bluuebunny/crossref_metadata_2025 huggingface dataset:
Theory of the sunpillar
Authors: White R.
Published: February 2007

The radiance distribution predicted by classical physical models of the sunpillar is calculated. It is shown that the loci of constant tip angle calculated by Stuchtey differ from loci of constant radiance only as a result of variation of the reflection coefficient with arc distance from the source. At least at low source elevations, crystals oscillating about a horizontal equilibrium position are more efficient as pillar producers than those rotating about horizontal C6 axes, but there is no conclusive evidence that the latter never produce pillars. It is shown that the variation of the cross‐sectional area of the incident beam has important effects, such as production of a minimum radiance at the source. Neglect of this factor by recent workers casts doubt on some of their conclusions.

No evidence is found to suggest that the classical models need replacement: tentative suggestions are made that diffraction, transfer effects, and modes other than external reflection in crystals rotating about horizontal C6 axes may enhance the formation of pillars at higher source elevations.
The angular distribution and interception of diffuse solar radiation below overcast skies
Authors: Steven M. D. , Unsworth M. H.
Published: February 2007

The angular distribution of diffuse radiation was measured from May 1976 to May 1977 with a set of purpose‐built actinometers. Radiance distributions for 98 overcast hours were fitted well by an expression similar to the conventional function for a ‘standard overcast sky’ (SOC). However, values of the coefficient b averaged 1.23, significantly smaller than for the SOC (b = 2). Expressions for the interception of diffuse irradiance by sloping planes are discussed.
A case study of a subsynoptic disturbance in a polar outbreak
Authors: Oerlemans J.
Published: February 2007

A subsynoptic disturbance, occurring in an outbreak of polar air over northwestern Europe, is studied by using routine observational data. The disturbance showed a characteristic length scale of about 150km, a time scale of 4 hours, caused a surface pressure drop of only 1 mb, but brought up to 20 mm of precipitation in the coastal region of the Netherlands.

The observational analysis suggests that the disturbance was a warm‐core system driven by latent heat release and triggered by forced lifting along the coastline, which activated a weak trough filled with potentially unstable air.
The blocking of middle latitude westerly winds by planetary waves
Authors: Austin J. F.
Published: February 2007

A study of hemispheric data suggests that the splitting of westerly winds by blocking anticyclones is initially due to simple interference between stationary planetary waves with very large amplitudes but normal phases. A simple model is then used to investigate Green's hypothesis that the blocking anticyclone, once formed, can be maintained by baroclinic waves travelling in the split jet. The results are in good agreement with observations, particularly the vertical structure. A resonance seems likely at zonal wavenumber four.
A two‐dimensional numerical study of horizontal roll vortices in the neutral atmospheric boundary layer
Authors: Mason P. J. , Sykes R. I.
Published: February 2007

The dynamics of large‐scale, horizontal roll vortices in the neutral planetary boundary layer are investigated by means of a two‐dimensional numerical model. The rolls are assumed to be two‐dimensional and are calculated explicitly, while small‐scale turbulence is parametrized by a mixing‐length hypothesis. Although buoyancy effects are never negligible in the atmosphere, the assumptions of the turbulence modelling are supported by atmospheric observations of large eddies which are highly elongated in a direction close to that of the geostrophic wind, together with the observed partitioning of turbulence energy between the large eddies and a distinctly smaller scale. The results indicate a strong sensitivity to the roll orientation, and also the presence of a slow variation on the Coriolis timescale.
Diurnal and seasonal variation of convection in the wind‐mixed layer of the ocean
Authors: Woods J. D.
Published: February 2007

Buoyant convection in the wind‐mixed layer of the ocean provides a source of turbulent kinetic energy that is restricted to a layer whose depth, C, is equal to, or less than, H, the depth of the mixed layer. At night C = H, and convection contributes significantly to deepening the mixed layer by turbulent entrainment. During most of the day, when the rate of heat input by the Sun exceeds the rate of surface heat loss by evaporation, conduction and long wave radiation, C < H and convection does not contribute to entrainment. When C < H, C = D, the thermal compensation depth, whose magnitude is independent of any consequences of turbulent mixing or entrainment. D can be calculated from a model of solar energy absorption in the ocean. Such a model is used to determine the diurnal variation of D at selected locations. The shape of this variation (a ‘top hat’ distribution) suggests the use of two parameters Dmin (the noon value) and P10 (the proportion of each day for which D < 10m). The seasonal and meridional variations of these parameters reveal the corresponding variations in the contribution of convectively generated turbulent kinetic energy to mixed layer deepening by turbulent entrainment. The parameter P10 can be used to take account of the daytime quenching of convection by the Sun in climate models that do not resolve the diurnal cycle.
Mass divergence in tropical weather systems Paper II: Large‐scale controls on convection
Authors: McBride John L. , Gray William M.
Published: February 2007

Composited summertime deep cumulus convective weather systems are investigated over three tropical oceans. The occurrence and intensity of the convection are governed by various large‐scale forcing mechanisms, such as ITCZ convergence, easterly waves, and differences in diabatic heating between the disturbance and its clear or mostly cloud‐free surrounding region. Vertical profiles of divergence and vertical velocity for the weather systems are analysed from the point of view of trying to establish the relative importance of each large scale control on convection.

The dominant forcing in the Western Pacific and the GATE region is ITCZ convergence, with important secondary roles being played by differential radiative‐convective heating, easterly waves and convective feedback. In the Western Atlantic trade wind region there is no contribution from ITCZ convergence; the vertical motion of a typical weather system is consequently much weaker than in the other regions.

In all three regions the contribution to vertical velocity from frictional convergence is shown to be quite small.

A large diurnal modulation of convection is observed. In the GATE region a diurnal variation is documented for the low‐level convergence into the ITCZ. A physical mechanism is proposed to explain this phenomenon.

It is shown that the actual observed vertical motion of the summer‐time tropical oceanic weather systems comes from a combination of forcing mechanisms and does not result from any individual large‐scale control acting by itself. It is important that any idealized picture of tropical weather systems be broad and flexible enough to account for the large regional and multi‐scale forcing mechanism differences which exist. A simple ‘easterly wave’ or ‘ITCZ’ model of organized tropical convection is inadequate.
The dynamics of stratospheric warmings generated by a general circulation model of the troposphere and stratosphere
Authors: O'Neill A.
Published: February 2007

A feature of integrations with a general circulation model of the troposphere and stratosphere was that events were spontaneously produced closely resembling stratospheric sudden warmings. One example is examined in detail by means of momentum, heat and energy budget calculations, after a description of its main synoptic developments. Brief reference is made to two other model‐simulated warmings. It is argued that Matsuno's simplest idealization of stratospheric sudden warmings does not provide an adequate description of these events. The appearance of equatorward eddy momentum fluxes was found to have played a central role in their evolution. A mechanism is proposed by means of which such a flux reversal might occur in the stratosphere. The model warmings were apparently instigated by planetary‐scale changes in the troposphere, rather than by internal hydrodynamic instability.
On the entropy balance of the earth‐atmosphere system
Authors: Nicolis By G. , Nicolis C.
Published: February 2007

The entropy balance associated with a Budyko‐Sellers climatic model is developed. It is shown that different regimes, associated with decreasing, as well as increasing values of entropy production (which measures the rate of dissipation in the system) in the course of time are possible. This immediately poses the problem of stability of steady states of the climatic system. An explicit criterion of climatic stability is thus derived, which is expressed in terms of thermodynamic quantities related to excess entropy production. The results are illustrated on simple cases involving diffusive energy transport. A comparison with Paltridge's minimum entropy exchange principle is also attempted.
A model of the quasi‐biennial oscillation on an equatorial beta‐plane
Authors: Plumb R. Alan , Bell Robert C.
Published: February 2007

An equatorial beta‐plane model of equatorial wave‐mean flow interaction in the lower stratosphere is described. Kelvin and mixed Rossby‐gravity waves generated by a specified forcing on the lower boundary radiate upward through the model domain where they are subject to thermal and mechanical dissipation. The wave‐mean flow feedback problem is solved on the assumption that changes in the zonal mean state occur slowly enough for the wave field to be steady, locally in time.

A quasi‐biennial oscillation develops in the model with a period of about 1000 days. A novel feature of this model is the incorporation of latitudinal structure and of thermal and meridional wind components of the oscillation. The influence of the meridional circulation on the structure of the zonal wind oscillation is discussed in some detail.
A Simple Model for the Synoptic Analysis of Cold Fronts
Authors: Browning K. A. , Monk G. A.
Published: February 2007

A split cold front model, with an over‐running upper front (or humidity front) ahead of a surface front (or humidity front), is shown to provide a useful representation of the principal characteristics of many katafronts. The use of such a model leads to more consistent analyses, especially of cloud and rain, than are achieved using unmodified classical concepts.
Units For Column Amounts of Ozone and Other Atmospheric Gases
Authors: Basher Reid E.
Published: February 2007

This paper points out the unsatisfactory character of the units for expressing column amounts of atmospheric ozone (i.e., total ozone), it examines a number of alternative units suitable for column amounts of not only ozone but any gas in any planetary atmosphere, and it argues the case for using the SI derived unit of mole per square metre.
A data assimilation experiment and the global circulation during the FGGE special observing periods
Authors: Lyne W. H. , Swinbank R. , Birch N. T.
Published: February 2007

During the two Special Observing Periods of the First GARP Global Experiment (FGGE) a data assimilation experiment was conducted at the UK Meteorological Office. Throughout the period of the experiment observational data were assimilated dynamically into a numerical forecast model at the four main synoptic hours of each day.

In the first part of the paper, the method of data assimilation is briefly described. Following that, a description is given of the zonally averaged atmospheric circulation, as diagnosed from our analyses. The results of this study are in generally good agreement with those found by previous workers.
Profile relationships in the superadiabatic surface layer
Authors: Webb E. K.
Published: February 2007

By analyses of profile data for the superadiabatic surface layer from the Johns Hopkins group (U.S.A.) in 1953 and from Swinbank and co‐workers (Australia) in 1962–64, mean profile forms which give a close representation of the data are adopted.

For the profiles of U, θ, and r the gradients initially pass from z−1 dependence through a smooth transition at z/|L| ⋍ 0.03 towards z−4/3 dependence. The corresponding ‘1m4/3’ form adopted is broadly similar to the O'KEYPS, Businger‐Dyer, and BWIB (Kansas) U profiles, but its change of gradient with increasing instability is a little more rapid than for these three. This form is maintained, with KH/KM remaining constant, up to z/|L| ⋍ 0.27.

For larger z/|L|, KH/KM increases, |∂θ/|z and |∂r/∂z| falling relatively more rapidly and ∂U/∂ relaively less rapidly. Intermittent near‐vanishing of |∂θ/∂z| is observed over intervals of a few minutes. Approaching z/|L| values around 1 (the data limit) the profiles, while not tightly defined by the data, are suitably represented by a continuing z−4/3 dependence with KH /Km nearly twice its near‐neutral value.

The analyses and results are independent of the von Kármán constants ku kθ and kr (a possible dependence on the ratio kθ/ku in relation to the stability parameter being avoided by use of a modified from of Obukhov's lenght). The do not provide values of these three constants and the adopted forms remain unaffected by any future assignment of these values.
A variational theorem for circulation integrals applied to inviscid symmetric flows with variable stability and shear
Authors: Emanuel Kerry A.
Published: February 2007

Conditional symmetric instability has recently been proposed as an explanation for rain and cloud bands which are imbedded in larger regions of precipitation associated with extratropical cyclones (Bennetts and Hoskins 1979). In their paper, Bennetts and Hoskins discuss the use of a circulation integral for calculating growth rates of the instability. The purpose of this note is to demonstrate that the growth rates so calculated will be exact eigenvalues of the associated linear perturbation equations provided that these growth rates are maximized with respect to the path of integration. A simple application is presented and compared with results of a numerical experiment using the Bennetts and Hoskins model. The circulation integral method appears to be a simple way of assessing the potential for conditional symmetric instability in the atmosphere.
Aircraft observations of marine stratocumulus during JASIN
Authors: Slingo A. , Nicholls S. , Schmetz J.
Published: February 2007

Observations of marine stratocumulus obtained by three research aircraft during the JASIN experiment are presented. Detailed measurements were made of the thermodynamic, cloud physics and radiation fields. These showed an essentially well‐mixed boundary layer with cloud liquid water contents close to their adiabatic values. The mean drop radius increased steadily towards cloud top, where evidence of inhomogeneous mixing was found. Both the longwave and shortwave radiative components of the cloud layer energy budget were measured. Good agreement was obtained between the observations and several radiation schemes and in particular the measured cloud shortwave absorption was close to the theoretical values. At midday, the net longwave cooling of the cloud layer was found to be approximately compensated by the shortwave warming, although the regions of net warming and cooling were separated vertically, thereby promoting convection within the cloud. The wider implications of these results to studies of stratiform cloud are discussed.
A new distribution for annual extremes of enivromental variables
Authors: Challenor P. G.
Published: February 2007

A new distribution is proposed for the annual maxima of enviromental variables such as wind speed or wave height. This distribution overcomes some of the problems caused by seasonal variation in environmental data. A method of estimating its parameteres is given together with some examples of its application.
An internal symmetric computational instability
Authors: Hollingsworth A. , Kållberg P. , Renner V. , Burridge D. M.
Published: February 2007

A rapidly growing instability in energy‐enstrophy conserving finite difference forms of the primitive equations is described. The instability is unusual in that it is purely internal. It arises because the linearized forms of the equations do not conserve momentum. The modifications necessary to control the instability are discussed.
Simulation of the atmospheric response to soil moisture anomalies over Europe
Authors: Rowntree P. R. , Bolton J. A.
Published: February 2007

Positive and negative anomalies in the initial soil moisture over central Europe were introduced in turn into integrations of a global model of the July atmosphere. The results show that by modifying the partitioning of the turbulent fluxes such anomalies can have major effects on the modelled rainfall, humidity and temperature during the following 50 days over the anomaly area and that the anomalies can propagate into adjacent land areas. Similar results were obtained with a lower‐resolution version of the model.

The effects on rainfall were already important on day 3 of the integration over the anomaly area and on day 6 over parts of Scandinavia outside the anomaly area.

The experiment was repeated with a version of the model in which the prevailing flow over Europe was less weak with increased moist westerly flow from the Atlantic. In this case the dry anomaly persisted for only about 20 days before it became too weak to affect the evaporation significantly.
Laboratory studies of the charging of soft‐hail during ice crystal interactions
Authors: Jayaratne E. R. , Saunders C. P. R. , Hallett J.
Published: February 2007

A laboratory investigation of electric charge transfer during the impact of vapour‐grown ice crystals and supercooled water droplets upon a simulated soft‐hailstone target has shown that the magnitude of the charge transferred to the riming surface when crystals separate from it is a function of temperature, crystal dimension, relative velocity, liquid water content, and impurity content of the water droplets and hence the impurity content of the riming target. The sign of the charge transfer depends on temperature, liquid water content and droplet and rime impurity content.

In the absence of crystals, no charge transfer was detected during riming. In the absence of supercooled water droplets, crystals impacting at 10ms1 on an evaporating rime target produced a small negative charge on the rime of less than − 0.25fC per separating crystal. When the target surface grew by vapour diffusion it gained a small positive charge during such interactions. Much larger charges and completely different charge transfer behaviour was noted during riming. The target became positively charged at high liquid water contents and temperatures above a critical value, but negatively charged at lower temperatures or with lower liquid water contents. The critical sign reversal temperature at a liquid water content of 1 gm−3 was about − 20°C. At − 10°C with a liquid water content of 2gm−3, a 125 μm crystal impacting at 3ms−1 charged the target by +101C upon separation. The charge transfer increased sharply with impact speed and crystal size. Warming the positively charging rime to cause it to evaporate failed to reverse the sign of the charge transfer. Experiments with impurities showed that the sign reversal temperature increased if the droplets contained contaminants at concentrations found in cloud water.

It is suggested that there are two distinct charge transfer processes during crystal interactions with an ice target, the dominant one requiring the presence of supercooled water droplets. Careful control and knowledge of the microphysical properties of the clouds used in these experimental simulations has permitted an examination of charge transfer under many of the conditions used in previous studies. The results provide an understanding of the differences and a reconciliation between some of the previously disparate findings in terms of the two distinct charge transfer regimes.
The effects of horizontal diffusion on baroclinic development in a spectral model
Authors: Macvean M. K.
Published: February 2007

Linear and non‐linear effects on baroclinic development of linear horizontal diffusion of the form K▽2q, where q is an integer and k, in general, a constant, are investigated using a spectral model and a climatological wintertime northern hemisphere basic state.

An eigenvalue approach shows the greatest linear sensitivity to the diffusion to be in the high zonal wavenumbers (m > 12) which, in the absence of diffusion or with the more scale‐selective formulations (large q), are more unstable than the intermediate wavenumbers (m = 7–9). The ▽4 diffusion with a decay time scale of 6 h on the shortest retained scale, which has traditionally been used in our model, is shown to affect significantly the linear stability of almost all wavenumbers, whereas the more scale‐selective formulations have only a very small effect on m < 12.

Without dissipation, integrations exhibit grossly physically unrealistic features after about nine days even with very high resolution, indicating the crucial role played by dissipation in non‐linear baroclinic development. Non‐linear integrations with dissipation show great sensitivity to the formulation used. Typically about three times more energy is converted from potential to kinetic with a ▽6 than with a ▽4 diffusion having a similar decay time scale on the smallest retained spatial scale. It appears that as the dissipation time scale on the large spatial scales is increased while retaining sufficiently short time scales near the truncation limit to prevent the accumulation of energy there, so the amount of potential to kinetic energy conversion approaches a limiting value equal to about five times the energy conversion with the ▽4 diffusion. In the limiting case the amount of energy lost to dissipation is equivalent to about 45% of that converted from potential to kinetic energy, again emphasizing the crucial role played by dissipation.

It is found that if the decay time scale on the short spatial scales is too long then the integrations exhibit large truncation dependency. This provides a criterion for judging the suitability of a particular choice of dissipation, which might have quite general applicability.
Spectral representation of the vertical structure of turbulence in the convective boundary layer
Authors: Berkowicz R. , Prahm L. P.
Published: February 2007

A model is presented of the vertical structure of turbulence in the convective boundary layer. The model is based on representation of the fluctuating velocity components by trigonometric orthogonal functions. The vertical velocity component is represented by a sine and the horizontal by a cosine expansion. The sine representation of the w component ensures that w = 0 at z = 0. The expansion coefficients are uncorrelated if the sine functions are eigenfunctions of the vertical correlation function.

A simple form, based on the inertial sub‐range, is adapted for the energy spectrum.

Constant parameters of the model are evaluated from the surface layer measurements of the wind velocity variances, and it is shown that the model agrees with the measured vertical structure of variances through the whole boundary layer.
Some simple analytical solutions to the problem of forced equatorial long waves
Authors: Heckley W. A. , Gill A. E.
Published: February 2007

This paper describes some simple time‐dependent analytical solutions to the problem of a sudden ‘switch‐on’ of heating localized about the equator. The equations used are the shallow water equations on the β plane under the ‘long wave approximation’. It is shown that, with a reasonable amount of dissipation, it can take several days for a steady state response to be established. In the early stages, the circulation differs considerably from that of the steady state. The long time scale for the adjustment process has implications in numerical weather prediction since it implies that if the initial data are not in balance with the model's forcing, it may take several days for a balance to be established, by which time the quasi‐stationary tropical flow may have been irrevocably changed.
On stochastic perturbation of simple blocking models
Authors: Benzi Roberto , Hansen Anthony R. , Sutera Alfonso
Published: February 2007

In this paper we discuss some problems connected with the stochastic dynamics generated by the effect of small random perturbations acting on prototype equations governing large‐scale flows. We focus on the features of the process which occur when the main instability is the result of orographic forcing on simple barotropic motion. In particular, we study a three‐mode truncation of barotropic flow over topography when a stochastic white forcing acts on the system. We found analytical estimates of the exit times from the attraction domains of the steady solutions of the model. The exit times were found to range widely as a function of the zonal forcing.

In addition, we included a representation of the large‐scale forcing in terms of a red process. We found that as a function of the variance of the perturbing process and the model's parameters, the bimodal nature of the barotropic model can be obscured.
Towards the monitoring and prediction of north‐east Brazil droughts
Authors: Hastenrath Stefan , Wu Ming‐Chin , Chu Pao‐Shin
Published: February 2007

The purpose of this study is to develop a method of forecasting the March‐April and March‐September rainfall in northern north‐east Brazil. Earlier diagnostic research has documented that the droughts of northern north‐east Brazil are heralded by identifiable departures in the quasi‐permanent circulation systems in the tropical Atlantic sector.

Dynamically plausible predictor candidates are selected on the basis of the aforementioned diagnostic results and simple correlation. These include sea surface temperature, pressure and wind in key areas of the tropical Atlantic, pre‐season rainfall in north‐east Brazil and northern South America, and an index of the Southern Oscillation. Candidates for the period 1921–30 plus 1946–56 are entered into a stepwise multiple regression scheme. This yields regression equations for each of the months October through February preceding the March/April peak of the rainy season. For January, these explain about 65% of the rainfall variance.

The regression equations are then used to produce forecasts for each of the years 1958 through 1972, in terms of a seasonal rainfall index for the northern Nordeste as a whole. The forecasts account for 41–62% of the variance of observed rainfall, with which they are thus highly correlated. Unusually abrupt seasonal evolution of general circulation anomalies, as well as noise in predictors may contribute to forecast failures. However, the severe 1958 drought was eminently predictable from the methodology developed here. The most important predictors are atmospheric and oceanic parameters in key areas of the tropical Atlantic; the timely acquisition of such observations consistent with a reference climatology being essential for the implementation of the prediction method on an operational basis.
A radiation boundary condition for multi‐dimensional flows
Authors: Raymond William H. , Kuo H. L.
Published: February 2007

A radiation boundary condition designed for use at open or computational boundaries with multidimensional flows is formulated and tested on three two‐dimensional problems. Two of the problems simulate a simple wave propagation and possess analytic solutions so that the effectiveness of the boundary condition can be measured in terms of a r.m.s. error. A more subjective analysis must be used in the final problem, which is the simulation of an atmospheric cold front. The proposed radiation boundary condition utilizes the phase velocity projection in each coordinate direction or equivalently a weighted configuration of the phase velocities. This approach greatly reduces the sensitivity on the spatial derivative normally displayed in the phase velocity calculations. Comparisons are made against the traditionally used Sommerfeld radiation condition and various numerical schemes are tested. For multi‐dimensional flows the proposed radiation boundary condition is found to give significant improvement over the traditional Sommerfeld radiation condition.
A numerical model investigation of the storm surge of 31 January and 1 February 1953 in the North Sea
Authors: Flather Roger A.
Published: February 2007

Two numerical finite difference models, developed for operational surge prediction, are used to simulate the great storm surge of 31 January/1 February 1953 in the North Sea. The required meteorological forcing is derived from an analysis of observed atmospheric pressures.

Both models reproduce the surges observed at the coast with good accuracy. A discussion of the surge generation and development is presented on the basis of the model solutions. Estimates of the work done by the wind stress and by gradients of atmospheric pressure indicate that the surge was generated within the North Sea primarily by the wind. Much of the surge generation took place in a few areas of limited extent, where the strongest winds and strong currents occurred in a favourable combination.
Solar heating of the oceans—diurnal, seasonal and meridional variation
Authors: Woods J. D. , Barkmann W. , Horch A.
Published: February 2007

Solar heating is an important factor in modelling the upper boundary layer of the ocean. It influences not only the temperature, but also the depth of the mixed layer and must be taken into account in circulation dynamics. The study reported in this paper was designed to reveal the principal features of the global climatology of solar heating in the ocean, with such applications in mind. The meridional, seasonal and diurnal variations of the vertical distribution of solar heating inside the ocean, expressed in terms of I(z), the rate of heat accumulation below depth z, and †(z) = (1/c). dzI(z), the rate of temperature rise, are calculated for given values of cloud cover and seawater turbidity (expressed in terms of Jerlov's water types) using a model that incorporates a new parametrization of I(z)/I(0), which is shown to be more accurate than previous versions. At present there exist no reliable global climatologies of cloud cover and seawater turbidity, so the values of the corresponding parameters are held constant in each computation, which is then repeated using parameter sets covering the full ranges from clear to overcast sky, clear to turbid ocean water. It is found that uncertainty in cloud cover is more important in the mixed layer, and uncertainty in seawater turbidity is more important below. The results presented in this paper are mainly concerned with solar heating below the mixed layer. It is calculated that the annual temperature rise can exceed 1 K and the annual heat accumulation can exceed 100 MJ/m2 below the mixed layer in the tropics. At higher latitudes solar heating produces similar heating rates in summer, but the stored heat is extracted locally in winter when the mixed layer depth exceeds the maximum depth of solar heating, defined here by a daily temperature rise of 1 mK or a heat flux of 86.4 KJ/m2d (=1 W/m2). The sensitivity of the seasonal and meridional variations of the maximum depth of solar heating to cloud cover and seawater turbidity is investigated. The actual change of temperature due to solar heating in the seasonal thermocline at Ocean Weather Station ‘C’ is calculated using Bunker's monthly mean cloud cover and Jerlov's seawater turbidity. Extension of such calculations to the whole of the World Ocean must await the publication of global climatologies of cloud cover and seawater turbidity, which are expected to be derived from satellite observations during the next decade.

A solar heating climatology is a prerequisite for computation of the thermal response of the ocean to CO2 pollution of the atmosphere. The implications of the results obtained from the present study are discussed. An early rise in tropical sea surface temperature seems likely, but exact prediction will be hindered by uncertainty in the turbidity of the tropical ocean.
The dynamics of stratocumulus: Aircraft observations and comparisons with a mixed layer model
Authors: Nicholls S.
Published: February 2007

Results are presented from a detailed case study of daytime stratocumulus over the North Sea using an instrumented aircraft. The measurements include turbulence fluctuation data, radiation fluxes and droplet spectra and were made both in and out of cloud. The mean structure is discussed and a diagnostic, one‐dimensional, mixed layer model is formulated to predict the variation of turbulent fluxes with height and to assess the sensitivity of these solutions to various physical processes. These predictions are compared with observations enabling the effectiveness of some widely used model assumptions to be tested.

Water transport by gravitational settling is found to be important throughout the depth of the cloud. Mixing within the cloud is driven by convection generated primarily by radiative effects: strong cooling from cloud top with warming beneath although the net heating of the cloud layer is close to zero. Turbulent kinetic energy is exported downwards via negatively buoyant elements from the zone of intense cooling near cloud top into the body of the cloud. This appears to be achieved by the combined action of the turbulent transport and velocity‐pressure correlation terms in the turbulent kinetic energy balance equation which are therefore important in maintaining mixing in the lower part of the cloud.

The cloud and sub‐cloud layers are found to be decoupled, i.e. they appear as two separated mixed layers. Reasons for this are examined and some further consequences for cloud evolution are investigated. In the particular case studied, potential instability could be generated in the lower layer leading to low‐level cumulus formation. These rise into the stratocumulus layer thereby reconnecting the two previously separated regions. The implications for stratiform cloud modelling are discussed and some recommendations for future work made.
Atmospheric flow over a succession of nearly two‐dimensional ridges and valleys
Authors: Mason P. J. , King J. C.
Published: February 2007

Observations of neutral static stability flow in a nearly two‐dimensional valley are presented. Upstream and downstream of the valley are a succession of similar ridges and valleys. The valley sides have a slope of about 32° to the horizontal and when the wind is nearly perpendicular to the valley there is a marked flow separation.

The observations of mean flow and turbulence are compared with numerical simulations of flow over periodic two‐dimensional ridges and valleys.
Random walk modelling of diffusion in inhomogeneous turbulence
Authors: Thomson D. J.
Published: February 2007

The use of random walk techniques in diffusion modelling has been very successful when the turbulence is not too inhomogeneous. In this paper the difficulties associated with inhomogeneous turbulence are investigated, and a random walk model is developed which ensures the correct steady state distribution of particles in phase space. The model requires the generation of random numbers from non‐Gaussian distributions, and this makes the model difficult to apply in cases of severe inhomogeneity. An alternative model is presented which overcomes this problem in cases where the velocity distribution is Gaussian.
Charge separation in a Florida thunderstorm
Authors: Hill R. D.
Published: February 2007

A simple explanation of the location and movement of the breakdown region in a Florida storm is given in terms of the separation resulting from dynamical equilibria between aerodynamic and electrostatic forces on charged particles.
An observational study of the structure of stratiform cloud sheets: Part II. Entrainment
Authors: Nicholls S. , Turton J. D.
Published: February 2007

The structure near the top of stratocumulus clouds is investigated with the aid of aircraft data to determine possible processes influencing entrainment. An expression is derived to predict the buoyancy fluctuations which can be produced by mixing at the interface due to evaporative cooling, and the relative influence of cooling by radiation and by evaporation near cloud top is discussed. It is argued that the effects of evaporative cooling are not properly considered in current assessments of the stability of inversions to entrainment. A method for taking this into account in a more realistic manner is proposed.

Entrainment rates derived directly from aircraft flux measurements are compared with various prediction methods. These include the predictions from four entrainment models. These predictions are found to vary widely for a given set of conditions but are generally smaller than the observationally derived values. Some of the consequences of underestimating entrainment and various other shortcomings of the models which are exposed by the comparisons are also discussed.
Alleviation of a systematic westerly bias in general circulation and numerical weather prediction models through an orographic gravity wave drag parametrization
Authors: Palmer T. N. , Shutts G. J. , Swinbank R.
Published: February 2007

Systematic westerly biases in the northern hemisphere wintertime flow of the Meteorological Office 15‐layer operational model and 11‐layer general circulation model are described. Evidence that the failure to parametrize subgrid‐scale orographic gravity wave drag may account for such biases is presented. This evidence is taken from aircraft studies, surface pressure drag measurements, and studies of the zonally averaged momentum budget. A parametrization scheme is described in which the surface stress is proportional to the near‐surface wind speed and static stability, and to the variance of subgrid‐scale orography. The stress is absorbed in the vertical by considering the influence of such gravity wave activity on static stability and vertical wind shear. A Richardson‐number‐dependent wave breaking formulation is devised, and the vertical stress profile determined by a saturation hypothesis whereby the breaking waves are maintained at marginal stability. It is shown that wave breaking preferentially occurs in the boundary layer and in the lower stratosphere.

Results from a simple zonally symmetric model show how the adjustment to thermal wind balance with a wave drag in the stratosphere, warms polar regions by adiabatic descent, and decelerates the mean westerlies in the troposphere.

The influence of the parametrization scheme on integrations of the 11‐layer model is described, and found to be generally beneficial.

In a discussion of the reasons why this problem has only recently emerged, it is suggested that the satisfactory northern hemisphere winter circulations of previous, coarser general circulation models were due to a compensation implied by underestimating both the surface drag, and the horizontal flux of momentum by explicitly resolved large‐scale eddies.
Concerning the effect of surface drag on the circulation of a baroclinic planetary atmosphere
Authors: James I. N. , Gray L. J.
Published: February 2007

Extended runs of a multi‐level spectral numerical model of atmospheric flow on a hemisphere have been used to investigate the effect of surface drag variations on the nature of the time‐averaged flow. It is shown that the most drastic effect of reducing the surface drag is to permit strong horizontal, barotropic shears in the zonal mean flow, which inhibit baroclinic instability. This superficially paradoxical result is implicit in many earlier calculations, most clearly in the lifecycle description of nonlinear baroclinic instability, but appears not to be widely appreciated. The kinetic energy spectra are interpreted in terms of the wavenumber of the most baroclinically unstable normal modes KR, the Rhines blocking wavenumber Kβ, and a limiting wavenumber based on surface drag KD. The changes in KR and the smaller changes in Kβ as drag is reduced account satisfactorily for the changes found in the kinetic energy spectra.
The impact of ensemble forecasts on predictability
Authors: Murphy J. M.
Published: February 2007

An estimate of the mean effect of ensemble‐averaging on forecast skill, under idealized ‘perfect model’ conditions, is obtained from a set of eight independent 50‐day winter ensemble forecast experiments made with a hemispheric version of the Meteorological Office (UKMO) 5‐level general circulation model. Each ensemble forecast consisted of seven individual integrations. Initial conditions for these were obtained by adding spatially correlated perturbations to a given wintertime analysis, and a further integration created in the same manner was used to represent nature, giving the perfect model approach.

The ensemble‐mean forecast shows a clear improvement in amplitude and phase skill compared with individual forecasts, the period of significant predictability for daily fields being increased by 50%. The improvement in skill is consistent with simple theoretical estimates based on the perfect model assumption. These calculations are used to deduce how ensemble‐mean forecast skill should vary with the size of ensemble. The superiority of the ensemble‐mean is maintained when forecasts are spatially smoothed or time‐averaged.

The spread of an ensemble distribution can in principle give an a priori indication of forecast skill. A moderate level of correlation between ensemble spread and the forecast skill of the ensemble‐mean is found on the hemispheric scale.

The extent to which the potential benefits of ensemble forecasting may be achieved in reality depends on the model's practical forecast skill. Since the practical skill of the 5‐level model is rather low, an ensemble‐mean forecast is on average no better than an individual forecast up to the normal limit of deterministic predictability. However, in four experiments where the individual forecasts show skill beyond this point, the ensemble‐mean forecast does give increased skill.

Spatial variations in both the practical and perfect model skills of an ensemble‐mean anomaly field are found to be related to corresponding variations in the statistical significance of the anomaly field. For example, the average perfect model skill, in regions where the ensemble‐mean anomaly is significantly different from zero, exceeds the full field skill in all experiments for forecast days 1–15, and in all but two cases for days 16–30.
Detailed description of deformation‐induced semi‐geostrophic frontogenesis
Authors: Davies H. C. , Muller J. C.
Published: February 2007

A variant of the classical deformation‐induced surface frontogenesis problem is studied in the semigeostrophic limit. The particular flow system examined is the response of a baroclinic, semi‐infinite, stratified, uniform potential vorticity atmosphere to an imposed deformation flow field.

This flow system permits the derivation of: a class of simple analytical solutions in geostrophic space; an explicit functional solution of the Sawyer‐Eliassen ageostrophic circulation equation; and a self‐contained nonlinear equation in real space for the surface development of the along‐front wind component.

The derived relationships are utilized to examine various frontogenetic aspects of this model system. These include the time scale of frontal development, the co‐occurrence of different frontal types, the slope and movement of frontal features, the structure and strength of the ageostrophic circulation, and the degree of validity of the semi‐geostrophic approximation.
Measurements of the size dependence of cloud droplet deposition at a hill site
Authors: Gallagher M. W. , Choularton T. W. , Morse A. P. , Fowler D.
Published: February 2007

Measurements have been made of the deposition of cloud droplets and dissolved chemical species to moorland vegetation. Measurements of liquid water fluxes have been made by the gradient technique using Knollenberg FSS probes and by a weighing lysimeter. Deposition velocities for cloud liquid water in the range 0.1 to 0.24 g m −3 at a height of 1.0m were close to those for momentum. The FSS probes enabled the determination of deposition velocities as a function of droplet size. It was found that the deposition velocity increased with droplet radius from 2.5 μm to 6.5 μm radius. Between 6.5 and 12.5 μm radius, deposition velocities exceeded those for momentum, but decreased sharply between 12.5 and 15.5 μm.

In periods of thin cloud (〈0.1 g m −3〉), evaporation from the surface occurred despite continued deposition.
Numerical models of the raingauge exposure problem, field experiments and an improved collector design
Authors: Folland C. K.
Published: February 2007

A new design of conventional raingauge is proposed that minimizes the most important source of systematic error in point rainfall measurements: that due to the interaction of the wind with the gauge (the ‘raingauge exposure’ problem). Basic physical principles, numerical simulations and extensive field trials in England and Denmark are also used to provide a consistent quantitative description of the wind‐induced errors of conventional cylindrical gauges.

It is shown that an inverted conical collector with a large semi‐vertical angle may provide the basis of an adequate solution of the raingauge exposure problem. The need to minimize outsplash demands that the collector should have a fairly large diameter and take the form of a flat champagne glass. A small diameter flat champagne glass collector has been tested for over 31/2 years against a standard 5′’ cylindrical gauge, a 5′ pit gauge and an inverted conical gauge at the Institute of Hydrology, Wallingford. The insensitivity of the losses of the ‘flat champagne glass’‐shaped collector to wind speed at low rates of rainfall is confirmed, as well as the need for a larger collector diameter to minimize outsplash. A combination of a ‘steep champagne glass’ gauge and a ‘flat champagne glass’ gauge might lead to even better estimates of rainfall in high winds, notably on ships, and could provide a low‐cost approach to improving snowfall measurements on land.
The annual cycle of oceanic heat storage and oceanic meridional heat transport
Authors: Hsiung Jane , Newell Reginald E. , Houghtby Thomas
Published: February 2007

The global climatology of monthly heat storage and heat transport are presented using data from the Master Oceanographic Observations Data Set for the period 1949–1980. Heat storage and the rate of heat storage change are calculated for the Pacific, Atlantic and Indian Oceans. Heat transports on a monthly basis are calculated using surface energy flux data from a previous study together with the computed heat storage change. There is a large annual cycle in heat storage change at almost all latitudes with the largest amplitude in latitudes where the western boundary currents dominate. There is also a considerable annual cycle in the heat transport estimates. The heat transport in the Atlantic is northward most of the year except in January when there is significant southward transport. The largest northward transport is found at 5°N during March with a magnitude of about 2.0PW (1 PW =1015W). In the Pacific Ocean, northward transport dominates north of 20°N with a maximum of about 1.5PW at 25°N in July‐September. South of 20°N, there is significant southward transport in July through September with a maximum of 1.5PW at 10°S in August. Overall, the largest northward transport in the Pacific is about 2.1 PW in March at 10°N. Heat transport in the Indian Ocean is mostly southward with a maximum of about 2.5 PW in May at 20°S except for a small northward transport in January‐February north of 5°S. Although the uncertainties involved with the transport estimates are large, the transport values presented here compare rather well with direct estimates and with model results.