bluuebunny/crossref_metadata_2025 huggingface dataset:
The anisotropy of diffuse solar radiation determined from shade‐ring measurements
Authors: Steven M. D.
Published: February 2007

When a shade‐ring is used to shield a pyranometer from direct solar radiation, a correction to the measured diffuse radiation is necessary to account for diffuse radiation occluded by the ring. The shade‐ring correction depends on the dimensions and positioning of the ring and on the anisotropy of sky radiance. In this analysis these factors are separated using a simple model of sky radiance as the sum of a uniform background and a circumsolar component. The predictions of the model are tested against standard distributions of radiance and measured shade‐ring corrections for clear skies. The model is also used to describe average distributions of radiance for cloudy skies and is in good agreement with the limited measurements available. This analysis enables measurements of solar radiation to be partitioned more exactly into direct and diffuse components and may also be applicable to the estimation of diffuse irradiance on other shaded surfaces.
Baroclinic instability in a long wave environment. Part II: Ageostrophic energy conversions
Authors: Grotjahn Richard
Published: February 2007

A linear, quasi‐geostrophic, numerical, spectral, initial value, model is used to examine the stability properties of two long wave flows on a mid‐latitude β plane. In the ‘barotropic case’ the long wave amplitude is independent of height; in the ‘baroclinic case’ the long wave amplitude varies from zero at the bottom and top to a maximum near the model's ‘tropopause’. The initial structure of the unstable short waves incorporates the eigenfunctions for a similar, straight‐flow version of this problem. The solutions rapidly converge to a repeatable structure.

This report emphasizes the eddy energetics and especially its horizontal variation due to the long wave. In deriving the eddy energy equation three ‘ageostrophic’ conversions are found along with the two quasigeostrophic conversions. While the ageostrophic conversions vanish on a volume average, they are locally important; they redistribute the eddy properties in each of the three directions; none are described by any previous author. The vertical ageostrophic conversion is the difference between the conversion terms shared by the eddy available potential and eddy kinetic energy equations. The horizontal ageostrophic conversions steer the eddies along storm tracks which would not be anticipated on the basis of the basic flow's direction or vertical shear. These conversions create significant cross‐isobaric motion of eddies even though the long wave amplitude is modest compared with the zonal mean flow. This motion is not reproduced in a standard barotropic model wherein the ageostrophic conversions are all identically zero. On a global average the eddies are baroclinically growing; the barotropic conversion is small and damping.
Intercomparison of planetary‐scale diagnostics derived from separate satellite and radiosonde time‐mean temperature fields
Authors: Miles T. , Chapman W. A.
Published: February 2007

The planetary‐scale components of the extratropical northern hemisphere troposphere‐stratosphere 1973–74 winter circulation are diagnosed using separate time‐mean temperature fields based on radiosonde and satellite observations. Meridional cross‐sections of zonal wind together with, for zonal wavenumbers 1, 2 and 3, the streamfunction amplitude, phase and Eliassen‐Palm flux are displayed, with the relative accuracy of the satellite‐derived diagnostics assessed through comparison with the ‘ground‐truth’ radiosonde information. The satellite and radiosonde diagnostics compare most favourably in terms of zonal wind speed and shear, direction of wave propagation and meridional wave structure—all of which are closely related to the differential properties of the atmospheric temperature field. The intensity of the satellite‐derived patterns of tropospheric wave propagation is underestimated due to the effects of spatial smoothing and residual cloud contamination present in the satellite radiance measurements.
Mean state of the troposphere over south‐east Asia and the East Indies, December 1978
Authors: Mower R. N. , Chu J.‐H. , Martin D. W. , Auvine B.
Published: February 2007

The mean state of the troposphere over south‐east Asia from 8–30 December 1978 is described.

In the lower troposphere, a jet was observed over the South China Sea extending south‐westward from the Philippines. Evidence suggests that this north‐easterly jet was supported by downstream blocking induced by the Borneo land mass. At the equator, the jet had the appearance of being channelled between the highlands of Borneo and Sumatra. On its left flank, near the west coast of Borneo, was a semi‐permanent and quasi‐stationary vortex. Being warm core, this vortex was probably maintained, in part, by latent heat release in cumulus convection.

By partitioning the data into means at 00 and 12 GMT, the intensity of the rising branch of the Hadley circulation over Borneo was shown to increase by a factor of two, from −100 × 10−5 to −200 × 10−5mb s−1, from 00 to 12 GMT.
A new formula for latent heat of vaporization of water as a function of temperature
Authors: Henderson‐Sellers B.
Published: February 2007

Existing formulae and approximations for the latent heat of vaporization of water, Lv, are reviewed. Using an analytical approximation to the saturated vapour pressure as a function of temperature, a new, temperature‐dependent function for Lv is derived.
Super‐rotation and diffusion of axial angular momentum: II. A review of quasi‐axisymmetric models of planetary atmospheres
Authors: Read P. L.
Published: February 2007

The role of angular momentum quasi‐conservation in generating and maintaining axisymmetric flows characterized by a net global and/or local super‐rotation is reviewed. The conditions under which angular momentum per unit mass, m, can be diffused by Newtonian viscosity against its mean gradient in the meridional plane in a compressible, spherical fluid shell are explored in detail, and integral constraints on the form of the diffusive flux of m are derived for a steady flow. The implications of the results for simple models of planetary and stellar atmospheres are explored, with particular reference to the analogy between molecular and ‘eddy’ viscosity (as represented in mixing‐length theory), and various mixing hypotheses for eddy transfer properties. The results are relevant to zonally‐averaged models of the circulation of planetary and stellar atmospheres since, in the absence of molecular viscosity, the Eliassen—Palm flux of m must obey similar integral constraints to those applicable to the viscous flux in a purely axisymmetric system. Despite the non‐Newtonian character of realistic eddies in a baroclinic atmosphere, therefore, studies of simple axisymmetric systems with viscous diffusion can shed some useful insight into ways in which atmospheres, such as those of the earth, Venus, Titan, the Jovian planets and the sun, may acquire significant components of super‐rotation and prograde differential rotation, especially (though not exclusively) near the equator.
Comments on the computational stability of Gadd's adjustment and advection schemes for a split explicit model
Authors: Riphagen H. A. , Burger A. P.
Published: February 2007

In this note we investigate two‐level difference schemes for the solution of the adjustment and advection stages of a split atmospheric model by means of von Neumann stability analyses. The stability properties of the schemes proposed by Gadd are discussed using generalized formulations of which his schemes are special cases. In particular it is shown (in one and two dimensions) that the criterion for linear computational stability for the modified Lax—Wendroff advection scheme can be relaxed somewhat if Gadd's choice of parameter is extended to allow negative as well as positive values.
Convectively forced internal gravity waves: Results from two‐dimensional numerical experiments
Authors: Clark Terry L. , Hauf Thomas , Kuettner Joachim P.
Published: February 2007

Two‐dimensional numerical simulations were performed to investigate the nature of tropospheric internal gravity waves of the type which are observed to occur above active thermal convection over an unstable boundary layer. These gravity waves are believed to be excited by a combination of pure thermal forcing and by the boundary layer eddies and cumulus clouds acting as obstacles to the flow in the presence of mean environmental wind shear.

Large amplitude internal gravity waves were obtained in the simulations with amplitudes and horizontal scales similar to the 12 June 1984 aircraft observations over western Nebraska. This was a day with strong wind shear in the lowest 3 km above the ground and with scattered cumulus clouds topping the boundary layer. The simulations show that there is significant difference between the early time solutions (as might be predicted by linear theory) and late time solutions for the boundary layer eddy structure. A layer interaction occurs in which gravity waves of the stable layer are excited by the boundary layer convection. There is evidence to suggest that this layer interaction occurs both with and without shear but that it is stronger in the presence of low‐level shear. Results indicate that shear (or the obstacle) effect is a more efficient generator of gravity waves than is the pure thermal forcing. The simulations show that the gravity waves initially forced by the boundary layer eddies lead to a feedback mechanism that acts to organize the boundary layer eddies and the cumulus convection. The solutions suggest that the character of fair weather convection (moist or dry) is a non‐local problem involving at times the full depth of the troposphere.

The clouds produced in the simulations have very little influence on the wave field or boundary layer eddy structure as they are relatively small cumuli. On the other hand the clouds are strongly influenced by the interactions between the wave and eddy fields. Upshear growth of cumulus clouds similar to that which is frequently observed in nature is reproduced in the simulations. The development of feeder (‘feeder’ is used here in a dynamical sense only) clouds on (typically) the upshear side of the cloud is found to be a result of the interaction between the gravity wave field and the dry and moist convection. The relative phase velocity between the gravity waves and the cloud plays a crucial role in determining the character of the cumulus cloud growth in the present simulations. These simulations suggest that the dynamics both internal and external to the boundary of a cumulus cloud is a complicated mix between wave dynamics and the usually considered convection dynamics. A brief discussion of the implications of the present results to cloud boundary baroclinic instability dynamics is also presented.
Objective quality control of observations using Bayesian methods. Theory, and a practical implementation
Authors: Lorenc A. C. , Hammon O.
Published: February 2007

This work attempts to provide a theoretical framework for the quality control of data from a large variety of types of observations, with different accuracies and reliabilities. Bayes' theorem is introduced, and is used in a simple example with Gaussian error distributions to derive the well‐known formula for the combination of data with errors. A simple model is proposed whereby the error in each datum is either from a known Gaussian distribution, or a gross error, in which case the observation gives no useful information. Bayes' theorem is applied to this, and it is shown that usual operational practice, which is to reject outlying data and to treat the rest as if their errors are Gaussian, is a reasonable approximation to the correct Bayesian analysis. Appropriate rejection criteria are derived in terms of the observational error and the prior probability of a gross error.

These ideas have been implemented in a computer program to check pressure, wind, temperature and position data from ships, weather ships, buoys and coastal synoptic reports. Historical information on the accuracies and reliabilities of various classifications of observation is used to provide prior estimates of observational errors and the prior probabilities of gross error. The latter are then updated in the light of information from a current forecast, and from nearby observations (allowing for the inaccuracies and possible gross errors in these) to give new estimates. The final probabilities can be used to reject or accept the data in an objective analysis.

Results from trials of this system are given. It is shown to be possible using an archive generated by the system to update the prior error statistics necessary to make the method truly objective. Some practical case studies are shown, and compared with careful human quality control.
Observations within two regions of charge during initial thunderstorm electrification
Authors: Dye J. E. , Jones J. J. , Weinheimer A. J. , Winn W. P.
Published: February 2007

Airborne electric field measurements in two small thunderstorms in New Mexico show the existence of a narrow region of charge in each storm during the early stage of electrification. In one case a net negative region of charge was observed at about 7 km (‐ 12°C) about 1.5 km below the radar cloud top, when the electric field was only 600 V m−1, and could be accounted for by total charge of −0–01 C with a maximum net space charge density of −0.15 nCm−3, assuming spherical symmetry. This region of charge was about 500 m across and appears to have been associated with an updraught‐downdraught transition zone. In the other cloud, a region of net positive charge, also about 500m across, was detected at 7.7 km (‐20°C) about 500 m below the radar cloud top, when the electric field was about 2000 Vm−1. In both regions of charge, supercooled liquid water and ice particles including graupel were present, and ice particle concentrations, sizes, and collision rates were at a relative maximum, suggesting that the charge generation occurred via a precipitation‐based mechanism.
Estimates of cyclone track predictability. I: Tropical cyclones in the Australian region
Authors: Fraedrich K. , Leslie L. M.
Published: February 2007

A nonlinear systems analysis is applied to the tracks of 249 tropical cyclones (with a six‐hour sampling time) in the Australian tropics for the period 1959–1980. First estimates are obtained of the degree of their chaotic, or irregular, behaviour. The degree of chaos is estimated by normalizing all trajectories to a common initial position and measuring the average rate at which initially close pairs of pieces of trajectories diverge.

It was found from the correlation integrals calculated for the tropical cyclone tracks that the dimensionality of the underlying dynamical processes appears to be between six and eight, and that the time scale for e‐folding error growth is about one day. The results of this study therefore suggest that the movement of Australian region tropical cyclones is predictable deterministically up to about 24 hours in advance. Beyond that limit, consideration must be given to statistically based techniques.

These findings were supported further by comparing the rate of growth of the observed Australian region tropical cyclone position variance with that derived from a random walk model superimposed on a mean drift. The correspondence was very close, with both the empirical and theoretical position variances growing linearly in time after approximately the first 18 to 24 hours, confirming that stochastic models have a role to play in forecasts beyond 24 hours.
Three‐dimensional numerical experiments on convectively forced internal gravity waves
Authors: Hauf Thomas , Clark Terry L.
Published: February 2007

Results are presented of thermally forced dry and moist convection and the associated gravity wave fields from three‐dimensional numerical simulations using a non‐hydrostatic anelastic model. This paper extends earlier two‐dimensional simulations to include effects of the third spatial dimension employing a very similar environmental speed‐shear case for the study. The present simulations produce scattered fair weather cumuli in agreement with observations. In many important respects, the physical response is quite similar to that obtained in the earlier two‐dimensional calculations. The near‐uniform surface sensible heat flux results in Rayleigh modes filling the convective boundary layer (CBL) to begin with, whereas later, after convective motions start interacting with the overlying stable layer, larger horizontal scale deep modes become evident and in some cases dominant. The eigenfunction structure of these dominant forced normal modes consists of boundary layer eddies in the lower levels and gravity waves above. They are important organizers of the cumulus convection. As in the earlier two‐dimensional simulations, the efficiency of gravity wave excitation was found to be very sensitive to the mean wind shear in the region spanning the CBL and the overlying stable layer. The dominant horizontal wavelength in the shear direction ranges between 10 and 15 km in the free atmosphere whereas it peaks at about 6 km in the CBL.

The strong difference between the preferred directions of alignment for the eddies in the CBL (rolls aligned with the mean shear) and the overlying waves (aligned with lines of constant phase normal to the shear) results in overall broken conditions. The boundary layer motions are organized in broken ‘varicose‐like’ rolls aligned approximately with the mean shear. The overlying waves show a somewhat more scattered pattern. This scattered‐type dominant forced modal response combined with the nonlinear effect of the clouds themselves results in a cloud pattern revealing a high degree of randomness. This cloud field randomness occurs in spite of a near‐zero horizontal wavenumber structure to the surface sensible heat flux.

Exchanges of momentum between convective and mean motions in the CBL result in strongly curved stress profiles and a mixing out of the initial boundary layer shear. Sensitivity tests were performed where the mixing of momentum was partially compensated by the addition of low‐level pressure gradient terms.
The chemistry of a mesoscale rainband
Authors: Hegg Dean A. , Rutledge Steven A. , Hobbs Peter V. , Barth Mary C. , Hertzman Owen
Published: February 2007

A diagnostic modelling study is presented of the chemistry of a cold‐frontal rainband observed on the mid‐Atlantic coast of the United States. Both the chemical and physical data sets were sufficient to allow realistic evaluation of the 2‐D model employed. Utilizing the model, assessment is made of the importance of in‐cloud chemical processes to the deposition on the ground of selected trace chemicals; in‐cloud sulphate production was found to be of great importance. The effects of the rainband on the vertical transport of trace chemicals, and the effects of precipitation‐formation mechanisms on chemical scavenging, are also discussed.
Seasonal variability of temperature, salinity, velocity, vorticity and sea level in the central Gulf of California, as inferred from historical data
Authors: Ripa P. , Marinone S. G.
Published: February 2007

Historical data from 17 cruises into the Guaymas basin, central part of the Gulf of California, are globally analysed. Temperature, across‐gulf integrated velocity, sea level, and salinity have a significant seasonal cycle. The last variable has an important semi‐annual component, presenting four extremes per year. The others are mainly annual, reaching their maximum values in summer and their minima in winter.

By interaction with the atmosphere, the central gulf is gaining heat and losing fresh water during the whole year, on a monthly‐mean time scale (the only exception is an unimportant gain of fresh water in August). This implies net heat and salinity fluxes into the open ocean, through the gulf's mouth.

Temperature and salinity vertical gradients at the surface are well correlated with atmospheric forcing, through turbulent diffusion of heat and salt. Average velocity correlates with the along‐gulf wind stress with zero lag, as if frictionally dragged in a surface layer, with a short relaxation time. Seasonal sea‐level changes are mainly caused by the thermal expansion of the water column.

Horizontal processes (advection and diffusion) are as important as, or more important than, vertical diffusion for both heat and salt balances in the upper layers.

Vorticity, the across‐gulf derivative of the velocity field, does not show a seasonal cycle. Its contribution to the instantaneous geostrophic velocity and surface elevation near each coast is larger than that due to the across‐gulf average velocity.
Airflow over a two‐dimensional escarpment. I: Observations
Authors: Pitts R. O. , Lyons T. J.
Published: February 2007

Observations of forced flow over the highly asymmetric topography of the Darling Scarp, Western Australia, are presented. These are typified by shallow surface gradient winds of 10–20 m s−1 with an inherent critical level aloft and a stable layer produced through nocturnal radiational cooling. The observations highlight a topographically induced hydraulic jump, lee waves or vertically propagating hydrostatic waves depending on the upwind stability and wind profile. The vertically propagating waves and hydraulic jump are observed to produce accelerated flow down the escarpment.
Regimes in the wintertime circulation over northern extratropics. I: Observational evidence
Authors: Molteni Franco , Tibaldi Stefano , Palmer T. N.
Published: February 2007

Regimes of the northern extratropical circulation in winter are identified in this paper as clusters of atmospheric states in a low‐dimensional phase space generated by the leading EOFs of eddy geopotential fields. In order to define the clusters, our algorithm seeks points corresponding to local maxima for the density of atmospheric states; subsequently, a cluster is defined around each density maximum as that portion of the phase space in which the observed density can be locally approximated by a unimodal function. Two analyses were performed, using a 5‐dimensional and a 3‐dimensional space respectively, and they provided consistent results. Six clusters were found. the largest cluster includes 40% of the fields in our sample; its centroid is close to the climatological winter state, but it possesses a positive projection on the Pacific‐North American (PNA) pattern. the other five clusters represent anomalous flow regimes and include 52% of the fields. One of them shows a low amplitude of the planetary waves; the remaining four represent states with large wave amplitude but different phases. the variability between clusters accounts for the bimodality in the amplitude of planetary waves detected in previous observational studies. Our analysis reveals that this bimodality is much enhanced in the region of the phase space where the PNA index is negative, and the separation among the clusters is stronger. Finally, frequencies of transitions between clusters are presented, which show an asymmetric behaviour in the transitions between regimes with low and high amplitude of planetary waves.
Nieuwstadt's stable boundary layer revisited
Authors: Derbyshire Stephen H.
Published: February 2007

Currently no theory of the stably stratified atmospheric boundary layer (SBL) is generally accepted as definitive even for idealized cases. Nieuwstadt's theory, though a promising candidate, faces objections relating to upper boundary and steady‐state conditions, internal wave effects and the consistency of the model outside the strong‐stability limit. the aim of this paper is to examine the objections, improve the model, draw further deductions and compare with numerical models.

We shall deduce from Nieuwstadt's model that Bo = RfcG2|f|/√3, where Bo is the surface buoyancy‐flux, G the geostrophic wind speed, f the Coriolis parameter and Rfc the critical value of the flux Richardson number Rf. Higher‐order expansion shows this value is an upper bound corresponding to the stable limit L/h → 0. This is consistent with a similar bound on Bo derived from independent energy arguments. By contrast, within present idealizations there is no bound on the surface cooling rate.

Comparisons with a new series of large eddy simulations (LES) and other numerical models support the interpretation of Nieuwstadt's SBL as an idealized limiting case. the theory explains from first principles the observed sensitivity to small slopes. Interaction between sharp inversions and slopes may cause turbulence in thin layers. Coupling with the surface boundary conditions is a likely cause of intermittent turbulence, and explains features of Brost and Wyngaard's second‐order closure study. the formal singularity at the top of the SBL gives time‐scales for approach to inertial and heat equilibrium. Both, for separate reasons, are O(|f|−1).

A natural extension to moderately stable and near‐neutral conditions agrees well with numerical model results, and provides a complete prediction of SBL structure from given surface heat flux and synoptic pressure gradient. the near‐neutral regime is narrow at high Rossby number. Comparison with LES supports both the local scaling approach and the gross predictions of the theory. the model gives insight into the limitations of Rossby‐similarity formulae. In particular, restrictions of domain size hd (or similar background stability effects) may not become negligible even when h/hd → 0. Hence matching to neutral conditions is important in predicting even quite stable boundary layers. Wave effects, though non‐local in some respects, do not fundamentally change the Nieuwstadt picture for the mean structure. Retention in the model of the original value for Rfc is recommended even if wave radiation perturbs local Rf.

In summary, the adapted Nieuwstadt theory seems to provide a definitive framework for the idealized SBL, from which other ‘perturbation’ effects may be assessed. the value of Bo/G2|f| gives a criterion for (a) quasi‐steady or (b) intermittent SBL regimes.
Comments on ‘Numerical models of the raingauge exposure problem, field experiments and an improved collector design’ by C. K. Folland (October 1988, 114, 1485–1516)
Authors: Sevruk B.
Published: February 2007

Direct field comparisons of precipitation gauges (PG) are difficult and costly. For this reason the development of correction models such as Folland's, based on wind tunnel tests, is very valuable. This note draws attention to some new wind tunnel data on the airflow over several types of gauge. the new data are in much better agreement with those of Robinson and Rodda (1969) than with those of Green and Helliwell (1972) supporting Folland's preference for the former data. In addition, the new data provide support for Folland's assumption that the airflows over the 5‐inch and Hellmann gauges are similar, though this appears to be more or less accidental.
The information content of data from satellite sounding systems: A simulation study
Authors: Eyre J. R.
Published: February 2007

The potential of the TIROS Operational Vertical Sounder (TOVS) and the Advanced TOYS (ATOVS) for providing information on the vertical profiles of atmospheric temperature and humidity is investigated. the theoretical performances of these systems are examined primarily in terms of their ability to contribute information to a numerical weather prediction (NWP) system. the covariance of retrieval error expected from an optimal nonlinear inversion scheme, which retrieves simultaneously all the atmospheric and surface variables which significantly affect the measured radiances, is calculated theoretically and compared with the covariance of error assumed in the prior information which constrains the inversion. Performance is assessed for different combinations of radiometers, corresponding to present and future operational systems, and for different sets of assumed conditions: in the atmospheric profile, the cloud conditions, the surface characteristics and the assumed prior knowledge of these variables.

The potential performance of ATOVS is shown to be a considerable improvement on the present TOVS in many respects. the value of retaining a combined infrared and microwave system is confirmed. It is demonstrated that, although the ability of these sounding systems to improve on an NWP model's analyses of temperature and humidity is rather limited, the potential for improving on the analysis of layer quantities is much greater.

Some new ideas are presented for extending current retrieval methods to accommodate the additional problems presented by ATOVS data. Methods are proposed for treating the effects of microwave surface emissivity and cloud liquid water in a simultaneous inversion.
The Meteorological Office analysis correction data assimilation scheme
Authors: Lorenc A. C. , Bell R. S. , Macpherson B.
Published: February 2007

A major revision is described to the analysis component of the Meteorological Office data assimilation scheme for both the global and the regional numerical weather prediction models. Initialization by repeated insertion of data and divergence damping is retained but a more flexible modified successive‐correction scheme replaces optimum interpolation for the analysis step. The scheme is derived as an approximate iterative solution to a general equation for the optimal analysis. Within iterations, the analyses of each model variable are performed sequentially and multivariate increment fields are derived for dynamical balance at intermediate stages. The horizontal influence area of data is substantially increased, giving a beneficial impact in data‐sparse regions. All data contribute to the analysis at each gridpoint within their radius of influence. The observation weights are recalculated every model time‐step, allowing asynoptic data to be used at their true valid time, though this feature is found to have only slight impact on forecasts. Each analysis is factorized into a vertical step, then into a horizontal step. The structure of each is described in detail, with system parameters listed in appendices.

Results of two kinds are presented. Sensitivity experiments highlight the effects of some individual constituents of the scheme; operational trials against the old scheme reveal the impact of the whole system. The new analyses are better balanced and lead to a better short‐period forecast, especially for the wind field. Medium‐range impact is clearly beneficial in the southern hemisphere but ambiguous in the northern hemisphere. Precipitation forecasts in the regional model are improved.
Concentration fluctuation measurements in a dispersing plume at a range of up to 1000 m
Authors: Mylne K. R. , Mason P. J.
Published: February 2007

A series of tracer experiments studying concentration fluctuations in a dispersing plume of pollutant in the atmosphere at ranges of between 50m and 1000m is described. Experiments were conducted on three different field sites in near‐neutral or slightly convective meteorological conditions. The results show time series which are characterised by the intermittent occurrence of periods of fluctuating non‐zero concentrations, interspersed by periods of essentially zero concentration. The spectrum of concentration fluctuations is found to display inertial subrange behaviour, characterised by a −2/3 power law when nS(n) is plotted against frequency n, where S(n) is the variance (of the fluctuation) per unit frequency interval. The spectral peak frequency varies with distance from the source. In all cases the clipped‐normal probability density function (PDF) provides a reasonable fit to the concentration PDF. The exponential PDF is less flexible in fitting a wide range of experimental conditions, but is slightly superior for some short range examples. In the alongwind direction it is found that, although there is a rapid initial decrease in fluctuation intensity with distance, the intensity seems to approach an approximately constant non‐zero value at long range. In cross‐sections of the plume the variation of fluctuation statistics is dominated by the varying proportion of time during which the concentration is essentially zero. Conditional statistics, calculated from significantly non‐zero concentrations only, show only slight variations across the plume.
Atmospheric gravity waves and the ‘Rissaga’ phenomenon
Authors: Monserrat S. , Ibbetson A. , Thorpe A. J.
Published: February 2007

The atmospheric forcing of large sea‐level oscillations (up to 1.5 m amplitude with about a 10 min period) in some bays and inlets of the Balearic Islands is described using simultaneous measurements of pressure and sea‐level elevation in the port of Ciutadella (at the end of a large and narrow inlet) during 5–7 July 1989. The influence of atmospheric pressure oscillations on these large sea‐level variations, locally known as ‘rissaga’, is investigated. Coherence and cross‐correlation functions reveal that the large oscillations in the inlet are associated with and probably forced by a 10 min gravity wave in the atmosphere. The effect of longer‐period waves on the inlet is seen in the well‐known inverted barometric effect, forcing small oscillations in the water level of just some centimetres. These latter waves have a period of around 50 min and an unusually large amplitude of some 3 mb. The phase spectrum between sea level and pressure suggests that the inlet may behave as a damped harmonic oscillator which, during rissaga, is externally forced by the atmospheric pressure oscillations.
Morphology and density of ice accreted on cylindrical collectors at low values of impaction parameter. I: Fixed deposits
Authors: Levi L. , Nasello O. B. , Prodi F.
Published: February 2007

The morphology and density of ice depositions grown on fixed cylinders are studied in the ranges of Macklin's parameter, X, and of the Stokes number, K, of 0.5 <X<7μ ms−1 K−1 and 0.8° K £3.2 respectively.

For K<3, the deposit morphology was characterized by concave growth fronts and inward direction of lateral feathers. For larger K‐values, the growth fronts changed to convex profiles, and ice feathers to an outward direction.

The density was measured in the first millimetre of rime, using the X‐ray micrography technique. The stagnation‐region density (θ = 0) was found to depend on X only, the ρ(X) curve being independent of the K‐value. For X<2, ρdecreased slowly down to ρ ±; 0.3gcm−3, probably approaching a limiting value.

The results ‘off the stagnation region’ depend on K. For K<3, air gaps appear between lateral feathers and ρdecreases with the angular distance, from the stagnation line. For K > 4, the air‐gap effect was reduced to the very flanks of the deposit, and ρwas nearly independent of θ. The possible dependence of the observed behaviour on the shadow effect, generating air gaps, and on the supposed variation with θ of the droplet impact speed is discussed. It is shown that experimental aspects of deposit morphology agree well enough with stochastic model results obtained by previous authors, some differences probably being related to the oversimplification of the model, where the droplet spectrum is represented by the median volume droplet radius.

Relations between the morphology and density of the deposit and between these properties and the parameters X and K are discussed.
Observation of a quasi‐periodic oscillation due to gravity waves in a shallow radiation fog
Authors: Duynkerke Peter G.
Published: February 2007

Quasi‐periodic oscillations with a period of about 40 min are observed in visibility, temperature, water vapour, wind speed and long‐wave radiation during the fully developed stage of a shallow radiation fog. These oscillations were observed up to a height of about 60 m, whereas the fog extended up to a height of about 30 m. An interpretation of the oscillations is given in terms of gravity waves, which are rather typical under these very stable conditions.
A Preliminary Morphology of Precipitation Systems In Tropical Northern Australia
Authors: Keenan T. D. , Carbone R. E.
Published: February 2007

A preliminary morphology of convective systems observed in the vicinity of Darwin, Australia is presented. Several types of tropical convection during monsoonal and break‐season flow are identified using specific examples and compared with a range of systems observed worldwide in the general context of the bulk Richardson number. A significant spectrum of convective activity ranging from low shear and low Convective Available Potential Energy (CAPE), to high shear and moderate to high CAPE, typical of mid‐latitude severe storms, is identified. Significant differences between monsoonal flow and break‐season systems are apparent. the formation of convective lines perpendicular to the low‐level shear and the apparent balance between advancing shallow cold pools and the shear are ubiquitous features. Convective‐scale downdraughts and mesoscale descent appear to be responsible for redistribution of 700 mb momentum to the planetary boundary layer (PEL).

The motion of the break‐season squall lines appears to result from an equilibrium between the cold pool and the undisturbed environmental PEL flow. Deviations from this balance are hypothesized, to result in the observed ‘propagating’ and ‘non‐propagating’ or slow‐moving modes. Examples of these modes are given, showing the classical continuous development of new cells in a balanced state along the leading edge of long‐lived squalls and, in the propagating case, the discontinuous development of new cells on the cold pool ahead of the squall.
Two‐Layer Solutions to Long's Equation For Vertically Propagating Mountain Waves: How Good Is Linear Theory?
Authors: Durran Dale R.
Published: February 2007

Finite‐amplitude solutions for infinitely deep stratified flow over orography are obtained for upstream conditions with uniform wind and a two‐layer static‐stability structure. the flow within each layer is governed by Long's equation. the governing equations are solved using a semi‐analytic model with an open (radiation) boundary condition in the upper layer. A simplified, computationally efficient form of the finite‐amplitude radiation upper‐boundary condition is presented and utilized in the two‐layer calculations.

When the higher stability is in the lower layer, finite‐amplitude effects may produce large‐amplitude waves in circumstances where linear theory predicts a weak response. the relationship between the continuously stratified system and shallow‐water hydraulic theory is explored, and an expression is derived for the effective Froude number in unbounded continuously stratified flow. In those cases producing the strongest finite‐amplitude response, the flow undergoes a transition from subcritical to supercritical just to the lee of the mountain crest.

When the higher stability is in the upper layer, the finite‐amplitude response is also sensitive to the location of the stability interface. the dependence of the solution on the height of the interface is, once again, different from that predicted by linear theory. the interface height that produces the strongest response increases as the mountain height increases. These results support the idea, originally suggested through the analysis of linear multi‐layer models, that the vertical phase shift between the ground and the tropopause has an important influence on the strength of downslope windstorms. the optimal phase shift for finite‐amplitude waves cannot, however, be reliably determined from linear theory.
The Parametrization of Rainfall Interception In GCMs
Authors: Johannes Dolman A. , Gregory David
Published: February 2007

Experiments with a one‐dimensional version of the Meteorological Office's 11‐layer GCM are used to derive a simple calibrated subgrid parametrization of rainfall interception by the Amazonian rain forest. Two interception parametrizations, both incorporating subgrid variability of precipitation within a GCM grid box are presented and tested, and compared with observations made at a single location. the 1‐D model is a fully interactive atmospheric column and incorporates all the GCM physics but is forced at its boundaries by climatic data. the interception losses appear to be sensitive to what fraction of area in the GCM grid box is covered by the rain and to the precise formulation of the interception process. the implications for the parametrization of the wet area in a GCM grid box are discussed and suggestions are made as to how to parametrize the fraction of wetted area in a GCM grid box.
Exergy in meteorology: Definition and properties of moist available enthalpy
Authors: Marquet Pascal
Published: February 2007

The exergy of the dry atmosphere can be considered as another aspect of the meteorological theories of available energies. The local and global properties of the dry available enthalpy function, also called flow exergy, were investigated in a previous paper.

The concept of exergy is well defined in thermodynamics, and several generalizations to chemically reacting systems have already been made. Similarly, the concept of moist available enthalpy is presented in this paper in order to generalize the dry available enthalpy to the case of a moist atmosphere. It is a local exergy‐like function which possesses a simple analytical expression where only two unknown constants are to be determined, a reference temperature and a reference pressure.

The moist available enthalpy,am, is defined in terms of a moist potential change in total entropy. The local functionamcan be separated into temperature, pressure and latent components. The latent component is a new component that is not present in the dry case. The moist terms have been estimated using a representative cumulus vertical profile. It appears that the modifications brought by the moist formulation are important in comparison with the dry case. Other local and global properties are also investigated and comparisons are made with some other available energy functions used in thermodynamics and meteorology.
Entrainment in cumulus clouds, II: Drop size variability
Authors: Telford James W. , Kim Kyung‐Eak , Keck Thomas S. , Hallett John
Published: February 2007

A new analysis method for examining the effects of entrained subsaturated air on cloud drop sizes is described. This, independently, gives information about height changes following dilution with subsaturated air, and hence about whether the cloud parcels received the subsaturated air when higher or lower than their present level. The physical details of the entrainment mechanisms are first discussed to provide a basis for interpreting the data.

Drop‐size distributions, measured by a Forward Scattering Spectrometer Probe and obtained from aircraft cloud samples at constant altitude in convective clouds in Montana, are used to analyse these cloud processes. The volume averaged drop radii for each parcel (i.e. each measured spectrum), in several clouds for several horizontal traverses in each, are plotted against concentration, given here in the form of drop numbers per unit mass of air. This concentration from parcel to parcel depends on the fraction of subsaturated air which has been entrained. The entity‐type entrainment mixing mechanism provides a satisfactory explanation for all the examples found in this study, where the average drop radius changes very little with drop‐number concentration from parcel to parcel. Alternative mechanisms are discussed and found to be insufficient.
On the dynamics of convection waves
Authors: Sang J. G.
Published: February 2007

The linearized system of atmospheric equations is solved analytically for a two‐layer model to study the characteristics of convection waves. The solutions show that convection waves induced by a disturbance are composed of trapped waves with selected wave numbers that are determined by the interaction between the convective boundary layer and the overlying stable layer. The explicit relation of wave‐number selection defines the pattern of the vortices in the convection waves, which may appear as cloud streets, cellular convections or convective thermals. The wave‐number selection explains the formation of longitudinal and cellular convections and the transition between them. The analytical solutions of the convection waves reveal, theoretially, the structure of the large vortices in the convective boundary layer, and bring to light the effects of the upper stable layer on these vortices.
The exchange of atmospheric ammonia with vegetated surfaces. I: Unfertilized vegetation
Authors: Sutton M. A. , Flower D. , Moncriefe J. B.
Published: February 2007

Micrometeorological techinques have been used to measure the surface/atmosphere exchange of gaseous ammonia (NH3) over semi‐natural and unfertilized plant communities. A range of temperate ecosystems have been examined, including moorland, cut meadows and coniferous forest. Apart from a recently harrowed calcareous grassland, where deposition velocities, Vd (1 m), were in the range 1–11 mm s−1, ammonia was found to deposit rapidly (Vd typically 15–20 mm s−1 for short vegetation) with surface resistances, Rc, not significantly different from zero. Limited measurements over forest were consistent with a larger Vd (mean 66 mm s‐1) as a consequence of increased turbulence over the aerodynamically rough surface.

These observations show that, except for the harrowed calcareous site, ammonia is efficiently deposited to leaf surfaces, and that the mean air concentration at the surfaces approaches zero. The results are used as a basis for estimating annual dry deposition of ammonia to semi‐natural and unfertilized vegetation. For example sites in the British Isles, subject to typical background air concentrations, the estimated annual dry deposition of ammonia is in range 2‐41 kg fixed nitrogen per hectare, showing that this is a major source of fixed atmospheric nitrogen.
Bayesian quality control using multivariate normal distributions
Authors: Ingleby N. Bruce , Lorenc Andrew C.
Published: February 2007

An expression for the probability density of any distribution of observed values (given background values of known accuracy) is derived from the properties of multivariate normal distributions. This is used in the quality control of observations—‘good’ and ‘bad’ observations are assumed to have errors from a normal distribution and from a distribution giving no useful information respectively.

Three Methods of quality control are presented and compared; two of these are based on the probability density derived above, and the third is based on a related maximum probability analysis. They differ in the optimality principal used: Individual Quality Control finds the most likely quality (i.e. good or bad) for each observation, given information from all the others; Simultaneous quality Control finds the most likely combination of qualities; while Variational Quality Control is based on a variational analysis which finds the most likely true values. The quality control should be considered as part of the ‘analysis’ process of using the observations; these approaches to quality control are considered as approximations to a system giving the ‘best’ analysis, based on minimizing a Bayesian loss function. Approximation are also necessary in their practical implementations; the effect of these on various operational schemes is discussed.

The multi‐observation framework used includes the ‘background’ check as a special case, and it is extended to deal with observations with common sources of gross error. Applications to multi‐level checks, bias checks and checks for known error patterns are sketched.

As a by‐product the standard statistical interpolation formulae are derived from the properties of normal distributions, thus demonstrating the implicit dependence of statistical interpolation on the normal distribution.