Showing papers in "International Journal of Climatology in 2007"

Linking climate change modelling to impacts studies: recent advances in downscaling techniques for hydrological modelling

Abstract: There is now a large published literature on the strengths and weaknesses of downscaling methods for different climatic variables, in different regions and seasons. However, little attention is given to the choice of downscaling method when examining the impacts of climate change on hydrological systems. This review paper assesses the current downscaling literature, examining new developments in the downscaling field specifically for hydrological impacts. Sections focus on the downscaling concept; new methods; comparative methodological studies; the modelling of extremes; and the application to hydrological impacts. Consideration is then given to new developments in climate scenario construction which may offer the most potential for advancement within the ‘downscaling for hydrological impacts’ community, such as probabilistic modelling, pattern scaling and downscaling of multiple variables and suggests ways that they can be merged with downscaling techniques in a probabilistic climate change scenario framework to assess the uncertainties associated with future projections. Within hydrological impact studies there is still little consideration given to applied research; how the results can be best used to enable stakeholders and managers to make informed, robust decisions on adaptation and mitigation strategies in the face of many uncertainties about the future. It is suggested that there is a need for a move away from comparison studies into the provision of decision-making tools for planning and management that are robust to future uncertainties; with examination and understanding of uncertainties within the modelling system. Copyright © 2007 Royal Meteorological Society

Empirical orthogonal functions and related techniques in atmospheric science: A review

Abstract: Climate and weather constitute a typical example where high dimensional and complex phenomena meet. The atmospheric system is the result of highly complex interactions between many degrees of freedom or modes. In order to gain insight in understanding the dynamical/physical behaviour involved it is useful to attempt to understand their interactions in terms of a much smaller number of prominent modes of variability. This has led to the development by atmospheric researchers of methods that give a space display and a time display of large space-time atmospheric data. Empirical orthogonal functions (EOFs) were first used in meteorology in the late 1940s. The method, which decomposes a space-time field into spatial patterns and associated time indices, contributed much in advancing our knowledge of the atmosphere. However, since the atmosphere contains all sorts of features, e.g. stationary and propagating, EOFs are unable to provide a full picture. For example, EOFs tend, in general, to be difficult to interpret because of their geometric properties, such as their global feature, and their orthogonality in space and time. To obtain more localised features, modifications, e.g. rotated EOFs (REOFs), have been introduced. At the same time, because these methods cannot deal with propagating features, since they only use spatial correlation of the field, it was necessary to use both spatial and time information in order to identify such features. Extended and complex EOFs were introduced to serve that purpose. Because of the importance of EOFs and closely related methods in atmospheric science, and because the existing reviews of the subject are slightly out of date, there seems to be a need to update our knowledge by including new developments that could not be presented in previous reviews. This review proposes to achieve precisely this goal. The basic theory of the main types of EOFs is reviewed, and a wide range of applications using various data sets are also provided. Copyright © 2007 Royal Meteorological Society

HISTALP—historical instrumental climatological surface time series of the Greater Alpine Region

Abstract: This paper describes the HISTALP database, consisting of monthly homogenised records of temperature, pressure, precipitation, sunshine and cloudiness for the ‘Greater Alpine Region’ (GAR, 4–19°E, 43–49°N, 0–3500m asl). The longest temperature and air pressure series extend back to 1760, precipitation to 1800, cloudiness to the 1840s and sunshine to the 1880s. A systematic QC procedure has been applied to the series and a high number of inhomogeneities (more than 2500) and outliers (more than 5000) have been detected and removed. The 557 HISTALP series are kept in different data modes: original and homogenised, gap-filled and outlier corrected station mode series, grid-1 series (anomaly fields at 1° × 1°, lat × long) and Coarse Resolution Subregional (CRS) mean series according to an EOF-based regionalisation. The leading climate variability features within the GAR are discussed through selected examples and a concluding linear trend analysis for 100, 50 and 25-year subperiods for the four horizontal and two altitudinal CRSs. Among the key findings of the trend analysis is the parallel centennial decrease/increase of both temperature and air pressure in the 19th/20th century. The 20th century increase (+1.2 °C/+ 1.1 hPa for annual GAR-means) evolved stepwise with a first peak near 1950 and the second increase (1.3 °C/0.6hPa per 25 years) starting in the 1970s. Centennial and decadal scale temperature trends were identical for all subregions. Air pressure, sunshine and cloudiness show significant differences between low versus high elevations. A long-term increase of the high-elevation series relative to the low-elevation series is given for sunshine and air pressure. Of special interest is the exceptional high correlation near 0.9 between the series on mean temperature and air pressure difference (high-minus low-elevation). This, further developed via some atmospheric statics and thermodynamics, allows the creation of ‘barometric temperature series’ without use of the measures of temperature. They support the measured temperature trends in the region. Precipitation shows the most significant regional and seasonal differences with, e.g., remarkable opposite 20th century evolution for NW (9% increase) versus SE (9% decrease). Other long- and short-term features are discussed and indicate the promising potential of the new database for further analyses and applications. Copyright © 2006 Royal Meteorological Society.

Class—A Canadian land surface scheme for GCMS. I. Soil model

Abstract: A new GCM land surface scheme is introduced, incorporating three soil layers with physically based calculations of heat and moisture transfers at the surface and across the layer boundaries. Snow-covered and snow-free areas are treated separately. The energy balance equation is solved iteratively for the surface temperature; the surface infiltration rate is calculated using a simplified theoretical analysis allowing for surface ponding. Snow cover is modelled as a discrete ‘soil’ layer. The results generated by CLASS are compared with those of an older model incorporating the force-restore method for the calculation of surface temperature and a bucket-type formulation for the ground moisture. Several month-long test runs are carried out in stand-alone mode. It is shown that the surface temperature in the old scheme responds more slowly to diurnal forcing and more quickly to longer term forcing than that modelled by CLASS, while its one-layer representation of soil moisture proves incapable of reproducing changes in the surface fluxes owing to surface variations of moisture content. Finally, the lumped treatment of snow and soil in the old scheme results in an extremely fast disappearance of the snow pack under certain conditions.

Different methods for estimating the mean radiant temperature in an outdoor urban setting

Abstract: The mean radiant temperature (Tmrt) is one of the most important meteorological parameters governing human energy balance. In this paper, three different methods of obtaining the Tmrt in an outdoor urban setting are compared. Method A is based on integral radiation measurements and angular factors, method B is based on measurements with a 38-mm flat grey globe thermometer and in method C makes use of the Rayman 1.2 software is used. Measurements were performed in a large open square in a high latitude city—Goteborg, Sweden—during clear to overcast weather conditions in October 2005 and in July and August 2006. Results show that the difference between Method A and Method B was generally relatively small. Most of the discrepancy, caused by rapid changes in radiation, temperature and wind speed was smoothed out using 5 min mean values. By systematically and empirically changing the mean convection coefficient, the accuracy of Method B was improved and a new equation expressing the Tmrt was obtained. With this new equation the 38 mm flat grey globe thermometer could successfully be used to estimate the Tmrt in an outdoor urban setting provided that the wind speed and the air and globe temperatures are measured accurately. The study also shows that the flat grey colour of the globe thermometer slightly underestimates the level of short-wave radiation (i.e. sunshine). Method C works very well during the middle of the day in July, i.e. at high sun elevations. However, the model considerably underestimates the Tmrt in the morning and evening in July and during the whole day in October, i.e. at low sun elevations. In outdoor urban settings where thermal comfort researchers or urban planners and designers require an easy and reliable method of estimating mean radiant temperature, the 38 mm flat grey globe thermometer provides a good and cheap solution. Copyright © 2007 Royal Meteorological Society

Statistical and dynamical downscaling of the Seine basin climate for hydro-meteorological studies

Abstract: Two downscaling methods designed for the study of the hydrological impact of climate change on the Seine basin in France are tested for present climate First, a multivariate statistical downscaling (SD) methodology based on weather typing and conditional resampling is described Then, a bias correction technique for dynamical downscaling based on quantile–quantile mapping is introduced To evaluate the end-to-end SD methodology, the atmospheric forcing derived from the large-scale circulation (LSC) of the ERA40 reanalysis by SD is used to force a hydrological model Simulated discharges reproduce historical values reasonably well Next, the dynamical and statistical approaches are compared using the Meteo–France ARPEGE general circulation model in a variable resolution configuration (resolution around 60 km over France) The ARPEGE simulation is downscaled using the two methodologies, and hydrological simulations are performed Regarding downscaled temperature and precipitation, the statistical approach is more efficient in reproducing the temporal and spatial autocorrelation properties The simulated river discharges from the two approaches are nevertheless very similar: the two methods reproduce well the seasonal cycle and the daily distribution of streamflows Finally, the results of the study are discussed from a practical impact study perspective Copyright © 2007 Royal Meteorological Society

Review of urban climate research in (sub)tropical regions

Abstract: Over the last 50 years the developing world, much of which is located in (sub)tropical regions, has seen a dramatic growth of its urban population associated with serious degradation of environmental quality. The total number of (sub)tropical urban climate studies, however; is still small (<20% of all urban climate studies). The available work is further biased towards descriptive studies rather than process work that seeks to indicate the physical climatology of (sub)tropical cities. The available results allow for a preliminary comparison with data from temperate latitudes. Urban heat island (UHI) intensities are generally lower compared to those of temperate cities with comparable population and show a seasonal variation with lower (higher) intensities during the wet (dry) season. (Sub)tropical population-based relations may exist but insufficient appropriate data is available to confirm a logarithmic relationship or systematic differences between different climate types. The (sub)tropical energy balance studies are biased towards dry, clear sky conditions. The amount of net radiation dissipated by sensible heat during daytime is about 40% which is similar to values observed in (sub)urban areas of cities located in temperate climates. Energy partitioning is modulated by water availability and higher percentage of vegetation promotes latent heat flux at the expense of surface heat storage. The apparent strong influence of vegetation and water availability on the energy partitioning irrespective of the climate type, suggests vegetation to be an effective means to reduce heat storage uptake during daytime and hence has the potential to effectively mitigate the nocturnal heat island. It is important to ensure that the rapidly expanding cities of the developing world incorporate climatological concerns in their design to provide a better living and working environment for a large segment of the world's inhabitants. Copyright © 2007 Royal Meteorological Society

Appropriate application of the standardized precipitation index in arid locations and dry seasons

Abstract: The Standardized Precipitation Index (SPI) is now widely used throughout the world in both a research and an operational mode. For arid climates, or those with a distinct dry season where zero values are common, the SPI at short time scales is lower bounded, referring to non-normally distributed in this study. In these cases, the SPI is always greater than a certain value and fails to indicate a drought occurrence. The nationwide statistics based on our study suggest that the non-normality rates are closely related to local precipitation climates. In the eastern United States, SPI values at short time scales can be used in drought/flood monitoring and research in any season, while in the western United States, because of its distinct seasonal precipitation distribution, the appropriate usage and interpretation of this index becomes complicated. This would also be the case for all arid climates. From a mathematical point of view, the non-normally distributed SPI is caused by a high probability of no-rain cases represented in the mixed distribution that is employed in the SPI construction. From a statistical point of view, the 2-parameter gamma model used to estimate the precipitation probability density function and the limited sample size in dry areas and times would also reduce the confidence of the SPI values. On the basis of the results identified within this study, we recommend that the SPI user be cautious when applying short-time-scale SPIs in arid climatic regimes, and interpret the SPI values appropriately. In dry climates, the user should focus on the duration of the drought rather than on just its severity. It is also worth noting that the SPI results from a statistical product of the input data. This character makes it difficult to link the SPI data to the physical functioning of the Earth system. Copyright © 2006 Royal Meteorological Society.

Drought forecasting using artificial neural networks and time series of drought indices

Abstract: Drought forecasting is a critical component of drought risk management. The paper describes an approach to drought forecasting, which makes use of Artificial Neural Network (ANN) and predicts quantitative values of drought indices—continuous functions of rainfall which measure the degree of dryness of any time period. The indices used are the Effective Drought Index (EDI) and the Standard Precipitation Index (SPI). The forecasts are attempted using different combinations of past rainfall, the above two drought indices in preceding months and climate indices like Southern Oscillation Index (SOI) and North Atlantic Oscillation (NAO) index. A number of different ANN models for both EDI and SPI with the lead times of 1 to 12 months have been tested at several rainfall stations in the Tehran Province of Iran. The best models in both cases have been found to include, among the others, a corresponding drought index value from the same month of the previous year. Both best models have the R2 values of 0.66-0.79 for a lead time of 6 months, but it is also shown that the EDI forecasts are superior to those of the SPI for all lead times and at all rainfall stations. The better performance of the EDI model is illustrated by its more accurate prediction of the overall pattern of ‘dry’ and ‘wet’ conditions. The structure of the model inputs (previous rain and drought indices) does not vary with the lead time, which makes the models very convenient for the operational purposes. The final forecasting models can be utilized by drought early warning systems, which are emerging in Iran at present. Copyright © 2007 Royal Meteorological Society

Synoptic aspects of the central chile rainfall variability associated with the southern oscillation

Abstract: Central Chile winter (June, July, August (JJA)) rainfall shows positive anomalies during the developing stage of warm events of the Southern Oscillation. Conversely, cold events correspond quite closely to dry conditions. A synoptic characterization of major storms during the most recent warm events is presented. Dry months during coldevent years are described in terms of average 500-hPa contour anomaly fields. Significant departures from this general behaviour are also discussed. It is found that major winter storms associated with warm events are related to blocking highs frequently located around the Bellingshausen Sea (9OOW) within hemispheric circulation anomaly patterns where zonal wavenumbers 4 and 3 dominate. This phenomenon seems consistent with observed teleconnection wavetrains stemming from the anomalous atmospheric heat source above the equatorial Pacific during ENS0 events. Cold years, often immediately preceding or following a warm event, bring dry conditions in the study area owing to a well-developed south-east subtropical anticyclone with enhanced zonal westerly flow at middle latitudes. Frequency distributions of 500-hPa daily blocking indices (BI) at 90°W, derived from 1980 to 1987 European Centre for Medium Range Weather Forecasts hemispheric analyses, show a significant departure towards positive BI values for the available warm-event winters; the opposite being also true. However, the JJA rainfall variability at Santiago (33.53) also seems to be related to the regional strength of the south-east Pacific anticyclone, as represented by seasonal 500-hPa geopotential anomalies at Puerto Montt, Chile (413"s). The apparent relationship between the phases of the Southern Oscillation (SO) and rainfall anomalies in central Chile (30-35"s) has been reported by several authors. Rubin (1955), while taking into consideration pressure anomalies in the Southern Hemisphere, found that the precipitation in central Chile stays below normal during the positive phase of the SO, namely when the south-east Pacific subtropical anticyclone is stronger than average. The strength and position of the aforementioned anticyclone has also been related by Pittock (1980) to the interannual rainfall variability in the central part of Chile. Quinn and Neal (1983), in an attempt to relate long series of annual precipitation in Santiago (33*5"S, 70.7"W) and in Valparaiso (33.0°S, 71.6"W) with an El Niiio index, obtained a good correspondence of the interannual rainfall variability with area averaged sea-surface temperatures in the south-eastern tropical Pacific. More recently, Aceituno (1987) has correlated the pressure, temperature, wind, and precipitation fields with a Southern Oscillation Index (SOI), concluding that the tendency to positive rainfall anomalies in central Chile during the negative phase of the SO is associated with a weak and northerly displaced southeast Pacific subtropical anticyclone; together with an overall increase in baroclinicity at the subtropical latitudes produced by tropospheric cooling in the southern part of South America and a corresponding warming in tropical latitudes. At present, the Chilean rainfall anomaly seems to have gained a place in the world-wide sequence of major climatic anomalies related 0899-84 1 8/9 1/0 10063-1 4$07.OO 0 1991 by the Royal Meteorological Society

Use of the gamma distribution to represent monthly rainfall in Africa for drought monitoring applications

Abstract: Evaluating a range of scenarios that accurately reflect precipitation variability is critical for water resource applications. Inputs to these applications can be provided using location- and interval-specific probability distributions. These distributions make it possible to estimate the likelihood of rainfall being within a specified range. In this paper, we demonstrate the feasibility of fitting cell-by-cell probability distributions to grids of monthly interpolated, continent-wide data. Future work will then detail applications of these grids to improved satellite-remote sensing of drought and interpretations of probabilistic climate outlook forum forecasts. The gamma distribution is well suited to these applications because it is fairly familiar to African scientists, and capable of representing a variety of distribution shapes. This study tests the goodness-of-fit using the Kolmogorov–Smirnov (KS) test, and compares these results against another distribution commonly used in rainfall events, the Weibull. The gamma distribution is suitable for roughly 98% of the locations over all months. The techniques and results presented in this study provide a foundation for use of the gamma distribution to generate drivers for various rain-related models. These models are used as decision support tools for the management of water and agricultural resources as well as food reserves by providing decision makers with ways to evaluate the likelihood of various rainfall accumulations and assess different scenarios in Africa. Copyright © 2006 Royal Meteorological Society

Urban shading—a design option for the tropics? A study in Colombo, Sri Lanka

Abstract: Recent urban microclimate studies in Colombo, Sri Lanka, indicate that the maximum daily temperature within street canyons decreases with increasing height to width (H/W) ratio, but higher H/W ratio negatively affects street-level wind flow. There is also evidence pointing to the cooling effect of sea breeze. The nocturnal heat island is small in contrast to daytime urban-rural differences. In this paper, we use the software ENVI-met to simulate the effect of different urban design options on air and surface temperatures, as well as on outdoor thermal comfort. The latter is expressed as the physiologically equivalent temperature (PET), an index based on air and radiant temperatures as well as wind and humidity. It is found that high albedo at street level gives the lowest air temperature during daytime, although the reduction is only about 1 degrees C. The lowest daytime mean radiant temperatures result from high H/W ratios of streets. This has a positive effect on thermal comfort; the increase of H/W ratio from about 1 to 3 leads to a decrease in PET by about 10 degrees C. Differences in air and surface temperatures, as well as PET, are small during the night. The results show that strategies that lead to better air temperature mitigation may not necessarily lead to better thermal comfort. However, shade enhancement through increased H/W ratios is clearly capable of significant reductions in PET, and thus, improved outdoor thermal comfort. Consequently, a critical urban design task in the humid tropics will be to guide the rapid urban growth towards efficient 'shade growth'.

Prediction of seasonal rainfall in the north nordeste of Brazil using eigenvectors of sea-surface temperature

Abstract: Two statistical techniques, multiple linear regression and linear discriminant analysis, are compared for making hindcasts and real-time forecasts of north Nordeste (north-east Brazil) wet season rainfall using only information about sea-surface temperature The predictors are strengths of large-scale patterns of sea-surface temperature measured just prior to the season being predicted The patterns are created from non-rotated covariance eigenvectors of seasonal, basin scale, sea-surface temperature anomalies for the Atlantic and Pacific Oceans in the years 1901–1980 Separate prediction models are constructed for 1912–1948 and 1949–1985; hindcasts using a given model are made and verified for the set of years not included in the model construction However, this strategy does not provide hindcast periods that are completely independent of the model construction period (the ‘training period’), because the eigenvector patterns were based on 1901–1980 data So for the multiple linear regression technique, a second version of the eigenvectors was used to provide predictor sea-surface temperature patterns derived from data that were independent of one of the testing periods The results show that skill was maintained when the predictor eigenvector patterns were also independent of the testing period Hindcast skill is measured with the help of a new type of score, which calculates the linear error of the hindcast in a historical cumulative probability distribution of the seasonal rainfall (linear error in probability space, LEPS) Well-known scores are also used to allow easy comparisons with the skill of tropical forecasting techniques published elsewhere At least 50 per cent of north Nordeste wet season rainfall variance appears to be predictable using sea-surface temperature data alone, with no significant overall bias Finally, the paper reviews the skill of four, real time, experimental seasonal forecasts issued for the wet season March to May for each year between 1987 and 1990

Association between Australian rainfall and the Southern Annular Mode

Abstract: In this study, we explore the relationships between seasonal Australian rainfall and the Southern Annular Mode (SAM). We produce two seasonal indices of the SAM: the Antarctic Oscillation Index (AOI), and an Australian regional version (AOIR) using ERA-40 mean sea-level pressure (MSLP) reanalysis data. The seasonal rainfall data are based on gridded monthly rainfall provided by the Australian Bureau of Meteorology. For the period 1958–2002 a significant inverse relationship is found between the SAM and rainfall in southern Australia, while a significant in-phase relationship is found between the SAM and rainfall in northern Australia. Furthermore, widespread significant inverse relationships in southern Australia are only observed in winter, and only with the AOIR. The AOIR accounts for more of the winter rainfall variability in southwest Western Australia, southern South Australia, western and southern Victoria, and western Tasmania than the Southern Oscillation Index. Overall, our results suggest that changes in the SAM may be partly responsible for the current decline in winter rainfall in southern South Australia, Victoria, and Tasmania, but not the long-term decline in southwest Western Australian winter rainfall. Copyright © 2006 Royal Meteorological Society.

A note on the temporal and spatial variability of rainfall in the drought-prone Amhara region of Ethiopia

Abstract: The aim of this study is to characterise rainfall variability and trend in the drought-prone Amhara Regional State of Ethiopia using standard rainfall statistical descriptors. A review of previous studies of Ethiopian rainfall shows different conclusions between studies about the existence of trends primarily due to their use of different periods of analysis. Various rainfall indicator series are presented and analysed for trend on annual, seasonal and daily time steps (including wet-day amounts and probabilities, percentiles and dry spell lengths). Two periods are used for analysis: 1975–2003 (12 stations) to optimise station density and 1961–2003 (five stations) to optimise record length in this relatively poorly monitored region. A complex picture of rainfall variability emerges from the analysis, both in terms of spatial variability and temporal variability, from decadal to daily timescales. The results generally support those of the previous studies in Ethiopia with the additional findings that: (1) High levels of spatial variability exist at subregional scales in Ethiopia that are unlikely to be fully explained by large-scale climate influences; (2) Choice of study period strongly influences the results of trend analysis in this region due to the effects of decadal variability (particularly because the 1980s was the driest decade and the 1990s the wettest decade on record); (3) Annual rainfall in the region recovered during the 1990s, although 2001–2003 were average or slightly lower; and (4) There are no consistent emergent patterns or trends in daily rainfall characteristics in this part of Ethiopia. Copyright © 2007 Royal Meteorological Society

The effects of deforestation on the hydrological cycle in Amazonia: a review on scale and resolution

Abstract: This paper reviews the effects of deforestation on the hydrological cycle in Amazonia according to recent modeling and observational studies performed within different spatial scales and resolutions. The predictions that follow from future scenarios of a complete deforestation in the region point to a restrained water cycle, while the simulated effects of small, disturbed areas show a contrasting tendency. Differences between coarsely spatially averaged observations and finely sampled data sets have also been encountered. These contrasts are only partially explained by the different spatial resolutions among models and observations, since they seem to be further associated with the weakening of precipitation recycling under scenarios of extensive deforestation and with the potential intensification of convection over areas of land-surface heterogeneity. Therefore, intrinsic and interrelated scale and heterogeneity dependencies on the impact of deforestation in Amazonia on the hydrological cycle are revealed and the acknowledgement of the relevance of these dependencies sets a few challenges for the future. Copyright © 2007 Royal Meteorological Society

Wave climate changes in the North Atlantic and North Sea

Abstract: This article presents a review of all available data describing long-term trends in the wave climate of the North Atlantic and North Sea, using both visual estimates and instrumental measurements of wave height. Long series of measurements from Seven Stones Light Vessel (1962–1986), sited off the south-west tip of England, and Ocean Weather Station Lima (1975–1988), at 57°N,20°W, are both examined for evidence of trends in mean wave height; the former data set is also examined for evidence of trends in annual maximum wave height and extreme (50-year return value) wave height. All available data show an increase in mean wave height over the whole of the North Atlantic in recent years, possibly since 1950, of about 2 per cent per year. There are insufficient data to say with confidence whether maxima or extremes have also risen.

Half-century seasonal relationships between the Southern Annular mode and Antarctic temperatures

Abstract: In this short communication we examine the relationship between the Southern Hemisphere Annular Mode (SAM) and Antarctic near-surface temperatures using data from Antarctic stations for 1957-2004. This near half-century period is significantly longer than analysed in previous studies. Furthermore, the four seasons are considered independently while the longer datasets allow the temporal stability of the relationship to be investigated. A general pattern of positive (negative) correlations between the strength of the SAM and temperatures in the northern Antarctic Peninsula (East Antarctica) is shown to be valid for the last half century but detailed differences are established between the seasons. These include a seasonal change in the sign of the relationship at one station, while at others there are single seasons when temperatures there are or, in some cases, are not significantly related to the SAM. Generally, SAM-temperature correlations are stronger across Antarctica in austral autumn and summer. Estimates of the contribution that trends in the SAM have made to Antarctic near-surface temperature change between 1957 and 2004 are greatest in autumn: in this season they exceed 1°C at half the 14 stations examined with a maximum change of –1.4°C. There does not appear to have been any significant long-term change in the strength of SAM-temperature relationships over the period examined, even with the onset of ozone depletion. However, on an annual basis, the long-term relationship between the SAM and near-surface temperatures can be disrupted and even reversed at some stations although coastal East Antarctica appears stable in this respect. These findings give support to the exploitation of appropriate ice core data to determine longer-term changes in the SAM based upon transfer-functions derived from recent data.

Climate regionalization and rotation of principal components

Abstract: Climate regionalization studies have made intensive use of eigenvector analysis in recent literature. This analysis provides a motivation for examination of the efficacy and validity of variations of principal component analysis (PCA) for such tasks as an eigenvector-based regionalization. Specifically, this study applies the results of an earlier statistical comparison of rotational schemes to monthly Pennsylvanian precipitation data (1958–1978) to analyse differences among the various solutions. Unrotated, orthogonally rotated, and obliquely rotated solutions (eight in total) are compared in order to assess the model and locational consistency among and within these solutions. Model correspondence and consistency are measured by a congruence coefficient used to match (i) the principal components (PC) of the total domain for the selected benchmark pattern with PCs from the total domains of the remaining seven solutions, and (ii) each PC of the total domain with PCs of 25 randomly selected subdomain pairs (a set of 10 and 11 years of data). Locational or geographical consistency among the PC patterns is determined by quantifying the changes in area and area boundary defined by a threshold loading. The results from the Pennsylvanian data indicated that substantial differences in regionalization arose solely from the choice of a particular rotation algorithm, or lack thereof. Oblique rotations were generally found to be the most stable, whereas the orthogonally rotated and unrotated solutions were less stable. The quantitative areal differences among rotation schemes and the unrotated solution illustrate the inherent danger in blindly applying any given solution if physical interpretation of the regionalization is important. The quantitative areal and boundary differences may particularly influence PCA over global spatial domains.

Review of scenario selection and downscaling methods for the assessment of climate change impacts on hydrology in the United States pacific northwest

Abstract: This paper reviews methods that have been used to evaluate global climate simulations and to downscale global climate scenarios for the assessment of climate impacts on hydrologic systems in the Pacific Northwest, USA. The approach described has been developed to facilitate integrated assessment research in support of regional resource management. Global climate model scenarios are evaluated and selected based on historic 20 th Century simulations. A statistical downscaling method is then applied to produce a regional data set. To facilitate the use of climate projections in hydrologic assessment, additional statistical mapping may be applied to generate synthetic station time series. Finally, results are presented from a regional climate model that indicate important differences in the regional climate response from what is captured by global models and statistical downscaling. 1. Introduction Some of the most important anticipated impacts of climate change are expressed through hydrologic processes such as streamflow, snowpack, and flooding. Modeling these impacts requires high- resolution regional data for future scenarios of temperature and precipitation. The science of climate change at global and regional scales is quite advanced and climate simulations are typically downscaled to as fine as 10-50 km grids or to station locations. While there remains significant research to be done to fully understand climate dynamics at these scales and to bolster confidence in future scenarios, the current climate modeling is adequate for many applications in hydrology. A principal challenge is linking global climate simulations to existing computational tools and institutional mechanisms within an integrated assessment. For example, under global climate change, system impact assessment is complicated by the constantly shifting underlying climate trends within large year-to-year variability (Arnell, 1996). The analysis of water resource systems and their reliability, yield, and specific event frequency, generally assumes a static state that can be described statistically using a time series of historic events and depends on using the observed record of the past to estimate the probability of future events. The observed record is assumed to be statistically stationary so that all events are equally probable and these probabilities are assumed to carry into the future. Typically, climate projections are based on transient simulations from multiple projected emissions scenarios and climate models. While this approach can generate a large number of projections based on various models and emissions scenarios, it does not correspond well to the current approach in resource management. This paper reviews methods developed by the Climate Impacts Group (CIG) at the University of Washington for integrated assessment of climate change impacts in the Pacific Northwest, United States. This research focuses on four diverse yet connected natural systems of the Pacific Northwest (fresh water, forests, salmon and coasts) and the socioeconomic and/or political systems associated with each. Hydrologic processes are central to the climate impacts in all sectors; thus, downscaling climate scenarios for hydrologic simulations forms the basis for quantitative analyses. Many of the approaches we have developed are based on empirical corrections to simulated climate data. These corrections are based on a relationship between the observed statistics of a parameter and the simulation of that parameter for equivalent climate conditions. This relationship is then used to correct the simulation of that parameter for future climate conditions. In its simplest form, that relationship could be a simple perturbation to correct a bias. In the quantile mapping, however, the full probability distribution is taken into account. For example, the temperature simulated by a given model for present-day conditions at a given location may be 5°C too cold compared with observations. For the future climate, one would add 5°C to all values simulated at that location to correct this bias. The bias may be simply a lapse-rate correction for unresolved topography or it may stem from a deficiency in the

Dust storm frequency in Asia: Climatic control and variability

Abstract: The dimension and environmental impact of the dust storm phenomenon has been realized only recently. There are few previous works on the climatic control of dust storm frequency, especially in the field of time series analysis. Actually, there is a poor correspondence of climatic parameter mean values and dust storm data. Although generally negative, rainfall — dust-storm correlations do not reveal the physical causes of dust storm generation, as is shown by lagged cross-correlation and spectral analysis. Temperature data may indicate seasonal variation of dust storms by extreme values, but they are no reliable defining factor owing to high persistence over shorter time series. The same is found for mean wind speeds, but negative interrelations with atmospheric pressure point to the importance of cyclogenesis and convective cells in dust storm generation in Asia. Three different seasonality patterns are described: (i) a single dust storm maximum in spring typical of summer rain areas; (ii) a single storm maximum in summer in areas with bimodal (winter and monsoonal) rainfall; (iii) an extended spring and summer dust storm maximum in areas with unimodal winter rains. Additional data from northern Africa fit into this pattern. Spectral energy of dust storm, rainfall, and temperature series for northern India and northern China (1976–1986) is mainly concentrated in the wavelength of seasonal variations, but some supra-seasonal signals indicate fluctuations between 3·6 and 5·5 years with various phase dislocations. In China, this interannual variability is supposed to be linked with dynamic shifts in circumpolar vortex dynamics.

ENSO and the South China Sea summer monsoon onset

Abstract: This paper investigates the relationship between the onset date of the South China Sea summer monsoon (SCSSM) and the El Nino/Southern Oscillation (ENSO). The monsoon onset date (MOD) is defined on the basis of the switch of the 850-hPa zonal winds over the South China Sea (SCS) from easterly to westerly for two consecutive pentads. The ENSO signal is represented by the ocean heat content (OHC), which is proportional to the depth of the 20 °C isotherm. It is found that, in years associated with a warm (cold) ENSO event or the year after, the monsoon tends to have a late (an early) onset and the intensity of the SCSSM also tends to be weaker (stronger). During a 2-year period prior to the onset, anomalies of OHC have an obvious eastward propagation. The 850-hPa flow east of the Philippines, specifically the strength of the subtropical high, is also found to be critical in determining the MOD. The link between these two results appears to be the propagation of cold (warm) subsurface water into the western North Pacific (WNP), which strengthens (weakens) the subtropical high, and hence a late (an early) SCSSM onset. Copyright © 2006 Royal Meteorological Society.

Trends in daily rainfall in the Iberian Peninsula from 1951 to 2002

Abstract: An analysis of 22 sites of daily precipitation records over the period 1951–2002 for the Iberian Peninsula is presented. Annual and seasonal total precipitation (P), number of wet days (N), precipitation intensity (I), the 95th percentile (P95), and percentage of rain falling on days with rainfall above the 95th percentile (%) are investigated. The annual and seasonal trends for these variables and for all 22 rain gauges are analysed, using the Mann–Kendall statistic, and a linear regression model. Moreover, a t-test is applied to the difference between the means of two subperiods, respectively 1951–1976 and 1977–2002. Principal results indicate a decreasing trend in P, I, and P95 for several northern and southern stations in winter; P, I, and P95 in some southern stations in spring, I and P95 in some southern stations in summer, and I in some northern and southern stations in autumn. The general behaviour is a decrease in the daily intensity of rainfall, while the number of wet days does not reveal pronounced changes. This pattern is valid for both annual and seasonal values of the indices. The decreasing trend found for I in winter and annual series for some localities may be related to the predominance of the positive phase of the North Atlantic Oscillation (NAO), but it is necessary to find other mechanisms for those stations and seasons not linked directly to NAO. Copyright © 2006 Royal Meteorological Society.

Anthropogenic heat release in an old European agglomeration (Toulouse, France)

Abstract: The anthropogenic heat release, QF, has been estimated for the old European agglomeration of Toulouse (France) from February 2004 to March 2005 in the frame of the CAPITOUL experiment. Surface energy balance (SEB) measurements have been conducted at a downtown site, over a dense urban area. A method is proposed to estimate QF at the local scale around this site from observations, as the daily residual term of the SEB equation. The values obtained from this method are in agreement with what can be expected: QF estimates are around 70 W m−2 during winter and 15 W m−2 during summer. On a larger scale (that of the agglomeration), an energy consumption inventory was conducted for the period of the field campaign with a 1-day temporal resolution and a 100-m spatial resolution. The estimates of QF obtained with this second method were analysed at the local scale around the measurements site, and compared with estimates computed from the energy budget observations. For the winter period, both estimates are in good agreement. For the summer period, the method based on SEB measurements seems to underestimate QF which is estimated around 30 W m−2 from the inventory. The simultaneous estimate of QF, with these two independent methods is a strength of this study. At the scale of the agglomeration, the basal state of energy consumption (observed during the summer period) varies between 25 W m−2 for the densest areas to less than 5 W m−2 for the residential suburban areas. In the regions crossed by the major roads, the traffic is the major source during summer. Then during the winter period, QF can reach 100 W m−2 in the densest areas of Toulouse whereas it ranges between 5 and 25 W m−2 in the suburban areas. Copyright © 2007 Royal Meteorological Society

Drought reconstruction for North Central China from tree rings: the value of the Palmer drought severity index

Abstract: We present a drought reconstruction for north central China based on a tree ring-width chronology developed from two sites of the Chinese pine (Pinus tabulaeformis) in the northern Helan Mountains. The drought reconstruction, spanning 1788–1999 A.D., was developed by calibrating tree-ring data with the Palmer drought severity index (PDSI), an index that describes the regional moisture condition properly. The reconstruction was verified with independent data, and accounts for 45.7% of the actual PDSI variance during their common period (1941–1999). The full reconstruction indicates that the regional drought variability was relatively stable during the nineteenth century, but became more variable and persistent during the twentieth century. The drought epoch in the late 1920s was the most severe one in our reconstruction. In contrast to a wetting trend in the western area of northwest China, a clear drying trend has occurred in north central China since mid-1930s. The multitaper method (MTM) spectral analysis indicates the existence of some decadal (∼11.4 year) and interannual (9.1, 6.8, 4.0, 2.7 and 2.1–2.0 year) cycles, which may potentially be the fingerprints of some proposed climate change forcings. Copyright © 2006 Royal Meteorological Society

Changes in European drought characteristics projected by the PRUDENCE regional climate models

Abstract: One of the key features of global climate change will be perturbations to the hydrological regime across Europe. To date, assessments of the impacts of future change have generally used results from only one climate model, thus underestimating the range of possible change projected by different climate models. Here, the skill of six regional climate models (RCMs) in reproducing the mean precipitation for the 1961–1990 period for six catchments across Europe is compared and their projections of changes in future precipitation are assessed. A simple drought index based on monthly precipitation anomalies is also described and used to assess the models. Considerable variation in model skill in reproducing monthly mean precipitation and drought statistics is observed, with model errors in the reproduction of drought events independent of those for the mean, suggesting that the models have difficulties in reproducing the observed persistence of low monthly rainfall totals. In broad terms, the models indicate decreases in summer and increases in winter precipitation across Europe. On the regional scales required for impacts analysis, considerable model uncertainty is demonstrated for future projections, particularly for drought frequency. Although increases in the frequency of long-duration droughts are identified for catchments in southern Europe, the magnitude of this change is not certain. In contrast, for a catchment in northern England, such events are likely to become less frequent. For shorter-duration droughts, future changes encompass the direction of change. For stakeholders in each of the regions, these changes and uncertainties pose different challenges for the management of water resources. For the scientific community, the challenge raised is how to incorporate this uncertainty in climate change projections in a way that allows those groups to make informed decisions based on model projections. It is suggested that probabilistic scenarios for specific hydrological impacts offer considerable potential to achieve this. Copyright © 2007 Royal Meteorological Society

Cities as agents of global change

Abstract: Cities, as places where human activities are concentrated, are frequently cited as the chief causes of, and solutions to, anthropogenic global change. In this article, I review the climatology literature that examines the relationship between cities and atmospheric changes at all scales. Despite the volume of literature on this theme, there is little overall coherence. In part, this is a result of the varying operational definitions of the city and the difficulty in obtaining pertinent information. Rather than attempt to provide a comprehensive review of the literature that focuses on cities and global change, this article categorises published research on the relationship between urban areas and climate changes at all scales into common themes. Copyright © 2007 Royal Meteorological Society

On the critical values of the standard normal homogeneity test (SNHT)

Abstract: The use of the standard normal homogeneity test (SNHT) for homogenization of climatological records and studying changes in their patterns has increased in recent years. The critical values of this test were originally developed for sample sizes ranging from 10 to 250 using relatively short Monte Carlo simulations (MCS). The objective of this paper is to improve the critical values of the SNHT and extend them to large sample sizes. The critical values, along with their standard errors, are developed for 108 sample sizes ranging from 10 to 50 000 using 30 replicates of one million samples for each sample size. These critical values mimic the tails of the SNHT statistic better and therefore are more accurate, and would be useful for making correct statistical inference for climate data homogenization and assessment of climate variability in future studies. Copyright © 2006 Royal Meteorological Society

Drought index evaluation for assessing future wheat production in Greece

Abstract: The main objectives of this study were (1) to evaluate the SPI, and three variations of the Palmer Drought Severity Index (PDSI) (the original PDSI (Orig-PDSI), a self-calibrated version (SC-PDSI) and a modified scheme employing Priestlay–Taylor's approach to compute potential evapotranspiration (PET) instead of Thornthwaite's method) and their respective moisture anomaly indices for assessing rainfed common wheat and durum wheat yield in two pilot crop regions in north and central Greece, and (2) to assess the vulnerability of wheat production to climate change, using the most appropriate drought index, with future scenarios provided by the Hadley Centre regional climate model HadRM3. The yield models that performed best at high-drought risk years (the Orig-PDSI index in the northern region and the SC-PDSI in the southern region, explaining 82.5 to 84.7% and 92% of the measured yield variability, respectively) were also the most effective at predicting the observed wheat yields when soil moisture was not an important yield-limiting factor. However, the strength of the relationship between the drought indices and the crop yields was much weaker. Improving the representation of PET in the PDSI algorithm did not improve the model's performance. The sensitivity of the two types of wheat to HadRM3 projections for the high-drought risk years differed dramatically between the two pilot districts, with extremely decreased yields of 3.14 tn ha−1 expected in the southern district and much smaller changes expected in the northern district (−4.6 vs + 6.7% for durum wheat and common wheat, respectively). For the low-drought risk years, the yield models in the northern region predicted lower yields by 30 to 60 kg ha−1. A positive yield response by 30 kg ha−1 was found for the southern district. Copyright © 2006 Royal Meteorological Society

Comparison of interpolation methods for mapping climatic and bioclimatic variables at regional scale

Abstract: Climatic data and bioclimatic indexes have been used to study plants, animals and ecosystem distribution. GIS-based maps of climatic and bioclimatic data can be obtained by interpolating values observed at measurement stations. Since no single method can be considered as optimal for all observed regions, a major task is to propose comparisons between results obtained using different methods applied to the same data set of climate variables. We compared three methods that have been proved to be useful at regional scale: 1 - a local interpolation method based on de-trended inverse distance weighting (D-IDW), 2 - universal kriging (i.e. simple kriging with trend function defined on the basis of a set of covariates) which is optimal (i.e. BLUP, best linear unbiased predictor) if spatial association is present, 3 - multilayer neural networks trained with backpropagation (representing a complex nonlinear fitting). Long-term (1955–1990) average monthly data were obtained from weather stations measuring precipitation (201 sites) and temperature (102 sites). We analysed twelve climatic variables (temperature and precipitation) and nine bioclimatic indexes. Terrain variables and geographical location have been used as predictors of the climate variables: longitude, latitude, elevation, aspect, slope, continentality and estimated solar radiation. Based on the root mean square errors from cross-validation tests, we ranked the best method for each variable data set. Universal kriging with external drift obtained the best performances for seventeen variables of the twenty-one analysed, neural network interpolator has proven to be more efficient for three variables and D-IDW for only one. Based on these results, we used the universal kriging estimates to produce the climatic and bioclimatic maps aimed at defining the bioclimatic envelope of species. Copyright © 2007 Royal Meteorological Society