bluuebunny/crossref_metadata_2025_split huggingface dataset:
Analysis of an ex‐tropical cyclone after its reintensification as a warm‐core extratropical cyclone
Authors: Browning K. A. , Panagi P. , Vaughan G. 
Published: February 2007

AbstractEx‐hurricane Lili reintensified as an extratropical cyclone before travelling across the data‐rich region of the British Isles on 28 October 1996. The cyclone centre passed close to a Mesosphere‐Stratosphere‐Troposphere (MST) radar, providing continuous profiles of wind etc. which were used to evaluate diagnostics from the mesoscale version of the operational UK Meteorological Office Unified Model. The paper presents a mesoanalysis of the mature extratropical cyclone using model output together with radar and satellite observations. The combined analysis reveals a vertically extensive warm core three‐quarters surrounded by a low‐level jet reaching over 40 m s−1. There was an associated eye, relatively free of cloud and partly surrounded by a hook cloud producing extensive heavy rain, which was itself encircled by cooler dry‐intrusion air. The stratospheric part of the dry intrusion (and its potential‐vorticity (PV) anomaly) descended within a tropopause fold around the cloud hook generally to below 400 hPa, with small pockets penetrating significantly lower. The cyclone's reintensification as an extratropical cyclone was related to its interaction with the stratospheric PV anomaly. This interaction commenced immediately after the decay of the strong moist ascent and associated deep column of diabatically generated positive PV that had characterized the earlier tropical‐cyclone phase. Following reintensification, the dry‐intrusion air entered the eye region of the extratropical cyclone over a deep layer. The mesoscale model represented many aspects of the cyclone structure well but it underestimated the dryness of the dry‐intrusion air entering the eye. The MST radar vividly depicted the region of moist boundary‐layer air responsible for the hook cloud rising up into the region of the lowered tropopause.
Remotely and Regionally Forced Pre‐Monsoon Signals Over Northern India and Neighbourhood
Authors: Krishnan R. , Mujumdar M. 
Published: February 2007

AbstractA detailed diagnostic examination of the National Centre for Environmental PredictiodNational Centre for Atmospheric Research Reanalysis data supplemented by a theoretical study is undertaken with the goal of investigating the dynamics of pre‐monsoon stationary wave anomalies occurring over northern India and adjoining region prior to weak Indian summer monsoons. It is seen from the analysis that the rotational wind anomalies preceding weak monsoon years are primarily forced by tropical divergent wind anomalies located over the tropical central‐eastern Pacific ocean, south Indian ocean and locally over north‐west India. the divergent wind anomalies over the central‐eastern Pacific ocean can be linked to anomalous changes in the tropical Pacific sea surface temperature arising due to El Nioñ/Southern Oscillation (ENSO). the upper level divergent wind forcing overlying the south Indian ocean seems to be locally connected to the warm sea surface temperature anomalies over that region.Results of simple experiments using a forced divergent barotropic vorticity model establish that the maintenance of rotational wind anomalies over north India prior to weak monsoons is attributable to the existence of anomalous divergent wind forcing over the aforementioned regions in the tropics. A normal mode experiment shows that the climatological basic flow alone is not sufficient to support these pre‐monsoon vorticity perturbations. Furthermore, calculation of vorticity generation terms suggests that the divergent wind advection and Rossby source terms make significant contributions to the production of rotational anomalies over north India during the pre‐monsoon months. Based on the above results, it is conjectured that energy injection into the subtropics via Rossby wave dispersion could be a foremost dynamical mechanism which efficiently sustains rotational wind perturbations over north India and neighbourhood. an assessment of the relative importance of the ENSO and non‐ENS0 divergent forcings, shows that the latter tend to exert a larger influence on the pre‐monsoon signals over north‐west India.The implications of using the pre‐monsoon signals for long range forecasting of Indian summer monsoon activity are determined by two factors viz., (i) the robustness and intensity of the pre‐monsoon signals and (ii) the persistence of the signals during the following months. the first factor in turn depends on the manner in which the different forcing anomalies co‐ordinate. the amplitude of the pre‐monsoon signals can be large (small) depending on whether the ENSO and non‐ENSO boundary forcings act in a mutually co‐operative (competitive) manner. the second factor, which is related to the maintenance of anomalous quasi‐stationary patterns during the following months, depends crucially on how the different boundary forcings evolve during the summer monsoon season.
Quasi‐lagrangian energetics of an intense mediterranean cyclone
Authors: Michaelides Silas C. , Prezerakos Nicholas G. , Flocas Helena A. 
Published: February 2007

AbstractStudies of the energetics of synoptic‐scale systems and similar kinds of investigation have traditionally used a Eulerian framework. In this study, the energetics of a synoptic‐scale system have been considered using a quasi‐Lagrangian method, in order to isolate the disturbance under consideration within a volume which moves together with the system at each stage of its development.Applying a Lagrangian framework implies that the dimensions of the computational area can be modified on the basis of predetermined criteria. In this study, an area surrounding a depression as shown on the surface analysis, has been selected. This area moves together with the centre of the depression. the energetics results obtained using such a quasi‐Lagrangian scheme are compared to those obtained by using a Eulerian framework.The synoptic‐scale system studied here is a wintertime frontal depression, the greatest development of which occurred in the central Mediterranean on 7 December 1991. This depression moved east accompanied by significant temperature changes, heavy precipitation and gale force winds.
The dynamics of heat lows
Authors: Rácz Zsuzsanna , Smith Roger K. 
Published: February 2007

AbstractA numerical model is used to investigate dynamical aspects of the structure and evolution of a heat low in an idealized flow configuration with an area of land surrounded by sea. of particular interest is the evolution of the distributions of relative vorticity and potential vorticity. While the heat low has a minimum surface pressure in the late afternoon following strong solar heating of the land, the relative vorticity is strongest in the early morning hours following a prolonged period of low‐level convergence. Thus the heat low is not approximately in quasi‐geostrophic balance. the low‐level convergence is associated with the sea‐breeze and later with the nocturnal low‐level jet. the effects of differing sea area, land area and Coriolis parameter on various aspects of the heat low are investigated.Although a cyclonic vortex, the heat low is characterized by an anticyclonic potential‐vorticity anomaly relative to its environment throughout much of the lower troposphere on account of the greatly reduced static stability in the convectively well‐mixed boundary layer; however, the surface temperature maximum over land corresponds with a cyclonic potential‐vorticity anomaly at the surface. the reduced static stability in the mixed layer has the further consequence that the horizontal components of relative vorticity and horizontal potential‐temperature gradient make a non‐negligible contribution and of opposite sign to the potential vorticity in certain flow regions.Two processes associated with the flow evolution in the model appear to be fundamental to understanding a range of low‐level atmospheric phenomena over the arid interior of Australia: these are the deep convective mixing over land during the daytime and the development of a nocturnal low‐level jet, which leads to convergence in the trough. Such phenomena include the diurnal behaviour of dry cold fronts and the generation of nocturnal wind surges and bores. It is reasonable to assume that similar processes operate in other arid regions of the world where deep convective mixing over land produces local maxima of diabatic heating in the lower atmosphere.
Observations of stratocumulus cloud break‐up over land
Authors: Price Jeremy D. 
Published: February 2007

AbstractDetailed measurements were made during three episodes of boundary‐layer stratocumulus clouds occumng at Cardington, Bedfordshire, UK, using the UK Meteorological Oftice tethered‐balloon system. no cases were of cloud which dispersed or partly dispersed during the day, and the other was of persistent cloud. Analysis of these data reveals that one cloud deck dispersed owing to solar heating of the boundary layer warming the cloud above its dew‐point, and the other dispersed because of a combination of the above and entrainment of drier and warmer air into the top of the cloud caused by strong wind shear. This was the only incidence where entrainment was seen to affect significantly the morphology of a cloud deck. Data for the persistent case suggest that insufficient solar radiation penetrated into the boundary layer to cause significant warming and that the dynamics of the cloud were governed by radiative cooling at cloud top. the differences in the initial morphology of the cloud decks that caused them to evolve differently are discussed.
Simulation of fog with the ECMWF prognostic cloud scheme
Authors: Teixeira João 
Published: February 2007

AbstractA prognostic cloud scheme has been operational at the European Centre for Medium‐Range Weather Forecasts (ECMWF) since 1995. In this paper the performance of the cloud scheme in simulating fog is assessed. A case‐study is performed with the one‐column version of the ECMWF model in order to analyse how the model reproduces the main mechanisms of fog generation and dissipation. A climatology of the model's fog is produced using data from the ECMWF Re‐Analysis and compared with climatological data. High resolution (T213L31) operational forecasts with the prognostic cloud scheme are compared with synoptic reports of visibility for Europe. In this context the relation between fog and temperature, on the one hand, and fog and wind speed, on the other, is explored in some detail.The comparison between the simulated and the observed visibility, in the one‐column model case‐study, shows that the evolution of fog is properly simulated. the analysis of the behaviour of the different parametrized physical processes suggests that the subtle balance between the various processes, fundamental for a realistic fog simulation, is achieved. This study also shows that in radiation fog a cooling process like radiation, that only changes the temperature without changing the moisture, is rather essential in order to reach saturation. the comparison between the model's fog climatology and the climatological data shows that the model is able to reproduce most of the major fog areas, particularly over the ocean.The analysis of the results from high‐resolution fog forecasts for Europe and the comparison with synoptic observations of visibility shows that the model simulates realistically some of the fog over Europe, particularly at 00 and 06 UTC. At 12 UTC the fog in the model is usually underestimated. In general, the number of situations where the visibility is between 2 km and 11 km is underestimated by the model. It is shown that the model is always slightly closer to saturation than the observations and this might be helping the model in producing fog. the model reproduces the observed behaviour of fog as a function of temperature and wind speed rather realistically.
Construction of correlation functions in two and three dimensions
Authors: Gaspari Gregory , Cohn Stephen E. 
Published: February 2007

AbstractThis article focuses on the construction, directly in physical space, of simply parametrized covariance functions for data‐assimilation applications. A self‐contained, rigorous mathematical summary of relevant topics from correlation theory is provided as a foundation for this construction. Covariance and correlation functions are defined, and common notions of homogeneity and isotropy are clarified. Classical results are stated, and proven where instructive. Included are smoothness properties relevant to multivariate statistical‐analysis algorithms where wind/wind and wind/mass correlation models are obtained by differentiating the correlation model of a mass variable. the Convolution Theorem is introduced as the primary tool used to construct classes of covariance and cross‐covariance functions on three‐dimensional Euclidean space R3. Among these are classes of compactly supported functions that restrict to covariance and cross‐covariance functions on the unit sphere S2, and that vanish identically on subsets of positive measure on S2. It is shown that these covariance and cross‐covariance functions on S2, referred to as being space‐limited, cannot be obtained using truncated spectral expansions. Compactly supported and space‐limited covariance functions determine sparse covariance matrices when evaluated on a grid, thereby easing computational burdens in atmospheric data‐analysis algorithms.Convolution integrals leading to practical examples of compactly supported covariance and cross‐covariance functions on R3 are reduced and evaluated. More specifically, suppose that gi and gj are radially symmetric functions defined on R3 such that gi(x) = 0 for |x| > di and gj(x) = 0 for |xv > dj, O < di,dj ≦, where |. | denotes Euclidean distance in R3. the parameters di and dj are ‘cut‐off’ distances. Closed‐form expressions are determined for classes of convolution cross‐covariance functions Cij(x,y) := (gi * gj)(x‐y), i ≠ j, and convolution covariance functions Cii(x,y) := (gi * gi)(x‐y), vanishing for |x ‐ y| > di + dj and |x ‐ y| > 2di, respectively, Additional covariance functions on R3 are constructed using convolutions over the real numbers R, rather than R3. Families of compactly supported approximants to standard second‐ and third‐order autoregressive functions are constructed as illustrative examples. Compactly supported covariance functions of the form C(x,y) := Co(|x ‐ y|), x,y ∈ R3, where the functions Co(r) for r ∈ R are 5th‐order piecewise rational functions, are also constructed. These functions are used to develop space‐limited product covariance functions B(x, y) C(x, y), x, y ∈ S2, approximating given covariance functions B(x, y) supported on all of S2 × S2.
Lightning activity as an indicator of climate change
Authors: Reeve N. , Toumi R. 
Published: February 2007

AbstractData from the Optical Transient Detector lightning sensor are analysed to investigate the hypothesis that global lightning activity will increase should the average global temperature increase. It is shown that changes in global monthly land lightning activity are well correlated with changes in global monthly land wet‐bulb temperatures. the correlation is strongest in the northern hemisphere and weak in the southern hemisphere. the conclusion is that a high land‐area to sea‐area ratio is necessary for a good correlation. Contrary to expectation, the tropics show no correlation. the results predict that a change in the average land wet‐bulb temperature of the globe of just 1K would result in a change in lightning activity of about 40%.
An economical second‐order advection scheme for numerical weather prediction
Authors: Malguzzi P. , Tartagione N. 
Published: February 2007

AbstractA very simple second‐order Eulerian scheme for the advection equation, based on a forward (backward) time integration on even (odd) grid points, is studied. the proposed scheme is similar, but not equivalent, to the so‐called ‘hopscotch method’, developed in the 1960s and early 1970s for the advection‐diffusion equation, and is stable up to Courant number 2.0. It is shown that, in the case of the advection equation, the proposed scheme has the same advantages yielded by the forward‐backward scheme in the case of the shallow‐water equations; in particular, it is equivalent to the application of centred time and space differencing on the Eliassen grid. the new scheme, unlike the classical leapfrog scheme, can be coupled to the forward‐backward integration of the gravity‐wave problem in primitive‐equation models.With the aid of the proposed scheme, an explicit version of the atmospheric, Bologna Limited‐Area Model (developed in recent years at the FISBAT Institute of the National Council of Research of Italy) is devised, and a comparison with the semi‐implicit version of the same model is performed. the explicit version runs with a double time step, achieving the same accuracy with significantly less computer time and storage. It is suggested that the new time scheme is particularly suitable for numerical weather prediction on massively parallel computing machines based on SIMD (Single Instruction Multiple Data) and distributed memory architecture, which strongly penalize non‐local algorithms.
A cloud‐detection scheme for use with satellite sounding radiances in the context of data assimilation for numerical weather prediction
Authors: English S. J. , Eyre J. R. , Smith J. A. 
Published: February 2007

AbstractA scheme for detecting cloud‐affected radiances is described. the method is used to determine the probability of cloud‐free conditions given the observations and the prior knowledge we have about the atmosphere from a numerical weather prediction (NWP) model. This is achieved using a likelihood method. It combines the strengths of some alternative methods (e.g. comparison of infra‐red and microwave channels sounding the lower troposphere and comparison of infra‐red window channels with sea surface temperature) in a powerful and flexible method. It is powerful because it uses different types of information simultaneously. It is flexible because it makes no assumption about which instrument is being processed, or what type of prior information (NWP, climatology etc.) is used. Therefore, it can readily be extended to new situations and data types (e.g. Advanced TIROS Operational Vertical Sounder (ATOVS)). It is suitable for use on general cloud‐detection problems, using combined microwave and infra‐red data. It has been tested using TIROS Operational Vertical Sounder (TOVS) radiances. the new method has been compared with an alternative cloud‐detection method tailored specifically for TOVS and has been developed to a level of robustness adequate for operational use. the new method gave very similar results to the alternative method, especially over the ocean. the differences that did occur have been investigated by comparing with cloud information derived from the Advanced Very High Resolution Radiometer (AVHRR). Both the alternative method and the new scheme were found to have deficiencies when dealing with very low cloud. Some cloud missed by the existing scheme is identified by the new scheme. Over land, cloud detection is more difficult. the two schemes disagree more often, but validation using AVHRR is also more difficult because of increased surface heterogeneity and more variable emissivity and surface temperature errors. the new method is therefore shown to perform at least as well as an alternative method in operational use, whilst gaining the flexibility required for future systems. the implications for ATOVS are discussed.
Impact of a simplified physical package in 4D‐var analyses of fastex situations
Authors: Janisková M. , Veersé F. , Thépaut J.‐N. , Desroziers G. , Pouponneau B. 
Published: February 2007

AbstractIn the context of the FASTEX (Fronts and Atlantic Storm‐Track EXperiment), this paper presents a number of tests and validations of the four‐dimensional variational (4D‐Var) data assimilation system under development at Météo‐France, over the Intensive Observation Periods IOP3 and IOP17, in particular the impact of adding a simplified physical package within the incremental approach. the impact of using a 6‐hour 4D‐Var system (instead of a standard optimum interpolation or a 3D‐Var scheme) is positive for the description and the forecast of Atlantic cyclones. Moreover, despite the lack of tuning and remaining deficiencies in the parametrization schemes. the analysis and prediction of Atlantic cyclones seem better handled with a 4D‐Var system including physical processes in the assimilating model.
Analysis and model dependencies in medium‐range ensembles: Two transplant case‐studies
Authors: Harrison M. S. J. , Palmer T. N. , Richardson D. S. , Buizza R. 
Published: February 2007

AbstractThere are numerous occasions on which operational forecasts from different meteorological centres diverge in the medium range. These differing evolutions may be due to the different analyses used by the centres or may be a result of the different models used to produce the forecasts; or a combination of both factors may be involved. Ensemble forecasts in routine production at the European Centre for Medium‐Range Weather Forecasts (ECMWF) are designed to address the problem of the impact of small analysis differences on forecast evolution. the initial perturbations, generated using the singular‐vector technique, are designed to maximise spread within the ensemble while being consistent with possible analysis errors. the aim of the current work is to examine, in the context of ensemble forecasting, the effects of model and analysis differences on forecast evolution. Two detailed case‐studies have been performed using the ECMWF model at T63 resolution and the UK Meteorological Office (UKMO) Unified Model run at a comparable resolution. In each case four 33‐member ensembles were produced using all possible combinations of ECMWF and UKMO models and analyses. the same initial perturbations, derived using the singular‐vector technique with the ECMWF model, were used for all four ensemble sets.Significant differences between all four ensemble sets are found in each case‐study. the relative impact of model and analysis dependencies varies with geographical region and forecast range; however the greater overall impact is, for both region and range, from the model differences. the varying evolutions of the four ensemble sets are shown to have a substantial impact on synoptic features in the predicted 500 hPa height fields, such that a forecaster would have issued different predictions depending on which ensemble data were available. the synoptic differences are consistent with the statistically significant model and analysis dependencies found using analysis of variance applied to anomaly correlations and to the distribution of ensemble members in phase space. It is demonstrated that the calculation of the initial perturbations is relatively insensitive to differences between the initial analyses of the two centres, but that the dispersion of initial conditions resulting from the 16 singular vectors used to generate the perturbations does not cover the dispersion arising from the analysis fields. In both of the case‐studies each of the four ensemble sets contains information not available in any of the other ensembles. These results raise the possibility that, to maximise the information available to the forecaster, both models and both analyses should be used in the generation of ensemble predictions.