1,153 research outputs found
A simple model for a transverse dune field
We present a simple one-dimensional model to describe the evolution of a
transverse dune field. The model is characterized by the distances between the
dunes and their heights, which determine the inter-dune sand flux. The model
reproduces the observation that the dunes in a given field have approximately
all the same height. We find that this result is independent of the initial
configuration of the field, as well as of coalescence effects between migrating
dunes. The time for the final state to be reached is studied as a function of
the relevant phenomenological parameters
Modelling a Dune Field
We present a model to describe the collective motion of barchan dunes in a
field. Our model is able to reproduce the observation that a typical dune stays
confined within a stripe. We also obtain some of the pattern structures which
ressemble those observed from aerial photos which we do analyse and compare
with the specific field of La\^ayounne.Comment: 15 pages, 13 figure
Annual cycle of subsurface thermal structure in the Tropical Atlantic ocean
The subsurface thermal structure in the Tropical Atlantic ocean (30°N-20°S, East of 80°W) is studied on the basis of an extensive data bank of subsurface soundings. Calendar monthly maps are presented showing mixed layer depth, base of thermocline, thermocline thickness, and vertical temperature gradient across the thermocline. These maps are complemented by vertical cross sections depicting mixed layer depth, base of thermocline, and selected isotherms : a zonal profile along the equator (50°W-10°E), a meridional transect across the Eastern Atlantic (4°N-18°S), and a meridional section across the Central Atlantic (30°N-18°S). (D'après résumé d'auteur
Compliance ist rechtliches Risikomanagement
Nicht das Risiko selbst, sondern der Umgang damit birgt die Gefahr. Um dieser zu begegnen und Synergien zu nutzen, müssen Compliance und Risikomanagement aufeinander abgestimmt werden
Stability of transverse dunes against perturbations; a theoretical study using dune skeleton model
The dune skeleton model is a reduced model to describe the formation process
and dynamics of characteristic types of dunes emerging under unidirectional
steady wind. Using this model, we study the dependency of the morphodynamics of
transverse dunes on the initial random perturbations and the lateral field
size. It was found that i) an increase of the lateral field size destabilizes
the transverse dune to cause deformation of a barchan, ii) the initial random
perturbations decay with time by the power function until a certain time;
thereafter, the dune shapes change into three phases according to the amount of
sand and sand diffusion coefficient, iii) the duration time, until the
transverse dune is broken, increases exponentially with increasing the amount
of sand and sand diffusion coefficient. Moreover, under the condition without
the sand supply from windward ground, the destabilization of transverse dune in
this model qualitatively corresponds to the subaqueous dunes in water tank
experiments.Comment: 7pages, 8figure
Teleconnection mechanisms of northeast Brazil droughts: modeling and empirical evidence
Targeted numerical modelling experimaents are conducted to complement the previous empirical diagnostics of circulation mechanisms leading from sea surface temperature (SST) departures in the equatorial Pacific in January to anomalies in the March-April rainy season of Brazil's Nordeste. A weak interhemispheric northward directed SST gradient in the Atlantic favors a more southerly position of the hydrostatically controlled low pressure trough, embedded in which is the Intertropical Convergence Zone (ITCZ), which is the main rainbearing system for the Nordeste. In addition, anomalously warm waters in the equatorial Pacific in January tend to be followed by Nordeste drought. The underlying chain of causalities has been explored by empirical diagnostics and numerical modelling. During El Nino years, an upper-tropospheric wave train extends from the equatorial eastern Pacific to the tropical North Atlantic, affecting the patterns of upper-tropospheric topography and divergence, and hence of vertical motion over the Atlantic. This leads to a weaker meridional pressure gradient on the equatorward flank of the North Atlantic subtropical high, weaker North Atlantic tradewinds, an anomalously far northerly ITCZ position and thus Nordeste drought. The previous empirical diagnostics are overall supported by the modelling experiments
Long-term trends in precipitation and temperature across the Caribbean
This study considers long-term precipitation and temperature variability across the Caribbean using two gridded data sets (CRU TS 3.21 and GPCCv5). We look at trends across four different regions (Northern, Eastern, Southern and Western), for three different seasons (May to July, August to October and November to April) and for three different periods (1901–2012, 1951–2012 and 1979–2012). There are no century-long trends in precipitation in either data set, although all regions (with the exception of the Northern Caribbean) show decade-long periods of wetter or drier conditions. The most significant of these is for the Southern Caribbean region which was wetter than the 1961–1990 average from 1940 to 1956 and then drier from 1957 to 1965. Temperature in contrast shows statistically significant warming everywhere for the periods 1901–2012, 1951–2012 and for over half the area during 1979–2012. Data availability is a limiting issue over much of the region and we also discuss the reliability of the series we use in the context of what is known to be available in the CRU TS 3.21 data set. More station data have been collected but have either not been fully digitized yet or not made freely available both within and beyond the region
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