bluuebunny/crossref_metadata_2025_split huggingface dataset:
A vegetation‐atmosphere interaction study for Amazonia deforestation using field data and a ‘single column’ model
Authors: da Rocha Humberto R. , Nobre Carlos A. , Bonatti José P. , Wright Ivan R. , Sellers Piers J. 
Published: February 2007

AbstractThe recent high deforestation rates in Amazonia have raised the question of how climate, mainly the precipitation patterns, might be affected as pasture land replaces the primary forest. This work initially shows how the dry season soil moisture and water stress in typical Amazonian pasture modifies the energy partitioning at the surface. Low‐level moisture convergence is a primary mechanism to trigger convective cloud formation and precipitation. The relationship between moisture convergence and local surface evaporation in generating precipitation in Amazonia is studied utilizing a one‐dimensional ‘single column’ model (SiB‐1D). SiB‐1D couples a surface‐vegetation model (SiB) to a physical parametrization of deep convection (Kuo scheme), radiation, turbulent diffusion and large‐scale precipitation. Model simulations for short periods (2 days) show its ability to calculate the Amazonian surface energy‐balance components and boundary‐layer dynamics when compared with field observations from the Rondǒnia Boundary‐Layer Experiment and the Anglo‐Brazilian Amazonian Climate Observation Study. The model was further used to test the sensitivity of the deep convection scheme to a range of typical low‐level moisture convergence situations in a second experiment, and a third test investigated the local convective precipitation generated over forest and grass vegetation as a function of available soil moisture. Results showed that the rainfall over forest vegetation appeared to be almost insensitive to soil water stress whereas reduced precipitation was generated over pasture. When available soil water fell below a threshold of 60% the calculated precipitation over the pasture sites rapidly declined. Although these results are confined by the short integration period and the initial atmospheric profiles, they help to strengthen the notion that deforestation reduces evaporation and convective precipitation, especially during the dry season: a result already indicated by some previous general‐circulation model experiments.
Potential vorticities in semi‐geostrophic theory
Authors: Roulstone Ian , Sewell Michael J. 
Published: February 2007

AbstractA family of kinematically possible motions in shallow‐water versions of semi‐geostrophic theory is devised, with variable Coriolis parameter, for which two distinct measures of potential vorticity take the same value. Conditions are described for which this value is conserved following the actual motion of a particle. The results are related to the existing literature, in particular for a constant Coriolis parameter.
Stratospheric inertia–gravity waves generated in a numerical model of frontogenesis. II: Wave sources, generation mechanisms and momentum fluxes
Authors: Reeder Michael J. , Griffiths Morwenna 
Published: February 2007

AbstractUsing a dry two‐dimensional numerical model, we investigate the source regions and generation mechanisms for stratospheric inertia–gravity waves accompanying frontogenesis. Attention is focused on run 3 from Part I of this paper (Griffiths and Reeder 1996), here referred to as our ‘standard calculation’.Ray‐tracing techniques are used to identify the source regions of the waves. Although these results are strictly valid only in the far field away from the frontal region, they suggest that the waves originate in the frontal region. The ray equations also show that the horizontal wavelength decreases exponentially with time because of the action of the basic‐state deformation, whereas the vertical wavelength increases linearly with time because of the shear of the basic‐state cross‐front wind.It is argued that the cross‐front circulation may be viewed, for the most part, as the unsteady source of stratospheric gravity‐waves in the standard calculation. This position is supported by a linear forced‐wave calculation, in which the forcing is provided by the cross‐front circulation from the standard calculation. Comparison of the vertical‐motion fields in the stratosphere for the standard and forced‐wave calculations show good agreement. An estimate of the nonlinear interaction between the frontal circulation and the wave field is made. It is shown that the back‐reaction of the waves on the forcing may be neglected, at least, qualitatively.The time–space mean of the vertical flux of horizontal momentum in the stratosphere accompanying the modelled waves is approximately 1.4 × 10−4 N m−2 where the average is taken over −2000 km ⩽ x ⩽ 3000 km, 15 km ⩽ z ⩽ 29 km and 24 h ⩽ t ⩽ 48 h.
Lee waves over the English Lake District
Authors: Vosper S. B. , Mobbs S. D. 
Published: February 2007

AbstractThis paper presents the results of a field experiment which was conducted for three weeks during November 1991. The aim was to investigate the properties of internal gravity waves in the troposphere and lower stratosphere which were generated by the mountains of the Lake District in north‐west England, UK. The main technique used was that of multiple sounding using radiosondes.Throughout the experiment measurements obtained from groups of five radiosondes launched nearly simultaneously indicate that the waves are quasi‐steady and trapped within the troposphere. This is illustrated by a single detailed case study for 26 November in which the radiosonde ascent‐rate fluctuations are as large as 6.5 m s−1 peak to peak. These measurements are supported by aircraft observations.Simple techniques for analysing the structure of the wave‐field are presented and these include the calculation of a phase‐line tilt and phase speed and comparisons with simple analytical and numerical solutions for trapped gravity waves.A three‐dimensional linear numerical model is used to simulate the waves observed on 26 November and three other cases also. The model predictions are shown to compare well with the observations. This is particularly true in the mid‐troposphere, where the model reproduces both the positions and amplitudes of the peaks and troughs in the vertical‐velocity field.Spectral analysis of the local orography and examination of typical variations of the wind and buoyancy frequency with height show that gravity waves generated by the orography in the Lake District will frequently be trapped in the troposphere.
Variational analysis of humidity information from TOVS radiances
Authors: McNally A. P. , Vesperini M. 
Published: February 2007

AbstractThe impact of assimilating TOVS radiance data in the European Centre for Medium‐Range Weather Forecasts humidity analysis is evaluated. It has been found that the introduction of a one‐dimensional variational analysis scheme (1DVAR) applied to the TOVS radiances significantly improves the representation of many aspects of the hydrological cycle. The theoretical information content of the TOVS radiance data is discussed and found to be consistent with significant changes observed in the mid upper‐tropospheric moisture fields when the radiance data are assimilated. In particular, a tendency of the model (without radiance assimilation) to produce a tropical humidity structure that is far too dry is removed, and excessively moist conditions in the southern sub‐tropics are improved. It is argued that the latter problem originates from the use of operational NESDIS retrieved products in the analysis. The humidity adjustments caused by the assimilation of TOVS radiances are accompanied by significant changes in the model dynamics, especially the description of the tropical Hadley circulation. One such case is described in detail, where the moistening of the tropics and drying of the sub‐tropics has resulted in a stronger mean analysed meridional circulation in the Atlantic. The analysis changes are also shown to improve the medium‐range forecasting of humidity, together with some associated benefit in the prediction of cloud and precipitation.
On four‐dimensional variational assimilation of ozone data in weather‐prediction models
Authors: Riishøjgaard Lars Peter 
Published: February 2007

AbstractRoutinely made satellite observations of total ozone contain information about the dynamical state of the atmosphere in the regions where the ozone gradients are strong. It is argued that four‐dimensional variational assimilation (4D‐VAR) of these data with a meteorological model may be a way of extracting this information in order to use it for determining the initial condition of the model. In the first part of this paper the problem of assimilating ozone data in a variational context is discussed. In the second part a series of simple experiments is presented in which ozone pseudo‐observations are assimilated with a barotropic vorticity‐equation model. The aim of these experiments is to examine whether the atmospheric flow at a given level near the tropopause can be reconstructed by a 4D‐VAR scheme using a series of simulated observations of passive tracer mixing ratios. Both data produced by the assimilating model itself and data generated by a more realistic three‐dimensional model have been tested. It is found that when started from a purely zonal flow field, 4D‐VAR can to a large extent reconstruct the flow field, using only observations of the mixing ratio of the tracer. The quality of the result depends on the resolution of the model and on the choice of the length of the assimilation window.
Dynamics of cumulus rising into stratocumulus as observed during the first ‘Lagrangian’ experiment of ASTEX
Authors: de Roode Stephan R. , Duynkerke Peter G. 
Published: February 2007

AbstractThe Atlantic Stratocumulus Transition Experiment took place during June 1992 in the eastern Atlantic Ocean in an area extending south‐east of the Azores to Madeira. One of the goals of this experiment was to study the physical processes responsible for the transition from stratocumulus to shallow cumulus clouds. The cloud type most frequently observed was cumulus rising into stratocumulus. Two ‘Lagrangian’ experiments were carried out to study the evolution of the boundary layer and to observe any transition in the cloud characteristics downstream.Here we concentrate on the last flight of the first ‘Lagrangian’ experiment during which cumulus cloud was penetrating thin and broken stratocumulus from below. The immediate cause of the cumulus convection was condensation at the top of the well‐mixed surface‐based boundary layer. The lifting condensation level was observed to be at about 500 m and above this height there was a conditionally unstable layer up to 1600 m. In the upper part of the conditionally unstable layer the horizontally averaged thermodynamical variables showed that the air was very close to saturation, indicating that the upper part of the cloud deck consisted of broken clouds. The vertical structure of mean variables, variances, fluxes and turbulent kinetic energy budget is discussed. One of the findings is that the cumulus convection is an important factor in counteracting the gradual dissipation of the stratocumulus deck in that it supplies moisture to the latter. Furthermore, the net longwave radiative loss at cloud top was observed to be about 40 ± 15 W m−2, which tended to destabilize the cloud layer from above. The skewness factor S showed that the upward convection was quite intense owing to the rising cumuli, the factor S reaching a maximum of about 2 ± 0.7 in the middle of the cloud layer. The skewness was found to be slightly negative near the cloud top.
High‐resolution numerical‐model integrations to validate gravity‐wave‐drag parametrization schemes: A case‐study
Authors: Broad Adrian S. 
Published: February 2007

AbstractA three‐dimensional non‐hydrostatic numerical model is used to simulate the lee‐wave event IOP3 (1800 GMT 14 October 1990 to 1200 GMT 15 October 1990) from the PYREX mountain experiment. Using high resolution with a horizontal gridlength of 1.5 km and a realistic terrain‐height specification the resulting model vertical‐velocity fields are compared directly with observations from aircraft flights. The overall agreement of the model‐simulated wave structure with the aircraft data is very encouraging, particularly given the high degree of nonlinearity and finite amplitude wave activity observed during this event. Given the very restrictive three‐dimensional coverage of the observations from the field experiment, the numerical‐model results are used as surrogate observational data to give exhaustive three‐dimensional data over the whole model volume. The model integrations are subsequently used to compute areally averaged surface‐pressure drags and height profiles of the vertical flux of horizontal momentum. Assuming these wave stresses to be an accurate representation of the real atmospheric stresses, they are compared, as a validation procedure, against grid‐box flux values taken from a gravity‐wave‐drag parametrization scheme in a model with resolution 2.5° latitude by 3.75° longitude. The drag scheme in question compares favourably with the model domain‐averaged stresses.
Sensitivity of a simulated tropical squall line to long‐wave radiation
Authors: Dharssi I. , Kershaw R. , Tao W. K. 
Published: February 2007

AbstractA two‐dimensional cloud‐resolving model is used to simulate a tropical squall line and to examine its sensitivity to long‐wave radiation. Model results show that the magnitude of the sensitivity to long‐wave radiation depend on the organization and structure of the squall line. For one simulation, the inclusion of long‐wave radiation increased surface precipitation by nearly a third. Long‐wave radiation is also found to enhance the mid‐level rear inflow and upper‐level outflow of the squall line. Sensitivity experiments are performed to examine the role of cloud‐top cooling and cloud‐base warming, differential cooling between clear and cloudy regions, and domain‐wide cooling.
Ambiguity removal and assimilation of scatterometer data
Authors: Stoffelen By A. , Anderson D. 
Published: February 2007

AbstractThe ERS‐1 scatterometer has proved to be a source of high quality ocean surface wind data, but a problem remains, namely the dual directional ambiguity of the solutions. an ambiguity removal scheme, called PRESCAT, is described based on our experience (a) that information on wind direction retrieval skill is an important input to ambiguity removal, (b) that wind‐vector filtering is beneficial compared to wind‐direction filtering, and (c) that already meteorological forecast information enables us to remove correctly approximately 95% of all ambiguities. the performance of the ambiguity filter is very good compared with other operational ambiguity removal schemes. Furthermore, a statistical interpolation analysis system called ‘buddy’ check is used effectively to identify and remove the few (approx. 0.1%) wrongly selected solutions.Assimilation of scatterometer winds has a beneficial impact on analyses and short‐range forecasts, probably mainly from improvements on the subsynoptic scales. On the wider temporal and spatial scales, scatterometer winds were also found beneficial, but only in the absence of satellite temperature soundings (SATEMs). In assimilation experiments in which the latter were included, the scatterometer did not provide a beneficial impact on the medium‐range forecast. Moreover, the conventional observations, including SATEMs are shown to have adverse effects on the surface‐wind analysis. We believe that both the redundancy and the adverse effects on the surface‐wind field are explained by the rigid formulation of the 6‐hour‐forecast error structure; the forecast error is assumed to be flow‐independent, and information on the special meteorological conditions in the atmospheric planetary boundary layer is lacking. to make observational systems more useful and complementary for numerical weather prediction the effects of the structure functions have to be investigated more precisely. In an adaptive four‐dimensional variational assimilation scheme the effect of the assumptions on forecast‐error structure will be less. We show that, in a variational framework, scatterometer backscatter measurements are difficult to assimilate directly. Instead, we derive and illustrate an alternative procedure to assimilate retrieved winds rather than backscatter measurements.
Observations of isolated wave‐turbulence interactions in the stable atmospheric boundary layer
Authors: Edwards N. R. , Mobbs S. D. 
Published: February 2007

AbstractTurbulence measurements of the stable atmospheric boundary layer made at Halley station in Antarctica are presented. the interaction between small‐scale turbulence and larger solitary wave disturbances is investigated. Spectral methods are used to separate the wave and turbulence parts of the flow, and the appropriateness of this approach is discussed. Both individual wave events and an average of 18 wave events are studied. the transport of wave‐induced velocity by wave‐induced turbulence is found to be mostly upwards, whilst a strong downward turbulent heat flux accompanies each wave. Waves are often found to be associated with a burst of low‐level turbulence which occurs later than the perturbation in the mean shear. the effect of the turbulence on the wave is shown to be small, whilst the instantaneous local effect of the wave on the turbulence appears to be approximately linear in the wave amplitude. Comparisons are made between the observations and numerical predictions. These tend to confirm the need for at least second‐order closure schemes for the turbulence modelling.
An observational study of the evening transition boundary‐layer
Authors: Grant A. L. M. 
Published: February 2007

AbstractData collected during the evening transition period are described. Well‐defined flux profiles are established rapidly after the surface sensible‐heat flux changes sign. the sensible‐heat flux profile is strongly curved, so that significant cooling only occurs within the lower half of the boundary layer.The turbulent kinetic energy flux is a significant term in the turbulent kinetic‐energy balance in the upper half of the boundary layer early in the transition period. Later the magnitude of the turbulent kinetic‐energy flux decreases to small values, so that the turbulence in the upper half of the boundary layer begins to decay. the time‐scale of this decay is of the order of u*/h, and should also be the time‐scale for the changes in the boundary‐layer depth.Local scaling does not appear to be valid, except in a shallow layer near the surface. This is because the turbulence time‐scale is comparable to the adjustment time‐scale of the mean flow over much of the boundary layer.
Seasonal heat and freshwater budgets of the upper ocean in the North‐east Atlantic
Authors: McCulloch M. E. , Leach H. 
Published: February 2007

AbstractHeat‐ and freshwater‐content changes were calculated from two repeated sections at 48°N, 18°W and 48°N, 26°W, surveyed during the Vivaldi cruise of the research ship RRS Charles Darwin in May 1991. Best estimates of the air‐sea heat‐fluxes agreed within error with the observed heat‐content changes, suggesting that diabatic forcing dominated the heat budget. This was confirmed by a numerical model which showed that Ekman and geostrophic advection and eddy diffusion together contributed less than 20 W m−2 to the heat budget.However, the model also showed that, near the oceanic polar front under westerly winds, Ekman advection of cold, fresh water southwards over the front can cause ‐40 W m−2 and 4 mm d−1 heat‐ and freshwater‐content changes in the upper 53 m, compared to air‐sea fluxes of typically 67 W m−2 and less than 1 mm d−1. the Ekman advection contributed little to the model's density‐budget as the advective cooling and freshening had opposing effects. However, an air‐sea interaction model showed that the advective surface‐cooling may be reduced because it triggers an extra air‐sea heat flux and the remaining advective freshening then makes an important contribution to the density budget.
The impact of increased resolution on predictability studies with singular vectors
Authors: Buizza R. , Gelaro R. , Molteni F. , Palmer T. N. 
Published: February 2007

AbstractThe dominant singular vectors of the tangent propagator of the European Centre for Medium‐Range Weather Forecast (ECMWF) numerical weather‐prediction model are an essential component of the ECMWF Ensemble Prediction System (EPS). These singular vectors describe the principal finite‐time linear instabilities of the northern extratropical atmospheric circulation.The impact of increasing the horizontal resolution of the tangent model from T21 to T42 on three different types of initial perturbation, which make use of these singular vectors, is considered. the increase of resolution allows the possibility of describing more accurately instabilities with an upscale cascade of energy from subsynoptic to synoptic scales. Two of the perturbations are referred to as the pseudo‐inverse and sensitivity vectors. These are both diagnostic, and involve estimating from the short‐range forecast error the component of initial error in the unstable subspace. the third type of perturbation is used to construct the set of initial states for the ensemble prediction scheme. Linear and nonlinear integrations are described using these different types of perturbation.All the results point to the conclusion that the higher‐resolution calculations lead to more accurate results. Specifically, it is shown that significant amounts of energy in the pseudo‐inverse and sensitivity vectors reside in scales smaller than T21, and that the skill of the EPS is improved when generating the initial perturbations using T42 singular vectors. This is found to be especially true in a number of cases when the control forecast was particularly poor.As a consequence, the predictability of synoptic‐scale disturbances in the atmosphere on time‐scales of a few days is likely to be determined by errors in the initial state on subsynoptic scales.In addition to these conclusions, it is proved that the analysis of the relationship between the EPS and the pseudo‐inverse perturbations can be used to define the initial amplitude of the EPS perturbations.
Parametrization of momentum transport by convection. I: Theory and cloud modelling results
Authors: Kershaw R. , Gregory D. 
Published: February 2007

AbstractThe theory of the mass‐flux approach to parametrization of convective momentum‐transport is presented. A cloud‐resolving model is used to simulate momentum transport by ensembles of deep convective clouds in two very different regimes: a mid‐latitude cold‐air outbreak, and tropical convection forced by convergence. Idealized, unidirectional, wind‐profiles are used to simplify interpretation of the results. Diagnostics relevant to the parametrization problem are presented, and it is shown that, for both regimes, the approximations inherent in the parametrization equations are reasonable. the pressure gradients inside clouds play an important role in determining horizontal velocities in the clouds. For the cold‐air outbreak with linear shear, the results suggest that these pressure gradients are proportional to the shear and to the up/downdraught mass‐fluxes. Sensitivity studies suggest that the results are not very sensitive to resolution or parametrization of subgrid‐scale processes, giving some confidence that the results are reasonably accurate. For the tropical case with a low‐level jet, the pressure gradients change sign with the wind shear. In part II of this paper, these results will be used to develop and validate a mass‐flux parametrization of convective momentum‐transport which is tested in single column and global versions of the Meteorological Office Unified Model.