Regional Contrast of Mesoscale Convective System Structure prior to and during Monsoon Onset across South America

Abstract

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.

Rainforest-initiated wet season onset over the southern Amazon

“…In situ observations indicate that maximum ET leads the late dry season increase in rainfall (26)(27)(28); however, it has been unclear whether modest increases in ET can contribute sufficient moisture above the ABL at regional scales. The potential influences of aerosols on the dry-to-wet season transition are an additional source of uncertainty (29), because the aerosol climatologies used by most reanalyses neglect or underestimate seasonal and interannual variations in aerosol loading in this region (30,31). It is therefore necessary to examine the dry-to-wet season transition by using observable quantities.…”

CMIP5 Simulations of Low-Level Tropospheric Temperature and Moisture over the Tropical Americas

“…Thermodynamic instability increases as the onset of the rainy season approaches and remains large throughout the wet season (Fu et al 1999;Fu and Li 2004;Fisch et al 2004). Convective activity intensifies on broad ranges of spatial and temporal scales to redistribute the excess of moist static energy accumulated near the surface (Adams and Comrie 1997;Rickenbach et al 2011). Land-atmosphere processes control evapotranspiration and play a significant role on the onset and maintenance of the SAMS (Fu and Li 2004) and NAMS (Gutzler and Preston 1997;Gutzler 2000).…”

Influence of summertime mesoscale convective systems on the heat balance and surface mixed layer dynamics of a large Amazonian hydroelectric reservoir

“…During the wet season (austral summer), the rainfall can exceed 400 mm month −1 , with a peak in January. This period corresponds to the occurrence of the SACZ and ITCZ over South America and the Amazon region, when it is possible to observe an increase in cloud clusters and MCS activity over these areas [ Liebmann et al ., ; Rickenbach et al ., ].…”