Sistema Regional de Información
en línea para Revistas Científicas de América Latina,
el Caribe, España y Portugal

The irregular occurrence of cold temperatures (frost) in central México (~19-23º N) produces high agricultural losses each year; the greatest effect is on cold-sensitive crops, which has important socio-economic implications for the region. There is a lack of information on frost-related studies regarding the onset and duration of frosts in central México, especially in response to the El Niño (EN) phenomenon. Due to the land’s irregular topography, the weather stations were grouped into 300 m range heights, resulting in five regions: I, III, and IV-VI with elevations from 875 to 2999 masl. Daily minimum temperatures (≤ 2ºC) from 50 weather stations and for each year from 1960 through 1998 were analyzed. The EN effect was evaluated by region comparing the frost-free period (FFP) for EN vs. neutral (N) events. Our results (although not statistically significant) show that the first frost occurs during EN years in the five regions under study; the last frost occurs in the highest regions (IV-VI) during N years. Also, during EN, the FFP is shorter than for N years, except in region VI where it was shorter during N years. The probability of frost occurrence at a 20% level was determined for stations with the shortest FFP. Regression analysis between frost periods and elevation show that frost occurrence is better correlated with elevation than with EN and N events; areas with higher elevation showed shorter FFP compared to areas with lower elevation.

A sequential extraction approach has been applied to analyze Cd, Cr, Cu, Mn, Ni, Pb, V and Zn in dust samples from different schools chosen according with their location in areas with different concentrations of total suspended particles (TSP). The aim of this work is to provide information about the chemical fractionation of metal content. Hence, information about the origin, mode of occurrence, mobilization and transport of metals could be obtained. Total and partial metal concentrations were further determined by analyzing the samples after acid digestion procedure. The results compare very well for both methods. The metals are associated with various chemical forms as distinguished by sequential chemical extraction. The carbonate and Fe-Mn oxide phases dominate for Pb, Zn and Cu, whereas for Ni the organic phase is most important. V, Mn, Cr, Cd are mainly present in the oxide phases, however they are too in a residual phase but in minor proportion. The different sources for the elements studied are supported by the results from multivariate (PCA) analysis.

The bivariate logistic model with two-component extreme value marginal distributions (BTCEV) is applied to provide a regional at-site wind speed estimate. The maximum likelihood estimators of the parameters were obtained numerically by using a multivariable constrained optimization algorithm. A total of 45 sets of largest annual wind speeds gathered of stations located in The Netherlands were selected to apply the model. Results were compared with those obtained by the univariate distributions: Gumbel (G), Generalized Extreme Value (GEV), Reverse Weibull (RW) and two-component extreme value (TCEV); the bivariate distributions with marginals G, GEV and RW; and three regional methods: station-year, index flood (index-wind) and L-moments. In general, a significant improvement occurs, measured through the use of a goodness-of-fit test, when estimating the parameters of the marginal distribution with the bivariate distributions instead of its univariate and regional counterpart, and differences between at-site and regional at-site design events can be significant as return period increases. Results suggest that it is very important to consider the bivariate joint estimation option when analyzing extreme wind speeds, especially for short samples.

The following study explores climatic change scenarios of extreme temperature and atmospheric humidity for the 2020 and 2050 decades. They were created for México through the GFDLR30, ECHAM4 and HadCM2 general circulation models. Base scenario conditions were associated with the normal climatological conditions for the period 1961-1990, with a database of 50 surface observatories. It was necessary to empirically estimate the missing data in approximately half of the pressure measurements. For the period 1961-1990, statistical models of the monthly means of maximum and minimum temperatures and atmospheric humidity (relative and specific) were obtained from the observed data of temperature, solar radiation and precipitation. Based on the simulations of the GFDLR30, ECHAM4 and HADCM2 models, a future scenario of monthly means of maximum and minimum temperatures and humidity in climatic change conditions was created. The results shown are for the representative months of winter (January) and summer (July).

A homogenized database of minimum temperatures from 135 observatories distributed around Europe was analyzed in order to estimate trends and to determine the main synoptic weather patterns that contribute to an extremely cold day (ECD). An ECD is defined as a day whose minimum temperature is within the lowest 5th centile of the daily temperature series for each observatory; these values represent temperatures below which public health concerns may be expected. The period from 1 January 1955 to 31 December 1998 was chosen for this analysis because it represents the period for which the greatest number of measuring stations with complete time series was available. The relationship between the occurrence of an ECD and the general circulation of the atmosphere was based on a statistical analysis in which a coefficient of effectiveness for each synoptic pattern was obtained. In order to simplify the manipulation of the data, a rotated principal components (RPC) analysis was applied and the synoptic patterns with the greatest contribution to the ECD were obtained. Trends for each synoptic pattern were studied in order to examine the relationship among the ECD.  

Multifractal turbulence formalism has been used to perform an analysis for scales from 1 hour to almost 6 months of the time structure of the hourly rainfall series recorded during twenty-four years in Córdoba, a location in southern Spain. The parameters of the universal multifractal model were estimated and the theoretical moments scaling exponent function was obtained exhibiting an acceptable agreement with the empirical function for a range of moments. The universal multifractal model shown itself to be a suitable tool for describing the statistics of the rainfall series recorded in Córdoba.

Since the 1950’s-1960’s a large portion of Argentina has experienced a long period of high precipitation with important changes a 30-year period annual averages. Opposite conditions have taken place throughout the twentieth century in a large portion of Chile, with declining tendencies in annual precipitations. If such conditions prevail, the socio-economic implications of these phenomena could eventually gain importance due to the expansion of grain crops in the semi-arid borders of Argentina and a decline in the availability of reservoir water used for irrigation or energy generation in the central region of Chile and western Argentina. An accurate diagnosis of these phenomena is very important for applied purposes in medium term planning that should be undertaken by private and state-owned companies. These diagnoses are also important from a methodological point of view, in order to find out whether these changes in long-term averages are constant and represent a climatic change or if they are low frequency interdecadal fluctuations that would mean a return to opposite conditions. We suggest that the two intense La Ni˜na events that took place during 1988- 1989 and 1995-1996 caused a decline in the precipitation trends over a large region of semi-arid Argentina and intensified droughts in Central Chile. A regionalized local precipitation analysis of the 1931-1932/1998-1999 agricultural periods over large areas was performed. Afterwards, long-term variations for homogeneous areas were analysed through their nonlinear trends and low frequency fluctuations in a regional scale. Seven inter-annual patterns with three different types of tendencies have been found. One of them shows an increasing linear trend throughout the analysed period; another one shows an increasing trend, but also a decrease after the 1980’s, and a third one shows a continuous decrease. Regions with continuous trends include medium latitude semi-arid regions in Argentina and Chile, where an intense human impact is observed. An 18-26-year-quasi-period that affects four of the selected regions and has a clearer signal in the Cuyo region has been found. This long period signal is in phase with the oscillations of other atmospheric variables in South America and with precipitation in South Africa.

For accurate determination of the three-dimensional structure of atmospheric circulation, it is very important to assimilate every observation from all available sources, especially over the data sparse oceanic regions. During the Indian Ocean Experiment Intensive Field Phase -1999 (INDOEX-IFP 99) several special observations were taken over the Indian Ocean region, which gave an opportunity to study this region more thoroughly. In this study these special observations, along with atmospheric motion vectors that form METEOSAT-5(63ºE), have been assimilated along with other conventional as well as non-conventional observations received at the National Centre for Medium RangeWeather Forecasting (NCMRWF) through the Global Telecommunication System (GTS) in its global analysesforecast system. The aim of this study is to assess the impact of the additional data on the Global Data Assimilation system of NCMRWF as well as to create better initial conditions for its future use in modeling studies. Impact of INDOEX data sets on analyses and subsequent 5-day forecast has been studied on the monthly mean fields. Mean wind field shows the strengthening of trade winds in the lower levels and the strengthening of the wind around sub-tropical anticyclone in the higher levels. It is found that the impact is stronger over the data sparse oceanic region and in the higher levels compared to the lower levels. The impact of these data sets is studied on different initial conditions as well as forecast as part of case studies. Significant positive impacts, specially on analysis, are noticed over those oceanic regions where some well-defined synoptic system persisted.

The overall goal of Pan American CLIVAR Research on the North American Monsoon System is to determine the sources and limits of predictability of warm season precipitation over North America, with emphasis on time scales ranging up to seasonal-to-interannual. To achieve this goal, several research objectives have been identified including (1) a better understanding of the key components of the monsoon system and their temporal and spatial variability, (2) a better understanding of the role of this system within the global water cycle (3) improved observational data sets, and (4) improved simulation and monthly-to-seasonal prediction of the monsoon and regional water resources. The purpose of this paper is to review the recent progress made towards achieving these objectives and to highlight some of the future challenges based on gaps in our understanding.

In order to improve the results given by Autoregressive Moving-Average (ARMA) modeling for the monthly accumulated rainfall series taken at 19 observatories of the Iberian Peninsula, a Discrete Linear Transfer Function Noise (DLTFN) model was applied taking the local pressure series (LP), North Atlantic sea level pressure series (SLP) and North Atlantic sea surface temperature (SST) as input variables, and the rainfall series as the output series. In all cases, the performance of the DLTFN models, measured by the explained variance of the rainfall series, is better than the performance given by the ARMA modeling. The best performance is given by the models which take the local pressure as the input variable, followed by the sea level pressure models and the sea surface temperature models. Geographically speaking, the models fitted to those observatories located in the west of the Iberian Peninsula work better than those on the north and east of the Peninsula. Also, it was found that there is a region located between 0ºN and 20º N, which shows the highest cross-correlation between SST and the peninsula rainfalls. This region moves to the west and northwest off the Peninsula when the SLP series are used.