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

Daily European Centre for Medium Range Weather Forecasts (ECMWF) analyses for the 1980-1989 period are used to study cyclonic vortices at the upper tropospheric levels (CVULs) in the vicinity of northeast Brazil. The occurrence of CVULs shows significant interannual and seasonal variability, with the highest frequency found during the Southern Hemisphere summer months. CVULs develop mostly over oceanic areas, with the closed circulation at 200-hPa and the cold core at 300-hPa, and remain confined to the middle and upper troposphere. The CVUL mean lifetime ranges from 4 days in June to 11 days in February. Approximately 57% of the Southern Hemisphere summer CVULs originate accordingly to the formation mechanism so far known, hi 27% of the Southern Hemisphere summer cases, the vortex originates associated with the occurrence of an upper level anticyclonic circulation over southwestern Atlantic and southeastern Brazil, in turn associated with stationary cold fronts (South Atlantic Convergence Zone-SACZ). CVULs can present regular or irregular trajectories. Midlatitude trough associated with frontal systems and an inter-hemispheric bifurcation of the upper level westerlies in the northwest of South America can cause such irregular displacements. For the CVULs with regular displacement, it is common to observe an amplification of the ridge axis in the southeastern sector of the Bolivian High, which can cause the upper level anticyclone in the southwestern Atlantic and southeastern Brazil.

During the pre-monsoon period, the frequency of occurrence of low level jet (LLJ) and of nocturnal thunderstorms (NCTS) over the Gangetic Plain of West Bengal, a region in the eastern part of India have been studied from both pilot balloon observations and numerical simulations. Observational study indicates that the formation of LLJ and NCTS is significant over inland stations. Composite diagrams for the vertical structure of LLJ show that the core wind speed is observed within the layers between 300 m and 600 m above ground. Sodar observations reveal that vertical shear of horizontal wind has positive lower value below the axis of LLJ whereas it has negative and higher values above the axis. This conclusion differs slightly from those obtained from pilot balloon data because of poor resolution in the later data. Model study also supports the formation of LLJ in the evening because of land/sea contrast. Model-derived vertical velocities are positive over the places of occurrence of nocturnal thunderstorms. The horizontal shear of wind speed beneath the core of LLJ indicates strong relative vorticity and hence convergence at lower levels. This production of convergence by LLJ helps the formation of nocturnal thunderstorms.

A two-level quasi-nondivergent model containing 12 spectral components on a rectangular beta-plane is used to simulate a number of atmospheric phenomena. The nonlinear model contains two components that describe the zonal flow at each level permitting zonal winds with two maxima and two minima. The eddy fields at the two levels contain four components selected in such a way that the resulting eddy fields have transports of both sensible heat and momentum. The model permits a full description of energy generations, conversions and dissipations, because the eddy components are selected with such wave numbers that interactions take place between, the eddies and the zonal fields. Diabatic heating, topographical effects and dissipation of kinetic energy are included in the model. A limitation of the model is that it contains only one wave number in the zonal direction. The model is used to illustrate nonlinear developments of baroclinic waves on various horizontal scales in a case of forcing on the zonal components alone. With a long channel it is possible to simulate the development of long stationary waves forced by topography and/or heating. For special definitions of the heating on both the zonal and the eddy modes one may simulate the formation and maintenance of blocking situations as a result of interactions between the zonal components and the eddies. The eddy components will normally go into periodic or almost periodic motion in the phase domain unless the model is forced by heating, topography and friction. These unforced motions and their periods are investigated. We also show that the type of atmospheric circulation may change significantly as a function of the position of the maximum heating in the south-north direction, illustrating a change from single to double jets and the resulting change in the intensity and position of the waves.

A group of 60 runoff and 15 precipitation time-series, all belonging to a hydrological unit in Southwest Mexico, in the Pacific watershed was analyzed. The internal homogeneity of the time series was tested and then they were completed for the calculation period using suitable statistical methods. Three homogeneous regions were defined in the studied zone through correlation matrices. The climatic decadal cycles were determined with the help of the integrated differences curves. The anthropogenic impact on the runoffs was estimated through the homogeneity curves. The length of the cycles were ranged from 30 to 40 years. The structure of the decadal cycles permitted the estimation of the duration of deficiency, abundance, and normality runoff periods in each region. The cyclical structure of the regional runoffs can be explained by precipitation. The anthropogenic impact was important in the middle of the general streamflow, where the two main streamflows connect through Chapala Lake. It was demonstrated that, in spite of the anthropogenic impact on part of the hydrologic system, it continues to be naturally regulated by climatic factors and still constitutes a hydrological unit. The estimation of the lengths of different phases of the decadal cycle can be used for the management of the regional water resources.

The one-dimensional shallow water equations permit steady state solutions for a given forcing that is independent of time. When the forcing is sufficiently small one obtains three periodic solution of which two have numerically large velocities and the third a somewhat lower velocity. Examples of solutions for simple forcing patterns are given. The nonlinear one-dimensional equations are simplified in the usual way by neglecting the advection term in the continuity equation and by replacing the geopotential by a constant when undifferentiated. It is shown that these equations have only one steady state solution which is similar to the low velocity solution in the more general system. The approximations act thus as a filter of large velocity solutions. Solutions of the simplified equations are obtained by formulating the equations in wave number space. Examples indicate that a maximum wave number of 30 is sufficient to obtain solutions of sufficient accuracy.

Estimates for both biogenic emissions of non methane biogenic hydrocarbons and nitric oxide in the Valley of Mexico Basin are reported. factors affecting the estimates and the possibility that episodic emissions may be much higher than these estimates are discussed.

The hemispheric averages of detrended mean monthly surface temperature anomalies are more highly persistent in summer than in winter. This property of the season is strikingly evident in the Northern Hemisphere, where summer months correlate highly with each other, and positively with the same summer months in the following year, while winter months have low correlations with all months, falling to zero, for the same months in the following year. The month to month variability in the persistence of temperature is presented as evidence to suggest that Northern Hemisphere summers are likely to exhibit stable properties while Northern Hemisphere winters may occasionally exhibit chaotic flow behaviour. The longer term persistence of summer temperature anomalies from one summer to the following summer may be explained by the fact that summers retain their memory for several years due to the heat capacity of the oceans, but this memory is temporarily destroyed in winter by the chaos induced by baroclinic instability of the upper level westerly flow.

Generally, the nonlinear dynamical systems (NDSa) described by systems of nonlinear ordinary differential equations, are sensitive to structural changes of the latter. Even simple additive constants can change the global behaviour of a given system, e.g. (1.4) below. To study directly this effect we choose two popular (NDSa) - the Lorenz classical system (1.2), (1.3) and the Wiin-Nielsen system (1.6), both having various meteorological applications. Moreover, we ask and give an answer to the nonstandard problem - what should be the additive constant forcing of these systems in order to have a desired number of real fixed points with desired stability properties? Some preliminary numerical results are also presented.

This paper describes the effect of parameterization on the NWP/CIMA model. The main features of the model are that it is formulated in terms of the primitive equations and that physical processes such as surface fluxes of momentum, heat and moisture, large scale and convective precipitation, and a surface temperature diurnal cycle have been included. The parameterizations used in the model were tested on a wide variety of case studies for the southern region of South America. An example a cyclogenesis over Buenos Aires that developed into a dipole of high - low pressure is described here. The results show that vertical diffusion is the main responsible for the changes in temperature, geopotential heights and precipitation. The first two variables are also affected by large scale condensation.

In this work, the impact of Sea Surface Temperatures (SSTs) during 1987 and 1988 on the Northern Hemisphere summer mean monsoon is studied using an Atmospheric General Circulation Model. Four model experiments were carried out to assess the relative impacts of observed SST fields of global(El), tropical(E2), tropical Pacific(E3) and tropical central and eastern Pacific(E4) oceans. It was seen that in El, the model captured a number of important observed circulation and precipitation features of 1987 and 1988 monsoons. The upper tropospheric outflow during 1988 was strong over India, Indian ocean and western Australia and the inflow was strong over central and eastern Pacific oceans. During 1987, the upper tropospheric velocity potential minimum over western Pacific shifted east by 20º of its 1988 position. The Tropical Easterly Jet (TEJ) over equatorial Africa was stronger in 1988 than in 1987. The precipitation maximum over western Pacific shifted to central and eastern Pacific during 1987. Over southern India, south west Indian ocean and parts of Sahel, there was increased rainfall during 1988 compared to 1987. However, it was less over northeast India in 1988 than in 1987. In experiments E2, E3 and E4, the large-scale structures of the upper tropospheric velocity potential difference between 1987 and 1988 were similar to those in El. However, some important changes were seen in E3 and E4. The divergent flow during 1988 was weaker over Indian ocean and monsoon region with the center of velocity potential differences shifted to the east in E3 and E4 than in El. Consistent changes were seen in the wind fields also. The TEJ was weaker during both the years 1987 and 1988 in E3 and E4 than in El.