Showing papers in "International Journal of Climatology in 1998"

A precipitation climatology of the alps from high-resolution rain-gauge observations

Abstract: A new precipitation climatology covering the European Alps is presented. The analysis covers the entire mountain range including adjacent foreland areas and exhibits a resolution of about 25 km. It is based on observations at one of the densest rain-gauge networks over complex topography world-wide, embracing more than 6600 stations from the high-resolution networks of the Alpine countries. The climatology is determined from daily analyses of bias-uncorrected, quality controlled data for the 20 year period 1971‐1990. The daily precipitation fields were produced with an advanced distance-weighting scheme commonly adopted for the analysis of precipitation on a global scale. The paper describes the baseline seasonal means derived from the daily analysis fields. The results depict the mesoscale distribution of the Alpine precipitation climate, its relations to the topography, and its seasonal cycle. Gridded analysis results are also provided in digital form. The most prominent Alpine effects include the enhancement of precipitation along the Alpine foothills, and the shielding of the inner-Alpine valleys. A detailed analysis along a section across the Alps also demonstrates that a simple precipitation‐height relationship does not exist on the Alpine scale, because much of the topographic signal is associated with slope and shielding rather than height effects. Although systematic biases associated with the rain-gauge measurement and the topographic clustering of the stations are not corrected for, a qualitative validation of the results, using existing national climatologies shows good agreement on the mesoscale. Furthermore a comparison is made between the present climatology and the Alpine sections of the global climatology of Legates and Willmott and the Greater European climatology from the Climate Research Unit (University of East Anglia). Results indicate that the pattern and magnitude of analysed Alpine precipitation critically depend upon the density of available observations and the analysis procedure adopted. © 1998 Royal Meteorological Society.

Homogeneity adjustments of in situ atmospheric climate data: a review

Abstract: Long-term in situ observations are widely used in a variety of climate analyses. Unfortunately, most decade- to century-scale time series of atmospheric data have been adversely impacted by inhomogeneities caused by, for example, changes in instrumentation, station moves, changes in the local environment such as urbanization, or the introduction of different observing practices like a new formula for calculating mean daily temperature or different observation times. If these inhomogeneities are not accounted for properly, the results of climate analyses using these data can be erroneous. Over the last decade, many climatologists have put a great deal of effort into developing techniques to identify inhomogeneities and adjust climatic time series to compensate for the biases produced by the inhomogeneities. It is important for users of homogeneity-adjusted data to understand how the data were adjusted and what impacts these adjustments are likely to make on their analyses. And it is important for developers of homogeneity-adjusted data sets to compare readily the different techniques most commonly used today. Therefore, this paper reviews the methods and techniques developed for homogeneity adjustments and describes many different approaches and philosophies involved in adjusting in situ climate data. © 1998 Royal Meteorological Society.

Spatial and temporal patterns of annual precipitation variability over the Iberian Peninsula

Abstract: In this study we have examined the spatial and temporal variability of the annual precipitation observations over the Iberian Peninsula (IP) for 47 years and 51 stations. Empirical orthogonal functions (EOFs) were obtained in order to characterise the variability. Four regional precipitation regimes have been identified and the corresponding principal components (PCs) were subjected to spectral analysis in order to obtain the structure of the temporal variations. The relationship between the precipitation and circulation patterns is also investigated. The four leading PCs of annual precipitation are associated with the following patterns: East Atlantic (EA); North Atlantic Oscillation (NAO); Southern Oscillation Index (SOI); Scandinavia (SCAND). The spectra of the precipitation PCs show statistically significant oscillations coherent with those found in the time series of the teleconnection indices. A reconstruction of the time series as a function of the PCs is provided in order to obtain a characterisation of precipitation climatology over the IP. © 1998 Royal Meteorological Society.

Spatial and temporal patterns of precipitation in Spain for the period 1880–1992

Abstract: The longest series of precipitation records in Spain are analysed using the principal component analysis (PCA) method. EOF analysis was used to spatially summarise the rainfall data and to enable clarification of the role of the dominant circulation regimes affecting the region. Three significant EOFs have been obtained in general, except for summer, when four EOFs are found. The first EOF is associated with Andalusia and the Spanish interior, the second and third EOFs with the Mediterranean and Cantabric coasts, alternatively, depending on the season. The analysis of the principal components series using a moving average and the Mann‐Kendall test, shows significant long term decreases in precipitation for the Mediterranean and interior regions (at least in some seasons), and an increase in precipitation for the Northern coastal region. More of these changes can be related to variations in the large scale circulation features over Western Europe and North Atlantic. The results are also compared with GCM outputs. © 1998 Royal Meteorological Society.

Trends in total rainfall, heavy rain events and number of dry days in Australia, 1910–1990

Abstract: Trends in heavy rainfall, total rainfall and number of dry days in Australia have been analysed using daily rainfall records at 125 stations. Summer and winter halves of the year were considered separately for the period 1910–1990. The summer half-year is defined as November–April, while the winter-half is May–October. Heavy rainfall is defined as the 90th and 95th percentiles of daily rainfall in each half-year. The magnitude of trends was derived from linear regression while statistical significance was determined by Kendall-Tau and field significance tests. Increasing trends in heavy rainfall and total rainfall have occurred during the summer half-year, but only 10–20% of stations have statistically significant trends. During the winter half-year, heavy rainfall and total rainfall have also increased, except in far southwest Western Australia and inland Queensland. There has been a reduction in the number of dry days in both halves of the year, except in far southwest Western Australia and at a few stations in eastern Australia where there has been an increase in the number of dry days in the winter half-year. Changes in the number of dry days were statistically significant at over 50% of stations. Hence there are regions showing coherent increases and decreases in rainfall which may be due to systematic changes in climate during the last century. Trends were averaged over three broad regions with adequate station coverage. There has been a general decrease in dry days with an increase in total and heavy rainfall intensity in the northeast and southeast, and a decrease in total and heavy rainfall in the southwest. These rainfall changes are related to changes in other climate variables such as temperature and cloud cover in Australia. © 1998 Royal Meteorological Society

The influence of green areas on nocturnal temperatures in a high latitude city (Göteborg, Sweden)

Abstract: The air temperature pattern in three urban parks and their surrounding built-up areas was studied over a one and a half year period in Goteborg, Sweden. The measurements were made at mobile and permanent stations on nights with clear skies and light winds. The maximum temperature difference found between a park and a built-up area was 5.9°C (summer), and the extension of the cool park climate into the built-up area was over 1100 m from the park border. Both the extension and the magnitude of the temperature difference depended on the size of the park and the distance from the park border. The green area cooled at a faster rate than the built-up area, although there were large variations within each area. At the rural station, located at an open site, the cooling was less than at the open part of the green area. Differences in sky obstruction between the sites explained some of the variations in cooling and temperature. However, the relationship between the urban-park temperature difference and sky view factor was not statistically significant. © 1998 Royal Meteorological Society

Global historical climatology network (GHCN) quality control of monthly temperature data

Abstract: All geophysical data bases need some form of quality assurance. Otherwise, erroneous data points may produce faulty analyses. However, simplistic quality control procedures have been known to contribute to erroneous conclusions by removing valid data points that were more extreme than the data set compilers expected. In producing version 2 of the global historical climatology network's (GHCN's) temperature data sets, a variety of quality control tests were evaluated and a specialized suite of procedures was developed. Quality control traditionally relies primarily on checks for outliers from both a time series and spatial perspective, the latter accomplished by comparisons with neighbouring stations. This traditional approach was used, and it was determined that there are many data problems that require additional tests to detect. In this paper a suite of quality control tests are justified and documented and applied to this global temperature data base, emphasizing the logic and limitations of each test. © 1998 Royal Meteorological Society.

A 30‐year (1964–1993) daily rainfall data base for the Spanish Mediterranean regions: first exploratory study

Abstract: A dense daily precipitation data base, extending from 1964 to 1993, has been created for the Mediterranean regions of Spain. It is composed of complete and homogeneous series at 410 raingauge stations (347 in the coastal fringe of peninsular Spain, and 63 in the Balearic Islands). The region offers an interesting scenario for mesoclimatological studies on time and spatial rainfall variability: geomorphologically, it is characterized by important coastal relief units and complex distribution of sea and land masses, leading to different exposures to the rain-bearing maritime winds; climatically, the western Mediterranean is subject to strong seasonal variability, since it is a transition zone between the midlatitude low pressure belt and the subtropical highs as a result of its latitude (between 36° and 44° N). In this study, we exploit the data base and present a first pluviometric characterization of the area by means of yearly and seasonal mean products. The results reveal clear and coherent spatial patterns that we interpret, based on typical storm tracks and land, sea, and relief distributions. In addition, a partition of the 30-year period into three decades (1964-1973, 1974-1983, 1984-1993) has been considered in order to assess the possible existence of any trend. A successive drying of the most sensitive areas to the winter Atlantic depressions (western Catalonia, and central and west Andalucia) is observed. In contrast, the second analysed decade is appreciably drier than the other two in the areas more dependent on the Mediterranean disturbances. The occurrence of anomalous autumns being the most responsible. This fact emphasizes the fundamental importance of the autumn season for the pluviometric balance of the considered area, especially in its eastern part where the major amount of precipitation during this season is produced by convective systems.

Towards the prediction of the East Africa short rains based on sea‐surface temperature–atmosphere coupling

Abstract: It is shown that the July–September sea-surface temperature (SST) pattern contains moderately strong relationships with the October–December (OND) seasonal rainfall total averaged across East Africa 15°S–5°N, 30°–41.25°E. The relations can be described by using three rotated global SST empirical orthogonal functions (EOFs), mainly measuring aspects of SST patterns in the tropical Pacific (related to El Nino/Southern Oscillation), tropical Indian and, to a lesser extent, tropical Atlantic. Confidence in the relationships is raised because the three EOFs correlate significantly with OND near-surface divergence over the tropical Pacific, Indian and Atlantic Oceans (extending into Northern mid-latitudes), as well as with the rainfall in East Africa and also with rainfall across southern and western tropical Africa. For the East African region, multiple linear regression (MLR) and linear discriminant analysis prediction models are tested. The predictors are pre-rainfall season values of the three rotated SST EOFs. The predictors use information through September. Validating MLR hindcasts using a 1945–1966 (1967–1988) training period and a 1967–1988 (1945–1966) testing period between 30 to 60% of the area-averaged rainfall variance is explained. To achieve unbiased estimates of the expected skill of a forecast system, it is safest to keep model training and testing periods completely separate. The above strategy achieves this in the most important step of ensuring that the models fit the SST predictors to the rainfall predictand using years independent of the testing period. However, the EOFs were calculated over 1901–1980, so for hindcasts prior to 1981, the EOFs describe the SST variability a little better than could be achieved in real-time, which could inflate skill estimates. Tests in the years 1981–1994, independent of the 1901–1980 eigenvector analysis period, do produce similar levels of skill, but a few more forecast years are needed to confirm this result. It is shown that the mean verification at each individual location within East Africa is somewhat lower, which is important to consider for some applications. The need to monitor the prediction relationships and update the models is emphasised. Furthermore, these forecasts only become available as the OND season is underway, though some evidence is found for one of the EOF predictors having skill as early as June. © 1998 Royal Meteorological Society

Development of daily rainfall scenarios for southeast spain using a circulation-type approach to downscaling

Abstract: A method for downscaling from the relatively coarse General Circulation Model (GCM) spatial scale to the finer spatial scale required for impact assessment has been developed and tested in the Guadalentin Basin, southeast Spain. The method uses a circulation-type approach and relates large-scale patterns of a predictor variable, gridded sea level pressure, to local values of a surface climate variable (daily rainfall at six stations). The large-scale patterns are defined using an automated version of the Lamb Weather Type classification scheme, originally developed for the British Isles. It is demonstrated that this scheme can be successfully transferred to another region, southeast Spain. The 14 basic circulation types are combined into eight groups. These provide a legitimate basis for downscaling because each has a characteristic pressure pattern which produces the expected type and direction of flow over the study region. Furthermore, a set of consistent and distinct relationships is identified between these circulation types and daily rainfall in the Guadalentin Basin. The ability of the GCM to reproduce the observed circulation types is assessed before applying these relationships to control and perturbed-run GCM output using a statistical weather generator. The effects of the GCM’s failure to reproduce the observed frequency of the circulation types are detectable in the weather generator output. The GCM changes in SLP and circulation-type frequency between the control and perturbed-runs are generally small. Nonetheless the weather generator results indicate significant changes in the number of rain days in spring and summer. These scenarios are presented as illustrative results rather than as reliable predictions. It is concluded that the circulation-type based approach to downscaling offers great potential. © 1998 Royal Meteorological Society.

Influence of geopotential heights, cyclone frequency and southern oscillation on rainfall variations in Turkey

Abstract: Normalized rainfall of Turkey tended to decrease in many annual and winter series and to increase in some spring and summer series during 1930–1993. Low-frequency fluctuation of regional winter rainfall series was generally dominated by cycles of 3–3.2, 6–7, 7–8.4, and 14–21 years. Spring rainfall series depicted high-frequency oscillations with cycles of 2–2.2 years, and longer cycles of 4.2–4.7 years. Mean 700 and 500 hPa geopotential heights over Turkey generally experienced positive anomalies from late 1970s to early 1990s, and showed an upward trend in winter and summer. Significant negative correlations were found between geopotential height and rainfall anomalies in winter over most of Turkey. Cycles of 2–2.2 and 3.2–3.8 years in spring rainfalls appeared to be associated with similar oscillations of spring geopotential heights. Cycles of 13 years in both winter geopotential series reflected in a similar cycle of 14 years in annual and winter rainfall. The number of depressions reaching Turkey tended to decrease for about 10 years. Increased frequencies and intensities of dry conditions in the last ca. 20 years may have been related to increased geopotential heights and decreased frequency of depressions over Turkey. Signs of warm minus cold event winter anomalies during various stages of the Southern Oscillation revealed the existence of some coherent regions without significant signals. Most of the selected 48 stations had a positive sign anomaly during year −1 warm and cold events. The cold event rainfall means showed a coherent region of significantly increased rainfall conditions over the central-west and central parts of Turkey. Slightly wetter than normal warm event conditions during year +1 were observed in many stations. Most of warm and cold event responses were characterized by a decreased rainfall. Drier than long-term average conditions were significant at some stations during year +1 cold events. Warm minus cold event differences had an opposite signal between year −1 and year 0 (+1) in many stations. Opposition of composite anomalies was evident in most stations between year −1 and year +1 cold events. © 1998 Royal Meteorological Society

Doubled CO2 precipitation changes for the Susquehanna Basin: down‐scaling from the Genesis general circulation model

Abstract: Artificial neural nets are used in an empirical down-scaling procedure to derive daily subgrid-scale precipitation from generalcirculation model (GCM) geopotential height and specific humidity data. The neural net-based transfer functions aredeveloped using a 2  625  gridded data assimilation product from the Goddard Space Flight Center, applied to a 464 matrixof grid-cells centred on the Susquehanna river basin. The down-scaled precipitation is a close match to the observed data(temporal correlations at individual grid-points range from 06to084). Doubled CO 2 climate change scenarios are producedby applying the same transfer functions to the geopotential height and specific humidity fields from 16CO 2 and 26CO 2 simulations of version II of the GENESIS climate model. The analysis indicates a 32 per cent increase in spring and summerrainfall over the basin, resulting from changes in both moisture availability and the orientation of the storm track over theregion. The down-scaled precipitation increases, derived from the change in the GCM’s circulation and humidity fields, areconsiderably larger than the change in the model’s actual computed precipitation. # 1998 Royal Meteorological Society. Int.J. Climatol., 18, 65–76 (1998)No. of Figures: 10. No. of Tables: 1. No of References: 15.

Regional climate change in Portugal: precipitation variability associated with large‐scale atmospheric circulation

Abstract: Four major circulation patterns, associated with daily precipitation in Portugal, are classified from daily sea level pressure fields over the northeastern Atlantic and western Europe, based on the K-means clustering algorithm coupled with principal component analysis A rainy pattern is clearly identified with a probability of rain of 746%, as well as two distinct dry patterns, one prevailing in summer and the other occurring frequently in winter; a blocking-like pattern with a probability of rain of 368% has also been identified These patterns are quasi-stationary, normally persisting for 1 week and sometimes even for 1 month, especially the dry ones; they represent the principal weather regimes associated with precipitation in Portugal Interannual variations in monthly precipitation associated with the circulation patterns are also investigated; results show that these variations match fluctuations in the frequencies of occurrence of both the rainy and the dry patterns The decreasing trend of March monthly rainfall in southern Portugal is closely related to corresponding trends in the frequencies of both the rainy pattern and the summer dry pattern Long term trends are not significant either in other monthly rainfall sequences or in the frequencies of different circulation patterns Interannual variations seem, in most months, to be quasi-periodic Singular spectrum analysis (SSA) is performed on these sequences to detect quasi-periodic oscillations Relationships between oscillations in rainfall and in frequencies of occurrence of circulation patterns are studied Results show that four weather circulation patterns or weather regimes are important for investigating regional climate change in Portugal and its relationship with variability of large-scale atmospheric circulation © 1998 Royal Meteorological Society

Atmospheric circulation patterns and spatial climatic variations in Beringia

Abstract: Analyses of more than 40 years of climatic data reveal intriguing spatial variations in climatic patterns for Beringia (North-eastern Siberia and Alaska), aiding the understanding of the hierarchy of climatic controls that operate at different spatial scales within the Arctic. A synoptic climatology, using a subjective classification methodology on January and July sea level pressure, and July 500 hPa height anomaly patterns, identified 13 major atmospheric circulation patterns (26 pairs consisting of 13 synoptic/temperature and 13 synoptic/precipitation comparisons) that occur over Beringia. Composite anomaly maps of circulation, temperature, and precipitation described the spatial variability of surface climatic responses to circulation. Results indicate that nine synoptic pairs yield homogeneous surface climatic anomaly patterns throughout most of Beringia. However, many of the surface climatic responses illustrate heterogeneous anomaly patterns as a result of variations in circulation controls, such as troughing over East Asia and the Pacific subtropical high superimposed over topography, with small shifts in atmospheric circulation dramatically altering spatial variations of anomaly patterns. Distinctive contrasts in climatic responses, as suggested from ten synoptic pairs, are clearly evident for Western Beringia versus Eastern Beringia. These results offer important implications for scholars interested in assessing late Quaternary climatic change in the region from interannual to millennial timescales. © 1998 Royal Meteorological Society

A study of national trend and variation in UK floods

Abstract: Trends in UK flood behaviour are examined using extensive peaks‒over‒threshold and annual maxima data from 890 gauging stations. The analysis focuses on changes that have occurred at the national scale; data were therefore pooled across all available sites. Two annual series are examined, representing flood size and frequency of flood occurrence. Results are presented for the 40‒year period 1941–1980, for the 50‒year period 1941–1990 (annual maxima only), and for some limited data for 1870–1995. Three main tests for trend (linear regression, normal scores regression and Spearman's correlation) were applied using (i) conventional theoretical significance levels, and (ii) a permutation approach. Test results were generally significant under the conventional approach but were not significant under the permutation approach. This difference occurs because some of the independence assumptions required for the conventional approach are not satisfied. There is a high year‒to‒year variation inherent in the data and, relative to this variation, any underlying trends are not significant. Regional and seasonal effects are considered but no significant trends emerge. Although no trends in national flood behaviour are seen, the year‒to‒year variations in UK floods appear to show some systematic behaviour (fluctuations) when smoothed using locally weighted regression. Further investigation suggests that these fluctuations are driven climatically; similar fluctuations are evident in UK annual rainfall values. The influence of climatic variation on the pattern of flood occurrences also means that, at the national scale, progressive changes in the UK flood regime, e.g. arising from land‒use change, are hidden. Over the period 1940 to 1980 smoothed curves suggest increased fluctuation, with emergence of some possible cyclic tendencies. However, in the context of the limited data going back to 1870, this increased fluctuation is not atypical. Overall, no evidence for climate change emerges for the periods for which data are currently available, although climate clearly has a marked effect on floods. The observed climate‒linked fluctuations indicate that inferences from short flood records may differ appreciably from those obtained from longer series. This could have potentially important implications for flood‒risk studies and trend detection. ©1998 Royal Meteorological Society.

Changes in rainfall characteristics in northern Nigeria

Abstract: This study examines recent changes in several rainfall characteristics in northern Nigeria. The records at 25 locations were analyzed for the occurrence of abrupt changes and trends using the Pettitt and the Mann-Kendall tests. Variables analyzed included annual total rainfall and number of rain days, the dates of onset, termination and duration of the rainy season as well as monthly rainfall, monthly number of rain days and various categories of rainfall above certain intensities. An abrupt change occurred in the time series of annual rainfall, number of rain days and affected areas north of latitude 11° N. However, the sub-periods prior to and after the change points may be considered to be homogenous. The series of variables related to the duration of the rainy season exhibit no significant trends or jumps. It is concluded that recent changes in rainfall over the Sahel were driven by a reduction in the frequency of rain days of high rainfall intensities during the months of August and September. The fact that the high intensity rainfall does not contribute significantly to crop growth may explain the continuation of agricultural activities in the Sahel despite massive reductions in annual rainfall. © 1998 Royal Meteorological Society.

Comparison of geostatistical methods for estimating the areal average climatological rainfall mean using data on precipitation and topography

Abstract: The results of estimating the areal average climatological rainfall mean in the Guadalhorce river basin in southern Spain are presented in this paper. The classical Thiessen method and three different geostatistical approaches (ordinary kriging, cokriging and kriging with an external drift) have been used as estimators and their results are compared and discussed. The first two methods use only rainfall information, while cokriging and kriging with an external drift use both precipitation data and orographic information (easily accessible from topographic maps). In the case study presented, kriging with an external drift seems to give the most coherent results in accordance with cross-validation statistics. If there is a correlation between climatological rainfall mean and altitude, it seems logical that the inclusion of topographic information should improve the estimates. Kriging with an external drift has the advantage of requiring a less demanding variogram analysis than cokriging. © 1998 Royal Meteorological Society.

Predicting rainfall statistics in England and Wales using atmospheric circulation variables

Abstract: Regression models are developed to predict point rainfall statistics, with potential application to downscaling general circulation model (GCM) output for future climates. The models can be used to predict the mean daily rainfall amount and the proportion of dry days for each calendar month at any site in England and Wales, and use the following explanatory variables: (i) geographical (altitude, geographic coordinates, and distance from nearest coast); and (ii) atmospheric circulation variables (mean values of air-flow indices derived from mean sea-level pressure grids). Values predicted by the models, for 10-km grid squares covering the whole of England and Wales, are in reasonable agreement with the 1961-1990 climatology of Barrow et al. (1993). The potential use of the models in hydrological climate change impact studies is discussed.

On the characteristics of the rainfall events in the Sahel with a view to the analysis of climatic variability

Abstract: The lack of appropriate data has long been a major obstacle to the study of the Sahelian rainfall at the event scale. In this paper, use is made of the EPSAT-Niger recording rain-gauge data to characterize the convective rain events of the central Sahel. Although some considerations lead to the identification of two main types of mesoscale convective systems, it is shown here that the most relevant stratification in terms of statistical analysis of the event rainfall distribution is between the events of the margins and those of the core of the rainy season. In fact, the average storm rain-depth appears to be non-stationary in time, with storm rain-depths slightly higher in the core of the rainy season than on the margins. Separation between the core and the margins thus allows the fitting of an exponential model to the observed storm rain-depth distributions of each period (core and margins), although a better fit would certainly be obtained if a proper modelling of the time non-stationarity was carried out. It is then shown that there is little, if any, correlation between the mean storm rain-depth of a given year and the overall abundance of the corresponding rainy season. This is a validation of previous works, which reached the same conclusion using daily rainfall data only. One major result of this work is that the statistics characterizing the rain events in the Sahel display little fluctuations, either in space or from year-to-year, as compared with those observed for the total seasonal rainfall. Each year and at each station the average storm rainfall remains close to 12 mm during the margins of the rainy season and close to 15 mm in the core. During the same period, the average seasonal rainfall over the study area ranged from 400 to 660 mm and for any given year the ratio between the maximum and the minimum point seasonal rainfall was of the order of 2. It is therefore concluded that the main source of rainfall variability in the Sahel is linked to the variability in the number of events rather than in the magnitude of these events. © 1998 Royal Meteorological Society

Urban-rural temperature differences in Buenos Aires

Abstract: The hourly temperature differences between Buenos Aires City and Ezeiza Airport were calculated using 3 years of data. This paper describes statistical results on the Buenos Aires urban heat island and how it varies with days of the week, seasons, cloud cover, direction and speed of wind. The average value of the maximal heat island fell in winter from 4.6°C with light winds and little sky coverage to 3.6°C with windy and cloudy conditions. Strong winds from the city toward rural areas and winds from the river over the city facilitated the occurrence of an inverse heat island (rural area warmer than city). The inverse heat island occurred 20% of the time over a total of 22 000 hours studied. Another important factor seems to be the great activity of the city; the average value of the maximal heat island fell between weekdays and weekends 1°C with weak winds and little sky coverage conditions. The hourly average values were analyzed depending on the days of the week. © 1998 Royal Meteorological Society

The effects of the Southwest Indian Ocean tropical cyclones on Ethiopian drought

Abstract: Tropical cyclones are one of the prominent weather systems that are generated over the tropical oceans. The cyclones that develop in the Southwest Indian Ocean (SWIO) usually travel west then southwest and finally recurve to southeast, generally before reaching the East African coast. However, it is shown in this study how SWIO-tropical cyclones/depressions can indirectly affect Ethiopian weather. Using correlation and composite analyses, interannual and intraseasonal rainfall variations in Ethiopia were compared with the frequency and time of occurrence of the tropical cyclones. Years showing the consecutive occurrence of several tropical depressions over the SWIO coincide with the drought years of Ethiopia. Reciprocally, years of abnormally low frequency of tropical cyclones are associated with heavy rainfall in Ethiopia. Belg rainfall (‘small rains’, February–May) is much more influenced by the cyclonic activity than Kiremt rainfall (‘big rains’, June–September), which occurs outside the cyclonic season of the Southeast Indian Ocean. On a daily basis, rainfall activity during the Belg period is significantly reduced when a tropical depression is observed in the SWIO, before picking up again a few days later. No systematic time-lag was found between the cyclone occurrence and the decrease in daily rainfall amounts. However, at interannual time-scales, a higher (lower) frequency of tropical depressions during the months of November–January tends to be followed by abnormally low (high) Belg rainfall. One of the mechanisms suggested to explain this seasonal persistence in the frequency of tropical depressions involves sea-surface temperatures in the SWIO, from where many of the depressions originate. The atmospheric processes accounting for the teleconnection between Ethiopia rainfall and tropical depressions were examined. Monthly anomalies of upper air winds for years showing frequent occurrence of tropical depressions proved significant not only over the SWIO, but also the Northern Hemisphere above Northeast Africa. In addition to a slightly enhanced cross-equatorial flow, diverting moisture advection towards the active centre of the system, much stronger than usual equatorial easterlies in the upper troposphere and a reduced southward excursion of the subtropical jet stream result in weaker convective activity over Ethiopia. © 1998 Royal Meteorological Society

Behaviour of the centres of action above the Atlantic since 1881. Part I: Characteristics of seasonal and interannual variability

Abstract: Based on generated time series of the central pressure, latitudinal and longitudinal position of the Iceland Low, Azores High, Inter Tropical Convergence Zone and South Atlantic Subtropical High, the behaviour of these atmospheric centres of action since 1881 is examined with regard to the intra- and interannual variations, persistence of anomalies, linear trends and interactions. The critical analysis of these time series as well as those of the pressure gradients between the neighbouring centres suggests that the existing non-periodic fluctuations of the surface atmospheric circulation on the time-scale of one or more decades suppress the long-term changes. These fluctuations are similar to the ‘internal’ variations of the climate system, recognized in the climate simulations of coupled ocean–atmosphere models. © 1998 Royal Meteorological Society.

A method for finding sea breeze days under stable synoptic conditions and its application to the Swedish west coast

Abstract: A method for selecting sea breeze days in an archipelago was developed with special application to the West Coast of Sweden. The primary criterion for identifying a sea breeze was the occurrence of a distinct change in surface wind direction within a 24 h period. The data used in the study were gathered using hourly meteorological records from field stations situated on the West Coast. Six different filters were included in the method; each of them developed from empirical knowledge of the physical processes responsible for the occurrence of a sea breeze system. Using this method it is possible to create a data set of sea breeze days, which does not use distance to the shoreline as a necessary condition for classification. © 1998 Royal Meteorological Society.

Tree-ring evidence for long-term precipitation changes in subtropical South America

Abstract: In recent years there has been a notable increase in the number of tree-ring chronologies for the temperate and cold regions of the Americas. In comparison, few advances have been reported for the American tropics and subtropics, where the absence of seasonality appears to be the main reason for the lack of well-defined growth bands in most species. Distinct, annually formed tree-rings have recently been reported for subtropical montane trees on the eastern slope of the Andes (22–28°S). Six absolute-dated chronologies from Juglans australis and Cedrela lilloi at the upper treeline (between 1700 and 2000 m) in the montane forest of north-western Argentina were selected to explore the potential of these records to infer decade- to century-scale climatic variations in the subtropics. These tree-rings capture a significant percentage of the variances in regional temperature and precipitation records and appear to be suitable to reconstruct decade-long changes in large-scale circulation over the South American subtropics. In particular, tree-growth at xeric sites has been strongly influenced by precipitation changes, which in turn respond to alternating patterns of zonal versus meridional flows over subtropical South America. The upper treeline records indicate that the increase in precipitation during the past three decades, caused by an enhanced transport of humid air masses from the Brazilian–Bolivian lowland tropics to the semiarid subtropics, has been unprecedented for the past 200 years. Although this precipitation increase may reflect natural variability in the subtropics, it is also consistent with 2×CO2 climatic simulations from five general circulation models. There is a general agreement among model results about a noticeable increase in precipitation in north-western Argentina due to an intensification of the water transport across subtropical South America in response to a southward displacement of the continental low and an increasing warming at these latitudes. A larger network of tree-ring chronologies will aid efforts to understand long-term climatic interactions between tropical and subtropical regions in South America, and shed light on the role of natural versus anthropogenic forcings on regional climatic changes. © 1998 Royal Meteorological Society

Behaviour of the centres of action above the atlantic since 1881. part ii: associations with regional climate anomalies

Abstract: This paper constitutes the second part of the analysis of the behaviour of centres of action above the Atlantic since 1881. The present study examines the applicability of the generated time series of the central pressure and geographical location of the Iceland Low (IL), Azores High (AH), Inter Tropical Convergence Zone (ITCZ) and South Atlantic Subtropical High (SSH) to the description of the surface circulation and its influence on the climate anomalies above the Atlantic and adjacent land areas in the period 1950—1989. Using the normalized anomalies of the pressure gradient between the cores of the AH and IL (AH—IL gradient) as a NAG (North Atlantic Oscillation) index, it can be shown that t his index is a practicable tool for the detection of characteristic spatial anomaly patterns in different climate variables above the North Atlantic and surrounding land areas. The ‘strong’ NAG (the AH-IL gradient above ‘normal’) in winter leads to a warming above northern and central Europe as well as to the development of the dipole-like anomaly patterns in the wind speed, sea-surface temperature, surface and tropospheric temperature and other variables above the western and eastern North Atlantic. The composite analysis for the ‘weak’ NAG reveals inverse patterns. The application of the characteristic parameters of the ITCZ and of the two subtropical highs as predictors of the rainfall variability in northeast Brazil and the Sahel for the period 1950-1983 shows that for the most part, these variations can be explained by the anomalies of the latitude position and intensity of the ITCZ. © 1998 Royal Meteorological Society. Int. J. Climatol., 18, 23-36 (1998) No. of Figures: 3. No. of Tables: 5. No. of References: 71.

A climatology of ozone mini-holes over the northern hemisphere

Abstract: A comprehensive climatology of ozone mini-hole occurrence over mid-latitudes of the northern hemisphere is constructed, based on daily measurements of total column ozone from the Nimbus-7 TOMS (total ozone mapping spectrometer) satellite, using version-7 data, over a 14-year period. An ozone mini-hole is a synoptic-scale area of strongly reduced total column ozone amount, which undergoes a cycle of growth and decay, in direct association with tropospheric weather systems. A computer-based analytical method for searching for local minima in the daily TOMS ozone fields is employed to build up a catalogue of mini-hole events, tracking location and intensity. Geographical and seasonal variations in the resulting data are summarised and possible trends over the data period are assessed. Ozone mini-hole activity exhibits a strong annual cycle, peaking in the late winter. Mini-holes are shown to be generally more frequent over the N. Atlantic/European sector than the N. Pacific/N. American sector. An alternative method of defining a threshold is also discussed in which ozone minima are viewed as anomalies from local climatological mean ozone levels. Mini-hole activity is seen to exhibit a close spatial correlation to each of the storm-track regions of the hemisphere. © 1998 Royal Meteorological Society

Drought tendencies in Hungary

Abstract: Drought is a major climatic hazard, which is expressed as relative to a long term average. Drought is a recurrent feature in Hungary's climate and it can cause significant damage to the country's agriculture. Its importance is emphasized by recurrent drought events in the 1980s and 1990s. In this study, we examine long time series of the Palmer Drought Severity Index (PDSI) to characterize statistical features of drought in Hungary. Two statistical tests are presented which are appropriate tools for studying normal and extreme climatic features or events that can be characterized with thresholds. The two tests were applied to the PDSI series in order to capture changes in drought occurrence: (i) whether drought events tend to concentrate at one end of the time series, and (ii) whether droughts tend to recur in subsequent years. On a country-wide scale, results indicate that more droughts happened at the end of the time series, especially moderate and severe droughts. Moreover, moderate and severe droughts tend to occur in subsequent years. Most of the stations show the same results as the country-wide series, although some of them failed to indicate significant change. There are spatial differences, but at most of the stations a general drying tendency is apparent. © 1998 Royal Meteorological Society.

The mean structure and temporal variability of the semiannual oscillation in the southern extratropics

Abstract: The semiannual oscillation of the pressure and mid-tropospheric baroclinicity in the southern extratropics has been studied using 21 years (1973–1993) of numerical Southern Hemisphere analyses and long series of pressure data from a number of mid- and high latitude stations. Using recent data and techniques not previously documented, this study has verified that the semiannual oscillation is an important but highly variable feature of the annual cycle of pressure and meridional temperature gradient. In the pressure, the half-yearly wave attains its greatest amplitudes in the mid-latitude ocean basins and on the Antarctic periphery, with a minimum near 55°S. The semiannual oscillation of the temperature gradient is strongest near 60°S, where it explains typically 50% of the mean annual variance of monthly data, and the harmonic maxima (strongest gradients) occur during the transition seasons. Analysis of the temporal behaviour of the half-yearly wave has revealed considerable variability on interannual to decadal time scales. The comparison and correlation of the temporal variability of the mid- and high latitude components of the semiannual oscillation of the pressure has revealed that these are statistically related and the strength of the relationship is greatest for variations occurring on longer time scales. In both the mid- and high latitudes the temporal variability of the semiannual oscillation of pressure has been found to be statistically related to the variability of the high latitude temperature gradient. These observations suggest that the differing annual cycles of temperature in the mid- and high southern latitudes not only give rise to the semiannual oscillation of the pressure in the mean, but, in addition, the variability of this dynamic phenomenon is linked to the variability of the thermal gradients. These findings suggest that a key to understanding the temporal variability of pressure in the southern extratropics on annual to decadal time scales lies in the understanding of the large scale variability of the temperature gradients. © 1998 Royal Meteorological Society

Synoptic associations of winter climate and snowfall variability in New England, USA, 1950–1992

Abstract: Seasonal snowfall in New England has exhibited wide variations over the last few decades, the underlying causes of which have not been identified previously. In this paper, compositing and statistical analyses examine how interactions among the large-scale atmospheric circulation, cyclonic activity and storm track preferences, and western Atlantic sea-surface temperatures contribute to winter climate variability and exceptional winter snowfall totals in New England. High winter snowfall totals are associated with a meridional circulation regime, as indicated by a negative North Atlantic Oscillation (NAO) index and negative 700 hPa height anomalies over the eastern USA. Anomalously high surface pressure over northern Canada promotes advection of cold air into the region, and the preferred storm track is along or just to the south of the New England coast. Low winter snowfall totals are associated with no unique set of circumstances, but a positive NAO index and more zonal circulation is indicated, along with a weaker surface high over Canada, and a storm track over interior New England rather than along the coast. There is no direct association with the Pacific-North American index, although this may modulate the association with the NAO. In extreme southern New England, high (low) winter snowfall totals are associated with negative (positive) sea-surface temperature anomalies along the Atlantic seaboard.

Relationships between extreme daily precipitation and topography in a mountainous region: a case study in Scotland

Abstract: The spatial pattern of precipitation is known to be highly dependent on meteorological conditions and relief. But the relationships between precipitation and topography in mountainous areas are not very well known, partly because of the complex topography in these regions, and partly because of the sparsity of information available to study such relationships in high elevation areas. Moreover, studies are usually focused on mean annual precipitation, and so the patterns of extreme precipitation at short time steps, like daily, remain difficult to model. Daily annual maximum precipitation for 1003 gauges in Scotland, the most mountainous region of the United Kingdom, are studied to investigate the relationships between the median of the daily rainfall annual maximum, RMED, and the topography. A set of 14 topographical variables, some of them defined with respect to one of eight cardinal directions, are calculated from a 1×1 km digital terrain model (DTM). A particular effort has been made to improve the definition of some of the topographical variables suggested in the literature, either to provide a better physical definition or to better reflect the spatial variability of the topography. Single and multiple regression analyses have been made in some parts of the Highlands, leading to a 4-parameter model. This model is a mixture of geographical parameters (distance from the sea in opposing directions) and of topographical parameters (obstruction against the prevailing winds, and roughness between the main moisture source and the gauge). Special care has been taken to define a model whose physical sense is consistent with the meteorological conditions and whose parameters are not too interdependent. © 1998 Royal Meteorological Society