bluuebunny/crossref_metadata_2025 huggingface dataset:
Aircraft observations of the ekman layer during the joint air‐sea interaction experiment
Authors: Nicholls S.
Published: July 2010

Results are presented from an observational study of the mid‐latitude atmospheric boundary layer over the sea. the data were obtained mainly by instrumented aircraft as part of the Joint Air‐Sea Interaction Experiment (JASIN).

The eight occasions chosen for study are characterized by small surface buoyancy fluxes and the conditions are well described as near neutral and barotropic. In the absence of low‐level inversions, a well‐mixed Ekman layer is observed on each occasion but is limited to a depth of approximately 0.2u */f. Accurate measurements of horizontal gradients (including pressure) together with improved wind observations and turbulent flux measurements enabled each of the terms of the momentum balance to be evaluated throughout the depth of this layer. These terms were found to balance quite closely, were well described by Ekman scaling and were consistent with the requirements of the measured turbulent kinetic energy balance. the latter suggests that the main shear production terms are dissipated locally and that little is exported to upper levels in the Ekman layer to enable significant deepening by entrainment although only a slight increase in instability appears to be needed to alter this balance. Conditions in which such boundary layer structure might be observed are suggested. Values of various coefficients used in schemes for relating surface fluxes to mean quantities (CD, CE, Cg, A, B) are derived and compared with previous measurements.

Spectral analysis reveals that most of the turbulent transport is confined to a distinct high wavenumber region whose characteristics vary as a function of Ekman layer depth and stability parameters. This is superimposed on larger‐scale fluctuations which do not vary appreciably within the Ekman layer and which therefore dominate the variances in the upper regions as the intensity of the smaller‐scale turbulence decreases strongly with height.

Finally, further implications of this interpretation are discussed with particular reference to the heat and water vapour balance. These imply that removal of water vapour from the Ekman layer is accomplished by transfer related to cloud activity, marking a significant change in the mechanism of turbulent transport at the top of this layer. the relationship between these and similar results obtained concurrently by different methods is also discussed.
Mesoscale analysis of a polar trough interacting with a polar front
Authors: Browning K. A. , Hill F. F.
Published: July 2010

The interaction of a polar trough with a polar front can lead to a characteristic satellite cloud pattern commonly referred to as an instant occlusion. the detailed structure and development of an event rather resembling an instant occlusion is analysed in this paper using routine data supplemented by additional radiosondes, soundings and cloud imagery from NOAA‐7 and Meteosat, and radar network pictures. the event presented differs from an instant occlusion as described by Anderson and others in that a band of polar front cloud developed simultaneously with the band of cloud associated with the polar trough rather than being a pre‐existing feature. Thus we refer to it under the different name of pseudo‐occlusion in order to keep this distinction in mind. A conceptual model of the pseudo‐occlusion is developed in which the key elements are (i) a moist, relatively warm, low‐level jet associated with the polar trough, referred to as the ‘polar trough conveyor belt’, and (ii) a major upper‐level jet streak associated with the polar front, referred to as the ‘polar front conveyor belt’. the two conveyor belts intersect almost at right angles. Precipitation associated with the polar front is predominantly from stratiform cloud, whereas that associated with the polar trough is a mixture of stratiform and convective. Just on the poleward side of the upper jet the cloud top drops abruptly. Here, dry air having recently descended beneath the tropopause fold, suppresses the depth of the moist air associated with the polar trough conveyor belt. This leads to a minimum in the intensity of precipitation between the rain areas associated with the trough and the front. Although the air overrunning the trough just on the poleward side of the polar front is dry as a result of earlier descent, the air in this location is likely to have begun ascending as part of the indirect circulation in the exit region of the upper level jet streak. the polar trough low‐level jet probably forms part of this circulation.
An airborne study of the microphysical structure of cumulus clouds
Authors: Hill T. A. , Choularton T. W.
Published: July 2010

Data are presented from aircraft passes through cumulus clouds studied during a cooperative convective precipitation experiment on the plains of Montana in 1981. It is found that small concentrations of large drops are produced which exceed the size predicted by growth in a closed parcel model. These concentrations are higher in young, rapidly ascending turrets than in older clouds and coalescence‐size drops are found in turrets less than ten minutes old. It is concluded that dry air entrainment is responsible for the production of these large drops in ascending turrets.
On the detection of temperature changes induced by increasing atmospheric carbon dioxide
Authors: Parker D. E.
Published: July 2010

Radiosonde data for approximately the past 25 years have been used to search for the lower‐stratospheric cooling and lesser upper‐tropospheric warming expected, according to many modelling studies, as a result of the increasing concentration of carbon dioxide in the atmosphere. There are no clear indications that the hypothesized temperature changes have occurred. Settling of volcanic dust after the 1963 eruption of Agung, and inhomogeneity in the data resulting from changes in design of radiosondes, may have caused observed loẃer‐stratospheric temperatures to fall in the late 1960s at some stations. Even in the absence of instrumental shortcomings, a further 35 years of radiosonde data, or 20‐25 years of satellite stratospheric sounding data, are a prerequisite for detection of temperature trends of the size predicted by global circulation models for the present rate of increase of atmospheric carbon dioxide concentration.
A comparison of observed and forecast energetics over North America
Authors: Baker Wayman E. , Brin Yevgenia
Published: July 2010

The observed kinetic energy balance is calculated over North America and compared with that computed from forecast fields for the 13‐15 January 1979 cyclone. the FGGE upper‐air rawinsonde network serves as the observational database while the forecast energetics are derived from a numerical integration with the GLAS fourth‐order general circulation model initialized at 00 GMT 13 January.

Maps of the observed and predicted kinetic energy and eddy conversion are in good qualitative agreement, although the model eddy conversion tends to be 2 to 3 times stronger than the observed values. Both the forecast and observations exhibit the lower and upper tropospheric maxima in vertical profiles of kinetic energy generation and dissipation typically found in cyclonic disturbances. an interesting time lag is noted in the observational analysis with the maximum observed kinetic energy occurring 12 h later than the maximum eddy conversion over the same region.
On systematic errors in parametrized calculations of longwave radiation transfer
Authors: Morcrette Jean‐Jacques , Fouquart Yves
Published: July 2010

A detailed narrow band model of longwave radiation transfer in a clear‐sky atmosphere is used to study the impact on cooling rate profiles and radiative fluxes of some approximations commonly used in GCM‐type radiation codes.

Systematic errors in the results arise when economies are made in the vertical and spectral resolution. A coarse discretization of the vertical profiles of temperature and humidity overestimates atmospheric absorption (by as much as +5 W m−2), as does (by as much as +5 W m−2) the use of mean values of the temperature and humidity in each model layer. Overestimation of the absorption (up to +7 W m−2) also results from the use of the strong line approximation for H2O and CO2, or of the weak line approximation for O3.

Decreasing the order of the vertical quadrature used for the vertical integration tends to decrease the absorption. Effects of the water vapour continuum absorption outside the 800‐1250 cm−1 window region cannot be neglected, as this absorption accounts respectively for 21 and 8 W m−2 of the downward radiation at the surface for a tropical and a subarctic winter atmosphere, and decreases the corresponding outgoing radiation at the top of the atmosphere by 5 and 0.3 W m−2 respectively. Neglecting N2O and CH3 decreases absorption by about 5 W m−2. Restricting CO2 absorption to the 500‐800 cm−1 interval, and O3 absorption to the 970‐1110 cm−1 interval further decreases by 5 W m−2 the absorption in the tropical atmosphere.

For transmission functions described by statistical models, a widening of the spectral intervals reinforces the influence of the strong lines and leads to an overestimation of the absorption, which is partly compensated by a weaker temperature dependence.
Systematic errors of the ecmwf operational forecasting model in tropical regions
Authors: Heckley William A.
Published: July 2010

The performance of the ECMWF forecasting system in tropical prediction is discussed in terms of objective scores and systematic errors. the temperature, humidity and wind errors appear to be mutually consistent. the forecasts exhibit a cooling in the tropical stratosphere, warming at and below the tropical tropopause and a cooling in the mid and lower tropical troposphere. the analyses appear too humid in the lower troposphere, while the forecasts show a drying. Precipitation and evaporation are both too weak; a new energy balance within the model is slowly established through cooling, this new balance having a reduced total energy (Lq + cqT). the associated wind errors are seen to have a highly baroclinic structure and a meridional extent such as to affect significantly the baroclinicity of the mid‐latitude flow, particularly over the North and South Atlantic. Systematic errors are large compared to the variability of the fields themselves and show a remarkable temporal consistency from month to month.
An airborne study of vertical structure and microphysical variability within a small cumulus
Authors: Blyth A. M. , Latham J.
Published: July 2010

On 27 July 1981, as part of the CCOPE experiment, the University of Wyoming King Air research aeroplane made penetrations at six levels ranging from 590mb (‐4.9°C) to 770mb (7.2°C) through a nonprecipitating, unglaciated cumulus cloud whose top was at about 560 mb (‐7°C) and base about 780mb (8°C). the entire flight sequence—from the top downwards—took about six minutes. Principal measurements were of droplet size distribution N(d), number concentration N and liquid water content L, measured at 10 Hz (≃10m spatial resolution); drop‐count N′ (50Hz, ≃2m); temperature T. Also we calculated the fraction F of cloud base air in the mixture sampled (1Hz, ≃100m). the cloud was substantially subadiabatic throughout the volume studied. Paluch analyses, the observed air motions near cloud top, buoyancy profiles and the observed distribution of L and F with altitude demonstrated that the cloud was diluted principally by entrainment from cloud top. Considerable scatter was found in all microphysical and thermodynamic variables on all spatial scales. Large variations in L were primarily attributable to fluctuations in N and N′. Bimodal spectra were sometimes found at the interface between statistically smooth and highly variable regions. the breadth of the droplet spectra and the size of the largest droplets in the condensate spectrum were not systematically related to the degree of dilution. Calculations showed that a small fraction of the droplets (≃100 litres−1) exhibited superadiabatic growth of up to 4 m̈m diameter at the intermediate levels, but not near cloud base or cloud top. Support for the idea that droplets in lower concentrations (≃10 litre−1) grew even faster was provided by 1‐D probe measurements of droplets of up to 75 m̈m diameter in the central and upper regions of the cloud.
Evaporation from sparse crops‐an energy combination theory
Authors: Shuttleworth W. James , Wallace J. S.
Published: July 2010

A one‐dimensional model is adopted to describe the energy partition of sparse crops. Theoretical development of this model yields a combination equation which describes evaporation in terms of controlling resistances associated with the plants, and with the soil or water in which they are growing. the equation provides a simple but physically plausible description of the transition between bare substrate and a closed canopy. Although the aerodynamic transfer resistances for incomplete canopies have, as yet, no experimental justification, typical values, appropriate to a specimen agricultural crop and soil, are shown to have limited sensitivity in the model. Processes which require further study if the equation is to be used to calculate evaporation throughout a crop season are also discussed.
Steady convection in pressure coordinates
Authors: Moncrieff M. W.
Published: July 2010

Conservative properties of steady flow in non‐hydrostatic pressure coordinates are used to formulate theoretical models of deep convection. the pressure coordinate analysis extends the range of analytic solutions previously obtained in height coordinates, because the effect of the variation of density with height is included and is more useful for comparison with observations and the implementation of dynamical cloud models in convective parametrization schemes. Previous models are made more realistic by including a non‐constant thermodynamic lapse rate. In the case of steering‐level convection, this is shown to be a simple reinterpretation of the constant lapse rate model, while for propagating convection there are more fundamental implications; downstream propagating solutions cannot exist if the net work done by the pressure field is zero, while upstream propagation, although dynamically possible, is not observed.
Remarks on the predictability properties of two‐ and three‐dimensional flow
Authors: Vallis Geoffrey K.
Published: July 2010

This paper discusses the relationship between certain well‐posedness results for the equations of motion and simple phenomenological arguments pertaining to the predictability of the flow fields. the barotropic and quasi‐geostrophic equations have been shown to be well‐posed under certain conditions. From this, it has been inferred that Lorenz's conjecture of finite predictability time for any scale of initial error is false. Such an inference is justified in the error energy may be made small by confining the error to small scales of motion. It is shown that this may be achieved, given finite bounds on the vorticity. It is also shown that there is no contradiction between essentially Kolmogorovian phenomenological arguments and the well‐posedness of the equations, since the conditions uder wich a finite predictability time is predicted by the former are those then phenomenological trubulence arguments and rigorous existence proofs imply that the predictability time of a barotropic fluid may be made as long as we wish if the initial error scale can be made small enough. Both sets of arguments also imply that enstrophy dissipation tends to zero as viscosity tends to zero. For a three‐dimensional fluid phenomenology implies a finite predictability time, and non‐zero energy dissipation as viscosity tends to zero, implying the ill‐posendness of the three‐dimensional Euler equations.
Secondary ice particle production during rime growth: The effect of drop size distribution and rimer velocity
Authors: Mossop S. C.
Published: July 2010

The rate of production of secondary ice particles during the growth of rime in a supercooled cloud at about ‐5°C is known to depend mainly upon the rate of accretion of large drops (diameter ⩾25 m̈m). New experiments have defined more clearly how ‘splinter’ production is affected by the rate of accretion of small drops (diameter ⩽12m̈m).

The relationship between secondary ice production and rimer velocity is explored over a wider velocity range than before. At low velocities, splinter production persists to 0.55 ms−1 and probably lower. There is no evidence of a low‐velocity cut‐off as previously claimed. At high velocities, splinter production persists to velocities >5ms−1. This may have important implications for secondary ice production in hail clouds.
Observations of shade‐ring corrections for diffuse sky radiation measurements at the dead sea
Authors: Stanhill G.
Published: July 2010

The shade‐ring correction factor needed for measurements of diffuse irradiance at the Dead Sea averaged 1.41. Under normal near cloudless conditions, the anisotropy factor for sky radiance at the Dead Sea was in mid‐winter 14% greater than that calculated on the basis of clear sky radiance distribution measured in central England, rising to a 28% excess in mid‐summer. the relative strength of the circumsolar radiance calculated from the Dead Sea measurements was 1.54 compared with the value of 1.10 found to fit the data from nine other stations. the calculated angular width of the circumsolar zone at the Dead Sea, 0.63 radians, was, however, similar to that derived from data at other stations. Two possible reasons for the high degree of anisotropy are discussed. the correction factor was not influenced by the degree of cloud cover until over half the sky was clouded. On the very infrequent occasions of completely overcast skies the shade‐ring factor averaged 1.16, agreeing with the value calculated solely on the basis of the geometric view factor. the error in measurement of diffuse irradiance at the Dead Sea caused by uncertainties in the value of the shade‐ring correction was less than 5%, smaller than the uncertainty in the basic reference method of measurement.
A lagrangian analysis of scalar transfer in vegetation canopies
Authors: Raupach M. R.
Published: July 2010

To study the dispersion of scalar trace constituents such as heat, water vapour or CO2 from extensive sources within vegetation canopies, a Lagrangian approach is essential because of the need to account for the influence of travel time (as a fraction of the Lagrangian time scale) on the dispersion properties of marked fluid particles. A simple, analytic Lagrangian theory is developed for a canopy in which the turbulence is homogeneous. For a given scalar source distribution, predictions are obtained for mean scalar concentration, vertical flux, terms in the balance equation for vertical flux, and vertical diffusivities at first, second and third order. the predictions agree remarkably well with measurements of dispersion from an elevated plane source within a model plant canopy in a wind tunnel. the theory also explains the countergradient heat and CO2 fluxes observed in real forest canopies.
Evolution of potential vorticity in tropical cyclones
Authors: Wayne H. , Schubert  , Alworth Brian T.
Published: July 2010

We explore the simultaneous use of potential radius and potential temperature coordinates in the Eliassen balanced vortex model of tropical cyclones. This results in a simple flux form for the equation governing the inverse potential vorticity. the fluxes are accomplished by R and θ, which are given in terms of the angular momentum and heat sources. If the sources are specified functions of R and θ, analytic solutions for the time evolution of the potential vorticity distribution can be obtained. the solutions illustrate how latent heat release generates potential vorticity at low levels, destroys it at upper levels, and through vertical advection causes a deepening of the low‐level potential vorticity maximum and a pinching off of the upper‐level potential vorticity minimum. to recover the tangential wind and mass fields from the known potential vorticity, one must solve a second‐order nonlinear partial differential equation. Numerical solutions of this equation result in tangential wind fields which are similar in many ways to observed tropical cyclones.
On semi‐geostrophic flow over synoptic‐scale topography
Authors: Cullen M. J. P. , Chynoweth S. , Purser R. J.
Published: July 2010

Existing semi‐geostrophic theories of flow over a two‐dimensional orographic ridge are extended using a geometrical interpretation of the equations. the method of solution allows information to be gained even when there are local violations of the scaling assumptions. the results show that the occurrence of many observed aspects of the flow, such as downslope winds, drag and flow separation, can be predicted. Quantitative assessment requires a three‐dimensional version of the model.
Comparison of conventional and transformed in the troposphere
Authors: Pfeffer Richard L.
Published: July 2010

The effect of synoptic‐scale transient eddies on the mean zonal current in the troposphere and lower stratosphere is examined using conventional and transformed Eulerian diagnostics. In each case the partial solution of the Eliassen‐Kuo diagnostic equation for the meridional circulation is obtained using as forcing functions only transient eddy (TE) fluxes of heat and momentum. This solution is then used to calculate the Coriolis force and the advective terms in the zonal mean momentum equation. the ‘transient‐eddy‐induced’ zonal momentum tendency is defined as the sum of these terms and the eddy flux terms in the zonal momentum equation. Although conventional and transformed Eulerian diagnostics give the same transient‐eddy‐induced momentum and kinetic energy tendencies, the interpretation of the mechanisms involved is different.

It is found that the TE flux convergence of momentum, which is regarded as the torque in the conventional diagnostics, correlates better with the TE‐induced zonal momentum tendency in the troposphere and lower stratosphere than does the transient E‐P flux divergence, which is regarded as the effective torque in the transformed diagnostics. In the latter formalism, the Coriolis force acting on the residual meridional circulation is larger in magnitude than the E‐P flux divergence and is of opposite sign. the net zonal wind acceleration is given by an extremely small difference between these two large quantities and is largely opposite to the E‐P flux divergence. Moreover. the role of the TE heat flux in this formalism is found to be negligible, with the poleward heat flux at the boundary and in the interior inducing accelerations of almost equal magnitude and opposite sign.

Some insight into the role of the transient E‐P flux divergence and its components is sought by examining a form of potential vorticity equation for the zonally averaged current, in which the residual meridional circulation has been eliminated. This relationship reveals the importance of the horizontal derivatives of the E‐P flux divergence in forcing zonal wind accelerations, and elucidates most vividly the role of the rotational stratification parameter in diminishing the impact of wave action at high latitudes in the middle troposphere. Examination of the numerical influence function corresponding to the differential operator which operates on the zonal momentum tendency in this equation reveals why the TE flux of heat, which peaks at a higher latitude and lower altitude than the TE flux convergence of momentum, is much less effective in accelerating the zonal current.
Impact of satellite‐based data on fgge general circulation statistics
Authors: Salstein David A. , Rosen Richard D. , Baker Wayman E. , Kalnay Eugenia
Published: July 2010

To assess the influence that data from satellites and other FGGE special observing systems can have on analyses of the large‐scale circulation, the NASA GLA analysis/forecast system was run in two different parallel modes. In the first, data from all of the FGGE observing systems were utilized (denoted the FGGE analysis), and in the second (the NOSAT analysis) only conventional upper air and surface reports were used. Global wind and temperature fields extracted from month‐long assimilations during the first Special Observing Period are similar in the two cases but showed some differences, especially over the southern hemisphere oceans where satellites play a significant role in filling data voids. Winds are a little stronger in general in the NOSAT case, suggesting a damping effect from the inclusion of satellite‐based data.

The GLA model was also integrated for the same period without any data inserted (model CLIMATE run). an independent objective analysis based only on rawinsonde and pilot balloon data (STATION analysis) was performed as well. Stronger zonal winds in the model CLIMATE run and differences in features between the NOSAT and STATION cases attest to some influence of the GLA model and analysis technique on the resulting fields.

Momentum and heat budget computations were performed for the FGGE, NOSAT and STATION cases. Integrated values of kinetic and potential energy terms in both zonal mean and eddy forms, as well as various conversions between terms, though similar among the cases, demonstrate overall a small decrease in the vigour of the circulation with the inclusion of satellite observations.
On Co2 climate sensitivity and model dependence of results
Authors: Mitchell J. F. B. , Wilson C. A. , Cunnington W. M.
Published: July 2010

The regional response of climate models to small perturbations is shown to be highly dependent on the unperturbed simulation. an experiment in which CO2 concentrations are doubled and sea surface temperatures are enhanced by 2 K has been carried out with two general circulation models which differ considerably in their control climates. the resulting changes in tropical precipitation in each model simulation are related to the increase in atmospheric water vapour which leads to enhanced precipitation in the main regions of low‐level atmospheric convergence. Since these regions of convergence occur in slightly different locations in the unperturbed simulations, the distribution of changes is also different.

Differences in control simulations must be taken into account when comparing results from different models (for example, on doubling atmospheric CO2); otherwise unduly pessimistic conclusions may be reached concerning the consistency of model results. One may be able to make subjective allowance for the effect of known deficiencies in the unperturbed simulation on the model's response before using the simulated changes in, for example, impact studies.

A detailed examination of one of the experiments reveals that the change in precipitation is limited by the heat balance of the atmosphere, and indicates the importance of treating accurately the radiative perturbation due to changes in water vapour. the magnitude of the model's response is shown to be consistent with that found in three‐dimensional climate models which include a simple representation of the ocean.
Large‐Eddy simulations of the neutral‐static‐stability planetary boundary layer
Authors: Mason P. J. , Thomson D. J.
Published: July 2010

A series of large‐eddy simulations of the neutral‐static‐stability planetary boundary layer has been conducted. the simulations differ in the size of the computational domain and the scales represented by the subgrid‐scale turbulence parametrization. the flow eddies are always irregular but show an elongation in the direction of the local velocity shear vector. the dominant horizontal scales of the eddies increase with height in the boundary layer and become much larger than the boundary layer depth. All of this wide range of scales of motion is important in determining the gross boundary layer characteristics. the larger scales of motion have dynamics influenced by both the vertical and horizontal components of the earth's rotation. the flow statistics are compared with previous simulations and with high‐order closure models.
Water Vapour Transport Over Africa and the Atlantic Ocean During Summer 1979
Authors: Cadet D. L. , Nnoli N. O.
Published: July 2010

The water vapour transport over Africa and the eastern Atlantic Ocean during the 1979 summer is studied. the analysis is based on wind fields from the European Centre for Medium Range Weather Forecasts and humidity fields derived from a three‐layer precipitable water data set.

The evolution of the bi‐weekly mean fields of water vapour flux in the surface to 850 mb layer shows the progressive penetration of moisture over West Africa during the summer. At low levels during the maximum activity of the African monsoon, the southerly flow penetrates up to 20°N and there is a large belt of westerlies around 10°N. the fluxes between 700 and 500 mb suggest the importance of the African easterly jet (AEJ) as a major artery for moisture for West Africa.

Vertically integrated fluxes across different sections have been analysed. Across the equator at the longitudes of the Gulf of Guinea the flux shows a decreasing trend due to the northward propagation of the intertropical discontinuity over west Africa. Near 22°N, the flux is mainly from the north at the beginning of the summer and becomes southerly during surges of the monsoon. the flux from West Africa toward the Atlantic Ocean is large, indicating that moisture from the Gulf is transported upward and advected westward by the AEJ. the fluxes oscillate with two main periods, around 3‐5 days and 5‐7 days.

Water vapour budgets have been estimated for boxes located over the Gulf of Guinea and West Africa. the Gulf of Guinea is a source of water vapour transported northward and westward by the trade winds. Over West Africa at low levels, moisture comes from the Gulf of Guinea. In the mid‐troposphere there is a strong easterly moisture flux associated with the AEJ and originating from central Africa. This moisture seems to come initially from the Gulf of Guinea. Because it is mainly divergent, the mid‐tropospheric flux may have little influence on rainfall over West Africa. the budget study also emphasizes the importance of evapotranspiration over the belt north of the Gulf of Guinea where water vapour is recycled and transported northward.
Two‐Dimensional Model Studies of Equatorial Dynamics and Tracer Distributions
Authors: Gray L. J. , Pyle J. A.
Published: July 2010

A parametrization of Kelvin wave absorption in the equatorial stratosphere has been included in a two‐dimensional model of the atmosphere in order to improve the simulation of the semi‐annual oscillation. Additionally, a more detailed treatment of the momentum deposition associated with gravity wave breaking in the mesosphere has replaced the previous Rayleigh friction scheme. the model exhibits a semi‐annual reversal of the zonal winds at the equator in the stratosphere, with the westerly phase descending with time, as observed. the inclusion of the improved gravity wave parametrization has led to a number of significant improvements; in particular, the model also exhibits a semi‐annual oscillation in the mesosphere which is out of phase with that in the stratosphere. Meridional circulations associated with the westerly phase of the semiannual oscillation produce ‘double peak’ structures in a number of modelled tracer fields, including N2O, CH4 and H2O which compare well with observations by the SAMS and LIMS satellite experiments. These structures consist of a distinct minimum at the equator and double maxima in the subtropics; they are present for several months around each equinox. Inclusion of the more realistic westerly forcing has pointed to the need for additional easterly forcing in the model around each solstice. Further supporting evidence for the importance of momentum deposition associated with planetary waves in the easterly phase of the semi‐annual oscillation comes from the difference in double peaks between the northern hemisphere spring and autumn, as suggested in an earlier paper.
Eddy‐Forced Coherent Structures As A Prototype of Atmospheric Blocking
Authors: Haines Keith , Marshall John
Published: July 2010

The relevance of nonlinear free solutions of the equivalent barotropic vorticity equation known as ‘modons’ or ‘vortex pairs’ is reconsidered as a prototype of atmospheric blocking. It is argued that modons are highly appropriate reference solutions from which to consider the dynamics of blocks accounting for their local nature, their stability and longevity.

Nonlinear resonance studies with an equivalent barotropic numerical model on a β‐plane channel demonstrate the ease and vigour with which vortex pairs can be excited by a vorticity‐forcing function of the appropriate form. Localized nonlinear structures which can be identified with modons exist on blocking timescales even in parameter ranges which allow stationary Rossby waves. Thecircumstances in which synoptic systems can provide an appropriate forcing function are investigated by studying the interaction between a modon and a train of travelling vortices. It is shownthat the anomalous transfer of potential vorticity induced by the straining of synoptic systems propagating in a diffluent jet is in a sense to maintain the modon against dissipation. Time sequences of the potential vorticity field from the equivalent barotropic model bear a remarkable resemblance to isentropic potential vorticity maps observed during a blocking episode and suggestthat much of the detail can be understood in terms of the passive deformation of baroclinically inactive weather systems.
Impact of Aircraft Wind Data On Ecmwf Analyses and Forecasts During the Fgge Period, 8‐19 November 1979
Authors: Baede A. P. M. , Kållberg P. , Uppala S.
Published: July 2010

This paper describes the results of an observing system experiment, set up to assess the impact of aircraft wind data on ECMWF analyses and forecasts. Two parallel data assimilation runs for an 11‐day period during the Global Weather Experiment (FGGE) in November 1979 show that the ECMWF system is able to extract valuable and sometimes essential information from such single‐level data. the impact was particularly significant in the tropics, though limited elsewhere. Especially for the aviation community it is interesting to note that a positive impact of aircraft data was found on both wind analyses and 6‐hour forecasts along flight tracks. On average an improvement of about 1 m s−1 is found for both wind components but locally and in individual cases this may be considerably larger.

In order to assess the possible redundancy of aircraft and satellite data outside the tropics, the with‐ and without‐aircraft data assimilations were repeated with satellite winds and satellite temperatures excluded from the data base. This experiment confirmed our suspicion but suggested that the redundancy may be less serious in operational non‐FGGE conditions and may also be a consequence of the particular synoptic situation during the period.

From an intercomparison with a similar experiment carried out at the Meteorological Office it is inferred that a multi‐variate scheme with a proper vertical coupling is advantageous in extracting and directing information to the rotational modes of the model.

A quality control on the data showed that the reliability of the automatically transmitted aircraft data (ASDAR and AIDS) is high. Therefore the results of this experiment may be unfavourably biased by the more abundant but less reliable conventional aircraft (AIREP) data. This leads us to conclude that a future system of aircraft data, based entirely on ASDAR, will be a valuable contribution to the global observing system.
The Development and Verification of A Cloud Prediction Scheme For the Ecmwf Model
Authors: Slingo J. M.
Published: July 2010

This paper describes the development of a fractional cloud cover scheme which was implemented operationally in the ECMWF medium range forecast model in May 1985. the scheme is based on a diagnostic approach in which cloudiness is related empirically to the large‐scale model variables, including convective activity. an example of the performance of the scheme is given, showing that a fair degree of skill is achieved in forecasting tropical and extratropical cloudiness. an attempt to verify the results using retrieved cloudiness from Nimbus 7 is described. Considerable difficulties were experienced due to the different height classifications of clouds in the model and in the satellite data. the importance of cloud radiative properties as well as cloud amount is considered briefly, based on a comparison of model earth radiation budget diagnostics with similar data from the NOAA polar‐orbiting satellites. the sensitivity of the simulated outgoing long‐wave radiance to changes in the prescribed cloud liquid water content is discussed. Two examples of the effects of cloudradiation interaction on boundary layer processes are described which demonstrate the importance of an integrated approach to the treatment of clouds, radiation and turbulent fluxes.
A Study of the Diurnal Variation of Stratocumulus Using A Multiple Mixed Layer Model
Authors: Turton J. D. , Nicholls S.
Published: July 2010

A multiple mixed layer model of the cloud‐topped boundary layer is developed to investigate the diurnal variation of stratocumulus. A simple representation of the microphysical properties of the cloud layer is included which enables high resolution, interactive radiation calculations to be made. A constraint on the buoyancy flux profile is introduced which permits the previously well‐mixed layer to separate into two independently driven layers, thereby avoiding many of the unrealistic aspects of single‐layer models and enabling the model to reproduce features seen in observational studies. the diagnosis of this decoupling, which generally occurs during the morning, is discussed and is shown to have a seasonal and latitudinal sensitivity due to its dependence on cloud layer short‐wave absorption.

Comparisons of results with those obtained when separation is not permitted show significant differences. In particular, the cloud layer displays a much enhanced diurnal variation in thickness when separation is allowed, with the minimum occurring in the afternoon. These results are shown to produce much better agreement with observations. the likely consequences of separation for boundary layer evolution are discussed. the surface energy balance can be quite strongly affected, suggesting that it is important to resolve this diurnal variation, especially in areas where stratocumulus is a dominant climatological feature.

The sensitivity of the results to model assumptions are investigated and the limitations of the model are also assessed.
A Study of the Short‐Wave Radiative Properties of Marine Stratus: Aircraft Measurements and Model Comparisons
Authors: Hignett P.
Published: July 2010

Aircraft measurements of the short‐wave albedo of a marine stratus cloud are described and compared with results from radiative transfer models of plane‐parallel clouds. the albedos are measured in the visible and near‐infrared parts of the solar spectrum using clear dome and red dome pyranometers. Using all the available liquid water content data and the measured microphysical properties, two model clouds are constructed to represent the maximum and minimum liquid water paths encountered during the experiment. Model albedo calculations based on these clouds are compared with the range of albedos measured just above cloud top. the natural variation of liquid water path at the cloud edge is exploited as a further qualitative assessment of the models' behaviour.
An Efficient Two‐Time‐Level Semi‐Lagrangian Semi‐Implicit Integration Scheme
Authors: Temperton Clive , Staniforth Andrew
Published: July 2010

The semi‐implicit semi‐Lagrangian integration technique enables numerical weather prediction models to be run with much longer timesteps than permitted by a semi‐implicit Eulerian scheme. the choice of timestep can then be made on the basis of accuracy rather than stability requirements. to realize the full potential of the technique, it is important to maintain second‐order accuracy in time; this has previously been achieved by applying it in the context of a three‐time‐level integration scheme. In this paper we present a two‐time‐level version of the technique which yields the same level of accuracy for half the computational effort. Unlike other efficient two‐time‐level schemes, ours does not rely on operator splitting.

We apply this scheme to a variable‐resolution barotropic finite‐element regional model with a minimum gridlength of 100 km, using timesteps of up to three hours. the results are verified against a control run with uniformly high resolution, and are shown to be of similar accuracy to those of a semi‐implicit Eulerian integration with a timestep of 10 minutes.
Angular Momentum Variation In A Translating Cyclone
Authors: Skubis Steven , Molinari John
Published: July 2010

An expression for the time rate of change of relative angular momentum in a moving cyclone is derived which allows a kinematic interpretation of the effects of motion. One term arises from the convergence of flux of momentum by the storm motion vector as the storm moves through an environmental gradient of momentum, and has a counterpart in the equation for the time change of any scalar in a moving volume. A second term, which appears because angular momentum depends upon distance from the axis of rotation, arises purely due to the motion of the axis. This latter term is shown to be proportional to the deviation of the storm motion vector from the basic current, and to make a positive contribution when the storm moves to the left of the mean flow.

The Lagrangian terms were calculated for an observed rapidly moving hurricane about to make landfall in a baroclinic environment. Their overall contribution was negative and equal in magnitude to the largest terms in the Eulerian momentum budget, indicating that the rapid observed decay of the storm was influenced not only by its landfall but also by unfavourable environmental interactions as the storm moved into a region of reduced cyclonic momentum, the results suggest that the effects of translation cannot be neglected for a vortex which is rapidly moving or embedded in a non‐uniform environment.
Diurnal Variations In Flow Over A Succession of Ridges and Valleys
Authors: Mason P. J.
Published: July 2010

Observations of flow in a nearly two‐dimensional valley located within a succession of similar ridges and valleys are presented. Measurements of the mean flow have been made at all times of the day and are found to be well characterized by a suitably defined Froude number. the unstable and neutral daytime flow shows a flow separation whilst when the Froude number has a value of about two (usually around evening and dawn transitions) the flow is attached and the valley well ventilated. For smaller values of Froude number the flow speed in the valley reduces and a characteristic ‘drainage’ flow occurs. In addition to mean flow measurements and soundings of temperature structure, measurements were also made of turbulence energy and stress. This allows an estimate of the effective value of the surface roughness, zo, and an indication of the stability dependence of the momentum transfer to be made.
A Model of Drizzle Growth In Warm, Turbulent, Stratiform Clouds
Authors: Nicholls S.
Published: July 2010

A model to describe the growth of precipitation sized drops in layers of warm, stratiform cloud is developed which combines the effects of stochastic turbulent diffusion with explicit microphysical calculations. the representation of diffusion is consistent with measured turbulence statistics, reproduces standard results and is used to define an ensemble of Lagrangian trajectories within which the effects of phase change, stochastic coalescence and vertical exchange due to gravitational settling can be calculated. Mean drop size distributions are predicted as functions of height within the cloud.

The inclusion of realistic levels of turbulence is found to produce greatly increased numbers of larger (r >100 μm) drops throughout the depth of the cloud. the modelled steady‐state drop size distributions show good agreement with corresponding aircraft measurements in a case study. the effects of changing the turbulence intensity, cloud depth and liquid water content on the distribution of larger drops are investigated and implications for the cloud layer water balance and the cloud radiative properties are also considered.
Wave Structure and Evolution In Baroclinic Flow Regimes
Authors: Weng H.‐Y. , Barcilon A.
Published: July 2010

A uniform potential vorticity model, modified by two Ekman layers of different strengths, is used to study analytically and numerically the structure and evolution of strongly nonlinear, baroclinic waves. the model consists of a single zonal wave with its lowest two meridional modes, and of the mean flow correction with its lowest four meridional modes. Each meridional mode consists of a baroclinic and a barotropic pattern.

Travelling steady waves, amplitude vacillation and structural vacillation are found in the model. Steady waves are energetically balanced between baroclinicity and Ekman dissipation through nonlinearity. In a typical amplitude vacillation, the wave potential energy vacillates with time, via the interference of nonlinear barotropic and baroclinic patterns of the lowest meridional mode of the wave. In a typical structural vacillation, the distribution of wave kinetic energy in the meridional direction vacillates with time, via interference of the lowest two nonlinear meridional modes of the wave. the results are qualitatively consistent with those observed in annulus experiments. the mechanisms of amplitude and structural vacillations may be responsible for blocking in middle and high latitudes.
Lateral Boundaries and the Heat‐Induced Flow In the Tropical Atmosphere
Authors: Phlips P. J. , Gill A. E.
Published: July 2010

The effect on the heat‐induced circulation of lateral vertical boundaries is studied by means of a simple model. the basic state in the absence of boundaries is a Hadley circulation driven by a narrow band of heating at a specified latitude. Lateral friction is included in the model. This has little effect on the meridional circulation but smooths out the zonal flow. the eastern boundary modifies the Hadley flow through planetary waves needed to make the flow at the boundary meridional, and the flow pattern is similar to that of the east Pacific trades. the western boundary generates a Kelvin wave in order to make the total flux normal to the boundary vanish. This can give rise to patterns similar to those encountered in the Indian Ocean.
Variational Assimilation of Meteorological Observations With the Adjoint Vorticity Equation. I: Theory
Authors: Talagrand Olivier , Courtier Philippe
Published: July 2010

The following variational approach is taken to the problem of assimilation of meteorological observations: find the solution of the assimilating model which minimizes a given scalar function measuring the ‘distance’ between a model solution and the available observations. It is shown how the ‘adjoint equations’ of the model can be used to compute explicitly the ‘gradient’ of the distance function with respect to the model's initial conditions. the computation of one gradient requires one forward integration of the full model equations over the time interval on which the observations are available, followed by one backward integration of the adjoint equations. Successive gradients thus computed are introduced into a descent algorithm in order to determine the initial conditions which define the minimizing model solution.

The theory is applied to the vorticity equation. Successful numerical experiments performed on a Haurwitz wave are described.