Descripción: This work proposes to employ the number of dry days (days without precipitation) as a variable of study, and to analyse their spatial and temporal variability in Argentina. Climatological aspects of dry days, such as their annual mean values and its seasonal cycle, were discussed and compared with precipitation features in the country. Linear trends in the annual number of dry days (ANDD) were identified for the period 1960-2005. Most of the regions exhibited decreasing trends, but few stations showed significant ones. The most important trends were present in the Central-West region and over the Patagonian coast and their magnitudes indicated a decrease of two to six dry days per decade. These trends coincide with the observed increase of accumulated precipitation in part of the country during the second half of the 20th century. To identify long-term fluctuations in the ANDD, a low pass filter, a wavelet analysis and a cubic polynomial fit was applied to the longest time series of the selected locations. Through these methodologies, periodicities were identified in the inter-annual and inter-decadal ranges for the last 80 years. The most important significant cycles were observed in the range of 10-24 years, but also, depending on the region, significant inter-annual variations were found. © 2012 Royal Meteorological Society.

Descripción: We study in this paper the time evolution of the spreading process of a small drop in contact with a flat dry surface, using asymptotic techniques. We reduced the problem by solving a quasisteady self-similar macroscopic problem and matched with the precursor region solution, where the van der Waals forces are important. A final nonlinear third-order ordinary differential equation has been solved numerically using shooting methods based on the fourth-order Runge-Kutta techniques. We obtained how the capillary number changes when the drop size decreases with time. The evolution process then diverges slightly from that obtained using the spherical cap approximation. The influence of gravity is also considered for both hanging and sitting drops. © 1998 The American Physical Society.

Descripción: We numerically and theoretically investigate the evolution of the ridges and rifts produced by the convergent and divergent motions of two substrates over which an initially uniform layer of a Newtonian liquid rests. We put particular emphasis on the various asymptotic self-similar and quasi-self-similar regimes that occur in these processes. During the growth of a ridge, two self-similar stages occur; the first takes place in the initial linear phase, and the second is obtained for a large time. Initially, the width and the height of the ridge increase as t 1/2. For a very large time, the width grows as t 3/4, while the height increases as t 1/4. On the other hand, in the process of formation of a rift, there are three self-similar asymptotics. The initial linear phase is similar to that for ridges. The second stage corresponds to the separation of the current in two parts, leaving a dry region in between. Last, for a very large t, each of the two parts in which the current has separated approaches the self-similar viscous dam break solution. © 2008 American Institute of Physics.

Descripción: The Little Ice Age (LIA) is a climate episode between the 16th and middle 19th centuries, characterized in Europe by colder temperatures and occasionally stormy weather. In certain areas of Europe, long instrumental observations record the Little Ice Age partially; however, in the pampean region meteorological data only started about one hundred years ago. The objective of this contribution is to provide new evidences about the Little Ice Age in the Pampean plain based on lake cores. Short cores of Chascomús and Monte lakes were analyzed and, according to AMS data, they span the last 500 years. The Chascomús core consists of three fining upward sequences. Sediment logical and biological proxies demonstrate a benign period from the end of the 15th century. Around 1700 AD lake level decreased and a dry period started and lasted for almost 150 years. After 1850 AD higher lake levels and increased moisture dominate the pampean scenario. The del Monte lake core is also composed of three fining upwards sequences, and AMS chronology assigned a basal age between 1441-1494 AD. Although the model age needs improvement, the events of Chascomús and del Monte lakes seem to be correlated and show coherent regional tendencies of humidity and droughts during the last 500 years.

Descripción: Water resources management and agriculture planning models require a statistical synthesis of some rainfall features, in particular those representing dry atmospheric conditions. The bigger the basin, the more important these features become, as is the case of the La Plata Basin (LPB). This paper focuses on the precipitation variability in the large LPB in South America, analysing the number of months per year with low rainfall and the sequences of months with low rainfall, their theoretical distributions and stability, which are needed as input for the models mentioned above. Long time series are used to analyse the low-frequency variability and the relative importance of decadal variability. Changes are evident in the number of months per year with low rainfall, with a decrease of about 20% in the period after 1970. Theoretical distribution models (binomial and geometric) are fitted to these empirical distributions, and the regional variability of the fitting parameters is shown. In practically the entire region, the goodness-of-fit of the two theoretical models considered is statistically satisfactory. The temporal variability of the parameters of the theoretical binomial (p) and geometric (α) distributions is analysed, in excluding sub-periods of 10 and 5 years, respectively. The results show low-frequency variability overlapped on a decadal variability, with low homogeneous regional behaviour. The distribution models have proven to be efficient for frequency adjustments of the rainfall properties studied. These results are an acceptable and necessary input to decision models in LPB. They also make it possible to infer effects of climate change. Copyright © 2008 Royal Meteorological Society.

Descripción: The Euterpe edulis embryo consists of a prominent single cotyledon, a very short radicle-hypocotyl axis and an epicotyl. The epicotyl is obliquely angled with respect to the cotyledon; consequently it corresponds to one of the two categories recognized for palm seeds by DeMason (1988). Parenchyma, protoderm and procambium can be distinguished on the basis of position and shape of their cells, which are highly vacuolated with one central vacuole and the cytoplasm restricted to a thin parietal layer. Initial cells from both apical meristems are also vacuolated but they have small vacuoles distributed around the nuclei. Silica occurs in cell walls of some protodermal cells. Raphides, silica bodies and tannins all occur occasionally in vacuoles, especially in the basal cotyledon region. Most embryo cells lack storage reserves and exhibit an active state, with numerous mitochondria, RER cisternae and Golgi apparatus, indicating a strategy of continuous development without the interposition, at maturity, of a dry state. The endosperm consists of living cells with very large nuclei and thickened cell walls. Similar to the endosperm of other studied palm species, their cells exhibit a quiescent appearance with lipid, protein, minerals (in the cytoplasm) and mannans (in the cell walls) as the insoluble storage reserves. © 2004 The Linnean Society of London.