bluuebunny/crossref_metadata_2025_split huggingface dataset:
Observed winter cyclone tracks in the northern hemisphere in re‐analysed ECMWF data
Authors: Sickmöller Michaela , Blender Richard , Fraedrich Klaus 
Published: February 2007

AbstractThe observed cyclone activity in the northern‐hemisphere winter (DJF) is investigated in 1979‐97 European Centre for Medium‐Range Weather Forecasts re‐analyses. The cyclone trajectories are determined automatically by a next‐neighbour search of minima identified in the 1000 hPa geopotential‐height field (z1000). These are compared with the traditional storm track, defined by the root mean square of the band‐pass filtered 500 hPa geopotential‐height anomaly. The analysis covers the North Atlantic and North Pacific basins. The trend of the cyclone density (distribution) is similar to that of the storm track in the North Atlantic, but opposite in the North Pacific. In the Atlantic, the total number of cyclones with low central z1000 increases, due to the northward shift towards lower regional climatological pressure, whereas the number of cyclones with large gradients decreases. In the Pacific, the number of intense (weak) cyclones with large (low) gradients increases (decreases).The cyclone‐track variability is investigated by a cluster analysts of the relative cyclone trajectories. The three centroids corresponding to north‐eastward and zonally propagating cyclones and nearly stationary ones are very similar in both ocean basins. The winter‐mean cluster occupation numbers of the two propagating cyclone clusters are positively correlated with one another in the Atlantic, and negatively in the Pacific. The three Atlantic cyclone clusters can be linked to particular aspects of the central European climate.The cyclonic activity can be related to teleconnection patterns which are dominant during northern hemisphere winter. During high North Atlantic Oscillation index winters (deeper Icelandic lows), the Atlantic cyclone density shifts northward and is associated with more stationary cyclones. During El Niño warm‐event winters, the Pacific cyclone density is shifted northward and their propagation is oriented more zonally as in high Pacific/North American index phases (deeper Aleutian lows). In the North Atlantic propagating cyclones occur less and stationary ones more frequently in El Nino winters. The correlations between the north‐east ward propagating cyclones in the Pacific and the north‐eastward and zonally propagating ones in the Atlantic suggest an interaction between the two storm tracks.
Combined use of sensitivity information and observations to improve meteorological forecasts: A feasibility study applied to the 'Christmas storm' case
Authors: Hello Gwenaëlle , Lalaurette Frančcois , Thépaut Jean‐Noël 
Published: February 2007

AbstractAn analysis correction method is presented that uses directly the sensitivity information coming from gradient fields at initial time to perform an alternative forecast. The method consists in imposing, at initial time, a 'sensitive' structure to the perturbation which is first computed by adjoint tools. The tuning of the perturbation amplitude is obtained by reducing the distance to the observations at the locations where the sensitive structure is of significant amplitude.The method is tested on a severe meteorological event which was operationally badly forecast even at very short ranges. Several sets of homogeneous observations are considered and the method proves its efficiency for part of them.
Observations of tropical convection events using Indian MST radar: First results
Authors: Jain A. R. , Rao Y. Jaya , Patra A. K. , Rao P. B. , Viswanathan G. , Subramanian S. K. 
Published: February 2007

AbstractMesoscale convective events, associated with severe weather, play a significant role in troposphere‐stratosphere coupling processes. Deep convection occurs very often at tropical latitudes. The convection events result in the transport of energy from the lower troposphere to the upper troposphere and stratosphere, and may also affect global circulation. In the Indian tropical region, meteorological convection events are a common occurrence during the months of May to December. In the present study, very‐high‐frequency radar located at Gadanki (13.45°N, 79.18°E), a tropical station in India, is used to make observations of winds, turbulence and stable‐layer structures, during the passage of convective events over the radar site under different synoptic situations.Radar observations of vertical velocity show enhanced up and down draughts during the passage of convection events. Height and time cross‐sections of radar‐observed signal‐to‐noise ratio clearly show distinct regions of enhanced signal intensity, indicating height and time Structures of enhanced atmospheric turbulence associated with the convection. It is also noted that stable‐layer structures associated with the tropical tropopause are not well defined during the passage of convection. This suggests ‘weakening’ of stable‐layer structures during the convective activity. The present observations also show enhanced mass flux, both in upward and downward directions, through the tropopause, indicating mass exchange between troposphere and stratosphere during the period of convective activity.
Decadal precipitation variability over Europe and its relation with surface atmospheric circulation and sea surface temperature
Authors: Rimbu Norel , Treut Hervf Le , Janicot Serge , Boroneant Constanta , Laurent Carine 
Published: February 2007

AbstractThe relationships between decadal (>5‐year period) observed winter precipitation (PP) variability over Europe and atmospheric circulation and sea surface temperature (SST) during the period 1950–1995 are investigated. More than 48% of the decadal precipitation variability is described by two modes: a meridional dipolar pattern (high PP over northern Europe and low over southern Europe, the Mediterranean and most of central Europe) explaining about 32% of the variance, and an alternating pattern in latitude over Europe (high in central Europe and low in far north‐western Europe and the Mediterranean) explaining about 16% of the variance. The first mode is strongly related to the North Atlantic Oscillation.A canonical correlation analysis between regional precipitation and sea‐level pressure fields shows that a PP pattern similar to the first decadal PP mode is associated with a north‐westerly circulation anomaly pattern while a PP pattern similar to the second decadal PP mode is associated with a monopolar circulation anomaly pattern over most parts of Europe.The correlation map between the time series of expansion coefficients of the first PP mode and sea surface temperature emphasizes a global pattern which resembles the linear trend coefficients pattern of SST for the analysed period. The second PP mode is associated with an SST anomaly pattern similar to that of the SST anomalies characterizing ‘decadal El Niňo Southern Oscillation’ SST pattern.Composite maps of global 500 mb geopotential height based on the time series of expansion coefficients of PP modes emphasize global patterns compatible with SST patterns. After removing the linear trend from the data, the first PP pattern remains almost unchanged but it emphasizes mainly regional connections. The second spatial pattern of the second PP mode and its connections are not affected significantly by removing the trend.
A study of probability distributions of boundary‐layer humidity and associated errors in parametrized cloud‐fraction
Authors: Price Jeremy D. 
Published: February 2007

AbstractObservations of boundary‐layer‐humidity probability‐distributions made with the Meteorological Office tethered‐balloon facility have been derived from nine representative days between 1996 and 1999. Measurements include those made at several heights simultaneously. Four types of distribution have been defined, including skewed and multimodal types. These have been analysed on scales relevant to sub‐grid parametrizations. Four distribution functions have been fitted to the observations and their accuracy in predicting cloud fraction assessed. It was found that a standard beta‐type distribution gave the best representation of the observed humidity‐distribution. In addition, it was found that for grid scales greater than about 5 km, advection plays a significant role in determining the form of the humidity distribution, and that this must be accounted for in any parametrizations. The cloud‐fraction error arising from errors in the mean total specific‐humidity and saturation specific‐humidity is also discussed. That from the former is found to be relatively large.
Residual cumulus parametrization
Authors: Yano Jun‐Ichi 
Published: February 2007

AbstractA novel approach to the parametrization of convective heating‐moistening is proposed. It is observationally known that convective heating is balanced overall by adiabatic cooling due to the resolved‐scale upward motion (large‐scale forcing) during the convective situations, with a corresponding balance also found in the moisture budget. A parametrization may be tuned to satisfy this first‐order balance. However, a small difference between these two terms defines the actual temporal evolution of the temperature and the moisture fields. Hence, the errors due to the cumulus parametrization are often the same order of magnitude as the temporal change itself, in spite of this first‐order tuning. In the present paper, it is proposed that this small ‘residual’ difference be directly represented by a cumulus parametrization. This idea is pursued by adopting a simple Newtonian relaxation formulation in the same spirit as the Betts‐Miller cumulus parametrization. Both the relaxation time‐scale and the reference profile are defined by the optimal fitting of convective‐forcing data diagnosed over the Intensive Flux Array domain during the TOGA‐COARE period for both the total and the residual convective heating parametrizations. Performance of these two types of parametrizations under this optimization is compared using single‐column model experiments.
The O3N2O relation from balloon‐borne observations as a measure of Arctic ozone loss in 1991/92
Authors: Müller Rolf , McKenna Daniel S. , Schmidt Ulrich , Engel Andreas , Proffitt Michael H. 
Published: February 2007

AbstractWe analyse balloon‐borne measurements of ozone (O3) and nitrous oxide (N2O) in the Arctic, obtained during the European Arctic Stratospheric Ozone Experiment campaign in winter and early spring 1991/92. Using N2O as a long‐lived tracer, we identify chemical ozone depletion in the Arctic vortex in the presence of ozone variations caused by dynamical effects. Substantial chemical ozone loss of about 25% locally over the height range of 15‐21 km, corresponding to the potential‐temperature range of 400‐550 K, is deduced for late winter. Mixing processes can be ruled out as the major cause of the observed anomaly in the O3‐N2O relation by considering the concurrently measured chloroflurocarbon‐11‐N2O relation. The chemical ozone loss derived from the balloon‐borne measurements is in agreement with the loss derived, following the same methodology, from satellite (Halogen Occultation Experiment) vortex observations. Consistent with the dynamical development of the polar vortex and with the observed chlorine activation, the major fraction of the ozone decline occurred before February 1992. The reduced ozone levels persisted over the lifetime of the polar vortex until late March 1992.
Explicit simulation of cumulus ensembles with the GATE phase III data: Budgets of a composite easterly wave
Authors: Xu Kuan‐Man , Randall David A. 
Published: February 2007

AbstractThe mass, heat and moisture budgets of a composite easterly wave over the eastern tropical Atlantic are analysed from simulations of a convection‐resolving model (CRM). The large‐scale data from the GARP (Global Atmospheric Research Program) Atlantic Tropical Experiment (GATE) phase III are used to drive the CRM simulations. The simulated, composite easterly wave is reasonably comparable with the observed, except for the lack of shallow convection embedded in the simulated easterly waves. The mesoscale stratiform precipitation processes are found to be an inseparable component of cumulus convection embedded in the easterly waves. Their main differences from convective processes are twofold: (1) the distinct vertical structures of the budget components due to mesoscale updraughts and downdraughts, and (2) the phase lags relative to the convective processes in the temporal evolution of the easterly waves. Thus, theoretical studies should include both convective and mesoscale stratiform precipitation processes in order to further understanding of diabatic processes in the dynamics of the easterly waves and possibly other waves in the tropics. In addition, the budget analysis presented in this study also provides some details for improving parametrizations of cumulus convection.
A review on the use of the adjoint method in four‐dimensional atmospheric‐chemistry data assimilation
Authors: Wang K.‐Y. , Lary D. J. , Shallcross D. E. , Hall S. M. , Pyle J. A. 
Published: February 2007

AbstractIn this paper we review a theoretical formulation of the adjoint method to be used in four‐dimensional (4D) chemistry data assimilation. The goal of the chemistry data assimilation is to combine an atmospheric‐chemistry model and actual observations to produce the best estimate of the chemistry of the atmosphere. The observational dataset collected during the past decades is an unprecedented expansion of our knowledge of the atmosphere. The exploitation of these data is the best way to advance our understanding of atmospheric chemistry, and to develop chemistry models for chemistry‐climate prediction. The assimilation focuses on estimating the state of the chemistry in a chemically and dynamically consistent manner (if the model allows online interactions between chemistry and dynamics). In so doing, we can: produce simultaneous and chemically consistent estimates of all species (including model parameters), observed and unobserved; fill in data voids; test the photochemical theories used in the chemistry models. In this paper, the Hilbert space is first formulated from the geometric structure of the Banach space, followed by the development of the adjoint operator in Hilbert space. The principle of the adjoint method is described, followed by two examples which show the relationship of the gradient of the cost function with respect to the output vector and the gradient of the cost function with respect to the input vector. Applications to chemistry data assimilation are presented for both continuous and discrete cases. The 4D data variational adjoint method is then tested in the assimilation of stratospheric chemistry using a simple catalytic ozone‐destruction mechanism, and the test results indicate that the performance of the assimilation method is good.
FASTEX IOP 18: A very deep tropopause fold. II: Quasi‐geostrophic omega diagnoses
Authors: Donnadille Jérǒme , Cammas Jean‐Pierre , Mascart Patrick , Lambert Dominique 
Published: February 2007

AbstractA set of two companion papers is dedicated to the documentation of the life cycle of a very deep tropopause fold (820 hPa) during the Intensive Observing Period 18 of the Fronts and Atlantic Storm‐Track EXperiment (FASTEX). In this second part, diagnoses of vertical motion using a Q‐vector partitioning in the natural coordinate system that follows the geostrophic wind are analysed. The partitioning allows the evaluation of vertical motions associated with forcing mechanisms such as confluence and diffluence, thermal advection by the horizontal geostrophic shear (shear advection) and curvature of the flow. The synoptic situation involves the formation of an intense upper‐level jet streak when an Arctic trough and a southern ridge move into phase with each other. Results assessed in the course of the tropopause‐fold life cycle show that subsiding vertical motions associated with each of the forcing mechanisms (confluence, shear advection and curvature) overlap on the cyclonic‐shear side of the entrance region of the jet streak. It is shown that an additional effect of shear advection over the confluence, a necessary ingredient for the development of deep tropopause folds in two‐dimensional contexts, takes place in the present case‐study. However, the forcing mechanisms that contribute mostly to subsiding vertical motions over the warm side of the upper‐level frontal zone (i.e. with a maximum frontogenetic effect) are, in order of importance, the shear advection and the curvature.
Water's two height scales: The moist adiabat and the radiative troposphere
Authors: Mapes Brian E. 
Published: February 2007

AbstractThe temperature structure of the tropical troposphere resembles a moist adiabat, with a lapse‐rate transition toward dry adiabatic where water becomes scarce at an altitude Hma ∼ 8 km (350 hPa). Infrared emission by water vapour cools a deeper layer, extending up to Hrad ∼ 14 km (160 hPa). Five consequences of these unequal heights are reviewed.1. Upper‐tropospheric relative humidity is often low, highly variable, and bimodal, due to the rapidity of drying by radiative subsidence.2. Large‐scale divergent circulations (e.g. equatorial v wind) exhibit a two‐celled vertical structure, with an elevated convergence layer near 8‐10 km in the rising branch.3. The dominant deep convective heating process changes from latent heating at low levels to eddy heat‐flux convergence in the upper troposphere. This requires a substantial updraught‐environment temperature difference, which leads to large entrainment near Hma, yielding stratiform anvil clouds which also contribute radiative heating.4. The rising branches of deep (∼Hrad) vertical circulations export more heat than they import as moisture, so that large‐scale tropical dynamics can be characterized by a ‘gross moist stability’.5. Divergent motions with a vertical wavelength ∼8 km, corresponding to Kelvin or gravity wave speeds of ∼15 m s−1, are excited by simple (e.g. uniform) heating profiles extending through the lapse‐rate change near Hma.
The zonal drift associated with time‐dependent particle motion on the earth
Authors: Paldor Nathan 
Published: February 2007

AbstractThe zonal drift associated with the time‐dependent particle velocity on the rotating earth, such as the inertial motion–westward in midlatitudes and eastward in the tropics–is shown to result from the conservation of angular momentum. This finding follows from a transformation of Newton's second law of motion on the earth into a dynamical system where the angular momentum is a system variable, a procedure similar to that used in the description of a spinning top's dynamics. In near‐geostrophic zonal motion (i.e. time‐dependent motion in the presence of a fixed meridional pressure gradient), where the angular momentum is conserved, and for angular momentum that is below a threshold value, the dynamical system possesses two elliptic points located off the equator. The fixed points of the dynamical system correspond to the steady, geostrophic, zonal motion so the time‐dependent velocity of the small‐amplitude motion (when the latitudinal particle excursion is not too large) deviates only slightly from the geostrophic velocity. The explicit expressions derived for the drift near the elliptic points imply that in near‐geostrophic, eastward (westward) motion in midlatitudes the geostrophic, steady, speed provides an overestimate (underestimate) of the temporally averaged drift. This difference between the steady motion and the average of the periodic motion characterizes nonlinear systems. At large values of the angular momentum a single elliptic point exists on the equator and the drift is directed eastwards. This drift, too, can be quantified near the fixed point associated with a fixed zonal velocity on the equator, where the Coriolis force vanishes.At large oscillation amplitude in midlatitudes, when the oscillation encompasses a large latitude band and the instantaneous velocity deviates significantly from the geostrophic velocity, the zonal drift is strongly affected by the presence of a hyperbolic point at the equator. Explicit expressions for the zonal drift are not available in this case, but numerical results show that the effect of this nonlinearity is to greatly enhance the westward drift in midlatitudes compared with the small‐amplitude case.
Conditional symmetric instability, frontogenetic forcing and rain‐band organization
Authors: Lemaǐtre Y. , Scialom G. , Protat A. 
Published: February 2007

AbstractThe mesoscale organization and the thermodynamic structures of cloud bands and rain bands observed on 9 January 1988 during the seventh Intensive Observation Period of the European field experiment project FRONTS 87 are investigated using both radiosounding measurements and dual Doppler‐radar data.The combined role of the conditional symmetric instability (CSI) and of the frontogenetic forcing in this banded organization is quantitatively assessed. It is shown that the observations are in agreement with the subsequent scenario, cloud bands initiated by the large‐scale forcing in the presence of a weakly CSI unstable atmosphere evolve rapidly into CSI rolls. The detailed thermodynamics within one of these rain bands is shown to be in very good agreement with the ones predicted by the CSI theory. A different dynamic behaviour between the cloud bands and the rain bands is, however, shown. Some additional role is suspected to be played by microphysics processes, in particular by snow evaporation, in the reinforcement of CSI ascending motions.
Radiative influences on the structure and lifetime of cirrus clouds
Authors: Dobbie Steven , Jonas Peter 
Published: February 2007

AbstractOur goal in this work is to determine the role of radiation in cirrus clouds. How it impacts the formation and evolution of cloud structure, whether or not it initiates convective instability, and what effect it has on the lifetime of these clouds. In this study, we simulate cirrus clouds using the Met Office large‐eddy simulation model with a broad‐band solar and infrared ice‐specific radiation scheme.We find that radiation can have quite an effect on cirrus clouds. Results for runs with and without radiation show many dramatic differences. In general, radiatively influenced clouds are observed to be much more dynamic and inhomogeneous. The radiation is observed to strongly enhance cellular structure within the cloud layer, and a Fourier analysis of the horizontal ice‐water path (IWP) shows that this cellular enhancement gives rise to inhomogeneity length‐scales roughly related to the thickness of the layer. The Fourier amplitudes for the radiative cases are usually two or three times larger in magnitude than the non‐radiative cases. The inhomogeneity length‐scales do appear in the non‐radiative cases but they are very weak. The radiatively driven clouds in these simulations often had horizontally averaged IWPs larger in magnitude by more than double compared to the non‐radiative cases once the radiation had taken effect, and the cloud lifetime was increased by between 30 minutes and 2 hours.To evaluate if radiation is causing convective instability, we derive and implement a radiation stability number, Rsn. For radiative instability to occur, Rsn must satisfy the condition 0 < Rsn < 1. We also present similar stability numbers for latent, and the combined radiation and latent processes. By evaluating these numbers throughout the domain during the simulation, we can determine when these diabatic processes will overcome the thermal stratification of the layer and induce convective instability. These processes are found frequently to induce instability for the radiative inclusive case. We find that radiative and latent instability usually occurs in updraught regions, and that latent heating initially enhances the development of the plumes prior to the radiation. We also find that the magnitude of the latent heating is also strongly coupled to the effects of radiation, since stronger cell structure induced by radiation results in stronger latent heating at cloud top and less precipitation at cloud base.