bluuebunny/crossref_metadata_2025 huggingface dataset:
A model of the development of the droplet spectrum in a growing cumulus turret
Authors: Hill T. A. , Choularton T. W.
Published: February 2007

A model is presented which describes the growth of the droplet spectrum in a growing cumulus turret in which the vertical velocities are determined by buoyancy forces. It is found that if the entrainment of dry air occurs continuously and homogeneously across the cloud then no enhanced growth of the largest droplets in the spectrum occurs unless the environment is assumed to be saturated with a lower concentration of CCN than in the air entering cloud base. This model is used as the basis for a comparison between recent cloud microphysical models, with a view to resolving some of the discrepancies in the results associated with them. Emphasis is placed on the propensity of a particular formulation to produce large droplets. If entrainment is allowed to occur discontinuously and non‐uniformly across the cloud then substantial enhanced growth can occur, in agreement with observations, provided that each part of the cloud experiences several distinct entrainment events. It is found that a highly buoyant turret is required to provide the momentum needed for the continual ascent of these regions.
A study of wintertime circulation anomalies during past El Niño events using a high resolution general circulation model. II: Variability of the seasonal mean response
Authors: Palmer T. N. , Mansfield D. A.
Published: February 2007

In this paper the response of a (relatively) high‐resolution 11‐layer general circulation model (GCM) to a number of different sea surface temperature (s.s.t.) anomalies in the tropical East and West Pacific is studied. The integrations were all run for 540 days in a perpetual January mode and had either mean or envelope orography.

The s.s.t. anomalies included an enhanced composite El Niño anomaly, the observed anomalies during December 1982, and December‐February 1982–3, and a weak (∼1 K) positive s.s.t. anomaly in the tropical West Pacific.

The objective of this study is to assess whether the observed variability in wintertime circulation anomalies between past El Niño events can be modelled by considering the 90‐day mean (‘interannual’) variability of the integration with the composite El Niño s.s.t. anomaly. It is concluded that such observed variability cannot be modelled in this way and that variations in tropical Pacific s.s.t. anomaly from one event to another are crucial in accounting for observed extratropical variability over the Pacific and North America. In particular it is shown that the model's response to the 1982/3 anomaly and the weak West Pacific anomaly are significantly different from the response to the composite El Niño anomaly over the Pacific and North America, and agree well with the appropriate observations for 1982/3 and 1976/7 respectively. It is also concluded that the relatively weak extratropical response during the winter of 1972/3 was related to the fact the s.s.t. anomalies were confined to the East Pacific. By contrast the response to the weak West Pacific anomaly was very strong and statistically significant over the Pacific and North America.
On the relative dispersion of two particles in homogeneous stationary turbulence and the implications for the size of concentration fluctuations at large times
Authors: Thomson D. J.
Published: February 2007

A model for the separation of two particles in homogeneous stationary turbulence is derived by considering the analogous process of one‐particle dispersion in inhomogeneous turbulence. Numerical simulations suggest that the intensity of concentration fluctuations tends to zero at large times.
On the development of orographic cyclones
Authors: Radinovic Djuro
Published: February 2007

Certain characteristics of cyclone development are compared and, besides the two types (type A and type B) described by S. Petterssen and S. J. Smebye, orographic cyclone development of a third type (type C) is identified. The analysis of dynamic features, thermal structure and kinetic energy change of cyclones which developed in the lee of the Alps during the special observing period of the ALPEX support the view that orographic cyclogenesis should be considered as a specific mechanism of extratropical cyclone development.
A further study of the surface zonal flow predicted by an eddy flux parametrization scheme
Authors: Wu G.‐X. , White A. A.
Published: February 2007

The problem of calculating the surface zonal flow using Green's large‐scale eddy flux parametrizations in spherical geometry is re‐examined. In a 2‐level, quasi‐geostrophic (QG1) model, low‐latitude easterlies and mid‐latitude westerlies of reasonable intensity can be obtained by assuming a more realistic baroclinicity than that applied in a study reported by White in 1977. This result also depends on the use of a more realistic value for the latitude average of a certain transfer coefficient; but no detailed treatment of its spatial variation is found to be necessary. Some further solutions are obtained using a strictly inconsistent formulation in which the Coriolis parameter is allowed its true latitude variation in all terms (in contrast to the QG1 model). In line with the conclusions of White's earlier study it seems in this case that detailed specification of the relevant transfer coefficient's spatial variation would be needed in order to produce a realistic surface zonal flow. Regarding the surface flow problem as a test of Green's parametrization scheme, we conclude that the latter performs quite well if QG1 approximations are applied to the Coriolis parameter. However, further refinement of the scheme is evidently required for good performance when these approximations are not applied.
Analysis methods for numerical weather prediction
Authors: Lorenc A. C.
Published: February 2007

Bayesian probabilistic arguments are used to derive idealized equations for finding the best analysis for numerical weather prediction. These equations are compared with those from other published methods in the light of the physical characteristics of the NWP analysis problem; namely the predetermined nature of the basis for the analysis, the need for approximation because of large‐order systems, the underdeterminacy of the problem when using observations alone, and the availability of prior relationships to resolve the underdeterminacy.

Prior relationships result from (1) knowledge of the time evolution of the model (which together with the use of a time distribution of observations constitutes four‐dimensional data assimilation); (2) knowledge that the atmosphere varies slowly (leading to balance relationships); (3) other nonlinear relationships coupling parameters and scales in the atmosphere.

Methods discussed include variational techniques, smoothing splines, Kriging, optimal interpolation, successive corrections, constrained initialization, the Kalman‐Bucy filter, and adjoint model data assimilation. They are all shown to relate to the idealized analysis, and hence to each other. Opinions are given on when particular methods might be more appropriate. By comparison with the idealized method some insight is gained into appropriate choices of parameters in the practical methods.
On the consequences of non‐negative absolute vorticity
Authors: Lunnon R. W.
Published: February 2007

It is shown that if the flow on an isobaric surface is such that the wind and mass fields satisfy the gradient wind equation, then the gradient of the Bernoulli function (the sum of the kinetic energy per unit mass and the geopotential) is everywhere perpendicular to the local wind vector and that, furthermore, if the absolute vorticity is non‐negative, the direction of increase of the Bernoulli function bears the same relation to the wind vector as does the direction of increase of the geopotential to the geostrophic wind. Applications of this relationship are discussed.
Linear wavetrains in models of the stratosphere
Authors: Marks C. J.
Published: February 2007

Experiments with a primitive equation model of the stratosphere and mesosphere are described which investigate the effect on the stratosphere of the growth of localized, stationary forcing in the troposphere. Two basic states are used which are representative of stratospheric flow in early and late winter in the southern hemisphere. Simple mechanistic models are then introduced to examine the wave phenomena which occur during the first 10 days of the experiments when the dynamics are known to be linear.

The localized forcing grows to a peak amplitude of 600 gpm and half‐width of 15° of latitude, values typical of the mid‐latitude upper troposphere. Stratospheric travelling waves are excited with phase velocities which are sensitive to the zonal mean winds and to the speed of the switch‐on of the forcing, and wave activity spreads quickly from the localized forcing with a typical vertical group velocity of 10 km d−1. Strong horizontal curvature in the basic states can profoundly affect the vertical and horizontal structures of the wavetrains: mid‐latitude regions of negative northward potential vorticity gradient in the stratosphere (as seen in the southern hemisphere in late winter) lead to the confinement and amplification of waves in high latitudes. Three‐dimensional diagnostics of wave activity propagation reveal wave reflections missed by zonally‐averaged EP flux diagnostics.

Significant wave reflection and trapping occur on the flow from late winter, eventually producing a steady linear wavetrain with an intense local centre of circulation in the upper stratosphere, the maximum anomaly being ∼1000gpm at 1mb. It is argued that such wavetrains are unrealistic because nonlinear ageostrophic winds drive the dynamics away from any linear steady state.
Stability of cloud‐topped boundary layers
Authors: Kuo Hung‐Chi , Schubert Wayne H.
Published: February 2007

According to the thermodynamic theory of cloud top evaporative instability, persistent stratocumulus should be observed only when the cloud top jumps in equivalent potential temperature θ and total water mixing ratio r satisfy the stability condition Δθ > k(L/cp)Δr, where k ± 0.23. Using observations of persistent mid‐latitude and subtropical stratocumulus we find that the above stability condition is violated in many cases. In an attempt to understand how stratocumulus can persist under apparently unstable conditions we first review the thermodynamic instability theory and then develop a dynamical framework using a two‐dimensional Boussinesq moist convection model with spectral discretization and with resolution sufficient to simulate cloud top processes. Idealized initial value experiments confirm that, when the above condition is violated, evaporative instability leads to cloud breakup through sequential dissections of existing cloud. However, initial conditions close to the critical stability line (e.g. Δθ, (L/cp)Δr = –6K, –15 K) lead to cloud breakup with a characteristic cloud half‐life of several hours, while physically realistic initial conditions far from the critical stability line (e.g. Δθ, (L/cp)Δr = –16 K, –25 K) lead to more rapid breakup, with a cloud half‐life on the order of 1/2 hour. When evaporative instability is so weak that the associated cloud half‐life is as long as several hours, other physical processes (such as surface evaporation) can apparently moisten the boundary layer rapidly enough to mask the cloud breakup process.
Extension of explicit radiance observations by the Stratospheric Sounding Unit into the lower stratosphere and lower mesosphere
Authors: Nash J.
Published: February 2007

It is shown that radiance observations centred at four additional levels in the stratosphere and lower mesosphere can be synthesized utilizing the measured limb‐brightening of the radiance observations of the Stratospheric Sounding Unit (SSU). These additional radiance measurements double the vertical range of explicit radiance observations from the SSU. As a series of Stratospheric Sounding Units have been flown on the TIROS‐N series of NOAA satellites, observations of zonal mean radiance, with higher vertical resolution than has generally been obtainable with nadir‐viewing satellite radiometers, are available since 1979 from the lower stratosphere to the lower mesosphere.

The additional radiances have allowed operational monitoring of the long term climatology of the lower mesosphere by means of satellite observations to commence.
A Lagrange multiplier approach for the metric terms of semi‐Lagrangian models on the sphere
Authors: Coté J.
Published: February 2007

The horizontal momentum equation of the shallow‐water equations on the sphere is written in 3‐D vector form using the undetermined Lagrange multiplier method. The direct time discretization of this equation is proposed as an alternative to the usual approach, i.e. time discretization after explicit removal of the constraint by the use of generalized coordinates. The proposed procedure is applicable to any coordinate system and eliminates an instability associated with the metric term in a spherical grid spectral semi‐Lagrangian model. For a polar‐stereographic grid finite‐element model, equivalent results and a gain in efficiency are obtained. The formalism can be extended to multilevel models.
Turbulent shear flows over low hills
Authors: Hunt J. C. R. , Leibovich S. , Richards K. J.
Published: February 2007

A general analysis is developed for turbulent shear flows over two‐ and three‐dimensional hills with low‐slopes which allows for a wide range of upwind velocity profiles, such as those caused by wakes of upwind hills, roughness changes, or changes in stratification. In this paper the atmosphere is assumed to be neutrally stable and the half‐lengths of the hills, L, are large compared with their heights, H, which are very large compared with the roughness length zo. The general structure of the solution is defined by dividing the flow into two regions, each of which is divided into two sublayers: an inviscid outer region composed of an upper layer in which there is potential flow when the atmosphere is neutrally stable, and a middle layer in which the wind shear dominates; and an inner region of thickness l ≤ L in which the effects of perturbation shear stresses are confined. The latter region is divided into two: a shear stress layer where the shear stresses, although weak, determine that the maximum of the perturbation velocity is located in this layer; and an inner surface layer of thickness ls where the shear stress gradient varies rapidly and the perturbation velocity tends to zero. The details of the middle layer are given here for different kinds of upwind profiles, including logarithmic, ‘power law’ and linear profiles. It is shown that the analysis can be extended to allow for nonlinear inertial effects in the middle layer. Analytical solutions are derived for the inner region as asymptotic expansions in δ = [ln(l/zo)]−1, which is assumed to be small, and this shows that ls ∼ zo(l/zo)1/2.

The results of the analytical model are compared with our own and with previously published numerical computations of the full equations (applying the same assumptions used for calculating the turbulent shear stresses as used in the analytical work), which have largely been validated against full‐scale measurements. These results confirm that the relative increase of surface stress is significantly greater than the increase of wind speed near the surface except when there is no upwind shear (as for example in a logarithmic boundary layer when the roughness length tends to zero).

Finally, the paper shows that the outer regions of laminar (or constant eddy viscosity) and of turbulent flows over hills are broadly similar, but that the effects of the flow in the inner region on the outer regions are much smaller in the latter case.
A practical Lagrangian method for relating scalar concentrations to source distributions in vegetation canopies
Authors: Raupach M. R.
Published: February 2007

The transfer of scalar atmospheric constituents within vegetation canopies cannot be predicted from simple gradient‐diffusion theory. To replace gradient‐diffusion theory in this context, an analytic Lagrangian theory is developed which predicts the concentration field of a scalar atmospheric constituent emanating from a spatially extensive source in an inhomogeneous turbulent flow. In this ‘localized near‐field’ theory, the mean scalar concentration C is expressed as the sum of a diffusive far‐field contribution Cf which obeys gradient‐diffusion theory, and a non‐diffusive near‐field contribution Cn which is calculated for each source element by assuming the turbulence to be locally homogeneous. It follows that Cf provides the large‐scale background variation and Cn the detailed local structure of the C profile. The theory applies to both non‐advective and advective situations. Comparison with random‐flight predictions of C shows that the assumptions of the theory (which are exact in homogeneous turbulence) are adequate for inhomogeneous turbulence typical of real canopies. The random‐flight predictions also show that vertical velocity skewness (not accounted for in the localized near‐field theory but typically of order −0.5 to −1 in canopies) has only a small effect on C.
Numerical experiments on urban heat island intensity
Authors: Richiardone R. , Brusasca G.
Published: February 2007

Numerical experiments with a two‐dimensional model have been performed to study diurnal urban circulation and heat islands in calm conditions. Vertical structures of rural and urban planetary boundary layers, heat island intensities and circulations are examined in different conditions of stability, surface heat fluxes and city width. A simple expression to predict diurnal evolution of the urban‐rural temperature difference is proposed.
Mean inversion strength of the convective boundary layer over the oceans
Authors: Betts Alan K.
Published: February 2007

An equilibrium energy budget for the oceanic convective boundary layer shows that the mean inversion strength in balance with a radiatively driven subsidence is typically ≈ 9 K, in agreement with the widely observed value.
The large‐scale atmospheric structure accompanying New England coastal frontogenesis and associated North American east coast cyclogenesis
Authors: Bell Gerald D. , Bosart Lance F.
Published: February 2007

An eleven‐case composite is constructed of the large‐scale flow configuration accompanying wintertime New England coastal frontogenesis events which are associated with Atlantic coast cyclogenesis in the United States.

The pre‐coastal‐front environment is dominated by anticyclonic activity in the lower troposphere over the eastern Great Lakes. The cold anticyclone moves eastward north of New England while slowly strengthening, and is eventually responsible for the cold south‐easterly flow that participates in the developing coastal front circulation. Coastal frontogenesis commences in an increasingly favourable synoptic‐scale environment for ascending motion and pressure falls with both the passage of the 500 mb ridge axis and the onset of strong 850 mb warm air advection beneath the entrance region of a broad confluence zone at 500 mb. The synoptic‐scale forcing for ascent over the developing coastal front is maximized from the cyclonic shear side of the jet in the base of the 500 mb trough to the anticyclonic shear side of the jet entrance region in the downstream confluence zone.

Clearly identifiable departures in both the global‐scale (wavenumbers 0–3) and large synoptic‐scale (wavenumbers 4–8) waves from the long term climatological mean 1000–500 mb thickness field are observed during the evolution of the Atlantic coast cyclogenesis event. The major deviations from climatology in the wavenumbers 0–3 thickness deviation fields are negative anomalies in eastern Asia, the northern Soviet Union and the United States, and positive anomalies over Alaska and Greenland. The global‐scale thickness waves show substantial evolution in their transience and amplitude characteristics following the onset of the cyclogenesis event. The wavenumbers 4–8 thickness anomaly analyses reveal initially quasi‐stationary amplification during the onset of cyclogenesis. As Atlantic coast cyclone development proceeds, however, a pronounced positive thickness anomaly is generated in the western Atlantic and extends into New England. Similarly, wavenumbers 4–8 thickness anomalies from eastern North America to western Europe amplify and become more progressive.
The importance of surface pressure changes in the response of the atmosphere to zonally‐symmetric thermal and mechanical forcing
Authors: Haynes P. H. , Shepherd T. G.
Published: February 2007

The classical problem of the response of a balanced, axisymmetric vortex to thermal and mechanical forcing is re‐examined, paying special attention to the lower boundary condition. The correct condition is DΦ/Dt = 0, where Φ is the geopotential and D/Dt the material derivative, which explicitly accounts for a mass redistribution as part of the mean‐flow response. This redistribution is neglected when using the boundary condition Dp/Dt = 0, which has conventionally been applied in this problem. It is shown that applying the incorrect boundary condition, and thereby ignoring the surface pressure change, leads to a zonal wind acceleration δū/δt that is too strong, especially near the surface. The effect is significant for planetary‐scale forcing even when applied at tropopause level.

A comparison is made between the mean‐flow evolution in a baroclinic life‐cycle, as simulated in a fully nonlinear, primitive‐equation model, and that predicted by using the simulated eddy fluxes in the zonally‐symmetric response problem. Use of the correct lower boundary condition is shown to lead to improved agreement.
Vertical subsynoptic momentum flux in the atmosphere over central Europe
Authors: Shao Yaping , Hantel Michael
Published: February 2007

Using the synoptic ALPEX (Alpine Experiment) data, a diagnostic study of vertical SSMF (subsynoptic momentum flux) is presented. The errors in the data are partially eliminated by the diagnostic model. It is found that the vertical SSMF has the order of magnitude of 1 N/m2 in the troposphere with its maximum at about 600 to 400hPa. In the synoptic momentum budgets, SSMF is closely related to ageostrophic components. Systematic distributions and changes with time of the SSFM in a cyclogenesis process are found. Slant convective activity is a possible mechanism responsible for the intensive SSMF found in the cold area of the cyclone.
The tropical circulation: Simple model versus general circulation model
Authors: Robertson Andrew W. , Frankignoul Claude
Published: February 2007

An attempt is made to simulate two general‐circulation‐model‐produced February climatologies in the tropics (from the GISS and the LMD GCMs), using a linear steady‐state multi‐layer primitive equations model which is forced thermally. A hypothesis‐testing strategy is used to quantify the simulations of the target climatologies taking into account the natural variability of the GCM fields. When the zonal‐mean flow and the diabatic heating field are prescribed from the GCM, the linear model is found to be generally consistent with both GCMs at the 5% level of statistical significance. the linear model simulation of the GCM is most successful for the vertical velocity. the horizontal wind field is moderately well simulated at low and high levels, but poorly in the mid troposphere. More than 40% of the spatial variance at 850 and 950mb between 30°S and 15°N is explained in the LMD GCM climatology whereas the corresponding figure for the GISS climatology is 34% at 850mb, falling off toward the surface. the linear model's skill at simulating the LMD climatology near the ground is little diminished when the forcing is restricted to the trade‐cumulus layer only. However, midtropospheric heating is found to be important for the zonal component of the flow at low levels, especially close to the equator. Sensitivity, especially at upper levels, was found to the inclusion of the zonal component of the mean wind in the linearization, but little to the mean meridional circulation. Sensitivity to the linear model dissipation is also discussed.
The development of narrow baroclinic zones and other small‐scale structure in the stratosphere during simulated major warmings
Authors: Fairlie T. D. A. , Fisher M. , O'Neill A.
Published: February 2007

Numerical simulations of stratospheric major warmings with a multi‐level, primitive equation model of the stratosphere and mesosphere reveal several examples of processes giving rise to small‐scale structure in the simulated flow. For a simulation of the warming of 1984/85, the following examples are cited: (1) the maintenance of the Aleutian high by injections of air with low values of potential vorticity; (2) the erosion of the main cyclonic vortex as it is stripped of air with high values of potential vorticity; (3) the merger of a pair of anticyclones at the height of the warming; (4) differential advection of potential vorticity in a vertically sheared flow; and (5) the generation in the stratosphere of narrow baroclinic zones with strong temperature gradients in the horizontal and in the vertical. For a simulation of the warming of 1979, the example given is of the in situ generation in the stratosphere of inertio‐gravity waves near a developing baroclinic zone.

The dynamics involved in the formation of narrow baroclinic zones in the simulated stratosphere is studied in some detail. It is shown that the ageostrophic flow (mainly the vertical motion) provides a positive feedback on the frontogenetic effect of the geostrophic (large‐scale) flow. Similarities and differences with frontogenesis at the ground and in the upper troposphere are enumerated.
A numerical study of tropical cyclone motion using a barotropic model. I: The role of vortex asymmetries
Authors: Smith Roger K. , Ulrich Wolfgang , Dietachmayer Gary
Published: February 2007

The motion of an initially symmetric vortex on a beta plane and the motion of initially asymmetric vortices on a beta planes are studied using a nondivergent. barotropic model. It is assumed that there is no basic flow, the effects of a basic flow will be addressed in part II. Parameter values such as vortex size and strength and computational domain size are chosen for their relevance to tropical cyclones. the simultaneous evolution of the asymmetric vorticity and streamfunction fields is investigated in terms of a partitioning of the flow introduced by Kasahara and Platzman in which all asymmetries are regarded as part of the vortex environment. It is shown that in this partitioning the asymmetric streamflow provides a 'steering current' for the vortex to a very close approximation.

For early times, typically 24 h for the parameters chosen, the development of the asymmetries in the vortices studied can be largely understood in terms of a simple analytical theory in which the vortex centre is fixed and the initial absolute vorticity distribution is rearranged by advection by the (initial) symmetric circulation.

The shearing effect of this mechanism alone would appear to preclude the establishment of a true steady state in the flows considered. the numerical calculation for the initially symmetric vortex on a beta plane is used to assess averaging procedures for computing the environmental wind field of a tropical cyclone from observed wind data. the results show that the averaging regions that have been commonly employed in observational studies are too large to properly characterize the environmental flow at the cyclone centre and they suggest caution in making inferences from these studies in relation to physical mechanisms such as beta drift.

The calculations for initially asymmetric vortices on an f plane show that the effects of the asymmetry on motion depend strongly on the scale of the asymmetry compared with that of the vortex. When the scale of the asymmetry is relatively small, the asymmetry is rapidly degraded by tangential shear and its effect on motion is not sustained. In contrast, larger asymmetries have a more persistent effect on motion. When the motion takes place on a beta plane, asymmetries induced by the advection of planetary vorticity ultimately dominate those present initially and, irrespective of the scale of the initial asymmetry, the vortex track finally turns to the north‐west as in the case of the initially symmetric vortex. Finally, as a case of extreme asymmetry we study the mutual interaction of a strong vortex and a weak one. the behaviour is similar to that just described, the weak vortex merging with the stronger one. the analytical theory also proves useful in understanding the evolution of initial vortex asymmetries.

The interpretations of the calculations add insight into the dynamics of tropical cyclone motion, complementing the findings of recent studies by Chan and Williams and Fiorino and Elsberry.
A multi‐level limited‐area slow‐equation model: Application to initialization
Authors: Lynch Peter , McDonald A.
Published: February 2007

A multi‐level limited‐area model based on the slow equations is formulated, and applied to the problem of initializing data for a primitive‐equation forecast. the slow equations comprise a filtered system whose linear solutions correspond to the low‐frequency atmospheric motions. There are no solutions corresponding to the high‐frequency gravity‐wave solutions of the primitive equations.

The model is used to initialize data for a primitive‐equation forecast. It is successful in eliminating high‐frequency noise without causing any significant changes in the forecast. the method converges well even when all vertical modes are initialized.
The sudden breakdown of an unusually strong cyclone in the stratosphere during winter 1988/89
Authors: Fairlie T. D. A. , O'Neill A. , Pope V. D.
Published: February 2007

In February 1989, a dramatic major warming occurred in the stratosphere of the northern hemisphere after an unusually cold January. the stratospheric westerly vortex was split when an anticyclone developed over Europe, apparently in response to cyclogenesis upstream in the troposphere. At the peak of the warming, the circulation resembled that of the warming of February 1979, save for a rotation of flow patterns. Very low temperatures in the lower stratosphere before the warming indicate that polar stratospheric clouds were present, so that heterogeneous chemical reactions with the potential for ozone destruction may have taken place.
Midwinter warmings in the southern hemisphere stratosphere in 1988
Authors: Hirota I. , Kuroi K. , Shiotani M.
Published: February 2007

An analysis was made of intense midwinter warmings which took place in the southern hemisphere stratosphere during August‐September 1988.

By using the southern hemispheric data‐set up to the 1 mb level provided by the Japan Meteorological Agency, it is found that the 30mb temperature increase over eastern Antarctica, about 60 K for 10 days from 22 August to 1 September, is associated with the amplification of a quasi‐stationary planetary wave of zonal wavenumber 1. the enhancement of wave 1 appears quasi‐periodically with a period of about 2 weeks during the midwinter of 1988.

Clear evidence is presented to show that the quasi‐periodic amplification of wave 1 is due to the wave‐wave interaction between the quasi‐stationary forced wave of wavenumber 1 and the eastward travelling wave of wavenumber 2. It is emphasized, therefore, that the mechanism of warming in this case is quite different from that of the transient vertical propagation of a planetary wave forced from below as observed in the northern hemisphere winter.
Convection in high stratiform cloud bands, some numerical experiments
Authors: Ryan Brian F. , Tripoli Gregory J. , Cotton William R.
Published: February 2007

The Colorado State University Regional Atmospheric Modelling System is used to test the hypothesis that unstable convection might be initiated by virga settling from the stratiform region of the conveyor belt into the pre‐frontal airmass below. Both three‐dimensional and two‐dimensional numerical experiments show that evaporative cooling at cloud base and evaporative and radiative cooling at cloud top play important roles in the initiation and organization of high‐based squall lines. However, the formation of virga is neither a necessary nor a sufficient condition for these squall lines. Sensitivity experiments reveal that the simulated squall‐line circulation is mildly sensitive to the size of raindrops assumed to settle from the cloud layer. the circulation, however, is a great deal more responsive to the destabilization caused by cloud‐top radiative cooling.
A model to estimate the source area contributing to turbulent exchange in the surface layer over patchy terrain
Authors: Schmid H. P. , Oke T. R.
Published: February 2007

The characteristics of turbulence in the surface layer are well understood over spatially homogeneous and smooth surfaces. Current theoretical knowledge is based on measurements over such ‘ideal’, flat and homogeneous surfaces. Over heterogeneous surfaces of a patchy nature (e.g. urban areas, mixed agricultural crops, forest etc.) turbulence is subject to considerable spatial variability over a wide range of scales.

A ‘source area’ is defined as the portion of the upstream surface containing the effective sources and sinks contributing to the turbulent exchange processes at a given point in the surface layer. It is noted that the term ‘source area’ refers to sources and sinks on the surface only and does not consider production or dissipation within the atmosphere. the contributions of surface elements are treated as ‘plumes’ of heat or vapour or momentum‐deficiency. A small‐perturbation approach to diffusion theory and a plume‐diffusion model are used in a numerical source‐area model (SAM) to estimate this region, depending on meteorological scaling parameters and the reference location. This model identifies a ‘maximum effect source location’ and the upwind, downwind and lateral dimensions of regions containing various levels of the total contributing effect. It accounts for the different source weight of each point on the surface, depending on its position relative to the reference location. Although the source area is not directly measurable, the results of SAM compare favourably with observed effects of changing source areas.

A sensitivity test of SAM results and non‐linear regression leads to a set of simple equations (mini‐SAM) approximating SAM. This short‐cut model provides easily obtainable estimates of the source‐area dimensions and the relative importance of surface elements for a given measurement of turbulent exchange.
The response of a general circulation model to cloud longwave radiative forcing. II: Further studies
Authors: Slingo J. M. , Slingo A.
Published: February 2007

Following on from the first part of this study, the impact of cloud longwave forcing on the general circulation has been studied further using a series of 510‐day, constant January integrations with the NCAR Community Climate Model. The sensitivity of the global response to the vertical profile of the forcing has been assessed by replacing the cloud prediction scheme by that used in the ECMWF model. The results confirm those reported in Part I and emphasize the influence of tropical cirrus clouds on the local thermal structure and on the strength of the subtropical jets. The impact on the model's hydrological cycle of the cloud longwave forcing associated with boundary‐layer clouds is also shown to be important.

Several integrations are described which assess the relative importance of the three major tropical forcing maxima over Indonesia, South Africa and South America in determining both the local and remote responses. For each region, the perturbation to the upper tropospheric diabatic heating by the radiative effects of the cirrus clouds excites an anticyclonic vorticity pair, located near the longitude of the forcing and almost symmetric about the equator. The influence of these anticyclones on the upper tropospheric circulation is substantial. The results also indicate that a perturbation to the diabatic heating over South America, provided in this case by the cloud longwave forcing, may have an important effect on the Walker circulation and on the extra‐tropical flow. The implications of these results for the problem of deforestation are discussed.
Simplification of the Kalman filter for meteorological data assimilation
Authors: Dee Dick P.
Published: February 2007

We propose a new statistical method of data assimilation that is based on a simplification of the Kalman filter equations. The forecast error covariance evolution is approximated simply by advecting the mass‐error covariance field, deriving the remaining covariances geostrophically, and accounting for external model‐error forcing only at the end of each forecast cycle. This greatly reduces the cost of computation of the forecast error covariance, which is the central and most expensive aspect of the Kalman filter algorithm. In simulations with a linear, one‐dimensional shallow‐water model and data generated artificially, the performance of the simplified filter is compared with that of the Kalman filter and the optimal interpolation (OI) method. These experiments are designed to isolate the effect of simplifying the evolution of the forecast error covariance. The simplified filter produces analyses that are nearly optimal, and represents a significant improvement over OI.
An analytic theory of tropical‐cyclone motion in a barotropic shear flow
Authors: Smith Roger K.
Published: February 2007

The recent analytic theory for the barotropic motion of an initially symmetric vortex on a beta plane at rest presented by Smith and Ulrich is extended to the case of a horizontal shear flow, enabling the precise effects of shear on vortex motion to be isolated. These effects are characterized by the contribution of the shear‐related terms in the vorticity equation to the evolution and structure of the wave‐number‐1 component of the vortex asymmetry. The analysis proceeds on the assumption that a general shear flow may be adequately approximated by the first three terms of a double Taylor‐series expansion about the initial vortex centre. The manner in which the linear and quadratic terms of this expansion affect the asymmetries is illustrated in detail for a zonal shear flow with a linear or quadratic variation in the meridional direction.

For tropical‐cyclone‐scale vortices, the theory shows excellent agreement with equivalent numerical calculations for a period of one or two days. As well as providing insight into the effects of horizontal shear on tropical‐cyclone motion, the calculations have the potential to assist in the design of ‘bogus’ vortices for the initialization of dynamically based tropical‐cyclone forecast models.
Application of Dobson and Smith's solar radiation model to German Bight data
Authors: Moll A. , Radach G.
Published: February 2007

The model for global radiation at the sea surface proposed by Dobson and Smith was tested for the Inner German Bight, North Sea with an 18‐year time series of 3‐hourly measurements of cloudiness and daily global radiation. The model, with unchanged parameters that were originally derived for oceanic stations, is suitable for use in coastal seas and seems best suited for simulating global radiation for long time‐scales. Modelled monthly means of global radiation deviate from the measured monthly means by less than 5% except for June and July (11%), two months with large solar insolation. For short time‐scales, root‐mean‐square errors from 14% (for 10 days) to 24% (for 1 day) occur. The application of the bulk model will be particularly useful to researchers working in the marine coastal areas of northern Europe.
Characteristics of the tropical intraseasonal oscillation in the NCAR community climate model
Authors: Slingo J. M. , Madden R. A.
Published: February 2007

The structure of the intraseasonal oscillation in a perpetual January integration of the NCAR Community Climate Model (CCM) is analysed and compared with observations. The simulated oscillation is identified as an eastward‐moving perturbation in the equatorial velocity‐potential field, with predominantly zonal wave‐number‐1 structure. Analysis of its vertical and horizontal structure, based on composites, shows good agreement with the observed oscillation from station data, and from numerical weather prediction model analyses.

The simulated intraseasonal oscillation is evident in the wind fields at all longitudes but its appearance in the precipitation and outgoing long‐wave radiation is confined to the eastern hemisphere. The perturbation to the zonal wind component is characterized by an out‐of‐phase behaviour between the lower and upper troposphere which is most coherent in the convective regions of the Indian Ocean and West Pacific. The stream‐function anomalies in these regions are suggestive of a coupled Rossby‐Kelvin wave, whilst in the western hemisphere the oscillation is more characteristic of a Kelvin wave.

The role of cloud‐radiation interaction was studied using a parallel integration in which the cloud long‐wave forcing in the atmosphere was removed. This was found to influence the strength of the oscillation but had no significant impact on the period.

In common with other model simulations of the intraseasonal oscillation, the period was found to be too short, near 25 days. Reasons for this are discussed in the context of a similar analysis of a perpetual July integration in which a much slower (period near 54 days) and less coherent oscillation was identified.
The Three‐Dimensional Nature of ‘Symmetric’ Instability
Authors: Jones Sarah C. , Thorpe Alan J.
Published: February 2007

The importance of three‐dimensional effects in a flow with negative potential vorticity is considered here; this is the three‐dimensional counterpart of symmetric instability. the term symmetric instability refers to a two‐dimensional flow with negative potential vorticity which develops roll circulations aligned along the thermal wind. the requirement for exact two‐dimensionality is here relaxed by using three‐dimensional numerical simulations. Two classes of simulation are described; both take a two‐dimensional basic flow.

In the first part we explore the response of a flow with uniform negative potential vorticity to localized initial perturbations. It is found that circulations become established which elongate at an angle to the thermal wind. This angle, it is shown, is determined by viscous effects and is in accord with the structure of linear viscous tilted modes for this flow. If, as happens on occasions, such an angle formed by rainbands to the thermal wind is observed in nature, it gives an indication of the importance of viscous effects. In the cases presented here the orientation is such that the rolls tilt towards the warm air as viewed in the direction of the thermal wind; i.e. they are rotated anticyclonically relative to the front. It is shown that these structures, when at finite amplitude, are stable to perturbations imposed along their length.

In the second part localized regions of instability are produced which are confined along the thermal wind direction by prescribing in these zones a reduced static stability for vertical motion. This represents the effects of latent‐heat release and it is only in these zones that the effective potential vorticity is negative. It is found that if the length of the zone along the thermal wind direction is smaller than about twice the roll wavelength across that direction then the growth rate is substantially reduced; otherwise it is relatively unaffected by this confinement. It therefore appears that a flow with negative potential vorticity is not only unstable in the pathological two‐dimensional case.
Iterative Analysis Using Covariance Functions and Filters
Authors: Lorenc Andrew
Published: February 2007

An earlier review paper of analysis methods for numerical weather prediction showed how many methods could be derived from the same basic analysis equation, expressing the Bayesian probability that any state is the true state. Prior probabilities depend on the error covariance of the background prior estimate, and these covariances figure in most solutions. This paper extends the analysis of iterative solution methods (one of which is known as the ‘Successive Correction’ method). In a simple example the methods are compared with a direct solution: the ‘Optimal Interpolation’ method. It is shown that the iterative methods can be equally ‘optimal’. If the number of iterations is limited, or the background weighting is omitted, then the methods normalizing the increments in observation space are shown to be more reliable than those normalizing in grid‐point space. the traditional successive‐correction method has a grid‐point space normalization.

The covariances in these methods perform a filtering function. It is possible to replace them by a filter acting on the analysis increments. Iterative methods using such a filter are derived, and are shown to correspond exactly to the iterative methods using covariance functions. A simple and efficient recursive filter is described and applied to the same example. the analysis using a two‐pass filter is almost identical to that using a ‘second‐order auto‐regressive’ covariance function. A filter with many passes corresponds to a Gaussian‐shaped covariance function.

Approximate filters can be devised to model the effect of observational‐error correlations, with an accuracy adequate in view of the lack of knowledge of the real correlations. With this filter, the iterative methods can be extended to deal effectively with data from remote‐sensing instruments.

Published iterative methods (many originally empirically derived) are reviewed, and fitted into the optimal theory. Practical applications are discussed.
Water‐Vapour Continuum Absorption In the Tropics: Aircraft Measurements and Model Comparisons
Authors: Kilsby C. G. , Edwards D. P. , Saunders R. W. , Foot J. S.
Published: February 2007

Measurements of water‐vapour continuum absorption in the 10‐12 μm window have been made in the tropical atmosphere using a multi‐channel narrow field‐of‐view radiometer mounted on an aircraft. Comparisons with two radiative transfer models (GENLN 2 and LOWTRAN7) are made which show that the model underestimates the absorption by some thirty percent. Two methods of modifying the existing empirical continuum parametrization to obtain better agreement with the aircraft measurements are assessed: one involves increasing the self‐broadened absorption coefficient, the other increasing the temperature‐dependence parameter.