Showing papers in "International Journal of Climatology in 1990"

Mean seasonal and spatial variability in gauge‐corrected, global precipitation

Abstract: Using traditional land-based gauge measurements and shipboard estimates, a global climatology of mean monthly precipitation has been developed. Data were obtained from ten existing sources, screened for coding errors, and redundant station records were removed. The edited data base contains 24,635 spatially independent terrestrial station records and 2223 oceanic grid-point records. A procedure for correcting gauge-induced biases is presented and used to remove systematic errors caused by wind, wetting on the interior walls of the gauge, and evaporation from the gauge. These ‘corrected’ monthly precipitation observations were then interpolated to a 0·5° of latitude by 0·5° of longitude grid using a spherically based interpolation procedure. Bias-corrected spatial distributions of the annual mean and intraannual variance are presented along with a harmonic decomposition of the intra-annual variance.

Characteristics of the low‐level temperature inversion along the Alaskan Arctic coast

Abstract: The climatological characteristics of low-level tropospheric temperature inversion along the Alaskan Arctic coast are examined from a 10-year record of surface and upper-air meteorological data at Barrow and Barter Island, Alaska. The meteorology at the two stations is found to be remarkably similar; surface-based inversions occur nearly half of the time, but even in the winter months inversions are frequently based up to 200 m above the surface. Median inversion depths range from 250m to 850m, with median temperature differences across the inversion ranging from 2°C to 11°C. In March and April when the inversion is strongest, the lowest levels frequently erode. This indicates that the transient character of the inversion reduces its effectiveness as a barrier to vertical mixing. The inversion depth closely follows the annual cloud-cover cycle, demonstrating that the development and maintenance of the inversion is a result of complex interactions between radiative forcing, synoptic activity, and sea-ice dynamics.

A search for accelerations in records of European mean sea level

Abstract: Long records from European tide gauges have been inspected for evidence of modifications in the rate of change, or ‘accelerations’, of mean sea level (MSL). In general, no evidence was found for MSL accelerations significantly different from zero over the period 1870 to the present, although non-zero accelerations were observed at individual stations. Changes in surface air pressure may have been responsible for an overall slightly negative acceleration observed in northern Europe. In order to extend the study to time-scales longer than a century, data from the oldest European MSL records at Brest, Sheerness, Amsterdam and Stockholm starting in 1807, 1834, 1700 and 1774, respectively, were also investigated with the result that a positive acceleration of order 0·4 (mm year−1) per century appears to be typical of European Atlantic coast and Baltic MSL over the last few centuries. Although of interest as an indicator of past climate change, this small low-frequency MSL acceleration is an order of magnitude less than that anticipated over the next few decades as a result of greenhouse warming. A conceptual study of possible future Newlyn (UK) tide gauge data has shown that the MSL acceleration anticipated from the global warming should be apparent in the records by the early part of the next century.

Decadal-scale changes of the circulation in the tropical atlantic sector associated with Sahel drought

Abstract: Trends in the general circulation setting during 1948–1983 accompanying the progressive aggravation of drought in sub-Saharan Africa are investigated from ship observations over the tropical Atlantic and surface station records in western Africa. Decadal-scale circulation changes in the Atlantic sector include a pressure rise over the tropical North Atlantic; southward displacement of the near-equatorial wind confluence and associated maximum cloud belt; acceleration of the north-east trades and possibly a strengthening of the South Atlantic trades, along with increasing cloudiness in the equatorial zone; increasing cloudiness over the tropical North Atlantic and Central American seas; and cooling of surface waters in a band across the tropical North Atlantic contrasting with warming in the South Atlantic. These changes occurred in all seasons, but were most pronounced at the height of boreal summer. Records at land stations in western Africa show that near the coasts the temperature and pressure trends are consistent with the adjacent ocean areas. In the interior of sub-Saharan Africa, warming trends prevail in boreal summer, presumably as a direct consequence of reduced inflow of cool maritime monsoon air; the concomitant downward pressure trends at continental stations in sub-Saharan Africa may reflect the hydrostatic effect of increasing temperature. In contrast to boreal summer, slight cooling trends and pressure rises prevail in the interior during winter. In the course of the past four decades, the near-equatorial wind confluence over the eastern Atlantic migrated southward by 200 km in July–August and 150 km for the rainy season as a whole. Given the steep meridional precipitation gradients across the Sahel, these circulation shifts are large enough to account for the observed downward trend in Sahel rainfall.

Development of an automated spatial synoptic climatological classification

Abstract: A spatial synoptic climatological index is developed that both characterizes the dynamic and thermodynamic surface weather regime and is capable of assessing and analysing the impact of synoptic-scale variations on numerous environmental parameters. Based upon surface weather data for the contiguous USA for 1984, daily maps are produced that show those areas under the influence of similar weather conditions. The index is developed by applying principal components analysis to a daily matrix of 141 stations by 24 weather elements (chosen to represent the thermal-moisture and surface flow characteristics). The resulting component scores are grouped using a two-stage clustering technique. This yields a map showing lobes of air having relatively homogeneous weather characteristics. These lobes (clusters) are spatially coherent and correspond closely to surface weather map features. To allow for daily tracking of the clusters and identification days or weeks after disappearance, the means of each of the daily clusters are grouped across days. The resulting 90-cluster categorization describes all of the major synoptic situations that occurred in 1984. Some of the common categories include maritime and continental polar air masses, frontal passages, and continental high dominance. These groups demonstrate seasonality, regionality, and high within-category similarity. The resulting maps are useful since they are surface weather maps yet add clarity for environmental interpretation.

Convection and rainfall over Mexico and their modulation by the Southern Oscillation

Abstract: The role of the Southern Oscillation (SO) in the interannual variability of convection and rainfall over Mexico is studied by correlation analysis of long-term precipitation records, a satellite-derived set of highly reflective clouds (HRC), upper-air soundings, surface ship observations, and an SO index defined as high for anomalously high low pressure at Tahiti/Darwin. Highly reflective clouds and rainfall have a similar annual cycle and interannual variability, except in certain regions and seasons with prevalence of stratiform cloudiness. During the boreal winter (November-April) dry season, the low SO phase (or El NinMo) is, in much of the country, characterized by increased precipitation associated with the enhanced influence of mid-latitude westerlies. However, in the Isthmus of Tehuantepec, most exposed to Norte invasions, precipitation is larger in the high SO phase. During the boreal summer (May-October) rainy season, precipitation is more abundant in the high SO phase, when the northward retraction of the North Atlantic High, weaker trades, and a northward displaced near-equatorial trough are conducive to enhanced ascending motion over the area.

Occurrence and characteristics of lower tropospheric ice crystals in the arctic

Abstract: This paper discusses the occurrence and characteristics of small ice crystals that are encountered in the lower troposphere of the Arctic during the cold half of the year, when large reductions in visibility are present. Because of the difficulties in observing these ice crystals, both from surface and satellite observations, and owing to the ambiguities of cloud classification in the polar regions, these ice crystals are not included in current cloud climatologies. A summary is presented of what is known about the Arctic ice crystal climatology, formation mechanisms, and physical properties. Data obtained from the University of Washington Convair C-131A research aircraft during April 1983 and 1986 are presented, including the frequency of occurrence of ice crystals as a function of height, temperature, and relative humidity. Radiative transfer calculations were made using the observed ice crystal size distributions. The radiative transfer through the ice crystal layers is inferred to have a substantial impact on visibility in the Arctic, the vertical temperature structure of the Arctic troposphere, and on the surface energy balance.

Southern Oscillation influences on tropical cyclone activity in the Australian/south‐west Pacific region

Abstract: Tropical cyclone activity within the Australian (105°–165°E) and south‐west Pacific (145°E–150°W approximately) regions is examined during seasons affected by contrasting extremes of the Southern Oscillation (SO). In the Australian region, the most pronounced and consistent contrast of tropical cyclone frequency variability was within the eastern Australian region (145°–165°E) where anti‐ENSO (ENSO) conditions were associated with a higher (lower) incidence of tropical cyclogenesis. This relationship appears to be a manifestation of tropical cyclone behaviour in the wider south‐west Pacific region, wherein tropical cyclone origin points exhibit distinct areal patterning associated with times of extreme SO states. Intra‐seasonal temporal distributions of the Australian region tropical cyclogenesis and the south‐west Pacific tropical cyclone incidence are also examined. Copyright

Historical evidence for Southern Oscillation-southern African rainfall relationships

Abstract: Summer rainfall over the central area of southern Africa is known to be modulated with the phase changes of the Southern Oscillation such that rainfall is above normal during high-phase (cold event) and below normal during low-phase (warm event) summers. This relationship holds for the period 1935–1986; the temporal stability of Southern Oscillation–southern African rainfall associations on longer time-scales has not been tested. A documentary-derived rainfall chronology has been developed for the southern parts of the summer rainfall area for the 100-year period 1820–1920, enabling the extension of the rainfall record backward some 60 years before reliable meteorological records are available for the region. Comparison of the documentary series with both early Southern Oscillation chronologies and earliest southern African rainfall records suggests that the nature of the relationship between summer rainfall over the subcontinent and the Southern Oscillation has remained essentially unaltered since at least 1820.

Analysis of the Kuwait city urban heat island

Abstract: Twenty-three years of maximum and minimum air temperature data are analysed for selected stations in and near Kuwait City, Kuwait. The results of a series of statistical analyses reveal the general lack of a well-developed heat island in this rapidly expanding urban area. This finding is in stark contrast to the identification of large heat islands in the growing arid-land cities of North America. The similarities in the urban-rural landscape of the Kuwait City environment and its close proximity to a large water body are suggested as explanations for the observed temporal and spatial temperature patterns.

European grosswetter during the warm and cold extremes of the El Niñco/Southern Oscillation

Abstract: El Niňo/Southern Oscillation (ENSO) warm and cold events affect the synoptic climatology of the north-eastern Atlantic-European sector. The classification of cyclonic and anticyclonic European Grosswetter (1881–1987) is analysed for its response on 26 warm and 21 cold ENSO episodes. Bi-monthly ranked composites computed over idealized 2–year ENSO warm (cold) episodes show more days of cyclonic (anticyclonic) steering over Europe. This signal is largest in the winter months of January and February following the year of a warm or cold event. The distributions of the occurrence of cyclonic and anticyclonic Grosswetter days are significantly different for warm and cold event winters: (i) there is more variability between individual warm event winter months, whereas the response to cold episodes is relatively uniform; (ii) on average, cyclonic Grosswetter days are experienced on 60 per cent (46 per cent) of the 58 warm (cold) event winter. days-about 70 per cent (90 per cent) of the warm (cold) event winters realize more than 30 (< 36) days of cyclonic steering. Qualitatively corresponding results are obtained at a representative central European location for sunshine duration and the sum of daily negative temperatures, which characterize the winter strength.

On the annual and seasonal patterns of rainfall fluctuations in sub‐saharan West Africa

Abstract: Monthly rainfall data for 16 stations in Nigeria were collected for the period 1922–1985. From this, the annual, the wet season, the onset, the peak, and the retreat rainfall data were derived. The patterns of fluctuations of rainfall were analysed therefrom. The series were first examined for trends and then for periodic fluctuations. Trend analysis revealed that a majority of the series exhibit negative trends, and in some cases the trends were significant. For all the series analysed, there is the general tendency towards increasing aridity. Spectral analysis indicates prominent periods of between 2-and 8-year cycles. Other periodicities observed include 16-, 18-, and 21-year cycles.

Satellite climatology of ‘polar lows’ and broadscale climatic associations for the Southern Hemisphere

Abstract: A climatology of polar air cloud vortices (so-called ‘polar lows’) is derived for the Southern Hemisphere using sets of medium resolution (5–4 km) DMSP (Defense Meteorological Satellite Program) imagery. The climatology is for the winter season (June through to September) and covers the 7–year period 1977–1983. Both comma cloud and spiraliform polar air signature types are considered, as identified in recent satellite studies for the Northern Hemisphere. The comma clouds dominate over a wide range of ocean latitudes, whereas the spiraliform systems occur less frequently (about 1:10 for the hemisphere), and show maximum frequencies in sea ice latitudes around mid-winter (about 1:3). Within-season variations in the locations of maximum occurrence of polar air systems are consistent with the large-scale changes in the longwaves associated with the semi-annual oscillation of surface pressures. Interannual variations in polar low occurrence for the seven winters appear connected, at least in part, with changes in the amplitude of sea-level pressure (SLP) wavenumber one. When the seasonal cycle is amplified, as in the year of a ‘warm’ ENSO (El Nino/Southern Oscillation) event (e.g. 1982), large numbers of polar lows are observed south-east of Australia and around New Zealand. This contrasts with the situation for year (−1) of a warm event (e.g. 1981), when the annual cycle of the trough is suppressed. At that time, reduced frequencies of cold air outbreaks in the New Zealand area are accompanied by fewer polar lows. On a seasonally averaged basis, and in most winters, there is a positive relationship between the regional extent of the Antarctic sea ice, the longitudes of preferred occurrence of cold air outbreaks and the incidence of polar lows. This effect may be enhanced just downstream of areas of strongest oceanic heat loss to the atmosphere.

some aspects of daily rainfall distribution over India during the south-west monsoon season

Abstract: This paper presents the results of an analysis of the daily rainfall at 365 Indian stations for the 80-year period, 1901–1980. The rainfall data relate to the south-west monsoon season June to September (122 days), which accounts for the major part of the annual rainfall over most parts of the country. For each of the stations the rain-days are arranged in ascending order of rain amount, and the association between the cumulated percentage rain amount (x) and the cumulated percentage number of rain-days (y), designated as the normalized rainfall curve (NRC), is calculated. It has been shown in an earlier study that x and y are related by the equation x = y exp [— b(100—y)c], where b and c are empirical constants that depend on the coefficient of variation (CV) of the rainfall series. This equation has been utilized to study various parameters of the daily rainfall distribution. The coefficient of variation of the daily rainfall series varies between 100 per cent and 230 per cent at individual stations, with nearly half the number of stations having CV values in the range 130–150 per cent. The number of days of significant rainfall (days with rainfall greater than the mean intensity per rain-day) constitute about 30 per cent of the total number of rain-days and account for about 75 per cent of the seasonal rainfall at almost all the stations.

A contribution to the study of action centres in the North Atlantic

Abstract: The North Atlantic High appears notably weakened from the beginning of the examined period (1873–1980) until the beginning of this century. From then until 1930, it exhibits an increased central pressure. From the beginning of the period until 1914, the North Atlantic High exhibits a significant trend of intensification and at the same time a trend of displacement to the west. From 1915 until 1963 it exhibits a trend of weakening and a trend of eastward displacement. The North Atlantic Low has not exhibited a significant trend of its central pressure, but it has shown a small trend of displacement to the west. There is generally a negative correlation between the pressures of these two centres of action and a positive correlation between latitudinal position of the centres. The meridional component of the pressure gradient created between the two action centres, appears to have low values during epochs of low frequency of westerlies and high values when the westerlies are more frequent.

Drought over Zimbabwe in a regional and global context

Abstract: This study attempts to identify meteorological conditions that lead to dry and wet episodes over Zimbabwe. Correlation analyses are carried out between the Southern Oscillation Index and rainfall over south-eastern central Africa. The results show significant correlation between the Southern Oscillation (SO) and seasonal rainfall over the region. The results also indicate that the SO signal is stronger over the south-east of Zimbabwe and along the coastal areas of Mozambique. El NinTo events seem to be associated with below normal rainfall over Zimbabwe, while cold events, which are the opposite extreme of the SO, are accompanied by above average rainfall. Three-month seasonal (December–February) deviations of 1000–mbar temperature, surface pressure, and the 500-mbar geopotential height from the long-term mean are analysed during wet and drought years. The anomaly pressure gradient force is directed towards the subcontinent during wet periods and towards the ocean area during dry episodes. A similar pattern is observed at the 500-mbar level. A comparison of wet and dry seasons in relation to the temperature anomaly field points to an association between warmer 1000-mbar surface temperatures around the African subcontinent and above normal seasonal rainfall. Correlation analysis shows that a high incidence of tropical cyclones in the Mozambique Channel is associated with below normal rainfall over Zimbabwe.

USA corn yields, the El Niñmo and agricultural drought: 1867–1988

Abstract: The magnitude of spatial and lagged correlation between the sea-surface temperature anomalies of the eastern tropical Pacific Ocean (El NinMo) and corn yield in the USA is presented. The record is not homogeneous in that the correlation level varies quite strongly over time. The period between 1910 and 1950 shows completely uncorrelated behaviour. In the most recent 40 years the correlation is found to be strongest in the region just south of the Great Lakes in the states of Illinois, Iowa, and Indiana. Correlation maps are presented that show the degree of association for the corn yield in those 41 states for which records are available. For the entire record the highest correlations occur with the sea-surface temperature anomalies appearing after the corn is harvested in September to February of the following year. Since corn yields are a proxy for summer agricultural drought in the central part of the USA, the droughts during the period 1910 to 1950 must have had some other cause.

Principal component analysis of the lamb catalogue of daily weather types: part 2, seasonal frequencies and update to 1987

Abstract: Principal component analysis is applied to seasonal weather-type frequencies from the Lamb Catalogue of Daily Weather Types for the British Isles. Four combinations of the major weather types are identified, which represent around 70 per cent of the variance of the data set. Long-term trends in the seasonal principal component time series are described and relationships with temperature and precipitation are identified. It is shown that a pronounced change in the character of the synoptic circulation over the British Isles has occurred in recent decades. A decline in the frequency of westerly weather types has been compensated for by an increase in the frequency of pure cyclonic and anticyclonic types and, dependent on season, directional types. This development suggests greater variability in the atmospheric circulation and related climatic parameters. The westerly decline has not been as marked in autumn. In this season, though, there has been a notable shift towards greater southerliness at the expense of northerly weather types.

Temporal and spatial characteristics of annual precipitation over the balkans in the twentieth century

Abstract: A study of the annual totals of precipitation is presented for 12 stations in the Balkan area for a period of 92 years (1894–1985) The homogeneity of the precipitation series is examined, and their statistical characteristics are analysed. An abrupt climatic change is found at the station of Belgrad, and a statistically significant trend of annual precipitation is found for various periods for the stations of Kerkyra, Patras, Hvar, and Sarajevo. Persistence is only ascertained for the Belgrad station. The application of principal component analysis has yielded three groups of stations. Group A includes northern and continental stations, group B includes stations of the Ionian and Adriatic Seas, and group C includes the stations of Thessaloniki and Athens. The most important period of fluctuations covers the years 1933–1953, when a humid period is followed by a dry period. The most recent 5-year period can be described as a very dry one. Finally, comparison between rainfall fluctuations and types of circulation shows that the predominance of meridional circulation over the Balkans results in an increase of precipitation, whilst a zonal circulation results in a decrease of precipitation.

Onset of thunderstorms and precipitation over Northern Nigeria

Abstract: An empirical prediction scheme for the start of thunderstorms, and the onset and expected amount of rainfall for any year at Kano, Nigeria, is presented, using 8 years' upper air data. The years investigated include two drought years (1973 and 1974) and 3 years of above average rainfall (1978, 1980, and 1988). Agriculturally sufficient and reliable rainfall begins 5–6 weeks after the vertical wind shear below the African Easterly Jet, ΔUL(surface, 700 hPa), and the mid-tropospheric shear ΔUM (700–400 hPa), simultaneously satisfy the condition −20≤UL≤−5 ms−1 and 0≤UM ≤10 ms−1, as previously put forward by Omotosho. The total precipitation for any year is found to be well correlated with the total moisture anomaly (specific humidity) from the week of the above-critical wind shear to the week of rainfall onset. This makes it possible to estimate the expected rainfall total at the very onset of the rains.

Space–time variability of rainfall in April and October–November over East Africa during the period 1932–1983

Abstract: The space–time variability of precipitation over East Africa during the two rainy seasons (April and October–November) is investigated using a network of 56 stations for the period 1972–1983 and 32 stations for the period 1932–1983 The analysis relies on a model of two-mode variance with rotation, which determines homogeneous groups of stations having a common time evolution During April, three components represent a large part of the variance with their cores (i) near the Zanzibar channel, (ii) the Kenyan-Tanzanian highlands and eastern Lake Victoria, and (iii) the southern part of the Tanzanian highlands During the October–November rainy season, rainfall is more homogeneously distributed The first component takes account of 60 per cent of the variance and encompasses a large part of the domain, except southern inland Tanzanian stations and western and northern Ugandan stations Spectral analysis of the time series associated with the space component reveals peaks around 5–6 years, which appears to be the major periodicity over East Africa

Regionalization and spatial estimation of Ethiopian mean annual rainfall

Abstract: This study aims at developing methods that will lead to the generation of a reliable mean annual precipitation data base for Ethiopia. Multiple regression models have been formulated that explain the mean annual rainfall as a function of elevation and geographical location. The estimations, based on yearly values from a data set of 63 Ethiopian rainfall stations with records between 1969 and 1985, were developed for the whole country as well as for the already existing Food and Agricultural Organization (FAO) rainfall pattern regions and a new zonation derived by principal component and common factor analyses (PCA/CFA). In the PCA/CFA study, monthly rainfall values between 1968 and 1985 for 43 stations were used. The optimal zonation was derived by testing 36 different combinations resulting in different rainfall pattern regions. The alternatives tested were: correlation and covariance dispersion matrices, PCA and CFA eigentechni-ques, unrotated and rotated (Varimax and Direct Oblimin) components/factors and number of possible significant components/factors (3, 7, and 11). Principal component analysis of covariance matrix, Varimax rotation and seven extracted components gave by far the best relationship between mean annual rainfall, elevation, and geographical location. Models explaining at least 72 per cent of the variation in rainfall were constructed for regions covering about 98 per cent of the country, which is better than models based on the FAO rainfall pattern regions (69 per cent explained variation for 81 per cent of the country) and a model for the whole country (66·5 per cent explained variation).

Upper air trough axis orientation and the spatial distribution of rainfall over Israel

Abstract: Since Israel is located far from large oceans, availability of moisture may be a limiting factor in producing rainfall by synoptic-scale disturbances. Thus, moisture evaporating from the nearby Mediterranean Sea may become an important factor in producing precipitation in the region. A simple scheme has been designed to test the hypothesis that rainfall over Israel is largely affected by the trajectories of air prior to the rain event. The relationship between air trajectories before a rain event and the intensity and spatial distribution of rain over Israel has been examined. We discovered that a large percentage of rain events is associated with maritime air trajectories. Since the Israeli coastline bends sharply towards the west near latitude 31 °N this means that the orientation of synoptic disturbances responsible for precipitation to the north of 31°N and to the south of this latitude are markedly different: troughs in the lower troposphere that are oriented from north-east to south-west produce relatively more rain over the south of the country, while troughs that are tilted from north-west to south-east or from north to south produce relatively more rainfall over north and central Israel.

Caribbean tropical storm activity over the past four centuries

Abstract: This paper examines the frequency and distribution of tropical cyclones and hurricanes throughout the Caribbean using data derived from written accounts, chronologies, and published charts. Problems of incomplete and inaccurate data are overcome by conducting two levels of analysis; a 5° grid square analysis for modern charted data (after 1871) and a subregional analysis for data stretching back to the late fifteenth century. Significant variations in favoured tracks and levels of cyclone activity are identified for the charted and pre-charted period. High levels of cyclone activity are suggested for the whole or part of the Caribbean during the 1770s to 1780s, 1810s and 1930s to 1950s while troughs in activity are noted around the 1650s, 1740s, 1860s and during the early twentieth century. A noticeable drift eastward in favoured tracks is reported from the mid-twentieth century onwards, while data available so far this decade suggests a strong midlatitude (15–25 °N) preference by cyclones and hurricanes.

Atmospheric synoptic conditions associated with the initiation of north‐west African depressions

Abstract: This study presents a discussion of the main characteristics, and at the same time gives a detailed classification, of the north-west African depressions within the framework of the close relationship between the 500-hPa flow and the origin, development, and movement of surface depressions. The classification relies mainly on the similarities and differences present in the 500-hPa flow patterns when each north-west African depression first appears on the surface synoptic chart. For a detailed discussion and, especially for a record of the synoptic conditions that are favourable for the formation of north-west African depressions, mean 500-hPa hight and anomaly charts and mean temperature charts, for the years 1970–1974, have been drawn for each subcategory into which the 79 cases fall. For each of these subcategories, mean charts have also been drawn for 850-hPa hight and temperature, mean sea-level (MSL) pressure, and MSL pressure anomalies. The classification resulted in the north-west African depression being divided in five main categories A, B, C, D, and ST. Category A is divided into four subcategories, the depressions of which appear only during spring. Categories B and C are divided into two subcategories, B1, B2 and C1, C2, and these too occur only in spring, except for subcategory C2, which also appears in autumn. Category D is also divided into two subcategories, which appear only during winter. Category ST (stationary depressions) includes only four cases out of 79 north-west African depressions. These are shallow, warm-cored atmospheric circulation systems having no correspondence with the 500-hPa flow and which appear in late spring and early autumn. The study showed that the north-west African depressions do not exhibit significant differences from other extratropical depressions. Their origin, development, and movement largely follow (except for category ST) Sutcliffe's and Petterssen's traditional theories of development.

Deterministic signals in USA precipitation records: Part I

Abstract: Evidence for the 18–6-year luni-solar MN signal is found in 220 out of 288 rain-gauge records (76 per cent detection ratio) in the states of California, Colorado, Kansas, Missouri, North Carolina, Ohio. Oregon, Washington, and West Virginia, with a mean period of 18·4 ± 1·5 years. This detection ratio is less than that for the nation, which is 1015 out of 1219 records, or 83 per cent. The solar cycle Sc signal was found in 70 per cent of these records, with a period of 10·6 ± 0·5 years. For Colorado, and states directly to the east as far as Illinois, minima in most 19-year Mn wavetrains were generally in phase with epochs 1898·9 through to 1954·7 (epochs are dates of maximum in tidal forcing). However, by 1973-3 virtually all records east of Colorado had experienced a 180° phase switch and wave maxima were in phase with this epoch. In Oregon and Washington most MN waves recorded maxima in wetness at epoch 1898·9 through to 1973·3, and thus have historically been out of phase with the continental interior. California records displayed evidence for two 180° phase switches in the past 100 years. The apparent reason for this is that California is near the edge of a fairly stable continental interior cell that has historically been dry at epochs; and when the dry cell waxes or wanes in areal extent, frequent 180° phase switches are produced in areas adjacent to it. The phenomena described have been found in other regions of the USA, and in records of air temperature and air pressure world-wide.

An ice-cover climatology for lake erie and lake superior for the winter seasons 1897 1898 to 1982 1983

Abstract: Observations of mid-lake Great Lakes ice cover are sparse prior to the decade of the 1960s. In an effort to provide an historical perspective of mid-lake ice cover back to the turn of the century, daily average ice cover for Lakes Erie and Superior over 86 winters (1897-1898 to 1982-1983) was reconstructed using empirical-statistical ice-cover models developed in an earlier study. Long term average maximal monthly ice cover occurs in February and is 68 per cent for Lake Erie and 40 per cent for Lake Superior. Mid-lake ice formation occurs about 1 month earlier on both lakes during severe winters. Average maximal monthly ice cover during severe and during mild winters is 95 per cent and 14 per cent for Lake Erie, 87 per cent and 17 per cent for Lake Superior. Severe winters are associated with lower 700-mbar heights over the eastern USA compared with mild ice-cover winters. Analysis of total winter ice cover indicates three ice cover regimes: (i) a high ice-cover regime from the late 1890s to early 1920s; (ii) a low ice-cover regime from the early 1920s to late 1950s; and (iii) a high ice-cover regime from the late 1950s to the early 1980s. Ice-cover climatologies developed during the 1960s and 1970s are not representative of ice covers in the low ice-cover regime of the 1920s to late 1950s. Spectral analysis of the reconstructed total winter ice cover suggests interannual variations in ice cycles that correspond with the 2-3-year interannual variation in atmospheric variables known as the quasi-biennial oscillation.

The Greek heatwaves of 1987 and 1988

Abstract: Heatwaves, which occurred in both July 1987 and July 1988 in Greece, are considered from the point of view of human discomfort. Hourly values of dry bulb temperature and relative humidity at Thessaloniki, Larissa, and Athens were used to define various limits of discomfort defined by Terjung, Herrington, Thom, and Tromp. On both occasions the heatwave was most severe in Thessaloniki, although both Greek and British press reports concentrated on morbidity and mortality in Athens. Finally, the synoptic situation at the time of each episode is briefly described.

Deterministic signals in precipitation records from the American corn belt

Abstract: Maximum entropy spectrum analysis of 120 yearly total precipitation records in the Corn Belt (States of Illinois, Indiana, Iowa, and Missouri) yield evidence for the 18·6-year luni-solar term in 109 instances, while the 10- to 11-year solar cycle signal was found in 100 records. For Illinois, minima in the 19-year wavetrains were highly correlated with epochs 1898·9, 1917·5, and 1936·1. For eighteen stations wave minima were still correlated with the next epoch, epoch 1954·7; however, a bistable phase switch of 180° then occurred and wave maxima came into phase with epoch 1973·3. In two instances in northern Illinois the switch occurred a cycle earlier at epoch 1954·7, and in five instances a half cycle earlier at mid-epoch 1964·0. O'Brien and Currie (1989) have provided a construct in mathematical physics that explains how bistability can occur at air-pressure recording stations, and Currie (1987a) has provided many examples of bistability in air pressure and air temperature data from around the world.

Dust storms in the Canadian Prairies

Abstract: Although dust storms are quite frequent in the Canadian Prairies, no scientific studies of the nature of dust storms of this area exist, to the authors' knowledge. This work is a preliminary step in depicting temporal and spatial variations of dust storms in the Canadian Prairies. Dust storms occur at least once to as often as five times per year, on average, in the agricultural prairies. This frequency indicates the risk of occurrence of the various impacts of dust storms, such as serious soil erosion. Dust storm frequencies exhibit a pronounced peak in the spring and a weak secondary peak in August. The spatial pattern of dust storms indicates a low risk of occurrence in the northern agricultural prairies and the highest risk in the south central prairies. Knowledge of these patterns in space and time should facilitate the control or avoidance of such impacts of dust storms as wind erosion of soils, crop damage, air pollution, and general environmental degradation. Studies should be carried out to further investigate the relationships of climatic factors with dust storms as well as their linkages with other environmental aspects.