bluuebunny/crossref_metadata_2025 huggingface dataset:
An idealized study of African easterly waves. I: A linear view
Authors: Thorncroft C. D. , Hoskins B. J.
Published: February 2007

The linear instability problem of the African easterly jet has been investigated using the primitive equations on a sphere. This has included an examination of the linear growth mechanisms and structure using diagnostics traditionally employed for mid latitudes, such as Eliassen‐Palm (EP) fluxes and potential vorticity. It has been shown that a growing normal mode on an African easterly jet is characterized by divergent EP fluxes in the region of the jet, implying both barotropic and baroclinic energy conversions. The linear instability is dominated by the interaction between positive and negative potential vorticity gradients at the level of the jet and, as found in previous studies, the normal modes grow mainly through barotropic energy conversions. Many of the synoptic features associated with the normal modes are in good agreement with those observed, except for the vertical‐velocity pattern, which has a ‘checkerboard’ structure in the vertical and is much weaker.

Changing the jet latitude while keeping the baroclinicity constant changed the growth rates very little, and although barotropic energy conversions remained dominant, the most unstable modes became more baroclinic when the jet was more poleward. The most unstable modes, which grow on a thinner jet, have a larger growth rate, a smaller wavelength and stronger barotropic energy conversions. The ‘checkerboard’ pattern in the vertical velocity persists with the normal modes which grow on these jets.

The effect of including diabatic effects in the linear problem has also been examined. First, a simple boundary‐layer scheme was found to have very little effect on the normal modes. With simply parametrized latent heat release however, the growth rates of the most unstable modes were increased slightly and the modes became less dominated by barotropic energy conversions. Also, an asymmetry is found between the ascent and descent regions in the wave, with the length scale of the updraught contracted relative to that of the downdraught. The unrealistic ‘checkerboard’ pattern in the vertical velocity is almost removed and the amplitude is increased. The structure of the normal mode, using a CISK‐type scheme for the latent heating, has more in common with the observed structures over west Africa than the structure of the dry modes.

It is suggested that African easterly waves may arise through a mixed barotropic/baroclinic instability mechanism where the role of latent heating is important in increasing the baroclinic energy conversions relative to the barotropic energy conversions, and also in determining the synoptic structure.
An idealized study of African easterly waves. II: A nonlinear view
Authors: Thorncroft C. D. , Hoskins B. J.
Published: February 2007

The nonlinear behaviour of an easterly‐wave normal mode has been examined. Significant differences from normal‐mode character become apparent after saturation of the instability. The easterly wave is initially dominated by barotropic energy conversions (CK) but later grows mainly through baroclinic energy conversions (CE). The eddy kinetic energy of the wave peaks and then decays with both CK and CE contributing to this decay. The energy conversions associated with the nonlinear growth of the wave have more in common with those found in the GATE study over west Africa and GCM integrations than do the previous linear studies of African easterly waves.

The life cycle is described using EP flux diagnostics and Rossby wave propagation ideas. After the linear growth period, characterized by divergent, mostly horizontal, EP flux vectors and barotropic growth, the life cycle becomes dominated mostly by vertical EP fluxes associated with vertical wave propagation from the jet region both above and below the wave, and by baroclinic growth. This results in increased wave amplitudes in the upper troposphere and at low levels. This mechanism for increased amplitudes at upper levels may help to explain the observed winds in the waves at these levels—a feature not found in linear studies.

The effect of including simple parametrizations of different diabatic processes is also examined. The main effect of including boundary‐layer fluxes is merely to damp the large surface amplitudes which occur in the nonlinear part of the life cycle. Including a restoration of the basic‐state results in stronger growth and larger amplitudes at all levels.

Two life cycles with different latent heating parametrizations are also examined. One has a CISK scheme driven by low‐level ascent, and the other a scheme based on the conservation of a prescribed value of the equivalent potential temperature. In each case CE becomes comparable to CK but does not dominate as in the basic life cycle. This is due to significant changes in the synoptic structure of the wave when latent heating is included: in particular the vertical velocity field changes dramatically. Indeed, with the CISK scheme it compares well with GATE observations, with a deep ascent region present throughout the troposphere ahead of the main cyclonic region. The maximum is also larger than in the dry case, although still somewhat less than observed.

Two life cycles were also examined with both CISK and boundary‐layer fluxes included; the second also including zonal flow restoration. In the case of both life cycles CE dominates over CK in the nonlinear part of the life cycle, unlike the case with CISK alone. The structures found are similar to those with CISK alone except that the surface amplitude is considerably weakened by the boundary layer. However, the magnitude of the ascent is increased with the boundary layer and increased further with the restoration, making it comparable with observed values.
Storm tracks in a high‐resolution GCM with doubled carbon dioxide
Authors: Hall Nicholas M. J. , Hoskins Brian J. , Valdes Paul J. , Senior Catherine A.
Published: February 2007

Transient eddy activity has been diagnosed for the winter season of two ten‐year equilibrium GCM integrations carried out at the UK Meteorological Office, one with present‐day levels of carbon dioxide concentration (control) and one with doubled carbon dioxide (2CO2). On synoptic timescales, indicators of storm‐track activity such as eddy kinetic energy are shifted northwards and intensified downstream in the 2CO2 experiment relative to the control. This effect is particularly marked for the Atlantic storm track. These patterns correspond closely to changes in relevant diagnostics of the time mean, such as the Eady growth rate and the diabatic heating. In the latter, the downstream intensification is effected by a decrease in the sensible heating and an increase in the latent heating in a warmer, moister atmosphere. Changes in the jet structure are consistent with changes in baroclinicity and eddy forcing. The total zonal‐mean poleward energy transport is only slightly different between the two experiments. However, the component of this transport due to transient eddies has a different character when the carbon dioxide is doubled. In particular, the moist contribution to the transient energy flux is enhanced.
Real‐time correction of weather radar data for the effects of bright band, range and orographic growth in widespread precipitation
Authors: Kitchen M. , Brown R. , Davies A. G.
Published: February 2007

A new method of correcting data from the United Kingdom operational weather radar network is described. The physically based scheme is designed to produce estimates of instantaneous precipitation rate at the surface by compensating for the effects of the bright band, range and low‐level orographic growth. In a preliminary study, the characteristic shape of the vertical profile of reflectivity factor was examined using a climatological dataset derived from range—height indication scans recorded by a high‐resolution radar. The results were then used to construct an idealized reflectivity factor profile. In the correction procedure, the heights of significant turning points in the profile are diagnosed from relevant meteorological data at each radar pixel. The parametrized profile is weighted by the radar‐beam power profile and the surface precipitation rate found by an iterative method in real‐time. The scheme has some important advantages over the alternative correction strategies. Detailed quantitative evaluation in typical cases of mainly frontal rainfall over southern England suggested that root‐mean‐square errors in estimates of instantaneous precipitation rates over 5 km pixels were reduced to less than half compared to the same radar data subject to only a fixed range correction.
A strategy for operational implementation of 4D‐Var, using an incremental approach
Authors: Courtier P. , Thépaut J.‐N. , Hollingsworth A.
Published: February 2007

An order of magnitude reduction in the cost of four‐dimensional variational assimilation (4D‐Var) is required before operational implementation is possible. Preconditioning is considered and, although it offers a significant reduction in cost, it seems that it is unlikely to provide a reduction as large as an order of magnitude. An approximation to 4D‐Var, namely the incremental approach, is then considered and is shown to produce the same result at the end of the assimilation window as an extended Kalman filter in which no approximations are made in the assimilating model but in which instead a simplified evolution of the forecast error is introduced. This approach provides the flexibility for a cost—benefit trade‐off of 4D‐Var to be made.
A new conservation law of the shallow‐water equations
Authors: Egger Joseph
Published: February 2007

A new conservation law is derived from the inviscid shallow‐water equations. This law governs the spreading of a fixed quantity of fluid. The symmetries of the Lagrangian of shallow‐water flow are determined. The links of these symmetries to the new law and to other conservation laws are discussed.
Three‐dimensional tracer initialization and general diagnostics using equivalent PV latitude–potential‐temperature coordinates
Authors: Lary D. J. , Chipperfield M. P. , Pyle J. A. , Norton W. A. , Riishøjgaard L. P.
Published: February 2007

This paper describes a simple, but effective, procedure for producing three‐dimensional tracer fields and general diagnostics based on an equivalent PV latitude–potential‐temperature coordinate system. The tracer fields generated by this method are shown to agree quantitatively with independently measured profiles and total columns. Using this initialization makes short model simulations of around a week or so much more realistic. It can be used to assimilate non‐global measurements, such as sonde or solar‐occultation satellite data, or to transform two‐dimensional model data into three‐dimensional fields.
A generalized Ekman layer profile with gradually varying eddy diffusivities
Authors: Grisogono Branko
Published: February 2007

The classical analytical solution for the Ekman layer flow has been generalized by introducing an assigned non‐rapidly‐varying eddy diffusivity K(z). Asymptotic expressions for the wind components u, v obtained via the WKB method retain the form and, to a large extent, the simplicity of the classical case in which K is kept constant. Three examples of idealized K‐profiles are used to compare the numerical, WKB and constant‐K solutions for Ekman profiles.
Spectral and bispectral characteristics of wind variability at Darwin, Australia observed by a VHF radar wind profiler
Authors: May Peter T. , Ecklund Warner L. , Hess G. D.
Published: February 2007

A year of wind data measured by a 50 MHz wind profiler at Darwin, Australia are analysed. The behaviour of vertical motions in the precipitation‐free boundary layer is shown to be different from the free troposphere. However, these differences are not reflected in the spectral characteristics of the horizontal wind fields. There are no significant differences between the horizontal‐velocity spectra above and within the mixed layer. The power spectra of the horizontal winds show little seasonal differences for the monsoon, dry and transition seasons as well as for the data set overall. The bispectral characteristics are also similar, with the strongest interactions between the lowest frequencies, and a weak self‐interaction peak at low frequencies. A cross‐peak in the zonal wind above 5 km is also present for frequency sums corresponding to a period of around 10 days. There are no detectable nonlinear interactions at high frequencies. The wind field is not strongly polarized except at the very lowest frequencies (i.e. seasonal changes). Thus there appears to be universality of the spectral characteristics observed despite the expected variations in the forcing in different seasons and stability regimes. These observations have interesting implications for internal‐wave models and two‐dimensional turbulence models of mesoscale variability.
Modelling of interaction processes between two raindrops in an electrical environment
Authors: Georgis Jean‐François , Coquillat Sylvain , Chauzy Serge
Published: February 2007

The behaviour of two raindrops of opposite polarity interacting in an electric field has been investigated numerically with a view to eventually identifying the microphysics and electrical conditions favourable for the natural triggering of cloud discharges. In the past, a few studies have been carried out on the subject of distortion and disintegration of drop‐pairs but in none of them has the aerodynamic pressure featured in the equation for the pressure balance between the two drops. Consequently in what follows we have expanded and modified the earlier model for the distortion of an isolated raindrop (Coquillat and Chauzy 1993) in which the effect of the aerodynamic pressure is taken into account. the action of this pressure on each drop leads to oblate shapes, whereas in the earlier studies both drops became prolate.

The mutual approach of two drops of opposite polarity and/or an increase in the ambient electric field do not always have the same effect on the distortion of each drop, so revealing the highly nonlinear character of the interaction processes between two hydrometeors. In fact the local electric fields do not necessarily increase (they can even decrease) as the distance, d, between them decreases and/or as the ambient field increases. Therefore it does not follow that the corona emission may be triggered more easily from a drop‐pair than from an isolated drop.

We show that a short air gap between the two drops is not equivalent to a strong ambient field. In particular, a pair of closely separated drops may coexist in the weakest and in the strongest intensities of the ambient electric field, but not in the mid intensities. We finally discuss the consequences of cloud electrification.
Surface transfer of heat and momentum over an inhomogeneous vegetated land surface
Authors: Hopwood W. P.
Published: February 2007

Tethered‐balloon data collected over an inhomogeneous, semi‐rural area of central England are presented. the data are compared with previous surface data to assess how representative fixed‐point flux‐measurements are of area averages. the differences between the data and ‘established’ surface‐layer results are relatively small with departures of about 10% to 20%. This supports the concept of a blending height above which the flow becomes horizontally homogeneous. Effective roughness lengths for momentum and heat transfer are derived. the ratio zeff0m/zeff 0t is found to be about 80 compared to z0m/z0t ≈︁10 observed for homogeneous terrain. For different wind directions the relationship between the effective roughness length and the upwind surface is investigated using recent theories on the area‐averaging of surface roughness lengths. It is found that the method of averaging transfer coefficients gives realistic values of the effective roughness lengths when compared to observations.
Lagrangian four‐dimensional variational data assimilation of chemical species
Authors: Fisher M. , Lary D. J.
Published: February 2007

For the first time, the method of four‐dimensional variational data assimilation is applied to the analysis of chemically active trace species. By combining observations with a numerical model to analyse simultaneously several species over a period of a few days, the analysis method is able to exploit information which is not available to conventional analysis techniques. Moreover, effective use can be made of asynoptic observations even for species which have strong diurnal cycles. Synoptic analyses are produced. A Lagrangian approach is adopted, allowing a separation of dynamics and chemistry which considerably reduces the computational expense of the method.
On the formation of ozone laminae at the edge of the Arctic polar vortex
Authors: Orsolini Yvan J.
Published: February 2007

Ozone vertical profiles commonly display thin‐layered structures, or laminae, in the lower stratosphere of the extratropics, predominantly during winter and spring. the mechanism for their formation is still uncertain.

In this study, high‐resolution isentropic transport of a passive tracer on synoptic time‐scales is performed with an off‐line transport model, which is forced by winds derived from global meteorological analyses or forecasts. the study focuses on a meteorological situation which occurred in late January 1992. On isentropic surfaces, the tracer distribution, initialized as the analysed potential vorticity, is eroded into filamentary structures in the mid‐latitude surf‐zone. Repeated poleward intrusions of mid‐latitude air are shown to lead to the formation of convoluted filamentary structures at high latitudes.

By performing isentropic advection on many closely spaced independent levels, the vertical structure of these tracer filaments can be studied. They are shown to be part of tracer sheets which are vertically tilted in the shear zone in the vicinity of the polar jet. Thin laminar structures in the tracer vertical profiles appear as the result of isentropic wrapping and vertical shearing of such tracer sheets. A comparison is made with ozone laminae, found in balloon‐borne soundings during the European Arctic Stratospheric Ozone Experiment.
Generalized Adams–Bashforth time integration schemes for a semi‐Lagrangian model employing the second‐derivative form of the horizontal momentum equations
Authors: Purser R. James , Leslie Lance M.
Published: February 2007

We present a generic class of semi‐implicit time‐integration methods, the ‘Generalized Adams–Bashforth’ schemes, for the simultaneous treatment in a semi‐Lagrangian model of the equations of horizontal momentum and kinematics in a rotating environment. The salient feature of the approach is that it deals directly with Lagrangian parcel momentum in terms of the parcel's second time‐derivative of position. The classical Adams–Bashforth methods can be generalized to accommodate equations of second‐derivative form and, as we demonstrate, can be formulated in such a way that the further important refinement of a semi‐implicit handling of the fastest gravity modes follows in a natural way.

The principal advantages expected of this unified approach over the more conventional separate semi‐Lagrangian treatment of kinematics and momentum are: (i) greater economy of storage at a given order of accuracy, (ii) smaller truncation errors at a given order of accuracy. Tests were run with a full‐physics three‐dimensional regional semi‐Lagrangian forecast model applied on a daily basis to archived operational data over a period of three months. Verifications based on the 48 hour forecasts confirm that the expected benefits of the new schemes are also realized in practice.
Bounds on the growth rate and phase velocity of instabilities in non‐divergent barotropic flow on a sphere: A semicircle theorem
Authors: Thuburn John , Haynes Peter H.
Published: February 2007

Bounds are presented for the growth rate and angular phase velocity of unstable modes in non‐divergent barotropic flow on a sphere. The bounds are given in terms of the minimum and maximum basic‐flow angular velocity. The result is analogous to the earlier results of Howard and of Pedlosky for plane parallel shear flow and for quasi‐geostrophic flow on a ß‐plane. An improvement to the earlier ß‐plane result, giving tighter bounds, is also presented.
Long‐term behaviour of precipitating tropical cloud systems: A numerical study
Authors: Grabowski Wojciech W. , Moncrieff Mitchell W. , Kiehl Jeffrey T.
Published: February 2007

Results of a 24‐day, two‐dimensional integration of tropical cloud systems forced by large‐scale ascent, surface fluxes and radiation in a typical sheared tropical environment are presented. A non‐hydrostatic, cloud‐resolving numerical model containing sophisticated microphysical parametrizations as well as turbulence, surface flux and short/long‐wave radiative representations was used. A predominant cloud‐system hierarchy was identified: fast westward‐moving mesoscale convective systems, producing extensive cirrus anvils and a strong radiative effect; and slow‐moving regions of enhanced precipitation, causing a significant modification of the fast‐moving cloudsystem behaviour on the time‐scale of about one day. The experimental set‐up was similar to that used by Sui et al. The demonstrated episodic convective activity and the fundamental role of organized deep convection by and large agrees with their analysis. However, despite many similarities, the results for the mean thermodynamic statistical equilibrium are dramatically different: a warm and humid regime, as opposed to the cold and dry regime of Sui et al.

High relative humidities and very high upper‐tropospheric cirrus cloud amount led to a strong greenhouse effect with low outgoing long‐wave radiation at the top of the atmosphere (120–150 W m−2). At the same time, these clouds screened the ocean surface from the solar radiation and caused very low solar radiation absorption at the surface (only about 30 W m−2 when averaged over a diurnal cycle). The surface net cloud forcing was very large, about −200 W m−2 which is in accord with earlier findings of Ramanathan and Collins. The above numbers should be considered as an upper limit of effects of deep convection on radiative fluxes in the tropics because of the warm and moist regime demonstrated in the experiment.
MANDOP analysis and airborne Doppler radar for mesoscale studies
Authors: Dou Xiankang , Scialom Georges , Lemaǐtre Yvon
Published: February 2007

The development, these last ten years, of airborne Doppler radars able to follow precipitating systems during a significant part of their lifetime and, more recently, of radars equipped with dual‐beam antennas allowing the sampled area to be extended (avoiding the necessity of performing two consecutive flights over the same area in order to obtain the three‐dimensional wind field), has increased the need for new wind‐field analyses flexible enough to output wind fields at several scales of motion, in particular at mesoscale. Numerous retrieval techniques valid for ground‐based radars exist. Some of them are devoted to convective‐scale studies, others, such as the MANDOP analysis, are devoted to mesoscale meteorology. The application of these techniques to airborne Doppler‐radar data is subject to new problems linked to the influence of the flight parameter's accuracy onto the sensitivity of the retrieved wind field (in particular the vertical‐velocity field). The knowledge of the effect of this sensitivity is of crucial importance for mesoscale motions with very weak vertical velocities.

In this context, the paper describes the application of the MANDOP analysis (a new analysis devoted to mesoscale meteorology and valid for any kind of Doppler‐radar data but until now only tested for ground‐based radar data) to the case of airborne dual‐beam radar data, with special attention brought to the accuracy of the wind‐field retrieval for mesoscale motions.

The study of these problems, and of the expected accuracy on the retrieved wind fields, is performed by means of simulated data. An application of the method to real data extracted from the CaPE experiment data set is also made. This application is twofold. First, a comparison is made between fields retrieved from airborne radar data and several combinations of airborne and ground‐based radar data, illustrating the flexibility of the analysis and validating its application at mesoscale on dual‐beam radar data; then a comparison is made between MANDOP and an analysis devoted to convective‐scale studies, the COPLANE analysis, on the same airborne data set, illustrating the capabilities of the MANDOP analysis to be applied within convective areas and thus to be used to scrutinize a precipitating system at various scales with the same airborne Doppler‐radar data set.
Monsoons and the dynamics of deserts
Authors: Rodwell Mark J. , Hoskins Brian J.
Published: February 2007

The existence of subtropical deserts, such as the Sahara, has often been attributed to the annual‐mean, zonal‐mean Hadley circulation which shows strong descent in the subtropics. However, the zonal‐mean Hadley circulation shows considerable evolution over the course of the year with very strong subtropical descent during winter, but practically no zonal‐mean subtropical descent during summer when rainfall over the eastern Sahara and the Mediterranean is least. Charney (1975) proposed a biosphere‐albedo feedback mechanism whereby local anthropogenic effects related to over‐grazing could affect the radiative balance, enhancing summertime diabatic descent and leading to desertification of the subtropics in general. The present study, which uses an idealized model, suggests a monsoon‐desert mechanism for desertification whereby remote diabatic heating in the Asian monsoon region can induce a Rossby‐wave pattern to the west. Integral with the Rossby‐wave solution is a warm thermal structure that interacts with air on the southern flank of the mid‐latitude westerlies causing it to descend. This adiabatic descent is localized over the eastern Sahara and Mediterranean, and over the Kyzylkum desert to the south‐east of the Aral Sea, by the mountains of north Africa and south‐west Asia. Trajectories indicate that the monsoon‐desert mechanism does not represent a simple ‘Walker‐type’ overturning cell. Instead, the descending air is seen to be mainly of mid‐latitude origin. It is speculated that the monsoon‐forced adiabatic descent may result in clear air and, therefore, a local diabatic enhancement which effectively doubles the strength of descent. With this mechanism, desertification can be forced by remote changes in monsoon strength rather than by local effects. This conclusion is supported by the observed dramatic strengthening of descent over the Mediterranean and east Sahara during the onset of the Asian monsoon and, on the longer timescale, by relating prehistoric lake‐levels to Milankovitch‐monsoon forcing. The latter may help to explain the perceived discrepancies between the palaeoclimate of the eastern Sahara and the strength of a ‘tropic‐wide’ monsoon. The monsoon‐desert mechanism may not be confined to the Asian monsoon alone and the existence of other monsoon‐climate regions over the globe may, in a similar way, explain the observed summertime strengthening of the oceanic sub‐tropical anticyclones and the existence of western continental deserts and ‘Mediterranean‐type’ climate regions.
Diagnosing the global hydrological cycle from routine atmospheric analyses
Authors: Dodd James P. , James Ian N.
Published: February 2007

This paper discusses the extent to which atmospheric analyses produced routinely by forecasting centres can be used to diagnose the global hydrological cycle. These analyses directly represent the storage and large‐scale advection of water vapour through a combination of observations and a short model forecast. In this work, the analyses produced by the European Centre for Medium‐Range Weather Forecasts are used. Due to changes in the model formulation, which have a profound effect on the analyses, only results from three years of these initialized analyses are presented. The equations representing the conservation of water and water vapour in the atmosphere are derived and discussed, including consideration of the approximations commonly made. Diagnostics of the storage and transport of water vapour have been calculated. The results are qualitatively similar to diagnostics from other sources. In addition, with regard to attempts to derive a global relationship between the surface specific humidity and the total column water vapour, special consideration is given to the variations in the vertical distribution of water vapour. The profile of water vapour is found to be very sensitive to the competing processes responsible for the water balance in the atmosphere. Although surface fluxes of water cannot be directly derived from these analyses, a technique is introduced which puts a lower bound on the evaporation and precipitation that would be consistent with the analyses. The results of this technique are rather encouraging, and comparisons with other climatologies of surface water fluxes could provide another way of validating these atmospheric analyses. A diagnostic characterizing the time‐scale of the the small‐scale processes responsible for surface evaporation and precipitation is also introduced. This time‐scale is found to vary widely but is of the order of one day where variability in the hydrological cycle is important.
Cold pools in shear
Authors: Parker Douglas J.
Published: February 2007

Gravity currents exist in many forms and develop within many types of flow. The dynamics of long‐lived cumulonimbus are thought to be strongly influenced by the interaction of a spreading pool of cold, low‐level air, which may act as a gravity current, and the ambient sheared flow in the boundary layer. In interpreting two‐ and three‐dimensional numerical models of such phenomena, the behaviour of the vorticity dynamics in the ambient flow and the spreading cold air is commonly discussed. In order to investigate these vorticity ideas from a gravitycurrent perspective, three‐dimensional numerical simulations have been performed in which a cold pool spreads from an isolated source in the presence of a horizontal flow that is sheared with height. It has been found that the shear of the incoming ambient flow does not qualitatively alter the horizontal spread of the cold pool, and that the location of the region of maximum horizontal convergence remains on the along‐wind axis: the actual strength of the ambient wind, which is shown analytically to be measured by the wind strength at the head height, has a greater bearing on the cold‐pool shape than the shear. Consequently, it must be the details of the moist convection which play a crucial role in determining the cold pool's development in sheared flows. As such, this model is a general indication of the robustness of gravity‐current theory, even in the presence of mean shear. The vortex structure on the flanks of the spreading cold air is found to be significantly modified, with a reversal in the dominant sign of vorticity and an increase in its intensity, when such mean shear exists.
Measurement of the pressure field on a mountain
Authors: Vosper S. B. , Mobbs S. D.
Published: February 2007

Four microbarographs, accompanied by wind vanes and anemometers, were deployed on a mountain called Black Combe (height 600 m) in Cumbria during a field experiment that took place in Novemberer 1991. the aims of the experiment were to measure the small flow‐induced pressure differences across the mountain and relate these to the local wind. These pressure differences may be used to calculate directly the drag exerted by the atmosphere on Black Combe.

The mean pressure difference between pairs of sites calculated over periods when the wind speed is small is due to the hydrostatic pressure difference caused by the height differences between the instruments. Removing this component reveals pressure differences across the mountain of up to 2 hPa. the main result of the experiment is the high correlation of the pressure differences with differences of the quantity pu2/2, where p is the air density and u is the wind speed. It is shown that this result holds for a stratified fluid when far upstream the streamlines originate from levels of similar wind speed.

Although reliable data were only obtained at three of the four stations, making the assumption that the dynamic pressure varies linearly across the mountain surface enables an estimate of the drag to be made. the average drag exerted on Black Combe over the period of the experiment is estimated to be 3.6 Pa.
A yardstick for global entropy‐flux
Authors: O'brien D. M.
Published: February 2007

The flux of entropy carried from the planet by radiation was estimated, from Earth Radiation Budget Experiment data, by Stephens and O'Brien. the present paper provides the numerical context for Stephens and O'Brien's results by deriving: (1) the maximum possible flux of entropy, subject only to the constraint that the planet be in energy balance with incoming solar radiation; (2) the latitude profiles of albedo and brightness temperature that maximize the entropy flux for a planet without horizontal transport of energy; (3) the entropy flux for a planet with such efficient transport that the brightness temperature is uniform from equator to pole. In each case, the entropy fluxes are computed as functions of the planetary albedo. the entropy fluxes in models (2) and (3) are found to be almost indistinguishable numerically. the result is extended to a family of latitude profiles for albedo and brightness temperature which interpolate between the dynamical extremes represented by models (2) and (3), and it is shown that all members of the family have essentially the same entropy‐flux. This suggests that measurements of the planetary entropy‐flux derived from satellite radiances may not enable us to distinguish easily between different atmospheric and oceanic circulations. the entropy flux for planet earth reported by Stephens and O'Brien is observed to lie close to the maximum.
Effects of airmass type on the interaction between warm stratocumulus and underlying cumulus clouds in the marine boundary‐layer
Authors: Martin G. M. , Johnson D. W. , Jonas P. R. , Rogers D. P. , Brooks I. M. , Barlow R. W.
Published: February 2007

Decoupling of the marine boundary‐layer beneath stratocumulus clouds and the formation of cumulus clouds at the top of a surface‐based mixed layer (SML) have frequently been observed and modelled. Observations of the cumulus‐stratocumulus interaction during a Lagrangian study as part of the Atlantic Stratocumulus Transition Experiment (ASTEX), detailed in Martin et al. (1995), were made in an airmass which, although it was over the sea, was highly polluted, since it had recently come from industrial Europe. These observations suggested that the interaction was associated with significant changes in the stratocumulus thickness, microphysics and radiative properties. However, it was suggested that such changes may vary according to the type of airmass in which the cumulus—stratocumulus interaction was taking place. In this paper, two further case studies of the interaction in airmasses from different locations, exhibiting different thermodynamic properties and different levels of pollution, but being generally cleaner than that in the ASTEX Lagrangian study, are analysed to try to assess the influence of airmass type on the interaction. Although the cloud liquid‐water content in the stratocumulus generally increases locally in a region of cumulus cloud penetration, the changes in the droplet spectrum which result from mixing between the cumulus and stratocumulus droplets depend on the individual droplet spectra in the two cloud types. This, in turn, is influenced by the aerosol characteristics in the boundary layer, the updraught velocities associated particularly with the cumulus clouds, and the actual and relative cloud thickness of the cumulus and stratocumulus. the effects of penetrating cumulus clouds on the droplet‐effective‐radius in the stratocumulus, and hence on the cloud radiative properties, may therefore differ significantly between boundary layers in which the interaction is occurring.
Climatic variability of cloud radiative forcing
Authors: Weare Bryan C.
Published: February 2007

Multiple‐regression models have been developed relating interannual departures of the Earth Radiation Budget Satellite net, long‐wave, and short‐wave cloud radiative forcing primarily to variations in the International Satellite Cloud Climatology Project cloud amounts for low, middle and high clouds, and cloud water. These models are used to evaluate the effects on cloud radiative forcing of specified and observed changes in cloud properties.

The calculated changes of cloud forcing due to 16.5% increases in low‐, middleand high‐cloud amounts and 25% increases in cloud water are compared with those of a radiative‐transfer model. the two methods have results which agree with respect to the signs of the responses and the order of the significance of the independent variables for net cloud forcing. Overall, variations in cloud water have the largest effect on net cloud forcing; those in high cloud have the largest effect on long‐wave cloud forcing; those in cloud water make the largest contributions to short‐wave cloud forcing.

Using the statistical models forced by one‐standard‐deviation variations in cloud properties, variations in high cloud are shown to have increased importance relative to the results for the 16.5 and 25% perturbations. In addition, variations in clear‐sky planetary albedo and long‐wave flux are also found to be important at higher latitudes.

In a small sample of observed short‐term climate‐change scenarios, compensations amongst the effects of the variables alter the magnitude, and in two cases the sign, of the change in net cloud forcing relative to those resulting from uniform variations of the cloud properties.
A simple‐model study of the interaction between cumulus and stratocumulus clouds in the marine boundary layer
Authors: Martin G. M. , Jonas P. R.
Published: February 2007

Decoupling of the marine boundary layer beneath stratocumulus clouds and the formation of cumulus clouds at the top of a surface‐based mixed layer have frequently been observed and modelled. However, little is known of the details of the interaction between the two cloud types, how this affects the microphysical and radiative properties of the stratocumulus, and how the cloud‐topped marine boundary layer evolves in such cases. Recent observational studies suggest that the microphysical and geometrical characteristics of the stratocumulus layer can be modified significantly by interaction with cumulus clouds. In this paper, a preliminary assessment of the relative contribution of different processes and initial boundary‐layer conditions, as suggested by the observations, to the outcome of the interaction between cumulus and stratocumulus clouds is carried out using a one‐dimensional entraining parcel model, which has been modified to simulate a cumulus cloud which penetrates a stratocumulus layer. the results suggest that the change in droplet size at the stratocumulus top as a result of mixing with the penetrating cumulus clouds is particularly sensitive both to the amount of pollution in the air mass and to the vertical structure of the boundary layer, and that these may be more important in determining the outcome of the interaction than the local temperature. A simple coalescence calculation is then used to show that the introduction of cumulus cloud droplets into a stratocumulus layer may alter the potential for drizzle formation in the cloud layer by introducing droplets which are of a different size from those already existing in the stratocumulus. However, it is clear that more detailed dynamical, microphysical and radiative‐transfer models are required, in conjunction with further observations, if these effects are to be parametrized for use in large‐scale numerical models.
A finite‐volume integration method for computing pressure gradient force in general vertical coordinates
Authors: Lin Shian‐Jiann
Published: February 2007

A finite‐volume integration method is proposed for computing the pressure gradient force in general vertical coordinates. It is based on fundamental physical principles in the discrete physical space, rather than on the common approach of transforming analytically the pressure gradient terms in differential form from the vertical physical (i.e., height or pressure) coordinate to one following the bottom topography. The finite‐volume discretization is compact, involving only the four vertices of the finite volume. The accuracy of the method is evaluated statically in a two‐dimensional environment and dynamically in three‐dimensional dynamical cores for general circulation models. The errors generated by the proposed method are demonstrated to be very low in these tests.
Pollution transport by convective clouds in a mesocale model
Authors: Gimson N. R.
Published: February 2007

The effects of cumulus convection on the dispersion of passive tracers are considered in the framework of a mesoscale model, where the subgrid‐scale clouds are parametrized. the parametrization uses a ‘mass‐flux’ scheme, where guidance on vertical fluxes and cloud cover is obtained from a cloud‐resolving large‐eddy model run, and applied to the one‐dimensional time‐dependent vertical transport model. A set of three one‐dimensional schemes is presented, the simplest of which contains approximations which are appropriate to, and commonly used in large‐scale models. On the mesoscale, however, it is postulated that these approximations are no longer valid and a less idealised scheme is necessary.

The schemes are compared using chosen initial tracer profiles, and their differences demonstrated in terms of the tracer evolution. It is found that the schemes diverge as the mass flux or grid‐box area are decreased. There is some sensitivity to mixing between the clouds and their surrounding environment, where differences between the schemes are decreased if entrainment dominates and increased if detrainment dominates at in‐cloud levels.

The results imply a necessity for the scheme with fewest approximations when the host code is at mesoscale resolution, and this is implemented in the UK Meteorological Office non‐hydrostatic model.

A case is studied, 23 October 1992, in which convective activity is present in the cold air over the United Kingdom behind a frontal system which passed through on the previous day. the dispersion from hypothetical emissions of conserved tracers is modelled, and results compared both with and without the convective transport parametrization scheme. the convection is widespread over the Atlantic Ocean, the Irish Sea and the North Sea, and more intense during the afternoon. the intersection of a mid‐level pollutant plume with the convection over the Irish Sea leads to a rapid vertical spread, both to the surface and cloud‐top levels, which are mainly between 4 km and 5 km. It is then transported in this redistributed form over the UK mainland, and over the North Sea where it encounters more maritime convection. It is also found that pollutants initially below the convective‐cloud‐base level are, through the effects of boundary‐layer turbulent diffusion, lifted to cloud‐top levels.
The information content of clear sky IASI radiances and their potential for numerical weather prediction
Authors: Prunet Pascal , Thépaut Jean‐Noël , Cassé Vincent
Published: February 2007

The estimation‐error covariance matrix associated with the analysis of the atmospheric state is used in one particular meteorological situation to examine the potential benefit of radiance data for numerical weather prediction. The gain of information content obtained from simulated Infrared Atmospheric Sounding Interferometer (IASI) data is studied and compared with the current information present in the TIROS Operational Vertical Sounder (TOVS) radiances.

Nineteen independent items of information on a typical temperature/humidity profile are available from the IASI data, compared to six from the TOVS data. In terms of temperature, fine‐scale structures associated with a vertical resolution of 1 km are estimated with a 0.7 K error standard deviation. The gain of information for specific humidity is of the same order of magnitude as for temperature. Typically, humidity structures associated with 1 km vertical resolution are estimated with a relative error of 16%.

The projection of the analysis‐error covariance matrix on atmospheric‐error structures (relevant for numerical weather prediction) gives a measure of the impact of the use of radiances on the observability of such structures. The results indicate that IASI data would be a decisive source of information for the analysis of such structures.

Finally, a preliminary sensitivity study suggests that the degradation due to radiance noise associated with possible modifications of the IASI instrument, hardly affects the quality of the analysis.
On modified rotational and divergent eddy fluxes and their application to blocking diagnoses
Authors: Nakamura Mototaka
Published: February 2007

Rotational and divergent eddy fluxes, introduced by Marshall and Shutts in the early 1980s, are modified to account for significant divergence in the mean flow and self advection by large‐amplitude eddies. Use of the modified rotational and divergent eddy fluxes is demonstrated by diagnosing two blocking episodes with potential vorticity.
Coupled processes for equatorial Pacific interannual variability
Authors: Pontaud Marc , Thual Olivier
Published: February 2007

Diagnostics for coupled processes relevant to equatorial Pacific interannual variability are defined, using a simplified analytical model, and applied to general circulation model simulations. The analytical model is a simplification of the Neelin model (Neelin 1991). The atmospheric component is represented by a direct zonal thermal circulation. The oceanic dynamics take into account a shear layer and a Kelvin wave. Analytical calculations lead to an unstable slow‐propagating coupled mode, referred to as the TK‐mode. All the processes of this mode are explicitly identified, as are their contributions to the resulting total growth rate. These contributions are computed in forced oceanic (covering the 1979–93 period) and in coupled ocean/atmosphere general circulation model simulations. The results are sensitive to the parameters but allow relative comparisons between general circulation model simulations.

The forced oceanic simulation suggests that the total instability is nearly constant all over the equatorial Pacific basin, but the central part is dominated by the zonal advective coupled process, while the eastern part is controlled by both zonal and vertical advective coupled processes. The thermocline coupled process appears largest in the eastern part of the basin, but its contribution is still weaker than the other two.

The coupled simulation appears globally more unstable than the forced one. The total instability is larger in the eastern part of the Pacific basin. This result is consistent with the location and the frequent occurrence of the simulated interannual signals in the eastern part of the basin.

The seasonal cycle of the forced simulation shows maximum instability between July and November. This could suggest an approach towards the understanding of the seasonal phase lock of the onset of warm El Niño/Southern Oscillation events. Possible interactions between the delayed oscillator mechanism or the westerly wind bursts with the TK‐mode processes are discussed. In order to understand the real system, these diagnostics should be applied to the observations and not just models. Nonetheless, analysis of coupled processes allows objective comparisons between different general circulation model simulations, and the results presented in this paper should be considered as a first step in this direction.