bluuebunny/crossref_metadata_2025_split huggingface dataset:
The Middle Atmosphere In the Absence of Dynamical Heat Fluxes
Authors: Shine K. P. 
Published: July 2010

AbstractA new radiative transfer scheme, developed for use in a three‐dimensional dynamical model of the stratosphere and mesosphere, is used to consider the hypothetical state of the middle atmosphere in the absence of dynamical heat fluxes. Most previous work in this area has considered radiative equilibrium temperatures using a fixed solar forcing. This equilibrium state is shown to be different from a state calculated using a seasonal cycle of solar forcing; in particular, the winter pole is found to be warmer and the latitudinal temperature gradient across the edge of the polar night is found to be smaller. This difference is due to two processes. Firstly, latitudes just polewards of the edge of the polar night at solstice are in darkness for only a short period. Secondly, radiative time scales lengthen greatly as temperatures decrease, so that equilibrium cannot be reached before the sun reappears.Recent ozone climatologies are used to impose a realistic annual cycle of ozone on the model. It is found that the simulation of the lower stratosphere is in considerably better agreement with observations than previous radiation‐only calculations. the better simulation is shown to be, in most cases, due to the use of the new ozone climatology. It is also shown that a realistic ozone distribution is necessary for a good simulation of the polar stratopause at the summer solstice.A comparison of the observed and calculated annual cycle of temperature in the polar regions of the southern hemisphere lower stratosphere indicates that this region is under strong radiative control. This contrasts with an implied strong dynamical control for the same region in the northern hemisphere.
The Formation and Movement In the Stratosphere of Very Dry Air
Authors: Robinson G. D. , Schoen M. G. Atticks 
Published: July 2010

AbstractStatistics of the saturation mixing ratio of H2 O at the temperature minimum and at 100 mb are extracted from about 16 000 radiosonde ascents between latitudes 20°N and 20°N on 140 days, sampling the year December 1978 through November 1979 (FGGE). ‘Window’ radiance temperatures measured by satellite (TIROS and Japanese GMS) between 15°N and 15°N latitude 85° to 185° longitude, at the highest available resolution (about 5 km × 5 km) on one day (Julian day 28 1979), are analysed to extract frequencies of occurrence of temperatures in ranges below −70°C. (Radiosondes and cloud imagery on this day are representative of conditions in these latitudes in the southern summer.) Low radiance temperatures are rare, the frequency of occurrence in the range −86°C to −90°C is 2.5×10−4, whereas about 1 in 4 of the radiosonde ascents in the area at this time have temperatures below −86°C. It is estimated that the cloud turrets with these low radiance temperatures could provide a sufficient source of the driest stratospheric air, but the temperature structure required, by Danielsen's hypothesis, for subsequent dehydration is not observed by radiosonde.All published in situ observations of H2 O mixing ratio in the tropical and northern temperate stratosphere are examined in conjunction with the zonal means observed by the LIMS satellite in December 1978 and May 1979. Constraints imposed by the observations on descriptions of air motion within the stratosphere are discussed.
Potential vorticity in models of the ocean circulation
Authors: Bryan Kirk 
Published: July 2010

AbstractExisting observations suffice to give a qualitative description of the wind‐driven and thermohaline components of the ocean circulation, but a hierarchy of analytical and numerical models is now needed for in coupled ocean‐atmosphere models of the earth's climate. Potentialvorticity is a more appropriate diagnostic field variable than angular momentum for the ocean circulation because of the complicated geometry of basins. Patterns of potential vorticity onsurfaces of constant density help validate ocean circulation models, and give physical insight into how the oceancirculation works.High resolution models suggest that the lateral mixing of potential vorticity by mesoscale eddies along isopycnal surfaces is of the same order as large‐scale advection, and that the assumption of inviscid, potential‐vorticity‐conserving flow in the thermocline is not appropriate for the real ocean. A satisfactory test of this conjecture will require the extensive measurements planned for the World Ocean Circulation Experiment.Models also indicate that the transport of water mass properties by mesoscale eddies is largely a mixing along isopycnal surfaces. The wcak temperaturc gradients along isopycnal surfaces in most areas of the ocean limit the effectiveness of the mesoscale eddies in transporting significant amounts of heat across latitude circles.
A Diagnostic Study of the Dynamics of the Northern Hemisphere Winter of 1985‐86
Authors: Hoskins Brian J. , Sardeshmukh Prashant D. 
Published: July 2010

AbstractFebruary 1986 was notable for a dramatic blocking pattern in the eastern N Atlantic with part of NW Europe having its coldest February in 300 years. However, the previous December and January had been mild in the same region and the hemispheric anomaly field was characterized by a pattern which could be viewed as a wave‐train emanating from the tropical western Pacific. Such an extreme change within a single winter season is unusual and must be understood if reliable monthly and seasonal forecasts are ever to be produced.The tropospheric flow field, isentropic distributions of potential vorticity, and diabatic heating calculated as a residual in the thermodynamic equation are used to illustrate the similarities and differences in the two parts of the winter and to suggest possible reasons for these differences. the mean vorticity equation is also used to investigate the importance of the change in the zonal flow, the transients and the mean divergence. It is concluded that the change in the fluxes due to the synoptic weather systems was crucial, but that a catalyst for the February block could have been provided by an unusual diabatic forcing in the S American‐Caribbean region. Implications are drawn for the ingredients which must be included in dynamical models for predicting on the monthly to seasonal time‐scales.
The Effect of the Hadley Circulation On the Meridional Propagation of Stationary Waves
Authors: Watterson Ian G. , Schneider Edwin K. 
Published: July 2010

AbstractThe effect on the meridional propagation of stationary planetary waves of the inclusion of the zonal mean Hadley circulation in linearized models of the atmosphere is examined. In a forced barotropic vorticity equation model a meridional wind in the basic state enables wave propagation through tropical easterlies in the direction of the wind. This is shown to be consistent with the WKB solution.In a multi‐level linearized steady primitive equation model with an idealized meridional circulation tropical forcing can still produce response through easterlies although propagation from one mid‐latitude to the other is much less than in the barotropic result. This is explained through WKB and wave‐tracing methods for a simplified three‐dimensional vorticity equation. the near non‐divergence of the unforced tropical motion permits a wave to travel largely within a layer of favourable winds.In a twelve‐level model using observed January and July zonal mean conditions the Hadley circulation changes the response to earth topographic forcing mainly in the subtropics. However, tropical heat forcing can now produce a large mid‐latitude response, in the winter hemisphere especially. the resulting subtropical jets suggest a mechanism for the maintenance of those observed over Australia during the northern monsoon season and adjacent to the anomalous forcing of the 1983 El Niño.
Simulated Sudden Stratospheric Warming; Synoptic Evolution
Authors: Blackshear W. T. , Grose W. L. , Turner R. E. 
Published: July 2010

AbstractAn analysis is presented of a sudden stratospheric warming event which occurred spontaneously during a general circulation model simulation of the global atmospheric circulation. Two separate warming pulses exhibit the same dynamical evolution with a ‘cycle’ of about two weeks. Two distinct phases of the warming cycle are apparent: (1) the generation of an intense localized warm cell in conjunction with significant adiabatic heating associated with cross‐isobar flow which has been induced by vertically propagating long wave disturbances; and (2) the northward transport of that warm cell via advection by the essentially geostrophic windfield corresponding to an intense, offset polar cyclone, in conjunction with a strong Aleutian anticyclone. During the first warming pulse in January, a moderate Aleutian anticyclone was in place prior to the warming cycle and was intensified by interaction with an eastward travelling anticyclone induced by the differential advection of the warm cell. the second warming pulse occurred in early February with a strong Aleutian anticyclone already established. In contrast to the January event, the warming in February culminated with reversal of the zonal westerlies to easterlies over a significant depth of the stratosphere.
On the Relation of 6.7 μm Water Vapour Features to Isentropic Distributions of Potential Vorticity
Authors: Manney Gloria L. , Stanford John L. 
Published: July 2010

AbstractCurrently available isentropic maps of potential vorticity (IPV maps) are of low resolution. In this note, we examine the possibility of using 6.7 μm satellite‐derived water vapour measurements to obtain higher resolution information about ipv distributions. While attempts to relate the two data sets analytically have so far proved unsuccessful, a qualitative comparison shows very similar features in both. Our results suggest that, over limited areas, it may be possible to use 6.7 μm brightness temperatures to obtain approximate contours for water vapour mixing ratio on a particular isentropic surface in the upper troposphere. For areas free of high clouds, these contours are expected to parallel the ipv contours on this surface. In a case study, the resulting map exhibits dramatically increased horizontal resolution compared with present ipv maps, detailing an intriguing upper‐tropospheric feature at sub‐synoptic scales.
The Effect of Variable Boundary Conditions On the Accuracy of Limited Area Numerical Forecasts
Authors: Hamilton J. E. M. 
Published: July 2010

AbstractWe present the results of an extensive parallel run experiment to determine the effect of variable boundary conditions on the accuracy of forecasts produced by the limited area primitive equation model of the Irish Meteorological Service. the boundary values were extracted from ECMWF forecasts starting from an earlier analysis time (sometimes 12 hours earlier and sometimes 24 hours earlier). the experiment was run for a total of 247 cases (covering the period July 1985 to January 1986). the results show that the use of variable boundary values improves the 24‐hour forecast over the full area of the chart but has only a negligible (but positive) impact over the central section of the forecast area.
Interpretation of Satellite Imagery of A Rapidly Deepening Cyclone
Authors: Young M. V. , Monk G. A. , Browning K. A. 
Published: July 2010

AbstractModel output and observational data are used to interpret, in detail, satellite images of a distinctive cloud system several hundred kilometres long which was observed before and during rapid cyclonic development that occurred over England. Conceptual models are forwarded which account for the structure of the major cloud areas and upper‐level moisture fields (as observed by water vapour imagery) immediately prior to and following the onset of vigorous cyclogenesis.The cloud system corresponded to what R. B. Weldon refers to as a baroclinic leaf cloud and was characterized by ‘ana’ cold frontal ascent. the baroclinic leaf lay within an area of enhanced ascent ahead of a major upper‐level trough. A jet streak which originated upstream, propagated rapidly around the base of the upper trough. A tongue of dry air centred on the left flank of this jet streak lay immediately upstream of the leaf cloud and had a major influence on its subsequent development. This intrusion of dry air was well represented by the Meteorological Office fine mesh model, which showed it as a pronounced tongue of recently descended air with high potential vorticity, which originated in the upper troposphere and lower stratosphere. Rapid cyclogenesis occurred as the dry intrusion overran low‐level air of high θw within a warm conveyor belt at the southern tip of the leaf cloud. Interaction of the dry intrusion and baroclinic leaf cloud is shown to be particularly important in accounting for the detailed distribution of weather. It also suggests a way of pinpointing the location and timing of vigorous cyclogenesis.
Radar Observations and Modelling of Warm Rain Initiation
Authors: Caylor I. J. , Illingworth A. J. 
Published: July 2010

AbstractDifferential radar reflectivity data are presented which indicate that some initial echoes of convective clouds arise from a small number of very large raindrops; these raindrops may be supercooled. Typically raindrop concentrations are three orders of magnitude lower than the average Marshall‐Palmer drop size distribution found in mature clouds. an explanation of these observations in terms of the sweep‐out of cloud droplets by a low concentration of ultra‐giant nuclei of radii between 30 and 100 μm is offered. A simple model of this process, initiated using the average background levels of these nuclei, gives good agreement with the radar data. the model is not critically dependent upon the choice of collection efficiencies of the nuclei for the cloud droplets nor on the precise form of the ultra‐giant nucleus spectrum. the ultra‐giant nuclei seem capable of explaining the appearance of the raindrops without invoking complex mixing processes within clouds or appealing to the stochastic nature of the collisions between cloud droplets.
Modelling Sulphate Deposition Onto Hills By Washout and Turbulence
Authors: Hill T. A. , Jones A. , Choularton T. W. 
Published: July 2010

AbstractIt has recently been shown from model predictions and measurements at Great Dun Fell, that the variation of sulphate deposition with altitude associated with rainfall is much larger than had previously been thought. In this paper the model of deposition by the seeder‐feeder mechanism is extended to cover a wider range of atmospheric conditions and hill sizes and the results are compared with a model of turbulent deposition. It is shown that turbulent deposition rates are a factor of about 5 to 10 less than by the seeder‐feeder mechanism. the patterns of both types of deposition are strongly affected by atmospheric stability and wind speed. the model of deposition by washout of a hill cap cloud by rain predicts that for a steep hill, characteristic length 2 km, the maximum rainfall and sulphate deposition rates are downstream of the hill summit. In this case, the raindrop trajectory through the highest liquid water content region is short, but the condensation rate is large due to strong updraughts. Hence the greatest effect is the wind‐drift of the raindrops and consequently the rainfall and deposition maxima are displaced downstream. On a longer hill, about 10 km in length, cloud water is not so rapidly resupplied due to reduced updraught. the rainfall rate in the region of the summit is greater than for a steep hill because the region of high water content is large in horizontal extent and the effects of wind‐drift are small. As a result, the maximum rainfall and deposition rates occur somewhat upstream of the hill summit, depending on atmospheric stability. the generation of sulphate by the oxidation of sulphur dioxide within cloud can greatly influence the deposition patterns due to both turbulence and washout. If 1 p.p.b.v. of H2O2 is assumed present in the atmosphere prior to cloud formation (a summer concentration) with a higher concentration of SO2, then the sulphate loading within the cloud will rise by several μgm−3 and deposition will increase commensurately. the other main oxidant, ozone, may also contribute significantly to sulphate production. In this case, however, there are no limitations on its availability to inhibit sulphate generation; instead it is found that there is a rapid decrease in the rate of reaction below about pH 4.5. Its effectiveness is therefore strongly dependent on the buffering capacity of the cloud droplet system.
Experiments On Tropical Circulation With A Simple Moist Model
Authors: Davey M. K. , Gill A. E. 
Published: July 2010

AbstractA simple model of the circulation of the tropical atmosphere is constructed where the circulation is determined entirely by the sea surface temperature (s.s.t.) pattern, or rather by a forcing field which is closely related to s.s.t. It is an equatorial beta‐plane model with an assumed simple baroclinic vertical structure. the moisture and precipitation fields are allowed to evolve freely so there are feedbacks between the latent heating, which contributes to driving the motion, and the low‐level velocity field which produces the moisture convergence required to generate precipitation.Experiments with idealized Pacific s.s.t. distributions produce qualitatively realistic precipitation patterns for January (a patch in the west Pacific over the equator) and July (a zonal strip north of the equator).Analytic solutions for zonally symmetric flow are given to illustrate the basic dynamics and to study the parameter dependence of the precipitation. an analysis of zonal variations shows that a good simplifying approximation for this component is to neglect Rayleigh friction while retaining Newtonian cooling.The effect of changing the zonal contrast in the prescribed forcing is to gradually alter the circulation from Hadley‐like to Walker‐like. the model suggests that along the equator a relatively small contrast is needed to break a zonal band of precipitation.
Variational Assimilation of Meteorological Observations With the Adjoint Vorticity Equation. Ii: Numerical Results
Authors: Courtier Philippe , Talagrand Olivier 
Published: July 2010

AbstractThe adjoint variational approach to data assimilation described in the first part of this paper is used, with the same vorticity equation model, to assimilate northern hemisphere radiosonde observations of wind and geopotential distributed over a 24‐hour period.Except over the eastern Pacific Ocean, where no observations are available, the variational assimilation reconstructs all structures of the flow resolvable by the model to an accuracy of about 30 m for geopotential heights and 8 m s−1for wind vectors. A particular structure, the Aleutian depression, is reconstructed even though it was not covered by the available observations.The assimilation produces unrealistic small‐scale noise which can be reduced by adding an appropriate smoothing term to the distance function minimized in the variational process. Detailed study of the minimization strongly suggests that the distance function varies quadratically with respect to the model's initial conditions. This implies that the tangent linear equation of the model suffices to describe the 24‐hour evolution of the forecast error.
Dynamics of a ‘bomb‐like’ deepening secondary cyclone from airborne doppler radar
Authors: Lemaǐtre Y. , Protat A. , Scialom G. 
Published: July 2010

AbstractDuring the second field phase of the COAST (Coastal Observations and Simulations with Topography) experiment, an extremely deep secondary cyclone was sampled over the open Pacific Ocean by an airborne Doppler radar. This secondary low led a few hours later to strong damage from central California to Washington as it moved northward along the western US coast. the present paper gives mesoscale dynamical insight into this deep secondary low. It reveals a warm‐seclusion situation, with the colder airflow that cyclonically encircles the low centre in quasi gradient‐wind‐balance state. A conceptual representation of the mesoscale airflow circulation in the low centre area is given. This study also validates new analysis methods developed to access three‐dimensional dynamical fields at mesoscale from airborne Doppler radar measurements within secondary cyclones. These methods will be used intensively to process airborne radar data gathered during the field phase of the FASTEX (Fronts and Atlantic Storm‐Track Experiment) project so as to scrutinize multiscale processes involved in secondary cyclogenesis over the Atlantic.
The impact of Greenland on cyclone evolution in the North Atlantic
Authors: Kristjánsson J. E. , Mcinnes H. 
Published: July 2010

AbstractIn order to understand better the role of Greenland's orography in determining the position and strength of the Icelandic low, we have carried out a number of experiments using a numerical weather prediction model. In those experiments, Greenland's orography was modified, and the impact on cyclone developments over the North Atlantic was investigated. We have focused on one fairly characteristic winter case from January 1995, but another one from January 1993 gave qualitatively similar results.We have found evidence that the deepening of baroclinic cyclones near Iceland is hampered by the presence of Greenland's orography. This has been shown to be related to the inability of cold air from the north to cross over Greenland, leading to a distortion of the thermal field associated with the disturbance and a halting of the progression of the cold front. In the January 1995 case, a characteristic secondary cyclone forming between Greenland and Iceland was shown to be entirely orographic, in that it was absent in runs where Greenland's orography was removed. the results are considered in the context of recent theoretical studies of flow splitting in stratified flow impinging on high mountains.
Stochastic representation of model uncertainties in the ECMWF ensemble prediction system
Authors: Buizza R. , Milleer M. , Palmer T. N. 
Published: July 2010

AbstractA stochastic representation of random error associated with parametrized physical processes (‘stochastic physics’) is described, and its impact in the European Centre for Medium‐Range Weather Forecasts Ensemble Prediction System (ECMWF EPS) is discussed. Model random errors associated with physical parametrizations are simulated by multiplying the total parametrized tendencies by a random number sampled from a uniform distribution between 0.5 and 1.5. A number of diagnostics are described and a choice of parameters is made. It is shown how the scheme increases the spread of the ensemble, and improves the skill of the probabilistic prediction of weather parameters such as precipitation. A choice of stochastic parameters is made for operational implementation. the scheme was implemented successfully in the operational ECMWF EPS on 21 October 1998.