bluuebunny/crossref_metadata_2025_split huggingface dataset:
On the interaction of tropical‐cyclone‐scale vortices. II: Discrete vortex patches
Authors: Ritchie Elizabeth A. , Holland Greg J. 
Published: February 2007

AbstractThe interaction of vortex patches in relation to the observed scales and features reported in Part I of this paper is investigated. It is found that the initial approach of compound vortices, such as tropical cyclones, arises from distortion of their, weaker, outer vorticity fields. Care needs to be taken in applying simple concepts, such as the propagation of each vortex on the gradient of its neighbour. For similar vortices, the initial interaction consists of mutual orbit and approach with only minor fluctuations in shape. Once the vortices approach to within a critical separation distance, rapid merger occurs. A simplified expression for this critical distance is derived and it indicates that binary tropical cyclones must approach to within 150‐300 km before merger of the cores can occur. When vortices have substantial differences in size or intensity, the smaller/weaker system shears into the outer circulation of the other and no core merger occurs. It is suggested that this is typical of the interaction between tropical cyclones and both the monsoon trough and embedded mesoscale convective complexes.The motion of interacting vortices consists largely of a mutual orbit, which scales according to their relative sizes and intensities. An important finding, however, is that higher‐frequency meanders may develop. This indicates that a tropical cyclone interacting with a monsoonal shear zone may develop internal asymmetries and a trochoidal oscillations as a direct result of such interaction. Further, weak mesoscale vortices within the cyclone circulation may continue to cause a track meander long after they have been sheared beyond the resolving capacity of atmospheric observations.
Assimilation of TOVS radiance information through one‐dimensional variational analysis
Authors: Eyre J. R. , Kelly G. A. , McNally A. P. , Andersson E. , Persson A. 
Published: February 2007

AbstractIn recent years difficulties have been experienced in exploiting satellite sounding data in numerical weather prediction (NWP) in the form of independently retrieved temperature and humidity profiles. Attention has now focused on methods through which the information in the radiance measurements may be assimilated more directly into the NWP system%.A scheme known as ‘one‐dimensional variational analysis’ (1DVAR) has been developed at the European Centre for Medium‐range Weather Forecasts as a method for extracting information from TIROS Operational Vertical Sounder radiances for use in the operational data‐assimilation system. The 1DVAR scheme is based on variational principles applied to the analysis of the atmospheric profile at a single location, using a forecast profile and its error covariance as constraints. The details of the scheme are presented. Errors in 1DVAR products are correlated with those of the short‐range forecast which serves as a background for the subsequent three‐dimensional analysis. Methods for addressing this aspect of the assimilation problem are discussed.The characteristics of 1DVAR products and their impact on the analysis are described. A series of forecast impact experiments has been conducted and has demonstrated consistent positive impacts on forecast skill in the northern hemisphere.
Ice particle concentrations and precipitation development in small continental cumuliform clouds
Authors: Rangno Arthur L. , Hobbs Peter V. 
Published: February 2007

AbstractMaximum ice particle concentrations (IM) in modest (≦3.7 km deep) continental cumuliform clouds, with tops with temperatures between −6 and −25 °C, were found to be better correlated with the broadness of the droplet spectrum near cloud top (r = 0.78) than with cloud‐top temperature (r = 0.58). Also, the broader the droplet spectrum the warmer was the cloud top at which ice first appeared.Stratification into three cloud‐base‐temperature (TB) categories, cool (0 °C ≦ TB −8 °C), cold (−8°C < TB < 0°C), and very cold (TB ≦ −8°C), produced correlations between IM and cloud‐top temperature (TT) of 0.71, 0.88, and 0.89, respectively. The best‐fit lines for these relationships shift to higher IM values as TB increases; this also reflects the effect on IM of the broadness of the droplet spectrum, since the size of the largest drops increases as TB increases. When the clouds contained drops with diameters ≧25 μm, ice particle formation was sudden and prolific. High concentrations of ice particles appeared coincident with, or very soon after, the formation of graupel; these high concentrations were observed in clouds at ambient temperatures between ‐11 and −28°C, including some clouds with TB <0°C.For clouds with similar droplet spectra and TT, the width of the cloud also affected IM; narrow clouds (<2 km wide) formed less ice than wider, multi‐turreted clouds.Continental cumulus clouds have to be about 50% wider and about 5 degC colder at their tops than maritime cumulus to have the same chance of producing a radar echo. However, the difference in width to produce a radar echo disappears for clouds with widths >4 km, although continental clouds still need to be about 5 degC colder at cloud top than maritime clouds to produce a radar echo.Several extant theories for high ice particle concentrations in clouds are examined but none provides a satisfactory explanation for the observations.
Use of cloud‐cleared radiances in three/four‐dimensional variational data assimilation
Authors: Andersson E. , Pailleux J. , Thépaut J.‐N. , Eyre J. R. , McNally A. P. , Kelly G. A. , Courtier P. 
Published: February 2007

AbstractThe direct use of TOVS (TIROS Operational Vertical Sounder) cloud‐cleared radiances in a three/four‐dimensional variational data assimilation scheme is described. This scheme uses a fast radiative transfer model and its adjoint. Radiances are used together with all the other observational data. Global spectral fields of mass, wind and humidity are analysed simultaneously under certain mass/wind balance constraints which control the degree to which gravity waves enter into the analysis. In this way the need for a subsequent initialization is avoided. The scheme thus combines retrieval, analysis and initialization in one step and makes it possible to achieve a more optimal combination of the information contained in the radiances, the conventional data and the background (a six‐hour forecast).At spectral truncation T63, a global three‐dimensional variational analysis (3D‐Var) of TOVS radiances and conventional data is compared with the ECMWF operational Optimum Interpolation scheme, which uses TOVS radiance information in the form of profiles of temperature and humidity, retrieved using a one‐dimensional variational method. The results of 3D‐Var are in good agreement with the operational analysis at the same resolution.In an application of the four‐dimensional scheme (4D‐Var), data covering a period of 24 hours were used simultaneously. In 4D‐Var consistency in time is ensured through the evolution of the forecast model and its adjoint. Using the adiabatic version of the ECMWF forecast model (spectral resolution T42) we show that 4D‐Var is able to extract additional information from the dynamics of the model. In particular we show an impact on the tropical wind field from the use of the humidity‐sensitive TOVS channels.
Potential vorticity and the electrostatics analogy: Quasi‐geostrophic theory
Authors: Bishop Craig H. , Thorpe Alan J. 
Published: February 2007

AbstractPotential vorticity (PV) is a property of the atmosphere which is characterized by the divergence of a vector field. It is illuminating to draw an analogy between electrical charges and PV anomalies. Here the field induced by quasi‐geostrophic PV charges is chosen to be a quantity that does not depend on conditions imposed at boundaries at a finite distance from the anomaly, or on the mean static‐stability or density profiles. This field describes ‘action‐at‐a‐distance’. Elementary PV charges involve circulation about a vertical axis as well as a vertically oriented temperature dipole. A further consequence of this electrostatics analogy is that the atmosphere is analogous to an anisotropic dielectric material and hence the existence of a ‘bound’ PV charge is implied. The mean static stability measures this dielectric property of the atmosphere, and the dielectric tensor for the quasi‐geostrophic system is here presented. The bound‐charge concept provides an elegant physical picture of how vertical gradients in the mean static‐stability parameter, such as occur at the tropopause, affect the flow produced by PV anomalies.The problem of attribution, namely attributing parts of the flow to particular PV features, is considered in the light of the electrostatics analogy. Imposing conditions at boundaries is equivalent to including possibly spurious PV anomalies exterior to such boundaries.
The prospects for seasonal forecasting—A review paper
Authors: Palmer T. N. , Anderson D. L. T. 
Published: February 2007

AbstractThe evidence for predictability of interannual fluctuations in the atmosphere and oceans is reviewed. The more linear nature of tropical dynamics is contrasted with the chaotic nature of extratropical circulations. The role of the largest interannual fluctuation, the El Niño Southern Oscillation, which has its origins in the tropical Pacific, but extends to influence half the globe, is the focus of much of the review. It is argued that the statistics of the chaotic regime behaviour of the extratropics are influenced by such forcing from the tropics.Seasonal predictions can be made with empirical or physically based models. The skill of both is reviewed but most consideration is given to the latter. Such models have both atmospheric and oceanic components but there is a wide range in the complexity of these modules. Developments in both atmospheric and oceanic models, needed to improve seasonal forecasts, are discussed. It is shown that predictions are sensitive to initial conditions as well as model formulation, implying the need for ensemble integrations similar to those currently under development for medium‐range weather forecasting. The benefits of developing a seasonal‐climate prediction capability are considered, including connections with weather forecasting on the one hand and climate change on the other.This is not an exhaustive review of extended‐range predictions. Monthly forecasting is not considered and seasonal predictability is only discussed for the tropics and northern extratropics, with some focus on Europe.
Mean climate and transience in the tropics of the UGAMP GCM: Sensitivity to convective parametrization
Authors: Slingo Julia , Blackburn Mike , Betts Alan , Brugge Roger , Hodges Kevin , Hoskins Brian , Miller Martin , Steenman‐Clark Lois , Thuburn John 
Published: February 2007

AbstractThe sensitivity of the UK Universities Global Atmospheric Modelling Programme (UGAMP) General Circulation Model (UGCM) to two very different approaches to convective parametrization is described. Comparison is made between a Kuo scheme, which is constrained by large‐scale moisture convergence, and a convective‐adjustment scheme, which relaxes to observed thermodynamic states. Results from 360‐day integrations with perpetual January conditions are used to describe the model's tropical time‐mean climate and its variability. Both convection schemes give reasonable simulations of the time‐mean climate, but the representation of the main modes of tropical variability is markedly different. The Kuo scheme has much weaker variance, confined to synoptic frequencies near 4 days, and a poor simulation of intraseasonal variability. In contrast, the convective‐adjustment scheme has much more transient activity at ail time‐scales. The various aspects of the two schemes which might explain this difference are discussed. The particular closure on moisture convergence used in this version of the Kuo scheme is identified as being inappropriate.
Comments on ‘Linke and Unsworth‐Monteith turbidity parameters in Athens’ by H. D. Kambezidis, D. H. Founda and N. S. Papanikolaou (January B, 1993, 119, 367‐374)
Authors: Cracknell A. P. , Varotsos C. A. 
Published: February 2007

AbstractThe Notes and correspondence section of this Journal recently contained an article by Kambezidis et al. in which there is a quotation from a paper by Bais et al. saying that the total ozone over Greece during the period 1982‐88 shows a slight increase. This is in contradiction to our recent findings which show that for the period in question, using both ground data and satellite data, there has been a mean annual decrease of total ozone over Greece of about 0.5% per annum; a result that is consistent with the existing literature since 1991.
A numerical modelling study of the geostrophic adjustment process following deep convection
Authors: Shutts G. J. , Gray M. E. B. 
Published: February 2007

AbstractA high resolution numerical model based on the anelastic equations of motion is used to simulate the geostrophic adjustment process that follows the growth of a single, deep convective plume. Interest focuses on the form of the balanced state and the proportions of energy which go into gravity waves, dissipation and balanced flow. Three simulations are described: a non‐rotating case, an intermediate rotation case (f= 10−3S−1) and a high rotation case (f= 2 × 10−3S‐1). The use of these artificially high rotation rates (in the context of the terrestial atmosphere) is regarded as a device for shortening the adjustment time thereby rendering the objective of the study computationally feasible. The model simulations are two‐dimensional and use an idealized specification of the moist process in which precipitation is instantaneous and no cloud is permitted. The initial state of the model atmosphere is calm and horizontally‐stratified except for a warm, moist bubble near the surface. Integrations are carried out for 50000 s by which time quasi‐steady flow states are achieved.In the rotating cases, the final quasi‐balanced state takes the form of a well‐defined lens‐shaped region of uniform absolute momentum at the neutral buoyancy level and an intense, vertical shear line front extending upwards from the surface. The response is characterized by two length scales: the Rossby radius of deformation based on the depth of the convection, and a mesoscale cloud scale related to the amount of mass which convects and the rotation rate, amongst other factors.It is found that ‐ 30% of the kinetic energy released in convection is captured in balanced flow‐ considerably larger than indicated in earlier studies and not dependent on the high rotation rates used here. Simple scaling arguments are used to account for the efficiency of balanced energy retention as a function of the amount of mass that convects and to define an upper bound on the amount of mass that can be convected through a single plume in a rotating environment. This is tentatively identified with a maximum possible scale for mesoscale convective systems.An analytic model for the balanced state is presented and shown to fit the simulated flow very well. The success of this model underlines the secondary role played by mixing in the numerical model.
Three‐dimensional quasi‐geostrophic contour dynamics, with an application to stratospheric vortex dynamics
Authors: Dritschel D. G. , Saravanan R. 
Published: February 2007

AbstractA new, versatile, and efficient numerical algorithm for three‐dimensional quasi‐geostrophic calculations using contour dynamics/surgery is described and illustrated. The numerical algorithm models the fluid as dissipationless (in contrast with conventional numerical models having artificial subgrid diffusivities) and is based on the Lagrangian representation in fluid dynamics, allowing one to focus resolution on the most dynamically active parts of the flow, i.e. regions of high potential‐vorticity gradients. The algorithm is generally applicable to a wide range of idealized atmospheric and oceanic flows. Important effects of compressibility, variable stratification, surface temperature gradients and topography are included.It is applied in this paper to study how a three‐dimensional barotropic vortex responds to topographic forcing at the bottom boundary or tropopause. Two regimes of wave breaking are found: the first is local wave breaking, which occurs near the lower boundary for strong topographic forcing; the second is remote wave breaking, which occurs at the upper levels for weak topographic forcing. The local wave breaking corresponds closely to the wave breaking seen in single‐layer calculations, if the layer depth is chosen equal to a density scale‐height. Rather than having an aspect ratio equal to Prandtl's ratio (as one might expect from geostrophic turbulence), features resulting from wave breaking in a compressible atmosphere with weak vertical shear tend to have a nearly barotropic structure.
Tides in the Extended UGAMP General Circulation Model
Authors: Jackson D. R. 
Published: February 2007

AbstractIn this paper the effect of tides on the zonal mean flow in the Extended UK Universities Global Atmospheric Modelling Programme General Circulation Model (EUGCM) is examined. Previous modelling studies have shown that omitting the diurnal cycle of solar radiation gives rise to changes in the zonal wind of the order of 10–20 m s−1 near the mesopause, and to larger changes in the lower thermosphere. In the EUGCM, which has a top level near the mesopause, turning off the diurnal cycle gives rise to model zonal winds in July that are up to 65 m s−1 stronger near the equatorial mesopause. It is shown that such zonal wind changes are reduced by up to 20 m s−1 when the model horizontal diffusion is reduced by a factor of 100, but are increased by up to 10 m s−1 when the model vertical diffusion is switched off above the lower stratosphere. Such large differences in zonal wind between diurnal and non‐diurnal runs cannot be attributed to excessive model tidal amplitudes, since they are found to be in good general agreement with both observations and other modelling studies. An exception is the diurnal tide in the subtropics, which decreases with height above about 0.01 mb instead of increasing to the model top. It is shown that this behaviour is insensitive to changes in the model diffusion, but it is suggested that it could be caused by spurious reflections at the model top.
Radiation and polar lows
Authors: Craig George C. 
Published: February 2007

AbstractIt is found that, in axisymmetric simulations of a convective polar low, long‐wave radiative cooling increases both growth rate and maximum intensity. This result is due to cooling of the system's environment relative to its warm core. The magnitude of the increase in growth rate is approximately 30%, but is sensitive to the parametrization of cloud microphysics and radiative properties. A steady‐state Carnot cycle was not obtained.
On the interaction of tropical‐cyclone‐scale vortices. IV: Baroclinic vortices
Authors: Wang Yuqing , Holland Greg J. 
Published: February 2007

AbstractThe binary interaction of tropical cyclones is investigated using a three‐dimensional primitive‐equation model. The extended anticyclonic circulations in the upper troposphere merge at very large vortex‐separation distances. For the cyclonic component in the lower troposphere, we find three fundamental modes of interaction separated by two critical separation distances: a mutual approach separation (MAS) and a mutual merger separation (MMS). We suggest that failure to identify these modes may have caused some confusion in interpreting previous baroclinic interaction studies.The MAS delineates vortices that approach each other from those that move on divergent orbits. The approach phase consists of steady radial movement and gradual acceleration, with deformation of the outer vorticity structure of each vortex but little change to their cores. In contrast to barotropic studies, the MAS is much larger than the radius at which the potential‐vorticity gradient of each vortex changes sign. Vortex tilting associated with the vertical shear of the azimuthal winds from the opposing vortex and secondary circulations associated with diabatic heating increases the mutual vortex attraction. The presence of an earth‐vorticity gradient reduces this attraction slightly, but also introduces considerable sensitivity to vortex orientation. When all physical processes are included, we find an MAS of around 1000 km with a scatter of several hundred km, which agrees well with observational studies.Approach occurs with little change in the vortex cores until they reach the MMS. Merger then occurs very rapidly, usually within several hours, and follows that described in parts II and III for barotropic vortices. The MMS is approximately three times the equivalent vortex‐patch radius for the cyclones; it is slightly reduced by diabatic heating, but it is largely independent of the earth‐vorticity gradient. The vortices experience a slight weakening during the approach and initial merger stages. However, with diabatic heating, rapid intensification follows merger; such intensification may have implications for rapid development of tropical cyclones.
The evolution of vortices in vertical shear. I: Initially barotropic vortices
Authors: Jones Sarah C. 
Published: February 2007

AbstractThe behaviour of initially‐barotropic vortices in vertically‐sheared environmental flows is investigated. the strength and structure of the vortices used are representative of tropical cyclones. the calculations are performed using a primitive‐equation numerical model on an f‐plane. It is found that the initial response of the vortex to the vertical shear is to tilt in the plane of the shear. As soon as a tilt is established, the upper‐ and lower‐level centres begin to rotate cyclonically about the mid‐level centre. This rotation can be understood in terms of upper‐ and lower‐level potential‐vorticity anomalies which are displaced in the horizontal relative to one another. the flow associated with the vertical projection of each anomaly advects the other anomaly, leading to the observed cyclonic rotation. the rotation rate decreases with time, so that the direction of tilt becomes constant, but the magnitude of the tilt continues to increase. We argue that the observed rotation acts to oppose the destructive action of the vertical shear on the vortex, even in the absence of diabatic processes.The role of the vertical circulation is considered in detail. It is shown that the vertical circulation develops in a manner which is consistent with the model flow remaining balanced. It is found that the mesoscale nature of the vertical circulation leads to a distortion of the axisymmetric vortex. This results in the inner core having a smaller vertical tilt than the outer region. the vertical circulation does not act on a large enough scale to explain why the vortex is not destroyed by the vertical shear.The behaviour of the vortex is found to depend on various parameters. Results are presented where the vertical shear, the strength and size of the vortex, the Coriolis parameter, and the static stability are varied. With the exception of the vertical shear, altering any of these parameters alters the vertical penetration of a potential‐vorticity anomaly. the results show that increasing the penetration depth leads to an increase in the rotation rate of the upper‐ and lower‐level vortex centres about the mid‐level centre, and to a reduction in the magnitude of the vertical tilt.