bluuebunny/crossref_metadata_2025 huggingface dataset:
Lagrangian flow in the middle atmosphere
Authors: Sutton Rowan
Published: February 2007

The Lagrangian view of air flow in the middle atmosphere is studied by computing tens of thousands of particle trajectories in a global primitive‐equation model. This approach focuses attention on the long‐term fate of air and provides a high‐resolution view of air transport. Simulations are carried out for both the northern and southern winters to permit an inter‐hemispheric comparison. The first part of the paper concentrates on transport in the meridional plane. Trajectories followed for many months show patterns which are well organized on a global scale. Ensembles of particles followed for a much shorter time are used to calculate a Lagrangian mean meridional circulation which shows close similarities to published diabatic circulations calculated from observational data. Maps of meridional dispersion reveal a broad mid‐latitude band, with well defined boundaries, in which rapid dispersion occurs. A more detailed visualization of transport in and around the southern polar vortex in late winter shows that small scales are generated in this mid‐latitude region, but not in the vortex nor at low latitudes. The ability of the vortex to retain a labelled air mass for many months is clearly demonstrated, and the relevance of the results to understanding springtime ozone depletion is discussed.
Influence of strong gradients of wind and humidity on frontal ageostrophic circulations
Authors: Lagouvardos K. , Lemaitre Y.
Published: February 2007

The impact is studied of strong gradients of wind and humidity on the ageostrophic circulation developed around a conceptual cold‐frontal system. The simulated basic wind and temperature fields are chosen to best represent the frontal observations made during the Fronts 87 field experiment and, in particular, to take account of the presence of strong low‐level jets. The numerical solution of the diagnostic Sawyer—Eliassen equation in both its adiabatic and diabatic forms permitted us to test the impact of the wind and humidity structure on the corresponding ageostrophic circulation. In particular, strong negative shear above the low‐level jet core enhanced subsiding motions on the warm side of the front, which restricted the horizontal extent of the cloud cover. On the other hand, the presence of well‐marked horizontal humidity gradients aloft, at the rear edge of the cloud cover, enhanced the ascending motions ahead of the frontal surface, creating locally a secondary maximum of upward motions. The degree of stability within the prefrontal boundary layer was found to be of major importance with regard to the intensification of the upright convection near the leading edge of the front. Moreover, the incorporation of a parametrization scheme, representing the Ekman pumping at the top of the boundary layer, enhanced the diagnosed circulation ahead of the front. A comparison with observations made during two case‐studies of Fronts 87 seems to confirm these results.
Correction of instrumental biases in historical sea surface temperature data
Authors: Folland C. K. , Parker D. E.
Published: February 2007

We describe a physically based empirical technique for correcting historical sea surface temperature measurements for time‐varying biases. The corrections are based on models of heat and moisture transfers from uninsulated (canvas) and partially insulated (wooden) sea temperature buckets exposed on deck. One of the canvas bucket models is tested using measurements on board ship and published wind‐tunnel measurements. The method gives geographically and seasonally varying bias corrections through the period 1856 to 1941. The corrections are fairly insensitive to uncertainties such as the size of the bucket or the details of its exposure on deck. A discussion of the history of sea surface temperature observations provides a background to the procedure.

The resulting globally and seasonally averaged sea surface temperature corrections increase from 0.11 degC in 1856 to 0.42 degC by 1940. The corrections are compatible with recent measurements made at sea of the errors of canvas buckets. Global and hemispheric time series of corrected sea surface temperature and night marine air temperature data show good agreement: more detailed verifications of the corrections will be reported elsewhere.
A general circulation model study of a tropopause‐folding event at middle latitudes
Authors: Cox B. D. , Bithell M. , Gray L. J.
Published: February 2007

The UK Universities' Global Atmospheric Modelling Programme General Circulation Model (UGCM) has been run at high horizontal resolution to investigate a tropopause‐folding event at middle latitudes. Geopotential‐height, potential‐vorticity and specific‐humidity distributions were diagnosed from the model and showed a realistic representation of the synoptic development of the cut‐off low associated with the tropopause fold. the model also successfully portrayed the frontogenesis associated with tropopause folding and represented both the upper‐level frontal region and the corresponding jet‐streaks as they progressed through the region of the developing trough. A series of vertical cross‐sections at the ends of the jet streaks demonstrated that a fold could be captured by the UGCM using potential vorticity and specific humidity, although more small‐scale features were evident in the specific‐humidity contours than in the corresponding potential‐vorticity fields. the corresponding baroclinic zone was also resolved, although both the slope and thickness were rather larger than sometimes observed. Both the spatial and temporal extent of the fold compared well with observations. In addition the model simulated the ageostrophic winds in the entrance, exit and middle‐streak regions. A direct circulation was obtained at the entrance and an indirect circulation at the exit region of the jet streak, in good agreement with theory and with observational data. A combination of confluence, cold advection and curvature was responsible for the forcing of the ageostrophic motion at the jet entrance, whereas the forcing at the jet exit was attributed mainly to diffluence and curvature. Q vectors were employed to investigate the vertical motion in the vicinity of the fold, and showed the precise positions of descent corresponding to the fold along the entire length of the jet streak. Finally a tropopause fold around a jet that was not subject to the development of strong baroclinic instabilities was reproduced by the model: it was smaller in size than the fold of the cyclonic trough. A Q‐vector analysis confirmed the position of the small fold along the jet and revealed large‐scale ascending motion that may have been responsible for the small size of fold. the possibility of using the model to yield a realistic estimate of the transfer of air from the stratosphere to the troposphere in a tropopause‐folding event at mid‐latitudes is discussed.
Gravity‐wave drag parametrization over complex terrain: The effect of critical‐level absorption in directional wind‐shear
Authors: Shutts Glenn
Published: February 2007

Most schemes to parametrize gravity‐wave drag effectively assume that the total wave‐stress is associated with a single wave‐vector pointing in the direction of the stress vector. However, the wave stress over real terrain is an integral over all azimuthal directions of wave‐vector contributions. In unidirectional flow, all of these modes will have a critical line at the same height—if one exists. However, if the wind turns montonically with height, by whatever degree, critical‐level absorption will occur at all heights for some portion of the wave spectrum. the resultant critical‐line wave‐drag is always normal to the local wind direction and the wave stress itself turns with height. This contrasts with the height‐independent stress assumption used in parametrizing gravity‐wave drag outside regions where the wave stress is saturated.

A practical approach to computing the gravity‐wave stress over anisotropic orography, which includes this selective critical‐level absorption effect, will be presented. This will be supported by an analytical model calculation for constant‐shear flow with wind turning.
Parametrization of momentum transport by convectively generated gravity waves
Authors: Kershaw R.
Published: February 2007

A parametrization equation for momentum flux due to convectively generated gravity waves is derived and tested numerically. the wave momentum flux just above the convective layer is shown to depend directly on the shear and the intensity of convection, and inversely on the stability of the air above the convection. Simulations of ensembles of precipitating convective clouds are used to validate the parametrization and to estimate the efficiency of wave generation by convection in one particular convective regime: an idealized cold‐air outbreak with unidirectional shear.
Simulations of the potential vorticity structure and budget of fronts 87 IOP8
Authors: Persson P. Ola G.
Published: February 2007

Dropsonde observations in the cold fronts sampled during the joint British‐French Fronts 87 Experiment along the west coast of Europe have shown that the dry and moist potential vorticity fields (qd and qe, respectively) have significant anomalies on the mesoscale. the processes associated with these anomalies during a 12 h period encompassing the observation time have been examined by simulating the qd and qe structures observed near the cold front of IOP8 using the Météo‐France mesoscale PERIDOT model, and by developing and using an Eulerian potential vorticity budget module. High‐resolution dropsonde, rawinsonde, and Doppler radar observations are used as verification of the simulated potential vorticity structures. Initial and boundary conditions are obtained from three different analysis systems.

The simulated potential vorticity structures are sensitive to the initial conditions because the analyses over the data‐sparse Atlantic Ocean vary significantly from one analysis system to another. of the seven persistent mesoscale potential vorticity features identified in the observations, five were simulated reasonably well using at least one of the initial analyses, and examined with the potential vorticity budget. the low‐level, pre‐frontal, positive qd anomaly along the length of the cold front is maintained through a balance between Lagrangian source/sink terms, dominated by differential diabatic heating, and vertical advection within the main frontal updraught. This balance maintains the qd anomaly in a region of near‐zero qd tendency between areas of qd generation near the surface and qd destruction in the middle troposphere. A mid‐tropospheric region of negative qe directly behind the surface cold front appears to be produced by a weak but persistent generation by the solenoid term in the region of strong humidity gradients along the back edge of the mid‐tropospheric cloud shield. This position, and the model evolution, suggest that the solenoid‐created region of negative qe can lead to enhanced mid‐tropospheric updraughts through conditional symmetric instability or a combination of conditional symmetric instability and large‐scale forcing. the magnitudes of regions of negative qd and qe on the anticyclonic side of the upper‐level jet decrease throughout the simulation due to horizontal momentum diffusion, while a post‐frontal, low‐level, negative qe region is generated principally by temperature and moisture diffusion.
Numerical experiments on radiation and tropical cyclones
Authors: Craig George C.
Published: February 2007

Numerical simulations of tropical cyclones were carried out using an axisymmetric model with explicit convection and detailed microphysical and radiative parametrizations. The simulated systems showed a modest increase in intensity when radiative heating and cooling were included. This could be attributed to differential cooling of the environment of the system relative to its warm core, where cloud cover prevented radiative cooling. However the results were strongly influenced by the initial conditions and by the evolution of the water vapour field in the environment of the cyclone. This indicates that conclusive results concerning the effects of radiation on tropical cyclones are unlikely to be obtained from such idealized simulations, but will require studies that adequately represent the initiation and environmental interactions.
A numerical model of the cloud‐topped planetary boundary‐layer: Radiation, turbulence and spectral microphysics in marine stratus
Authors: Bott A. , Trautmann T. , Zdunkowski W.
Published: February 2007

A numerical model of the cloud‐topped planetary boundary‐layer is presented. The model is one‐dimensional with special emphasis on a detailed description of cloud microphysical processes. Aerosols and cloud droplets are treated in a two‐dimensional particle‐distribution whereby the activation of aerosols is calculated explicitly by solving the droplet‐growth equation at all relative humidities. Atmospheric radiation is determined with a δ‐two‐stream radiation scheme. Turbulent fluxes are parametrized as a function of the turbulent kinetic energy. Numerical results are presented which are obtained by utilizing measurements made over the North Sea. The interaction between radiation, turbulence and cloud processes for maintaining the quasi‐steady state of stratiform cloud fields is emphasized.
The behaviour of a cloud ensemble in response to external forcings
Authors: Guichard F. , Redelsperger J.‐L. , Lafore J.‐P.
Published: February 2007

The behaviour of a population of tropical moderate precipitating clouds is investigated with a cloud‐resolving model over a period equivalent to 2–3 days. The response of the system is analysed as a function of external forcings, comprising shortwave and longwave radiation, large‐scale ascent effects and surface fluxes. Radiative and largescale ascent processes enhance the convective activity. In all the experiments, an accumulation of humidity is observed in the upper part of the cloud layer and in the region above. In the case of weak large‐scale ascent, a diurnal cycle of convection is found, having maximum activity during the night and minimum activity around noon. Depending on the anvil cloud coverage, a modulation of this cycle is found whose cause can be explained through an infrared radiative feedback. The anvil deck also has a diurnal cycle but phase shifted by six hours with reference to the convective cycle.
Microphysical and short‐wave radiative structure of wintertime stratocumulus clouds over the Southern Ocean
Authors: Boers R. , Jensen J. B. , Krummel P. B. , Gerber H.
Published: February 2007

Results are presented from an aircraft measurement campaign carried out over the Southern Ocean near the north‐west coast of Tasmania, Australia. The microphysical and radiative characteristics of marine stratocumulus cloud sheets were sampled on four days, three of which are considered baseline days when air parcels had traversed long distances over the ocean without having been exposed to anthropogenic sources of pollution. Clouds were depleted by drizzle, with cloud liquid‐water often smaller than 50% of the expected adiabatic value. Horizontal variability in liquid‐water content associated with non‐drizzle droplets was primarily caused by variations in the droplet number concentration, which was found to be among the lowest ever recorded (10 to 40 cm −3 for clouds of up to 300 m deep). Cloud albedos integrated over the solar spectrum varied from 40 to 60%, which roughly agreed with radiative‐transfer computations. Data from one non‐baseline day were also obtained and compared with the data obtained during baseline conditions. The functional dependence of cloud optical depth on liquid‐water path was found to be much stronger on this day than the others, due to the smaller size of the cloud droplets present. Calculations show that on the days when drizzle was intense, the cloud optical depth could have been reduced by as much as 50% by shifts to larger values of the cloud droplet effective radius.
The Brewer‐Dobson Circulation In the Light of High Altitude In Situ Aircraft Observations
Authors: Tuck A. F. , Baumgardner D. , Chan K. R. , Dye J. E. , Elkins J. W. , Hovde S. J. , Kelly K. K. , Loewenstein M. , Margitan J. J. , May R. D. , Podolske J. R. , Proffitt M. H. , Rosenlof K. H. , Smith W. L. , Webster C. R. , Wilson J. C.
Published: February 2007

Fast response in situ measurements of a suite of chemical species made from the NASA ER2 high altitude aircraft, between 60°N and 70°S at potential temperatures up to 530 K from March to November 1994 at longitudes 115° W to 150°E, are considered for the view they offer of the Brewer‐Dobson circulation in the lower stratosphere and upper troposphere. In the southern hemisphere, where most of the flights occurred, comparisons are made with measurements taken in August/September 1987 at longitudes 120° W to 60° W to examine temporal and longitudinal differences. Interpretations made suggest conceptual modifications to the simple construct of advection in a two dimensional long‐term mean.
Scaling the turbulent transport of chemical compounds in the surface layer under neutral and stratified conditions
Authors: Galmarini S. , Vilà‐Guerau De Arellano J. , Duynkerke P. G.
Published: February 2007

A model of the vertical transport of chemically reactive species is presented which is applicable to all possible stability conditions in the atmospheric surface‐layer. the non‐dimensional formulation adopted allows any chemical scheme to be included. an equation hierarchy is presented for a fully second‐order closure description of the process. the model describes comprehensively the turbulence/chemistry process by calculating the mean concentrations, the turbulent fluxes and concentration covariances of the chemical species. It explicitly takes into account the chemical transformation in first‐ and second‐order variable equations. the model is applied to the NO‐NO2‐O3 cycle and to the so‐called ‘night‐time’ chemical scheme of nitrogen oxides. an analysis of the flux budget indicates that the relative importance of the chemistry term in the flux equation varies with distance from the surface. For atmospheric neutral conditions, it is suggested that the chemistry term in the flux equation can be used to make an a priori evaluation of the possible effect of chemistry upon the flux of a chemical species. the model also shows that under stable conditions, despite the greatly reduced activity of turbulence, differences can occur between the turbulent transport of an inert tracer and that of a chemically reactive species. Under any stratification and for typical surface‐layer fluxes of nitrogen oxides, the intensity of segregation is very low and can in general be neglected in model calculations.
On the question of enhanced absorption of solar radiation by clouds
Authors: Francis P. N. , Taylor J. P. , Hignett P. , Slingo A.
Published: February 2007

Some recent papers have claimed that clouds cause the atmosphere to absorb significantly more solar radiation than current understanding would predict. In this paper aircraft measurements of the broad‐band and narrow‐band radiation fields in and around different cloud types are presented. These measurements have been made over a number of years by the C‐130 aircraft of the UK Meteorological Office, in several different locations around the globe. Our observations show no evidence of this ‘enhanced’ cloud absorption, and they can be explained, both qualitatively and quantitatively, by currently accepted theory. the suggestion that such enhanced absorption is due to one particular mechanism, that of increased condensed‐phase water absorption in the near‐infrared, is examined. This hypothesis is rejected as being completely at variance with the observations.
Modelling isolated wave‐turbulence interactions in the stable atmospheric boundary layer
Authors: Edwards N. R. , Mobbs S. D.
Published: February 2007

The interaction between a large‐scale isolated wave and turbulence is considered in the context of the stable atmospheric boundary layer. A two‐dimensional model with a second‐moment turbulence closure is used to simulate the interaction numerically. Results obtained using other closures are discussed. the initial form of the wave‐like disturbance is suggested by observations, which are described in a companion paper. the turbulence is shown to exert only a small influence on the wave, while the turbulence itself remains close to production‐dissipation equilibrium. Wave‐like components of the turbulence fluxes vary roughly linearly with wave amplitude, but overall the wave is strongly nonlinear and depends heavily on the background flow. Only one term in the equations for the quadratic turbulence quantities varies in a simple way with wave amplitude; this term represents a net transfer of energy from the wave to the turbulence.
A numerical model of the cloud‐topped planetary boundary‐layer: Impact of aerosol particles on the radiative forcing of stratiform clouds
Authors: Bott Andreas
Published: February 2007

In a numerical sensitivity study, the microphysical model of stratus MISTRA, is used to investigate the impact of aerosol particles on the evolution of stratiform clouds. Four model runs are presented, each for a different type of background aerosol. Two include aerosol particle size‐distributions which are typical of marine and rural continental air masses; a third represents a mixture of marine and rural continental aerosol particles, and the fourth rural continental aerosol particles with a reduced solubility in water.

The results show that the microphysical structure of layer clouds was strongly affected by the physico‐chemical properties of the aerosol particles from which the cloud droplets grew. the relatively low concentration of the typical marine aerosol resulted in clouds with a low concentration of large droplets. By contrast, the rural continental and mixed aerosols yielded higher concentrations of relatively small cloud‐droplets. Because of their low solubility in water, relatively few of the aerosol particles simulated in the fourth run of the model became activated, so that the microstructure of the cloud was again similar to that with marine aerosol. the different size‐distributions of the cloud droplets had a direct influence on the radiative forcing of the clouds. When there were many small droplets, the reflectivity of the clouds was distinctly higher than when there were fewer, but larger, cloud‐droplets. In all simulations, the effective radii of the cloud droplets showed a strong diurnal variation.
The singular vectors of a coupled ocean‐atmosphere model of Enso. I: Thermodynamics, energetics and error growth
Authors: Moore A. M. , Kleeman R.
Published: February 2007

The singular vectors of a dynamical system are the most rapidly growing perturbations that can exist in the system before nonlinearity becomes important. We have studied the singular vectors of an intermediate coupled ocean‐atmosphere model of El Niño/Southern Oscillation (ENSO) in an effort to understand the dynamics responsible for the growth of small perturbations in the tropics. In particular, we have examined how the singular vectors of the observed seasonal cycle are influenced by various thermodynamic processes which operate in the upper‐ocean mixed‐layer, and which affect sea‐surface temperature (SST). These processes include vertical movements of the main thermocline, zonal advection and vertical upwelling. the main findings are:

the singular vector spectrum is dominated by the fastest growing member, regardless of which thermodynamic processes are active;

the growth factors of the singular vectors exhibit a strong seasonal dependence;

the western and central parts of the Pacific Ocean are the areas most often favoured for growth by the singular vectors, and the general criteria that must be met for singular‐vector growth have been determined;

at a given time of year, the growth factors of the singular vectors are sensitive to different combinations of thermodynamic processes, although the configuration of the dominant singular‐vector wind field after optimal growth has been achieved is very similar in all cases. This suggests that in the tropics, the coupled system has a preferred response that the ocean thermodynamics controlling SST conspire to produce, regardless of which processes are operating in the mixed layer;

the observed ENSO cycle is likely to influence singular‐vector growth significantly.

These results and ideas have important ramifications for the growth of errors and uncertainties in models during ENSO forecasts, and for the way this influences the predictability of ENSO. They also have important ramifications for the way perturbations grow in the real coupled ocean‐atmosphere system.
Using forecast sensitivity patterns to improve future forecast skill
Authors: Pu Zhao‐Xia , Kalnay Eugenia , Derber John C. , Sela Joseph G.
Published: February 2007

A simple, relatively inexpensive technique has been developed for using past forecast errors to improve the future forecast skill. the method uses the forecast model and its adjoint and can be considered as a simplified 4‐dimensional variational (4‐D VAR) system. One‐or two‐day forecast errors are used to calculate a small perturbation (sensitivity perturbation) to the analyses that minimizes the forecast error. the longer forecasts started from the corrected initial conditions, although better than the original forecasts, are still significantly worse than the shorter forecasts started from the latest analysis, even though they both had access to information covering the same period.

As a much less expensive alternative to 4‐D VAR, the adjusted initial conditions from one or two days ago are used as a starting point for a second iteration of the regular NCEP analysis and forecast cycle until the present time (t = O) analysis is reached. Forecast experiments indicate that the new analyses result in improvements to medium‐range forecast skill, and suggest that the technique can be used in operations, since it increases the cost of the regular analysis cycle by a maximum factor of about 4 to 8, depending on the length of the analysis cycle that is repeated. Several possible operational configurations are also tested.

The model used in these experiments is the NCEP's operational global spectral model with 62 waves triangular truncation and 28 ő‐vertical levels. an adiabatic version of the adjoint was modified to make it more consistent with the complete forecast model, including only a few simple physical parametrizations (horizontal diffusion and vertical mixing). This adjoint model was used to compute the gradient of the forecast error with respect to initial conditions.
A numerical study of the effects of land proximity and changes in sea surface temperature on hurricane tracks
Authors: Dengler Klaus
Published: February 2007

A three‐layer shallow‐water model with a convection parametrization is used to study the track deflections of slow‐moving hurricanes on a β‐plane as they approach straight coastlines of different orientations with respect to north, or as they cross to a cooler or warmer body of water. In the landfall experiments, the land surface is level and is characterized by an abrupt increase of the drag coefficient and zero evaporation. the tracks of a 72 h integration on a β‐plane are compared with an ocean control experiment.

It is found that the divergent flow in the boundary layer in regions of offshore winds is responsible for the observed track deflections. In this region middle‐layer air descends into the boundary layer and increases the positive potential‐vorticity anomaly in the middle layer. In the case of land conditions to the west of the vortex, the maximum potential‐vorticity anomaly extends less to the north of the vortex centre and results in a more southerly track than in the ocean control case. If land conditions are specified in the north, the positive potential‐vorticityasymmetry maximum extends more to the north‐west and results in a northward deflection of the track. On the other hand, land conditions to the north‐west of the initial vortex centre do not significantly change the location of the asymmetry, and therefore the track deflection is small. the magnitude of the track deflections, as well as the drift speed, increases when the roughness of the land is increased. It is significant that track deflections commence about 20 h before landfall, reaching a value of about 150 km at the time of landfall.

Compared with landfalling hurricanes, track deflections of model hurricanes which drift into a region of different sea surface temperature (SST) are smaller and are a result of intensity changes of the vortex circulation. the maximum wind speed at the time of landfall increases if a region of higher SST is located adjacent to the coastline.
The structure of katabatic flows down a simple slope
Authors: Papadopoulos K. H. , Helmis C. G. , Soilemes A. T. , Kalogiros J. , Papageorgas P. G. , Asimakopoulos D. N.
Published: February 2007

An analysis of the katabatic flow structure is presented, based on data collected along a simple slope over a two‐month period. Four meteorological stations were deployed along the nearly two‐dimensional east slope of Mt. Hymettos in Greece. The study focuses on the momentum and energy budgets of the flows during carefully selected stationary periods with negligible ambient winds. In order to achieve a proper selection, the mean features of the observed flows are presented along with their response to external winds. The observed profiles are successfully compared with the theoretical Prandtl profiles. Bulk quantities of the katabatic layer are compared with proposed experimental values in order to check the representativity of the observed flows. These comparisons offer a further test of theoretical considerations and assist in discussions of processes affecting mean‐flow features.

The flows were ∼ 20 m deep, with velocities 1‐2 m s−1. During steady‐state periods the katabatic acceleration was mainly balanced by the surface friction, the contribution of advective terms being small. The estimated interfacial drag was less important than the surface drag, which proved large compared with usually suggested values. Within the experimental uncertainties, values of the drag coefficient of the order of 10−2 seem appropriate. The steady‐state energy balance showed that the downslope advection balanced the turbulent heat flux to the ground surface.
Particle trajectories above sinusoidal terrain
Authors: Stout J. E. , Janowitz G. S.
Published: February 2007

As heavy particles fall towards windswept topography, their motion is governed partly by gravitational forces and partly by fluid forces resulting from the relative motion of the particles through the flow field. Topographically induced perturbations of the flow field distort particle paths and ultimately modify deposition patterns at the surface. Here, we calculate trajectories of particles falling toward a series of low‐amplitude hills. Particle motion is obtained by simplifying the dynamical equations of particle motion to kinematic form and then applying perturbation techniques. This simplification is possible when the timescale of fluid motions is much longer than the characteristic response‐time of the particles. It is shown that, under the right atmospheric conditions, alternating regions of convergent and divergent particle‐paths will occur. an extension of the trajectory analysis yields expressions that predict the point of surface impact as functions of the initial release point. This leads to a method for predicting the point along the surface where deposition is either a maximum or a minimum.
Mode selection, wave breaking and parametric sensitivity in the quasi‐biennial oscillation
Authors: Li X. , Read P. L. , Andrews D. G.
Published: February 2007

An analytical model is developed for the propagation of equatorially trapped waves on an equatorial β‐plane in a uniform zonal flow in the presence of Rayleigh friction and Newtonian cooling. By comparing solutions obtained from this model under conditions appropriate to the troposphere and stratosphere, it is shown that wave modes which are most likely to contribute to the excitation of zonal flow oscillations occur in a ‘spectral window’, for which modes propagate freely in the vertical in the troposphere but are strongly damped in the stratosphere. This spectral window leads to a mode‐selection criterion which accounts for the dominant modes observed in association with the main stratospheric quasi‐biennial oscillation (QBO), and other aspects of equatorial‐wave propagation in the tropical stratosphere and mesosphere.

Solutions from this model are also used to verify a new set of numerical models of equatorial‐wave propagation (using the ‘transformed Eulerian‐mean’ formulation) in the presence of zonal flows with general lateral and vertical shear and arbitrarily specified stratification. the latter numerical models are shown to simulate a realistic stratospheric QBO successfully, under conditions similar to those observed. the models are then used to investigate the probable role of large‐scale wave‐breaking processes in modifying the form of the QBO, via a convective parametrization scheme, which is shown to act in certain respects like Rayleigh friction. Finally, a sensitivity study is carried out to investigate the effect of varying parameters‐such as the vertical diffusion coefficient, buoyancy frequency and forcing by tropospheric wave‐on the form, strength and period of the QBO.
Long‐term fluctuations in the statistical properties of low‐frequency variability: Dynamical origin and predictability
Authors: Molteni Franco , Corti Susanna
Published: February 2007

The stability and predictability of the statistical properties of low‐frequency variability are investigated by studying the dynamical origin of long‐term changes arising, respectively, from the nonlinear response to variations in atmospheric forcing and from chaotic fluctuations in the energy partition between different variability patterns. Results from long integrations of a realistic quasi‐geostrophic (QG) model are analysed, and interpreted on the basis of the behaviour of simple nonlinear dynamical systems. It is shown that chaotic energy fluctuations not only modify the variance explained by different variability patterns, but also may either obscure or enhance the multimodality in the distribution of model states in periods as long as several decades. On the other hand, it is argued that significant variations in atmospheric statistics may be induced by anomalies in the atmospheric forcing originated from the lower boundary. QG simulations show that a strengthening of the planetary‐wave forcing due to the asymmetry in the divergent circulation between the western and eastern Pacific increases low‐frequency variability in the extratropical Pacific, and may turn the distribution of model states from unimodal to bimodal. Anomalies in low‐frequency variance related to changes in atmospheric forcing seem to be larger than those originated by internal dynamics, at least in certain periods and regions, suggesting a potential predictability of these statistics by coupled ocean‐atmosphere models.
Observations of a cut‐off low over southern Australia
Authors: Griffiths Morwenna , Reeder Michael J. , Low David J. , Vincent Robert A.
Published: February 2007

The structure and evolution of an intense cut‐off low is documented in detail unprecedented for the Australian region, by combining high time‐resolution radiosonde profiles with co‐located wind‐profiler observations and routine Australian Bureau of Meteorology analyses. The cut‐off low develops over the ocean south of Australia and subsequently interacts with a subtropical frontal system over the central part of the continent. Throughout the period of observation, the strongest temperature gradients lay through the subtropics. Moreover, the subtropical temperature gradients strengthened while the midlatitude temperature gradients associated with the cut‐off low weakened. The Australian Bureau of Meteorology's 150 km Regional Assimilation and Prognosis System analyses capture the broad‐scale structure and evolution of the system reasonably well. The system documented here is similar in structure to the instant occlusion investigated by Browning and Hill (1985), the major difference being that our cut‐off low develops poleward of a subtropical front. The cut‐off low was cold‐cored and produced a very deep tropopause fold. Over Adelaide, the tropopause descended to an altitude of about 5 km, and the temperature at 500 hPa fell to −30°C. The western edge of the low‐level cold dome was marked by a strong secondary warm front, which passed over Adelaide about 8 hours after the tropopause height minimum. The warm front was characterized at the surface by an absolute minimum in the pressure, and a temperature rise of about 7°C. Significantly, this increase in temperature took place in the early hours of the morning, against the diurnal trend. The cut‐off low produced very high rainfalls, resulting in flash flooding and much damage in the Adelaide Hills area. The most severe convection took place in the region behind the lowered tropopause, around the passage of the warm front.
Broadband solar fluxes and heating rates for atmospheres with 3D broken clouds
Authors: Barker Howard W. , Morcrette Jean‐Jacques , Alexander G. David
Published: February 2007

A 3D Monte Carlo photon transport algorithm is presented that computes broadband solar fluxes and heating rates. It treats attenuation by cloud droplets and gases separately and can produce 3D distributions of constituent absorptances. Underlying surfaces are accounted for and diurnal‐mean calculations can be achieved in the same time as typical single‐zenith‐angle experiments. Domain‐averaged fluxes and heating rate profiles are presented for two very different 3D cloud fields: (i) scattered, shallow cumuli inferred from Landsat imagery; and (ii) towering clouds simulated by a cloud‐resolving model. Plane‐parallel, homogeneous (PPH), independent column approximation (ICA), and clear‐sky versions of the 3D fields were generated and used as well.

For both cloud fields, total atmospheric absorptance depends very weakly on cloud geometry. Cloud geometry does, however, invoke major differences in surface absorptance and, hence, reflectance to space. At high sun, albedos for 3D clouds are less than corresponding PPH values, but are in almost perfect agreement with ICA estimates. This indicates that simple horizontal variability of cloud optical depth outweighs the impact of cloud sides. At very low sun 3D fields reflect most because of interception of radiation by cloud sides, while PPH and ICA albedos come into better agreement. For the towering cloud field, radiative fluxes are determined largely by clouds below 6 km, despite some clouds reaching 12 km. Heating rate profiles are also affected by cloud geometry. For most sun angles, PPH clouds exhibit anomalously large heating near cloud tops and anomalously small heating beneath clouds. On the other hand, profiles for 3D and ICA fields are very similar and depend much less on altitude; partly because of side illumination but also because the dense cores of inhomogeneous clouds are often radiatively‐shielded (unlike their PPH counterparts).

Finally, regular arrays of idealized cloud forms are used to demonstrate the potential ambiguity of using cloud radiative forcing ratios, R, as proxy measures for the impact of clouds on atmospheric absorptance. In essence, R depends not only on how clouds influence atmospheric absorption, but also on how they partition radiation between albedo and transmittance.
Transport of near‐tropopause air into the lower midlatitude stratosphere
Authors: Vaughan G. , Timmis C.
Published: February 2007

During the last week of January 1992 ozonesonde observations over Europe revealed a layer of very low ozone concentrations in the stratosphere—below 100 parts per billion (109) by volume in the potential‐temperature range 360–380 K. A coincident lidar observation revealed that the air was virtually free of volcanic aerosol, which filled the lower stratosphere at that time. The layer corresponded well with low potential vorticity (PV) in ECMWF analysis fields. Trajectory calculations confirmed a subtropical origin for the layer, and PV fields suggest that it was formed in a streamer of low‐PV air drawn from the troposphere over North America as the sub‐tropical jet stream turned northwards on 25 January. Ozone profiles on the equatorward flank of the subtropical jet stream contain similar mixing ratios to those seen in the layer over Europe in the same potential‐temperature range. The mass of the layer at its maximum extent is estimated as 6 × 1015 kg. Most of the air in the layer eventually returned to the subtropics after mixing with ambient midlatitude air.
Thermodynamic analytic fields from Doppler‐radar data by means of the MANDOP analysis
Authors: Protat A. , Lemaitre Y. , Scialom G.
Published: February 2007

The paper is devoted to scale interaction studies. It presents a new approach for retrieving mesoscale and convective‐scale thermodynamic fields from Doppler‐radar data in order to diagnose processes involved in these interactions. Thermodynamic fields are retrieved analytically, using the analytic form of the three‐dimensional wind field obtained by the MANDOP analysis in the non‐dissipative and stationary first‐order anelastic approximation of the equation of motion. Applications on simulated data and experimental data from the TOGA‐COARE show the capability of this new method to retrieve the pressure and temperature fields at various scales of motion, and in particular at mesoscale. This new analysis will be used intensively on Doppler‐radar data collected during the Fronts and Atlantic Storm Tracks Experiment in order to diagnose the multiscale interaction processes associated with frontal‐wave and/or secondary cyclogenesis.
The ECMWF implementation of three‐dimensional variational assimilation (3D‐Var). II: Structure functions
Authors: Rabier F. , McNally A. , Andersson E. , Courtier P. , Undén P. , Eyre J. , Hollingsworth A. , Bouttier F.
Published: February 2007

Structure functions for the 3D‐Var assimilation scheme of the European Centre for Medium‐Range Weather Forecasts are evaluated from statistics of the differences between two forecasts valid at the same time. Results compare satisfactorily with those reported in the existing literature. Non‐separability of the correlation functions is a pervasive feature. Accounting for non‐separability in 3D‐Var is necessary to reproduce geostrophic characteristics of the statistics, such as the increase of length‐scale with height for the horizontal correlation of the mass variable, sharper vertical correlations for wind than for mass and shorter horizontal length‐scales for temperature than for mass. In our non‐separable 3D‐Var, the vertical correlations vary with total wave‐number and the horizontal correlation functions vary with vertical level.
Stratospheric water vapour and tropical tropopause temperatures in Ecmwf analyses and multi‐year simulations
Authors: Simmons A. J. , Untch A. , Jakob C. , Kållberg P. , Undén P.
Published: February 2007

Stratospheric humidity analyses produced operationally by the European Centre for Medium‐Range Weather Forecasts (ECMWF) are discussed for the period since late January 1996 when the practice of resetting the upperlevel specific humidity to a fixed value at each analysis time was abandoned. Near‐tropopause analyses are in reasonable overall agreement with independent observations. Very low humidities occur in conjunction with deep convection and a particularly cold tropopause over the equatorial western Pacific during the northern winter. Drying occurs also in the cold core of the Antarctic polar‐night vortex. the lower stratosphere is moistened in the outer tropics and subtropics in summer and autumn, predominantly in the northern hemisphere. Changes associated with the latest occurrence of El Niño are illustrated.

Analysed temperatures near the tropical tropopause are generally in good agreement with corresponding radiosonde measurements, with standard‐level biases of the order of 0.5 degC or less. the past two years are the coldest by about 1 degC in a series of tropical mean 100 hPa analyses extending back to 1979. A cooling trend of about 0.6 degC per decade is seen in the global means of the 100 hPa analyses.

Moisture is spread zonally and upward from the tropical tropopause as the data assimilation proceeds, but the rate of upward transfer is much faster than observed. Substantial lateral mixing can occur within the stratosphere over the course of a season. Moistening at middle and high latitudes due to mixing with more humid tropospheric air is confined, realistically, to a shallow layer at the base of the stratosphere.

The rate of upward transfer of tropical stratospheric moisture is much more realistic in a multi‐year simulation using a version of the model that has finer stratospheric resolution than the version used for the operational data assimilation. Temperatures at the tropical tropopause and in the Antarctic polar night are accurately simulated, apart from excessive persistence of cold south‐polar temperatures in late winter and early spring. the latter is conducive to drying the model stratosphere; lack of a parametrization of moistening due to methane oxidation is an obvious deficiency in this regard.
The marine atmospheric boundary layer during semaphore. I: Mean vertical structure and non‐axisymmetry of turbulence
Authors: Lambert D. , Durand P.
Published: February 2007

This paper describes the vertical structure of the marine atmospheric boundary layer, analysed using aircraft measurements in the Azores region during SEMAPHORE (Structure des Echanges Mer‐Atmosphère, Propriétés des Hétérogénéités Océaniques: Recherche Expérimentale). the flights were performed in the middle of the solar day, during near‐anticyclonic conditions, with weak‐to‐moderate winds and weak surface sensible‐heat flux, over a homogeneous oceanic area. the boundary layer was characterized by a mixed layer, driven by surface fluxes, which was not coupled to the overlying, broken stratocumulus, layer. This decoupling is demonstrated by the shape of the continuous profile of the dissipation rate of turbulent kinetic energy, computed from the continuous slant profiles performed by the aircraft. This method makes it possible to determine the thickness of the mixed layer, which is a relevant scale for the parametrization of the profiles. the turbulence field is analysed through the wavelength of the spectrum peak of the vertical velocity, computed on 392 runs more or less parallel or perpendicular to the mean wind. It demonstrates a stretching of the most energetic eddies along the mean wind. the consequences of this non‐axisymmetric behaviour of the turbulent field on the flux estimates from aircraft data are discussed.