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Vadlamudi Brahmananda Rao
and
Valdo Da Silva Marques

Abstract

Water vapor characteristics over northeast Brazil for two contrasting years are discussed. During the wet year 1974, the precipitation efficiencies were less than 20% over the interior dry region and during the dry year 1976, they were reduced to 10% or less. The reduction is mainly due to low precipitation in 1976. Calculation of water vapor flux showed that in both wet and dry years, the flux is inward in the east sector and outward in the west sector. This suggests the importance of the South Atlantic Ocean in providing water vapor for NE Brazil. During the wet year, the lower tropospheric flow is convergent and in the northwest part, the direction of flow is more northerly/less southerly. During the dry year the opposite occurred. This indicates the possible role of the Intertropical Convergence Zone in the interannual variations of climate over NE Brazil.

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Manoel Alonso Gan
and
Vadlamudi Brahmananda Rao

Abstract

The influence of the Andes Cordillera on transient disturbances is investigated in this study using a lag-correlation analysis. This analysis shows that the unfiltered geopotential height data have a wavelike pattern moving to the east while tilting to the west in the vertical. When the wave approaches the Andes Cordillera, it exhibits orographic effects such as anticyclonic turning of a low-level disturbance trajectory, a zonal trajectory in the upper levels, distortions of the isolines of correlation, and an elongation of maximum correlation on the lee side of the Andes. The anticyclonic turning of the trajectory in the low-attitude levels and a zonal trajectory in upper levels implies a decrease in the vertical tilt of the system on the windward side and an increase in the tilt on the lee side. The increase of baroclinicity on the lee side results in baroclinic development as predicted from a linearly obtained normal-mode solution in the presence of mountains.

A cross-correlation analysis of the high-pass-filtered disturbances shows an eastward phase propagation and a westward vertical tilt with height on the order of one-quarter wavelength between 1000- and 3OO-hPa levels. The horizontal structure and phase propagation show characteristics similar to the fastest-growing baroclinic normal mode in a two-layer, quasigeostrophic, β-plane, linear model with a mountain placed in the north-south direction. This shows that the high-pass-filtered anomalies over the South American region are associated with baroclinic disturbances influenced by the Andes Cordillera. The results further show that the interaction of these anomalies with the Andes Cordillera is responsible for Ice cyclogenesis. The composite maps show that the positive and negative high-pass-filtered anomalies have the same structure and paths of phase propagation. These anomalies intensify over the Pacific Ocean near the South American continent.

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Manoel Alonso Gan
and
Vadlamudi Brahmananda Rao

Abstract

The frequency of surface cyclogenesis over South America (approximately the area enclosed by 15°–50°S and 30°–90°W) has been calculated using, 10 years (1979–1988) of data. The frequency of cyclogenesis is more in winter than in any other season. Highest frequency (139) is found in the month of May and the lowest frequency (71) is found in the month of December. In addition to seasonal, the frequency of cyclogenesis shows interannual variation. The occurrence of cyclogenesis is more during the years of negative Southern Oscillation index (El Niño years) and less during the years of positive Southern Oscillation index. The years of higher (lower) cyclogenesis are found to be associated with higher (lower) rainfall. This explains the negative correlation between the precipitation over southern Brazil and the Southern Oscillation index.

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Dandu Govardhan
,
Vadlamudi Brahmananda Rao
, and
Karumuri Ashok

Abstract

In this paper, the authors suggest a dynamical mechanism involved in the revival of the summer monsoon after breaks. In this context, the authors carry out a diagnostic analysis using the datasets from National Centers for Environmental Prediction Reanalysis 2 for the period 1979–2007 to identify a robust mechanism that typifies breaks and subsequent revival of monsoon. The authors find during the peak of significant breaks an anomalous southward shift of the subtropical westerly jet stream, which is invariably accompanied by an anomalous northward shift of a stronger-than-normal easterly jet. These major changes during a break facilitate an instability mechanism, which apparently leads to formation of a synoptic disturbance. Formation of such a disturbance is critical to the subsequent revival of the summer monsoon in 61% of the observed break-to-active revivals.

Computations of energetics and correlation analysis carried out suggest an increase in the eddy kinetic energy at the expense of the mean kinetic energy during the breaks, in agreement with the formation of the synoptic disturbance. This demonstrates that barotropic instability in the presence of a monsoon basic flow is the primary physical mechanism that controls the revival of the summer monsoon subsequent to the break events.

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Mário Adelmo Varejão-Silva
,
Sergio H. Franchito
, and
Vadlamudi Brahmananda Rao

Abstract

A biosphere model based on BATS (Biosphere–Atmosphere Transfer Scheme) is coupled to a primitive equation global statistical–dynamical model in order to study the climatic impact due to land surface alterations. The fraction of the earth’s surface covered by each vegetation type according to BATS is obtained for each latitude belt. In the control experiment, the mean annual zonally averaged climate is well simulated when compared with observations. Deforestation and desertification experiments are performed. In the deforestation experiment, the evergreen broadleaf tree in the Amazonian region is substituted by short grass; in the desertification experiment the semidesert, and the tall grass and deciduous shrubs are substituted by desert and semidesert in the African continent, respectively.

The results show that in both the experiments there is a reduction in evapotranspiration and precipitation in the perturbed region and an increase in the soil surface temperature, the temperature of the foliage air layer, and the foliage temperature. Also, the latent heat flux decreased in the perturbed cases relative to the control case. To partially compensate for the decrease in latent heating, sensible heat flux increased in the perturbed cases compared with the control case. The changes in the deforestation case are greater in the latitude belt centered at 5°S, where in most part the Amazonian forest is situated. Otherwise, the changes in the desertification are greater in the latitude belt centered at 15°N. When there is also degradation of the African tropical forest (substitution of evergreen broadleaf trees by short grass), the greatest changes occur southward from that region (in the latitude belt centered at 5°N), and the magnitude of the changes are also increased. This shows the important role of the modification of tropical forest when there is degradation of the vegetation in the African region from 20°N to 0°°.

The results regarding the changes in the temperature and in the energy fluxes are in agreement with those of earlier experiments carried out with sophisticated general circulation models, which shows the usefulness of this kind of simple model.

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Prakki Satyamurthy
,
Vadlamudi Brahmananda Rao
, and
Antonio Divino Moura

Abstract

A linear stability analysis of the hyperbolic tangent profiles is made. A Boussinesq primitive equation model with high vertical resolution is used. Unstable modes of intermediate scales (Lx ≈ 1000 km) are generated when the curvature, d2ū/dz2, of the basic flow in the lower levels is negative. Even if the curvature in the lower levels is positive, intermediate-scale unstable modes appear for smaller static stability and shear (Richardson number not necessarily small) or for certain vertical distributions of diabatic heating due to the liberation of latent heat in the lower troposphere. The amplitude of the most unstable intermediate-scale wave is confined to the lower troposphere and its growth rate increases with the inclusion of diabatic heating.

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Srinivasa Rao Chapa
,
Vadlamudi Brahmananda Rao
, and
Gannabathula Sri Sesha Durga Prasad

Abstract

Cold cloud index (CCI) data derived from Meteosat infrared imagery are used to detect periodicities in convective activity in South America. The generally used Fourier transform (FT) cannot provide time-localized information but gives information on the average periodicity of oscillations over the entire time domain. As many events in the atmosphere are intermittent, wavelet transform (WT) is used to identify periodic events in CCI data.

First, the Morlet WT is applied to different combinations of time series data of known periodicities to demonstrate the advantage of WT over FT. Later it is applied to CCI data over four 9° square areas between the latitudes 4.5°N and 31.5°S, and longitudes 54°–45°W. Near the equator periodic convective activities are observed to be more prominent in the boreal summer than in the austral summer. Between the latitudes 4.5° and 22.5°S, 1-, 2–3-, approximately 5-, and 8–10-day oscillations are seen in the austral summer and seldom is any convective activity seen in the winter. In January semidiurnal variation of cloudiness is also observed for a few days. Farther south in the extratropics, approximately 10- and approximately 20-day periodic events, which refer to the baroclinic waves, are seen more prominently in the austral autumn and winter, and 1- and approximately 5-day oscillations are seen in the summer, perhaps due to convective cloudiness.

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