Raindrop Size Distribution and Radar Parameters in Coastal Tropical Rain Systems of Northeastern Brazil

Ricardo Sarmento Tenório Instituto de Ciências Atmosféricas, Universidade Federal de Alagoas, Maceió, Alagoas, Brazil

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Marcia Cristina da Silva Moraes Departamento de Meteorologia, Universidade Federal de Campina Grande, Campina Grande, Paraíba, Brazil

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Henri Sauvageot Laboratoire d’Aérologie, Observatoire Midi-Pyrénées, Université Paul Sabatier—Toulouse III, Toulouse, France

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Abstract

A 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.

Corresponding author address: Prof. Dr. Ricardo Sarmento Tenório, Sistema de Radar Meteorológico, ICAT, Universidade Federal de Alagoas, Cidade Universitária, Tabuleiro do Martins, Maceió, Alagoas 57072-970, Brazil. E-mail: ricardo.sarmento@pq.cnpq.br

Abstract

A 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.

Corresponding author address: Prof. Dr. Ricardo Sarmento Tenório, Sistema de Radar Meteorológico, ICAT, Universidade Federal de Alagoas, Cidade Universitária, Tabuleiro do Martins, Maceió, Alagoas 57072-970, Brazil. E-mail: ricardo.sarmento@pq.cnpq.br
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