This article describes results from a new four-year (2009-2012) radar-based precipitation climatology for the southeastern United States (SE USA). The climatology shows that a size-based classification between mesoscale precipitation features (MPF) and isolated precipitation reveals distinct seasonal and diurnal variability of precipitation. On average, from 70 to 90% of precipitation is associated with MPF, generally less in the summertime and in southern coastal regions. MPF precipitation has a relatively small seasonal cycle except in Florida and the warm offshore waters of the Gulf Stream. In contrast, isolated precipitation has a dramatic seasonal cycle that outlines the SE USA coastline whereas the MPF precipitation does not, consistent with a thermodynamic mechanism for onshore isolated storms in coastal regions. In summer, the isolated precipitation preferentially forms offshore at night, and dramatically 'flips' inland by early afternoon. In contrast, MPF precipitation has no clear diurnal variations except in the southern coastal region in the summer, likely associated with sea breeze convection organized on the mesoscale. These results suggest that the MPF versus isolated precipitation system framework provides a useful basis for future studies of large-scale and local controls on precipitation and resulting implications for long-range predictability of precipitation.

The evolution of monsoon onset across South America has complex temporal and regional variability that are controlled by local and remote land-ocean-atmosphere processes. In this study, a three-stage conceptual model for the onset of the South American monsoon season is proposed based on a rain threshold analysis and a rotated empirical orthogonal function (REOF) analysis of the Global Precipitation Climatology Project version 2 (GPCP-v2) dataset. This two-pronged approach allowed the identification of regions of South America that share a common seasonal cycle of rainfall variability and likely have a common mechanism for monsoon onset.According to this model, the first stage of onset starts around pentad 59 (October 18-22) when precipitation begins in the northwestern part of the continent and gradually progresses towards the south and southeast. The second stage is marked by the abrupt establishment of the South Atlantic Convergence Zone (SACZ). This stage occurs on average around pentad 61 (October 28-November 1). The third stage of monsoon onset involves the late arrival of the monsoon to the mouth of the Amazon River, associated with the slow migration of the Atlantic Intertropical Convergence Zone (ITCZ). This final stage of onset occurs on average by pentad 73 (December 27-31). This three-stage model of onset provides a useful framework for the study of regional differences in monsoon onset mechanisms, a subject that is further explored in two companion studies.

The onset of the South American monsoon season culminates with the abrupt establishment of the South American convergence zone (SACZ). The impact of cold fronts on the abrupt establishment of the SACZ is studied using an 11-year composite analysis of the dynamic and thermodynamic structures, intensity and propagation of cold fronts that occur prior to, during, and after monsoon onset in the SACZ. A significant change in the structure and propagation of cold fronts is observed at the time of monsoon onset in the SACZ, with cold fronts suddenly stalling and becoming stationary in southeastern Brazil. It is proposed that this regime change of the structure and propagation of cold fronts causes the abrupt onset of the monsoon in the SACZ. A mechanism for this sudden change in cold

In this study, a 10-yr (1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007) climatology of observations from the Tropical Rainfall Measuring Mission (TRMM) satellite is used to study regional mechanisms of monsoon onset across tropical and subtropical South America. The approach is to contrast regional differences in the structure, intensity, and rainfall of mesoscale convective systems (MCSs) prior to and after onset, in the context of thermodynamic conditions from the National Centers for Environmental Prediction (NCEP) reanalysis data. This is accomplished by analyzing the mean annual cycle time series, 10-yr frequency histograms, and 3-month-averaged values prior to and following onset in four regions of distinct rainfall variability. Observed MCS metrics and NCEP variables include lightning flash rate, convective rain fraction, height of the 30-dBZ isosurface, minimum 85-GHz polarization corrected temperature, and the fluxes of sensible and latent heat.The west-central Amazon region had a distinct maximum of MCS intensity 2 months prior to the monsoon onset date of each region, which was well correlated with surface sensible heat flux, despite the observation that thermodynamic instability was greatest after onset. At the mouth of the Amazon, the dry season rainfall minimum, the premonsoon maximum in MCS intensity metrics, and monsoon onset were all delayed by 2-3 months relative to the west-central Amazon. This delay in the annual cycle and comparatively large difference in pre-versus postonset MCSs, combined with previous work, suggest that the slow migration of the Atlantic Ocean intertropical convergence zone controls onset characteristics at the mouth of the Amazon. All metrics of convective intensity in the tropical regions decreased significantly following onset. These results, in the context of previous studies, are consistent with the hypothesis that thermodynamic, land surface, and aerosol controls on MCS intensity operate in concert with each other to control the evolution of precipitation system structure from the dry season to the wet season. The other two regions [the South Atlantic convergence zone (SACZ) and the south], associated with the well-documented dipole of intraseasonal rain variability, have a weaker and more variable annual cycle of all MCS metrics. This is likely related to the strong influence of baroclinic circulations and frontal systems in those regions. In the south, fewer but larger and more electrified MCSs prior to onset transition to more, smaller, and less electrified MCSs after onset, consistent with previous climatologies of strong springtime mesoscale convective complexes in that region.

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.