bluuebunny/crossref_metadata_2025_split huggingface dataset:
On the relationship between northern hemispheric weather regimes in wintertime and spring precipitation over China
Authors: Liu Hui , Tosi Ennio , Tibaldi Stefano 
Published: February 2007

AbstractIn this study, a possible relationship between global wintertime weather regimes in the northern hemisphere (NH) mid latitudes and the total precipitation over mainland China in the season following is investigated. Weather regimes are defined, as by Molteni et al. (1990), as clusters in the low‐dimensional phase space of the leading rotated EOFs of 500 hPa geopotential height, while the total precipitation over continental China is cumulated over different periods in the following spring (representative of a pre‐Mey‐Yu rainfall). Some distinct NH winter weather regimes are found to be highly correlated with the subsequent spring rainfall and so constitute a good basis for a method of quantitative prediction based on simple linear regression techniques.
The adjustment of a rotating, stratified fluid subject to localized sources of mass
Authors: Shutts Glenn 
Published: February 2007

AbstractThe time‐dependent response of a rotating, stratified fluid to localized, impulsive forcing is examined under the hydrostatic approximation and the assumption of two‐dimensionality. Analytic solutions of the linearized quasi‐Boussinesq equations are derived for quiescent and uniformly sheared flows when the axis of the forcing function lies in the direction of the mean wind shear. The mass forcing function, it is imagined, represents the effect of deep, penetrative convection at the cloud top.In the unsheared case, the phase lines of gravity waves emanating from the forcing region adopt a fanlike configuration symmetric about the vertical. The slopes of the phase lines become more horizontal as time progresses resulting in a vertical ‘fine structure’. As a consequence of this the value of the wave Richardson number may fall below 1/4 in the vicinity of the forcing function.When the basic‐state shear is chosen to be equal to the buoyancy frequency, the flow has neutral symmetric stability. In this case the response to impulsive heat and momentum forcing is biased towards modes with phase lines of orientation similar to that of the absolute momentum (or equivalently, potential temperature surfaces). A permanent, ageostrophic residual flow along these surfaces is revealed in the long‐time limit. In contrast, the response to mass forcing shows a far greater degree of symmetry about the vertical, with no residual ageostrophic flow.In general, for symmetrically stable flows, a geostrophic and hydrostatically balanced flow remains after the gravity‐wave energy has dispersed, consistent with the potential vorticity change caused by the mass forcing. However, the potential vorticity anomaly created is proportional in size to the basic state potential vorticity. For slantwise neutral flows therefore, mass forcing does not create a potential vorticity anomaly, and no balanced flow remains.
Dependence of the energy balance of the Greenland ice sheet on climate change: Influence of katabatic wind and tundra
Authors: Meesters Antoon 
Published: February 2007

AbstractThe summer‐time atmospheric circulation at the margin of a large ice sheet, with an extended adjacent tundra, is investigated with a two‐dimensional atmospheric model. The assumed topography corresponds to the section through west Greenland for which the Greenland Ice Margin Experiment (GIMEX) was performed. With this model, the local sensitivity of the ablation of the ice sheet to changes in the external parameters (which pertain to climate) is explored. The main focus is the sensitivity which is related to atmospheric dynamics. A comparison is made between the results with an exposed tundra and results with a snow‐covered tundra. In the latter case, the katabatic wind is strongly reduced close to the ice margin, so that the downward sensible‐heat flux for the ablation zone is more than halved. This shows that the influence of the exposed tundra on the energy balance of the ice sheet is important. Increasing the initial air temperatures leads to a stronger katabatic wind. As a consequence, the downward sensible‐heat flux at the ablation zone is strongly enhanced. Neglecting the change of the katabatic wind would lead to a much smaller growth of this flux, which shows that the presence of an exposed tundra increases the sensitivity of the energy balance of the ice sheet to climate change.For those runs the large‐scale wind is zero, but in the presence of an inland‐directed large‐scale wind a strong surface wind parallel to the glacier front is found to develop above the ablation zone, by which the sensible‐heat flux is increased. The advection of heated air from the tundra to the ice sheet plays a lesser role.
An observational and theoretical study of the radiative properties of cirrus: Some results from ICE'89
Authors: Francis P. N. , Jones A. , Saunders R. W. , Shine K. P. , Slingo A. , Sun Zhian 
Published: February 2007

AbstractObservations of the microphysical and radiative properties of two cirrus clouds are reported. The measurements were taken from the C‐130 aircraft of the UK Meteorological Research Flight as part of the field campaign of the International Cirrus Experiment, ICE'89.Cirrus microphysical properties are analysed in terms of ice water content and a suitably defined ice crystal ‘effective radius’. Estimates of the ice water content are derived from both a standard Particle Measuring Systems 2D‐C cloud probe and a Lyman‐α total water content (TWC) hygrometer. With both flights large discrepancies are seen to occur between the two instruments near the cloud base and top, with the ice water contents derived from the TWC probe being consistently greater than those from the 2D‐C probe. Evidence is presented which suggests that the differences near the cloud base are due to a significant number of large (>800 μm) crystals, which the 2D‐C probe is unable to sample. Near the cloud top the differences are assumed to be due to small (<100μm) crystals, which the 2D‐C probe is again unable to sample correctly. Thus more weight has been given to the TWC probe measurements of ice water content, with the measurements of ice crystal effective radius being adjusted accordingly.Model clouds, constructed from the microphysical measurements, are used as input to a number of radiation parametrizations, both in the long‐wave and short‐wave regions of the spectrum. In the long wave, model agreement with both narrow‐band and broad‐band measurements is reasonably good, although discrepancies exist near the cloud tops. Some evidence is presented which suggests that the measured effective radii from one of the flights are too large to explain the radiative characteristics of the cloud. The effect of long‐wave scattering is shown to be small. The inferred long‐wave broad‐band mass‐absorption coefficient varies between 0.002 m2g‐1 and 0.087 m2g−1, and it shows a marked inverse dependence on the ice crystal effective radius, as anticipated by simple theoretical calculations. In the short wave, agreement between the measured irradiances and those computed using several parametrizations is again reasonably good, although limited by the natural variability of the clouds. The net radiation budgets of both cloud layers are analysed, and the possibility of the existence of a relationship between ice water content and effective radius is considered.
Wave‐zonal‐flow interaction and ultra‐low‐frequency variability in a simplified global circulation model
Authors: James P. M. , Fraedrich K. , James I. N. 
Published: February 2007

AbstractThe interactions between baroclinic waves and the zonal‐mean mid‐latitude flow are investigated using a multilevel primitive‐equation model of moderate resolution with idealized surface friction and diabatic heating terms. Integrations corresponding to 100 years were performed under perpetual solstice conditions. Empirical Orthogonal Function (EOF) analyses were carried out on the zonal‐mean zonal wind, [u]. Adjustments to the winter hemisphere zonal jet due to the growth and decay of baroclinic waves can be described in terms of the first two EOF patterns, which exhibit substantial interannual variability. For a single lifecycle, such adjustments take the form of a circuit of a phase‐space whose axes are defined by the amplitudes of the two EOFs. The cumulative effects of multiple lifecycles lead to a low‐frequency circulation in the phase‐space, with a timescale of about 150 days. We suggest that this effect may be intrinsically related to the internal generation of ultra‐low‐frequency variability in the model. Normal‐mode lifecycle calculations, including dissipation, reveal similar phase‐space properties. The growth rates of normal‐mode disturbances are shown to be highly sensitive to the phase of the first EOF.
Short baroclinic waves of small amplitude in a moist environment
Authors: Fantini Maurizio , Buzzi Andrea 
Published: February 2007

AbstractBaroclinic instability at short wavelengths is investigated using a primitive‐equation non‐hydrostatic two‐dimensional numerical model in which the vertical shear is uniform and in which moisture is present in the lower part of the troposphere. The height of the transition between saturated and unsaturated air determines the vertical and horizontal scales of the most unstable modes. As the transition region is raised the most unstable wavelength increases, until the large‐scale (moist) Eady wave, extending to the tropopause, prevails.
Lagrangian flow in the middle atmosphere
Authors: Sutton Rowan 
Published: February 2007

AbstractThe Lagrangian view of air flow in the middle atmosphere is studied by computing tens of thousands of particle trajectories in a global primitive‐equation model. This approach focuses attention on the long‐term fate of air and provides a high‐resolution view of air transport. Simulations are carried out for both the northern and southern winters to permit an inter‐hemispheric comparison. The first part of the paper concentrates on transport in the meridional plane. Trajectories followed for many months show patterns which are well organized on a global scale. Ensembles of particles followed for a much shorter time are used to calculate a Lagrangian mean meridional circulation which shows close similarities to published diabatic circulations calculated from observational data. Maps of meridional dispersion reveal a broad mid‐latitude band, with well defined boundaries, in which rapid dispersion occurs. A more detailed visualization of transport in and around the southern polar vortex in late winter shows that small scales are generated in this mid‐latitude region, but not in the vortex nor at low latitudes. The ability of the vortex to retain a labelled air mass for many months is clearly demonstrated, and the relevance of the results to understanding springtime ozone depletion is discussed.
Influence of strong gradients of wind and humidity on frontal ageostrophic circulations
Authors: Lagouvardos K. , Lemaitre Y. 
Published: February 2007

AbstractThe impact is studied of strong gradients of wind and humidity on the ageostrophic circulation developed around a conceptual cold‐frontal system. The simulated basic wind and temperature fields are chosen to best represent the frontal observations made during the Fronts 87 field experiment and, in particular, to take account of the presence of strong low‐level jets. The numerical solution of the diagnostic Sawyer—Eliassen equation in both its adiabatic and diabatic forms permitted us to test the impact of the wind and humidity structure on the corresponding ageostrophic circulation. In particular, strong negative shear above the low‐level jet core enhanced subsiding motions on the warm side of the front, which restricted the horizontal extent of the cloud cover. On the other hand, the presence of well‐marked horizontal humidity gradients aloft, at the rear edge of the cloud cover, enhanced the ascending motions ahead of the frontal surface, creating locally a secondary maximum of upward motions. The degree of stability within the prefrontal boundary layer was found to be of major importance with regard to the intensification of the upright convection near the leading edge of the front. Moreover, the incorporation of a parametrization scheme, representing the Ekman pumping at the top of the boundary layer, enhanced the diagnosed circulation ahead of the front. A comparison with observations made during two case‐studies of Fronts 87 seems to confirm these results.
Correction of instrumental biases in historical sea surface temperature data
Authors: Folland C. K. , Parker D. E. 
Published: February 2007

AbstractWe describe a physically based empirical technique for correcting historical sea surface temperature measurements for time‐varying biases. The corrections are based on models of heat and moisture transfers from uninsulated (canvas) and partially insulated (wooden) sea temperature buckets exposed on deck. One of the canvas bucket models is tested using measurements on board ship and published wind‐tunnel measurements. The method gives geographically and seasonally varying bias corrections through the period 1856 to 1941. The corrections are fairly insensitive to uncertainties such as the size of the bucket or the details of its exposure on deck. A discussion of the history of sea surface temperature observations provides a background to the procedure.The resulting globally and seasonally averaged sea surface temperature corrections increase from 0.11 degC in 1856 to 0.42 degC by 1940. The corrections are compatible with recent measurements made at sea of the errors of canvas buckets. Global and hemispheric time series of corrected sea surface temperature and night marine air temperature data show good agreement: more detailed verifications of the corrections will be reported elsewhere.
A general circulation model study of a tropopause‐folding event at middle latitudes
Authors: Cox B. D. , Bithell M. , Gray L. J. 
Published: February 2007

AbstractThe UK Universities' Global Atmospheric Modelling Programme General Circulation Model (UGCM) has been run at high horizontal resolution to investigate a tropopause‐folding event at middle latitudes. Geopotential‐height, potential‐vorticity and specific‐humidity distributions were diagnosed from the model and showed a realistic representation of the synoptic development of the cut‐off low associated with the tropopause fold. the model also successfully portrayed the frontogenesis associated with tropopause folding and represented both the upper‐level frontal region and the corresponding jet‐streaks as they progressed through the region of the developing trough. A series of vertical cross‐sections at the ends of the jet streaks demonstrated that a fold could be captured by the UGCM using potential vorticity and specific humidity, although more small‐scale features were evident in the specific‐humidity contours than in the corresponding potential‐vorticity fields. the corresponding baroclinic zone was also resolved, although both the slope and thickness were rather larger than sometimes observed. Both the spatial and temporal extent of the fold compared well with observations. In addition the model simulated the ageostrophic winds in the entrance, exit and middle‐streak regions. A direct circulation was obtained at the entrance and an indirect circulation at the exit region of the jet streak, in good agreement with theory and with observational data. A combination of confluence, cold advection and curvature was responsible for the forcing of the ageostrophic motion at the jet entrance, whereas the forcing at the jet exit was attributed mainly to diffluence and curvature. Q vectors were employed to investigate the vertical motion in the vicinity of the fold, and showed the precise positions of descent corresponding to the fold along the entire length of the jet streak. Finally a tropopause fold around a jet that was not subject to the development of strong baroclinic instabilities was reproduced by the model: it was smaller in size than the fold of the cyclonic trough. A Q‐vector analysis confirmed the position of the small fold along the jet and revealed large‐scale ascending motion that may have been responsible for the small size of fold. the possibility of using the model to yield a realistic estimate of the transfer of air from the stratosphere to the troposphere in a tropopause‐folding event at mid‐latitudes is discussed.
Gravity‐wave drag parametrization over complex terrain: The effect of critical‐level absorption in directional wind‐shear
Authors: Shutts Glenn 
Published: February 2007

AbstractMost schemes to parametrize gravity‐wave drag effectively assume that the total wave‐stress is associated with a single wave‐vector pointing in the direction of the stress vector. However, the wave stress over real terrain is an integral over all azimuthal directions of wave‐vector contributions. In unidirectional flow, all of these modes will have a critical line at the same height—if one exists. However, if the wind turns montonically with height, by whatever degree, critical‐level absorption will occur at all heights for some portion of the wave spectrum. the resultant critical‐line wave‐drag is always normal to the local wind direction and the wave stress itself turns with height. This contrasts with the height‐independent stress assumption used in parametrizing gravity‐wave drag outside regions where the wave stress is saturated.A practical approach to computing the gravity‐wave stress over anisotropic orography, which includes this selective critical‐level absorption effect, will be presented. This will be supported by an analytical model calculation for constant‐shear flow with wind turning.
Parametrization of momentum transport by convectively generated gravity waves
Authors: Kershaw R. 
Published: February 2007

AbstractA parametrization equation for momentum flux due to convectively generated gravity waves is derived and tested numerically. the wave momentum flux just above the convective layer is shown to depend directly on the shear and the intensity of convection, and inversely on the stability of the air above the convection. Simulations of ensembles of precipitating convective clouds are used to validate the parametrization and to estimate the efficiency of wave generation by convection in one particular convective regime: an idealized cold‐air outbreak with unidirectional shear.
Simulations of the potential vorticity structure and budget of fronts 87 IOP8
Authors: Persson P. Ola G. 
Published: February 2007

AbstractDropsonde observations in the cold fronts sampled during the joint British‐French Fronts 87 Experiment along the west coast of Europe have shown that the dry and moist potential vorticity fields (qd and qe, respectively) have significant anomalies on the mesoscale. the processes associated with these anomalies during a 12 h period encompassing the observation time have been examined by simulating the qd and qe structures observed near the cold front of IOP8 using the Météo‐France mesoscale PERIDOT model, and by developing and using an Eulerian potential vorticity budget module. High‐resolution dropsonde, rawinsonde, and Doppler radar observations are used as verification of the simulated potential vorticity structures. Initial and boundary conditions are obtained from three different analysis systems.The simulated potential vorticity structures are sensitive to the initial conditions because the analyses over the data‐sparse Atlantic Ocean vary significantly from one analysis system to another. of the seven persistent mesoscale potential vorticity features identified in the observations, five were simulated reasonably well using at least one of the initial analyses, and examined with the potential vorticity budget. the low‐level, pre‐frontal, positive qd anomaly along the length of the cold front is maintained through a balance between Lagrangian source/sink terms, dominated by differential diabatic heating, and vertical advection within the main frontal updraught. This balance maintains the qd anomaly in a region of near‐zero qd tendency between areas of qd generation near the surface and qd destruction in the middle troposphere. A mid‐tropospheric region of negative qe directly behind the surface cold front appears to be produced by a weak but persistent generation by the solenoid term in the region of strong humidity gradients along the back edge of the mid‐tropospheric cloud shield. This position, and the model evolution, suggest that the solenoid‐created region of negative qe can lead to enhanced mid‐tropospheric updraughts through conditional symmetric instability or a combination of conditional symmetric instability and large‐scale forcing. the magnitudes of regions of negative qd and qe on the anticyclonic side of the upper‐level jet decrease throughout the simulation due to horizontal momentum diffusion, while a post‐frontal, low‐level, negative qe region is generated principally by temperature and moisture diffusion.
Numerical experiments on radiation and tropical cyclones
Authors: Craig George C. 
Published: February 2007

AbstractNumerical simulations of tropical cyclones were carried out using an axisymmetric model with explicit convection and detailed microphysical and radiative parametrizations. The simulated systems showed a modest increase in intensity when radiative heating and cooling were included. This could be attributed to differential cooling of the environment of the system relative to its warm core, where cloud cover prevented radiative cooling. However the results were strongly influenced by the initial conditions and by the evolution of the water vapour field in the environment of the cyclone. This indicates that conclusive results concerning the effects of radiation on tropical cyclones are unlikely to be obtained from such idealized simulations, but will require studies that adequately represent the initiation and environmental interactions.
A numerical model of the cloud‐topped planetary boundary‐layer: Radiation, turbulence and spectral microphysics in marine stratus
Authors: Bott A. , Trautmann T. , Zdunkowski W. 
Published: February 2007

AbstractA numerical model of the cloud‐topped planetary boundary‐layer is presented. The model is one‐dimensional with special emphasis on a detailed description of cloud microphysical processes. Aerosols and cloud droplets are treated in a two‐dimensional particle‐distribution whereby the activation of aerosols is calculated explicitly by solving the droplet‐growth equation at all relative humidities. Atmospheric radiation is determined with a δ‐two‐stream radiation scheme. Turbulent fluxes are parametrized as a function of the turbulent kinetic energy. Numerical results are presented which are obtained by utilizing measurements made over the North Sea. The interaction between radiation, turbulence and cloud processes for maintaining the quasi‐steady state of stratiform cloud fields is emphasized.
The behaviour of a cloud ensemble in response to external forcings
Authors: Guichard F. , Redelsperger J.‐L. , Lafore J.‐P. 
Published: February 2007

AbstractThe behaviour of a population of tropical moderate precipitating clouds is investigated with a cloud‐resolving model over a period equivalent to 2–3 days. The response of the system is analysed as a function of external forcings, comprising shortwave and longwave radiation, large‐scale ascent effects and surface fluxes. Radiative and largescale ascent processes enhance the convective activity. In all the experiments, an accumulation of humidity is observed in the upper part of the cloud layer and in the region above. In the case of weak large‐scale ascent, a diurnal cycle of convection is found, having maximum activity during the night and minimum activity around noon. Depending on the anvil cloud coverage, a modulation of this cycle is found whose cause can be explained through an infrared radiative feedback. The anvil deck also has a diurnal cycle but phase shifted by six hours with reference to the convective cycle.