bluuebunny/crossref_metadata_2025_split huggingface dataset:
On the symmetric circulation of a moving hurricane
Authors: Smith Roger K. , Weber Harry C. , Kraus Annette 
Published: February 2007

AbstractThe 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

AbstractIn 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

AbstractAn 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.
How large must surface inhomogeneities be before they influence the convective boundary layer structure? A case study
Authors: Shen Shaohua , Leclerc Monique Y. 
Published: February 2007

AbstractA high‐resolution large‐eddy simulation model has been used to investigate the effect of scale in surface inhomogeneities on the turbulence structure of the convective boundary layer (CBL). Surface heat fluxes were varied sinusoidally in two dimensions with wavelengths of 250, 500 and 1000 m, corresponding to about 0.27 zi, 0.55 zi and 1.1zi (zi is the boundary layer depth). the geostrophic wind speed was 1 ms−1. Horizontally‐averaged statistics of the CBL above surface heat‐flux inhomogeneities of different two‐dimensional scales were compared. the phase‐average method was used to study the horizontal variation of turbulence and the mean circulation in the CBL.Results from horizontally‐averaged statistics show that surface inhomogeneities of small horizontal extent (x < zi, influence the horizontally‐averaged variances, covariances and third moments. the surface heterogeneities of larger scales produce more vigorous energy transport in the CBL and larger temperature variances near the ground than do those of smaller scales. There is more entrainment over the region of high heat flux than over the region of low heat flux. the magnitude of entrainment and the mean circulation both increase with increasing scale of heterogeneities present at the ground.The effects on the turbulence structure of the two‐dimensional surface inhomogeneities differ from those in the one‐dimensional case reported by Hadfield et al. (1991, 1992). the most pronounced difference occurs when the scale of the surface heterogeneities is comparable to the height of the boundary layer; the v‐variance is larger than the u‐variance in the boundary layer. Although the scope of this case‐study is restricted to a limited number of typical scenarios, it does provide evidence in support of the fact that even small‐scale inhomogeneities in the surface thermal forcing can modify the CBL structure very differently than could an equivalent but uniform field. the sensitivity of the CBL turbulence structure to surface thermal perturbations with scales smaller than the boundary layer depth is also discussed.
A computational study of the relationships linking lightning frequency and other thundercloud parameters
Authors: Baker Marcia B. , Christian Hugh J. , Latham John 
Published: February 2007

AbstractIn an effort to optimize the value of global‐scale measurements obtained with the NASA/MSFC satelliteborne Lightning Imaging System (LIS), a simple computational model of thundercloud electrification has been developed, from which it is possible to derive crude relationships between lightning frequency f (which LIS will measure) and cloud parameters such as radar reflectivity Z, precipitation rate P, updraught speed w, cloud radius R, ice‐crystal concentration i and graupel‐pellet concentration Ng. Electric field‐growth is assumed to occur via the non‐inductive charging mechanism, for both Fletcher and Hallett‐Mossop types of glaciation mechanisms. A simple criterion is used to distinguish between cloud‐to‐ground and intracloud lightning discharges. f is found to be especially sensitive to w in situations where, as updraught speed increases, the temperature at balance level, Tbal, of the upper boundary of the charging zone falls. In these circumstances N1 and the sizes of the ice hydrometeors are significantly increased, with a corresponding enhancement of the effectiveness of charge transfer.Over a wide range of conditions, f is found to be roughly proportional to the first power of the parameters R1 Ni Ng and Z and (in some circumstances) to at least the sixth power of w. the relationship between f and P depends critically on whether or not w and Tbal are strongly linked. Hallett‐Mossop glaciation is capable of producing inverted‐polarity lightning from thunderclouds; Fletcher glaciation is not.
Vortex axisymmetrization: Dependence on azimuthal wave‐number or asymmetric radial structure changes
Authors: Smith Gerald B. , Montgomery Michael T. 
Published: February 2007

AbstractIntense vortices in the atmosphere and ocean exhibit a high degree of axisymmetry despite persistent asymmetric forcing from their environment. to further elucidate vortex axisymmetrization a variety of idealized initial‐value models for barotropic non‐divergent flow is considered. to ensure basic understanding, disturbance evolution is first examined in a rectilinear system of simple shear. Particular emphasis is placed on identifying how inviscid disturbance‐evolution depends on the zonal wave‐number and on the meridional structure of the initial conditions.Insight acquired from the rectilinear problem is then applied to a bounded Rankine vortex. Here, the dependency of disturbance evolution on the azimuthal wave‐number is of special interest. Recent development of a low‐frequency balance theory for rapidly rotating (large Rossby number) vortices has provided observational evidence that the low‐azimuthal‐wave‐number asymmetries, especially wave number one, are dominant in the nearvortex region. the results of this work provide further theoretical evidence of an inviscid wave‐number‐selection mechanism that preferentially damps the high‐wave‐number asymmetries.The radial structure and location of the initial conditions are found to be important factors in determining how rapidly a disturbance is compressed or elongated. This in turn controls the rate of disturbance growth or decay. For swirling flows, a definition of an effective shear is proposed that accounts for both the radial variations of the initial condition and the radial variation of the angular velocity. Using the reciprocal of this effective shear, time‐scales for a disturbance to decay to half its initial energy, the half‐life, are calculated for initial conditions and symmetric wind‐profiles that are found in hurricanes.Simple‐shear flow and the bounded Rankine vortex do not admit discrete modal solutions since there is no mean‐state vorticity‐gradient to support them. the unbounded Rankine vortex is examined to investigate how the presence of discrete modes (Rossby edge‐waves) associated with the radial vorticity‐gradient of the Rankine swirl modifies the continuous spectrum solutions presented here.
Entropy and climate. II: Simple models
Authors: O'brien D. M. , Stephens G. L. 
Published: February 2007

AbstractThe possibility that the climate of planet earth might be a state of maximum dissipation was explored by Paltridge through the development of a simple energy‐balance model of climate. In this paper we examine the assumptions in Paltridge's model and show that the model can be reduced analytically to a model involving trivial numerical computation. This step exposes the interplay of energy balance, dynamics, and extremum principles. In particular, we highlight the role of a secondary extremum‐principle, related to convective activity, which postulates that cloud cover and surface temperature conspire to maximize the sum of sensible‐and latent‐heat fluxes. We show that this convection hypothesis leads to simple algebraic relations (between cloud cover, surface temperature and horizontal convergence of energy) which could be tested against satellite data. We examine a single‐box version of Paltridge's model that exhibits remarkable temperature‐regulation through adjustment of cloud cover, and show that the regulation follows from the convection hypothesis rather than the maximum‐dissipation hypothesis. Next we investigate the relevance to climate of a theorem on maximum dissipation derived by Ziegler. We reconcile the maximum‐ and minimum‐dissipation theorems of Ziegler and Prigogine in the context of a simple model. Finally, we speculate how the principle of maximum dissipation might be applied in a climate model.
Summer thermal lows in the Iberian peninsula: A three‐dimensional simulation
Authors: Portela Adelaida , Castro Manuel 
Published: February 2007

AbstractA summary of some of the most relevant climatological characteristics of summer thermal lows in the Iberian peninsula is given followed by a description of a high‐resolution numerical model which has been used to simulate the mesoscale system of a typical summer's day. The results from this model show the appearance of small low‐pressure centres before noon, one of which prevails over the others and intensifies during the afternoon. Other small‐size structures related to this system, and not evident in the analysis from the ECMWF (European Centre for Medium‐Range Weather Forecasts) forecast model, appear in the simulation, such as the existence of areas with strong convergence near the surface, and the deviation towards the interior of the peninsula of the surface winds as the depression intensifies. Also, the structures associated with certain variables, such as potential temperature, vertical velocity and potential vorticity, become more noticeable in the model's simulation compared with the ECMWF analysis. Finally, a detailed description of the dynamical development of the simulated surface low‐pressure system is presented, taking all these findings into consideration.
A shallow‐water semi‐geostrophic model on a sphere
Authors: Mawson Mark H. 
Published: February 2007

AbstractThe f‐plane semi‐geostrophic equations have provided much insight into atmospheric motions and we wish to see if more can be gained by removing the f‐plane restriction. As a first step we begin by considering the shallow‐water semi‐geostrophic equations on a sphere and describe a numerical solution procedure for them based on a semi‐Lagrangian predictor–corrector method. It is demonstrated, numerically, that the method appears to converge to a true solution of the semi‐geostrophic equations for fixed resolution, and this suggests that there is a unique solution to the continuous equations. The results from several idealized and real‐data test problems are presented and show that the scheme is stable and accurate. The results are compared with those obtained from a shallow‐water semi‐Lagrangian primitive‐equation model. The simulations are broadly similar but the semi‐geostrophic solution maintains stronger anticyclones than the primitive‐equation model. This is in agreement with results expected after considering f‐plane baroclinic wave simulations with the effects of spherical geometry taken into account.
Variational retrieval of humidity profile, wind speed and cloud liquid‐water path with the SSM/I: Potential for numerical weather prediction
Authors: Phalippou L. 
Published: February 2007

AbstractSeveral regression algorithms have been proposed to retrieve geophysical parameters from the Special Sensor Microwave/Imager (SSM/I) radiances. Their performances are generally limited by a simplified handling of nonlinearities and/or by the poor quality of the a priori information. In this paper, a variational method is proposed for retrieving the atmospheric humidity profile, the wind speed and the cloud liquid‐water path from SSM/I observations over ocean. This method is based on nonlinear optimal estimation theory. The first guess is derived from a European Centre for Medium‐Range Weather Forecasts forecast, and the forecast‐error covariance is used as a constraint. The geophysical variable space is mapped into the radiance space through a radiative‐transfer model which permits an accurate representation of nonlinearities. This method has been applied to several orbits and the results for one of them are presented and discussed. It is argued that the variational approach is a simple optimal way of extracting information from SSM/I radiances, exploiting the high quality a priori information available from a numerical weather‐prediction model. The retrieved humidity profiles are found to compare well with the total precipitable water estimated from a regression algorithm, while avoiding local bias in very dry and very wet conditions. It is also shown that the potential wind speed‐cloud ambiguity is removed through the use of a high‐quality wind speed first guess.
Impact of orographically induced gravity‐wave drag in the GLA GCM
Authors: Zhou Jiayu , Sud Y. C. , Lau K.‐M. 
Published: February 2007

AbstractThe influence of gravity‐wave drag (GWD) parametrization has been investigated in the Goddard Laboratory for Atmospheres general‐circulation model with the resolution of 4° latitude × 5° longitude in the horizontal and 17 sigma layers in the vertical. The simulations showed several overall beneficial effects in the extended‐range forecast as well as the seasonal simulation for both winter and summer conditions. The key results include the following:
The mid‐latitude westerlies, which were shifted poleward (equatorward) in northern (southern) winter, have been corrected. Consequently, the systematic bias of low (high) sea level pressure over the north (south) polar region has been largely eliminated.
There is a significant improvement in the simulation of the winter hemisphere stationary waves; this can be attributed to the GWD induced Rossby‐wave propagation. Over the northern Pacific region, as the storm track is simulated better, precipitation distribution improves remarkably.
In cooperation with the improved regional zonal‐mean flow, an adjustment of the mean meridional circulation is found over the Asian summer‐monsoon region, leading to an improvement in the precipitation distribution.
Spurious absorption of gravity waves at the model top (10 mb) has been removed by applying an open upper‐boundary condition. The vertical propagation of the tropospherical gravity wave into the upper stratosphere and mesosphere over mid‐latitudes of the northern hemisphere in winter is consistent with observational studies as well as the middle‐atmospheric dynamics.
A comparison of ozone and thermal tropopause heights and the impact of tropopause definition on quantifying the ozone content of the troposphere
Authors: Bethan S. , Vaughan G. , Reid S. J. 
Published: February 2007

AbstractA comparison has been conducted of the height and sharpness of the tropopause as revealed by temperature and ozone profiles. In the study, 628 ECC‐type ozonesonde profiles from four stations in northern Europe were used. Two tropopauses were defined for each profile: a thermal tropopause and an ozone tropopause defined in terms of both mixing ratio and vertical gradient of mixing ratio. On average, the ozone tropopause lay 800 m below the thermal. Large differences in tropopause height were associated with indefinite thermal tropopauses which were, in turn, often associated with cyclonic conditions (some corresponding to profiles taken within the stratospheric polar vortex). On almost all profiles the thermal tropopause was the higher of the two, and of the 15 profiles that did not fit this pattern, two‐thirds were associated with anticyclonic flow in the upper troposphere. It is also shown that the tropopause definition impacts greatly on the evaluation of the ozone content of the troposphere. Where the thermal tropopause is indefinite in character, on average 27% of the ozone found below the thermal tropopause lies above the ozone tropopause.
The Asian monsoon and predictability of the tropical ocean–atmosphere system
Authors: Lau K.‐M. , Yang Song 
Published: February 2007

AbstractThe influence of the Asian monsoon on the predictability of the tropical ocean–atmosphere is studied using long‐term records of monsoon rainfall, sea‐level pressure, sea surface temperature (SST) and surface wind data. Results indicate that the Asian monsoon plays an important role in mediating interannual variations of SST, surface pressure and surface wind in a quasi‐biennial oscillation of the tropical ocean–atmosphere as found in previous studies. During the boreal spring, monsoon‐related atmospheric and oceanic anomalies change most rapidly, while the east–west gradients in sea‐level pressure and SST across the equatorial Pacific are at a minimum. The so‐called ‘spring predictability barrier’ (SPB) is associated with the phase‐locking of interannual anomalies to the annual cycle variation of the tropical ocean–atmosphere.Results suggest that the Asian monsoon defines two climatic states in the tropical ocean–atmosphere system, corresponding to the warm and cold phases of the tropical biennial oscillation. A weak Asian monsoon is associated with enhanced memory for predictions starting from late boreal spring through to summer and with a well‐defined SPB for those starting from boreal fall to winter. On the other hand, a strong monsoon is associated with a recovery of memory at long lags (> 4–6 months) from the boreal fall through to the following spring, indicating a partial breakdown or ‘tunnelling’ effect for the SPB. The above results may be exploited to improve seasonal‐to‐interannual predictions of the monsoon–ocean–atmosphere system.
Gravity‐wave drag on two mountains
Authors: Vosper Simon 
Published: February 2007

AbstractThe generation of linear internal gravity waves by the flow over two idealized mountains, whose summits are aligned in the same direction as the mean wind, is studied. In particular, the sensitivity of the wave drag to the downwind distance between the mountains is examined by using a three‐dimensional numerical model based on the linearized steady‐state Boussinesq equations of motion. Both two‐ and three‐dimensional orography is considered. When the upstream conditions are such that the wind and buoyancy‐frequency do not vary with height, and the mountains are wide enough for non‐hydrostatic effects to be unimportant, the model predicts the same drag dependence on two‐dimensional ridge spacing as was derived analytically by Grisogono et al. The corresponding three‐dimensional numerical result predicted by the model is qualitatively very similar to the two‐dimensional result. When the mountains are narrow and the waves are non‐hydrostatic the drag dependence on mountain spacing is more complicated; maxima and minima in the drag can occur when the distance between the two mountains is such that waves generated by the mountain furthest upstream are in phase (maxima in the drag), or out of phase (minima in the drag), with the waves generated by the downstream mountain. It is shown that this is an important effect when the waves are trapped by upstream profiles whose wind speed varies with height.
Infrared heating rates in the stratosphere due to volcanic sulphur dioxide
Authors: Zhong Wenyi , Haigh Joanna D. , Toumi Ralf , Bekki Slimane 
Published: February 2007

AbstractA narrow‐band radiative‐transfer model is used to assess potential infrared cooling rates due to SO2 injected into the stratosphere by volcanic eruptions. In addition to SO2 profiles representing the eruptions of El Chichón and Pinatubo, two hypothetical SO2 profiles following a massive volcanic eruption are used. The net radiative impact of the SO2 depends on the vertical distribution as well as the altitude and magnitude of its injection. For broader distributions the magnitudes of the infrared heating rates are comparable with those due to heating caused by the absorption of solar ultraviolet by SO2 tending to enhance the heating of the lower stratosphere and reduce it in the upper stratosphere. These results are explained in terms of the relative strengths of the SO2 bands and of those of H2O and O3 overlapping in the same spectral regions.