ALBERT

Notes: Abstract. We consider the problem of upscaling transient real gas flow through heterogeneous bounded reservoirs. One of the commonly used methods for deriving effective permeabilities is based on stochastic averaging of nonlinear flow equations. Such an approach, however, would require rather restrictive assumptions about pressure-dependent coefficients. Instead, we use Kirchhoff transformation to linearize the governing stochastic equations prior to their averaging. The linearized problem is similar to that used in stochastic analysis of groundwater flow. We discuss the effects of temporal localization of the nonlocal averaged Darcy's law, as well as boundary effects, on the upscaled gas permeability. Extension of the results obtained by means of small perturbation analysis to highly heterogeneous porous formations is also discussed.

Notes: Abstract. Some of the considerable statistical content of modified Bessel functions of the second kind and of Tricomi's confluent hypergeometric function is illustrated. Moment solutions for the parameters of exponential class distribution functions based on both are derived. Unlike the generalised inverse Gaussian, the Tricomi exponential distribution is little known but it emerges that it is of wide applicability, highly flexible and has the gamma distribution as a special case.

Notes: Abstract.  The use of complicated models of the cancer process has been limited by the need to have a mathematical framework allowing for a general expansion of the theory of carcinogenesis while maintaining numerically tractable results. A system of ordinary differential equations is derived for calculating tumor incidence in multistage models of carcinogenesis. The resulting system of equations is applicable to nonhomogenous processes (i.e. ones in which the rates change with time, age, etc.). In addition, a solution is given for the case of instantaneous initiation. These formulae can easily be coupled with physiologically-based pharmacokinetic models to link biomarkers of exposure to cancer risks. An example is presented.

Notes: Abstract. In recent years, comprehensive geographic data sets of metropolitan areas and individual-level, georeferenced data are becoming more available to social scientists. At the same time, tools for performing spatial analysis in a GIS environment have also become more available. These developments provide many new opportunities for the analysis and theoretical understanding of disaggregate human spatial behavior. This paper examines how these developments may enable the researcher to represent complex urban and cognitive environments more realistically, and to overcome the limitations of aggregate spatial data framework. It explores their implications for the theoretical and methodological development in geography and other social science disciplines.

Notes: Abstract. This paper summarizes contributions of GIS in epidemiology, and identifies needs required to support spatial epidemiology as science. The objective of spatial epidemiology is to identify disease causes and correlates by relating spatial disease patterns to geographic variation in health risks. GIS supports disease mapping, location analysis, the characterization of populations, and spatial statistics and modeling. Although laudable, these accomplishments are not sufficient to fully identify disease causes and correlates. One reason is the failure of present-day GIS to provide tools appropriate for epidemiology. Two needs are most pressing. First, we must reject the static view: meaningful inference about the causes of disease is impossible without both spatial and temporal information. Second, we need models that translate space-time data on health outcomes and putative exposures into epidemiologically meaningful measures. The first need will be met by the design and implementation of space-time information systems for epidemiology; the second by process-based disease models.

Notes: Abstract. This paper shows an explanatory tool (a FORTRAN program linked to GIS software), called VCM (Variable Clumping Method), for detecting statistically significant `multi-level clumps' in the distribution of points. The multi-level clumps imply a set of clumps whose clumping radius varies from a small radius to a large radius (in spirits, VCM is similar to the K-function method). Compared with the ordinary (fixed radius) clumping method, VCM can detect overall significant clumps in the distribution of points.

Notes: Abstract. The spatial aggregation problem – also termed the modifiable areal unit problem – has attracted regular attention in spatial statistics and econometrics. In this study econometric aggregation analysis is used to investigate the formal composition of meso-areal parameters given micro-areal underlying relations with spatial dependence. Impact on stochastic terms (possible meso-areal spatial autocorrelation) is also studied. Finally consequences for meso-areal estimation are derived, the general finding having been that spatial aggregation leads to meso-region specific parameter values, with the estimation problems this implies.

Notes: Abstract. A consistent theme in recent work on developing exploratory spatial data analysis (ESDA) has been the importance attached to visualization techniques, particularly following the pioneering development of packages such as SPIDER and REGARD (Haslett et al. 1990).  The focus on visual techniques is often justified in two ways: (a) the power of modern graphical interfaces means that graphics is no longer a way of simply presenting results in the form of maps or graphs, but a tool for the extraction of information from data; (b) graphical, exploratory methods are felt to be more intuitive for non-specialists to use than methods of numerical spatial statistics enabling wider participation in the process of getting data insights. However, little work has been done to decide which visualization tools might be needed to support ESDA, or how they can be most effectively implemented.  This paper briefly reviews a theoretical framework that has been suggested for addressing these issues, which comprises two elements: firstly a data model, based on the distinction between rough and smooth properties of spatial data, that defines what an analyst is looking for in data (Haining et al. 1998) and secondly a theoretical model for assessing the quality of visualisation tools (Cleveland 1994). The emphasis of this paper is the use of the theoretical framework to structure an assessment of SAGE, a software system that has been written for the spatial statistical analysis (including both exploratory and confirmatory data analysis) of area based data linked to a GIS. The aim of the assessment is to identify the desirable features of the system (that might be employed in other systems) whilst also highlighting what the additional requirements are thereby contributing to the development of systems for ESDA that contain good quality scientific visualization tools for exploratory spatial data analysis.

Notes: Abstract. This paper describes the functionality and architecture of SpaceStat, the SpaceStat Extension for ArcView and the DynESDA Extension for ArcView. It compares the features of these packages to five other software implementations for spatial data analysis. Some ideas are formulated on generic requirements and future directions pertaining to computing environments for spatial data analysis.

Notes: Abstract. It is past time for us to examine the relationship that exists between what we commonly call spatial analysis on the one hand, and geographic information systems or GIS on the other. GIS technology is widely used but seems to incorporate only a portion of the tools found in spatial analysis. Spatial analysis can be accused of overly simplistic approaches that severely limit its practical utility. This paper suggests that discarding myopic views and meeting upon a common ground of increased knowledge of modern computing concepts and techniques, including object-orientation, can prove extremely fruitful to both sides.

Notes: Abstract. This paper is concerned with the intersection of GIS and spatial analysis, its accessibility for scientists who may be less methodologically oriented than others, and its use in substantive research driven by theoretical, conceptual, and empirical issues central to one's discipline. GIS/SA use in this manner has lagged considerably. The case is documented, that there may be shifts in this trend is noted, and suggestions for moving ahead more quickly are put forth. These include instructional efforts, software development, and substantive research that will provide a demonstration effect, guide, and direction for others.

Notes: Abstract. Spatial analysis mostly developed in an era when data was scarce and computational power was expensive. Consequently, traditional spatial analysis greatly simplifies its representations of geography. The rise of geographic information science (GISci) and the changing nature of scientific questions at the end of the 20th century suggest a comprehensive re-examination of geographic representation in spatial analysis. This paper reviews the potential for improved representations of geography in spatial analysis. Existing tools in spatial analysis and new tools available from GISci have tremendous potential for bringing more sophisticated representations of geography to the forefront of spatial analysis theory and application.

Notes: Abstract. Geographical Information Science is essentially computational geography and has its own research program, namely all aspects of formal models for spatial natural processes and the interaction of humans with the environment in space and time. This is not a question of technology and technology-related research; but technology influences what questions can be researched effectively. Collection of data in the field and the simulation of field experience through Virtual Reality are just two questions of how spatial reality and human experience are linked. The focus on human spatial cognition is similarly found in software engineering for interoperable Geographic Information Systems.

Notes: Abstract. The discussion is structured around two themes, namely spatial data analysis and spatial structure/process analysis. With respect to spatial data analysis, I concentrate on the issues related to geographical information systems (GIS), as well as spatial data mining and knowledge discovery in databases. With reference to spatial structure/process analysis, the emphasis is placed on the analysis of complex systems.

Notes: Abstract. This paper provides a description of SAGE, a software package linked to the Arc/Info GIS that can be used to undertake spatial statistical analysis of area based data. The paper is written from the perspective of the user who wishes to undertake exploratory and confirmatory spatial data analysis. The paper discusses design aspects of the package and also the statistical analysis philosophy underlying its contents. The paper describes the statistical analyses which SAGE can perform and details on how it performs them together with some illustrative examples. Detail on visualisation aspects of SAGE are discussed in a separate paper (Haining et al. 2000). The wider contribution of the paper is to build on earlier developments in this area and identify the needs of software packages if they are to enable users to implement effective spatial statistical analysis.

Notes: Abstract. Recently, there is an increasing demand in spatial planning for models based on the demographic concepts of birth and death of firms. This article describes the structure of a spatial demographic simulation model of firms, and its application within The Netherlands. The model structure is essentially of the familiar demographic cohort component type, where an initial cohort of firms ages in a number of discrete steps, and where in each step additions and subtractions to and from the population are modelled using birth, death and migration components. Apart from the central processes of birth, death and migration, the type of economic activity and firm size are highly important for understanding firm behaviour over time. The article describes the transition functions for each of the demographic components and for firm growth. In addition, some empirical results are presented of a number of model simulations in The Netherlands. The results were partly validated using observed economic demographic data. It is concluded that a substantial amount of work remains to be done in this new field. The model presented here has direct implications for the research agenda of the study of the demography of the firm.

Notes: Abstract. Cole (1989, 1997, 1999) advocates the introduction of expenditure lags and the fullest possible closure of single-region input-output models. Jackson et al. (1997, 1999) claim that closing also with regard to the Rest-of-the-World leads to inconsistencies and zero exogenous demand, which makes impact studies impossible. Using somewhat different arguments I agree with them: endogenous interregional feedbacks are conceptually impossible outside a full interregional model. Two hardly discussed points, however, remain for further research. First, closing with regard to all other regional demand is precarious too, as it empirically and theoretically amplifies the one-sidedness of the demand-driven input-output model. Realistic impact studies ask for models including supply-side aspects. Second, adding expenditure lags is an improvement, but the way in which this can be done and should be done requires further theoretical development.

Notes: Abstract. This is the second part of a two-part analysis of optimal spatial search begun in Harwitz et al. (1998). In the present article, two explicit computational procedures are developed for the optimal spatial search problem studied in Part I. The first uses reservation prices with continuous known distributions of prices and is illustrated for three stores. The second does not use reservation prices but assumes known discrete distributions. It is a numerical approximation to the first and also a tool for examining examples with larger numbers of stores.

Notes: Abstract. This article investigates regional variations in firm formation in Finland between 1989 and 1993, and estimates the effects of regional factors on firm formation utilising panel and cross-sectional data. Panel data evidence shows that the average size of firms and establishments in the subregions tends to explain firm formation in Finland most robustly. Cross-sectional results for Finland and several other countries tend to show that demand growth is also an important factor explaining regional firm formation. Panel data results appear to differ from the cross-sectional ones.

Notes: Abstract. This article analyzes the extent to which human capital differences can explain the differences in gross state product (GSP) per capita levels between the richer and poorer states of the US. It uses 1990 Census and Bureau of Economic Analysis data on educational attainment, wage levels of different segments of the labor force, and GSP to compare New York – our representative rich state – with the poorest third of the states. The findings indicate that human capital differences explain at least 49% of the observed difference in GSP per capita between New York and each of the poor states.

Notes: Abstract. The dynamical responses of a shoreline over long-term (years or decades) is a non-linear and time-dependent random process. It is affected by both longshore and cross-shore sediment transports. The former tends to cause cumulative changes in the mean shoreline position while the latter usually only leads to beach profile fluctuations relative to the moving mean beach profile. Due to the time-dependency of the process the life-cycle approach is ideally suited to formulate the probability distribution of extreme shoreline erosion. A model based on such approach and using standard Monte Carlo simulation techniques has been reported by Dong and Chen (1999). In this paper a simplified procedure is developed by introducing the assumption that the longshore and cross-shore processes are statistically independent. This then allows the probability distribution of the extreme erosion to be calculated in terms of the marginal probability distributions of the maximum recessions due to purely longshore and purely cross-shore transport. This method was applied to two idealised shoreline configurations and its usefulness for engineering applications is evaluated by comparison with the full Monte Carlo method.

Notes: Abstract.  Models of dose–response for environmental pollutants generally do not include explicit consideration of the stochastic nature of the spatial pattern of dose delivered to an organ or tissue, or the correlation between events leading to a final health endpoint (such as cancer). The result can be significant errors in risk calculations when these stochastic properties contribute as strongly to the dose–response relationship as do the dose–response relationships for individual cells. The present paper considers the issue of stochasticity of dose and events (initiation, promotion and inactivation) for the case of carcinogenicity following exposure to environmental pollutants, using the case of irradiation by high LET emitters such as radon and progeny from water or air. The model is based on the concepts of hit probabilities and effect-specific track length probabilities (probability of damage per unit track length), and is applied first to in vitro data and then to predictions in vivo. It is shown that inhomogeneity of dose throughout an irradiated tissue or organ volume, and correlation between initiation, promotion and inactivation, can lead to significant differences in predicted risk.

Notes: Abstract.  The selection of optimal management strategies for environmental contaminants requires detailed information on the risks imposed on populations. These risks are characterized by both inter-subject variability (different individuals having different levels of risk) and by uncertainty (there is uncertainty about the risk associated with the Yth percentile of the variability distribution). In addition, there is uncertainty introduced by the inability to agree fully on the appropriate decision criteria. This paper presents a methodology for incorporating uncertainty and variability into a multi-medium, multi-pathway, multi-contaminant risk assessment, and for placing this assessment into an optimization framework to identify optimal management strategies. The framework is applied to a case study of a sludge management system proposed for North Carolina and the impact of stochasticity on selection of an optimal strategy considered. Different sets of decision criteria reflecting different ways of treating stochasticity are shown to lead to different selections of optimal management strategies.

Notes: Abstract.  Although hydrological time series for different sites in a given region are usually correlated and that climate changes should have a regional impact on water resources, very little has appeared in the literature about multivariate change-point analysis. This paper generalizes the univariate Bayesian approach for the detection of a single shift in the mean level to study a change in the mean-vector of a sequence of multivariate normal vectors. Two different problems are considered: the first one is the estimation of the unknown regional change-point under the hypothesis that a shift occurred, while the second one is the overall assessment of change versus no change. This method is illustrated by an application to streamflow data series for six rivers situated in the Northern Québec Labrador region.

Notes: Abstract.  The variability of rainfall is a key component determining how the continental surfaces react to the atmospheric forcing. When studying the impact of climatic fluctuations onto the water resources, it is thus of paramount importance to evaluate to which extent the atmospheric models used in this kind of studies are able to reproduce the variability of the rain process, both in space and time. First among these are the general circulation models (GCM) with coarse resolution, which has two consequences: (i) a simplified parametrisation of convection; (ii) a scale of representation of rainfields which is not adequate when it comes to use them as inputs to hydrologic models. Since linking GCM's and regional hydrologic models is the corner stone of impact studies, it is necessary to analyse the consequences of this gap in scales and to find ways of bridging it. As a preliminary step in that direction, a comparative analysis of the observed and of the GCM rainfall variabilities is carried out for a tropical semi-arid zone of West Africa displaying a high sensitivity to climatic fluctuations. Over tropical regions the GCM used here (LMD-6) has a space resolution of 1.6° in latitude and of 3.75° in longitude. The comparative study shows that the errors of the GCM rainfall outputs may be traced down to two fundamental shortcomings: (i) a wrong seasonal cycle, probably linked to problems in representing the large scale circulation; (ii) an unrealistic simulation of the mesoscale convective systems that are responsible for 90% of the rainfall over this area. This latter problem is especially damaging from an hydrological point of view, as shown from a detailed analysis of high resolution rainfall observations. Even though it is possible to design rainfall desaggregation models producing realistic small scale rainfields from large scale rainfields, such models are of limited utility as long as atmospheric models are not able to produce a realistic climatology in term of number and magnitude of convective systems.

Notes: Abstract.  Environmental companies bidding on multiple projects are presented with the opportunity for diversification in several areas of the environmental market but, also, with the risk of exposure to uncertain events that can result in major financial loss and/or make the company liable to future claims. Accordingly, each corporation determines its maximum financial involvement in a project by evaluating a project's risk characteristics. Further, a company usually takes a global perspective on the total amount it is prepared to lose without compromising its operational viability. This frequently translates into taking less than 100% working interest in a contract and having to determine the combination of working interests for the projects that satisfy both the financial constraints to risk and the requirement to maximize the total profit. This study provides a procedure that can be easily implemented numerically to quantitatively assess the participation in a number of projects under exponential and parabolic utility models. In particular, the parabolic utility model lends itself to analytic expressions for the working interests. Application of the method is illustrated in the case of three projects arranged at an increasing order of expected return, tolerance to risk, uncertainty, and potential to financial loss. Depending on the global risk tolerance, a greater interest is taken in the least uncertain (but least profitable) project, riskier projects considered only after full participation has been achieved in safer projects.

Notes: Abstract.  Over a decade ago, point rainfall models based upon Poisson cluster processes were developed by Rodriguez-Iturbe, Cox and Isham. Two types of point process models were envisaged: the Bartlett–Lewis and the Neyman–Scott rectangular pulse models. Recent developments are reviewed here, including a number of empirical studies. The parameter estimation problem is addressed for both types of Poisson-cluster based models. The multiplicity of parameters which can be obtained for a given data set using the method of moments is illustrated and two approaches to finding a best set of parameters are presented. The use of a proper fitting method will allow for the problems encountered in regionalisation to be adequately dealt with. Applications of the point process model to flood design are discussed and finally, results for a model with dependent cell depth and duration are given. Taking into account the spatial features of rainfall, three multi-site models are presented and compared. They are all governed by a master Poisson process of storm origins and have a number of cell origins associated with each storm origin. The three models differ as to the type of dependence structure between the cell characteristics at different sites. Analytical properties are presented for these models and their ability to represent the spatial structure of a set of raingauge data in the South-West of England is examined. Continuous spatial-temporal models are currently being developed and results are presented for a model in which storm centres arrive in a homogeneous Poisson process in space-time, and cells follow them in time according to a Bartlett–Lewis type cluster. Examples of simulations using this model are shown and compared with radar data from the South-West of England. The paper concludes with a summary of the main areas in which further research is required.

Notes: Abstract.  Permissibility of a covariance function (in the sense of Bochner) depends on the norm (or metric) that determines spatial distance in several dimensions. A covariance function that is permissible for one norm may not be so for another. We prove that for a certain class of covariances of weakly homogeneous random fields, the spatial distance can be defined only in terms of the Euclidean norm. This class includes commonly used covariance functions. Functions that do not belong to this class may be permissible covariances for some non-Euclidean metric. Thus, a different class of covariances, for which non-Euclidean norms are valid spatial distances, is also discussed. The choice of a coordinate system and associated norm to describe a physical phenomenon depends on the nature of the properties being described. Norm-dependent permissibility analysis has important consequences in spatial statistics applications (e.g., spatial estimation or mapping), in which one is concerned about the validity of covariance functions associated with a physically meaningful norm (Euclidean or non-Euclidean).

Notes: Abstract.  An efficient numerical solution for the two-dimensional groundwater flow problem using artificial neural networks (ANNs) is presented. Under stationary velocity conditions with unidirectional mean flow, the conductivity realizations and the head gradients, obtained by a traditional finite difference solution to the flow equation, are given as input-output pairs to train a neural network. The ANN is trained successfully and a certain level of recognition of the relationship between input conductivity patterns and output head gradients is achieved. The trained network produced velocity realizations that are physically plausible without solving the flow equation for each of the conductivity realizations. This is achieved in a small fraction of the time necessary for solving the flow equations. The prediction accuracy of the ANN reaches 97.5% for the longitudinal head gradient and 94.7% for the transverse gradient. Head-gradient and velocity statistics in terms of the first two moments are obtained with a very high accuracy. The cross covariances between head gradients and the fluctuating log-conductivity (log-K) and between velocity and log-K obtained with the ANN approach match very closely those obtained by a traditional numerical solution. The same is true for the velocity components auto-covariances. The results are also extended to transport simulations with very good accuracy. Spatial moments (up to the fourth) of mean-concentration plumes obtained using ANNs are in very good agreement with the traditional Monte Carlo simulations. Furthermore, the concentration second moment (concentration variance) is very close between the two approaches. Considering the fact that higher moments of concentration need more computational effort in numerical simulations, the advantage of the presented approach in saving long computational times is evident. Another advantage of the ANNs approach is the ability to generalize a trained network to conductivity distributions different from those used in training. However, the accuracy of the approach in cases with higher conductivity variances is being investigated.

Notes: Abstract. The probability plots (graphical approach) are used to fit the probability distribution to given series, to identify the outliers and to assess goodness of fit. The graphical approach requires probability of exceedence or non exceedence of various events. This is obtained through the use of plotting position formula. In literature many plotting position formulae have been reported. All of the many existing formulae provide different results particularly at the tails of the distribution and hence there is need of unbiased plotting position formulae for different distributions. Expression for the largest expected order statistics is found in a simple form. Using exact plotting position from Gumbel order statistics a new unbiased plotting position formula has been developed for the Gumbel distribution. The developed formula better approximates the exact plotting positions as compared to other existing formulae.

Notes: Abstract. We calculate explicitly some expectations (integrals over Hilbert spaces) of functionals of Gaussian random fields. Our work is inspired by the formulas for the harmonic oscillator. We also show how to do similar calculations by considering continuum limits of sequences of coupled harmonic oscillators. The results obtained in this work have applications in the study of random media, renormalization and scaling, and in engineering perturbation analysis.

Notes: Abstract. The sampling error formalism by North and Nakamoto (1989) has been widely referenced in research papers on sampling using space-borne sensors or ground-borne sensors. However, their formalism is found to not only underestimate the sampling error, especially for the raingauge network case, but also not be applicable for the cases of using a line of raingauges or microwave attenuation measurements. In this paper, the sampling error formalism has been revised and applied to the same sampling design and the same rainrate model as in North and Nakamoto (1989) for the comparison. The sampling error estimated using the revised formula was found to be more than 50% higher than that by North and Nakamoto (1989). For the case of using a line of raingauges we found that the sampling error converges to a certain value, not zero as in North and Nakamoto formalism, as the number of gauges increases. The microwave attenuation measurements case, which is the same as the case of using a line of infinite raingauges, also gives non-zero sampling errors. Finally, the combined sampling using both satellite and ground-borne sensors (e.g., raingauge network, a line of raingauges, or microwave attenuation measurements) was reviewed to check their design orthogonality and estimated the sampling errors for the combination of satellite and raingauge network case to see its behavior depending on various settings of these two different measurements.

Notes: Abstract.  The North Atlantic Oscillation (NAO) is the most important mode of variability in the northern hemisphere (NH) atmospheric circulation. Put simply, the NAO measures the strength of the westerly winds blowing across the North Atlantic Ocean between 40°N and 60°N. The NAO is not a regional, North Atlantic phenomenon, however, but rather is hemispheric in extent. Based on 60 years of data from 1935 to 1995, Hurrell (1996) estimates that the NAO accounts for 31% of the variance in hemispheric winter surface air temperature north of 20°N. The present article provides an overview of the NAO, its role in the atmospheric circulation, its close relationship to the Arctic Oscillation of Thompson and Wallace (1998), and its influence on the underlying North Atlantic Ocean. Some discussion is also given on the dynamics of the NAO, the possible role of ocean surface temperature, and recent evidence that the stratosphere plays an important role in modulating the NAO.

Notes: Abstract.  A brief review of the state of seasonal forecasting at the end of the twentieth century is given. The physical basis of seasonal predictability is examined, and the implications of this for forecast strategies considered. The range of methods used for seasonal forecasting is described, with its division into empirical and numerical strategies, and methods for creating multi-model forecasts are discussed. Numerical prediction of climate anomalies is a new and emerging field of human endeavour, and some of its particular challenges are highlighted. Finally, the importance of the development of applications of seasonal forecasts is stressed, and the non-trivial nature of this task is noted.

Notes: Abstract.  Since 1995, Météo France has engaged important research works concerning seasonal forecasting within the framework of projects of the European Union. One of these projects is described here. The main goal of the project PROVOST was to evaluate the potential of predictability on seasonal and monthly time-scales of some Global Climate Models (GCM) running separately ensembles of integrations, or gathered in a multimodel ensemble of predictions. The result is that the deterministic skill and the probabilistic skill are both improved by increasing the ensemble size and by merging different models.

Notes: Abstract  It is well known that the computation of higher order statistics, like skewness and kurtosis, (which we call C-moments) is very dependent on sample size and is highly susceptible to the presence of outliers. To obviate these difficulties, Hosking (1990) has introduced related statistics called L-moments. We have investigated the relationship of these two measures in a number of different ways. Firstly, we show that probability density functions (pdf ) that are estimated from L-moments are superior estimates to those obtained using C-moments and the principle of maximum entropy. C-moments computed from these pdf's are not however, contrary to what one may have expected, better estimates than those estimated from sample statistics. L-moment derived distributions for field data examples appear to be more consistent sample to sample than pdf 's determined by conventional means. Our observations and conclusions have a significant impact on the use of the conventional maximum entropy procedure which typically uses C-moments from actual data sets to infer probabilities.

Notes: Abstract. The land surface elevation distribution will serve as fundamental input data to any wetland flow model. As an alternative to the traditional smooth function approach to represent or interpolate elevation data, we explore the use of Levy monofractals and universal multifractals as a means for defining a statistically equivalent topography. The motivation behind this effort is that fractals, like natural topography, are irregular, they offer a way to relate elevation variations measured at different scales, and the relationships are of a statistical nature. The study site was a riparian wetland near Savannah, GA, that contained beavers, and a total of four elevation transects were examined. The elevation increments showed definite non-Gaussian behavior, with parameter values, such as the Hurst coefficient and Lévy index (α), depending on the question of presence of beaver activity. It was obvious that the data were highly irregular, especially the transects influenced by beavers. Significantly different α values were obtained depending on whether the entire data set or just the tails were examined, which demonstrated inability of the monofractal model to reflect fully the irregularity of wetland data. Further analysis confirmed definite multifractal scaling, and it is concluded that the multifractal model is superior for this data set. Universal multifractal parameters are calculated and compared to those obtained previously for more typical terrain. Although it is difficult to consider a unique universal multifractal parameter α for the entire wetland, multifractal-like scaling was evident in each transect as reflected by the nonlinear behaviors of the scaling functions. We demonstrate a good agreement between theory and measurements up to a critical order of statistical moments, q D , close to 3.5 and obtain realistic unconditioned simulations of multifractal wetland topography based on our parameter estimates. Future work should be devoted to conditioning multifractal realizations to data and to obtaining larger data sets so that the question of anisotropy may be studied.

Notes: Abstract. This study is to evaluate the applicability of estimating the one-dimensional horizontal hydraulic diffusivity of an unconfined aquifer with time-dependent fluctuation of lateral head and vertical recharge boundaries using observed water level spectra. Different models of boundary condition are imposed to evaluate the statistical significance between the calculated hydraulic diffusivity (ξ′) with the given hydraulic diffusivity (ξ). The auto-spectra of the water level in observation wells tapping the same aquifer are closely related to those at the disturbed boundaries. For an aquifer with a constant hydraulic diffusivity, the water level fluctuation in the monitoring wells is linearly related to the water level spectra observed at the boundaries. The spectral density function of aquifer hydraulic head varies inversely with specific yield (S y) and directly with recharge. Given small variation in water level spectra at the disturbed boundaries, the water level fluctuation in the aquifer is affected by the recharge condition and the aquifer spectral density function is sensitive to S y. Using an iterative technique to estimate ξ from 1400 sets of given parameters, 99% of the ξ′/ξ values deviated within only one order of magnitude with the model length (L) being equal to 1 km and 10 km. For L equal to 100 m, approximately 82% of the ξ′/ξ population falls within two orders of magnitude. Therefore, spectral analysis of aquifer hydraulic head response can be used to estimate the hydraulic diffusivity of an unconfined aquifer which is affected by periodic variations in recharge and head at boundaries.

Notes: Abstract. Stochastic models are often fitted to historical data in order to produce streamflow scenarios. These scenarios are used as input data for simulation/optimization models that support operational decisions for water resource systems. The streamflow scenarios are sampled from probability distributions conditioned on the available information, such as recent streamflow data. In this paper we introduce a procedure for further conditioning the probability distributions by considering the recent measurements of climatic variables, such as sea temperatures, that are used to describe the occurrence of El Niño. We adopt an auto-regressive model and use the “El Niño information” to refine the parameter estimation process for each time step. The corresponding methodology is tested for the monthly energy time series, “inflowing” to the power plants of Colombia. This is a linear combination of streamflow values for the 18 most important rivers of the country.

Notes: Abstract.  The inherent variability of morphological and physiological parameters can cause significant statistical variations in the deposition patterns of inhaled particles in the human lungs. From a dosimetric point of view, the two major sources of variability are (i) the statistical distribution of particles deposited among all airways in a given generation, caused by the biological variability of the lung structure, and (ii) the local distribution of particles deposited within a given airway bifurcation resulting from the inhomogeneity of flow patterns and associated deposition mechanisms. Due to the stochastic nature of particle transport within the lungs in general, and within airway bifurcations in particular, this variability can be described mathematically by stochastic models. In addition to average values, stochastic models also provide information about the statistical distributions of deposition patterns, reflecting intra- and intersubject variability in particle deposition.

Notes: Abstract.  Because of the nonlinear processes and interactions of the hydroclimatic system, a given hydroclimatic event has an associated probability distribution of possible hydrologic response that changes in space and time. An initial approach in quantifying these evolving probability distributions for use in water resources planning utilizes a simplified climate model. The simplified climate model incorporates the salient physics of the hydroclimatic system for the midlatitudes of the Northern Hemisphere. Using a Monte Carlo format with random initial conditions for the model state variables, hydrologic system response associated with a selected hydroclimatic event is quantified. A case study is presented that utilizes results from the simplified climate model to provide probabilistic seasonal forecasts for water resources planning.

Notes: Abstract.  A possible cause of nonstationarity in time series is the existence of some abrupt modification of their statistical parameters, and especially of a sudden change of the mean. Series with such a change exhibit a strong temporal persistence, with high values of the Hurst coefficient, but with poor possibilities to fit any autoregressive model. Some classical tests (Pettitt, 1979; Buishand, 1982) enable to find a possible change point of the mean and then to split the original nonstationary series into two stationary sub-series. The Bayesian procedure defined by Lee and Heghinian (1977) supposes the “a-priori” existence of a change of the mean somewhere in the series and yields at each time step an “a-posteriori” probability of mean change. But these classical tests and procedures consider only one change point in the original series. To go further and to explore the theoretical multiple singularity models defined by Klemeš (1974) and Potter (1976), a segmentation procedure of time series has been designed. This procedure yields an optimal partition (from a least squares point of view) of the original series into as many subseries as possible, all differences between two contiguous means remaining simultaneously significant. This last requirement is ensured using the Scheffe test of contrasts. The main problem has been to master the combinatory explosion while exploring the tree of all possible segmentations of a series. Some applications of the procedure to hydrometeorological time series are reviewed and some possible improvements are presented.

Notes: Abstract.  Variations in the Earth's climate have had considerable impact on society sectors such as energy, agriculture, fisheries, water resources, and environmental quality. This natural climate variability must be documented and understood in order to assess its potential impacts, its predictability and relationships with human-induced changes. Understanding the mechanisms responsible for natural variability proceeds through a strategy based on the use of a hierarchy of climate models and careful data analysis. In this paper, we examine primarily climate fluctuations on interannual-to-decadal time scales and their climate signature in terms of precipitation and temperature. First, space and time characteristics of two of the major variability modes, the Southern Oscillation (and its associated teleconnection patterns) and the North Atlantic Oscillation, are documented with a focus onto the midlatitudes of the Northern Hemisphere. Then, the current hypothesis regarding the nature of these modes (forced, coupled or internal) are reviewed based on both simulation results and statistical data analyses. Next, we address the potential predictability of seasonal surface temperature and land precipitation using an ensemble of atmospheric model simulations forced by observed sea surface temperatures. Finally, we review the relationships between the atmospheric variability modes and the recent low-frequency trends and suggest a possible influence of anthropogenic effects on these low-frequency variations.

Notes: Abstract.  Optimal selection of sampling strategies is considered for the prediction of spatio-temporal processes in a state-space-model framework. General conditions are assumed in relation to the basic elements of the problem: modelling space-time interaction, formulating prediction objectives, defining the type and structure of sampling configurations, and formulating optimality criteria. An empirical study, involving a diversity of cases selected within two different examples, is carried out with the aim of illustrating some aspects of interest inherent to the problem considered, with special emphasis on highlighting the important effect of the space-time interaction structure on the ratios of information associated with different possible sampling configurations.

Notes: Abstract.  An estimate of a distribution obtained from a sample by any method of classical statistics may be erroneous when the sample is not representative of the population. A subjective distribution elicited from an expert may be miscalibrated when information is scanty and experience limited. The Bayesian Improver of a Distribution (BID) exploits a coherence principle and improves, in the ex ante sense, an initial estimate of a continuous distribution by using (i) the known distribution of a related variate and (ii) information about the dependence structure between the two variates. The theory of BID is developed into an applied (ABID) procedure. The ABID estimator is applicable to any continuous, monotone likelihood ratio dependent variates with arbitrary, strictly increasing marginal distributions, parametric or nonparametric; it is analytic in form and easy to implement via statistical or judgmental methods; it converges to the true distribution, provided the initial estimator does, as the sample size n→∞; it outperforms the initial estimator in the expected Kolmogorov–Smirnov distance for all n; and it offers the greatest gains when n is small – precisely when improved estimates are needed most.

Notes: Abstract. Geographic boundary analysis is a relatively new approach unfamiliar to many spatial analysts. It is best viewed as a technique for defining objects – geographic boundaries – on spatial fields, and for evaluating the statistical significance of characteristics of those boundary objects. This is accomplished using null spatial models representative of the spatial processes expected in the absence of boundary-generating phenomena. Close ties to the object-field dialectic eminently suit boundary analysis to GIS data. The majority of existing spatial methods are field-based in that they describe, estimate, or predict how attributes (variables defining the field) vary through geographic space. Such methods are appropriate for field representations but not object representations. As the object-field paradigm gains currency in geographic information science, appropriate techniques for the statistical analysis of objects are required. The methods reviewed in this paper are a promising foundation. Geographic boundary analysis is clearly a valuable addition to the spatial statistical toolbox.¶ This paper presents the philosophy of, and motivations for geographic boundary analysis. It defines commonly used statistics for quantifying boundaries and their characteristics, as well as simulation procedures for evaluating their significance. We review applications of these techniques, with the objective of making this promising approach accessible to the GIS-spatial analysis community. We also describe the implementation of these methods within geographic boundary analysis software: GEM.

Notes: Abstract. This paper examines why GIS has made little contribution to the dissemination of spatial analysis procedures. In the future, it is anticipated that GIS is more likely to embrace procedures which are exploratory and local and which are capable of performing multiple roles. To facilitate this, there is a need to develop more interdisciplinary interaction, new texts, and integrated software.

Notes: Abstract. The relationship between spatial analysis and GIS has been a debating point for over a decade. Some see GIS as leading the way to a new era in which the desire and the ability to analyse spatial data is widespread, not just in geography but throughout all disciplines in which spatial data are encountered. Others see GIS as simply providing a medium for the recycling of out-dated spatial analytical techniques and models. This paper describes a role for GIS through the development of new forms of ‘local’ or ‘context-dependent’ spatial analytical methods in which the focus is on exceptions to the general trend represented by the more traditional ‘global’ methods.

Notes: Abstract. As the attribute spaces available to geography expand, new challenges are posed in comprehending and analysing data. This article introduces two complementary approaches to analysis that show promise in addressing data with high attribute dimensionality: inductive learning and visualisation. Whilst neither of these techniques are yet as robust or generally available as many accepted parametric techniques, they are nevertheless able to provide insight, and in the case of inductively-based classifiers and approximation methods, have been shown to outperform traditional approaches in some geographic settings. Some problems with parametric inferential statistics are briefly mentioned, followed by descriptions of inductive and visual analysis methods, and some of the important research that remains to be done before they can take a more prominent role.

Notes: Abstract: The main aim of this paper is to provide a theoretical background and empirical evidence on the importance of the cultural sector for competitiveness and development of the regional economy. It is argued that the fall of barriers to trade and the movement of capital and labor in the European Single Market will result in a transformation of the forces shaping its spatial structure. Consequently, the theory of international trade looses its capacity to explain the volume and structure of exchange between regions, which are or belong to different states within the European Union. On the other hand, the theory of spatial distribution of economic activities becomes increasingly relevant for explaining movements of the factors of production and patterns of interregional specialisation.  Based on a case study, the paper analyses the influence of “soft” location factors, in particular cultural activities, on the competitive position of producers located in different urban regions. The “cultural multiplier” approach is used to measure direct income flows resulting from such activities while the concept of externalities is employed to analyse the indirect effects.

Notes: Abstract. The decision to move job and the decision to move residence are closely related dynamic discrete choices, as both moves involve a change of commuting distance. This suggests that labour and residential mobility are mutually dependent and should be simultaneously analysed, based on a theory which incorporates these dynamic decisions explicitly. In the present paper, we base our analysis on search theory and choose the point of departure that individuals maximise utility by moving through different labour market and housing market states, while taking into consideration that moving from one state to another is costly. Based on these assumptions, a search model is constructed and the optimality conditions are derived.  One of main conclusions based on the search model is that the effect of factors which cause housing market imperfections (viz. the residential moving costs and the residence arrival rate) have ambiguous effects on he job acceptance and job search behaviour of employed individuals. For some interesting cases however, the effect of the residential moving costs on the job acceptance behaviour can be derived. In contrast, the effect of housing market imperfections on the labour market behaviour of nonemployed persons is determined. We find for example that housing policies which discourage nonemployed persons to move residence (e.g. housing subsidies) also decrease the probability of becoming employed and may therefore unintentionally increase the number of nonemployed persons.

Notes: Abstract. The initiation of innovation in lagging regions has become one of the most pressing issues in regional policy. Several studies have attempted to identify the factors that influence the creation and development of product innovation in firms located in lagging regions. The identification of these factors could assist regional decision-makers in promoting technological innovation in such regions.  The research question investigated in this study is whether the effectiveness of such regional policies is related to the degree of regional innovation potential and innovativness. This paper tries to deal with this central question by implementing an extended empirical model developed by the author. The Extended Model combines two prevailing probability models: LOGIT and Bayesian decision theory.  The data analyzed in the paper were collected from a field survey of a sample of 211 industrial firms located in the northern region of Israel. In the first stage of the analysis, the model was used to identify variables influencing product innovation; the second stage investigates changes in the probability of producing innovations in the defined region. The results of the analyses point to the effectiveness of a regional policy that could promote and support the creation of factors fostering technological innovation in selected industries in lagging regions.

Notes: Abstract. The causative-matrix method to analyze temporal change assumes that a matrix transforms one Markovian transition matrix into another by a left multiplication of the first matrix; the method is demand-driven when applied to input-output economics. An extension is presented without assuming the demand-driven or supply-driven hypothesis. Starting from two flow matrices X and Y, two diagonal matrices are searched, one premultiplying and the second postmultiplying X, to obtain a result the closer as possible to Y by least squares. The paper proves that the method is deceptive because the diagonal matrices are unidentified and the interpretation of results is unclear.

Notes: Summary  In this paper we describe the results of several numerical experiments performed with the limited area model LAMBO, based on a 1989 version of the NCEP (National Center for Environmental Prediction) ETA model, operational at ARPA-SMR since 1993. The experiments have been designed to assess the impact of different horizontal resolutions and initial conditions on the quality and detail of the forecast, especially as regards the precipitation field in the case of severe flood events. For initial conditions we developed a mesoscale data assimilation scheme, based on the nudging technique. The scheme makes use of upper air and surface meteorological observations to modify ECMWF (European Centre for Medium Range Weather Forecast) operational analyses, used as first-guess fields, in order to better describe smaller scales features, mainly in the lower troposphere. Three flood cases in the Alpine and Mediterranean regions have been simulated with LAMBO, using a horizontal grid spacing of 15 and 5 km and starting either from ECMWF initialised analysis or from the result of our mesoscale analysis procedure. The results show that increasing the resolution generally improves the forecast, bringing the precipitation peaks in the flooded areas close to the observed values without producing many spurious precipitation patterns. The use of mesoscale analysis produces a more realistic representation of precipitation patterns giving a further improvement to the forecast of precipitation. Furthermore, when simulations are started from mesoscale analysis, some model-simulated thermodynamic indices show greater vertical instability just in the regions where strongest precipitation occurred.

Notes: Summary  Two numerical models are used to investigate aspects of thunderstorm dynamics and thunderstorm initiation in the northern Alpine foreland. The first, an isentropic model of airflow over and around the Alps, is used to investigate flow patterns favourable for the initiation of deep convection in the region. It is found that a stably-stratified southerly flow towards the Alps leads to a southwesterly flow in the Alpine foreland, a situation most often found during thunderstorm periods, and to the formation of a gravity wave in the lee of the Alps. This wave is accompanied by raised isentropes which, in reality, would lead to a reduction in static stability and convective inhibition as well as an increase in convective available potential energy. The second model, a cloud model, is used to study the development of an observed squall line over southern Bavaria. The model is initialized with wind, temperature and moisture profiles from a radiosonde sounding ahead of the squall line and the squall line is initiated by an array of thermal bubbles. The model simulation is used to interpret the evolution of the squall line.

Notes: Summary  A semi-geostrophic model of frontal passage over topography was developed to examine the effects of the interaction of a well developed front with an isolated mountain, and the subsequent orographically induced flow evolution. The analytic representation of the primary wave and its frontal structure gives us the ability to control the environmental and geometrical constraints and perform an exploration of parameter space. A number of problems appearing because of this approach are discussed. The results shown relate the characteristics of the orographic perturbation to varying amplitude of the primary wave and varying frontal intensity, but suggest that the representation of flow blocking by the mountain,which this model lacks, is crucial for the representation of frontal intensification in the lee of orography.

Notes: Summary  An analysis of the kinetic energy budget is made for a cyclonic development over the Mediterranean. Horizontal flux convergence constitutes a major energy source. Generation of kinetic energy via cross-contour flow is a persistent sink except only a weak energy source for the decay period of our case study. Dissipation of kinetic energy, computed as a residual, has local maxima both in the lower troposphere and near the jet stream level. By investigating the relative importance of divergent and non-divergent components in the kinetic energy budget we found that when calculating the magnitude of the kinetic energy one may safely replace the total wind with the non-divergent one. But the horizontal flux convergence of kinetic energy and the generation of kinetic energy are sensitive to the magnitude of the divergent wind. A large increase in the kinetic energy of the total flow fields and the nondivergent component of the flows occurs over the southeastern Mediterranean with the existence of the subtropical jet. This indicates that the subtropical jet steadily receives energy from divergent flow. Maximum energy conversion and transport occur near the time of maximum storm intensity while smaller values are observed during the development and decay stages.

Notes: Summary  In this study an attempt is made to investigate comprehensively the dynamics of a case of cyclogenesis over the Aegean Sea within the context of the potential vorticity. At early stages the cyclogenesis is manifested by a large scale development at the upper levels over Adriatic Sea and Yugoslavia associated with an upper tropospheric potential vorticity anomaly. At later stages a smaller scale development was generated over Aegean Sea associated with a low-level potential vorticity anomaly and a surface warm anomaly. By means of a two-dimensional potential vorticity inversion it is demonstrated that the scale, the position and the strength of the involved anomalies contribute to the surface development, however, the low-level potential vorticity anomaly seems to constitute the most significant feature, more likely to be associated with condensation.

Notes: Summary  In this paper, a method of phase space in modeling the climatological series is put forward on the correlation dimension, the theory of phase space and the direct retrieval technique. Then time series of monthly mean temperature are used to build dynamic models of regional climate in phase space in which some dynamic characteristics can be discovered. The results illustrate that the methodology holds promise for a wide variety of applications. Furthermore, the evolution characteristic and the interior interaction mechanism of the regional climate of southern China are also discussed.

Notes: Summary  This paper presents a trend coefficient as a characteristic index with which to investigate the 1951–1998 secular trend variation (STV) of northern 500-hPa height on a seasonal basis. Evidence suggests that low (high) latitude 500 hPa geopotential height displays a remarkably positive (negative) trend; the positive anomaly of the height is maintained over NW Africa, the western Mediterranean Sea and North Africa, mid-Pacific at lower latitudes, Lake Baikal and area to the east and NW North America; the height drops significantly over the Aleutians and North Pacific, the area to the south of the Greenland and NE North America and the sector to the north of the Arabian Sea. Further, analysis of abrupt change shows that the 500-hPa circulations experience noticeable sudden change twice, once around the early 1960s and again in 1976, during which period the height exhibits great differences between the Pacific and Atlantic. In the first period it drops (rises) over the mid-Atlantic at lower latitudes (NW Africa); in the second period the height rises (drops) over the mid-Pacific at lower latitudes and Lake Baikal (the North Pacific). Statistical tests indicate that in the 1976 case the low-latitude positive abrupt change is even more noticeable compared to the negative of the North Pacific, thus representing a large-scale jumping event of the northern 500-hPa height.

Notes: Summary  A shallow-water model is used to examine the motion of a barotropic vortex on an equatorial beta plane in the presence of a large-scale mountain range that represents the Sierra Madre of Mexico. The model is initialized with the analyzed large-scale winds of two Atlantic storms, Hurricane Gilbert (1988) and Tropical Storm Hermine (1980). The study is focused on understanding the motion of tropical cyclones in the western Gulf of Mexico. Vortices representative of each storm are found to experienc e southward deflections as they approach the Sierra Madre. The deflections are attributed to the divergent production of relative vorticity associated with vortex stretching as fluid columns are advected over the mountains. While variations of the depth and stratification of the model atmosphere are also found to influence the track of the vortex, the presence of the mountains is shown to produce the most dominant effect. Additional informatio n on the motion of the vortex is also determined through an examination of the asymmetric potential vorticity field.

Notes: Summary The motivation for this study came from recent results of an Atmospheric Model Inter-comparison Project (AMIP) coordinated by the Lawrence Livermore Laboratory at Livermore, California. That project included a review of seasonal monsoon simulations from 13 different atmospheric models over the world. Most of the models used a horizontal resolution of roughly 300 km. The seasonal monsoon simulations from these models varied significantly. The poor performance by these models stems in part from the use of the coarse resolution. The purpose of this note is to show that by using the same model physics and lower boundary conditions, such as snow/ice cover and sea surface temperatures, the use of the higher horizontal resolution does have a stronger positive impact on the skill of monthly rainfall when compared to a lower horizontal resolution. In this note we present the results of such a comparison between the horizontal resolutions of T42 and T170. These studies are carried out for the prescribed lower boundary specification of sea surface temperatures and snow/ice cover with the help of an Atmospheric General Circulation Model.

Notes: Summary Three different Special Sensor Microwave Imager (SSM/I) rain rate algorithms are evaluated as a means of improving both the physical initialization and the hurricane forecast output of the Florida State University Global Spectral Model (FSU GSM). These SSM/I rain rate algorithms are known as Cal/Val, NESDIS, and GPROF 4.0. In addition the Tropical Rainfall Measuring Mission (TRMM) TMI – 2A12 rain rate algorithm is validated, and its impact on FSU GSM hurricane forecasts is also studied. Validation results of the Cal/Val rain rate algorithm show a bias toward gross underestimation. Both the NESDIS and GPROF 4.0 algorithms produce robust rain rates, in agreement with surface based observations. However, the NESDIS SSM/I rain rate algorithm proves to be the most consistent and accurate in this study. Surface rain rates as estimated by the TRMM/TMI – 2A12 algorithm can be inconsistent, mainly due to satellite observational coverage gaps. The impact of different magnitudes of rain on the FSU GSM is significant. In theory, the application of more accurate and consistent rain rates should produce an improvement in model-calculated latent heat release and cumulus parameterization. The net effect is a more representative, modeled global circulation and improved hurricane track prediction. This research has shown that the use of NESDIS SSM/I rain rates in the physical initialization of the FSU GSM provides the most accurate hurricane track forecasts.

Notes: Summary Errors produced by a nonlinear predictive scheme contain information about both the observations and the prediction system. Therefore, its error history would be expected to contribute to increasing the skill of the predictions if it is included in the forecast. In this study an error recycling procedure is developed for tropical cyclone track prediction. Errors are defined here as differences between the model forecast and the best track position. Error histories are incorporated into a nonlinear analogue, or simplex, forecast scheme and applied to tropical cyclone track prediction, using the archives of observed position data associated with the forecast errors. Various forecast experiments of the cyclone tracks are performed: standard simplex predictions using observed positions only; simplex predictions improved by error forecasts based on libraries of both observations and the recycled forecast errors; and, finally, predictions that include NWP-model forecasts and their errors as predictors. The resulting gains in skill of predictions out to 72 hours ahead are found to be substantial.

Notes: Summary  The fluctuations of intensity of the Tropical Easterly Jet (TEJ) and its association with the Indian summer monsoon rainfall have been examined using the diagnostics from NCEP/NCAR (National Centre for Environmental Prediction/National Centre for Atmospheric Research) reanalyses project for the period 1986 to 1994. The intensity of TEJ is found to be well correlated with India summer monsoon rainfall. The TEJ is weaker/stronger during the El Niño/La Niña year of 1987/1988 and is associated with deficient (excess) summer monsoon rainfall over India. A numerical study was carried out for the same period using the Centre for Ocean-Land-Atmosphere studies General Circulation Model (COLA GCM, T30L18) with observed Sea-Surface Temperature (SST). The GCM simulates the TEJ with reasonable accuracy. The strong interannual variability of TEJ during the El Niño/La Niña years of 1987/1988 are well simulated in the GCM. Like observations, the intensity of the TEJ is positively correlated with the summer monsoon rainfall over India in the model simulation. The intensity of Tibetan anticyclone and diabatic heating over the Tibetan Plateau diminished during the El Niño-year of 1987. The divergence centre in the upper troposphere associated with Asian monsoon becomes weaker and shifts eastward during the weak monsoon season of 1987. However, the opposite happens for the strong monsoon season of 1988. Also the middle and upper tropospheric meridional temperature gradient between the Tibetan High and Indian Ocean region decreased (increased) during the weak(strong) monsoon season of 1987 (1988).

Notes: Summary ¶On 18 June 1997 two simultaneous supercells 50 km apart swept the Po valley W-NW to E-SE. An exceptional hailfall lasted for more than 3 hours over a strip 200 km wide. There are no records of companion supercells over northern Italy, a fact that attributes to the present observations a considerable meteorological interest. The forcing due to a baroclinic wave disturbance created the synoptic conditions favourable for storm development. A closer mesoscale analysis conducted using the Local Analysis and Prediction System (LAPS) indicates that the interaction of the storm systems with the low-level frontal high-humidity band is a key aspect of their evolution. The operational polarimetric Doppler weather radar of S. Pietro Capofiume was used for a volumetric analysis of the storm system focusing on the microphysical and dynamical structure, and wind patterns. Reflectivity and Doppler wind fields document the transition from multicell to supercell phase for one of the storms. Differential reflectivity (ZDR) fields are examined. Positive ZDR columns are detected in connection with the storms strong updraft. Flare echoes hint to the presence of substantial hailshafts. The spectrum width field is used to investigate internal motions. Limited dual Doppler analysis is carried out and results are discussed in terms of storm evolution properties.

Notes: Summary ¶We have investigated the effects of shear and sharp gradients in static stability and demonstrated how a mountain wave and its associated surface winds can be strongly influenced. Linear theory for two-dimensional, nonrotating stratified flow over an isolated mountain ridge with positive shear and constant static stability shows that the horizontal wind speeds on both the lee and upslope surfaces are suppressed by positive shear. The critical F(=U/Nh where U is the basic wind speed, N the Brunt-Vaisala frequency, and h the mountain height) for the occurrence of wave breaking decreases when the strength of the positive shear increases, while the location for the wave-induced critical level is higher in cases with larger positive shear. The linear theory is then verified by a series of systematic nonlinear numerical experiments. Four different flow regimes are found for positive shear flow over a two-dimensional mountain. The values of critical F which separate the flow regimes are lower when the strength of the positive shear is larger. The location of stagnation aloft from numerical simulations is found to be quite consistent with those predicted by linear theory. We calculate the strongest horizontal wind speed on the lee surface (U max), the smallest horizontal wind speed on the upslope surface (U min), the reflection (Ref), and the transmission (Tran) coefficients for different combinations of the stability ratio between the upper and lower layers (i.e. and z 1 (interface height) in a two-layer atmosphere from linear analytical solutions. Both Ref and Tran are found to be functions of log( ) but not the interface height (z 1). Ref is larger when is much different from 1, no matter whether it is larger or smaller than 1. However, Tran decreases when log( ) increases and approaches 0 when log( ) is large. The magnitude of the largest U max (smallest U min) increases (decreases) as the absolute value of log( ) increases. It is found that the largest U max occurs when the nondimensional z 1 is near for cases with a less stable upper layer or when z 1 is near for cases with a more stable upper layer. These results are confirmed by nonlinear numerical simulations. We find that linear theory is very useful in qualitative analysis of the possibility of high-drag state for different stability profiles. The location of stagnation aloft in a two-layer atmosphere from numerical simulations agrees very well with those predicted by linear theory. The above findings are applied to investigate the Boulder severe downslope windstorm of 11 January 1972. We find that the windstorm cannot develop if the near mountain-top inversion is located at a higher altitude (e.g.,  km). However, if there exists a less stable layer right below the tropopause, the windstorm can develop in the absence of a low-level inversion. These results indicate the importance of partial reflection due to the structured atmosphere in influencing the possibility of severe downslope windstorms, although partial reflection may not be the responsible mechanism for the generation of windstorms.

Notes: Summary  Following a recent approach of Fox-Rabinovitz an iterative Matsuno or a “Super-Matsuno” style scheme is applied as a filter in the Eta Model. In contrast to Fox-Rabinovitz, we however apply the scheme not for all of the model’s time-differencing but for its adjustment terms only. These distinctions compared to the original Fox-Rabinovitz’-method are made easy to implement by the split time differencing approach of the Eta, and at the same time would appear clearly appropriate for the “initialization” purpose. In addition, while Fox-Rabinovitz emphasizes the use of the method within a long time-scale data assimilation framework, we are focusing on the impact of the method in a short-range forecasting environment/time-scale. After a short one hour “initialization” procedure is completed, standard model integration is continued, now very much free of noise. The Super-Matsuno style scheme is found to balance initially unbalanced external and internal modes and to significantly reduce the high-frequency noise during the first 6 time steps. In a control case noise also reduces in amplitude as integration proceeds, but at a much slower rate. The model integration results with and without “initialization” after 6 hours are however very similar. Even so, it is to be expected that small differences, given that they have resulted from the removal of spurious initial noise, have to be beneficial.

Notes: Summary ¶Intra-mountain summertime precipitation was studied in the Alps in a 40×20 km2 area centered around Innsbruck, Austria, from June through September 1997. An observational network with a mean separation distance of 9 km and forecasts from the ECMWF model were used to examine the role the strong forcing from the lower boundary plays in creating “hot spots” for the formation of thunderstorms and the location of heavy precipitation as well as systematic precipitation patterns for different weather situations, which can be used to downscale forecasts from global scale routine numerical weather prediction models.

Notes: Summary  Quality and limitations of current quantitative precipitation simulations are determined using the Deutschland-Modell of Deutscher Wetterdienst (DWD). The model independent validation data comprise the regular rain-gauge network of DWD, taylored composites of an international radar network, Doppler winds from the DLR research radar, and meteorological timeseries at a ground station. Area skill scores are compared for convectively dominated as well as more synoptically forced situations in the northern Alpine region. Inspection of the temporal evolution of the components comprising the atmospheric water budget and of the precipitation partition gives insight in the different nature of excitation mechanisms of heavy precipitation. The simulations are found to exhibit growing potential for follow-on hydrological applications, while a real break-through appears to necessitate the coming generation of non-hydrostatic operational forecast models with increased spatial resolution.

Notes: Summary  This paper concerns the use of airborne or ground-based Doppler radars to observe precipitating systems over complex orography. As nearly all of the previous experiments involving Doppler radars were conducted over flat surfaces over the continents or the oceans, new techniques are needed firstly to separate ground clutter from meteorological signal and, in the case of airborne Doppler observations, to deduce navigational errors. Secondly, it is necessary to take the atmospheric circulation induced by orography into account in the three-dimensional wind field analysis. Variational techniques are presented to solve these problems. The proposed methods are tested with simulated ground-based and airborne Doppler radar observations for analytic flows over analytic terrains and for numerically simulated wind and reflectivity fields for the Brig event (22 September 1993) of heavy precipitation over the southern flank of the Alps (Cosma and Richard, 1998), and with actual airborne Doppler data relative to weak snow showers over the Rocky Mountains on 12 March 1995.

Notes: Summary  An hourly averaged climatology at 0.05 ° horizontal resolution over the Lake Tanganyika region was created by making simulations with a mesoscale model (HIRLAM) using a high resolution physiography to represent the surface. Initial and boundary values were interpolated from ECMWF analyses. Climatologies for a typical dry season month (July 1994) and wet season month (March 1994) were created by 7-day segmenting. Model results were validated by utilizing a special coastal observation network. A number of experiments were made with changes to the physiography (mountains/no mountains, lake/no lake). The results reveal local channelling and blocking effects of the near-surface southeasterly trade winds by the high mountain chains in the region of the East African rift. Furthermore, surface winds display regular diurnal cycles in many places, due to slope winds over hills and lake-land-type breezes near the coast. The diurnal coastal winds (defined by the observation network) are reasonably well simulated. Precipitation patterns display the semi-annual march of the ITCZ across the area, plus considerable topographic effects. There is high evaporation from lakes and wetlands during the windy dry season, while evaporation from the moist land surface dominates the rainy season.

Notes: Summary The University of New South Wales (UNSW) High Resolution numerical weather prediction model (HIRES) is run routinely, on a daily basis, at a horizontal resolution of 25 km. The output is made available to the New South Wales (NSW) regional office of the Australian Bureau of Meteorology in Sydney. HIRES has been used to forecast mean wind direction and speed at a height of 12 metres for a number of events in the past. The opportunity was used in December 1997 to run the model for the annual Sydney to Hobart yacht race area. For the 1997 Sydney to Hobart yacht race the model was run at 25 km horizontal resolution and the output was made available to all competitors on the morning of the race, namely December 26th. It was also decided by the authors to expand the verification to include all observations available both from land and sea within the model domain an well as those available from a moving single point at sea, namely a yacht. After the event, the model was run once at the increased resolution of 10 km, out to 5 days ahead. Both model runs were subjected to detailed verification by one of the authors (KLB) who participated in the race aboard the maxi-yacht Nicorette and who carried out a pre-arranged observational program during the race. Surface synoptic weather maps prepared in the NSW office of the Bureau of Meteorology were also consulted in order to extend the verification scheme. The model predicted winds were verified on a six-hourly basis utilising instrumentation on the yacht as well as surface observations plotted in standard World Meterological Organisation (WMO) format on surface synoptic weather maps. The yacht carried wind sensors situated on top of the mast at a height of 30 metres above the water. The authors were most interested in the accuracy of the wind velocity forecast by the model. It is important to note that forecasting for points over the ocean at widely separated time intervals represents a very difficult challenge. The verifications reveal that the model gave overall forecast guidance of very good to excellent quality and was particularly accurate early in the race, when a Southerly Buster event occurred during the evening of the first day. However, a caveat should be added that this forecast, despite its remarkable accuracy, is not a claim that accurate very high resolution regional prediction is a solved problem. Rather, it is a demonstration that in particular instances current models are now capable of achieving high levels of skill a number of days ahead.

Notes: Summary ¶The interannual variability of broad-scale Asian summer monsoon was studied using a general circulation model (GCM) and NCEP (National Center for Environmental Prediction) data set during 1979–95. In the GCM experiment, the main emphasis was given to isolate the individual role of surface boundary conditions on the existence of winter-spring time circulation anomalies associated with the interannual variability of Asian summer monsoon. In order to understand the role of sea-surface temperatures (SSTs) alone on the existence of precursory signals, we have conducted 17 years numerical integration with a GCM forced with the real-time monthly averaged SSTs of 1979 to 1995. In this experiment, among the many surface boundary conditions only SSTs are varying interannually. The composite circulation anomalies simulated by the GCM have good resemblance with the NCEP circulation anomalies over subtropical Asia. This suggests that the root cause of the existence of winter-spring time circulation anomalies associated with the interannual variability of Asian summer monsoon is the interannual variability of SST. Empirical Orthogonal Functions (EOFs) of 200-mb winds and OLR were constructed to study the dynamic coupling between SST anomalies and winter-spring time circulation anomalies. It is found that the convective heating anomalies associated with SST anomalies and stationary eddies undergo systematic and coherent interannual variations prior to summer season. We have identified Matsuno-Gill type mode in the velocity potential and stream function fields. This suggests the existence of dynamic links between the SST anomalies and the precursory signals of Asian summer monsoon.

Notes: Summary ¶The knowledge of the dependence pattern of daily weather variables, which tend to persist in time, is important in developing techniques for simulating weather data. The dependence of daily maximum and minimum temperatures is presented for 17 stations in the three main climatic zones (humid, sub-humid and semi-arid) of Nigeria. Daily maximum and minimum temperatures have an average lag-one serial correlation coefficient of 0.833 and 0.802, respectively, depending more on latitude followed by elevation of the site. The correlation between maximum and minimum temperature averaged 0.387 and also varied with season and location. The results of the Student’s T-test show that persistence within maximum and minimum temperatures and interdependence between the two meteorological variables did not differ significantly, 95% of the times, at most of the sites. The implication of these in developing limited area models for weather forecasting and regional climate studies has been stressed.

Notes: Summary  Two organized mesoscale convective systems (MCSs) developed sequentially along the Meiyu front over the Yangzi-Huai River basin and caused severe flooding over eastern China during 12–13 June 1991. In this paper, the structure and evolution of these MCSs are studied with a high-resolution (18 km) numerical simulation using the Fifth Generation Penn-State/NCAR Mesocale Model (MM5). The model reproduced the successive development of these two MCSs along the Meiyu front. The evolution of these MCSs was recorded clearly on satellite-derived cloud-top black body temperature (T bb ) maps. A mesoscale low-level jet (mLLJ) and a mesoscale upper-level jet (mULJ) were simulated, respectively, to the south and east of each of these two MCSs. Our analyses shows that the mLLJ and mULJ were formed as a responses to the intense convection associated with the MCS. The mLLJs transported warm, moist air with equivalent potential temperature greater than 352 K into the MCSs, and strong low-level convergence can be identified on the left-front end of the mLLJ. This strong convergence was associated with intense upward motion in the MCS with speed up to 80 cm s−1. Much of inflow into the MCSs extends up to the middle and upper troposphere, and ventilated through the mULJ. The development of the MCSs was also associated with substantial increase in potential vorticity (PV). The build up of PV in the lower-level along the Meiyu front was in turn related to a local intensification of the frontal equivalent potential temperature gradient, suggesting a relationship between the MCSs and the local enhancement and cyclogenesis of the front. In a sensitivity experiment without the effect of latent heating, a series of ascent centers with average separation of about 300 km were simulated. This result suggests that the initial formation of the MCSs along the Meiyu front could occur in absence of moist-diabatic process. Since the horizontal velocity gradient across the Meiyu front near the synoptic-scale low-level jet (LLJ) was quite large while the corresponding temperature gradient across the frontal zone was rather weak, we speculate that barotropic process may be responsible for triggering these MCSs along the Meiyu front.

Notes: Summary  Data measured during the TRACT field campaign at various stations along a 7.5° steep and west-northwest facing slope of the Black Forest mountain range (Germany) are used to analyze the thermal structure and the momentum budget associated with thermally induced slope winds. Acceleration of the air close to the ground is found to be directed nearly vertically downward during the night and nearly vertically upward during the day, rather than parallel to the slope. This means that during the night the airflow is deflected by the slope surface in the down-slope direction, whereas during daytime stable stratification above the heated slope layer is required to establish up-slope flow parallel to the slope. The diurnal cycle of the momentum budget of the along-slope wind component near the surface is analyzed in detail with respect to the driving forces (buoyancy and pressure gradient force) and friction. It is found that a small imbalance between forcing and friction is responsible for the diurnal change in slope flow intensity. The along-slope components of the horizontal pressure gradient force and the buoyancy force are shown to have the same order of magnitude. This means that for small to moderate slope angles the pressure gradient force cannot be neglected as is done in some analytical slope wind models. The reaction time of the slope flow to changes in forcing is estimated to be in the range of 30 to 120 seconds, which confirms the empirically known fact that slope winds react very quickly.

Notes: Summary  A new methodology for rainfall retrievals from indirect measurements is proposed and illustrated using IR brightness temperature and radar rainfall observations collected during TOGA-COARE. Since (1) rain rate has a mixed distribution with a delta-function for a zero rain and lognormal distribution for nonzero and (2) the least squares method which is used to calculate regression coefficients gives a priori consistent estimates only for normally distributed data, it is proposed to convert the rain rate to a normally distributed set and only after that to develop a retrieval method and estimate the skill of this method. Consideration of the physics of clouds and cloud ensembles, the goal to minimize errors in the radar data, and the desire to remove the influence of cirrus clouds lead us to use: a) minimum of IR brightness temperatures over a 1° × 1° area and a 3 hour interval as a predictor, and b) radar rainfall, averaged over 3 hours over a 1° × 1° area, with the radar in its center, as the truth. Results using the TOGA-COARE data show that the correlation of the rain rate transformed to normal distribution is significantly higher with minimum temperature than with the fraction of area covered by high clouds. The sizes of heavy rainfall areas obtained using the new methodology are reasonable. The regression coefficients should change with latitude, season and location. Taken together, the results indicate that it is possible, in principle, to retrieve rainfall from IR satellite observations and obtain reliable rainfall data. To realize this goal it is necessary to process radar and IR data using the new methodology for different latitudes, seasons, over land and ocean.

Notes: Summary.  Background and key results of the EU-funded project HERA are presented. The identification of the growing possibilities to use forecast precipitation fields as input for hydrological models is followed by a broad assessment of the state of the art regarding the determination of the atmospheric part of the hydrological cycle, with the geographical focus on the mountainous region of the Alps in the heart of Europe. This includes the construction of (north)Alpine radar composites for nine episodes; a first systematic cross-validation on a daily basis of four operational forecasting models with a trans-national gridded observat ional dataset; several detailed case studies involving research versions of operational forecast models and the latest advances in model resolution and microphysical parameterizations; new algorithms for Doppler radar retrievals over complex terrain; and synthetic modelling studies with governing parameters derived from the selected cases to investigate some basic processes in isolation. Finally, implications for proto-type applicatio ns of forecast areal precipitation fields in the hydrological modelling of mountainous catchments are given.

Notes: Summary  A new automatic tracking method for convective cells and cell complexes is introduced. The method uses a simple Gaussfilter and a selection procedure to define displacement vectors of the specific system. The change of the half width of the filter results in a separation of the system’s scale. The proposed method is applied on two data sources: lightning density and RADAR reflectivity. They describe different properties of convective cells and cell complexes. A strong connection of the storm tracks to topography becomes also evident.

Notes: Summary.  Mesoscale convective precipitation systems in the Alpine region are studied by analyzing radar and rain gauge data. The data from weather radars in Austria, France, Germany, and Switzerland are combined into a composite. Availability of radar data restricts the study mainly to the northern part of the Alpine region. Mesoscale convective systems (MCS) occur often in this region and are comparable to large systems observed in the USA. Seven precipitation events lasting one to six days from the years 1992–1996 are examined in detail. They all moved west to east and showed no diurnal preference in formation or dissipation. They reach sizes of 2 − 6 · 104 km2. MCS with leading-line trailing-stratiform structure tended to be larger and more intense. A 25-year set of rain gauge data indicates that a giant MCS (covering more than 4 · 104 km2 with more than 30 mm/day) occurs every 6 years in the northern Alpine region. MCS occur more frequently in the southern Alpine region.

Notes: Summary  Several episodes of heavy precipitation, which occurred in the region south of the Alps, have been simulated by means of the mesoscale model BOLAM3. Each case was run at 3 different resolutions, up to 4 km grid interval. The quantitative precipitation “forecast” fields are compared with available rain data. In general, satisfactory results are obtained in terms of spatial distribution and timing of precipitation, except in cases in which pre-frontal convection is dominant. The diagnostics of phenomena involved in orographic precipitation identify the different mesoscale atmospheric features associated with the interaction with topography, like the formation of low level jets, convergence zones, rainbands, and organized convective systems. These appear as “ingredients” common to all the cases considered and are shown to be sensitive to orographic forcing, as well as to the latent heat exchange processes.

Notes: Summary  The precipitation forecasts of four operational numerical weather prediction models over the Alpine region are evaluated and intercompared for two periods of interest to the Mesoscale Alpine Programme (MAP). The new analysis of Alpine rainfall of Frei and Schaär (1998) is used to validate the models. It is found that the models have a tendency to overestimate the total precipitation and the frequency of intense rain events over high orography. The skill scores show good consistency between models, except for the ability to forecast light rain or heavy rain events. The partition between convective and stratiform rainfall is rather variable between the models.

Notes: Summary This study proposes a method that can be used to provide guidelines to aircraft reconnaissance for hurricane observations. The method combines numerical weather prediction (NWP) model with a statistical approach to target adaptive observations over areas where the hurricane predictions are very sensitive to the initial analysis for the NWP-model. A single model experiment is performed using regular initial analysis, while 50 other ensemble runs are performed from randomly perturbed initial states. Under the perfect model assumption, the single model experiment serves as a true state. The method first computes the forecast error variances at a certain verification time, e.g. hour 48, and then locates the maximum centers of variances. After the locations of the maximum forecast error variances are known, various correlations of different variables between these maximum variance points and the perturbation fields at the target time, e.g. hour 12, are calculated to identify those locations at the target time, over where the observational errors might be responsible for the growth of forecast error variances at the verification time. Statistically, these correlation fields indicate where the most sensitive areas are at the target time, i.e. where the need for additional observations is suggested. Hurricane Fran of 1996 is used to test the proposed method. The reason for choosing this case is that, during the first 48 hour forecast, the track forecast from NWP-model was very close to the best track. Two additional experiments were designed to examine the method. One experiment updates predicted variables at the target time (12 h) over the areas, to where the proposed method indicates the forecast would be sensitive. The updating combines observations (or truth) with the first guess (predicted) fields. Another experiment also modifies predicted variables at the target time (12 h), but over the areas where the method indicates the forecast errors are less correlated to. The results show that the modification has greatly reduced the forecast error variances at the verification time (48 h) in the first experiment, however it has a very little impact on the variance fields at the forecast hour (48 h) in the second experiment. It is very clear from our experiments, that the proposed method is able to identify sensitive areas, where additional observations can help to reduce hurricane forecast errors from an NWP-model.

Notes: Summary A severe localized windstorm, with near-surface winds 〉 60 ms−1, occurred in an isolated valley within the Alpine mountains (〉 1800 m) of central Norway on 31 January 1995. A multi-scale numerical simulation of the event was performed with the Naval Research Laboratory (NRL)’s Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS), configured with four nested grids telescoping down to 1-km horizontal resolution. The windstorm occurred in response to topographic blocking and deformation of a lower-tropospheric warm front and attendant jet (〉 35 ms−1 at 2 km). The key findings are: i) mountain wave resonance and amplification arising from the interaction of the surface-based front and jet with complex orography, ii) sensitivity of the wave response to differential diabatic heating (vertical) gradients above the front, and iii) trapped response within the layer of large frontal stratification in the lower troposphere and subsequent amplification consistent with the theoretically-established two-layer windstorm analogue of Durran (1986).

Notes: Summary Forecasting the tracks of hurricanes is a problem of immense importance. It is a major scientific exercise in solving the complicated set of mathematical equations that govern the behavior of atmospheric flow in general and hurricanes in particular. Moreover, hurricanes rank as the most devastating of all natural phenomena, in terms of loss of life and destruction of property. Hitherto, unlike many other atmospheric and oceanic systems, hurricanes have defied rapid advances in prediction of their motion, and progress has been of the order of a mere one percent or so reduction per annum in mean 48 hour forecast position errors over the past two decades. A research program aimed at estimating inherent and actual mean absolute forecast position errors, has produced an apparent paradox. Despite the fact that the equations governing hurricane motion are a complex, coupled, nonlinear set of dynamical equations, there is very strong evidence for the existence of an underlying simple, linear, invariant behavior. The original aim of the research program was to determine the lower limits of mean hurricane forecast position errors and to quantify them out to 72 hour leadtime. The appearance of the paradox meant that the focus shifted first to examining and explaining the paradox. Attention then turns to showing that the mean forecasts errors are still a very large 40 to 50 percent lower than the mean position errors currently being achieved in practice by state-of-the-art models numerical weather prediction (NWP) models.

Notes: Summary An Australian circular mesoscale convective system (MCS) is examined using available surface and upper air analyses as well as satellite imagery. The MCS formed over central South Australia on 5 February 1997 and lasted approximately nine hours. It is found that MCS generation occurred following anomalous southward penetration of the monsoon trough over Australia. This penetration into southern Australia resulted in an input of extremely moist and unstable tropical air over the region which, together with the development of complex of shallow lows and troughs within the main monsoon trough, led to generation of the MCS. During the lifespan of the MCS, rainfall amounts in excess of 100 mm (and up to 175 mm over a four hour period at certain locations) were recorded with accompanying flash flooding and severe damage. A low in the middle levels of the atmosphere was responsible for the eventual decay of the storm. North to north-westerly winds winds around this low continually advected cloud away from the MCS towards the south and south east. This removal of cloud mass eventually led to dissipation of the MCS as it tracked away from the zone of maximum surface heating. Despite this storm just failing to meet the size criterion for mesoscale convective complex (MCC) status, it is very similar to “typical” MCCs found elsewhere in the world in terms of its lifetime and nocturnal nature. Although mesoscale storms of this type are not rare in Australia, MCS’s in South Australia make up only a small proportion of the total number of systems over south eastern Australia. These factors, in conjunction with the anomalous southward penetration of the monsoon trough and associated synoptic conditions, make this storm somewhat unusual.

Notes: Summary  The growth rates of amplifying mid-tropospheric perturbations in tropical North Africa is known to reduce with increased vertical shear in the troposphere. This phenomenon leads to a reduction in the frequency of generation of squall lines – the main rain-producing mechanism in tropical North Africa – because squalls are initiated by amplifying modes of African Easterly Waves (AEW). Ultimately, therefore, tropical North Africa experiences a shortfall, with respect to long-term averages, in annual rainfall. Weakening of AEW intensity is shown to be linked with the warming up to the sea-surface temperatures (SST) of the South Atlantic, Pacific and Indian Oceans. These findings are consistent with the observed reduction in the incidence of intense hurricanes along the entire westem Atlantic in Sahelian dry years. It is shown that the frequency of occurrence of Atlantic tropical storms and hurricanes is unaffected by the dryness or otherwise in the Sahel, but the paths of the storms are determined by the zonal exit point, from the African continental land mass to the Atlantic, of West African disturbance lines. These results have applications, and implications, in the level of preparedness for the economic impacts of Atlantic storms and hurricanes.

Notes: Summary. ¶A hydrostatic numerical model is used to simulate the lee wave event IOP3 (0000 GMT to 1200 GMT 15th October 1990) from the PYREX mountain experiment. Results from integrations at different horizontal resolutions are used to investigate the effect on surface pressure drag and the vertical flux of horizontal momentum due to orographically forced gravity waves. In particular, results showing the dependence on resolution of the partitioning between resolved and parametrized wave drag and fluxes are presented. With the model horizontal gridlength changing from 50 km to 10 km the majority of wave momentum flux changes from being parametrized to becoming resolved. More significantly, there is a change in the profile of flux with height. At 50 km resolution the largest inferred mean flow decelerations are at lower stratospheric level due to the parametrization scheme. At 10 km resolution this is shifted, with less deceleration high up and more wave deceleration lower down in the troposphere. Numerical weather prediction models are now beginning to take account of such low level drag with beneficial results.

Notes: Summary ¶Closed ideal thermodynamic cycles are used to analyze the atmospheric upward heat convection process which is compared to the Brayton gas-turbine cycle. The heat to work conversion efficiency of the atmosphere is shown to be close to the Carnot efficiency calculated using the average temperatures at which heat is received and given up for hot and cold source temperatures, respectively. The efficiency is independent of whether the lifting process is discontinuous or continuous, and nearly independent of whether the heat is transported as sensible or as latent heat. The total energy given up by an air mass going through any process is shown to be equal to the reduction in its static-energy. For a given sounding, there is a definite quantity of work which can be produced per unit mass of air raised. The paper investigates how updraft and sounding properties affect the work produced when air is raised and shows that the work can be transferred downward.

Notes: Summary. ¶The airflow over an idealized orography with two mountain peaks and a valley between is investigated using a non-linear numerical model. The flow is assumed to be two-dimensional and nonrotational. Surface friction is neglected. This setup is a first step in studying the modifications a finely structured “real” topography introduces to the well-studied flow over one isolated obstacle. The sensitivity of the flow behavior to the valley width is examined for the case of specified mountain volume as well as constant non-dimensional mountain height. Flow patterns for linear, weakly nonlinear, wave breaking and upstream blocking cases are examined. Whereas the nondimensional mountain height is still the main measure of the nonlinearity of the flow, the differing steepness of upslope and downslope caused by the separating valley, strengthens nonlinear effects. It also modifies wave breaking and upstream blocking. For wide enough valleys wave breaking regions can form above both peaks.

Notes: Abstract.  The changes in the atmospheric response (e.g., in the variables of state, the amount of cloud-and precipitating particles, the water- and energy fluxes) to concurrent land-use changes are exemplarly examined for various landscape scenarios since the cessation of open-pit mining. The principle of superposition is applied to detect areas where the atmospheric response is either enhanced or diminished according to concurrent land-use changes as compared to simple land-use changes. The results show that nonlinear behavior of a given quantity may occur in areas without significant changes in this quantity and that a quantity may be significantly changed without finding nonlinearity. Moreover, the concurrent land-use changes do not necessarily yield a stronger atmospheric response than simple land-use changes. In addition to the size of the patches, where land-use changes, the contrast in the hydrologic and thermal behavior of the changes is decisive in measuring the magnitude of the response. Enhancement and diminish ment seem also to depend on the land-use adjacent to the altered land-use.

Notes: Summary ¶Simulations of two heavy rainfall events south of the Alps have been conducted with a non-hydrostatic model at horizontal resolutions ranging from 40 to 2.5 km. Emphasis is placed on the assessment of two recent model developments: a detailed explicit mixed phase microphysical scheme and a two-way interactive grid nesting method. In the high-resolution simulations, the impact of the ice phase was found to be quite substantial, accelerating the propagation of the surface front and modifying the spatial distribution of the rainfall. Simulations carried out with two-way interactive grid nesting yielded more realistic overall precipitation patterns, particularly at the finest horizontal resolution.