensional, vertically‐coherent character of the flow. Ongoing extensions of the algorithm to the generally ageostrophic flows characteristic of planetary fluid dynamics are outlined.
Effect of relative humidity on the electrical conductivity of marine air
Authors: Kamra A. K. , Deshpande C. G. , Gopalakrishnan V. 
Published: February 2007

AbstractMeasurements of the atmospheric electric conductivity made in the equatorial Indian Ocean and Arabian Sea in August and September 1991 show that the value of conductivity decreases from 2.3 × 10−14 mho m−1 in the equatorial Indian Ocean, where relative humidity of the surface air is 70‐80%, to 1.1 × 10−14 mho m−1 in the region of the Somali current, where relative humidity of the surface air increases to 80‐90%. the inverse relationship observed between conductivity and relative humidity, in spite of showing a large scatter, is stronger in the case of negative than positive conductivity. the sharp increase in the sizes of ions and marine aerosol particles when the relative humidity exceeds 75‐80% is proposed as the cause of the observed decrease in conductivity in the region of the Somali current.
Relations between interannual and intraseasonal monsoon variability as diagnosed from AMIP integrations
Authors: Ferranti L. , Slingo J. M. , Palmer T. N. , Hoskins B. J. 
Published: February 2007

AbstractMonsoon variability on intraseasonal and interannual time‐scales is analysed using data from five 10‐year European Centre for Medium‐Range Weather Forecasts Atmospheric Model Intercomparison Project integrations, which differ only in their initial conditions. the results show that monsoon fluctuations within a season and within different years have a common dominant mode of variability. the spatial pattern of the common dominant mode in precipitation has a pronounced zonal structure, with one band of anomalous rainfall extending from 20°N to 5°N, covering most of the land areas, with the other band, of opposite sign, lying between 5°N and 10°S, mostly over the Indian Ocean. This mode therefore describes both the active/break monsoon spells associated with fluctuations of the Tropical Convergence Zone (TCZ) between the continental and the oceanic regime and the principal pattern of interannual variability of monsoon rainfall. In the observations the oscillations between active and break monsoon spells have similar behaviour, although the model is deficient in representing the rainfall variability over India.On the intraseasonal time‐scale the transition between the two regimes seems to have a chaotic nature. In addition the probability density function of the principal mode is bimodal for the years in which this mode is particularly dominant. These two results indicate a possible similarity with the Lorenz 3‐component chaotic model. Northward‐propagating convective regions, simulated by the model, are not clearly associated with the phase transitions of the TCZ regime. the timing of the monsoon onset appears to be modulated by the phase of the El Niño/Southern Oscillation during the preceding season, consistent with observational studies.The results suggest that the dominant mode may also represent some components of the observed monsoon variability. the interannual fluctuations of the dominant mode exhibit only a weak level of reproducibility compared with the relatively large predictability of a broad‐scale monsoon wind‐shear index.
Biosphere feedback on regional climate in tropical North Africa
Authors: Xue Yongkang 
Published: February 2007

AbstractThe impact of land‐surface degradation over the Sahel area on seasonal variations of atmospheric and hydrological components over tropical north Africa are investigated in a general‐circulation model numerical experiment, together with the mechanisms involved. The model was integrated for four years with and without vegetation change over the Sahel region with different initial atmospheric conditions. The results demonstrate that the degradation of the land surface can have a significant impact on the Sahelian regional climate. It increases the surface air temperature and reduces the precipitation, runoff and soil moisture over the Sahel region during the July‐August‐September (JAS) season. The impact is not only limited to the specified desertification area and the JAS season but is found also to the south of this area and extends into the October‐November‐December season. The changes in the annual rainfall cycle and the JAS mean surface temperature over the Sahel area are consistent with the observed climate anomalies of the past 40 years. The changes in rainfall to the south of the Sahel including eastern Africa are also in line with the observed anomalies.The reduction in total diabatic heating rate and relative increase in subsidence motion in the upper troposphere are consistnet with the rainfall anomalies. The variations in convective heating rate, which were caused by changes in latent‐heat flux from the land surface and moisture flux convergence in the atmosphere, are the dominating factors in this process. The radiative cooling is a secondary effect. The influence of the initial conditions on the simulation of the soil water balance is also analysed.