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Keywords:

  • monsoon;
  • convection;
  • South America

Abstract

Isolated precipitating convection, though a minor contributor to total rainfall in the tropics, is important to regional and seasonal climate variability because its diabatic heating structure is characteristic of the convectively inactive phase of tropical interseasonal oscillations. This study extends a previous analysis of mesoscale convective system (MCS) variability in the South America monsoon system to examine regional differences in the annual cycle of the rainfall and vertical structure of isolated convection over the 10 year period of 1998–2007. The goal is to document the annual variation of shallow and deep isolated convection in order to provide a more complete picture of monsoon onset across South America. Over the 10 year period of 1998–2007, the average rain contribution from isolated convection (compared to MCSs) ranged from 1 to 8% of the total rainfall in the four regions depending on the season. The contribution of rainfall by isolated convection was on the high end of that range during the pre-monsoon months of June to August in the tropical regions. Of the isolated convection rainfall, 4–9% was attributable to shallow ‘warm rain’, with the largest fraction after monsoon onset (December to February) in the coastal Mouth of Amazon region. Mean annual time series of conditional rainfall, lightning activity, 85 GHz ice-scattering signature, and radar echo depth for isolated convective features all suggested that an oceanic regime strongly influences isolated convection at the Mouth of Amazon region in northeastern Brazil. In the interior tropical regions, there was a clear pre-monsoon (August to September) maximum in the vertical intensity of deep isolated convection as indicated by lightning, ice scattering, and radar echo depth. Copyright © 2012 Royal Meteorological Society