bluuebunny/crossref_metadata_2025 huggingface dataset:
Four‐dimensional variational data assimilation using the adjoint of a multilevel primitive‐equation model
Authors: Thepaut Jean‐Noéul , Courtier Philippe
Published: February 2007

The aim of the paper is to demonstrate the numerical feasibility of 4‐D variational assimilation using a multilevel primitive‐equation model. The experiments consist in minimizing the distance between the model solution and the observations. The gradient of the cost function thus defined is computed by integrating the adjoint of the model.

Here, assimilations are performed using model‐generated observations. In a preliminary set of experiments, assimilations were performed assuming that observations consisting of a full‐model‐state vector are available only at the end of the assimilation period. The numerical convergence of the method is proved and the results are meteorologically realistic. The use of the Machenhauer nonlinear normal mode initialization scheme and its adjoint turned out to have hastened the convergence and to have controlled to some extent the amount of gravity waves appearing in the solution. We identify a loss of conditioning of the minimization problem with an increase in the length of the assimilation period. The presence of horizontal diffusion in the model has the effect of degrading the convergence.

The second set of experiments evaluates the impact of observations distributed over the whole assimilation period. Through different senarios of sets of observations, we demonstrate the efficiency of the 4‐D variational approach in extracting the information contained in the dynamics of the model, together with the information contained in the observations. In particular, observing only the small scales of the flow leads to a good reconstruction of both small scales and large scales. Observations of the mass‐field evolution lead to a good reconstruction of the vorticity field in mid latitudes but less so in the tropics.

Increased resolution in the model in the experiments was found to have a negative impact on the speed of convergence of the minimization algorithm.
Three‐Dimensional Mesoscale Circulation Within A Convective Post‐Frontal System. Possible Role of Conditional Symmetric Instability For Triggering Convective Motion
Authors: Lemaitre Y. , Scialom G.
Published: February 2007

The role of the Conditional Symmetric Instability (CSI) in the formation of convective systems in the cold airmass located at the rear of a secondary cold‐frontal system is evaluated. This study is based on observations made on 4 June 1984 during the French experiment LANDES‐FRONTS 84. In particular, the mesoscale three‐dimensional circulation deduced from Doppler‐radar data within one of these convective systems is studied in detail and compared with that predicted by the CSI theory. In spite of the fact that, on the one hand, notable convective‐scale motion masks the mesoscale circulation, and that, on the other hand, the synoptic‐scale circulation is poorly described, this comparison shows that, among the various theories involved at the mesoscale, only CSI gives a proper account of the observed mesoscale circulation.
Rossby Waves and El NiñO During 1935‐46
Authors: Bigg Grant R. , Inoue Masamichi
Published: February 2007

The late 1930s and early 1940s saw strong climatic anomalies in the tropical Pacific basin. In this paper we combine an ocean‐modelling aproach to the investigation of the Pacific climate of this period with a multi‐parameter data analysis. It is shown that a large and deep pool of warm water in the western Pacific in the late 1930s acted as an energy source for the strong and prolonged El Niño event of the early 1940s and its rapidly following successor. Remote off‐equatorial Rossby‐wave forcing plays a prominent role in the generation of these events.
Observation of High Ice Particle Concentrations In Convective Cells and Cloud Glaciation Evolution
Authors: Gayet J.‐F. , Soulage R. G.
Published: February 2007

During an investigation into convective cloud at a single temperature level near −6°C, observations by aircraft have found ice crystal concentrations significantly in excess (up to 600 L−1) of those to be expected from primary ice formation (i.e. ice multiplication). Despite the fact that the main conditions for the operation of the Hallet‐Mossop secondary ice multiplication process were fulfilled, the corresponding rate of ice production, as calculated, appeared to be lower than was observed in the cloud. This suggests that other ice‐enhancement processes might be at work during the ice‐forming stage.

A small‐scale (100m) analysis of the data revealed that ice‐multiplication processes operated only in the fringes of the updraught regions of growing cells. It was also observed that a sudden occurrence of the ice usually coincided with the decay of the updraught. From these observations and from an evaluation of the amount of heat released by riming and vapour deposition processes, we suggest that the cell cores are auto‐seeded by the mixing or recycling of numerous ice crystals and that the rapid glaciation tends to be ineffective in increasing the buoyancy of the cell.
Gravity‐Wave Observations Using an Array of Microbarographs In the Alearic Islands
Authors: Monserrat S. , Thorpe A. J.
Published: February 2007

Time series of surface pressure from an array of microbarographs in the Balearic Islands (western Mediterranean) are used to obtain wavelength, phase speed and direction of propagation of gravity waves on the Balearic Islands. These pressure waves are commonly found during the summer and they force the large seal‐level oscillations observed in the inlet of Ciutadella; a phenomenon locally known as ‘rissaga’. Four events are described here, each of which has a duration of between 30 and 48 hours. Radiosonde data from Palma de Mallorca are used to relate the waves to the vertical structure of the atmosphere. the various source mechanisms of these waves and also the possibility of the existence of a duct layer trapping the wave energy near the surface are discussed. the waves are found to have no significant variation of phase speed with frequency and so they are basically non‐dispersive. This is an indicator pointing either to a local generation of gravity waves by wind‐shear instability or to the existence of neutral trapped waves, in the lower troposphere.
Comments On ‘Observations of Two Regions of Charge During Initial Thunderstorm Electrification’ By J. E. Dye, J. J. Jones, A. J. Weinheimer and W. P. Winn (July 1988, 114, 1271‐1290)
Authors: Moore C. B. , Vonnegut B.
Published: February 2007

Dye et al., in a 1984 thunderstorm study, identified regions containing electric charge in two growing clouds from measurements of the atmospheric electric fields taken, during penetrations of these clouds, by an instrumented aeroplane. There are, however, problems concerning the locations they assigned to these charges relative to the radar echoes from the clouds. In one case the authors identified the pressure altitude reported by the aeroplane pilot with the altitude of the radar echo top; this introduced an error of about 450m. In another case, the location given for the charged region relative to the cloud echoes is not compatible with the pilot's report of an updraught. While the authors measurements add to our knowledge of cloud electrification, some of their interpretation is questionable, and other explanations for the charged regions are possible.
Vibrational Temperatures and Radiative Cooling of the Co2 15 Mum Bands In the Middle Atmosphere
Authors: LÓPez‐Puertas M. , LÓPez‐Valverde M. A. , Taylor F. W.
Published: February 2007

Altitude and latitude distributions of the populations of the bending and symmetric stretching levels of CO2 and of the cooling rates induced by the bands originating from them in the middle atmosphere (40‐120km), have been calculated for the CIRA 1986 reference atmosphere using a non‐LTE radiative transfer model. A sensitivity study of the more uncertain parameters that strongly affect these quantities has been undertaken. These factors are the carbon dioxide and atomic oxygen volume mixing ratios, and the rate coefficient for the deactivation of the bending mode of CO2 by atomic oxygen. the model makes use of the most recent values of these parameters: the abundances of CO2 in the upper mesosphere and lower thermosphere as measured by, and derived from, rocket and satellite instruments; the global distribution of atomic oxygen density calculated by Rees and Fuller‐Rowell (1988) and the measurements of Thomas (1990); and the fast rate for the deactivation of CO2(0, v2, 0) by O(3P), recently derived by Sharma and Wintersteiner (1990). Among the most important results are the following: (i) the CO2 15 μm fundamental band is close to LTE (Tv‐ Tk ≤ 6K) up to about 100km altitude; (ii) the first and second hot bands exhibit high vibrational temperatures, especially in the daytime summer mesopause region; (iii) the cooling rates by the CO2 15μm bands in the lower thermosphere can be very large, depending on the CO2 abundance. Finally, our results have been compared with other recent non‐LTE models in order to quantify the effects of different radiative transfer schemes on the calculations of vibrational temperatures and cooling rates.
Point Vortices In A Low‐Order Model of Barotropic Flow On the Sphere
Authors: Egger Joseph
Published: February 2007

A low‐order model of global barotropic flow with intrinsic long‐term variability is presented. the model is based on the barotropic vorticity equation with Newtonian forcing of the zonal flow and realistic orography. Subgrid eddy motion is represented by point vortices which move within the low‐order flow of the model and affect it through their own circulation. the vortices are inserted at locations of maximum flow speed and are removed after a prescribed lifetime has elapsed. In the absence of Newtonian forcing, mid‐latitude westerlies and strong polar easterlies are induced if there are only cyclonic vortices. Anticyclonic vortices create equatorial easterlies.

If ‘realistic’ Newtonian forcing is added, two stationary orographic troughs dominate the model's mean flow in the northern hemisphere. Cyclones are generated downstream of the troughs, and pronounced storm tracks extend eastward from these genesis areas. the model atmosphere exhibits slow variability of a realistic order of magnitude with maxima over the ‘oceans’. This long‐term variability is due to the interaction of long‐lived circulation patterns, like blocks, with the point vortices. Moreover, cyclone generation is important in inducing slow motions. In the southern hemisphere there is little variation of flow statistics with longitude. All in all the low‐order model is surprisingly successful in simulating the observed patterns of atmospheric transience. This suggests that a low‐order model with point vortices might be a useful tool in climate research.
The Role of Diffusive Effects On Potential Vorticity In Fronts
Authors: Cooper Iain M. , Thorpe Alan J. , Bishop Craig H.
Published: February 2007

Theoretical studies of two‐dimensional fronts have shown that frictional processes at the time of occlusion and/or diabatic effects imply low‐level, positive potential vorticity (PV) anomalies. It is shown here that significant amounts of PV can be produced in an evolving baroclinic wave (taken to be the two‐dimensional semigeostrophic Eady wave) purely owing to the presence of a no‐slip, insulating lower boundary. A simple analytic expression is given for the time evolution of the mass‐weighted volume‐average PV, (PV), up to the point of frontal collapse. For the most unstable linear normal mode it is found, at the time of the formation of the frontal discontinuity, that the PV has increased by approximately a third of its basic‐state value.

The bulk formulation for surface fluxes of heat and momentum is used to illustrate the general role of boundary‐layer processes in modifying PV locally. Turbulent fluxes which decrease linearly from maximum values at the surface to zero at the top of the planetary boundary layer (PBL) are used to recover a simple analytical form for the PV generation tendency. the magnitudes of the PV anomalies produced by flows typical of mid‐latitude cyclones and associated fronts are highlighted by vertically integrating the tendency equation over the PBL. It is shown that the generation of PV in the PBL depends on the vertical differences of velocity and potential temperature over the depth of the PBL; and on the vertical vorticity and horizontal potential‐temperature gradient at the top of the PBL. For example, there is a local source of PV if there is cooler air at the top of the boundary layer to the right of the surface wind vector, i.e. if the thermal wind has a component opposing the wind at anemometer level. the production of PV anomalies by surface processes, using this formulation, is independent of the occlusion process.

Purely horizontal mixing of heat in the vicinity of a potential‐temperature anomaly is examined to predict the magnitude of local PV anomalies, consistent with the internal ‘eddy’ viscosity often used in numerical models.

Numerical integrations of a two‐dimensional Eady wave with a detailed surface‐layer description and stability‐dependent eddy viscosity coefficients also illustrate that significant growth in the PV occurs before the warm sector is lifted off the surface. There is a direct correspondence between this growth and the decrease of the horizontally‐averaged surface potential temperature, as predicted from the no‐slip, insulating boundary theory. Also, the local PV anomalies predicted by the bulk PBL representation are shown to be in close agreement with those produced in the numerical simulation, which uses a more comprehensive scheme.
Four‐Dimensional Assimilation In the Presence of Baroclinic Instability
Authors: Rabier Florence , Courtier Philippe
Published: February 2007

Current operational assimilation methods have revealed deficiencies in cases of strong baroclinic development. Baroclinic conditions are therefore appropriate for evaluating the potential for improvement which could be achieved through the implementation of a fully four‐dimensional data assimilation. In this paper the behaviour of a variational scheme is investigated for a typical baroclinic instability problem, where a wave develops and retroacts on to the basic zonal flow. the tangent‐linear model is shown to lead to a good approximation of the time evolution of the wave over a range of 48 hours, even at the most intense cyclogenesis period. For the assimilation experiments the twin experiment approach is applied. Only part of the flow evolution is observed, either the zonal component or the eddy component. In either case the method proves successful in reconstructing the unobserved part of the flow, taking advantage of the nonlinear coupling that exists between those components. However, nonlinearities can lead to difficulties when the range of validity of the tangent‐linear model is exceeded or when the cost function exhibits multiple minima.
Four‐Dimensional Data Assimilation: Comparison of Variational and Sequential Algorithms
Authors: Rabier F. , Courtier P. , Ehrendorfer Martin
Published: February 2007

The problem of data assimilation that is concerned with the complete and accurate specification of the atmospheric state based upon observations and other types of information can be approached either by variational or sequential algorithms. While variational techniques proceed by the global fitting of an assimilating model to the available information, sequential assimilation involves a statistical minimum mean‐square‐error estimation approach.

In this paper both algorithms are compared in a systematic manner with regard to assimilation/forecast accuracy, computational efficiency, and storage requirements based on a limited series of observing‐systems simulation experiments. the barotropic vorticity equation on a rotating sphere is used as the assimilating model.

The results indicate that under a variety of conditions the variational algorithm performs at least as well as the sequential algorithm. the variational algorithm is also found to be more successful than the sequential algorithm in the reconstruction of the physical fields in data‐void regions. Limitations and possible extensions, as well as operational implications of this work, are briefly discussed.
Measurements of Dry Deposition of No2 to A Dutch Heathland Using the Eddy‐Correlation Technique
Authors: Coe H. , Gallagher M. W.
Published: February 2007

Nitrogen dioxide (NO2) fluxes were determined over a dry heather moor by the eddy‐correlation technique using a sonic anemometer and two Scintrex LMA‐3 chemiluminescent analysers. Using power spectral analysis virtually all the fluctuations in NO2 concentration were of frequencies less than 1 Hz at the measuring height of 5.32m. Even with the non‐ideal response of the LMA‐3 little information about NO2 concentration was lost. Co‐spectral analysis showed there was no loss of flux at high frequencies arising from the slow response of the LMA‐3.

NO2 was deposited at a constant rate of 0.02 μg m‐2 ‐1 for nearly all of the measuring period. the deposition velocity maintained a mean value of 1 mm s‐1 throughout with minimum values nearing zero and maximum values of 3.5 mms‐1. There was no diurnal variation in canopy resistances to NO2. Mean values were 548 s m‐1 and 400 sm‐1 with and without the inclusion of several brief periods when the resistance increased markedly. Except for a brief period on 1 May the deposition was to the leaf cuticle as temperature and water stress caused the stomata to be closed. During the afternoon of 1 May there was some stomatal activity when the canopy resistance to water vapour fell and NO2 flux increased.

Finally a simulation of the cumulative sampling method was performed on the data set as described in Businger and Oncley (1990). They calculated a constant of proportionality, b, and showed it to have a value of 0.6 when considering heat and water‐vapour fluxes. A value of b was calculated for NO2 flux using the data set presented here and was found to be 0.58.
A Quasi‐Analytic Model of Droplet and Ccn Lifetimes, Optical and Chemical Characteristics of Ice‐Free Cumulus Clouds
Authors: Baker Marcia B. , Latham John
Published: February 2007

We have formulated a semi‐analytic model of the microphysical and passive chemical development within entraining ice‐free cumulus clouds growing in surroundings characteristic of Hawaii and Montana in summertime. Calculations were made of liquid‐water contents, age distributions of droplets and cloud condensation nuclei (CCN), optical parameters (effective radius and optical depth), the concentration of a passive chemical tracer in both cloud and rainwater; and the variability of the concentration of cloud‐water tracer over the droplet spectrum, all as functions of altitude within the clouds, entrainment scale, droplet number concentration and altitudinal variability in environmental tracer concentration.

All of these microphysical and chemical parameters were found to be sensitive to the entrainment scale II. Droplets at any level above cloud base may have been formed on primary nuclei or entrained nuclei, either of which may already have activated droplets which subsequently evaporated as a consequence of entrainment. the average nuclear ages are greater (typically by about 50%) than that of the average droplets, the latter being in conformity with those inferred from observations. the effective radius at a given level was only mildly dependent on II, whereas the optical depth increased rapidly as II increased. the concentration of tracer in both cloud water and rainwater decreased markedly as the entrainment scale increased.
Comparisons of Multi‐Year Statistics of Selected Variables From the Coads and the Nimbus‐7 and Ecmwf Data Sets
Authors: Weare Bryan C.
Published: February 2007

Coarse spatial resolution monthly statistics of selected variables of the Consolidated Ocean‐Atmosphere Data Set (COADS) have been compared with Nimbus‐7 satellite analyses and the European Centre for Medium‐range Weather Forecasts (ECMWF) operational analyses. Comparisons are made for the important cloud/climate variables of total cloudiness, surface air temperature and surface relative humidity. to ease these comparisons the COADS variances have been adjusted to account for small numbers of observations, and the COADS temperature and humidity data have been transformed to the 1000 mb level of the ECMWF analyses.

The results show that the Nimbus‐7 cloud fractions tend to underestimate total cloudiness for much of the earth, especially in regions of substantial low cloud cover. After account has been made for the different observation heights, the COADS and the ECMWF annual mean temperature statistics are found to be in generally good agreement. However, the annual mean ECMWF relative humidities are significantly lower over large portions of the oceans.

The interrelations between the Nimbus‐7 and the COADS total cloudiness and the ECMWF and the COADS lower‐level temperature and relative humidity are compared. Significant regional correlation patterns are evident in all cloud‐surface data pairs. On the other hand, some patterns of correlation using the Nimbus‐7 and the COADS cloud data are quite different, with those of the COADS generally appearing to be the more realistic.
Assessing the Sensitivity of A Land‐Surface Scheme to Parameters Used In Tropical‐Deforestation Experiments
Authors: Henderson‐Sellers A.
Published: February 2007

A series of factorial experiments is used to elucidate the factors and multifactor interactions important for the performance of one complex land‐surface scheme in a tropical environment. the BATS (Biosphere‐Atmosphere Transfer Scheme) is found to be sensitive to atmospheric factors which are currently poorly predicted by global climate models (GCMs) and, more importantly, to combinations of factors rendering tuning for good performance when coupled into today's global models (a worthless practice if the tropical area is subjected to different conditions such as those induced by deforestation). Similarly, two‐factor interactions, between coded ecological parameters (here roughness length and short wave, <0.7 μm, vegetation albedo), mean even if the BATS characterization and its driving GCM perform well in today's tropical‐forest environment, altering either or both parameters, as will occur in a deforestation simulation, must greatly diminish confidence in representations of future climate.
The Investigation of Regional Climate Anomalies With A Linear Barotropic Model and an Adjoint Technique
Authors: Robertson Andrew W.
Published: February 2007

The adjoint of a steady‐state linear barotropic model is presented as a tool for diagnosing regional upper‐level vorticity anomalies occurring in general circulation model (GCM) experiments. As a starting point, a simulation of a chosen GCM vorticity anomaly is made using a standard linear barotropic model forced by the corresponding anomalies in the GCM's upper‐level divergence and transient‐forcing fields. the associated adjoint model is then used to interpret this linear simulation over a chosen geographical region, in terms of the linear model's global forcing field. to do this, the adjoint model is forced by a local information source situated over the chosen region. the resulting adjoint solution consists of a global spatial weighting function which can then be used to identify regions of the forcing—local or remote—associated with the regional anomaly. the validity of barotropic linear theory can also be quantified as a by‐product.

As an illustration, the model is applied to a large interdecadal anomaly over Europe, arising in a 100‐year doubled CO2 integration of the Hamburg coupled ocean‐atmosphere GCM. Linear barotropic theory is found to account for over 60% of the European interdecadal anomaly amplitude in terms of anomalous divergence and the effects of transients. Although a large part of the effects of anomalous divergence are spatially concurrent with the European anomaly and just upstream of it, remote forcing regions over the east Pacific and tropical Atlantic are also implicated. Anomalous transients are found to play a secondary role. the linear results are compared with some one‐point correlation maps of the GCM's interdecadal variability.
The sea breeze and gravity‐current frontogenesis
Authors: Reible D. D. , Simpson J. E. , Linden P. F.
Published: February 2007

The development of a sea‐breeze circulation into a sea‐breeze front in often observed late in the afternoon. Measurements of horizontal temperature and humidity profiles in a number of sea‐breeze circulation, made using an instrumented light aircraft, are presented. On each of the four days for which data are presented an initial weak horizontal temperature gradient, extending over a distance of 5 to 10 kilometres, developed later in the into a sharp front only a few hundred metres across. The data are discussed in terms of simple gravity‐current frontogenesis models applied to the development of sharp sea‐breeze front from an initial diffuse transition between the air over the land sea. Examination of the equations for frontogenesis, and the result of some previous laboratory experiments on gravity‐current frontogenesis, lead to the proposal that, given sufficient contrast between land and sea temperatures, the formation of a sea‐breeze front depends on the balance between convergent horizontal winds which act to generate a front, and turbulent convective mixing over the land which tends to prevent its formation. the differences in driving force, wind convergence and turbulent intensity are discussed for each day on which data are available and are shown to be consistent with the theory proposed.
Entropy and climate. I: ERBE observations of the entropy production of the earth
Authors: Stephens G. L. , O'Brien D. M.
Published: February 2007

This paper introduces an approximate method for estimating the global distributions of the entropy fluxed flowing through the upper boundary of the climate systems, and provides an estimate of the entropy exchange between the earth and space and the entropy production of the planet. The entropy fluxes are those associated with the exchange of radiation between the earth and space and a specific method is described to derive these entropy fluxes from available measurements of the earth's radiation budget. The distribution of these entropy fluxes and the entropy production derived from these fluxes differ in significant ways from those reported previously. The major source of these differences arises from the incorrect estimate of entropy exchange associated with reflected solar radiation in these earlier studies. This problem is overcome by the application of a simple physical model for the entropy exchange. Radiative‐transfer simulations of the entropy exchange demonstrate that this model reasonably approximates the entropy exchange associated with reflected solar radiation, and that the long‐wave fluxes of entropy are closely approximated by the equivalent black‐body entropy flux. Entropy fluxes calculated from the Earth Radiation Budget Experiment measurements show how the long‐wave entropy flux densities dominate the total entropy fluxes at all latitudes compared with the entropy flux densities associated with reflected sunlight, although the short‐wave flux densities are important in the context of clear sky‐cloudy sky net entropy flux differences. It appears from the analysis presented in this paper that the entropy production of the planet is both constant for the 36 months of data considered and very near its maximum possible value. The mean value of this production is 0.68 × 1015 W K−1 and the amplitude of the annual cycle is approximately 1 to 2% of this value, which is within the uncertainty of our estimate. Climatic fluctuations of entropy production are shown to be less than about 2% of the total production and therefore cannot be detected using the method described in this paper. The production of global entropy is dominated entirely by radiative processes. It remains a considerable challenge to detect these climatic fluctuations above such a large background of entropy production.
A model of the development of droplet effective radius in convective cloud
Authors: Choularton T. W. , Bower K. N.
Published: February 2007

A model is presented of a growing cumulus turret which is used to investigate the sensitivity of the droplet effective radius in a convective cloud to variations in the cloud‐base temperature, the liquid‐water content, the vertical wind speed at cloud base, the cloud condensation nucleus (CCN) spectrum entering the cloud base, and dry‐air entrainment. It is found that the existence of large quantities of dry‐air entrainment has a major impact on the evolution of the cloud microphysics. Near to the cloud base the growth of newly nucleated droplets causes a rapid increase in the droplet effective radius with height. Higher in the cloud, dry‐air entrainment results in new nucleation of small droplets at all heights in the cloud and the complete evaporation of some of the pre‐existing droplets. The effect of this is that is that the horizontally averaged effective radius becomes a very weak function of height. The value of the effective radius is most strongly influenced by the CCN spectrum entering through the cloud base. This mean that differences between oceanic and continental clouds are reflected in different values for the effective radius. The effects of entrainment do, however, tend to reduce the sensitivity of the effective radius to the initial CCN spectrum. In addition, the very broad droplet‐size distribution produced by the effects of entrainment may be very important in the development of the ice phase in these clouds. The droplent effective radius produced by the entraining model is less sensitive to temperature than the effective radius predicted by adiabatic growth. The result. The result is also insensitive to differences between the CCN distribution entering the cloud base and the CNC population entrained from the environment within reasonable limits.
An analogue‐dynamical long‐range numerical weather prediction system incorporating historical evolution
Authors: Jianping Huang , Yuhong Yi , Shaowu Wang , Jifen Chou
Published: February 2007

It is convenient to regard a predicted dynamical field as a small disturbance superimposed on a historical analogue field. A Statistical technique can then be used in combination with a dynamical forecast. Using this approach, a coupled atmosphere/earth‐surface analogue‐dynamical model has been formulated and applied to seasonal prediction. This approach facilitates the use of information contained both in the historical data‐set and in the initial field in improving the dynamical model (based solely on the latter) and shows some predictive skill.
Comments on ‘The density of natural ice accretions related to non‐dimensional icing parameters’ by K. F. Jones (January B 1990, 116, 477‐496)
Authors: Levi L. , Prodi F.
Published: February 2007

The results obtained by applying the various equations relating the accreted ice density, p, to the growth parameters are compared with each other and with direct experimental results. Special attention is paid to the equation recently proposed by Jones, in which, taking into account an extensive series of results derived from Mt Washington time experiments, an attempt was made to relate p to several thermal and dynamcial non‐dimensional parameters, πi. It is shown that this equation can lead to large errors, especially at low densities, it the parameter ranges are not those that prevailed in the Mr Washington experiments. This disparity arises from basic defects in the physical arguments used by Jones to establish the dependence of p on related phenomena.

Nevertheless, the attempt at relating p explicitly to the main system parameters through the dimensionless products πi is considered to be of interest as a possible basis for new studies directed at obtaining a more general equation for p(πi).
Reply to comments by L. Levi and F. Prodi: Another look at the dependence of ice accretion density on non‐dimensional parameters
Authors: Jones Kathleen F.
Published: February 2007

In response to the comments of Levi and Prodi I have made a comparison between their rime density data and a similar subset of my data. The choice of air temperature rather than accretion temperature to characterize droplet freezing is further explained, and the convective‐heat‐flux π‐product is modified by using the droplet diameter rather than the cylinder diameter as the length scale. A limitation to the regression analysis associated with the form of the function relating ice density to non‐dimensional parameters associated with ice accretion is discussed.
Entrainment in cumulus clouds. I: Thermodynamics and buoyancy
Authors: Telford James W. , Chai Steven K.
Published: February 2007

Conclusions drawn from thermodynamic composition diagrams are examined in terms of their applicability in real conditions. Displays show the temperature and wet‐bulb temperature of samples calculated at 1000 mb. Additional lines show total mixing ratio and saturation pressure. A point giving the composition where there is a minimum of liquid water in cloud can be taken as a saturation pressure point. Such a point can be selected with a PC ‘mouse’ on the monitor display of the thermodynamic diagram, whereupon the program draws the associated constant virtual‐temperature (density and buoyancy) or the constant temperature line.

Buoyancy can then be used to eliminate some sources of outside air as possible components for in‐cloud mixtures. Similarly, constant temperature lines associated with constant‐altitude‐aircraft cloud measurements provide an estimate of the composition which would remain after precipitation removed the condensed water and ice.

Examination of Cooperative Convective Precipitation Experiment data show that many cloud compositions consist of cloud‐base air mixed with air remaining from previous clouds. Sometimes this air is from clouds which have precipitated, even though the sampled cloud is not precipitating. Similarly, balloon soundings in clear air may show parcels containing most patches from previous clouds. Such soundings, not representing air near the cloud under study, cannot be used to give unambiguous information about the level of mixing. Studies based on balloon soundings often suggest that the entrained air originates from above the level where the cloud is sampled, but several claim otherwise. The moist air may be important in radiation studies.
Cloud processing of the cloud condensation nucleus spectrum and its climatological consequences
Authors: Bower K. N. , Choularton T. W.
Published: February 2007

A modeling study of the effects of aqueous phase cloud chemistry in changing the activated cloud condensation nucleus (CCN) spectrum is presented. The chemistry model is contained within a model of the development of a hill cap cloud. The aqueous phase oxidation of sulphur (IV) to sulphur (VI) by the oxidants ozone and hydrogen peroxide is considered for each of the explicit cloud‐droplet categories. All gases enter cloud droplets at a finite rate which is calculated in the model.

Significant modification of the CCN spectrum emerging downwind of the processing cloud is observed. This leads to the formation of strongly bimodal aerosol size distributions. The degree of modification is strongly correlated to the concentration of sulphur dioxide, and in oxidant limited situations tot eh concentration of hydrogen peroxide.

The cloud‐droplet chemistry is seen to have the largest effect on the smallest activated CCNs. This enables such particles to activate much more readily after cloud processing at critical supersaturations up to 20 times lower than originally required. A simple cloud model is used to investigate the sensitivity of this secondary activation.

With updraughts typical of a stratocumulus‐type cloud, droplet effective radii are seen to be lowered by up to 3 μ, 500 m above cloud base. This result is insensitive to the concentration of sulphur dioxide present in the processing cloud down to very low concentrations. If reproduced on a global scale this result cloud have important climatological consequences, as recently indicated by Slingo (1990). However, the reduction in effective radius is expected to be important only on local scales close to the sources of new aerosol, where the processing of the CCN spectrum by clouds may be occurring for the first time. Additional cloud processing will lead to changes in the direct radiative properties of the aerosols but will have little effect upon the radiative properties of clouds subsequently forming on them.
Nonlinear evolution of ordinary frontal waves induced by low‐level potential vorticity anomalies
Authors: Malardel S. , Joly A. , Courbet F. , Courtier P. H.
Published: February 2007

Linear, semi‐geostrophic (SG) theory reveals the instability of steady fronts with low‐level potential vorticity anomalies. Joy and Thorpe (1990) have shown in this context the most unstable normal modes to have sub‐synoptic wavelenghts. The present study uses a primitive equation (PE) model to construct, at these wavelenghts and along the same fronts, the PE normal modes and extends the evolution tot he nonliner regime. IT is shown that PE normal modes have a structure similar to the original SG modes at the same given wavenumber.

In the nonlinear experiments, two different kinds of behaviour are found, depending on the initial wavelenght of the perturbation, the frontal baroclinicity and the width of the potential vorticity anomaly. The first kind, and main finding of this study, is characterized by the inability of a barotropically unstable mode (in the energy sense) to lead to large pressure falls in the vortex. Such a mode, with its wavelenght smaller than the Rossby radius, is successful in breaking the frontal flow but saturates within two days. The other occurs when the wavelengh is larger than the Rossby radius. Then, it is shown that the initially significant barotropic contribution to the growth vanishes and the wave enters a phase of classical baroclinic growth. It is only when this second phase occurs that the frontal change in structure is accompanied by significant deepening of the surface low. IT saturates in a way similar to larger‐scale baroclinic waves, by increasing the upper‐level jet and shear.
An observational study on the energetics of the onsent monsoon vortex, 1979
Authors: George L. , Mishra S. K.
Published: February 2007

Time variations, of the vertically integrated, and vertical distribution of time‐mean zonal and eddy components of the kinetic and available potential energy, and the associated energy conversions, are examined in order to understand the formation and growth mechanisms associated with the monsoon onset vortex during 14‐17 June 1979. For the computations, the area 0‐22.5°N and 48.75°E‐78.75°E and the vertical layer 1000‐300 hPa are considered. The dynamical characteristics of the large‐scale monsoon current surrounding the onset vortex are also investigated. Strengthening of the meridional and vertical shear of the mean zonal wind is noticed during the period. The vortex is formed to the north of the westerly jet, in the zone of maximum cyclonic shear. The northward shift of the large‐scale monsoon current is noticed in association with northward movement of the onset vortex.

It is identified from the mean energetics that both barotropic and baroclinic energy conversions, and also the boundary terms, maintain the vortex. Baroclinic energy transfer to eddy kinetic energy (KE) takes place not only in the upper levels but also in the lower levels. Further, the energy conversion from zonal kinetic energy (KZ) to KE dominates over the energy conversion from eddy available potential energy (AE) to KE. The barotropic energy conversion due to the down‐gradient eddy transport of meridional momentum is more than that due to zonal momentum transport. The transfer of the vertical wave‐energy flux, due to the eddy pressure interaction, away from the middle level also contributes significantly to the maintenance of KE in the upper and lower levels. The generation rate of the zonal and eddy available potential energy and the frictional dissipation of KE are computed b the residue method; the values are 0.54 W m‐2, 0.25 W m−2 and 0.65 W m−2, respectively.

The meridional and vertical transports of momentum and heat are examined in relation to the structure of the vortex. The requirement of minimum vertical resolution for the vortex modelling studies is indicated.
Multi‐year statistics of selected variable from the ISCCP C2 data set
Authors: Waare Bryan C.
Published: February 2007

Coarse spatial resolution monthly statistics of selected variables of the International Satellite Cloud Climatology Project (ISCCP) C2 data set are analysed. The total cloud amount statistics are compared with those from conventional surface observations available from the Consolidated Ocean‐Atmosphere Data Set (COADS). These statistics include annual means and intra‐annual variances and correlations with the COADS sea surface temperature and surface relative humidity. In general the ISCCP C2 total cloud amount statistics are found to be in very good agreement with those of the COADS. In particular a number of correlation patterns are evident which are explainable in terms of known climatology, and which might be utilized as verification diagnostics for forecast and climate models.

Preliminary analyses of the ISCCP C2 cloud‐top pressure and cloud water content have also been carried out. The annual means and intra‐annual standard deviations of these parameters appears to be consistent with surface cloud‐type observations. Furthermore, the intra‐annual variations of cloud‐top pressure and cloud water all have broad regions with significant correlations with sea surface temperature.
Marine aerosol concentrations and estimated fluxes over the sea
Authors: Smith M. H. , Park P. M. , Consterdine I. E.
Published: February 2007

Aerosol particulate fluxes, generated at the sea surface by the action of the wind, have been derived from a comprehensive series of measurements taken in the Outer Hebrides, off the west coast of Scotland. These observations cover wind speeds from essentially zero to values in excess of 30 m s−1, and show that the particle production, over the radius range from 1 to 25 μm, is very strongly dependent upon the prevailing wind speed, especially for the larger particles. These fluxes, estimated for a height of about 10 m above the ocean surface, may be approximated by an expression comprising the sum of two log‐normal distributions (in particle radius R) of the form:

where the values of fI and R0i remain essentially unchanged with wind speed, but with the values of Ai being strongly dependent upon this parameter.

Alternative formulations of the aerosol flux, based upon combining field observations of the dependence of whitecap coverage upon prevailing wind speed with estimates of aerosol production from laboratory‐simulated whitecaps, provide poorer approximations to the aerosol observations, especially at the higher wind speeds when a substantial spume‐drop contribution may be expected.
Evolution of a mesoscale upper tropospheric vorticity maximum and comma cloud from a cloud‐free two‐dimensional potential vorticity anomaly
Authors: Browning K. A.
Published: February 2007

A diagnostic case study is presented in which output from the limited‐area forecast version of the Meteorological Office operational Unified Model was used together with water vapour and cloud imagery from the Meteosat satellite. The case chosen for study was an archetypally simple example in which a long, narrow strip of air with high potential vorticity (PV) in the upper troposphere—strictly a tropopause depression—became unstable and rolled up into fairly ling‐lived mesoscale vortices. For much of the time the vortices were cloud‐free and they were dynamically pure examples of very small upper‐air PV anomalies moving relative to the strata beneath. As such they provided a good test of theoretical deductions from the PV invertability principle and of the performance of the dynamical components of the forecast model.

The evolution of the mesoscale vortices as revealed in the satellite imagery was well handled by the Unified Model, indicating that the small scales were reproduced well by the model dynamics even though they were not fully, defined by the routine observations fed into the model. As the vortices began to form, the stretching deformation along the axis of the initial PV strip was very small, and the 900 km spacing of the vortices was broadly consistent with theoretical expectations for the wavelength of instabilities growing on such a PV strip. A detailed analysis of one of the mesoscale vortices throughout its 3‐day lifetime showed that its maximum potential vorticity and absolute vorticity decreased with time as it travelled within the circulation of large‐scale anticyclonic gyre. By the end of the second day, when cloud first formed, the vortex had become a weak upper tropospheric shear line, 500 km long, sandwiched between a pair of anticyclonic mesoscale vortices 900 km apart.

The Meteosat imagery revealed a characteristic two‐component structure both in the water‐vapour channel and, when the cloud eventually formed, in the infrared channel too. This structure, which resembled that seen on other occasions, is explained in terms of a particular combination of vertical motion and advection in the region of sharp humidity gradients alongside dry stratospheric air that had intruded into the upper troposphere.
Cumulus entrainment, fine‐scale mixing, and buoyancy reversal
Authors: Grabowski Wojciech W.
Published: February 2007

The formation of buoyancy reversal due to entrainment of dry environmental air, and its implication for cumulus dynamics, are discusses. Concepts originating from laboratory experiments with reacting turbulent flows, and from numerical simulations of homogeneous and isotropic turbulence, are applied to distinguish between large‐scale entraining eddies developing at the interface, subsequent development of smaller‐scale motions, and final homogenization by microscale processes. The physics of the microscale homogenization by molecular diffusion and sedimentation of cloud droplets is discussed. It is shown that, as a result of droplet sedimentation, much smaller negative buoyancy (buoyancy undershoots) may be generated on cloud microscale as compared with the value predicted by a classical nonlinear mixing diagram of cloudy and cloud‐free air.

Highly idealized numerical experiments aimed at simulating the temporal evolution of a small convective cloud were performed with and without the effects of molecular mixing that results in buoyancy reversal. It is argued that these experiments provide two limiting cases, and that the dynamics of a real cumulus cloud is located somewhere between them. The major effect of buoyancy reversal as suggested by these experiments is to increase the intermittency associated with cloud evolution. The temporal variation of cloud‐top height, maximum undraughts, and minimum downdraughts increases significantly when buoyancy reversal is allowed.

It is argued that results of numerical experiments, together with other laboratory and theoretical studies, cast serious doubts on the concept of cumulus entrainment being driven by the cloud‐top entrainment instability.