bluuebunny/crossref_metadata_2025_split huggingface dataset:
An idealized study of African easterly waves. I: A linear view
Authors: Thorncroft C. D. , Hoskins B. J. 
Published: February 2007

AbstractThe 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

AbstractThe 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

AbstractTransient 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

AbstractA 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

AbstractAn 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.
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

AbstractThis 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

AbstractThe 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

AbstractA 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

AbstractThe 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

AbstractTethered‐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

AbstractFor 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

AbstractOzone 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

AbstractWe 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

AbstractBounds 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

AbstractResults 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.