bluuebunny/crossref_metadata_2025 huggingface dataset:
A Numerical Investigation of Cloud‐Top Entrainment Instability and Related Experiments
Authors: Siems Steven T. , Bretherton Christopher S.
Published: February 2007

Buoyancy reversal occurs between two homogeneous fluids when as a result of non‐linear mixing the mixture becomes denser than either of the fluids. In 1980 both Randall and Deardorff suggested that such nonlinear mixing, which may occur across a stratocumulus cloud‐top as a consequence of evaporative cooling. could create an entrainment instability; the ensuing rapid entrainment could lead to stratocumulus break‐up. This break‐up mechanism is known as cloud‐top entrainment instability or CTEI.

In 1990 Shy and Breidenthal investigated buoyancy reversal within a laboratory environment and proposed a model for turbulent entrainment, and conditions under which buoyancy reversal may lead to an entrainment instability. They found that an instability could occur only if D (the ratio of the density difference between the most cooled mixed parcel and the lower fluid to the density difference across the unmixed layers) was approximately one or larger. This restriction of a strong buoyancy reversal for an instability (hence SBRI) is more stringent than Randall's condition, D ≥ 0, and is almost never satisfied at the inversion topping a marine stratocumulus layer.

We have used numerical simulations of a single eddy at an interface between two fluids to explore the entrainment model of Shy and Breidenthal. Consistent with SBRI. we find that in all cases a critical value of D, Dc, between 0.3 and 2.0 exists. For D Dc, kinetic energy is found to increase on the scale of the entraining eddy. We expand the domain to model the interaction of the multiple eddies with an entrainment interface. We find a similar Dc where unstable entrainment is exhibited, and a tendency for larger eddies to develop if Dc. > D. We explore the influence of the fluid and flow characteristics on Dc to bridge the gap between the laboratory and the atmosphere. We find that Dc is lower at Schmidt numbers of order unity, characteristic of the atmosphere, than for those equal to 103, typical of the laboratory fluid. In all cases Dc remains too large to be a factor in stratocumulus break‐up.
Nonlinear Balance and Potential‐Vorticity Thinking At Large Rossby Number
Authors: Raymond D. J.
Published: February 2007

Two rational approximations are made to the divergence, potential‐vorticity, and potential‐temperature equations resulting in two different nonlinear balance models. Semi‐balance is similar (but not identical to) the nonlinear balance model of Lorenz. Quasi‐balance is a simpler model which is equivalent to quasi‐geostrophy at low Rossby number and to the barotropic model at high Rossby number. Practical solutions involving methods that work for all Rossby numbers are outlined for both models. A variety of simple initial‐value problems are then solved with the aim of fortifying our insight into the behaviour of flows at large Rossby numbers. the flows associated with a potential‐vorticity anomaly at very large Rossby numbers differ in significant ways from the corresponding low Rossby number results. In particular, an isolated anomaly has zero vertical radius of influence at infinite Rossby number, while the induced tangential velocity in a horizontal plane containing the anomaly decreases inversely as, rather than inversely as the square of, the radius. the effects of heating and frictional forces are approached from a point of view somewhat different from that recently expressed by Haynes and McIntyre, though the physical content is the same.
A Three‐Dimensional Model of Airflow and Orographic Rainfall Enhancement
Authors: Dore A. J. , Choularton T. W.
Published: February 2007

A three‐dimensional model of airflow over hills has been used in conjunction with a two‐dimensional model of orographic rainfall enhancement. This development has allowed the accurate production of rainfall contour maps in areas of complex terrain. A comparison has been made, using a variety of atmospheric stability profiles, between the wind fields and rainfall distributions over a bell‐shaped hill and those over a two dimensional ridge of similar height. the results show that, in conditions of either stably stratified air or with an inversion layer aloft, three‐dimensional airflow effects can lead to much reduced rainfall over a bell‐shaped hill. It is shown that in such situations the use of a two‐dimensional approximation can introduce significant errors. an example is presented of the rainfall distribution in complex hill terrain over south Kintyre on the western coast of Scotland.
Simulation of Clear‐Sky Outgoing Longwave Radiation Over the Oceans Using Operational Analyses
Authors: Slingo A. , Webb M. J.
Published: February 2007

We have developed a system for the Simulation and Analysis of Measurements from Satellites using Operational a Nalyses (SAMSON). In this initial study, simulations of clear‐sky outgoing longwave radiation (OLR) over the oceans are compared with data from the Earth Radiation Budget Experiment (ERBE). the calculations were performed with a Malkmus narrow‐band radiation scheme, including the effects of all the radiatively important gases. Sea surface temperatures were taken from the National Meteorological Center's blended analyses and ozone data from the NIMBUS‐7 Solar Backscatter Ultra Violet experiment. Atmospheric temperatures and humidities came from operational analyses. Parallel simulations were performed with the analyses produced both by the European Centre for Medium‐range Weather Forecasts (ECMWF) and by the Meteorological Office.

Simulations for the period February 1985 to October 1986 are compared with ERBE data. Differences between the humidity data in the analyses have a significant impact on the calculated clear‐sky OLR. the changes to the ECMWF model in May 1985 reduce the differences compared with ERBE. After this time, the differences are consistent with two known systematic errors; a small overestimate in the ERBE data and a tendency for the ECMWF analyses to be too dry in the ITCZ and too moist in the sub‐tropics. the simulations with the Meteorological Office's analyses generally show slightly larger differences compared with ERBE. Tests show the sensitivity of the ECMWF results to diurnal sampling of the analyses and to the time resolution employed to calculate the monthly means. These tests reveal instances of extremely low humidities in the ECMWF analyses in the upper troposphere over the tropical east Pacific.

Over most of the domain, the simulations are within 5‐10 W m−2 of the ERBE data. the clear‐sky OLR can thus be computed from operational analyses with an accuracy comparable with that of ERBE. This indicates the potential of such simulations and has implications for the planning of future radiation‐budget observations.
An Optimal Nudging Data Assimilation Scheme Using Parameter Estimation
Authors: Zou X. , Navon I. M. , Ledimet F. X.
Published: February 2007

A new optimal nudging dynamical relaxation technique is tested in the framework of 4‐dimensional variational data assimilation, applied to an adiabatic T40 version of the National Meteorological Center (NMC) spectral model with 18 vertical layers. Several experiments are performed using the NMC operationally analysed data. the variational data assimilation algorithm is also employed in a parameter‐estimation mode to determine the vector of optimal nudging coefficients. Results of data‐assimilation experiments involving estimated nudging, optimal nudging and variational data assimilation are compared. Issues are addressed related to the dependence of the assimilation on the length of the assimilation period as well as to the ability of retrieving high‐quality model initial conditions.

The study outlines the ability to obtain optimal nudging coefficients, which can vary in space, in the framework of a parameter‐estimation approach using variational data assimilation. Based on our preliminary results the optimal nudging seems to be a most promising data‐assimilation scheme.
Variational assimilation of conventional meteorological observations with a multilevel primitive‐equation model
Authors: Thépart Jean‐Noël , Vasiljevic Drasko , Courtier Philippe , Pailleux Jean
Published: February 2007

The paper describes a four‐dimensional variational assimilation using a multilevel global primitive‐equation spectral model. The experiments consist in minimizing the distance between the model solution and conventional observations spread over 24 hours.

A first set of experiments was performed at low resolution (spectral truncation T21). The model solution converges to fit the observations satisfactorily over the whole assimilation period. The meteorological quality of the analyses, in terms of the large‐scale part of the flow, is comparable with the ECMWF operational analysis. Confirming early results, the control of gravity waves in the solution can be easily handled using a combination of a penalty term and a normal‐mode initialization scheme within the forward‐backward process of the variational assimilation. Experiments excluding data over an area with a strong baroclinic development show that the information contained in the dynamics of the model is used successfully in the analysis over this area. In addition, the dynamics are able to inter implicitly flow‐dependent structure functions.

Higher‐resolution experiments (spectral truncation T42) were performed and compared with optimum interpolation (OI) analyses performed under the same conditions. Although the lack of a first‐guess term in the cost function leads to small‐scale noise generation if the minimization is pursued, the quality of the variational assimilation is comparable with OI, and the error growth of the following 24‐hour forecast is smaller when performed from the variational analysis. Moreover, experiments excluding wind data from aircraft show a clear advantage for the variational approach against OI in using, in a consistent way, the information coming from the model and the observations.
Ensemble prediction using dynamically conditioned perturbations
Authors: Mureau R. , Molteni Franco , Palmer T. N.
Published: February 2007

We apply the technique developed in the companion paper by Molteni and Palmer (1993) as a means of providing dynamically conditioned perturbations for ensemble forecasting with a primitive‐equation model. Four wintertime initial states are chosen—three at random and one because of substantial development in the large‐scale flow within four days, which the control forecast completely missed. A set of singular vectors are created using a quasi‐geostrophic model linearized about basic states taken from data close to the chosen initial dates. These are interpolated onto the primitive‐equation‐model grid, and used as perturbations to the initial state. An ensemble forecast is made form the perturbed initial states. The dispersion of this ensemble is compared, for each date, with that from a second ensemble with initial perturbations constructed from 6‐hour‐forecast errors. Throughout the forecast period, it is found that the amplitude of the perturbations is noticeably larger using the singular vectors. The dispersion of the ensembles using the forecast‐error perturbations did not indicate that the control forecast form the case with substantial development was likely to be poor. By contrast, the (envelope) dispersion of the ensemble using the singular vectors was notably larger for this case than the other three. A number of members of this ensemble were particularly skilful in predicting weather‐related elements of the flow, such as low‐level temperature change. It is found that the evolution of perturbations which are initially localized over the western Pacific, or western Atlantic, can develop blocking‐like structures several days later over the eastern oceans. With the initial amplitudes used in this paper, the development of these structures is in part nonlinear.

The growth of the singular‐vector perturbations was not as large in the primitive‐model as in the quasi‐geostrophic model, probably due to interpolation problems exacerbated by inconsistent orographic representation. Further work to overcome these, and other, problems is indicated.
Linke and Unsworth‐Monteith turbidity parameters in Athens
Authors: Kambezidis H. D. , Founda D. H. , Papanikolaou N. S.
Published: February 2007

The Linke, TL, and Unsworth‐Monteith, TU, turbidity parameters were chosen to be studied in Athens. They were derived from measurements of direct solar radiation and exhibit similar behaviour in the decade 1975‐84. The increase in air pollution in Athens does not reveal itself in the trends of the turbidity coefficients during the decade. An interdependence between TL and TU was found and is expressed as a mathematical function. The seasonal, monthly and daily variation of the parameters are investigated.
Use of a flux‐limited scheme for vertical advection in a GCM
Authors: Thuburn John
Published: February 2007

A general method of constructing one‐dimensional total‐variation diminishing advection scehmes using flux limiters is presented. A simple scheme that uses the Van Leer limiter is demonstrated in some idealized tests. The scheme is compared with the more usual centred‐difference scheme in a global circulation model where the schemes are used to calculate the vertical advection of tracers, moisture, temperature and momentum. In climate simulations noise is generated by the centred‐difference scheme in tracer and temperature profiles near the tropopause, and in the moisture profile near the tropical boundary‐layer inversion; this is eliminated when the flux‐limited scheme is used. The use of this improved scheme for vertical advection is a cheap and simple way to remove these errors from global circulation models.
Estimation of area‐average rainfall for frontal rain using the threshold method
Authors: Cheng M. , Brown R.
Published: February 2007

The relationship between area‐average rainfall rate and the fractional area covered by radar echoes of intensity greater than a predefined rainfall‐rate threshold is investigated using quality‐controlled data from the United Kingdom and Irish radars for the period 26 January to 25 February 1990. During this period the precipitation was predominantly frontal, unlike previous studies that have concentrated upon convective precipitation. A high degree of correlation is found for the composite radar field and for the areas covered by individual radars within the composite. The correlation coefficient varies with rainfall‐rate threshold, peaking at a threshold of 2.0 mm h−1 when it is higher than 0.97 for all individual and composite radar data. The order of merit of the six thresholds investigated is 2.0, 1.0, 4.0, 0.5, 0.06 and 80. mm h−1. The spatial and temporal stability of the correlation over the area and period studied is high, especially for the threshold of 2.0 mm h−1%.

A linear‐regression relationship is used to estimate the area‐average rainfall from the observed fractional area. The average error in the estimation of instantaneous areal rainfall rate for the single thresholds of 1.0 and 2.0 mm h−1 is 18.0% to 20.0% for observed area‐average rainfall rates ⩾0.05 mm h−1. Use of combined thresholds of 1.0 and 2.0 mm h−1 reduces the average error to about 14%. Rainfall accumulations are estimated with greater accuracy than rainfall rates, the accuracy increasing with accumulation period. Five‐day areal‐average accumulations obtained using thresholds of 0.5, 1 and 2 mm h−1 have an average error in the range 8% to 11%. Over this period even a threshold of 0.06 mm h−1 produces a useful estimate, with an average error of 18%.
A comparison between four‐dimensional variational assimilation and simplified sequential assimilation relying on three‐dimensional variational analysis
Authors: Rabier Florence , Courtier Philippe , Pailleux Jean , Talagrand Olivier , Vasiljevic Drasko
Published: February 2007

The aim of this study is to make a strict comparison between two assimilation algorithms, sequential and four‐dimensional variational, on a 24‐hour period extracted from a baroclinic instability situation representative of mid‐latitude dynamics. In the case of linear dynamics, and under the hypothesis of a perfect model, these two four‐dimensional algorithms are known to lead to the same optimal estimate of the atmosphere at the end of the assimilation period, and both methods can be generalized in the nonlinear case. Because the full sequential algorithm is too resource‐demanding to be implemented as such, we shall test the four‐dimensional variational method (4D‐VAR), and a simplified sequential method based on three‐dimensional variational analysis (3D‐VAR), deliberately not exceeding the range of validity of the tangent‐linear model in the experiments. 4D‐VAR is then expected to be almost equivalent to the generalization of the sequential Kalman filter in the nonlinear case, i.e. the extended Kalman filter. As for the simplified sequential algorithm, it can be seen as an approximation of this full extended Kalman filter, for which the forecast error matrices are evaluated only approximately before each analysis, instead of being explicitly computed from the complete dynamical equations. In the four‐dimensional variational scheme, the consistency of the propagation of information with the dynamics is illustrated in an experiment assimilating some localized AIREP data. The large impact which these additional observations have over a large geographical area appears to be very beneficial for the quality of the analysis. Comparing the results of both methods in various configurations, we found that 4D‐VAR systematically behaved substantially better than the simplified sequential algorithm, and had a more accurate analysis at the end of the assimilation period and a much smaller error growth rate in subsequent forecasts. On the one hand, extremely bad specifications of initial forecast errors were found to be detrimental to both algorithms. On the other hand, the four‐dimensional variational algorithm proves to be more robust to the way by which gravity‐wave control is implemented.
On the interaction of tropical‐cyclone‐scale vortices. II: Discrete vortex patches
Authors: Ritchie Elizabeth A. , Holland Greg J.
Published: February 2007

The 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

In 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

Maximum 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

The 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

Potential 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

The 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

The 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

The 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

A 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

A 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

In 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

It 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

The 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

The 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.
On the symmetric circulation of a moving hurricane
Authors: Smith Roger K. , Weber Harry C. , Kraus Annette
Published: February 2007

The evolution of the symmetric circulation of a moving hurricane‐scale vortex on a beta‐plane is investigated and interpretations of this investigation are given in terms of vorticity fluxes. the study, which is fundamental to understanding vortex motion, is based largely on numerical integrations of the barotropic vorticity equation, using a finite‐difference method. In the absence of any large‐scale flow, the north‐westward drift of an initially symmetric, cyclonic vortex in the northern hemisphere is accompanied by a decrease in the tangential circulation at most radii, and a consequent deceleration of the azimuthally averaged tangential velocity. This behaviour is not simply a consequence of the increase in the Coriolis parameter over the domain of the cyclone as is often supposed, but may be explained as the sum of three effects: the outwards radial flux of relative vorticity associated with the asymmetric component of flow, the corresponding flux of planetary vorticity, and the rate of change in planetary vorticity due to the meridional displacement of the vortex. In particular, the net rate of change in absolute vorticity, the sum of the last two effects, makes the largest contribution to the circulation changes. the flux of absolute vorticity is associated, inter alia, with the generation of Rossby waves by the vortex. the behaviour is different from that in recent calculations by Carr and Williams in which the flux of planetary vorticity was omitted.

It is pointed out that in the analytic theory for vortex motion developed by the first author and co‐workers, an initially symmetric vortex with zero net relative circulation at large radial distances subsequently develops a finite negative relative circulation whose strength increases linearly with time; this is because the net flux of planetary vorticity at large radial distances is finite. In contrast, in the numerical model the flux is close to zero so that the symmetric circulation decays more rapidly with radius than the 1/(radius) decay rate in the analytic model. At inner radii there is good qualitative agreement between the predictions of the two methods, even though there are quantitative differences between them. It is noted that the occurrence of the finite circulation in the analytic theory violates a theorem of Flierl et al. and that it represents a limitation of the long‐term validity of the theory.
The pressure‐advection term and additive splitting in split‐explicit models
Authors: Janjiš Zaviša I. , Mesinger Fedor , Black Thomas L.
Published: February 2007

In this note two major points are made. First, pressure advection needs to be carried within the adjustment step of split schemes if conservation of energy in the transformation between kinetic and potential energy is to be made possible. Second, in contrast to several recently published views, additive split schemes, without incremental addition of the advection contributions, are not necessarily noisy, given that the presented scheme is not. an analysis in support of the first point and evidence in support of the second are presented. Finally, several additional remarks regarding the ratio of the adjustment and advection time steps and energy conservation in split‐explicit models are made.
The Australian nocturnal jet and diurnal variations of boundary‐layer winds over Mt. Isa in North‐eastern Australia
Authors: May Peter T.
Published: February 2007

An analysis of radiosonde data shows the nocturnal boundary‐layer jet over Australia to be continental in scale. the detailed temporal development of the jet is described using two years of boundary‐layer wind‐profiler data from Mt. Isa in north‐eastern Australia. A distinct isallobaric wind is found, which has a spectral peak near the inertial period. This is associated with the diurnal pressure‐oscillation and developments in the heat low over central and western Australia. However, it is not important for the development of the nocturnal jet. the nocturnal jet begins as a shallow disturbance and grows through the night. Most of the year the wind maximum is at about 500 m above the ground and has a well‐defined jet‐profile. In the summer‐monsoon season the vertical extent of the wind maximum increases, but has weaker vertical shear. the jet results from an inertial response, but is affected by other processes. the amplitude of the jet is less than would be expected from a straightforward response to the daytime ageostrophic flow and there is super‐inertial rotation and damping of the wind vector late in the night.