bluuebunny/crossref_metadata_2025_split huggingface dataset:
Bubble clouds and the dynamics of the upper ocean
Authors: Thorpe S. A. 
Published: February 2007

AbstractThis is a review of turbulence in the upper ocean close to the sea surface, particularly of the information that has been obtained from sonar observations of bubble clouds produced by breaking wind waves. These clouds provide tracers of the turbulent motions and are important, especially at high wind speeds, in the process of air‐sea gas transfer. the observations of bubble clouds are here related to other measurements of turbulence, particularly to direct measurements of currents and temperatures in lakes or at sea, and to laboratory studies. Some novel observations of bubble clouds and breaking waves, their frequency and relation to Langmuir circulation, are presented.There is now emerging a pattern of clues that point to the dominance of breaking surface gravity waves as a source of turbulence to a depth below the surface of 0.04 to 0.2 times the wavelength of the dominant breaking waves, although the effect of swell has yet to be clarified. the relative depth appears to increase with increasing values of W,1/c, where W10 is the wind speed and c the phase speed of the dominant waves. Below this region the generation of turbulence may be dominated by shear‐stress or convection. Here, turbulence is generally similar to that in the atmospheric boundary layer. There are, however, coherent motions on the scale of the mixing layer that persist for periods of a few minutes to an hour or so, to which the transport of a large part of the momentum and heat fluxes can be attributed, and which strongly affect the dispersion of buoyant particles or bubbles. These motions deserve special study to establish their contribution to heat and momentum transport, and hence to determine if, or when, they should be specifically represented in models of the upper ocean devised, for example, to describe the dispersion of passive and non‐passive tracers or the air‐sea transfer of gases.
The accuracy of rainfall estimates by radar as a function of range
Authors: Fabry F. , Austin G. L. , Tees D. 
Published: February 2007

AbstractHigh‐spatial‐resolution data were collected for nearby convective storms by using a volume scanning weather radar with 200m resolution, and for stratiform events by using a high‐resolution (20m) vertically pointing radar. Errors in the estimation of rainfall by the scanning radar, due to the radar beam intersecting or overshooting features such as the bright band, were simulated by using these high‐resolution images. It was found that these effects define a maximum useful range for the radar that is a strong function of the prevailing meteorological conditions (e.g. bright‐band height, intense cell diameter and height).Sudden changes in bright‐band height over short times and the large scatter observed in its thickness limit the accuracy with which corrections for the vertical profile of reflectivity may be precalculated in stratiform rain events. A scheme for using a vertically pointing radar to produce these corrections in real time is proposed. the convective events show enough significant storm‐to‐storm variability that caution is required in extending the useful range of the radar too far.
The Boreal Forests and Climate
Authors: Thomas G. , Rowntree P. R. 
Published: February 2007

AbstractThe sensitivity of modelled northern hemisphere climate to modification of the snow‐covered surface albedo is investigated using the United Kingdom Meteorological Office (UKMO) general circulation model (GCM). the UKMO GCM is a global, primitive‐equation model with 11 layers in the atmosphere. Surface processes in the model are highly parametrized, with bulk aerodynamic formulation of the surface fluxes and a ‘bucket’ soil‐moisture accounting method. the experiment represents in a highly simplified fashion the role of the boreal forests in reducing the surface albedo under snow‐covered conditions. A parametrization of snow‐covered land was developed which allows the maximum albedo attainable with a snow cover to be prescribed as a function of vegetation type. In the standard version of the model the maximum snow‐covered surface albedo attainable is 0.60, which exceeds observed values for the forested areas of the northern hemisphere. the model was integrated twice, with different albedos representing forested and deforested conditions. the sensitivity of the heat and hydrologic budgets for the northern hemisphere and deforested areas is discussed. A detailed analysis of the deforested regions reveals systematic reductions in temperature of up to 2.8 K. Precipitation shows a systematic decrease in the affected regions. the largest decreases occur generally in the months with largest evaporation changes. For the case of no masking by forest vegetation (equivalent to boreal deforestation) the model produces a significant change in the pattern of snow‐melt. the removal of forest affects both the magnitude and the timing of spring snow‐melt, and consequently also the runoff. There are delays in snow‐melt‐induced runoff peaks by a month and increases in the magnitudes by on average 32%. the role of large‐scale advection is investigated by comparison with the results from a single‐column‐model experiment. A realistic representation of the snow‐covered surface albedo is evidently a requirement for simulations of the northern hemisphere climate.
Non‐Linear Three‐Dimensional Effects On Gravity‐Wave Drag: Splitting Flow and Breaking Waves
Authors: Miranda P. M. A. , James I. N. 
Published: February 2007

AbstractA study has been made of some aspects of frictionless stratified flow past three‐dimensional isolated mountains. the study uses a three‐dimensional non‐hydrostatic numerical model to investigate the behaviour of the flow as a function of the Froude number, and produces a picture of the dependence of the gravity‐wave drag on the Froude number for a wide range of that parameter. At the same time, the results of the numerical experiments clarify the behaviour of the flow in the transition from high to low Froude number, showing the relative importance of wave breaking and flow splitting in the transitional regime.
Solar Radiative Transfer Through Clouds Possessing Isotropic Variable Extinction Coefficient
Authors: Barker Howard W. 
Published: February 2007

AbstractSolar radiative fluxes were computed for heterogeneous clouds using an extension of the Monte Carlo method of photon transport which assumes that clouds possess isotropic variability. Hence, computation of fluxes for three‐dimensional (3‐D) clouds can be achieved with only an extended, characteristic one‐dimensional (1‐D) transect of extinction coefficient, β. These are easily obtained by aircraft measurements. In order to validate the new scheme, fluxes for 3‐D, stochastic multiplicative cascade clouds were computed by the conventional approach to Monte Carlo simulation. 1‐D transects through these clouds were then strung together and used in the new scheme. Not only do both methods of calculation yield statistically identical flux estimates, but they also produce virtually identical distributions of photon optical pathlengths and number of scattering events. Furthermore, the new technique may require an order of magnitude less computation time, depending on the desired level of accuracy.Cloud microphysical data obtained by aircraft were used to represent characteristic transects of β, and corresponding fluxes were computed with the new scheme. Results suggest that internal inhomogeneity reduces cloud albedo below homogeneous values by about 5‐10% for overcast and isolated cubic clouds. Also, it is predicted that for overcast clouds of optical depth τ < (>) ≈ 40, inhomogeneous clouds absorb less (more) radiation relative to their homogeneous counterparts. Three individual and trivial modifications of a plane‐parallel, homogeneous two‐stream solution of the radiative‐transfer equation appear to be capable of capturing the essential effects of inhomogeneity. This is promising for incorporation of inhomogeneous cloud effects into existing climate‐model solar‐radiation routines. Little evidence, however, was found to suggest that the effects of inhomogeneity alone can explain the spectral aspects of the cloud absorption/albedo anomaly problem.
Impact of Cloud Longwave Radiative Surface Forcing On Interception Loss From A Coniferous Forest Canopy
Authors: Lockwood J. G. 
Published: February 2007

AbstractDownward longwave radiation at the surface is greater in a cloudy atmosphere than in a clear one. This additional longwave radiative forcing at the surface has an average daily value in Britain of around 40 W m−2, which is small compared to observed fine‐weather shortwave radiative fluxes. However in British rainfall events, the intensity of the incoming shortwave radiation is normally around 100 W m−2 or less during the day and zero at night. A numerical experiment using a multilayer model of a coniferous forest is described that suggests that during typical rainfall under British conditions the additional cloud longwave radiative forcing has a significant impact on interception loss from coniferous forest canopies. Interception loss is reduced from about 31% of rainfall, with normal cloud longwave radiative surface forcing, to about 15% with the cloud longwave forcing removed. This result is confirmed using a single‐layer model of a coniferous forest over an 80‐day period.
Two paradigms of baroclinic‐wave life‐cycle behaviour
Authors: Thorncroft C. D. , Hoskins B. J. , McIntyre M. E. 
Published: February 2007

AbstractTwo idealized baroclinic wave‐6 life cycles examined here suggest a framework of opposite extremes (a) in which to view the behaviour of real synoptic‐scale disturbances in middle latitudes, and (b) in which to interrelate the synoptic and wave‐theoretic viewpoints, using the ‘saturation‐propagation‐saturation’ (SPS) picture of wave‐6 life‐cycle behaviour. The two life cycles. dented by LC1 and LC2, are higher‐resolution versions of the Simmons‐Hoskins ‘basic’ and ‘anomalous’ cases (showing strong and weak late decay of eddy kinetic energy, EKE). They illustrate, in varying degrees. Two extreme types of behaviour here disignated ‘anticyclonic’ and ‘cyclonic’, and epitomized by strongly contrasting upper‐air trough behaviour. ‘Anticyclonic’ behaviour dominates the late stages of LC1 and is characterized by backward‐tilted, thinning troughs being advected anticyclonically and equatorward, as in the commoner cases of planetary‐scale mid‐stratospheric ‘Rossby‐wave breaking’. ‘Cyclonic’ behaviour dominates LC2 and is characterized by forward‐tilted, broadening troughs wrapping themselves up cyclonically and poleward. producing major cut‐off cyclones in high latitudes. These morphologies are visualized by upper‐air maps of potential temperature on the nominal tropopause, defined as a constant‐potential‐vorticity surface. Some atmospheric mid‐latitude disturbances examined here, using the same visualization applied to operational analyses, show the same two extreme types of trough behaviour together with intermediate cases.The SPS picture is re‐examined, using Eliassen‐palm and refractive‐index cross‐sections. It is shown, in particular, by reference to a wave‐activity theorem of Haynes, that the late stages of LC2 can be looked upon as a remarkably clear, and morphologically novel, large‐amplitude counterpart of the nonlinear reflection scenario of Rossby‐wave critical‐layer theory. The late stages of LC1, by constrast, look more akin to an nonlinear critical‐layer absorption scenario. LC2 exhibits region of largely undular PV contours adjacent to a nonlinear of irreversiby deformen PV contous. In the latter region PV rearrangement, and hence absorption of Rossby‐wave activity, has largely ceased. This accounts for the more persistent EKEE in the LC2 case.
Predictability and finite‐time instability of the northern winter circulation
Authors: Molteni Franco , Palmer T. N. 
Published: February 2007

AbstractThe finite‐time instability and associated predictability of atmospheric of atmospheric circulations are defined in terms of the largest singular values, and associated singular vectors, of the linear evolution operator determined form given equations of motion. These quantities are calculated in both a barotropic and a three‐level quasi‐geostrophic model, using as basic states realistic large‐scale northern wintertime flows that represent the climatological state, regime composites, and specific realizations of these regimes. for time‐invariant basic states, the singular vectors are compared with the corresponding normal‐mode solutions; it is shown that the perturbations defined (at the initial time) by the singular vectors have much larger growth rates than the normal modes, and possess a more localized spatial structure.The regimes studied have opposite values of the Pacific/North American (PNA) index, and growth rates for the barotropic basis states appear to confirm earlier studies that the barotropic instability of the negative PNA states may be larger than the corresponding positive PNA states. The evolution of the singular‐vector perturbations, with emphasis on the vertical structure, is compared for time‐evolving and time‐invariant baroclinic basis states; the effects of nonlinearity are also discusses. It is shown that, in the baroclinic model, interactions between synoptic‐scale eddies in the time‐evolving basic state and in the perturbation field are fundamental for studying the predictability of transitions in the large‐scale circulation. Consequently results obtained from linear calculations using very smooth basic states cannot properly account for such predictability.These results form the basis of a technique used to initialize ensembles of forecasts made with a primitive‐equation model, and are described in the companion paper (Mureau et al. 1993).
Atmospheric water‐vapour transport to Antarctica inferred from radiosonde data
Authors: Connolley W. M. , King J. C. 
Published: February 2007

AbstractData from 16 radiosonde stations are used to study the transport of water vapour in the antarctic atmosphere. Total column moisture (TCM), winds and moisture flux measurements are presented, together with an analysis of their reliability. Annual TCM values are similar—about 4 kg m−2—at all stations around the coast of East Antarctica but are much smaller on the East Antarctic Plateau. Over a period of 6 years the interannual variation (standard deviation) of the TCM is about 10% of the mean value.At the coastal stations moisture fluxes reflect the predominantly zonal easterly flow in the lower troposphere, and their meridional components are generally small. As a result of interannual variations in the strength of the atmospheric circulation, and to a lesser extent in the TCM, interannual variability of the fluxes is high, suggesting that there may be large interannual variability in the precipitation over Antarctica.From the data a water‐vapour budged for East Antarctica is computed. The annual accumulation rate obtained agrees surprisingly well with glaciological estimates. However, the uncertainties are considerable as a result of measurement errors and the representativity of the stations available. The conclusion is that the data are more suitable for evaluating the regional performance of circulation models from which systematic budget estimates may be derived.
Sensitivity of the Extended UGAMP General Circulation Model to the specification of gravity‐wave phase speeds
Authors: Jackson D. R. 
Published: February 2007

AbstractTwo simulations made using the Extended UGAMP General Circulation Model (EUGCM) are compared. The first uses a gravity‐wave parametrization scheme that only includes orographically forced gravity waves with zero phase speeds, whilst the second uses a similar scheme, except that it includes a gravity wave with a phase speed of 20 ms−1. Results from the first simulation show that wintertime westerlies are reasonably estimated, but summertime and equatorial easterlies are excessive owing to an absence of gravity‐wave drag in these region. This absence of gravity‐wave drag also means that the summertime mean meridional circulation is too weak, and summer mesospheric model temperatures are accordingly warmer than observations by up to 35 degC. Inclusion of gravity waves with phase speeds of 20 ms−1 reduces summer and equatorial easterlies and decreases summer mesophere temperatures because, unlike zero phase‐speed waves, these waves can propagate into the summer and equatorial middle atmosphere and thus of the 20 ms−1 gravity wave also improves the simulation of the semiannual oscillation both at the stratopause and at the mesopause.
Interannual variability of the stratospheric circulation in the southern hemisphere
Authors: Shiotani Masato , Shimoda Naoki , Hirota Isamu 
Published: February 2007

AbstractThe variability from year‐to‐year of the seasonal evolution in the southern hemisphere stratosphere over a period of 10 years, from 1980 to 1989, is investigated using the global geopotential height derived from the data from the Stratospheric Sounding Unit on board the TIROS‐NOAA satellites. As a measures of interannual variability, the variance of the zonal mean geostrophic wind over the 10 years was calculated for each day. Although the major warming hardly occurs in the southern hemisphere stratosphere, large variances can be observed during the winter and spring. The maximum variances appear at low latitudes in late autumn, move towards mid latitudes in midwinter and remain near 60°S in spring. The seasonal movement of the zonal mean westerly jet in the southern hemisphere stratosphere can be classified into two categories in terms of the location of the maximum westerlies at the 1 mb level in midwinter, namely
HLJ (high‐latitude‐jet) years with maximum westerlies around 50°S,
LLJ (low‐latitude‐jet) years with maximum westerlies around 40°S.
During the late winter the core of the westerly jet moves polewards earlier in HLJ years in LLJ years. In association with this earlier movement, the growth of the amplitudes of planetary waves 1 and 2 during the period from winter to spring occurs earlier in HLJ years than in LLJ years. In autumn, however, the wave‐1 amplitude, only for LLJ years, develops vigorously in the stratosphere and even the troposphere, before the seasonal movement of the zonal mean westerly jet branches off into the two categories. The appearances of HLJ and LLJ years occur in groups of a few years; this is quite different from the behaviour of the equatorial quasi‐biennial oscillation.
Cyclone behaviour response to changes in winter southern hemisphere sea‐ice concentration
Authors: Simmonds Ian , Wu Xingren 
Published: February 2007

AbstractThe sensitivity of climatological cyclone behaviour in the southern hemisphere to reductions in winter antarctic sea‐ice concentration is studies in general‐circulation‐model (GMC) experiments. An increase in the number of cyclones over most of the Weddell an Ross Seas is a response common to all cases of reduction of sea‐ice concentration from 100%. There is a tendency, particularly at lower ice concentrations, for more cyclones to be found in the latitude band of about 10 to 15° of latitude width centered on the antarctic coast, and a belt of fewer cyclones to the north. The structure of the changes in cyclogenesis assumes a much lesser zonally oriented form, and all experiments induce enhanced cyclogenesis in the western Weddell Sea. For the most part thee is an increase in cyclogenesis south of about 65°S, although some longitudinal sectors show up as regions of decreased cyclogenesis even through the sea‐ice concentration has been reduced there.The intensity of the extratropical cyclones has been quantified as the difference between the Mean depth of the systems and the climatological average pressure for each particular sea‐ice configuration. Perhaps contrary to what might have been expected, this ‘relative’ central pressure of lows exhibits an increase over most of the sea‐ice area (i.e. the cyclones become weaker in a relative sense). In an earlier study it was found that the climatological response to imposed anomalies of sea‐ice concentration was that the lowering of the concentration of antarctic sea ice resulted in a lowering of sea‐level pressure, particularly close to the continent. The results presented here strongly suggest that the climatological decreases in pressure result essentially from increases in cyclone numbers as opposed to the possibility that the cyclones are more intense.We have attempted to relate the regions of cyclogenesis with a measure of baroclinicity in the control simulation. We have also examined the relationship between changes in these parameters in response to anomalous sea‐ice concentration. Despite some similarity between theses, our study emphasizes the important role being played by other factors, including surface heat fluxes and topography, in these experiments. This study demonstrates the utility of GCMs in the study of high‐latitude cyclone sensitivity and suggests it is a valuable tool to be used in tandem with observational‐based analyses.
The Pressure force induced by neutral, turbulent flow over hills
Authors: Wood Nigelo , Mason Pual 
Published: February 2007

AbstractThe pressure force induced by neutral, turbulent flow over small‐scale hills is considered. The influence that the level of turbulence closure has on liner, analytical expressions for the force is evaluated. The two‐dimensional linear expressions are then extended to three dimensions and a heuristic extension to moderate and steep slopes, for which nonlinear effects are important, is proposed. The predictions of these various expressions are compared with results from a three‐dimensional numerical model employing a 1 1/2‐order turbulence‐closure model, and very good agreement is found.The numerical‐model results supports the idea that, sufficiently far above the hills, the areally averaged velocity profile varies approximately logarithmically with height but with an enhanced, or effective, roughness length and a friction velocity equal to the square root of the total surface force per unit area and unit density. Using the theoretical results for the pressure force, discussed above, and approximate expression for the effective roughness length is obtained that agrees well with values from the model, and also matches with existing expressions that are valid for two‐dimensional hills with either very low or very large slopes. Qualitative agreement is also found with the limited number of observed values of roughness length in hilly terrain. These results suggest that the effective roughness length is an accurate and convenient method of parametrizing the drag due to subgrid‐scale orography in large‐scale numerical‐weather‐prediction and climate models.
Dynamical structure of a wide cold‐frontal cloudband observed during FRONTS 87
Authors: Lagouvardos K. , Lemaitre Y. , Scialom G. 
Published: February 2007

AbstractConditional symmetric instability (CSI) is examined as a possible mechanism for generating cloudbands within a cold‐frontal system. The data used in this study were collected during the Franco‐British field experiment FRONTS 87. A dense network of rawinsounding measurements was used to test the various hypotheses underlying CSI, such as the sign of the moist potential vorticity, the relative slope of absolute momentum and moist isentropic surfaces or the alignment of bands with the vertical shear of the wind inside the unstable domain. Radar data provided from the French dual‐Doppler radar system RONSARD was used to retrieve the three‐dimensional airflow within a cloudband observed during the last frontal event of FRONTS 87. A new retrieval method, called MANDOP, was used to estimate the circulations within this band and to test whether the retrieved circulations were consistent with the CSI theoretical expectation. A roll‐like circulation present in the vicinity of the observed cloudband seems to support the hypothesis that CSI was the formative mechanism.