bluuebunny/crossref_metadata_2025 huggingface dataset:
A numerical study of radiation fog with an explicit formulation of the microphysics
Authors: Brown R.
Published: February 2007

A model of radiation fog is described containing an explicit formulation of the microphysics of the condensation process. The coupling between the radiative and microphysical processes within the model means that apart from the specification of the turbulence and CCN spectrum it is deterministic.

Results are presented which indicated that:

 The drop‐size distribution is sensitive to the CCN concentration but not to the presence of large CCN.

 Radiative loss from the drops can have a significant effect upon their growth.

 The drop‐size distribution is very sensitive to reductions in the condensation coefficient to a value below 0.033.

Differences between observed and model drop‐size distributions are attributed to an underestimate in the model of the level of mixing within a mature fog.
Measurements of nitric oxide in the stratosphere of the Southern Hemisphere
Authors: Roy C. R. , Galbally I. E. , Ridley B. A.
Published: February 2007

Vertical profiles of NO in the stratosphere above Mildura, Australia (34°S, 142°E), were obtained in December 1977 using a balloon‐borne chemiluminescent detector. The average mixing ratio increased from about 0.2 ppbv at 10 km to 0.45 at 20 km and to about 7 ppbv at 33 km. The results are in good agreement with a profile obtained in the USA (32°N, 96°W) during October 1977 with the same instrument. It is suggested that the stratospheric NO distribution may, at low latitudes, be symmetric about the equator. The NO measurements and earlier chemical‐filter HNO3 measurements made at Mildura are compared to recent chemical model predictions.
A non‐linear atmospheric long wave model incorporating parametrizations of transient baroclinic eddies
Authors: White A. A. , Green J. S. A.
Published: February 2007

Numerical experiments with a simple spectral model of tropospheric long waves are described. The model is based on the 2‐parameter, β‐plane, quasi‐geostrophic equations and a coarse truncation is applied. Transient baroclinic eddy motion is represented parametrically using transfer coefficient expressions which are extensions of those proposed by Green in 1970 for zonal average formulations. The long waves are forced directly by externally specified diabatic and orographic functions. Zonal average diabatic forcing is also applied. When the eddy flux parametrizations act on the zonal fields only, realistic steady states are obtained if a coarse meridional truncation is applied, but explicit baroclinic instability occurs if higher meridional modes are included. Explicit baroclinic instability is suppressed if the parametrizations act also on the long wave fields; and in this case it is found that diabatic forcing of the largest scales of motion is the most important generator of stationary long waves in the model. Various reformulations are considered, and the problem of devizing better transient eddy parametrizations is discussed.
Mesoscale observations of upstream blocking
Authors: Manins P. C. , Sawford B. L.
Published: February 2007

Analysis of detailed tethered balloon soundings simultaneously undertaken at two sites in a valley shows that blocked (i.e. near stagnant) flow upwind of the ridges is characterized by a critical internal Froude number, Fr = uaH−1(gγa/θr)−1/2 of approximately 1.6. For Fr above this value the air in the valley is swept out by ambient wind. Otherwise blocking to near ridge height is observed. Here ua is ambient wind speed measured above ridge height H, g is acceleration due to gravity, θr a reference potential virtual temperature and γa the ambient vertical gradient of this quantity.
A note on the paper by P. J. Sellers and J. G. Lockwood, ‘A computer simulation of the effects of differing crop types on the water balance of small catchments over long time periods’
Authors: Shuttleworth W. J. , Gash J. H. C. , Sellers P. J. , Lockwood J. G.
Published: February 2007

Arguments are presented suggesting that the difference between predicted interception loss given by the multi‐layer MANTA model and the single source Rutter model, reported by Sellers and Lockwood (1981), are primarily related to differences in the treatment of aerodynamic transfer in saturated conditions. The reinterpretation is important in that, assuming the treatment used in the MANTA model is correct, changes in effective source position generate an unexpectedly large change in effective aerodynamic resistance. Attention is also drawn to features present in results given by the MANTA model operating in dry conditions which require further investigation.
Airflow over hills of moderate slope
Authors: Carruthers D. J. , Choularton T. W.
Published: February 2007

A three‐layer model is presented. This describes the flow on an inversion capped boundary layer over hills of moderate size and slope. The linearized vorticity equation is solved using non‐linear boundary conditions. The pressure field developed in the boundary layer is used to calculate a modified inner region velocity field (Jackson and Hunt 1975).

A low level inversion with height I, ≲ 2h is found to decrease the speed‐up near the ground. When I ≳ 2h the flow of the inversion layer shows an increasing tendency to be subcritical; this leads to a much increased speed‐up. Increasing the stability of air above the inversion increases the asymmetry of the flow. This causes an upwind lull and a subsequent rapid increase in the flow speed with very high wind speeds at the summit and downwind.

Comparison of the theory with simple observations made on Great Dun Fell, Cumbria shows encouraging agreement.
Aircraft measurements of wave momentum flux over the Colorado Rocky Mountains
Authors: Lilly Douglas K. , Nicholls J. M. , Kennedy Patrick J. , Klemp Joseph B. , Chervin Robert M.
Published: February 2007

Results are presented from a programme designed to measure the momentum flux associated with standing gravity waves over the central Rocky Mountains of North America. The purpose was to determine momentum losses from the westerlies as they cross a mountainous section of the northern hemisphere where wave drag is believed to be substantial. The data from one to three instrumented aircraft, operating on 20 flight days, mostly during January to March of 1973, are in reasonably good conformity with linear wave theory and also correlate well with surface measurements of downslope winds. Downward westerly momentum flux averaged 0.5 ‐ 1.0 dynes cm−2 (0.05 ‐ 0.1 Pa). The mean westerly flow on the flight days was near the climatological mean for the period of the project, although the mean flow over this period as a whole was much weaker than the climatological mean.
The daytime planetary boundary layer: A new interpretation of Wangara data
Authors: Manins P. C.
Published: February 2007

The efficacy of a simple encroachment model of mixed‐layer growth is explored by simulating two disparate days of the Wangara Experiment. The inclusion of thermal winds is shown to be essential if temperature, height and vector wind of the mixed layer are to be predicted with accuracy. In strong wind shears, additional growth due to the stress between the mixed layer and the air above evidently occurs and this is included by a new closure, invoking mean‐flow energetics (termed ‘the Froude dynamics’ approach).

Simulation of Day 12 supports the new model showing that even when averaged over (say) the central four hours of that day, the interfacial stress is expected to have been 30% larger than the surface stress. In conditions of convective heating and light winds the model predicts that negligible entrainment occurs. Growth is by encroachment only. This is confirmed by the simulation of Day 33.

Working from the turbulence kinetic energy equation, the closure problem for the Froude dynamics model is solved by a ‘catastrophe’ interpretation in which the normal state is one of no entrainment. Turbulence kinetic energy generated at the surface is dissipated and used to redistribute fluid within the mixed layer. Only when the Froude number, Fr, of the mixed layer exceeds a value of unity does entrainment occur such that the normal state is restored. Here Fr = |Δu|2θvo/gh ΔθV and Δu is the velocity jump, Δθv the virtual potential temperature jump (reference value θvo) at the top of the mixed layer which is of height h; g is acceleration due to gravity. There are no adjustable or tunable parameters involved.
Field studies of the effect of entrainment upon the structure of clouds at Great Dun Fell
Authors: Baker M. B. , Blyth A. M. , Carruthers D. J. , Choularton T. W. , Fullarton G. , Gay M. J. , Latham J. , Mill C. S. , Smith M. H. , Stromberg I. M. , Caughey S. J. , Conway B. J.
Published: February 2007

A description is given of experiments and calculations performed to investigate the effect of the entrainment of undersaturated air upon the microphysical structure of cap clouds at Great Dun Fell, Cumbria. Three case studies are presented: in I the properties of the cloud were essentially adiabatic; II provided evidence for mixing intermediate between the homogeneous and extreme inhomogeneous types; in III the mixing was extremely inhomogeneous. Consistent with this picture, the time constant for turbulent erosion of the entrained parcels of air is significantly greater in II than in III. Spectral changes observed in II and III agree well with predictions from a diffusive model of mixing (Baker and Latham 1982).
The climatological minimum in tropical outgoing infrared radiation: Contributions of humidity and clouds
Authors: Warren Stephen G. , Thompson Starley L.
Published: February 2007

Satellite observations of outgoing terrestrial infrared (IR) radiation as a function of latitude exhibit a minimum near the equator 20–40 Wm−2smaller than peaks in the subtropics. We attempt to dissect the causes of the dip through calculations with a spectrally‐detailed multi‐level radiative transfer model. Roughly one third of the dip can be attributed to the latitudinal variation of atmospheric water vapour; the remainder apparently is due to latitudinal variations in cloud amount and (especially) cloud‐top height. However, inclusion of clouds as given by published climatologies enhances the clear‐sky dip only slightly. Thus, about one half of the dip is essentially unexplained. We suspect the explanation is that near‐equatorial cirrus and cumulonimbus heights are too low in the cloud climatology, emphasizing the need for a better cloud climatology.

Since tropospheric humidity variations have a strong effect on clear‐sky outgoing IR, empirical studies which correlate cloud with IR variations may overestimate the effect of clouds on outgoing IR if cloud amount is correlated with relative humidity.

The effect of humidity variations on outgoing IR suggests that a measure of tropospheric humidity be incorporated explicitly in the parametrization of outgoing IR for simple climate models.
Stratospheric water vapour over Australia
Authors: Hyson P.
Published: February 2007

Measurements of stratospheric water vapour over Australia (Mildura, 34°S 140°E) during the period 1977–1979 indicate a mean mixing ratio between 15 and 21 km of 2°4p.p.m.m. The record since 1973 shows a quasi‐biennial oscillation with a peak to peak amplitude of 2 p.p.m.m. in the 15–21 km height interval. The latter appears well correlated with 50–30 mb equatorial westerlies.
Inland convection and energy transfers in a sea breeze model
Authors: Richiardone R. , Pearson R. A.
Published: February 2007

Availability of potential energy in inland convection and in a sea breeze circulation are compared using a formulation of P. C. Manins and J. S. Turner for the convectively mixed atmospheric boundary layer, and a numerical model of a sea breeze. Entrainment processes during the mixed layer growth appear to affect both the convective scale and the mesoscale, decreasing the amount of potential energy available to the convective scale yet increasing the potential energy available at the mesoscale. The kinetic energy content of the sea breeze cell is explained in terms of a constant efficiency for the conversion of available potential energy inside a cell whose horizontal dimension increases in time. Results of numerical experiments indicate that the current value of the temporally integrated buoyancy flux at the ground is the most important parameter for the sea breeze flow. This parameter determines the strength of the flow as well as the inland movement of the sea breeze front.
Solar radiation in dense saharan aerosol in Northern Nigeria
Authors: Brinkman A. W. , McGregor James
Published: February 2007

Measurements of normal incidence and global irradiance were made during the periods November to December 1977 and November 1978 to February 1979. Aerosol density was monitored in terms of turbidity, derived from the normal incidence record. A daily mean turbidity was calculated and used to trace variations in aerosol density over the season. Values ranged from 0–09 to 2–16 with average values of 0–44 and 0–47 for the two measurement periods respectively. A simple model was devised which related the diffuse irradiance to the turbidity and a parameter which described the angular dependence of scattering. This model demonstrated that significant differences existed between the optical properties of the local background aerosol and the dense Saharan aerosol periodically transported by the Harmattan wind. One such period of dense aerosol, when the visibility was reduced throughout most of Nigeria, is presented as a case study. The reduction was particularly severe in the north, where visibility fell below 500 m and an associated drop in air temperature of up to 6 °C occurred. Diurnal variations in turbidity were observed during periods of dense aerosol, although during the clearer periods diurnal variations were not evident. Some broad‐band spectral measurements of normal incidence irradiance were taken which indicated that extinction in both the dense Saharan and background aerosols was spectrally neutral.
A random walk model of dispersion in the diabatic surface layer
Authors: Ley Anne J. , Thomson D. J.
Published: February 2007

A two‐dimensional random walk atmospheric dispersion model applicable to a passive pollutant within the surface layer in all stability conditions is described. Numerical concentration profiles from the model are shown to be in good agreement with experimental data. Lagrangian statistics from the model are compared with Lagrangian similarity theory, and the variation of the similarity constant c with stability is studied. The similarity properties of the root‐mean‐square cloud height σ2 are also investigated.
Stratospheric tracer transport: A modified diabatic circulation model
Authors: Rogers C. F. , Pyle J. A.
Published: February 2007

We describe a model for stratospheric tracers in which transport consists of advection by the mean diabatic circulation and chemical eddy transport arising from longitudinal variations in in situ sources and sinks. The ozone distribution is reproduced well by the model and all species show interesting variations with longitude. The limitations of the model are discussed; we speculate that the most important of these is the neglect of transient contributions.
Production of laboratory clouds
Authors: Mossop S. C.
Published: February 2007

A cloud can be produced by injecting steam into a large plastic‐walled chamber. The liquid water content and drop size distribution of the cloud are found to depend chiefly on the power input to the steam boiler and the size of the boiler orifice respectively.
Aircraft measurements of the humidity in the lower stratosphere from 1977 to 1980 between 45°N and 65°N
Authors: Foot J. S.
Published: February 2007

Results are presented of measurements of humidity in the lower stratosphere from a Canberra aircraft of the Meteorological Research Flight using a manually operated frost point hygrometer. From 1977 to 1980, observations were taken routinely between 45 and 65°N so it has proved possible to study both temporal and latitudinal variations. The most striking feature emerging from these analyses is the decrease in variability with increasing latitude. Overall the results appear to indicate that (a) the subtropical jet stream is an important source as well as sink of water vapour to this region; (b) that there may be a sink of water vapour in polar regions; and (c) that the influence of sudden warming events can be detected in water vapour changes at latitudes greater than 50°N. No explanation for an apparently greater probability of unusually high humidity in the north in this range has been possible.
Transport of water vapour in a stratosphere‐troposphere general circulation model. I: Fluxes
Authors: Allam R. J. , Tuck A. F.
Published: February 2007

The fluxes of water vapour in the upper troposphere and lower stratosphere of a general circulation model have been calculated. Although the results are probably affected by some model characteristics, particularly the vertical resolution, they suggest that ingress of water vapour to the stratosphere may occur by large‐scale eddy motions above the core of the subtropical jet stream, as well as by vertical motion through the tropical tropopause. The latter motion occurs predominantly in the equatorial western Pacific and Indian Ocean, and is connected with the monsoon circulation.
On the non‐hydrostatic equations in pressure and sigma coordinates
Authors: Miller M. J. , White A. A.
Published: February 2007

The non‐hydrostatic equations governing the inviscid, adiabatic motion of a perfect gas are formulated using pressure as vertical coordinate; and M. J. Miller's 1974 approximate quasi‐non‐hydrostatic pressure coordinate equations are derived by applying a systematic scale analysis and power series expansion. The derivation makes clear that these equations are the pressure coordinate counterparts of the anelastic height coordinate equations obtained by Y. Ogura and N. A. Phillips in 1962. The two sets cannot be interconverted by coordinate transformation and so they are not physically equivalent; but the differences are small at the order of validity of both sets. Consideration of a quasi‐hydrostatic approximation emphasizes the non‐hydrostatic character of Miller's equations. Sigma coordinate quasi‐non‐hydrostatic equations are obtained by direct transformation of the pressure coordinate forms, and consistent energy equations are derived for both sets. Convenient diagnostic partial differential equations for the geopotential field are obtained for both pressure and sigma coordinate forms. As shown by Miller, the quasi‐non‐hydrostatic formulation does not permit vertically propagating acoustic waves. Horizontally propagating acoustic waves (Lamb waves) are in general allowed, but can be removed from the pressure coordinate system by applying suitable boundary conditions. Some aspects of the treatment of the Lamb wave problem are corrected in this study. The quasi‐non‐hydrostatic sigma coordinate system permits Lamb waves, but it may still be considered suitable for convective and (especially) mesoscale modelling with or without orography. The possible use of the quasi‐non‐hydrostatic system in large‐scale theory and modelling is also discussed.
On the accuracy of general circulation statistics calculated from FGGE data—a comparison of results from two sets of analyses
Authors: Lorenc A. C. , Swinbank R.
Published: February 2007

By comparing statistics from two independent sets of analyses, and relating the differences seen to known characteristics of the analysis system, an indication is given of the likely accuracy of typical general circulation statistics calculated from FGGE data. The analyses compared are those made by the European Centre for Medium Range Weather Forecasts using the main delayed mode observations, and those made in near real time by the Meteorological Office FGGE analysis system. Examples are given of the accuracy of both zonal means and geographical distributions, for basic time mean fields, for derived fields such as divergence, variances and fluxes, and for budget calculations.

It was found that qualitative agreement between the two sets of statistics was very good, even in the tropics and southern hemisphere. Quantitative agreement was also in general good; most of the systematic differences could be traced to known characteristics of the analysis systems.
The response of a nonlinear, time‐dependent, baroclinic model of the atmosphere to tropical thermal forcing
Authors: Grose W. L. , Blackshear W. T. , Turner R. E.
Published: February 2007

A multi‐level, sigma coordinate, primitive equation atmospheric model has been utilized to study both the tropical and extratropical response to an isolated region of steady thermal forcing in the tropics. The nonlinear response during the first 28 days of the simulation is described. The response can be generally characterized by two distinct components. The first component is a quasi‐stationary disturbance which extends eastward and poleward away from the source region along a ‘great circle’ path. The structure of this disturbance is essentially barotropic away from the source region. The second component is a growing baroclinic wave propagating zonally at mid‐latitudes. Significantly, this disturbance is apparently the result of baroclinic instability induced by the quasi‐stationary wavetrain. The discussion is predominantly heuristic in form and relies heavily on graphical presentation and quasi‐geostrophic theory to interpret the response and individual components of the thermodynamic energy and momentum equations.
An application of the Langevin equation for inhomogeneous conditions to dispersion in a convective boundary layer
Authors: De Baas Anne F. , Van Dop Han , Nieuwstadt Frans T. M.
Published: February 2007

Dispersion in one dimension is simulated by the Langevin equation dW = −(W/TL)dt + dμ, where W is the velocity of the particle (hypothetical fluid element), TL the Lagrangian time scale and dμ the random velocity increment induced by forces exerted by the turbulence on the particle during dt. The moments of dμ in the Langevin equation in inhomogeneous conditions can be determined, by requiring that for large times the density distribution of the particles is the same as that of the air. In our numerical experiment the Langevin equation with the above‐defined moments is applied to diffusion in the convective boundary layer. Profiles of the moments of the vertical turbulence velocities, U(z), n = 1, 2, 3, are based on measurements and scaled by convective scaling; TL is assumed constant. Particles are released at several heights, with an initial velocity distribution that has the same moments as the Eulerian turbulence velocity distribution at that height. At the boundaries reflection conditions are imposed. Our results are extensively compared with water‐tank experiments of Willis and Deardorff, wind‐tunnel experiments of Poreh and Cermak, field experiments by Briggs and a model of Baerentsen and Berkowicz. The mean height and variance of the particles, the concentration field as a function of down‐wind distance and height, and ground level concentrations are presented. They agree very well with observations of dispersion in the convective boundary layer.
On the local rate of turning of wind direction
Authors: Lecluyse A. , Neumann J.
Published: February 2007

From the equations of horizontal motion an equation is derived for the local rate of change of wind direction. The derivation retains the nonlinear and frictional terms, in addition to the horizontal pressure gradient and Coriolis terms. The resulting equation can be briefly described as comprising two groups: in the first ‘group’ we have the constant term −f(f= Coriolis parameter; −f= rate of directional turning of ‘inertial’ wind); the second group is the sum of several variable terms which thus ‘modulate’ the fundamental term −f. The modulation can result in changes both in the magnitude and sense of rotation.

In the discussion, the relative importance of the various terms is considered for two flows: sea breeze and synoptic‐scale flow, including an estimate of magnitude of contribution of the frictional term.
Development of a revised longwave radiation scheme for an atmospheric general circulation model
Authors: Slingo A. , Wilderspin R. C.
Published: February 2007

This paper describes the development of a new version of the longwave part of the radiation scheme used in the Meteorological Office 11‐layer atmospheric general circulation model. The work was initiated when comparisons with other radiation schemes revealed systematic errors in the longwave fluxes. A detailed investigation of these errors was carried out by examining the sensitivity of the scheme to many of the details of its formulation. Various improvements were made, notably to the treatment of the temperature dependence of the Planck function and the water vapour continuum absorption. It is demonstrated that the fluxes are extremely sensitive to the formulation of the continuum absorption and suggested that more observations are needed to refine the values of the absorption coefficients. Special attention was paid to the treatment of carbon dioxide absorption, as the model is being used to continue research on the effect on climate of increasing CO2 concentrations. A simple scaling of the absorber amounts is applied in order to include the temperature dependence of the CO2 absorption. It is demonstrated that fluxes from the new version of the scheme and the sensitivity of the fluxes to changing CO2 concentrations in realistic atmospheres are in good agreement with those from more detailed models.
A simple model of the tropical atmosphere driven by a circulation‐dependent forcing
Authors: Weare Bryan C.
Published: February 2007

A linearized shallow water model is forced by a circulation‐dependent heating term. This heating is derived from a consideration of the moisture budget equation and includes terms representing vertical motion, horizontal advection and evaporation. The model is iterated to achieve steady states. The impact of various terms of the heating expression to initial surface layer specific humidity perturbations near 2.5°N and 22.5°S is explored for a hypothetical mean specific humidity field representing the tropical Pacific Ocean. The vertical motion and advection terms in the heating equation lead to a smaller scale of heating than the initial perturbation. The evaporation term tends, on the other hand, to maintain or increase the spatial scale of the heating. For the initial perturbation at 2.5°N the final heating region is often shifted westward of its original position but remains at the same latitude. The 22.5°S perturbation leads to a heating zone equatorward of the original perturbation. The possible impact of these processes on El Nino theories is briefly discussed.
An observational study of the structure of stratiform cloud sheets: Part I. Structure
Authors: Nicholls S. , Leighton J.
Published: February 2007

Results from six flights with an instrumented aircraft in marine, stratiform cloud‐topped boundary layers are presented. The flights took place around the U.K. in a variety of conditions, both during the day and at night. The data comprise simultaneous, high resolution turbulence, cloud microphysics and radiation measurements.

On five of the flights, turbulent mixing in the cloud layer was found to be primarily maintained by convection resulting from cloud top radiative cooling. In four cases, this mixed layer was observed to be decoupled from the surface, i.e. the cloud‐induced mixing did not extend down to the surface, and the cloud formed part of a detached turbulent layer. One flight in high surface wind conditions displayed a significantly different structure. Possible reasons for this are suggested.

The mean microphysical features of the different cloud layers are compared, as are the measured and theoretically calculated radiation fluxes. The latter show good general agreement, but some systematic differences are apparent. Again, possible causes of these disparities are discussed.

Features of the turbulent velocity field are presented and a number of similarities between data from the convective cases and cloud‐free boundary layers driven by heating from below are found. Values of the entrainment velocity are estimated directly from the measured cloud top water fluxes in all but one case, together with an estimate of the errors incurred. The measured vertical turbulent fluxes are found to be reasonably well predicted by diagnostic mixed‐layer models using the measured boundary conditions as long as the heights of the mixed‐layer boundaries are also specified from observations. Such a model is also used to help explain the main observed features of the mixed‐layer structure and the differences between the various cases.
Sea‐ice and the antarctic winter circulation: A numerical experiment
Authors: Mitchell J. F. B. , Hills T. S.
Published: February 2007

A numerical experiment has been conducted to test the sensitivity of a global general circulation model to changes in sea‐ice extents in the antarctic during winter. Three 112‐day integrations have been made in which all the antarctic sea‐ice poleward of 66°S was removed, commencing on 10 June in the second, third and fourth years of a control integration. There was a large increase in sensible heat flux over the anomaly, a warming over the antarctic confined to the lowest atmospheric layer, and a 2ms−1 reduction in the westerly flow around the periphery of the (new) sea‐ice margin, in broad agreement with a previous study by I. Simmonds. The increased heating over the anomaly was accompanied by a decrease in surface pressure which was not found by Simmonds, and possible explanations of this discrepancy are considered. The results are related to previous work on the effect of increased CO2 using prescribed changes in sea surface temperature and sea‐ice extents. The implications of the results for the parametrization of the heat flux through sea‐ice cover, and for coupled ocean‐atmosphere models are discussed.
The water vapour budget of the stratosphere studied using LIMS and SAMS satellite data
Authors: Jones R. L. , Pyle J. A. , Harries J. E. , Zavody A. M. , Russell J. M. , Gille J. C.
Published: February 2007

Monthly zonal mean observations of H2O and CH4 made by the limb infrared monitor of the stratosphere (LIMS) and the stratospheric and mesospheric sounder (SAMS) instruments on Nimbus 7 have been used to investigate whether the H2O mixing ratios in the stratosphere are consistent with a source via the oxidation of CH4. While both sets of data show considerable seasonally varying structure, total hydrogen (neglecting molecular hydrogen) is relatively featureless with a mean value over the stratosphere of 6.0±0.35 p.p.m.v. (1σ) for the five‐month period studied. The uniformity of the total hydrogen fields points to the validity of the CH4 oxidation hypothesis.

The derived fields of total hydrogen are used to deduce a mean H2O mixing ratio for air as it enters the stratosphere of 2.7±0.35 p.p.m.v. (1σ) from which a desiccation temperature may be deduced (for example. −87.2°C at 60 mb).
The skill of dynamical long‐range forecasts, including the effect of sea surface temperature anomalies
Authors: Mansfield D. A.
Published: February 2007

The Meteorological Office 5‐level general circulation model has been used to investigate the skill of wintertime, long‐range (50‐day) forecasts. In spite of its low vertical resolution and relatively simple parametrization (by present day standards), its winter climatology is shown to be realistic not only in terms of the time‐mean flow but also in the behaviour of transients. In order to eliminate the less predictable synoptic scales, 15‐day means have been used to assess the skill at longer ranges. Even with climatological sea surface temperatures, the largest‐scale features of the flow are found on average to have a small but significant correlation with the real atmospheric evolution out to about 20 days. However, the skill is far from uniformly distributed between the 17 forecasts studied. Three forecasts exhibited some skill throughout the 50 days, while in five forecasts the anomaly correlation fell to zero within 10 days. The use of observed tropical sea surface temperatures improved the forecasts in four out of five years. Examination of the model response to tropical sea surface temperature anomalies shows that it is particularly sensitive to anomalies in the west Pacific and to a lesser extent the west Atlantic. In this region anomalies as small as 1 K or less, similar to the uncertainty in their measurement, can have marked mid‐latitude effects.
On the scale of baroclinic instability in deep, compressible atmospheres
Authors: Read P. L.
Published: February 2007

A simple model of linearized, inviscid baroclinic instability in an adiabatic, hydrostatic, compressible atmosphere of arbitrary (though finite) depth, based on the well‐known Eady model, is used to investigate the variation of growth rates and favoured horizontal length scales as functions of δ, the ratio of the model depth D to the density scale height Hs. Both geometric height coordinates (with w = 0 horizontal boundary conditions) and log‐pressure (with ω = 0 boundary conditions) are considered. For δ > 3 and a given zonal velocity scale, growth rates are significantly reduced relative to comparable instabilities in an incompressible fluid (δ = 0), and may be suppressed altogether in a laterally‐bounded channel for large enough δ at a given value of static stability. Where instability does occur, the length scales favoured are longer than for an incompressible fluid, and are generally comparable to a deformation radius based on D (rather than Hs). The relationship between these results and those obtained in comparable recent studies (which have found scales comparable to a deformation radius based on Hs to be important) is examined. Some implications for the role of baroclinic instability in the atmospheres of the major planets are also discussed.
Simulations of linear and nonlinear disturbances in the stratosphere
Authors: O'Neill A. , Pope V. D.
Published: February 2007

The aim is to study and contrast the dynamics of linear and nonlinear, large‐scale disturbances generated in the stratosphere by a localized perturbation in the troposphere. The perturbation is applied at the lower boundary of a primitive equation model of the stratosphere and mesosphere. The experiments are diagnosed using a range of dynamical quantities; in particular isentropic maps of Ertel's potential vorticity reveal where and how the stratospheric disturbance becomes nonlinear.

The simulations are contrasted with an idealized theoretical model of a nonlinear critical layer, and the relevance for our experiments of ‘wave breaking’ and irreversible mixing of potential vorticity is discussed. Apparent polar focusing of Eliassen‐Palm fluxes is related to localized changes in a widening ‘buckling zone’. Wave amplitudes are far too large to apply simple ideas of wave propagation.

The notion of pre‐conditioning as it has been applied to the stratosphere is challenged; there are occasions when it is an oversimplification to regard the troposphere as a wavemaker forcing changes in the stratosphere according to the tenets of wave, mean‐flow theory. Preceding a major warming, the flow is asymmetric about any axis (even in the much‐studied case of February 1979), and strong warmings develop through the nonlinear interaction of large, deep vortices associated with a tropospheric asymmetry. The nonlinearity of the flow is also manifested in an anticorrelation in time between zonal harmonics one and two over a deep layer, and by a phase shift of harmonics as amplitudes change. Associated changes in zonal mean winds give the appearance of distinct minor and major warmings, though a synoptic view allows systematic changes to be seen which caution against dividing the evolution into separate events.
Baroclinic waves and frontogenesis with an embedded zone of small moist symmetric stability
Authors: Xu Qin
Published: February 2007

Three‐dimensional semigeostrophic moist baroclinic waves and frontogenesis are studied both analytically and numerically. It is found that a slantwise zone of small (either unconditional or quasi‐conditional) moist symmetric stability embedded in a dry and baroclinic basic state traps the energy of baroclinic waves, forming a strong meridional structure with maximum wave amplitude in the moist zone and exponential decay of the wave amplitude away from the moist zone. The unstable waves absorb energy from both the moist and dry baroclinic basic state, but the strongest energy conversion is in the moist zone, so growth rates decrease with the width of the moist zone. As the moist zone becomes infinitely narrow (wide) the model degenerates into a dry (moist) Eady model. The smaller the moist symmetric stability, the larger the growth rate and the wavenumber of the most unstable wave. The valid range of the model parameters depends on the semigeostrophic approximation, which sets a limit to the smallness of the moist symmetric stability.

In semigeostrophic space, the moist zone changes shape due to geostrophic deformation. As the wave develops in real space, the surface pressure low (high) shifts northward (southward) due to geostrophic deformation, tightens (expands) due to ageostrophic convergence (divergence), and goes on to form a coupled warm‐cold frontal system.

A numerical model of the semi‐Lagrangian finite element method is developed. The method is shown to be accurate in computing wave growths and efficient in resolving frontal structures, although the semigeostrophic approximation is found to become poor or even invalid locally in the narrow (meso‐β‐scale) cold frontal region.
The seasonal evolution of planetary waves in the southern hemisphere stratosphere and troposphere
Authors: Randel William J.
Published: February 2007

Seasonality in planetary wave structure in the southern hemisphere stratosphere and troposphere (1000‐1 mb) is studied based on eight years (1979–1986) of daily geopotential data. Emphasis is placed on the distinctive seasonality in the stratosphere, along with tracing the stratospheric fluctuations into the troposphere. Time filtering is used to separate stationary and transient components. Maximum geopotential height variance is observed in the middle to upper stratosphere in late winter‐spring, with a distinct secondary, smaller maximum in late fall‐early winter; these maxima sandwich the strongest mid‐winter zonal winds. This sequence occurs approximately one month earlier in the upper stratosphere. Two corresponding peaks are observed in stratospheric zonal mean wind and temperature fluctuations. The two maxima in stratospheric wave variance are not in agreement with observed seasonal changes in quasi‐geostrophic refractive index of the zonal mean flow.

The stratospheric geopotential variance maxima result from nearly equal amounts of stationary and transient zonal wave‐1 fluctuations, with transient wave 2 also contributing during late winter. Stationary wave 1 shows intriguing seasonal evolution: the early winter maximum is predominantly equivalent barotropic, whereas that of late winter‐spring is highly baroclinic in the stratosphere. Stratospheric poleward fluxes of heat and momentum both exhibit pronounced maxima during September‐October; these result primarily from the stationary wave‐1 contributions, and the transients assume a secondary role. Although not as pronounced as the stationary wave seasonality, seasonal differences in transient wave behaviour are also documented, including three‐dimensional propagation characteristics and wave‐mean flow interaction diagnostics.
Dynamical analyses of marine atmospheric boundary layer structure near the Gulf Stream oceanic front
Authors: Wai Mickey Man‐Kui , Stage Steven A.
Published: February 2007

The effects of the sea surface temperature (s.s.t.) front at the edge of the Gulf Stream on the marine atmospheric boundary layer (MABL) are investigated using a numerical model to study the modification effects of an oceanic front on the MABL structure. The situation simulated is flow from over cold shelf water to over the warm water of the Gulf Stream. The initial temperature and humidity profiles of the air are specified to be near neutral over the cold water and are therefore typical of undisturbed conditions. The differential in s.s.t. across the oceanic front creates a horizontal variation in the surface perturbation pressure and the stability. The surface perturbation pressure and turbulent fluxes modulate the flow and produce horizontal variations in horizontal wind components with associated vertical motions. A thermally direct cell is produced as a result of the s.s.t. difference across the front. The isotherms slope upward towards the warm water. Entrainment of inversion layer air and upward vertical motion over the warm water cause the MABL to be deeper there. A layer of cloud forms over warm water and is associated with mixed layer deepening rather than lowering of the condensation level. Turbulent fluxes in the MABL show considerable spatial variation. Surface stress is much larger over the front and over the warm water than over the cold water. This is mostly caused by wind speed changes associated with the front. Changes in the drag coefficient due to changes in surface roughness and stability are much less important.

Mean budgets for temperature and total water indicate that there is a balance between horizontal advection and turbulent flux divergence. The U momentum budget shows that once the geostrophic balance terms are subtracted, the balance is mainly between the pressure gradient force associated with the induced temperature field and turbulent friction, with horizontal advection and the Coriolis force acting on the geostrophic departure playing minor roles. The V momentum budget shows a balance between horizontal advection, Coriolis force and friction.

Although there are few data for comparison, the results are in qualitative agreement with observations in the area. This study shows that the s.s.t. front at the Gulf Stream edge produces marked local changes in the nearby atmospheric surface layer.
A study of topographically induced multiple equilibria and low‐frequency variability. I. Idealized topography
Authors: Mak Mankin
Published: February 2007

It is shown that both the steady and the vacillatory states of a two‐layer, quasi‐geostrophic, forced dissipative topographic model with a high spectral resolution can be truly intransitive. There exists only an odd number of equilibria in this geophysical system, as predicted by Benjamin's general theorem for the Navier‐Stokes equation. The overall findings constitute a generalization of previous results for a barotropic model.

The analyses specifically establish for the case of a single‐wave topography that, in response to a barotropic forcing, there exist: (1) a low threshold value of the topographic height required for the existence of hysteresis: (2) a higher threshold value for the occurrence of weakly vacillating states with a high frequency; (3) a detuning effect of the nonlinearity due to an increase of the equivalent resonance frequency; (4) a baroclinization effect of an asymmetric friction; and (5) a robustness of the multiple equilibria for an additional weak baroclinic forcing.

Under near‐resonance conditions of the topographic waves for the case of a simple multi‐wave topography, as defined according to the linear dynamics, none of the equilibria is stable. The strong wave‐wave interaction in such a situation sustains a dynamically different pronounced vacillation with a dominant long period in the order of tens of days. The existence of multiple vacillations with distinctly different structure and period is established. Outside that range of parametric conditions, there is only one stable equilibrium state.

It is demonstrated that the time‐mean wavy flow of a decidedly nonlinear time‐dependent response can be reproduced, in a high degree of resemblance, by tuning the dissipation parameters in a counterpart linearized model that makes use of the ‘known’ mean zonal flow. It would be misleading, however, to conclude, on the basis of such an agreement, that the linear dynamics really accounts for the actual time‐mean wavy flow. This example serves to reveal the potential dangers in interpreting an observed mean flow as consisting of linear planetary waves on a pre‐determined zonally symmetric basic state.
Description and performance of an hourly nowcasting and very short‐range forecasting system
Authors: Heijboer L. C. , Timmerman H. , van der Hoek A.
Published: February 2007

Since the disaster in The Netherlands caused by a mesoscale gale on 12 May 1983, an hourly nowcasting and very short‐range forecasting system has been developed using all available hourly surface observations of pressure and wind over the North Sea and the surrounding countries.

Hourly analyses and prognoses are produced using hourly interpolated background fields and lateral boundaries from the fine mesh United Kingdom prognoses, which are received from Bracknell twice a day based on 00 and 12 GMT up to 36 hours in advance at 3‐hour intervals.

The computational area consists of a grid of 35 × 35 points with a grid distance of 50 km covering the North Sea and surroundings. The update and forecast model is a two‐parameter model based on filtered equations with the geopotential height fields at 500 and 1000 mb as input and output. It is called VIMOLA (vertically integrated model on limited area). A new method is used for the analysis at 1000 mb. The observations of pressure and wind are directly assimilated onto the computational grid, and in this way wind measurements are converted into pressure gradients. All 10‐minute‐mean winds are taken into account. The analysis system is called APW (assimilation of pressure and wind).

Every hour analyses and prognoses are produced up to nine hours in advance. The prognoses are merged with the fine mesh U.K. prognoses, i.e. the final prognoses are a linear combination of VIMOLA prognoses and U.K. prognoses. For one hour in advance no merging takes place and after nine hours the final prognoses are identical to the U.K. prognoses.

For The Netherlands and surrounding regions winds computed from the forecasts have been verified against the observations. Winds from the U.K. prognoses have been verified in the same way. The U.K. prognoses show a very good quality and a small error growth during the first 24 hours. However, the hourly incorporation of the observations gives an improvement, especially for the first hours of the forecast period.

Because no upper air observations are needed, a very short data cut‐off time can be applied (20–45 minutes after the time of observation). In practice, the system is suitable for operational gale warnings and tidal surge forecasts. Due to the hourly character, the system is able to detect temporal variations in the surface wind field such as the diurnal cycle over land and air mass variability. Practical examples are given in this paper, including some results obtained for the storm of 15–16 October 1987, which hit the south‐eastern part of England.