bluuebunny/crossref_metadata_2025_split huggingface dataset:
On the recirculation of the subtropical gyre
Authors: Marshall John , Nurser George 
Published: February 2007

AbstractA hydrographic section through the Gulf Stream and its recirculation to the south is interpreted in terms of an analytical model based on the baroclinic Fofonoff gyre, in which layers shielded from forcing are homogenized to a uniform value of potential vorticity. Given this simple paradigm of the circulation, the process of anticyclogenesis is studied in which seasonal changes in the volume of low potential vorticity waters accompany a steepening of the main thermocline and an intensification of the recirculating gyre. The consequences of the gyre intensification for the structure of the bowl, the circulation within it and the deep abyssal flow are investigated. It is shown that as the anticyclogenesis proceeds, there is a general southward shift of the current systems; the bowl of the circulation strikes the ocean floor further south and there is an intensification of the depth‐independent recirculation tight in to the axis of the Stream. These changes in the structure of the recirculation are broadly in accord with observed seasonal variability associated with wintertime mode water production.
A cubic spline Galerkin scheme for the vertical discretization of atmospheric models
Authors: Steppeler J. 
Published: February 2007

AbstractA Galerkin finite element discretization for the vertical coordinate using high‐order basis functions is introduced. The basis functions are cubic splines, assuming continuity of the spline and its first and second derivatives. Locality of the scheme is required, allowing each basis function to be different from zero on four grid intervals only. An efficient solution of the resulting Galerkin finite element equations is achieved by introducing a collocation grid.A number of test integrations were done using the ECMWF spectral model with resolution T63 for horizontal discretization. The scheme was also used in a data assimilation experiment where it reduced the first guess error.
The antarctic winter; simulations with climatological and reduced sea‐ice extents
Authors: Mitchell J. F. B. , Senior C. A. 
Published: February 2007

AbstractThe southern hemisphere climatology of a recent version of the Meteorological Office 11‐layer general circulation model is presented. The simulated climate is compared with observational data, including Meteorological Office operational analyses for the period 1983‐1987. The surface temperatures and depth and position of the antarctic circumpolar trough are substantially more realistic than found in earlier climate models, but the high latitude upper troposphere is still colder than observed giving an excessively strong westerly jet. The sensitivity of the model to changes in the distribution of sea‐ice is investigated and compared with that found in other models. In a first experiment, all southern hemisphere sea‐ice equatorward of 67.5°S was replaced by sea at 271.2 K, and the surface roughness length was reduced from 10−1m to 10−4m. In a second experiment only the roughness length was changed. It is found that the change in surface roughness contributes substantially to the response to reduced sea‐ice extents. The specification of surface roughness over sea‐ice in numerical models is discussed.
The effects of rotation on flow of a single layer over a ridge
Authors: Baines P. G. , Leonard B. P. 
Published: February 2007

AbstractThe effects of the Coriolis force on the hydrostatic flow of a single lower layer over a two‐dimensional ridge are described. For short periods the flows resemble those of the non‐rotating case, but after a few hours (in the atmospheric context) effects of rotation become significant. For steady‐state flow over ridges and plateaux, critical conditions near the crest of the obstacle can control the flow in the same manner as for conventional hydraulics. However, steady disturbances are only manifested a finite distance upstream. For initial Froude number Fo < 1, upstream disturbances decay on the length scale (1 – F)1/2L, where L is the Rossby deformation radius. For Fo > 1, upstream bores (where they exist) become arrested at a finite distance ahead of the barrier. When flow on the lee side is supercritical, steady lee‐side disturbances consist of nonlinear inertial oscillations. Obstacle shape is important in determining upstream and downstream flow. The results are useful for understanding atmospheric flows over mesoscale ridges.
A composite diagnosis in sigma coordinates of the atmospheric energy balance during intense cyclonic activity
Authors: Prezerakos N. G. , Michaelides S. C. 
Published: February 2007

AbstractFollowing the recent trend for composite analysis of the energetics of cyclonic systems, a rapidly deepening Saharan depression is studied within this framework. This case refers to a cyclone which was the prevailing feature in the synoptic analyses over the Mediterranean for several days in mid March 1981. This system was marked with a range of interesting phenomena over the central and eastern Mediterranean, such as very strong surface winds, rising and transportation of desert dust to large distances, and coloured precipitation. The synoptic dynamics of the evolution of the system is briefly presented and a reasoning for its unusually rapid deepening is provided.The forms of energy considered here are kinetic energy, and sensible and latent heats. The diagnostic analysis of the budgets of these forms of energy has been carried out using the sigma coordinate system. Bearing in mind the explosive nature of the cyclogenesis studied here, the analysis is separated into two parts: one associated with the deepening phase of the cyclone and the other with its filling phase. The results are presented and discussed as time‐height cross‐sections of the energy contents; as time‐averaged vertical distributions of budget components; and as vertically averaged budgets. The analysis revealed contrasting differences in the energy budgets in each phase of the development of the system.
Warm and occluded fronts in two‐dimensional moist baroclinic instability
Authors: Joly Alain , Thorpe Alan J. 
Published: February 2007

AbstractThe two‐dimensional Eady mode of baroclinic instability exhibits a strong cold front structure after a period of development using semi‐geostrophic equations excluding moist processes. Indeed the surface wave has a single centre of cyclonic vorticity on the cold side of the warm air anomaly. Here results are presented from a semi‐geostrophic numerical model including moist effects which predicts an Eady wave with a cold and a warm front associated with two centres of surface vorticity located on either side of the warm anomaly. The ascent regions are taken to be saturated and close to moist neutral which in this baroclinic environment corresponds to a condition of zero equivalent potential vorticity; the descent is taken to be unsaturated.This development of a structure resembling a warm front occurs only when the moist effects become dominant within a pre‐existing, weak, dry Eady mode. Using the same condensation representation starting from an extremely small, and random, amplitude disturbance a ‘pure’ moist Eady mode develops with a single vorticity maximum in the form of a cold front. In nature it is likely that often the moisture becomes dominant after a period, albeit sometimes a short period, of essentially unsaturated growth, although observational evidence is limited. Therefore the process of warm frontogenesis described here may be a contributory factor to those minority of mid‐latitude cyclones which exhibit a strong warm front. The model indicates that the warm front moves more slowly than the cold front, resulting in a cold occlusion process.These investigations require a detailed examination of the initial growth of small amplitude disturbances near the limit of neutral moist ascent, i.e. pure moist modes. Results of previous such calculations using an analytical 2‐level model are confirmed here using a numerical multi‐level model. The more accurate estimates obtained here show that the finite growth rate in the limit of zero equivalent potential vorticity has a value of 1.9 times the dry value compared with the 2.4‐fold increase given by the 2‐level model.
Quasi‐trajectory analysis of a sea‐breeze front
Authors: Holland Greg J. , McBride John L. 
Published: February 2007

AbstractObservations from a parachute‐borne omegasonde that executed a trajectory through, along, and up and over the head of a sea breeze are analysed. This sea‐breeze head is shown to have similar characteristics to those of a gravity current. A deep lobe/cleft combination was observed at the front where continental air was entrained into the sea‐breeze air. Observed vertical motion near the sea‐breeze front exceeded 5 ms−1 and the sonde rose into a cloud that had formed in rising continental air over this front. An evaporatively cooled downdraught from this cloud was observed to be entrained through the turbulent wake region and deep into the sea‐breeze head.
The haar of north‐east Scotland
Authors: Findlater J. , Roach W. T. , McHugh B. C. 
Published: February 2007

AbstractCase studies of three sea fog (haar) situations off the coast of NE Scotland are presented. The studies are based on aircraft and mini‐sonde observations and satellite imagery, and are used to assess the roles of physical and dynamical factors in developing and advecting sea fog in order to improve guidance to forecasters at coastal outstations. Two fogs had formed 2–3 days previously west of Ireland and had been advected round the N coast of Scotland, but the second fog had lifted into stratus by the time the aircraft had reached it. The third fog was warmer than the sea and appeared to be in the development/advection stage characterized by much stronger winds and lower liquid water contents.Analysis and some modelling of the one‐dimensional heat and water budgets of sea fog suggest that after a period of development during transport over a relatively cold sea, initially ‘warm’ advection fog tends to become a diurnally modulated, self‐maintaining ‘cold’ radiation fog held at a temperature 1–2 degC below sea surface temperature. In other words, radiative cooling has depressed the temperature of the fog far enough below sea surface temperature for this cooling to become balanced by latent and sensible convective heat input from the sea surface.The aircraft observations demonstrated the convective control of the vertical structure of ‘cold’ sea fog, and the influence of diurnally forced wind systems (in this case, a heat low formed over the Scottish mainland) in moving and moulding the fog.
The effect of solar radiation on occult deposition over hills
Authors: Gallagher M. W. , Choularton T. W. 
Published: February 2007

AbstractA model of the turbulent deposition of cloud droplets to the surface from cap clouds enveloping a ridge has been extended to include the effects of the evaporation of deposited cloud water on the surface of vegetation. It is shown that evaporation driven by solar radiation penetrating the cloud increases the concentration of dissolved species on the leaf surfaces. The concentration effect tends to be a maximum close to cloud base both upwind and downwind of the hill, where deposition rates are low and the optical depth of the cloud is large. In a supercritical flow regime, however, deposition rates are very large on the lee side of the hill where winds are strong. In this region the cloud also has its smallest vertical depth, increasing the radiation density at the ground. It is this region, just to the lee of the summit, where phytotoxic effects are likely to be a maximum.
Estimation of skill and uncertainty in regional numerical models
Authors: Anthes Richard A. , Kuo Ying‐Hwa , Hsie Eirh‐Yu , Low‐Nam Simon , Bettge Thomas W. 
Published: February 2007

AbstractThe results from 72‐hour simulations and forecasts from the Penn State/NCAR limited‐area model, in which a number of numerical and physical factors are varied, are analysed to understand the contribution to model error or uncertainty introduced by these factors. The factors include the initial conditions, horizontal resolution (80 km and 160 km) and domain size, lateral boundary conditions, and physical parametrizations. The results are compared with those of a global forecast model (the NCAR Community Climate Model). The simulations and forecasts are verified both for 12 individual cases and for the ensemble average of the 12 cases, using several objective measures of skill. The differences in these skill scores between different models are tested for their statistical significance.The use of observed lateral boundary conditions (LBC) exerts a strong control on the growth of errors over a domain size 3600 × 4800 km2. Objective measures of error show little growth beyond about 36 hours, so that 72‐hour errors are nearly as low as the 36‐hour errors. On these time and space scales, the quality of the LBC is more important than any other factor tested in the temporal evolution of the model errors. These results show that the large‐scale atmospheric motions have a major effect on the evolution of small‐scale features in the model.Small variations in initial conditions have little effect on the model skill beyond 12–24 hours. Of all the factors examined, small uncertainties or errors in the initial conditions have the least effect on model skill beyond 12 hours.Latent heating effects associated with condensation and precipitation, and sensible and latent heat fluxes from the surface have a statistically significant effect on model skill. However, relatively simple parametrizations of these effects produce nearly the same skill as do more complex schemes for the cases studied here.There is a large variation in model skill from case to case; some cases are easier to forecast than others. Also, there is much greater variation in different model performances for a single case than when averaged over all 12 cases.The concept of climatological use and verification of regional models is introduced. The climatological skill of the control model version which includes all physical processes appears to be quite good. The skill scores of the ensemble average simulation are considerably better than the average scores for individual cases, the model has small bias errors, and the horizontal structure of the model‐simulated atmosphere is similar to the observed structure for scales of motion resolved by the upper‐air observational network over the United States.
A numerical study of the parametrization of deep tropical convection
Authors: Gregory D. , Miller M. J. 
Published: February 2007

AbstractA two‐dimensional mesoscale cloud model is used to simulate an ensemble of deep tropical convective clouds using GATE data. Random heating perturbations near the sea surface initiate the development of a population of clouds which grow under the influence of specified large‐scale, radiative and surface forcing. Budgets of convective heating and moistening are presented and the model found to be a reasonable representation of the atmosphere.The model is used to investigate the parametrization of cumulus convection in large‐scale models. Equations governing the effect of convection upon the large‐scale atmosphere are derived in full and compared with approximate forms used by previous workers. These forms are the basis of many parametrization schemes. Simple representations of cumulus heating (Q1), cumulus moistening (Q2), and dynamical transports for use in parametrization schemes in large‐scale atmospheric models are studied. The importance of shallow convection in determining the vertical structure of Q2 is recognized. The dependence of convective rainfall upon atmospheric structure and large‐scale processes (the closure problem) is investigated.The importance of downdraughts is emphasized and difficulties in the calculation of Q2 identified. Study of parametric forms for cumulus momentum transports indicates that the ‘cumulus friction’ hypothesis is inadequate. It is suggested that present closure schemes are too restrictive.
The role of diabatic heating in maintaining the upper‐tropospheric baroclinic zone in the South Pacific
Authors: Robertson Franklin R. , Vincent Dayton G. , Kann Deirdre M. 
Published: February 2007

AbstractA modified set of ECMWF FGGE level III‐b analyses is used to examine the four‐dimensional structure of the region containing the South Pacific convergence zone (SPCZ) during a period when two cyclones form in the tropics and propagate south‐eastward along the SPCZ into middle latitudes. During this period. 10–18 January 1979, the atmosphere is characterized by a highly baroclinic state which, together with the existence of an upper tropospheric subtropical jet, supports the growth and development of the observed cyclone activity. In a previous study, it was found that convective activity was prominent during the life cycle of each cyclone.The goal of the present study is to examine the significance of kinematic and thermodynamic processes in maintaining the upper baroclinic region coincident with the subtropical jet. This baroclinic zone lies just poleward of the warm core associated with the convectively‐active SPCZ. A partitioned form of the fronto‐genetical function is used to diagnose the adiabatic and diabatic contributions to the maintenance of baroclinicity in the vicinity of the SPCZ. A major balance is found between the frontogenetical contribution by differential diabatic heating and the opposing diabatic tilting processes. In contrast to the results of most mid‐latitude cyclone studies, the adiabatic contributions from the deformation and tilting terms appear to be of lesser importance. Also, the contribution from the deformation associated with the horizontal branch of the diabatically forced circulation is found to be relatively unimportant. The latter finding is in disagreement with some previously reported numerical results concerned with middle latitude moist frontogenesis. Possible reasons for these differences are discussed.
Reflection and attenuation of equatorial waves in the stratosphere and mesosphere
Authors: Reddy C. A. , Vijayan Lekshmi 
Published: February 2007

AbstractQuantitative theoretical estimates are made of the reflection and radiative damping of the Kelvin and Rossby‐gravity (RG) waves propagating upward in the 20–90 km altitude region. An atmosphere with realistic temperature, wind and cooling rate coefficient values is approximated by a model of vertically stacked, thin, homogeneous layers. The amplitude reflection coefficients are shown for different wave periods (T) and zonal wavenumbers (kx). Combining the attenuation due to radiative cooling with the growth due to density decrease, the net changes of wave amplitudes with height in the 20–90 km region are shown for various wave modes.For Kelvin waves the reflected wave energy is generally less than 15% of the incident wave energy, but the reflected wave amplitude can be 20%‐30% of the incident wave amplitude for many modes, at 20 km height. In contrast, many RG wave modes suffer strong or total reflection at various heights if they are not severely damped at lower heights. Calculations for January and July wind conditions reveal a good deal of sensitivity of RG wave reflection to wind conditions.Radiative damping is found to be severe in the mesosphere for Kelvin waves with T ⩾ 6–8 days, depending on wind conditions; it is severe even in the lower stratosphere for RG waves with T > 4 days. Considerable changes in the damping rates and wave amplitudes at various heights take place from January to July owing to wind changes. Kelvin waves in the 2–8‐day period range and RG waves in the 1.5‐3‐day range can propagate into the upper mesosphere/lower thermosphere without large attenuation, depending upon wind conditions.It is shown that our theoretical results can explain several observed characteristics of equatorial waves in the stratosphere and mesosphere, including the recent results on wave disturbances in satellite‐measured temperatures. Further modelling studies based on our methodology can lead to a good physical insight and quantitative interpretations of the SAO‐ and the QBO‐generation processes.
Precipitation development in a New Mexico thunderstorm
Authors: Raymond D. J. , Blyth A. M. 
Published: February 2007

AbstractObservations were made of a thunderstorm over Langmuir Laboratory for Atmospheric Research on 2 August 1984. Four Doppler radars were closely spaced, allowing observations to be taken at high elevation angles. As a result, the vertical component of the precipitation particle velocity was accurately measured. In conjunction with the vertical air velocity, obtained by integration of the continuity equation, the particle terminal velocity field was computed.Backward integration of particle trajectories showed that most precipitation embryos originated near the tops of thunderstorm precursor clouds when these cloud tops were 7–8 km above sea level. Early and mature phase precipitation grew by accretion at levels below 7 km, and resulted in relatively high density graupel or hail. Late phase precipitation underwent growth mainly above 8 km, and was sufficiently small in diameter and low in density to undergo rapid melting at the freezing level.
The effects of oppositely sloping boundaries with Ekman dissipation in a nonlinear baroclinic system
Authors: Weng H.‐Y. 
Published: February 2007

AbstractA modified Eady channel model with oppositely sloping top and bottom Ekman layers is used to examine analytically and numerically the effect of the slope, Δ, with dissipation, δ, on baroclinic flows in linear and nonlinear systems. the spectral wave solution is truncated up to six components.In the linear system, when δ = 0, waves are dispersive and move westward owing to Δ. the maximum value of Δ for which unstable waves may exist, ΔCmax, is equal to the slope of the isentropes of the basic state. Δ exhibits only a weak stabilizing influence on short waves, while exhibiting a strong stabilizing influence on long waves when it exceeds a critical value. When δ ≠ = 0, a small δ has a destabilizing effect on short waves by shifting the short‐wave cut‐off in the shorter wave direction and extending Δcmax to a larger value. However, the combined effect of Δ and δ strongly stabilizes long waves. the most unstable wavenumber may be shifted to a higher one by increasing Δ, in both inviscid and viscous cases. the relation between the most unstable wavenumber and Δ is similar to some annulus experiments with oppositely sloping boundaries. It is also similar to the relation between the most unstable wavenumber and β in β‐plane models.In a nonlinear system without wave‐wave interaction, Δ stabilizes the flow even for small δ and reduces the domain of vacillation and aperiodic flow regimes, while enlarging the domain of single‐wave steady state. It also qualitatively agrees with the results from β‐plane dissipative systems. Within the single‐wave steady state regime, the preferred nonlinear wavenumber may also be shifted to a higher one by increasing Δ. When wave‐wave interaction is allowed, a variety of flow regimes is observed in four truncated systems. There are single‐wave steady states, multi‐wave steady states where wave dispersion or structural vacillation may occur, multi‐wave vacillation, and aperiodic flows. In contrast to the system without wave‐wave interaction, the domain of vacillation is enlarged. A ‘frequency‐locking’ mechanism is found in multi‐wave steady states, where all three conditions required for a resonant triad are strictly satisfied.