No abstract

International audienceA dataset on raindrop size distribution (DSD) gathered in a coastal site of the Alagoas state in northeastern Brazil is used to analyze some differences between continental and maritime rainfall parameters. The dataset is divided into two subsets. One is composed of rainfall systems coming from the continent and moving eastward (i.e., offshore), representing the continental subset. The other is composed of rainfall systems that developed over the sea and are moving westward (i.e., inshore), representing the maritime subset. The mean conditional rain rate (i.e., for rain rate R > 0) is found to be higher for maritime (4.6 mm h−1) than for continental (3.2 mm h−1) conditions. The coefficient of variation of the conditional rain rate is lower for the maritime (1.75) than for the continental (2.25) subset. The continental and maritime DSDs display significant differences. For drop diameter D smaller than about 2 mm, the number of drops is higher for maritime rain than for continental rain. This reverses for D > 2 mm, in such a way that radar reflectivity factor Z for the maritime case is lower than for the continental case at the same rain rate. These results show that, to estimate precipitation by radar in the coastal area of northeastern Brazil, coefficients of the Z-R relation need to be adapted to the direction of motion of the rain-bearing system, inshore or offshore

A frequência de trovoadas a partir de dados de satélite TRMM e do DECEA foi estudada para o período de 10 anos (de 1998 até 2007), onde se observou que as trovoadas foram mais frequentes nos meses de verão, com maior ocorrência no mês de março. Em anos de El Niño ocorre uma diminuição de cerca de 20% dos casos de trovoadas em Alagoas, mas nos anos normais e de La Niña observa-se um aumento dos dias de trovoadas de até 80%. As trovoadas foram relacionadas, em baixos níveis, aos Alísios com curvatura ciclônica e em altos níveis à presença de VCAN (Vórtice Ciclônico em Altos Níveis), CJNEB (Corrente de Jato no Nordeste do Brasil) e Anticiclone sobre o Estado de Alagoas. As trovoadas se formaram predominantemente a noroeste do VCAN e na região da entrada (lado quente) e na saída (lado frio) da CJNEB. As análises termodinâmicas mostraram que as curvas de temperatura do ar e de temperatura do ponto de orvalho observadas foram semelhantes às curvas previstas, construídas pelas trajetórias das parcelas de ar pelo modelo HYSPLIT, nas previsões até 48 e 24h de antecedência, respectivamente. Os índices LI e CAPE+ previram uma atmosfera muito instável com 48h de antecedência.

Usually a single literature-suggested Z-R relationship, where Z the radar reflectivity factor and R the rain rate, is used for weather radar data interpretation. It is desirable to calculate a Z-R relationship by precipitation type to improve the accuracy of quantitative rainfall rate in case of coexistence of different precipitation types, such as, in the area of precipitation produced from a Mesoscale Convective System (MCS). In general, in the MCS trailing anvil, the stratiform precipitation does not fall as drizzle. Rather, the rainfall can assume significant intensity (~10 mm h-1) with showery character. For that reason, in this study, the precipitations were classified into convective and stratiform type, to produce optimum rainfall estimates. Therefore, Z-R relationships were developed for the Eastern Coast of Northeastern Brazil (NEB) using rainfall raindrop size distribution (DSD) data collected with a disdrometer RD-69, aiming their utilization to start the operation of a weather radar system. In this study, due to operation and maintenance facilities, the disdrometer was installed in the Campus of the Universidade Federal de Alagoas (The Federal University of Alagoas) in Maceió in 2001, 12 months before the complete installation of a new radar system. The DSD was stratified by rainfall rate classes. It is found that the DSD are clearly dependent on the parameters of the analytical distribution functions are, and show a marked monthly variability. The parameters of the frequency distributions are dependent on R. The forms of DSDs are similar but the amount of droplets in each one very strongly. This may be possible due to the short period of data collection or to the intraseasonal rainfall variability. The general relationship for the Eastern Coast of NEB was found to be Z = 176.5 R1.29, with correlation coefficient equal to 0.83. This equation is in accordance with the ones for stratiform rain reported in the literature. We found that the convective rain observed is produced by convective cells imbedded into stratiform cloud layers. However, when separating stratiform and convective rainfall we found that the linear coefficient of the Z-R relation is significantly smaller for convective rainfall than for stratiform one (65 and 167 respectively), but the exponential coefficient is higher for convective than for stratiform (1.69 and 1.26 respectively).