Distribuição de gotas de chuva e a relação Z-R para radar na costa lest do Nordeste do Brasil

Abstract

The large variability of precipitacion intensity and structural characteriztics is the one of the major difficulties for modeling rainfall field. In particular, the raindrop size distributions (DSD) are not yet well known in global manner. Precipitation measurement using passive or active microwaves from space or ground based radar and others scientific domains involves hypotheses about DSD. Then a universal knowledge of DSD characteristics is needed. A tem months dataset collected with a Joss-Waldvogel disdrometer RD69 at Maceió, on the Atlantic Coast of Northeast of Brazil (9° 33' 17,24" S 35° 46' 54,84" W with 80m of altitude), is used to analyze the DSD. First, the DSDs were stratified in eight rain rate classes and then fitted to analytical distributions. The shape of the DSDs is found similar from one class to another, despite that the quantities of raindrop did not settle so much. This result can be justified due to the short period of study or probable explanation caused by the interseasonal variability of the rain intensity. The correlation coefficients between the parameters of na exponential and log-normal distributions, as a function of the rain rate has presented a good results, mainly when they were compared with others results gotten in similar researches. The stratification of the DTGs in rainfall class shows clearly the dependence of the majority of the paramenters of the analytical functions and it was also observed a strong monthly varioation. One notices that all the parameters of the the used distributions (exponantial and log-normal) are dependents of the rain rate R, with exception of σ(geometric standard deviation), that practically remained Constant in all the adjustments. With the purpose to calculate the Z-R relation, distinguishing convective and stratiform rainfall, it was considered only the rain rate (R<mm h-1 – stratiform; R3 10 mm h-1 - convective). In this condition, the results show that the correlation coefficients of the stratiform rain present higher values, than the one for convective rains. The referring general equation to the summation of the tem months data. Without considering the criterion of convective and stratiform rains, got results that is similar to the value found for stratiform rains. This result also was observed when the monthly data had been used. It can be concluded that during the period of study there was a large number of meteorological systems responsible for stratiform rains. However,, analyzing the hyetograms, it was possible to verify that, in all the studied representative events, the convective áreas (cells) were always “imbedded” in the stratiform áreas. Therefore, in this region, a Z-R relation that represents a mixing rainfall must be considered.