bluuebunny/crossref_metadata_2025 huggingface dataset:
A study of meteorological patterns associated with a lake confined by mountains – the Dead Sea case
Authors: Segal M. , Mahrer Y. , Pielke R. A.
Published: February 2007

A planned project to generate hydroelectric power by carrying Mediterranean water into the Dead Sea has stimulated a preliminary numerical model study of the regional meteorological patterns and the evaluation of some possible local climatic modifications. Predicted mesoscale meteorological patterns associated with two frequent synoptic conditions over the region are presented. Emphasis has been focussed on illustrating and studying two general aspects of the thermal circulations generated by a lake which is confined between mountains (LMC): the effect of external flows on the LMC structure and the effects of lake water temperature on the LMC intensity.
On the initial formation of deep fronts
Authors: Manton M. J.
Published: February 2007

A perturbation analysis is used to describe the initial intensification of a baroclinic temperature field by the shearing action of an upper‐level transverse wave. The perturbation parameter ε is the ratio of the across‐front to along‐front length scales. The interaction of the horizontal shear of the large‐scale wave with the horizontal temperature gradient is found to generate a vertical velocity field, which is steady in time to O(ε2). Conditions are sought for the formation of anafronts and katafronts.
Diagnostics of hydrometeorological anomalies in the Zaire (Congo) basin
Authors: Hirst Anthony C. , Hastenrath Stefan
Published: February 2007

Interannual variations in the large‐scale circulation over the tropical Atlantic are studied in relation to hydrometeorological anomalies in the Zairee (Congo) basin. Composite departure patterns of atmospheric and oceanic fields in the Atlantic are constructed for extremely dry and for extremely wet years in the Zaire river catchment. These maps are complemented by isocorrelate charts between river level and selected elements in 5° blocks of the Atlantic.

Water levels of the Zaire river in the first and second halves of the year are positively related to sea surface temperature off Angola, and to humidity and to actual and convective instability at Luanda, approximately upwind from the Zaire catchment. However, lapse rates at the Angola coast are strongly controlled by upper air conditions: cold air aloft may enhance instability and thus rainfall. Unlike during the first semester, sea surface temperature anomalies during the second half of the year do not seem to be forced by prior behaviour of wind stress in the equatorial Atlantic, nor by local wind speed and cloudiness. Equatorial south‐westerlies flowing across the Angola—Gabon coast toward the Zaire catchment are strong/weak during second semesters of high/low water level. Rainfall over the Zaire basin appears to be modulated by a zonal circulation along the Atlantic equator.
A study of dewfall in an arid region: An analysis of wangara data
Authors: Hicks Bruce B.
Published: February 2007

Micrometeorological flux‐gradient relations have been used to deduce average evaporation rates for most hours of the Wangara experiment, using net radiation and ground heat transfer values. At night, dewfall is found to have been a common occurrence, at rates that averaged about 0·015mmh−1 and sometimes approached (but never exceeded) the value of about 0·07mmh−1 that has been proposed as a natural limit. The average nocturnal dewfall was about 0·22 mm. During the 44 days of the experiment, about 20mm of rain fell at the central site, augmented by about 9mm of dewfall. Evaporation amounted to about 26mm of water.
Simultaneous measurements of the turbulent and microphysical structure of nocturnal stratocumulus cloud
Authors: Caughey S. J. , Kitchen M.
Published: February 2007

Observations from tethered balloon‐borne instrumentation of small‐scale air motion, temperature fluctuations and microphysical structure occurring within nocturnal stratocumulus are presented and discussed. The cloud top was marked by a very sharp and thin inversion layer whose vertical displacement was monitored with an acoustic sounder. The velocity field indicates the presence of strong downwards convection within the cloud, in which radiatively cooled air is transported away from the region near cloud top in discrete plumes. Droplet data show the preferential location of larger cloud droplets (> 7 μm radius) within the convection cells. Some aspects of the droplet observations are explained in terms of a simple Lagrangian model. Although entrainment is excluded from the model the predictions are consistent with the observations. Entrainment effects at cloud top result primarily in droplet concentration fluctuations with little change in the shape of the droplet spectrum, which the model suggests is mainly determined by the effects of radiation on droplet growth by condensation and the effects of vertical motion. Finally, some characteristics of turbulence through the cloud layer, and boundary layer generally, are examined and related to observations from the cloud‐free convective boundary layer and data from laboratory experiments.
Preliminary results of an investigation of the spatial distribution of fog droplets by a holographic method
Authors: Kozikowska Anna , Haman Krzysztof , Supronowicz Jan
Published: February 2007

Holograms of natural fogs have been made. A fog volume of 22·5 cm3 has been analysed from one hologram. Tests have been carried out to see whether the obtained spatial distribution of droplets could be approximated by Poisson statistics, mainly with negative results.

A continuation of this type of research and a deeper theoretical study of the effects on coalescence processes of a non‐random spatial distribution of droplets is suggested.
A study of mesoscale convective bands behind cold fronts. Part II: Cloud and microphysical structure
Authors: Bennetts D. A. , Ryder P.
Published: February 2007

This paper reports observations made over the North Atlantic of a system of convective cloud bands found within the cold air behind an occluded front. The observations were made from an instrumented aircraft which also released a pattern of dropsondes, measuring temperature, humidity and wind as functions of pressure.

The microphysical observations are interpreted by use of a three‐dimensional numerical model of deep convection with particular emphasis on the ice budget. The observations cannot be reconciled with a straightforward application of the theory of droplet freezing due to the activation of ice nuclei. Alternative freezing mechanisms involving the ingestion of the remnants of previously glaciated cloud and ice multiplication by the Hallett‐Mossop effect are considered.
Transfer coefficient eddy flux parametrizations in a simple model of the zonal average atmospheric circulation
Authors: White A. A. , Green J. S. A.
Published: February 2007

Baroclinic eddy parametrizations of the type proposed by Green in 1970 are applied in a simple climate model which is the zonal average counterpart of the long wave model used by the authors in a recent paper. Three parametrization schemes, each with different specifications of transfer coefficients, are examined. Diabatic forcing is represented by a sinusoidal function of latitude, and the performance of each scheme is gauged by the realism of the eddy flux and zonal mean fields which it produces in the steady state. In broad agreement with numerical simulations of baroclinic eddy life cycles, the best results are obtained when a certain condition of maximum available potential energy release is relaxed. The main feature of the study is the systematic application of fundamental constraints on the eddy vorticity fluxes: these are used to determine free parameters in the functional specification of the transfer coefficients.
An update of the Arosa ozone series to the present using a statistical instrument calibration
Authors: Dütsch H. U.
Published: February 2007

Results of the statistical method for determining and continuously adjusting the so‐called instrumental constants of two Dobson spectrophotometers operated in parallel for eight years at Arosa, Switzerland, are discussed. It is found that year‐to‐year variations of the constant are largely due to instrumental change and may have only a very slight contribution‘if any’from variations in the solar spectrum. Haze influence on the determination of the constants and on single wavelength ozone measurements is found; the latter has a strong seasonal variation. It is also found that the nickel sulphate filter used to eliminate stray light within the instrument has a small temperature dependence, which introduces a slight error in the determination of the instrumental constant with the filter and also in the measured seasonal variation of total ozone.

The Arosa C wavelength total ozone series beginning in 1926 is extended to 1982. Secular variations during the last 20 years demonstrate a remarkable constancy of the C—AD difference. This establishes the validity of the statistical method for monitoring the instrumental constant. The haze‐produced seasonal variation of the C—AD difference has hardly changed during that period, showing that the haze influence on C wavelength pair measurements has hardly increased with time.
Further theoretical studies of the role of splintering in cumulus glaciation
Authors: Koenig L. Randall , Murray Francis W.
Published: February 2007

The hypothesis that rime splintering can account for the presence of more ice particles in certain clouds than active ice‐forming nuclei is tested in a group of numerical experiments using a cloud model incorporating dynamics and bulk microphysics. Both warm base and cold base clouds were simulated with several sets of microphysical parameters. A second part of the study examined the dependence of the rate of glaciation on the concentration of natural ice‐forming nuclei. It is found that inclusion of a splintering mechanism leads to simulations that are more in accord with field observation, especially as regards the sensitivity of ice crystal concentration and rapidity of glaciation to the early concentration of cloud droplets and the dispersion of their size distribution. The rate of glaciation is highly sensitive to the properties of the supercooled liquid phase and accordingly to cloud base temperature (rapid for warm base clouds, slow for cold base clouds) but insensitive to the concentration of natural ice‐forming nuclei.
Comments on paper ‘the effect of gaussian particle‐pair distribution functions in the statistical theory of concentration fluctuations in homogeneous turbulence’ by B. L. Sawford (Q.J. April 1983, 109, 339–353)
Authors: Egbert Gary D. , Baker Marcia B.
Published: February 2007

In this brief note we examine the necessity of distinguishing between the use of forward and reversed transition densities in the computation of second moments of concentration fields advocated by Sawford. In contrast to Sawford, we show that the forward densities are always correct and that for incompressible flow the forward and reversed formulations are equivalent. This equivalence implies a simple constraint on particle‐pair separation densities for incompressible flow.
The response of the upper ocean to solar heating. I: The mixed layer
Authors: Woods J. D. , Barkmann W.
Published: February 2007

The 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

A 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

A 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

A 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

A 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

A 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

In 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.
A new convective adjustment scheme. Part I: Observational and theoretical basis
Authors: Betts A. K.
Published: February 2007

A new convective adjustment scheme is proposed, based on the simultaneous relaxation of temperature and moisture fields towards observed quasi‐equilibrium thermodynamic structures, with a relaxation time of order two hours. Separate schemes are used for deep and shallow (non‐precipitating) convection.
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

A 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

The 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

Data 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

A 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

The 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

The 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

The 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

It 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

An 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

A 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

An 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.
On the recirculation of the subtropical gyre
Authors: Marshall John , Nurser George
Published: February 2007

A hydrographic section through the Gulf Stream and its recirculation to the south is interpreted in terms of an analytical model based on the baroclinic Fofonoff gyre, in which layers shielded from forcing are homogenized to a uniform value of potential vorticity. Given this simple paradigm of the circulation, the process of anticyclogenesis is studied in which seasonal changes in the volume of low potential vorticity waters accompany a steepening of the main thermocline and an intensification of the recirculating gyre. The consequences of the gyre intensification for the structure of the bowl, the circulation within it and the deep abyssal flow are investigated. It is shown that as the anticyclogenesis proceeds, there is a general southward shift of the current systems; the bowl of the circulation strikes the ocean floor further south and there is an intensification of the depth‐independent recirculation tight in to the axis of the Stream. These changes in the structure of the recirculation are broadly in accord with observed seasonal variability associated with wintertime mode water production.
A cubic spline Galerkin scheme for the vertical discretization of atmospheric models
Authors: Steppeler J.
Published: February 2007

A Galerkin finite element discretization for the vertical coordinate using high‐order basis functions is introduced. The basis functions are cubic splines, assuming continuity of the spline and its first and second derivatives. Locality of the scheme is required, allowing each basis function to be different from zero on four grid intervals only. An efficient solution of the resulting Galerkin finite element equations is achieved by introducing a collocation grid.

A number of test integrations were done using the ECMWF spectral model with resolution T63 for horizontal discretization. The scheme was also used in a data assimilation experiment where it reduced the first guess error.
The antarctic winter; simulations with climatological and reduced sea‐ice extents
Authors: Mitchell J. F. B. , Senior C. A.
Published: February 2007

The southern hemisphere climatology of a recent version of the Meteorological Office 11‐layer general circulation model is presented. The simulated climate is compared with observational data, including Meteorological Office operational analyses for the period 1983‐1987. The surface temperatures and depth and position of the antarctic circumpolar trough are substantially more realistic than found in earlier climate models, but the high latitude upper troposphere is still colder than observed giving an excessively strong westerly jet. The sensitivity of the model to changes in the distribution of sea‐ice is investigated and compared with that found in other models. In a first experiment, all southern hemisphere sea‐ice equatorward of 67.5°S was replaced by sea at 271.2 K, and the surface roughness length was reduced from 10−1m to 10−4m. In a second experiment only the roughness length was changed. It is found that the change in surface roughness contributes substantially to the response to reduced sea‐ice extents. The specification of surface roughness over sea‐ice in numerical models is discussed.
The effects of rotation on flow of a single layer over a ridge
Authors: Baines P. G. , Leonard B. P.
Published: February 2007

The effects of the Coriolis force on the hydrostatic flow of a single lower layer over a two‐dimensional ridge are described. For short periods the flows resemble those of the non‐rotating case, but after a few hours (in the atmospheric context) effects of rotation become significant. For steady‐state flow over ridges and plateaux, critical conditions near the crest of the obstacle can control the flow in the same manner as for conventional hydraulics. However, steady disturbances are only manifested a finite distance upstream. For initial Froude number Fo < 1, upstream disturbances decay on the length scale (1 – F)1/2L, where L is the Rossby deformation radius. For Fo > 1, upstream bores (where they exist) become arrested at a finite distance ahead of the barrier. When flow on the lee side is supercritical, steady lee‐side disturbances consist of nonlinear inertial oscillations. Obstacle shape is important in determining upstream and downstream flow. The results are useful for understanding atmospheric flows over mesoscale ridges.
A composite diagnosis in sigma coordinates of the atmospheric energy balance during intense cyclonic activity
Authors: Prezerakos N. G. , Michaelides S. C.
Published: February 2007

Following the recent trend for composite analysis of the energetics of cyclonic systems, a rapidly deepening Saharan depression is studied within this framework. This case refers to a cyclone which was the prevailing feature in the synoptic analyses over the Mediterranean for several days in mid March 1981. This system was marked with a range of interesting phenomena over the central and eastern Mediterranean, such as very strong surface winds, rising and transportation of desert dust to large distances, and coloured precipitation. The synoptic dynamics of the evolution of the system is briefly presented and a reasoning for its unusually rapid deepening is provided.

The forms of energy considered here are kinetic energy, and sensible and latent heats. The diagnostic analysis of the budgets of these forms of energy has been carried out using the sigma coordinate system. Bearing in mind the explosive nature of the cyclogenesis studied here, the analysis is separated into two parts: one associated with the deepening phase of the cyclone and the other with its filling phase. The results are presented and discussed as time‐height cross‐sections of the energy contents; as time‐averaged vertical distributions of budget components; and as vertically averaged budgets. The analysis revealed contrasting differences in the energy budgets in each phase of the development of the system.
Warm and occluded fronts in two‐dimensional moist baroclinic instability
Authors: Joly Alain , Thorpe Alan J.
Published: February 2007

The two‐dimensional Eady mode of baroclinic instability exhibits a strong cold front structure after a period of development using semi‐geostrophic equations excluding moist processes. Indeed the surface wave has a single centre of cyclonic vorticity on the cold side of the warm air anomaly. Here results are presented from a semi‐geostrophic numerical model including moist effects which predicts an Eady wave with a cold and a warm front associated with two centres of surface vorticity located on either side of the warm anomaly. The ascent regions are taken to be saturated and close to moist neutral which in this baroclinic environment corresponds to a condition of zero equivalent potential vorticity; the descent is taken to be unsaturated.

This development of a structure resembling a warm front occurs only when the moist effects become dominant within a pre‐existing, weak, dry Eady mode. Using the same condensation representation starting from an extremely small, and random, amplitude disturbance a ‘pure’ moist Eady mode develops with a single vorticity maximum in the form of a cold front. In nature it is likely that often the moisture becomes dominant after a period, albeit sometimes a short period, of essentially unsaturated growth, although observational evidence is limited. Therefore the process of warm frontogenesis described here may be a contributory factor to those minority of mid‐latitude cyclones which exhibit a strong warm front. The model indicates that the warm front moves more slowly than the cold front, resulting in a cold occlusion process.

These investigations require a detailed examination of the initial growth of small amplitude disturbances near the limit of neutral moist ascent, i.e. pure moist modes. Results of previous such calculations using an analytical 2‐level model are confirmed here using a numerical multi‐level model. The more accurate estimates obtained here show that the finite growth rate in the limit of zero equivalent potential vorticity has a value of 1.9 times the dry value compared with the 2.4‐fold increase given by the 2‐level model.
Normal mode initialization and atmospheric tides
Authors: Wergen W.
Published: February 2007

It is shown that the standard normal mode initialization scheme mishandles atmospheric tides. A suitable modification is proposed. The performance of the revised scheme is discussed.
Quasi‐trajectory analysis of a sea‐breeze front
Authors: Holland Greg J. , McBride John L.
Published: February 2007

Observations from a parachute‐borne omegasonde that executed a trajectory through, along, and up and over the head of a sea breeze are analysed. This sea‐breeze head is shown to have similar characteristics to those of a gravity current. A deep lobe/cleft combination was observed at the front where continental air was entrained into the sea‐breeze air. Observed vertical motion near the sea‐breeze front exceeded 5 ms−1 and the sonde rose into a cloud that had formed in rising continental air over this front. An evaporatively cooled downdraught from this cloud was observed to be entrained through the turbulent wake region and deep into the sea‐breeze head.
The haar of north‐east Scotland
Authors: Findlater J. , Roach W. T. , McHugh B. C.
Published: February 2007

Case studies of three sea fog (haar) situations off the coast of NE Scotland are presented. The studies are based on aircraft and mini‐sonde observations and satellite imagery, and are used to assess the roles of physical and dynamical factors in developing and advecting sea fog in order to improve guidance to forecasters at coastal outstations. Two fogs had formed 2–3 days previously west of Ireland and had been advected round the N coast of Scotland, but the second fog had lifted into stratus by the time the aircraft had reached it. The third fog was warmer than the sea and appeared to be in the development/advection stage characterized by much stronger winds and lower liquid water contents.

Analysis and some modelling of the one‐dimensional heat and water budgets of sea fog suggest that after a period of development during transport over a relatively cold sea, initially ‘warm’ advection fog tends to become a diurnally modulated, self‐maintaining ‘cold’ radiation fog held at a temperature 1–2 degC below sea surface temperature. In other words, radiative cooling has depressed the temperature of the fog far enough below sea surface temperature for this cooling to become balanced by latent and sensible convective heat input from the sea surface.

The aircraft observations demonstrated the convective control of the vertical structure of ‘cold’ sea fog, and the influence of diurnally forced wind systems (in this case, a heat low formed over the Scottish mainland) in moving and moulding the fog.
The effect of solar radiation on occult deposition over hills
Authors: Gallagher M. W. , Choularton T. W.
Published: February 2007

A model of the turbulent deposition of cloud droplets to the surface from cap clouds enveloping a ridge has been extended to include the effects of the evaporation of deposited cloud water on the surface of vegetation. It is shown that evaporation driven by solar radiation penetrating the cloud increases the concentration of dissolved species on the leaf surfaces. The concentration effect tends to be a maximum close to cloud base both upwind and downwind of the hill, where deposition rates are low and the optical depth of the cloud is large. In a supercritical flow regime, however, deposition rates are very large on the lee side of the hill where winds are strong. In this region the cloud also has its smallest vertical depth, increasing the radiation density at the ground. It is this region, just to the lee of the summit, where phytotoxic effects are likely to be a maximum.