bluuebunny/crossref_metadata_2025_split huggingface dataset:
A Lagrangian‐based analysis of extratropical cyclones. II: A detailed case‐study
Authors: Wernli Heini 
Published: February 2007

AbstractA detailed case‐study analysis is undertaken of the occurrence of significant coherent ensembles of trajectories (CETs) that accompany Atlantic cyclogenesis. The approach is based upon the Lagrangian‐based analysis scheme introduced in Part I.The analysis reveals the existence of several distinct moist ascending (and dry descending) CETs. Two types of moist ascending CETs associated with strong condensational processes ascend from the boundary layer of the warm sector, either in the close vicinity of the cyclone centre, or in the trailing cold‐frontal region. The latter is the main contributor to the precipitation along the cold front during the early phase of the development. Another type of moist CETs—distinguished by maximum water vapour flux—travels at low levels through the warm sector and ascends to mid‐tropospheric levels in the warm‐frontal region, where it produces strong precipitation around the time of maximum storm intensity.CETs with an upper‐level origin are linked to the classical dry intrusion. Different CETs correspond to: strong descent of upper‐level tropospheric air that penetrated to the lower troposphere, the movement of stratospheric air toward the centre of the maturing cyclone, and an event of stratosphere‐to‐troposphere exchange upstream of the upper‐level trough.Two different visualizations of time sequences of the moist ascending CETs indicate that they represent well‐defined ‘flexible tubes’ or airstreams that exist over a time period comparable with the duration of the main cyclone intensification. They provide a translating, flexible but coherent ‘river bed’ for the quasi‐continuous ascent of moist boundary‐layer air to the upper troposphere. These ‘tubes’ are akin to the concept of warm conveyor belts, and the present analysis method objectively diagnoses their geometrical and physical characteristics and potential‐vorticity pattern.Some consideration is also given to the relationship of CETs and ‘tropospheric rivers’, and to possible shortcomings of the approach.
The contribution of cloud inhomogeneities and droplet concentration to the albedo of broken‐cloud fields
Authors: Coley P. F. , Jonas P. R. 
Published: February 2007

AbstractCloud fields from a Large‐Eddy Simulation (LES) combined with a Monte Carlo radiative‐transfer model have been used to examine the behaviour of clouds with regard to the monochromatic albedo of short‐wave solar radiation. the LES cloud fields have an inhomogeneous structure in two dimensions and contain highly irregular concentrations of liquid water. the calculations show that the albedo sensitivity to the cloud‐field orientation becomes notably important for solar zenith angles above 30° for the LES clouds used. Replacing each cloud point with the in‐cloud mean water content, increases the cloud‐field albedo, compared to the original cloud‐field albedo, in many cases by more than 30%. Calculations have also been made with differing droplet concentrations throughout the cloud fields. It is found that changes in the cloud‐field albedo with droplet concentration are influenced by the general degree of inhomogeneity in the cloud structure. the original irregular LES cloud fields are more sensitive to changes in the droplet concentration than their equivalent plane‐parallel clouds with the same total water content and mean vertical water‐content profiles.
On the maintenance of potential vorticity in isentropic coordinates
Authors: Edouard Sandrine , Vautard Robert , Brunet Gilbert 
Published: February 2007

AbstractWe present a diagnostic study of the maintenance of the potential vorticity (PV) on isentropic surfaces, in the troposphere to the lower stratosphere, using ten years of analyses produced by the European Centre for Medium‐Range Weather Forecasts. After a brief three‐dimensional description of the general circulation in isentropic coordinates, we examine the budgets of the PV evolution equation. By decomposing the flow into its mean, high‐frequency transient and low‐frequency transient parts, we assess the role of these various scales of motion in the maintenance of the mean PV distribution. the contribution of vortical and thickness fluxes is also investigated. One interesting result is the key role of the divergent part of the transient flow along the tropopause in the midlatitudes, which creates a large PV sink region. We give here a tentative explanation.The PV maps are also used to generate a climatology of a wave activity called pseudomomentum, that indicates four centres of wave activity over the Atlantic and Pacific. the first two are located in the northern hemisphere storm tracks and the two others in the upper tropospheric subtropical areas between the 310 K and the 350 K levels.
Seasonal forecasting
Authors: Carson D. J. 
Published: February 2007

AbstractExamples are used to provide an appreciation of the most recent developments in real‐time experimental and operational seasonal forecasting and also of some of the wide issues surrounding the provision and use of such forecasts. The stimulus provided by the study of the El Niño Southern Oscillation phenomenon is discussed, especially with respect to operational seasonal forecasting in the US National Weather Service. The Meteorological Office's seasonal‐forecasting activities and objectives are then reviewed briefly, leading to a description of, and some tentative results from, a major European collaborative project on Prediction Of Climate Variations On Seasonal to Interannual Time‐scales. A wider international perspective is given by considering the relevant projects of the Word Climate Programme and also the proposal for an international research institute for seasonal‐to‐international climate prediction. In conclusion, some further general issues are raised for consideration.
Aircraft observations of the surface energy balance in TOGA‐COARE
Authors: Grant A. L. M. , Hignett P. 
Published: February 2007

AbstractThe TOGA‐COARE (Tropical Ocean/Global Atmosphare Coupled Ocean‐Atomspahere Response Experiment) was in an international observational campaign designed to study the processes that occur in the Pacific warm pool region. Data from the Meteorologiacal Research Flight C130 aircraft are presented to illustrate some of the characteristics of the surface energy balance in this region. The dominant terms in the balance are the turbluent latent‐heat flux and the solar flux. The C130 moisture flux data are compared with a parametrization based on data from the RV Moana Wave, one of the ships that took part in the COARE. It is found that there is no evidence for a systematic difference between the aircraft and ship flux estimates. In addition to the comparison with the COARE parametrization the effect of gustiness due to convective clouds on the surface latent‐heat flux is estimated for three flights, and the thermal balance of the mixed layer in undisturbed conditions described. The data suggest that direct solar absorption is an important part of the thermal balance, and the implications for diurnal variations in the mixed‐layer temperature are considered.The variation in the downwelling solar flux with cloud cover is compared with simple parametrizations, which are found to agree reasonably well with the present data. The correlation between the long‐wave and short‐wave cloud forcings at the surface are described, to illustrate the different effects of different cloud types on the surface radiative balance.
Troposphere to stratosphere transport at low latitudes as studies using HALOE observations of water vapour 1992–1997
Authors: Jackson D. R. , Driscoll S. J. , Highwood E. J. , Harries J. E. , Russell J. M. 
Published: February 2007

AbstractIn this paper we present observations of water vapour (version 18 retrievals) in the equatorial tropopause region made by the Halogen Occulatation Experiment (HALOE) instrument, which files on the Upper Atmosphere Research Satellite. These data make an important new contribution to the observationl knowledge of this region, since water vapour here has largely been observed to date using in situ instruments, and the only global longitudinally‐resolving satellite observations to be reported are those from the Stratospheric Aerosol and Gas Experiment II (SAGE II). Firstly, the HALOE water vapour climatology is presented. Comparison of HALOE and SAGE II climatologies shows considerable agreement. An exception is over Asia in June, July and August (JJA), where the HALOE fields show a distinct region of moist air (associated with convection due to the Asian summer monsoon), whereas no such feature was reported in the SAGE II observations. This suggests that HALOE is sensitive to water vapour variability near the tropopause over regions of strong convection. Secondly, the troposphere to stratosphere transport of air at low latitudes is investigated. Pressure/longitude sections near the equator show that dry air with water vapour mixing ratios of less than 2.4 parts per million by volume (p.p.m.v.) enters the stratosphere in December, January and February, then travels upwards and polewards in the subsequent seasons. A more detailed examination of pressure/longitude sections indicates episodes when dry air also enters the equatorial stratosphere in March, April and May and JJA at times of strong convenction. There is also evidence of entry of air with higher water vapour mixing ratios, of between 3.6 and 4.0 p.p.m.v, into the stratosphere in September, October and November, chiefly at 10°N and 20°N. Further examination of the region of moist air over Asia in JJA (mentioned above) shows that there is clear cross‐isentropic upward transport in the 375–410 K region of unsaturated air. The HALOE water vapour data set reported here provides an important new insight into low latitude processes near the equatorial tropopause.
Data assimilation in the presence of forecast bias
Authors: Dee Dick P. , Da Silva Arlindo M. 
Published: February 2007

AbstractStatistical‐analysis methods are generally derived under the assumption that forecast errors are strictly random and zero in the mean. If the short‐term forecast, used as the background field in the statistical‐analysis equation, is in fact biased, so will the resulting analysis be biased. The only way to account properly for bias in a statistical analysis is to do so explicitly, by estimating the forecast bias and then correcting the forecast prior to analysis.We present a rigorous method for estimating forecast bias by means of data assimilation, based on an unbiased subset of the observing system. The result is a sequential bias estimation and correction algorithm, whose implementation involves existing components of operational statistical‐analysis systems. The algorithm is designed to perform on‐line, in the context of suboptimal data‐assimilation methods which are based on approximate information about forecast‐ and observation‐error covariances. The added computational cost of incorporating online bias estimation and correction into an operational system roughly amounts to one additional solution of the statistical‐analysis equation, for a limited number of observations. Off‐line forecast‐bias estimates based on previously produced assimilated‐data sets can be produced as well, using an existing analysis system.We show that our sequential bias estimation algorithm fits into a broader theoretical framework provided by the separate‐bias estimation approach of estimation theory. In this framework the bias parameters are defined rather generally and can be used to describe systematic model errors and observational bias as well. We illustrate the performance of the algorithm in a simulated data‐assimilation experiment with a one‐dimensional forced dissipative shallow‐water model. A climate error is introduced into the forecast model via topographic forcing. while random errors are generated by stochastic forcing. In this simple setting our algorithm is well able to estimate and correct the forecast bias caused by this systematic error, and the climate error in the assimilated‐data set is virtually eliminated as a result.
Explicit forecasting of supercooled liquid water in winter storms using the MM5 mesoscale model
Authors: Reisner J. , Rasmussen R. M. , Bruintjes R. T. 
Published: February 2007

AbstractAn explicit microphysical parametrization including ice physics was developed for use in the NCAR/Penn State Mesoscale Model Version 5 (MM5). This scheme includes three options of increasing complexity to represent the hydrometeor species. The scheme is evaluated by comparing model simulations with two well observed winter storms that occurred during the Winter Icing and Storms Project. The evaluation focused on the prediction of supercooled liquid water (SLW), which is of particular importance to aircraft icing. The intercomparisons showed that:
The double‐moment microphysical scheme, in which both ice mixing ratios and number concentrations were predicted, performed best, with close agreement to the observed fields.
The single‐moment schemes, in which the mixing ratio of ice species are predicted and number concentration specified, performed reasonably well if a diagnostic equation for No, s, the Y‐intercept of the assumed exponential snow distribution, is allowed to vary with snow mixing ratio.
Accurate microphysical simulations of SLW in shallow upslope clouds and cyclonic storms required accurate simulations of the kinematic and thermodynamic structure and evolution of the storms.
Though the two storms were dynamically different, the SLW formed through a balance of the condensational growth of cloud water and the depletion of cloud water by deposition and riming of snow and/or graupel for both storms. The results of this study suggest that accurate prediction of SLW over limited areas of the country may be possible using the current microphysical parametrization and high‐resolution grids (δχ <10 km).
Climatology of gravity waves in a forest
Authors: Lee Xuhui , Barr Alan G. 
Published: February 2007

AbstractThe objectives of this study are to establish a climatology of gravity waves in a forest and to gain insights into the dynamics of this motion type. The site is part of the BOREAS (Boreal Ecosystem—Atmosphere Study) flux monitoring network. The analysis relies mainly on 5 Hz temperature time series observed with two arrays of fine‐wire thermocouples deployed in the vertical (up to 40 m above the ground) and horizontal (separation up to 150 m) from July to November 1996. Supporting measurements include mean wind speed and direction at two heights above the forest.Waves of various intensities are found over 40% of the night‐time observations, emphasizing the need to understand its role in transporting momentum and masses between the vegetation and the atmosphere. These waves are shear‐generated. They propagate in the direction of the mean wind at a speed that is most likely to fall between the mean wind speeds at z/h = 1.2 and 1.8, where z is the height above the ground and h is the mean tree height. The median values of wave speed, wavelength, wave vertical displacement, and wave frequency are 1.61 m s−1, 75 m, 10 m and 0.0214 Hz, respectively. The wave motion remains coherent in the horizontal over less than one wavelength but is in general persistent in time.
The tropical tropopause
Authors: Highwood E. J. , Hoskins B. J. 
Published: February 2007

AbstractThe physical meaning of several different tropical tropopause definitions is examined using atmospheric data from a variety of sources, and model output. The conventional lapse‐rate definition of the tropopause appears to have little physical relevance in the tropics, although it is easy to use operationally. A four year ‘climatology’ of the tropical tropopause from the European Centre for Medium‐Range Weather Forecasts (ECMWF) analyses is presented. The zonal mean picture shows the annual cycle in properties that has been related to the extratropical stratospheric wave pump. However, there are large and important zonal asymmetries. These include a relatively low pressure and temperature at the tropopause near the west Pacific heating region during December—January—February, and a striking region with low pressure on the tropopause over India during June—July—August. Results from a baroclinic model with imposed diabatic heating are used to support the hypothesis that both these features can be attributed to the direct response of the atmosphere to a large‐scale region of tropospheric diabatic heating. It is proposed that the stratospheric pump provides the general picture for the upper troposphere/lower stratosphere region, but that tropospheric convection is crucial in determining the important zonal asymmetries in this region.
Case‐studies of developing east Pacific easterly waves
Authors: Raymond Davi D. J. , López‐Carrillo Carlos , Cavazos Lucio López 
Published: February 2007

AbstractThis paper contains a study of developing east Pacific easterly waves observed during the summer of 1991 with aircraft in situ and radar data, and geosynchronous infrared satellite imagery. The circulation and divergence profiles, surface momentum and entropy fluxes, as well as 700 mb and 975 mb saturated and actual equivalent potential temperatures, were measured repeatedly for three disturbances that evolved into east Pacific hurricanes. As the disturbances develop, the level of maximum updraught mass flux decreases, which leads to increased convergence at low levels. This process appears to be coupled to an increase in the mid‐level relative humidity, which in turn suppresses the development of convective downdraughts. Comparison of the respective tendencies of convergence and surface friction to spin up and spin down a developing disturbance suggests that friction is unimportant in the early stages of development. However, as the surface circulation develops, the two effects become comparable in magnitude if surface friction is deposited through a fairly deep layer of the troposphere. The observations appear to rule out boundary‐layer Ekman balance in these developing systems, though more‐accurate observations of the eddy fluxes of absolute vorticity into such systems are needed to answer this question with certainty.
Air‐sea fluxes inferred from an upper ocean heat budget north‐east of the Azores
Authors: McCulloch M. E. , Leach H. 
Published: February 2007

AbstractThe contributions to the seasonal heat budget of the upper 500 m of the North Atlantic between 33° and 60° N and 40° and 10° W are calculated using climatology and wind‐stress data. Near the polar front, advection and diffusion are as important in the seasonal heat budget as the air‐sea heat fluxes. In contrast, along a line extending between the Azores and the British Isles the seasonal air‐sea heat fluxes are balanced primarily by heat‐content changes. Therefore, here, the air‐sea fluxes can be inferred from the heat budget with an error of only 23 W m−2 because the large errors in estimates of advection and diffusion are avoided. The inferred fluxes in this area are compared with three bulk‐formulae estimates.
Momentum fluxes due to three‐dimensional gravity‐waves: Implications for measurements and numerical modelling
Authors: Vosper S. B. , Mobbs S. D. 
Published: February 2007

AbstractA three‐dimensional wave‐action equation for linear internal gravity‐waves is derived. When the basic‐state flow does not turn with height, this equation can be used to derive the three‐dimensional version of a theorem of Eliassen and Palm. This states that, for steady flows in the absence of dissipation, the vertical flux of both horizontal components of momentum is independent of height. When the basic‐state flow exhibits turning with height, analysis of the wave‐action equation indicates that, in general, the wave field cannot be regarded as steady and the three‐dimensional Eliassen and Palm theorem does not apply. It is also shown that the wave‐action equation has serious implications for the interpretation of atmospheric measurements of the momentum flux: in order to make accurate measurements of the momentum flux arising from three‐dimensional orographically forced gravitywaves, it is necessary to average ρmu'w' (where ρm is the basic‐state density and u' and w' are the horizontal and vertical wave‐induced velocity‐perturbations respectively) on horizontal planes downstream of the orography, rather than along lines in the downstream direction (as has previously been done with aircraft measurements). Using a three‐dimensional linear numerical model, we demonstrate the importance of this in even the most simple idealized cases.
Extremal principle of entropy production in the climate system
Authors: Pujol T. , Llebot J. E. 
Published: February 2007

AbstractPaltridge found reasonable values for the most significant climatic variables through maximizing the material transport part of entropy production by using a simple box model. Here, we analyse Paltridge's box model to obtain the energy and the entropy balance equations separately. Derived expressions for global entropy production, which is a function of the radiation field, and even its material transport component, are shown to be different from those used by Paltridge. Plausible climatic states are found at extrema of these parameters. Feasible results are also obtained by minimizing the radiation part of entropy production, in agreement with one of Planck's results. Finally, globally averaged values of the entropy flux of radiation and material entropy production are obtained for two dynamical extreme cases: an earth with uniform temperature, and an earth in radiative equilibrium at each latitudinal point.
Distribution of precipitation and wet deposition around an Island mountain in South‐West Poland
Authors: Blaś M. , Dore A. J. , Sobik M. 
Published: February 2007

AbstractA comparison has been made between measurements of precipitation intensity and chemical composition and a three‐dimensional model of airflow and orographic rainfall enhancement. the ‘seeder‐feeder’ effect is shown to be of importance in rainfall distribution, wet deposition of pollutants and the modification of the chemical composition of rainfall over an isolated island mountain. the results show increases in rainfall of between 30% and 83% over the hill, with the smallest increase corresponding to the presence of a strong inversion layer. the peak rainfall enhancement occurred between 0.5 and 2 km downstream of the summit.The measured concentrations of major ions in the feeder‐cloud droplets were greater than those in seeder rain by factors of between two and five. the scavenging of polluted cap cloud by rain drops resulted in an increase in wet‐deposition rate by factors of between three and five, with the peak deposition occurring approximately 2 km downstream of the hill summit.
A model study of the atmospheric boundary layer in the Mars pathfinder lander conditions
Authors: Savijarvi Hannu 
Published: February 2007

AbstractThe diurnal cycle in the Mars Pathfinder sol 3‐6 observed temperatures is compared with results from a one‐dimensional high‐resolution atmospheric boundary‐layer model for Mars. the agreement is in general quite good, the model catching the observed cycle well with realistic regolith (soil) parameter values. the model processes driving the temperature changes, such as the surface radiation and heat budgets, and the atmospheric radiative and turbulent heating rates, are presented and discussed.The model includes moisture, and its hydrologic cycle is described. It indicates a dryish soil and a quite dry daytime well‐mixed boundary layer up to 5 km with specific‐humidity values around 0.18 g kg−1. the relative humidity increases strongly in the evening and saturation is reached by 04 local time near the surface. the very thin radiation fog thus formed is dissolved soon after sunrise. There is weak frost formation at night and sublimation during daytime.