bluuebunny/crossref_metadata_2025_split huggingface dataset:
The response of the upper ocean to solar heating. I: The mixed layer
Authors: Woods J. D. , Barkmann W. 
Published: February 2007

AbstractThe results of two earlier papers on convection in the mixed layer and on the solar heating profile are here introduced into a one‐dimensional model in order to investigate the following consequences of the daily cycle of solar heating in the upper ocean:1. the daytime convection depth becomes less than the turbocline depth;2. the convective power supply to turbulence in the mixed layer is reduced;3. the mixed layer below the convection layer becomes stably stratified;4. the depth of the turbocline is reduced, leaving a diurnal thermocline between it and the top of the seasonal thermocline;5. the heat content and potential energy of the diurnal and seasonal thermoclines are increased, slowing down the subsequent nocturnal descent of the turbocline.These diurnal changes are illustrated by integrating a one‐dimensional model forced by the astronomical cycle of solar heating and seasonal variation of surface meteorology derived from Bunker's climatology. The model is integrated for 18 months to show the seasonal modulation of the diurnal cycle. Nocturnal convection plays a dominant role. The convection depth closely follows the thermal compensation depth during the day when they are less than the turbocline depth. Integrating the model with a 24‐hour time step leads to large errors in the seasonal variation of mixed layer temperature and depth, and in the source term of isopycnic potential vorticity. The errors are reduced by using two time steps per day, one for the daytime when convection is quenched, the other for the night when it is active. A novel parametrization based on tuning the daily equivalent solar elevation to surface temperature further reduces the error. This parametrization is used to investigate the sensitivity of the seasonal cycles of mixed layer depth and temperature to: (1) seasonality in the surface fluxes; (2) systematic changes in the net annual solar heating; (3) random changes in the seasonal cycles of solar heating induced (i) monthly and (ii) daily. The sensitivity to uncertainty in seawater turbidity is investigated in the same way. The profile of isopycnic potential vorticity subducted into the thermocline depends on the vernal correlation of mixed layer depth and density, so gyre circulation is sensitive to solar heating in spring.
Nonlinear effects on heat‐induced circulation of the tropical atmosphere
Authors: Gill A. E. , Phlips P. J. 
Published: February 2007

AbstractA nonlinear model of the atmospheric response to an isolated region of heating centred on the equator is constructed by expanding in a small amplitude parameter. In order to emphasize nonlinear effects relative to frictional ones, the asymptotics of the small‐friction limit are first derived, and this limiting solution is used as the first term in the expansion with respect to the amplitude parameter. Compared with the linear solution. the nonlinear solution has more inflow from the west at the surface and a reduced easterly outflow at the tropopause. Moreover the pressure is reduced when velocities are large because of the Bernoulli effect. The conclusions are in good general agreement with the numerical results of Nobre and the observational study of Sardeshmukh and Hoskins.
The microstructure of hill cap clouds
Authors: Carruthers D. J. , Choularton T. W. 
Published: February 2007

AbstractA cloud microphysical model and a model of airflow over hills are used to examine the microstructure of hill cap clouds. The lifetime of the clouds and the distance travelled by the clouds are both assumed to be sufficiently small so that condensation alone is considered to be important; the coalescence of drops is neglected. The effects of changing (i) the height of cloud base, (ii) humidity fluctuations below cloud base and (iii) incloud turbulence, are described. Process (i) is found to be important, its effect on the cloud microstructure being as great as the effect of changes in the CCN distribution or in the mean wind. Droplet loss to ground, radiative cooling at cloud top and entrainment of dry air are also discussed; these processes may all significantly affect the cloud droplet distribution.
A comparison between frontogenesis in the two‐dimensional Eady model of baroclinic instability and summertime cold fronts in the Australian region
Authors: Reeder Michael J. , Smith Roger K. 
Published: February 2007

AbstractA two‐dimensional, anelastic, numerical model with a simple turbulence parametrization is integrated with Eady's linear normal mode solution for an unstable baroclinic wave as initial condition. The structure of the surface front which develops after five days of model integration is compared with data on summertime cold fronts, or ‘cool changes’, in south‐eastern Australia obtained during recent field experiments of the Australian Cold Fronts Research Programme. It is shown that the ridge‐trough structure of an amplifying baroclinic wave and its attendant surface front captures many of the important features of ‘cool changes’, together with the broad‐scale flow in which they develop. Indeed, it is argued that the model provides a useful theoretical framework in which the dynamics of the Australian summertime ‘cool change’ may be understood.
A model of the orographic enhancement of snowfall by the seeder‐feeder mechanism
Authors: Choularton T. W. , Perry S. J. 
Published: February 2007

AbstractA model has been constructed of the orographic enhancement of snowfall by the seeder‐feeder mechanism. It is found that the collection of water from the feeder cloud is much more efficient than with rainfall under similar conditions, producing significantly larger enhancements of up to three times the seeder cloud precipitation rate. Over long hills growth is mostly by vapour diffusion, with riming dominating over shorter hills. It is found that the peak snowfall enhancement is strongly dependent on hill height, wind speed and feeder cloud depth over a long hill. These sensitivities are much weaker or are absent over short hills due to the large effects of wind drift.
A random walk model of dispersion in turbulent flows and its application to dispersion in a valley
Authors: Thomson D. J. 
Published: February 2007

AbstractA random walk model of the dispersion of a passive tracer in a general three‐dimensional turbulent flow is developed. The model is used to simulate dispersion in the Sirhowy valley, South Wales. The results show encouraging agreement with the experimentally measured concentrations, which are generally much lower than those typical of dispersion over flat ground. The simulations show that, for releases from the summit on the upwind side of the valley, the low ground‐level concentrations in the valley are primarily a result of the mean flow and not the high turbulence levels in the valley. The simulations have been unable to explain the differences observed between experiments which were carried out in similar conditions; it is possible that the differences are simply a result of the inherent variability in the dispersion. The simulations have enabled estimates of a., and oz to be made; these quantities are difficult to obtain from the experimental data because of the lack of sufficient elevated samplers and the large horizontal area covered by the cloud.
The statistical properties of general atmospheric circulation: Observational evidence and a minimal theory of bimodality
Authors: Benzi R. , Malguzzi P , Speranza A. , Sutera A. 
Published: February 2007

AbstractIn this paper we address the problem of constructing a minimal model of atmospheric circulation able to describe the bulk of the statistical properties of the general circulation in middle latitudes. After a brief reexamination of the Charney‐DeVore theory of multiple equilibria, we compare the statistical properties that can be deduced from it with those resulting from analysis of observations of the northern hemisphere middle‐latitude circulation at 500 mb. While the prediction concerning the amplitude of the waves seems to be confirmed by the presence of a clear bimodality of the statistical distribution, we find no trace of bimodality in the zonal wind. We show how the theory can be brought into agreement with observations by introducing nonlinearity into the wave equation in such a way that the Charney‐DeVore resonance curve is bent, producing different states corresponding to the same value of the zonal wind. We finally address ourselves to the question of what physical processes determine the observed unimodal statistics of zonal wind, a subject of future work.
Diabatic modification of airflow over a mesoscale orographic ridge: A model study of the coupled response
Authors: Davies Huw C. , Schár Christoph 
Published: February 2007

AbstractA theoretical model is used to study the combined influence upon mesoscale atmospheric flow of orography and the diabatic effects of non‐precipitating clouds. In the model a CISK‐like representation is adopted for non‐precipitating convective clouds. The resulting coupled response is examined in the limit of small amplitude, steady, hydrostatic perturbations of a flow incident upon a ridge (or a train of ridges) in a non‐rotating environment.It is shown that the combined effect of the two forcing processes can be significantly different from that of orography acting alone. In particular in certain situations an enhanced (indeed resonant) response can occur with strong winds on the lee slope and a concomitant large surface pressure drag. Physical considerations based upon the CISK instability concept indicate that for the present model the converse flow response with reduced or even negative values of the pressure drag is usually (but not always) an inadmissible flow configuration.These results help to clarify the disparate results obtained in earlier studies, and, unlike those studies, suggest that diabatic effects might on occasions have a major role in inducing strong surface lee‐side winds.
Ducted coastal ridging over S.E. Australia
Authors: Holland Greg J. , Leslie Lance M. 
Published: February 2007

AbstractThe intense and rapidly established coastal ridging which is a regular occurrence over south‐eastern Australia during the warm seasons, is investigated using a detailed case study and mesoscale numerical model simulations. We suggest that the ridge is initiated by a forced Kelvin‐type edge wave which forms on the south‐western edge of the Great Dividing Range and is ducted, in an anticlockwise sense, around the coast. The ridge is then stabilized by inertial modification and decays over a synoptic time scale of a few days. Our results apply to similar phenomena in other parts of the world, notably southern Africa and South America.
Observations of boundary layer structure made during the 1981 KONTUR experiment
Authors: Grant A. L. M. 
Published: February 2007

AbstractData collected by two aircraft over the German Bight during the 1981 KONTUR experiment are described. Turbulence data were obtained throughout the depth of the mixed layer in near‐neutral to slightly convective conditions. Similarity theory generally orders the data well and shows the effect of stability on turbulence structure under these conditions. The humidity flux profile, however, is found to depend on conditions at the top of the mixed layer.Spectral analysis is used to investigate the variation of turbulence length scales with height and the mixed layer eddy structure. Differences in spectra from alongwind and acrosswind sampling directions are found, particularly near the surface. Spectra of the transverse wind component show that two‐dimensional roll circulations were generally present in the mixed layer and became more pronounced with increasing instability.
Roughness‐dependent geographical interpolation of surface wind speed averages
Authors: Wieringa J. 
Published: February 2007

AbstractA two‐layer boundary layer model is presented for regional assessment of seasonal‐average surface wind climate from station data. In the surface layer the wind measurements are objectively corrected for imperfect station exposure, using gustiness‐derived roughness values for each azimuth sector. This produces at 60 m a mesoscale wind speed, Um, representative for a 5×5km2 area block. The regional averaging of Um in the Ekman layer is done by geostrophic similarity methods, referring to a mesoscale block roughness obtained from maps through terrain classification. All wind data for the flat Netherlands are analysed, using a 30‐year series of annual windiness indices for time‐span adjustment of non‐simultaneous data series. The evaluation leads to objectively interpolated seasonal surface wind maps, supplemented by wind distributions and extreme design wind speeds. Possible extension of the methodology to complex terrain is discussed.
Properties of ice accreted in two‐stage growth
Authors: Prodi F. , Santachiara G. , Franzini A. 
Published: February 2007

AbstractThe hypothesis that hailstones may grow in two stages by the soaking and refreezing of a previously formed porous structure, has been explored by obtaining artificial accretions on rotating cylinders in a wind tunnel, in controlled laboratory conditions. Once the first stage of growth was obtained, the inner plastic embryo was extracted and slices were cut for a series of analyses; the remaining pieces of the deposit were rejoined and a new layer was grown onto it or, alternatively, the deposit was partially melted in a flow of warm air, and other slices were sawn for another set of analyses.Changes in accreted ice characteristics due to soaking have been determined for local density, crystallographic parameters and morphology.While deposits obtained at high updraught velocity (26 ms−1) were already at a high density in the first stage, the lower density of deposits grown at lower impact velocities (10 ms−1 or less) was increased, due to water penetration, to values similar to those observed in natural hailstones. The local density measurements were obtained by X‐ray contact micrography. This technique enabled us to record even minor density variations crucial for correct interpretation and analysis.All crystallographic parameters: the average grain size, σ, the mean maximum crystal length, l, and the mean maximum crystal width, w, showed an increase after penetration due to both annealing during the time spent at 0°C and the filling of voids.In deposits grown at impact speeds ⩽10 ms−1 the appearance and shape of the first stage deposits are considerably changed as a result of soaking, and the changes introduced are more important the lower the density of the first stage. The dry growth lobes are smoothed and the ice made translucent, radial lines of bubbles are formed within the channels between lobes, and sometimes features are propagated to the external layer. Features similar to those found in natural hailstones have been observed. The lobe structure defined in the first stage, though modified by soaking, retains its general characteristics.Evidence of two‐stage growth in natural hailstones is given and the origin of features such as cuspidated lobes, radial lines of bubbles and rounded air bubbles is explained.The evidence that two‐stage growth may be a frequent occurrence in hailstones is changing the criteria for natural hailstone interpretation by introducing new criteria and suggesting modifications to those currently adopted.
The density of accreted ice
Authors: Prodi F. , Levi L. , Pederzoli P. 
Published: February 2007

AbstractThe density of accreted ice has been experimentally investigated by performing local measurements with a technique based on X‐ray micrography of calibrated slices.Accreted ice deposits were grown on fixed cylinders, which allowed an investigation of the accretion process in steadier and more regular conditions at the stagnation point, avoiding oscillations in local surface temperature. Further data used were obtained from previous work on rotating cylinders.The results show that the values of the local ice density found using X‐ray micrography are unexpectedly high compared with previous results. This discrepancy is discussed and mostly related to the different techniques used and different estimations of the air between lobes penetrating deeply inside the deposit.The consequences of these results in various situations of natural ice accretions are discussed, as well as on understanding basic mechanisms of low density ice formation due to collision and freezing of droplets on an ice substrate.
The role of molecular hydrogen and methane oxidation in the water vapour budget of the stratosphere
Authors: le Texier H. , Solomon S. , Garcia R. R. 
Published: February 2007

AbstractThe detailed photochemistry of methane oxidation has been studied in a coupled chemical/dynamical model of the middle atmosphere. The photochemistry of formaldehyde plays an important role in determining the production of water vapour from methane oxidation. At high latitudes, the production and transport of molecular hydrogen is particularly important in determining the water vapour distribution. It is shown that the ratio of the methane vertical gradient to the water vapour vertical gradient at any particular latitude should not be expected to be precisely two, due both to photochemical and dynamical effects. Modelled H2O profiles are compared with measurements from the Limb Infrared Monitor of the Stratosphere (LIMS) experiment at various latitudes. Molecular hydrogen is shown to be responsible for the formation of a secondary maximum displayed by the model water vapour profiles in high latitude summer, a feature also found in the LIMS data.
The formation of areally‐averaged roughness lengths
Authors: Mason P. J. 
Published: February 2007

AbstractIt is argued that the best area average of the roughness length zo in heterogeneous terrain is that which, if it applied in homogeneous terrain, would produce the correct spatial average value of the surface stress. A heuristic argument is presented to show that this effective value of z0, zeff0, can be obtained by averaging drag coefficients based on a ‘blending’ height. The blending height of about L/200, where L is the horizontal scale of the roughness variations, is the characteristic height at which the flow changes from equilibrium with the local surface to independence of horizontal position. These calculations are compared with numerical simulations of planetary boundary layer flow over variations in roughness length and show good agreement. The values of surface stress which result are always greater than those which would be deduced by assuming a local flow equilibrium through the whole depth of the boundary layer. For variations of z0 on short length scales, zeff0 approaches the largest values of z0 within the averaging area.
Eddy structure in the convective boundary layer—new measurements and new concepts
Authors: Hunt J. C. R. , Kaimal J. C. , Gaynor J. E. 
Published: February 2007

AbstractAn analysis is developed for the eddy structure in the convective boundary layer (CBL) both in terms of thermals and downdraughts and in terms of moments, spectra and cross‐correlations of the vertical component of velocity, w. Using previous measurements of the area of thermals, we calculate the contributions from the average velocity 〈WU〉 in updraughts and (wD) in downdraughts to the total variance w̄2 and the third moment w̄3. We conclude that the variability in 〈WU〉 (i.e. variance and skewness) produces more than half the contributions to w̄2 and w̄3. The mean value of 〈WD〉 is the major contributor to the mode w̄2 in the probability distribution of w, but small‐scale turbulence is also important. The results of a statistical theory of eddies near the ground is explained in physical terms, in particular the cross‐correlation of w at two points (normalized on w̄2 at the reference height Zr) in the lower part of the CBL, Rzzr = z/z, where zr > z. The new theory for the cospectrum of w2 and w accounts for the contribution by the thermals at large scales and predicts that the eddies at small scaies are anisotropic, leading to a universal cospectral form that falls off as k−2 at the high frequency end (k being the wavenumber).Measurements at the Boulder Atmospheric Observatory in convective conditions are presented for a range of zi/L values (zi is depth of the CBL and L the Monin‐Obukhov length). The results for w̄3 and w̄3 plotted in mixed layer variables, agree with measurements made over the sea and over flat land for z < 0.5zi. The mode, w̌ shows some variation with height, and reaches a maximum value of 0.5w*, where w* is the convection velocity. Rzzr agrees with the theoretical prediction to within ±20%. The cospectra of w at z and zr also conform to the theoretical form but with large scatter at low frequencies. All these measurements, except for w̄2 are effectively functions of z/zi and independent of zi/L, even when |z/L| w̄3 1, because they are controlled by large‐scale motions. It is shown that the small scales provide an increasing proportion of w̄3 as z/zi decreases. This explains why large eddy simulations (LES) give incorrect values of w̄3 but correct values of w3/δz. The implications of these results for diffusion calculations and LES modelling are discussed.
The response of a general circulation model to cloud longwave radiative forcing. I: Introduction and initial experiments
Authors: Slingo A. , Slingo J. M. 
Published: February 2007

AbstractA new version of the NCAR Community Climate Model (CCM1) is used to study the effect of cloud radiative forcing on model simulations. Previous attempts to determine the role played by clouds in climate and the general circulation of the atmosphere are reviewed first. The concept of cloud radiative forcing is discussed and the forcings in the shortwave and longwave spectral regions are contrasted. At low latitudes, the cloud longwave forcing is primarily within the atmosphere, rather than at the surface. It is thus appropriate to study its effect with a model which employs fixed sea surface temperatures (s.s.t.s).The impact of cloud longwave forcing is studied in 510‐day integrations for constant January conditions. The experiments isolate the forcing by tropical and extra‐tropical clouds. Tropical cloud forcing warms the tropical upper troposphere and accelerates the subtropical jets. There are subtle interactions between the forcing, the clear‐sky longwave heating and the latent heating in the tropics. These additional diabatic heating terms oppose the forcing in some regions and enhance it in others.The tropical cloud forcing strengthens the precipitation maxima at low latitudes. There are also changes in the extra‐tropical flow, including the excitation of a pattern in the 200 mb geopotential height differences which is similar to those found in previous studies of the effect of s.s.t. anomalies. This confirms that the forcing may be as important as latent heat release in determining the atmospheric response to such anomalies. The mode is excited with comparable strength by the extra‐tropical cloud forcing. Finally, some concerns regarding the generality of these results and their applicability to other models and the real atmosphere are discussed. These concerns suggest the need for sensitivity studies, some of which are already in progress.
Early winter in the northern stratosphere
Authors: Juckes M. N. , O'Neill A. 
Published: February 2007

AbstractAn account is given of the evolution and structure of the circulation in the stratosphere and upper troposphere of the northern hemisphere in the early winter of 1982/83. An early winter warming is described and it is shown to have some features in common with Canadian warmings. A tentative connection is made between the intensification in the troposphere of a trough/ridge pattern associated with the East Asian low and the establishment of the Aleutian high as a persistent feature extending through the whole depth of the stratosphere. Processes leading to the formation of the so‐called surf‐zone are considered, and aspects of zonalmean diagnostics are related to developments on synoptic maps.