Um estudo do fluxo horizontal do vapor d'água atmosférico e o seu armazenamento sobre o Nordeste do Brasil.

Abstract

Mean monthly computations for total precipitable water, the water vapour flux and its divergence for the year 1975 have been made for the drought prone region of Northeast Brazil 0o -20°S and 30° -50°W. Monthly rainfall charts for the same period have also been prepared and analysed. The total water vapour in an atmospheric column naturally depends on how the circulations patterns, specially lower layer convergence transfers it to higher regions. The precipitable water is thus seen to depend on the seasons, proximity of the oceans and the al titude of the station. The magnitude of the zonal water vapour flux is much larger than that of the meridional, because of the predominance of the zonal circulation of air flow over the region. It is found that divergence of the water vapour flux in general governs the precipitation patterns and its amount, inspite of the fact that .in the strict sense, this divergence equals evaporation minus precipitation in the balance equation. Upper air data for 12 radiosonde stations of the region has been used, and the daily values of the observed wind field and specific humidity fields for each individual station at various pressure leveis in the atmosphere have been used for analysis. Monthly means have been computed and the results integrated from the surface to 500 mb to obtain the total precipitable water, the meridio nal and zonal vapour transports and thus the total vector transport field. Finally, the water vapour divergence field has been obtained for the 4 typical months of the year, using a two degree grid and the finite difference method. The relevant equations have been adapted in the followying form for the purpose of computations of the followying. (To see the equations we recommend downloading the file). For the year under study, the monthly rainfall patterns resemble the normal monthly rainfall for almost ali the months except that the month of January shows somewhat higher rainfall amonts in the state of Bahia, Paraíba and their neighbourhood and the month of March depits below normal rainfall in Minas Gerais and the interior southwest. The mean monthly precipitable water, the water vapour flux, and its di vergence are therefore representative of the normal patterns of these quantities for each month. Since most of the water vapour occurs in the lower layers, the water vapour flux field is governed by the convergence and divergence pat terns i.e. the cyclonic and anticyclonic air flow in the lower atmosphere, in addition to seasonal normal air flow fields. Since Atlantic coastal regions are the main source of water vapour, its flux naturally depends on the position and intensity of the south Atlantic high, as also on the position of the I.T.C Z. and southern subtropical convergence zone, bath of which have seasonal nor th-soirth shifts. In summer a heat low develops over the land surface, and in winter fchere is a weak anticyclone. The combined effect of these flows is seen in the monthly water vapour flux fields. The área of Northeast Brazil, under this study, is most of the year is under the influence of the southern subtropical high pressure belt enlongated east-west over Atlantic. The region is thus located on the margin of semipermanent high pressure cells. The large divergence associated with such flows, leads to weak moisture flux along the vertical, and consequent droughts. The water vapour flux divergence fields for four typical months of the year (for each season] i.e. March, June, September and December, have been com puted and analysed. Al though the data is insufficient for a broad generalization, it is found that the convergence patterns are associated with rainfall, and the divergence patterns occur over rain deficient or dry regions. It is clear that similar studies covering more ex,tensive regions of Brazil with a larger number of upper air stations, for a number of years should give us better insight into the role of water vapour and its divergence fields and its application for water budget and precipitation characteristics over extensive geographical areas.