bluuebunny/crossref_metadata_2025 huggingface dataset:
Radar measurements of critical‐layer absorption in mountain waves
Authors: Worthington R. M. , Thomas L.
Published: February 2007

The conditions controlling the vertical propagation of mountain waves in the troposphere and lower stratosphere have been examined using radar observations at 46.5 MHz of vertical‐velocity perturbations at Aberystwyth (52.4°N, 4.1°W) and radiosonde observations from Aberporth, some 50 km to the south‐west. Attention has been paid to the influence of the variation with height of the mean wind vector (U), the Brunt–Väisälä frequency and the Scorer parameter. Four case‐studies show the absorption of mountain waves at heights where the mean wind component normal to the wave front vanishes; this corresponds to U = 0 if the wind direction does not change with increasing height, and to the wind vector being orthogonal to the wave vector if the wind rotates with increasing height. For two cases, enhanced turbulence is found in the upper few hundred metres of the mountain‐wave field, directly below the height at which the waves disappear; for a third case, the enhancement extends over a broader range of heights, and for the fourth no clear identification of the enhancement is possible. The frequency power‐spectra of the vertical‐velocity oscillations show a ωn dependence, with n approaching −5/3 for regions of mountain‐wave activity below the critical layers, and n = 0 for heights above the critical layers. Statistically, mountain waves generated by low‐level easterly winds are often confined to tropospheric heights. An analysis of a large sample of radiosonde data indicates that critical‐layer absorption associated with rotation of the background wind in addition to wave reflection and trapping, plays an important role in this confinement. The results are discussed in terms of the momentum flux associated with gravity waves, their turbulent breakdown near the critical layer, and the implications of the frequency spectra below and above such layers.
Southern hemisphere climate system recovery from ‘instantaneous’ sea‐ice removal
Authors: Wu Xingren , Simmonds Ian , Budd W. F.
Published: February 2007

A coupled atmosphere–sea‐ice model has been used to investigate the temporal response to instantaneous Antarctic sea‐ice removal under the present external climate conditions. In all cases the system was seen to be transitive, in that the sea ice eventually grew back to the present climate conditions. Here we examine the timescale and nature of the ice and atmospheric relaxation back to ‘control’ conditions. Several experiments were carried out, either supposing that all the sea ice was removed at certain times of year, or that it was removed only over certain large areas. The model in each case was run for six realizations for the equivalent of several years to a new equilibrium.

For the ‘whole sea‐ice removal’ experiment the maximum impact was found to occur when the removal was made at the time of sea‐ice maximum (September) and least reaction near the minimum. In both cases the return to normal occured by the middle of the following winter. The atmospheric responses comprised reduced surface pressure over the area from which ice had been removed, greatly increased turbulent fluxes, reduced difference between precipitation and evaporation (P — E) over the ocean and increased (P — E) over the Antarctic continent. The air temperature changes spread through the troposphere and tended to reduce the circumpolar westerlies north of and over the sea‐ice zone. The reaction to the regional sea‐ice removals was more local and depended on where the region was located relative to the mean circulation pattern around and over Antarctica.
A study of the behaviour of semi‐Lagrangian models in the presence of orography
Authors: Li Yong , Bates J. R.
Published: February 2007

Orography is incorporated into two global shallow‐water numerical models, the first (which is termed the (u, u) model) based on a semi‐Lagrangian vector discretization of the momentum equation, the second (which is termed the PV–D model) based on the semi‐Lagrangian advection of potential vorticity. Both models are two‐time‐level and use the finite‐difference approach.

Linear analyses of both models are performed for the idealized situation of stationary flow over orography on an f‐plane. In extension of the analysis of Rivest et al. the effects of spatial discretization are included. Spurious orographic resonance in the sense of infinite amplitude is eliminated by the damping effects of spatial interpolation, but spurious enhanced responses in the neighbourhood of the points at which resonance previously occurred still remain. The linear analyses indicate similar spurious enhanced responses for both models, which is confirmed by the numerical experiments.

The spurious orographic noise in both models can be eliminated by a sufficient amount of uncentring, though it is shown that uncentring of either first order or second order can have negative impacts on the large‐scale Rossby waves. The PV–D model formulation, however, allows the use of a scheme in which only the divergence and continuity equations are uncentred, which minimizes such negative impacts.
The mass flux across the tropopause: Quasi‐geostrophic theory
Authors: Juckes M. N.
Published: February 2007

The mean meridional circulation in the troposphere is driven by eddy momentum fluxes associated with baroclinic waves. the dynamics of these waves can be described qualitatively by quasi‐geostrophic theory. Here, the flow in the vicinity of the storm tracks is modelled using f‐plane quasi‐geostrophic theory with the tropopause represented as a nearly horizontal surface separating a troposphere of uniform potential vorticity from a similarly uniform stratosphere. Combining an analysis of the mean meridional mass‐flux associated with decaying eddies and the cross‐tropopause mass‐flux generated in the same process, it is possible to estimate the net flux both in the zonal mean and in a quasi‐Lagrangian mean framework. the quasi‐Lagrangian mean is here defined as an average along potential temperature contours on the tropopause. Consistent with earlier numerical and observational studies, it is found that the zonal mean mass‐flux is upwards poleward of the jet, whereas the quasi‐Lagrangian mean is downwards.
Entropy of a convecting water‐air system and the interpretation of cloud morphogenesis
Authors: Duane Gregory S. , Curry Judith A.
Published: February 2007

Cloud‐top entrainment in the stratocumulus‐to‐cumulus transition can be viewed as an entropy‐decreasing, self‐organizing process. an expression is derived for the entropy of a two‐phase convecting water‐air system which locates the entropy deficit in the region of the convection itself, generalizing a similar formulation of Dutton's for the one‐component case. This expression is applied to a buoyancy‐reversal model that simulates entrainment through convection driven by evaporative cooling. the overall entropy budget suggests that the evaporation/precipitation cycle is an essential ingredient of the stratocumulus‐to‐cumulus transition, underlying the difference between the behaviour of this system and that of the buoyancy‐reversal system. the cumulus cloud system is thus rendered as a dissipative structure. the class of dynamical mechanisms that can explain the stratocumulus‐to‐cumulus transition, together with the associated high entrainment rates, is restricted to those which depend on the water cycle. the relocalized entropy expression, in conjunction with the second law of thermodynamics, implies an upper bound on the possible extent of change in cloud form for given flux of water substance through the evaporation/precipitation cycle over the history of cloud morphogenesis.
Identification of cyclone‐track regimes in the North Atlantic
Authors: Blender R. , Fraedrich K. , Lunkeit F.
Published: February 2007

A Langrangian‐type climatology of North Atlantic cyclones is established based on the high‐resolution European Centre for Medium‐Range Weather Forecasts data‐set of the 1000 hPa height‐field. First, an algorithm is introduced to identify mid‐latitude cyclones and cyclone paths with as few constraints as possible. Cluster analysis of relative cyclone displacements yields three types of cyclone tracks characterizing stationary‐, north‐eastward‐and zonally‐travelling storms. the internal Lagrangian statistics of these cyclone‐track types reveal representative life‐cycles for central pressure and geopotential‐height gradients and a power‐law scaling behaviour of cyclone displacements. Finally, a basic climatology of North Atlantic cyclone‐track regimes is deduced in terms of a time‐series and circulation statistics.
Some limits to the variable‐mesh solution and comparison with the nested‐lam solution
Authors: Caian Mihaela , Geleyn Jean‐François
Published: February 2007

The solution at high resolution of a global spectral model with a conformally‐variable mesh, obtained through a stretched coordinate, is compared as fairly as possible to the ‘classical’ solution of a limited‐area nested model. We compare the global, variable‐mesh, ARPEGE model at hyper‐stretched values (which is integrated operationally at Météo‐France with a ‘low’ value of stretching) with its limited‐area version, the ALADIN model. Having the same physics, the same grid‐point dynamics and a high code‐compatibility, they allow clean comparisons on real situations. to conclude on their efficiency and accuracy, comparisons are made choosing configurations of the two models having either the same resolution or the same computational price.

The limits of the variable‐mesh strategy on the sphere are also investigated through a set of experiments with increasing stretching. We found that there is indeed a limit to the stretching factor in the interval [7, 9]: a stretching factor c = 7 keeps an accurate spectral solution and gives satisfying results, even when compared with the limited‐area model at the same computational price. We show that the value c = 10.5 cannot be reached without paying the price of a spectral deformation of the fields which becomes too obvious. Additional tests showed the occurrence of this deformation, even if less pronounced, at the stretching value c = 8.75. At the same resolution ALADIN does not manifest any similar weakness.

Although the solution remains stable for at least 72 hours, its accuracy when increasing the stretching above the critical value becomes questionable. This problem seems to be due to a particular behaviour of the truncation error, which does not decrease with increasing resolution through stretching, thus leading to an energy accumulation in the tail of the spectra.

As a conclusion of this study, it follows that one can extend the idea of Courtier and Geleyn, currently used in operations at Météo‐France for c = 3.5, to ‘medium’ stretched configurations up to c = 7 at least (at the current computer's power). Above a value found to be about c = 9, our results show that increasing the resolution does not produce any improvement and, hence, that the use of a limited‐area model appears a better choice for very high resolutions.
Dynamical modelling of wintertime lidar observations in the arctic : Ozone laminae and ozone depletion
Authors: Orsolini Y. J. , Hansen G. , Hoppe U.‐P. , Manney G. L. , Fricke K. H.
Published: February 2007

A series of lower stratospheric ozone profiles taken by a lidar during February 1995 in northern Norway are examined. the instrument location allowed continuous monitoring of rapidly evolving layered ozone structures, or laminae, near the polar vortex edge.

Observations under the vortex edge revealed laminae extending up to 20 km. Moreover, when the lidar was sampling vortex air, a thicker layer, characterized by unusually low mixing ratios, was also observed, thereby confirming satellite observations of Arctic ozone depletion during the winter 1994/95.

To unravel the respective role of dynamics and chemistry in generating these ozone‐depleted layers requires better understanding of trace constituent lamination and filamentation through numerical modelling.

High‐resolution modelled realistic transport of idealized tracers and satellite‐derived ozone on many isentropes is therefore used to reconstruct fine‐scale three‐dimensional tracer fields. Tilted tracer or ozone sheets, peeled off near the vortex edge, lead to the formation of laminae in profiles. the model shows remarkable success in reproducing laminae intensification, thickening and lofting, analogous to the ones observed by the lidar over the course of a few hours.
Broadening of convective cells
Authors: Dörnbrack Andreas
Published: February 2007

The turbulent convection in a dry Boussinesq fluid above a heated surface is studied in a domain of large aspect ratio and long duration. Large‐eddy simulations are used to investigate the influence of the dynamical character of the lower boundary and the effect of cooling at the top on the growth of convective cells. the aim is to identify physical mechanisms that lead to the broadening of convective cells even under conditions where moisture processes play no role. Cooling at the top produces a vertically uniform heat‐flux, and it acts to enhance the total kinetic energy of the whole flow. Essentially, there are two competitive effects: first, the prescribed heat‐flux at the upper (lower) boundary produces very warm (very cool) fluid that is forced to rise (sink). the resulting large temperaturefluctuations are transported by large‐scale motions and cause broad temperature‐variances independent of height. On the other hand, the strong turbulent mixing (mainly near the boundaries where the turbulent kinetic energy is maximum) tries to homogenize the flow structure in such a way that the resulting temperature‐distribution is uniform. Except directly close to the walls, horizontal and vertical temperature‐gradients are reduced. Additionally, a trend to form an organized large‐scale horizontal drift in one direction near the bottom and in the opposite direction near the top was found for runs without surface friction. This horizontal streaming motion has two effects: firstly, the turbulent mixing is enhanced due to larger shear and, secondly, separated thermals approach faster and are able to merge more easily, forming gradually growing cells. an adiabatic upper boundary condition leads to a heat‐flux profile that is linear and decreasing with height, and to a moderate temporal growth of thermal structures. A considerable scale‐reduction of the temperature structures occurs because of friction at the lower surface.
Short‐wave optical properties of precipitating water clouds
Authors: Savijärvi Hannu , Arola Antti , Räisänen Petri
Published: February 2007

Water cloud drop‐size distributions for non‐precipitating clouds are taken from various sources. the optical depth, single‐scattering co‐albedo and asymmetry parameter are calculated using Mie theory. By averaging over four spectral intervals in the short‐wave regime, a linear parametrization for the co‐albedo and the asymmetry parameter is made as a function of the cloud‐layer effective drop radius, re, in each interval; re can be related to the cloud‐layer liquid‐water content (LWC). the optical depth is parametrized quadratically on re−1. This gives an excellent fit. These parametrizations are useful for general‐circulation model (GCM) and weather‐forecast models, which produce estimates of cloud‐layer LWC.

Large‐drop effects are studied by adding typical raindrop size distributions to the stratusand cumulonimbus‐type cloud‐drop distributions. the re of these precipitating clouds can be related to the rain intensity, R. It turns out that (1) the asymmetry parameter is insensitive to R, (2) the optical depth follows the quadratic parametrization on re−1, and (3) the co‐albedo increases nearly linearly with re (or R), so absorption of a cloud layer increases with R. A cloud layer with large drops can thus absorb more solar radiation than one without large drops. A simple four‐band GCM‐type parametrization of the large‐drop effect is presented.
The Berlin troposphere‐stratosphere‐mesosphere GCM: Climatology and forcing mechanisms
Authors: Langematz Ulrike , Pawson Steven
Published: February 2007

The climatological structure of the Berlin troposphere‐stratosphere‐mesosphere general circulation model is presented and compared with observations. the zonal‐mean zonal wind and temperature display some successes for the simulation in that all of the climatological features are captured, but there are some systematic errors regarding their location and strength. In particular, the location and structure of the polar‐night jet in the northern winter is incorrect: it is located too far polewards at high levels and is too weak in the middle and lower stratosphere. On the positive side, there is a separation between the polar‐night jet and the subtropical jet. the temperature in the winter polar upper troposphere and lower stratosphere is too high, presumably arising from the treatment of radiative processes. Consistent with hydrostatic balance, the 30 hPa (and other) isobaric surface lies too high. the polar vortex is both too asymmetric and too weak. This leads to an underestimate of the modelled zonal velocity, which is consistent with the poleward (rather than equatorward) propagation of the stationary planetary waves, in the lower and middle stratosphere. In the upper stratosphere the poleward heat and momentum fluxes due to these waves are too strong. Despite these systematic errors, the seasonal evolution of both hemispheres is quite well simulated: the large variability and the strong, episodic forcing of the northern winter contrasts well with the relatively stable southern winter and the breakdown of the southern polar vortex in springtime. the magnitude of the variability in the northern hemisphere is too small in the model and it is proportionally too large in the early winter.
Stability of equatorially symmetric and asymmetric climates under annual solar forcing
Authors: Xie Shang‐Ping
Published: February 2007

A simple coupled ocean‐atmosphere model is developed to investigate the maintenance of the latitudinal asymmetries of the sea surface temperature (SST) field and the intertropical convergence zone (ITCZ). the model is of the minimum degree of freedom necessary to describe latitudinal asymmetries, computing only the difference between the hemispherical SST maxima. A wind‐evaporation‐SST feedback is found to destabilize the equatorial symmetry established by the annual‐mean solar radiation. By considering the nonlinear SST threshold for deep convection, equatorially asymmetric steady states are reached, with a single ITCZ off the equator.

The annual component of solar radiation is anti‐symmetric about the equator, acting to force convection in the summer hemisphere independent of symmetry‐breaking ocean‐atmosphere interactions. A multiple time‐scales analysis is applied to separate the annual cycle and the slowly evolving annual‐mean climatology. It is found that the stability of an annual‐mean state varies with annual forcing amplitude. As forcing amplitude varies, the model displays three flow regimes: (1) an asymmetric regime for small seasonal forcing, featuring a permanent ITCZ in one hemisphere and a seasonal one in the other; (2) a mixed regime for intermediate forcing, with both symmetric and asymmetric solutions being stable; and (3) a symmetric regime for large annual forcing where the ITCZ migrates between the hemispheres following the seasonal march of the sun.
Modelling of stratospheric intrusions within a mid‐latitude synoptic‐scale disturbance
Authors: Cox B. D. , Bithell M. , Gray L. J.
Published: February 2007

The results of a run of the UK Universities' Global Atmospheric Modelling Programme (UGAMP) General Circulation Model (GCM) is presented, initialized with European Centre for Medium‐Range Weather Forecasts data for 6 October 1990. the dates correspond with observations of a trough made during a Transport of Ozone and Stratosphere/Troposphere Exchange campaign. This anticyclonic wrapping system is compared with a cyclonic wrapping system studied in a previous paper. the fate of stratospheric air is followed using potential vorticity (PV) and specific humidity. the run gives good agreement with radiosonde data for the development of the trough, which involves a tropopause fold and a ‘streamer’, both with regard to the position and evolution of a layer of dry air. Analysis of the height of the tropopause shows that PV values (in PV units) in the range 1‐5 agree well with the thermal tropopause outside the trough, but in order to capture the layer of dry air seen in the radiosonde data, a value of PV of order 1 is required within the trough. Use of PV values in the range 3‐4 may therefore not follow the evolution of dry stratospheric air entering the troposphere, even though such values may on average follow the thermal tropopause. Most of the dry region is visible as a dark band in the Meteosat data, although the lowest levels of dry air (at approx. 800 mb) are too low to be seen by the satellite. Thus Meteosat observations may not capture all processes involved in stratosphere‐troposphere exchange, and the images must be interpreted with caution when using them to follow the movement of dry stratospheric air.
Sensitivity of ozone and temperature to vertical resolution in a gcm with coupled stratospheric chemistry
Authors: Austin John , Butchart Neal , Swinbank Richard
Published: February 2007

Results are presented from a general‐circulation model with comprehensive stratospheric photochemistry and which includes the coupling between radiative heating and simulated ozone. Each model integration covers the 60‐day period beginning 15 January during which episodes of polar stratospheric clouds (PSCs) are normally observed in the Arctic. Results from two versions of the model with different numbers of atmospheric levels near and above the tropopause are compared with observations. In the 19‐level model the ozone transport is poorly simulated and, in particular, there is a significant increase in the tropospheric column. In contrast, in the 49‐level model the simulated ozone distribution is in good general agreement with observations and reproduces well the steep vertical gradients in ozone mixing ratios in the tropical lower stratosphere, and the weak vertical gradients in the high‐latitude middle stratosphere. This version also maintains a virtually constant tropospheric ozone column. Since in the 49‐level model the ozone distribution is well simulated, including this ozone in the radiation calculation has only a moderate influence on the results. However, with the 19‐level model, it substantially increases the global stratospheric temperature error. Increasing the number of levels improves the simulation of stratospheric temperatures but the zonal‐mean temperatures in both versions of the model are generally lower than observed. Despite this, the modelled frequency of occurrences of PSCs is less than observed because of the underprediction of the zonally asymmetric component of the temperature distribution. the results suggest that if coupled chemistry‐climate simulations are to proceed, it is important to have both a high upper boundary and good vertical resolution in the lower stratosphere to ensure a realistic meridional circulation and a more accurate representation of ozone transport across the tropopause.
Radar observations of wind‐shear splitting within evolving atmospheric kelvin‐helmholtz billows
Authors: Chapman D. , Browning K. A.
Published: February 2007

The high‐resolution radar at Chilbolton was used to measure the velocity structure of large‐amplitude Kelvin‐Helmholtz billows within a precipitating warm‐frontal zone. A deep layer of frontal shear was observed to split into thin layers of more concentrated shear displaying the characteristic braided structure previously reported within clear‐air radar reflectivity patterns and attributed to a splitting of the thermal stratification. the billows were situated within a region of strong wind veer with height and they were orientated at right angles to the mean flow such that air parcels within them were tending to follow helical trajectories up the inclined frontal zone. Radar scans normal to the axes of the billows at different distances from their origin, close to the position of the surface warm front, provided an indication of the evolution of the split‐shear structure.
The ‘1/3’ power law between effective radius and liquid‐water content
Authors: Liu Yangang , Hallett John
Published: February 2007

Radiative properties of water clouds are parametrized in terms of the effective radius, liquid‐water content and total droplet concentration. A new general ‘1/3’ power law between effective radius and the ratio of liquid‐water content to total droplet concentration is formulated theoretically. The prefactor in the ‘1/3’ power law depends upon, and relates, cloud droplet size distributions to physical mechanisms such as entrainment and mixing. It shows that the ‘1/3’ power law holds in general for non‐precipitating water clouds with any entrainment and mixing, including clouds with monodispersed cloud droplet size distributions as special cases. The formulation builds upon a system theory that is based on Shannon's maximum entropy principle. The theoretical ‘1/3’ power law is further demonstrated using observational data. The existence of such a ‘1/3’ power law indirectly justifies the system theory generalized in this paper.
Evaporation and radiation interception by neighbouring plants
Authors: Wallace J. S.
Published: February 2007

This paper presents the ERIN model, for estimating evaporation and radiation interception by neighbouring species of plants. the model contains a comparatively simple theoretical framework which couples transpiration and the interception of light in multi‐species mixtures, such as those which occur in many natural and man‐made plant communities including grasslands, savannahs, crop/weed mixtures and the results of intercropping and agroforestry. the light‐interception sub‐model can estimate the individual amounts of radiation intercepted by the components of a multi‐species mixture based on a knowledge of the canopy height, leaf area and extinction coefficient of each constituent. In homogeneous plant mixtures, the light sub‐model in ERIN compares well with other more complex models of light interception. A simple method for dealing with spatially heterogeneous canopies is also included, based on a knowledge of the canopy cover and the clumping of the foliage of each species within the canopy. the transpiration sub‐model in ERIN allows the evaporation from each component of a multi‐species mixture to be calculated. It also has the advantage that it allows for microclimatic interactions between the different canopies and that it is coupled to the competition for light. the way in which the fluxes of heat and water vapour from one canopy can affect the microclimate and transpiration from another canopy is also illustrated.
The effects of different radiation parametrizations on cloud evolution
Authors: Loughlin P. E. , Trautmann T. , Bott A. , Panhans W. G. , Zdunkowski W.
Published: February 2007

With the use of the microphysical stratus model (MISTRA), investigations into the effects of variations in the radiative‐transfer parametrizations on cloud development have been undertaken. Two radiative‐transfer schemes were coupled with the microphysical‐thermodynamical section of MISTRA, one based on the exponential sumfitting method and the other based on the correlated k‐distribution method of determining gaseous absorption properties. Model runs were initiated with parameter values in accordance with measurements made over the North Sea and comparisons made between model runs where only the radiation schemes were altered. Results indicated that differences between the two schemes had a significant effect on cloud evolution. Alterations were then made to the correlated k‐distribution method in an attempt to match the results from the original radiation method and so determine why there were differences between the two sets of results.

The results showed that infrared scattering played an important role in cloud development as it extended the infrared cooling rates into the cloud top when compared with a case with no infrared scattering, and therefore helped to offset solar heating during daytime. Exclusion of infrared scattering produced a large increase in total cloud‐top cooling rates, almost 100%, but produced only minimal changes in other bulk cloud properties of typically 3%, partially due to the net increase in heating immediately below cloud top. the inclusion of e‐type water‐vapour absorption was also investigated. Results showed that by reducing the wavelength range where this component was included from 8.0‐35.7 μm to 8.0‐12.5 μm produced only a small change, 3%, in the downward infrared fluxes above the cloud. However, this was matched by larger changes in bulk cloud properties of typically 7%. This indicates that cloud development is very sensitive to changes in the downward infrared radiation field above the cloud. Most importantly, the changes in the radiation fields reported are smaller than the values given as experimental errors and what could be considered reasonable theoretical uncertainty.
Improvement in stratosphere simulation with a hybrid σ‐ϑ coordinate GCM
Authors: Zhu Z. , Schneider E. K.
Published: February 2007

This study demonstrates the practical use of a smoothly varying hybrid sigma‐isentropic (σ‐ϑ) coordinate in a general‐circulation model (GCM). Changes have been made to the vertical scheme developed in previous work in order to overcome a numerical problem associated with the crossing of levels. the hybrid coordinate, which changes gradually from a sigma coordinate at the bottom to a nearly isentropic one in the stratosphere, was implemented into the GCM. Multi‐year integrations were successfully conducted with the σ‐ϑ coordinate model.

Improvements in the simulated stratospheric and upper‐tropospheric circulation are found compared with a control run using the original sigma‐pressure coordinate GCM. the greatest improvements are significant reductions of the cold biases in the winter polar stratosphere and of the wind speed of the polar‐night jet. These changes are shown to be related to a significant increase of the poleward eddy heat flux in the middle‐latitude stratosphere. Diagnostics show the wave kinetic energy, stationary‐wave phase structure, and EP flux are changed by using the nearly isentropic coordinate in the stratosphere. Possible reasons for the improvements of the stratospheric simulation are discussed, including the more accurate representation of the advection processes, and the effect of the isentropic model top.
On the concept of exergy and available potential energy
Authors: Kucharski Fred
Published: February 2007

A systematic derivation of the concepts of available potential energy of Lorenz and Van Mieghem from an extended concept of exergy is performed. This procedure leads to an extension of Van Mieghem's global theory to a local formulation which can be used in limited‐area models. In the limit of small amplitude motion it is shown that the extended exergy reduces to the available potential energy of Boussinesq‐approximated models. Moreover, Shepherd's concept of pseudoenergy is extended in order to deal with diabatic and irreversible processes.
Superposing semi‐geostrophic potential‐vorticity anomalies
Authors: Birkett H. R. , Thorpe A. J.
Published: February 2007

A numerical method for inversion of the semi‐geostrophic potential‐vorticity equation is described. It is used to examine the validity of a strictly linear superposition principle when applied to this high‐order balance model. These results are important when aiming to extend rigorously the value of recent ‘attribution’ ideas. Experiments conclude that the nonlinear components of the inversion have only a very short‐range effect, with a magnitude which increases with the magnitude of the potential‐vorticity anomalies.

For balance of higher‐order than quasi‐geostrophic these results show that, whilst flow and temperature of an anomaly are nonlinear functions of potential‐vorticity magnitude, spatially separated anomalies are quasi‐linearly superposable.
Effects of entrainment and droplet sedimentation on the microphysical structure of status and stratocumulus clouds
Authors: Considine Geoffrey , Curry Judith A.
Published: February 2007

A new cloud model has been developed based upon a reconsideration of cloud‐top entrainment mechanisms. A finite‐thickness entrainment layer at cloud top allows the cloudy air to lose liquid water to dry entrained air by sedimentation. The model use a steady‐state ensemble of one‐dimensional updraughts and downdraughts, combined with a simplified traetment of droplet sedimentation, autoconversion, and collection of cloud drops by drizzle, to examine vertical profiles of cloud liquid‐water content and droplet concentration. The model is demonstrated both for idealized cases and in comparison with a nocturnal stratocumulus case from the ASTEX. The sedimentation flux divergence in the entrainment layer at cloud top is shown to have a large impact on the cloud microphysics.
Microphysical and short‐wave radiative structure of stratocumulus clouds over the Southern Ocean: Summer results and seasonal differences
Authors: Boers R. , Jensen J. B. , Krummel P. B.
Published: February 2007

Six case‐studies are presented of stratocumulus clouds observed during the summer phase of the Southern Ocean Cloud EXperiment (SOCEX). The experiment was conceived to investigate the microphysical properties of clouds in the unpolluted background region over the Southern Ocean. The case‐studies show evidence of decoupled stratocumulus layers, i.e. situations where cumulus clouds were covered by a uniform sheet of stratocumulus cloud. Optical depths of the clouds ranged from 2 to 23. Cloud droplet concentrations varied from 50 to 180cm−3, and mostly decreased with altitude, indicating the importance of droplet coalescence in reducing the number of large cloud droplets. Drizzle was observed on four of the six flights.

A comparison of the summer results with those published from the winter phase of SOCEX indicates that the droplet concentration in summer was three times that of the concentration in winter. The differences between summer and winter are so large that errors in sampling and measurement uncertainties hardly influence the results. Because these results were obtained under conditions where the air parcels had traversed long distances over the unpolluted Southern Ocean, it suggests a natural cause, the most likely being oxidation products of oceanic dimethylsulphide acting as cloud condensation nuclei.

Model results show that the natural variation in droplet number concentration is responsible for a natural variation in cloud albedo of around 30% at a constant solar zenith angle. This may indicate that there is a natural negative albedo feedback counteracting the enhanced greenhouse effect. The feedback links increased emission of dimethylsulphide at higher oceanic temperatures to an increase in cloud condensation nuclei, and thus to an increase in cloud albedo.
Potential vorticity inversion of a two‐dimensional steady flow: Application to symmetric instability
Authors: Arbogast Philippe , Joly Alain
Published: February 2007

The potential vorticity inversion concept is extended to obtain two‐dimensional symmetrically unstable basic states. The goal is to invert a lens‐like patch of negative potential vorticity embedded within a large area of uniform positive potential vorticity to create the basic state. In order to circumvent the non‐ellipticity in areas of negative potential vorticity, a variational formulation of the invertibility principle is proposed. The functional incorporates the thermal wind balance as a constraint between the mass and the wind field. This condition is necessary and sufficient to define a steady solution to the inversion problem that is suitable to serve as a basic state. The uniqueness of this solution, however, is not addressed. Inversions with different potential vorticity distributions are performed, enabling us to highlight the dependence of the behaviour of the symmetric rolls (initial linear growth rate, wavelength) on the shape of the negative potential vorticity area (aspect ratio, amplitude and surface). Particular attention is paid to defining the domain of parameters (frontal environment and anomalies) that enables symmetric instability in a convectively‐ and inertially‐stable flow. Results from the stability analysis for embedded zones are compared to those obtained for uniform potential vorticity in the linear regime. Results are generally confirmed and new ones are found. It is shown, for example, that the linear growth rate increases when the unstable surface decreases. The inversion technique is finally extended to the moist (saturated) case. The variational approach is, therefore, shown to be able to handle areas of negative potential vorticity, opening new areas for a more systematic use of the concept of inversion in synoptic and dynamic meteorology.
Skill assessment for ENSO using ensemble prediction
Authors: Moore A. M. , Kleeman R.
Published: February 2007

A crucial component of any prediction system is the ability to estimate the predictive skill of a forecast so that the degree of confidence that can be placed in an individual forecast can be assessed. In this paper we have used ensemble prediction techniques to develop a means of estimating a priori the predictive skill of forecasts of El Niñ Southern Oscillation (ENSO) using an intermediate coupled ocean–atmosphere model. Each member of an ensemble forecast is perturbed using either noise forcing fields or perturbations that are known to increase the low‐frequency variability of the coupled model. These are respectively the so‐called stochastic optimals and optimal perturbations of the coupled system; they are added to the model to mimic the presence of initial‐condition errors and high‐frequency stochastic noise and their effect on the predictability of the coupled system in the tropics.

By performing ensemble predictions in hindcast mode, we have identified a usable relation between the skill of a model hindcast and the spread of the ensemble measured relative to some control hindcast. The practical nature of ensemble prediction is demonstrated by computing the relationship between model skill and the spread of an ensemble from hindcasts of ENSO for the period 1972–86, and then comparing the actual hindcast skills for the period 1987–93 with those suggested by the skill‐spread relation from the preceding period. The relationship that we find between the model skill and spread of an ensemble appears to be robust in the sense that it is relatively insensitive to variations in the ensemble prediction procedure and to changes in some model parameters. However, crucial to the success of the ensemble predictions is the use of perturbations in the ensembles that are known to increase the low‐frequency variability of the coupled model. These perturbations can efficiently probe the probability density function of possible coupled‐model states. If randomly chosen perturbations are used in the ensembles, however, no practical relation between model skill and the spread of an ensemble emerges.
The oceanic mesoscale convective system observed with airborne Doppler radars on 9 February 1993 during TOGA–COARE: Structure, evolution and budgets
Authors: Roux Frank
Published: February 2007

Data, collected on 9 February 1993 during the Tropical Ocean and Global Atmosphere Programme—Coupled Ocean‐Atmosphere Response Experiment (TOGA‐COARE) with the Doppler radars on board the two National Oceanic and Atmospheric Administration WP‐3D and the National Center for Atmospheric Research Electra aircraft, are analysed to study the structure and evolution of an equatorial oceanic mesoscale convective system (MCS). Observations lasted more than five hours, a period during which the cloud‐cluster characteristics changed from convective to stratiform. This mesoscale system was approximately aligned along the tropospheric wind shear, but it was nearly perpendicular to the low‐level shear.

Series of three‐dimensional fields of reflectivity, air‐velocity components, retrieved pressure and temperature perturbations show similarities with previously observed convective systems over the tropical oceans. In particular, although the cloud top reached 18 km altitude, relatively weak vertical motions and temperature perturbations resulted from a moderate convective instability. In the low levels, a cold front‐to‐rear flow intensified and deepened as the overall structure evolved from convective to stratiform. The vertical profiles of mean horizontal divergence evolved from low‐level convergence in the cross‐line direction for the earliest analyses to mid‐level convergence from all directions during the later stratiform phase. Vertical momentum fluxes were important and led to an increase in the cross‐line wind shear and a decrease in the along‐line one.

Analysis of radiosounding data from the nearest stations of the TOGA‐COARE array shows that the period during which airborne Doppler observations were conducted was characterized by relatively strong large‐scale ascent and ‘apparent’ sources of heat, moisture and momentum. These ‘diagnosed’ values and the mean radarderived ones agree fairly well. This shows that the early convective and late stratiform phases of the MCS are equally important for the ‘apparent’ sources of heat and momentum.
The effective roughness length for scalar transfer in neutral conditions over hilly terrain
Authors: Hewer F. E. , Wood N.
Published: February 2007

Numerical weather prediction models are required to represent the area‐averaged effects of hills on temperature and water vapour in the atmospheric boundary layer. The transfer of these scalars within the boundary layer is affected by bouyancy. However, we have simplified the problem by simulating the behaviour of a passive scalar in a neutrally stratified turbulent boundary layer over hills. The numerical model used for these simulations was fully nonlinear and flows were simulated over a range of slopes, including ones where separation occurred.

The area‐averaged effects of hills are small over low slopes but significant at steeper slopes due to the impact of the separated region. The area‐averaged scalar concentration varies logarithmically with height, but in a region that is shallower than the logarithmic wind region. We have found that, using an effective roughness length for scalar transfer, ZOSeff, the effects of hills on area‐averaged scalars may be parametrized. ZOSeff is smaller than the local value of roughness length for scalar transfer, and for hills of certain wavelengths and roughness it is three orders of magnitude smaller. A simplified interpretation of the physical processes simulated by the numerical model has been used to derive an expression for ZOSeff in terms of hill characteristics only. This parametrization should be useful for representing the influence of hills on scalar transfer in numerical weather prediction and climate models.
The retrieval of cirrus cloud properties from aircraft multi‐spectral reflectance measurements during EUCREX'93
Authors: Francis P. N. , Hignett P. , Macke A.
Published: February 2007

We present details of a scheme for retrieving cirrus cloud optical thickness and effective particle size from nadir‐viewing reflectance measurements. Two near‐infrared wavelengths are used, one with negligible ice absorption (1.04 μm) and one with significant ice absorption (1.55 μm). Four ice crystal shapes are used in the scheme: ice spheres, hexagonal columns, hexagonal plates and randomized polycrystals. We highlight the sensitivity of the retrievals to the shape of the phase function, which follows from the particular ice crystal shape assumed.

Five aircraft‐based retrievals are presented, all cases occurring during the 1993 intensive field campaign of the European Cloud Radiation Experiment (EUCREX'93). The retrieved effective sizes are compared with the in situ measurements taken from the same cases. In these comparisons, account has to be taken of the fact that (a) in situ replicator data indicate that the presence of small (<100 μm) ice crystals can have a significant effect on the effective crystal size, and (b) the measured crystal sizes consistently show a systematic decrease with height in all cases, the effective sizes at cloud base being typically two or three times larger than at cloud top. Taking these considerations into account, we conclude that the assumption of polycrystals gives the most consistent agreement with the in situ measurements.
Assessment of a gravity‐wave drag parametrization with PYREX data
Authors: Beau Isabelle , Bougeault Philippe
Published: February 2007

An original methodology is developed to assess the behaviour of gravity‐wave drag parametrizations. It relies on the computation of differences between numerical simulations at various horizontal resolutions. The terms to parametrize are extracted from comparisons between the full momentum budgets of the high‐resolution and low‐resolution simulations. The parametrization under assessment is firstly calibrated off‐line to reproduce at best the residual vertical momentum flux, then it is introduced in the numerical model for an on‐line verification simulation. The methodology is applied to the ECMWF parametrization (in its 1990 version), using four meteorological situations well documented during the PYREX field experiment. The results show that the classical formulation, limiting the effect of orographic waves to the vertical divergence of the momentum flux, is well founded. However, it is shown that the optimal coefficients of the tested scheme are in general much larger than the value recommended at the time, and quite variable from one case to another. The variability of the low‐level blocking effect with the Froude number of the incident flow is suggested as a possible explanation.
Four‐dimensional variational assimilation of SSM/I precipitable water content data
Authors: Filiberti M.‐A. , Eymard L. , Rabier F. , Courtier P. , Thépaut J. N.
Published: February 2007

Satellite microwave radiometers provide measurements of precipitable water content (PWC) over the oceans, with a horizontal resolution of a few tens of kilometres. These data represent the water vapour content integrated over the atmospheric column. We assess the value of this source of information for numerical weather prediction systems. We used the European Centre for Medium‐Range Weather Forecasts Integrated Forecasting System with a four‐dimensional variational method to assimilate the Special Sensor Microwave/Imager (SSM/I) PWC data over a 24‐hour period. Our experiments use an incremental variational method. Comparison with observations is made using a multi‐level global primitive‐equation T106 spectral model with physical parametrizations. Minimization is performed using a T63 adiabatic dynamics model which includes only simplified physics (horizontal diffusion, a simple surface drag and a vertical diffusion scheme).

Comparing the control and assimilation experiments with aircraft and other data shows that the use of PWC data from SSM/I improves the analysis. We also obtain a slight improvement in short‐range forecasts of almost all parameters when SSM/I‐PWC data are used in the assimilation.