bluuebunny/crossref_metadata_2025 huggingface dataset:
Surface conditions associated with anomalous rainfall in the guinea coastal region
Authors: Wagner Richard G. , da Silva Arlindo M.
Published: February 2007

This paper examines surface atmospheric and oceanic conditions associated with anomalous rainfall at the Guinea coast. For time‐scales shorter than 5 years, rainfall in the Guinea coast is practically not correlated with rainfall in the Sahel, although both time series show a well‐defined drying trend in the last 30 years or so. While interannual variability of Sahel rainfall is associated with a sea‐surface temperature (SST) pattern anti‐symmetric about the Equator, anomalous rainfall near the Guinea coast in associated with an SST pattern roughly symmetric about the Equator. During wet years near the Guinea coast this SST pattern corresponds to a less developed cold tougue in the boreal summer.

Using the Comprehensive Ocean‐Atmosphere Data Set surface fields we examine key terms of the oceanic heat budget. Anomalous shortwave radiation and latent heat fluxes seem to play an important role in the decaying of the SST anomaly. During wet years anomalous atmospheric winds act to relax the trade winds, which probably reduces the equatorial thermocline slope and contributes to the observed warming of the equatorial waters in the east.

We also investigate the relationship between the surface winds and the underlying SST using numerical and analytical solutions of a simple model. The anomalous winds are shown to be consistent with the hydrostatic adjustment of the tropical boundary layer.
An analysis of the circulation characteristics and energy budget of a dry, asymmetric, east‐west urban canyon. II. Energy budget
Authors: Arnfield A. John , Mills Gerald M.
Published: February 2007

This study presents partial energy budget evaluations for a dry east‐west oriented urban canyon in Columbus, Ohio, based on canyon‐top measurements of net radiation and the turbulent and circulation‐related components of the sensible heat flux. While net radiation data were similar to those reported in previous studies, the sensible heat exchanges were small in absolute magnitude, showed a “flat” diurnal regime and were uncorrelated with the net radiation. These results suggest a minimal degree of coupling by heat exchange between the canyon air space and the urban boundary layer above roof level. Speculations are offered on the nature of the unmeasured residual in the canyon energy budget. Evidence suggests that both a substrate heat flux and a significant advective term (through the canyon ends and top) may be instrumental in accounting for the residual. However, definitive conclusions were precluded by the absence of simultaneous evaluations of all the energy budget components. The need for such studies at canyon sites with diverse structures and fabrics is emphasized.
Towards a classification of tropical rainfall stations
Authors: Jackson I. J. , Weinand H.
Published: February 2007

As part of a long‐term project examining short‐period tropical rainfall characteristics, a preliminary classification of stations for a wide range of localities based on such characteristics is presented. The approach adopted, using principal component analysis and a clustering procedure, functioned well and the resulting classification would not have been anticipated simply by using general annual and seasonal variables alone. Interpretation of the components allowed the important characteristics of each group of stations to be described. Although the stations used did not represent every region of the tropics, the range of groups with a wide variety of characteristics suggests that they are indicative of conditions applicable to many areas. Although the data base was for varying periods and record lengths, it appeared to be adequate for the purpose of the analysis.
Relationships between the pacific/north american teleconnection patterns and precipitation events in the south‐eastern USA
Authors: Henderson Keith G. , Robinson Peter J.
Published: February 2007

Precipitation event attributes, defined as the amount, duration, and frequency of individual rainstorms, vary considerably from place to place and season to season. Event variability is caused by a combination of large‐scale and local climate processes. Correlations of seasonal time series show that the Pacific/North American (PNA) teleconnection pattern and local rainfall event attributes are significantly related in specific areas of the south‐eastern USA in some seasons. Shifts in the zonal/meridional flow over North America affect both the number of events and the event size. The total number of events in some regions changes by as many as 10 to 12 events per season with shifts in continental‐scale circulation. An analysis of attribute distributions shows that the percentage of total events falling into selected intensity categories may also change by as much as 10 per cent between flow regimes. Relationships have both temporal and spatial patterns. Shifts in continental‐scale circulation affect event attributes most strongly in winter, with more events occurring in zonal years. Events with greater amounts and durations are found east of the Appalachians in meridional years and west of the mountains in zonal years.
Exploring the possibility of forecasting monthly ‐ 700 hPa geopotential fields over india
Authors: Prasad K. D. , Singh S. V.
Published: February 2007

The principal component regression approach is used for forecasting the monthly 700 hPa geopotential fields over India, for July, from the geopotential fields observed during June. The data used are for the 23‐year period from 1958 to 1980. The multiple linear regression equations are developed on 22 years of data using the principal components as predictors and keeping one randomly selected year for independent testing. The forecasts and validation experiments are carried out for seven representative years. The results show some skill in forecasting the monthly height fields over India.
Spatial regression methods in dendroclimatology: A review and comparison of two techniques
Authors: Cook Edward R. , Briffa Keith R. , Jones Philip D.
Published: February 2007

We review and compare two alternative spatial regression methods used in dendroclimatology to reconstruct climate from tree rings. These methods are orthogonal spatial regression (OSR) and canonical regression (CR). Both the OSR and CR methods have a common foundation in least‐squares theory and converge to the same solution when all p candidate tree‐ring predictors of climate are forced into the model. However, the perfomance of OSR and CR may differ when only subsets p′ < p predictors are used. Theory cannot predict how either method is likely to perform when best‐subset selection is applied, especially with regards to reconstruction accuracy. Consequently, empirical comparisons of OSR and CR are made using three tree‐ring and climate networks from western Europe and eastern North America that have been used in previous dendroclimatic studies. These comparisons rely on a suite of regression model verification statistics to validate the accuracy of the climatic reconstructions produced by the best‐subset models. The results indicate little real difference between OSR and CR, with each performing equally good or bad depending on the amount of recoverable climatic information in the tree rings. Canonical regression may perform slightly better in high signal‐to‐noise cases; conversely, OSR may perform slightly better when the signal‐to‐noise ratio is low. None of these apparent differences are large enough to select one method in preference to the other, however, and many more comparisons would be needed to determine if such indications are generally valid.
Estimating continental and terrestrial precipitation averages from rain‐gauge networks
Authors: Willmott Cort J. , Robeson Scott M. , Feddema Johannes J.
Published: February 2007

Influences of varying rain‐gauge networks on continental and terrestrial precipitation averages (derived from data observed on those networks) are evaluated. Unsystematically and systematically designed station networks are considered, the latter being represented by the NCAR World Monthly Surface Station Climatology, which contains hand‐picked but time‐varying networks that date back to the 1800s. Biases arising from spatially uneven and temporally variable precipitation‐observing networks can be significant.

For all the continents, except South America, sparse rain‐gauge networks produce overestimates of continental mean precipitation. Mean precipitation for South America, in contrast, is underestimated substantially by low densities of observing stations. Sampling errors tend to be large in areas of high precipitation and in regions with strong spatial precipitation gradients (e.g. in the Sahel). These patterns occur whether the station network has been selected systematically (as in the NCAR network) or unsystematically.

Systematic sampling of mean precipitation (at the NCAR station locations), however, suggests that many yearly NCAR station networks are adequate for estimating continental average precipitation. As early as 1890, NCAR networks for Australia resolve continental average precipitation accurately. Not until 1960, however, do NCAR networks for South America begin to resolve continental mean precipitation adequately. Regional and continental NCAR network errors also tend to cancel one another, often giving accurate yearly estimates of terrestrial mean precipitation.
Validation of tropical ocean‐atmosphere heat fluxes‐from marine data against a satellite‐based method
Authors: Michael K. J. , Nunez M. , Meyers G.
Published: February 2007

Ocean‐atmosphere heat fluxes were calculated from marine weather reports over a mesoscale region of the Coral Sea near Townsville, Australia. These heat fluxes were compared, as 5‐week averages, with a set of heat fluxes derived over the same region from a combination of satellite and surface data. These satellite‐ and surface‐derived fluxes were calibrated and validated against accurate, independent surface heat fluxes, as described in a previous paper.

Significant differences were noted between the two sets of heat fluxes. The discrepancies in the net solar radiation are attributed to insufficient marine weather reports and/or improper cloud parameterization in the formulae applied to the marine data. Some large differences were observed in the outgoing turbulent heat fluxes, and these were traced to higher water‐air specific humidity gradients in the marine data. These two discrepancies cancel so that the long‐term (45 week) averages of the total ocean‐atmosphere heat flux, at 52 W m−2 for the satellite‐based technique and 34 W m−2 for the shipboard method, differ by only 18 W m−2.

On the basis of these systematic differences it was concluded that the heat fluxes derived from satellite and surface data in this study were superior in quality and reliability to heat fluxes calculated from marine weather reports. Further studies are required to test this result in other locations.
Prediction of the interannual variations of tropical cyclone movement over regions of the western north pacific
Authors: Chan Johnny C. L.
Published: February 2007

The interannual variability of tropical cyclone (TC) movement has been found to be rather significant in certain regions of the western North Pacific. Westward and northwestward moving TCs are found to occur mostly at low latitudes and have the largest interannual variations in the region just east of the Philippines and over the northern part of the South China Sea. The area east of the Ryukyu Islands and south of Japan is identified as having the largest interannual variation of northward moving TCs.

Correlations are made between the annual number of occurrences of TCs in these prescribed regions and the principal components of the monthly mean 850‐ and 500‐hPa zonal wind patterns over the western North Pacific (for the months of November of the previous year to April of the current year). Westward‐moving cyclones are found to correlate well with the 850‐hPa zonal wind patterns in January and March. Using the principal components associated with these patterns, prediction equations are then developed using the total (dependent) sample and the jackknife method (simulating an independent sample). The predictions made with both the dependent and “independent” samples are found to be very good.

For north‐westward moving cyclones, no prediction equation is developed because principal components of the 850‐ and 500‐hPa zonal winds found to be significant can explain less than 30 per cent only of the total variance. However, the February 500‐hPa zonal winds correlate well with northward‐moving cyclones in the region east of the Ryukyu Islands and south of Japan. Predictions made using the principal components associated with this flow pattern for both the dependent and “independent” samples give rather good results.

Because all the predictors are between January and March, they can be used operationally to predict the annual number of occurrences of TCs in the predefined regions. As TCs in these regions are likely either to move into the South China Sea or affect Japan, these results should prove useful in the seasonal prediction of TC occurrence in these latter areas.
The effects of EL NIñO‐southern oscillation on precipitation in san josé, costa rica
Authors: Waylen Peter R. , Quesada Marvin E. , Caviedes César N.
Published: February 2007

The effects of interannual variations in El Niño‐Southern Oscillation (ENSO) upon annual, seasonal and monthly precipitation totals of San José, Costa Rica, over the period 1882–1987 are analysed using both a Southern Oscillation index (SOI) and a classification of El Niño and anti‐El Niño years. The analyses confirm the posited regional positive relationship between annual precipitation and the SOI and suggest that estimates of the probabilities of annual rainfall could be made successfully by using a mixed probability distribution that incorporates changes in both the mean and variance of precipitation under each of the three classes of El Niño activity. Strong seasonality is identified in the effects of ENSO, particularly in the months of July, August, and October, whereas the month of September seems unaffected. Monthly lead lag analyses imply that there may also be a reversal of the nature of association between the SOI and precipitation in the following year.
Climatic‐scale vegetation—cloud interactions during drought using satellite data
Authors: Carleton Andrew M. , Travis David , Arnold David , Brinegar Richard , Jelinski Dennis E. , Easterling David R.
Published: February 2007

Land‐surface‐atmosphere interactions for the 1988 summer drought in the Mid‐west USA are studied at climatic scales (104 km2; >5 days) using polar orbiter satellite radiance information on vegetation activity and cloud conditions. The emphasis is on documenting the occurrences of ‘organized’ land‐cover types and cumulus convection, and evaluating their mutual associations. For purpose of comparison, similar analyses are performed for the more moisture sufficient (non‐drought) summer of 1987. The satellite retrievals and analysis methods are calibrated using station meteorological data on precipitation and surface moisture, and also land‐cover maps.

In early summer (June) 1988, Mid‐west USA land surfaces were generally highly stressed (high shortwave reflectance, high surface temperatures), particularly for the Corn Belt. Moreover, the spatial variation of the land‐cover signal was greater than in June 1987, and also when compared with later that summer (August 1988). Statistical analysis suggests a significant role for the land surface conditions of early summer in the spatial patterns of deep convection over the remainder of the season. This involves generally higher (lower) frequencies of convective cloud days for surfaces having a high relative density of forest vegetation (crops). Because broadly similar results are obtained for the summer of 1987, the possibility exists that the vegetation‐convective‐cloud interaction may be a feature of Mid‐west USA summer climate.

The diurnal dependence of cloud‐forming processes for agricultural and natural vegetation regions is assessed using GOES cloud imagery at high temporal resolution for a “typical” convective day in early summer 1988. Associations with station radiosonde data support the hypothesis that spatial differences in land cover influence the generation and timing of convective cloudiness for the Mid‐west region.
Measurements of albedo variation over natural vegetation in the sahel
Authors: Allen S. J. , Wallace J. S. , Gash J. H. C. , Sivakumar M. V. K.
Published: February 2007

This paper reports ground‐based measurements of albedo at two different sites in south‐western Niger: agricultural fallow at the ICRISAT Sahelian Center (ISC), Sadoré, and a type of open natural forest (tiger‐bush), 6 km south‐west of ISC. The vegetative cover at each site consisted of two clearly defined components, for which separate measurements were made. The four different component surfaces spanned the entire range of plant cover density normally occurring in the southern Sahel, ranging from zero (bare soil) to a dense, closed canopy of woody shrubs. Continuous automated observations of albedo were made over a period of 15 months, from July 1989 to September 1990, including both wet and dry seasons. The data are analysed on hourly, monthly, and annual time‐scales in order to demonstrate how albedo varies in response to the density of plant cover, soil‐surface moisture content, solar zenith angle, and the proportion of diffuse light in the incoming solar radiation. Large annual variation in monthly mean albedo was observed at both sites (increases from the wet to dry season of 0.065 and 0.057 for the fallow and tiger‐bush, respectively). At the fallow site the annual variation in albedo resulted mainly from the wet to dry season cycle of leaf growth and loss. At the tiger‐bush site the primary cause of annual variation in albedo was the frequent wetting of the extensive bare soil component that occurred during the rainy season. The significance of these results for global climate modelling is assessed briefly.
Long‐range prediction of hawaiian winter rainfall using canonical correlation analysis
Authors: Chu Pao‐Shin , He Yuxiang
Published: February 2007

Hawaiian rainfall is teleconnected to short‐term climate variability in the Pacific Ocean. The summer Southern Oscillation Index (SOI) and summer sea‐level pressure (SLP) over the North Pacific are used as predictors, and the following winter rainfall indices from three islands of Hawaii are used as predictands. To consolidate the large data array of the SLP field prior to prediction experiments, lagged correlation and empirical orthogonal function analyses are used.

Canonical correlation analysis has been used for predicting Hawaiian winter rainfall. Among many schemes tested, predictions. Cross‐validation techniques have also been used to estimate the “overall” forecast skill of various schemes, and the results are consistent with those from prediction experiments. Winter rainfall in Hawaii can be predicted with a good degree of success two seasons in advance using the summer SOI and the first four eigenmodes of summer SLP over the North Pacific as predictors.
Linkage between eurasian winter snow cover and regional chinese summer rainfall
Authors: Yang Song , Xu Lizhang
Published: February 2007

An inverse relationship has been documented by previous studies between antecedent Eurasian winter snow cover (EWSC) and all‐India summer monsoon rainfall. Because both China and India are strongly influenced by the Asian monsoon system, it may be reasonable to ask whether a similar relationship exists between the EWSC and summer rainfall over China. This study seeks an answer to this question.

Only a weak interannual relationship is found between the EWSC and summer rainfall over China as a whole. This weak EWSC‐rainfall linkage occurs only when the averaged rainfall over the entire country is used. By dividing the country into several geographical regions based on observed features in the interannual variability of rainfall, we find much stronger regional rainfall‐EWSC relationships. A strong in‐phase EWSC‐rainfall relationship is found in southern and northern China, whereas an inverse EWSC‐rainfall relationship appears in the western, central, and north‐eastern parts of China. Thus, EWSC may be a useful predictor for regional Chinese summer rainfall, but not for rainfall over the country as a whole.
Mesoclimatology of winter temperature and precipitation in the po valley of northern italy
Authors: Cacciamani Carlo , Nanni Sandro , Tibaldi Stefano
Published: February 2007

An attempt is made to characterize the mesoscale climatological structures of the daily maximum and minimum temperature anomalies (with respect to monthly climatology) and total daily precipitation of the Emilia Romagna region of northern Italy (essentially the Po Valley) by means of multivariate statistical analysis techniques. Firstly, a definition of homogeneous seasons is attempted, based on the analysis of the month‐to‐month variability of the temporal variance of the parameters, derived from 24 years of daily data (1961–1984). This definition is then adjusted following a monthly empirical orthogonal functions (EOF) analysis of the spatial structures of variability of the same fields. Having defined “seasons” in this way, attention is concentrated on the cold season, an “sextended winter” ranging from October‐November to March‐April. Empirical orthogonal functions of both temperature anomalies and precipitation parameters are then recomputed for this extended season. Separate cluster analyses of the low‐dimensionality subspaces defined by the first three EOF coefficients (the principal components, PC) of the two parameters reveals the existence of winter weather ‘regimes’. Projections of these ‘regimes’ on geopotential height‐fields analyses in the middle (500 hPa) and low (mean sea‐level pressure) troposphere are qualitatively consistent with synoptic knowledge of the typical ‘weather types’ (grosswetterlagen) of the Mediterranean area. A comparison between the cluster analyses of temperature and precipitation shows that mutually compatible thermal and pluviometric regimes can be defined, providing a unified description of the space‐time mesoscale variability of the observed parameters.
Spatial and temporal characteristics of maximum dry spells in Southern Norway
Authors: Perzyna Gregor
Published: February 2007

The paper deals with the spatial and temporal characteristics of summer maximum dry spells in south‐east Norway, and the influence they exert on river low flows. The analysis was conducted at two spatial scales: at a catchment scale of the order of tenths of kilometres, and at a regional scale of the order of hundreds of kilometres. At both scales, the spatial persistence of maximum dry spells differs distinctly between summer seasons. During ‘wet seasons’ the short dry spells are temporally spread, show no spatial trend, and drought conditions are suppressed. During ‘dry seasons’, the spatial timing of maximum dry spells tends to be uniform over the whole study area. At the catchment scale, however, prolonged, single‐season dry spells can be disturbed by convective rainfall events. The long‐term, statistical parameters of maximum dry‐spell periods showed stable and uniform spatial distribution over the study area. The mean of maximum dry‐spell periods and the coefficient of variation have been mapped regionally. The regional values of the parameters can be used as climatic input to stochastic low‐flow‐prediction models. It was shown that for Norwegian conditions the distribution function of maximum dry spells followed the extreme value type 1 distribution.
Urban albedo derived from direct measurements and landsat 4 TM satellite data
Authors: Soler M. R. , Ruiz C.
Published: February 2007

The main aim of this study is to examine the effect of urban structure on the albedo value. The urban area chosen for this study was Barcelona. On the basis of measurements made in July 1989, and without consideration of the urban structure, the albedo values of various urban structures was determined.

Simultaneously, from analysis of Landsat 4 TM images, thus taking account of the urban effect, a method for the calculation of albedo was developed. These results are compared.
CLimatological characteristics of very low humidity in Northern Crete due to Downslope southerly winds
Authors: Prezerakos N. G.
Published: February 2007

In this paper an attempt is made to study the climatological characteristics of very low relative humidity (below or equal to 10 per cent) in northern Crete, especially at Heraklio, by utilizing data from a 34‐year period (1956–1989). Specifically, after selecting the very low humidity days based on a previously determined definition and the synoptic situation with which they are associated, the following aspects were examined: (i) the interannual and monthly variation, (ii) the mean duration of very low humidity occurrence on each day at Heraklio and its monthly frequency, (iii) the mean values of the meteorological elements (temperature, wind, wind gusts, pressure, total and low‐cloud amounts, and visibility) for the time of synoptic observations reporting very low humidity at Heraklio and for the corresponding times of the day gust before it for both Heraklio and Tibaki, that is, on the northern and southern coasts of Crete. Also by comparing Heraklio's and Tibaki's mean values of the meteorological elements it was possible to determine the contribution of the Cretan relief characteristics to the very low humidity occurrence at Heraklio.
Moisture condition in the South‐Eastern USA and teleconnection patterns
Authors: Yin Zhi‐Yong
Published: February 2007

The purpose of this study is to investigate the relationship between the moisture condition and teleconnection patterns including the Pacific‐North American (PNA) pattern, North Atlantic Oscillation (NAO), and the El Niño‐Southern Oscillation (ENSO). The February and August Palmer's Drought Severity Index (PDSI) was used to represent the moisture condition of the cold and warm seasons in the south‐eastern USA. The teleconnection patterns were represented by the indices of PNAI, NAOI, and SOI. Composite maps for extreme values of the teleconnection indices revealed that spatial variation in the moisture condition is associated with different phases of the teleconnection patterns. Correlation analysis indicated that the PNA, NAO, and ENSO are related mostly to the subregional moisture conditions. Principal component analysis defined subregions of homogeneous moisture regimes. Then regression analysis was performed on the scores of the principal components of the February and August PDSI, using the teleconnection indices as the independent variables. The area with its cold‐season moisture condition best explained by the teleconnection patterns is central and south Florida. The teleconnection indices also explained significant portions of the variation in the cold‐season moisture condition of Mississippi and Tennessee. The teleconnection patterns in the previous autumn probably can be used to predict the moisture condition in the following winter. Compared with the cold season, the summer teleconnection patterns are less strongly related to the warm‐season moisture condition. Nevertheless, the relationships between the moisture condition and the teleconnection patterns are significant statistically. In addition, the teleconnection patterns in the previous spring offered some explanations to the spatial patterns in the warm‐season moisture condition in the region.
Solar‐irradiance variations and regional precipitation fluctuations in the Western USA
Authors: Perr Charles A.
Published: February 2007

Changes in total solar irradiance can be linked to changes in regional precipitation. A possible mechanism responsible for this linkage begins with the absorption of varying amounts of solar energy by the tropical oceans, which creates ocean temperature anomalies. These anomalies are then transported by major ocean currents to locations where the stored energy is released into the atmosphere, thereby altering atmospheric pressure and moisture patterns, which can ultimately affect regional precipitation.

Correlation coefficients between annual differences in empirically modelled solar‐irradiance variations and annual State‐divisional precipitation in the USA for the period 1950–1988 were computed with lag times of 0 to 7 years. The most significant correlations occur in the Pacific North‐west, with a lag time of 4 years, which is approximately equal to the travel time of water within the Pacific gyre from the western tropical Pacific Ocean to the Gulf of Alaska. Precipitation in the Desert South‐west correlates significantly with solar irradiance lagged 3 and 5 years, which suggests a link with ocean‐water temperature anomalies transported by the Equatorial Counter Current as well as the North Pacific gyre. With the correlations obtained, droughts coincide with periods of negative irradiance differences (dry high‐pressure development), and wet periods coincide with periods of positive differences (moist low‐pressure development).
Sensitivity analysis of extreme precipitation events
Authors: Katz Richard W. , Acero Juan Garrido
Published: February 2007

A sensitivity analysis of the frequency of extreme precipitation events is performed. Extreme events in the form of total precipitation (e.g. over a month or season) exceeding a relatively large threshold (i.e. extremely wet conditions) or falling below a relatively small threshold (i.e. extremely dry conditions) are treated. To allow for the positive skewness of the distribution of total precipitation, a power transformation to normality is used. The relative sensitivities of extreme precipitation events to the median and to the scale parameter of this distribution are examined. It is shown that these relative sensitivities are both generally greater the more extreme the event. Moreover, the relative sensitivity to the scale is greater than that corresponding to the median for extreme wet events, whereas the opposite is true for extreme dry events. These theoretical results are illustrated through use of a network of precipitation observations in Spain.
Minimum sample size for synoptic weather type classification. Application to winter period data recorded on the catalan coast (North‐East Spain)
Authors: Lana X. , Fern Mills G. Ández
Published: February 2007

Characteristics of weather types obtained from a Davis and Kalkstein classification process can depend on a variety of factors. One of them is the number of components retained after principal component analysis (PCA) of the original data. The classification algorithm itself and the number of elements to be classified also play important roles in the final results. The main objective of the present paper is to describe a methodology leading to a deduction of the minimum sample size that must be considered in order to obtain a reliable synoptic weather type classification. Some rules related to the number of factors to be retained after a PCA and the optimal number of groups of our classification procedure are discussed. Finally, the main features of the winter weather types obtained from Barcelona Airport data are briefly presented.
Regionalization of Indian monsoon rainfall and long‐term variability signals
Authors: Iyengar R. N. , Basak P.
Published: February 2007

Large regions in India with homogeneous variability of monsoon rainfall are identified with the help of principal component analysis. Four major regions, called principal regions, are first demarcated. Even though the variance explained by the first PC is only 21 per cent, it can be used to organize 45 per cent of the surface area into a coherent region. A novel sequential methodology wherein at every step the grouped stations are sieved out is also proposed. This helps in identifying 10 sequentially homogeneous regions accounting for 91 per cent of the total area under consideration. The four principal regional rainfall series reflect the interannual variability originally present in the data set. These are mutually uncorrelated and hence represent four independent monsoon anomaly patterns also. The first and the fourth principal regions contain temporal signals well discriminated from white noise.
Reconstruction of New Zealand climate indices back to AD 1731 using dendroclimatic techniques: Some preliminary results
Authors: Salinger M. J. , Palmer J. G. , Jones P. D. , Briffa K. R.
Published: February 2007

Reconstructions of temperature and zonal and meridional flow over New Zealand are presented for the period back to AD 1731. The reconstructions have made use of ring‐width chronologies from five endemic species (three different genera); the first time multispecies reconstructions have been attempted in New Zealand. The chronologies were standardized using techniques that attempted to maintain as much of the low‐frequency variance in the tree‐ring series as possible, so that long‐term climatic trends could be detected. In independent periods verification, the reconstructions of temperature explain between 30 and 33 per cent of the variance, whereas zonal and meridional flow reconstructions explain 31–43 per cent and 18–39 per cent of the variance, respectively.
Climate of two locations on the Southwestern corner of Hudson Bay: AD 1720–1729
Authors: Ball Timothy
Published: February 2007

Lack of historic climate data is a severe limit to advancing understanding of climate and improving general circulation models. Diaries maintained by the Hudson's Bay Company at Churchill and York Factory on Hudson Bay are analysed to determine a synoptic climate for the decade 1720–1729, a critical decade within the Little Ice Age. Subjective data are converted by a coding system to objective numerical data for computer analysis. Instrumental temperatures were not available this early, however, wind direction, number of days with precipitation events including rain, snow, freezing rain, and hail were recorded. In addition, first day of rain in spring, snow in autumn and number of days with thunder are analysed. Weather conditions during the decade are presented for other parts of the world. Results show similarities with these climate records. Overall the conditions reflect Little Ice Age conditions. The first pentad is markedly different than the second, which has greater variation in all variables. This pattern parallels sunspot activity. The decade has a meridional pattern, however, this is more pronounced in the second pentad.
Impact of the greenhouse effect on sea‐ice characteristics and snow accumulation in the polar regions
Authors: Hunt B. G. , Gordon H. B. , Davies H. L.
Published: February 2007

An extensive analysis has been made of the simulated sea‐ice behaviour for current and doubled carbon dioxide levels for both polar regions. The sea‐ice variations were computed as a component of the overall performance of a global climatic model. This model simulated the major climatic processes, but used an elementary oceanic representation known as theQ‐flux parameterization. For simplicity, dynamical processes were omitted from the sea‐ice formulation. For current conditions the sea‐ice extent, thickness, seasonal and interannual variability were reasonably simulated, particularly for the Northern Hemisphere. The major deficiency was the lack of regions with very thick sea‐ice, which is known to be generated in the real world by dynamical interactions. Very substantial reductions occurred in the sea‐ice thickness, and to a lesser extent in sea‐ice area, under greenhouse conditions, with the major impact being in summer. Water mass accumulation over the great ice‐sheets agreed moderately well with limited observations for control conditions. An increase in accumulation rate was simulated for both polar regions in the greenhouse experiment. The results indicate that, overall, a first‐order representation of sea‐ice behaviour is obtainable with a thermodynamics‐only sea‐ice parameterization.
Variability of the African convection centre as viewed by outgoing longwave radiation records and relationships with sea‐surface temperature patterns
Authors: Moron Vincent
Published: February 2007

The main African convection centre (ACC) is delimited by the 5°‐square areas with outgoing longwave radiation less than 235 W m−2 in the domain 17·5°N‐17·5°S and 18°W–46°E. Four parameters (latitude and longitude of the centre of gravity, mean intensity, and extension) were analysed during the period June 1974 to December 1991. The relationships between the variability of the ACC and the main sea‐surface temperature (SST) patterns were then studied. Correlation and composite analyses demonstrate that the first principal component of SST, representing mainly the variability of the eastern and the central tropical Pacific, is associated strongly with the extension and the mean intensity of the ACC. Both are strongest when cold conditions prevail in the eastern and central Pacific (= La Niña event) and smallest when warm conditions occur in the eastern and central Pacific (= El Niño event). This relationship is strongest during the boreal summer, when ACC is elongated from the Atlantic Ocean to the Ethiopian highlands. The second principal component, representative of the south‐eastern and equatorial Atlantic, is also linked with the extension and the longitudinal position of the ACC (greatest extension/westernmost position when the equatorial and South Atlantic is anomously warm, and inversely). The intermonthly variability of the latitude seems more noisy and less related to the large‐scale SST patterns than the other parameters.
Recent frost date trends in the north‐eastern USA
Authors: Cooter Ellen J. , Leduc Sharon K.
Published: February 2007

Uncertainty in projections of future climate, along with a need for policy makers and management to respond to change in climate, highlight the importance of the detection of trends in climatological time series. Selected indicators of regional climate status and change may well be related to indicators of ecological condition. Climate factors are known to stress ecosystems. With the identification and description of trends in biologically important climate descriptors this knowledge could be used to provide scenarios of biological changes. One aspect of climate which stresses forest ecosystems is described by the annual date of the last hard spring‐freeze (minimum temperature ⩽ −2·2°C). Averaged for New England, there was a significant linear trend indicating an earlier initiation of frost‐free conditions (negative slope) for the period 1961–1990.

Significant time trends were detected locally at stations throughout much of the New England regions as well. Temporal discontinuities in trend, spatial relationships, and urban influence were considered. The presence of trends in time does not appear to be related to geographical location. The direction of trend (positive or negative) exhibits some geographical coherency, which may be related to climatological variables such as percentage of possible sunshine and cloud cover.
Sea‐surface temperature — South African rainfall associations, 1910–1989
Authors: Mason S. J.
Published: February 2007

The main features of sea‐surface temperature variability in the South Atlantic and the south‐west Indian Oceans are identified and their interaction with the Southern Oscillation discussed. Most of the variance is explained by coherent features of variability in the South Atlantic Ocean. Tropical and subtropical features dominate the variance, but this may be partly a reflection of data availability. Many of the principal components are associated with rainfall over southern Africa and the strongest associations occur with sea‐surface temperatures in the western equatorial Indian Ocean, the Agulhas system, and the central South Atlantic Ocean.
Complex demodulation and the estimation of the changing continentality of Europe's climate
Authors: Thompson R.
Published: February 2007

Six long records of European mean monthly temperature have been analysed for changes in continentality of climate using the time series technique of complex demodulation. Variations in amplitude and phase of the annual temperature cycle as estimated by the complex demodulation, in combination with low‐pass filtering, are found to be coherent across much of Europe, from distinctly maritime to distinctly continental locations. The most significant departure of continentality found during the last 460 years is the maritime period of the 1920s when cool summers accompanied mild winters along with a retardation of the seasons throughout Europe.
The effect of station drift on the estimation of shifts in meteorological records due to site changes
Authors: Rhoades D. A. , Neill W. A.
Published: February 2007

The presence of station drift can seriously affect tests of significance of the shift due to a site change when neighbouring stations are used to adjust a meteorological series. The series of differences between a station and its neighbours (on which such tests are based) are often moderately autocorrelated; this is evidence that station drift effects commonly exist. Two types of station drift, both of which appear to be present in real meteorological series, are considered here: transient drift, modelled as a first‐order autoregressive process, and enduring drift, modelled as a random walk. A method is given to estimate the size of each of these effects in real data sets, using data from the periods between site changes, and hence to correct the variance of the estimated shift for station drift. The optimal length of the adjustment period can also be determined; it depends mainly on the size of the enduring station drift.
Global and terrestrial precipitation: A comparative assessment of existing climatologies
Authors: Legates David R.
Published: February 2007

Over the past 20 years, several global and terrestrial precipitation climatologies have been developed. These climatologies have been used extensively, for example, to evaluate general circulation model simulations of the present‐day climate, as input fields in global hydrological studies, and to validate satellite precipitation algorithms. If these climatologies differ significantly, the results of these applications may be affected adversely.

In this paper, three global precipitation climatologies—developed by Schutz and Gates, Jaeger, and Legates and Willmott—are examined comparatively. Additionally, three terrestrial precipitation archives—compiled by Eischeidet al., Leemans and Cramer, and Hulme—are compared with the Jaeger and the Legates and Willmott climatologies to assess precipitation over land areas and the importance of the time period of record on long‐term averages.

Results indicate that seasonal precipitation estimates from the Schutz and Gates climatology are considerably smaller than either of the other two climatologies and its annual average estimate is much smaller than most estimates of global precipitation made since 1960. Zonally, however, the smaller estimates are predominantly located in lower latitudes although the general spatial pattern is similar to that of the other two global climatologies. Spatial gradients and areas of precipitation maxima are much smaller in the Schutz and Gates climatology, which accounts for its lower estimates. Qualitatively, the Jaeger climatology agrees well with that of Legates and Willmott although Legates and Willmott's estimates are much greater along the Inter Tropical Convergence Zone. Oceanic estimates vary considerably which underscores the uncertainties in estimating oceanic precipitation. Over land, the Legates and Willmott climatology compares favourably with the observation‐based estimates from the terrestrial archives, leading to the conclusion that observation‐based climatologies are consistent regardless of the time period of record and are preferable to climatologies compiled subjectively.
Seasonal and regional variations of tropospheric temperatures in the Northern Hemisphere 1976–1990
Authors: Weber Gerd Rainer
Published: February 2007

The 300–1000 hPa monthly average thickness data for the Northern Hemisphere (NH) published by the synoptic climatology group of the Free University of Berlin are analysed on a seasonally aggregated basis and with respect to regional variations. It is shown that the rise in tropospheric temperatures between 1976 and 1990 began over the tropical oceans—when temperatures in the mid‐latitudes were still falling—continued over continental areas of the mid‐latitudes in the late 1970s, fell in the of the first half of the 1980s and continued again in the second half of the 1980s. Temperatures over the mid‐latitude oceans rose slowly in the late 1970s, fell in the early 1980s and rose strongly in the late 1980s. High‐latitude tropospheric temperatures showed no significant change throughout the period.

In terms of seasonal variations, the largest warming relative to the late 1970s occurred over mid‐latitude continents in winter. However, compared with the base period used here, 1951–1960, the largest warming in absolute terms occurred over the low‐latitude oceans in summer and autumn and over mid‐latitude continents in spring. Low‐latitude warming was particularly strong over the Pacific Ocean. There was some significant warming over mid‐latitude continents in summer in the 1980s, which was of the same magnitude as the cooling in previous decades. The high latitudes are generally still colder than during the 1950s even though there has been some warming there as well since the 1970s. Around latitude 35°N, a ring of minimum warming appears in all seasons except winter.

In the tropics, tropospheric warming is larger than surface warming; in the mid‐latitudes it is of comparable magnitude and in the higher latitudes it is smaller than surface warming.
Characteristic surface energy balances for different climate types
Authors: Kraus Helmut , Alkhalaf Adnan
Published: February 2007

An attempt is made to answer the question of whether the different climate types of the Köppen‐Geiger classification are also characterized by distinct annual courses of the surface energy balance terms. For this purpose the KöppenGeiger climate map was linked with Budyko's atlas of the heat balance of the Earth. For every climate area shown on the Köppen‐Geiger map (and classified by a certain combination of up to three letters) the annual course of the energy balance terms was extracted from Budyko's atlas and plotted as an energy balance diagram. Then the annual courses of the energy balance terms of all areas classified as equable by Köppen‐Geiger (i.e. being characterized by the same combination of up to three letters) and independent of where in the world they occur could be compared. It became evident from this that each climate type (characterized by a certain letter combination) shows a distinct structure for its energy balance diagram. Thus there is an unambiguous relationship between the Köppen‐Geiger classification and the distinctive structure of the annual course of the components of the surface energy balance. There is only one exception: the BSh climate, with its transitional character throughout the world.