bluuebunny/crossref_metadata_2025_split huggingface dataset:
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

AbstractAn 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

AbstractThe 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

AbstractUsing 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

AbstractAn 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

AbstractNumerical 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

AbstractThe 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

AbstractA 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

AbstractIn 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

AbstractAn 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

AbstractThe 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

AbstractThe 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

AbstractFollowing 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

AbstractWe 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

AbstractThe 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

AbstractThe 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

AbstractThe 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.