TITLE:
Convective Rainfall Systems in the La Plata Basin
AUTHORS:
Augusto J. Pereira Filho, Richard E. Carbone, John D. Tuttle
KEYWORDS:
Satellite-Derived Rainfall, Organized Convection, La Plata Basin, Diurnal Cycle
JOURNAL NAME:
Atmospheric and Climate Sciences,
Vol.4 No.4,
October
31,
2014
ABSTRACT: Hourly rainfall estimates from integrated satellite data are used to build a dynamically based climatology of convectively generated rainfall across the La Plata Basin in South America and adjacent oceans. Herein, the focus of this manuscript is on 20S to 35S, including the Andes cordillera. Emphasis is placed on rainfall resulting from organized convective regimes which are known to produce the majority of seasonal rainfall in Southern South America and other continents. The statistical characteristics of individual events are quantified and examined with respect to regional atmospheric conditions. Among the factors considered are steering winds and wind shear, convective available potential energy (CAPE), localized sensible and latent heat sources over mountains and wetlands (Chaco), and the occurrence of baroclinic waves such as mid-latitude jet stream transient disturbances. Forcing and convective triggering mechanisms are inferred from the diagnosis of systematic patterns as evidenced in the continental diurnal cycle and longer periods of natural variability. The diurnal cycle of rainfall is especially informative with respect to the frequency and phase of rainfall associated with long-lived propagating rainfall “episodes”. Similar to findings in tropical northern Africa and tropical northern Australia, there is a strong presence of organized convection, which can propagate zonally hundreds to thousands of km as a coherent sequence of mesoscale convective systems. Convective triggering is often associated with elevated terrain, the Andes, and the La Plata basin region, which is especially rich in moist static energy. The passage of baroclinic waves over the Andes is consistent with eastward propagating clusters of convection, within which westward-propagating systems also reside. These organized convective systems over the La Plata Basin are analyzed with hourly rainfall estimates with CMOPRH method. Rainfall estimates at 8-km spatial resolution were obtained between December 2002 and June 2008. Very few data are missing so it is one of the most complete, longest and highest resolution data sets available to date that allows a comprehensive description of spatial and temporal distribution of convection from its hourly to interannual variability over the region. In this work, diurnal, intra and inter seasonal and interannual cycles are obtained and examined in the light of episodes of organized convection. Daily, monthly and yearly spatial patterns of rainfall accumulation over the La Plata Basin region vary both inter- and intra-seasonally and are forced by underlying dynamic and thermodynamics mechanisms. Time-longitude diagrams of CMORPH hourly rainfall are used to describe the genesis, structure, longevity, phase speed and inferences of the underlying dynamics and thermodynamics of episodes of organized convection. The episodes of organized convection are analyzed in terms of their duration, span, phase speed, starting and ending time, starting and ending longitude, month and year through frequency distribution analysis. Most episodes of organized convection move eastward across the La Plata Basin with variable phase speeds. Basic descriptive statistics indicate that the La Plata eastward propagating average phase speed is 13.0 m·s-1.